The Boundary Layers in Fluids with Little Friction
NASA Technical Reports Server (NTRS)
Blasius, H.
1950-01-01
The vortices forming in flowing water behind solid bodies are not represented correctly by the solution of the potential theory nor by Helmholtz's jets. Potential theory is unable to satisfy the condition that the water adheres at the wetted bodies, and its solutions of the fundamental hydrodynamic equations are at variance with the observation that the flow separates from the body at a certain point and sends forth a highly turbulent boundary layer into the free flow. Helmholtz's theory attempts to imitate the latter effect in such a way that it joins two potential flows, jet and still water, nonanalytical along a stream curve. The admissibility of this method is based on the fact that, at zero pressure, which is to prevail at the cited stream curve, the connection of the fluid, and with it the effect of adjacent parts on each other, is canceled. In reality, however, the pressure at these boundaries is definitely not zero, but can even be varied arbitrarily. Besides, Helmholtz's theory with its potential flows does not satisfy the condition of adherence nor explain the origin of the vortices, for in all of these problems, the friction must be taken into account on principle, according to the vortex theorem.
Friction dynamics of confined weakly adhering boundary layers.
Mazuyer, Denis; Cayer-Barrioz, Juliette; Tonck, André; Jarnias, Frédéric
2008-04-15
The nanotribological behavior of self-assembled monolayers is investigated. The latter accommodate friction through transient relaxation and dilatancy effects whose kinetics depends on the structure of the confined layers. Thus, the molecular ordering onto the surfaces controls the level and the stability of the friction coefficient. Moreover, the behavior of these systems is theoretically accounted for using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces with an additional viscous term. PMID:18327959
Effects of Riblets on Skin Friction in High-Speed Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Duan, Lian; Choudhari, Meelan M.
2012-01-01
Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction at supersonic speeds. Zero-pressure gradient boundary layers with an adiabatic wall, a Mach number of M1 = 2.5, and a Reynolds number based on momentum thickness of Re = 1720 are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric V- groove riblets with nominal spacings of 20 and 40 wall units. The DNS results confirm the few existing experimental observations and show that a drag reduction of approximately 7% is achieved for riblets with proper spacing. The influence of riblets on turbulence statistics is analyzed in detail with an emphasis on identifying the differences, if any, between the drag reduction mechanisms for incompressible and high-speed boundary layers.
Skin-Friction Measurements in a 3-D, Supersonic Shock-Wave/Boundary-Layer Interaction
NASA Technical Reports Server (NTRS)
Wideman, J. K.; Brown, J. L.; Miles, J. B.; Ozcan, O.
1994-01-01
The experimental documentation of a three-dimensional shock-wave/boundary-layer interaction in a nominal Mach 3 cylinder, aligned with the free-stream flow, and 20 deg. half-angle conical flare offset 1.27 cm from the cylinder centerline. Surface oil flow, laser light sheet illumination, and schlieren were used to document the flow topology. The data includes surface-pressure and skin-friction measurements. A laser interferometric skin friction data. Included in the skin-friction data are measurements within separated regions and three-dimensional measurements in highly-swept regions. The skin-friction data will be particularly valuable in turbulence modeling and computational fluid dynamics validation.
Boundary Layer Skin Friction in the Vicinity of Multiple Jets in Crossflow
NASA Astrophysics Data System (ADS)
Peterson, Sean; Plesniak, Michael
2000-11-01
Multiple jets in crossflow are commonly used for gas turbine film cooling and other industrial applications. Various flow structures, such as the pair(s) of counter-rotating vortices, influence the jet trajectory, separation and reattachment phenomena, and ultimately the surface skin friction and heat transfer. This study utilized a single row of five "short" injection hole jets separated in the spanwise direction by three jet diameters, which were injected into a crossflow at a 90-degree inclination. When short holes are used, i.e. L/D = 1 or less, the external flowfield is influenced by the flow within the holes and supply plenum, especially by separation events. A Fringe Imaging Skin Friction (FISF) technique was employed to investigate the effect of velocity ratio (0.5 to 1.0) and plenum flow direction on the skin friction in the boundary layer between and downstream of the injection holes. Interference fringes produced by a thin oil film deposited on the surface were imaged by a digital camera/frame grabber and processed to infer the skin friction distributions. Correlation between the skin friction distributions and other reported flow characteristics, and their relation to film cooling effectiveness, will be discussed.
NASA Technical Reports Server (NTRS)
Richmond, Robert Chafee (Inventor); Schramm, Jr., Harry F. (Inventor); Defalco, Francis G. (Inventor)
2013-01-01
A wear and/or friction reducing additive for a lubricating fluid in which the additive is a combination of a moderately hydrophilic single-phase compound and an anti-wear and/or anti-friction aqueous salt solution. The aqueous salt solution produces a coating on boundary layer surfaces. The lubricating fluid can be an emulsion-free hydrophobic oil, hydraulic fluid, antifreeze, or water. Preferably, the moderately hydrophilic single-phase compound is sulfonated castor oil and the aqueous salt solution additionally contains boric acid and zinc oxide. The emulsions produced by the aqueous salt solutions, the moderately hydrophilic single-phase compounds, or the combination thereof provide targeted boundary layer organizers that significantly enhance the anti-wear and/or anti-friction properties of the base lubricant by decreasing wear and/or friction of sliding and/or rolling surfaces at boundary layers.
High frequency radar measurements of friction velocity in the marine boundary layer
NASA Astrophysics Data System (ADS)
Meadows, Lorelle Annise
The purpose of this dissertation research is to determine the utility of using a multi-frequency high frequency (BF) radar operating at decameter wavelengths to obtain estimates of the friction velocity in the sea and to relate these estimates to measurements obtained on the air side of the interface. This is accomplished through the detection of changes in the phase velocity of surface gravity waves induced by flow in the water. In situ measurements of this sort are difficult to obtain due to the harsh environment which exists at the air-sea interface, and are usually restricted to point measurements obtained at moored instrument platforms or onboard ships. The use of the HF radar to obtain such information bypasses these difficulties and provides synoptic coverage in near real time. In this work, the Levenberg-Marquardt method of non-linear least squares is used to determine the near surface current profile based on a theoretical model of the effect of the current on the phase velocity of a surface gravity wave. The results of this analysis are compared to in situ measurements of wind velocity and calculations of friction velocity obtained by the Innovative Coastal-ocean Observing Network group as part of the Monterey Bay Aquarium Research Institute (MBARI) Ocean Observing System Upper-water-column Science Experiment over Monterey Bay, California. In particular, a comparison is made with the Flux buoy measurements and calculations of the Boundary Layer Studies Group of the Naval Postgraduate School and the MBARI M1 buoy measurements. In addition, several methods to determine the variance in the HF data are tested to ensure data quality, and the adequacy of the HF frequencies for friction velocity retrieval are evaluated. The results show that our estimates of the friction velocity compare well with in situ measurements during moderate wind conditions, in excess of about 5 m/s and less than about 12 m/s. Further evaluation of the present method for obtaining friction velocities under higher wind conditions is warranted. In addition, development of a higher order wave-current interaction theory and/or wave boundary layer flow theory would enhance this capability.
NASA Technical Reports Server (NTRS)
Richmond, Robert Chaffee (Inventor); Schramm, Jr., Harry F. (Inventor); Defalco, Francis G. (Inventor)
2015-01-01
Lubrication additives of the current invention require formation of emulsions in base lubricants, created with an aqueous salt solution plus a single-phase compound such that partitioning within the resulting emulsion provides thermodynamically targeted compounds for boundary layer organization thus establishing anti-friction and/or anti-wear. The single-phase compound is termed "boundary layer organizer", abbreviated BLO. These emulsion-contained compounds energetically favor association with tribologic surfaces in accord with the Second Law of Thermodynamics, and will organize boundary layers on those surfaces in ways specific to the chemistry of the salt and BLO additives. In this way friction modifications may be provided by BLOs targeted to boundary layers via emulsions within lubricating fluids, wherein those lubricating fluids may be water-based or oil-based.
Boundary-layer Development and Skin Friction at Mach Number 3.05
NASA Technical Reports Server (NTRS)
Brinich, Paul F; Diaconis, Nick S
1952-01-01
Experimental and theoretical results are presented for boundary layer studies consisting of Schlieren observations and momentum surveys made on hollow cylinder models with their axes aligned parallel to the stream. Results were obtained for three model diameters and for natural and artificially induced turbulent boundary layer flows.
NASA Technical Reports Server (NTRS)
Bertram, Mitchel H.; Feller, William V.
1959-01-01
A procedure based on the method of similar solutions is presented by which the skin friction, heat transfer, and boundary-layer thickness in a laminar hypersonic flow with pressure gradient may be rapidly evaluated if the pressure distribution is known. This solution, which at present is. restricted to power-law variations of pressure with surface distance, is presented for a wide range of exponents in the power law corresponding to both favorable and adverse pressure gradients. This theory has been compared to results from heat-transfer experiments on blunt-nose flat plates and a hemisphere cylinder at free-stream Mach numbers of 4 and 6.8. The flat-plate experiments included tests made at a Mach number of 6.8 over a range of angle of attack of +/- 10 deg. Reasonable agreement of the experimental and theoretical heat-transfer coefficients has been obtained as well as good correlation of the experimental results over the entire range of angle of attack studied. A similar comparison of theory with experiment was not feasible for boundary-layer-thickness data; however, the hypersonic similarity theory was found to account satisfactorily for the variation in boundary-layer thickness due to local pressure distribution for several sets of measurements.
NASA Astrophysics Data System (ADS)
MacRorie, Michael; Pauley, Wayne R.
1993-01-01
The interaction between propagating spanwise vortices and a turbulent boundary layer was studied experimentally. The experimental techniques include hotwire anemometry and smoke visualization. The results focus on the relationship between the passage of vortex structures and the response of the boundary layer in terms of unsteady mean velocity, wall shear, and turbulence quantities. Both positive and negative circulation vortices were studied at three different heights above the test surface. The results indicate that the height of the vortex above the surface has an effect on the wall shear response. However, vortex height and strength are related in this experiment. A phase lag between the passage of the vortex center and the peak wall shear stress response is demonstrated. This phase lag was found to increase with streamwise distance. An examination of the response of the mean and turbulent velocity fields in the boundary layer shows that the phase lag is confined to a region close to the wall.
NASA Technical Reports Server (NTRS)
Frankl, F.; Voishel, V.
