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Sample records for wake vortex model

  1. Review of Idealized Aircraft Wake Vortex Models

    NASA Technical Reports Server (NTRS)

    Ahmad, Nashat N.; Proctor, Fred H.; Duparcmeur, Fanny M. Limon; Jacob, Don

    2014-01-01

    Properties of three aircraft wake vortex models, Lamb-Oseen, Burnham-Hallock, and Proctor are reviewed. These idealized models are often used to initialize the aircraft wake vortex pair in large eddy simulations and in wake encounter hazard models, as well as to define matched filters for processing lidar observations of aircraft wake vortices. Basic parameters for each vortex model, such as peak tangential velocity and circulation strength as a function of vortex core radius size, are examined. The models are also compared using different vortex characterizations, such as the vorticity magnitude. Results of Euler and large eddy simulations are presented. The application of vortex models in the postprocessing of lidar observations is discussed.

  2. Multi-Model Ensemble Wake Vortex Prediction

    NASA Technical Reports Server (NTRS)

    Koerner, Stephan; Ahmad, Nash'at N.; Holzaepfel, Frank; VanValkenburg, Randal L.

    2015-01-01

    Several multi-model ensemble methods are investigated for predicting wake vortex transport and decay. This study is a joint effort between National Aeronautics and Space Administration and Deutsches Zentrum fuer Luft- und Raumfahrt to develop a multi-model ensemble capability using their wake models. An overview of different multi-model ensemble methods and their feasibility for wake applications is presented. The methods include Reliability Ensemble Averaging, Bayesian Model Averaging, and Monte Carlo Simulations. The methodologies are evaluated using data from wake vortex field experiments.

  3. On point vortex models of exotic bluff body wakes

    NASA Astrophysics Data System (ADS)

    Stremler, Mark A.; Basu, Saikat

    2014-12-01

    Exotic vortex wakes, in which three or more vortices are generated during each shedding cycle, are frequently found in the wake of an oscillating bluff body. Two common examples are P+S wakes (with 3 vortices) and 2P wakes (with 4 vortices). We consider mathematical models of these wakes consisting of N = 3 or 4 point vortices with constant strengths in an inviscid fluid that is otherwise at rest in a singly-periodic domain. By enforcing constraints on the vortex strengths and, in the case of N = 4, on the symmetry of the vortex locations, the mathematical models reduce to integrable Hamiltonian systems. We compare the point vortex trajectories with two exotic wake patterns reported in the literature. Results support the use of point vortex modeling to investigate vortex dynamics in exotic wakes and suggest the need for additional classification of experimental wake patterns.

  4. Wake Vortex Inverse Model User's Guide

    NASA Technical Reports Server (NTRS)

    Lai, David; Delisi, Donald

    2008-01-01

    NorthWest Research Associates (NWRA) has developed an inverse model for inverting landing aircraft vortex data. The data used for the inversion are the time evolution of the lateral transport position and vertical position of both the port and starboard vortices. The inverse model performs iterative forward model runs using various estimates of vortex parameters, vertical crosswind profiles, and vortex circulation as a function of wake age. Forward model predictions of lateral transport and altitude are then compared with the observed data. Differences between the data and model predictions guide the choice of vortex parameter values, crosswind profile and circulation evolution in the next iteration. Iterations are performed until a user-defined criterion is satisfied. Currently, the inverse model is set to stop when the improvement in the rms deviation between the data and model predictions is less than 1 percent for two consecutive iterations. The forward model used in this inverse model is a modified version of the Shear-APA model. A detailed description of this forward model, the inverse model, and its validation are presented in a different report (Lai, Mellman, Robins, and Delisi, 2007). This document is a User's Guide for the Wake Vortex Inverse Model. Section 2 presents an overview of the inverse model program. Execution of the inverse model is described in Section 3. When executing the inverse model, a user is requested to provide the name of an input file which contains the inverse model parameters, the various datasets, and directories needed for the inversion. A detailed description of the list of parameters in the inversion input file is presented in Section 4. A user has an option to save the inversion results of each lidar track in a mat-file (a condensed data file in Matlab format). These saved mat-files can be used for post-inversion analysis. A description of the contents of the saved files is given in Section 5. An example of an inversion input file, with preferred parameters values, is given in Appendix A. An example of the plot generated at a normal completion of the inversion is shown in Appendix B.

  5. Updated Results for the Wake Vortex Inverse Model

    NASA Technical Reports Server (NTRS)

    Robins, Robert E.; Lai, David Y.; Delisi, Donald P.; Mellman, George R.

    2008-01-01

    NorthWest Research Associates (NWRA) has developed an Inverse Model for inverting aircraft wake vortex data. The objective of the inverse modeling is to obtain estimates of the vortex circulation decay and crosswind vertical profiles, using time history measurements of the lateral and vertical position of aircraft vortices. The Inverse Model performs iterative forward model runs using estimates of vortex parameters, vertical crosswind profiles, and vortex circulation as a function of wake age. Iterations are performed until a user-defined criterion is satisfied. Outputs from an Inverse Model run are the best estimates of the time history of the vortex circulation derived from the observed data, the vertical crosswind profile, and several vortex parameters. The forward model, named SHRAPA, used in this inverse modeling is a modified version of the Shear-APA model, and it is described in Section 2 of this document. Details of the Inverse Model are presented in Section 3. The Inverse Model was applied to lidar-observed vortex data at three airports: FAA acquired data from San Francisco International Airport (SFO) and Denver International Airport (DEN), and NASA acquired data from Memphis International Airport (MEM). The results are compared with observed data. This Inverse Model validation is documented in Section 4. A summary is given in Section 5. A user's guide for the inverse wake vortex model is presented in a separate NorthWest Research Associates technical report (Lai and Delisi, 2007a).

  6. Evaluation of Fast-Time Wake Vortex Models using Wake Encounter Flight Test Data

    NASA Technical Reports Server (NTRS)

    Ahmad, Nashat N.; VanValkenburg, Randal L.; Bowles, Roland L.; Limon Duparcmeur, Fanny M.; Gloudesman, Thijs; van Lochem, Sander; Ras, Eelco

    2014-01-01

    This paper describes a methodology for the integration and evaluation of fast-time wake models with flight data. The National Aeronautics and Space Administration conducted detailed flight tests in 1995 and 1997 under the Aircraft Vortex Spacing System Program to characterize wake vortex decay and wake encounter dynamics. In this study, data collected during Flight 705 were used to evaluate NASA's fast-time wake transport and decay models. Deterministic and Monte-Carlo simulations were conducted to define wake hazard bounds behind the wake generator. The methodology described in this paper can be used for further validation of fast-time wake models using en-route flight data, and for determining wake turbulence constraints in the design of air traffic management concepts.

  7. Wake Vortex Encounter Research

    NASA Technical Reports Server (NTRS)

    Wilson, Jeffrey M.

    1995-01-01

    The National Aeronautics and Space Administration (NASA) is conducting research to improve airport capacity by reducing the separation distance between aircraft. The limiting factor in reducing separation distances and improving airport capacity is the wake vortex hazard. The ability to accurately model wake vortices and predict the outcome of a vortex encounter is critical in developing a system to safely improve airport capacity. This is the focus of the wake vortex research being done at NASA Langley Research Center (LaRC). This paper will concentrate on two topics. The first topic is the control system developed for the Boeing 737 freeflight model in support of vortex encounter tests to be conducted in the 30- by 60- foot tunnel at NASA Langley Research Center later this year. The second topic discussed is the limited degree of freedom (DOF) trajectory generation study that is being conducted to determine the relative severity of a multitude of paths through a wake vortex.

  8. NASA wake vortex research

    NASA Technical Reports Server (NTRS)

    Stough, H. P., III; Greene, George C.; Stewart, Eric C.; Stuever, Robert A.; Jordan, Frank L., Jr.; Rivers, Robert A.; Vicroy, Dan D.

    1993-01-01

    NASA is conducting research that will enable safe improvements in the capacity of the nation's air transportation system. The wake-vortex hazard is a factor in establishing the minimum safe spacing between aircraft during landing and takeoff operations and, thus, impacts airport capacity. The ability to accurately model the wake hazard and determine safe separation distances for a wide range of aircraft and operational scenarios may provide the basis for significant increases in airport capacity. Current and planned NASA research is described which is focused on increasing airport capacity by safely reducing wake-hazard-imposed aircraft separations through advances in a number of technologies including vortex motion and decay prediction, vortex encounter modeling, wake-vortex hazard characterization, and in situ flow sensing.

  9. Development of a rotor wake-vortex model, volume 1

    NASA Technical Reports Server (NTRS)

    Majjigi, R. K.; Gliebe, P. R.

    1984-01-01

    Certain empirical rotor wake and turbulence relationships were developed using existing low speed rotor wave data. A tip vortex model was developed by replacing the annulus wall with a row of image vortices. An axisymmetric turbulence spectrum model, developed in the context of rotor inflow turbulence, was adapted to predicting the turbulence spectrum of the stator gust upwash.

  10. Wake Vortex Minimization

    NASA Technical Reports Server (NTRS)

    1977-01-01

    A status report is presented on research directed at reducing the vortex disturbances of aircraft wakes. The objective of such a reduction is to minimize the hazard to smaller aircraft that might encounter these wakes. Inviscid modeling was used to study trailing vortices and viscous effects were investigated. Laser velocimeters were utilized in the measurement of aircraft wakes. Flight and wind tunnel tests were performed on scale and full model scale aircraft of various design. Parameters investigated included the effect of wing span, wing flaps, spoilers, splines and engine thrust on vortex attenuation. Results indicate that vortives may be alleviated through aerodynamic means.

  11. Evaluation of a Wake Vortex Upset Model Based on Simultaneous Measurements of Wake Velocities and Probe-Aircraft Accelerations

    NASA Technical Reports Server (NTRS)

    Short, B. J.; Jacobsen, R. A.

    1979-01-01

    Simultaneous measurements were made of the upset responses experienced and the wake velocities encountered by an instrumented Learjet probe aircraft behind a Boeing 747 vortex-generating aircraft. The vortex-induced angular accelerations experienced could be predicted within 30% by a mathematical upset response model when the characteristics of the wake were well represented by the vortex model. The vortex model used in the present study adequately represented the wake flow field when the vortices dissipated symmetrically and only one vortex pair existed in the wake.

  12. Modeling of Wake-vortex Aircraft Encounters. Appendix B

    NASA Technical Reports Server (NTRS)

    Smith, Sonya T.

    1999-01-01

    There are more people passing through the world's airports today than at any other time in history. With this increase in civil transport, airports are becoming capacity limited. In order to increase capacity and thus meet the demands of the flying public, the number of runways and number of flights per runway must be increased. In response to the demand, the National Aeronautics and Space Administration (NASA), in conjunction with the Federal Aviation Administration (FAA), airport operators, and the airline industry are taking steps to increase airport capacity without jeopardizing safety. Increasing the production per runway increases the likelihood that an aircraft will encounter the trailing wake-vortex of another aircraft. The hazard of a wake-vortex encounter is that heavy load aircraft can produce high intensity wake turbulence, through the development of its wing-tip vortices. A smaller aircraft following in the wake of the heavy load aircraft will experience redistribution of its aerodynamic load. This creates a safety hazard for the smaller aircraft. Understanding this load redistribution is of great importance, particularly during landing and take-off. In this research wake-vortex effects on an encountering 10% scale model of the B737-100 aircraft are modeled using both strip theory and vortex-lattice modeling methods. The models are then compared to wind tunnel data that was taken in the 30ft x 60ft wind tunnel at NASA Langley Research Center (LaRC). Comparisons are made to determine if the models will have acceptable accuracy when parts of the geometry are removed, such as the horizontal stabilizer and the vertical tail. A sensitivity analysis was also performed to observe how accurately the models could match the experimental data if there was a 10% error in the circulation strength. It was determined that both models show accurate results when the wing, horizontal stabilizer, and vertical tail were a part of the geometry. When the horizontal stabilizer and vertical tail were removed there were difficulties modeling the sideforce coefficient and pitching moment. With the removal of only the vertical tail unacceptable errors occurred when modeling the sideforce coefficient and yawing moment. Lift could not be modeled with either the full geometry or the reduced geometry attempts.

  13. Model Validation of Wake-Vortex/Aircraft Encounters

    NASA Technical Reports Server (NTRS)

    Pete, Kimberly R.; Vicroy, Dan D.; Smith, Sonya T.

    2000-01-01

    Wake-vortex effects on an 10% scale model of the B737-100 aircraft are calculated using both strip theory and vortex-lattice methods. The results are then compared to data taken in the 30ft x 60ft wind tunnel at NASA Langley Research Center (LaRC). The accuracy of the models for a reduced geometry, such with the horizontal stabilizer and the vertical tail removed, is also investigated. Using a 10% error in the circulation strength and comparing the model's results with the experiment illustrates the sensitivity of the models to the vortex circulation strength. It was determined that both strip theory and the vortex lattice method give accurate results when all the geometrical information is used. When the horizontal stabilizer and vertical tail were removed there were difficulties modeling the sideforce coefficient and pitching moment. With the removal of only the vertical tail unacceptable errors occurred when modeling the sideforce coefficient and yawing moment. Lift could not be accurately modeled with either the full geometry or the reduced geometry.

  14. Vortex Wake Geometry of a Model Tilt Rotor in Forward Flight

    NASA Technical Reports Server (NTRS)

    Yamauchi, G. K.; Johnson, W.; Wadcock, A. J.

    2002-01-01

    The vortex wake trajectory from one rotor of a 0.25-scale V-22 tiltrotor model was measured for four test conditions in the NASA Ames 40- by 80-Foot Wind Tunnel. Vortex wake images were acquired using a laser light sheet and video camera. Wake trajectories were constructed by extracting vortex positions from the video images. Wake trajectories were also calculated using the comprehensive analysis CAMRAD II. Measured and calculated wake geometries exhibit similar trends when advance ratio is varied at fixed thrust or when thrust is varied at fixed advance ratio.

  15. Coupling of a free wake vortex ring near-wake model with the Jensen and Larsen far-wake deficit models

    NASA Astrophysics Data System (ADS)

    van Heemst, J. W.; Baldacchino, D.; Mehta, D.; van Bussel, G. J. W.

    2015-06-01

    This paper presents a simple physical model to improve the currently used far-wake deficit models in the wind industry. The main improvement is deemed on the determination of the wake deficit in the near-wake. A Vortex Ring Model (VRM) is used to calculate the induced velocities in the near-wake, which are then coupled to the Jensen far-wake model and the Larsen far-wake model based on the concept of Eddy Viscosity (EV). The inviscid near-wake VRM is based on the shedding of discrete tip vortex rings released from a uniformly loaded actuator disc. The model is validated against wind tunnel measurements from experiments with a two- bladed turbine and a circular metal mesh with a uniform porosity to represent an actuator disc. The VRM shows a good agreement with the experimental data with respect to the wake deficit evolution. The VRM is coupled with two well-known engineering type far-wake models: the Jensen and Larsen wake deficit models. The results of the coupling of the VRM and the more elaborated Larsen far-wake model are compared against a 3D Large Eddy Simulation (LES) CFD model. This comparison shows the effect of different near-wake models on the development of centreline velocities in the far-wake. The centreline velocity deficit predicted by the VRM-Larsen model more closely matches LES calculations in comparison with the reference Larsen model.

  16. Documentation for Three Wake Vortex Model Data Sets from Simulation of Flight 587 Wake Vortex Encounter Accident Case

    NASA Technical Reports Server (NTRS)

    Switzer, George F.

    2008-01-01

    This document contains a general description for data sets of a wake vortex system in a turbulent environment. The turbulence and thermal stratification of the environment are representative of the conditions on November 12, 2001 near John F. Kennedy International Airport. The simulation assumes no ambient winds. The full three dimensional simulation of the wake vortex system from a Boeing 747 predicts vortex circulation levels at 80% of their initial value at the time of the proposed vortex encounter. The linked vortex oval orientation showed no twisting, and the oval elevations at the widest point were about 20 meters higher than where the vortex pair joined. Fred Proctor of NASA?s Langley Research Center presented the results from this work at the NTSB public hearing that started 29 October 2002. This document contains a description of each data set including: variables, coordinate system, data format, and sample plots. Also included are instructions on how to read the data.

  17. Evaluation of Fast-Time Wake Vortex Prediction Models

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Hamilton, David W.

    2009-01-01

    Current fast-time wake models are reviewed and three basic types are defined. Predictions from several of the fast-time models are compared. Previous statistical evaluations of the APA-Sarpkaya and D2P fast-time models are discussed. Root Mean Square errors between fast-time model predictions and Lidar wake measurements are examined for a 24 hr period at Denver International Airport. Shortcomings in current methodology for evaluating wake errors are also discussed.

  18. Free Wake Techniques for Rotor Aerodynamic Analylis. Volume 2: Vortex Sheet Models

    NASA Technical Reports Server (NTRS)

    Tanuwidjaja, A.

    1982-01-01

    Results of computations are presented using vortex sheets to model the wake and test the sensitivity of the solutions to various assumptions used in the development of the models. The complete codings are included.

  19. The NASA-Langley Wake Vortex Modelling Effort in Support of an Operational Aircraft Spacing System

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.

    1998-01-01

    Two numerical modelling efforts, one using a large eddy simulation model and the other a numerical weather prediction model, are underway in support of NASA's Terminal Area Productivity program. The large-eddy simulation model (LES) has a meteorological framework and permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, humidity, and atmospheric turbulence. Results from the numerical simulations are being used to assist in the development of algorithms for an operational wake-vortex aircraft spacing system. A mesoscale weather forecast model is being adapted for providing operational forecast of winds, temperature, and turbulence parameters to be used in the terminal area. This paper describes the goals and modelling approach, as well as achievements obtained to date. Simulation results will be presented from the LES model for both two and three dimensions. The 2-D model is found to be generally valid for studying wake vortex transport, while the 3-D approach is necessary for realistic treatment of decay via interaction of wake vortices and atmospheric boundary layer turbulence. Meteorology is shown to have an important affect on vortex transport and decay. Presented are results showing that wake vortex transport is unaffected by uniform fog or rain, but wake vortex transport can be strongly affected by nonlinear vertical change in the ambient crosswind. Both simulation and observations show that atmospheric vortices decay from the outside with minimal expansion of the core. Vortex decay and the onset three-dimensional instabilities are found to be enhanced by the presence of ambient turbulence.

  20. Development of a rotor wake/vortex model. Volume 2: User's manual for computer program

    NASA Technical Reports Server (NTRS)

    Majjigi, R. K.; Gliebe, P. R.

    1984-01-01

    The principal objective was to establish a verified rotor wake/vortex model for specific application to fan and compressor rotor-stator interaction and resulting noise generation. A description and flow chart of the Rotor Wake/Vortex Model computer program, a listing of the program, definitions of the input/output parameters, a sample input/output case, and input files for Rotor 55, the JT15D rotor, and Rotor 67, Stage 1 are provided.

  1. Wind turbine wake stability investigations using a vortex ring modelling approach

    NASA Astrophysics Data System (ADS)

    Baldacchino, Daniel; van Bussel, Gerard J. W.

    2014-12-01

    In the present study, a simple inviscid vortex ring (VR) modelling approach is used to represent the developing rotor wake. This allows a straightforward investigation and comparison of the impact of uniform, yawed and sheared flow conditions on the development of the rotor wake, with the additional possibility of including ground effect. The effect of instabilities on the development of the wake is manually introduced in the form of perturbations of strength, ring position and size. The phenomenon of vortex filament interaction or leapfrogging, could play a role in the observation of unsteady phenomena and is therefore also addressed. Such a study is hence performed in light of recent conflicting views on the causes of wake meandering: is the observed dynamic wake behaviour a result of large scale turbulent forcing or do more subtle and intrinsic wake instabilities play a role? This study concludes that the presence of the ground and external perturbations, most notably changes in the wake pitch and the rotor thrust coefficient, can significantly affect the steady development of the wake. The mutual vortex pairing instability, whilst displaying interesting periodic behaviour, does not correlate with periodic wake behaviour reported by Medici et al. [1]. However, in the absence of unsteady inflow, it is shown that the wake of a Horizontal Axis Wind Turbine (HAWT) is certainly prone to displaying unstable, dynamic behaviour caused by these additional factors.

  2. Passive Wake Vortex Control

    SciTech Connect

    Ortega, J M

    2001-10-18

    The collapse of the Soviet Union and ending of the Cold War brought about many significant changes in military submarine operations. The enemies that the US Navy faces today and in the future will not likely be superpowers armed with nuclear submarines, but rather smaller, rogue nations employing cheaper diesel/electric submarines with advanced air-independent propulsion systems. Unlike Cold War submarine operations, which occurred in deep-water environments, future submarine conflicts are anticipated to occur in shallow, littoral regions that are complex and noisy. Consequently, non-acoustic signatures will become increasingly important and the submarine stealth technology designed for deep-water operations may not be effective in these environments. One such non-acoustic signature is the surface detection of a submarine's trailing vortex wake. If a submarine runs in a slightly buoyant condition, its diving planes must be inclined at a negative angle of attack to generate sufficient downforce, which keeps the submarine from rising to the surface. As a result, the diving planes produce a pair of counter-rotating trailing vortices that propagate to the water surface. In previous deep-water operations, this was not an issue since the submarines could dive deep enough so that the vortex pair became incoherent before it reached the water surface. However, in shallow, littoral environments, submarines do not have the option of diving deep and, hence, the vortex pair can rise to the surface and leave a distinct signature that might be detectable by synthetic aperture radar. Such detection would jeopardize not only the mission of the submarine, but also the lives of military personnel on board. There has been another attempt to solve this problem and reduce the intensity of trailing vortices in the wakes of military submarines. The research of Quackenbush et al. over the past few years has been directed towards an idea called ''vortex leveraging.'' This active concept works by placing shape memory alloy (SMA) control surfaces on the submarine's diving planes and periodically oscillating them. The modulated control vortices generated by these surfaces interact with the tip vortices on the diving planes, causing an instability to rapidly occur. Though several numerical simulations have been presented, experimental verification does not appear to be available in the open literature. The authors address this problem through a concept called passive wake vortex control (PWVC), which has been demonstrated to rapidly break apart a trailing vortex wake and render it incoherent. PWVC functions by introducing unequal strength, counter-rotating control vortices next to the tip vortices. The presence of these control vortices destabilizes the vortex wake and produces a rapidly growing wake instability.

  3. A three dimensional vortex wake model for missiles at high angles on attack

    NASA Technical Reports Server (NTRS)

    Sheffield, J. S.; Deffenbaugh, F. D.

    1980-01-01

    A three dimensional model for the steady flow past missile and aircraft nose shaped bodies is presented based on augmenting a potential solution with a wake composed of vortex filaments. The vortex positions are determined by the requirement that they, in some sense, align with the flow. The aerodynamic loads on the body are compared with experimental values and used to evaluate the model. The vortex positions compare well with flow visualization results for slender bodies at high angles of attack. The approximations in the wake near the body cause peaks in the force distributions more severe than in the measured values. For given vortex strengths and body attachment points multiple steady vortex positions were not found.

  4. A vortex pair model for plume downwash into stack wakes

    NASA Astrophysics Data System (ADS)

    Johnston, C. R.; Wilson, D. J.

    A model is proposed that uses a streamwise vortex pair embedded in the plume to produce a downwash velocity that varies with downwind distance and competes with the trajectory centerline plume rise velocity. This streamwise vortex pair is an idealization of the connection back to the stack tip of the vertical von Karman vortices shed from the stack. The model is compared with the measurements of trajectories from non-buoyant momentum jets injected from a circular tube with subcritical external Reynolds number into a uniform cross flow in a water channel. Fluorescent dye and laser sheet-lighting were used to define the trajectories over a range of effluent to crosswind velocity ratios from 0.25 to 8.0. With an empirical function to account for the efficiency with which the jet entrains vorticity from the stack-tip vortex pair, the model accurately predicts the plume downwash effects, and accounts for the continuous downward deflection of the jet trajectory.

  5. Nested contour-dynamic models for axisymmetric vortex rings and vortex wakes

    NASA Astrophysics Data System (ADS)

    O'Farrell, Clara; Dabiri, John O.

    2013-11-01

    Jetting swimmers, such as squid and jellyfish, propel themselves by forming vortex rings. It is known that vortex rings cannot grow indefinitely, but rather ``pinch off'' once they reach their physical limit, and that a decrease in efficiency of fluid transport is associated with pinch-off. Previously, the Norbury family of vortices has been used as a model for axisymmetric vortex rings, and the response of this family to shape perturbations has been characterized. We improve upon the Norbury models, using nested patches of vorticity to construct a family of models for vortex rings generated by a piston-cylinder apparatus at different stroke ratios. The perturbation response of this family is considered by the introduction of a small region of vorticity at the rear of the vortex, which mimics the addition of circulation to a growing vortex ring by a feeding shear layer. Model vortex rings are found to either accept the additional circulation or shed it into a tail, depending on the perturbation size. A change in the behavior of the model vortex rings is identified at a stroke ratio of three. We hypothesize that this change in response is analogous to pinch-off, and that pinch-off might be understood and predicted based on the perturbation responses of model vortex rings.

  6. Vortex-wake interactions of a flapping foil that models animal swimming and flight.

    PubMed

    Lentink, David; Muijres, Florian T; Donker-Duyvis, Frits J; van Leeuwen, Johan L

    2008-01-01

    The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda*=4-24) at a constant dimensionless flapping amplitude (A*=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda*<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von Kármán-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically. PMID:18165254

  7. Models of Wake-Vortex Spreading Mechanisms and Their Estimated Uncertainties

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.; Hardy, Gordon H.; Meyn, Larry A.

    2006-01-01

    One of the primary constraints on the capacity of the nation's air transportation system is the landing capacity at its busiest airports. Many airports with nearly-simultaneous operations on closely-spaced parallel runways (i.e., as close as 750 ft (246m)) suffer a severe decrease in runway acceptance rate when weather conditions do not allow full utilization. The objective of a research program at NASA Ames Research Center is to develop the technologies needed for traffic management in the airport environment so that operations now allowed on closely-spaced parallel runways under Visual Meteorological Conditions can also be carried out under Instrument Meteorological Conditions. As part of this overall research objective, the study reported here has developed improved models for the various aerodynamic mechanisms that spread and transport wake vortices. The purpose of the study is to continue the development of relationships that increase the accuracy of estimates for the along-trail separation distances available before the vortex wake of a leading aircraft intrudes into the airspace of a following aircraft. Details of the models used and their uncertainties are presented in the appendices to the paper. Suggestions are made as to the theoretical and experimental research needed to increase the accuracy of and confidence level in the models presented and instrumentation required or more precise estimates of the motion and spread of vortex wakes. The improved wake models indicate that, if the following aircraft is upwind of the leading aircraft, the vortex wakes of the leading aircraft will not intrude into the airspace of the following aircraft for about 7s (based on pessimistic assumptions) for most atmospheric conditions. The wake-spreading models also indicate that longer time intervals before wake intrusion are available when atmospheric turbulence levels are mild or moderate. However, if the estimates for those time intervals are to be reliable, further study is necessary to develop the instrumentation and procedures needed to accurately define when the more benign atmospheric conditions exist.

  8. Investigation of Wake-Vortex Aircraft Encounters

    NASA Technical Reports Server (NTRS)

    Smith, Sonya T.

    1999-01-01

    The National Aeronautics and Space Administration is addressing airport capacity enhancements during instrument meteorological conditions though the Terminal Area Productivity (TAP) program. The major goal of the TAP program is to develop the technology that will allow air traffic levels during instrument meteorological condition to approach those achieved during visual operations. The Reduced Spacing Operations (RSO) subelement of TAP at the NASA Langley Research Center (LaRC) will develop the Aircraft Vortex Spacing System (AVOSS). The purpose of the AVOSS is to integrate current and predicted weather conditions, wake vortex transport and decay knowledge, wake vortex sensor data, and operational definitions of acceptable strengths for vortex encounters to produce dynamic wake vortex separation criteria. The proposed research is in support of the wake vortex hazard definition component of the LaRC AVOSS development research. The research program described in the next section provided an analysis of the static test data and uses this data to evaluate the accuracy vortex/wake-encounter models. The accuracy of these models has not before been evaluated using experimental data. The research results also presented the first analysis of the forces and moments imparted on an airplane during a wake vortex encounter using actual flight test data.

  9. Vortex interactions and decay in aircraft wakes

    NASA Technical Reports Server (NTRS)

    Bilanin, A. J.; Teske, M. E.; Dupdonaldson, C.; Williamson, G. G.

    1977-01-01

    The dynamic interaction of aircraft wake vortices was investigated using both inviscid and viscous models. For the viscous model, a computer code was developed using a second-order closure model of turbulent transport. The phenomenon of vortex merging which results in the rapid aging of a vortex wake was examined in detail. It was shown that the redistribution of vorticity during merging results from both convective and diffusive mechanisms.

  10. Wake Vortex Prediction Models for Decay and Transport Within Stratified Environments

    NASA Technical Reports Server (NTRS)

    Switzer, George F.; Proctor, Fred H.

    2002-01-01

    This paper proposes two simple models to predict vortex transport and decay. The models are determined empirically from results of three-dimensional large eddy simulations, and are applicable to wake vortices out of ground effect and not subjected to environmental winds. The results, from the large eddy simulations assume a range of ambient turbulence and stratification levels. The models and the results from the large eddy simulations support the hypothesis that the decay of the vortex hazard is decoupled from its change in descent rate.

  11. Numerical Modeling Studies of Wake Vortex Transport and Evolution Within the Planetary Boundary Layer

    NASA Technical Reports Server (NTRS)

    Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.; Han, Jongil

    2000-01-01

    The fundamental objective of this research is study behavior of aircraft wake vortices within atmospheric boundary layer (ABL) in support of developing the system, Aircraft VOrtex Spacing System (AVOSS), under NASA's Terminal Area Productivity (TAR) program that will control aircraft spacing within the narrow approach corridors of airports. The purpose of the AVOSS system is to increase airport capacity by providing a safe reduction in separation of aircraft compared to the now-existing flight rules. In our first funding period (7 January 19994 - 6 April 1997), we have accomplished extensive model development and validation of ABL simulations. Using the validated model, in our second funding period (7 April 1997 - 6 April 2000) we have investigated the effects of ambient atmospheric turbulence on vortex decay and descent, Crow instability, and wake vortex interaction with the ground. Recognizing the crucial influence of ABL turbulence on wake vortex behavior, we have also developed a software generating vertical profiles of turbulent kinetic energy (TKE) or energy dissipation rate (EDR), which are, in turn, used as input data in the AVOSS prediction algorithms.

  12. Numerical Modeling Studies of Wake Vortex Transport and Evolution Within the Planetary Boundary Layer

    NASA Technical Reports Server (NTRS)

    Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.; Shen, Shaohua

    1998-01-01

    In support of the wake vortex effect of the Terminal Area Productivity program, we have put forward four tasks to be accomplished in our proposal. The first task is validation of two-dimensional wake vortex-turbulence interaction. The second task is investigation of three-dimensional interaction between wake vortices and atmospheric boundary layer (ABL) turbulence. The third task is ABL studies. The, fourth task is addition of a Klemp-Durran condition at the top boundary for TASS model. The accomplishment of these tasks will increase our understanding of the dynamics of wake vortex and improve forecasting systems responsible for air safety and efficiency. The first two tasks include following three parts: (a) Determine significant length scale for vortex decay and transport, especially the length scales associated with the onset of Crow instability (Crow, 1970); (b) Study the effects of atmospheric turbulence on the decay of the wake vortices; and (c) Determine the relationships between decay rate, transport properties and atmospheric parameters based on large eddy simulation (LES) results and the observational data. These parameters may include turbulence kinetic energy, dissipation rate, wind shear and atmospheric stratification. The ABL studies cover LES modeling of turbulence structure within planetary boundary layer under transition and stable stratification conditions. Evidences have shown that the turbulence in the stable boundary layer can be highly intermittent and the length scales of eddies are very small compared to those in convective case. We proposed to develop a nesting grid mesh scheme and a modified Klemp-Durran conditions (Klemp and Wilhelmson, 1978) at the top boundary for TASS model to simulate planetary boundary layer under stable stratification conditions. During the past year, our group has made great efforts to carry out the above mentioned four tasks simultaneously. The work accomplished in the last year will be described in the next section.

  13. A dynamic prescribed vortex wake model for the FAST/AeroDyn wind energy conversion simulation code

    NASA Astrophysics Data System (ADS)

    Currin, Hugh D.

    2007-12-01

    A Dynamic Prescribed Vortex Wake model for analysis of horizontal axis wind turbines has been developed. This model extends the HAWTDAWG steady state prescribed wake code to dynamic flows. This extension assumes wake vortices follow prescription functions valid at the time each vortex is generated. This allows modeling of dynamic wake effects known to exist. This assumption is supported through analysis and comparison to UAE Phase VI test data. The new Dynamic Prescribed Vortex Wake model is built into AeroDyn as a third aerodynamic model and uses the FAST structural dynamic model. It implements the wind models and dynamic stall model in AeroDyn. FAST structural degrees of freedom are implemented. Comparisons are made to UAE Phase VI wind tunnel data, and to the other two AeroDyn models, Blade Element Momentum and Generalized Dynamic Wake. Both steady, to verify the base model, and dynamic, to validate the extension to dynamic flow, conditions are considered. Both axial and yawed flow are analyzed. Dynamic UAE test data analyzed include rapid pitch, Sequence Q, and yaw release, Sequence E. The Dynamic Prescribed Wake model compares favorably to test data and to other AeroDyn models. Small rapid dynamic response is noted in each model and the test data. The new Dynamic Prescribed Vortex Wake model shows promise. Release of the code for experimental use and further validation is recommended.

  14. NASA Wake Vortex Research for Aircraft Spacing

    NASA Technical Reports Server (NTRS)

    Perry, R. Brad; Hinton, David A.; Stuever, Robert A.

    1996-01-01

    The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements during instrument meteorological conditions through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations (RSO) subelement at the NASA Langley Research Center is developing an Aircraft Vortex Spacing System (AVOSS). AVOSS will integrate the output of several inter-related areas to produce weather dependent, dynamic wake vortex spacing criteria. These areas include current and predicted weather conditions, models of wake vortex transport and decay in these weather conditions, real-time feedback of wake vortex behavior from sensors, and operationally acceptable aircraft/wake interaction criteria. In today's ATC system, the AVOSS could inform ATC controllers when a fixed reduced separation becomes safe to apply to large and heavy aircraft categories. With appropriate integration into the Center/TRACON Automation System (CTAS), AVOSS dynamic spacing could be tailored to actual generator/follower aircraft pairs rather than a few broad aircraft categories.

  15. Numerical modeling studies of wake vortex transport and evolution within the planetary boundary layer

    NASA Technical Reports Server (NTRS)

    Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.

    1994-01-01

    The proposed research involves four tasks. The first of these is to simulate accurately the turbulent processes in the atmospheric boundary layer. TASS was originally developed to study meso-gamma scale phenomena, such as tornadic storms, microbursts and windshear effects in terminal areas. Simulation of wake vortex evolution, however, will rely on appropriate representation of the physical processes in the surface layer and mixed layer. This involves two parts. First, a specified heat flux boundary condition must be implemented at the surface. Using this boundary condition, simulation results will be compared to experimental data and to other model results for validation. At this point, any necessary changes to the model will be implemented. Next, a surface energy budget parameterization will be added to the model. This will enable calculation of the surface fluxes by accounting for the radiative heat transfer to and from the ground and heat loss to the soil rather than simple specification of the fluxes. The second task involves running TASS with prescribed wake vortices in the initial condition. The vortex models will be supplied by NASA Langley Research Center. Sensitivity tests will be performed on different meteorological environments in the atmospheric boundary layer, which include stable, neutral, and unstable stratifications, calm and severe wind conditions, and dry and wet conditions. Vortex strength may be varied as well. Relevant non-dimensional parameters will include the following: Richardson number or Froude number, Bowen ratio, and height to length scale ratios. The model output will be analyzed and visualized to better understand the transport, decay, and growth rates of the wake vortices. The third task involves running simulations using observed data. MIT Lincoln Labs is currently planning field experiments at the Memphis airport to measure both meteorological conditions and wake vortex characteristics. Once this data becomes available, it can be used to validate the model for vortex behavior under different atmospheric conditions. The fourth task will be to simulate the wake in a more realistic environment covering a wider area. This will involve grid nesting, since high resolution will be required in the wake region but a larger total domain will be used. During the first allocation year, most of the first task will be accomplished.

  16. Vortex motion in the near-wake region behind a single gas bubble in a liquid-solid fluidized bed - The pendulum model for wake size prediction

    SciTech Connect

    Tsuchiya, K.; Fan, L.S.

    1987-01-01

    A mechanistic model, denoted as the pendulum model, which interrelates the frequency of vortex shedding and the size of the bubble wake is developed based on secondary motion of a single gas bubble rising in liquids and/or liquid-solid suspensions. In the model, the bubble and its primary wake are regarded as a single semi-rigid body steadily rocking at the vortex shedding frequency. Inherent model parameters, including bubble geometric parameters, the bubble rise velocity and the vortex shedding frequency, are expressed in terms of the bubble size to permit a priori prediction of the wake size. The predicted wake sizes are compared satisfactorily with the experimental data reported in the literature over a wide range of the bubble Reynolds number.

  17. Recent NASA Wake-Vortex Flight Tests, Flow-Physics Database and Wake-Development Analysis

    NASA Technical Reports Server (NTRS)

    Vicroy, Dan D.; Vijgen, Paul M.; Reimer, Heidi M.; Gallegos, Joey L.; Spalart, Philippe R.

    1998-01-01

    A series of flight tests over the ocean of a four engine turboprop airplane in the cruise configuration have provided a data set for improved understanding of wake vortex physics and atmospheric interaction. An integrated database has been compiled for wake characterization and validation of wake-vortex computational models. This paper describes the wake-vortex flight tests, the data processing, the database development and access, and results obtained from preliminary wake-characterization analysis using the data sets.

  18. Comparisons of Crosswind Velocity Profile Estimates Used in Fast-Time Wake Vortex Prediction Models

    NASA Technical Reports Server (NTRS)

    Pruis, Mathew J.; Delisi, Donald P.; Ahmad, Nashat N.

    2011-01-01

    Five methods for estimating crosswind profiles used in fast-time wake vortex prediction models are compared in this study. Previous investigations have shown that temporal and spatial variations in the crosswind vertical profile have a large impact on the transport and time evolution of the trailing vortex pair. The most important crosswind parameters are the magnitude of the crosswind and the gradient in the crosswind shear. It is known that pulsed and continuous wave lidar measurements can provide good estimates of the wind profile in the vicinity of airports. In this study comparisons are made between estimates of the crosswind profiles from a priori information on the trajectory of the vortex pair as well as crosswind profiles derived from different sensors and a regional numerical weather prediction model.

  19. Vortex Wake Geometry of a Model Tilt Rotor in Forward Flight

    NASA Technical Reports Server (NTRS)

    Wadcock, Alan J.; Yamauchi, Gloria K.; Rutkowski, Michael (Technical Monitor)

    2001-01-01

    A full-span 0.25-scale V-22 tiltrotor was tested in the NASA Ames 40-by 80-Foot Wind Tunnel in November 2000. The main objective of the test was to acquire a comprehensive database to validate tiltrotor analyses. Figure 1 shows the model installed in the Ames 40- by 80-Foot Wind Tunnel. Rotor and vehicle performance measurements were taken in addition to wing pressures, acoustics, and flow visualization. A dual acoustic traverse system was installed to measure blade-vortex interaction (BVI) noise levels and directivity. Test conditions included hover and forward flight in helicopter mode. Angle-of-attack and thrust sweeps for three tunnel speeds were acquired before model problems caused the premature conclusion of the test. The test will resume in the Ames 80- by 120-Foot Wind Tunnel in late 2001. This paper will focus on the wake geometry measurements that were acquired during the test. The wake geometry measurements were a small subset of a larger matrix of planned measurements designed to study the development and structure of the dual vortex system generated during BVI conditions. The present paper will provide wake geometry data for four test conditions. In addition, the data will be compared with previously acquired wake measurements from an isolated tiltrotor

  20. Validation of a vortex ring wake model suited for aeroelastic simulations of floating wind turbines

    NASA Astrophysics Data System (ADS)

    de Vaal, J. B.; Hansen, M. O. L.; Moan, T.

    2014-12-01

    In order to evaluate aerodynamic loads on floating offshore wind turbines, advanced dynamic analysis tools are required. As a unified model that can represent both dynamic inflow and skewed inflow effects in it basic formulation, a wake model based on a vortex ring formulation is discussed. Such a model presents a good intermediate solution between computationally efficient but simple momentum balance methods and computationally expensive but complete computational fluid dynamics models. The model introduced is shown to be capable of modelling typical steady and unsteady test cases with reasonable accuracy.

  1. Implementation and validation of a slender vortex filament code: Its application to the study of a four-vortex wake model

    NASA Astrophysics Data System (ADS)

    Margerit, D.; Brancher, P.; Giovannini, A.

    2004-01-01

    A computational code EZ-vortex is developed for the motion of slender vortex filaments of closed or open shape. The integro-differential equations governing the motion of the vortex centre lines are either the Callegari and Ting equations, which are the leading order solution of a matched asymptotic analysis, or equivalent forms of these equations. They include large axial velocity and nonsimilar profiles in the vortical cores. The fluid may be viscous or inviscid. This code is validated both against known solutions of these equations and results from linear stability analyses. The linear and non-linear stages of a perturbed two-vortex wake and of a four-vortex wake model are then computed.

  2. Three-Phased Wake Vortex Decay

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Ahmad, Nashat N.; Switzer, George S.; LimonDuparcmeur, Fanny M.

    2010-01-01

    A detailed parametric study is conducted that examines vortex decay within turbulent and stratified atmospheres. The study uses a large eddy simulation model to simulate the out-of-ground effect behavior of wake vortices due to their interaction with atmospheric turbulence and thermal stratification. This paper presents results from a parametric investigation and suggests improvements for existing fast-time wake prediction models. This paper also describes a three-phased decay for wake vortices. The third phase is characterized by a relatively slow rate of circulation decay, and is associated with the ringvortex stage that occurs following vortex linking. The three-phased decay is most prevalent for wakes imbedded within environments having low-turbulence and near-neutral stratification.

  3. ASRS Reports on Wake Vortex Encounters

    NASA Technical Reports Server (NTRS)

    Connell, Linda J.; Taube, Elisa Ann; Drew, Charles Robert; Barclay, Tommy Earl

    2010-01-01

    ASRS is conducting a structured callback research project of wake vortex incidents reported to the ASRS at all US airports, as well as wake encounters in the enroute environment. This study has three objectives: (1) Utilize the established ASRS supplemental data collection methodology and provide ongoing analysis of wake vortex encounter reports; (2) Document event dynamics and contributing factors underlying wake vortex encounter events; and (3) Support ongoing FAA efforts to address pre-emptive wake vortex risk reduction by utilizing ASRS reporting contributions.

  4. Wake Vortex Advisory System (WakeVAS) Concept of Operations

    NASA Technical Reports Server (NTRS)

    Rutishauser, David; Lohr, Gary; Hamilton, David; Powers, Robert; McKissick, Burnell; Adams, Catherine; Norris, Edward

    2003-01-01

    NASA Langley Research Center has a long history of aircraft wake vortex research, with the most recent accomplishment of demonstrating the Aircraft VOrtex Spacing System (AVOSS) at Dallas/Forth Worth International Airport in July 2000. The AVOSS was a concept for an integration of technologies applied to providing dynamic wake-safe reduced spacing for single runway arrivals, as compared to current separation standards applied during instrument approaches. AVOSS included state-of-the-art weather sensors, wake sensors, and a wake behavior prediction algorithm. Using real-time data AVOSS averaged a 6% potential throughput increase over current standards. This report describes a Concept of Operations for applying the technologies demonstrated in the AVOSS to a variety of terminal operations to mitigate wake vortex capacity constraints. A discussion of the technological issues and open research questions that must be addressed to design a Wake Vortex Advisory System (WakeVAS) is included.

  5. Interfacing comprehensive rotorcraft analysis with advanced aeromechanics and vortex wake models

    NASA Astrophysics Data System (ADS)

    Liu, Haiying

    This dissertation describes three aspects of the comprehensive rotorcraft analysis. First, a physics-based methodology for the modeling of hydraulic devices within multibody-based comprehensive models of rotorcraft systems is developed. This newly proposed approach can predict the fully nonlinear behavior of hydraulic devices, and pressure levels in the hydraulic chambers are coupled with the dynamic response of the system. The proposed hydraulic device models are implemented in a multibody code and calibrated by comparing their predictions with test bench measurements for the UH-60 helicopter lead-lag damper. Predicted peak damping forces were found to be in good agreement with measurements, while the model did not predict the entire time history of damper force to the same level of accuracy. The proposed model evaluates relevant hydraulic quantities such as chamber pressures, orifice flow rates, and pressure relief valve displacements. This model could be used to design lead-lag dampers with desirable force and damping characteristics. The second part of this research is in the area of computational aeroelasticity, in which an interface between computational fluid dynamics (CFD) and computational structural dynamics (CSD) is established. This interface enables data exchange between CFD and CSD with the goal of achieving accurate airloads predictions. In this work, a loose coupling approach based on the delta-airloads method is developed in a finite-element method based multibody dynamics formulation, DYMORE. To validate this aerodynamic interface, a CFD code, OVERFLOW-2, is loosely coupled with a CSD program, DYMORE, to compute the airloads of different flight conditions for Sikorsky UH-60 aircraft. This loose coupling approach has good convergence characteristics. The predicted airloads are found to be in good agreement with the experimental data, although not for all flight conditions. In addition, the tight coupling interface between the CFD program, OVERFLOW-2, and the CSD program, DYMORE, is also established. The ability to accurately capture the wake structure around a helicopter rotor is crucial for rotorcraft performance analysis. In the third part of this thesis, a new representation of the wake vortex structure based on Non-Uniform Rational B-Spline (NURBS) curves and surfaces is proposed to develop an efficient model for prescribed and free wakes. NURBS curves and surfaces are able to represent complex shapes with remarkably little data. The proposed formulation has the potential to reduce the computational cost associated with the use of Helmholtz's law and the Biot-Savart law when calculating the induced flow field around the rotor. An efficient free-wake analysis will considerably decrease the computational cost of comprehensive rotorcraft analysis, making the approach more attractive to routine use in industrial settings.

  6. Wake Vortex Research in the USA (WakeNet-USA)

    NASA Technical Reports Server (NTRS)

    Lang, Steve; Bryant, Wayne

    2006-01-01

    This viewgraph presentation reviews the cooperative work that FAA and NASA are engaged in to safely increase the capacity of the National Airspace System by studying the wake vortex operations. Wake vortex avoidance is a limiting factor in defining separation standards in the airport terminal area and could become a reducing separation standards in en route airspace.

  7. Application of laser velocimetry to aircraft wake-vortex measurements

    NASA Technical Reports Server (NTRS)

    Ciffone, D. L.; Orloff, K. L.

    1977-01-01

    The theory and use of a laser velocimeter that makes simultaneous measurements of vertical and longitudinal velocities while rapidly scanning a flow field laterally are described, and its direct application to trailing wake-vortex research is discussed. Pertinent measurements of aircraft wake-vortex velocity distributions obtained in a wind tunnel and water towing tank are presented. The utility of the velocimeter to quantitatively assess differences in wake velocity distributions due to wake dissipating devices and span loading changes on the wake-generating model is also demonstrated.

  8. Dynamics of the vortex wakes of flying and swimming vertebrates.

    PubMed

    Rayner, J M

    1995-01-01

    The vortex wakes of flying and swimming animals provide evidence of the history of aero- and hydrodynamic force generation during the locomotor cycle. Vortex-induced momentum flux in the wake is the reaction of forces the animal imposes on its environment, which must be in equilibrium with inertial and external forces. In flying birds and bats, the flapping wings generate lift both to provide thrust and to support the weight. Distinct wingbeat and wake movement patterns can be identified as gaits. In flow visualization experiments, only two wake patterns have been identified: a vortex ring gait with inactive upstroke, and a continuous vortex gait with active upstroke. These gaits may be modelled theoretically by free vortex and lifting line theory to predict mechanical energy consumption, aerodynamic forces and muscle activity. Longer-winged birds undergo a distinct gait change with speed, but shorter-winged species use the vortex ring gait at all speeds. In swimming fish, the situation is more complex: the wake vortices form a reversed von Kármán vortex street, but little is known about the mechanism of generation of the wake, or about how it varies with speed and acceleration or with body form and swimming mode. An unresolved complicating factor is the interaction between the drag wake of the flapping fish body and the thrusting wake from the tail. PMID:8571221

  9. Sensitivity of vortex-shedding in unsteady wake based on a self-consistent model

    NASA Astrophysics Data System (ADS)

    Meliga, Philippe; Boujo, Edouard; Gallaire, François

    2015-11-01

    An adjoint method is used to obtain sensitivity maps for the limit-cycle frequency and amplitude of the Bénard-von Kàrmàn vortex street in the unsteady wake past a circular cylinder. The sensitivity analysis is based on a semi-linear self-consistent model recently introduced by Mantic, Arratia & Gallaire (2014), so as to properly take into account the effect of control not only on the mean flow but also on finite-amplitude fluctuations which couple back into the mean flow equation through steady Reynolds stresses. The sensitivity is computed with respect to steady forcing, synchronous time-harmonic forcing, as well as localized feedback. For each case, the obtained results are compared to those stemming from the classical mean flow approach in which the feeding back of the fluctuations on to the mean flow is simply overlooked. As an illustration, the method is applied to passive control by means of a small control cylinder, and the results are discussed in light of the seminal experiments of Strykowski & Sreenivasan (1990).

  10. Wake Vortex Algorithm Scoring Results

    NASA Technical Reports Server (NTRS)

    Robins, R. E.; Delisi, D. P.; Hinton, David (Technical Monitor)

    2002-01-01

    This report compares the performance of two models of trailing vortex evolution for which interaction with the ground is not a significant factor. One model uses eddy dissipation rate (EDR) and the other uses the kinetic energy of turbulence fluctuations (TKE) to represent the effect of turbulence. In other respects, the models are nearly identical. The models are evaluated by comparing their predictions of circulation decay, vertical descent, and lateral transport to observations for over four hundred cases from Memphis and Dallas/Fort Worth International Airports. These observations were obtained during deployments in support of NASA's Aircraft Vortex Spacing System (AVOSS). The results of the comparisons show that the EDR model usually performs slightly better than the TKE model.

  11. Analysis of vortex wake encounter upsets

    NASA Technical Reports Server (NTRS)

    Johnson, W. A.; Teper, G. L.

    1974-01-01

    The problem of an airplane being upset by encountering the vortex wake of a large transport on takeoff or landing is currently receiving considerable attention. This report describes the technique and results of a study to assess the effectiveness of automatic control systems in alleviating vortex wake upsets. A six-degree-of-freedom nonlinear digital simulation was used for this purpose. The analysis included establishing the disturbance input due to penetrating a vortex wake from an arbitrary position and angle. Simulations were computed for both a general aviation airplane and a commercial jet transport. Dynamic responses were obtained for the penetrating aircraft with no augmentation, and with various command augmentation systems, as well as with human pilot control. The results of this preliminary study indicate that attitude command augmentation systems can provide significant alleviation of vortex wake upsets; and can do it better than a human pilot.

  12. An unsteady vortex lattice method model of a horizontal axis wind turbine operating in an upstream rotor wake

    NASA Astrophysics Data System (ADS)

    Hankin, D.; Graham, J. M. R.

    2014-12-01

    An unsteady formulation of the vortex lattice method, VLM, is presented that uses a force- free representation of the wake behind a horizontal axis wind turbine, HAWT, to calculate the aerodynamic loading on a turbine operating in the wake of an upstream rotor. A Cartesian velocity grid is superimposed over the computational domain to facilitate the representation of the atmospheric turbulence surrounding the turbine and wind shear. The wake of an upstream rotor is modelled using two methods: a mean velocity deficit with superimposed turbulence, based on experimental observations, and a purely numeric periodic boundary condition. Both methods are treated as frozen and propagated with the velocity grid. Measurements of the mean thrust and blade root bending moment on a three bladed horizontal axis rotor modelling a 5 MW HAWT at 1:250 scale were carried out in a wind tunnel. Comparisons are made between operation in uniform flow and in the wake of a similarly loaded rotor approximately 6.5 diameters upstream. The measurements were used to validate the output from the VLM simulations, assuming a completely rigid rotor. The trends in the simulation thrust predictions are found to compare well with the uniform flow case, except at low tip speed ratios where there are losses due to stall which are yet to be included in the model. The simple wake model predicts the mean deficit, whilst the periodic boundary condition captures more of the frequency content of the loading in an upstream wake. However, all the thrust loads are over-predicted. The simulation results severely overestimate the bending moment, which needs addressing. However, the reduction in bending due to the simple wake model is found to reflect the experimental data reasonably well.

  13. Feasibility of wake vortex monitoring systems for air terminals

    NASA Technical Reports Server (NTRS)

    Wilson, D. J.; Shrider, K. R.; Lawrence, T. R.

    1972-01-01

    Wake vortex monitoring systems, especially those using laser Doppler sensors, were investigated. The initial phases of the effort involved talking with potential users (air traffic controllers, pilots, etc.) of a wake vortex monitoring system to determine system requirements from the user's viewpoint. These discussions involved the volumes of airspace to be monitored for vortices, and potential methods of using the monitored vortex data once the data are available. A subsequent task led to determining a suitable mathematical model of the vortex phenomena and developing a mathematical model of the laser Doppler sensor for monitoring the vortex flow field. The mathematical models were used in combination to help evaluate the capability of laser Doppler instrumentation in monitoring vortex flow fields both in the near vicinity of the sensor (within 1 kilometer and at long ranges(10 kilometers).

  14. Vortex Wakes of Subsonic Transport Aircraft

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.; Nixon, David (Technical Monitor)

    1999-01-01

    A historical overview will be presented of the research conducted on the structure and modification of the vortices generated by the lifting surfaces of subsonic transport aircraft. The seminar will describe the three areas of vortex research; namely, the magnitude of the hazard posed, efforts to reduce the hazard to an acceptable level, and efforts to develop a systematic means for avoiding vortex wakes. It is first pointed out that the characteristics of lift-generated vortices are related to the aerodynamic shapes that produce them and that various arrangements of surfaces can be used to produce different vortex structures. The largest portion of the research conducted to date has been directed at finding ways to reduce the hazard potential of lift-generated vortices shed by subsonic transport aircraft in the vicinity of airports during landing and takeoff operations. It is stressed that lift-generated vortex wakes are so complex that progress towards a solution requires application of a combined theoretical and experimental research program because either alone often leads to incorrect conclusions. It is concluded that a satisfactory aerodynamic solution to the wake-vortex problem at airports has not yet been found but a reduction in the impact of the wake-vortex hazard on airport capacity may become available in the foreseeable future through wake-vortex avoidance concepts currently under study. The material to be presented in this overview is drawn from articles published in aerospace journals that are available publicly.

  15. Measurements and modeling of flow structure in the wake of a low profile wishbone vortex generator

    NASA Technical Reports Server (NTRS)

    Wendt, B. J.; Hingst, W. R.

    1994-01-01

    The results of an experimental examination of the vortex structures shed from a low profile 'wishbone' generator are presented. The vortex generator height relative to the turbulent boundary layer was varied by testing two differently sized models. Measurements of the mean three-dimensional velocity field were conducted in cross-stream planes downstream of the vortex generators. In all cases, a counter-rotating vortex pair was observed. Individual vortices were characterized by three descriptors derived from the velocity data; circulation, peak vorticity, and cross-stream location of peak vorticity. Measurements in the cross plane at two axial locations behind the smaller wishbone characterize the downstream development of the vortex pairs. A single region of stream wise velocity deficit is shared by both vortex cores. This is in contrast to conventional generators, where each core coincides with a region of velocity deficit. The measured cross-stream velocities for each case are compared to an Oseen model with matching descriptors. The best comparison occurs with the data from the larger wishbone.

  16. Wake Vortex Study at Wallops Island

    NASA Technical Reports Server (NTRS)

    1990-01-01

    The air flow from the wing of this agricultural plane is made by a technique that uses colored smoke rising from the ground. The swirl at the wingtip traces the aircraft's wake vortex, which exerts a powerful influence on the flow field behind the plane. Because of wake vortex, the Federal Aviation Administration (FAA) requires aircraft to maintain set distances behind each other when they land. A joint NASA-FAA program aimed at boosting airport capacity, however, is aimed at determining conditions under which planes may fly closer together. NASA researchers are studying wake vortex with a variety of tools, from supercomputers to wind tunnels to actual flight tests in research aircraft. Their goal is to fully understand the phenomenon, then use that knowledge to create an automated system that could predict changing wake vortex conditions at airports. Pilots already know, for example, that they have to worry less about wake vortex in rough weather because windy conditions cause them to dissipate more rapidly.

  17. Wake-Vortex Hazards During Cruise

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.; James, Kevin D.; Nixon, David (Technical Monitor)

    1998-01-01

    Even though the hazard posed by lift-generated wakes of subsonic transport aircraft has been studied extensively for approach and departure at airports, only a small amount of effort has gone into the potential hazard at cruise altitude. This paper reports on a studio of the wake-vortex hazard during cruise because encounters may become more prevalent when free-flight becomes available and each aircraft, is free to choose its own route between destinations. In order to address the problem, the various fluid-dynamic stages that vortex wakes usually go through as they age will be described along with estimates of the potential hazard that each stage poses. It appears that a rolling-moment hazard can be just as severe at cruise as for approach at airports, but it only persists for several minutes. However, the hazard posed by the downwash in the wake due to the lift on the generator aircraft persists for tens of minutes in a long narrow region behind the generating aircraft. The hazard consists of severe vertical loads when an encountering aircraft crosses the wake. A technique for avoiding vortex wakes at cruise altitude will be described. To date the hazard posed by lift-generated vortex wakes and their persistence at cruise altitudes has been identified and subdivided into several tasks. Analyses of the loads to be encounter and are underway and should be completed shortly. A review of published literature on the subject has been nearly completed (see text) and photographs of vortex wakes at cruise altitudes have been taken and the various stages of decay have been identified. It remains to study and sort the photographs for those that best illustrate the various stages of decay after they are shed by subsonic transport aircraft at cruise altitudes. The present status of the analysis and the paper are described.

  18. Civil aircraft vortex wake. TsAGI's research activities

    NASA Astrophysics Data System (ADS)

    Chernyshev, S. L.; Gaifullin, A. M.; Sviridenko, Yu. N.

    2014-11-01

    This paper provides a review of research conducted in TsAGI (Central AeroHydrodynamic Institute) concerning a vortex wake behind an airliner. The research into this area of theoretical and practical importance have been done both in Russia and in other countries, for which these studies became a vital necessity at the end of the 20th century. The paper describes the main methods and ratios on which software systems used to calculate the evolution of a vortex wake in a turbulent atmosphere are based. Verification of calculation results proved their acceptable consistency with the known experimental data. The mechanism of circulation loss in a vortex wake which is based on the analytical solution for the problem of two vortices diffusing in a viscous fluid is also described. The paper also describes the model of behavior of an aircraft which has deliberately or unintentionally entered a vortex wake behind another aircraft. Approximated results of calculations performed according to this model by means of artificial neural networks enabled the researchers to model the dynamics of an aircraft in a vortex wake on flight simulators on-line.

  19. Simulation of Wake Vortex Radiometric Detection via Jet Exhaust Proxy

    NASA Technical Reports Server (NTRS)

    Daniels, Taumi S.

    2015-01-01

    This paper describes an analysis of the potential of an airborne hyperspectral imaging IR instrument to infer wake vortices via turbine jet exhaust as a proxy. The goal was to determine the requirements for an imaging spectrometer or radiometer to effectively detect the exhaust plume, and by inference, the location of the wake vortices. The effort examines the gas spectroscopy of the various major constituents of turbine jet exhaust and their contributions to the modeled detectable radiance. Initially, a theoretical analysis of wake vortex proxy detection by thermal radiation was realized in a series of simulations. The first stage used the SLAB plume model to simulate turbine jet exhaust plume characteristics, including exhaust gas transport dynamics and concentrations. The second stage used these plume characteristics as input to the Line By Line Radiative Transfer Model (LBLRTM) to simulate responses from both an imaging IR hyperspectral spectrometer or radiometer. These numerical simulations generated thermal imagery that was compared with previously reported wake vortex temperature data. This research is a continuation of an effort to specify the requirements for an imaging IR spectrometer or radiometer to make wake vortex measurements. Results of the two-stage simulation will be reported, including instrument specifications for wake vortex thermal detection. These results will be compared with previously reported results for IR imaging spectrometer performance.

  20. Simulation of wake vortex radiometric detection via jet exhaust proxy

    NASA Astrophysics Data System (ADS)

    Daniels, Taumi S.

    2015-06-01

    This paper describes an analysis of the potential of an airborne hyperspectral imaging IR instrument to infer wake vortices via turbine jet exhaust as a proxy. The goal was to determine the requirements for an imaging spectrometer or radiometer to effectively detect the exhaust plume, and by inference, the location of the wake vortices. The effort examines the gas spectroscopy of the various major constituents of turbine jet exhaust and their contributions to the modeled detectable radiance. Initially, a theoretical analysis of wake vortex proxy detection by thermal radiation was realized in a series of simulations. The first stage used the SLAB plume model to simulate turbine jet exhaust plume characteristics, including exhaust gas transport dynamics and concentrations. The second stage used these plume characteristics as input to the Line By Line Radiative Transfer Model (LBLRTM) to simulate responses from both an imaging IR hyperspectral spectrometer or radiometer. These numerical simulations generated thermal imagery that was compared with previously reported wake vortex temperature data. This research is a continuation of an effort to specify the requirements for an imaging IR spectrometer or radiometer to make wake vortex measurements. Results of the two-stage simulation will be reported, including instrument specifications for wake vortex thermal detection. These results will be compared with previously reported results for IR imaging spectrometer performance.

  1. An Operational Wake Vortex Sensor Using Pulsed Coherent Lidar

    NASA Technical Reports Server (NTRS)

    Barker, Ben C., Jr.; Koch, Grady J.; Nguyen, D. Chi

    1998-01-01

    NASA and FAA initiated a program in 1994 to develop methods of setting spacings for landing aircraft by incorporating information on the real-time behavior of aircraft wake vortices. The current wake separation standards were developed in the 1970's when there was relatively light airport traffic and a logical break point by which to categorize aircraft. Today's continuum of aircraft sizes and increased airport packing densities have created a need for re-evaluation of wake separation standards. The goals of this effort are to ensure that separation standards are adequate for safety and to reduce aircraft spacing for higher airport capacity. Of particular interest are the different requirements for landing under visual flight conditions and instrument flight conditions. Over the years, greater spacings have been established for instrument flight than are allowed for visual flight conditions. Preliminary studies indicate that the airline industry would save considerable money and incur fewer passenger delays if a dynamic spacing system could reduce separations at major hubs during inclement weather to the levels routinely achieved under visual flight conditions. The sensor described herein may become part of this dynamic spacing system known as the "Aircraft VOrtex Spacing System" (AVOSS) that will interface with a future air traffic control system. AVOSS will use vortex behavioral models and short-term weather prediction models in order to predict vortex behavior sufficiently into the future to allow dynamic separation standards to be generated. The wake vortex sensor will periodically provide data to validate AVOSS predictions. Feasibility of measuring wake vortices using a lidar was first demonstrated using a continuous wave (CW) system from NASA Marshall Space Flight Sensor and tested at the Volpe National Transportation Systems Center's wake vortex test site at JFK International Airport. Other applications of CW lidar for wake vortex measurement have been made more recently, including a system developed by the MIT Lincoln Laboratory. This lidar has been used for detailed measurements of wake vortex velocities in support of wake vortex model validation. The first measurements of wake vortices using a pulsed, lidar were made by Coherent Technologies, Inc. (CTI) using a 2 micron solid-state, flashlamp-pumped system operating at 5 Hz. This system was first deployed at Denver's Stapleton Airport. Pulsed lidar has been selected as the baseline technology for an operational sensor due to its longer range capability.

  2. Vortex wake alleviation studies with a variable twist wing

    NASA Technical Reports Server (NTRS)

    Holbrook, G. T.; Dunham, D. M.; Greene, G. C.

    1985-01-01

    Vortex wake alleviation studies were conducted in a wind tunnel and a water towing tank using a multisegmented wing model which provided controlled and measured variations in span load. Fourteen model configurations are tested at a Reynolds number of one million and a lift coefficient of 0.6 in the Langley 4- by 7-Meter Tunnel and the Hydronautics Ship Model Basin water tank at Hydronautics, Inc., Laurel, Md. Detailed measurements of span load and wake velocities at one semispan downstream correlate well with each other, with inviscid predictions of span load and wake roll up, and with peak trailing-wing rolling moments measured in the far wake. Average trailing-wing rolling moments are found to be an unreliable indicator of vortex wake intensity because vortex meander does not scale between test facilities and free-air conditions. A tapered-span-load configuration, which exhibits little or no drag penalty, is shown to offer significant downstream wake alleviation to a small trailing wing. The greater downstream wake alleviation achieved with the addition of spoilers to a flapped-wing configuration is shown to result directly from the high incremental drag and turbulence associated with the spoilers and not from the span load alteration they cause.

  3. Analog Processing Assembly for the Wake Vortex Lidar Experiment

    NASA Technical Reports Server (NTRS)

    Stowe, Edwood G.