1943-01-01
In the present report an investigation is made on a flat plate in a two-dimensional compressible flow of the effect of compressibility and heating on the turbulent frictional drag coefficient in the boundary layer of an airfoil or wing radiator. The analysis is based on the Prandtl-Karman theory of the turbulent boundary later and the Stodola-Crocco, theorem on the linear relation between the total energy of the flow and its velocity. Formulas are obtained for the velocity distribution and the frictional drag law in a turbulent boundary later with the compressibility effect and heat transfer taken into account. It is found that with increase of compressibility and temperature at full retardation of the flow (the temperature when the velocity of the flow at a given point is reduced to zero in case of an adiabatic process in the gas) at a constant R (sub x), the frictional drag coefficient C (sub f) decreased, both of these factors acting in the same sense.
NASA Astrophysics Data System (ADS)
McNaughton, K. G.; Clement, R. J.; Moncrieff, J. B.
2007-06-01
We report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection". We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (zi ɛo)2/3, where zi is inversion height and ɛo is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z ɛo)2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow. We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z1/2 zi1/2). That is, the lengths of the thermal structures depend on both the lengths of the transporting eddies, ~9z, and the progressive aggregation of the plumes with height into the larger-scale structures of the CBL. This aggregation depends, in top-down fashion, on zi. The whole system is therefore highly organized, with even the smallest structures conforming to the overall requirements of the whole flow.
NASA Astrophysics Data System (ADS)
Rodrguez-Lpez, Eduardo; Bruce, Paul J. K.; Buxton, Oliver R. H.
2015-04-01
The present paper describes a method to extrapolate the mean wall shear stress, , and the accurate relative position of a velocity probe with respect to the wall, , from an experimentally measured mean velocity profile in a turbulent boundary layer. Validation is made between experimental and direct numerical simulation data of turbulent boundary layer flows with independent measurement of the shear stress. The set of parameters which minimize the residual error with respect to the canonical description of the boundary layer profile is taken as the solution. Several methods are compared, testing different descriptions of the canonical mean velocity profile (with and without overshoot over the logarithmic law) and different definitions of the residual function of the optimization. The von Krmn constant is used as a parameter of the fitting process in order to avoid any hypothesis regarding its value that may be affected by different initial or boundary conditions of the flow. Results show that the best method provides an accuracy of for the estimation of the friction velocity and for the position of the wall. The robustness of the method is tested including unconverged near-wall measurements, pressure gradient, and reduced number of points; the importance of the location of the first point is also tested, and it is shown that the method presents a high robustness even in highly distorted flows, keeping the aforementioned accuracies if one acquires at least one data point in . The wake component and the thickness of the boundary layer are also simultaneously extrapolated from the mean velocity profile. This results in the first study, to the knowledge of the authors, where a five-parameter fitting is carried out without any assumption on the von Krmn constant and the limits of the logarithmic layer further from its existence.
NASA Technical Reports Server (NTRS)
Howard, F. G.; Strokowski, A. J.
1978-01-01
Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased the skin friction, but the incremental improvement in skin-friction reduction decreased as the number of slots was increased. Cooling effectiveness was found to improve, for a given total mass injection, as the number of slots was increased from one to four. Comparison with previously reported step-slot data, however, indicated that step slots with tangential injection are more effective for both reducing skin friction and cooling than the present flush-slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction data and with boundary-layer profile data.
NASA Technical Reports Server (NTRS)
Loitsianskii. L. G.
1956-01-01
The fundamental, practically the most important branch of the modern mechanics of a viscous fluid or a gas, is that branch which concerns itself with the study of the boundary layer. The presence of a boundary layer accounts for the origin of the resistance and lift force, the breakdown of the smooth flow about bodies, and other phenomena that are associated with the motion of a body in a real fluid. The concept of boundary layer was clearly formulated by the founder of aerodynamics, N. E. Joukowsky, in his well-known work "On the Form of Ships" published as early as 1890. In his book "Theoretical Foundations of Air Navigation," Joukowsky gave an account of the most important properties of the boundary layer and pointed out the part played by it in the production of the resistance of bodies to motion. The fundamental differential equations of the motion of a fluid in a laminar boundary layer were given by Prandtl in 1904; the first solutions of these equations date from 1907 to 1910. As regards the turbulent boundary layer, there does not exist even to this day any rigorous formulation of this problem because there is no closed system of equations for the turbulent motion of a fluid. Soviet scientists have done much toward developing a general theory of the boundary layer, and in that branch of the theory which is of greatest practical importance at the present time, namely the study of the boundary layer at large velocities of the body in a compressed gas, the efforts of the scientists of our country have borne fruit in the creation of a new theory which leaves far behind all that has been done previously in this direction. We shall herein enumerate the most important results by Soviet scientists in the development of the theory of the boundary layer.
Boundary-layer transition and global skin friction measurement with an oil-fringe imaging technique
NASA Technical Reports Server (NTRS)
Monson, Daryl J.; Mateer, George G.; Menter, Florian R.
1993-01-01
A new oil-fringe imaging fkin friction (FISF) technique to measure skin friction on wind tunnel models is presented. In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced proportional to local skin friction. After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer. Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions.
Boundary-layer transition and global skin friction measurement with an oil-fringe imaging technique
NASA Technical Reports Server (NTRS)
Monson, Daryl J.; Mateer, George G.; Menter, Florian R.
1993-01-01
A new oil-fringe imaging system skin friction (FISF) technique to measure skin friction on wind tunnel models is presented. In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced in proportion to local skin friction. After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer. Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions.
NASA Astrophysics Data System (ADS)
Aljallis, Elias; Sarshar, Mohammad Amin; Datla, Raju; Sikka, Vinod; Jones, Andrew; Choi, Chang-Hwan
2013-02-01
In this paper, we report the measurement of skin friction drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin friction drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to 30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the drag in the turbulent flow. The results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.
Boundary layers and resistance on liquid motion with only slight friction
NASA Technical Reports Server (NTRS)
1980-01-01
The laws of fluid motion are examined systematically for the case where friction is assumed to be very slight. Calculations are carried out with the appropriate differential equation and practical investigations are illustrated.
NASA Astrophysics Data System (ADS)
Hua, Huichun; Su, Xiaohong
2015-05-01
This paper is devoted to the analysis of the unsteady magnetohydrodynamic (MHD) boundary layer flow and heat transfer on a permeable stretching sheet embedded in a moving incompressible viscous fluid. The combined effects of Ohmic heating, thermal radiation, frictional heating and internal heat absorption/generation are taken into account. The governing time dependent nonlinear boundary layer equations are converted into a systemof nonlinear ordinary differential equations by similarity transformations. Some analytical results that give the characteristics of the velocity field in the boundary layer are presented and proved. The governing equations are then solved by using the shooting technique along with the fourth order Runge-Kutta method. The analytical properties proved in this paper are consistent with those obtained by the numerical method. Furthermore, the effects of the various parameters on the velocity and temperature fields are presented graphically and discussed in detail.
Wahidi, R.; Chakroun, W.; Al-Fahed, S.
2005-11-01
Skin-friction coefficient of turbulent boundary layer flow over a smooth-wall with transverse square grooves was investigated. Four grooved-wall cases were investigated. The four grooved-wall configurations are single 5mm square grooved-wall, and 5mm square grooves spaced 10, 20 and 40 element widths apart in the streamwise direction. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient determined from the velocity profile increases sharply just downstream of the groove. This overshoot is followed by an undershoot and then relaxation back to the smooth-wall value. This behavior is observed in most grooved-wall cases. Integrating the skin-friction coefficient in the streamwise direction indicates that there is an increase in the overall drag in all the grooved-wall cases.
NASA Technical Reports Server (NTRS)
Barr, P. K.
1980-01-01
An analysis is presented of the reliability of various generally accepted empirical expressions for the prediction of the skin-friction coefficient C/sub f/ of turbulent boundary layers at low Reynolds numbers in zero-pressure-gradient flows on a smooth flat plate. The skin-friction coefficients predicted from these expressions were compared to the skin-friction coefficients of experimental profiles that were determined from a graphical method formulated from the law of the wall. These expressions are found to predict values that are consistently different than those obtained from the graphical method over the range 600 Re/sub theta 2000. A curve-fitted empirical relationship was developed from the present data and yields a better estimated value of C/sub f/ in this range. The data, covering the range 200 Re/sub theta 7000, provide insight into the nature of transitional flows. They show that fully developed turbulent boundary layers occur at Reynolds numbers Re/sub theta/ down to 425. Below this level there appears to be a well-ordered evolutionary process from the laminar to the turbulent profiles. These profiles clearly display the development of the turbulent core region and the shrinking of the laminar sublayer with increasing values of Re/sub theta/.
NASA Astrophysics Data System (ADS)
Klein, E. C.; Flesch, L. M.; Holt, W. E.
2009-12-01
We present dynamic solutions for the seismogenic layer of the India-Eurasia collision zone. We exploit a forward dynamic modeling approach, where body force distributions, inferred lateral variations in linear effective viscosity, and known far-field velocity boundary conditions are defined. Body forces are the differences in gravity potential energy per unit area (GPE), which are calculated as the depth integral of vertical stress from the surface down to a common depth reference (20 km below sea level). In our models of the seismogenic layer, depth-integrated viscosities are proportional to the assumed long-term friction on faults and inversely proportional to the long-term strain rates. Lateral variations in linear effective viscosity of the modeled layer span over 5 orders of magnitude. The velocity boundary conditions are defined using long-term plate motion estimates. Self-consistent dynamic strain rate tensor solutions to the force-balance equations were solved and tested for best-fit match with kinematic strain rate and velocity fields of central and east Asia defined by a large set of Quaternary fault observations. We investigate models using a range of long-term fault friction coefficients from 0.02 - 1.0 under hydrostatic pore pressure conditions. Assessment of fitness of the dynamic solutions to deformation indicators is evaluated using four measures. The forward dynamic strain rate tensor styles are scored by misfit to the kinematic strain rate tensor styles inferred from Kostrov summation of Quaternary fault observations. The ratio of dynamic strain rates to kinematic model strain rates are compared. Dynamic model velocity fields are scored via reduced chi-square misfit with the long-term kinematic model velocity field defined by Quaternary fault observations. Dynamic model velocity fields are scored via reduced chi-square misfit with GPS velocity observations. Preliminary results indicate that models defined with low fault friction (0.05 < ? < 0.40) achieve a better fit to Quaternary deformation indicators than models with models with high fault friction coefficients. Such models indicate that deviatoric stresses associated with internal crustal buoyancies dominate over deviatoric stresses associated with velocity boundary conditions within the Tibetan Plateau. Conversely, deviatoric stresses associated with boundary conditions dominate over deviatoric stresses associated with internal crustal buoyancies within Tarim Basin and the Tien Shan.
NASA Astrophysics Data System (ADS)
Roach, P. E.
1987-09-01
A simple wake model which described the qualitative and quantitative skin friction reducing potential of manipulators was developed. The device drag may be ascertained with a reasonable degree of certainty for flat plate manipulators. Consideration of the average friction drag shows that the optimum device height is 0.6 delta. From net drag considerations, this optimum height is found to be 0.55 delta. Device trailing edge chamfer is shown to be very important, having a first-order influence upon the drag reduction potential. Further work is necessary, particularly to more accurately determine the influence of such devices under flight conditions. The parasitic drag of airfoil devices at flight-type Reynolds numbers and the influence of device geometry both warrant further investigation. The analysis suggests that net drag reductions of 20 percent are feasible, possibly even under flight conditions.
NASA Astrophysics Data System (ADS)
Matthews, R. K.
The prediction of the boundary layer transition Reynolds number for the design of hypersonic vehicles is considered. The boundary layer state (laminar or turbulent) that approaches the vehicle control surfaces can significantly affect the control surface effectiveness. In addition, the heating rates associated with turbulent boundary layers are often ten times higher than those of laminar boundary layers. Unfortunately, the methodology to predict transition has eluded the aerodynamicist for over three decades, and there are still many unanswered questions. Many parameters that affect transition are considered and numerous references for those who are interested in specializing in this topic are provided. It is emphasized that during wind tunnel testing it is very important to know the boundary layer state. Typically, heat transfer distributions can provide this information; however, it is often necessary to artificially trip the flow to induce a turbulent boundary layer. The methodology of using trip spheres is discussed, and illustrative data are presented.
Heat Transfer Through Turbulent Friction Layers
NASA Technical Reports Server (NTRS)
Reichardt, H.
1943-01-01
The "general Prandtl number" Pr(exp 1) - A(sub q)/A Pr, aside from the Reynolds number determines the ratio of turbulent to molecular heat transfer, and the temperature distribution in turbulent friction layers. A(sub q) = exchange coefficient for heat; A = exchange coefficient for momentum transfer. A formula is derived from the equation defining the general Prandtl number which describes the temperature as a function of the velocity. For fully developed thermal boundary layers all questions relating to heat transfer to and from incompressible fluids can be treated in a simple manner if the ratio of the turbulent shear stress to the total stress T(sub t)/T in the layers near the wall is known, and if the A(sub q)/A can be regarded as independent of the distance from the wall. The velocity distribution across a flat smooth channel and deep into the laminar sublayer was measured for isothermal flow to establish the shear stress ratio T(sub t)/T and to extend the universal wall friction law. The values of T(sub t)/T which resulted from these measurements can be approximately represented by a linear function of the velocity in the laminar-turbulent transition zone. The effect of the temperature relationship of the material values on the flow near the wall is briefly analyzed. It was found that the velocity at the laminar boundary (in contrast to the thickness of the laminar layer) is approximately independent of the temperature distribution. The temperature gradient at the wall and the distribution of temperature and heat flow in the turbulent friction layers were calculated on the basis of the data under two equations. The derived formulas and the figures reveal the effects of the Prandtl number, the Reynolds number, the exchange quantities and the temperature relationship of the material values.
The Atmospheric Boundary Layer
ERIC Educational Resources Information Center
Tennekes, Hendrik
1974-01-01
Discusses some important parameters of the boundary layer and effects of turbulence on the circulation and energy dissipation of the atmosphere. Indicates that boundary-layer research plays an important role in long-term forecasting and the study of air-pollution meteorology. (CC)
NASA Astrophysics Data System (ADS)
Geredeli, Pelin G.; Lasiecka, Irena; Webster, Justin T.
In this paper dynamic von Karman equations with localized interior damping supported in a boundary collar are considered. Hadamard well-posedness for von Karman plates with various types of nonlinear damping are well known, and the long-time behavior of nonlinear plates has been a topic of recent interest. Since the von Karman plate system is of "hyperbolic type" with critical nonlinearity (noncompact with respect to the phase space), this latter topic is particularly challenging in the case of geometrically constrained, nonlinear damping. In this paper we first show the existence of a compact global attractor for finite energy solutions, and we then prove that the attractor is both smooth and finite dimensional. Thus, the hyperbolic-like flow is stabilized asymptotically to a smooth and finite dimensional set.
Boundary Layer Theory. Part 1; Laminar Flows
NASA Technical Reports Server (NTRS)
Schlichting, H.
1949-01-01
The purpose of this presentation is to give you a survey of a field of aerodynamics which has for a number of years been attracting an ever growing interest. The subject is the theory of flows with friction, and, within that field, particularly the theory of friction layers, or boundary layers. As you know, a great many considerations of aerodynamics are based on the so-called ideal fluid, that is, the frictionless incompressible fluid. By neglect of compressibility and friction the extensive mathematical theory of the ideal fluid (potential theory) has been made possible.
The process of separation in the turbulent friction layer
NASA Technical Reports Server (NTRS)
Gruschwitz, E
1933-01-01
The separation of the flow which occurs at large angles of attack on the suction side of an airplane wing is attributable to phenomena in the flowing fluid layer adjacent to the surface; the fluid particles slowed up by the friction on the surface can no longer advance against an unduly great pressure rise. It is of vital importance that there exist two types of flow - laminar and turbulent - in the fluid layer flowing in the immediate vicinity of a body. According to Prandtl, by whom the whole theory was developed, we speak in the first case of a laminar boundary layer, in the second, of a turbulent friction layer. (author)
Turbulent boundary layers over nonstationary plane boundaries
NASA Technical Reports Server (NTRS)
Roper, A. T.
1976-01-01
Methods of predicting integral parameters and skin-friction coefficients of turbulent boundary layers developing over moving-ground-planes are evaluated using test information from three different wind tunnel facilities at the NASA Langley Research Center. These data include test information from the VSTOL tunnel which is presented for the first time. The three methods evaluated were: (1) relative integral parameter method, (2) relative power law method, and (3) modified law of the wall method. Methods (1) and (2) can be used to predict moving-ground-plane shape factors with an expected accuracy of + or - 10%. They may also be used to predict moving-ground-plane displacement and momentum thicknesses with lower expected accuracy. This decrease in accuracy can be traced to the failure of approximations upon which these methods are based to prove universal when compared with VSTOL tunnel test results.
Boundary Layer Relaminarization Device
NASA Technical Reports Server (NTRS)
Creel, Theodore R. (Inventor)
1993-01-01
Relamination of a boundary layer formed in supersonic flow over the leading edge of a swept airfoil is accomplished using at least one band, especially a quadrangular band, and most preferably a square band. Each band conforms to the leading edge and the upper and lower surfaces of the airfoil as an integral part thereof and extends perpendicularly from the leading edge. Each band has a height of about two times the thickness of the maximum expected boundary layer.
Friction microprobe investigation of particle layer effects on sliding friction
NASA Astrophysics Data System (ADS)
Blau, P. J.
Interfacial particles (third-bodies), resulting from wear or external contamination, can alter and even dominate the frictional behavior of solid-solid sliding in the absence of effective particle removal processes (e.g., lubricant flow). A unique friction microprobe, developed at Oak Ridge National Laboratory, was used to conduct fine-scale friction studies using 1.0 mm diameter stainless steel spheres sliding on several sizes of loose layers of fine aluminum oxide powders on both aluminum and alumina surfaces. Conventional pin-on-disk experiments were conducted to compare behavior with the friction microprobe results. The behavior of the relatively thick particle layers was found to be independent of the nature of underlying substrate, substantiating previous work by other investigators. The time-dependent behavior of friction, for a spherical macrocontact starting from rest, could generally be represented by a series of five rather distinct phases involving static compression, slider breakaway, transition to steady state, and dynamic layer instability. A friction model for the steady state condition, which incorporates lamellar powder layer behavior, is described.
Friction microprobe investigation of particle layer effects on sliding friction
Blau, P.J.
1993-01-01
Interfacial particles (third-bodies), resulting from wear or external contamination, can alter and even dominate the frictional behavior of solid-solid sliding in the absence of effective particle removal processes (e.g., lubricant flow). A unique friction microprobe, developed at Oak Ridge National Laboratory, was used to conduct fine- scale friction studies using 1.0 mm diameter stainless steel spheres sliding on several sizes of loose layers of fine aluminum oxide powders on both aluminum and alumina surfaces. Conventional, pin-on-disk experiments were conducted to compare behavior with the friction microprobe results. The behavior of the relatively thick particle layers was found to be independent of the nature of underlying substrate, substantiating previous work by other investigators. The time-dependent behavior of friction, for a spherical macrocontact starting from rest, could generally be represented by a series of five rather distinct phases involving static compression, slider breakaway, transition to steady state, and dynamic layer instability. A friction model for the steady state condition, which incorporates lamellar powder layer behavior, is described.
Boundary layer simulator improvement
NASA Technical Reports Server (NTRS)
Praharaj, Sarat C.; Schmitz, Craig P.; Nouri, Joseph A.
1989-01-01
Boundary Layer Integral Matrix Procedure (BLIMPJ) has been identified by the propulsion community as the rigorous boundary layer program in connection with the existing JANNAF reference programs. The improvements made to BLIMPJ and described herein have potential applications in the design of the future Orbit Transfer Vehicle engines. The turbulence model is validated to include the effects of wall roughness and a way is devised to treat multiple smooth-rough surfaces. A prediction of relaminarization regions is examined as is the combined effects of wall cooling and surface roughness on relaminarization. A turbulence model to represent the effects of constant condensed phase loading is given. A procedure is described for thrust decrement calculation in thick boundary layers by coupling the T-D Kinetics Program and BLIMPJ and a way is provided for thrust loss optimization. Potential experimental studies in rocket nozzles are identified along with the required instrumentation to provide accurate measurements in support of the presented new analytical models.