    1995-01-01

    The Federal Aviation Administration (FAA) and NASA have initiated a joint study in the development of reliable means of tracking, detecting, measuring, and predicting trailing wake-vortices of commercial aircraft. Being sought is an accurate model of the wake-vortex hazard, sufficient to increase airport capacity by reducing minimum safe spacings between planes. Several means of measurement are being evaluated for application to wake-vortex detection and tracking, including Doppler RADAR (Radio Detection and Ranging) systems, 2-micron Doppler LIDAR (Light Detection And Ranging) systems, and SODAR (Sound Detection And Ranging) systems. Of specific interest there is the lidar system, which has demonstrated numerous valuable capabilities as a vortex sensor Aerosols entrained in the vortex flow make the wake velocity signature visible to the lidar, (the observable lidar signal is essentially a measurement of the line-of-sight velocity of the aerosols). Measurement of the occurrence of a wake vortex requires effective reception and monitoring of the beat signal which results from the frequency-offset between the transmitted pulse and the backscattered radiation. This paper discusses the mounting, analysis, troubleshooting, and possible use of an analog processing assembly designed for such an application.

  4. Tip Vortex and Wake Characteristics of a Counterrotating Open Rotor

    NASA Technical Reports Server (NTRS)

    VanZante, Dale E.; Wernet, Mark P.

    2012-01-01

    One of the primary noise sources for Open Rotor systems is the interaction of the forward rotor tip vortex and blade wake with the aft rotor. NASA has collaborated with General Electric on the testing of a new generation of low noise, counterrotating Open Rotor systems. Three-dimensional particle image velocimetry measurements were acquired in the intra-rotor gap of the Historical Baseline blade set. The velocity measurements are of sufficient resolution to characterize the tip vortex size and trajectory as well as the rotor wake decay and turbulence character. The tip clearance vortex trajectory is compared to results from previously developed models. Forward rotor wake velocity profiles are shown. Results are presented in a form as to assist numerical modeling of Open Rotor system aerodynamics and acoustics.

  5. A new methodology for free wake analysis using curved vortex elements

    NASA Technical Reports Server (NTRS)

    Bliss, Donald B.; Teske, Milton E.; Quackenbush, Todd R.

    1987-01-01

    A method using curved vortex elements was developed for helicopter rotor free wake calculations. The Basic Curve Vortex Element (BCVE) is derived from the approximate Biot-Savart integration for a parabolic arc filament. When used in conjunction with a scheme to fit the elements along a vortex filament contour, this method has a significant advantage in overall accuracy and efficiency when compared to the traditional straight-line element approach. A theoretical and numerical analysis shows that free wake flows involving close interactions between filaments should utilize curved vortex elements in order to guarantee a consistent level of accuracy. The curved element method was implemented into a forward flight free wake analysis, featuring an adaptive far wake model that utilizes free wake information to extend the vortex filaments beyond the free wake regions. The curved vortex element free wake, coupled with this far wake model, exhibited rapid convergence, even in regions where the free wake and far wake turns are interlaced. Sample calculations are presented for tip vortex motion at various advance ratios for single and multiple blade rotors. Cross-flow plots reveal that the overall downstream wake flow resembles a trailing vortex pair. A preliminary assessment shows that the rotor downwash field is insensitive to element size, even for relatively large curved elements.

  6. A Probabilistic Wake Vortex Lateral Transport Model Using Data from SFO and DEN

    NASA Technical Reports Server (NTRS)

    Mellman, George R.; Delisi, Donald P.

    2008-01-01

    In a previous report, we considered the behavior of the lateral position of vortices as a function of time after vortex formation for Out of Ground Effects (OGE) data for aircraft landing at San Francisco International Airport (SFO). We quantified the spread in lateral position as a function of time and examined how predictable lateral position is under a variety of assumptions. The combination of spread and predictability allowed us to derive probability distribution functions (PDFs) for lateral position given observed crosswind (CW) velocities. In this study, we examine the portability of these PDFs with respect to other landing sites. To this end, we consider OGE data obtained by the Federal Aviation Administration for landings at Denver International Airport (DEN) between 04/05/2006 and 06/03/2006. We consider vortices from both B733 (Boeing 737 models 200-500) and B757 (Boeing 757) aircraft. The data set contains 635 B733 landings and 506 B757 landings. The glide slope altitude for these measurements was 280 m, determined by the average initial vortex observation adjusted for a 3-second delay in the initial observation. The comparable SFO altitude was 158 m. We note that the principal mechanism for lateral transport in the OGE regime is advection by the ambient wind. This implies that a simple crosswind correction may be effective in explaining much of the variation in the lateral transport data. In this study, we again consider the use of ASOS data and average Lidar crosswind data over the vortex altitude range to predict vortex location as a function of time.

  7. Wake Vortex Transport in Proximity to the Ground

    NASA Technical Reports Server (NTRS)

    Hamilton, David W.; Proctor, Fred H.

    2000-01-01

    A sensitivity study for aircraft wake vortex transport has been conducted using a validated large eddy simulation (LES) model. The study assumes neutrally stratified and nonturbulent environments and includes the consequences of the ground. The numerical results show that the nondimensional lateral transport is primarily influenced by the magnitude of the ambient crosswind and is insensitive to aircraft type. In most of the simulations, the ground effect extends the lateral position of the downwind vortex about one initial vortex spacing (b(sub o)) in the downstream direction. Further extension by as much as one b(sub o) occurs when the downwind vortex remains 'in ground effect' (IGE) for relatively long periods of time. Results also show that a layer-averaged ambient wind velocity can be used to bound the time for lateral transport of wake vortices to insure safe operations on a parallel runway.

  8. Meteorology and Wake Vortex Influence on American Airlines FL-587 Accident

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Hamilton, David W.; Rutishauser, David K.; Switzer, George F.

    2004-01-01

    The atmospheric environment surrounding the crash of American Airlines Flight 587 is investigated. Examined are evidence for any unusual atmospheric conditions and the potential for encounters with aircraft wake vortices. Computer simulations are carried out with two different vortex prediction models and a Large Eddy Simulation model. Wind models are proposed for studying aircraft and pilot response to the wake vortex encounter.

  9. Wake vortex alleviation using rapidly actuated segmented Gurney flaps

    NASA Astrophysics Data System (ADS)

    Matalanis, Claude G.

    All bodies that generate lift also generate circulation. The circulation generated by large commercial aircraft remains in their wake in the form of trailing vortices. These vortices can be hazardous to following aircraft due to their strength and persistence. To account for this, airports abide by spacing rules which govern the frequency with which aircraft can use their runways when operating in instrument flight rules. These spacing rules are the limiting factor on increasing airport capacity. We conducted an experimental and computational study to assess the potential for using rapidly actuated segmented Gurney flaps, also known as Miniature Trailing Edge Effectors (MiTEs), for active wake vortex alleviation. Wind tunnel tests were performed on a half-span model NACA 0012 wing equipped with an array of 13 independent MITE pairs. The chord-based Reynolds number was around 350,000. Each MiTE could extend 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing. Pressure profiles and a five-hole probe survey in the near wake were used to examine the influence that the MiTEs had upon the wing aerodynamics and the vortex rollup process. Particle image velocimetry was used to measure the static and time-dependent response of the vortex in the intermediate wake to various MiTE actuation schemes. These results were used to form complete initial conditions for vortex filament computations of the far wake evolution. Results from these computations showed that the perturbations created by MiTEs could be used to excite a variety of three-dimensional inviscid vortex instabilities. Finally, the research performed on MiTEs led to the invention of a more practical wake alleviation device: the spanwise actuating Gurney flap. Prototype tests showed that this device could produce similar perturbations to the MiTEs.

  10. Perspective: Numerical simulation of wakes and blade-vortex interaction

    SciTech Connect

    Dong, B. . Dept. of Engineering Science and Mechanics); Mook, D.T.

    1994-03-01

    A method for simulating incompressible flows past airfoils and their wakes is described. Vorticity panels are used to represent the body, and vortex blobs (vortex points with their singularities removed) are used to represent the wake. The procedure can be applied to the simulation of completely attached flow past an oscillating airfoil. The rate at which vorticity is shed from the trailing edge of the airfoil into the wake is determined by simultaneously requiring the pressure along the upper and lower surface streamlines to approach the same value at the trailing edge and the circulation around both the airfoil and its wake to remain constant. The motion of the airfoils is discretized, and a vortex is shed from the trailing edge at each time step. The vortices are convected at the local velocity of fluid particles, a procedure that renders the pressure continuous in an inviscid fluid. When the vortices in the wake begin to separate they are split into more vortices, and when they begin to collect they are combined. The numerical simulation reveals that the wake, which is originally smooth, eventually coils, or wraps, around itself, primarily under the influence of the velocity it induces on itself, and forms regions of relatively concentrated vorticity. Although discrete vortices are used to represent the wake, the spatial density of the vortices is so high that the computed velocity profiles across a typical region of concentrated vorticity are quite smooth. Although the computed wake evolves in an entirely inviscid model of the flowfield, these profiles appear to have a viscous core. As an application, a simulation of the interaction between vorticity in the oncoming stream and a stationary airfoil is also discussed.

  11. Vortex Core Size in the Rotor Near-Wake

    NASA Technical Reports Server (NTRS)

    Young, Larry A.

    2003-01-01

    Using a kinetic energy conservation approach, a number of simple analytic expressions are derived for estimating the core size of tip vortices in the near-wake of rotors in hover and axial-flow flight. The influence of thrust, induced power losses, advance ratio, and vortex structure on rotor vortex core size is assessed. Experimental data from the literature is compared to the analytical results derived in this paper. In general, three conclusions can be drawn from the work in this paper. First, the greater the rotor thrust, t h e larger the vortex core size in the rotor near-wake. Second, the more efficient a rotor is with respect to induced power losses, the smaller the resulting vortex core size. Third, and lastly, vortex core size initially decreases for low axial-flow advance ratios, but for large advance ratios core size asymptotically increases to a nominal upper limit. Insights gained from this work should enable improved modeling of rotary-wing aerodynamics, as well as provide a framework for improved experimental investigations of rotor a n d advanced propeller wakes.

  12. Mathematical models for exotic wakes

    NASA Astrophysics Data System (ADS)

    Basu, Saikat; Stremler, Mark

    2014-11-01

    Vortex wakes are a common occurrence in the environment around us; the most famous example being the von Kármán vortex street with two vortices being shed by the bluff body in each cycle. However, frequently there can be many other more exotic wake configurations with different vortex arrangements, based on the flow parameters and the bluff body dimensions and/or its oscillation characteristics. Some examples include wakes with periodic shedding of three vortices (`P+S' mode) and four vortices (symmetric `2P' mode, staggered `2P' mode, `2C' mode). We present mathematical models for such wakes assuming two-dimensional potential flows with embedded point vortices. The spatial alignment of the vortices is inspired by the experimentally observed wakes. The idealized system follows a Hamiltonian formalism. Model-based analysis reveals a rich dynamics pertaining to the relative vortex motion in the mid-wake region. Downstream evolution of the vortices, as predicted from the model results, also show good correspondence with wake-shedding experiments performed on flowing soap films.

  13. Proceedings of the NASA First Wake Vortex Dynamic Spacing Workshop

    NASA Technical Reports Server (NTRS)

    Creduer, Leonard (Editor); Perry, R. Brad (Editor)

    1997-01-01

    A Government and Industry workshop on wake vortex dynamic spacing systems was conducted on May 13-15, 1997, at the NASA Langley Research Center. The purpose of the workshop was to disclose the status of ongoing NASA wake vortex R&D to the international community and to seek feedback on the direction of future work to assure an optimized research approach. Workshop sessions examined wake vortex characterization and physics, wake sensor technologies, aircraft/wake encounters, terminal area weather characterization and prediction, and wake vortex systems integration and implementation. A final workshop session surveyed the Government and Industry perspectives on the NASA research underway and related international wake vortex activities. This document contains the proceedings of the workshop including the presenters' slides, the discussion following each presentation, the wrap-up panel discussion, and the attendees' evaluation feedback.

  14. Flow visualization of vortex interactions in multiple vortex wakes behind aircraft

    NASA Technical Reports Server (NTRS)

    Ciffone, D. L.; Lonzo, C., Jr.

    1975-01-01

    A flow visualization technique was developed which allows the nature of lift-generated wakes behind aircraft models to be investigated. The technique was applied to models being towed underwater in a ship model basin. Seven different configurations of a small-scale model of a 747 transport aircraft were used to allow observation of typical vortex interactions and merging in multiple vortex wakes. It was established that the motion of the wake vortices is often sensitive to small changes in either wing span loading or model attitude. Landing gear deployement was found to cause a far-field reformation of vorticity behind a model configuration which dissipated concentrated vorticity in the near-field wake. Alleviation of wake vorticity is achievable by configuring the wing span loading to cause the wake vortices to move in paths that result in their interactions and merging. The vortices shed from the horizontal stabilizer always moved down rapidly into the wake and merged with the other vortices, primarily the inboard flap vortices.

  15. Implementation and validation of a wake model for vortex-surface interactions in low speed forward flight

    NASA Technical Reports Server (NTRS)

    Komerath, Narayanan M.; Schreiber, Olivier A.

    1987-01-01

    The wake model was implemented using a VAX 750 and a Microvax II workstation. Online graphics capability using a DISSPLA graphics package. The rotor model used by Beddoes was significantly extended to include azimuthal variations due to forward flight and a simplified scheme for locating critical points where vortex elements are placed. A test case was obtained for validation of the predictions of induced velocity. Comparison of the results indicates that the code requires some more features before satisfactory predictions can be made over the whole rotor disk. Specifically, shed vorticity due to the azimuthal variation of blade loading must be incorporated into the model. Interactions between vortices shed from the four blades of the model rotor must be included. The Scully code for calculating the velocity field is being modified in parallel with these efforts to enable comparison with experimental data. To date, some comparisons with flow visualization data obtained at Georgia Tech were performed and show good agreement for the isolated rotor case. Comparison of time-resolved velocity data obtained at Georgia Tech also shows good agreement. Modifications are being implemented to enable generation of time-averaged results for comparison with NASA data.

  16. Wake Vortex Field Measurement Program at Memphis, Tennessee: Data Guide

    NASA Technical Reports Server (NTRS)

    Campbell, S. D.; Dasey, T. J.; Freehart, R. E.; Heinrichs, R. M.; Mathews, M. P.; Perras, G. H.; Rowe, G. S.

    1997-01-01

    Eliminating or reducing current restrictions in the air traffic control system due to wake vortex considerations would yield increased capacity, decreased delays, and cost savings. Current wake vortex separation standards are widely viewed as very conservative under most conditions. However, scientific uncertainty about wake vortex behavior under different atmospheric conditions remains a barrier to development of an adaptive vortex spacing system. The objective of the wake vortex field measurement efforts during December, 1994 and August, 1995 at Memphis, TN were to record wake vortex behavior for varying atmospheric conditions and types of aircraft. This effort is part of a larger effort by the NASA Langley Research Center to develop an Aircraft Vortex Spacing System (AVOSS) as an element of the Terminal Area Productivity (TAP) program. The TAP program is being performed in concert with the FAA Terminal Air Traffic Control Automation (TATCA) program and ATC Automation. Wake vortex behavior was observed using a mobile continuous-wave (CW) coherent laser Doppler radar (lidar) developed at Lincoln Laboratory. This lidar features a number of improvements over previous systems, including the first-ever demonstration of an automatic wake vortex detection and tracking algorithm.

  17. Vortex cloud model for body vortex shedding and tracking

    NASA Technical Reports Server (NTRS)

    Mendenhall, Michael R.; Perkins, Stanley C., Jr.

    1986-01-01

    The discrete vortex cloud approach models a missile airframe's vortex wake by means that are capable of treating a variety of configurations over a range of flow conditions. Attention is given to the sheets of vorticity formed on the lee side of a missile at moderate angles-of-attack. While three-dimensional attached flow models are used to represent the missile body, two-dimensional, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body under the influence of the freestream and the separation vortex wake are used to calculate aerodynamic loads on the body. The separation vortex wake is represented by clouds of discrete vortices in cross flow planes normal to the body axis.

  18. Evaluation of the discrete vortex wake cross flow model using vector computers. Part 1: Theory and application

    NASA Technical Reports Server (NTRS)

    1979-01-01

    The current program had the objective to modify a discrete vortex wake method to efficiently compute the aerodynamic forces and moments on high fineness ratio bodies (f approximately 10.0). The approach is to increase computational efficiency by structuring the program to take advantage of new computer vector software and by developing new algorithms when vector software can not efficiently be used. An efficient program was written and substantial savings achieved. Several test cases were run for fineness ratios up to f = 16.0 and angles of attack up to 50 degrees.

  19. Vortex shedding in the wake of a step cylinder

    NASA Astrophysics Data System (ADS)

    Morton, Chris; Yarusevych, Serhiy

    2010-08-01

    Flow past a circular cylinder with a single stepwise discontinuity in diameter was investigated numerically for the diameter ratio D/d=2 and two Reynolds numbers, ReD=150 and 300. The primary focus was on vortex shedding and vortex interactions occurring in the cylinder wake. In agreement with previous experimental findings, three distinct spanwise vortex cells were identified in the step-cylinder wake: a single vortex shedding cell in the wake of the small cylinder (the S-cell) and two vortex shedding cells in the wake of the large cylinder, one in the region downstream of the step (the N-cell) and the other away from the step (the L-cell). Due to the differences in vortex shedding frequencies, complex vortex connections occurred in two vortex interaction regions located between the adjacent cells. However, distinct differences in vortex splitting and vortex dislocations were identified in the two regions. The region at the boundary between the S-cell and the N-cell was relatively narrow and its spanwise extent did not fluctuate significantly. In this region, vortex dislocations manifested as half-loop connections between two S-cell vortices of opposite sign. In contrast, the region at the boundary between the N-cell and the L-cell exhibited transient behavior, with large scale vortex dislocations causing cyclic variation in the extent of N-cell vortices. Spectral analysis of velocity data showed that the presence of the N-cell was continuous through all simulations. For ReD=300, small scale streamwise vortices forming in the wake of the large cylinder weaken the primary spanwise vortices and vortex connections, complicating vortex dynamics in the step-cylinder wake. However, no significant Reynolds number effect on the average spanwise extent of the vortex cells and the two transition regions between neighboring cells was observed. Finally, formation of N-cell vortices was shown to be linked to downwash fluctuations near the step.

  20. Analysis of the Radar Reflectivity of Aircraft Vortex Wakes

    NASA Technical Reports Server (NTRS)

    Shariff, Karim; Wray, Alan; Yan, Jerry (Technical Monitor)

    2000-01-01

    Radar has been proposed as a way to track wake vortices to reduce aircraft spacing and tests have revealed radar echoes from aircraft wakes in clear air. The results are always interpreted qualitatively using Tatarski's theory of weak scattering by isotropic atmospheric turbulence. The goal of the present work was to predict the value of the radar cross-section (RCS) using simpler models. This is accomplished in two steps. First, the refractive index is obtained. Since the structure of the aircraft wakes is different from atmospheric turbulence, three simple mechanisms specific to vortex wakes are considered: (1) Radial density gradient in a two-dimensional vortex, (2) three-dimensional fluctuations in the vortex cores, and (3) Adiabatic transport of the atmospheric fluid in a two-dimensional oval surrounding the pair of vortices. The index of refraction is obtained more precisely for the two-dimensional mechanisms than for the three-dimensional ones. In the second step, knowing the index of refraction, a scattering analysis is performed. Tatarski's weak scattering approximation is kept but the usual assumptions of a far-field and a uniform incident wave are dropped. Neither assumption is generally valid for a wake that is coherent across the radar beam. For analytical insight, a simpler approximation that invokes, in addition to weak scattering, the far-field and wide cylindrical beam assumptions, is also developed and compared with the more general analysis. The predicted RCS values for the oval surround the vortices (mechanism C) agree with the experiments of Bilson conducted over a wide range of frequencies. However, the predictions have a cut-off away from normal incidence which is not present in the measurements. Estimates suggest that this is due to turbulence in the baroclinic vorticity generated at the boundary of the oval. The reflectivity of a vortex itself (mechanism A) is comparable to that of the oval (mechanism C) but cuts-off at frequencies lower than those considered in all the experiments to date. The RCS of a vortex happens to peak at the frequency (about 49 MHz) where atmospheric radars (known as ST radars) operate and so the present prediction could be verified in the future. Finally , we suggest that hot engine exhaust could increase RCE by 40 db and reveal vortex circulation, provided its mixing with the surroundings is prevented in the laminarising flow of the vortices.

  1. Wake Vortex Detection: Phased Microphone vs. Linear Infrasonic Array

    NASA Technical Reports Server (NTRS)

    Shams, Qamar A.; Zuckerwar, Allan J.; Sullivan, Nicholas T.; Knight, Howard K.

    2014-01-01

    Sensor technologies can make a significant impact on the detection of aircraft-generated vortices in an air space of interest, typically in the approach or departure corridor. Current state-of-the art sensor technologies do not provide three-dimensional measurements needed for an operational system or even for wake vortex modeling to advance the understanding of vortex behavior. Most wake vortex sensor systems used today have been developed only for research applications and lack the reliability needed for continuous operation. The main challenges for the development of an operational sensor system are reliability, all-weather operation, and spatial coverage. Such a sensor has been sought for a period of last forty years. Acoustic sensors were first proposed and tested by National Oceanic and Atmospheric Administration (NOAA) early in 1970s for tracking wake vortices but these acoustic sensors suffered from high levels of ambient noise. Over a period of the last fifteen years, there has been renewed interest in studying noise generated by aircraft wake vortices, both numerically and experimentally. The German Aerospace Center (DLR) was the first to propose the application of a phased microphone array for the investigation of the noise sources of wake vortices. The concept was first demonstrated at Berlins Airport Schoenefeld in 2000. A second test was conducted in Tarbes, France, in 2002, where phased microphone arrays were applied to study the wake vortex noise of an Airbus 340. Similarly, microphone phased arrays and other opto-acoustic microphones were evaluated in a field test at the Denver International Airport in 2003. For the Tarbes and Denver tests, the wake trajectories of phased microphone arrays and lidar were compared as these were installed side by side. Due to a built-in pressure equalization vent these microphones were not suitable for capturing acoustic noise below 20 Hz. Our group at NASA Langley Research Center developed and installed an infrasonic array at the Newport News-Williamsburg International Airport early in the year 2013. A pattern of pressure burst, high-coherence intervals, and diminishing-coherence intervals was observed for all takeoff and landing events without exception. The results of a phased microphone vs. linear infrasonic array comparison will be presented.

  2. Vortex cloud model for body vortex shedding and tracking

    NASA Technical Reports Server (NTRS)

    Mendenhall, Michael R.; Perkins, Stanley C., Jr; Lesieutre, Daniel J.

    1992-01-01

    The present engineering prediction method addresses the aerodynamic characteristics and associated flowfields of circular and noncircular bodies at large incidence angles and arbitrary roll angles. 3D attached-flow models are used to represent the body, and 2D, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body, under the influence of both the freestream and the separation vortex wake, is used to calculate aerodynamic loads. The separation vortex wake is represented by clouds of discrete vortices in crossflow planes normal to the body axis.

  3. Two Dimensional Wake Vortex Simulations in the Atmosphere: Preliminary Sensitivity Studies

    NASA Technical Reports Server (NTRS)

    Proctor, F. H.; Hinton, D. A.; Han, J.; Schowalter, D. G.; Lin, Y.-L.

    1998-01-01

    A numerical large-eddy simulation model is currently being used to quantify aircraft wake vortex behavior with meteorological observables. The model, having a meteorological framework, permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, and humidity. The addition of grid-scale turbulence as an initial condition appeared to have little consequence. Results show that conventional nondimensionalizations work very well for vortex pairs embedded in stably stratified flows. However, this result is based on simple environments with constant Brunt-Vaisala frequency. Results presented here also show that crosswind profiles exert important and complex interactions on the trajectories of wake vortices. Nonlinear crosswind profiles tended to arrest the descent of wake vortex pairs. The member of the vortex pair with vorticity of same sign as the vertical change in the ambient along-track vorticity may be deflected upwards.

  4. Wake Vortex Tracking Using a 35 GHz Pulsed Doppler Radar

    NASA Technical Reports Server (NTRS)

    Neece, Robert T.; Britt, Charles L.; White, Joseph H.; Mudukutore, Ashok; Nguyen, Chi; Hooper, Bill

    2005-01-01

    A 35 GHz, pulsed-Doppler radar system has been designed and assembled for wake vortex detection and tracking in low visibility conditions. Aircraft wake vortices continue to be an important factor in determining safe following distances or spacings for aircraft in the terminal area. Currently, under instrument meteorological conditions (IMC), aircraft adhere to conservative, fixed following-distance guidelines based primarily on aircraft weight classifications. When ambient conditions are such that vortices will either drift or dissipate, leaving the flight corridor clear, the prescribed spacings are unnecessarily long and result in decreased airport throughput. There is a potential for significant airport efficiency improvement, if a system can be employed to aid regulators and pilots in setting safe and efficient following distances based on airport conditions. The National Aeronautics and Space Administration (NASA), the Federal Aviation Agency, and Volpe National Transportation Systems Center have promoted and worked to develop systems that would increase airport capacity and provide for safe reductions in aircraft separation. The NASA Aircraft Vortex Spacing System (AVOSS), a wake vortex spacing system that can provide dynamic adjustment of spacings based on real-time airport weather conditions, has demonstrated that Lidar systems can be successfully used to detect and track vortices in clear air conditions. To fill the need for detection capability in low-visibility conditions, a 35 GHz, pulsed-Doppler radar system is being investigated for use as a complimentary, low-visibility sensor for wake vortices. The radar sensor provides spatial and temporal information similar to that provided by Lidar, but under weather conditions that a Lidar cannot penetrate. Currently, we are analyzing the radar design based upon the data and experience gained during the wake vortex Lidar deployment with AVOSS at Dallas/Fort Worth International Airport. As part of this study, two numerical models were utilized in system simulations. The results of this study improve our understanding of the method of detection, resolution requirements for range and azimuth, pulse compression, and performance prediction. Simulations applying pulse compression techniques show that detection is good in heavy fog to greater than 2000 m. Both compressed and uncompressed short pulses show the vortex structure. To explore operational challenges, siting and scanning strategies were also analyzed. Simulation results indicate that excellent wake vortex detection, tracking and classification is possible in drizzle (+15 dBZ) and heavy fog (- 13 dBZ) using short pulse techniques (<99ns) at ranges on the order of 900 m, with a modest power of 500 W output. At 1600 m, detection can be expected at reflectivities as low as -13 dBZ (heavy fog). The radar system, as designed and built, has the potential to support field studies of a wake vortex spacing system in low-visibility conditions ranging from heavy fog to rain, when sited within 2000m of the flight path.

  5. Coherent Pulsed Lidar Sensing of Wake Vortex Position and Strength, Winds and Turbulence in the Terminal Area

    NASA Technical Reports Server (NTRS)

    Brockman, Philip; Barker, Ben C., Jr.; Koch, Grady J.; Nguyen, Dung Phu Chi; Britt, Charles L., Jr.; Petros, Mulugeta

    1999-01-01

    NASA Langley Research Center (LaRC) has field tested a 2.0 gm, 100 Hertz, pulsed coherent lidar to detect and characterize wake vortices and to measure atmospheric winds and turbulence. The quantification of aircraft wake-vortex hazards is being addressed by the Wake Vortex Lidar (WVL) Project as part of Aircraft Vortex Spacing System (AVOSS), which is under the Reduced Spacing Operations Element of the Terminal Area Productivity (TAP) Program. These hazards currently set the minimum, fixed separation distance between two aircraft and affect the number of takeoff and landing operations on a single runway under Instrument Meteorological Conditions (IMC). The AVOSS concept seeks to safely reduce aircraft separation distances, when weather conditions permit, to increase the operational capacity of major airports. The current NASA wake-vortex research efforts focus on developing and validating wake vortex encounter models, wake decay and advection models, and wake sensing technologies. These technologies will be incorporated into an automated AVOSS that can properly select safe separation distances for different weather conditions, based on the aircraft pair and predicted/measured vortex behavior. The sensor subsystem efforts focus on developing and validating wake sensing technologies. The lidar system has been field-tested to provide real-time wake vortex trajectory and strength data to AVOSS for wake prediction verification. Wake vortices, atmospheric winds, and turbulence products have been generated from processing the lidar data collected during deployments to Norfolk (ORF), John F. Kennedy (JFK), and Dallas/Fort Worth (DFW) International Airports.

  6. Simulation of Rotary-Wing Near-Wake Vortex Structures Using Navier-Stokes CFD Methods

    NASA Technical Reports Server (NTRS)

    Kenwright, David; Strawn, Roger; Ahmad, Jasim; Duque, Earl; Warmbrodt, William (Technical Monitor)

    1997-01-01

    This paper will use high-resolution Navier-Stokes computational fluid dynamics (CFD) simulations to model the near-wake vortex roll-up behind rotor blades. The locations and strengths of the trailing vortices will be determined from newly-developed visualization and analysis software tools applied to the CFD solutions. Computational results for rotor nearwake vortices will be used to study the near-wake vortex roll up for highly-twisted tiltrotor blades. These rotor blades typically have combinations of positive and negative spanwise loading and complex vortex wake interactions. Results of the computational studies will be compared to vortex-lattice wake models that are frequently used in rotorcraft comprehensive codes. Information from these comparisons will be used to improve the rotor wake models in the Tilt-Rotor Acoustic Code (TRAC) portion of NASA's Short Haul Civil Transport program (SHCT). Accurate modeling of the rotor wake is an important part of this program and crucial to the successful design of future civil tiltrotor aircraft. The rotor wake system plays an important role in blade-vortex interaction noise, a major problem for all rotorcraft including tiltrotors.

  7. An Improved Wake Vortex Tracking Algorithm for Multiple Aircraft

    NASA Technical Reports Server (NTRS)

    Switzer, George F.; Proctor, Fred H.; Ahmad, Nashat N.; LimonDuparcmeur, Fanny M.

    2010-01-01

    The accurate tracking of vortex evolution from Large Eddy Simulation (LES) data is a complex and computationally intensive problem. The vortex tracking requires the analysis of very large three-dimensional and time-varying datasets. The complexity of the problem is further compounded by the fact that these vortices are embedded in a background turbulence field, and they may interact with the ground surface. Another level of complication can arise, if vortices from multiple aircrafts are simulated. This paper presents a new technique for post-processing LES data to obtain wake vortex tracks and wake intensities. The new approach isolates vortices by defining "regions of interest" (ROI) around each vortex and has the ability to identify vortex pairs from multiple aircraft. The paper describes the new methodology for tracking wake vortices and presents application of the technique for single and multiple aircraft.

  8. Wake Vortex Advisory System (WakeVAS) Evaluation of Impacts on the National Airspace System

    NASA Technical Reports Server (NTRS)

    Smith, Jeremy C.; Dollyhigh, Samuel M.

    2005-01-01

    This report is one of a series that describes an ongoing effort in high-fidelity modeling/simulation, evaluation and analysis of the benefits and performance metrics of the Wake Vortex Advisory System (WakeVAS) Concept of Operations being developed as part of the Virtual Airspace Modeling and Simulation (VAMS) project. A previous study, determined the overall increases in runway arrival rates that could be achieved at 12 selected airports due to WakeVAS reduced aircraft spacing under Instrument Meteorological Conditions. This study builds on the previous work to evaluate the NAS wide impacts of equipping various numbers of airports with WakeVAS. A queuing network model of the National Airspace System, built by the Logistics Management Institute, Mclean, VA, for NASA (LMINET) was used to estimate the reduction in delay that could be achieved by using WakeVAS under non-visual meteorological conditions for the projected air traffic demand in 2010. The results from LMINET were used to estimate the total annual delay reduction that could be achieved and from this, an estimate of the air carrier variable operating cost saving was made.

  9. Numerical Simulation of the Aircraft Wake Vortex Flowfield

    NASA Technical Reports Server (NTRS)

    Ahmad, Nashat N.; Proctor, Fred H.; Perry, R. Brad

    2013-01-01

    The near wake vortex flowfield from a NACA0012 half-wing was simulated using a fully unstructured Navier-Stokes flow solver in three dimensions at a chord Reynolds number of 4.6 million and a Mach number of approximately 0.15. Several simulations were performed to examine the effect of boundary conditions, mesh resolution and turbulence scheme on the formation of wingtip vortex and its downstream propagation. The standard Spalart-Allmaras turbulence model was compared with the Dacles-Mariani and Spalart-Shur corrections for rotation and curvature effects. The simulation results were evaluated using the data from experiment performed at NASA Ames' 32in x 48in low speed wind tunnel.

  10. A three dimensional unsteady iterative panel method with vortex particle wakes and boundary layer model for bio-inspired multi-body wings

    NASA Astrophysics Data System (ADS)

    Dhruv, Akash; Blower, Christopher; Wickenheiser, Adam M.

    2015-03-01

    The ability of UAVs to operate in complex and hostile environments makes them useful in military and civil operations concerning surveillance and reconnaissance. However, limitations in size of UAVs and communication delays prohibit their operation close to the ground and in cluttered environments, which increase risks associated with turbulence and wind gusts that cause trajectory deviations and potential loss of the vehicle. In the last decade, scientists and engineers have turned towards bio-inspiration to solve these issues by developing innovative flow control methods that offer better stability, controllability, and maneuverability. This paper presents an aerodynamic load solver for bio-inspired wings that consist of an array of feather-like flaps installed across the upper and lower surfaces in both the chord- and span-wise directions, mimicking the feathers of an avian wing. Each flap has the ability to rotate into both the wing body and the inbound airflow, generating complex flap configurations unobtainable by traditional wings that offer improved aerodynamic stability against gusting flows and turbulence. The solver discussed is an unsteady three-dimensional iterative doublet panel method with vortex particle wakes. This panel method models the wake-body interactions between multiple flaps effectively without the need to define specific wake geometries, thereby eliminating the need to manually model the wake for each configuration. To incorporate viscous flow characteristics, an iterative boundary layer theory is employed, modeling laminar, transitional and turbulent regions over the wing's surfaces, in addition to flow separation and reattachment locations. This technique enables the boundary layer to influence the wake strength and geometry both within the wing and aft of the trailing edge. The results obtained from this solver are validated using experimental data from a low-speed suction wind tunnel operating at Reynolds Number 300,000. This method enables fast and accurate assessment of aerodynamic loads for initial design of complex wing configurations compared to other methods available.

  11. Wake Vortex Transport and Decay in Ground Effect: Vortex Linking with the Ground

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Hamilton, David W.; Han, Jongil

    2000-01-01

    Numerical simulations are carried out with a three-dimensional Large-Eddy Simulation (LES) model to explore the sensitivity of vortex decay and transport in ground effect (IGE). The vortex decay rates are found to be strongly enhanced following maximum descent into ground effect. The nondimensional decay rate is found to be insensitive to the initial values of circulation, height, and vortex separation. The information gained from these simulations is used to construct a simple decay relationship. This relationship compares well with observed data from an IGE case study. Similarly, a relationship for lateral drift due to ground effect is constructed from the LES data. In the second part of this paper, vortex linking with the ground is investigated. Our numerical simulations of wake vortices for IGE show that a vortex may link with its image beneath the ground, if the intensity of the ambient turbulence is moderate to high. This linking with the ground (which is observed in real cases)gives the appearance of a vortex tube that bends to become vertically oriented and which terminates at the ground. From the simulations conducted, the linking time for vortices in the free atmosphere; i.e., a function of ambient turbulence intensity.

  12. Design of airport wake vortex monitoring system based on 1.5-μm pulsed coherent Doppler lidar

    NASA Astrophysics Data System (ADS)

    Wu, Yong-Hua; Hu, Yi-Hua; Xu, Shi-Long; Li, Jin-Ming; Dai, Ding-Chuan

    2011-07-01

    To shun the vortex hazard, the airport wake vortex monitoring system based on 1.5-μm pulsed coherent Doppler lidar is designed successfully in this paper. Based on the realistic analytical model, the wake vortex generated by airbus A340 under typical flight condition is simulated. Then the principle of airport wake vortex monitoring is introduced, and the work flow of the monitoring system is also presented. Moreover, based on the mechanism of vortex coherent detection and typical system parameters, both detection SNR and detection precision are obtained through numerical simulations. When the system outputs 2 J energy, the coherent detection SNR at 10 km distance is up to 23.452, and detection precision can reach 0.328 m/s. With the wake vortex monitoring experiment of A340, some vortex parameters are estimated. Due to these results comparatively coinciding with the previous simulation conclusions, the ability of Doppler lidar for full-scale wake vortex characterization and real time measurement is demonstrated. The study shows that the wake vortex detection based on 1.5-μm pulsed coherent Doppler lidar has the advantages of high accuracy and far distance, and the designed airport wake vortex monitoring system has proved to be effective and feasible, which has significant development and application prospect in the aspect of assuring flight security and increasing airport capacity.

  13. Numerical Study of Wake Vortex Interaction with the Ground Using the Terminal Area Simulation System

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Han, Jongil

    1999-01-01

    A sensitivity study for the in-ground effect on aircraft wake vortices has been conducted using a validated large eddy simulation model. The numerical results are compared with observed data and show good agreement for vortex decay and lateral vortex transport. The vortex decay rate is strongly influenced by the ground, but appears somewhat insensitive to ambient turbulence. In addition, the results show that the ground can affect the trajectory and descent-rate of a wake vortex pair at elevations up to about 3 b(sub o) (where b(sub o) is the initial vortex separation). However, the ground does not influence the average circulation of the vortices until the cores descend to within about 0.6 b(sub o), after which time the ground greatly enhances their rate of demise. Vortex rebound occurs in the simulations, but is more subtle than shown in previous numerical studies.

  14. Application of Wind Tunnel Free-Flight Technique for Wake Vortex Encounters

    NASA Technical Reports Server (NTRS)

    Brandon, Jay M.; Jordan, Frank L., Jr.; Stuever, Robert A.; Buttrill, Catherine W.

    1997-01-01

    A wind tunnel investigation was conducted in the Langley 30- by 60-Foot Tunnel to assess the free-flight test technique as a tool in research on wake vortex encounters. A typical 17.5-percent scale business-class jet airplane model was flown behind a stationary wing mounted in the forward portion of the wind tunnel test section. The span ratio (model span-generating wingspan) was 0.75. The wing angle of attack could be adjusted to produce a vortex of desired strength. The test airplane model was successfully flown in the vortex and through the vortex for a range of vortex strengths. Data obtained included the model airplane body axis accelerations, angular rates, attitudes, and control positions as a function of vortex strength and relative position. Pilot comments and video records were also recorded during the vortex encounters.

  15. Turbulence Climatology at Dallas/Ft.Worth (DFW) Airport: Implications for a Departure Wake Vortex Spacing System

    NASA Technical Reports Server (NTRS)

    Perras, G. H.; Dasey, T. J.

    2000-01-01

    Potential adaptive wake vortex spacing systems may need to rely on wake vortex decay rather than wake vortex transport in reducing wake separations. A wake vortex takeoff-spacing system in particular will need to rely on wake decay. Ambient turbulence is the primary influence on wake decay away from the ground. This study evaluated 18 months of ambient turbulence measurements at Dallas/Ft. Worth (DFW) Airport. The measurements show minor variation in the turbulence levels at various times of the year or times of the day for time periods when a departure system could be used. Arrival system operation was also examined, and a slightly lower overall turbulence level was found as compared to departure system benefit periods. The Sarpkaya model, a validated model of wake vortex behavior, was applied to various turbulence levels and compared to the DFW turbulence statistics. The results show that wake vortices from heavy aircraft on takeoff should dissipate within one minute for the majority of the time and will rarely last two minutes. These results will need to be verified by wake vortex measurements on departure.

  16. Fluid Forces and Vortex Wakes of a Flapping Foil

    NASA Astrophysics Data System (ADS)

    Schnipper, Teis; Andersen, Anders; Bohr, Tomas; Honoré Walther, Jens

    2009-11-01

    We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil with pitching oscillations in a two-dimensional free stream. We vary the frequency and amplitude of the oscillations and observe von K'arm'an wake, inverted von K'arm'an wake, and wakes in which two vortex pairs form per oscillation period. We find a close correspondence between the numerically determined vortex structures and the thickness variations that visualize the flow in the soap film.footnotetextSchnipper, Andersen, and Bohr, J. Fluid Mech. 633, 411--423 (2009). Numerically we obtain systematic maps with 25 x40 simulations in the frequency and amplitude plane of both wake type and average forces and moments, and we discuss the drag-thrust transition in relation to the changes in wake structure. Finally, we investigate the time evolution of the fluid forces and its link to the vortex formation near the round leading edge and the vortex shedding at the sharp trailing edge.

  17. The vortex street wakes of a rotating circular cylinder

    SciTech Connect

    Cheng, M.; Chew, Y.T.; Luo, S.C.

    1994-12-31

    The evolution of the vortex stress behind a rotating circular cylinder in uniform free stream is investigated numerically at high Reynolds number. The ratio of the cylinder surface velocity to the free stream velocity, {alpha} is in the range 0 {le} {alpha} {le} 3. The method used to calculate the flow can be considered as a combination between the diffusion-vortex method and the vortex-in-cell method. The lift and drag forces exerted by the fluid on the cylinder surface as well as the Strouhal number of vortex shedding, are determined together with the flow patterns in the wake. It was found that the vortex shedding and wake development behind the cylinder vary significantly depending on the magnitude of the rotation parameter {alpha}. When {alpha} {le} 2, the vortex street behind the cylinder in the near-wake inclines as a whole towards the direction of rotation as {alpha} increases. The Karman vortex street structure begins to deteriorate as soon as {alpha} exceeds 2 and finally disappears for {alpha} = 3.

  18. Developments and Validations of Fully Coupled CFD and Practical Vortex Transport Method for High-Fidelity Wake Modeling in Fixed and Rotary Wing Applications

    NASA Technical Reports Server (NTRS)

    Anusonti-Inthra, Phuriwat

    2010-01-01

    A novel Computational Fluid Dynamics (CFD) coupling framework using a conventional Reynolds-Averaged Navier-Stokes (BANS) solver to resolve the near-body flow field and a Particle-based Vorticity Transport Method (PVTM) to predict the evolution of the far field wake is developed, refined, and evaluated for fixed and rotary wing cases. For the rotary wing case, the RANS/PVTM modules are loosely coupled to a Computational Structural Dynamics (CSD) module that provides blade motion and vehicle trim information. The PVTM module is refined by the addition of vortex diffusion, stretching, and reorientation models as well as an efficient memory model. Results from the coupled framework are compared with several experimental data sets (a fixed-wing wind tunnel test and a rotary-wing hover test).

  19. Effects of spoilers and gear on B-747 wake vortex velocities

    NASA Technical Reports Server (NTRS)

    Luebs, A. B.; Bradfute, J. G.; Ciffone, D. L.

    1976-01-01

    Vortex velocities were measured in the wakes of four configurations of a 0.61-m span model of a B-747 aircraft. The wakes were generated by towing the model underwater in a ship model basin. Tangential and axial velocity profiles were obtained with a scanning laser velocimeter as the wakes aged to 35 span lengths behind the model. A 45 deg deflection of two outboard flight spoilers with the model in the landing configuration resulted in a 36 percent reduction in wake maximum tangential velocity, altered velocity profiles, and erratic vortex trajectories. Deployment of the landing gear with the inboard flaps in the landing position and outboard flaps retracted had little effect on the flap vortices to 35 spans, but caused the wing tip vortices to have: (1) more diffuse velocity profiles; (2) a 27 percent reduction in maximum tangential velocity; and (3) a more rapid merger with the flap vortices.

  20. Rotor Wake Vortex Definition Using 3C-PIV Measurements: Corrected for Vortex Orientation

    NASA Technical Reports Server (NTRS)

    Burley, Casey L.; Brooks, Thomas F.; vanderWall, Berend; Richard, Hughues Richard; Raffel, Markus; Beaumier, Philippe; Delrieux, Yves; Lim, Joon W.; Yu, Yung H.; Tung, Chee

    2003-01-01

    Three-component (3-C) particle image velocimetry (PIV) measurements, within the wake across a rotor disk plane, are used to determine wake vortex definitions important for BVI (Blade Vortex Interaction) and broadband noise prediction. This study is part of the HART II test program conducted using a 40 percent scale BO-105 helicopter main rotor in the German-Dutch Wind Tunnel (DNW). In this paper, measurements are presented of the wake vortex field over the advancing side of the rotor operating at a typical descent landing condition. The orientations of the vortex (tube) axes are found to have non-zero tilt angles with respect to the chosen PIV measurement cut planes, often on the order of 45 degrees. Methods for determining the orientation of the vortex axis and reorienting the measured PIV velocity maps (by rotation/projection) are presented. One method utilizes the vortex core axial velocity component, the other utilizes the swirl velocity components. Key vortex parameters such as vortex core size, strength, and core velocity distribution characteristics are determined from the reoriented PIV velocity maps. The results are compared with those determined from velocity maps that are not corrected for orientation. Knowledge of magnitudes and directions of the vortex axial and swirl velocity components as a function of streamwise location provide a basis for insight into the vortex evolution.

  1. Feasibility of an onboard wake vortex avoidance system

    NASA Technical Reports Server (NTRS)

    Bilanin, Alan J.; Teske, Milton E.; Curtiss, Howard C., Jr.

    1987-01-01

    It was determined that an onboard vortex wake detection system using existing, proven instrumentation is technically feasible. This system might be incorporated into existing onboard systems such as a wind shear detection system, and might provide the pilot with the location of a vortex wake, as well as an evasive maneuver so that the landing separations may be reduced. It is suggested that this system might be introduced into our nation's commuter aircraft fleet and major air carrier fleet and permit a reduction of current landing separation standards, thereby reducing takeoff and departure delays.

  2. Computational hydrodynamics of animal swimming: boundary element method and three-dimensional vortex wake structure.

    PubMed

    Cheng, J Y; Chahine, G L

    2001-12-01

    The slender body theory, lifting surface theories, and more recently panel methods and Navier-Stokes solvers have been used to study the hydrodynamics of fish swimming. This paper presents progress on swimming hydrodynamics using a boundary integral equation method (or boundary element method) based on potential flow model. The unsteady three-dimensional BEM code 3DynaFS that we developed and used is able to model realistic body geometries, arbitrary movements, and resulting wake evolution. Pressure distribution over the body surface, vorticity in the wake, and the velocity field around the body can be computed. The structure and dynamic behavior of the vortex wakes generated by the swimming body are responsible for the underlying fluid dynamic mechanisms to realize the high-efficiency propulsion and high-agility maneuvering. Three-dimensional vortex wake structures are not well known, although two-dimensional structures termed 'reverse Karman Vortex Street' have been observed and studied. In this paper, simulations about a swimming saithe (Pollachius virens) using our BEM code have demonstrated that undulatory swimming reduces three-dimensional effects due to substantially weakened tail tip vortex, resulting in a reverse Karman Vortex Street as the major flow pattern in the three-dimensional wake of an undulating swimming fish. PMID:11733166

  3. NWRA AVOSS Wake Vortex Prediction Algorithm. 3.1.1

    NASA Technical Reports Server (NTRS)

    Robins, R. E.; Delisi, D. P.; Hinton, David (Technical Monitor)

    2002-01-01

    This report provides a detailed description of the wake vortex prediction algorithm used in the Demonstration Version of NASA's Aircraft Vortex Spacing System (AVOSS). The report includes all equations used in the algorithm, an explanation of how to run the algorithm, and a discussion of how the source code for the algorithm is organized. Several appendices contain important supplementary information, including suggestions for enhancing the algorithm and results from test cases.

  4. Large Eddy Simulation of Aircraft Wake Vortices in a Homogeneous Atmospheric Turbulence: Vortex Decay and Descent

    NASA Technical Reports Server (NTRS)

    Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.

    1999-01-01

    The effects of ambient turbulence on decay and descent of aircraft wake vortices are studied using a validated, three-dimensional: large-eddy simulation model. Numerical simulations are performed in order to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally-stratified atmosphere. Simulations are conducted for a range of turbulence intensities, by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level, which is consistent with field observations. Based on the results from the numerical simulations, simple decay models are proposed as functions of dimensionless ambient turbulence intensity (eta) and dimensionless time (T) for the circulation averaged over a range of radial distances. With good agreement with the numerical results, a Gaussian type of vortex decay model is proposed for weak turbulence: while an exponential type of Tortex decay model can be applied for strong turbulence. A relationship for the vortex descent based on above vortex decay model is also proposed. Although the proposed models are based on simulations assuming neutral stratification, the model predictions are compared to Lidar vortex measurements observed during stable, neutral, and unstable atmospheric conditions. In the neutral and unstable atmosphere, the model predictions appear to be in reasonable agreement with the observational data, while in the stably-stratified atmosphere, they largely underestimate the observed circulation decay with consistent overestimation of the observed vortex descent. The underestimation of vortex decay during stably-stratified conditions suggests that stratification has an important influence on vortex decay when ambient levels of turbulence are weak.

  5. Simulating Wake Vortex Detection with the Sensivu Doppler Wind Lidar Simulator

    NASA Technical Reports Server (NTRS)

    Ramsey, Dan; Nguyen, Chi

    2014-01-01

    In support of NASA's Atmospheric Environment Safety Technologies NRA research topic on Wake Vortex Hazard Investigation, Aerospace Innovations (AI) investigated a set of techniques for detecting wake vortex hazards from arbitrary viewing angles, including axial perspectives. This technical report describes an approach to this problem and presents results from its implementation in a virtual lidar simulator developed at AI. Threedimensional data volumes from NASA's Terminal Area Simulation System (TASS) containing strong turbulent vortices were used as the atmospheric domain for these studies, in addition to an analytical vortex model in 3-D space. By incorporating a third-party radiative transfer code (BACKSCAT 4), user-defined aerosol layers can be incorporated into atmospheric models, simulating attenuation and backscatter in different environmental conditions and altitudes. A hazard detection algorithm is described that uses a twocomponent spectral model to identify vortex signatures observable from arbitrary angles.

  6. Measurements of Aircraft Wake Vortex Separation at High Arrival Rates and a Proposed New Wake Vortex Separation Philosophy

    NASA Technical Reports Server (NTRS)

    Rutishauser, David; Donohue, George L.; Haynie, Rudolph C.

    2003-01-01

    This paper presents data and a proposed new aircraft wake vortex separation standard that argues for a fundamental re-thinking of international practice. The current static standard, under certain atmospheric conditions, presents an unnecessary restriction on system capacity. A new approach, that decreases aircraft separation when atmospheric conditions dictate, is proposed based upon the availability of new instrumentation and a better understanding of wake physics.

  7. Wake Vortex Influence on Ambient Potential Temperature

    NASA Technical Reports Server (NTRS)

    1997-01-01

    The two-dimensional version of the Terminal Area Simulation System (TASS) was used to numerically simulate the interaction of wake vortices from closely separated aircraft. The aircraft parameters and separations are taken from observed data at an actual airport. The wake vortices are generated near the runway threshold for four successive aircraft. The ambient conditions are characterized by light crosswinds and stable stratification. This movie shows the effect that the vortices have upon the ambient potential temperature field.

  8. Development and testing of laser Doppler system components for wake vortex monitoring. Volume 1: Scanner development, laboratory and field testing and system modeling

    NASA Technical Reports Server (NTRS)

    Wilson, D. J.; Krause, M. C.; Coffey, E. W.; Huang, C. C.; Edwards, B. B.; Shrider, K. R.; Jetton, J. L.; Morrison, L. K.

    1974-01-01

    A servo-controlled range/elevation scanner for the laser Doppler velocimeter (LDV) was developed and tested in the field to assess its performance in detecting and monitoring aircraft trailing vortices in an airport environment. The elevation scanner provides a capability to manually point the LDV telescope at operator chosen angles from 3.2 deg. to 89.6 deg within 0.2 deg, or to automatically scan the units between operator chosen limits at operator chosen rates of 0.1 Hz to 0.5 Hz. The range scanner provides a capability to manually adjust the focal point of the system from a range of 32 meters to a range of 896 meters under operator control, or to scan between operator chosen limits and at rates from 0.1 Hz to 6.9 Hz. The scanner controls are designed to allow simulataneous range and elevation scanning so as to provide finger scan patterns, arc scan patterns, and vertical line scan patterns. The development and testing of the unit is discussed, along with a fluid dynamic model of the wake vortex developed in a laser Doppler vortex sensor simulation program.

  9. Numerical Study of Wake Vortex Behavior in Turbulent Domains with Ambient Stratification

    NASA Technical Reports Server (NTRS)

    Switzer, George F.; Proctor, Fred H.

    2000-01-01

    A three-dimensional large eddy simulation model is used to investigate the sensitivity of ambient stratification with turbulence on the behavior of aircraft wake vortices. Modeled ambient turbulence levels range from very weak to moderate, and stratification levels range from strongly stable to unstable. The results of profound significance from this study are: 1) very little sensitivity between vortex linking time and the level of stratification, 2) the mean vortex separation remained nearly constant regardless of stratification and turbulence (at least prior to linking), 3) the wake vortices did not rise regardless of the level of stratification, and 4) for very strong stratification, the vortex stopped descending and quickly dissipated even before vortex linking could occur. These results are supported by experimental data and are contrary to conclusions from other numerical studies that assume laminar flow and/or relatively-low Reynolds numbers.

  10. Aircraft Wake Vortex Measurements at Denver International Airport

    NASA Technical Reports Server (NTRS)

    Dougherty, Robert P.; Wang, Frank Y.; Booth, Earl R.; Watts, Michael E.; Fenichel, Neil; D'Errico, Robert E.

    2004-01-01

    Airport capacity is constrained, in part, by spacing requirements associated with the wake vortex hazard. NASA's Wake Vortex Avoidance Project has a goal to establish the feasibility of reducing this spacing while maintaining safety. Passive acoustic phased array sensors, if shown to have operational potential, may aid in this effort by detecting and tracking the vortices. During August/September 2003, NASA and the USDOT sponsored a wake acoustics test at the Denver International Airport. The central instrument of the test was a large microphone phased array. This paper describes the test in general terms and gives an overview of the array hardware. It outlines one of the analysis techniques that is being applied to the data and gives sample results. The technique is able to clearly resolve the wake vortices of landing aircraft and measure their separation, height, and sinking rate. These observations permit an indirect estimate of the vortex circulation. The array also provides visualization of the vortex evolution, including the Crow instability.

  11. A Candidate Wake Vortex Strength Definition for Application to the NASA Aircraft Vortex Spacing System (AVOSS)

    NASA Technical Reports Server (NTRS)

    Hinton, David A.; Tatnall, Chris R.

    1997-01-01

    A significant effort is underway at NASA Langley to develop a system to provide dynamical aircraft wake vortex spacing criteria to Air Traffic Control (ATC). The system under development, the Aircraft Vortex Spacing System (AVOSS), combines the inputs of multiple subsystems to provide separation matrices with sufficient stability for use by ATC and sufficient monitoring to ensure safety. The subsystems include a meteorological subsystem, a wake behavior prediction subsystem, a wake sensor subsystem, and system integration and ATC interfaces. The proposed AVOSS is capable of using two factors, singly or in combination, for reducing in-trail spacing. These factors are wake vortex motion out of a predefined approach corridor and wake decay below a strength that is acceptable for encounter. Although basic research into the wake phenomena has historically used wake total circulation as a strength parameter, there is a requirement for a more specific strength definition that may be applied across multiple disciplines and teams to produce a real-time, automated system. This paper presents some of the limitations of previous applications of circulation to aircraft wake observations and describes the results of a preliminary effort to bound a spacing system strength definition.

  12. Aircraft wake two-vortex system at turbulent equilibrium

    NASA Astrophysics Data System (ADS)

    Winckelmans, Gregoire; de Visscher, Ivan; Bricteux, Laurent

    2012-11-01

    We consider a two-vortex system (2VS) started from a 2-D initial condition of given energy (two opposite sign vortices, each with an algebraic circulation profile and with a relatively tight core, rc, compared to the distance, b0, center to center: rc /b0 = 0 . 05). The 2VS is submitted to a very weak and realistic atmospheric turbulence background (of energy < 0 . 01 that of the 2VS) so that it is excited to go unstable. The flow then generates, by non-linear interactions, instabilities and much more turbulence and eventually reaches a statistical equilibrium: a 2VS still with tight cores, with significant turbulence in the vortex oval, yet still laminar in the inner part of the cores, and which slowly decays in time. This state of equilibrium is quite universal (as confirmed by various sensitivity analyses). It is then of great importance to the physics and modeling of fully formed aircraft wake vortices and is characterized: spectrum, vorticity field, circulation profile and core size of the vortices in cross-planes and for the mean (i.e., longitudinal average). The two-scales Proctor-Winckelmans profile model is also compared to the data: it fits well the inner part 0 < r /b0 < 0 . 04 and the outer part 0 . 16 < r /b0 < 0 . 5 of the profile, but is still poor in between.

  13. Trailing Vortex Measurements in the Wake of a Hovering Rotor Blade with Various Tip Shapes

    NASA Technical Reports Server (NTRS)

    Martin, Preston B.; Leishman, J. Gordon

    2003-01-01

    This work examined the wake aerodynamics of a single helicopter rotor blade with several tip shapes operating on a hover test stand. Velocity field measurements were conducted using three-component laser Doppler velocimetry (LDV). The objective of these measurements was to document the vortex velocity profiles and then extract the core properties, such as the core radius, peak swirl velocity, and axial velocity. The measured test cases covered a wide range of wake-ages and several tip shapes, including rectangular, tapered, swept, and a subwing tip. One of the primary differences shown by the change in tip shape was the wake geometry. The effect of blade taper reduced the initial peak swirl velocity by a significant fraction. It appears that this is accomplished by decreasing the vortex strength for a given blade loading. The subwing measurements showed that the interaction and merging of the subwing and primary vortices created a less coherent vortical structure. A source of vortex core instability is shown to be the ratio of the peak swirl velocity to the axial velocity deficit. The results show that if there is a turbulence producing region of the vortex structure, it will be outside of the core boundary. The LDV measurements were supported by laser light-sheet flow visualization. The results provide several benchmark test cases for future validation of theoretical vortex models, numerical free-wake models, and computational fluid dynamics results.

  14. Near wake vortex dynamics of a hovering hawkmoth

    NASA Astrophysics Data System (ADS)

    Aono, Hikaru; Shyy, Wei; Liu, Hao

    2009-02-01

    Numerical investigation of vortex dynamics in near wake of a hovering hawkmoth and hovering aerodynamics is conducted to support the development of a biology-inspired dynamic flight simulator for flapping wing-based micro air vehicles. Realistic wing-body morphologies and kinematics are adopted in the numerical simulations. The computed results show 3D mechanisms of vortical flow structures in hawkmoth-like hovering. A horseshoe-shaped primary vortex is observed to wrap around each wing during the early down- and upstroke; the horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring with an intense jet-flow present in its core, forming a downwash. The doughnut-shaped vortex rings of the wing pair eventually break up into two circular vortex rings as they propagate downstream in the wake. The aerodynamic yawing and rolling torques are canceled out due to the symmetric wing kinematics even though the aerodynamic pitching torque shows significant variation with time. On the other hand, the time-varying the aerodynamics pitching torque could make the body a longitudinal oscillation over one flapping cycle.

  15. Viscous effects on a vortex wake in ground effect

    NASA Technical Reports Server (NTRS)

    Zheng, Z.; Ash, Robert L.

    1992-01-01

    Wake vortex trajectories and strengths are altered radically by interactions with the ground plane. Prediction of vortex strength and location is especially important in the vicinity of airports. Simple potential flow methods have been found to yield reasonable estimates of vortex descent rates in an otherwise quiescent ambient background, but those techniques cannot be adjusted for more realistic ambient conditions and they fail to provide satisfactory estimates of ground-coupled behavior. The authors have been involved in a systematic study concerned with including viscous effects in a wake-vortex system which is near the ground plane. The study has employed numerical solutions to the Navier-Stokes equations, as well as perturbation techniques to study ground coupling with a descending vortex pair. Results of a two-dimensional, unsteady numerical-theoretical study are presented in this paper. A time-based perturbation procedure has been developed which permits the use of analytical solutions to an inner and outer flow domain for the initial flow field. Predictions have been compared with previously reported laminar experimental results. In addition, the influence of stratification and turbulence on vortex behavior near the ground plane has been studied.

  16. A Parametric Study of Accelerations of an Airplane Due to a Wake Vortex System

    NASA Technical Reports Server (NTRS)

    Stewart, Eric C.

    1999-01-01

    A study was conducted using strip theory to systematically investigate the effects of progressively more complete descriptions of the interaction of an airplane with a wake vortex system. The emphasis was in roll-dominant, parallel, vortex encounters. That is, the simulated airplane's longitudinal axis was nearly parallel to the rotation axis of the vortex system for most of the results presented. The study began with a drag-less rectangular wing in the flow field of a single vortex and progressed to a complete airplane with aerodynamic surfaces possessing taper, sweep, dihedral, and stalling and immersed in the flow field of a vortex pair in ground effect. The effects of the pitch, roll, and yaw attitudes of the airplane on the calculated accelerations were also investigated. The airplane had the nominal characteristics of a Boeing 757, and the vortex flow field had the nominal characteristics of the wake of a Boeing 767. The Bumham-Hallock model of a vortex flow field was used throughout the study. The data are presented mainly in terms of contours of equal acceleration in a two-dimensional area centered on the vortex pair and having dimensions of 300 feet by 300 feet.

  17. Use of Individual Flight Corridors to Avoid Vortex Wakes

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.

    2001-01-01

    Vortex wakes of aircraft pose a hazard to following aircraft until the energetic parts of their flow fields have decayed to a harmless level. It is suggested here that in-trail spacings between aircraft can be significantly and safely reduced by designing an individual, vortex-free flight corridor for each aircraft. Because each aircraft will then have its own flight corridor, which is free of vortex wakes while in use by the assigned aircraft, the time intervals between aircraft operations can be safely reduced to the order of seconds. The productivity of airports can then be substantially increased. How large the offset distances between operational corridors need to be to have them vortex free, and how airports need to be changed to accommodate an individual flight-corridor process for landing and takeoff operations, are explored. Estimates are then made of the productivity of an individual flight-corridor system as a function of the in-trail time interval between operations for various values of wake decay time, runway width, and the velocity of a sidewind. The results confirm the need for short time intervals between aircraft operations if smaller offset distances and increased productivity are to be achieved.

  18. On the Development of Turbulent Wakes from Vortex Streets

    NASA Technical Reports Server (NTRS)

    Roshko, Anatol

    1953-01-01

    Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hotwire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At R = 40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. The range R = 150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream. It was found that in the stable range the vortex street has a periodic spanwise structure. The dependence of shedding frequency on velocity was successfully used to measure flow velocity. Measurements in the wake of a ring showed that an annular vortex street is developed.

  19. Numerical Study of a Long-Lived, Isolated Wake Vortex in Ground Effect

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.

    2014-01-01

    This paper examines a case observed during the 1990 Idaho Falls Test program, in which a wake vortex having an unusually long lifetime was observed while in ground effect. A numerical simulation is performed with a Large Eddy Simulation model to understand the response of the environment in affecting this event. In the simulation, it was found that one of the vortices decayed quickly, with the remaining vortex persisting beyond the time-bound of typical vortex lifetimes. This unusual behavior was found to be related to the first and second vertical derivatives of the ambient crosswind.

  20. NASA Langley Research Center Wake Vortex Research Supporting VAMS

    NASA Technical Reports Server (NTRS)

    Rutishauser, David

    2002-01-01

    NASA researchers have designed a system to predict aircraft wake turbulence on final approach, so airliners can be spaced more safely and efficiently. This technology, known as the Aircraft VOrtex Spacing System (AVOSS), demonstrates an integration of technologies that provides weather-dependent dynamic aircraft spacing for wake avoidance in a real-time relevant environment. AVOSS was successfully demonstrated at Dallas Fort-Worth Airport in July 2000. The demonstration represented the culmination of 6 years of field-testing, data collection, and development.

  1. Wake vortex measurements of bodies at high angle of attack

    NASA Technical Reports Server (NTRS)

    Owen, F. K.; Johnson, D. A.

    1978-01-01

    Three-dimensional laser velocimeter measurements have been made of the wake vortices of a slender tangent-ogive body which had nose and body fineness ratios of 3.5 and 12, respectively. Data were obtained for an angle of attack to seminose angle ratio of 2.3 at a free-stream Mach number of 0.6 and unit Reynolds number of 2 million/ft. Details of the mean flow field are presented and features of the turbulent and unsteady nature of the vortex flow field are discussed. Problems associated with obtaining meaningful vortex measurements in high-speed flows are addressed.

  2. Characterizing the Hazard of a Wake Vortex Encounter

    NASA Technical Reports Server (NTRS)

    Vicroy, Dan D.; Brandon, Jay; Greene, George; Rivers, Robert; Shah, Gautam; Stewart, Eric; Stuever, Robert

    1998-01-01

    The National Aeronautics and Space Administration (NASA) is conducting research with the goal of enabling safe improvements in the capacity of the nation's air transportation system. The wake vortex upset hazard is an important factor in establishing the minimum safe spacing between aircraft during landing and take-off operations, thus impacting airport capacity. Static and free-flight wind tunnel tests and flight tests have provided an extensive data set for improved understanding of vortex encounter dynamics and simulation. Piloted and batch simulation studies are also ongoing to establish a first-order hazard metric and determine the limits of an operationally acceptable wake induced upset. This paper outlines NASA's research in these areas.

  3. Evaluation of the discrete vortex wake cross flow model using vector computers. Part 2: User's manual for DIVORCE

    NASA Technical Reports Server (NTRS)

    Deffenbaugh, F. D.; Vitz, J. F.

    1979-01-01

    The users manual for the Discrete Vortex Cross flow Evaluator (DIVORCE) computer program is presented. DIVORCE was developed in FORTRAN 4 for the DCD 6600 and CDC 7600 machines. Optimal calls to a NASA vector subroutine package are provided for use with the CDC 7600.

  4. Vortex age as a wake turbulence scaling parameter

    NASA Technical Reports Server (NTRS)

    Marshall, J. R.; Marchman, J. F., III

    1973-01-01

    Research which was conducted to determine the significance of vortex age as a scaling parameter in wake turbulence development and dissipation is reported. Tests were conducted at three angles of attack, three free stream speeds, and seven downstream positions from 2 to 30 chordlengths using an NACA 0012 wing and a five hole yawhead pitot probe. The end surface of the wing tip was flat. Speeds were selected to give a predetermined range of vortex ages. The complete velocity structure of the vortex was measured at each station and speed. The resulting plots of maximum tangential velocity and vortex core diameter versus downstream distance and vortex age indicate that vortex age is not a self sufficient scaling parameter. In addition to the expected effect of lift coefficient there is also a definite free stream speed influence at high wing angles of attack. The exact cause and nature of this effect is not fully understood, but it does not appear to be explainable in terms of Mach number or Reynolds number; however, the influence of tip edge shape on spanwise flow separation appears to be an important factor.

  5. Rotor Vortex Wake in Close Proximity of Walls in Hover

    NASA Astrophysics Data System (ADS)

    Konus, Mehmet Fatih; Savas, Omer

    2014-11-01

    Expanding flight envelopes of rotorcraft raise concerns about their behavior in very close proximity of walls or corners where the separation between the wall and the rotor disk can almost vanish. A series of experiments are conducted in a water tank to study the hover behavior of the wake of a 25-cm diameter three-bladed rotor at 8 rev/s. Particle image velocimetry, strain gage force balance measurements and flow visualization are employed. The vortex wake, which is axisymmetric on the average in an unbounded surrounding, is distorted increasingly with decreasing separation from a wall or corner. The vortex wake bends toward the wall and into the corner. The individual helical filaments off the rotor tips are distorted and closely follow the wall. Intermittent reversed vortical flow regions appear upstream of the rotor disk in the proximity of the wall. The mean streamlines indicate that the wake is bent toward the wall or into the corner. The component of the thrust vector along the axis of the rotor decreases. These observation suggest that the thrust vector progressively deviates from the geometric axis of the rotor.

  6. A self-consistent model for the saturation dynamics of the vortex shedding around the mean flow in the unstable cylinder wake

    NASA Astrophysics Data System (ADS)

    Mantič-Lugo, Vladislav; Arratia, Cristóbal; Gallaire, François

    2015-07-01

    The supercritical instability leading to the Bénard-von Karman vortex street in a cylinder wake is a well known example of supercritical Hopf bifurcation: the steady solution becomes linearly unstable and saturates into a periodic limit cycle. Nonetheless, a simplified physical formulation accurately predicting the transition dynamics of the saturation process is lacking. Building upon our previous work, we present here a simple self-consistent model that provides a clear description of the saturation mechanism in a quasi-steady manner by means of coupling the instantaneous mean flow with its most unstable eigenmode and its instantaneous amplitude through the Reynolds stress. The system is coupled for different oscillation amplitudes, providing an instantaneous mean flow as function of an equivalent time. A transient physical picture is described, wherein a harmonic perturbation grows and changes in amplitude, frequency, and structure due to the modification of the mean flow by the Reynolds stress forcing, saturating when the flow is marginally stable. Comparisons with direct numerical simulations show an accurate prediction of the instantaneous amplitude, frequency, and growth rate, as well as the saturated mean flow, the oscillation amplitude, frequency, and the resulting mean Reynolds stresses.

  7. Influence of structural flexibility on the wake vortex pattern of airfoils undergoing harmonic pitch oscillation

    NASA Astrophysics Data System (ADS)

    Monnier, B.; Naguib, A. M.; Koochesfahani, M. M.

    2015-04-01

    Reported herein is an investigation of the influence of the structural flexibility of sinusoidally pitching airfoils on the pattern of vorticity shed into the wake. For rigid airfoils, it is well known that, depending on the oscillation frequency and amplitude, this pattern takes the form of the classical or reverse von Kármán vortex street. The pattern may be characterized by the vortex circulation ( Γ o ), vortex-to-vortex streamwise and cross-stream spacing ( a and b, respectively), and vortex core radius ( R). In the present work, these four parameters are obtained from particle image velocimetry measurements in the wake of airfoils consisting of a rigid "head" and flexible "tail" at chord Reynolds number of 2010 for different tail flexibilities. The results show that flexible airfoils exhibit the switch from classical to reverse von Kármán vortex street (i.e., change in the sign of b) at a reduced frequency of oscillation lower than their rigid counterpart. At a given oscillation frequency, the Strouhal number at which this switch occurs is smallest for a given airfoil structural flexibility; which becomes stiffer with increasing frequency. Using Strouhal number based on the actual trailing edge oscillation amplitude, reasonable scaling is found of the dependence of not only b but also Γ o , a and R on the motion and structure parameters for all airfoils investigated. These results are complemented with analyses using a vortex array model, which together with the identified scaling of the wake vortex parameters, provide basis for the computation of the net thrust acting on the airfoil.

  8. Cavitation and Wake Structure of Unsteady Tip Vortex Flows.

    NASA Astrophysics Data System (ADS)

    Hart, Douglas Payton

    Unsteady flows are prevalent in virtually every fluid application yet, because of their intrinsic complexity, few attempts have been made to measure them or explain their behavior. This thesis presents an experimental study of one of the simplest unsteady flow induced effects, the periodic change in angle of attack of a lifting surface. Of particular interest is the influence this effect has on the tip vortex structure of a finite aspect ratio hydrofoil and the part it plays in the inception of cavitation. An aspect ratio 2.3 hydrofoil was reflection-plane mounted to the test section floor of the Caltech Low Turbulence Water Tunnel and harmonically oscillated in pitch near its center of pressure. Observations of the growth and collapse of surface and tip vortex cavitation were made along with detailed observations of the interaction of the tip vortex formation with the spanwise wake structure. Measurements of the cavitation inception number for surface cavitation and tip vortex cavitation were made relative to the phase of the hydrofoil and the reduced frequency, k = omegac/2U_{infty }, of oscillation. Studies of the oscillation -induced spanwise trailing vortex structures and the Karman vortex street generated by the boundary layer were made of a two-dimensional hydrofoil. Laser Doppler Velocimetry (LDV) measurements were taken of the tip vortex velocity profile and the flow at the trailing edge of both the two - and the three-dimensional hydrofoils at reduced frequencies ranging from 0.5 to 2.0. Dynamic changes in bound circulation and shed vorticity in the streamwise and spanwise directions relative to the freestream were calculated from these measurements at three locations along the span of the foil. The results of these measurements are compared to theoretical flow calculations and related to measurements of the cavitation inception number in the tip vortex region of the three -dimensional foil.

  9. Validation of Vortex-Lattice Method for loads on wings in lift-generated wakes

    NASA Technical Reports Server (NTRS)

    Rossow, J.

    1994-01-01

    A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by use of measurements made in the 80- by 120-foot wind tunnel of the lift, rolling-moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling moment distributions are used to determine the accuracy of the vortex-lattice code. It was found that the vortex-lattice method is very reliable as long as the span of the encountering of following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts. The increase in deviation of the computed from the measured loads with size of the following wing is attributed to the increase in distortion of the structure of the vortex wake as it approaches and passes the larger following wings.

  10. Large-Eddy Simulations and Lidar Measurements of Vortex-Pair Breakup in Aircraft Wakes

    NASA Technical Reports Server (NTRS)

    Lewellen, D. C.; Lewellen, W. S.; Poole, L. R.; DeCoursey, R. J.; Hansen, G. M.; Hostetler, C. A.; Kent, G. S.

    1998-01-01

    Results of large-eddy simulations of an aircraft wake are compared with results from ground-based lidar measurements made at NASA Langley Research Center during the Subsonic Assessment Near-Field Interaction Flight Experiment field tests. Brief reviews of the design of the field test for obtaining the evolution of wake dispersion behind a Boeing 737 and of the model developed for simulating such wakes are given. Both the measurements and the simulations concentrate on the period from a few seconds to a few minutes after the wake is generated, during which the essentially two-dimensional vortex pair is broken up into a variety of three-dimensional eddies. The model and experiment show similar distinctive breakup eddies induced by the mutual interactions of the vortices, after perturbation by the atmospheric motions.

  11. Flight test investigation of the vortex wake characteristics behind a Boeing 727 during two-segment and normal ILS approaches

    NASA Technical Reports Server (NTRS)

    Garodz, L. J.

    1975-01-01

    A series of flight tests were performed to evaluate the vortex wake characteristics of a Boeing 727 (B727-200) aircraft during conventional and two-segment ILS approaches. Flights of the B727, equipped with smoke generators for vortex marking, were flown wherein its vortex wake was intentionally encountered by a Lear Jet model 23 (LR-23) or a Piper Twin Comanche (Pa-30); and its vortex location during landing approach was measured using a system of photo-theodolites. The tests showed that at a given separation distance there were no differences in the upsets resulting from deliberate vortex encounters during the two types of approaches. Timed mappings of the position of the landing configuration vortices showed that they tended to descend approximately 91 meters (300 feet) below the flight path of the B727. The flaps of the B727 have a dominant effect on the character of the trailed wake vortex. The clean wing produces a strong, concentrated vortex. As the flaps are lowered, the vortex system becomes more diffuse. Pilot opinion and roll acceleration data indicate that 4.5 nautical miles would be a minimum separation distance at which roll control could be maintained during parallel encounters of the B727's landing configuration wake by small aircraft.

  12. Video images of smoke dispersion in the near wake of a model building. Part 1. Temporal and spatial scales of vortex shedding

    SciTech Connect

    Huber, A.H.

    1988-01-01

    In a wind-tunnel study, recorded video images of smoke dispersion in the wake of a rectangular-shaped building were analyzed. A continuous source of smoke was emitted at floor level, midway along the leeward side of the building. Smoke was observed to build up within a region adjacent to the building. Then the smoke was periodically swept away by vortices shed from the leeward building sides and roof. The applicability of analyzing time series of smoke scattered light for selected quantized video picture elements was favorably demonstrated. Temporal and spatial scales of vortex-shedding of smoke puffs near the center of the wake were quantified through the calculation of spectral densities and autocorrelations. The dominant frequencies (cycles/second) near the center of the wake flow were found to be 0.1-0.3 times the ratio of the velocity to the building height. Estimated integral length scales near the center of the wake flow were found to be 1-2 times the building height.

  13. Validation of Vortex-Lattice Method for Loads on Wings in Lift-Generated Wakes

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.

    1995-01-01

    A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by the use of measurements made in the 80 by 120 ft Wind Tunnel of the lift, rolling moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling-moment distributions the vortex-lattice method is very reliable as long as the span of the encountering or following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts.

  14. On the Development of Turbulent Wakes from Vortex Streets

    NASA Technical Reports Server (NTRS)

    Roshko, Anatol

    1954-01-01

    Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hot-wire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At r=40 to 150, called the stable range, regular vortex streets are formed and no turbulent velocity fluctuations accompany the periodic formation of vortices. The range r=150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. The transition first occurs in the range r=150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations.

  15. Generation of vortex rings by nonstationary laser wake field

    SciTech Connect

    Tsintsadze, N.L.; Murtaza, G.; Shah, H.A.

    2006-01-15

    A new concept of generating quasistatic magnetic fields, vortex rings, and electron jets in an isotropic homogeneous plasma is presented. The propagation of plasma waves, generated by a relativistically intense short pulse laser, is investigated by using the kinetic model and a novel nonpotential, time-dependent ponderomotive force is derived by obtaining a hydrodynamic equation of motion. This force can in turn generate quasistatic magnetic fields, vortex rings, and electron jets. It is also shown that the vortex rings can become a means for accelerating electrons, which are initially in equilibrium. The conservation of canonical momentum circulation and the frozen-in condition for the vorticity is discussed. The excitation of the vortex waves by the modulation of the amplitude of the plasma waves is considered. These vortex waves, which generate a lower hybrid mode propagating across the generated magnetic field, are also investigated.

  16. Spectral Characteristics of Wake Vortex Sound During Roll-Up

    NASA Technical Reports Server (NTRS)

    Booth, Earl R., Jr. (Technical Monitor); Zhang, Yan; Wang, Frank Y.; Hardin, Jay C.

    2003-01-01

    This report presents an analysis of the sound spectra generated by a trailing aircraft vortex during its rolling-up process. The study demonstrates that a rolling-up vortex could produce low frequency (less than 100 Hz) sound with very high intensity (60 dB above threshold of human hearing) at a distance of 200 ft from the vortex core. The spectrum then drops o rapidly thereafter. A rigorous analytical approach has been adopted in this report to derive the spectrum of vortex sound. First, the sound pressure was solved from an alternative treatment of the Lighthill s acoustic analogy approach [1]. After the application of Green s function for free space, a tensor analysis was applied to permit the removal of the source term singularity of the wave equation in the far field. Consequently, the sound pressure is expressed in terms of the retarded time that indicates the time history and spacial distribution of the sound source. The Fourier transformation is then applied to the sound pressure to compute its spectrum. As a result, the Fourier transformation greatly simplifies the expression of the vortex sound pressure involving the retarded time, so that the numerical computation is applicable with ease for axisymmetric line vortices during the rolling-up process. The vortex model assumes that the vortex circulation is proportional to the time and the core radius is a constant. In addition, the velocity profile is assumed to be self-similar along the aircraft flight path, so that a benchmark vortex velocity profile can be devised to obtain a closed form solution, which is then used to validate the numerical calculations for other more realistic vortex profiles for which no closed form solutions are available. The study suggests that acoustic sensors operating at low frequency band could be profitably deployed for detecting the vortex sound during the rolling-up process.

  17. NASA AVOSS Fast-Time Wake Prediction Models: User's Guide

    NASA Technical Reports Server (NTRS)

    Ahmad, Nash'at N.; VanValkenburg, Randal L.; Pruis, Matthew

    2014-01-01

    The National Aeronautics and Space Administration (NASA) is developing and testing fast-time wake transport and decay models to safely enhance the capacity of the National Airspace System (NAS). The fast-time wake models are empirical algorithms used for real-time predictions of wake transport and decay based on aircraft parameters and ambient weather conditions. The aircraft dependent parameters include the initial vortex descent velocity and the vortex pair separation distance. The atmospheric initial conditions include vertical profiles of temperature or potential temperature, eddy dissipation rate, and crosswind. The current distribution includes the latest versions of the APA (3.4) and the TDP (2.1) models. This User's Guide provides detailed information on the model inputs, file formats, and the model output. An example of a model run and a brief description of the Memphis 1995 Wake Vortex Dataset is also provided.

  18. Development of a Wake Vortex Spacing System for Airport Capacity Enhancement and Delay Reduction

    NASA Technical Reports Server (NTRS)

    Hinton, David A.; OConnor, Cornelius J.

    2000-01-01

    The Terminal Area Productivity project has developed the technologies required (weather measurement, wake prediction, and wake measurement) to determine the aircraft spacing needed to prevent wake vortex encounters in various weather conditions. The system performs weather measurements, predicts bounds on wake vortex behavior in those conditions, derives safe wake spacing criteria, and validates the wake predictions with wake vortex measurements. System performance to date indicates that the potential runway arrival rate increase with Aircraft VOrtex Spacing System (AVOSS), considering common path effects and ATC delivery variance, is 5% to 12% depending on the ratio of large and heavy aircraft. The concept demonstration system, using early generation algorithms and minimal optimization, is performing the wake predictions with adequate robustness such that only 4 hard exceedances have been observed in 1235 wake validation cases. This performance demonstrates the feasibility of predicting wake behavior bounds with multiple uncertainties present, including the unknown aircraft weight and speed, weather persistence between the wake prediction and the observations, and the location of the weather sensors several kilometers from the approach location. A concept for the use of the AVOSS system for parallel runway operations has been suggested, and an initial study at the JFK International Airport suggests that a simplified AVOSS system can be successfully operated using only a single lidar as both the weather sensor and the wake validation instrument. Such a selfcontained AVOSS would be suitable for wake separation close to the airport, as is required for parallel approach concepts such as SOIA.

  19. A flight evaluation of methods for predicting vortex wake effects on trailing aircraft

    NASA Technical Reports Server (NTRS)

    Robinson, G. H.; Larson, R. R.

    1972-01-01

    The results of four current analytical methods for predicting wing vortex strength and decay rate are compared with the results of a flight investigation of the wake characteristics of several large jet transport aircraft. An empirical expression defining the strength and decay rate of wake vortices is developed that best represents most of the flight-test data. However, the expression is not applicable to small aircraft that would be immersed in the vortex wake of large aircraft.

  20. Stability of a wind turbine wake subject to root vortex perturbations

    NASA Astrophysics Data System (ADS)

    Smith, David; Blackburn, Hugh; Sheridan, John

    2013-11-01

    Results for DNS of a wind turbine wake will be presented. The Tjæborg wind turbine geometry is modelled using a spectral element solver in coupled to an actuator line model described by Sørensen and Shen (2002). The actuator line model considers the flow over the turbine by calculating body forces derived from two-dimensional airfoil data and flow velocity localised at the blade. Using such a model, Ivanell et al. (2010) identified instabilities in the tip vortex for sinusoidal perturbations that reduced the streamwise spacing between tip vortices. In work to be presented we consider perturbations to the blade-root vortex of the turbine. We examine whether perturbations to the root vortex can excite instability mechanisms in the tip vortex and potentially modify tip vortex downstream extents. We also explore how changes to the spacing between root and tip vortices modifies these effects. Ivanell et al. (2010) J Wind Energy 13, Sørensen and Shen. (2002) J Fluids Eng 124. Supported by Australian Research Council grant DP1096444.

  1. An Investigation of Candidate Sensor-Observable Wake Vortex Strength Parameters for the NASA Aircraft Vortex Spacing System (AVOSS)

    NASA Technical Reports Server (NTRS)

    Tatnall, Chistopher R.

    1998-01-01

    The counter-rotating pair of wake vortices shed by flying aircraft can pose a threat to ensuing aircraft, particularly on landing approach. To allow adequate time for the vortices to disperse/decay, landing aircraft are required to maintain certain fixed separation distances. The Aircraft Vortex Spacing System (AVOSS), under development at NASA, is designed to prescribe safe aircraft landing approach separation distances appropriate to the ambient weather conditions. A key component of the AVOSS is a ground sensor, to ensure, safety by making wake observations to verify predicted behavior. This task requires knowledge of a flowfield strength metric which gauges the severity of disturbance an encountering aircraft could potentially experience. Several proposed strength metric concepts are defined and evaluated for various combinations of metric parameters and sensor line-of-sight elevation angles. Representative populations of generating and following aircraft types are selected, and their associated wake flowfields are modeled using various wake geometry definitions. Strength metric candidates are then rated and compared based on the correspondence of their computed values to associated aircraft response values, using basic statistical analyses.

  2. Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure.

    PubMed

    Flammang, Brooke E; Lauder, George V; Troolin, Daniel R; Strand, Tyson

    2011-12-22

    Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns. PMID:21543357

  3. Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure

    PubMed Central

    Flammang, Brooke E.; Lauder, George V.; Troolin, Daniel R.; Strand, Tyson

    2011-01-01

    Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns. PMID:21543357

  4. Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel.

    PubMed

    Henningsson, P; Spedding, G R; Hedenström, A

    2008-03-01

    In this paper we describe the flight characteristics of a swift (Apus apus) in cruising flight at three different flight speeds (8.0, 8.4 and 9.2 m s(-1)) in a low turbulence wind tunnel. The wingbeat kinematics were recorded by high-speed filming and the wake of the bird was visualized by digital particle image velocimetry (DPIV). Certain flight characteristics of the swift differ from those of previously studied species. As the flight speed increases, the angular velocity of the wingbeat remains constant, and so as the wingbeat amplitude increases, the frequency decreases accordingly, as though the flight muscles were contracting at a fixed rate. The wings are also comparatively inflexible and are flexed or retracted rather little during the upstroke. The upstroke is always aerodynamically active and this is reflected in the wake, where shedding of spanwise vorticity occurs throughout the wingbeat. Although the wake superficially resembles those of other birds in cruising flight, with a pair of trailing wingtip vortices connected by spanwise vortices, the continuous shedding of first positive vorticity during the downstroke and then negative vorticity during the upstroke suggests a wing whose circulation is gradually increasing and then decreasing during the wingbeat cycle. The wake (and implied wing aerodynamics) are not well described by discrete vortex loop models, but a new wake-based model, where incremental spanwise and streamwise variations of the wake impulse are integrated over the wingbeat, shows good agreement of the vertical momentum flux with the required weight support. The total drag was also estimated from the wake alone, and the calculated lift:drag ratio of approximately 13 for flapping flight is the highest measured yet for birds. PMID:18281334

  5. Dynamic Wind Loads and Vortex Structures in the Wake of a Wind Turbine

    NASA Astrophysics Data System (ADS)

    Hu, Hui; Yang, Zifeng; Sarkar, Partha

    2010-11-01

    We report an experimental study to characterize the dynamic wind loads and evolution of wake vortex flow structures downstream of a horizontal axis wind turbine (HAWT). The experiments were conducted in a wind tunnel with a wind turbine model placed in a boundary layer flow developed over rough and smooth surfaces in order to study the effects of roughness and the resulting velocity and turbulence fields on the wake characteristics and fatigue loads acting on the wind turbine. In addition to measuring dynamic wind loads (both aerodynamic forces and moments) acting on the wind turbine model using a six-component load cell, a high-resolution Particle Image Velocimetry (PIV) system was used to make phase-locked flow field measurements to quantify the time-evolution of the wake vortex and turbulence flow structures shedding from wind turbine blades. The detailed flow field measurements were correlated with the wind load measurements to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on wind turbines.

  6. An experimental investigation of bending wave instability modes in a generic four-vortex wake

    SciTech Connect

    Babie, Brian M.; Nelson, Robert C.

    2010-07-15

    An experimental study of a planar wake consisting of four vortices that simulate the trailing vortex wakes generated by transport airplanes in either takeoff or landing configurations is presented. The objective of this study was to examine naturally occurring wake instabilities. Specifically, the focus of the study was centered on bending wave instabilities of which the Crow instability represents a particular case. A unique method of generating a four-vortex wake was developed for this study. The four-vortex wake generating device permitted direct variation of the spacing between vortices as well as control over the vortex circulation strength. Two quantitative flow visualization experiments were instrumental in identifying wake configurations that were conducive to the rapid growth of bending wave modes and in the identification of the long-wavelength mode. Detailed experiments were also conducted to examine the flow structure in the near-field or roll-up region using a four sensor, hot-wire probe that could measure all three velocity components in the wake simultaneously. The results of both the flow visualization and hot-wire experiments indicate that the long-wavelength mode and the first short-wavelength mode likely dominate the far-field wake physics and may potentially be utilized in a wake control strategy.

  7. Vortex Formation in the Wake of Dark Matter Propulsion

    NASA Astrophysics Data System (ADS)

    Robertson, G. A.; Pinheiro, M. J.

    Future spaceflight will require a new theory of propulsion; specifically one that does not require mass ejection. A new theory is proposed that uses the general view that closed currents pervade the entire universe and, in particular, there is a cosmic mechanism to expel matter to large astronomical distances involving vortex currents as seen with blazars and blackholes. At the terrestrial level, force producing vortices have been related to the motion of wings (e.g., birds, duck paddles, fish's tail). In this paper, vortex structures are shown to exist in the streamlines aft of a spaceship moving at high velocity in the vacuum. This is accomplished using the density excitation method per a modified Chameleon Cosmology model. This vortex structure is then shown to have similarities to spacetime models as Warp-Drive and wormholes, giving rise to the natural extension of Hawking and Unruh radiation, which provides the propulsive method for space travel where virtual electron-positron pairs, absorbed by the gravitational expansion forward of the spaceship emerge from an annular vortex field aft of the spaceship as real particles, in-like to propellant mass ejection in conventional rocket theory.

  8. A comparison of airborne wake vortex detection measurements with values predicted from potential theory

    NASA Technical Reports Server (NTRS)

    Stewart, Eric C.

    1991-01-01

    An analysis of flight measurements made near a wake vortex was conducted to explore the feasibility of providing a pilot with useful wake avoidance information. The measurements were made with relatively low cost flow and motion sensors on a light airplane flying near the wake vortex of a turboprop airplane weighing approximately 90000 lbs. Algorithms were developed which removed the response of the airplane to control inputs from the total airplane response and produced parameters which were due solely to the flow field of the vortex. These parameters were compared with values predicted by potential theory. The results indicated that the presence of the vortex could be detected by a combination of parameters derived from the simple sensors. However, the location and strength of the vortex cannot be determined without additional and more accurate sensors.

  9. Thrust Production and Wake Structure of an Actuated Lamprey Model

    NASA Astrophysics Data System (ADS)

    Buchholz, James; Smits, Alexander

    2004-11-01

    Thrust generation is studied for a flexible lamprey model which is actuated periodically to produce a streamwise traveling wave. Shape memory alloy actuators are used to achieve this deformation. The flow field is investigated using DPIV and flow visualization for a range of Strouhal numbers based on peak-to-peak amplitude of the trailing edge. The vortex kinematics in the spanwise and streamwise planes are examined, and a three-dimensional unsteady vortex model of the wake will be discussed.

  10. Three-dimensional vortex wake structure of flapping wings in hovering flight.

    PubMed

    Cheng, Bo; Roll, Jesse; Liu, Yun; Troolin, Daniel R; Deng, Xinyan

    2014-02-01

    Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity. PMID:24335561

  11. Three-dimensional vortex wake structure of flapping wings in hovering flight

    PubMed Central

    Cheng, Bo; Roll, Jesse; Liu, Yun; Troolin, Daniel R.; Deng, Xinyan

    2014-01-01

    Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity. PMID:24335561

  12. Separation of Lift-Generated Vortex Wakes Into Two Diverging Parts

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.; Brown, Anthony P.

    2010-01-01

    As part of an ongoing study of the spreading rate of lift-generated vortex wakes, the present investigation considers possible reasons as to why segments of lift-generated wakes sometimes depart from the main part of the wake to move rapidly in either an upward or downward direction. It is assumed that deficiencies or enhancements of the lift carry over across the fuselage-shrouded wing are the driving mechanism for departures of wake-segments. The computations presented first indicate that upwardly departing wake segments that were observed and photographed could have been produced by a deficiency in lift carryover across the fuselage-shrouded part of the wing. Computations made of idealized vortex wakes indicate that upward departure of a wake segment requires a centerline reduction in the span loading of 70% or more, whether the engines are at idle or robust thrust. Similarly, it was found that downward departure of wake segments is produced when the lift over the center part of the wing is enhanced. However, it was also found that downward departures do not occur without the presence of robust engine-exhaust streams (i.e., engines must NOT be at idle). In those cases, downward departures of a wake segment occurs when the centerline value of the loading is enhanced by any amount between about 10% to 100%. Observations of condensation trails indicate that downward departure of wake segments is rare. Upward departures of wake segments appears to be more common but still rare. A study to determine the part of the aircraft that causes wake departures has not been carried out. However, even though departures of wake segments rarely occur, some aircraft do regularly shed these wake structures. If aircraft safety is to be assured to a high degree of reliability, and a solution for eliminating them is not implemented, existing guidelines for the avoidance of vortex wakes [1,3] may need to be broadened to include possible increases in wake sizes caused by vertical departures of wake segments. Further study may indicate that it is not possible to modify existing aircraft enough to prevent wake departures. Wake-avoidance guidelines must then be adjusted to provide the desired degree of safety. It appears that steps to avoid upwardly moving wake segments have already been incorporated into the avoidance procedures used for aircraft on approach to runways at the Frankfurt Airport [6,7]. The uncertainty in the prospects for compromises in flight safety caused by rapidly upwardly or downwardly moving wake segments suggest that it be specified that aircraft do not fly above or below each other during operations in the airport vicinity where aircraft are likely to be closely spaced [20].

  13. Unsteady vortex lattice techniques applied to wake formation and performance of the statically thrusting propeller

    NASA Technical Reports Server (NTRS)

    Hall, G. F.

    1975-01-01

    The application is considered of vortex lattice techniques to the problem of describing the aerodynamics and performance of statically thrusting propellers. A numerical lifting surface theory to predict the aerodynamic forces and power is performed. The chordwise and spanwise loading is modelled by bound vortices fixed to a twisted flat plate surface. In order to eliminate any apriori assumptions regarding the wake shape, it is assumed the propeller starts from rest. The wake is generated in time and allowed to deform under its own self-induced velocity field as the motion of the propeller progresses. The bound circulation distribution is then determined with time by applying the flow tangency boundary condition at certain selected control points on the blades. The aerodynamics of the infinite wing and finite wing are also considered. The details of wake formation and roll-up are investigated, particularly the localized induction effect. It is concluded that proper wake roll-up and roll-up rates can be established by considering the details of motion at the instant of start.

  14. Two Dimensional Parametric Studies of Wake Vortex Interaction with the Atmosphere

    NASA Technical Reports Server (NTRS)

    Proctor, Fred

    1997-01-01

    Results from parametric runs using two-dimensional TASS are presented. First, a set of experiments are presented that examine the sensitivity of the aircraft initiation height for an "in ground effect" case with weak crosswind. Interaction between the ground and the wake vortex produces an oscillatory rebound whose phase and amplitude are a function of the generation height. A second set of experiments are presented which examine the influence on crosswind shear. Shear layers, such as may be found between the nocturnal stable layer and the residual layer, can act to deflect vortices upward. Further investigation reveals that the second derivative of the crosswind can differentially reduce the descent speed of each member of a vortex pair, causing tilting of the vortex pair. If sufficiently large, the second derivative of crosswind can deflect the vortex pair upwards, with the sign of the second derivative determining which of the two vortices rises to a higher altitude. Linear shear, on the other hand, caused no change in the descent speed of the vortices; thus having no effect on the orientation of the vortices. Observed and model data from an actual case are presented in support of the conclusion regarding the influence of shear on rising vortices.

  15. The timing of vortex shedding in a cylinder wake imposed by periodic inflow perturbations

    NASA Astrophysics Data System (ADS)

    Konstantinidis, E.; Balabani, S.; Yianneskis, M.

    2005-10-01

    The interaction of vortex shedding from a circular cylinder with an inflow which has low-amplitude periodic velocity oscillations (perturbations) superimposed upon it, was investigated experimentally by means of particle image velocimetry. The experiments were made at three perturbation frequencies across the lock-on range in which the vortex shedding frequency is synchronized with the subharmonic of the imposed frequency. The basic wake pattern in this range is antisymmetric vortex shedding, i.e. the familiar 2S mode. The timing of vortex shedding is defined with respect to the cross-flow oscillation of the wake which is found to play a critical role. Quantitative analysis of the phase-referenced patterns of vorticity distribution in the wake shows that a vortex is actually shed from the cylinder when the cross-flow oscillation of the wake is strongest, marked by a sudden drop in the computed vortex strength. At the middle of the lock-on range, shedding occurs near the minimum inflow velocity in the cycle or, equivalently, during the forward stroke of a cylinder oscillating in-line with the flow. It is argued that the imposed timing of vortex shedding relative to the cylinder motion induces a negative excitation from the fluid, which might explain why the in-line response of a freely vibrating cylinder exhibits two positive excitation regions separated by the lock-on region found in forced oscillations.

  16. An investigation of the increase in vortex induced rolling moment associated with landing gear wake

    NASA Technical Reports Server (NTRS)

    Patterson, J. C., Jr.; Jordan, F. L., Jr.

    1975-01-01

    Flight tests were conducted to verify the results found in ground base facilities of the effect of span lift load variation as well as the vortex attentuation of the high energy jet engine exhaust through proper thrust programming. During these flight tests a large increase in vortex strength was experienced as a result of extending the landing gear. Tests in the Langley Vortex Research Facility indicate that the wake produced by the landing gear may possibly form an aerodynamic endplate or reflection plane at the inboard edge of each inboard flap which increases the effective aspect ratio of the flap and thereby increases the strength of the flap outer edge vortex.

  17. Evaluation of Fast-Time Wake Models Using Denver 2006 Field Experiment Data

    NASA Technical Reports Server (NTRS)

    Ahmad, Nash’at N.; Pruis, Matthew J.

    2015-01-01

    The National Aeronautics and Space Administration conducted a series of wake vortex field experiments at Denver in 2003, 2005, and 2006. This paper describes the lidar wake vortex measurements and associated meteorological data collected during the 2006 deployment, and includes results of recent reprocessing of the lidar data using a new wake vortex algorithm and estimates of the atmospheric turbulence using a new algorithm to estimate eddy dissipation rate from the lidar data. The configuration and set-up of the 2006 field experiment allowed out-of-ground effect vortices to be tracked in lateral transport further than any previous campaign and thereby provides an opportunity to study long-lived wake vortices in moderate to low crosswinds. An evaluation of NASA's fast-time wake vortex transport and decay models using the dataset shows similar performance as previous studies using other field data.

  18. Wake Geometry Measurements and Analytical Calculations on a Small-Scale Rotor Model

    NASA Technical Reports Server (NTRS)

    Ghee, Terence A.; Berry, John D.; Zori, Laith A. J.; Elliott, Joe W.

    1996-01-01

    An experimental investigation was conducted in the Langley 14- by 22-Foot Subsonic Tunnel to quantify the rotor wake behind a scale model helicopter rotor in forward level flight at one thrust level. The rotor system in this test consisted of a four-bladed fully articulated hub with blades of rectangular planform and an NACA 0012 airfoil section. A laser light sheet, seeded with propylene glycol smoke, was used to visualize the vortex geometry in the flow in planes parallel and perpendicular to the free-stream flow. Quantitative measurements of wake geometric proper- ties, such as vortex location, vertical skew angle, and vortex particle void radius, were obtained as well as convective velocities for blade tip vortices. Comparisons were made between experimental data and four computational method predictions of experimental tip vortex locations, vortex vertical skew angles, and wake geometries. The results of these comparisons highlight difficulties of accurate wake geometry predictions.

  19. Control of wake and vortex shedding behind a porous circular obstacle by exerting an external magnetic field

    NASA Astrophysics Data System (ADS)

    Bovand, M.; Rashidi, S.; Dehghan, M.; Esfahani, J. A.; Valipour, M. S.

    2015-07-01

    In this article the finite volume method (FVM) is carried out to simulate the flow around and through a two-dimensional porous cylinder. An external magnetic field is used to control the wake behind the bluff body and also to suppress the vortex shedding phenomena. The Darcy-Brinkman-Forchheimer model has been used for modeling the flow in the porous medium. Effects of Stuart (N), Reynolds (Re) and Darcy (Da) numbers on the flow behavior have been investigated. The results show that the critical Stuart number for suppress vortex shedding decreases with increasing the Darcy numbers. Also, the Stuart number for disappearance the re-circulating wake increases with increased Reynolds number for both porous and solid cylinders.

  20. Rotor Wake Vortex Definition: Initial Evaluation of 3-C PIV Results of the Hart-II Study

    NASA Technical Reports Server (NTRS)

    Burley, Casey L.; Brooks, Thomas F.; vanderWall, Berend; Richard, Hughes; Raffel, Markus; Beaumier, Philippe; Delrieux, Yves; Lim, Joon W.; Yu, Yung H.; Tung, Chee

    2002-01-01

    An initial evaluation is made of extensive three-component (3C) particle image velocimetry (PIV) measurements within the wake across a rotor disk plane. The model is a 40 percent scale BO-105 helicopter main rotor in forward flight simulation. This study is part of the HART II test program conducted in the German-Dutch Wind Tunnel (DNW). Included are wake vortex field measurements over the advancing and retreating sides of the rotor operating at a typical descent landing condition important for impulsive blade-vortex interaction (BVI) noise. Also included are advancing side results for rotor angle variations from climb to steep descent. Using detailed PIV vector maps of the vortex fields, methods of extracting key vortex parameters are examined and a new method was developed and evaluated. An objective processing method, involving a center-of-vorticity criterion and a vorticity 'disk' integration, was used to determine vortex core size, strength, core velocity distribution characteristics, and unsteadiness. These parameters are mapped over the rotor disk and offer unique physical insight for these parameters of importance for rotor noise and vibration prediction.

  1. An approximate model of vortex decay in the atmosphere

    NASA Technical Reports Server (NTRS)

    Greene, G. C.

    1985-01-01

    An approximate analysis of atmospheric effects on wake vortex motion and decay is presented. The effects of density stratification, turbulence, and Reynolds number are combined in a single model so that the relative importance of different parameters can be determined. Predicted wake motion is shown to be in good agreement with limited data from both ground facility and flight test measurements taken under low turbulence conditions. Wake decay was found to depend strongly on both density stratification and turbulence. For typical levels of turbulence, wake decay was found to result from the 'Crow instability' except under strongly stratified conditions.

  2. Periodic vortex shedding in the supersonic wake of a planar plate

    NASA Technical Reports Server (NTRS)

    Xing, W. F.; Marenbach, G.

    1985-01-01

    Vortex sheets in the wake have been mainly studied in incompressible flows and in the transonic region. Heinemann et al. (1976) have shown that for the subsonic region the Strouhal number is nearly independent of the Mach number. Motallebi and Norbury (1981) have observed an increase in the Strouhal number in transonic supersonic flow at Mach numbers up to 1.25. The present investigation is concerned with an extension of the studies of vortex shedding to higher supersonic Mach numbers, taking into account questions regarding the possibility of a generation of stable von Karman vortex paths in the considered Mach number range. It is found that the vortex sheet observed in a supersonic wake behind a rough plate is only stable and reproducible in cases involving a certain surface roughness and certain aspects of trailing edge geometry.

  3. Improving actuator disk wake model

    NASA Astrophysics Data System (ADS)

    Costa Gomes, V. M. M. G.; Palma, J. M. L. M.; Silva Lopes, A.

    2014-06-01

    The wind energy industry has traditionally relied on simple wake models for estimating Wind Turbine (WT) wake losses. Despite limitations, low requirements in terms of detailed rotor information makes their use feasible, unlike more complex models, such as Blade Element Method (BEM) or Actuator Line. Froude's Actuator Disk (AD) does not suffer the simpler model's limitation of prescribing the wake through a closed set of equations, while sharing with them the low rotor data requirements. On the other hand they require some form of parametrization to close the model and calculate total thrust acting on the flow. An Actuator Disk model was developed, using an iterative algorithm based on Froude's one-dimensional momentum theory to determine the WT's performance, proving to be successful in estimating the performance of both machines in undisturbed flow and in the wake of an upstream machines. Before Froude's AD limitations compared to more complex rotor models, load distributions emulating those of a BEM model were tested. The results show that little impact is obtained at 3 rotor diameters downstream and beyond, agreeing with common definition of a far-wake that starts at 1-2 diameters downstream, where rotor characteristics become negligible and atmospheric flow effects dominate.

  4. A study of the interaction between a wake vortex and an encountering airplane

    NASA Technical Reports Server (NTRS)

    Stewart, E. C.

    1993-01-01

    An analytical study of the mutual interaction between a wake vortex and an encountering airplane has been conducted. The interaction was calculated on the basis of conservation of angular momentum of the wake vortex and the encountering airplane in the roll axis. For the nominal conditions assumed in the study, the interaction reduced the roll response in the lust second by 20 percent compared to the classical method of calculating the response. However, depending on the conditions assumed, the reduction varied from 0 to 30 percent which could be highly significant in determining separation standards for airplanes. Controlled experiments are needed to determine the exact amount of interaction.

  5. Determination of Bragg scatter in an aircraft-generated wake vortex system for radar detection

    NASA Astrophysics Data System (ADS)

    Myers, Theodore Jon

    1997-12-01

    Remote detection and tracking of wingtip generated wake vortices is important for hazard avoidance especially near airports. Aircraft that fly through these hazardous vortices experience sudden induced roll. Experiments have demonstrated that there is sufficient radar cross section for remote detection at frequencies ranging from VHF to C band (100 MHz to 5 GHz). The mechanism that yields this radar cross section is Bragg scattering from the index of refraction variations due to the atmospheric water vapor and potential temperature fields being mixed by the wake vortex system. Refractive index variations of the size that correspond to half the operating radar wavelength produce the observed radar return. Previous analysis has postulated turbulence within the wake vortex to be the generator of the index of refraction variations. In this work, a new mechanism is identified that does not assume turbulence within the wake vortex system. This 'laminar flow mechanism' causes refractive index structuring that stretches into successively smaller spirals over time as the wake vortex system swirls and descends through the stratified atmosphere. The results are quantitatively consistent with experimental data. Results indicate that this new mechanism has a sharply peaked doppler spectrum which is encouraging for coherent detection by doppler radar.

  6. Progress Towards the Investigation of Technical Issues Relevant to the Design of an Aircraft Wake Vortex Advisory System (WakeVAS)

    NASA Technical Reports Server (NTRS)

    Rutishauser, David K.

    2003-01-01

    Wake vortex separations applied to aircraft during instrument operations have been shown to potentially introduce inefficiencies in air traffic operations during certain weather conditions conducive to short duration wake hazards between pairs of landing aircraft. NASA Langley Research Center (LaRC) demonstrated an integration of technologies that provided real-time observations and predictions of aircraft wake behavior, from which reduced wake spacing from the current criteria was derived. In order to take this proof of concept to an operational prototype system, NASA has been working in cooperation with the FAA and other government and industry members to design operational concepts for a Wake Vortex Advisory System (WakeVAS). In addition to concept development, open research issues are being addressed and activities to quantify system requirements and specifications are currently underway. This paper describes the technological issues relevant to WakeVAS development and current NASA efforts to address these issues.

  7. Spanwise vortex dislocation in the wake of segmented blunt trailing edge

    NASA Astrophysics Data System (ADS)

    Deshpande, P. J.; Sharma, S. D.

    2012-10-01

    Dislocation of the Karman vortex is forced in the near wake behind a two dimensional blunt trailing edge aerofoil to induce strong three dimensionality to weaken the Karman vortex and inhibit its periodic shedding by segmenting the trailing edge in a novel way, different from a rectangular segmented trailing edge tried in the past. Symmetrical trapezoidal prismatic blocks, with the major and the minor sides being equal to 4 and 2 base heights, respectively, are attached to the base at regular intervals along the span which could be varied in order to render multiple wavelengths of spanwise discontinuity to ensure that at least one of the modes of dislocation is triggered independent of the Reynolds number. Hot-wire measurements confirm effectiveness of the trailing edge configurations with trapezoidal prismatic blocks in creating controlled dislocation along the span which annihilates the Karman vortices and suppresses their periodic shedding completely. Flow visualization in a water tunnel reinforces the hot-wire results and clearly shows that the classical Karman vortex street, seen behind the plain base model, disappears when segmented trailing edge is used. In comparison, the rectangular segmented trailing edge is found to attenuate the Karman vortex strength only partially. However, the base pressure measurements have shown improvement in reducing the associated base drag only by 3-4%. The segmented trailing edges designed for the present study are found to generate strong streamwise vortices that effectively transfer energy from the Karman vortices resulting in suppression of the unsteadiness but, perhaps, lower pressures in their core may be restricting the further rise in the base pressure due to the induced effect.

  8. Characterizing the wake vortex signature for an active line of sight remote sensor. M.S. Thesis Technical Report No. 19

    NASA Technical Reports Server (NTRS)

    Heil, Robert Milton

    1994-01-01

    A recurring phenomenon, described as a wake vortex, develops as an aircraft approaches the runway to land. As the aircraft moves along the runway, each of the wing tips generates a spiraling and expanding cone of air. During the lifetime of this turbulent event, conditions exist over the runway which can be hazardous to following aircraft, particularly when a small aircraft is following a large aircraft. Left to themselves, these twin vortex patterns will converge toward each other near the center of the runway, harmlessly dissipating through interaction with each other or by contact with the ground. Unfortunately, the time necessary to disperse the vortex is often not predictable, and at busy airports can severely impact terminal area productivity. Rudimentary methods of avoidance are in place. Generally, time delays between landing aircraft are based on what is required to protect a small aircraft. Existing ambient wind conditions can complicate the situation. Reliable detection and tracking of a wake vortex hazard is a major technical problem which can significantly impact runway productivity. Landing minimums could be determined on the basis of the actual hazard rather than imposed on the basis of a worst case scenario. This work focuses on using a windfield description of a wake vortex to generate line-of-sight Doppler velocity truth data appropriate to an arbitrarily located active sensor such as a high resolution radar or lidar. The goal is to isolate a range Doppler signature of the vortex phenomenon that can be used to improve detection. Results are presented based on use of a simplified model of a wake vortex pattern. However, it is important to note that the method of analysis can easily be applied to any vortex model used to generate a windfield snapshot. Results involving several scan strategies are shown for a point sensor with a range resolution of 1 to 4 meters. Vortex signatures presented appear to offer potential for detection and tracking.

  9. History of Wake Vortex Research: Problems and Accomplishments

    NASA Technical Reports Server (NTRS)

    Greene, George C.

    1997-01-01

    Significant progress has been made in understanding vortex behavior but much remains to be done. The primary challenge is to bring "science" into operational use. Success will require cooperation from a diverse group of organizations.

  10. Aircraft wake vortex velocity measurements using a scanning CO2 laser Doppler velocimeter

    NASA Technical Reports Server (NTRS)

    Dimarzio, C. A.; Sonnenschein, C. M.; Jeffreys, H. B.

    1975-01-01

    A CO2 laser Doppler velocimeter was employed in the study of pairs of counterrotating vortices trailing aircraft in an airport air space. A laser positioned on an extended runway centerline scans a vertical plane perpendicular to the centerline. Vortex location, measurement of vortex transport, and measurement of the properties of aircraft wake vortex flow fields are achieved via spectral analysis of the data. Highest amplitude in the spectrum, the associated maximum velocity, the highest velocity above the amplitude threshold, and the total number of frequency (velocity) cells above thresholds are studied as parameters in analysis of the vortex-associated flow field. The profile of the radial variation of tangential velocity is studied, and two special problems are examined: location of the vortex center and error introduced by crosswind.

  11. Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna)

    PubMed Central

    Wolf, M.; Ortega-Jimenez, V. M.; Dudley, R.

    2013-01-01

    Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8–26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds. PMID:24174113

  12. Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna).

    PubMed

    Wolf, M; Ortega-Jimenez, V M; Dudley, R

    2013-12-22

    Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8-26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds. PMID:24174113

  13. Transient Vortex Structures in the Near Wake of a Wing during Pitch Up/Down Maneuvers

    NASA Astrophysics Data System (ADS)

    Graff, Emilio; Grivel, Morgane; Williams, David

    2012-11-01

    The vorticity distribution in the wake of a thin airfoil reflects the lift and bound circulation history of the wing. During a pitch-up maneuver from 0 degrees to some higher angle of attack (assuming attached flow), a ``starting vortex'' is formed in the wake whose circulation is opposite in strength to the bound circulation in the wing. However, a finite time is required for the starting vortex to fully develop, and if the wing pitches down to a smaller angle of attack before the first starting vortex has reached full strength then an imbalance in the wake circulation occurs. The delay time between the up/down pitch motions and the maximum angle of attack determine which additional vortices must be formed to satisfy Kelvin's theorem. In addition to the irrotational flow vortices that form, vorticity associated with the viscous boundary layers also accumulates into discrete vortices that accompany each ``starting vortex.'' The complicated distributions of vortices and their evolution in the wake are examined with detailed PIV, smoke-visualization, and numerical simulations at Re = 240 to 70,000. The support for David Williams by AFOSR Grant FA9550-09-1-0189 is gratefully acknowledged.

  14. Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation

    PubMed Central

    Bomphrey, Richard J.; Henningsson, Per; Michaelis, Dirk; Hollis, David

    2012-01-01

    Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread. PMID:22977102

  15. Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation.

    PubMed

    Bomphrey, Richard J; Henningsson, Per; Michaelis, Dirk; Hollis, David

    2012-12-01

    Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread. PMID:22977102

  16. Airloads, wakes, and aeroelasticity

    NASA Technical Reports Server (NTRS)

    Johnson, Wayne

    1990-01-01

    Fundamental considerations regarding the theory of modeling of rotary wing airloads, wakes, and aeroelasticity are presented. The topics covered are: airloads and wakes, including lifting-line theory, wake models and nonuniform inflow, free wake geometry, and blade-vortex interaction; aerodynamic and wake models for aeroelasticity, including two-dimensional unsteady aerodynamics and dynamic inflow; and airloads and structural dynamics, including comprehensive airload prediction programs. Results of calculations and correlations are presented.

  17. Method of radial velocities for the estimation of aircraft wake vortex parameters from data measured by coherent Doppler lidar.

    PubMed

    Smalikho, I N; Banakh, V A; Holzäpfel, F; Rahm, S

    2015-09-21

    The method of radial velocities (RV) is applied to estimate aircraft wake vortex parameters from measurements conducted with pulsed coherent Doppler lidar (PCDL). Operations of the Stream Line lidar and the 2-µm PCDL are simulated numerically to analyze the accuracy of the estimated wake vortex parameters with the RV method. The RV method is also used to estimate wake vortex trajectories and circulation from lidar measurements at Tomsk and Munich airports. The method of velocity envelopes and the RV method are compared employing data gathered with the 2-µm PCDL. The domain of applicability of the RV method is determined. PMID:26406749

  18. Investigation and Optimization of Blade Tip Winglets Using an Implicit Free Wake Vortex Method

    NASA Astrophysics Data System (ADS)

    Lawton, Stephen; Crawford, Curran

    2014-06-01

    Novel outer-blade geometries such as tip winglets can increase the aerodynamic power that can be extracted from the wind by tailoring the relative position and strengths of trailed vorticity. This design space is explored using both parameter studies and gradient-based optimization, with the aerodynamic analysis carried out using LibAero, a free wake vortex-based code introduced in previous work. The starting design is the NREL 5MW reference turbine, which allows comparison of the aerodynamic simulation for the unmodified blade with other codes. The code uses a Prandtl-Weissinger lifting line model to represent the blade, and vortex filaments as the flow elements. A fast multipole method is implemented to accelerate the influence calculations and reduce the computational cost. This results in higher fidelity aerodynamic simulations that can capture the effects of novel geometries while maintaining sufficiently fast run-times (on the order of an hour) to allow the use of optimization. Gradients of the objective function with respect to design variables are calculated using the complex step method which is accurate and efficient. Since the vortex structure behind the rotor is being resolved in detail, insight is also gained into the mechanisms by which these new blade designs affect performance. It is found that adding winglets can increase the power extracted from the wind by around 2%, with a similar increase in thrust. It is also possible to create a winglet that slightly lowers the thrust while maintaining very similar power compared to the standard straight blade.

  19. Mesoscale Simulation Data for Initializing Fast-Time Wake Transport and Decay Models

    NASA Technical Reports Server (NTRS)

    Ahmad, Nashat N.; Proctor, Fred H.; Vanvalkenburg, Randal L.; Pruis, Mathew J.; LimonDuparcmeur, Fanny M.

    2012-01-01

    The fast-time wake transport and decay models require vertical profiles of crosswinds, potential temperature and the eddy dissipation rate as initial conditions. These inputs are normally obtained from various field sensors. In case of data-denied scenarios or operational use, these initial conditions can be provided by mesoscale model simulations. In this study, the vertical profiles of potential temperature from a mesoscale model were used as initial conditions for the fast-time wake models. The mesoscale model simulations were compared against available observations and the wake model predictions were compared with the Lidar measurements from three wake vortex field experiments.

  20. Turbulent vortex streets and the entrainment mechanism of the turbulent wake

    NASA Technical Reports Server (NTRS)

    Papailiou, D. D.; Lykoudis, P. S.

    1974-01-01

    The results of an experimental investigation of a turbulent vortex street in the range from 1000 to 20,000 are presented. The vortex street was created by the motion of a circular cylinder in a motionless fluid (mercury). Photographs obtained showed that the turbulent street, created by the vortex shedding behind the cylinder, persisted at longer downstream distances and higher Reynolds numbers than previously reported in the literature. A theory was developed to account for the experimental measurements pertaining to the change of the geometrical characteristics, (the distance between the two rows of vortices and the longitudinal distance between two consecutive vortices on the same row), of the street in the downstream direction. The implications of the structure of the vortex street on the entrainment mechanism of the turbulent wake are discussed.

  1. Characteristics of wake vortex generated by a Boeing 727 jet transport during two-segment and normal ILS approach flight paths

    NASA Technical Reports Server (NTRS)

    Kurkowski, R. L.; Barber, M. R.; Garodz, L. J.

    1976-01-01

    A series of flight tests was conducted to evaluate the vortex wake characteristics of a Boeing 727 (B727-200) aircraft during conventional and two-segment ILS approaches. Twelve flights of the B727, which was equipped with smoke generators for vortex marking, were flown and its vortex wake was intentionally encountered by a Lear Jet model 23 (LR-23) and a Piper Twin Comanche (PA-30). Location of the B727 vortex during landing approach was measured using a system of photo-theodolites. The tests showed that at a given separation distance there were no readily apparent differences in the upsets resulting from deliberate vortex encounters during the two types of approaches. Timed mappings of the position of the landing configuration vortices showed that they tended to descend approximately 91 m(300 ft) below the flight path of the B727. The flaps of the B727 have a dominant effect on the character of the trailed wake vortex. The clean wing produces a strong, concentrated vortex but as the flaps are lowered, the vortex system becomes more diffuse. Pilot opinion and roll acceleration data indicate that 4.5 n.mi. would be a minimum separation distance at which roll control of light aircraft (less than 5,670 kg (12,500 lb) could be maintained during parallel encounters of the B727's landing configuration wake. This minimum separation distance is generally in scale with results determined from previous tests of other aircraft using the small roll control criteria.

  2. Parallel direct numerical simulation of wake vortex detection using monostatic and bistatic radio acoustic sounding systems

    NASA Astrophysics Data System (ADS)

    Boluriaan Esfahaani, Said

    A parallel two-dimensional code is developed in this thesis to numerically simulate wake vortex detection using a Radio Acoustic Sounding System (RASS). The Maxwell equations for media with non-uniform permittivity and the linearized Euler equations for media with non-uniform mean flow are the main framework for the simulations. The code is written in Fortran 90 with the Message Passing Interface (MPI) for parallel implementation. The main difficulty encountered with a time accurate simulation of a RASS is the number of samples required to resolve the Doppler shift in the scattered electromagnetic signal. Even for a 1D simulation with a typical scatterer size, the CPU time required to run the code is far beyond currently available computer resources. Two solutions that overcome this problem are described. In the first the actual electromagnetic wave propagation speed is replaced with a much lower value. This allows an explicit, time accurate numerical scheme to be used. In the second the governing differential equations are recast in order to remove the carrier frequency and solve only for the frequency shift using an implicit scheme with large time steps. The numerical stability characteristics of the resulting discretized equation with complex coefficients are examined. A number of cases for both the monostatic and bistatic configurations are considered. First, a uniform mean flow is considered and the RASS simulation is performed for two different types of incident acoustic field, namely a short single frequency acoustic pulse and a continuous broadband acoustic source. Both the explicit and implicit schemes are examined and the mean flow velocity is determined from the spectrum of the backscattered electromagnetic signal with very good accuracy. Second, the Taylor and Oseen vortex models are considered and their velocity field along the incident electromagnetic beam is retrieved. The Abel transform is then applied to the velocity profiles determined by both explicit and implicit schemes, and the radial variation of the wake vortex velocity is reconstructed. The effect of the transmitter beam width on the results is also examined. The parallel performance of the simulations on several platforms is investigated. It is found that the code is nearly perfectly scalable on the tested platforms and for the number of processors considered.

  3. Prediction and control of vortex-dominated and vortex-wake flows

    NASA Technical Reports Server (NTRS)

    Kandil, Osama

    1993-01-01

    This progress report documents the accomplishments achieved in the period from December 1, 1992 until November 30, 1993. These accomplishments include publications, national and international presentations, NASA presentations, and the research group supported under this grant. Topics covered by documents incorporated into this progress report include: active control of asymmetric conical flow using spinning and rotary oscillation; supersonic vortex breakdown over a delta wing in transonic flow; shock-vortex interaction over a 65-degree delta wing in transonic flow; three dimensional supersonic vortex breakdown; numerical simulation and physical aspects of supersonic vortex breakdown; and prediction of asymmetric vortical flows around slender bodies using Navier-Stokes equations.

  4. Analytical model of rotor wake aerodynamics in ground effect

    NASA Technical Reports Server (NTRS)

    Saberi, H. A.

    1983-01-01

    The model and the computer program developed provides the velocity, location, and circulation of the tip vortices of a two-blade helicopter in and out of the ground effect. Comparison of the theoretical results with some experimental measurements for the location of the wake indicate that there is excellent accuracy in the vicinity of the rotor and fair amount of accuracy far from it. Having the location of the wake at all times enables us to compute the history of the velocity and the location of any point in the flow. The main goal of out study, induced velocity at the rotor, can also be calculated in addition to stream lines and streak lines. Since the wake location close to the rotor is known more accurately than at other places, the calculated induced velocity over the disc should be a good estimate of the real induced velocity, with the exception of the blade location, because each blade was replaced only by a vortex line. Because no experimental measurements of the wake close to the ground were available to us, quantitative evaluation of the theoretical wake was not possible. But qualitatively we have been able to show excellent agreement. Comparison of flow visualization with out results has indicated the location of the ground vortex is estimated excellently. Also the flow field in hover is well represented.

  5. Estimation of aircraft wake vortex parameters from data measured by a Stream Line lidar

    NASA Astrophysics Data System (ADS)

    Smalikho, I. N.; Banakh, V. A.

    2015-11-01

    A method for estimation of aircraft wake vortex parameters (coordinates of axis and circulation of vortices) from raw data measured by a pulsed coherent Doppler lidar "Stream Line" has been offered. By numerical simulation we found optimal measurement parameters, with which it is possible to obtain information about the wake vortices, despite the low level of echo signal inherent to this lidar. The method was tested in an experiment at the airfield of Tomsk airport. The results of the experimental data processing are consistent with theoretical calculations for the type of aircrafts involved in this experiment.

  6. Self-consistent mean flow description of the nonlinear saturation of the vortex shedding in the cylinder wake.

    PubMed

    Mantič-Lugo, Vladislav; Arratia, Cristóbal; Gallaire, François

    2014-08-22

    The Bénard-von Kármán vortex shedding instability in the wake of a cylinder is perhaps the best known example of a supercritical Hopf bifurcation in fluid dynamics. However, a simplified physical description that accurately accounts for the saturation amplitude of the instability is still missing. Here, we present a simple self-consistent model that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields. The model is formally constructed by a set of coupled equations governing the mean flow together with its most unstable eigenmode with finite size. The saturation amplitude is determined by requiring the mean flow to be neutrally stable. Without requiring any input from numerical or experimental data, the resolution of the model provides a good prediction of the amplitude and frequency of the vortex shedding as well as the spatial structure of the mean flow and the Reynolds stress. PMID:25192100

  7. Self-Consistent Mean Flow Description of the Nonlinear Saturation of the Vortex Shedding in the Cylinder Wake

    NASA Astrophysics Data System (ADS)

    Mantič-Lugo, Vladislav; Arratia, Cristóbal; Gallaire, François

    2014-08-01

    The Bénard-von Kármán vortex shedding instability in the wake of a cylinder is perhaps the best known example of a supercritical Hopf bifurcation in fluid dynamics. However, a simplified physical description that accurately accounts for the saturation amplitude of the instability is still missing. Here, we present a simple self-consistent model that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields. The model is formally constructed by a set of coupled equations governing the mean flow together with its most unstable eigenmode with finite size. The saturation amplitude is determined by requiring the mean flow to be neutrally stable. Without requiring any input from numerical or experimental data, the resolution of the model provides a good prediction of the amplitude and frequency of the vortex shedding as well as the spatial structure of the mean flow and the Reynolds stress.

  8. N-S Simulations of Crow-Type Instabilities in Vortex Wake

    NASA Technical Reports Server (NTRS)

    Zheng, Z. C.; Baek, K.

    1999-01-01

    Three-dimensional Navier-Stokes simulations of the Crow instability of wake vortices are conducted using large-eddy simulations without background turbulence. Sinusoidal displacement has been specified as the initial perturbation for the vortex system. The results have shown that the minimum Crow instability wavelength is about one vortex spacing shorter than predicted by Crow's linear stability theory. The planar- standing-wave-angle value and the amplitude indifference behavior agree with Crow's analysis. Simulations with periodicity in the axial direction have indicated minor influence of axial flow on the Crow instability.

  9. Computation of rotor aerodynamic loads with a constant vorticity contour free wake model

    NASA Technical Reports Server (NTRS)

    Quackenbush, Todd R.; Wachspress, Daniel A.; Boschitsch, Alexander H.

    1991-01-01

    An analytical method is presented which facilitates the study of isolated rotors with an improved approach to wake simulation. Vortex filaments are simulated along contours of constant sheet strength for the sheet of vorticity resulting from each rotor blade. Curved vortex elements comprise the filaments which can be distorted by the local velocity field. Called the Constant Vorticity Contour wake model, the approach permits the simulation of the blades' wakes corresponding to the full span of the rotor blade. The discretization of the wake of the rotor blade produces spacing and structure that are consistent with the spatial and temporal variations in the loading. A vortex-lattice aerodynamic model of the blade is also included which introduces a finite-element structural model of the blade and consideration of the force and moment trim analysis. Results of the present version of the simulation, called RotorCRAFT, are found to correlate well with H-34 flight-test data.

  10. Flow visualization of the wake of a transport aircraft model with lateral-control oscillations

    NASA Technical Reports Server (NTRS)

    Jordan, F. L., Jr.

    1983-01-01

    An exploratory flow visualization study conducted in the Langley Vortex Research Facility to investigate the effectiveness of lateral control surface oscillations as a potential method for wake vortex attenuation on a 0.03 scale model of a wide body jet transport aircraft is described. Effects of both asymmetric surface oscillation (control surfaces move as with normal lateral control inputs) and symmetric surface oscillation (control surfaces move in phase) are presented. The asymmetric case simulated a flight maneuver which was previously investigated on the transport aircraft during NASA/FAA flight tests and which resulted in substantial wake vortex attenuation. Effects on the model wake vortex systems were observed by propelling the model through a two dimensional smoke screen perpendicular to the model flight path. Results are presented as photographic time histories of the wake characteristics recorded with high speed still cameras. Effects of oscillation on the wake roll up are described in some detail, and the amount of vortex attenuation observed is discussed in comparative terms. Findings were consistent with flight test results in that only a small amount of rotation was observed in the wake for the asymmetric case. A possible aerodynamic mechanism contributing to this attenuation is suggested.

  11. Aircraft Wake Vortex Spacing System (AVOSS) Performance Update and Validation Study

    NASA Technical Reports Server (NTRS)

    Rutishauser, David K.; OConnor, Cornelius J.

    2001-01-01

    An analysis has been performed on data generated from the two most recent field deployments of the Aircraft Wake VOrtex Spacing System (AVOSS). The AVOSS provides reduced aircraft spacing criteria for wake vortex avoidance as compared to the FAA spacing applied under Instrument Flight Rules (IFR). Several field deployments culminating in a system demonstration at Dallas Fort Worth (DFW) International Airport in the summer of 2000 were successful in showing a sound operational concept and the system's potential to provide a significant benefit to airport operations. For DFW, a predicted average throughput increase of 6% was observed. This increase implies 6 or 7 more aircraft on the ground in a one-hour period for DFW operations. Several studies of performance correlations to system configuration options, design options, and system inputs are also reported. The studies focus on the validation performance of the system.

  12. Numerical Modeling Studies of Wake Vortices: Real Case Simulations

    NASA Technical Reports Server (NTRS)

    Shen, Shao-Hua; Ding, Feng; Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.

    1999-01-01

    A three-dimensional large-eddy simulation model, TASS, is used to simulate the behavior of aircraft wake vortices in a real atmosphere. The purpose for this study is to validate the use of TASS for simulating the decay and transport of wake vortices. Three simulations are performed and the results are compared with the observed data from the 1994-1995 Memphis field experiments. The selected cases have an atmospheric environment of weak turbulence and stable stratification. The model simulations are initialized with appropriate meteorological conditions and a post roll-up vortex system. The behavior of wake vortices as they descend within the atmospheric boundary layer and interact with the ground is discussed.

  13. Wake Sensor Evaluation Program and Results of JFK-1 Wake Vortex Sensor Intercomparisons

    NASA Technical Reports Server (NTRS)

    Barker, Ben C., Jr.; Burnham, David C.; Rudis, Robert P.

    1997-01-01

    The overall approach should be to: (1) Seek simplest, sufficiently robust, integrated ground based sensor systems (wakes and weather) for AVOSS; (2) Expand all sensor performance cross-comparisons and data mergings in on-going field deployments; and (3) Achieve maximal cost effectiveness through hardware/info sharing. An effective team is in place to accomplish the above tasks.

  14. Commercial aircraft wake vortices

    NASA Astrophysics Data System (ADS)

    Gerz, Thomas; Holzpfel, Frank; Darracq, Denis

    2002-04-01

    This paper discusses the problem of wake vortices shed by commercial aircraft. It presents a consolidated European view on the current status of knowledge of the nature and characteristics of aircraft wakes and of technical and operational procedures of minimizing and predicting the vortex strength and avoiding wake encounters. Methodological aspects of data evaluation and interpretation, like the description of wake ages, the characterization of wake vortices, and the proper evaluation of wake data from measurement and simulation, are addressed in the first part. In the second part an inventory of our knowledge is given on vortex characterization and control, prediction and monitoring of vortex decay, vortex detection and warning, vortex encounter models, and wake-vortex safety assessment. Each section is concluded by a list of questions and required actions which may help to guide further research activities. The primary objective of the joint international efforts in wake-vortex research is to avoid potentially hazardous wake encounters for aircraft. Shortened aircraft separations under appropriate meteorological conditions, whilst keeping or even increasing the safety level, is the ultimate goal. Reduced time delays on the tactical side and increased airport capacities on the strategic side will be the benefits of these ambitious ventures for the air transportation industry and services.

  15. An Evaluation of the Measurement Requirements for an In-Situ Wake Vortex Detection System

    NASA Technical Reports Server (NTRS)

    Fuhrmann, Henri D.; Stewart, Eric C.

    1996-01-01

    Results of a numerical simulation are presented to determine the feasibility of estimating the location and strength of a wake vortex from imperfect in-situ measurements. These estimates could be used to provide information to a pilot on how to avoid a hazardous wake vortex encounter. An iterative algorithm based on the method of secants was used to solve the four simultaneous equations describing the two-dimensional flow field around a pair of parallel counter-rotating vortices of equal and constant strength. The flow field information used by the algorithm could be derived from measurements from flow angle sensors mounted on the wing-tip of the detecting aircraft and an inertial navigation system. The study determined the propagated errors in the estimated location and strength of the vortex which resulted from random errors added to theoretically perfect measurements. The results are summarized in a series of charts and a table which make it possible to estimate these propagated errors for many practical situations. The situations include several generator-detector airplane combinations, different distances between the vortex and the detector airplane, as well as different levels of total measurement error.