Boundary layer simulator improvement
NASA Technical Reports Server (NTRS)
Praharaj, S. C.; Schmitz, C.; Frost, C.; Engel, C. D.; Fuller, C. E.; Bender, R. L.; Pond, J.
1984-01-01
High chamber pressure expander cycles proposed for orbit transfer vehicles depend primarily on the heat energy transmitted from the combustion products through the thrust wall chamber wall. The heat transfer to the nozzle wall is affected by such variables as wall roughness, relamarization, and the presence of particles in the flow. Motor performance loss for these nozzles with thick boundary layers is inaccurate using the existing procedure coded BLIMPJ. Modifications and innovations to the code are examined. Updated routines are listed.
NASA Technical Reports Server (NTRS)
Adamson, T. C., Jr.; Liou, M. S.; Messiter, A. F.
1980-01-01
An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.
Boundary layer transition studies
NASA Technical Reports Server (NTRS)
Watmuff, Jonathan H.
1995-01-01
A small-scale wind tunnel previously used for turbulent boundary layer experiments was modified for two sets of boundary layer transition studies. The first study concerns a laminar separation/turbulent reattachment. The pressure gradient and unit Reynolds number are the same as the fully turbulent flow of Spalart and Watmuff. Without the trip wire, a laminar layer asymptotes to a Falkner & Skan similarity solution in the FPG. Application of the APG causes the layer to separate and a highly turbulent and approximately 2D mean flow reattachment occurs downstream. In an effort to gain some physical insight into the flow processes a small impulsive disturbance was introduced at the C(sub p) minimum. The facility is totally automated and phase-averaged data are measured on a point-by-point basis using unprecedently large grids. The evolution of the disturbance has been tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. At first, the amplitude decays exponentially with streamwise distance in the APG region, where the layer remains attached, i.e. the layer is viscously stable. After separation, the rate of decay slows, and a point of minimum amplitude is reached where the contours of the wave packet exhibit dispersive characteristics. From this point, exponential growth of the amplitude of the disturbance is observed in the detached shear layer, i.e. the dominant instability mechanism is inviscid. A group of large-scale 3D vortex loops emerges in the vicinity of the reattachment. Remarkably, the second loop retains its identify far downstream in the turbulent boundary layer. The results provide a level of detail usually associated with CFD. Substantial modifications were made to the facility for the second study concerning disturbances generated by Suction Holes for laminar flow Control (LFC). The test section incorporates suction through interchangeable porous test surfaces. Detailed studies have been made using isolated holes in the impervious test plate that used to establish the Blasius base flow. The suction is perturbed harmonically and data are averaged on the basis of the phase of the disturbance, for conditions corresponding to strong suction and without suction. The technique was enhanced by using up to nine multiple probes to reduce the experimental run-time. In both cases, 3D contour surfaces in the vicinity of the hole show highly 3D TS waves which fan out in the spanwise direction forming bow-shaped waves downstream. The case without suction has proved useful for evaluating calculation methods. With suction, the perturbations on the centerline are much stronger and decay less rapidly, while the TS waves in the far field are similar to the case without suction. Downstream, the contour surfaces of the TS waves develop spanwise irregularities which eventually form into clumps. The spanwise clumping is evidence of a secondary instability that could be associated with suction vortices. Designers of porous surfaces use Goldsmith's Criterion to minimize cross-stream interactions. It is shown that partial TS wave cancellation is possible, depending on the hole spacing, disturbance frequency and free-stream velocity. New high-performance Constant Temperature Hot-Wire Anemometers were designed and built, based on a linear system theory analysis that can be extended to arbitrary order. The motivation was to achieve the highest possible frequency reponse while ensuring overall system stability. The performance is equal to or superior to commercially available instruments at about 10% of the cost. Details, such as fabrication drawings and a parts list, have been published to enable the instrument to be construced by others.
Vortex boundary-layer interactions
NASA Technical Reports Server (NTRS)
Bradshaw, P.
1986-01-01
The interaction of a turbulent boundary layer (on a flat plate) with a strong artificially-generated longitudinal vortex, which may or may not actually enter the boundary layer, was studied. Experiments, including extensive hot-wire measurements, were completed for the case in which the vortex does enter the boundary layer, and measurements for the somewhat simpler cases in which the boundary layer and vortex remain distinct are now in progress. Contours of total pressure (recently acquired) and of turbulent kinetic energy at various downstream positions are presented to show the overall development of the vortex imbedded in the boundary layer.
Diverging boundary layers with zero streamwise pressure gradient
NASA Technical Reports Server (NTRS)
Pauley, Wayne R.; Eaton, John K.; Cutler, Andrew D.
1989-01-01
The effects of spanwise divergence on the boundary layer forming between a pair of embedded streamwise vortices with the common flow between them directed toward the wall was studied. Measurements indicate that divergence controls the rate of development of the boundary layer and that large divergence significantly retards boundary layer growth and enhances skin friction. For strongly diverging boundary layers, divergence accounts for nearly all of the local skin friction. Even with divergence, however, the local similarity relationships for two-dimensional boundary layers are satisfactory. Although divergence modifies the mean development of the boundary layer, it does not significantly modify the turbulence structure. In the present experiments with a zero streamwise pressure gradient, it was found that spanwise divergence dit not significantly affect the Reynolds stress and the turbulent triple product distributions.
Progress in modeling hypersonic turbulent boundary layers
NASA Technical Reports Server (NTRS)
Zeman, Otto
1993-01-01
A good knowledge of the turbulence structure, wall heat transfer, and friction in turbulent boundary layers (TBL) at high speeds is required for the design of hypersonic air breathing airplanes and reentry space vehicles. This work reports on recent progress in the modeling of high speed TBL flows. The specific research goal described here is the development of a second order closure model for zero pressure gradient TBL's for the range of Mach numbers up to hypersonic speeds with arbitrary wall cooling requirements.
NASA Astrophysics Data System (ADS)
Costigliola, V.
2010-09-01
It has long been known that specific atmospheric processes, such as weather and longer-term climatic fluctuations, affect human health. The biometeorological literature refers to this relationship as meteorotropism, defined as a change in an organism that is correlated with a change in atmospheric conditions. Plenty of (patho)physiological functions are affected by those conditions - like the respiratory diseases - and currently it is difficult to put any limits for pathologies developed in reply. Nowadays the importance of atmospheric boundary layer and health is increasingly recognised. A number of epidemiologic studies have reported associations between ambient concentrations of air pollution, specifically particulate pollution, and adverse health effects, even at the relatively low concentrations of pollution found. Since 1995 there have been over twenty-one studies from four continents that have explicitly examined the association between ambient air pollutant mixes and daily mortality. Statistically significant and positive associations have been reported in data from various locations around the world, all with varying air pollutant concentrations, weather conditions, population characteristics and public health policies. Particular role has been given to atmospheric boundary layer processes, the impact of which for specific patient-cohort is, however, not well understood till now. Assessing and monitoring air quality are thus fundamental to improve Europe's welfare. One of current projects run by the "European Medical Association" - PASODOBLE will develop and demonstrate user-driven downstream information services for the regional and local air quality sectors by combining space-based and in-situ data with models in 4 thematic service lines: - Health community support for hospitals, pharmacies, doctors and people at risk - Public information for regions, cities, tourist industry and sporting event organizers - Compliance monitoring support on particulate matter for regional environmental agencies - Local forecast model evaluation support for local authorities and city bodies. Giving value to the above listed aspects, PASODOBLE objectives are following: - Evolution of existing and development of new sustainable air quality services for Europe on regional and local scales - Development and testing of a generic service framework for coordinated input data acquisition and customizable user-friendly access to services - Utilization of multiple cycles of delivery, use and assessment versus requirements and market planning in cooperation with users - Promotion and harmonisation of best practise tools for air quality communities. Further European multidisciplinary projects should be created to better understand the most prevalent atmospheric factors to be impacted in predictive, preventive and personalised medicine considered as the central concept for future medicine.
Boundary and mixed lubrication friction modeling under forming process conditions
NASA Astrophysics Data System (ADS)
Meinders, V. T.; Hol, J.; van den Boogaard, A. H.
2013-12-01
A multi-scale friction model for large-scale forming simulations is presented. A framework has been developed for the boundary and mixed lubrication regime, including the effect of surface changes due to normal loading, sliding and straining the underlying bulk material. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. To account for the lubricant effects special hydrodynamic contact elements have been developed. Pressure degrees of freedom are introduced to capture the pressure values which are computed by a finite element discretization of the 2D averaged Reynolds equations. The boundary friction model and the hydrodynamic friction model have been coupled to cover the boundary and mixed lubrication regime. To prove the numerical efficiency of the multi-scale friction model, finite element simulations have been carried out on a top hat section. The computed local friction coefficients show to be dependent on the punch stroke, punch speed and location in the product, and are far from constant. The location and range of friction coefficient values are in the order of what to expect from practice. The agreement between the numerical results and the experiments for different lubrication types and amount of lubrication is good. The multi-scale friction model proves to be stable, and compared to a Coulomb-based FE simulation, with only a modest increase in computation time.
Modeling the urban boundary layer
NASA Technical Reports Server (NTRS)
Bergstrom, R. W., Jr.
1976-01-01
A summary and evaluation is given of the Workshop on Modeling the Urban Boundary Layer; held in Las Vegas on May 5, 1975. Edited summaries from each of the session chairpersons are also given. The sessions were: (1) formulation and solution techniques, (2) K-theory versus higher order closure, (3) surface heat and moisture balance, (4) initialization and boundary problems, (5) nocturnal boundary layer, and (6) verification of models.
NASA Technical Reports Server (NTRS)
Pappas, C C
1954-01-01
Local heat-transfer rates on the surface of a heated flat plate at zero incidence to an air stream flowing at Mach numbers of 1.69 and 2.27 are presented. The Reynolds number range for both Mach numbers was 1 million to 10 million. Surface temperatures were maintained near recovery temperature. It was found that the variation of heat transfer with Mach number was in agreement with previously reported variations of directly measured skin friction with Mach number on unheated bodies. The variation with Mach number of the average skin coefficient, as determined from impact-pressure surveys, was in agreement with that from other momentum loss measurements but differed from the variation obtained from directly measured skin friction as reported by others. (author)
Vortex boundary-layer interactions
NASA Technical Reports Server (NTRS)
Bradshaw, P.