  16. Visualization on fish's wake

    NASA Astrophysics Data System (ADS)

    Li, Xuemin; Lu, Xiyun; Yin, Xiezhen

    2002-05-01

    In this paper an experiment on wake of Goldfish swimming unrestricted was conducted in a water tunnel. Method of color liquid was used to visualize the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the wake and the Strouhal frequency of the fish is in the range 0.25-0.35. A 3D vortex ring chain model was presented.

  17. Wake transition and vortex street interaction in flows generated by traveling localized Lorentz forces in a shallow electrolyte layer

    NASA Astrophysics Data System (ADS)

    Roman, Joel; Cuevas, Sergio

    2014-11-01

    We present an experimental and numerical study of the vortex street produced by a traveling localized Lorentz force, namely a magnetic obstacle, in a thin layer of electrolyte. The Lorentz force is generated by the interaction a localized magnetic field created by a small permanent magnet which travels with a uniform velocity underneath a rectangular container and a uniform D.C. current applied transversally to the motion of the magnet. We find that by increasing the Reynolds number (based on the velocity of the magnet) the wake generated by the magnetic obstacle presents a transition from the Bénard-von Kármán (BvK) wake to the reversed BvK wake. In addition, we analyze the flow past a pair magnetic obstacles side-by-side in a thin layer of electrolyte by varying the separation between the magnets and the intensity of the applied current. The attention is focused in the interference of the wakes created by the magnetic obstacles. Numerical simulations based on a quasi-two dimensional numerical model present a satisfactory agreement with experimental results. Work supported by CONACYT, Mexico under Project 131399. J. Roman acknowledges a grant from CONACYT.

  18. Coupled wake boundary layer model of windfarms

    NASA Astrophysics Data System (ADS)

    Stevens, Richard; Gayme, Dennice; Meneveau, Charles

    2014-11-01

    We present a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a windfarm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall windfarm boundary layer structure. Wake models capture the effect of turbine positioning, while the top-down approach represents the interaction between the windturbine wakes and the atmospheric boundary layer. Each portion of the CWBL model requires specification of a parameter that is unknown a-priori. The wake model requires the wake expansion rate, whereas the top-down model requires the effective spanwise turbine spacing within which the model's momentum balance is relevant. The wake expansion rate is obtained by matching the mean velocity at the turbine from both approaches, while the effective spanwise turbine spacing is determined from the wake model. Coupling of the constitutive components of the CWBL model is achieved by iterating these parameters until convergence is reached. We show that the CWBL model predictions compare more favorably with large eddy simulation results than those made with either the wake or top-down model in isolation and that the model can be applied successfully to the Horns Rev and Nysted windfarms. The `Fellowships for Young Energy Scientists' (YES!) of the Foundation for Fundamental Research on Matter supported by NWO, and NSF Grant #1243482.

  19. Coherent vortex structures in the wake of a stationary disk and of a disk excited to vibration

    NASA Astrophysics Data System (ADS)

    Scholz, Dietmar

    1987-09-01

    Coherent votex structures were studied by comparing the large scale coherent flow structure in the wake of a rigid vertical circular disk with the turbulent structures behind the externally driven disk. The periodic vortex structures in these wakes can be decomposed via cross-correlation measurements with hot wires and microphones, particularly by using representations of coherences, phase relations, and azimuthal modes. In the very near wake, coherent periodic pressure oscillations due to pulsing of the recirculation bubble are superimposed onto pressure fluctations of the naturally occurring helical vortex structures which dominate in the entire flow region outside the recirculation bubble.

  20. Initialization and Simulation of Three-Dimensional Aircraft Wake Vortices

    NASA Technical Reports Server (NTRS)

    Ash, Robert L.; Zheng, Z. C.

    1997-01-01

    This paper studies the effects of axial velocity profiles on vortex decay, in order to properly initialize and simulate three-dimensional wake vortex flow. Analytical relationships are obtained based on a single vortex model and computational simulations are performed for a rather practical vortex wake, which show that the single vortex analytical relations can still be applicable at certain streamwise sections of three-dimensional wake vortices.

  1. Runway Wake Vortex, Crosswind, and Visibility Detection with a Scintillometer at Schiphol Airport

    NASA Astrophysics Data System (ADS)

    van Dinther, D.; Hartogensis, O. K.; Holtslag, A. A. M.

    2015-12-01

    We evaluate the performance and investigate the capability of a scintillometer to detect wake vortices, crosswind and visibility near an airport runway. An experiment is carried out at Schiphol airport (Amsterdam, The Netherlands), where an optical scintillometer is positioned alongside a runway. An algorithm is developed to detect wake vortices, and also the strength of the wake vortex, from the variance in the scintillation signal. The algorithm shows promising results in detecting wake vortices and their strengths during the night. During the day, the scintillometer signal is dominated by environmental turbulence and wake vortices are no longer detectable. The crosswind measured by the scintillometer is compared with wind-speed and wind-direction data at the airport. Our results show that, after applying an outlier filter, the scintillometer is able to measure the crosswind over the short time period of 3 s required for aviation applications. The outlier filter does not compromise the capability of the scintillometer to obtain the maximum 3 s crosswind over a 10-min time frame correctly. Finally, a transmission method is used to obtain the visibility from the scintillometer signal, which is then compared with that obtained from a visibility sensor. The scintillometer is able to identify periods of low visibility correctly, although it shows a high amount of scatter around the exact visibility value.

  2. Flight Test Analysis of the Forces and Moments Imparted on a B737-100 Airplane During Wake Vortex Encounters

    NASA Technical Reports Server (NTRS)

    Roberts, Chistopher L.

    2001-01-01

    Aircraft travel has become a major form of transportation. Several of our major airports are operating near their capacity limit, increasing congestion and delays for travelers. As a result, the National Aeronautics and Space Administration (NASA) has been working in conjunction with the Federal Aviation Administration (FAA), airline operators, and the airline industry to increase airport capacity without sacrificing public safety. One solution to the problem is to increase the number of airports and build new. runways; yet, this solution is becoming increasingly difficult due to limited space. A better solution is to increase the production per runway. This solution increases the possibility that one aircraft will encounter the trailing wake of another aircraft. Hazardous wake vortex encounters occur when an aircraft encounters the wake produced by a heavier aircraft. This heavy-load aircraft produces high-intensity wake turbulence that redistributes the aerodynamic loads of trailing smaller aircraft. This situation is particularly hazardous for smaller aircraft during takeoffs and landings. In order to gain a better understanding of the wake-vortex/aircraft encounter phenomena, NASA Langley Research Center conducted a series of flight tests from 1995 through 1997. These tests were designed to gather data for the development a wake encounter and wake-measurement data set with the accompanying atmospheric state information. This data set is being compiled into a database that can be used by wake vortex researchers to compare with experimental and computational results. The purpose of this research is to derive and implement a procedure for calculating the wake-vortex/aircraft interaction portion of that database by using the data recorded during those flight tests. There were three objectives to this research. Initially, the wake-induced forces and moments from each flight were analyzed based on varying flap deflection angles. The flap setting alternated between 15 and 30 degrees while the separation distance remained constant. This examination was performed to determine if increases in flap deflection would increase or decrease the effects of the wake-induced forces and moments. Next, the wake-induced forces and moments from each flight were analyzed based on separation distances of 1-3 nautical miles. In this comparison, flap deflection was held constant at 30 degrees. The purpose of this study was to determine if increased separation distances reduced the effects of the wake vortex on the aircraft. The last objective compared the wake-induced forces and moments of each flight as it executed a series of maneuvers through the wake-vortex. This analysis was conducted to examine the impact of the wake on the B737 as it traversed the wake horizontally and vertically. Results from the first analysis indicated that there was no difference in wake effect at flap deflections of 15 and 30 degrees. This conclusion is evidenced in the cases of the wake-induced sideforce, rolling moment, and yawing moment. The wake-induced lift, drag, and pitching moment cases yielded less conclusive results. The second analysis compared the wake-induced forces and moments at separation distances of 1-3 nautical miles. Results indicated that there was no significant difference in the wake-induced lift, drag, sideforce, or yawing moment coefficients. The analysis compared the wake-induced forces and moments based on different flight maneuvers. It was found that the wake-induced forces and moments had the greatest impact on out-to-in and in-to-out maneuvers.

  3. A method for modeling finite-core vortices in wake-flow calculations

    NASA Technical Reports Server (NTRS)

    Stremel, P. M.

    1984-01-01

    A numerical method for computing nonplanar vortex wakes represented by finite-core vortices is presented. The approach solves for the velocity on an Eulerian grid, using standard finite-difference techniques; the vortex wake is tracked by Lagrangian methods. In this method, the distribution of continuous vorticity in the wake is replaced by a group of discrete vortices. An axially symmetric distribution of vorticity about the center of each discrete vortex is used to represent the finite-core model. Two distributions of vorticity, or core models, are investigated: a finite distribution of vorticity represented by a third-order polynomial, and a continuous distribution of vorticity throughout the wake. The method provides for a vortex-core model that is insensitive to the mesh spacing. Results for a simplified case are presented. Computed results for the roll-up of a vortex wake generated by wings with different spanwise load distributions are presented; contour plots of the flow-field velocities are included; and comparisons are made of the computed flow-field velocities with experimentally measured velocities.

  4. Low-Dimensional Model of a Cylinder Wake

    NASA Astrophysics Data System (ADS)

    Luchtenburg, Mark; Cohen, Kelly; Siegel, Stefan; McLaughlin, Tom

    2003-11-01

    In a two-dimensional cylinder wake, self-excited oscillations in the form of periodic shedding of vortices are observed above a critical Reynolds number of about 47. These flow-induced non-linear oscillations lead to some undesirable effects associated with unsteady pressures such as fluid-structure interactions. An effective way of suppressing the self-excited flow oscillations is by the incorporation of closed-loop flow control. In this effort, a low dimensional, proper orthogonal decomposition (POD) model is based on data obtained from direct numerical simulations of the Navier Stokes equations for the two dimensional circular cylinder wake at a Reynolds number of 100. Three different conditions are examined, namely, the unforced wake experiencing steady-state vortex shedding, the transient behavior of the unforced wake at the startup of the simulation, and transient response to open-loop harmonic forcing by translation. We discuss POD mode selection and the number of modes that need to be included in the low-dimensional model. It is found that the transient dynamics need to be represented by a coupled system that includes an aperiodic mean-flow mode, an aperiodic shift mode and the periodic von Karman modes. Finally, a least squares mapping method is introduced to develop the non-linear state equations. The predictive capability of the state equations demonstrates the ability of the above approach to model the transient dynamics of the wake.

  5. A Sensitivity Study of the Aircraft Vortex Spacing System (AVOSS) Wake Predictor Algorithm to the Resolution of Input Meteorological Profiles

    NASA Technical Reports Server (NTRS)

    Rutishauser, David K.; Butler, Patrick; Riggins, Jamie

    2004-01-01

    The AVOSS project demonstrated the feasibility of applying aircraft wake vortex sensing and prediction technologies to safe aircraft spacing for single runway arrivals. On average, AVOSS provided spacing recommendations that were less than the current FAA prescribed spacing rules, resulting in a potential airport efficiency gain. Subsequent efforts have included quantifying the operational specifications for future Wake Vortex Advisory Systems (WakeVAS). In support of these efforts, each of the candidate subsystems for a WakeVAS must be specified. The specifications represent a consensus between the high-level requirements and the capabilities of the candidate technologies. This report documents the beginnings of an effort to quantify the capabilities of the AVOSS Prediction Algorithm (APA). Specifically, the APA horizontal position and circulation strength output sensitivity to the resolution of its wind and turbulence inputs is examined. The results of this analysis have implications for the requirements of the meteorological sensing and prediction systems comprising a WakeVAS implementation.

  6. The Transition from Thick to Thin Plate Wake Physics: Whither Vortex Shedding?

    NASA Technical Reports Server (NTRS)

    Rai, Man Mohan

    2016-01-01

    The near and very near wake of a flat plate with a circular trailing edge is investigated with data from direct numerical simulations. Computations were performed for six different combinations of the Reynolds numbers based on plate thickness (D) and boundary layer momentum thickness upstream of the trailing edge (theta). Unlike the case of the cylinder, these Reynolds numbers are independent parameters for the flat plate. The separating boundary layers are turbulent in all the cases investigated. One objective of the study is to understand the changes in the wake vortex shedding process as the plate thickness is reduced (increasing theta/D). The value of D varies by a factor of 16 and that of theta by approximately 5 in the computations. Vortex shedding is vigorous in the low theta/D cases with a substantial decrease in shedding intensity in the large theta/D cases. Other shedding characteristics are also significantly altered with increasing theta/D. A visualization of the shedding process in the different cases is provided and discussed. The basic shedding mechanism is explored in depth. The effect of changing theta/D on the time-averaged, near-wake velocity statistics is also discussed. A functional relationship between the shedding frequency and the Reynolds numbers mentioned above is obtained.

  7. Vortex pairing in the wake of an oscillating bubble rising in a thin-gap cell

    NASA Astrophysics Data System (ADS)

    Ern, Patricia; Filella, Audrey; Roig, Véronique

    2015-11-01

    We investigate experimentally the oscillatory motion and wake of a bubble rising in a counter flow in a thin gap cell (3 mm) by shadowgraphy and PIV. The equivalent diameter d of the bubble in the plane of the cell is used to define the Archimedes number Ar =√{/gd3 } ν (ν is the kinematic viscosity and g the gravitational acceleration). The counter flow is characterized by the Reynolds number Recf based on the mean liquid velocity and the gap thickness. For 500 <= Ar <= 5500 and 0 <=Recf <= 200 , the mean vertical velocity of the bubble relative to the counter flow, Vbr, corresponds to the mean rising velocity in liquid at rest; and the frequency and the amplitude of the oscillatory motion superpose for all Recf when normalized with Vbr and the timescale d /Vbr . For a given size of the bubble (d 9 . 5 mm and Ar 2800) corresponding to a Reynolds number based on Vbr and d of about 1900, we then investigate in detail the wake associated to the bubble in several counter flows. As Recf increases, the number of vortices released increases. Furthermore, the wake of the bubble undergoes vortex pairing for 0 <=Recf <= 110), whereas no vortex pairing is observed for Recf >= 140 .

  8. Lidar and numerical studies on the different evolution of vortex pair and secondary wake in young contrails

    NASA Astrophysics Data System (ADS)

    Sussmann, Ralf; Gierens, Klaus M.

    1999-01-01

    Vortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross-section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios. In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake-gap opens between the wingtip vortices and the secondary wake. It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system. The vertical wake-gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind light aircraft migrate down too slowly. The secondary wake is composed of nonspherical particles larger than the ones in the wingtip vortices which are spherical particles and/or particles smaller than ≈0.5 μm. In most cases the secondary wake is the only part of a contrail that persists after vortex breakdown. This is because the ice in the vortex tubes evaporates due to adiabatic heating as the vortices travel downward. Only in the rare case of higher ambient ice supersaturation (>2%) do both parts of a contrail contribute to the persistent ice cloud. The number of ice crystals initially formed is typically reduced by a factor of 200 by evaporation (60% ambient humidity). This leads to a high population of interstitial particles. The results imply that formation of persistent contrails can be minimized by technical means.

  9. A family of vortex wakes generated by a thrush nightingale in free flight in a wind tunnel over its entire natural range of flight speeds.

    PubMed

    Spedding, G R; Rosén, M; Hedenström, A

    2003-07-01

    In view of the complexity of the wing-beat kinematics and geometry, an important class of theoretical models for analysis and prediction of bird flight performance entirely, or almost entirely, ignores the action of the wing itself and considers only the resulting motions in the air behind the bird. These motions can also be complicated, but some success has previously been recorded in detecting and measuring relatively simple wake structures that can sometimes account for required quantities used to estimate aerodynamic power consumption. To date, all bird wakes, measured or presumed, seem to fall into one of two classes: the closed-loop, discrete vortex model at low flight speeds, and the constant-circulation, continuous vortex model at moderate to high speeds. Here, novel and accurate quantitative measurements of velocity fields in vertical planes aligned with the freestream are used to investigate the wake structure of a thrush nightingale over its entire range of natural flight speeds. At most flight speeds, the wake cannot be categorised as one of the two standard types, but has an intermediate structure, with approximations to the closed-loop and constant-circulation models at the extremes. A careful accounting for all vortical structures revealed with the high-resolution technique permits resolution of the previously unexplained wake momentum paradox. All the measured wake structures have sufficient momentum to provide weight support over the wingbeat. A simple model is formulated and explained that mimics the correct, measured balance of forces in the downstroke- and upstroke-generated wake over the entire range of flight speeds. Pending further work on different bird species, this might form the basis for a generalisable flight model. PMID:12796450

  10. An all-fiber, modular, compact wind lidar for wind sensing and wake vortex applications

    NASA Astrophysics Data System (ADS)

    Prasad, Narasimha S.; Sibell, Russel; Vetorino, Steven; Higgins, Richard; Tracy, Allen

    2015-05-01

    This paper discusses an innovative, compact and eyesafe coherent lidar system developed for wind and wake vortex sensing applications. With an innovative all-fiber and modular transceiver architecture, the wind lidar system has reduced size, weight and power requirements, and provides enhanced performance along with operational elegance. This all-fiber architecture is developed around fiber seed laser coupled to uniquely configured fiber amplifier modules. The innovative features of this lidar system, besides its all fiber architecture, include pulsewidth agility and user programmable 3D hemispherical scanner unit. Operating at a wavelength of 1.5457 microns and with a PRF of up to 20 KHz, the lidar transmitter system is designed as a Class 1 system with dimensions of 30"(W) x 46"(L) x 60"(H). With an operational range exceeding 10 km, the wind lidar is configured to measure wind velocities of greater than 120 m/s with an accuracy of +/- 0.2 m/s and allow range resolution of less than 15 m. The dynamical configuration capability of transmitted pulsewidths from 50 ns to 400 ns allows high resolution wake vortex measurements. The scanner uses innovative liquid metal slip ring and is built using 3D printer technology with light weight nylon. As such, it provides continuous 360 degree azimuth and 180 degree elevation scan angles with an incremental motion of 0.001 degree. The lidar system is air cooled and requires 110 V for its operation. This compact and modular lidar system is anticipated to provide mobility, reliability, and ease of field deployment for wind and wake vortex measurements. Currently, this wind lidar is undergoing validation tests under various atmospheric conditions. Preliminary results of these field measurements of wind characteristics that were recently carried out in Colorado are discussed.

  11. An All-Fiber, Modular, Compact Wind Lidar for Wind Sensing and Wake Vortex Applications

    NASA Technical Reports Server (NTRS)

    Prasad, Narasimha S.; Sibell, Russ; Vetorino, Steve; Higgins, Richard; Tracy, Allen

    2015-01-01

    This paper discusses an innovative, compact and eyesafe coherent lidar system developed for wind and wake vortex sensing applications. With an innovative all-fiber and modular transceiver architecture, the wind lidar system has reduced size, weight and power requirements, and provides enhanced performance along with operational elegance. This all-fiber architecture is developed around fiber seed laser coupled to uniquely configured fiber amplifier modules. The innovative features of this lidar system, besides its all fiber architecture, include pulsewidth agility and user programmable 3D hemispherical scanner unit. Operating at a wavelength of 1.5457 microns and with a PRF of up to 20 KHz, the lidar transmitter system is designed as a Class 1 system with dimensions of 30"(W) x 46"(L) x 60"(H). With an operational range exceeding 10 km, the wind lidar is configured to measure wind velocities of greater than 120 m/s with an accuracy of +/- 0.2 m/s and allow range resolution of less than 15 m. The dynamical configuration capability of transmitted pulsewidths from 50 ns to 400 ns allows high resolution wake vortex measurements. The scanner uses innovative liquid metal slip ring and is built using 3D printer technology with light weight nylon. As such, it provides continuous 360 degree azimuth and 180 degree elevation scan angles with an incremental motion of 0.001 degree. The lidar system is air cooled and requires 110 V for its operation. This compact and modular lidar system is anticipated to provide mobility, reliability, and ease of field deployment for wind and wake vortex measurements. Currently, this wind lidar is undergoing validation tests under various atmospheric conditions. Preliminary results of these field measurements of wind characteristics that were recently carried out in Colorado are discussed.

  12. Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel

    PubMed Central

    Hedenström, A; Rosén, M; Spedding, G.R

    2005-01-01

    The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments. PMID:16849236

  13. Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel.

    PubMed

    Hedenström, A; Rosén, M; Spedding, G R

    2006-04-22

    The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments. PMID:16849236

  14. Flutter clearance flight tests of an OV-10A airplane modified for wake vortex flight experiments

    NASA Technical Reports Server (NTRS)

    Doggett, Robert V., Jr.; Rivera, Jose A., Jr.; Stewart, Eric C.

    1995-01-01

    The envelope expansion, flight flutter tests of a modified OV-10A aircraft are described. For the wake vortex research program, the airplane was modified to incorporate three forward-extending instrumentation booms, one extending forward from each wing tip and one from the right side of the fuselage. The booms were instrumented with sensors to measure the velocity and direction of local air flow. The flutter test results show that the modified OV-10A aircraft is free from flutter at speeds up to 330 KEAS at 5000 feet altitude.

  15. Secondary vortex street in the wake of two tandem circular cylinders at low Reynolds number

    NASA Astrophysics Data System (ADS)

    Wang, Si-Ying; Tian, Fang-Bao; Jia, Lai-Bing; Lu, Xi-Yun; Yin, Xie-Zhen

    2010-03-01

    The experiments on two tandem circular cylinders were conducted in a horizontal soap film tunnel for the Reynolds number Re=60 , 80, and 100 and the nondimensional center-to-center spacing Γ ranging in 1˜12 . The flow patterns were recorded by a high-speed camera and the vortex shedding frequency was obtained by a spatiotemporal evolution method. The secondary vortex formation (SVF) mode characterized by the formation of a secondary vortex street in the wake of the downstream cylinder was found at large Γ . Moreover, some typical modes predicted by previous investigations, including the single bluff-body, shear layer reattachment, and synchronization of vortex shedding modes, were also revisited in our experiments. Further, numerical simulations were carried out using a space-time finite-element method and the results confirmed the existence of the SVF mode. The mechanism of SVF mode was analyzed in terms of the numerical results. The dependence of the Strouhal number Sr on Γ was given and the flow characteristics relevant to the critical spacing values and the hysteretic mode transitions were investigated.

  16. Numerical Studies of Three-dimensional Breakdown in Trailing Vortex Wakes

    NASA Technical Reports Server (NTRS)

    Evans, P. F.; Hackett, J. E.

    1976-01-01

    Finite element, three dimensional relaxation methods are used to calculate the development of vortex wakes behind aircraft for a considerable downstream distance. The inclusion of a self-induction term in the solution, dependent upon local curvature and vortex core radius, permits calculation of finite lifetimes for systems for which infinite life would be predicted two dimensionally. The associated computer program is described together with single-pair, twin-pair, and multiple-pair studies carried out using it. It is found, in single-pair studies, that there is a lower limit to the wavelengths at which the Crow-type of instability can occur. Below this limit, self-induction effects cause the plane of the disturbance waves to rotate counter to the vortex direction. Self induction in two dimensionally generated twin spiral waves causes an increase in axial length which becomes more marked with decreasing initial wavelength. The time taken for vortex convergence toward the center plane is correspondingly increased. The limited parametric twin-pair study performed suggests that time-to-converge increases with increasing flap span. Limited studies of Boeing 747 configurations show correct qualitative response to removal of the outer flap and to gear deployment, as compared with wind tunnel and flight test experience.

  17. Information Requirements for Supervisory Air Traffic Controllers in Support of a Wake Vortex Departure System

    NASA Technical Reports Server (NTRS)

    Lohr, Gary W.; Williams, Daniel M.; Trujillo, Anna C.

    2008-01-01

    Closely Space Parallel Runway (CSPR) configurations are capacity limited for departures due to the requirement to apply wake vortex separation standards from traffic departing on the adjacent parallel runway. To mitigate the effects of this constraint, a concept focusing on wind dependent departure operations has been developed, known as the Wake Turbulence Mitigation for Departures (WTMD). This concept takes advantage of the fact that crosswinds of sufficient velocity blow wakes generated by aircraft departing from the downwind runway away from the upwind runway. Consequently, under certain conditions, wake separations on the upwind runway would not be required based on wakes generated by aircraft on the downwind runway, as is currently the case. It follows that information requirements, and sources for this information, would need to be determined for airport traffic control tower (ATCT) supervisory personnel who would be charged with decisions regarding use of the procedure. To determine the information requirements, data were collected from ATCT supervisors and controller-in-charge qualified individuals at Lambert-St. Louis International Airport (STL) and George Bush Houston Intercontinental Airport (IAH). STL and IAH were chosen as data collection sites based on the implementation of a WTMD prototype system, operating in shadow mode, at these locations. The 17 total subjects (STL: 5, IAH: 12) represented a broad-base of air traffic experience. Results indicated that the following information was required to support the conduct of WTMD operations: current and forecast weather information, current and forecast traffic demand and traffic flow restrictions, and WTMD System status information and alerting. Subjects further indicated that the requisite information is currently available in the tower cab with the exception of the WTMD status and alerting. Subjects were given a demonstration of a display supporting the prototype systems and unanimously stated that the WTMD status information they felt important was represented. Overwhelmingly, subjects felt that approving, monitoring and terminating the WTMD procedure could be integrated into their supervisory workload.

  18. Experimental investigation of the wake behind a model of wind turbine in a water flume

    NASA Astrophysics Data System (ADS)

    Okulov, V. L.; Naumov, I. N.; Kabardin, I.; Mikkelsen, R.; Sørensen, J. N.

    2014-12-01

    The flow behind the model of wind turbine rotor is investigated experimentally in a water flume using Particle Image Velocimetry. The study carried out involves rotors of three bladed wind turbine designed using Glauert's optimization. The transitional regime, generally characterized as in between the regime governed by stable organized vortical structures and the turbulent wake, develops from disturbances of the tip and root vorticies through vortex paring and further complex behaviour towards the fully turbulent wake. Our PIV measurements pay special attention to the onset of the instabilities. The near wake characteristics (development of expansion, tip vortex position, deficit velocity and rotation in the wake) have been measured for different tip speed ratio to compare with main assumptions and conclusions of various rotor theories.

  19. Vertical dispersion of an aircraft wake: Aerosol-lidar analysis of entrainment and detrainment in the vortex regime

    NASA Astrophysics Data System (ADS)

    Sussmann, Ralf

    1999-01-01

    Vertical dispersion of contrails in the vortex regime is investigated by focusing on the role of entrainment and detrainment of exhaust with respect to the pair of trailing vortices. A ground-based backscatter-depolarization lidar with an integrated CCD camera provides information on optical and geometrical parameters of the contrail in the time span between 5.7 and 50.3 s behind a B747-400 aircraft. This is combined with coincident airborne in situ measurements of turbulence and the vertical profiles of temperature and wind speed in a case study. The two wingtip vortices, separated by 47 m, are descending with an increasing speed (2.5-3.1 m/s for 10.8-47.8 s behind aircraft) in the weakly non-stably-stratified atmosphere. The turbulent vertical dissipation rate on the day of the study above southern Germany is a factor of 1000 higher than found typically above oceans at cruising altitude. At 4.2 s behind the aircraft, a diffuse secondary wake starts to evolve above the two wingtip vortices. After ≈ 50 s the secondary wake encloses a cross-sectional area (4410 m2) comparable to that of the primary wake (4620 m2) and a relative ice surface area of 1:5. The observed early onset of the secondary wake is conjectured to be due to turbulent detrainment of fluid out of the primary wake which can be enhanced by detrainment due to baroclinic forces later in the vortex regime evolution. By exclusion of other mechanisms of secondary wake formation, detrainment of fluid from the primary wake is concluded to be the precondition for secondary wake formation. Detrainment due to baroclinic forces, shear or turbulence is, in general, unlikely to be absent for typical atmospheric conditions. It is suggested that the ambient humidity level may determine when a secondary wake is visible above a vortex pair and when it is not.

  20. An experimental investigation of vortex stability, tip shapes, compressibility, and noise for hovering model rotors

    NASA Technical Reports Server (NTRS)

    Tangler, J. L.; Wohlfeld, R. M.; Miley, S. J.

    1973-01-01

    Schlieren methods of flow visualization and hot-wire anemometry for velocity measurements were used to investigate the wakes generated by hovering model propellers and rotors. The research program was directed toward investigating (1) the stability of the tip vortex, (2) the effects produced by various tip shapes on performance and tip vortex characteristics, and (3) the shock formation and noise characteristics associated with various tip shapes. A free-wake analysis was also conducted for comparison with the vortex stability experimental results. Schlieren photographs showing wake asymmetry, interaction, and instability are presented along with a discussion of the effects produced by the number of blades, collective pitch, and tip speed. Two hot-wire anemometer techniques, used to measure the maximum circumferential velocity in the tip vortex, are discussed.

  1. Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers

    PubMed Central

    Muijres, Florian T.; Bowlin, Melissa S.; Johansson, L. Christoffer; Hedenström, Anders

    2012-01-01

    Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a ‘feathered upstroke’ during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called ‘normal hovering’ as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body–tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability. PMID:21676971

  2. Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers.

    PubMed

    Muijres, Florian T; Bowlin, Melissa S; Johansson, L Christoffer; Hedenström, Anders

    2012-02-01

    Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a 'feathered upstroke' during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called 'normal hovering' as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body-tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability. PMID:21676971

  3. Atmospheric Boundary Layer Sensors for Application in a Wake Vortex Advisory System

    NASA Technical Reports Server (NTRS)

    Zak, J. Allen; Rutishauser, David (Technical Monitor)

    2003-01-01

    Remote sensing of the atmospheric boundary layer has advanced in recent years with the development of commercial off-the-shelf (COTS) radar, sodar, and lidar wind profiling technology. Radio acoustic sounding systems for vertical temperature profiles of high temporal scales (when compared to routine balloon soundings- (radiosondes) have also become increasingly available as COTS capabilities. Aircraft observations during landing and departures are another source of available boundary layer data. This report provides an updated assessment of available sensors, their performance specifications and rough order of magnitude costs for a potential future aircraft Wake Vortex Avoidance System (WakeVAS). Future capabilities are also discussed. Vertical profiles of wind, temperature, and turbulence are anticipated to be needed at airports in any dynamic wake avoidance system. Temporal and spatial resolution are dependent on the selection of approach and departure corridors to be protected. Recommendations are made for potential configurations of near-term sensor technologies and for testing some of the sensor systems in order to validate performance in field environments with adequate groundtruth.

  4. Wake effects on the aerodynamic performance of horizontal axis wind turbines

    SciTech Connect

    Afjeh, A.A.

    1984-08-01

    Success of vortex theories in the performance analysis of horizontal axis wind turbines depends greatly upon accurate specification of the geometry of the vortex wake. In this study, two methods of analysis are developed: a new simplified free wake method (SFW) and a prescribed wake method. In addition, an earlier wake model of helicopter rotors is extended for wind turbine applications. This method is referred to as the fast free wake method (FFW). The FFW was accomplished by partitioning the flow field downstream of the rotor into three regions: the near wake, modeled as a series of straight vortex lines; the intermediate wake, modeled as a number of vortex rings; and the far wake, taken to be a semi-infinite cylindrical wake. In the SFW, a new wake model is proposed. The model assumes that the wake is composed of an intense tip vortex and a diffused inboard wake, consistent with the experimentally observed wake of hovering helicopters. However, due to the complexity of the tip vortex formation and due to the lack of such experimental data for wind turbines, it was assumed that the vortex formation was almost immediate as opposed to the actual gradual rolling-up of the tip vortex. For the prescribed wake analysis the expansion of the wake must be known. Unfortunately, detailed wake measurements for wind turbines are sparse in number; hence, the method was demonstrated by assuming the wake expansion could be represented by an analytical expression.

  5. Overview of the preparation and use of an OV-10 aircraft for wake vortex hazards flight experiments

    NASA Technical Reports Server (NTRS)

    Stuever, Robert A.; Stewart, Eric C.; Rivers, Robert A.

    1995-01-01

    An overview is presented of the development, use, and current flight-test status of a highly instrumented North American Rockwell OV-10A Bronco as a wake-vortex-hazards research aircraft. A description of the operational requirements and measurements criteria, the resulting instrumentation systems and aircraft modifications, system-calibration and research flights completed to date, and current flight status are included. These experiments are being conducted by the National Aeronautics and Space Administration as part of an effort to provide the technology to safely improve the capacity of the nation's air transportation system and specifically to provide key data in understanding and predicting wake vortex decay, transport characteristics, and the dynamics of encountering wake turbulence. The OV-10A performs several roles including meteorological measurements platform, wake-decay quantifier, and trajectory-quantifier for wake encounters. Extensive research instrumentation systems include multiple airdata sensors, video cameras with cockpit displays, aircraft state and control-position measurements, inertial aircraft-position measurements, meteorological measurements, and an on-board personal computer for real-time processing and cockpit display of research data. To date, several of the preliminary system check flights and two meteorological-measurements deployments have been completed. Several wake encounter and wake-decay-measurements flights are planned for the fall of 1995.

  6. Flight Test Analysis of the Forces and Moments Imparted on a B737-100 Aircraft During Wake Vortex Encounters

    NASA Technical Reports Server (NTRS)

    Roberts, Christopher L.; Smith, Sonya T.; Vicroy, Dan D.

    2000-01-01

    Several of our major airports are operating at or near their capacity limit, increasing congestion and delays for travelers. As a result, the National Aeronautics and Space Administration (NASA) has been working in conjunction with the Federal Aviation Administration (FAA), airline operators, and the airline industry to increase airport capacity and safety. As more and more airplanes are placed into the terminal area the probability of encountering wake turbulence is increased. The NASA Langley Research Center conducted a series of flight tests from 1995 through 1997 to develop a wake encounter and wake-measurement data set with the accompanying atmospheric state information. The purpose of this research is to use the data from those flights to compute the wake-induced forced and moments exerted on the aircraft The calculated forces and moments will then be compiled into a database that can be used by wake vortex researchers to compare with experimental and computational results.

  7. Comparison of calculated and measured model rotor loading and wake geometry

    NASA Technical Reports Server (NTRS)

    Johnson, W.

    1980-01-01

    The calculated blade bound circulation and wake geometry are compared with measured results for a model helicopter rotor in hover and forward flight. Hover results are presented for rectangular tip and ogee tip planform blades. The correlation is quite good when the measured wake geometry characteristics are used in the analysis. Available prescribed wake geometry models are found to give fair predictions of the loading, but they do not produce a reasonable prediction of the induced power. Forward flight results are presented for twisted and untwisted blades. Fair correlation between measurements and calculations is found for the bound circulation distribution on the advancing side. The tip vortex geometry in the vicinity of the advancing blade in forward flight was predicted well by the free wake calculation used, although the wake geometry did not have a significant influence on the calculated loading and performance for the cases considered.

  8. Estimation of aircraft wake vortex parameters from data measured with a 1.5-μm coherent Doppler lidar.

    PubMed

    Smalikho, I N; Banakh, V A

    2015-07-15

    A strategy of measurement by a 1.5-μm pulsed coherent Doppler lidar "Stream Line" has been developed, and a method for estimation of aircraft wake vortices from the lidar data has been proposed. The principal possibility of obtaining the information about the vortex situation over an airport airfield with the Stream-Line lidar has been demonstrated. PMID:26176481

  9. Anomalous vortex shedding and wake profiles in quasi-two-dimensional flows

    NASA Astrophysics Data System (ADS)

    Fontana, Paul W.; Dams, Dominic A.

    2014-11-01

    Vortex shedding by circular cylinders in a vertical soap film channel exhibits anomalously low shedding frequencies compared with observations in conventional systems. Furthermore, the Strouhal number (St = fD /U∞ , where f is the shedding frequency, D the cylinder diameter, and U∞ the upstream flow speed) is not uniquely determined by the Reynolds number (Re = DU∞ / ν , where ν is the kinematic viscosity). We have previously argued that Ekman friction is a likely cause [Bull. Amer. Phys. Soc. 57(17), R10.7]. Other possibilities include gravity, which in this system acts as a forcing mechanism not typically present during vortex shedding measurements, surface tension effects, or variable-viscosity effects due to variations in film thickness. Theory to predict the shedding frequency is lacking and so it is unclear if or how each of these mechanisms should affect it, but understanding the anomaly may elucidate the shedding process. We present two-dimensional profiles of velocity, viscosity, and surface friction measured in the wake of the cylinder under several sets of flow parameters and discuss their implications for the various candidates. The results do not support variable viscosity as a cause.

  10. TURBULENT DIFFUSION BEHIND VEHICLES: EXPERI-MENTALLY DETERMINED INFLUENCE OF VORTEX PAIR IN VEHICLE WAKE

    EPA Science Inventory

    The wake of a moving vehicle was simulated using a wind tunnel with a moving floor. he vehicle models, both scale and block-shaped, were held in a fixed position while the floor moved at the upstream air speed. his simulates an automobile traveling on a straight highway in still ...

  11. Measurements of fish's wake by PIV

    NASA Astrophysics Data System (ADS)

    Li, Xuemin; Wu, Yanfeng; Lu, Xiyun; Yin, Xiezhen

    2003-04-01

    In this paper an experiment on measurements of the wake of Goldfish carassius auratus swimming unrestricted was conducted in a water tunnel. Color liquid was used to visualize the wake of the fish and PIV was used to measure velocity field of the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the fish's wake and the Strouhal frequency of the fish is about 0.35 udner the different experimental conditions. The distribution of velocity and vorticity in the wake of Goldfish was measured by PIV and formation of reverse Karman vortex street in the wake was studied in a model experiment.

  12. On vortex evolution in the wake of axisymmetric and non-axisymmetric low-aspect-ratio accelerating plates

    NASA Astrophysics Data System (ADS)

    Fernando, John N.; Rival, David E.

    2016-01-01

    Impulsively started, low-aspect-ratio elliptical and rectangular flat plates were investigated to determine the role of geometric asymmetries on vortex evolution. Dye visualizations, force measurements, and particle image velocimetry were used throughout to characterize the variation between shapes. For all the shapes studied, aspect ratio was observed to have the largest influence on force production and vortex evolution. Non-uniform curvature and edge discontinuities characteristic of ellipses (with aspect ratios other than one) and rectangles, respectively, play a secondary role. Furthermore, it was shown that stably attached vortex rings form behind the circular and square flat plates, which reduce the instantaneous drag force of each plate until the vortex rings break down. In contrast, all flat plates with aspect ratios other than one are subjected to fast-modulating elliptical vortex rings in the wake. These vortex rings increase the drag force of each plate until pinch-off occurs. Finally, pinch-off was identified with the streamwise pressure-gradient field and compared with formation numbers calculated using the circulation-based methodology, yielding good agreement for all plates with aspect ratios greater than one.

  13. Flight test investigation of the vortex wake characteristics behind a Boeing 727 during two-segment and normal ILS approaches (A joint NASA/FAA report)

    NASA Technical Reports Server (NTRS)

    Barber, M. R.; Kurkowski, R. L.; Garodz, L. J.; Robinson, G. H.; Smith, H. J.; Jacobsen, R. A.; Stinnett, G. W., Jr.; Mcmurtry, T. C.; Tymczyszyn, J. J.; Devereaux, R. L.

    1975-01-01

    Flight tests were performed to evaluate the vortex wake characteristics of a Boeing 727 aircraft during conventional and two-segment instrument landing approaches. Smoke generators were used for vortex marking. The vortex was intentionally intercepted by a Lear Jet and a Piper Comanche aircraft. The vortex location during landing approach was measured using a system of phototheodolites. The tests showed that at a given separation distance there are no readily apparent differences in the upsets resulting from deliberate vortex encounters during the two types of approaches. The effect of the aircraft configuration on the extent and severity of the vortices is discussed.

  14. Vortex dynamics in nonrelativistic Abelian Higgs model

    NASA Astrophysics Data System (ADS)

    Kozhevnikov, A. A.

    2015-11-01

    The dynamics of the gauge vortex with arbitrary form of a contour is considered in the framework of the nonrelativistic Abelian Higgs model, including the possibility of the gauge field interaction with the fermion asymmetric background. The equations for the time derivatives of the curvature and the torsion of the vortex contour generalizing the Betchov-Da Rios equations in hydrodynamics, are obtained. They are applied to study the conservation of helicity of the gauge field forming the vortex, twist, and writhe numbers of the vortex contour. It is shown that the conservation of helicity is broken when both terms in the equation of the vortex motion are present, the first due to the exchange of excitations of the phase and modulus of the scalar field and the second one due to the coupling of the gauge field forming the vortex, with the fermion asymmetric background.

  15. Calculation of asymmetric vortex separation on cones and tangent ogives based on a discrete vortex model

    NASA Technical Reports Server (NTRS)

    Chin, Suei; Lan, C. Edward; Gainer, Thomas G.

    1989-01-01

    The boundary value problem for vortex separation at zero sideslip on cones and tangent ogives is set up by means of a discrete vortex model. The nonlinear algebraic equations for the boundary value problem admit multiple, physically feasible solutions, including the symmetric and asymmetric vortex solutions. Multiple solutions are proposed as an alternative explanation of the existence of asymmetric vortex separation at zero sideslip.

  16. PIV and LDA measurements of the wake behind a wind turbine model

    NASA Astrophysics Data System (ADS)

    Naumov, I. V.; Mikkelsen, R. F.; Okulov, V. L.; Sørensen, J. N.

    2014-06-01

    In the present work we review the results of a series of measurements of the flow behind a model scale of a horizontal axis wind turbine rotor carried out at the water flume at Technical University of Denmark (DTU). The rotor is three-bladed and designed using Glauert theory for tip speed ratio λ =5 with a constant design lift coefficient along the span, CLdesign= 0.8. The measurements include dye visualization, Particle Image Velocimetry and Laser Doppler Anemometry. The wake instability has been studied in the range λ =3 - 9 at different cross-sections from the very near wake up to 10 rotor diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different processes and instabilities in the rotor wake. In the far wake a constant frequency corresponding to the Strouhal number was found for the long-scale instabilities. This Strouhal number is in good agreement with the well-known constant that usually characterizes the oscillation in wakes behind bluff bodies. From associated visualizations and reconstructions of the flow field, it was found that the dynamics of the far wake is associated with the precession (rotation) of a helical vortex core. The data indicate that Strouhal number of this precession is independent of the rotor angular speed.

  17. Simulating Virtual Terminal Area Weather Data Bases for Use in the Wake Vortex Avoidance System (Wake VAS) Prediction Algorithm

    NASA Technical Reports Server (NTRS)

    Kaplan, Michael L.; Lin, Yuh-Lang

    2004-01-01

    During the research project, sounding datasets were generated for the region surrounding 9 major airports, including Dallas, TX, Boston, MA, New York, NY, Chicago, IL, St. Louis, MO, Atlanta, GA, Miami, FL, San Francico, CA, and Los Angeles, CA. The numerical simulation of winter and summer environments during which no instrument flight rule impact was occurring at these 9 terminals was performed using the most contemporary version of the Terminal Area PBL Prediction System (TAPPS) model nested from 36 km to 6 km to 1 km horizontal resolution and very detailed vertical resolution in the planetary boundary layer. The soundings from the 1 km model were archived at 30 minute time intervals for a 24 hour period and the vertical dependent variables as well as derived quantities, i.e., 3-dimensional wind components, temperatures, pressures, mixing ratios, turbulence kinetic energy and eddy dissipation rates were then interpolated to 5 m vertical resolution up to 1000 m elevation above ground level. After partial validation against field experiment datasets for Dallas as well as larger scale and much coarser resolution observations at the other 8 airports, these sounding datasets were sent to NASA for use in the Virtual Air Space and Modeling program. The application of these datasets being to determine representative airport weather environments to diagnose the response of simulated wake vortices to realistic atmospheric environments. These virtual datasets are based on large scale observed atmospheric initial conditions that are dynamically interpolated in space and time. The 1 km nested-grid simulated datasets providing a very coarse and highly smoothed representation of airport environment meteorological conditions. Details concerning the airport surface forcing are virtually absent from these simulated datasets although the observed background atmospheric processes have been compared to the simulated fields and the fields were found to accurately replicate the flows surrounding the airport where coarse verification data were available as well as where airport scale datasets were available.

  18. Three-dimensional vortex wake structure of a flapping-wing micro aerial vehicle in forward flight configuration

    NASA Astrophysics Data System (ADS)

    Percin, M.; van Oudheusden, B. W.; Eisma, H. E.; Remes, B. D. W.

    2014-09-01

    This paper investigates the formation and evolution of the unsteady three-dimensional wake structures generated by the flapping wings of the DelFly II micro aerial vehicle in forward flight configuration. Time-resolved stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at several spanwise-aligned planes in the wake, so as to allow a reconstruction of the temporal development of the wake of the flapping wings throughout the complete flapping cycle. Simultaneous thrust-force measurements were performed to explore the relation between the wake formation and the aerodynamic force generation mechanisms. The three-dimensional wake configuration was subsequently reconstructed from the planar PIV measurements by two different approaches: (1) a spatiotemporal wake reconstruction obtained by convecting the time-resolved, three-component velocity field data of a single measurement plane with the free-stream velocity; (2) for selected phases in the flapping cycle a direct three-dimensional spatial wake reconstruction is interpolated from the data of the different measurement planes, using a Kriging regression technique. Comparing the results derived from both methods in terms of the behavior of the wake formations, their phase and orientation indicate that the spatiotemporal reconstruction method allows to characterize the general three-dimensional structure of the wake, but that the spatial reconstruction method can reveal more details due to higher streamwise resolution. Comparison of the wake reconstructions for different values of the reduced frequency allows assessing the impact of the flapping frequency on the formation and interaction characteristics of the vortical structures. For low values of the reduced frequency, it is observed that the vortex structure formation of instroke and outstroke is relatively independent of each other, but that increasing interaction occurs at higher reduced frequencies. It is further shown that there is a phase lag in the appearance of the structures for increasing flapping frequency, which is in correlation with the generation of the forces. Comparison of thrust generated during the instroke and the outstroke phases of the flapping motion in conjunction with the development of the wake structures indicates that wing-wing interaction at the start of outstroke (peel motion) becomes a dominant feature for reduced frequencies greater than 0.62.

  19. Comparison of the lifting-line free vortex wake method and the blade-element-momentum theory regarding the simulated loads of multi-MW wind turbines

    NASA Astrophysics Data System (ADS)

    Hauptmann, S.; Bülk, M.; Schön, L.; Erbslöh, S.; Boorsma, K.; Grasso, F.; Kühn, M.; Cheng, P. W.

    2014-12-01

    Design load simulations for wind turbines are traditionally based on the blade- element-momentum theory (BEM). The BEM approach is derived from a simplified representation of the rotor aerodynamics and several semi-empirical correction models. A more sophisticated approach to account for the complex flow phenomena on wind turbine rotors can be found in the lifting-line free vortex wake method. This approach is based on a more physics based representation, especially for global flow effects. This theory relies on empirical correction models only for the local flow effects, which are associated with the boundary layer of the rotor blades. In this paper the lifting-line free vortex wake method is compared to a state- of-the-art BEM formulation with regard to aerodynamic and aeroelastic load simulations of the 5MW UpWind reference wind turbine. Different aerodynamic load situations as well as standardised design load cases that are sensitive to the aeroelastic modelling are evaluated in detail. This benchmark makes use of the AeroModule developed by ECN, which has been coupled to the multibody simulation code SIMPACK.

  20. Delayed Orexin Signaling Consolidates Wakefulness and Sleep: Physiology and Modeling

    PubMed Central

    Diniz Behn, C. G.; Kopell, N.; Brown, E. N.; Mochizuki, T.; Scammell, T. E.

    2011-01-01

    Orexin-producing neurons are clearly essential for the regulation of wakefulness and sleep because loss of these cells produces narcolepsy. However, little is understood about how these neurons dynamically interact with other wake- and sleep-regulatory nuclei to control behavioral states. Using survival analysis of wake bouts in wild-type and orexin knockout mice, we found that orexins are necessary for the maintenance of long bouts of wakefulness, but orexin deficiency has little impact on wake bouts <1 min. Since orexin neurons often begin firing several seconds before the onset of waking, this suggests a surprisingly delayed onset (>1 min) of functional effects. This delay has important implications for understanding the control of wakefulness and sleep because increasing evidence suggests that different mechanisms are involved in the production of brief and sustained wake bouts. We incorporated these findings into a mathematical model of the mouse sleep/wake network. Orexins excite monoaminergic neurons and we hypothesize that orexins increase the monoaminergic inhibition of sleep-promoting neurons in the ventrolateral preoptic nucleus. We modeled orexin effects as a time-dependent increase in the strength of inhibition from wake- to sleep-promoting populations and the resulting simulated behavior accurately reflects the fragmented sleep/wake behavior of narcolepsy and leads to several predictions. By integrating neurophysiology of the sleep/wake network with emergent properties of behavioral data, this model provides a novel framework for investigating network dynamics and mechanisms associated with normal and pathologic sleep/wake behavior. PMID:18417630

  1. Laboratory measurements of vortex- and wake-induced vibrations of a tandem arrangement of two flexible risers

    NASA Astrophysics Data System (ADS)

    Liu, Huai-zeng; Wang, Fei; Jiang, Guo-sheng; Guo, Hai-yan; Li, Xiao-min

    2016-03-01

    The dynamic response of two flexible model risers in tandem arrangement immersed in a stepped current was analyzed. The risers, with an external diameter of 20 mm and a total length of 6200 mm, had an aspect ratio of 310. They were hinged to the support structure at the center-to-center distances away 3-12 times the external diameter. The top 1200 mm was exposed to a uniform current at a speed which was up to 0.9 m/s (the Reynolds number was 18000) and the rest in still water. The dynamic responses, which were obtained through the Fiber Bragg Grating strain gauges mounted on the surface, were analyzed by studying the cross-flow amplitudes and modal weights. The cross-flow vibration were observed up to the third mode, and the modal transformation from the second mode to the third mode was clearly observed. The experiment confirmed that the typical vortex-induced vibration (VIV) had occurred on the up-stream riser. But for the down-stream riser, the main excitation mechanism was wake-induced vibration (WIV). The modal transformation of WIV was more complex than that of VIV, which might be helpful for other researchers to study the interference effect.

  2. Locomotor forces on a swimming fish: three-dimensional vortex wake dynamics quantified using digital particle image velocimetry.

    PubMed

    Drucker; Lauder

    1999-01-01

    Quantifying the locomotor forces experienced by swimming fishes represents a significant challenge because direct measurements of force applied to the aquatic medium are not feasible. However, using the technique of digital particle image velocimetry (DPIV), it is possible to quantify the effect of fish fins on water movement and hence to estimate momentum transfer from the animal to the fluid. We used DPIV to visualize water flow in the wake of the pectoral fins of bluegill sunfish (Lepomis macrochirus) swimming at speeds of 0.5-1.5 L s(-)(1), where L is total body length. Velocity fields quantified in three perpendicular planes in the wake of the fins allowed three-dimensional reconstruction of downstream vortex structures. At low swimming speed (0.5 L s(-)(1)), vorticity is shed by each fin during the downstroke and stroke reversal to generate discrete, roughly symmetrical, vortex rings of near-uniform circulation with a central jet of high-velocity flow. At and above the maximum sustainable labriform swimming speed of 1.0 L s(-)(1), additional vorticity appears on the upstroke, indicating the production of linked pairs of rings by each fin. Fluid velocity measured in the vicinity of the fin indicates that substantial spanwise flow during the downstroke may occur as vortex rings are formed. The forces exerted by the fins on the water in three dimensions were calculated from vortex ring orientation and momentum. Mean wake-derived thrust (11.1 mN) and lift (3.2 mN) forces produced by both fins per stride at 0.5 L s(-)(1) were found to match closely empirically determined counter-forces of body drag and weight. Medially directed reaction forces were unexpectedly large, averaging 125 % of the thrust force for each fin. Such large inward forces and a deep body that isolates left- and right-side vortex rings are predicted to aid maneuverability. The observed force balance indicates that DPIV can be used to measure accurately large-scale vorticity in the wake of swimming fishes and is therefore a valuable means of studying unsteady flows produced by animals moving through fluids. PMID:10460729

  3. On Use of Global Positioning Technology for Solution of Wake Vortex Problem

    NASA Technical Reports Server (NTRS)

    Rossow, Vernon J.; Olson, Lawerence E. (Technical Monitor)

    1997-01-01

    Improved precision of the flight paths used by aircraft to approach and depart airports is becoming available when the Global Positioning System (GPS) is implemented at airports. An overview will be given of published information on how GPS precision guidance at airports can be used to avoid encounters with the lift-generated vortices shed by preceding aircraft during landing. It is pointed out that GPS provides two needed services to bring about improved avoidance capability. Firstly, GPS pseudolites are being built and installed at airports so that, when coupled with autopilot systems currently available on subsonic transport aircraft, the aircraft can make precision approaches for zero visibility landings. The same equipment can also be used for precision approaches for wake-vortex avoidance. Secondly, regular monitoring of atmospheric motions along the approach corridor can be obtained by use of GPS equipment on board aircraft that are in the flight corridors. That is, wind velocity is determined by use of GPS equipment to measure the ground speed of the aircraft and then combined with onboard instrumentation to measure the airspeed of the aircraft. The difference between the two measurements yields the wind velocity. When the measured wind velocities are transmitted to an airport ground station they are used to monitor whether air motions adverse for safety in the flight corridor are present. If any parts of the corridor are unsafe, the spacing of the aircraft, or the location of the flight corridor being used, is modified. It is estimated that the spacings between any combination of aircraft can then be safely reduced to a uniform 3 n. mi. Information to be presented is contained in an article published in the AIAA Journal of Aircraft, May-June 1996.

  4. Application of digital particle image velocimetry to insect aerodynamics: measurement of the leading-edge vortex and near wake of a Hawkmoth

    NASA Astrophysics Data System (ADS)

    Bomphrey, Richard J.; Lawson, Nicholas J.; Taylor, Graham K.; Thomas, Adrian L. R.

    2006-04-01

    Some insects use leading-edge vortices to generate high lift forces, as has been inferred from qualitative smoke visualisations of the flow around their wings. Here we present the first Digital Particle Image Velocimetry (DPIV) data and quantitative analysis of an insect’s leading-edge vortex and near wake at two flight speeds. This allows us to describe objectively 2D slices through the flow field of a tethered Tobacco Hawkmoth ( Manduca sexta). The near-field vortex wake appears to braodly resemble elliptical vortex loops. The presence of a leading-edge vortex towards the end of the downstroke is found to coincide with peak upward force production measured by a six-component force-moment balance. The topology of Manduca’s leading-edge vortex differs from that previously described because late in the downstroke, the structure extends continuously from wingtip across the thorax to the other wingtip.

  5. A simplified ring-vortex downburst model

    NASA Technical Reports Server (NTRS)

    Zhao, Yiyuan; Bryson, A. E.

    1990-01-01

    Three dimensional incompressible vortex rings are used to model downburst flow field, and modifications are made to express the induced velocities analytically. As a result, this three dimensional downburst model can be used in trajectory optimization, as well as in flight simulations. Two such vortex rings are superposed to match the JAWS AB Corridor wind profiles and the DFW Downburst wind profiles. Model parameters are determined through nonlinear least square fit.

  6. Vortex-Body Interactions: A Critical Assessment. Coupled Gap-Wake Instabilities/Turbulence: A Source of Noise

    NASA Technical Reports Server (NTRS)

    Rockwell, Donald

    1999-01-01

    This program has involved, first of all, a critical state-of-the-art assessment of vortex-body interactions. Then, efforts were focused on experimental investigation on coupled-wake instabilities and turbulence occurring in a two-cylinder system. An extensive review was undertaken on the effect of incident vortices on various types of bodies. These incident vortices have a length scale of the same order of magnitude as the scale of the body. The body can take on various forms, including, for example, a circular cylinder, a blade or a wing. The classes of vortex-body interaction that were critically assessed include: (1) Periodic distortion of the incident (primary) vortex and shedding of secondary vorticity from the surface of the body. (2) Modulated vortex distortion and shedding at a leading-edge or surface due to incidence of a complex system of vortices. (3) Vortex distortion and shedding in presence of body oscillation. (4) Three-dimensional vortex interaction and shedding. For all of these classes of vortex-body interaction, quantitative topologies of the vorticity distributions and streamline patterns were found to be central to a unified description of mechanisms of vortex distortion and shedding. In most cases, it was possible to define relationships between vortex interactions and unsteady loading at the body surface. This phase of the program was an experimental investigation of a two-cylinder system, which simulated a central aspect of a four-wheel bogie on a large-scale commercial aircraft. The overall aim of this experimental research program was to determine the crucial elements of the unsteadiness in the gap and near-wake regions as a function of time using cinema-based techniques. During the research program, various image evaluation techniques were employed. They involved assessment of instantaneous velocity fields, streamline topology and patterns of vorticity. Experiments were performed in a large-scale water channel using a high-resolution version of digital particle image velocimetry. The program has focused on acquisition of images of velocity and vorticity for varying gap widths between the two-cylinder system. As a result of analysis of a relatively large number of images, it is demonstrated that low frequency instabilities can occur in the gap region between the cylinder. These low frequency instabilities are hypothesized to influence the near-wake structure of the entire two-cylinder system. The nature of the unstable shear layers in the gap region involves generation of small-scale Kelvin-Helmholtz instabilities. These unsteady shear layers then impinge upon the upper and lower surfaces of the cylinders, thereby influencing both the unsteady structure and the time-averaged patterns of the near-wake. Initial efforts have focused on characterization of the patterns of instantaneous and averaged streamlines using topological concepts. The end result of this investigation is a series of documented instantaneous images. They will serve as a basis for various types of post-processing, which will lead to a fuller understanding of the instantaneous and time-averaged unstable-turbulent fields in the gap region and downstream of the two-cylinder system. This further assessment is the focus of a subsequent program.

  7. Inviscid double wake model for stalled airfoils

    NASA Astrophysics Data System (ADS)

    Marion, L.; Ramos-García, N.; Sørensen, J. N.

    2014-06-01

    An inviscid double wake model based on a steady two-dimensional panel method has been developed to predict aerodynamic loads of wind turbine airfoils in the deep stall region. The separated flow is modelled using two constant vorticity sheets which are released at the trailing edge and at the separation point. A calibration of the code through comparison with experiments has been performed using one set of airfoils. A second set of airfoils has been used for the validation of the calibrated model. Predicted aerodynamic forces for a wide range of angles of attack (0 to 90 deg) are in overall good agreement with wind tunnel measurements.

  8. Computer models of the spacecraft wake

    NASA Technical Reports Server (NTRS)

    Rubin, A. G.; Heinemann, M.; Tautz, M.; Cooke, D.

    1986-01-01

    Until recently, computations of space plasma flow over a spacecraft have been unstable for ratios of spacecraft dimension to Debye length typical of the low Earth orbit environment. Calculations are presented of the spacecraft/environment interaction based on two computer codes, MACH and POLAR. MACH, an inside-out particle tracking code, was developed for the purpose of validating the physics of POLAR in regimes where these are no comprehensive theoretical or experimental results. While the spacecraft which can be treated by MACH are restricted to simple geometries, the methodology is more fundamental than POLAR. MACH generates self-consistent solutions within the context of quasisteady Vlasov plasma flow and achieves Debye ratios previously unobtainable. POLAR uses a three-dimensional finite-element representation of the vehicle in a staggered mesh. The plasma sheath is modeled by outside-in particle tracking. Solutions for the plasma flow, wake and vehicle charging are obtained by Vlasov-Poisson iteration; charge stabilization techniques make the results virtually insensitive to the Debye ratio. POLAR reproduces the Laframboise static plasma solutions for sperical probes and fits the Makita-Kuriki probe data for spheres in a flowing plasma in regions where comparisons are valid. POLAR and MACH solutions for the particle and electrostatic potential structure of the wake of a charged disk in a low-altitude flow are shown for Mach numbers 4, 5, and 8. New features of the solutions include ion focussing in the wake and a definitive determination of the sheath edge in the wake which shows that the sheath is not an equipotential.

  9. The structure of the wake generated by a submarine model in yaw

    NASA Astrophysics Data System (ADS)

    Ashok, A.; Van Buren, T.; Smits, A. J.

    2015-06-01

    The turbulent wake of a submarine model in yaw was investigated using stereoscopic particle image velocimetry at The model (DARPA SUBOFF idealized submarine geometry) is mounted in a low-speed wind tunnel using a support that mimics the sail, and it is yawed so that the body moves in the plane normal to the support. The measurements reveal the formation of a pair of streamwise vortices that are asymmetric in strength. The weaker vortex quickly diffuses, and in the absence of further diffusion, the stronger vortex maintains its strength even at the furthest downstream location. It is suggested that the flow fields obtained here using a semi-infinite sail as a support will be similar to those obtained using a finite length sail since its tip vortex would not interact significantly with the body vortices present in the wake, at least for a considerable distance downstream of the stern Hence, a submarine in yaw is expected to generate wakes which are inherently more persistent than one in pitch, and the strong asymmetries in yaw are expected to produce a net rolling moment on the body.

  10. Computational simulation of turbulent vortex merger and decay. [in aircraft wakes

    NASA Technical Reports Server (NTRS)

    Raj, P.; Iversen, J. D.

    1979-01-01

    The interaction and eventual merger of corotational vortices and the decay of a single vortex have been studied by employing zero-, one- and two-equation turbulent-flow models in order to gain a better understanding of the role of turbulence. An implicit finite-difference procedure is used to integrate the unsteady, two-dimensional equations in a cross-plane. The zero- and one-equation formulations utilize a mixing-length model, which incorporates the streamline curvature effect by prescribing a spatially-varying mixing-length. In the two-equation model, the turbulence kinetic energy equation and a modified rate of dissipation equation which includes a streamline curvature correction are solved. Computational results of different models applied to various flow-configurations are presented and compared with available experimental data whenever possible.

  11. Wake effects of the aerodynamic performance of horizontal axis wind turbines

    SciTech Connect

    Afjeh, A.A.A.K.

    1984-01-01

    Success of vortex theories in the performance prediction of horizontal axis wind turbines largely depends upon accurate specification of the geometry of the vortex wake. In this study, two methods of vortex wake analysis are developed: a new simplified free wake method (SFW) and a prescribed wake method. In addition, an earlier wake model of helicopter rotors, referred to as the fast free wake method (FFW), is extended for wind turbine applications. In the FFW model, the flow field downstream of the rotor was partitioned into three regions: the near wake, modeled as a series of straight vortex lines; the intermediate wake, modeled as a number of vortex rings; and the far wake, taken to be a semi-infinite cylindrical wake. The methods of this work were compared with an existing unconstrained free wake analysis, with an existing rigid wake analysis, with a popular blade element momentum method and with existing experimental data. Airload parameters obtained by using the present methods were found to be in good agreement with those of a full free wake analysis. However, the computational times were greatly reduced. Furthermore, the predicted performance agrees well with the experimental data. Both the FFW and SFW methods out performed the rigid wake and the blade element momentum methods. A parametric study using the prescribed wake analysis indicated that at some expansion rates, corresponding to low wind conditions, the predicted power exceeded that of the rigid wake prediction and could in fact exceeds the Betz limit.

  12. A numerical study of the laminar necklace vortex system and its effect on the wake for a circular cylinder

    NASA Astrophysics Data System (ADS)

    Kirkil, Gokhan; Constantinescu, George

    2014-11-01

    Large Eddy Simulation is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter, D, smaller than 4,460. The study concentrates on the analysis of the structure of the HV system in the periodic breakaway sub-regime which is characterized by the formation of three main necklace vortices. For the relatively shallow flow conditions considered in this study (H/D 1, H is the channel depth), at times, the disturbances induced by the legs of the necklace vortices do not allow the SSLs on the two sides of the cylinder to interact in a way that allows the vorticity redistribution mechanism to lead to the formation of a new wake roller. As a result, the shedding of large scale rollers in the turbulent wake is suppressed for relatively large periods of time. Simulation results show that the wake structure changes randomly between time intervals when large-scale rollers are forming and are convected in the wake (von Karman regime), and time intervals when the rollers do not form.

  13. A Scanning laser-velocimeter technique for measuring two-dimensional wake-vortex velocity distributions. [Langley Vortex Research Facility

    NASA Technical Reports Server (NTRS)

    Gartrell, L. R.; Rhodes, D. B.

    1980-01-01

    A rapid scanning two dimensional laser velocimeter (LV) has been used to measure simultaneously the vortex vertical and axial velocity distributions in the Langley Vortex Research Facility. This system utilized a two dimensional Bragg cell for removing flow direction ambiguity by translating the optical frequency for each velocity component, which was separated by band-pass filters. A rotational scan mechanism provided an incremental rapid scan to compensate for the large displacement of the vortex with time. The data were processed with a digital counter and an on-line minicomputer. Vaporized kerosene (0.5 micron to 5 micron particle sizes) was used for flow visualization and LV scattering centers. The overall measured mean-velocity uncertainity is less than 2 percent. These measurements were obtained from ensemble averaging of individual realizations.

  14. Near wakes of advanced turbopropellers

    NASA Technical Reports Server (NTRS)

    Hanson, D. B.; Patrick, W. P.

    1989-01-01

    The flow in the wake of a model single rotation Prop-Fan rotor operating in a wind tunnel was traversed with a hot-wire anemometer system designed to determine the 3 periodic velocity components. Special data acquisition and data reduction methods were required to deal with the high data frequency, narrow wakes, and large fluctuating air angles in the tip vortex region. The model tip helical Mach number was 1.17, simulating the cruise condition. Although the flow field is complex, flow features such as viscous velocity defects, vortex sheets, tip vortices, and propagating acoustic pulses are clearly identified with the aid of a simple analytical wake theory.

  15. Wind tunnel measurements for dispersion modelling of vehicle wakes

    NASA Astrophysics Data System (ADS)

    Carpentieri, Matteo; Kumar, Prashant; Robins, Alan

    2012-12-01

    Wind tunnel measurements downwind of reduced scale car models have been made to study the wake regions in detail, test the usefulness of existing vehicle wake models, and draw key information needed for dispersion modelling in vehicle wakes. The experiments simulated a car moving in still air. This is achieved by (i) the experimental characterisation of the flow, turbulence and concentration fields in both the near and far wake regions, (ii) the preliminary assessment of existing wake models using the experimental database, and (iii) the comparison of previous field measurements in the wake of a real diesel car with the wind tunnel measurements. The experiments highlighted very large gradients of velocities and concentrations existing, in particular, in the near-wake. Of course, the measured fields are strongly dependent on the geometry of the modelled vehicle and a generalisation for other vehicles may prove to be difficult. The methodology applied in the present study, although improvable, could constitute a first step towards the development of mathematical parameterisations. Experimental results were also compared with the estimates from two wake models. It was found that they can adequately describe the far-wake of a vehicle in terms of velocities, but a better characterisation in terms of turbulence and pollutant dispersion is needed. Parameterised models able to predict velocity and concentrations with fine enough details at the near-wake scale do not exist.

  16. Preliminary comparison of model and prototype wakes. [building wake effects on atmospheric boundary layer

    NASA Technical Reports Server (NTRS)

    Logan, E., Jr.; Camp, D. W.

    1978-01-01

    Velocity and turbulence profiles previously measured in the wake of a long building 3.2 m high, located in the field, transverse to the wind and in an atmospheric boundary layer several hundred meters thick are compared with wake profiles at corresponding longitudinal stations for a scale model of the building located in a large meteorological wind tunnel having a boundary layer thickness of 0.61 m to assess the accuracy of full scale wake profile predictions based on model tests. Results are presented which show that disparities in nondimensional profiles result from differences in relative depth of logarithmic layers and in surface conditions.

  17. Secondary frequencies in the wake of a circular cylinder with vortex shedding

    NASA Technical Reports Server (NTRS)

    Abarbanel, Saul S.; Don, Wai Sun; Gottlieb, David; Rudy, David H.; Townsend, James C.

    1990-01-01

    A detailed numerical study of two-dimensional flow past a circular cylinder at moderately low Reynolds numbers was conducted using three different numerical algorithms for solving the time-dependent compressible Navier-Stokes equations. It was found that if the algorithm and associated boundary conditions were consistent and stable, then the major features of the unsteady wake were well-predicted. However, it was also found that even stable and consistent boundary conditions could introduce additional periodic phenomena reminiscent of the type seen in previous wind-tunnel experiments. However, these additional frequencies were eliminated by formulating the boundary conditions in terms of the characteristic variables. An analysis based on a simplified model provides an explanation for this behavior.

  18. Vortex-induced vibrations of two cylinders in tandem arrangement in the proximity–wake interference region

    PubMed Central

    BORAZJANI, IMAN; SOTIROPOULOS, FOTIS

    2009-01-01

    We investigate numerically vortex-induced vibrations (VIV) of two identical two-dimensional elastically mounted cylinders in tandem in the proximity–wake interference regime at Reynolds number Re = 200 for systems having both one (transverse vibrations) and two (transverse and in-line) degrees of freedom (1-DOF and 2-DOF, respectively). For the 1-DOF system the computed results are in good qualitative agreement with available experiments at higher Reynolds numbers. Similar to these experiments our simulations reveal: (1) larger amplitudes of motion and a wider lock-in region for the tandem arrangement when compared with an isolated cylinder; (2) that at low reduced velocities the vibration amplitude of the front cylinder exceeds that of the rear cylinder; and (3) that above a threshold reduced velocity, large-amplitude VIV are excited for the rear cylinder with amplitudes significantly larger than those of the front cylinder. By analysing the simulated flow patterns we identify the VIV excitation mechanisms that lead to such complex responses and elucidate the near-wake vorticity dynamics and vortex-shedding modes excited in each case. We show that at low reduced velocities vortex shedding provides the initial excitation mechanism, which gives rise to a vertical separation between the two cylinders. When this vertical separation exceeds one cylinder diameter, however, a significant portion of the incoming flow is able to pass through the gap between the two cylinders and the gap-flow mechanism starts to dominate the VIV dynamics. The gap flow is able to periodically force either the top or the bottom shear layer of the front cylinder into the gap region, setting off a series of very complex vortex-to-vortex and vortex-to-cylinder interactions, which induces pressure gradients that result in a large oscillatory force in phase with the vortex shedding and lead to the experimentally observed larger vibration amplitudes. When the vortex shedding is the dominant mechanism the front cylinder vibration amplitude is larger than that of the rear cylinder. The reversing of this trend above a threshold reduced velocity is associated with the onset of the gap flow. The important role of the gap flow is further illustrated via a series of simulations for the 2-DOF system. We show that when the gap-flow mechanism is triggered, the 2-DOF system can develop and sustain large VIV amplitudes comparable to those observed in the corresponding (same reduced velocity) 1-DOF system. For sufficiently high reduced velocities, however, the two cylinders in the 2-DOF system approach each other, thus significantly reducing the size of the gap region. In such cases the gap flow is entirely eliminated, and the two cylinders vibrate together as a single body with vibration amplitudes up to 50% lower than the amplitudes of the corresponding 1-DOF in which the gap flow is active. Three-dimensional simulations are also carried out to examine the adequacy of two-dimensional simulations for describing the dynamic response of the tandem system at Re = 200. It is shown that even though the wake transitions to a weakly three-dimensional state when the gap flow is active, the three-dimensional modes are too weak to affect the dynamic response of the system, which is found to be identical to that obtained from the two-dimensional computations. PMID:19693281

  19. Studies of aircraft wake chemistry and dispersion

    NASA Technical Reports Server (NTRS)

    Poppoff, I. G.; Farlow, N. H.; Anderson, L. B.

    1974-01-01

    Use of aerospace technology to study aircraft wakes is reviewed. It is shown how aerospace vehicles can be used to provide data for increased understanding of the atmosphere and of aircraft exhaust trails where knowledge is inadequate to evaluate fully the potential impact of the engine emissions. Models of aircraft near-field exhaust wakes are characterized by jet, vortex, and dispersion regimes. Wake growth in the jet regime is self-determined and rapid, whereas further spreading is inhibited in the vortex regime because of circulating vortex motion. Wake diffusion in the dispersion regime is initially influenced by aircraft induced turbulence but is dominated later by small-scale atmospheric turbulence. Computed fluid mechanical results show the importance of effects such as wake buoyancy, wind shear, turbulence, and traffic corridor exhaust buildup on dispersion of the wake. In the jet regime the exhaust characteristics and thermochemistry serve to illustrate initial chemical changes involving potential pollutant species.

  20. An experimental study of the unsteady vortex structures in the wake of a root-fixed flapping wing

    NASA Astrophysics Data System (ADS)

    Hu, Hui; Clemons, Lucas; Igarashi, Hirofumi

    2011-08-01

    An experimental study was conducted to characterize the evolution of the unsteady vortex structures in the wake of a root-fixed flapping wing with the wing size, stroke amplitude, and flapping frequency within the range of insect characteristics for the development of novel insect-sized nano-air-vehicles (NAVs). The experiments were conducted in a low-speed wing tunnel with a miniaturized piezoelectric wing (i.e., chord length, C = 12.7 mm) flapping at a frequency of 60 Hz (i.e., f = 60 Hz). The non-dimensional parameters of the flapping wing are chord Reynolds number of Re = 1,200, reduced frequency of k = 3.5, and non-dimensional flapping amplitude at wingtip h = A/C = 1.35. The corresponding Strouhal number (Str) is 0.33 , which is well within the optimal range of 0.2 < Str < 0.4 used by flying insects and birds and swimming fishes for locomotion. A digital particle image velocimetry (PIV) system was used to achieve phased-locked and time-averaged flow field measurements to quantify the transient behavior of the wake vortices in relation to the positions of the flapping wing during the upstroke and down stroke flapping cycles. The characteristics of the wake vortex structures in the chordwise cross planes at different wingspan locations were compared quantitatively to elucidate underlying physics for a better understanding of the unsteady aerodynamics of flapping flight and to explore/optimize design paradigms for the development of novel insect-sized, flapping-wing-based NAVs.

  1. Vertical Axis Wind Turbine flows using a Vortex Particle-Mesh method: from near to very far wakes

    NASA Astrophysics Data System (ADS)

    Backaert, Stephane; Chatelain, Philippe; Winckelmans, Gregoire; Kern, Stefan; Maeder, Thierry; von Terzi, Dominic; van Rees, Wim; Koumoutsakos, Petros

    2012-11-01

    A Vortex Particle-Mesh (VPM) method with immersed lifting lines has been developed and validated. The vorticity-velocity formulation of the NS equations is treated in a hybrid way: particles handle advection while the mesh is used to evaluate the differential operators and for the fast Poisson solvers (here a Fourier-based solver which simultaneously allows for unbounded directions and inlet/outlet boundaries). Both discretizations communicate through high order interpolation. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. LES of Vertical Axis Wind Turbine (VAWT) flows are performed, with a relatively fine resolution (128 and 160 grid points per blade) and for computational domains extending up to 6 D and 14 D downstream of the rotor. The wake complex development is captured in details, from the blades to the near wake coherent vortices, to the transitional ones, to the fully developed turbulent far wake. Mean flow statistics in planes (horizontal, vertical and cross) are also presented. A case with a realistic turbulent wind inflow is also considered. The physics are more complex than for HAWT flows. Computational resources provided by a PRACE award.

  2. Implementation and validation of a wake model for low-speed forward flight

    NASA Technical Reports Server (NTRS)

    Komerath, Narayanan M.; Schreiber, Olivier A.

    1987-01-01

    The computer implementation and calculations of the induced velocities produced by a wake model consisting of a trailing vortex system defined from a prescribed time averaged downwash distribution are detailed. Induced velocities are computed by approximating each spiral turn by a pair of large straight vortex segments positioned at critical points relative to where the induced velocity is required. A remainder term for the rest of the spiral is added. This approach results in decreased computation time compared to classical models where each spiral turn is broken down in small straight vortex segments. The model includes features such a harmonic variation of circulation, downwash outside of the blade and/or outside the tip path plane, blade bound vorticity induced velocity with harmonic variation of circulation and time averaging. The influence of various options and parameters on the results are investigated and results are compared to experimental field measurements with which, a resonable agreement is obtained. The capabilities of the model as well as its extension possibilities are studied. The performance of the model in predicting the recently-acquired NASA Langley Inflow data base for a four-bladed rotor is compared to that of the Scully Free Wake code, a well-established program which requires much greater computational resources. It is found that the two codes predict the experimental data with essentially the same accuracy, and show the same trends.

  3. Two dimensional thick center vortex model

    NASA Astrophysics Data System (ADS)

    Rafibakhsh, Shahnoosh; Ahmadi, Alireza

    2016-01-01

    The potential between static color source is calculated in the SU (3) gauge group by introducing a two dimensional vortex flux. To generalize the model, the length of the Wilson loop is equal to R oriented along the x axis, and the vortex flux is considered as a function of x and y. The comparison between the generalized model and the original one shows that the intermediate linear regime is increased significantly and better agreement with Casimir scaling is achieved. Furthermore, the model is applied to calculate the potential between baryons.

  4. Vortex microscope: analytical model and experiment

    NASA Astrophysics Data System (ADS)

    Masajada, Jan; Popiołek-Masajada, Agnieszka; Szatkowski, Mateusz; Plociniczak, Łukasz

    2015-11-01

    We present the analytical model describing the Gaussian beam propagation through the off axis vortex lens and the set of axially positioned ideal lenses. The model is derived on the base of Fresnel diffraction integral. The model is extended to the case of vortex lens with any topological charge m. We have shown that the Gaussian beam propagation can be represented by function G which depends on four coefficients. When propagating from one lens to another the function holds its form but the coefficient changes.

  5. Time-resolved vortex wake of a common swift flying over a range of flight speeds

    PubMed Central

    Henningsson, P.; Muijres, F. T.; Hedenström, A.

    2011-01-01

    The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s−1. The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of ‘wing root vortices’ are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings. PMID:21131333

  6. Time-resolved vortex wake of a common swift flying over a range of flight speeds.

    PubMed

    Henningsson, P; Muijres, F T; Hedenström, A

    2011-06-01

    The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s(-1). The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of 'wing root vortices' are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings. PMID:21131333

  7. Engineering models for merging wakes in wind farm optimization applications

    NASA Astrophysics Data System (ADS)

    Machefaux, E.; Larsen, G. C.; Murcia Leon, J. P.

    2015-06-01

    The present paper deals with validation of 4 different engineering wake superposition approaches against detailed CFD simulations and covering different turbine interspacing, ambient turbulence intensities and mean wind speeds. The first engineering model is a simple linear superposition of wake deficits as applied in e.g. Fuga. The second approach is the square root of sums of squares approach, which is applied in the widely used PARK program. The third approach, which is presently used with the Dynamic Wake Meandering (DWM) model, assumes that the wake affected downstream flow field to be determined by a superposition of the ambient flow field and the dominating wake among contributions from all upstream turbines at any spatial position and at any time. The last approach developed by G.C. Larsen is a newly developed model based on a parabolic type of approach, which combines wake deficits successively. The study indicates that wake interaction depends strongly on the relative wake deficit magnitude, i.e. the deficit magnitude normalized with respect to the ambient mean wind speed, and that the dominant wake assumption within the DWM framework is the most accurate.

  8. Flight test to determine feasibility of a proposed airborne wake vortex detection concept

    NASA Technical Reports Server (NTRS)

    Branstetter, James R.; Hastings, E. C., Jr.; Patterson, James C., Jr.

    1991-01-01

    This investigation was conducted to determine the radial extent at which aircraft mounted flow vanes or roll rate gyros can sense the circulatory flow field that exists around the lift induced vortex system generated by an aircraft in flight. The probe aircraft was equipped with wingtip sensors for measuring angle of attack and angle of sideslip, and with a fuselage mounted gyroscope for measuring roll rate. Analysis of flight test data indicated that the vortex was detectable at a lateral distance of about 105 feet (best results) using unsophisticated equipment. Measurements were made from the centerline of the probe aircraft to the center of the nearest vortex with the probe aircraft flying between one half and one and one half miles behind the vortex generating aircraft.

  9. Development and application of a method for predicting rotor free wake positions and resulting rotor blade air loads. Volume 1: Model and results

    NASA Technical Reports Server (NTRS)

    Sadler, S. G.

    1971-01-01

    Rotor wake geometries are predicted by a process similar to the startup of a rotor in a free stream. An array of discrete trailing and shed vortices is generated with vortex strengths corresponding to stepwise radial and azimuthal blade circulations. The array of shed and trailing vortices is limited to an arbitrary number of azimuthal steps behind each blade. The remainder of the wake model of each blade is an arbitrary number of trailing vortices. Vortex element end points were allowed to be transported by the resultant velocity of the free stream and vortex-induced velocities. Wake geometry, wake flow, and wake-induced velocity influence coefficients are generated by this program for use in the blade loads portion of the calculations. Blade loads computations include the effects of nonuniform inflow due to a free wake, nonlinear airfoil characteristics, and response of flexible blades to the applied loads. Computed wake flows and blade loads are compared with experimentally measured data. Predicted blade loads, response and shears and moments are obtained for a model rotor system having two independent rotors. The effects of advance ratio, vertical separation of rotors, different blade radius ratios, and different azimuthal spacing of the blades of one rotor with respect to the other are investigated.

  10. The Development of a Plan for the Assessment, Improvement and Deployment of a Radar Acoustic Sounding System (RASS) for Wake Vortex Detection

    NASA Technical Reports Server (NTRS)

    Morris, Philip J.; McLaughlin, Dennis K.; Gabrielson, Thomas B.; Boluriaan, Said

    2004-01-01

    This report describes the activities completed under a grant from the NASA Langley Research Center to develop a plan for the assessment, improvement, and deployment of a Radar Acoustic Sounding System (RASS) for the detection of wake vortices. A brief review is provided of existing alternative instruments for wake vortex detection. This is followed by a review of previous implementations and assessment of a RASS. As a result of this review, it is concluded that the basic features of a RASS have several advantages over other commonly used wake vortex detection and measurement systems. Most important of these features are the good fidelity of the measurements and the potential for all weather operation. To realize the full potential of this remote sensing instrument, a plan for the development of a RASS designed specifically for wake vortex detection and measurement has been prepared. To keep costs to a minimum, this program would start with the development an inexpensive laboratory-scale version of a RASS system. The new instrument would be developed in several stages, each allowing for a critical assessment of the instrument s potential and limitations. The instrument, in its initial stages of development, would be tested in a controlled laboratory environment. A jet vortex simulator, a prototype version of which has already been fabricated, would be interrogated by the RASS system. The details of the laboratory vortex would be measured using a Particle Image Velocimetry (PIV) system. In the early development stages, the scattered radar signal would be digitized and the signal post-processed to determine how extensively and accurately the RASS could measure properties of the wake vortex. If the initial tests prove to be successful, a real-time, digital signal processing system would be developed as a component of the RASS system. At each stage of the instrument development and testing, the implications of the scaling required for a full-scale instrument would be considered. It is concluded that a RASS system, developed for the specific application of wake vortex detection, could become part of a robust Aircraft Vortex Spacing System (AVOSS). This system, in turn, could contribute to Reduced Spacing Operations (RSO) in US airports and improvements in Terminal Area productivity (TAP).

  11. Sadovskii vortex in strain

    NASA Astrophysics Data System (ADS)

    Freilich, Daniel; Llewellyn Smith, Stefan

    2015-11-01

    Sadovskii vortices are patches of fluid with uniform vorticity surrounded by a vortex sheet. They were first constructed as models for wakes behind bluff objects. We investigate the Sadovskii vortex in a straining field and examine limiting cases to validate our computational method. One limit is the patch vortex in strain (Moore & Saffman, Aircraft wake turbulence and its detection 1971), where there is no vortex sheet. We solve this as a free-boundary problem, and show that a simple method using the Biot-Savart law quickly gives solutions for stable shapes. When used for the more elongated (stronger straining field) situations, the method also leads to new vortex shapes. In the hollow vortex case, where there is no vortex patch and the circulation is entirely due to the vortex sheet (Llewellyn Smith and Crowdy, J. Fluid Mech. 691 2012), we use the Birkhoff-Rott equation to calculate the velocity of the fluid on the vortex boundary. The combination of these two methods can then be used to calculate the shape and velocity field of the Sadovksii vortex in strain.

  12. Crosswind Shear Gradient Affect on Wake Vortices

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.; Ahmad, Nashat N.

    2011-01-01

    Parametric simulations with a Large Eddy Simulation (LES) model are used to explore the influence of crosswind shear on aircraft wake vortices. Previous studies based on field measurements, laboratory experiments, as well as LES, have shown that the vertical gradient of crosswind shear, i.e. the second vertical derivative of the environmental crosswind, can influence wake vortex transport. The presence of nonlinear vertical shear of the crosswind velocity can reduce the descent rate, causing a wake vortex pair to tilt and change in its lateral separation. The LES parametric studies confirm that the vertical gradient of crosswind shear does influence vortex trajectories. The parametric results also show that vortex decay from the effects of shear are complex since the crosswind shear, along with the vertical gradient of crosswind shear, can affect whether the lateral separation between wake vortices is increased or decreased. If the separation is decreased, the vortex linking time is decreased, and a more rapid decay of wake vortex circulation occurs. If the separation is increased, the time to link is increased, and at least one of the vortices of the vortex pair may have a longer life time than in the case without shear. In some cases, the wake vortices may never link.

  13. Computation of potential flows with embedded vortex rings and applications to helicopter rotor wakes

    NASA Technical Reports Server (NTRS)

    Roberts, T. W.

    1983-01-01

    A finite difference scheme for solving the motion of a number of vortex rings is developed. The method is an adaptation of the 'cloud-in-cell' technique to axisymmetric flows, and is thus a combined Eulerian-Lagrangian technique. A straightforward adaptation of the cloud-in-cell scheme to an axisymmetric flow field is shown to introduce a grid dependent self-induced velocity to each vortex ring. To correct this behavior the potential is considered to consist of two parts, a local and a global field. An improved difference formula is derived, allowing the accurate calculation of the potential at points near vortex locations. The local potential is then subtracted before calculating the velocity, leaving only the influences of the remaining vortices. The correct self-induced velocity is then explicitly added to the vortex velocity. Calculations of the motion of one and two vortex rings are performed, demonstrating the ability of the new method to eliminate the grid dependence of the self-induced velocity. The application of the method to the calculation of helicopter rotor flows in hover is attempted.

  14. Computation of vortex lock-in in the laminar wake of a circular cylinder using unsteady monopole sources

    SciTech Connect

    Pan, D.; Chin, Y.C.; Chang, C.H.

    1995-06-01

    The vortex lock-in in the laminar wake behind a circular cylinder induced by the unsteady monopole source is numerically simulated in this paper. The artificial compressibility method is employed to solve the incompressible Navier-Stokes equations. A high-order accuracy upwind flux-difference finite-volume scheme is used to discretize the flow field. The unsteady monopole source is simulated by a pulsating volume flux through the cylinder surface at a prescribed forcing frequency and amplitude. The forcing amplitude is set to a fixed value while the frequency is varied to search for the lock-in region. The flow field of the periodic lock-in state is examined in detail. Finally, the effects of a higher amplitude and a different source location are briefly investigated.

  15. Estimation of flow parameters of turbulent fluctuations and vortex motions based on randomly sampled velocity data in the near wake of a circular cylinder in a steady flow

    SciTech Connect

    Kong, D.

    1995-12-31

    Turbulent near wake flow is one of the key subjects for solving fluid dynamics-related problems in industrial practice, e.g. numerical simulation of gas explosions on offshore oil/gas production platforms. Flow velocities measured in the near-wake of bluff bodies contain information of mean flow, turbulent fluctuations and vortex motions. Flow parameters like RMS values of velocity fluctuations, Reynolds shear stresses, and auto-/spatial correlation of velocity fluctuations, derived by direct averaging of measured data, will include the total contributions of the periodic vortex motions and the random turbulent fluctuations. In the case of near-wake flows behind a cylinder at sub-critical Reynolds numbers, a vortex-street is formed in the wake and the vortex-shedding frequency is well defined. This offers a possibility for simplifying the decomposition of regular vortex motions and the random turbulent fluctuations. Velocity profiles and cross-stream spatial correlation of streamwise velocity were measured by positioning a TSI`s two-component LDA system in the region 2d to 4d downstream of a circular cylinder at Re = 35,000. The randomly sampled data were first interpolated linearly, and then re-sampled with a pertinent sampling frequency. The optimal FIR filters, designed using the Remez exchange algorithm, were applied to reject the digital signals in a narrow band around the vortex-shedding frequency. Various flow parameters associated with random turbulence were computed. The integration of the auto- and spatial correlation, obtained based on the filtered data gave a physically adequate estimation of the integral time and length scales of the turbulent fluctuations. Errors due to linear interpolation and filtering were discussed.

  16. Study of the far wake vortex field generated by a rectangular airfoil in a water tank

    NASA Technical Reports Server (NTRS)

    Lezius, D. K.

    1973-01-01

    Underwater towing experiments were carried out with a rectangular airfoil of aspect ratio 5.3 at 4 and 8 deg angles of attack and at chord-based Reynolds numbers between 2 x 100,000 and 7.5 x 100,000. Quantitative measurements by means of the hydrogen bubble technique indicated lower peak swirl velocities in the range of 100 to 1000 lenghts downstream than have been measured in wind tunnel of flight tests. The maximum circumferential velocity decayed whereas the turbulent eddy viscosity increased. This behavior and other known rates of vortex decay are explained in terms of an analytical solution for the vortex problem with time varying eddy viscosity. It is shown that this case corresponds to nonequilibrium turbulent vortex flow.

  17. The effect of single-horn glaze ice on the vortex structures in the wake of a horizontal axis wind turbine

    NASA Astrophysics Data System (ADS)

    Jin, Zhe-Yan; Dong, Qiao-Tian; Yang, Zhi-Gang

    2015-02-01

    The present study experimentally investigated the effect of a simulated single-horn glaze ice accreted on rotor blades on the vortex structures in the wake of a horizontal axis wind turbine by using the stereoscopic particle image velocimetry (Stereo-PIV) technique. During the experiments, four horizontal axis wind turbine models were tested, and both "free-run" and "phase-locked" Stereo-PIV measurements were carried out. Based on the "free-run" measurements, it was found that because of the simulated single-horn glaze ice, the shape, vorticity, and trajectory of tip vortices were changed significantly, and less kinetic energy of the airflow could be harvested by the wind turbine. In addition, the "phase-locked" results indicated that the presence of simulated single-horn glaze ice resulted in a dramatic reduction of the vorticity peak of the tip vortices. Moreover, as the length of the glaze ice increased, both root and tip vortex gaps were found to increase accordingly.

  18. Velocity and rolling-moment measurements in the wake of a swept-wing model in the 40 by 80 foot wind tunnel

    NASA Technical Reports Server (NTRS)

    Rossow, V. J.; Corsiglia, V. R.; Schwind, R. G.; Frick, J. K. D.; Lemmer, O. J.

    1975-01-01

    Measurements were made in the wake of a swept wing model to study the structure of lift generated vortex wakes shed by conventional span loadings and by several span loadings designed to reduce wake velocities. Variations in the span loading on the swept wing generator were obtained by deflecting seven flap segments on each side by amounts determined by vortex lattice theory to approximate the desired span loadings. The resulting wakes were probed with a three component, hot wire probe to measure velocity, and with a wing to measure the rolling moment that would be induced on a following aircraft. The experimental techniques are described herein, and the measured velocity and rolling moments are presented, along with some comparisons with the applicable theories.

  19. A mathematical model of the sleep/wake cycle.

    PubMed

    Rempe, Michael J; Best, Janet; Terman, David

    2010-05-01

    We present a biologically-based mathematical model that accounts for several features of the human sleep/wake cycle. These features include the timing of sleep and wakefulness under normal and sleep-deprived conditions, ultradian rhythms, more frequent switching between sleep and wakefulness due to the loss of orexin and the circadian dependence of several sleep measures. The model demonstrates how these features depend on interactions between a circadian pacemaker and a sleep homeostat and provides a biological basis for the two-process model for sleep regulation. The model is based on previous "flip-flop" conceptual models for sleep/wake and REM/NREM and we explore whether the neuronal components in these flip-flop models, with the inclusion of a sleep-homeostatic process and the circadian pacemaker, are sufficient to account for the features of the sleep/wake cycle listed above. The model is minimal in the sense that, besides the sleep homeostat and constant cortical drives, the model includes only those nuclei described in the flip-flop models. Each of the cell groups is modeled by at most two differential equations for the evolution of the total population activity, and the synaptic connections are consistent with those described in the flip-flop models. A detailed analysis of the model leads to an understanding of the mathematical mechanisms, as well as insights into the biological mechanisms, underlying sleep/wake dynamics. PMID:19557415

  20. Ribbon Surface Pressure and Wake Velocity Data for the Experimental Validation of a Vortex-Based Parachute Inflation Code

    SciTech Connect

    MCBRIDE,DONALD DEAN; CLARK JR,EDWARD LEE; HENFLING,JOHN F.

    2001-06-01

    An experiment to measure surface pressure data on a series of three stainless steel simulated parachute ribbons was conducted. During the first phase of the test, unsteady pressure measurements were made on the windward and leeward sides of the ribbons to determine the statistical properties of the surface pressures. Particle Image Velocimetry (PIV) measurements were simultaneously made to establish the velocity field in the wake of the ribbons and its correlation with the pressure measurements. In the second phase of the test, steady-state pressure measurements were made to establish the pressure distributions. In the third phase, the stainless steel ribbons were replaced with nylon ribbons and PIV measurements were made in the wake. A detailed error analysis indicates that the accuracy of the pressure measurements was very good. However, an anomaly in the flow field caused the wake behind the stainless steel ribbons to establish itself in a stable manner on one side of the model. This same stability was not present for the nylon ribbon model although an average of the wake velocity data indicated an apparent 2{degree} upwash in the wind tunnel flow field. Since flow angularity upstream of the model was not measured, the use of the data for code validation is not recommended without a second experiment to establish that upstream boundary condition.

  1. A vortex entrainment model applied to slender delta wings

    NASA Technical Reports Server (NTRS)

    Coe, P. L., Jr.

    1974-01-01

    A mathematical model of the vortex flow over a slender sharp-edged delta wing is proposed, and is shown to provide good agreement with the experiment. Although the technique requires experimental data in the form of the vortex core locations, it does account for the previously ignored mass entrainment of the vortex core.

  2. Airloads and Wake Geometry Calculations for an Isolated Tiltrotor Model in a Wind Tunnel

    NASA Technical Reports Server (NTRS)

    Johnson, Wayne

    2001-01-01

    Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 0.25-scale V-22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of measured and calculated performance for hover and helicopter mode operation, and airloads for helicopter mode. Calculated induced power, profile power, and wake geometry provide additional information about the aerodynamic behavior. An aerodynamic and wake model and calculation procedure that reflects the unique geometry and phenomena of tiltrotors has been developed. There are major differences between this model and the corresponding aerodynamic and wake model that has been established for helicopter rotors. In general, good correlation between measured and calculated performance and airloads behavior has been shown. Two aspects of the analysis that clearly need improvement are the stall delay model and the trailed vortex formation model.

  3. Helicopter rotor wake geometry and its influence in forward flight. Volume 1: Generalized wake geometry and wake effect on rotor airloads and performance

    NASA Technical Reports Server (NTRS)

    Egolf, T. A.; Landgrebe, A. J.

    1983-01-01

    An analytic investigation to generalize wake geometry of a helicopter rotor in steady level forward flight and to demonstrate the influence of wake deformation in the prediction of rotor airloads and performance is described. Volume 1 presents a first level generalized wake model based on theoretically predicted tip vortex geometries for a selected representative blade design. The tip vortex distortions are generalized in equation form as displacements from the classical undistorted tip vortex geometry in terms of vortex age, blade azimuth, rotor advance ratio, thrust coefficient, and number of blades. These equations were programmed to provide distorted wake coordinates at very low cost for use in rotor airflow and airloads prediction analyses. The sensitivity of predicted rotor airloads, performance, and blade bending moments to the modeling of the tip vortex distortion are demonstrated for low to moderately high advance ratios for a representative rotor and the H-34 rotor. Comparisons with H-34 rotor test data demonstrate the effects of the classical, predicted distorted, and the newly developed generalized wake models on airloads and blade bending moments. Use of distorted wake models results in the occurrence of numerous blade-vortex interactions on the forward and lateral sides of the rotor disk. The significance of these interactions is related to the number and degree of proximity to the blades of the tip vortices. The correlation obtained with the distorted wake models (generalized and predicted) is encouraging.

  4. Rotorcraft acoustic radiation prediction based on a refined blade-vortex interaction model

    NASA Astrophysics Data System (ADS)

    Rule, John Allen

    1997-08-01

    The analysis of rotorcraft aerodynamics and acoustics is a challenging problem, primarily due to the fact that a rotorcraft continually flies through its own wake. The generation mechanism for a rotorcraft wake, which is dominated by strong, concentrated blade-tip trailing vortices, is similar to that in fixed wing aerodynamics. However, following blades encounter shed vortices from previous blades before they are swept downstream, resulting in sharp, impulsive loading on the blades. The blade/wake encounter, known as Blade-Vortex Interaction, or BVI, is responsible for a significant amount of vibratory loading and the characteristic rotorcraft acoustic signature in certain flight regimes. The present work addressed three different aspects of this interaction at a fundamental level. First, an analytical model for the prediction of trailing vortex structure is discussed. The model as presented is the culmination of a lengthy research effort to isolate the key physical mechanisms which govern vortex sheet rollup. Based on the Betz model, properties of the flow such as mass flux, axial momentum flux, and axial flux of angular momentum are conserved on either a differential or integral basis during the rollup process. The formation of a viscous central core was facilitated by the assumption of a turbulent mixing process with final vortex velocity profiles chosen to be consistent with a rotational flow mixing model and experimental observation. A general derivation of the method is outlined, followed by a comparison of model predictions with experimental vortex measurements, and finally a viscous blade drag model to account for additional effects of aerodynamic drag on vortex structure. The second phase of this program involved the development of a new formulation of lifting surface theory with the ultimate goal of an accurate, reduced order hybrid analytical/numerical model for fast rotorcraft load calculations. Currently, accurate rotorcraft airload analyses are limited by the massive computational power required to capture the small time scale events associated with BVI. This problem has two primary facets: accurate knowledge of the wake geometry, and accurate resolution of the impulsive loading imposed by a tip vortex on a blade. The present work addressed the second facet, providing a mathematical framework for solving the impulsive loading problem analytically, then asymptotically matching this solution to a low-resolution numerical calculation. A method was developed which uses continuous sheets of integrated boundary elements to model the lifting surface and wake. Special elements were developed to capture local behavior in high-gradient regions of the flow, thereby reducing the burden placed on the surrounding numerical method. Unsteady calculations for several classical cases were made in both frequency and time domain to demonstrate the performance of the method. Finally, a new unsteady, compressible boundary element method was applied to the problem of BVI acoustic radiation prediction. This numerical method, combined with the viscous core trailing vortex model, was used to duplicate the geometry and flight configuration of a detailed experimental BVI study carried out at NASA Ames Research Center. Blade surface pressure and near- and far-field acoustic radiation calculations were made. All calculations were shown to compare favorably with experimentally measured values. The linear boundary element method with non-linear corrections proved sufficient over most of the rotor azimuth, and particular in the region of the blade vortex interaction, suggesting that full non-linear CFD schemes are not necessary for rotorcraft noise prediction.

  5. Propeller tip vortex interactions

    NASA Technical Reports Server (NTRS)

    Johnston, Robert T.; Sullivan, John P.

    1990-01-01

    Propeller wakes interacting with aircraft aerodynamic surfaces are a source of noise and vibration. For this reason, flow visualization work on the motion of the helical tip vortex over a wing and through the second stage of a counterrotation propeller (CRP) has been pursued. Initially, work was done on the motion of a propeller helix as it passes over the center of a 9.0 aspect ratio wing. The propeller tip vortex experiences significant spanwise displacements when passing across a lifting wing. A stationary propeller blade or stator was installed behind the rotating propeller to model the blade vortex interaction in a CRP. The resulting vortex interaction was found to depend on the relative vortex strengths and vortex sign.

  6. Preliminary flight test investigation of an airborne wake vortex detection concept

    NASA Technical Reports Server (NTRS)

    Verstynen, Harry A.; Patterson, James C., Jr.

    1990-01-01

    NASA and the FAA have conducted a brief flight-test investigation to furnish preliminary data on the feasibility of transport aircraft wake-vortices' detection, with a view to improving airport capacity by reducing the longitudinal in-trail spacing between aircraft on landing runs. Attention was given to the possibility that the detection of strong vortices at a sufficiently early stage might furnish reasonable warning of impending aerodynamic effects. Preliminary results indicate that while maximum detection distances obtainable with wingtip-mounted flow-angularity vanes are slightly lower than had been predicted, improved detector algorithms may render them more sensitive.

  7. Modelling wind turbine wakes using the turbulent entrainment hypothesis

    NASA Astrophysics Data System (ADS)

    Luzzatto-Fegiz, Paolo

    2015-11-01

    Simple models for turbine wakes have been used extensively in the wind energy community, both as independent tools, as well as to complement more refined and computationally-intensive techniques. Jensen (1983; see also Katić et al. 1986) developed a model assuming that the wake radius grows linearly with distance x, approximating the velocity deficit with a top-hat profile. While this model has been widely implemented in the wind energy community, recently Bastankhah & Porté-Agel (2014) showed that it does not conserve momentum. They proposed a momentum-conserving theory, which assumed a Gaussian velocity deficit and retained the linear-spreading assumption, significantly improving agreement with experiments and LES. While the linear spreading assumption facilitates conceptual modeling, it requires empirical estimates of the spreading rate, and does not readily enable generalizations to other turbine designs. Furthermore, field measurements show sub-linear wake growth with x in the far-wake, consistently with results from fundamental turbulence studies. We develop a model by relying on a simple and general turbulence parameterization, namely the entrainment hypothesis, which has been used extensively in other areas of geophysical fluid dynamics. Without assuming similarity, we derive an analytical solution for a circular turbine wake, which predicts a far-wake radius increasing with x 1 / 3, and is consistent with field measurements and fundamental turbulence studies. Finally, we discuss developments accounting for effects of stratification, as well as generalizations to other turbine designs.

  8. Knuckleball and Flying Disk: Boundary Layer Transitions, Separations and Vortex Wakes in Sports Aerodynamics

    NASA Astrophysics Data System (ADS)

    Higuchi, Hiroshi; Kiura, Toshiro; Goto, Yuichiro; Hiramoto, Riho

    2001-11-01

    In spite of their popularity, flow structures over common baseball and flying disks have not been studied in detail. A slowly rotating baseball is subject to erratic flight paths, and is known as a knuckleball. In the present experiment, the characteristic of force acting on a baseball was obtained and the velocity vector field near the surface of the ball and the wake were measured with the DPIV technique. The seam triggered the boundary layer transition or caused the boundary layer separation itself. The laminar/turbulent boundary layer separations were identified with specific ball orientations. Corresponding three-dimensional wake pattern and the side force result in unpredictable trajectories. In the second part of the talk, flow physics regarding a spin-stabilized flying disk is addressed. The roll-up of trailing vortices was visualized in detail and their vorticity field was measured with the DPIV. The vortical flow over the disk produced flow reattachment at a very high angle of attack. The boundary layer at low angles of attack was affected by the surface motion with asymmetric boundary layer transitions as evidenced by the flow visualization and the hot wire survey. The flow separation and attachment on the underside cavity were also affected by the rotation.

  9. Improvement of a near wake model for trailing vorticity

    NASA Astrophysics Data System (ADS)

    Pirrung, G. R.; Hansen, M. H.; Madsen, H. A.

    2014-12-01

    A near wake model, originally proposed by Beddoes, is further developed. The purpose of the model is to account for the radially dependent time constants of the fast aerodynamic response and to provide a tip loss correction. It is based on lifting line theory and models the downwash due to roughly the first 90 degrees of rotation. This restriction of the model to the near wake allows for using a computationally efficient indicial function algorithm. The aim of this study is to improve the accuracy of the downwash close to the root and tip of the blade and to decrease the sensitivity of the model to temporal discretization, both regarding numerical stability and quality of the results. The modified near wake model is coupled to an aerodynamics model, which consists of a blade element momentum model with dynamic inflow for the far wake and a 2D shed vorticity model that simulates the unsteady buildup of both lift and circulation in the attached flow region. The near wake model is validated against the test case of a finite wing with constant elliptical bound circulation. An unsteady simulation of the NREL 5 MW rotor shows the functionality of the coupled model.

  10. Flight Data Reduction of Wake Velocity Measurements Using an Instrumented OV-10 Airplane

    NASA Technical Reports Server (NTRS)

    Vicroy, Dan D.; Stuever, Robert A.; Stewart, Eric C.; Rivers, Robert A.

    1999-01-01

    A series of flight tests to measure the wake of a Lockheed C- 130 airplane and the accompanying atmospheric state have been conducted. A specially instrumented North American Rockwell OV-10 airplane was used to measure the wake and atmospheric conditions. An integrated database has been compiled for wake characterization and validation of wake vortex computational models. This paper describes the wake- measurement flight-data reduction process.

  11. Typhoon effect on Kuroshio and Green Island wake: a modelling study

    NASA Astrophysics Data System (ADS)

    Hsu, T.-W.; Chou, M.-H.; Hou, T.-H.; Liang, S.-J.

    2015-12-01

    Green Island located in the typhoon active eastern Taiwan coastal water is the potential Kuroshio power plant site. In this study, a high resolution (250-2250 m) shallow-water equations (SWEs) model is used to investigate the effect of typhoon on the hydrodynamics of Kuroshio and Green Island wake. Two wind induced flows, typhoon Soulik and Holland's wind field model, are studied. Simulation results of the typhoon Soulik indicate that salient characteristics of Kuroshio and downstream island wake seems less affected by the typhoon Soulik because typhoon Soulik is 250 km away Green Island and the wind speed near Green Island is small. Moreover, Kuroshio currents increase when flow is in the same direction as the counterclockwise rotation of typhoon, and vice versa. This finding is in favorable agreements with the TOROS observed data. The SWEs model, forced by the Kuroshio and Holland's wind field model, successfully reproduces the downstream recirculation and meandering vortex street. Numerical results unveil that the slow moving typhoon has a more significant impact on the Kuroshio and downstream Green Island wake than the fast moving typhoon does. Due to the counterclockwise rotation of typhoon, Kuroshio currents increase (decrease) in the right (left) of the moving typhoon's track. This rightward bias phenomenon is evident, especially when typhoon moves in the same direction as the Kuroshio mainstream.

  12. Near wake models for the hydrodynamics forces on a cylinder in periodic flow

    SciTech Connect

    Sibetheros, I.A.; Medeiros, E.F.; Miksad, R.W.

    1995-12-31

    The relationship between hydrodynamic forces and the reversing near wake flow field around a cylinder in oscillatory farfield flow was experimentally investigated. A computer controlled pressurized water tunnel was employed to generate a planar harmonic flow past a stationary cylinder of Keulegan-Carpenter numbers (KC) in the inertia-drag flow regime and at subcritical Reynolds numbers (Re). Streamwise (u-) and cross-stream (v-) velocities near the cylinder and u-velocities at the far flow field were measured using a three-component LDV system. Inline and lift forces on the cylinder were measured using a very sensitive six-component load cell. A modal phase averaging technique was applied to the velocity data to estimate averaged velocity cycles that distinguish between positive and negative flow modes. A positive (negative) flow mode signifies dominant vortex shedding and motion occurring from the top (bottom) of the cylinder, respectively. The force measurements were likewise phase averaged into positive and negative mode force cycles. u- and v-velocity profiles for KC = 10, 15, and 20, based on modal phase averages of velocity measurements, were used to estimate averaged wake u- and v-velocities. These wake velocities were used in the place of the undisturbed farfield u-velocity as inputs to Morison`s equation for the inline force, and to the quasi-steady model for the lift force. It was found that, for KC = 15 a significant improvement in the prediction of lift force was achieved using the wake-based lift force model, whereas a smaller improvement in the inline force prediction was obtained from the wake-based Morison`s equation.

  13. Comparing offshore wind farm wake observed from satellite SAR and wake model results

    NASA Astrophysics Data System (ADS)

    Bay Hasager, Charlotte

    2014-05-01

    Offshore winds can be observed from satellite synthetic aperture radar (SAR). In the FP7 EERA DTOC project, the European Energy Research Alliance project on Design Tools for Offshore Wind Farm Clusters, there is focus on mid- to far-field wind farm wakes. The more wind farms are constructed nearby other wind farms, the more is the potential loss in annual energy production in all neighboring wind farms due to wind farm cluster effects. It is of course dependent upon the prevailing wind directions and wind speed levels, the distance between the wind farms, the wind turbine sizes and spacing. Some knowledge is available within wind farm arrays and in the near-field from various investigations. There are 58 offshore wind farms in the Northern European seas grid connected and in operation. Several of those are spaced near each other. There are several twin wind farms in operation including Nysted-1 and Rødsand-2 in the Baltic Sea, and Horns Rev 1 and Horns Rev 2, Egmond aan Zee and Prinses Amalia, and Thompton 1 and Thompton 2 all in the North Sea. There are ambitious plans of constructing numerous wind farms - great clusters of offshore wind farms. Current investigation of offshore wind farms includes mapping from high-resolution satellite SAR of several of the offshore wind farms in operation in the North Sea. Around 20 images with wind farm wake cases have been retrieved and processed. The data are from the Canadian RADARSAT-1/-2 satellites. These observe in microwave C-band and have been used for ocean surface wind retrieval during several years. The satellite wind maps are valid at 10 m above sea level. The wakes are identified in the raw images as darker areas downwind of the wind farms. In the SAR-based wind maps the wake deficit is found as areas of lower winds downwind of the wind farms compared to parallel undisturbed flow in the flow direction. The wind direction is clearly visible from lee effects and wind streaks in the images. The wind farm wake cases are modeled by various types of wake models. In the EERA DTOC project the model suite consists of engineering models (Ainslie, DWM, GLC, PARK, WASP/NOJ), simplified CFD models (FUGA, FarmFlow), full CFD models (CRES-flowNS, RANS), mesoscale model (SKIRON, WRF) and coupled meso-scale and microscale models. The comparison analysis between the satellite wind wake and model results will be presented and discussed. It is first time a comprehensive analysis is performed on this subject. The topic gains increasing importance because there is a growing need to precisely model also mid- and far-field wind farms wakes for development and planning of offshore wind farm clusters.

  14. The dynamics of vortex streets in channels

    NASA Astrophysics Data System (ADS)

    Wang, Xiaolin; Alben, Silas

    2015-07-01

    We develop a model to numerically study the dynamics of vortex streets in channel flows. Previous work has studied the vortex wakes of specific vortex generators. Here, we study a wide range of vortex wakes including regular and reverse von Kármán streets with various strengths, geometries, and Reynolds numbers (Re) by applying a smoothed von Kármán street as an inflow condition. We find that the spatial structure of the inflow vortex street is maintained for the reverse von Kármán street and altered for the regular street. For regular streets, we identify a transition to asymmetric dynamics which happens when Re increases, or vortices are stronger, or vortex streets are compressed horizontally or extended vertically. We also determine the effects of these parameters on vortex street inversion.

  15. Vortex filament model and multifractal conjecture.

    PubMed

    Zybin, K P; Sirota, V A

    2012-05-01

    We develop a theory of turbulence based on the inviscid Navier-Stokes equation. We get a simple but exact stochastic solution (vortex filament model) which allows us to obtain a power law for velocity structure functions in the inertial range. Combining the model with the multifractal conjecture, we calculate the scaling exponents without using the extended self-similarity approach. The results obtained are shown to be in very good agreement with numerical simulations and experimental data. The role of more general stochastic solutions of the Navier-Stokes equation is discussed. PMID:23004872

  16. The effect of asymmetric vortex wake characteristics on a slender delta wing undergoing wing rock motion

    NASA Technical Reports Server (NTRS)

    Arena, A. S., Jr.; Nelson, R. C.

    1989-01-01

    An experimental investigation into the fluid mechanisms responsible for wing rock on a slender delta wing with 80 deg leading edge sweep has been conducted. Time history and flow visualization data are presented for a wide angle-of-attack range. The use of an air bearing spindle has allowed the motion of the wing to be free from bearing friction or mechanical hysteresis. A bistable static condition has been found in vortex breakdown at an angle of attack of 40 deg which causes an overshoot of the steady state rocking amplitude. Flow visualization experiments also reveal a difference in static and dynamic breakdown locations on the wing. A hysteresis loop in dynamic breakdown location similar to that seen on pitching delta wings was observed as the wing was undergoing the limit cycle oscillation.

  17. Wind-tunnel measurements in the wakes of structures

    NASA Technical Reports Server (NTRS)

    Woo, H. G. C.; Peterka, J. A.; Cermak, J. E.

    1977-01-01

    Detailed measurements of longitudinal mean velocity, turbulence intensity, space correlations, and spectra made in the wake of two rectangular scaled models in simulated atmospheric boundary-layer winds are presented. The model buildings were 1:50 scale models of two trailers. Results of a flow visualization study of the wake geometry are analyzed with some singular point theorems. Two hypothetical flow patterns of the detailed wake geometry are proposed. Some preliminary studies of the vortex wake, effects of the model size, model aspect ratios, and boundary layer characteristics on the decay rate and extent of the wake are also presented and discussed.

  18. Model-based control of vortex shedding at low Reynolds numbers

    NASA Astrophysics Data System (ADS)

    Illingworth, Simon J.

    2016-03-01

    Model-based feedback control of vortex shedding at low Reynolds numbers is considered. The feedback signal is provided by velocity measurements in the wake, and actuation is achieved using blowing and suction on the cylinder's surface. Using two-dimensional direct numerical simulations and reduced-order modelling techniques, linear models of the wake are formed at Reynolds numbers between 45 and 110. These models are used to design feedback controllers using H_∞ loop-shaping. Complete suppression of shedding is demonstrated up to Re = 110—both for a single-sensor arrangement and for a three-sensor arrangement. The robustness of the feedback controllers is also investigated by applying them over a range of off-design Reynolds numbers, and good robustness properties are seen. It is also observed that it becomes increasingly difficult to achieve acceptable control performance—measured in a suitable way—as Reynolds number increases.

  19. A comprehensive comparison of turbulence models in the far wake

    NASA Technical Reports Server (NTRS)

    Cimbala, John M.

    1993-01-01

    In the present study, the far wake was examined numerically using an implicit, upwind, finite-volume, compressible Navier-Stokes code. The numerical grid started at 500 equivalent circular cylinder diameters in the wave, and extended to 4000 equivalent diameters. By concentrating only on the far wake, the numerical difficulties and fine mesh requirements near the wake-generating body were eliminated. At the time of this writing, results for the K-epsilon and K-omega turbulence models at low Mach number have been completed and show excellent agreement with previous incompressible results and far-wake similarity solutions. The code is presently being used to compare the performance of various other turbulence models, including Reynolds stress models and the new anisotropic two-equation turbulence models being developed at NASA Langley. By increasing our physical understanding of the deficiencies and limits of these models, it is hoped that improvements to the universality of the models can be made. Future plans include examination of two-dimensional momentumless wakes as well.

  20. A study of the rotor wake of a small-scale rotor model in forward flight using laser light sheet flow visualization with comparisons to analytical models

    NASA Technical Reports Server (NTRS)

    Ghee, Terence A.; Elliott, Joe W.

    1992-01-01

    An experimental investigation was conducted in the 14 by 22 ft subsonic tunnel at NASA Langley Research Center to quantify the rotor wake behind a scale model helicopter rotor in forward flight (mu = 0.15 and 0.23) at one thrust level (C sub T = 0.0064). The rotor system used in the present test consisted of a four-bladed, fully articulated hub and utilized blades of rectangular planform with a NACA-0012 airfoil section. A laser light sheet, seeded with propylene glycol smoke, was used to visualize the flow in planes parallel and perpendicular to the freestream flow. Quantitative measurements of vortex location, vertical skew angle, and vortex particle void radius were obtained for vortices in the flow; convective velocities were obtained for blade tip vortices. Comparisons were made between the experimental results and the wake geometry generated by computational predictions. The results of these comparisons show that the interaction between wake vortex structures is an important consideration for correctly predicting the wake geometry.

  1. CFD modeling of wind turbine wake in wind farms

    NASA Astrophysics Data System (ADS)

    Sun, Lijian

    Wind energy is one of the most common and preferred renewable energy sources. Accurate predictions of atmospheric boundary layer flow, wind turbine induced wakes and their interaction are essential to maximize wind power output and efficiently harness wind energy. In this dissertation, a computational fluid dynamics (CFD) flow model is developed utilizing a three dimensional weighted essentially non-oscillatory (WENO) high order Finite Volume Model system including Large Eddy Simulation (LES) and the Actuator Line Method (ALM). The developed model system is thus able to accurately capture and simulate wind turbine wakes and their interaction with the atmospheric boundary layer, thereby providing insight into the phenomenon of turbine wake interaction and its effect on the external aerodynamic loads on wind turbines. This enables the wind energy production to be maximized and also minimizes turbine fatigue loading in the evaluation of wind farm layouts. By using LES model to simulate the Atmospheric Boundary Layer flow rather than the Reynolds-Averaged Navier-Stokes (RANS) model, the error introduced by turbulence modeling is reduced. The Actuator Line Model, ALM, is used to model the rotor by replacing the rotor with radially distributed body forces. It is more accurate than the actuator disc method as it captures the influence of the blade tip vortices. It can focus on a larger portion of the wake without resolving the actual wind turbine blades' geometry, thereby reducing computational cost. It is suitable and a promising method for wind turbine wake simulation. Classic non-trivial turbulent benchmark cases are used to validate the high order LES algorithms. Simulation results are compared with available results whenever possible, with good agreement observed. Results for the atmospheric boundary layer under neutral conditions are presented. By using LES coupled with the Actuator Line model, simulation results are obtained for detailed wake flow features around single wind turbine as well as wind turbine arrays.

  2. Perturbation response of model vortex rings and dipoles

    NASA Astrophysics Data System (ADS)

    O'Farrell, Clara; Dabiri, John O.

    2012-11-01

    Jetting swimmers, such as squid or jellyfish, propel themselves by forming axisymmetric vortex rings. It is known that vortex rings cannot grow indefinitely, but rather ``pinch off'' once they reach their physical limit, and that a decrease in efficiency of fluid transport is associated with pinch-off. In contrast, two-dimensional vortex dipoles have been found to grow well beyond the physical limit observed in axisymmetric vortex rings. Previously, the Norbury and Pierrehumbert families of vortices have been used as models for axisymmetric vortex rings and two-dimensional dipoles respectively, and the response of these two families to shape perturbations has been characterized. In this study, we improve upon the Norbury and Pierrehumbert models, using nested contours to obtain more realistic models for experimentally-generated vortex rings and dipoles. The resulting vortices are subjected to shape perturbations akin to those previously introduced to members of the Norbury and Pierrehumbert families, and their response is characterized.

  3. Review of Wind Turbine Wake Models and Future Directions (Presentation)

    SciTech Connect

    Churchfield, M. J.

    2013-08-01

    This presentation gives a brief overview to wind turbine wake modeling, ranging from models used in the 1980s up to the present. The presentation shows the strengths and weaknesses of various models and discusses the needs of the wind energy industry and research sectors. Both power production and loads analysis are discussed.

  4. Discrete-vortex model for the symmetric-vortex flow on cones

    NASA Technical Reports Server (NTRS)

    Gainer, Thomas G.

    1990-01-01

    A relatively simple but accurate potential flow model was developed for studying the symmetric vortex flow on cones. The model is a modified version of the model first developed by Bryson, in which discrete vortices and straight-line feeding sheets were used to represent the flow field. It differs, however, in the zero-force condition used to position the vortices and determine their circulation strengths. The Bryson model imposed the condition that the net force on the feeding sheets and discrete vortices must be zero. The proposed model satisfies this zero-force condition by having the vortices move as free vortices, at a velocity equal to at the local crossflow velocity at their centers. When the free-vortex assumption is made, a solution is obtained in the form of two nonlinear algebraic equations that relate the vortex center coordinates and vortex strengths to the cone angle and angle of attack. The vortex center locations calculated using the model are in good agreement with experimental values. The cone normal forces as well as center locations are in good agreement with the vortex cloud method of calculating symmetric flow fields.

  5. The impact of wake models on wind farm layout optimization

    NASA Astrophysics Data System (ADS)

    Schmidt, Jonas; Stoevesandt, Bernhard

    2015-06-01

    Results for nine gradient-based layout optimization runs of a wind farm with 25 turbines in flat terrain are presented, varying three different choices of the underlying wake model and three inflow scenarios. In all cases the AEP is maximised and the constraints are purely geometrical. A single inflow vector, a uniform wind rose and a realistic synthetic wind rose are studied, and the final layouts for the Jensen, the Ainslie and a CFD-based numerical wake model are compared. Prom this an estimate of the average variation of the turbine position due to the different wake models is obtained. All calculations were carried out with the in-house software flapFOAM.

  6. Effects of incoming wind condition and wind turbine aerodynamics on the hub vortex instability

    NASA Astrophysics Data System (ADS)

    Ashton, R.; Viola, F.; Gallaire, F.; Iungo, G. V.

    2015-06-01

    Dynamics and instabilities occurring in the near-wake of wind turbines have a crucial role for the wake downstream evolution, and for the onset of far-wake instabilities. Furthermore, wake dynamics significantly affect the intra-wind farm wake flow, wake interactions and potential power losses. Therefore, the physical understanding and predictability of wind turbine wake instabilities become a nodal point for prediction of wind power harvesting and optimization of wind farm layout. This study is focused on the prediction of the hub vortex instability encountered within wind turbine wakes under different operational conditions of the wind turbine. Linear stability analysis of the wake flow is performed by means of a novel approach that enables to take effects of turbulence on wake instabilities into account. Stability analysis is performed by using as base flow the time-averaged wake velocity field at a specific downstream location. The latter is modeled through Carton-McWilliams velocity profiles by mimicking the presence of the hub vortex and helicoidal tip vortices, and matching the wind turbine thrust coefficient predicted through the actuator disc model. The results show that hub vortex instability is promoted by increasing the turbine thrust coefficient. Indeed, a larger aerodynamic load produces an enhanced wake velocity deficit and axial shear, which are considered the main sources for the wake instability. Nonetheless, wake swirl also promotes hub vortex instability, and it can also affect the azimuthal wavenumber of the most unstable mode.

  7. Source Term Model for an Array of Vortex Generator Vanes

    NASA Technical Reports Server (NTRS)

    Buning, P. G. (Technical Monitor); Waithe, Kenrick A.

    2003-01-01

    A source term model was developed for numerical simulations of an array of vortex generators. The source term models the side force created by a vortex generator being modeled. The model is obtained by introducing a side force to the momentum and energy equations that can adjust its strength automatically based on a local flow. The model was tested and calibrated by comparing data from numerical simulations and experiments of a single low-profile vortex generator vane, which is only a fraction of the boundary layer thickness, over a flat plate. The source term model allowed a grid reduction of about seventy percent when compared with the numerical simulations performed on a fully gridded vortex generator without adversely affecting the development and capture of the vortex created. The source term model was able to predict the shape and size of the stream wise vorticity and velocity contours very well when compared with both numerical simulations and experimental data.

  8. Theoretical modelling of wakes from retractable flapping wings in forward flight.

    PubMed

    Parslew, Ben; Crowther, William J

    2013-01-01

    A free-wake method is used to simulate the wake from retractable, jointed wings. The method serves to complement existing experimental studies that visualise flying animal wakes. Simulated wakes are shown to be numerically convergent for a case study of the Rock Pigeon in minimum power cruising flight. The free-wake model is robust in simulating wakes for a range of wing geometries and dynamics without requiring changes to the numerical method. The method is found to be useful for providing low order predictions of wake geometries. However, it is not well suited to reconstructing 3d flowfields as solutions are sensitive to the numerical mesh node locations. PMID:23882442

  9. Theoretical modelling of wakes from retractable flapping wings in forward flight

    PubMed Central

    Crowther, William J.

    2013-01-01

    A free-wake method is used to simulate the wake from retractable, jointed wings. The method serves to complement existing experimental studies that visualise flying animal wakes. Simulated wakes are shown to be numerically convergent for a case study of the Rock Pigeon in minimum power cruising flight. The free-wake model is robust in simulating wakes for a range of wing geometries and dynamics without requiring changes to the numerical method. The method is found to be useful for providing low order predictions of wake geometries. However, it is not well suited to reconstructing 3d flowfields as solutions are sensitive to the numerical mesh node locations. PMID:23882442

  10. High resolution numerical modeling of mesoscale island wakes and sensitivity to static topographic relief data

    NASA Astrophysics Data System (ADS)

    Nunalee, C. G.; Horváth, Á.; Basu, S.

    2015-03-01

    Recent decades have witnessed a drastic increase in the fidelity of numerical weather prediction (NWP) modeling. Currently, both research-grade and operational NWP models regularly perform simulations with horizontal grid spacings as fine as 1 km. This migration towards higher resolution potentially improves NWP model solutions by increasing the resolvability of mesoscale processes and reducing dependency on empirical physics parameterizations. However, at the same time, the accuracy of high-resolution simulations, particularly in the atmospheric boundary layer (ABL), are also sensitive to orographic forcing which can have significant variability on the same spatial scale as, or smaller than, NWP model grids. Despite this sensitivity, many high resolution atmospheric simulations do not consider uncertainty with respect to selection of static terrain height dataset. In this paper, we use the Weather Research and Forecasting (WRF) model to simulate realistic cases of lower tropospheric flow over and downstream of mountainous islands using both the default global 30 s United States Geographic Survey terrain height dataset (GTOPO30) and the 3 s Shuttle Radar Topography Mission (SRTM) terrain height dataset. Our results demonstrate cases where the differences between GTOPO30-based and SRTM-based model terrain height are significant enough to produce entirely different orographic wake mechanics, such as vortex shedding vs. no vortex shedding. These results are also compared to MODIS visible satellite imagery and highlight the importance of considering uncertain static boundary conditions when running high-resolution mesoscale models.

  11. A Neuron-Based Model of Sleep-Wake Cycles

    NASA Astrophysics Data System (ADS)

    Postnova, Svetlana; Peters, Achim; Braun, Hans

    2008-03-01

    In recent years it was discovered that a neuropeptide orexin/hypocretin plays a main role in sleep processes. This peptide is produced by the neurons in the lateral hypothalamus, which project to almost all brain areas. We present a computational model of sleep-wake cycles, which is based on the Hodgkin-Huxley type neurons and considers reciprocal glutaminergic projections between the lateral hypothalamus and the prefrontal cortex. Orexin is released as a neuromodulator and is required to keep the neurons firing, which corresponds to the wake state. When orexin is depleted the neurons are getting silent as observed in the sleep state. They can be reactivated by the circadian signal from the suprachiasmatic nucleus and/or external stimuli (alarm clock). Orexin projections to the thalamocortical neurons also can account for their transition from tonic firing activity during wakefulness to synchronized burst discharges during sleep.

  12. Wake model for helicopter rotors in high speed flight

    NASA Technical Reports Server (NTRS)

    Johnson, Wayne R.

    1988-01-01

    Two alternative approaches are developed to calculate blade-vortex interaction airloads on helicopter rotors: second order lifting-line theory, and a lifting surface theory correction. The common approach of using a larger vortex core radius to account for lifting-surface effects is quantified. The second order lifting-line theory also improves the modeling of yawed flow and swept tips. Calculated results are compared with wind tunnel measurements of lateral flapping, and with flight test measurements of blade section lift on SA349/2 and H-34 helicopter rotors. The tip vortex core radius required for good correlation with the flight test data is about 20 percent chord, which is within the range of measured viscous core sizes for helicopter rotors.

  13. Modeling Vortex Generators in a Navier-Stokes Code

    NASA Technical Reports Server (NTRS)

    Dudek, Julianne C.

    2011-01-01

    A source-term model that simulates the effects of vortex generators was implemented into the Wind-US Navier-Stokes code. The source term added to the Navier-Stokes equations simulates the lift force that would result from a vane-type vortex generator in the flowfield. The implementation is user-friendly, requiring the user to specify only three quantities for each desired vortex generator: the range of grid points over which the force is to be applied and the planform area and angle of incidence of the physical vane. The model behavior was evaluated for subsonic flow in a rectangular duct with a single vane vortex generator, subsonic flow in an S-duct with 22 corotating vortex generators, and supersonic flow in a rectangular duct with a counter-rotating vortex-generator pair. The model was also used to successfully simulate microramps in supersonic flow by treating each microramp as a pair of vanes with opposite angles of incidence. The validation results indicate that the source-term vortex-generator model provides a useful tool for screening vortex-generator configurations and gives comparable results to solutions computed using gridded vanes.

  14. The modelling of symmetric airfoil vortex generators

    NASA Technical Reports Server (NTRS)

    Reichert, B. A.; Wendt, B. J.

    1996-01-01

    An experimental study is conducted to determine the dependence of vortex generator geometry and impinging flow conditions on shed vortex circulation and crossplane peak vorticity for one type of vortex generator. The vortex generator is a symmetric airfoil having a NACA 0012 cross-sectional profile. The geometry and flow parameters varied include angle-of-attack alfa, chordlength c, span h, and Mach number M. The vortex generators are mounted either in isolation or in a symmetric counter-rotating array configuration on the inside surface of a straight pipe. The turbulent boundary layer thickness to pipe radius ratio is delta/R = 0. 17. Circulation and peak vorticity data are derived from crossplane velocity measurements conducted at or about 1 chord downstream of the vortex generator trailing edge. Shed vortex circulation is observed to be proportional to M, alfa, and h/delta. With these parameters held constant, circulation is observed to fall off in monotonic fashion with increasing airfoil aspect ratio AR. Shed vortex peak vorticity is also observed to be proportional to M, alfa, and h/delta. Unlike circulation, however, peak vorticity is observed to increase with increasing aspect ratio, reaching a peak value at AR approx. 2.0 before falling off.

  15. Modeling Vortex Generators in the Wind-US Code

    NASA Technical Reports Server (NTRS)

    Dudek, Julianne C.

    2010-01-01

    A source term model which simulates the effects of vortex generators was implemented into the Wind-US Navier Stokes code. The source term added to the Navier-Stokes equations simulates the lift force which would result from a vane-type vortex generator in the flowfield. The implementation is user-friendly, requiring the user to specify only three quantities for each desired vortex generator: the range of grid points over which the force is to be applied and the planform area and angle of incidence of the physical vane. The model behavior was evaluated for subsonic flow in a rectangular duct with a single vane vortex generator, supersonic flow in a rectangular duct with a counterrotating vortex generator pair, and subsonic flow in an S-duct with 22 co-rotating vortex generators. The validation results indicate that the source term vortex generator model provides a useful tool for screening vortex generator configurations and gives comparable results to solutions computed using a gridded vane.