1985-01-01
The interaction of a turbulent boundary layer on a flat plate with a strong artificially generated longitudinal vortex which may or may not actually enter the boundary layer is studied. The vortices are generated by a delta wing suspended ahead of the test plate, so that the configuration is approximately that of a close coupled carnard with zero main-wing sweep and an invisible body. All necessary configuration and parametric checks are completed, and data acquisition and analysis on the first configuration chosen for detailed study, in which the vortex starts to merge with the boundary layer a short distance downstream of the leading edge of the test plate, are nearly complete.
Flow unsteadiness effects on boundary layers
NASA Technical Reports Server (NTRS)
Murthy, Sreedhara V.
1989-01-01
The development of boundary layers at high subsonic speeds in the presence of either mass flux fluctuations or acoustic disturbances (the two most important parameters in the unsteadiness environment affecting the aerodynamics of a flight vehicle) was investigated. A high quality database for generating detailed information concerning free-stream flow unsteadiness effects on boundary layer growth and transition in high subsonic and transonic speeds is described. The database will be generated with a two-pronged approach: (1) from a detailed review of existing literature on research and wind tunnel calibration database, and (2) from detailed tests in the Boundary Layer Apparatus for Subsonic and Transonic flow Affected by Noise Environment (BLASTANE). Special instrumentation, including hot wire anemometry, the buried wire gage technique, and laser velocimetry were used to obtain skin friction and turbulent shear stress data along the entire boundary layer for various free stream noise levels, turbulence content, and pressure gradients. This database will be useful for improving the correction methodology of applying wind tunnel test data to flight predictions and will be helpful for making improvements in turbulence modeling laws.
Boundary Layer Control on Airfoils.
ERIC Educational Resources Information Center
Gerhab, George; Eastlake, Charles
1991-01-01
A phenomena, boundary layer control (BLC), produced when visualizing the fluidlike flow of air is described. The use of BLC in modifying aerodynamic characteristics of airfoils, race cars, and boats is discussed. (KR)
Removing Boundary Layer by Suction
NASA Technical Reports Server (NTRS)
Ackeret, J
1927-01-01
Through the utilization of the "Magnus effect" on the Flettner rotor ship, the attention of the public has been directed to the underlying physical principle. It has been found that the Prandtl boundary-layer theory furnishes a satisfactory explanation of the observed phenomena. The present article deals with the prevention of this separation or detachment of the flow by drawing the boundary layer into the inside of a body through a slot or slots in its surface.
Frictional anisotropy under boundary lubrication: effect of surface texture.
Ajayi, O. O.; Erck, R. A.; Lorenzo-Martin, C.; Fenske, G. R.; Energy Systems
2009-06-15
The friction coefficient was measured under boundary lubrication with a ball-on-flat contact configuration in unidirectional sliding. The ball was smooth and hardened 52100 steel. Discs were made from case-carburized and hardened 4620, annealed 1080, and 1018 steels with directionally ground surfaces. A synthetic lubricant of stock polyalphaolefin was used for testing. During testing with each material, a frictional spike was observed whenever the ball slid parallel to the grinding ridge on the disc surface. The average friction coefficient for all tests was about 0.1, which is typical for the boundary lubrication regime. The magnitude of the frictional spikes, which reached as high as a friction coefficient of 0.25, and their persistence depended on the hardness of the disc surface. On the basis of elastohydrodynamic theory, coupled with the observation of severe plastic deformation on the ridges parallel to the sliding direction, the frictional spike could be due to localized plastic deformation on the disc surface at locations of minimal thickness for the lubricant fluid film. This hypothesis was further supported by lack of frictional spikes in tests using discs coated with a thin film of diamond-like carbon, in which plastic deformation is minimal.
Approximation theory for boundary layer suction through individual slits
NASA Technical Reports Server (NTRS)
Walz, A.
1979-01-01
The basic concepts of influencing boundary layers are summarized, especially the prevention of flow detachment and the reduction of frictional resistance. A mathematical analysis of suction through a slit is presented with two parameters, for thickness and for shape of the boundary layer, being introduced to specify the flow's velocity profile behind the slit. An approximation of the shape parameter produces a useful formula, which can be used to determine the most favorable position of the slit. An aerodynamic example is given.
Asymptotic similarity in turbulent boundary layers
NASA Astrophysics Data System (ADS)
Duncan, Richard D.
The turbulent boundary layer is one of the most fundamental and important applications of fluid mechanics. Despite great practical interest and its direct impact on frictional drag among its many important consequences, no theory absent of significant inference or assumption exists. Numerical simulations and empirical guidance are used to produce models and adequate predictions, but even minor improvements in modeling parameters or physical understanding could translate into significant improvements in the efficiency of aerodynamic and hydrodynamic vehicles. Classically, turbulent boundary layers and fully-developed turbulent channels and pipes are considered members of the same "family," with similar "inner" versus "outer" descriptions. However, recent advances in experiments, simulations, and data processing have questioned this, and, as a result, their fundamental physics. To address a full range of pressure gradient boundary layers, a new approach to the governing equations and physical description of wall-bounded flows is formulated, using a two variable similarity approach and many of the tools of the classical method with slight but significant variations. A new set of similarity requirements for the characteristic scales of the problem is found, and when these requirements are applied to the classical "inner" and "outer" scales, a "similarity map" is developed providing a clear prediction of what flow conditions should result in self-similar forms. An empirical model with a small number of parameters and a form reminiscent of Coles' "wall plus wake" is developed for the streamwise Reynolds stress, and shown to fit experimental and numerical data from a number of turbulent boundary layers as well as other wall-bounded flows. It appears from this model and its scaling using the free-stream velocity that the true asymptotic form of u'2 may not become self-evident until Retheta ? 275,000 or delta+ ? 105, if not higher. A perturbation expansion made possible by the novel inclusion of the scaled streamwise coordinate is used to make an excellent prediction of the shear Reynolds stress in zero pressure gradient boundary layers and channel flows, requiring only a streamwise mean velocity profile and the new similarity map. Extension to other flows is promising, though more information about the normal Reynolds stresses is needed. This expansion is further used to infer a three layer structure in the turbulent boundary layer, and modified two layer structure in fully-developed flows, by using the classical inner and logarithmic profiles to determine which portions of the boundary layer are dominated by viscosity, inertia, or turbulence. A new inner function for U+ is developed, based on the three layer description, providing a much more simplified representative form of the streamwise mean velocity nearest the wall.
The wave-induced boundary layer under long internal waves
NASA Astrophysics Data System (ADS)
Lin, Yuncheng; Redekopp, Larry G.
2011-08-01
The boundary layer formed under the footprint of an internal solitary wave is studied by numerical simulation for waves of depression in a two-layer model of the density stratification. The inviscid outer flow, in the perspective of boundary-layer theory, is based on an exact solution for the long wave-phase speed, yielding a family of fully nonlinear solitary wave solutions of the extended Korteweg-de Vries equation. The wave-induced boundary layer corresponding to this outer flow is then studied by means of simulation employing the Reynolds-averaged Navier-Stokes (RANS) formulation coupled with a turbulence closure model validated for wall-bounded flows. Boundary-layer characteristics are computed for an extensive range of environmental conditions and wave amplitudes. Boundary-layer transition, identified by monitoring the eddy viscosity, is correlated in terms of a boundary-layer Reynolds number. The frictional drag is evaluated for laminar, transitional, and turbulent cases, and correlations are presented for the friction coefficient plus relevant measures of the boundary-layer thickness.
Steady-state wear and friction in boundary lubrication studies
NASA Technical Reports Server (NTRS)
Loomis, W. R.; Jones, W. R., Jr.
1980-01-01
A friction and wear study was made at 20 C to obtain improved reproducibility and reliability in boundary lubrication testing. Ester-base and C-ether-base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk in a friction and wear apparatus. Conditions included loads of 1/2 and 1 kg and sliding velocities of 3.6 to 18.2 m/min in a dry air atmosphere and stepwise time intervals from 1 to 250 min for wear measurements. The wear rate results were compared with those from previous studies where a single 25 min test period was used. Satisfactory test conditions for studying friction and wear in boundary lubrication for this apparatus were found to be 1 kg load; sliding velocities of 7.1 to 9.1 m/min (50 rpm disk speed); and use of a time stepwise test procedure. Highly reproducible steady-state wear rates and steady-state friction coefficients were determined under boundary conditions. Wear rates and coefficients of friction were constant following initially high values during run-in periods.
Physics of magnetospheric boundary layers
NASA Technical Reports Server (NTRS)
Cairns, Iver H.
1995-01-01
This final report was concerned with the ideas that: (1) magnetospheric boundary layers link disparate regions of the magnetosphere-solar wind system together; and (2) global behavior of the magnetosphere can be understood only by understanding its internal linking mechanisms and those with the solar wind. The research project involved simultaneous research on the global-, meso-, and micro-scale physics of the magnetosphere and its boundary layers, which included the bow shock, the magnetosheath, the plasma sheet boundary layer, and the ionosphere. Analytic, numerical, and simulation projects were performed on these subjects, as well as comparisons of theoretical results with observational data. Other related activity included in the research included: (1) prediction of geomagnetic activity; (2) global MHD (magnetohydrodynamic) simulations; (3) Alfven resonance heating; and (4) Critical Ionization Velocity (CIV) effect. In the appendixes are list of personnel involved, list of papers published; and reprints or photocopies of papers produced for this report.
Nonparallel stability of boundary layers
NASA Technical Reports Server (NTRS)
Nayfeh, Ali H.
1987-01-01
The asymptotic formulations of the nonparallel linear stability of incompressible growing boundary layers are critically reviewed. These formulations can be divided into two approaches. The first approach combines a numerical method with either the method of multiple scales, or the method of averaging, of the Wentzel-Kramers-Brillouin (WKB) approximation; all these methods yield the same result. The second approach combined a multi-structure theory with the method of multiple scales. The first approach yields results that are in excellent agreement with all available experimental data, including the growth rates as well as the neutral stability curve. The derivation of the linear stability of the incompressible growing boundary layers is explained.
Physics of magnetospheric boundary layers
NASA Technical Reports Server (NTRS)
Cairns, I. H.