  16. CFD modeling and analysis of rotor wake in hover interacting with a ground plane

    NASA Astrophysics Data System (ADS)

    Kalra, Tarandeep Singh

    The action of the rotor wake on loose sediment on the ground is primarily responsible for inducing the rotorcraft brownout phenomenon. Therefore, any simulation of brownout must be capable of accurately predicting the velocity field induced by the rotor when it is operating in ground effect. This work attempts to use a compressible, structured, overset Reynolds-Averaged Navier-Stokes (RANS) based solver to simulate hovering rotors in ground effect (IGE) to demonstrate the capability of the code to provide accurate tip vortex flow field predictions, and provide a good understanding of the ground-wake interactions. The computations are performed for a micro-scale rotor (0.086m radius, aspect ratio of 4.387 operating at a tip Mach number of 0.08 and Reynolds number of 32,500) and a sub-scale rotor (0.408m radius, aspect ratio of 9.132 operating at a tip Mach number of 0.24 and Reynolds number of 250,000) in order to compare to experimental measurements. The micro-scale rotor has a rectangular tip shape and is simulated three rotor heights: 1.5R, 1.0R and 0.5R above ground (R = Rotor radius). The sub-scale rotor is simulated at one particular rotor height (i.e. 1R) but with four different tip shapes: rectangular, swept, BERP-like and slotted tip. Various mesh placement strategies are devised to efficiently capture the path of the tip vortices for both regimes. The micro-scale rotor simulations are performed using the Spalart Allmaras (S-A) turbulence model. The examination of the IGE tip vortex flow field suggests high degree of instabilities close to the ground. In addition, the induced velocities arising from the proximity of the rotor tip vortices causes flow separation at the ground. The sub-scale rotor simulations show a smeared out flow field even at early wake ages due to excessive turbulence levels. The distance function in the S-A turbulence model is modified using the Delayed Detached Eddy Simulation (DDES) approach and a correction to length scaling is included for anistropic grids. The resulting computational flow field after these modifications compares well with the experiments. The slotted tip is seen to diffuse the tip vortices at early wake ages through injection of momentum and increased turbulence, and generates the least amount of unsteady pressure variation at the ground plane when compared with other three tip shapes.

  17. A free wake method for performance prediction of VAWT

    NASA Astrophysics Data System (ADS)

    Ilin, S.; Dumitrescu, H.; Cardos, V.; Dumitrache, A.

    2012-09-01

    Based on the lifting line theory and a free vortex wake model, a method including dynamic stall effects is presented for predicting the performance of a three-dimensional vertical-axis wind turbine (VAWT). A vortex model is used in which the wake is composed of trailing streamwise and shedding spanwise vortices, whose strengths are equal to the change in the bound vortex strength as dictated by Helmholtz and Kelvin's theorems. Performance parameters are calculated by application of the Biot-Savart law along with the Kutta-Joukowski theorem and a semi-empirical dynamic stall model. Predictions are shown to compare favorably with existing experimental data.

  18. Computational Modeling of Vortex Generators for Turbomachinery

    NASA Technical Reports Server (NTRS)

    Chima, R. V.

    2002-01-01

    In this work computational models were developed and used to investigate applications of vortex generators (VGs) to turbomachinery. The work was aimed at increasing the efficiency of compressor components designed for the NASA Ultra Efficient Engine Technology (UEET) program. Initial calculations were used to investigate the physical behavior of VGs. A parametric study of the effects of VG height was done using 3-D calculations of isolated VGs. A body force model was developed to simulate the effects of VGs without requiring complicated grids. The model was calibrated using 2-D calculations of the VG vanes and was validated using the 3-D results. Then three applications of VGs to a compressor rotor and stator were investigated: 1) The results of the 3-D calculations were used to simulate the use of small casing VGs used to generate rotor preswirl or counterswirl. Computed performance maps were used to evaluate the effects of VGs. 2) The body force model was used to simulate large part-span splitters on the casing ahead of the stator. Computed loss buckets showed the effects of the VGs. 3) The body force model was also used to investigate the use of tiny VGs on the stator suction surface for controlling secondary flows. Near-surface particle traces and exit loss profiles were used to evaluate the effects of the VGs.

  19. Extension of an atmospheric dispersion model to include building wake effects

    SciTech Connect

    Weil, J.C.; Brower, R.P.; Corio, L.A.

    1999-07-01

    A modification to a dispersion model for the convective boundary layer (CBL) is proposed to deal with stack sources located on or near buildings and affected by the turbulent wake of the building. Wake effects are greatest within the near wake or cavity region close to the building. The approach is to combine an earlier wake model with the CBL model such that the appropriate concentration and dispersion limits are satisfied at short, intermediate, and large downwind distances.

  20. Characterization of a wind turbine model for wake aerodynamics studies

    NASA Astrophysics Data System (ADS)

    Cuzzola, Francesco; Aubrun, Sandrine; Leitl, Bernd

    2014-12-01

    A model wind turbine has been designed at the University of Hamburg within the scope of the FP7 fundend project WAUDIT. The purpose of the experiment described in this paper is to characterize the performances of two rotors by means of measuring the thrust coefficient Ct. Ct is a similarity parameter for the wake and is thought to be the most effective one. Its value has been directly measured using a force balance and indirectly calculated from the velocity profiles measured three diameters downstream of the rotor with hot wire anemometry. Results show that, in order to reproduce the wake behaviour, the matching of the Ct, which is a quantitative achievement, has to be integrated with measurements such as velocity profiles in the wake. In fact the velocity deficit illustrates the mechanism of transforming the axial momentum into torque assuring qualitatively the proper reproduction of the wake. This latter information assures that the achievement of a certain thrust force acting on the rotor is due to its performances in transforming the axial momentum into torque and not an effect of other phenomena such as a stall at the blades.

  1. Field measurements in the wake of a model wind turbine

    NASA Astrophysics Data System (ADS)

    Pol, Suhas; Taylor, Amelia; Bilbao, Argenis; Doostalab, Ali; Novoa, Santiago; Westergaard, Carsten; Hussain, Fazle; Sheng, Jian; Ren, Beibei; Giesselmann, Michael; Glauser, Mark; Castillo, Luciano

    2014-06-01

    As a first step to study the dynamics of a wind farm' we experimentally explored the flow field behind a single wind turbine of diameter 1.17 m at a hub height of 6.25 m. A 10 m tower upstream of the wind farm characterizes the atmospheric conditions and its influence on the wake evolution. A vertical rake of sonic anemometers is clustered around the hub height on a second tower' 6D downstream of the turbine. We present preliminary observations from a 1- hour block of data recorded in near-neutral atmospheric conditions. The ratio of the standard deviation of power to the inflow velocity is greater than three' revealing adverse effects of inflow turbulence on the power and load fluctuations. Furthermore' the wake defect and Reynolds stress and its gradient are pronounced at 6D. The flux of energy due to Reynolds stresses is similar to that reported in wind tunnel studies. The swirl and mixing produces a constant temperature wake which results in a density jump across the wake interface. Further field measurements will explore the dynamics of a model wind farm' including the effects of atmospheric variability.

  2. Modelling of Offshore Wind Turbine Wakes with the Wind Farm Program FLaP

    NASA Astrophysics Data System (ADS)

    Lange, Bernhard; Waldl, Hans-Peter; Gil Guerrero, Algert; Heinemann, Detlev; Barthelmie, Rebecca J.

    2003-01-01

    The wind farm layout program FLaP estimates the wind speed at any point in a wind farm and the power output of the turbines. The ambient flow conditions and the properties of the turbines and the farm are used as input. The core of the program is an axisymmetric wake model describing the wake behind one rotor. Here an approach based on the simplified Reynolds equation with eddy viscosity closure is chosen. The single-wake model is combined with a model for the vertical wind speed profile and a wind farm model, which takes care of the interaction of all wakes in a wind farm. The wake model has been extended to improve the description of wake development in offshore conditions, especially the low ambient turbulence and the effect of atmospheric stability. Model results are compared with measurements from the Danish offshore wind farm Vindeby. Vertical wake profiles and mean turbulence intensities in the wake are compared for single-, double- and quintuple-wake cases with different mean wind speed, turbulence intensity and atmospheric stability. It is found that within the measurement uncertainties the results of the wake model compare well with the measurements for the most important ambient conditions. The effect of the low turbulence intensity offshore on the wake development is modelled well for Vindeby wind farm. Deviations are found when atmospheric stability deviates from near-neutral conditions. For stable atmospheric conditions both the free vertical wind speed profile and the wake profile are not modelled satisfactorily.

  3. Confining bond rearrangement in the random center vortex model

    NASA Astrophysics Data System (ADS)

    Altarawneh, Derar; Höllwieser, Roman; Engelhardt, Michael

    2016-03-01

    We present static meson-meson and baryon-antibaryon potentials in Z (2 ) and Z (3 ) random center vortex models for the infrared sector of Yang-Mills theory, i.e., hypercubic lattice models of random vortex world surfaces. In particular, we calculate multiple Polyakov loop correlators corresponding to static meson-meson or baryon-antibaryon configurations in a center vortex background and observe that their expectation values follow the minimal area law, displaying bond rearrangement behavior, a characteristic expected for the confining dynamics of the strong interaction. The static meson-meson and baryon-antibaryon potentials are compared with theoretical predictions and lattice QCD simulations.

  4. Mathematical model of network dynamics governing mouse sleep-wake behavior.

    PubMed

    Behn, Cecilia G Diniz; Brown, Emery N; Scammell, Thomas E; Kopell, Nancy J

    2007-06-01

    Recent work in experimental neurophysiology has identified distinct neuronal populations in the rodent brain stem and hypothalamus that selectively promote wake and sleep. Mutual inhibition between these cell groups has suggested the conceptual model of a sleep-wake switch that controls transitions between wake and sleep while minimizing time spent in intermediate states. By combining wake- and sleep-active populations with populations governing transitions between different stages of sleep, a "sleep-wake network" of neuronal populations may be defined. To better understand the dynamics inherent in this network, we created a model sleep-wake network composed of coupled relaxation oscillation equations. Mathematical analysis of the deterministic model provides insight into the dynamics underlying state transitions and predicts mechanisms for each transition type. With the addition of noise, the simulated sleep-wake behavior generated by the model reproduces many qualitative and quantitative features of mouse sleep-wake behavior. In particular, the existence of simulated brief awakenings is a unique feature of the model. In addition to capturing the experimentally observed qualitative difference between brief and sustained wake bouts, the model suggests distinct network mechanisms for the two types of wakefulness. Because circadian and other factors alter the fine architecture of sleep-wake behavior, this model provides a novel framework to explore dynamical principles that may underlie normal and pathologic sleep-wake physiology. PMID:17409167

  5. Recent Laboratory and Numerical Trailing Vortex Studies

    NASA Technical Reports Server (NTRS)

    Delisi, Donald P.; Greene, George C.; Robins, Robert E.; Singh, Raminder

    1996-01-01

    Results from two laboratory studies and two numerical studies are presented. In the first laboratory study, measurements of the strength of vortices from a three-dimensional (3-D) model wing are presented. The measurements follow the vortices as they evolve in time from a two-dimensional (2-D) line vortex pair to the development and migration of 3-D vortex rings. It is shown that the resulting vortex rings can contain up to 40 percent of the initial vortex circulation. Thus, the formation of vortex rings may not necessarily signal the end of the wake hazard to following aircraft. In the second laboratory study, we present the results of an experiment which shows how the spanwise drag distribution affects wake-vortex evolution. In this experiment, we modified the spanwise drag distribution on a model wing while keeping the total lift and drag constant. The results show that adding drag on or near the centerline of the wing has a larger effect than adding drag at or near the wingtips. These measurements complement the results of NASA studies in the 1970s. In the first numerical study, results of 3-D numerical calculations are presented which show that the vortex Reynolds number has a significant influence on the evolution and migration of wake vortices. When the Reynolds number is large, 3-D vortex rings evolve from the initially 2-D line vortex pairs. These vortex rings then migrate vertically. When the Reynolds number is lower, the transition of vorticity from 2-D to 3-D is delayed. When the Reynolds number is very low, the vortices never transition to 3-D, and the vertical migration is significantly reduced. It is suggested that this effect may have been important in previous laboratory wake-evolution studies. A second numerical study shows the influence that vertical wind shear can have on trailing vortex evolution.

  6. System Identification of a Vortex Lattice Aerodynamic Model

    NASA Technical Reports Server (NTRS)

    Juang, Jer-Nan; Kholodar, Denis; Dowell, Earl H.

    2001-01-01

    The state-space presentation of an aerodynamic vortex model is considered from a classical and system identification perspective. Using an aerodynamic vortex model as a numerical simulator of a wing tunnel experiment, both full state and limited state data or measurements are considered. Two possible approaches for system identification are presented and modal controllability and observability are also considered. The theory then is applied to the system identification of a flow over an aerodynamic delta wing and typical results are presented.

  7. Cylinder wakes in flowing soap films.

    PubMed

    Vorobieff, P; Ecke, R E

    1999-09-01

    We present an experimental characterization of cylinder wakes in flowing soap films. From instantaneous velocity and thickness fields, we find the vortex-shedding frequency, mean-flow velocity, and mean-film thickness. Using the empirical relationship between the Reynolds and Strouhal numbers obtained for cylinder wakes in three dimensions, we estimate the effective soap-film viscosity and its dependence on film thickness. We also compare the decay of vorticity with that in a simple Rankine vortex model with a dissipative term to account for air drag. PMID:11970100

  8. Cylinder wakes in flowing soap films

    SciTech Connect

    Vorobieff, P.; Ecke, R.E. ); Vorobieff, P. )

    1999-09-01

    We present an experimental characterization of cylinder wakes in flowing soap films. From instantaneous velocity and thickness fields, we find the vortex-shedding frequency, mean-flow velocity, and mean-film thickness. Using the empirical relationship between the Reynolds and Strouhal numbers obtained for cylinder wakes in three dimensions, we estimate the effective soap-film viscosity and its dependence on film thickness. We also compare the decay of vorticity with that in a simple Rankine vortex model with a dissipative term to account for air drag. [copyright] [ital 1999] [ital The American Physical Society

  9. A Critical Review of the Transport and Decay of Wake Vortices in Ground Effect

    NASA Technical Reports Server (NTRS)

    Sarpkaya, T.

    2004-01-01

    This slide presentation reviews the transport and decay of wake vortices in ground effect and cites a need for a physics-based parametric model. The encounter of a vortex with a solid body is always a complex event involving turbulence enhancement, unsteadiness, and very large gradients of velocity and pressure. Wake counter in ground effect is the most dangerous of them all. The interaction of diverging, area-varying, and decaying aircraft wake vortices with the ground is very complex because both the vortices and the flow field generated by them are altered to accommodate the presence of the ground (where there is very little room to maneuver) and the background turbulent flow. Previous research regarding vortex models, wake vortex decay mechanisms, time evolution within in ground effect of a wake vortex pair, laminar flow in ground effect, and the interaction of the existing boundary layer with a convected vortex are reviewed. Additionally, numerical simulations, 3-dimensional large-eddy simulations, a probabilistic 2-phase wake vortex decay and transport model and a vortex element method are discussed. The devising of physics-based, parametric models for the prediction of (operational) real-time response, mindful of the highly three-dimensional and unsteady structure of vortices, boundary layers, atmospheric thermodynamics, and weather convective phenomena is required. In creating a model, LES and field data will be the most powerful tools.

  10. Modeling and simulation of a turbulent far wake

    NASA Technical Reports Server (NTRS)

    Cimbala, John M.

    1995-01-01

    Work continued on two projects which had been started during previous years. Both projects involve calculations of the subsonic, turbulent far wake of a two-dimensional object at a Reynolds number of 1000 (based on wake momentum thickness). This flow was used as a test case for direct comparison of various turbulence models and a direct numerical simulations (DNS) of this flow were undertaken. In the turbulence model comparison studies, for any particular model tested, a unique self-similar solution was obtained far enough downstream, regardless of inlet conditions. Furthermore, different turbulence models led to different far-wake equilibrium solutions. No turbulence model could correctly predict all features of the turbulent far wake. For example, the spreading rate and turbulent shear stresses were underpredicted by all the standard models (both two-equation and full Reynolds stress models). In cases where a more correct spreading rate was achieved, it was at the expense of the turbulent kinetic energy, which was overpredicted. In general, the Algebraic Dissipation Rate Model of Gatski and Speziale, 1992, when added to any of the standard models, improved the results dramatically. Also, full Reynolds stress closure models did a much better job at predicting the shapes of both the mean and turbulence profiles, but the spreading rate was not significantly improved over that predicted by the simpler two-equation models. There are two main conclusions from these studies: First, in a comparison such as this, it is not enough to compare just one parameter, like the spreading rate. A good prediction for one parameter does not necessarily imply good predictions for all parameters in a flow. Second, since no turbulence model could correctly predict the turbulent far wake, much of the important physics of turbulent free shear flows is apparently lost by the assumptions inherent in today's methods of turbulence modeling; turbulence models must be improved. Direct simulations of this flow were begun last year in order to provide a data base through which some of the deficiencies of the existing turbulence models could be identified. Quantities such as the pressure-strain correlation, turbulent diffusion, and the dissipation rate tensor can be easily calculated from the DNS results, whereas these quantities are nearly impossible to measure experimentally. Improvements to existing turbulence models (and development of new models) require knowledge about flow quantities such as these. During this summer, diagnostics codes were written which will calculate the parameters mentioned above, along with other single-point and multi-point statistics. The DNS calculations are still in progress at the time of this writing. When these calculations are complete, the diagnostics codes will be applied so that the results can aid turbulence modelers. In addition, the results will show whether or not there exists a universal equilibrium turbulent far wake, independent of initial conditions.

  11. Applying canopy flow model for estimation of wind turbine wake

    NASA Astrophysics Data System (ADS)

    Sogachev, A.; Joergensen, H. E.; Mann, J.; Frandsen, S.; Ott, S.

    2008-12-01

    For the planning of large offshore wind farm the optimal spatial placing of wind turbines as well as wind farms relatively to each other is highly important to reduce the wake losses of energy. Conventional instrumental investigations of airflow characteristics around and inside an offshore wind farm aimed at understanding of far-wake behavior are very difficult and expensive. Computational fluid dynamic (CFD) models can provide the information on spatial patterns of wind and turbulence and thus, help to develop the optimal wind farm design. With limited level of model resolution, however, there is still a problem of how to describe the effect of a wind turbine itself on air flow. Having this problem solved the joint effects of a given number of wind turbines could be easily estimated. In present work, to describe the influence of a wind turbine on the flow a coupled canopy-atmospheric boundary-layer model SCADIS is implemented. It has been shown that this model, based on two-equation closure and modified to account for plant drag, is able to simulate airflow through a wide range of vegetation reasonably. In the numerical experiment with SCADIS the turbine's rotor was replaced by a disk of limited thickness, with diameter (D) and location of real rotor but with properties of vegetation. Aerodynamic drag values for this rotor with some 'plant' surface density can be derived from the trust coefficient Cp of the wind turbine of interest. Model results were compared with measurements from the Danish offshore wind farm Vindeby consisted of 11 Bonus 450 kW turbines (hub height and rotor diameter are 38 m and 35 m, respectively). The comparison show that the approach can describe well the single- and double-wake cases (at distance 9.6D behind the last turbine), and quintuple-wake case (at distance 8.6D). Taking in account relatively low the computing time demands of the approach, it is a promising tool for further studies of wakes of offshore wind turbines of any size and composition.

  12. A Hybrid Vortex Sheet / Point Vortex Model for Unsteady Separated Flows

    NASA Astrophysics Data System (ADS)

    Darakananda, Darwin; Eldredge, Jeff D.; Colonius, Tim; Williams, David R.

    2015-11-01

    The control of separated flow over an airfoil is essential for obtaining lift enhancement, drag reduction, and the overall ability to perform high agility maneuvers. In order to develop reliable flight control systems capable of realizing agile maneuvers, we need a low-order aerodynamics model that can accurately predict the force response of an airfoil to arbitrary disturbances and/or actuation. In the present work, we integrate vortex sheets and variable strength point vortices into a method that is able to capture the formation of coherent vortex structures while remaining computationally tractable for control purposes. The role of the vortex sheet is limited to tracking the dynamics of the shear layer immediately behind the airfoil. When parts of the sheet develop into large scale structures, those sections are replaced by variable strength point vortices. We prevent the vortex sheets from growing indefinitely by truncating the tips of the sheets and transfering their circulation into nearby point vortices whenever the length of sheet exceeds a threshold. We demonstrate the model on a variety of canonical problems, including pitch-up and impulse translation of an airfoil at various angles of attack. Support by the U.S. Air Force Office of Scientific Research (FA9550-14-1-0328) with program manager Dr. Douglas Smith is gratefully acknowledged.

  13. Numerical simulations of a turbulent axial vortex

    NASA Astrophysics Data System (ADS)

    Qin, Jim Hongxin

    Although the vortex is present in most flows of engineering interest, the turbulent structure of the vortex is not well understood. Current prediction capabilities are especially weak for the vortex as well as other strongly rotating flows. The objective of this work is to aid the development of turbulence models for the vortex as well as strongly rotating flows in general by using direct numerical simulations of the vortex. The present study focuses on the turbulent axial vortex with and without an external strain field. The numerical simulations of a turbulent axial vortex without strain, i.e. an isolated vortex, have been performed by using a pseudo spectral method for compressible flow. The results qualitatively match well with the experimental data. The isolated vortex is stable unless the mean axial wake flow has sufficient magnitude. During the period of decay of disturbances, the mean tangential velocity profile exhibits anti-diffusion because a negative eddy viscosity develops near the center of the vortex. With the disturbance growth, the isolated vortex develops large-scale helical vortex structures, but they eventually disappear during the period of relaminarization. The details of turbulent statistics have been examined. The turbulent structure is related to the in stability of the isolated vortex. The budgets for the Reynolds stresses reveal that the production term is the primary source term, but the pressure strain, pressure transport, and turbulent transport terms also make a large contribution to the budgets for the Reynolds stresses.

  14. Modeling the plasma near-wakes

    NASA Astrophysics Data System (ADS)

    Liu, V. C.; Ying, S. J.

    1980-10-01

    An ambient unmagnetized plasma is considered in which a flat-based projectile of radius R and zero surface potential moves along its axis of symmetry at a constant mesothermal speed. The plasma cavity-filling process is likened to the transient flow associated with a radially imploding cylindrical shock seen immediately after the rupture of a cylindrical diaphragm of radius R. A modeling scheme is presented assuming that electron temperature remains constant and is much higher than the ion temperature in plasma; and that ions move in a self-consistent electric field with which the electrons are already in Boltzmann equilibrium. A figure illustrates the ion density distributions at consecutive times after the start of implosion and the existence of an ion wavefront as a result of charge separation is clearly seen.

  15. Oscillating airfoils and their wake

    NASA Technical Reports Server (NTRS)

    Send, W.

    1985-01-01

    The unsteady phenomena in the wake of an oscillating wing or rotor blade are examined theoretically using the Prandtl approximation of the vortex-transport equation. A mathematical model is developed and applied to such problems as the effect of winglets on the performance of fixed wings and the possibly of employing similar designs in rotor blades. Model predictions for several profiles are compared with published and experimental measurements, and good agreement is found. Graphs and diagrams are provided.

  16. Point vortex model for prediction of sound generated by a wing with flap interacting with a passing vortex.

    PubMed

    Manela, A; Huang, L

    2013-04-01

    Acoustic signature of a rigid wing, equipped with a movable downstream flap and interacting with a line vortex, is studied in a two-dimensional low-Mach number flow. The flap is attached to the airfoil via a torsion spring, and the coupled fluid-structure interaction problem is analyzed using thin-airfoil methodology and application of the emended Brown and Michael equation. It is found that incident vortex passage above the airfoil excites flap motion at the system natural frequency, amplified above all other frequencies contained in the forcing vortex. Far-field radiation is analyzed using Powell-Howe analogy, yielding the leading order dipole-type signature of the system. It is shown that direct flap motion has a negligible effect on total sound radiation. The characteristic acoustic signature of the system is dominated by vortex sound, consisting of relatively strong leading and trailing edge interactions of the airfoil with the incident vortex, together with late-time wake sound resulting from induced flap motion. In comparison with the counterpart rigid (non-flapped) configuration, it is found that the flap may act as sound amplifier or absorber, depending on the value of flap-fluid natural frequency. The study complements existing analyses examining sound radiation in static- and detached-flap configurations. PMID:23556563

  17. High-resolution numerical modeling of mesoscale island wakes and sensitivity to static topographic relief data

    NASA Astrophysics Data System (ADS)

    Nunalee, C. G.; Horváth, Á.; Basu, S.

    2015-08-01

    Recent decades have witnessed a drastic increase in the fidelity of numerical weather prediction (NWP) modeling. Currently, both research-grade and operational NWP models regularly perform simulations with horizontal grid spacings as fine as 1 km. This migration towards higher resolution potentially improves NWP model solutions by increasing the resolvability of mesoscale processes and reducing dependency on empirical physics parameterizations. However, at the same time, the accuracy of high-resolution simulations, particularly in the atmospheric boundary layer (ABL), is also sensitive to orographic forcing which can have significant variability on the same spatial scale as, or smaller than, NWP model grids. Despite this sensitivity, many high-resolution atmospheric simulations do not consider uncertainty with respect to selection of static terrain height data set. In this paper, we use the Weather Research and Forecasting (WRF) model to simulate realistic cases of lower tropospheric flow over and downstream of mountainous islands using the default global 30 s United States Geographic Survey terrain height data set (GTOPO30), the Shuttle Radar Topography Mission (SRTM), and the Global Multi-resolution Terrain Elevation Data set (GMTED2010) terrain height data sets. While the differences between the SRTM-based and GMTED2010-based simulations are extremely small, the GTOPO30-based simulations differ significantly. Our results demonstrate cases where the differences between the source terrain data sets are significant enough to produce entirely different orographic wake mechanics, such as vortex shedding vs. no vortex shedding. These results are also compared to MODIS visible satellite imagery and ASCAT near-surface wind retrievals. Collectively, these results highlight the importance of utilizing accurate static orographic boundary conditions when running high-resolution mesoscale models.

  18. Wind flow characteristics in the wakes of large wind turbines. Volume 1: Analytical model development

    NASA Technical Reports Server (NTRS)

    Eberle, W. R.

    1981-01-01

    A computer program to calculate the wake downwind of a wind turbine was developed. Turbine wake characteristics are useful for determining optimum arrays for wind turbine farms. The analytical model is based on the characteristics of a turbulent coflowing jet with modification for the effects of atmospheric turbulence. The program calculates overall wake characteristics, wind profiles, and power recovery for a wind turbine directly in the wake of another turbine, as functions of distance downwind of the turbine. The calculation procedure is described in detail, and sample results are presented to illustrate the general behavior of the wake and the effects of principal input parameters.

  19. Infrared imaging simulation and detection of ship wake

    NASA Astrophysics Data System (ADS)

    Yang, Li; Chen, Xuan; Chang, Shizheng; Xu, Enchi; Wang, Xingyu; Wang, Ye; Zhao, Xiaolong; Du, Yongchen; Kou, Wei; Fan, Chunli

    2015-10-01

    The thermal wake would be formed owing to the cooling water or exhaust heat discharged by ship, and the cold wake could be formed by the cool water in the lower part of sea stirred up by the ship propeller or vortexes. Owing to the difference of surface temperature and emissivity between the ship wake and the surrounding ocean the ship wake will be easily detected by the infrared detecting system. The wave of wake also could be detected by the difference of reflected radiance between the background and the Kelvin wake of ship. In this paper the simulating models of infrared imaging of ship wake are developed based on the selfradiation of wake, the reflected radiance of the sky and sun and the transmitted radiance of atmosphere, and the infrared imaging signatures of ship wake are investigated. The results show that the infrared imaging signatures of ship wake can be really simulated by the models proposed in this paper. The effects of the detecting height, the angle of view, the NETD of detector and the temperature of wake on the infrared imaging signatures of ship wake are studied. The temperature difference between the ship wake and surrounding ocean is a main fact which effects on the detecting distance. The infrared imaging signatures of ship wake in 8-14μm wave band is stronger than that in 2-5μm wave band whenever the temperature of ship wake is warmer or cooler than the surrounding ocean. Further, the infrared imaging of thermal wake is investigated in the homogenous water and temperature stratified water at different speed of a ship and different flow rate and depth of the discharged water in a water tank. The spreading and decaying laws of infrared signature of ship wake are obtained experimentally. The results obtained in this paper have an important application in the infrared remote sensing of ship wake.

  20. The development of a prescribed wake model for performance prediction in steady yawed flow

    SciTech Connect

    Robison, D.J.; Coton, F.N.; Galbraith, R.A.M.; Vezza, M.

    1995-09-01

    A new prescribed wake model for horizontal axis wind turbines (HAWTs) is presented. The model`s wake geometry is derived from simple prescriptive functions, based on momentum theory, defining the three-dimensional wake development from the near to the far field. The work described herein considers the analysis of both steady axial and yawed flow conditions. The detailed modelling of the yawed case is still in the initial stages, it is envisaged that this will eventually include fully unsteady aerodynamic effects. Model validation is by comparison with both experimental data and results from a free wake model.

  1. The Role of Wakes in Modelling Tidal Current Turbines

    NASA Astrophysics Data System (ADS)

    Conley, Daniel; Roc, Thomas; Greaves, Deborah

    2010-05-01

    The eventual proper development of arrays of Tidal Current Turbines (TCT) will require a balance which maximizes power extraction while minimizing environmental impacts. Idealized analytical analogues and simple 2-D models are useful tools for investigating questions of a general nature but do not represent a practical tool for application to realistic cases. Some form of 3-D numerical simulations will be required for such applications and the current project is designed to develop a numerical decision-making tool for use in planning large scale TCT projects. The project is predicated on the use of an existing regional ocean modelling framework (the Regional Ocean Modelling System - ROMS) which is modified to enable the user to account for the effects of TCTs. In such a framework where mixing processes are highly parametrized, the fidelity of the quantitative results is critically dependent on the parameter values utilized. In light of the early stage of TCT development and the lack of field scale measurements, the calibration of such a model is problematic. In the absence of explicit calibration data sets, the device wake structure has been identified as an efficient feature for model calibration. This presentation will discuss efforts to design an appropriate calibration scheme which focuses on wake decay and the motivation for this approach, techniques applied, validation results from simple test cases and limitations shall be presented.

  2. Wake Development of a Model Vertical Axis Wind Turbine

    NASA Astrophysics Data System (ADS)

    Kadum, Hawwa; Friedman, Sasha; Camp, Elizabeth; Cal, Rau'l.

    2015-11-01

    At the Portland State University wind tunnel facility, an experiment is conducted to observe the downstream development of the wake past a model vertical axis wind turbine (VAWT). The flow domain is composed of streamwise-spanwise planes at mid-height of the VAWT rotor and data is obtained via particle image velocimetry (PIV). The flow field is assessed by analyzing contours of mean velocities and the full Reynolds stress tensor. Furthermore, profiles of the aforementioned quantities and flow parameters are discussed in the context of downstream evolution/flow development.

  3. Implications of MAVEN Mars near-wake measurements and models

    NASA Astrophysics Data System (ADS)

    Luhmann, J. G.; Dong, Chuanfei; Ma, Yingjuan; Curry, S. M.; Mitchell, D.; Espley, J.; Connerney, J.; Halekas, J.; Brain, D. A.; Jakosky, B. M.; Mazelle, C.

    2015-11-01

    Mars is typically viewed as a member of the category of weakly magnetized planets, with a largely induced magnetosphere and magnetotail produced by the draped fields of the solar wind interaction. However, selected Mars Atmosphere and Volatile EvolutioN Mission (MAVEN) suprathermal electron and magnetic field observations in the near wake, sampled along its elliptical orbit during the early prime mission at altitudes ranging from its ~150 km periapsis to the tail magnetosheath, reinforce a picture seen in an MHD model where magnetic fields are rooted in the planet throughout much of the Martian magnetotail.

  4. Information Requirements for Supervisory Air Traffic Controllers in Support of a Mid-Term Wake Vortex Departure System

    NASA Technical Reports Server (NTRS)

    Lohr, Gary W.; Williams, Daniel M.; Trujillo, Anna C.; Johnson, Edward J.; Domino, David A.

    2008-01-01

    A concept focusing on wind dependent departure operations has been developed the current version of this concept is called the Wake Turbulence Mitigation for Departures (WTMD). This concept takes advantage the fact that cross winds of sufficient velocity blow wakes generated by "heavy" and B757 category aircraft on the downwind runway away from the upwind runway. Supervisory Air Traffic Controllers would be responsible for authorization of the Procedure. An investigation of the information requirements necessary to for Supervisors to approve monitor and terminate the Procedure was conducted. Results clearly indicated that the requisite information is currently available in air traffic control towers and that additional information was not required.

  5. Investigation of Some Wake Vortex Characteristics of an Inclined Ogive-Cylinder Body at Mach Number 2

    NASA Technical Reports Server (NTRS)

    Jorgensen, Leland H; Perkins, Edward W

    1958-01-01

    For a body consisting of a fineness-ratio-3 ogival nose tangent to a cylindrical afterbody 7.3 diameters long, pitot-pressure distributions in the flow field, pressure distributions over the body, and downwash distributions along a line through the vortex centers have been measured for angles of attack to 20 degrees. The Reynolds numbers, based on body diameter, were 0.15 x 10 to the 6th power and 0.44 x 10 to the 6th power. Comparisons of computed and measured vortex paths and downwash distributions are made. (author)

  6. Modeling of vortex generated sound in solid propellant rocket motors

    NASA Technical Reports Server (NTRS)

    Flandro, G. A.

    1980-01-01

    There is considerable evidence based on both full scale firings and cold flow simulations that hydrodynamically unstable shear flows in solid propellant rocket motors can lead to acoustic pressure fluctuations of significant amplitude. Although a comprehensive theoretical understanding of this problem does not yet exist, procedures were explored for generating useful analytical models describing the vortex shedding phenomenon and the mechanisms of coupling to the acoustic field in a rocket combustion chamber. Since combustion stability prediction procedures cannot be successful without incorporation of all acoustic gains and losses, it is clear that a vortex driving model comparable in quality to the analytical models currently employed to represent linear combustion instability must be formulated.

  7. Transition and Turbulence Modeling for Blunt-Body Wake Flows

    NASA Technical Reports Server (NTRS)

    Nance, Robert P.; Horvath, Thomas J.; Hassan, H. A.

    1997-01-01

    This study attempts t o improve the modeling and computational prediction of high- speed transitional wake flows. The recently developed kappa - zeta (Enstrophy) turbulence model is coupled with a newly developed transition prediction method and implemented in an implicit flow solver well-suited to hypersonic flows. In this model, transition onset is determined as part of the solution. Results obtained using the new model for a 70- deg blunted cone/sting geometry demonstrate better agreement with experimental heat- transfer measurements when compared to laminar calculations as well as solutions using the kappa - omega model. Results are also presented for the situation where transition onset is preselected. It is shown that, in this case, results are quite sensitive to location of the transition point.

  8. Measuring wind turbine wakes and unsteady loading in a micro wind farm model

    NASA Astrophysics Data System (ADS)

    Bossuyt, Juliaan; Meneveau, Charles; Meyers, Johan

    2014-11-01

    Very large wind farms, approximating the ``infinite'' asymptotic limit, are often studied with LES using periodic boundary conditions. In order to create an experimental realization of such large wind-turbine arrays in a wind tunnel experiment including over 100 turbines, a very small-scale turbine model based on a 3 cm diameter porous disk is designed. The porous disc matches a realistic thrust coefficient between 0.75--0.85, and the far wake flow characteristics of a rotating wind turbine. As a first step, we characterize the properties of a single model turbine. Hot-wire measurements are performed for uniform inflow conditions with different background turbulence intensity levels. Strain gage measurements are used to measure the mean value and power spectra of the thrust force, power output and wind velocity in front of the turbine. The dynamics of the wind turbine are modeled making it possible to measure force spectra at least up to the natural frequency of the model. This is shown by reproducing the -5/3 spectrum from the incoming flow and the vortex shedding signatures of an upstream obstruction. An array with a large number of these instrumented model turbines is placed in JHU's Corrsin wind tunnel, to study effects of farm layout on total power output and turbine loading. Work supported by ERC (ActiveWindFarms, Grant No: 306471), and by NSF (CBET-113380 and IIA-1243482).

  9. Hot jet/wake turbulent structure and laser propagaion. Part 1: Jet/wake turbulence modeling

    NASA Astrophysics Data System (ADS)

    Dash, Sandy M.; Kenzakowski, D. C.

    1994-10-01

    The prediction of laser propagation through the hot turbulent exhaust of an aircraft (or missile) requires knowledge of the detailed turbulent structure, and in particular, density/temperature fluctuations and inner/outer turbulent scales. The ability to predict the mean flow structure of round turbulent jets in an isolated environment has now become well established including hot jets and supersonic jets. However, the ability to predict density or temperature fluctuations and inner scales is poorly established, as is the ability to predict jet interactions with the aerodynamic vorticity (e.g. with the wing-tip vortex). A three-tier program of work has been in progress to enhance our understanding of the turbulent structure in hot jets and its influence on laser propagation. In Part 1 of this paper, the turbulence modeling activities will be described whose basis is the new data being generated at NASA LaRC, to be described in Part 2 of this paper. The kappa epsilon turbulence model and 'g' scalar-fluctuation extension have been implemented in this work. The coefficients of the g-equation have been revised in accordance with new temperature fluctuation data for hot, subsonic jets into still air. Data is now being gathered for hot jets in a co-flow which interact with wing-tip vortices. The inclusion of the g-equation into a Navier-Stokes framework and its analysis of this complex jet data for which detailed turbulence statistics will be available will comprise the main part of this paper. The utility of direct numerical simulation (LES approach to be described in paper by Sinha and Dash) to provide additional insight into this complex problem will also be discussed. Part 2 of this paper discusses the new data, the measurement techniques, and the complexities entailed in data reduction. Part 3 discusses the military applications (laser countermeasures), new laser propagation data through hot jets (as well as new atmospheric turbulence data), and predictive methodology.

  10. Impacts of Wake Effect and Time Delay on the Dynamic Analysis of Wind Farms Models

    ERIC Educational Resources Information Center

    El-Fouly, Tarek H. M.; El-Saadany, Ehab F.; Salama, Magdy M. A.

    2008-01-01

    This article investigates the impacts of proper modeling of the wake effects and wind speed delays, between different wind turbines' rows, on the dynamic performance accuracy of the wind farms models. Three different modeling scenarios were compared to highlight the impacts of wake effects and wind speed time-delay models. In the first scenario,…

  11. Impacts of Wake Effect and Time Delay on the Dynamic Analysis of Wind Farms Models

    ERIC Educational Resources Information Center

    El-Fouly, Tarek H. M.; El-Saadany, Ehab F.; Salama, Magdy M. A.

    2008-01-01

    This article investigates the impacts of proper modeling of the wake effects and wind speed delays, between different wind turbines' rows, on the dynamic performance accuracy of the wind farms models. Three different modeling scenarios were compared to highlight the impacts of wake effects and wind speed time-delay models. In the first scenario,

  12. Development of test methods for scale model simulation of aerial applications in the NASA Langley Vortex Research Facility. [agricultural aircraft

    NASA Technical Reports Server (NTRS)

    Jordan, F. L., Jr.

    1980-01-01

    As part of basic research to improve aerial applications technology, methods were developed at the Langley Vortex Research Facility to simulate and measure deposition patterns of aerially-applied sprays and granular materials by means of tests with small-scale models of agricultural aircraft and dynamically-scaled test particles. Interactions between the aircraft wake and the dispersed particles are being studied with the objective of modifying wake characteristics and dispersal techniques to increase swath width, improve deposition pattern uniformity, and minimize drift. The particle scaling analysis, test methods for particle dispersal from the model aircraft, visualization of particle trajectories, and measurement and computer analysis of test deposition patterns are described. An experimental validation of the scaling analysis and test results that indicate improved control of chemical drift by use of winglets are presented to demonstrate test methods.

  13. Too big to grow: the saturation mechanism of the von Karman vortex street captured by a self-consistent model

    NASA Astrophysics Data System (ADS)

    Mantic-Lugo, Vladislav; Arratia, Cristobal; Gallaire, Francois

    2013-11-01

    The supercritical instability leading to the Karman vortex street in a cylinder wake is a well studied problem: the steady solution becomes linearly unstable and saturates into a limit cycle. However a simple physical picture for understanding the saturation amplitude is still missing. We present a simple self-consistent model that captures the saturation mechanism. The model shows that the main nonlinear effects of the saturation process are retained by the coupling of the mean flow and perturbation equations through the Reynolds stress, which is built only with the first harmonic calculated as the most unstable eigenmode. A simple physical picture is revealed, wherein the perturbation amplitude is such that the modified mean flow is neutrally stable. The mean flow velocity field and the Reynolds stress spatial structure are thus well approximated in a self-consistent manner without any DNS data. Moreover, the results show an accurate vortex shedding frequency prediction when compared to experiments.

  14. Sound Generation by Aircraft Wake Vortices

    NASA Technical Reports Server (NTRS)

    Hardin, Jay C.; Wang, Frank Y.

    2003-01-01

    This report provides an extensive analysis of potential wake vortex noise sources that might be utilized to aid in their tracking. Several possible mechanisms of aircraft vortex sound generation are examined on the basis of discrete vortex dynamic models and characteristic acoustic signatures calculated by application of vortex sound theory. It is shown that the most robust mechanisms result in very low frequency infrasound. An instability of the vortex core structure is discussed and shown to be a possible mechanism for generating higher frequency sound bordering the audible frequency range. However, the frequencies produced are still low and cannot explain the reasonably high-pitched sound that has occasionally been observed experimentally. Since the robust mechanisms appear to generate only very low frequency sound, infrasonic tracking of the vortices may be warranted.

  15. Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

    NASA Astrophysics Data System (ADS)

    Kirkil, Gokhan; Constantinescu, George

    2015-07-01

    The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

  16. An extension of the local momentum theory to a distorted wake model of a hovering rotor

    NASA Technical Reports Server (NTRS)

    Kawachi, K.

    1981-01-01

    The local momentum theory is based on the instantaneous balance between the fluid momentum and the blade elemental lift at a local station in the rotor rotational plane. Therefore, the theory has the capability of evaluating time wise variations of air loading and induced velocity distributions along a helicopter blade span. Unlike a complex vortex theory, this theory was developed to analyze the instantaneous induced velocity distribution effectively. The boundaries of this theory and a computer program using this theory are discussed. A concept introduced into the theory is the effect of the rotor wake contraction in hovering flight. A comparison of this extended local momentum theory with a prescribed wake vortex theory is also presented. The results indicate that the extended local momentum theory has the capability of achieving a level of accuracy similar to that of the prescribed wake vortex theory over wide range variations of rotor geometrical parameters. It is also shown that the analytical results obtained using either theory are in reasonable agreement with experimental data.

  17. Vortex dynamics and scalar transport in the wake of a bluff body driven through a steady recirculating flow

    NASA Astrophysics Data System (ADS)

    Poussou, Stephane B.; Plesniak, Michael W.

    2012-09-01

    The air ventilation system in wide-body aircraft cabins provides passengers with a healthy breathing environment. In recent years, the increase in global air traffic has amplified contamination risks by airborne flu-like diseases and terrorist threats involving the onboard release of noxious materials. In particular, passengers moving through a ventilated cabin may transport infectious pathogens in their wake. This paper presents an experimental investigation of the wake produced by a bluff body driven through a steady recirculating flow. Data were obtained in a water facility using particle image velocimetry and planar laser induced fluorescence. Ventilation attenuated the downward convection of counter-rotating vortices produced near the free-end corners of the body and decoupled the downwash mechanism from forward entrainment, creating stagnant contaminant regions.

  18. Kinematics of flight and the relationship to the vortex wake of a Pallas' long tongued bat (Glossophaga soricina).

    PubMed

    Wolf, Marta; Johansson, L Christoffer; von Busse, Rhea; Winter, York; Hedenström, Anders

    2010-06-15

    To obtain a full understanding of the aerodynamics of animal flight, the movement of the wings, the kinematics, needs to be connected to the wake left behind the animal. Here the detailed 3D wingbeat kinematics of bats, Glossophaga soricina, flying in a wind tunnel over a range of flight speeds (1-7 m s(-1)) was determined from high-speed video. The results were compared with the wake geometry and quantitative wake measurements obtained simultaneously to the kinematics. The wingbeat kinematics varied gradually with flight speed and reflected the changes observed in the wake of the bats. In particular, several of the kinematic parameters reflected the differences in the function of the upstroke at low and high flight speeds. At lower flight speeds the bats use a pitch-up rotation to produce a backward flick which creates thrust and some weight support. At higher speeds this mechanism disappears and the upstroke generates weight support but no thrust. This is reflected by the changes in e.g. angle of attack, span ratio, camber and downstroke ratio. We also determined how different parameters vary throughout a wingbeat over the flight speeds studied. Both the camber and the angle of attack varied over the wingbeat differently at different speeds, suggesting active control of these parameters to adjust to the changing aerodynamic conditions. This study of the kinematics strongly indicates that the flight of bats is governed by an unsteady high-lift mechanism at low flight speeds and points to differences between birds and bats. PMID:20511529

  19. Modeling of Fine Stratified Wake and Internal Waves Structures Past Obstacles

    NASA Astrophysics Data System (ADS)

    Bardakov, R. N.; Chashechkin, Y. D.; Mitkin, V. V.

    2007-12-01

    The goal of the talk is to describe a fine flow structure formation in a continuously stratified fluid past uniformly moving obstacles basing on analytical and numerical analysis of set of fundamental governing equations and on visualization of flows by sensitive and high resolution instruments. Mathematical modeling is based on the set of fundamental governing equations including continuity, Navier-Stokes, transport of substances and the state equation with standard no-slip and no-flux boundary conditions for given particular geometry of the problem. At the first step transient diffusion induced flows on a motionless body of perfect shape (plane, cylinder or sphere) are calculated and basic components of the flow are identified. A fine structure of velocity and density fields of the diffusion-induced flow is analyzed. Flows produced by uniformly moving obstacles are calculated in linear approximation taking into account viscosity effects. Analytical calculations reveals set of regular and singular disturbed components describing internal waves, wake and upstream disturbances if a wide ranges of flow parameters. Large number of experiments with different towing bodies in stratified tanks was performed in a wide range of flow parameters. Conditions of well mutual correspondence of analytical, numerical and experimental results are defined. Formation of transverse streaky structures on obstacles and inside the wake and their transformation firstly into sequence of clusters and then into vortex systems is observed. Formation of singular soaring interfaces inside internal wave fields past 2D obstacles ad their gradual transformation into vortices is visualized. Soaring interfaces looks like infinitesimal analogues of shock waves. Due to physics of their formation soaring interfaces are acting as attractors and collectors of contaminants and producing a great impact on redistribution of concentration of diffusive clouds. Data extrapolation on the environmental conditions is discussed.

  20. Model for nodal quasiparticle scattering in a disordered vortex lattice

    NASA Astrophysics Data System (ADS)

    Maltseva, Marianna; Coleman, P.

    2009-10-01

    Recent scanning-tunneling experiments on Ca2-xNaxCuO2Cl2 by Hanaguri [Science 323, 923 (2009)] observe field-dependent quasiparticle interference effects which are sensitive to the sign of the d -wave order parameter. Their analysis of spatial fluctuations in the local density of states shows that there is a selective enhancement of quasiparticle scattering events that preserve the gap sign and a selective depression of the quasiparticle scattering events that reverse the gap sign. We introduce a model which accounts for this phenomenon as a consequence of vortex pinning to impurities. Each pinned vortex embeds several impurities in its core. The observations of recent experiments can be accounted for by assuming that the scattering potentials of the impurities inside the vortex cores acquire an additional resonant or Andreev scattering component, both of which induce gap sign preserving scattering events.

  1. NASA Aircraft Vortex Spacing System Development Status

    NASA Technical Reports Server (NTRS)

    Hinton, David A.; Charnock, James K.; Bagwell, Donald R.; Grigsby, Donner

    1999-01-01

    The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements during instrument meteorological conditions through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations (RSO) subelement at the NASA Langley Research Center is developing an Aircraft VOrtex Spacing System (AVOSS). AVOSS will integrate the output of several systems to produce weather dependent, dynamic wake vortex spacing criteria. These systems provide current and predicted weather conditions, models of wake vortex transport and decay in these weather conditions, and real-time feedback of wake vortex behavior from sensors. The goal of the NASA program is to provide the research and development to demonstrate an engineering model AVOSS in real-time operation at a major airport. The demonstration is only of concept feasibility, and additional effort is required to deploy an operational system for actual aircraft spacing reduction. This paper describes the AVOSS system architecture, a wake vortex facility established at the Dallas-Fort Worth International Airport (DFW), initial operational experience with the AVOSS system, and emerging considerations for subsystem requirements. Results of the initial system operation suggest a significant potential for reduced spacing.

  2. Modeling and simulation of high-speed wake flows

    NASA Astrophysics Data System (ADS)

    Barnhardt, Michael Daniel

    High-speed, unsteady flows represent a unique challenge in computational hypersonics research. They are found in nearly all applications of interest, including the wakes of reentry vehicles, RCS jet interactions, and scramjet combustors. In each of these examples, accurate modeling of the flow dynamics plays a critical role in design performance. Nevertheless, literature surveys reveal that very little modern research effort has been made toward understanding these problems. The objective of this work is to synthesize current computational methods for high-speed flows with ideas commonly used to model low-speed, turbulent flows in order to create a framework by which we may reliably predict unsteady, hypersonic flows. In particular, we wish to validate the new methodology for the case of a turbulent wake flow at reentry conditions. Currently, heat shield designs incur significant mass penalties due to the large margins applied to vehicle afterbodies in lieu of a thorough understanding of the wake aerothermodynamics. Comprehensive validation studies are required to accurately quantify these modeling uncertainties. To this end, we select three candidate experiments against which we evaluate the accuracy of our methodology. The first set of experiments concern the Mars Science Laboratory (MSL) parachute system and serve to demonstrate that our implementation produces results consistent with prior studies at supersonic conditions. Second, we use the Reentry-F flight test to expand the application envelope to realistic flight conditions. Finally, in the last set of experiments, we examine a spherical capsule wind tunnel configuration in order to perform a more detailed analysis of a realistic flight geometry. In each case, we find that current 1st order in time, 2nd order in space upwind numerical methods are sufficiently accurate to predict statistical measurements: mean, RMS, standard deviation, and so forth. Further potential gains in numerical accuracy are demonstrated using a new class of flux evaluation schemes in combination with 2nd order dual-time stepping. For cases with transitional or turbulent Reynolds numbers, we show that the detached eddy simulation (DES) method holds clear advantage over heritage RANS methods. From this, we conclude that the current methodology is sufficient to predict heating of external, reentry-type applications within experimental uncertainty.

  3. A prescribed wake rotor inflow and flow field prediction analysis, user's manual and technical approach

    NASA Technical Reports Server (NTRS)

    Egolf, T. A.; Landgrebe, A. J.

    1982-01-01

    A user's manual is provided which includes the technical approach for the Prescribed Wake Rotor Inflow and Flow Field Prediction Analysis. The analysis is used to provide the rotor wake induced velocities at the rotor blades for use in blade airloads and response analyses and to provide induced velocities at arbitrary field points such as at a tail surface. This analysis calculates the distribution of rotor wake induced velocities based on a prescribed wake model. Section operating conditions are prescribed from blade motion and controls determined by a separate blade response analysis. The analysis represents each blade by a segmented lifting line, and the rotor wake by discrete segmented trailing vortex filaments. Blade loading and circulation distributions are calculated based on blade element strip theory including the local induced velocity predicted by the numerical integration of the Biot-Savart Law applied to the vortex wake model.

  4. Model for Vortex Ring State Influence on Rotorcraft Flight Dynamics

    NASA Technical Reports Server (NTRS)

    Johnson, Wayne

    2004-01-01

    The influence of vortex ring state (VRS) on rotorcraft flight dynamics is investigated, specifically the vertical velocity drop of helicopters and the roll-off of tiltrotors encountering VRS. The available wind tunnel and flight test data for rotors in vortex ring state are reviewed. Test data for axial flow, nonaxial flow, two rotors, unsteadiness, and vortex ring state boundaries are described and discussed. Based on the available measured data, a VRS model is developed. The VRS model is a parametric extension of momentum theory for calculation of the mean inflow of a rotor, hence suitable for simple calculations and real-time simulations. This inflow model is primarily defined in terms of the stability boundary of the aircraft motion. Calculations of helicopter response during VRS encounter were performed, and good correlation is shown with the vertical velocity drop measured in flight tests. Calculations of tiltrotor response during VRS encounter were performed, showing the roll-off behavior characteristic of tiltrotors. Hence it is possible, using a model of the mean inflow of an isolated rotor, to explain the basic behavior of both helicopters and tiltrotors in vortex ring state.

  5. Model for Vortex Ring State Influence on Rotorcraft Flight Dynamics

    NASA Technical Reports Server (NTRS)

    Johnson, Wayne

    2005-01-01

    The influence of vortex ring state (VRS) on rotorcraft flight dynamics is investigated, specifically the vertical velocity drop of helicopters and the roll-off of tiltrotors encountering VRS. The available wind tunnel and flight test data for rotors in vortex ring state are reviewed. Test data for axial flow, non-axial flow, two rotors, unsteadiness, and vortex ring state boundaries are described and discussed. Based on the available measured data, a VRS model is developed. The VRS model is a parametric extension of momentum theory for calculation of the mean inflow of a rotor, hence suitable for simple calculations and real-time simulations. This inflow model is primarily defined in terms of the stability boundary of the aircraft motion. Calculations of helicopter response during VRS encounter were performed, and good correlation is shown with the vertical velocity drop measured in flight tests. Calculations of tiltrotor response during VRS encounter were performed, showing the roll-off behavior characteristic of tiltrotors. Hence it is possible, using a model of the mean inflow of an isolated rotor, to explain the basic behavior of both helicopters and tiltrotors in vortex ring state.

  6. Modelling on cavitation in a diffuser with vortex generator

    NASA Astrophysics Data System (ADS)

    Jablonsk, J.

    2013-04-01

    Based on cavitation modelling in Laval nozzle results and experience, problem with the diffuser with vortex generator was defined. The problem describes unsteady multiphase flow of water. Different cavitation models were used when modelling in Fluent, flow condition is inlet and pressure condition is outlet. Boundary conditions were specified by Energy Institute, Victor Kaplan's Department of Fluid Engineering, Faculty of Mechanical Engineering, Brno University of Technology. Numerical modelling is compared with experiment.

  7. Effect of nacelle on wake meandering in a laboratory scale wind turbine using LES

    NASA Astrophysics Data System (ADS)

    Foti, Daniel; Yang, Xiaolei; Guala, Michele; Sotiropoulos, Fotis

    2015-11-01

    Wake meandering, large scale motion in the wind turbine wakes, has considerable effects on the velocity deficit and turbulence intensity in the turbine wake from the laboratory scale to utility scale wind turbines. In the dynamic wake meandering model, the wake meandering is assumed to be caused by large-scale atmospheric turbulence. On the other hand, Kang et al. (J. Fluid Mech., 2014) demonstrated that the nacelle geometry has a significant effect on the wake meandering of a hydrokinetic turbine, through the interaction of the inner wake of the nacelle vortex with the outer wake of the tip vortices. In this work, the significance of the nacelle on the wake meandering of a miniature wind turbine previously used in experiments (Howard et al., Phys. Fluid, 2015) is demonstrated with large eddy simulations (LES) using immersed boundary method with fine enough grids to resolve the turbine geometric characteristics. The three dimensionality of the wake meandering is analyzed in detail through turbulent spectra and meander reconstruction. The computed flow fields exhibit wake dynamics similar to those observed in the wind tunnel experiments and are analyzed to shed new light into the role of the energetic nacelle vortex on wake meandering. This work was supported by Department of Energy DOE (DE-EE0002980, DE-EE0005482 and DE-AC04-94AL85000), and Sandia National Laboratories. Computational resources were provided by Sandia National Laboratories and the University of Minnesota Supercomputing.

  8. PREFACE: Wake Conference 2015

    NASA Astrophysics Data System (ADS)

    Barney, Andrew; Nørkær Sørensen, Jens; Ivanell, Stefan

    2015-06-01

    The 44 papers in this volume constitute the proceedings of the 2015 Wake Conference, held in Visby on the island of Gotland in Sweden. It is the fourth time this conference has been held. The Wake Conference series started in Visby, where it was held in 2009 and 2011. In 2013 it took place in Copenhagen where it was combined with the International Conference on Offshore Wind Energy and Ocean Energy. In 2015 it is back where it started in Visby, where it takes place at Uppsala University Campus Gotland, June 9th-11th. The global yearly production of electrical energy by wind turbines has grown tremendously in the past decade and it now comprises more than 3% of the global electrical power consumption. Today the wind power industry has a global annual turnover of more than 50 billion USD and an annual average growth rate of more than 20%. State-of-the-art wind turbines have rotor diameters of up to 150 m and 8 MW installed capacity. These turbines are often placed in large wind farms that have a total production capacity corresponding to that of a nuclear power plant. In order to make a substantial impact on one of the most significant challenges of our time, global warming, the industry's growth has to continue for a decade or two yet. This in turn requires research into the physics of wind turbine wakes and wind farms. Modern wind turbines are today clustered in wind farms in which the turbines are fully or partially influenced by the wake of upstream turbines. As a consequence, the wake behind the wind turbines has a lower mean wind speed and an increased turbulence level, as compared to the undisturbed flow outside the farm. Hence, wake interaction results in decreased total production of power, caused by lower kinetic energy in the wind, and an increase in the turbulence intensity. Therefore, understanding the physical nature of the vortices and their dynamics in the wake of a turbine is important for the optimal design of a wind farm. This conference is aimed at scientists and PhD students working in the field of wake dynamics. The conference covers the following subject areas: Wake and vortex dynamics, instabilities in trailing vortices and wakes, simulation and measurements of wakes, analytical approaches for modeling wakes, wake interaction and other wind farm investigations. Many people have been involved in producing the 2015 Wake Conference proceedings. The work by the more than 60 reviewers ensuring the quality of the papers is greatly appreciated. The timely evaluation and coordination of the reviews would not have been possible without the work of the section editors: Christian Masson, ÉTS, Fernando Porté-Agel, EPFL, Gerard Schepers, ECN Wind Energy, Gijs Van Kuik, Delft University, Gunner Larsen, DTU Wind Energy, Jakob Mann, DTU Wind Energy, Javier Sanz Rodrigo, CENER, Johan Meyers, KU Leuven, Rebecca Barthelmie, Cornell University, Sandrine Aubrun-Sanches, Université d'Orléans and Thomas Leweke, IRPHE-CNRS. We are also immensely indebted to the very responsive support from the editorial team at IOP Publishing, especially Sarah Toms, during the review process of these proceedings. Visby, Sweden, June 2015 Andrew Barney, Jens Nørkær Sørensen and Stefan Ivanell Uppsala University - Campus Gotland

  9. Numerical Analysis of Tip Cavitation on Marine Propeller with Wake Alignment Using a Simple Surface Panel Method “SQCM”

    NASA Astrophysics Data System (ADS)

    Kanemaru, T.; Ando, J.

    2015-12-01

    This paper presents the calculation method of tip cavitation with wake alignment. Tip cavitation consists of tip vortex cavitation and tip super cavitation which means the undeveloped and local super cavitation around blade tip. The feature of this study is that the method applies the wake alignment model in order to express the realistic phenomena of tip cavitation and predict the pressure fluctuation more accurately. In the present method, the wake sheet is deformed according to the induced velocity vector on the vortex lines. The singularity of the potential vortex can be removed by using the Rankine Vortex model. This paper shows the calculated results regarding cavitation pattern, pressure fluctuation etc. comparing with published experimental data and calculated results without wake alignment.

  10. A Reduced Order Model for Wake Surface-Wave Interaction

    NASA Astrophysics Data System (ADS)

    Pauley, Laura; Mehdizadeh, Amir

    2015-11-01

    Surface waves can change the radar and optical signatures initially produced by the wake of a towed or self-propelled object on the sea surface. To date, most investigations considered the effects of the wake on the surface waves. Here our intention is to study how surface waves affect the wake. The wake of a moving object can persist thousands of diameters downstream. Due to the extensive domain, a reduced order method (2D+t) is often used to sweep downstream through the wake development. The 2D+t computation approximates the development of the wake at a fixed location as an object moves past but applies cyclical boundary conditions in the streamwise direction. A modified (Parabolized) Navier-Stokes (PNS) method has the same numerical efficiency as the classical 2D+t method but includes additional streamwise gradient terms derived in the transformation from a moving reference frame to a fixed reference frame. The present paper aims to assess the capability of the 2D+t and PNS methods for a laminar/turbulent wake interacting with a surface wave described by the Stokes drift velocity distribution. Results from 3D simulations will be used for validation to find criteria where 2D+t and PNS methods deliver results with an acceptable level of accuracy.

  11. Wake Turbulence Mitigation for Arrivals (WTMA)

    NASA Technical Reports Server (NTRS)

    Williams, Daniel M.; Lohr, Gary W.; Trujillo, Anna C.

    2008-01-01

    The preliminary Wake Turbulence Mitigation for Arrivals (WTMA) concept of operations is described in this paper. The WTMA concept provides further detail to work initiated by the Wake Vortex Avoidance System Concept Evaluation Team and is an evolution of the Wake Turbulence Mitigation for Departure concept. Anticipated benefits about reducing wake turbulence separation standards in crosswind conditions, and candidate WTMA system considerations are discussed.

  12. Rotor Vortex Filaments: Living on the Slipstream's Edge

    NASA Technical Reports Server (NTRS)

    Young, Larry A.

    1997-01-01

    The purpose of this paper is to gain a better understanding of rotor wake evolution in hover and axial flow by deriving an analytical solution for the time dependent behavior of vortex filament circulation and core size. This solution is applicable only for vortex filaments in the rotor far-wake. A primarily inviscid vortex/shear layer interaction (where the slipstream boundary is modeled as a shear layer) has been identified in this analytical treatment. This vortex/shear layer interaction results in decreasing, vortex filament circulation and core size with time. The inviscid vortex/shear layer interaction is shown, in a first-order treatment, to be of greater magnitude than viscous diffusion effects. The rate of contraction, and ultimate collapse, of the vortex filament core is found to be directly proportional to the rotor inflow velocity. This new insight into vortex filament decay promises to help reconcile several disparate observations made in the literature and will, hopefully, promote new advances in theoretical modeling of rotor wakes.

  13. A Physiologically Based Model of Orexinergic Stabilization of Sleep and Wake

    PubMed Central

    Fulcher, Ben D.; Phillips, Andrew J. K.; Postnova, Svetlana; Robinson, Peter A.

    2014-01-01

    The orexinergic neurons of the lateral hypothalamus (Orx) are essential for regulating sleep-wake dynamics, and their loss causes narcolepsy, a disorder characterized by severe instability of sleep and wake states. However, the mechanisms through which Orx stabilize sleep and wake are not well understood. In this work, an explanation of the stabilizing effects of Orx is presented using a quantitative model of important physiological connections between Orx and the sleep-wake switch. In addition to Orx and the sleep-wake switch, which is composed of mutually inhibitory wake-active monoaminergic neurons in brainstem and hypothalamus (MA) and the sleep-active ventrolateral preoptic neurons of the hypothalamus (VLPO), the model also includes the circadian and homeostatic sleep drives. It is shown that Orx stabilizes prolonged waking episodes via its excitatory input to MA and by relaying a circadian input to MA, thus sustaining MA firing activity during the circadian day. During sleep, both Orx and MA are inhibited by the VLPO, and the subsequent reduction in Orx input to the MA indirectly stabilizes sustained sleep episodes. Simulating a loss of Orx, the model produces dynamics resembling narcolepsy, including frequent transitions between states, reduced waking arousal levels, and a normal daily amount of total sleep. The model predicts a change in sleep timing with differences in orexin levels, with higher orexin levels delaying the normal sleep episode, suggesting that individual differences in Orx signaling may contribute to chronotype. Dynamics resembling sleep inertia also emerge from the model as a gradual sleep-to-wake transition on a timescale that varies with that of Orx dynamics. The quantitative, physiologically based model developed in this work thus provides a new explanation of how Orx stabilizes prolonged episodes of sleep and wake, and makes a range of experimentally testable predictions, including a role for Orx in chronotype and sleep inertia. PMID:24651580

  14. Optimal two-dimensional models for wake flows

    NASA Astrophysics Data System (ADS)

    Balachandar, S.; Najjar, F. M.

    2001-01-01

    In the case of nominally two-dimensional (2D) cylinders of arbitrary cross section in cross flow, the three-dimensionality of the wake manifests in the form of quasi-streamwise vortices. These three-dimensional (3D) features profoundly influence lift and drag forces. However, a two-dimensional projection of such a flow, where the effects of three-dimensionality are modeled, will be computationally very attractive. One can consider the two-dimensional projection as the limiting case of large eddy simulation, where the spanwise direction has been completely averaged out. The transport equation for the span-averaged spanwise component of vorticity, ω¯z, is considered; the 3D effects to be modeled appear as a subgrid scale flux of torque. It is shown that simple minded eddy viscosity type models that assume the flux vector to be proportional to the spatial gradient of ω¯z are inadequate. Here we extend the optimal modeling formalism [Moser, Balachandar, and Adrian, Turbulence and Internal Flow/Unsteady Aerodynamics and Hypersonics Conference, Annapolis, MD, pp. 269-274 (1998); Langford and Moser, J. Fluid Mech. 398, 321 (1999)] to address issues pertaining to complex flows with multiple directions of inhomogeneity. We present optimal closures for subgrid flux modeled in terms of ω¯z distribution, based on linear and quadratic stochastic approximations. These ideas are tested using the database of flow over a flat plate held normal to a cross flow. It is observed that even the optimal model has about 70% normalized error, indicating that the subgrid flux is only about 30% deterministic. Furthermore, it is observed that local models are inadequate, but there exists a region of nonlocality for model dependence, expanding beyond which does not improve the estimate. Higher order nonlinearities however do not seem to improve the model's predictability.

  15. Remote measurement utilizing NASA's scanning laser Doppler systems. Volume 1. Laser Doppler wake vortex tracking at Kennedy Airport

    NASA Technical Reports Server (NTRS)

    Krause, M. C.; Wilson, D. J.; Howle, R. E.; Edwards, B. B.; Craven, C. E.; Jetton, J. L.