1993-01-01
The central ideas of this grant are that the magnetospheric boundary layers link disparate regions of the magnetosphere together, and the global behavior of the magnetosphere can be understood only by understanding the linking mechanisms. Accordingly the present grant includes simultaneous research on the global, meso-, and micro-scale physics of the magnetosphere and its boundary layers. These boundary layers include the bow shock, magnetosheath, the plasma sheet boundary layer, and the ionosphere. Analytic, numerical and simulation projects have been performed on these subjects, as well as comparison of theoretical results with observational data. Very good progress has been made, with four papers published or in press and two additional papers submitted for publication during the six month period 1 June - 30 November 1993. At least two projects are currently being written up. In addition, members of the group have given papers at scientific meetings. The further structure of this report is as follows: section two contains brief accounts of research completed during the last six months, while section three describes the research projects intended for the grant's final period.
Heat transfer to the transpired turbulent boundary layer.
NASA Technical Reports Server (NTRS)
Kays, W. M.
1972-01-01
This paper contains a summarization of five years work on an investigation on heat transfer to the transpired turbulent boundary layer. Experimental results are presented for friction coefficient and Stanton number over a wide range of blowing and suction for the case of constant free-stream velocity, holding certain blowing parameters constant. The problem of the accelerated turbulent boundary layer with transpiration is considered, experimental data are presented and discussed, and theoretical models for solution of the momentum equation under these conditions are presented. Data on turbulent Prandtl number are presented so that solutions to the energy equation may be obtained. Some examples of boundary layer heat transfer and friction coefficient predictions are presented using one of the models discussed, employing a finite difference solution method.
Behavior of turbulent boundary layers on curved convex walls
NASA Technical Reports Server (NTRS)
Schmidbauer, Hans
1936-01-01
The system of linear differential equations which indicated the approach of separation and the so-called "boundary-layer thickness" by Gruschwitz is extended in this report to include the case where the friction layer is subject to centrifugal forces. Evaluation of the data yields a strong functional dependence of the momentum change and wall drag on the boundary-layer thickness radius of curvature ratio for the wall. It is further shown that the transition from laminar to turbulent flow occurs at somewhat higher Reynolds Numbers at the convex wall than at the flat plate, due to the stabilizing effect of the centrifugal forces.
Stability of compressible boundary layers
NASA Technical Reports Server (NTRS)
Nayfeh, Ali H.
1989-01-01
The stability of compressible 2-D and 3-D boundary layers is reviewed. The stability of 2-D compressible flows differs from that of incompressible flows in two important features: There is more than one mode of instability contributing to the growth of disturbances in supersonic laminar boundary layers and the most unstable first mode wave is 3-D. Whereas viscosity has a destabilizing effect on incompressible flows, it is stabilizing for high supersonic Mach numbers. Whereas cooling stabilizes first mode waves, it destabilizes second mode waves. However, second order waves can be stabilized by suction and favorable pressure gradients. The influence of the nonparallelism on the spatial growth rate of disturbances is evaluated. The growth rate depends on the flow variable as well as the distance from the body. Floquet theory is used to investigate the subharmonic secondary instability.
Manipulation of the structure of a turbulent boundary layer
NASA Astrophysics Data System (ADS)
Lynn, Theodore Brooks
The manipulation of a turbulent boundary layer for the purpose of net drag reduction is an attractive topic for research, because even modest success will result in large energy savings. The focus is on passive manipulation, one of the simplest manipulation techniques. The most promising manipulator is the so-called BLADE device, consisting of two thin ribbons or foils suspended in the outer portion of the boundary layer. When the research was begun, there was significant controversy over the magnitude of the net drag reduction possible (20 percent) and the maximum skin friction reduction obtainable (50 percent). Accurate local skin-friction measurements were made using sublayer fences in a perturbed boundary layer. By comparing the direct measurements with those obtained by indirect methods, it was determined that the degree of drag reduction obtained depends on the method used to calculate the combined device drag and skin friction drag. Using auto and two-point correlation measurements as well as space-time correlations, the effects of BLADE were investigated on the turbulent structures in the boundary layer, comparing them with wire devices, which are not known to produce a net reduction in drag. The space-time correlation revealed that the most significant effect of the BLADE device was on the large structures (the dominant structures in the outer region of the boundary layer). The inner layer devices consisting of sublayer wires were also investigated. The results from both the inner and outer layer manipulations suggest that the effective alteration of a turbulent boundary layer depends on the scaling of the device.
Transition in hypersonic boundary layers
NASA Astrophysics Data System (ADS)
Zhang, Chuanhong; Zhu, Yiding; Chen, Xi; Yuan, Huijing; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2015-10-01
Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. It is found that the second-mode instability is a key modulator of the transition process. Although the second-mode is primarily an acoustic wave, it causes the formation of high-frequency vortical waves, which triggers a fast transition to turbulence.
Jupiter's deep magnetotail boundary layer
NASA Astrophysics Data System (ADS)
Nicolaou, G.; McComas, D. J.; Bagenal, F.; Elliott, H. A.; Ebert, R. W.
2015-06-01
In 2007 the New Horizons (NH) spacecraft flew by Jupiter for a gravity assist en route to Pluto. After closest approach on day of year (DOY) 58, 2007, NH followed a tailward trajectory that provided a unique opportunity to explore the deep jovian magnetotail and the surrounding magnetosheath. After DOY 132, 16 magnetopause crossings were observed between 1654 and 2429 Jupiter radii (Rj) along the dusk flank tailward of the planet. In some cases the crossings were identified as rapid transitions from the magnetotail to the magnetosheath and vice versa. In other cases a boundary layer was observed just inside the magnetopause. Solar Wind Around Pluto (SWAP) is an instrument on board NH that obtained spectra of low energy ions during the flyby period. We use a forward model including the SWAP instrument response to derive plasma parameters (density, temperature and velocity) which best reproduce the observations. We also vary the plasma parameters in our model in order to fit the observations more accurately on occasions where the measurements exhibit significant variability. We compare the properties of the plasma in the boundary layer with those of the magnetosheath plasma derived in our earlier work. We attempt to estimate the magnetic field in the boundary layer assuming pressure balance between it and the magnetosheath. Finally, we investigate several possible scenarios to assess if magnetopause movement and structure could cause the variations seen in the data.
Boundary layer receptivity and control
NASA Technical Reports Server (NTRS)
Hill, D. C.
1993-01-01
Receptivity processes initiate natural instabilities in a boundary layer. The instabilities grow and eventually break down to turbulence. Consequently, receptivity questions are a critical element of the analysis of the transition process. Success in modeling the physics of receptivity processes thus has a direct bearing on technological issues of drag reduction. The means by which transitional flows can be controlled is also a major concern: questions of control are tied inevitably to those of receptivity. Adjoint systems provide a highly effective mathematical method for approaching many of the questions associated with both receptivity and control. The long term objective is to develop adjoint methods to handle increasingly complex receptivity questions, and to find systematic procedures for deducing effective control strategies. The most elementary receptivity problem is that in which a parallel boundary layer is forced by time-harmonic sources of various types. The characteristics of the response to such forcing form the building blocks for more complex receptivity mechanisms. The first objective of this year's research effort was to investigate how a parallel Blasius boundary layer responds to general direct forcing. Acoustic disturbances in the freestream can be scattered by flow non-uniformities to produce Tollmien-Schlichting waves. For example, scattering by surface roughness is known to provide an efficient receptivity path. The present effort is directed towards finding a solution by a simple adjoint analysis, because adjoint methods can be extended to more complex problems. In practice, flows are non-parallel and often three-dimensional. Compressibility may also be significant in some cases. Recent developments in the use of Parabolized Stability Equations (PSE) offer a promising possibility. By formulating and solving a set of adjoint parabolized equations, a method for mapping the efficiency with which external forcing excites the three-dimensional motions of a non-parallel boundary layer was developed. The method makes use of the same computationally efficient formulation that makes the PSE currently so appealing. In the area of flow control, adjoint systems offer a powerful insight into the effect of control forces. One of the simplest control strategies for boundary layers involves the application of localized mean wall suction.
Fifty Years of Boundary-Layer Theory and Experiment
NASA Technical Reports Server (NTRS)
Dryden, Hugh L.
1955-01-01
The year 1954 marked the 50th anniversary of the Prandtl boundary-layer theory from which we may date the beginning of man's understanding of the dynamics of real fluids. A backward look at this aspect of the history of the last 50 years may be instructive. This paper (1) attempts to compress the events of those 50 years into a few thousand words, to tell in this brief space the interesting story of the development of a new concept, its slow acceptance and growth, its spread from group to group within its country of origin, and its diffusion to other countries of the world. The original brief paper of Prandtl (2) was presented at the Third International Mathematical Congress at Heidelberg in 1904 and published in the following year. It was an attempt to explain the d'Alembert paradox, namely, that the neglect of the small friction of air in the theory resulted in the prediction of zero resistance to motion. Prandtl set himself the task of computing the motion of a fluid of small friction, so small that its effect could be neglected everywhere except where large velocity differences were present or a cumulative effect of friction occurred This led to the concept of boundary layer, or transition layer, near the wall of a body immersed in a fluid stream in which the velocity rises from zero to the free-stream value. It is interesting that Prandtl used the term Grenzsehicht (boundary layer) only once and the term Ubergangsschicht (transition layer) seven times in the brief article. Later writers also used Reibungsschicht (friction layer), but most writers today use Grenzschicht (boundary layer).
Influences on the Height of the Stable Boundary Layer as seen in Large-Eddy Simulations
Kosovic, B; Lundquist, J K
2004-03-29
Numerical weather prediction (NWP) models and atmospheric dispersion models rely on parameterizations of planetary boundary layer height. In the case of a stable boundary layer, errors in boundary layer height estimation can result in gross errors in boundary-layer evolution and in prediction of turbulent mixing within the boundary layer. We use large-eddy simulations (LES) of moderately stable boundary layers to characterize the effects of various physical processes on stable boundary layers. The stable boundary layer height is assumed to be a function of surface friction velocity, geostrophic wind, Monin-Obukhov length, and the strength of the temperature inversion atop the stable boundary layer. This temperature inversion induces gravity waves with a frequency determined by the strength of the temperature inversion.
An approximate analytical solution of the laminar boundary layer equations
NASA Astrophysics Data System (ADS)
Yuan, Yi-Wu
1993-01-01
Using the pressure gradient as the new variable instead of the ordinary longitudinal coordinate x, Liu (1981) transformed the ordinary laminar boundary equations into a new form. On this basis, Liu obtained the frictional stress factor by using the graphical method. In this paper the same variable replacement is used, and an approximate analytical solution of the laminar boundary layer equations is obtained by the series method. Also obtained is a formula for the frictional stress factor. For the case of the main function without the term of the constant, a further simplification is made. The error of the frictional stress factor obtained is still less than 10 percent, compared with that of Liu.