    1976-01-01

    Test operations of the Scanning Laser Doppler System (SLDS) at Kennedy International Airport (KIA) during August 1974 through June 1975 are reported. A total of 1,619 data runs was recorded with a totally operational system during normal landing operations at KIA. In addition, 53 data runs were made during cooperative flybys with the C880 for a grand total of 1672 recorded vortex tracks. Test crews were in attendance at KIA for 31 weeks, of which 25 weeks were considered operational and the other six were packing, unpacking, setup and check out. Although average activity equates to 67 recorded landing operations per week, two periods of complete runway inactivity spanned 20 days and 13 days, respectively. The operation frequency therefore averaged about 88 operations per week.

  16. Contrail Formation in Aircraft Wakes Using Large-Eddy Simulations

    NASA Technical Reports Server (NTRS)

    Paoli, R.; Helie, J.; Poinsot, T. J.; Ghosal, S.

    2002-01-01

    In this work we analyze the issue of the formation of condensation trails ("contrails") in the near-field of an aircraft wake. The basic configuration consists in an exhaust engine jet interacting with a wing-tip training vortex. The procedure adopted relies on a mixed Eulerian/Lagrangian two-phase flow approach; a simple micro-physics model for ice growth has been used to couple ice and vapor phases. Large eddy simulations have carried out at a realistic flight Reynolds number to evaluate the effects of turbulent mixing and wake vortex dynamics on ice-growth characteristics and vapor thermodynamic properties.

  17. Wingtip Vortex-Augmented Turbopusher Propeller Thrust

    NASA Technical Reports Server (NTRS)

    Patterson, J. C., Jr.

    1985-01-01

    Thrust of propeller enhanced by tip vortex. Wingtip-Mounted Nacelle provides turboprop vortex velocity recovery. Thrust of turbopusher propeller increased by flow of lift-induced vortex. As result of weaker vortex, reduction in induced drag of wing afforded by propeller-wake mass injection into core of vortex, causing it to break down.

  18. Vortex sheet modeling with higher order curved panels. Ph.D Thesis Final Technical Report

    NASA Technical Reports Server (NTRS)

    Nagati, M. G.

    1985-01-01

    A numerical technique is presented for modeling the vortex sheet with a deformable surface definition, along which a continuous vortex strength distribution in the spanwise direction is applied, so that by repeatedly modifying its shape, its true configuration is approached, in the proximity of its generating wing. Design problems requiring the inclusion of a realistic configuration of the vortex sheet are numerous. Examples discussed include: control effectiveness and stability derivatives, longitudinal stability, lateral stability, canards, propellers and helicopter rotors, and trailing vortex hazards.

  19. LCS analysis of a biologically inspired wake

    NASA Astrophysics Data System (ADS)

    Green, Melissa; Smits, Alexander

    2008-11-01

    Particle Image Velocimetry (PIV) was used to investigate the wakes of rigid pitching panels with a trapezoidal panel geometry, chosen to model idealized fish caudal fins. Experiments were performed for Strouhal numbers from 0.23 to 0.65. The three dimensional flow field around the panel is reconstructed by integrating two-dimensional PIV results across the volume surrounding the panel. A Lagrangian coherent structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake. A classic reverse von Kármán vortex street pattern was observed along the mid-span of the near wake, but the complexity and three-dimensionality of the wake increases away from the mid-span as streamwise vortices interact with the swept edges of the panel.

  20. A model of the ion wake of Mars

    NASA Technical Reports Server (NTRS)

    Luhmann, J. G.

    1990-01-01

    Observations from the Phobos spacecraft suggest that, for the most part, the Martian magnetotail is induced instead of intrinsic. Test particle techniques are used to determine how the associated ion wake might appear to detectors behind Mars if it is produced primarily by planetary ions picked up by the solar wind in the dayside exosphere. The results suggest that low energy pickup ions populate the inner wake, while a more energetic component is present in the outer magnetosheath and adjacent solar wind.

  1. Fractional vortex molecules and vortex polygons in a baby Skyrme model

    NASA Astrophysics Data System (ADS)

    Kobayashi, Michikazu; Nitta, Muneto

    2013-06-01

    We construct a molecule of fractional vortices with fractional topological lump charges as a baby Skyrmion with the unit topological lump charge in the antiferromagnetic (or XY) baby Skyrme model, that is, an O(3) sigma model with a four-derivative term and an antiferromagnetic or XY-type potential term quadratic in fields. We further construct configurations with topological lump charges Q≤7 and find that bound states of vortex molecules constitute regular polygons with 2Q vertices as vortices, where the rotational symmetry SO(2) in real space is spontaneously broken into a discrete subgroup ZQ. We also find metastable and arrayed bound states of fractional vortices for Q=5, 6. On the other hand, we find for Q=7 that the regular polygon is metastable and the arrayed bound state is stable. We calculate binding energies of all configurations.

  2. Site Suitability Assessment with Dynamic Wake Meandering Model. A Certification Point of View.

    NASA Astrophysics Data System (ADS)

    Tomas Bayo, Ricard; Parro, Gema

    2015-04-01

    Establishment of large wind farms requires enormous investments putting steadily greater emphasis on optimal topology design and control of these. This requires not only an optimization of the power output, but also the development of strategies to cope with the higher loading expected. The cornerstone of such strategies is a realistic characterization and modelling of the wake flow field inside the wind farm, beyond Frandsen's equivalent turbulence method. Whereas Frandsen model has been mostly considered in the industry so far, it has not proved completely satisfactory when facing current problems such as wake effects on turbines placed at short distances or consequences of half wake for turbine loading. The objective of the present work is to address these questions from a certification point of view within the framework of Risoe's Dynamic Wake Meandering (DWM) model. The DWM model is based on the combination of three parts: modeling of quasi-steady wake deficits, a stochastic model of the downwind wake meandering and an added or self-generated wake turbulence. The analysis carried out is two-fold: First, a comparative study of the wake effects generated in Frandsen model as well as in various realizations of the DWM model is performed. For this purpose wake-induced loads are calculated using two different aeroelastic codes: HAWC2 and Bladed. Second, the applicability of DWM for the assessment of wind turbines under site-specific conditions is discussed and the conclusions summarized in a Recommended Practice. Clear prescriptions are thereby provided for the use of DWMM for site suitability assessments, including the aforementioned extreme situations, along with the interpretation of the future version of the IEC 61400-1 standards.

  3. The Vortex Lattice Method for the Rotor-Vortex Interaction Problem

    NASA Technical Reports Server (NTRS)

    Padakannaya, R.

    1974-01-01

    The rotor blade-vortex interaction problem and the resulting impulsive airloads which generate undesirable noise levels are discussed. A numerical lifting surface method to predict unsteady aerodynamic forces induced on a finite aspect ratio rectangular wing by a straight, free vortex placed at an arbitrary angle in a subsonic incompressible free stream is developed first. Using a rigid wake assumption, the wake vortices are assumed to move downsteam with the free steam velocity. Unsteady load distributions are obtained which compare favorably with the results of planar lifting surface theory. The vortex lattice method has been extended to a single bladed rotor operating at high advance ratios and encountering a free vortex from a fixed wing upstream of the rotor. The predicted unsteady load distributions on the model rotor blade are generally in agreement with the experimental results. This method has also been extended to full scale rotor flight cases in which vortex induced loads near the tip of a rotor blade were indicated. In both the model and the full scale rotor blade airload calculations a flat planar wake was assumed which is a good approximation at large advance ratios because the downwash is small in comparison to the free stream at large advance ratios. The large fluctuations in the measured airloads near the tip of the rotor blade on the advance side is predicted closely by the vortex lattice method.

  4. Analysis of Tip Vortices Identified in the Instantaneous Wake of a Horizontal-Axis Model Wind Turbine Placed in a Turbulent Boundary Layer

    NASA Astrophysics Data System (ADS)

    Jain, Akash; Mehdi, Faraz; Sheng, Jian

    2014-11-01

    The near-wake field, a short region characterized by the physical specifications of a turbine, is of particular interest for flow-structure interactions responsible for asymmetric loadings, premature structural breakdown, noise generation etc. Helical tip vortices constitute a distinctive feature of this region and are dependent not only on the turbine geometry but also on the incoming flow profile. High-spatial resolution PIV measurements are made in the wake of a horizontal-axis model wind turbine embedded in a neutrally stratified turbulent boundary layer. The data is acquired over consecutive locations up to 10 diameters downstream of the turbine but the focus here is on the tip vortices identified in the instantaneous fields. Contrary to previous studies, both top and bottom tip vortices are clearly distinguishable in either ensemble fields or instantaneous realizations. The streamwise extent of these vortices stretches from the turbine till they merge into the expanding mid-span wake. The similarities and differences in the top and bottom tip vortices are explored through the evolution of their statistics. In particular, the distributions of the loci of vortex cores and their circulations are compared. The information will improve our understanding of near wake vortical dynamics, provide data for model validation, and aid in the devise of flow control strategies.

  5. Influence of pitch motion on the turbulent mixing in the wake of floating wind turbine models

    NASA Astrophysics Data System (ADS)

    Rockel, Stanislav; Peinke, Joachim; Hoelling, Michael; Cal, Raúl Bayoán

    2014-11-01

    Offshore wind turbines use fixed foundations, which are economical in shallow water up to a depth of 50m. For deeper water areas floating support structures are feasible alternatives. The added degrees of freedom of a floating platform introduce additional oscillations to the wind turbine and therefore influence the aerodynamics at the rotor and its wake, respectively. The influence of platform pitch motion on the wake of an upstream wind turbine and a turbine positioned in the wake is investigated. Wind tunnel experiments were performed using classical bottom fixed wind turbine models and turbines in free pitch motion. Using 2D-3C particle image elocimetry (SPIV), wakes of both turbines were measured. In both cases - fixed and pitching - the inflow conditions were kept constant. The differences in the turbulent quantities of the wake of the upwind turbine for the fixed and oscillating case are investigated and their influence the wake of the downwind turbine. Our results show that platform pitch and oscillatory motions of the wind turbine have a strong impact on the shape of the fluctuating components of the wake. Also the turbulent mixing is changed by the oscillations, which is transferred to statistical quantities of higher order in the wake of the downwind turbine.

  6. Linear stability of the Moore-Saffman model for a trailing wingtip vortex

    NASA Astrophysics Data System (ADS)

    Feys, Jan; Maslowe, Sherwin A.

    2014-02-01

    This paper presents an investigation of the stability of a trailing vortex using mean flow profiles given by an approximate solution of the Navier-Stokes equations. The axial and tangential velocity profiles obtained from this solution, deduced by Moore and Saffman ["Axial flow in laminar trailing vortices," Proc. R. Soc. London, Ser. A 333, 491-508 (1973)], agree well with experiments involving wings at slight angles of attack. In particular, the Moore-Saffman profiles better describe the jet-like and wake-like axial flows near the center of the vortex than does the much-studied Batchelor vortex. We determine solutions numerically for these profiles and find that they are well suited to describe the flow at short and intermediate distances behind the wingtip. Growth rates for unstable perturbations are presented for different values of n, the wingtip loading parameter. These growth rates are shown to be somewhat larger than those obtained for the Batchelor vortex, and instability persists for larger values of the swirl. The largest amplification rates were found to occur near n = 0.5, the value corresponding to elliptic loading. This is within the range 0.44 < n < 1.0, where the core axial flow is wake-like. For n < 0.44, the flow in the vortex core is jet-like and the growth rates of unstable perturbations become progressively smaller, with all modes damped for n ≈ 0.25.

  7. A cockpit-display concept for executing a multiple glide-slope approach for wake-vortex avoidance

    NASA Technical Reports Server (NTRS)

    Abbott, T. S.

    1984-01-01

    A piloted simulation study was undertaken to determine the feasibility of utilizing a forward-looking display to provide information that would enable aircraft to rredue their in-trail separation interval, and hence increase airport capacity, through the application of multiple glide-path approach techniques. The primary objective of this study was to determine whether information could be satisfactorily provided on a head-up display (HUD) format to permit the pilot to conduct a multiple glide-slope approach while maintaining a prespecified in-trail separation interval. The tests were conducted in a motion-base cockpit simulator configured as a current-generation transport aircraft and included dynamic effects of the vortices generated by the lead aircraft. The information provided on the HUD included typical aircraft guidance information and the current and past positions of the lead aircraft. Additionally, the displayed information provided self-separation cues that allowed the pilot to maintain separation on the lead aircraft. Performance data and pilot subjective ratings and comments were obtained during the tests. The results of this study indicate that multiple glide-slope approaches, procedurally designed for vortex avoidance, are possible while maintaining pilot work load and performance within operationally acceptable limits. In general, it would seem that multiple glide-slope approaches are possible even under reduced in-trail separation conditions if the pilot is provided with adequate situational information.

  8. Model of Pressure Distribution in Vortex Flow Controls

    NASA Astrophysics Data System (ADS)

    Mielczarek, Szymon; Sawicki, Jerzy M.

    2015-06-01

    Vortex valves belong to the category of hydrodynamic flow controls. They are important and theoretically interesting devices, so complex from hydraulic point of view, that probably for this reason none rational concept of their operation has been proposed so far. In consequence, functioning of vortex valves is described by CFD-methods (computer-aided simulation of technical objects) or by means of simple empirical relations (using discharge coefficient or hydraulic loss coefficient). Such rational model of the considered device is proposed in the paper. It has a simple algebraic form, but is well grounded physically. The basic quantitative relationship, which describes the valve operation, i.e. dependence between the flow discharge and the circumferential pressure head, caused by the rotation, has been verified empirically. Conformity between calculated and measured parameters of the device allows for acceptation of the proposed concept.

  9. Dynamic parameters in models of atmospheric vortex structures

    NASA Astrophysics Data System (ADS)

    Dobryshman, E. M.; Kochina, V. G.; Letunova, T. A.

    2013-09-01

    Vortex simulation and the computation of fields of dynamic parameters of vortex structures (velocity, rotor velocity, and helicity) are carried out with the use of exact hydrodynamic equations in a cylindrical coordinate system. Components of centripetal and Coriolis accelerations are taken into account in the initial equations. Internal and external solutions are defined. Internal solutions ignore the disturbances of the pressure field, but they are considered in external solutions. The simulation is carried out so that the effect of accounting for spatial coordinates on the structure of the above fields is pronounced. It is shown that the initial kinetic energy of rotating motion transforms into the kinetic energy of radial and vertical velocity components in models with centripetal acceleration. In models with Coriolis acceleration, the Rossby effect is clearly pronounced. The method of an "inverse problem" is used for finding external solutions, i.e., reconstruction of the pressure field at specified velocity components. Computations have shown that tangential components mainly contribute to the velocity and helicity vortex moduli at the initial stage.

  10. Direct numerical simulations and modeling of a spatially-evolving turbulent wake

    NASA Astrophysics Data System (ADS)

    Cimbala, John M.

    1994-12-01

    Understanding of turbulent free shear flows (wakes, jets, and mixing layers) is important, not only for scientific interest, but also because of their appearance in numerous practical applications. Turbulent wakes, in particular, have recently received increased attention by researchers at NASA Langley. The turbulent wake generated by a two-dimensional airfoil has been selected as the test-case for detailed high-resolution particle image velocimetry (PIV) experiments. This same wake has also been chosen to enhance NASA's turbulence modeling efforts. Over the past year, the author has completed several wake computations, while visiting NASA through the 1993 and 1994 ASEE summer programs, and also while on sabbatical leave during the 1993-94 academic year. These calculations have included two-equation (K-omega and K-epsilon) models, algebraic stress models (ASM), full Reynolds stress closure models, and direct numerical simulations (DNS). Recently, there has been mutually beneficial collaboration of the experimental and computational efforts. In fact, these projects have been chosen for joint presentation at the NASA Turbulence Peer Review, scheduled for September 1994. DNS calculations are presently underway for a turbulent wake at Re(sub theta) = 1000 and at a Mach number of 0.20. (Theta is the momentum thickness, which remains constant in the wake of a two dimensional body.) These calculations utilize a compressible DNS code written by M. M. Rai of NASA Ames, and modified for the wake by J. Cimbala. The code employs fifth-order accurate upwind-biased finite differencing for the convective terms, fourth-order accurate central differencing for the viscous terms, and an iterative-implicit time-integration scheme. The computational domain for these calculations starts at x/theta = 10, and extends to x/theta = 610. Fully developed turbulent wake profiles, obtained from experimental data from several wake generators, are supplied at the computational inlet, along with appropriate noise. After some adjustment period, the flow downstream of the inlet develops into a fully three-dimensional turbulent wake. Of particular interest in the present study is the far wake spreading rate and the self-similar mean and turbulence profiles. At the time of this writing, grid resolution studies are underway, and a code is being written to calculate turbulence statistics from these wake calculations; the statistics will be compared to those from the ongoing PIV wake measurements, those of previous experiments, and those predicted by the various turbulence models. These calculations will lead to significant long-term benefits for the turbulence modeling effort. In particular, quantities such as the pressure-strain correlation and the dissipation rate tensor can be easily calculated from the DNS results, whereas these quantities are nearly impossible to measure experimentally. Improvements to existing turbulence models (and development of new models) require knowledge about flow quantities such as these. models do a very good job at prediction of the shape of the mean velocity and Reynolds stress profiles in a turbulent wake, but significantly underpredict the magnitude of the stresses and the spreading rate of the wake. &Thus, the turbulent wake is an ideal flow for turbulence modeling research. By careful comparison and analysis of each term in the modeled Reynolds stress equations, the DNS data can show where deficiencies in the models exist; improvements to the models can then be attempted.

  11. Direct numerical simulations and modeling of a spatially-evolving turbulent wake

    NASA Technical Reports Server (NTRS)

    Cimbala, John M.

    1994-01-01

    Understanding of turbulent free shear flows (wakes, jets, and mixing layers) is important, not only for scientific interest, but also because of their appearance in numerous practical applications. Turbulent wakes, in particular, have recently received increased attention by researchers at NASA Langley. The turbulent wake generated by a two-dimensional airfoil has been selected as the test-case for detailed high-resolution particle image velocimetry (PIV) experiments. This same wake has also been chosen to enhance NASA's turbulence modeling efforts. Over the past year, the author has completed several wake computations, while visiting NASA through the 1993 and 1994 ASEE summer programs, and also while on sabbatical leave during the 1993-94 academic year. These calculations have included two-equation (K-omega and K-epsilon) models, algebraic stress models (ASM), full Reynolds stress closure models, and direct numerical simulations (DNS). Recently, there has been mutually beneficial collaboration of the experimental and computational efforts. In fact, these projects have been chosen for joint presentation at the NASA Turbulence Peer Review, scheduled for September 1994. DNS calculations are presently underway for a turbulent wake at Re(sub theta) = 1000 and at a Mach number of 0.20. (Theta is the momentum thickness, which remains constant in the wake of a two dimensional body.) These calculations utilize a compressible DNS code written by M. M. Rai of NASA Ames, and modified for the wake by J. Cimbala. The code employs fifth-order accurate upwind-biased finite differencing for the convective terms, fourth-order accurate central differencing for the viscous terms, and an iterative-implicit time-integration scheme. The computational domain for these calculations starts at x/theta = 10, and extends to x/theta = 610. Fully developed turbulent wake profiles, obtained from experimental data from several wake generators, are supplied at the computational inlet, along with appropriate noise. After some adjustment period, the flow downstream of the inlet develops into a fully three-dimensional turbulent wake. Of particular interest in the present study is the far wake spreading rate and the self-similar mean and turbulence profiles. At the time of this writing, grid resolution studies are underway, and a code is being written to calculate turbulence statistics from these wake calculations; the statistics will be compared to those from the ongoing PIV wake measurements, those of previous experiments, and those predicted by the various turbulence models. These calculations will lead to significant long-term benefits for the turbulence modeling effort. In particular, quantities such as the pressure-strain correlation and the dissipation rate tensor can be easily calculated from the DNS results, whereas these quantities are nearly impossible to measure experimentally. Improvements to existing turbulence models (and development of new models) require knowledge about flow quantities such as these. Present turbulence models do a very good job at prediction of the shape of the mean velocity and Reynolds stress profiles in a turbulent wake, but significantly underpredict the magnitude of the stresses and the spreading rate of the wake. Thus, the turbulent wake is an ideal flow for turbulence modeling research. By careful comparison and analysis of each term in the modeled Reynolds stress equations, the DNS data can show where deficiencies in the models exist; improvements to the models can then be attempted.

  12. The wake of hovering flight in bats.

    PubMed

    Håkansson, Jonas; Hedenström, Anders; Winter, York; Johansson, L Christoffer

    2015-08-01

    Hovering means stationary flight at zero net forward speed, which can be achieved by animals through muscle powered flapping flight. Small bats capable of hovering typically do so with a downstroke in an inclined stroke plane, and with an aerodynamically active outer wing during the upstroke. The magnitude and time history of aerodynamic forces should be reflected by vorticity shed into the wake. We thus expect hovering bats to generate a characteristic wake, but this has until now never been studied. Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover at a feeder and using time-resolved stereoscopic particle image velocimetry in conjunction with high-speed kinematic analysis we show that hovering nectar-feeding bats produce a series of bilateral stacked vortex loops. Vortex visualizations suggest that the downstroke produces the majority of the weight support, but that the upstroke contributes positively to the lift production. However, the relative contributions from downstroke and upstroke could not be determined on the basis of the wake, because wake elements from down- and upstroke mix and interact. We also use a modified actuator disc model to estimate lift force, power and flap efficiency. Based on our quantitative wake-induced velocities, the model accounts for weight support well (108%). Estimates of aerodynamic efficiency suggest hovering flight is less efficient than forward flapping flight, while the overall energy conversion efficiency (mechanical power output/metabolic power) was estimated at 13%. PMID:26179990

  13. Modeling for Airframe Noise Prediction Using Vortex Methods

    NASA Astrophysics Data System (ADS)

    Zheng, Z. Charlie

    2002-12-01

    Various components of the airframe are known to be a significant source of noise. With the advent of technology in quieting modern engines, airframe generated noise competes and, in certain instances, surpasses the engine noise. Airframe noise is most pronounced during aircraft approach when the engines are operating at reduced thrust, and airframe components such as high-lift devices and landing gears are in deployed conditions. Recent experimental studies have reaffirmed that the most significant sources of high-lift noise are from the leading-edge slat and the side edges of flaps. Studies of flow field around these structures have consistently shown that there are complicated unsteady vortical flows such as vortex shedding, secondary vortices and vortex breakdown, which are susceptible to far-field radiated sound. The near-field CFD computational data have been used to calculate the far-field acoustics by employing Ffowcs-Williams/Hawkings equation using Lighthill's analogy. However, because of the limit of current computing capacity, it is very time consuming to generate unsteady Navier-Stokes (N-S) computational data for aeroacoustics. Although the N-S simulations are probably necessary to reveal many complex flow phenomena that are unsteady and fully nonlinear, these simulations are not feasible to be used for parametric design. purposes. The objective of this study is thus to develop theoretical models for airframe noise predictions which have quick turn-around computing time. Since it is known that vorticity is a major mechanism responsible for noise generation on high-lift devices, vortex methods have been chosen as modeling tools. Vortex methods are much faster in comparison with other numerical methods, yet they are able to incorporate nonlinear interactions between vortices. Obviously, as with any theoretical model, assumptions have to be made and justified when such models are used in complex flow. The merit and applicability of the models for aeroacoustics applications will be investigated. Issues related with conservation of vorticity, unsteady Kutta conditions and nonlinear frequency response to vortex shedding will be addressed.

  14. Model for particle balance in pumped divertors (pre-VORTEX)

    SciTech Connect

    Hogan, J.T.

    1990-08-01

    An internally consistent model for particle transport in an open divertor geometry has been developed. Embodied in a new code, pre-VORTEX, the model couples the particle balance in the plasma core, the scrape-off layer, the open divertor channels, and the vacuum'' regions. This mutual coupling is particularly important in determining the conditions required for high recycling in the divertor. The plasma core is considered to have a relatively quiescent core region and a less well confined edge-localized mode''(ELM) region. The scrape-off layer is modeled with one-dimensional parallel and perpendicular transport. A two-point divertor channel model is used; it is similar to previous models, but with the addition of new physical processes: hydrogen charge exchange, impurity thermal charge exchange, and flux-limited parallel transport. Wall recycling data are required to describe the differing recycling properties of the wall regions and the divertor plates. Given local plasma diffusivities and wall recycling properties, the model predicts the volume-averaged density and global particle confinement time. The input data are uncertain, and a major use for the model is to permit comparison with data. The final model, VORTEX, is intended for application to the analysis of divertor confinement experiments; it is coupled to a one-and-one-half--dimensional transport code and uses detailed geometric input from equilibrium fitting codes, experimentally measured core profiles, and such parameters as can be measured in the scrape-off layer. The pre-VORTEX model is compared as a stand-alone code with typical data from the DIII-D experiment and applied to the proposed DIII-D Advanced Divertor Project.

  15. Experimental and theoretical Doppler-lidar signatures of aircraft wake vortices

    NASA Technical Reports Server (NTRS)

    Koepp, F.; Krichbaumer, W. A.

    1986-01-01

    The DFVLR laser Doppler anemometer is a CO2 continuous wave homodyne system designed for boundary layer wind measurements. During the last three years, it was mainly used in the wake-vortex program at Frankfurt airport for determination of vortex strength, transport, and lifetime. The strategy for that special type of measurement was previously reported in detail along with single experimental results. Therefore, herein is given a short summary of the data concerning questions of air traffic control. In addition to the experimental activities a computer model describing wake-vortex behavior was installed. It allows the comparison of the measured data with the hydrodynamically predicted quantities. On the other hand, it leads to an improved procedure for future wake-vortex measurements.

  16. Three-dimensional wake of a biologically-inspired propulsor

    NASA Astrophysics Data System (ADS)

    Green, Melissa; Rowley, Clarence; Smits, Alexander

    2009-11-01

    Digital Particle Image Velocimetry (DPIV) was used to investigate the wakes of rigid pitching panels with a trapezoidal planform geometry, chosen to model idealized fish caudal fins. Experiments were performed for Strouhal numbers of 0.17 and 0.23. The three-dimensional unsteady vortex wake downstream of the panel trailing edge was visualized using spatially- and temporally-resolved two-component data. A Lagrangian Coherent Structure (LCS) analysis was employed in addition to Eulerian vortex identification criteria to investigate the generation and evolution of the wake. A reverse von K'arm'an vortex street pattern was observed near the mid-span immediately downstream of the panel trailing edge, but the complexity and three-dimensionality of the wake increases away from the mid-span as streamwise vortices interact with the swept edges of the panel. Farther downstream of the trailing edge, the wake was observed to shrink in the spanwise direction at both Strouhal numbers. In addition, a quantitative bifurcation in the LCS coincided with a qualitative transition of the wake structure observed with increasing Strouhal number.

  17. ENDOW (efficient development of offshore wind farms): modelling wake and boundary layer interactions

    NASA Astrophysics Data System (ADS)

    Barthelmie, Rebecca; Larsen, Gunner; Pryor, Sara; Jørgensen, Hans; Bergström, Hans; Schlez, Wolfgang; Rados, Kostas; Lange, Bernhard; Vølund, Per; Neckelmann, Søren; Mogensen, Søren; Schepers, Gerard; Hegberg, Terry; Folkerts, Luuk; Magnusson, Mikael

    2004-07-01

    While experience gained through the offshore wind energy projects currently operating is valuable, a major uncertainty in estimating power production lies in the prediction of the dynamic links between the atmosphere and wind turbines in offshore regimes. The objective of the ENDOW project was to evaluate, enhance and interface wake and boundary layer models for utilization offshore. The project resulted in a significant advance in the state of the art in both wake and marine boundary layer models, leading to improved prediction of wind speed and turbulence profiles within large offshore wind farms. Use of new databases from existing offshore wind farms and detailed wake profiles collected using sodar provided a unique opportunity to undertake the first comprehensive evaluation of wake models in the offshore environment. The results of wake model performance in different wind speed, stability and roughness conditions relative to observations provided criteria for their improvement. Mesoscale model simulations were used to evaluate the impact of thermal flows, roughness and topography on offshore wind speeds. The model hierarchy developed under ENDOW forms the basis of design tools for use by wind energy developers and turbine manufacturers to optimize power output from offshore wind farms through minimized wake effects and optimal grid connections. The design tools are being built onto existing regional-scale models and wind farm design software which was developed with EU funding and is in use currently by wind energy developers. Copyright

  18. Aerodynamic interaction between vortical wakes and lifting two-dimensional bodies

    NASA Technical Reports Server (NTRS)

    Stremel, Paul M.

    1989-01-01

    Unsteady rotor wake interactions with the empennage, tail boom, and other aerodynamic surfaces of a helicopter have a significant influence on its aerodynamic performance, the ride quality, and vibration. A numerical method for computing the aerodynamic interaction between an interacting vortex wake and the viscous flow about arbitrary two-dimensional bodies was developed to address this helicopter problem. The method solves for the flow field velocities on a body-fitted computational mesh using finite-difference techniques. The interacting vortex wake is represented by an array of discrete vortices which, in turn, are represented by a finite-core model. The evolution of the interacting vortex wake is calculated by Lagrangian techniques. The viscous flow field of the two-dimensional body is calculated on an Eulerian grid. The flow around circular and elliptic cylinders in the absence of an interacting vortex wake was calculated. These results compare very well with other numerical results and with results obtained from experiment and thereby demonstrate the accuracy of the viscous solution. The interaction of a rotor wake with the flow about a 4 to 1 elliptic cylinder at 45 degree incidence was calculated for a Reynolds number of 3000. The results demonstrate the significant variations in the lift and drag on the elliptic cylinder in the presence of the interacting rotor wake.

  19. Numerical Study of Tip Vortex Flows

    NASA Technical Reports Server (NTRS)

    Dacles-Mariani, Jennifer; Hafez, Mohamed

    1998-01-01

    This paper presents an overview and summary of the many different research work related to tip vortex flows and wake/trailing vortices as applied to practical engineering problems. As a literature survey paper, it outlines relevant analytical, theoretical, experimental and computational study found in literature. It also discusses in brief some of the fundamental aspects of the physics and its complexities. An appendix is also included. The topics included in this paper are: 1) Analytical Vortices; 2) Experimental Studies; 3) Computational Studies; 4) Wake Vortex Control and Management; 5) Wake Modeling; 6) High-Lift Systems; 7) Issues in Numerical Studies; 8) Instabilities; 9) Related Topics; 10) Visualization Tools for Vertical Flows; 11) Further Work Needed; 12) Acknowledgements; 13) References; and 14) Appendix.

  20. Modelling wake and boundary-layer interactions for EfficieNt Development of Offshore Windfarms (ENDOW)

    NASA Astrophysics Data System (ADS)

    Barthelmie, R.; Endow Partners

    2003-04-01

    The objective of the ENDOW project was to evaluate, enhance and interface wake and boundary-layer models for utilisation offshore. This resulted in a significant advance in the state of the art in both wake and marine boundary layer models leading to improved prediction of wind speed and turbulence profiles within large offshore wind farms. The model hierarchy formed the basis of design tools for use by wind energy developers and turbine manufacturers to optimise power output from offshore wind farms through minimised wake effects and optimal grid connections. The design tools were built onto existing regional scale models and wind farm design software which was developed with EU funding and is in use currently by wind energy developers. Part of the design tool evaluation included the issues of computational feasibility and ease of use (in addition to scientific and technical aspects). The project utilised databases from existing offshore wind farms (Vindeby and Bockstigen) to undertake the first comprehensive evaluation of offshore wake model performance. The six wake models vary in complexity from empirical solutions to the most advanced models based on solutions of the Navier-Stokes equations using eddy viscosity, k-epsilon or DES (detached eddy simulation) turbulence closure. The performance of the models was evaluated in comparison with measurements under a number of different wind speed, stability and turbulence conditions and the same criteria were applied for evaluation of model performance for predicting multiple wakes. Following this evaluation most of the models were enhanced with new parameterisations giving improved performance. Parallel research included comparison of a local-scale stability/roughness model with a mesoscale model focusing on boundary-layer development within and over a large offshore wind farm, and particularly the influence of large scale thermal flows. An experiment was also conducted to examine vertical wind speed profiles to hub-height and beyond in near-wake conditions and wake dispersion using SODAR to assist in model development and testing. Evaluation of the six wake models performance at different wake distances was based on this dataset. The paper will present the results of the project which concludes in February 2003. Final comparisons of the models will be given together with the results from the design tools for offshore wind farms.

  1. A Discretized Method for Deriving Vortex Impulse from Volumetric Datasets

    NASA Astrophysics Data System (ADS)

    Buckman, Noam; Mendelson, Leah; Techet, Alexandra

    2015-11-01

    Many biological and mechanical systems transfer momentum through a fluid by creating vortical structures. To study this mechanism, we derive a method for extracting impulse and its time derivative from flow fields observed in experiments and simulations. We begin by discretizing a thin-cored vortex filament, and extend the model to account for finite vortex core thickness and asymmetric distributions of vorticity. By solely using velocity fields to extract vortex cores and calculate circulation, this method is applicable to 3D PIV datasets, even with low spatial resolution flow fields and measurement noise. To assess the performance of this analysis method, we simulate vortex rings and arbitrary vortex structures using OpenFOAM computational fluid dynamics software and analyze the wake momentum using this model in order to validate this method. We further examine a piston-vortex experiment, using 3D synthetic particle image velocimetry (SAPIV) to capture velocity fields. Strengths, limitations, and improvements to the framework are discussed.

  2. Experimental examination of vortex-sound generation in an organ pipe: A proposal of jet vortex-layer formation model

    NASA Astrophysics Data System (ADS)

    Yoshikawa, Shigeru; Tashiro, Hiromi; Sakamoto, Yumiko

    2012-05-01

    Aero-dynamical models of sound generation in an organ pipe driven by a thin jet are investigated through an experimental examination of the vortex-sound theory. An important measurement requirement (acoustic cross-flow as an irrotational potential flow reciprocating sinusoidally) from the vortex-sound theory is carefully realized when the pipe is driven with low blowing pressures of about 60 Pa (jet velocities of about 10 m/s). Particle image velocimetry (PIV) is applied to measure the jet velocity and the acoustic cross-flow velocity over the mouth area at the same phase by quickly switching the jet drive and the loudspeaker-horn drive. The vorticity of the jet flow field and the associated acoustic generation term are evaluated from the measurement data. It is recognized that the model of the "jet vortex-layer formation" is more relevant to the sound production than the vortex-shedding model. The acoustic power is dominantly generated by the flow-acoustic interaction near the edge, where the acoustic cross-flow velocity takes larger magnitudes. The acoustic generation formula on the vortex sound cannot deny the conventional acoustical volume-flow model because of the in-phase relation satisfied between the acoustic pressure at the mouth and the acoustic volume flow into the pipe. The vortex layers formed along both sides of the jet act as the source of an accelerating force (through the "acceleration unbalance") with periodically alternating direction to oscillate the jet flow and to reinforce the acoustic cross-flow at the pipe mouth.

  3. Experimental modelling of satellite wakes in auroral arcs

    NASA Technical Reports Server (NTRS)

    Wild, N.; Stenzel, R. L.; Gekelman, W.

    1983-01-01

    Preliminary measurements have been made in a large laboratory discharge device configured to simulate plasma wake phenomena. A large relative electron drift impinging on a nonconducting disc gives rise to a perturbed density struture downstream, as well as a reflected beam of electrons upstream. Velocity distribution measurements were made using a novel energy analyzer with angular resolution.

  4. Active control of a cylinder wake flow by using a streamwise oscillating foil

    NASA Astrophysics Data System (ADS)

    Bao, Y.; Tao, J.

    2013-05-01

    In this study, numerical experiments are carried out to control the vortex shedding of a circular cylinder by utilizing an oscillating foil. The thin foil of elliptic shape undergoes prescribed harmonic oscillations in the streamwise direction in the near wake region. This simplified model is intended to study how wake dynamics are modified via localized wake disturbance, and then to stabilize the global wake instability. The results show that, at proper gap spacing, the oscillating foil can completely suppress the wake unsteadiness and recover the recirculating bubble type flow. The global instability suppression is then established on the imposition of local symmetry into the reversed flow behind the cylinder. It is revealed that the dynamic interaction between the main shears layer and oscillatory boundary layers is responsible for the wake stabilization mechanism. In addition, the kinematic/dynamic parameters related to foil motions and flow properties are widely discussed to reveal their effects on the performance of wake stabilization and drag reduction.

  5. Analysis of Wake VAS Benefits Using ACES Build 3.2.1: VAMS Type 1 Assessment

    NASA Technical Reports Server (NTRS)

    Smith, Jeremy C.

    2005-01-01

    The FAA and NASA are currently engaged in a Wake Turbulence Research Program to revise wake turbulence separation standards, procedures, and criteria to increase airport capacity while maintaining or increasing safety. The research program is divided into three phases: Phase I near term procedural enhancements; Phase II wind dependent Wake Vortex Advisory System (WakeVAS) Concepts of Operations (ConOps); and Phase III farther term ConOps based on wake prediction and sensing. The Phase III Wake VAS ConOps is one element of the Virtual Airspace Modelling and Simulation (VAMS) program blended concepts for enhancing the total system wide capacity of the National Airspace System (NAS). This report contains a VAMS Program Type 1 (stand-alone) assessment of the expected capacity benefits of Wake VAS at the 35 FAA Benchmark Airports and determines the consequent reduction in delay using the Airspace Concepts Evaluation System (ACES) Build 3.2.1 simulator.

  6. Dynamics of wake structure in clapping propulsion

    NASA Astrophysics Data System (ADS)

    Kim, Daegyoum; Gharib, Morteza

    2009-11-01

    Some animals such as insects and frogs use a pair of symmetric flaps for locomotion. In some cases, these flappers operate in close proximity or even touch each other. In order to understand the underlying physics of these kinds of motion, we have studied the wake structures induced by clapping and their associated thrust performance. A simple mechanical model with two acrylic plates was used to simulate the power stroke of the clapping motion and three-dimensional flow fields were obtained using defocusing digital particle image velocimetry. Our studies show that the process of vortex connection plays a critical role in forming a downstream closed vortex loop. Under some kinematic conditions, this vortex loop changes its shape dynamically, which is analogous to the process of an elliptical vortex ring switching its minor and major axis. As the length of the plate along the rotating shaft decreases to change an aspect ratio, the downstream motion of the vortex is retarded due to the outward motion of side edge vortices and less propulsive force is generated per the surface area of the plate. The impact of compliance and stroke angle of the plate on wake structures and thrust magnitudes are also presented.

  7. Vortex Flow Aerodynamics, volume 1

    SciTech Connect

    Campbell, J.F.; Osborn, R.F.; Foughner, J.T. Jr.

    1986-07-01

    Vortex modeling techniques and experimental studies of research configurations utilizing vortex flows are discussed. Also discussed are vortex flap investigations using generic and airplane research models and vortex flap theoretical analysis and design studies.

  8. Vortex Flow Aerodynamics, volume 1

    NASA Technical Reports Server (NTRS)

    Campbell, J. F. (Editor); Osborn, R. F. (Editor); Foughner, J. T., Jr. (Editor)

    1986-01-01

    Vortex modeling techniques and experimental studies of research configurations utilizing vortex flows are discussed. Also discussed are vortex flap investigations using generic and airplane research models and vortex flap theoretical analysis and design studies.

  9. Calculations of axisymmetric vortex sheet roll-up using a panel and a filament model

    NASA Technical Reports Server (NTRS)

    Kantelis, J. P.; Widnall, S. E.

    1986-01-01

    A method for calculating the self-induced motion of a vortex sheet using discrete vortex elements is presented. Vortex panels and vortex filaments are used to simulate two-dimensional and axisymmetric vortex sheet roll-up. A straight forward application using vortex elements to simulate the motion of a disk of vorticity with an elliptic circulation distribution yields unsatisfactroy results where the vortex elements move in a chaotic manner. The difficulty is assumed to be due to the inability of a finite number of discrete vortex elements to model the singularity at the sheet edge and due to large velocity calculation errors which result from uneven sheet stretching. A model of the inner portion of the spiral is introduced to eliminate the difficulty with the sheet edge singularity. The model replaces the outermost portion of the sheet with a single vortex of equivalent circulation and a number of higher order terms which account for the asymmetry of the spiral. The resulting discrete vortex model is applied to both two-dimensional and axisymmetric sheets. The two-dimensional roll-up is compared to the solution for a semi-infinite sheet with good results.

  10. A vortex-filament and core model for wings with edge vortex separation

    NASA Technical Reports Server (NTRS)

    Pao, J. L.; Lan, C. E.

    1981-01-01

    A method for predicting aerodynamic characteristics of slender wings with edge vortex separation was developed. Semiempirical but simple methods were used to determine the initial positions of the free sheet and vortex core. Comparison with available data indicates that: the present method is generally accurate in predicting the lift and induced drag coefficients but the predicted pitching moment is too positive; the spanwise lifting pressure distributions estimated by the one vortex core solution of the present method are significantly better than the results of Mehrotra's method relative to the pressure peak values for the flat delta; the two vortex core system applied to the double delta and strake wing produce overall aerodynamic characteristics which have good agreement with data except for the pitching moment; and the computer time for the present method is about two thirds of that of Mehrotra's method.

  11. Measurement of Aircraft Wake Vortices Using Doppler LIDAR

    NASA Astrophysics Data System (ADS)

    Ogasawara, Takeshi; Misaka, Takashi; Ogawa, Toshihiro; Obayashi, Shigeru; Yamada, Izumi

    In this research, the wake turbulence from actual passenger airplanes taking off from Sendai airport was measured with Electronic Navigation Research Institute's Doppler laser radar (lidar). First, the influence of the surrounding wind on the behavior of wake vortices was investigated. The wake vortices in the crosswind case disappear more quickly from the runway than those of the low surrounding wind and head wind cases. In addition, the wake vortices in the case of large crosswind move faster than those in small crosswind. Next, the correction factor was estimated by using the pseudo lidar measurements based on Computational Fluid Dynamics (CFD). The corrected data for weak surrounding wind case agreed with the existing wake vortex model.

  12. Calculation of symmetric and asymmetric vortex seperation on cones and tangent ogives based on discrete vortex models

    NASA Technical Reports Server (NTRS)

    Chin, S.; Lan, C. Edward

    1988-01-01

    An inviscid discrete vortex model, with newly derived expressions for the tangential velocity imposed at the separation points, is used to investigate the symmetric and asymmetric vortex separation on cones and tangent ogives. The circumferential locations of separation are taken from experimental data. Based on a slender body theory, the resulting simultaneous nonlinear algebraic equations in a cross-flow plane are solved with Broyden's modified Newton-Raphson method. Total force coefficients are obtained through momentum principle with new expressions for nonconical flow. It is shown through the method of function deflation that multiple solutions exist at large enough angles of attack, even with symmetric separation points. These additional solutions are asymmetric in vortex separation and produce side force coefficients which agree well with data for cones and tangent ogives.

  13. An Empirical Model for Vane-Type Vortex Generators in a Navier-Stokes Code

    NASA Technical Reports Server (NTRS)

    Dudek, Julianne C.

    2005-01-01

    An empirical model which simulates the effects of vane-type vortex generators in ducts was incorporated into the Wind-US Navier-Stokes computational fluid dynamics code. The model enables the effects of the vortex generators to be simulated without defining the details of the geometry within the grid, and makes it practical for researchers to evaluate multiple combinations of vortex generator arrangements. The model determines the strength of each vortex based on the generator geometry and the local flow conditions. Validation results are presented for flow in a straight pipe with a counter-rotating vortex generator arrangement, and the results are compared with experimental data and computational simulations using a gridded vane generator. Results are also presented for vortex generator arrays in two S-duct diffusers, along with accompanying experimental data. The effects of grid resolution and turbulence model are also examined.

  14. Wind tunnel investigation of helicopter-rotor wake effects on three helicopter fuselage models

    NASA Technical Reports Server (NTRS)

    Wilson, J. C.; Mineck, R. E.

    1975-01-01

    The effects of rotor wake on helicopter fuselage aerodynamic characteristics were investigated in the Langley V/STOL tunnel. Force, moment, and pressure data were obtained on three fuselage models at various combinations of windspeed, sideslip angle, and pitch angle. The data show that the influence of rotor wake on the helicopter fuselage yawing moment imposes a significant additional thrust requirement on the tail rotor of a single-rotor helicopter at high sideslip angles.

  15. An Aeroelastic Perspective of Floating Offshore Wind Turbine Wake Formation and Instability

    NASA Astrophysics Data System (ADS)

    Rodriguez, Steven N.; Jaworski, Justin W.

    2015-11-01

    The wake formation and wake stability of floating offshore wind turbines are investigated from an aeroelastic perspective. The aeroelastic model is composed of the Sebastian-Lackner free-vortex wake aerodynamic model coupled to the nonlinear Hodges-Dowell beam equations, which are extended to include the effects of blade profile asymmetry, higher-order torsional effects, and kinetic energy components associated with periodic rigid-body motions of floating platforms. Rigid-body platform motions are also assigned to the aerodynamic model as varying inflow conditions to emulate operational rotor-wake interactions. Careful attention is given to the wake formation within operational states where the ratio of inflow velocity to induced velocity is over 50%. These states are most susceptible to aerodynamic instabilities, and provide a range of states about which a wake stability analysis can be performed. In addition, the stability analysis used for the numerical framework is implemented into a standalone free-vortex wake aerodynamic model. Both aeroelastic and standalone aerodynamic results are compared to evaluate the level of impact that flexible blades have on the wake formation and wake stability.

  16. Design of an Aircraft Vortex Spacing System for Airport Capacity Improvement

    NASA Technical Reports Server (NTRS)

    Hinton, David A.; Charnock, James K.; Bagwell, Donald R.

    2000-01-01

    The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations element at the NASA Langley Research Center is developing an Aircraft VOrtex Spacing System (AVOSS). AVOSS will integrate the output of several systems to produce weather dependent, dynamic wake vortex spacing criteria. These systems provide current and predicted weather conditions, models of wake vortex transport and decay in these weather conditions, and real-time feedback of wake vortex behavior from sensors. The goal of the NASA program is to provide the research and development to demonstrate an engineering model AVOSS, in real-time operation, at a major airport. A wake vortex system test facility was established at the Dallas-Fort Worth International Airport (DFW) in 1997 and tested in 1998. Results from operation of the initial AVOSS system, plus advances in wake vortex prediction and near-term weather forecast models, "nowcast", have been integrated into a second-generation system. This AVOSS version is undergoing final checkout in preparation for a system demonstration in 2000. This paper describes the revised AVOSS system architecture, subsystem enhancements, and initial results with AVOSS version 2 from a deployment at DFW in the fall of 1999.

  17. Inter-Comparison of WRF Model Simulated Winds and MISR Stereoscopic Winds Embedded within Mesoscale von Kármán Wake Vortices

    NASA Astrophysics Data System (ADS)

    Horvath, A.; Nunalee, C. G.; Mueller, K. J.

    2014-12-01

    Several distinct wake regimes are possible when considering atmospheric flow past a steep mountainous island. Of these regimes, coherent vortex shedding in low-Froude number flow is particularly interesting because it can produce laterally focused paths of counter rotating eddies capable of extending downstream for hundreds of kilometers (i.e., a von Kármán vortex street). Given the spatial scales of atmospheric von Kármán vortices, which typically lies on the interface of the meso-scale and the micro-scale, they are uniquely challenging to model using conventional numerical weather prediction platforms. In this presentation, we present high resolution (1-km horizontally) numerical modeling results using the Weather Research and Forecasting (WRF) model, of multiple real-world von Kármán vortex shedding events associated with steep islands (e.g., Madeira island, Gran Canaria island, etc.). In parallel, we also present corresponding cloud-motion wind and cloud-top height measurements from the satellite-based Multiangle Imaging SpectroRadiometer (MISR) instrument. The MISR stereo algorithm enables experimental retrieval of the horizontal wind vector (both along-track and cross-track components) at 4.4-km resolution, in addition to the operational 1.1-km resolution cross-track wind and cloud-top height products. These products offer the fidelity appropriate for inter-comparison with the numerically simulated vortex streets. In general, we find an agreement between the instantaneous simulated cloud level winds and the MISR stereoscopic winds; however, discrepancies in the vortex street length and localized horizontal wind shear were documented. In addition, the simulated fields demonstrate sensitivity to turbulence closure and input terrain height data.

  18. A non-geometrically similar model for predicting the wake field of full-scale ships

    NASA Astrophysics Data System (ADS)

    Guo, Chunyu; Zhang, Qi; Shen, Yu

    2015-07-01

    The scale effect leads to large discrepancies between the wake fields of model-scale and actual ships, and causes differences in cavitation performance and exciting forces tests in predicting the performance of actual ships. Therefore, when test data from ship models are directly applied to predict the performance of actual ships, test results must be subjected to empirical corrections. This study proposes a method for the reverse design of the hull model. Compared to a geometrically similar hull model, the wake field generated by the modified model is closer to that of an actual ship. A non- geometrically similar model of a Korean Research Institute of Ship and Ocean Engineering (KRISO)'s container ship (KCS) was designed. Numerical simulations were performed using this model, and its results were compared with full-scale calculation results. The deformation method of getting the wake field of full-scale ships by the non-geometrically similar model is applied to the KCS successfully.

  19. Application of Three-Component PIV to a Hovering Rotor Wake

    NASA Technical Reports Server (NTRS)

    Yamauchi, Gloria K.; Lourenco, Luiz; Heineck, James T.; Wadcock, Alan J.; Abrego, Anita I.; Aiken, Edwin W. (Technical Monitor)

    2000-01-01

    The key to accurate predictions of rotorcraft aerodynamics, acoustics, and dynamics lies in the accurate representation of the rotor wake. The vortical wake computed by rotorcraft CFD analyses typically suffer from numerical dissipation before the first blade passage. With some a priori knowledge of the wake trajectory, grid points can be concentrated along the trajectory to minimize the dissipation. Comprehensive rotorcraft analyses based on lifting-line theory rely on classical vortex models and/or semi-empirical information about the tip vortex structure. Until the location, size, and strength of the trailed tip vortex can be measured over a range of wake ages, the analyses will continue to be adjusted on a trial and error basis in order to correctly predict blade airloads, acoustics, dynamics, and performance. Using the laser light sheet technique, tip vortex location can be acquired in a straightforward manner. Measuring wake velocities and vortex core size, however, has been difficult and tedious using point-measurement techniques such as laser velocimetry. Recently, the Particle Image Velocimetry (PIV) technique has proven to be an efficient method for acquiring velocity measurements over relatively large areas and volumes of a rotor wake. The work reported to date, however, has been restricted to 2-component velocity measurements of the rotor wake. Three-component velocity measurements of a hovering rotor wake were acquired at NASA Ames Research Center in May 1999. This experiment represents a major step toward understanding the detailed structure of a rotor wake. This paper will focus primarily on the experimental technique used in acquiring this data. The accuracy and limitations of the current technique will also be discussed. Representative velocity field measurements will be included.

  20. Venusian Polar Vortex reproduced in an Atmospheric General Circulation Model

    NASA Astrophysics Data System (ADS)

    Ando, Hiroki; Imamura, Takeshi; Takagi, Masahiro; Sugimoto, Norihiko; Kashimura, Hiroki

    The Venus atmosphere has a polar vortex rotating in the retrograde direction with a period of about three days. The vortex has a warm feature surrounded by a cold collar (e.g., Taylor et al. 1980; Piccioni et al. 2006). Although the Venusian polar vortex has been reported by many observations, its mechanism is still unknown. Elson (1982, 1989) examined the structure of the polar vortex by linear calculations. However, the background zonal wind assumed in the calculations was much stronger or weaker than those retrieved in the previous measurements (e.g., Peralta et al. 2008; Kouyama et al. 2012). Lee et al. (2010) and Yamamoto and Takahashi (2012) performed numerical simulations with general circulation models (GCMs) of the Venus atmosphere and obtained vertical structure in the polar region. However, the models included artificial forcing of Kelvin and/or Rossby waves. We have developed a new Venusian GCM by modifying the Atmospheric GCM For the Earth Simulator (Sugimoto et al. 2012; 2013). The basic equations of the GCM are primitive ones in the sigma coordinate on a sphere without topography. The model resolution is T42 (i.e., about 2.8 deg x 2.8 deg grids) and L60 (Deltaz is about 2 km). Rayleigh friction (sponge layer) in the upper layer (>80 km) is applied to prevent the reflection of waves, whose effect increases gradually with height. In the model, the atmosphere is dry and forced by the solar heating and Newtonian cooling. The vertical profile of the solar heating is based on Crisp (1986), and zonally averaged distribution is used. In addition diurnal component of the solar heating, which excites the diurnal and semi-diurnal tides, is also included. Newtonian cooling relaxes the temperature to the zonally uniform basic temperature which has a virtual static stability of Venus with almost neutral layers, and its coefficient is based on Crisp (1986). To prevent numerical instability, the biharmonic hyper-diffusion is included with 0.8 days of e-folding time for the truncation wavenumber. The coefficient of the vertical eddy diffusion is 0.15 m (2) s (-1) . A fast zonal wind in a solid-body rotation and the temperature field that balances with the zonal wind (gradient wind balance) is given as the initial state. Time integrations are performed until the solution achieves a statistically steady state. In this study we analyzed the data of 300 days of the last from getting to the quasi-steady state. The temporal and zonal mean wind and temperature fields are almost consistent with those obtained in previous studies (e.g., Kouyama et al. 2012; Tellmann et al. 2009). Barotropic or baroclinic instability occurs at the polar region, and zonal wavenumber one component is the most dominant in the polar vortex, followed by wavenumber two. It rotates the pole with the period of 4-5 days. These features are almost consistent with recent measurements (e.g. Lopez et al. 2013). We also calculated the potential vorticity distribution and found that it sometimes shows the filament structure, which is similar to the cloud morphology observed in recent imaging measurements (e.g. Piccioni et al. 2006). The vertical structure of each zonal wavenumber component has the common feature that the phase stands upright within the polar vortex and seems to shift at the top of the neutral stable layer. In this presentation, we will also compare the vertical structure seen in our calculation with that examined by radio occultation measurements in Venus Express mission.

  1. Three-dimensional topology and dynamical modelling of vortex shedding from finite surface-mounted bluff bodies

    NASA Astrophysics Data System (ADS)

    Bourgeois, Jason A.

    While the dynamically rich behaviour of fully turbulent wakes is very high dimensional, the most energetic, large scale coherent structures generated through instability processes are typically low dimensional and are thereby conducive to reduced order modeling procedures. These large scale eddies associated with the flow instability have the most anisotropic and geometry dependent topology and act as a source of kinetic energy in the cascade process, making them the most important to characterize. Dissipative small scale structure can then be modelled with reasonable accuracy by traditional means. The present study experimentally educes the coherent structures in the complex three-dimensional wake of a wall-mounted finite square-cross-section cylinder of aspect ratio h/d = 4 and 8 immersed in boundary layers of thickness delta/d = 0.72 and 2.6 at a Reynolds number of 12,000. Coherent structure eduction is carried out using phase averaging and a novel generalized phase averaging technique that incorporates proper orthogonal decomposition (POD) modes that are most important in the nonlinear instability saturation process. Global flow estimation and mode construction is undertaken using linear pressure-POD coefficient correlations, applicable to experimental investigations where practicality demands that subdomains of the global field are measured asynchronously. The large-scale coherent structures of the wakes investigated are analyzed in terms of their topology, their turbulent kinetic energy (amounting to roughly half the total fluctuation energy), and their influence on turbulence production. The educed coherent vortical structures are found to have either full-loop or half-loop topological structure depending on the boundary layer thickness, showing vortical connector strands connecting alternately shed vortices from either side of the obstacle. The structure provides an explanation of the dipole and quadrupole distributions of streamwise vorticity that have previously been observed in these types of three-dimensional wakes. The reduced order nonlinear Galerkin models derived for the dynamics of the coherent structures using the generalized phase average are shown to successfully account for the slow base flow transients, the instability saturation mechanism, and the excitation of the second harmonic modes. KEYWORDS: Full-loop vortex shedding, Half-loop vortex shedding, Finite wall-mounted bluff-bodies, Coherent structures, Trailing vortices, Reduced order modelling, Proper orthogonal decomposition, Linear stochastic estimation, Particle image velocimetry.

  2. Numerical Simulation of Wake Vortices Measured During the Idaho Falls and Memphis Field Programs

    NASA Technical Reports Server (NTRS)

    Proctor, Fred H.

    1996-01-01

    A numerical large-eddy simulation model is under modification and testing for application to aircraft wake vortices. The model, having a meteorological framework, permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, and humidity. As part of the validation process, model results are compared with measured field data from the 1990 Idaho Falls and the 1994-1995 Memphis field experiments. Cases are selected that represent different aircraft and a cross section of meteorological environments. Also included is one case with wake vortex generation in ground effect. The model simulations are initialized with the appropriate meteorological conditions and a post roll-up vortex system. No ambient turbulence is assumed in our initial set of experiments, although turbulence can be self generated by the interaction of the model wakes with the ground and environment.

  3. Stabilization of vortices in the wake of a circular cylinder using harmonic forcing.

    PubMed

    Chamoun, Georges C; Schilder, Frank; Brøns, Morten

    2011-06-01

    We explore whether vortex flows in the wake of a fixed circular cylinder can be stabilized using harmonic forcing. We use Föppl's point vortex model augmented with a harmonic point source-sink mechanism which preserves conservation of mass and leaves the system Hamiltonian. We discover a region of Lyapunov-stable vortex motion for an appropriate selection of parameters. We identify four unique parameters that affect the stability of the vortices: the uniform flow velocity, vortex equilibrium positions, forcing amplitude, and forcing frequency. We assess the robustness of the controller using a Poincaré section. PMID:21797478

  4. Numerical study on wake characteristics of high-speed trains

    NASA Astrophysics Data System (ADS)

    Yao, Shuan-Bao; Sun, Zhen-Xu; Guo, Di-Long; Chen, Da-Wei; Yang, Guo-Wei

    2013-12-01

    Intensive turbulence exists in the wakes of high speed trains, and the aerodynamic performance of the trailing car could deteriorate rapidly due to complicated features of the vortices in the wake zone. As a result, the safety and amenity of high speed trains would face a great challenge. This paper considers mainly the mechanism of vortex formation and evolution in the train flow field. A real CRH2 model is studied, with a leading car, a middle car and a trailing car included. Different running speeds and cross wind conditions are considered, and the approaches of unsteady Reynold-averaged Navier-Stokes (URANS) and detached eddy simulation (DES) are utilized, respectively. Results reveal that DES has better capability of capturing small eddies compared to URANS. However, for large eddies, the effects of two approaches are almost the same. In conditions without cross winds, two large vortex streets stretch from the train nose and interact strongly with each other in the wake zone. With the reinforcement of the ground, a complicated wake vortex system generates and becomes strengthened as the running speed increases. However, the locations of flow separations on the train surface and the separation mechanism keep unchanged. In conditions with cross winds, three large vortices develop along the leeward side of the train, among which the weakest one has no obvious influence on the wake flow while the other two stretch to the tail of the train and combine with the helical vortices in the train wake. Thus, optimization of the aerodynamic performance of the trailing car should be aiming at reducing the intensity of the wake vortex system.

  5. Numerical study on wake characteristics of high-speed trains

    NASA Astrophysics Data System (ADS)

    Yao, Shuan-Bao; Sun, Zhen-Xu; Guo, Di-Long; Chen, Da-Wei; Yang, Guo-Wei

    2013-11-01

    Intensive turbulence exists in the wakes of high speed trains, and the aerodynamic performance of the trailing car could deteriorate rapidly due to complicated features of the vortices in the wake zone. As a result, the safety and amenity of high speed trains would face a great challenge. This paper considers mainly the mechanism of vortex formation and evolution in the train flow field. A real CRH2 model is studied, with a leading car, a middle car and a trailing car included. Different running speeds and cross wind conditions are considered, and the approaches of unsteady Reynold-averaged Navier-Stokes (URANS) and detached eddy simulation (DES) are utilized, respectively. Results reveal that DES has better capability of capturing small eddies compared to URANS. However, for large eddies, the effects of two approaches are almost the same. In conditions without cross winds, two large vortex streets stretch from the train nose and interact strongly with each other in the wake zone. With the reinforcement of the ground, a complicated wake vortex system generates and becomes strengthened as the running speed increases. However, the locations of flow separations on the train surface and the separation mechanism keep unchanged. In conditions with cross winds, three large vortices develop along the leeward side of the train, among which the weakest one has no obvious influence on the wake flow while the other two stretch to the tail of the train and combine with the helical vortices in the train wake. Thus, optimization of the aerodynamic performance of the trailing car should be aiming at reducing the intensity of the wake vortex system.

  6. Demonstration of rapid-scan two-dimensional laser velocimetry in the Langley Vortex Research Facility for research in aerial applications

    NASA Technical Reports Server (NTRS)

    Gartrell, L. R.; Jordan, F. L., Jr.

    1977-01-01

    Tests were conducted to demonstrate a rapid scan two dimensional laser velocimeter (LV) measurement technique for aerial applications research. The LV system is capable of simultaneously measuring both vertical and axial flow velocity components in a near or far field vortex system. Velocity profiles were successfully measured in the wake vortex of a representative agricultural aircraft model, with the vortex system rapidly transporting in ground effect. Results indicate that the laser velocimetry technique can provide quantitative information of wake vortex characteristics in ground effect.

  7. Preliminary study of the three-dimensional deformation of the vortex in Karman vortex street

    NASA Astrophysics Data System (ADS)

    Ling, Guocan; Guo, Liang; Wu, Zuobin; Ma, Huiyang

    1992-03-01

    The mechanism for 3D evolution of the isolated Karman vortex and the thin-vortex filament in a circular cylinder wake is studied numerically using the LIA method. The results show that the vortex motion is unstable for small 3D disturbances in the separated wake of a circular cylinder. Karman vortex in the time-averaged wake flowfield wolves into a horseshoe-spoon-like 3D structure. The thin vortex filament deforms three-dimensionally in the braid and generates streamwise vortex structures which incline to the region maximum-deformation direction of the flowfield.

  8. Evaluation of the radial pressure distribution of vortex models and comparison with experimental data

    NASA Astrophysics Data System (ADS)

    Hommes, T.; Bosschers, J.; Hoeijmakers, H. W. M.

    2015-12-01

    An important input parameter for a recently developed semi-empirical prediction method for the noise generated by cavitating vortices is the diameter of the cavitating vortex. This diameter may be obtained from models of non-cavitating vortices that describe the radial distribution of the azimuthal velocity, for given vortex strength and size of the viscous core. The present paper discusses several vortex models that suit this purpose and compares the distribution of the azimuthal velocity as well as that of the pressure with detailed experimental data for the tip vortex of a rectangular wing obtained from literature.

  9. Vortex solutions of an Abelian Higgs model with visible and hidden sectors

    NASA Astrophysics Data System (ADS)

    Arias, Paola; Schaposnik, Fidel A.

    2014-12-01

    We study vortex solutions in a theory with dynamics governed by two weakly coupled Abelian Higgs models, describing a hidden sector and a visible sector. We analyze the radial dependence of the axially symmetric solutions constructed numerically and discuss the stability of vortex configurations for different values of the model parameters, studying in detail vortex decay into lower energy configurations. We find that even in a weak coupling regime vortex solutions strongly depend on the parameters of both the visible and hidden sectors. We also discuss on qualitative grounds possible implications of the existence of a hidden sector in connection with superconductivity and dark matter (dark strings).

  10. Dissipative N-point-vortex Models in the Plane

    NASA Astrophysics Data System (ADS)

    Shashikanth, Banavara N.

    2010-02-01

    A method is presented for constructing point vortex models in the plane that dissipate the Hamiltonian function at any prescribed rate and yet conserve the level sets of the invariants of the Hamiltonian model arising from the SE (2) symmetries. The method is purely geometric in that it uses the level sets of the Hamiltonian and the invariants to construct the dissipative field and is based on elementary classical geometry in ℝ3. Extension to higher-dimensional spaces, such as the point vortex phase space, is done using exterior algebra. The method is in fact general enough to apply to any smooth finite-dimensional system with conserved quantities, and, for certain special cases, the dissipative vector field constructed can be associated with an appropriately defined double Nambu-Poisson bracket. The most interesting feature of this method is that it allows for an infinite sequence of such dissipative vector fields to be constructed by repeated application of a symmetric linear operator (matrix) at each point of the intersection of the level sets.

  11. Instability of a two-step Rankine vortex in a reduced gravity QG model

    NASA Astrophysics Data System (ADS)

    Perrot, Xavier; Carton, Xavier

    2014-06-01

    We investigate the stability of a steplike Rankine vortex in a one-active-layer, reduced gravity, quasi-geostrophic model. After calculating the linear stability with a normal mode analysis, the singular modes are determined as a function of the vortex shape to investigate short-time stability. Finally we determine the position of the critical layer and show its influence when it lies inside the vortex.

  12. Wind Speed Estimation and Wake model Re-calibration for Downregulated Offshore Wind Farms

    NASA Astrophysics Data System (ADS)

    Göçmen Bozkurt, Tuhfe; Giebel, Gregor; Kjølstad Poulsen, Niels; Réthoré, Pierre-Elouan; Mirzaei, Mahmood

    2014-05-01

    In recent years, the wind farm sizes have increased tremendously and with increasing installed capacity, the wind farms are requested to downregulate from their maximum possible power more frequently, especially in the offshore environment. Determination of the possible (or available) power is crucial not only because the reserve power has considerable market value but also for wind farm developers to be properly compensated for the loss during downregulation. While the available power calculation is straightforward for a single turbine, it gets rather complicated for the whole wind farm due to the change in the wake characteristics. In fact, the wake losses generated by the upstream turbine(s) decrease during downregulation and the downstream turbines therefore see more wind compared to the normal operation case. Currently, the Transmission System Operators (TSOs) have no real way to determine exactly the available power of a whole wind farm which is downregulated. Therefore, the PossPOW project aims to develop a verified and internationally accepted way to determine the possible power of a down-regulated offshore wind farm. The first phase of the project is to estimate the rotor effective wind speed. Since the nacelle anemometers are not readily available and are known to have reliability issues, the proposed method is to use power, pitch angle and rotational speed as inputs and combine it with a generic Cp model to estimate the wind speed. The performance of the model has been evaluated for both normal operation and downregulation periods using two different case studies: Horns Rev-I wind farm and NREL 5MW single turbine. During downregulation, the wake losses are not as severe and the velocity deficits at the downstream turbines are smaller as if also the wake is "downregulated". On the other hand, in order to calculate the available power, the wakes that would have been produced normally (if the turbines were not curtailed) are of importance, not the downregulated wake. For this reason, the proposed methodology is to use the clear wind without the wake (downregulated or not) as inputs to the wake model. Then a dynamic wake model can be directly applied to estimate the velocity deficit row by row inside the wind farm and calculate the possible power output on the wind farm scale. Most of the computationally affordable wake models have only been used to acquire long term, statistical information and verified using 10-min averaged data. However for smaller averaging bins or real-time modeling, the dynamics of the flow inside the wind farm such as wind direction variability and wake meandering is much more significant. Therefore GCLarsen wake model, which has been implemented in WindPro and shown to perform also well on offshore in Wake benchmark work package in EERA-DTOC, is re-calibrated and validated for single wake case in Horns Rev-I offshore wind farm.