Unsteady turbulent boundary layers in swimming rainbow trout.
Yanase, Kazutaka; Saarenrinne, Pentti
2015-05-01
The boundary layers of rainbow trout, Oncorhynchus mykiss, swimming at 1.020.09?L?s(-1) (means.d., N=4), were measured by the particle image velocimetry (PIV) technique at a Reynolds number of 410(5). The boundary layer profile showed unsteadiness, oscillating above and beneath the classical logarithmic law of the wall with body motion. Across the entire surface regions that were measured, local Reynolds numbers based on momentum thickness, which is the distance that is perpendicular to the fish surface through which the boundary layer momentum flows at free-stream velocity, were greater than the critical value of 320 for the laminar-to-turbulent transition. The skin friction was dampened on the convex surface while the surface was moving towards a free-stream flow and increased on the concave surface while retreating. These observations contradict the result of a previous study using different species swimming by different methods. Boundary layer compression accompanied by an increase in local skin friction was not observed. Thus, the overall results may not support absolutely the Bone-Lighthill boundary layer thinning hypothesis that the undulatory motions of swimming fish cause a large increase in their friction drag because of the compression of the boundary layer. In some cases, marginal flow separation occurred on the convex surface in the relatively anterior surface region, but the separated flow reattached to the fish surface immediately downstream. Therefore, we believe that a severe impact due to induced drag components (i.e. pressure drag) on the swimming performance, an inevitable consequence of flow separation, was avoided. PMID:25750412
Outline of research on oscillating boundary layers
NASA Technical Reports Server (NTRS)
Cousteix, J.
1979-01-01
The state of the art in the field of unsteady boundary layers is outlined with emphasis on turbulent boundary layers. The unsteady flows considered are mainly periodic with the external velocity varying around a zero or nonzero mean time value. The principal results obtained on laminar boundary layers are also presented.
Turbulent boundary layer of an airfoil
NASA Technical Reports Server (NTRS)
Fediaevsky, K
1937-01-01
A need has arisen for a new determination of the velocity profiles in the boundary layer. Assuming that the character of the velocity distribution depends to a large extent on the character of the shear distribution across the boundary layer, we shall consider the nature of the shear distribution for a boundary layer with a pressure gradient.
A fast method to solve incompressible boundary layer interaction problems
NASA Astrophysics Data System (ADS)
Veldman, A. E. P.; Dijkstra, D.
1981-06-01
The mutual influence of pressure and displacement thickness is recognized by simultaneously updating both these quantities as the boundary layer is marched. A survey of hierarchical and nonhierarchical solutions is presented. The iteration technique was implemented in a full boundary layer approach and a triple-deck formulation. Results were calculated for the flat plate trailing edge interaction problem, for backward and forward facing steps and for the Carter/Wornom trough. In all cases fast convergence is obtained. Separation, reattachment, dividing streamline and skin friction are found to be virtually identical.
Modelling the transitional boundary layer
NASA Technical Reports Server (NTRS)
Narasimha, R.
1990-01-01
Recent developments in the modelling of the transition zone in the boundary layer are reviewed (the zone being defined as extending from the station where intermittency begins to depart from zero to that where it is nearly unity). The value of using a new non-dimensional spot formation rate parameter, and the importance of allowing for so-called subtransitions within the transition zone, are both stressed. Models do reasonably well in constant pressure 2-dimensional flows, but in the presence of strong pressure gradients further improvements are needed. The linear combination approach works surprisingly well in most cases, but would not be so successful in situations where a purely laminar boundary layer would separate but a transitional one would not. Intermittency-weighted eddy viscosity methods do not predict peak surface parameters well without the introduction of an overshooting transition function whose connection with the spot theory of transition is obscure. Suggestions are made for further work that now appears necessary for developing improved models of the transition zone.
Nonequilibrium chemistry boundary layer integral matrix procedure
NASA Technical Reports Server (NTRS)
Tong, H.; Buckingham, A. C.; Morse, H. L.
1973-01-01
The development of an analytic procedure for the calculation of nonequilibrium boundary layer flows over surfaces of arbitrary catalycities is described. An existing equilibrium boundary layer integral matrix code was extended to include nonequilibrium chemistry while retaining all of the general boundary condition features built into the original code. For particular application to the pitch-plane of shuttle type vehicles, an approximate procedure was developed to estimate the nonequilibrium and nonisentropic state at the edge of the boundary layer.
Study of boundary-layer transition using transonic-cone preston tube data
NASA Technical Reports Server (NTRS)
Reed, T. D.; Moretti, P. M.
1980-01-01
The laminar boundary layer on a 10 degree cone in a transonic wind tunnel was studied. The inviscid flow and boundary layer development were simulated by computer programs. The effects of pitch and yaw angles on the boundary layer were examined. Preston-tube data, taken on the boundary-layer-transition cone in the NASA Ames 11 ft transonic wind tunnel, were used to develope a correlation which relates the measurements to theoretical values of laminar skin friction. The recommended correlation is based on a compressible form of the classical law-of-the-wall. The computer codes successfully simulates the laminar boundary layer for near-zero pitch and yaw angles. However, in cases of significant pitch and/or yaw angles, the flow is three dimensional and the boundary layer computer code used here cannot provide a satisfactory model. The skin-friction correlation is thought to be valid for body geometries other than cones.
Application of algebraic-RNG eddy viscosity model to simulation of transitional boundary layer flow
NASA Technical Reports Server (NTRS)
Yakhot, Alexander; Kedar, Omer; Orszag, Steven A.
1992-01-01
An algebraic eddy-viscosity model is derived from the renormalization group (RNG) theory of turbulence. A new length scale, based on boundary layer characteristics (displacement thickness, shape factor), is proposed. The model was applied to transitional boundary layer flow over a flat plate. Integral characteristics, such as the total skin friction coefficient, and mean velocity profile across the boundary layer, are found to be in good agreement with experimental data.
Vortex boundary-layer interactions
NASA Technical Reports Server (NTRS)
Bradshaw, P.
1986-01-01
Parametric studies to identify a vortex generator were completed. Data acquisition in the first chosen configuration, in which a longitudinal vortex pair generated by an isolated delta wing starts to merge with a turbulent boundary layer on a flat plate fairly close to the leading edge is nearly completed. Work on a delta-wing/flat-plate combination, consisting of a flow visualization and hot wire measurements taken with a computer controlled traverse gear and data logging system were completed. Data taking and analysis have continued, and sample results for another cross stream plane are presented. Available data include all mean velocity components, second order mean products of turbulent fluctuations, and third order mean products. Implementation of a faster data logging system was accomplished.
Boundary layer theory and subduction
Fowler, A.C.
1993-12-01
Numerical models of thermally activated convective flow in Earth`s mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction, other than by the inclusion of artificial weak zones. Here we show, using a boundary layer argument, how the `rigid lid` style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction ot occur through the visoc-plastic failure of lithospheric rock. An explicit criterion for the failure of the lid is given, which is sensitive to the internal viscosity eta(sub a) below the lid. For numbers appropriate to Earth`s mantle, this criterion is approximately eta(sub a) greater than 10(exp 21) Pa s.
Large eddy simulation of boundary layer flow under cnoidal waves
NASA Astrophysics Data System (ADS)
Li, Yin-Jun; Chen, Jiang-Bo; Zhou, Ji-Fu; Zhang, Qiang
2016-02-01
Water waves in coastal areas are generally nonlinear, exhibiting asymmetric velocity profiles with different amplitudes of crest and trough. The behaviors of the boundary layer under asymmetric waves are of great significance for sediment transport in natural circumstances. While previous studies have mainly focused on linear or symmetric waves, asymmetric wave-induced flows remain unclear, particularly in the flow regime with high Reynolds numbers. Taking cnoidal wave as a typical example of asymmetric waves, we propose to use an infinite immersed plate oscillating cnoidally in its own plane in quiescent water to simulate asymmetric wave boundary layer. A large eddy simulation approach with Smagorinsky subgrid model is adopted to investigate the flow characteristics of the boundary layer. It is verified that the model well reproduces experimental and theoretical results. Then a series of numerical experiments are carried out to study the boundary layer beneath cnoidal waves from laminar to fully developed turbulent regimes at high Reynolds numbers, larger than ever studied before. Results of velocity profile, wall shear stress, friction coefficient, phase lead between velocity and wall shear stress, and the boundary layer thickness are obtained. The dependencies of these boundary layer properties on the asymmetric degree and Reynolds number are discussed in detail.
Large eddy simulation of boundary layer flow under cnoidal waves
NASA Astrophysics Data System (ADS)
Li, Yin-Jun; Chen, Jiang-Bo; Zhou, Ji-Fu; Zhang, Qiang
2015-09-01
Water waves in coastal areas are generally nonlinear, exhibiting asymmetric velocity profiles with different amplitudes of crest and trough. The behaviors of the boundary layer under asymmetric waves are of great significance for sediment transport in natural circumstances. While previous studies have mainly focused on linear or symmetric waves, asymmetric wave-induced flows remain unclear, particularly in the flow regime with high Reynolds numbers. Taking cnoidal wave as a typical example of asymmetric waves, we propose to use an infinite immersed plate oscillating cnoidally in its own plane in quiescent water to simulate asymmetric wave boundary layer. A large eddy simulation approach with Smagorinsky subgrid model is adopted to investigate the flow characteristics of the boundary layer. It is verified that the model well reproduces experimental and theoretical results. Then a series of numerical experiments are carried out to study the boundary layer beneath cnoidal waves from laminar to fully developed turbulent regimes at high Reynolds numbers, larger than ever studied before. Results of velocity profile, wall shear stress, friction coefficient, phase lead between velocity and wall shear stress, and the boundary layer thickness are obtained. The dependencies of these boundary layer properties on the asymmetric degree and Reynolds number are discussed in detail.
Friction and wear behaviour of Mo-W doped carbon-based coating during boundary lubricated sliding
NASA Astrophysics Data System (ADS)
Hovsepian, Papken Eh.; Mandal, Paranjayee; Ehiasarian, Arutiun P.; Sáfrán, G.; Tietema, R.; Doerwald, D.