  13. A numerical investigation of flow around octopus-like arms: near-wake vortex patterns and force development.

    PubMed

    Kazakidi, A; Vavourakis, V; Tsakiris, D P; Ekaterinaris, J A

    2015-01-01

    The fluid dynamics of cephalopods has so far received little attention in the literature, due to their complexity in structure and locomotion. The flow around octopuses, in particular, can be complicated due to their agile and dexterous arms, which frequently display some of the most diverse mechanisms of motion. The study of this flow amounts to a specific instance of the hydrodynamics problem for rough tapered cylinder geometries. The outstanding manipulative and locomotor skills of octopuses could inspire the development of advanced robotic arms, able to operate in fluid environments. Our primary aim was to study the hydrodynamic characteristics of such bio-inspired robotic models and to derive the hydrodynamic force coefficients as a concise description of the vortical flow effects. Utilizing computational fluid dynamic methods, the coefficients were computed on realistic morphologies of octopus-like arm models undergoing prescribed solid-body movements; such motions occur in nature for short durations in time, e.g. during reaching movements and exploratory behaviors. Numerical simulations were performed on translating, impulsively rotating, and maneuvering arms, around which the flow field structures were investigated. The results reveal in detail the generation of complex vortical flow structures around the moving arms. Hydrodynamic forces acting on a translating arm depend on the angle of incidence; forces generated during impulsive rotations of the arms are independent of their exact morphology and the angle of rotation; periodic motions based on a slow recovery and a fast power stroke are able to produce considerable propulsive thrust while harmonic motions are not. Parts of these results have been employed in bio-inspired models of underwater robotic mechanisms. This investigation may further assist elucidating the hydrodynamics underlying aspects of octopus locomotion and exploratory behaviors. PMID:24730546

  14. Inviscid Interactions Between Wake Vortices and Shear Layers

    NASA Technical Reports Server (NTRS)

    Zheng, Z. C.; Baek, K.

    1998-01-01

    Aircraft trailing vortices can be influenced significantly by atmospheric conditions such as crosswind, turbulence, and stratification. According to the NASA 1994 and 1995 field measurement program in Memphis, Tennessee, the descending aircraft wake vortices could stall or be deflected at the top of low-level temperature inversions that usually produce pronounced shear zones. Numerical simulations of vortex/shear interactions with ground effects have been performed by several groups. Burnham used a series of evenly spaced line vortices at a particular altitude to model the ground shear layer of the cross- wind. He found that the wind shear was swept up around the downwind vortex and caused the downwind vortex to move upward, and claimed that the effect was actually produced by the vertical gradient in the wind shear rather than by the wind shear directly, because uniformly distributed wind-shear vortices would have no effect on the trailing vortex vertical motion. Recently, Proctor et al. numerically tested the effects of narrow shear zones on the behavior of the vortex pair, motivated by the observation of the Memphis field data. The shear-layer sensitivity tests indicated that the downwind vortex was more sensitive and deflected to a higher altitude than its upwind counterpart. The downstream vortex contained vorticity of opposite sign to that of the shear. There was no detectable preference for the downwind vortex (or upwind vortex) to weaken (or strengthen) at a greater rate.

  15. Asymptotic wake behavior of swept, blunt trailing-edge airfoils

    SciTech Connect

    Selby, G.V.; Miandoab, F.H. . Dept. of Mechanical Engineering)

    1994-03-01

    The effect of base sweep and the addition of passive flow-control devices at constant base sweep angle (30 deg) on the asymptotic behavior of turbulent wakes produced by flatplate airfoils was experimentally examined. It was determined that values of the nondimensional streamwise velocity defect and wake thickness parameters for the grooved model with 30 deg swept base at fourteen base thickness downstream of the base at mid-span were closer to asymptotic values from empirical plane wake predictions than values for the 0, 30, and 45 deg swept baseline models and the 30 deg swept model with Wishbone vortex generators. The grooves apparently inhibited the three-dimensionality of the resulting wake flow.

  16. Recent Developments on Airborne Forward Looking Interferometer for the Detection of Wake Vortices

    NASA Technical Reports Server (NTRS)

    Daniels, Taumi S.; Smith, William L.; Kirev, Stanislav

    2012-01-01

    A goal of these studies was development of the measurement methods and algorithms necessary to detect wake vortex hazards in real time from either an aircraft or ground-based hyperspectral Fourier Transform Spectrometer (FTS). This paper provides an update on research to model FTS detection of wake vortices. The Terminal Area Simulation System (TASS) was used to generate wake vortex fields of 3-D winds, temperature, and absolute humidity. These fields were input to the Line by Line Radiative Transfer Model (LBLRTM), a hyperspectral radiance model in the infrared, employed for the FTS numerical modeling. An initial set of cases has been analyzed to identify a wake vortex IR signature and signature sensitivities to various state variables. Results from the numerical modeling case studies will be presented. Preliminary results indicated that an imaging IR instrument sensitive to six narrow bands within the 670 to 3150 per centimeter spectral region would be sufficient for wake vortex detection. Noise floor estimates for a recommended instrument are a current research topic.

  17. Wake effect on a uniform flow behind wind-turbine model

    NASA Astrophysics Data System (ADS)

    Okulov, V. L.; Naumov, I. V.; Mikkelsen, R. F.; Sørensen, J. N.

    2015-06-01

    LDA experiments were carried out to study the development of mean velocity profiles of the very far wake behind a wind turbine model in a water flume. The model of the rotor is placed in a middle of the flume. The initial flume flow is subjected to a very low turbulence level, limiting the influence of external disturbances on the development of the inherent wake instability. The rotor is three-bladed and designed using Glauert's optimum theory at a tip speed ratio λ = 5 with a constant of the lift coefficient along the span, CL= 0.8. The wake development has been studied in the range of tip speed ratios from 3 to 9, and at different cross-sections from 10 to 100 rotor radii downstream from the rotor. By using regression techniques to fit the velocity profiles it was possible to obtain accurate velocity deficits and estimate length scales of the wake attenuation. The data are compared with different analytical models for wind turbine wakes.

  18. Wind tunnel study of wake downwash behind A 6% scale model B1-B aircraft

    SciTech Connect

    Strickland, J.H.; Tadios, E.L.; Powers, D.A.

    1990-05-01

    Parachute system performance issues such a turnover and wake recontact may be strongly influenced by velocities induced by the wake of the delivering aircraft, especially if the aircraft is maneuvering at the time of parachute deployment. The effect of the aircraft on the parachute system is a function of the aircraft size, weight, and flight path. In order to provide experimental data for validation of a computer code to predict aircraft wake velocities, a test was conducted in the NASA 14 {times} 22 ft wind tunnel using a 5.78% model of the B-1B strategic bomber. The model was strut mounted through the top of its fuselage by a mechanism which was capable of pitching the model at moderate rates. In this series of tests, the aircraft was pitched at 10{degree}/sec from a cruise angle of attack of 5.3{degree} to an angle of attack of 11{degree} in order to simulate a 2.2g pullup. Data were also taken for the subsequent pitch down sequence back to the cruise angle of attack. Instantaneous streamwise and vertical velocities were measured in the wake at a number of points using a hot wire anemometer. These data have been reduced to the form of downwash coefficients which are a function of the aircraft angle of attack time-history. Unsteady effects are accounted for by use of a wake convection lag-time correlation. 12 refs., 59 figs., 4 tabs.

  19. Applying dynamic wake models to large swirl velocities for optimal propellers

    NASA Astrophysics Data System (ADS)

    Makinen, Stephen M.

    The dynamic wake model is applied to the optimal propeller systems originally studied by the classic aerodynamicists: Betz, Prandtl and Goldstein. Several modified forms of the model are theoretically developed to extend the applicable range to flight conditions with a large swirl velocity component. Dynamic wake model calculations accurately predict the inflow behavior for helicopter rotors, including axial flow for large tip-speed ratios, (OR/V infinity) ≥ 20. The swirl velocity is a prominent component for small tip-speed ratios (≤5), typical of forward flight for tiltrotor craft such as the V-22 Osprey and the BA609. Dynamic wake calculation results are compared to the closed-form solutions by Prandtl and Goldstein. The exact and approximate solutions correlate strongly for infinite blade cases and finite blade cases with a large tip-speed ratio. The original form of the He-Peters and Morillo-Peters dynamic wake models converge poorly for small tip-speed ratios, due to neglect of the swirl velocity. Derivations are presented for several adaptations of the models to account for the large apparent mass at the inboard blade region. A best modified form is chosen and the associated empirical factor is optimized to correlate well with Prandtl's solution. Error norms for the original and modified forms of the dynamic wake model are presented for propellers of various number of blades and a range of tip-speed ratios. The Goldstein solution is also studied in depth and conclusions are drawn for improving the dynamic wake model.

  20. Propulsive force calculations in swimming frogs. II. Application of a vortex ring model to DPIV data.

    PubMed

    Stamhuis, Eize J; Nauwelaerts, Sandra

    2005-04-01

    Frogs propel themselves by kicking water backwards using a synchronised extension of their hind limbs and webbed feet. To understand this propulsion process, we quantified the water movements and displacements resulting from swimming in the green frog Rana esculenta, applying digital particle image velocimetry (DPIV) to the frog's wake. The wake showed two vortex rings left behind by the two feet. The rings appeared to be elliptic in planform, urging for correction of the observed ring radii. The rings' long and short axes (average ratio 1.75:1) were about the same size as the length and width of the propelling frog foot and the ellipsoid mass of water accelerated with it. Average thrust forces were derived from the vortex rings, assuming all propulsive energy to be compiled in the rings. The calculated average forces (F(av)=0.10+/-0.04 N) were in close agreement with our parallel study applying a momentum-impulse approach to water displacements during the leg extension phase. We did not find any support for previously assumed propulsion enhancement mechanisms. The feet do not clap together at the end of the power stroke and no "wedge-action" jetting is observed. Each foot accelerates its own water mantle, ending up in a separate vortex ring without interference by the other leg. PMID:15802668

  1. Effects of a three-dimensional hill on the wake characteristics of a model wind turbine

    NASA Astrophysics Data System (ADS)

    Yang, Xiaolei; Howard, Kevin B.; Guala, Michele; Sotiropoulos, Fotis

    2014-11-01

    The spatial evolution of a turbine wake downwind of a sinusoidal hill is studied using large-eddy simulations and wind tunnel measurements. The computed flow fields behind the hill show good agreement with Particle Image Velocimetry measurements. It is observed that the turbine wake downwind of the hill recovers faster than the wake of the same turbine in the turbulent boundary layer flow (OT case) because of the increased entrainment of ambient flow into the turbine wake, which is due to enhanced turbulence convection in both the spanwise and vertical directions. It is also observed that the recovery rates of the available mean kinetic energy in the turbine wakes are nearly the same for turbine positions of 4D, 6D and 8D downwind of the hill (HT cases). The turbulence kinetic energy (TKE) in the turbine wake for all the HT cases exhibit significant increases as compared to that from the OT case. However, the profiles of the turbine-added TKE nearly collapse for all OT and HT cases (except in the turbine near wake region) when normalized by a characteristic velocity defined by the turbine thrust. A simple model for the turbine-added TKE in complex terrain is also proposed based on the new physical insights from the simulated cases. This work was supported by Department of Energy (DE-EE0002980, DE-EE0005482 and DE-AC04-94AL85000), the National Science Foundation (IIP-1318201) and partially by the IREE early career award (UMN). Computational resources were provided by MSI.

  2. Effects of a three-dimensional hill on the wake characteristics of a model wind turbine

    NASA Astrophysics Data System (ADS)

    Yang, Xiaolei; Howard, Kevin B.; Guala, Michele; Sotiropoulos, Fotis

    2015-02-01

    The spatial evolution of a turbine wake downwind of a three-dimensional sinusoidal hill is studied using large-eddy simulations and wind tunnel measurements. The computed flow fields behind the hill show good agreement with wind tunnel measurements. Three different heights of the hill, i.e., hhill = zh - 0.5D, ≈ zh and =zh + 0.5D (where zh is the turbine hub height and D is the diameter of the turbine rotor), were considered. The effect of the hill turbine spacing was investigated through a comparative analysis with the turbine wake results in the undisturbed turbulent boundary layer. It is observed that the turbine wakes downwind of the hill with hhill ≈ zh and hhill = zh + 0.5D recover faster because of the increased entrainment of ambient flow into the turbine wake, which is due to the enhanced turbulent transport in both spanwise and vertical directions. In comparison with the turbine only case, significant increases in the turbulence kinetic energy (TKE) in the turbine wake are observed for the hill-turbine cases with hhill ≈ zh and hhill = zh + 0.5D. A velocity scale UT, defined in terms of the thrust force acting on the turbine, is introduced for the turbine-added velocity deficit and TKE. For the turbine-added velocity deficit, UT is shown to be an appropriate scale at wake locations sufficiently far downwind of the turbine (i.e., greater than or equal to 8D). The vertical profiles of the turbine-added TKE normalized by UT 2 are shown to nearly collapse in the wake both for the turbine only and hill-turbine cases at all locations greater than 4D downwind of the turbine. A simple model for the turbine-added TKE in complex terrain is also proposed based on the new physical insights obtained from our simulations.

  3. A comparison of wake characteristics of model and prototype buildings in transverse winds

    NASA Technical Reports Server (NTRS)

    Logan, E., Jr.; Phataraphruk, P.; Chang, J.

    1978-01-01

    Previously measured mean velocity and turbulence intensity profiles in the wake of a 26.8-m long building 3.2 m high and transverse to the wind direction in an atmospheric boundary layer several hundred meters thick were compared with profiles at corresponding stations downstream of a 1/50-scale model on the floor of a large meteorological wind tunnel in a boundary layer 0.61 m in thickness. The validity of using model wake data to predict full scale data was determined. Preliminary results are presented which indicate that disparities result from differences in relative depth of logarithmic layers, surface roughness, and the proximity of upstream obstacles.

  4. Rotor Wake Development During the First Revolution

    NASA Technical Reports Server (NTRS)

    McAlister, Kenneth W.

    2003-01-01

    The wake behind a two-bladed model rotor in light climb was measured using particle image velocimetry, with particular emphasis on the development of the trailing vortex during the first revolution of the rotor. The distribution of vorticity was distinguished from the slightly elliptical swirl pattern. Peculiar dynamics within the void region may explain why the peak vorticity appeared to shift away from the center as the vortex aged, suggesting the onset of instability. The swirl and axial velocities (which reached 44 and 12 percent of the rotor-tip speed, respectively) were found to be asymmetric relative to the vortex center. In particular, the axial flow was composed of two concentrated zones moving in opposite directions. The radial distribution of the circulation rapidly increased in magnitude until reaching a point just beyond the core radius, after which the rate of growth decreased significantly. The core-radius circulation increased slightly with wake age, but the large-radius circulation appeared to remain relatively constant. The radial distributions of swirl velocity and vorticity exhibit self-similar behaviors, especially within the core. The diameter of the vortex core was initially about 10 percent of the rotor-blade chord, but more than doubled its size after one revolution of the rotor. According to vortex models that approximate the measured data, the core-radius circulation was about 79 percent of the large-radius circulation, and the large-radius circulation was about 67 percent of the maximum bound circulation on the rotor blade. On average, about 53 percent of the maximum bound circulation resides within the vortex core during the first revolution of the rotor.

  5. Studies on unsteady vortex motions including thermo-fluid interactions

    SciTech Connect

    Sundaram, P.

    1987-01-01

    An inviscid unsteady vortex dynamics study is presented to study the flow details in several free and bounded shear flows. The specific flows considered are the starting vortex behind sharp edges, the vortex street behind bluff-body wakes, the plane-mixing layers between two streams, and the wall bounded vortex street. Using the unsteady vortex dynamics models developed for each of these flows, and through the Lagrangian dynamics computational approach, the streakline and pathline pattern were computed for these flows. From the combined study of the computed streakline and pathline pattern, various flow-visualization details and the illusions created by the streaklines were identified and explained. With the accuracy of the unsteady vortex model for the vortex street wake having been verified by the good agreement obtained between the computed and flow visualization results, the model was used to investigate the different flow interactions present in this complex flow. This computational study highlights the important inviscid entrainment effects of convecting vortices by showing that the eruption-like appearance of the wall-layer streaklines observed in the flow visualization pictures is predominantly due to the inviscid entrainment effects and not merely caused by the viscous effects.

  6. Stratospheric polar vortex splits and displacements in the high-top CMIP5 climate models

    NASA Astrophysics Data System (ADS)

    Seviour, William J. M.; Gray, Lesley J.; Mitchell, Daniel M.

    2016-02-01

    Sudden stratospheric warming (SSW) events can occur as either a split or a displacement of the stratospheric polar vortex. Recent observational studies have come to different conclusions about the relative impacts of these two types of SSW upon surface climate. A clearer understanding of their tropospheric impact would be beneficial for medium-range weather forecasts and could improve understanding of the physical mechanism for stratosphere-troposphere coupling. Here we perform the first multimodel comparison of stratospheric polar vortex splits and displacements, analyzing 13 stratosphere-resolving models from the fifth Coupled Model Intercomparison Project (CMIP5) ensemble. We find a wide range of biases among models in both the mean state of the vortex and the frequency of vortex splits and displacements, although these biases are closely related. Consistent with observational results, almost all models show vortex splits to occur barotropically throughout the depth of the stratosphere, while vortex displacements are more baroclinic. Vortex splits show a slightly stronger North Atlantic surface signal in the month following onset. However, the most significant difference in the surface response is that vortex displacements show stronger negative pressure anomalies over Siberia. This region is shown to be colocated with differences in tropopause height, suggestive of a localized response to lower stratospheric potential vorticity anomalies.

  7. Computation of rotor aerodynamic loads in forward flight using a full-span free wake analysis

    NASA Technical Reports Server (NTRS)

    Quackenbush, Todd R.; Bliss, Donald B.; Wachspress, Daniel A.; Boschitsch, Alexander H.; Chua, Kiat

    1990-01-01

    The development of an advanced computational analysis of unsteady aerodynamic loads on isolated helicopter rotors in forward flight is described. The primary technical focus of the development was the implementation of a freely distorting filamentary wake model composed of curved vortex elements laid out along contours of constant vortex sheet strength in the wake. This model captures the wake generated by the full span of each rotor blade and makes possible a unified treatment of the shed and trailed vorticity in the wake. This wake model was coupled to a modal analysis of the rotor blade dynamics and a vortex lattice treatment of the aerodynamic loads to produce a comprehensive model for rotor performance and air loads in forward flight dubbed RotorCRAFT (Computation of Rotor Aerodynamics in Forward Flight). The technical background on the major components of this analysis are discussed and the correlation of predictions of performance, trim, and unsteady air loads with experimental data from several representative rotor configurations is examined. The primary conclusions of this study are that the RotorCRAFT analysis correlates well with measured loads on a variety of configurations and that application of the full span free wake model is required to capture several important features of the vibratory loading on rotor blades in forward flight.

  8. Transition and Turbulence Modeling for Blunt-Body Wake Flows

    NASA Technical Reports Server (NTRS)

    Nance, Robert P.; Horvath, Thomas J.; Hassan, H. A.

    1997-01-01

    Aerobraking has been proposed as an efficient means of decelerating spacecraft for planetary missions. Most current aerobrake designs feature a blunt forebody shielding the payload from the intense heat generated during atmospheric entry. Although this forebody will absorb the largest portion of the heat pulse, accurate prediction of heating in the near wake is of great importance, since large local heating values can occur at points of shear-layer impingement. In order to address the various issues associated with these blunt-body wake flowfields, the Advisory Group for Aerospace Research and Development (AGARD) formed Working Group 18 in 1992. One of the objectives of this activity was to examine real-gas effects in high-speed flow fields around a 70 deg. blunted cone. To date, many researchers have conducted experiments using this geometry in various facilities, such as the Large Energy National Shock (LENS) tunnel at Cubric/Calspan and the HEG shock tunnel at DLR-Goettingen. Several computational studies have also been conducted in concert with these tests. Many of the experimental results have indicated the possible presence of a transitional shear layer through a large increase in heat transfer downstream of the reattachment point. The presence of transition could in fact lead to much higher peak heating than if the separated flow is entirely laminar or turbulent. In the shock-tunnel tests, however, it is difficult to separate such viscous-flow phenomena from real-gas effects. In order to help make this distinction, Horvath et al. recently conducted a set of experiments in the NASA Langley 20-Inch Mach 6 Tunnel, and compared the results to laminar Navier-Stokes calculations. They found heat-transfer distributions similar to those obtained in the high-enthalpy facilities, with the measured peak heating along the sting support markedly greater than that predicted by the laminar computations. These trends point to the need to find transitional and turbulent computational solutions for these flowfields.

  9. High-resolution offshore wake simulations with the LES model PALM

    NASA Astrophysics Data System (ADS)

    Witha, B.; Steinfeld, G.; Heinemann, D.; Stütz, E.

    2012-04-01

    The turbulent wake of a wind turbine is important especially in wind farms, as it can affect the flow and power output of downstream turbines. Upstream turbines extract momentum of the mean flow so that the power output of subsequent turbines is reduced. On the other hand, the turbulence intensity is significantly enhanced which results in an increased load for downstream turbines. The marine atmospheric boundary layer is different from that onshore, especially in terms of a lower turbulence intensity and a higher wind speed due to the smaller roughness. So far, there has been little experience in simulating a realistic marine boundary layer. Models used for the design and energy yield prediction of offshore wind farms usually base upon onshore measurements. Several wind turbine models have been implemented in the LES model PALM: a simple uniformly loaded actuator disk model, an enhanced non-uniformly loaded actuator disk model which also accounts for rotational effects and an actuator line model. The comparison of the three turbine models for the wake of a single turbine shows, that the enhanced actuator disk model is a significant improvement and provides similarly good results as the computationally unfeasible actuator line model. With the enhanced actuator disk model simulations of a single wake have been conducted, using different inflow boundary conditions. The results have been compared with observations from the offshore test site "alpha ventus". So far, only cyclic inflow boundary conditions have been used for wake simulations, whose major drawback is the re-inflow of air already modified by the wind turbine. With non-cyclic boundary conditions, used for the first time in wake simulations, the inflow profile at the turbine remains undisturbed and constant in time. The additional application of a turbulent inflow results in a fully turbulent flow already at the inflow boundary, so that the model domain can be significantly reduced. Finally, results of a simulation of the offshore wind farm "alpha ventus" will be shown.

  10. A Hybrid Vortex Method for Two-Dimensional Flow Over Tube Bundles

    SciTech Connect

    Strickland, J.H.; Wolfe, W.P.

    1998-11-13

    A hybrid vortex method is presented for computing flows about objects that accurately resolves the boundary layer details while keeping the number of free vortices at a reasonable level. The method uses a wall layer model close to the body surface and discrete vortex blobs in the free wake. Details of the wall layer implementation are presented, and results of sample calculations are compared with known analytical solutions and with calculations from other vortex codes. These results show that the computed boundary layer details are accurate to approximately 0.3 percent of analytical solutions while using three orders of magnitude fewer vortices than other vortex simulations.

  11. Spatial evolution of a quasi-two-dimensional Kármán vortex street subjected to a strong uniform magnetic field

    NASA Astrophysics Data System (ADS)

    Hamid, Ahmad H. A.; Hussam, Wisam K.; Pothérat, Alban; Sheard, Gregory J.

    2015-05-01

    A vortex decay model for predicting spatial evolution of peak vorticity in a wake behind a cylinder is presented. For wake vortices in the stable region behind the formation region, results have shown that the presented model has a good capability of predicting spatial evolution of peak vorticity within an advecting vortex across 0.1 ≤ β ≤ 0.4, 500 ≤ H ≤ 5000, and 1500 ≤ ReL ≤ 8250. The model is also generalized to predict the decay behaviour of wake vortices in a class of quasi-two-dimensional magnetohydrodynamic duct flows. Comparison with published data demonstrates remarkable consistency.

  12. Helicopter rotor wake geometry and its influence in forward flight. Volume 2: Wake geometry charts

    NASA Technical Reports Server (NTRS)

    Egolf, T. A.; Landgrebe, A. J.

    1983-01-01

    Isometric and projection view plots, inflow ratio nomographs, undistorted axial displacement nomographs, undistorted longitudinal and lateral coordinates, generalized axial distortion nomographs, blade/vortex passage charts, blade/vortex intersection angle nomographs, and fore and aft wake boundary charts are discussed. Example condition, in flow ratio, undistorted axial location, longitudinal and lateral coordinates, axial coordinates distortions, blade/tip vortex intersections, angle of intersection, and fore and aft wake boundaries are also discussed.

  13. Velocity perturbations induced by the longitudinal vortices in a cylinder wake

    SciTech Connect

    Wu, J.; Welsh, M.C.; Sheridan, J.

    1996-09-01

    This paper presents data showing the three-dimensional vortical structures in the near wake region of circular cylinders. The in-plane velocity field was measured using a digital Particle Image Velocimetry (PIV) technique. The vortical structures are found to include inclined counter-rotating longitudinal vortices in the braids joining consecutive Karman vortices. A simple vortex-pair model is proposed to estimate velocity perturbation induced by the longitudinal vortices in the near wake region. The perturbation resulting from the longitudinal vortices is shown to induce spanwise velocity modulation and a velocity spike of a nominally two-dimensional vortex street.

  14. On the evolution of the wake structure produced by a low-aspect-ratio pitching panel

    PubMed Central

    BUCHHOLZ, JAMES H. J.; SMITS, ALEXANDER J.

    2009-01-01

    Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional von Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation. PMID:19746198

  15. On the evolution of the wake structure produced by a low-aspect-ratio pitching panel.

    PubMed

    Buchholz, James H J; Smits, Alexander J

    2005-12-01

    Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional von Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation. PMID:19746198

  16. Simulations of tip vortex cavitation flows with nonlinear k-ε model

    NASA Astrophysics Data System (ADS)

    Wang, Benlong; Liu, Zhihui; Peng, Xiaoxing; Liu, Dengcheng

    2015-12-01

    Effects of turbulence models on the simulations of tip vortex wetted flows and cavitating flows are studied. Two types RANS turbulence models, with and beyond the Boussinesq turbulent-viscosity hypothesis, are investigated by examining experimental results regarding strain rate and Reynolds shear stress distributions in the vortex region. The numerical results imply that the spatial phase shift between the mean strain rate and Reynolds stresses can be accurately modeled by the nonlinear k-ε turbulence model; the tip vortex cavitation region can only be predicted using the nonlinear turbulence model. The mechanism of the over-dissipation due to the turbulence model is analyzed in terms of the turbulence production, which is one of the dominant source terms in the transport equations of energy. The numerical results imply that the nonlinear k-ε model is a promising candidate for predicting tip vortex cavitation flows in practical applications at a reasonable computational cost.

  17. Transverse forces on a vortex in lattice models of superfluids

    NASA Astrophysics Data System (ADS)

    Sonin, E. B.

    2013-12-01

    The paper derives the transverse forces (the Magnus and the Lorentz forces) in the lattice models of superfluids in the continuous approximation. The continuous approximation restores translational invariance absent in the original lattice model, but the theory is not Galilean invariant. As a result, calculation of the two transverse forces on the vortex, Magnus force and Lorentz force, requires the analysis of two balances, for the true momentum of particles in the lattice (Magnus force) and for the quasimomentum (Lorentz force) known from the Bloch theory of particles in the periodic potential. While the developed theory yields the same Lorentz force, which was well known before, a new general expression for the Magnus force was obtained. The theory demonstrates how a small Magnus force emerges in the Josephson-junction array if the particle-hole symmetry is broken. The continuous approximation for the Bose-Hubbard model close to the superfluid-insulator transition was developed, which was used for calculation of the Magnus force. The theory shows that there is an area in the phase diagram for the Bose-Hubbard model, where the Magnus force has an inverse sign with respect to that which is expected from the sign of velocity circulation.

  18. PRELIMINARY STUDIES OF VIDEO IMAGES OF SMOKE DISPERSION IN THE NEAR WAKE OF A MODEL BUILDING

    EPA Science Inventory

    A scary of analyses of video images of smoke in a wind tunnel study of dispersion in the near wake of a model building is presented. The analyses provide information on both the instantaneous and the time- average patterns of dispersion. ince the images represent vertically-integ...

  19. A Heuristic Model of Media Conflict Systems in the Wake of Expose of Cover-Ups.

    ERIC Educational Resources Information Center

    Nager, Norman R.

    The purpose of this study was to stimulate scholarly investigation of a conflict systems model for news media and the interaction of sources in the wake of an expose. A search of the literature led to nine tentative axioms that focus attention on systemic conflict of media and sources related to cover-ups and suggested additional theory…

  20. Properties of the lunar wake predicted by analytic models and hybrid-kinetic simulations

    NASA Astrophysics Data System (ADS)

    Gharaee, Hossna; Rankin, Robert; Marchand, Richard; Paral, Jan

    2015-05-01

    An approximate model derived by Hutchinson (2008a), describing the interaction between the solar wind and the Moon, is applied to describe plasma in the lunar wake. The model accounts for plasma entering the wake region from two tangent points around the Moon. Cylindrical geometry is assumed, along with a strong constant magnetic field, and fixed transverse velocity and temperature. Under these approximations two angle-dependent equations for fluid flow are obtained, which can be solved using the method of characteristics to provide the density inside the wake region. It is demonstrated that the model valid under these assumptions provides excellent agreement with observations from the ARTEMIS mission and with large-scale hybrid-kinetic plasma simulations. The model provides a practical alternative to kinetic simulations and is generally useful for determining properties of the lunar wake under different solar wind conditions. It will be useful as well for predicting properties of the plasma environment around unmagnetized bodies that have not yet been visited by spacecraft.

  1. Modeling variable density turbulence in the wake of an air-entraining transom stern

    NASA Astrophysics Data System (ADS)

    Hendrickson, Kelli; Yue, Dick

    2015-11-01

    This work presents a priori testing of closure models for the incompressible highly-variable density turbulent (IHVDT) flows in the near wake region of a transom stern. This three-dimensional flow is comprised of convergent corner waves that originate from the body and collide on the ship center plane forming the ``rooster tail'' that then widens to form the divergent wave train. These violent free-surface flows and breaking waves are characterized by significant turbulent mass flux (TMF) at Atwood number At = (ρ2 -ρ1) / (ρ2 +ρ1) ~ 1 for which there is little guidance in turbulence closure modeling for the momentum and scalar transport along the wake. To whit, this work utilizes high-resolution simulations of the near wake of a canonical three-dimensional transom stern using conservative Volume-of-Fluid (cVOF), implicit Large Eddy Simulation (iLES), and Boundary Data Immersion Method (BDIM) to capture the turbulence and large scale air entrainment. Analysis of the simulation results across and along the wake for the TMF budget and turbulent anisotropy provide the physical basis of the development of multiphase turbulence closure models. Performance of isotropic and anisotropic turbulent mass flux closure models will be presented. Sponsored by the Office of Naval Research.

  2. Turbulence structures in wind turbine wake: Effects of atmospheric stratification

    NASA Astrophysics Data System (ADS)

    Bhaganagar, Kiran

    2014-11-01

    Turbulence structure in the wake behind full-scale horizontal-axis WT under the influence of realistic atmospheric turbulent flow conditions has been investigated using actuator-line-model based large-eddy-simulations. Wind turbine simulations have revealed that, in addition to wind shear and ABL turbulence, height-varying wind angle and low-level jets are ABL metrics that influence the structure of turbine wake. Turbulent mixing layer forms downstream of the WT, the strength and size of which decreases with increasing stability. Height dependent wind angle and turbulence are the ABL metrics influencing the lateral wake expansion. Further, ABL metrics strongly impact the evolution of tip and root vortices formed behind the rotor. Two factors play an important role in wake meandering: tip vortex merging due to the mutual inductance form of instability and the corresponding instability of the turbulent mixing layer. NSF CBET Energy for Sustainability.

  3. Wing-vortex interaction: unraveling the flowfield of a hovering rotor

    NASA Astrophysics Data System (ADS)

    Bhagwat, Mahendra J.; Caradonna, Francis X.; Ramasamy, Manikandan

    2015-01-01

    This paper focuses on one of the most prominent flow features of the hovering rotor wake, the close interaction of the tip vortex with a following blade. Such vortex interactions are fundamental determinants of rotor performance, loads, and noise. Yet, they are not completely understood, largely due to the lack of sufficiently comprehensive experimental data. The present study aims to perform such comprehensive measurements, not on hovering helicopter rotors (which hugely magnifies test complexity) but using fixed-wing models in controlled wind tunnel tests. The experiments were designed to measure, in considerable detail, the aerodynamic loading resulting from a vortex interacting with a semi-span wing, as well as the wake resulting from that interaction. The goal of the present study is to answer fundamental questions such as (a) the influence of a vortex passing below a wing on the lift, drag, tip vortex, and the wake of that wing and (b) the strength of the forming tip vortex and its relation to the wing loading and/or the tip loading. This paper presents detailed wing surface pressure measurements that result from the interaction of the wing with an interacting vortex trailing from an upstream wing. The data show large lift distribution changes for a range of wing-vortex interactions including the effects of close encounter with the vortex core. Significant asymmetry in the vortex-induced lift loading was observed, with the increase in wing sectional lift outboard of the interacting vortex (closer to the tip) being much smaller than the corresponding decrease inboard of the vortex.

  4. Conference on Capacity and Wake Vortices Held in Imperial College of Science, London on 11-14 September 2001. Abstracts of Presentations

    NASA Technical Reports Server (NTRS)

    Whitelaw, J. H.

    2001-01-01

    Partial Contents: The Effect of Aircraft Wake Vortex Separation on Air Transportation Capacity; The Pilots View of Wake Vortices - Capacity vs. Safety; Runway Capacity Constraints at Heathrow Airport; FAA's Research Strategy; Increasing Capacity by Wake Turbulence Avoidance Systems at Frankfurt/Main Airport; Improving Airport Capacity Using Vertical Flight; Recent Developments in Industrial Wake Vortex Research; Vortex Evolution and Characterization; PIV -Survey of the Vortex Wake Structure behind an Airbus A340 in a Towing Tank.

  5. Vortex shedding from struts in an annular exhaust diffuser

    SciTech Connect

    Fric, T.F.; Villarreal, R.; Auer, R.O.; James, M.L.; Ozgur, D.; Staley, T.K.

    1998-01-01

    Results from scale-model experiments and industrial gas turbine tests show that strut vortex shedding in an annular exhaust diffuser can effectively be modified by adding tapered chord to the struts. The struts are bluff bodies at full-speed, no-load conditions, when inlet swirl is close to 60 deg. Data from wind tunnel tests show that wake Strouhal number is 0.47, larger than that expected for an isolated cylinder wake. This value of Strouhal number agrees with those measured in full-scale exhaust diffusers. Wind tunnel tests showed that a strut with tapered chord most effectively reduced wake amplitudes and shifted shedding frequency. The tapered strut was also effective in reducing shedding amplitude in a scale-model diffuser. Finally, gas turbine tests employing a tapered strut showed significant reductions in unsteady pressure and noise. A major benefit of strut taper is a reduction of noise by uncoupling of vortex shedding from acoustic resonant response.

  6. Mesh sensitivity analysis on wind farms using CFD wind flow models and CFD wake models

    NASA Astrophysics Data System (ADS)

    Garcia Barcelo, Roger

    In the present framework, renewables seem to take every day more weight in the current energy policies. Among many types, wind energy, has already a great presence within green energies with great perspectives for the coming decades. A very effective tool for wind resource estimation is computational fluid dynamics (CFD). This technique solves the flow motion governing equations taking into account the recirculation effects and flow separation. Obviously, CFD results depend not only on modeling set up but also on achieving mesh independence. Meshing the computational domain must be the result of an optimized function taking into account two parameters: results accuracy and computational costs. In order to optimize the mesh, two sensitivity analysis are done through different battery cases. The geometrical parameters defining the mesh such as horizontal resolution, expansion of the different areas, height of the computational domain among others are analyzed. The main objective is to furnish a guideline of meshing parameters for CFDWind1.0 users through studies of wind flow over complex terrain and offshore. The results obtained over complex terrain shown very good accuracy and the grid geometry is optimized enough. Nevertheless, for wake model studies, the current modelisation can be improved and some further works are proposed. Keywords: CFD wind flow model, CFD wake model, Mesh sensitivity analysis, OpenFOAM, grid independence.

  7. Low-dimensional modelling of a transient cylinder wake using double proper orthogonal decomposition

    NASA Astrophysics Data System (ADS)

    Siegel, Stefan G.; Seidel, J.?Rgen; Fagley, Casey; Luchtenburg, D. M.; Cohen, Kelly; McLaughlin, Thomas

    For the systematic development of feedback flow controllers, a numerical model that captures the dynamic behaviour of the flow field to be controlled is required. This poses a particular challenge for flow fields where the dynamic behaviour is nonlinear, and the governing equations cannot easily be solved in closed form. This has led to many versions of low-dimensional modelling techniques, which we extend in this work to represent better the impact of actuation on the flow. For the benchmark problem of a circular cylinder wake in the laminar regime, we introduce a novel extension to the proper orthogonal decomposition (POD) procedure that facilitates mode construction from transient data sets. We demonstrate the performance of this new decomposition by applying it to a data set from the development of the limit cycle oscillation of a circular cylinder wake simulation as well as an ensemble of transient forced simulation results. The modes obtained from this decomposition, which we refer to as the double POD (DPOD) method, correctly track the changes of the spatial modes both during the evolution of the limit cycle and when forcing is applied by transverse translation of the cylinder. The mode amplitudes, which are obtained by projecting the original data sets onto the truncated DPOD modes, can be used to construct a dynamic mathematical model of the wake that accurately predicts the wake flow dynamics within the lock-in region at low forcing amplitudes. This low-dimensional model, derived using nonlinear artificial neural network based system identification methods, is robust and accurate and can be used to simulate the dynamic behaviour of the wake flow. We demonstrate this ability not just for unforced and open-loop forced data, but also for a feedback-controlled simulation that leads to a 90% reduction in lift fluctuations. This indicates the possibility of constructing accurate dynamic low-dimensional models for feedback control by using unforced and transient forced data only.

  8. Wake visualization behind multiple VAWTs in a wind tunnel using sPIV

    NASA Astrophysics Data System (ADS)

    Parker, Colin; Leftwich, Megan C.

    2014-11-01

    This work visualizes the wake behind multiple vertical axis wind turbines (VAWTs). The flow is visualized in a wind tunnel behind scaled model VAWTs driven at constant rotational velocity. The wake is visualized using stereo particle imaging velocimetry (sPIV) at the mid-plane downstream of the turbines. Syncing the sPIV system with the rotation of the turbine allows images to be taken at known phase angles. These images are then averaged to see the phase-averaged wake behind the VAWTs. Moving downstream, the averaged wake structure can be tracked by phase matching. Initially, data was taken in the near wake behind a single VAWT. As the blade turns normal to, and then back towards the free-stream, a vortex structure is shed into the wake and moves downstream. The out-of-plane velocity corresponding to this vortex pair shows the structures to be highly three-dimensional. Phase averaged wakes show distinct structures behind the turbine that move downstream with the free stream. Next, we measured the wake interactions behind a two turbine system. In this setup, a pair of counter rotating VAWTs is placed in the wind tunnel. We can vary the spacing and orientation between the counter rotating pair to compare changes in the downstream wind profile.

  9. Mathematical Models for Sleep-Wake Dynamics: Comparison of the Two-Process Model and a Mutual Inhibition Neuronal Model

    PubMed Central

    Skeldon, Anne C.; Dijk, Derk-Jan; Derks, Gianne

    2014-01-01

    Sleep is essential for the maintenance of the brain and the body, yet many features of sleep are poorly understood and mathematical models are an important tool for probing proposed biological mechanisms. The most well-known mathematical model of sleep regulation, the two-process model, models the sleep-wake cycle by two oscillators: a circadian oscillator and a homeostatic oscillator. An alternative, more recent, model considers the mutual inhibition of sleep promoting neurons and the ascending arousal system regulated by homeostatic and circadian processes. Here we show there are fundamental similarities between these two models. The implications are illustrated with two important sleep-wake phenomena. Firstly, we show that in the two-process model, transitions between different numbers of daily sleep episodes can be classified as grazing bifurcations. This provides the theoretical underpinning for numerical results showing that the sleep patterns of many mammals can be explained by the mutual inhibition model. Secondly, we show that when sleep deprivation disrupts the sleep-wake cycle, ostensibly different measures of sleepiness in the two models are closely related. The demonstration of the mathematical similarities of the two models is valuable because not only does it allow some features of the two-process model to be interpreted physiologically but it also means that knowledge gained from study of the two-process model can be used to inform understanding of the behaviour of the mutual inhibition model. This is important because the mutual inhibition model and its extensions are increasingly being used as a tool to understand a diverse range of sleep-wake phenomena such as the design of optimal shift-patterns, yet the values it uses for parameters associated with the circadian and homeostatic processes are very different from those that have been experimentally measured in the context of the two-process model. PMID:25084361

  10. A Framework for Quantitative Modeling of Neural Circuits Involved in Sleep-to-Wake Transition

    PubMed Central

    Sorooshyari, Siamak; Huerta, Ramón; de Lecea, Luis

    2015-01-01

    Identifying the neuronal circuits and dynamics of sleep-to-wake transition is essential to understanding brain regulation of behavioral states, including sleep–wake cycles, arousal, and hyperarousal. Recent work by different laboratories has used optogenetics to determine the role of individual neuromodulators in state transitions. The optogenetically driven data do not yet provide a multi-dimensional schematic of the mechanisms underlying changes in vigilance states. This work presents a modeling framework to interpret, assist, and drive research on the sleep-regulatory network. We identify feedback, redundancy, and gating hierarchy as three fundamental aspects of this model. The presented model is expected to expand as additional data on the contribution of each transmitter to a vigilance state becomes available. Incorporation of conductance-based models of neuronal ensembles into this model and existing models of cortical excitability will provide more comprehensive insight into sleep dynamics as well as sleep and arousal-related disorders. PMID:25767461

  11. Numerical Study of Vortex Matter using the Bose Model: First-Order Melting and Entanglement

    NASA Astrophysics Data System (ADS)

    Nordborg, Henrik; Vinokur, Valerii; Blatter, Gianni

    1998-03-01

    Despite its popularity as a theoretical model for vortices in type II superconductors, the 2D Bose model has only rarely been used as a basis for numerical studies of vortex phase diagram. Here we present an extensive numerical study of vortex lattice melting using the Bose model. For a clean system, we find a single first-order melting transition from a vortex lattice to an phentangled vortex liquid. The entropy jump per vortex and layer is given by Δ S ≈ 0.013 (1+t^2) \\varepsilon_0(0) d/T_m, where d is the layer spacing, Tm the melting temperature, and \\varepsilon0 the vortex line energy(H. Nordborg and G. Blatter, Phys. Rev. Lett. 79), 1925 (1997). The jump in density is Δ ρ ≈ 5.2× 10-4 /λ(T)^2. Both results are in good agreement with experimental results on YBCO. For the corresponding Bose system we have a single quantum phase transition from a Wigner crystal to a superfluid. We have also studied the effect of columnar defects in the vortex system, and present results on the Bose glass transition(D. R. Nelson and V. M. Vinokur Phys. Rev. Lett. 68), 2398 (1992). The case with a tilted magnetic field is especially interesting, since it corresponds to a Bose system with a non-Hermitian Hamiltonian(N. Hatano and D. R. Nelson, Phys. Rev. Lett. 77), 570 (1996).

  12. Source Term Model for Vortex Generator Vanes in a Navier-Stokes Computer Code

    NASA Technical Reports Server (NTRS)

    Waithe, Kenrick A.

    2004-01-01

    A source term model for an array of vortex generators was implemented into a non-proprietary Navier-Stokes computer code, OVERFLOW. The source term models the side force created by a vortex generator vane. The model is obtained by introducing a side force to the momentum and energy equations that can adjust its strength automatically based on the local flow. The model was tested and calibrated by comparing data from numerical simulations and experiments of a single low profile vortex generator vane on a flat plate. In addition, the model was compared to experimental data of an S-duct with 22 co-rotating, low profile vortex generators. The source term model allowed a grid reduction of about seventy percent when compared with the numerical simulations performed on a fully gridded vortex generator on a flat plate without adversely affecting the development and capture of the vortex created. The source term model was able to predict the shape and size of the stream-wise vorticity and velocity contours very well when compared with both numerical simulations and experimental data. The peak vorticity and its location were also predicted very well when compared to numerical simulations and experimental data. The circulation predicted by the source term model matches the prediction of the numerical simulation. The source term model predicted the engine fan face distortion and total pressure recovery of the S-duct with 22 co-rotating vortex generators very well. The source term model allows a researcher to quickly investigate different locations of individual or a row of vortex generators. The researcher is able to conduct a preliminary investigation with minimal grid generation and computational time.

  13. Controlling wake turbulence.

    PubMed

    Patnaik, B S V; Wei, G W

    2002-02-01

    This Letter introduces a control strategy for taming the wake turbulence behind a cylinder. An angular momentum injection scheme is proposed to synchronize the vertical velocity field. We show that the base suction, wake formation length, absolute instability, and the Kármán vortex street are effectively controlled by the angular momentum injection. A control equation is designed to implement the injection. The Navier-Stokes equations, along with the control equation, are solved. The occurrence of a new recirculation free zone is identified. PMID:11863732

  14. Arvicanthis ansorgei, a Novel Model for the Study of Sleep and Waking in Diurnal Rodents

    PubMed Central

    Hubbard, Jeffrey; Ruppert, Elisabeth; Calvel, Laurent; Robin-Choteau, Ludivine; Gropp, Claire-Marie; Allemann, Caroline; Reibel, Sophie; Sage-Ciocca, Dominique; Bourgin, Patrice

    2015-01-01

    Study Objectives: Sleep neurobiology studies use nocturnal species, mainly rats and mice. However, because their daily sleep/wake organization is inverted as compared to humans, a diurnal model for sleep studies is needed. To fill this gap, we phenotyped sleep and waking in Arvicanthis ansorgei, a diurnal rodent widely used for the study of circadian rhythms. Design: Video-electroencephalogram (EEG), electromyogram (EMG), and electrooculogram (EOG) recordings. Setting: Rodent sleep laboratory. Participants: Fourteen male Arvicanthis ansorgei, aged 3 mo. Interventions: 12 h light (L):12 h dark (D) baseline condition, 24-h constant darkness, 6-h sleep deprivation. Measurements and Results: Wake and rapid eye movement (REM) sleep showed similar electrophysiological characteristics as nocturnal rodents. On average, animals spent 12.9 h ± 0.4 awake per 24-h cycle, of which 6.88 h ± 0.3 was during the light period. NREM sleep accounted for 9.63 h ± 0.4, which of 5.13 h ± 0.2 during dark period, and REM sleep for 89.9 min ± 6.7, which of 52.8 min ± 4.4 during dark period. The time-course of sleep and waking across the 12 h light:12 h dark was overall inverted to that observed in rats or mice, though with larger amounts of crepuscular activity at light and dark transitions. A dominant crepuscular regulation of sleep and waking persisted under constant darkness, showing the lack of a strong circadian drive in the absence of clock reinforcement by external cues, such as a running wheel. Conservation of the homeostatic regulation was confirmed with the observation of higher delta power following sustained waking periods and a 6-h sleep deprivation, with subsequent decrease during recovery sleep. Conclusions: Arvicanthis ansorgei is a valid diurnal rodent model for studying the regulatory mechanisms of sleep and so represents a valuable tool for further understanding the nocturnality/diurnality switch. Citation: Hubbard J, Ruppert E, Calvel L, Robin-Choteau L, Gropp CM, Allemann C, Reibel S, Sage-Ciocca D, Bourgin P. Arvicanthis ansorgei, a novel model for the study of sleep and waking in diurnal rodents. SLEEP 2015;38(6):979–988. PMID:25409107

  15. Continuous parametric families of stationary and translating periodic point vortex configurations

    NASA Astrophysics Data System (ADS)

    O'Neil, Kevin A.

    The number of periodic arrangements of point vortices in two-dimensional fluid flow that are stationary is known to be finite for a generic choice of vortex circulations. When all circulations are the same in absolute value, however, stationary vortex street configurations have been associated with the zeros of certain trigonometric polynomials containing free complex parameters. The presence of these parameters may prove useful in constructing point vortex models of shear layers and wakes. In this paper it is shown that such a continuum of stationary configurations exists in a much wider class of point vortex street systems. The circulations may take on many values, not just two, providing increased flexibility in the modelling context. A simple method for computing these configurations is derived. The effects of symmetries on the solution polynomials are described, and illustrated with examples. In addition, novel translating vortex street configurations are found having arbitrary translation velocity and containing free parameters for vortex circulations 2.

  16. Experimental Study on the Effects of Winglets on the Wake of a Model Wind Turbine

    NASA Astrophysics Data System (ADS)

    Tobin, Nicolas; Hamed, Ali M.; Chamorro, Leonardo P.

    2015-11-01

    Wind tunnel particle image velocimetry was used to investigate the effects of winglets on the wake dynamics of a model wind turbine. The behavior of a turbine with downstream-facing winglets was compared with a turbine without winglets. The turbines were placed in a turbulent boundary layer that reached up to the hub height, allowing for investigation of behavior in both turbulent and uniform flow. The winglets did not significantly change the strength of the tip vortices in the region of uniform incoming flow. The tip vortices in the more turbulent region, however, decayed much faster, diminishing to near-zero within the first ~1.5 rotor diameters, whereas the upper tip vortices persisted potentially up to ~4 rotor diameters. The winglets also increased the power coefficient by 7.5 %, while increasing the coefficient of thrust by 10.0 %. The higher coefficient of thrust created a region of enhanced mean shear in the outer portion of the wake, leading to increased turbulence statistics in the far wake. The wingletted turbine had a similar wake deficit at 5 rotor diameters as the base turbine did at 1.5 rotor diameters, with potential implications for using wingletted turbines in wind farms.

  17. Experimental study on influence of pitch motion on the wake of a floating wind turbine model

    NASA Astrophysics Data System (ADS)

    Rockel, Stanislav; Cal, Raul Bayoan; Peinke, Joachim; Hoelling, Michael

    2013-11-01

    Wind energy has become a major contributor to energy from renewable sources and is still demanded to increase its portion to the overall energy supply. Offshore wind energy was found to have the highest potential to fulfill these demands, due to better and steadier wind conditions found on seas. Offshore wind turbines which have been installed lately use monopiles as foundations and are feasible in shallow water up to a depth of 50m. Such shallow areas are rare and often exploited, so floating support structures for offshore wind turbines in deep water are possible solutions. The additional degrees of freedom of a floating support structure will influence the aerodynamics at the rotor and its wake. Wind tunnel experiments were performed using a classical fixed turbine model and a streamwise oscillating turbine in free pitch motion. For both cases the turbines were operated under same inflow conditions and wakes up to 7 rotor diameters were measured using 2D-3C stereographic particle image velocimetry (SPIV). The obtained data was statistically analyzed and a direct comparison of the wake of the fixed and oscillating turbine was performed. Our results show that inclinations and oscillations of the turbine have a strong impact on the structure of the wake and its development.

  18. Calculations of longitudinal and transverse velocity structure functions using a vortex model of isotropic turbulence

    SciTech Connect

    He, G.; Doolen, G.D.; Chen, S.

    1999-12-01

    The longitudinal structure function (LSF) and the transverse structure function (TSF) in isotropic turbulence are calculated using a vortex model. The vortex model is composed of the Rankine and Burgers vortices which have the exponential distributions in the vortex Reynolds number and vortex radii. This model exhibits a power law in the inertial range and satisfies the minimal condition of isotropy that the second-order exponent of the LSF in the inertial range is equal to that of the TSF. Also observed are differences between longitudinal and transverse structure functions caused by intermittency. These differences are related to their scaling differences which have been previously observed in experiments and numerical simulations. {copyright} {ital 1999 American Institute of Physics.}

  19. SAR observation and numerical modeling of tidal current wakes at the East China Sea offshore wind farm

    NASA Astrophysics Data System (ADS)

    Li, XiaoMing; Chi, Lequan; Chen, Xueen; Ren, YongZheng; Lehner, Susanne

    2014-08-01

    A TerraSAR-X (TS-X) Synthetic Aperture Radar (SAR) image acquired at the East China Sea offshore wind farm presents distinct wakes at a kilometer scale on the lee of the wind turbines. The presumption was that these wakes were caused by wind movement around turbine blades. However, wind analysis using spaceborne radiometer data, numerical weather prediction, and in situ measurements suggest that the prevailing wind direction did not align with the wakes. By analyzing measurement at the tidal gauge station and modeling of the tidal current field, these trailing wakes are interpreted to have formed when a strong tidal current impinged on the cylindrical monopiles of the wind turbines. A numerical simulation was further conducted to reproduce the tidal current wake under such conditions. Comparison of the simulated surface velocity in the wake region with the TS-X sea surface backscatter intensity shows a similar trend. Consequently, turbulence intensity (T.I.) of the tidal current wakes over multiple piles is studied using the TS-X observation. It is found that the T.I. has a logarithmic relation with distance. Furthermore, another case study showing wakes due to wind movement around turbine blades is presented to discuss the differences in the tidal current wakes and wind turbine wakes. The conclusion is drawn that small-scale wakes formed by interaction of the tidal current and the turbine piles could be also imaged by SAR when certain conditions are satisfied. The study is anticipated to draw more attentions to the impacts of offshore wind foundations on local hydrodynamic field.

  20. Euler solutions for self-generated rotor blade-vortex interactions

    NASA Technical Reports Server (NTRS)

    Hassan, A. A.; Tung, C.; Sankar, L. N.

    1990-01-01

    A finite-difference procedure was developed, on the basis of the conservation form of the unsteady three-dimensional Euler equations, for the prediction of rotor blade-vortex interactions (BVIs). Numerical solution procedures were obtained for the analysis of the model parallel BVIs and the more realistic helicopter self-generated-rotor BVIs. It was found that, for self-generated subcritical interactions, the accuracy of the predicted leading edge pressures relied heavily on the user-specified vortex core radius and on the CAMRAD-code-predicted geometry of the interaction vortex elements and their relative orientation with respect to the blade. It was also found that the free-wake model used in CAMRAD to predict the tip vortex trajectory for use in the Euler solution yields lower streamwise and higher axial wake convective velocities than those inferred from the experimental data.

  1. Sound signature of propeller tip vortex cavitation

    NASA Astrophysics Data System (ADS)

    Pennings, Pepijn; Westerweel, Jerry; van Terwisga, Tom

    2015-12-01

    The design of an efficient propeller is limited by the harmful effects of cavitation. The insufficient understanding of the role of vortex cavitation in noise and vibration reduces the maximum efficiency by a necessary safety margin. The aim in the present study is to directly relate propeller cavitation sound to tip vortex cavity dynamics. This is achieved by a dedicated experiment in a cavitation tunnel on a specially designed two-bladed propeller using a high-speed video camera and a hydrophone. The sound signature of a tip vortex cavity is not evidently present in the sound spectrum above the tunnel background. The addition of a simulated wake inflow results in a high amplitude broadband sound. With a decrease in the free-stream pressure the centre frequency of this sound decreases as a result of a larger vortex cavity diameter. In the near future each blade passage in the high-speed video will be analyzed in detail. The frequency content of the cavity dynamics can then be directly related to the measured sound. An analytic model for vortex cavity dynamics resulting in a cavity eigenfrequency using a vortex velocity model can finally be evaluated as a design instrument for estimation of broadband sound from propeller cavitation.

  2. Singularity formation and nonlinear evolution of a viscous vortex sheet model

    NASA Astrophysics Data System (ADS)

    Sohn, Sung-Ik

    2013-01-01

    We study Dhanak's model [J. Fluid Mech. 269, 265 (1994)], 10.1017/S0022112094001552 of a viscous vortex sheet in the sharp limit, to investigate singularity formations and present nonlinear evolutions of the sheets. The finite-time singularity does not disappear by giving viscosity to the vortex sheet, but is delayed. The singularity in the sharp viscous vortex sheet is found to be different from that of the inviscid sheet in several features. A discontinuity in the curvature is formed in the viscous sheet, similarly as the inviscid sheet, but a cusp in the vortex sheet strength is less sharpened by viscosity. Exponential decay of the Fourier amplitudes is lost by the formation of the singularity, and the amplitudes of high wavenumbers exhibit an algebraic decay, while in the inviscid vortex sheet, the algebraic decay of the Fourier amplitudes is valid from fairly small wavenumbers. The algebraic decay rate of the viscous vortex sheet is approximately -2.5, independent of viscosity, which is the same rate as the asymptotic analysis of the inviscid sheet. Results for evolutions of the regularized vortex sheets show that the roll-up is weakened by viscosity, and the regularization parameter has more significant effects on the fine-structure of the core than does viscosity.

  3. Effects of incoming surface wind conditions on the wake characteristics and dynamic wind loads acting on a wind turbine model

    NASA Astrophysics Data System (ADS)

    Tian, Wei; Ozbay, Ahmet; Hu, Hui

    2014-12-01

    An experimental investigation was conducted to examine the effects of incoming surface wind conditions on the wake characteristics and dynamic wind loads acting on a wind turbine model. The experimental study was performed in a large-scale wind tunnel with a scaled three-blade Horizontal Axial Wind Turbine model placed in two different types of Atmospheric Boundary Layer (ABL) winds with distinct mean and turbulence characteristics. In addition to measuring dynamic wind loads acting on the model turbine by using a force-moment sensor, a high-resolution Particle Image Velocimetry system was used to achieve detailed flow field measurements to characterize the turbulent wake flows behind the model turbine. The measurement results reveal clearly that the discrepancies in the incoming surface winds would affect the wake characteristics and dynamic wind loads acting on the model turbine dramatically. The dynamic wind loads acting on the model turbine were found to fluctuate much more significantly, thereby, much larger fatigue loads, for the case with the wind turbine model sited in the incoming ABL wind with higher turbulence intensity levels. The turbulent kinetic energy and Reynolds stress levels in the wake behind the model turbine were also found to be significantly higher for the high turbulence inflow case, in comparison to those of the low turbulence inflow case. The flow characteristics in the turbine wake were found to be dominated by the formation, shedding, and breakdown of various unsteady wake vortices. In comparison with the case with relatively low turbulence intensities in the incoming ABL wind, much more turbulent and randomly shedding, faster dissipation, and earlier breakdown of the wake vortices were observed for the high turbulence inflow case, which would promote the vertical transport of kinetic energy by entraining more high-speed airflow from above to re-charge the wake flow and result in a much faster recovery of the velocity deficits in the turbine wake.

  4. Supersonic shock wave/vortex interaction

    NASA Technical Reports Server (NTRS)

    Settles, G. S.; Cattafesta, L.

    1993-01-01

    Although shock wave/vortex interaction is a basic and important fluid dynamics problem, very little research has been conducted on this topic. Therefore, a detailed experimental study of the interaction between a supersonic streamwise turbulent vortex and a shock wave was carried out at the Penn State Gas Dynamics Laboratory. A vortex is produced by replaceable swirl vanes located upstream of the throat of various converging-diverging nozzles. The supersonic vortex is then injected into either a coflowing supersonic stream or ambient air. The structure of the isolated vortex is investigated in a supersonic wind tunnel using miniature, fast-response, five-hole and total temperature probes and in a free jet using laser Doppler velocimetry. The cases tested have unit Reynolds numbers in excess of 25 million per meter, axial Mach numbers ranging from 2.5 to 4.0, and peak tangential Mach numbers from 0 (i.e., a pure jet) to about 0.7. The results show that the typical supersonic wake-like vortex consists of a non-isentropic, rotational core, where the reduced circulation distribution is self similar, and an outer isentropic, irrotational region. The vortex core is also a region of significant turbulent fluctuations. Radial profiles of turbulent kinetic energy and axial-tangential Reynolds stress are presented. The interactions between the vortex and both oblique and normal shock waves are investigated using nonintrusive optical diagnostics (i.e. schlieren, planar laser scattering, and laser Doppler velocimetry). Of the various types, two Mach 2.5 overexpanded-nozzle Mach disc interactions are examined in detail. Below a certain vortex strength, a 'weak' interaction exists in which the normal shock is perturbed locally into an unsteady 'bubble' shock near the vortex axis, but vortex breakdown (i.e., a stagnation point) does not occur. For stronger vortices, a random unsteady 'strong' interaction results that causes vortex breakdown. The vortex core reforms downstream of the rear stagnation point, and the reduced circulation distribution once again becomes self-similar in this region. A-new model of this interaction is proposed. Finally, a curve defining the approximate limits of supersonic vortex breakdown is presented.

  5. Application of a stochastic vehicular wake erosion model to determine PM2.5 exposure

    NASA Astrophysics Data System (ADS)

    Jazcilevich, Aron; Wellens, Ann; Siebe, Christina; Rosas, Irma; Bornstein, Robert D.; Riojas-Rodríguez, Horacio

    2012-06-01

    A stochastic model based on empirical probability distribution functions has been applied to estimate human exposure to emissions from the resuspension of road dust due to isolated wakes from moving vehicles. The model incorporates the turbulent nature of resuspension due to vehicular wake erosion phenomena, and is tested by in situ measurements. It statistically estimates exposure as a function of ambient temperature, traffic activity, human height, and breathe intake rates as a function of age and activity. The model is applied to evaluate whether communities in the Molango region in Mexico are affected by vehicular resuspension of Manganese (Mn) contaminated soil from on-road activities related to mining. Results showed that six and 10 year old children are at higher risk to vehicular resuspended dust exposure than adults, due to their lower height and higher respiratory intake rates during periods of light and moderate activity.

  6. Prescribed wake methodologies for wind turbine design codes

    SciTech Connect

    Galbraith, R.A.M.; Coton, F.N.; Robison, D.J.

    1995-12-31

    Prescribed wake performance assessment models have been developed successfully for both vertical (VAWT) and horizontal (HAWT) axis wind turbines. In the case of the VAWT model the Beddoes and Leishman dynamic stall model has been incorporated. This has resulted in a fully unsteady 3-D code, establishing extremely accurate performance prediction across a wide range of operating conditions. Comparison of performance estimates from the prescribed wake model with those from free wake models have shown excellent correlation. To date, the HAWT model has been developed for the consideration of steady axial and yawed inflows. In the axial flow case comparisons of predicted power output with field data and free wake predictions have shown excellent agreement. Full validation of the yawed flow model is currently underway, with very encouraging initial results. The capabilities of the HAWT model are currently being extended by the inclusion of the Beddoes and Leishman dynamic stall model. Consideration of the significant unsteady aerodynamic influences acting on HAWTs while operating in yaw will significantly improve the models performance. The power of this modelling technique is the significant reduction in the computational overhead it offers. The prescribed wake models offer performance estimates of comparable detail and accuracy to those from free vortex analyses in minutes rather than hours. As such these models are highly suited to design assessment, with particular application to fatigue load analysis.

  7. A Numerical Simulation Study to Develop an Acceptable Wake Encounter Boundary for a B737-100 Airplane

    NASA Technical Reports Server (NTRS)

    Vicroy, Dan D.; Nguyen, Truc

    1993-01-01

    The National Aeronautics and Space Administration (NASA) is conducting research with the goal of enabling safe improvements in the capacity of the nation's air transportation system. The wake-vortex upset hazard is an important factor in establishing the minimum safe spacing between aircraft during landing and take-off operations, thus impacting airport capacity. A batch simulation study was conducted to assess the sensitivity of various safe landing criteria in the development of an acceptable wake encounter boundary. A baseline six-degree-of-freedom simulation of a B737-100 airplane was modified to include a wake model and the vortex-induced forces and moments. The guidance and control input for the airplane was provided by an auto-land system. The wake strength and encounter geometry were varied. A sensitivity study was also conducted to assess the effects of encounter modeling methods and accuracy.

  8. VIDEO IMAGES OF SMOKE DISPERSION IN THE NEAR WAKE OF A BUILDING PART I. TEMPORAL AND SPATIAL SCALES OF VORTEX SHEDDING

    EPA Science Inventory

    In a wind-tunnel study, recorded video images of smoke dispersion in the wake of a rectangular-shaped building web analyzed. ontinuous source of smoke was emitted at floor level, midway along the leeward side of the building. moke was observed to build up within a region adjacent...

  9. Modeling the impact of impulsive stimuli on sleep-wake dynamics

    NASA Astrophysics Data System (ADS)

    Fulcher, B. D.; Phillips, A. J. K.; Robinson, P. A.

    2008-11-01

    A neuronal population model of the sleep-wake switch is extended to incorporate impulsive external stimuli. The model includes the mutual inhibition of the sleep-active neurons in the hypothalamic ventrolateral preoptic area (VLPO) and the wake-active monoaminergic brainstem populations (MA), as well as circadian and homeostatic drives. Arbitrary stimuli are described in terms of their relative effects on the VLPO and MA nuclei and represent perturbations on the normal sleep-wake dynamics. By separating the model’s intrinsic time scales, an analytic characterization of the dynamics in a reduced