2016-03-01
A molybdenum and tungsten doped carbon-based coating (Mo-W-C) was developed in order to provide low friction in boundary lubricated sliding condition at ambient and at high temperature. The Mo-W-C coating showed the lowest friction coefficient among a number of commercially available state-of-the-art DLC coatings at ambient temperature. At elevated temperature (200 °C), Mo-W-C coating showed a significant reduction in friction coefficient with sliding distance in contrast to DLC coatings. Raman spectroscopy revealed the importance of combined Mo and W doping for achieving low friction at both ambient and high temperature. The significant decrease in friction and wear rate was attributed to the presence of graphitic carbon debris (from coating) and 'in situ' formed metal sulphides (WS2 and MoS2, where metals were supplied from coating and sulphur from engine oil) in the transfer layer.
Supersonic separated turbulent boundary - layer over a wavy wall
NASA Technical Reports Server (NTRS)
Polak, A.; Werle, M. J.
1977-01-01
A prediction method is developed for calculating distributions of surface heating rates, pressure and skin friction over a wavy wall in a two-dimensional supersonic flow. Of particular interest is the flow of thick turbulent boundary layers. The surface geometry and the flow conditions considered are such that there exists a strong interaction between the viscous and inviscid flow. First, using the interacting turbulent boundary layer equations, the problem is formulated in physical coordinates and then a reformulation of the governing equations in terms of Levy-Lees variables is given. Next, a numerical scheme for solving interacting boundary layer equations is adapted. A number of modifications which led to the improvement of the numerical algorithm are discussed. Finally, results are presented for flow over a train of up to six waves at various flow conditions.
Influence of wall permeability on turbulent boundary-layer properties
NASA Technical Reports Server (NTRS)
Wilkinson, S. P.
1983-01-01
Experimental boundary-layer studies of a series of low pressure drop, permeable surfaces have been conducted to characterize their surface interaction with a turbulent boundary layer. The models were flat and tested at nominally zero pressure gradient in low speed air. The surfaces were thin metal sheets with discrete perforations. Direct drag balance measurements of skin friction indicate that the general effect of surface permeability is to increase drag above that of a smooth plate reference level. Heuristic arguments are presented to show that this type of behavior is to be expected. Other boundary-layer data are also presented including mean velocity profiles and conditionally sampled streamwise velocity fluctuations (hot wire) for selected models.
Frictional rheology of a confined adsorbed polymer layer.
Cayer-Barrioz, Juliette; Mazuyer, Denis; Tonck, André; Yamaguchi, Elaine
2009-09-15
The sliding dynamics of a confined adsorbed polymer layer is investigated at the nanoscale. A combined mechanical and physical approach is used to model the rheology and structure of the adsorbed layer. The confinement at short distances governs the nanotribological behavior of the polymer layer formed close to the surface. It appears that the Amontons' proportionality between frictional and normal stresses does not hold here: the higher the contact pressure, the lower the friction. Besides, the sliding stress is strongly dependent on the velocity: it increases with the sliding velocity. Using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces theoretically accounts for the behavior of this system. This approach allows us to correlate the frictional properties to the molecular organization on the surfaces. PMID:19572533
Microgravity Effects on Plant Boundary Layers
NASA Technical Reports Server (NTRS)
Stutte, Gary; Monje, Oscar
2005-01-01
The goal of these series of experiment was to determine the effects of microgravity conditions on the developmental boundary layers in roots and leaves and to determine the effects of air flow on boundary layer development. It is hypothesized that microgravity induces larger boundary layers around plant organs because of the absence of buoyancy-driven convection. These larger boundary layers may affect normal metabolic function because they may reduce the fluxes of heat and metabolically active gases (e.g., oxygen, water vapor, and carbon dioxide. These experiments are to test whether there is a change in boundary layer associated with microgravity, quantify the change if it exists, and determine influence of air velocity on boundary layer thickness under different gravity conditions.
Turbulent oceanic western-boundary layers at low latitude
NASA Astrophysics Data System (ADS)
Quam Cyrille Akuetevi, Cataria; Wirth, Achim
2013-04-01
Low latitude oceanic western-boundary layers range within the most turbulent regions in the worlds ocean. The Somali current system with the Great Whirl and the Brazilian current system with its eddy shedding are the most prominent examples. Results from analytical calculations and integration of a one layer reduced-gravity fine resolution shallow water model is used to entangle this turbulent dynamics. Two types of wind-forcing are applied: a remote Trade wind forcing with maximum shear along the equator and a local Monsoon wind forcing with maximum shear in the vicinity of the boundary. For high values of the viscosity (> 1000m2s-1) the stationary solutions compare well to analytical predictions using Munk and inertial layer theory. When lowering the friction parameter time dependence results. The onset of instability is strongly influenced by inertial effects. The unstable boundary current proceeds as a succession of anti-cyclonic coherent eddies performing a chaotic dynamics in a turbulent flow. The dynamics is governed by the turbulent fluxes of mass and momentum. We determine these fluxes by analyzing the (potential) vorticity dynamics. We demonstrate that the boundary-layer can be separated in four sub-layers, which are (starting from the boundary): (1) the viscous sub-layer (2) the turbulent buffer-layer (3) the layer containing the coherent structures and (4) the extended boundary layer. The characteristics of each sub-layer and the corresponding turbulent fluxes are determined, as are the dependence on latitude and the type of forcing. A new pragmatic method of determining the eddy viscosity, based on Munk-layer theory, is proposed. Results are compared to observations and solutions of the multi-level primitive equation model (DRAKKAR).
An experimental investigation of turbulent boundary layers along curved surfaces
NASA Technical Reports Server (NTRS)
So, R. M. C.; Mellor, G. L.
1972-01-01
A curved wall tunnel was designed, and an equilibrium turbulent boundary layer was set up on the straight section preceding the curved test section. Turbulent boundary layer flows with uniform and adverse pressure distributions along convex and concave walls were investigated. Hot-wire measurements along the convex surface indicated that turbulent mixing between fluid layers was very much reduced. However, the law of the wall held and the skin friction, thus determined, correlated well with other measurements. Hot-wire measurements along the concave test wall revealed a system of longitudinal vortices inside the boundary layer and confirmed that concave curvature enhances mixing. A self-consistent set of turbulent boundary layer equations for flows along curved surfaces was derived together with a modified eddy viscosity. Solution of these equations together with the modified eddy viscosity gave results that correlated well with the present data on flows along the convex surface with arbitrary pressure distribution. However, it could only be used to predict the mean characteristics of the flow along concave walls because of the existence of the system of longitudinal vortices inside the boundary layer.
Structure of the low latitude boundary layer
NASA Technical Reports Server (NTRS)
Sckopke, N.; Paschmann, G.; Haerendel, G.; Sonnerup, B. U. O.; Bame, S. J.; Forbes, T. G.; Hones, E. W., Jr.; Russell, C. T.
1980-01-01
Observations at high temporal resolution of the frontside magnetopause and plasma boundary layer, made with the LASL/MPE fast plasma analyzer onboard the ISEE 1 and 2 spacecraft, revealed a complex quasiperiodic structure of some of the observed boundary layers. A cool tailward streaming boundary layer plasma was seen intermittently, with intervening periods of hot tenuous plasma which has properties similar to the magnetospheric population. While individual encounters with the boundary layer plasma last only a few minutes, the total observation time may extend over one hour or more.
Boundary layers of the earth's outer magnetosphere
NASA Technical Reports Server (NTRS)
Eastman, T. E.; Frank, L. A.
1984-01-01
The magnetospheric boundary layer and the plasma-sheet boundary layer are the primary boundary layers of the earth's outer magnetosphere. Recent satellite observations indicate that they provide for more than 50 percent of the plasma and energy transport in the outer magnetosphere although they constitute less than 5 percent by volume. Relative to the energy density in the source regions, plasma in the magnetospheric boundary layer is predominantly deenergized whereas plasma in the plasma-sheet boundary layer has been accelerated. The reconnection hypothesis continues to provide a useful framework for comparing data sampled in the highly dynamic magnetospheric environment. Observations of 'flux transfer events' and other detailed features near the boundaries have been recently interpreted in terms of nonsteady-state reconnection. Alternative hypotheses are also being investigated. More work needs to be done, both in theory and observation, to determine whether reconnection actually occurs in the magnetosphere and, if so, whether it is important for overall magnetospheric dynamics.
Atmospheric tides on Venus. III - The planetary boundary layer
NASA Technical Reports Server (NTRS)
Dobrovolskis, A. R.
1983-01-01
Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the bottom of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from about 5 to about 1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed.
Turbulent boundary-layer structure of flows over freshwater biofilms
NASA Astrophysics Data System (ADS)
Walker, J. M.; Sargison, J. E.; Henderson, A. D.
2013-12-01
The structure of the turbulent boundary-layer for flows over freshwater biofilms dominated by the diatom Tabellaria flocculosa was investigated. Biofilms were grown on large test plates under flow conditions in an Australian hydropower canal for periods up to 12 months. Velocity-profile measurements were obtained using LDV in a recirculating water tunnel for biofouled, smooth and artificially sandgrain roughened surfaces over a momentum thickness Reynolds number range of 3,000-8,000. Significant increases in skin friction coefficient of up to 160 % were measured over smooth-wall values. The effective roughnesses of the biofilms, k s, were significantly higher than their physical roughness measured using novel photogrammetry techniques and consisted of the physical roughness and a component due to the vibration of the biofilm mat. The biofilms displayed a k-type roughness function, and a logarithmic relationship was found between the roughness function and roughness Reynolds number based on the maximum peak-to-valley height of the biofilm, R t. The structure of the boundary layer adhered to Townsend's wall-similarity hypothesis even though the scale separation between the effective roughness height and the boundary-layer thickness was small. The biofouled velocity-defect profiles collapsed with smooth and sandgrain profiles in the outer region of the boundary layer. The Reynolds stresses and quadrant analysis also collapsed in the outer region of the boundary layer.
Particle motion in atmospheric boundary layers of Mars and Earth
NASA Technical Reports Server (NTRS)
White, B. R.; Iversen, J. D.; Greeley, R.; Pollack, J. B.
1975-01-01
To study the eolian mechanics of saltating particles, both an experimental investigation of the flow field around a model crater in an atmospheric boundary layer wind tunnel and numerical solutions of the two- and three-dimensional equations of motion of a single particle under the influence of a turbulent boundary layer were conducted. Two-dimensional particle motion was calculated for flow near the surfaces of both Earth and Mars. For the case of Earth both a turbulent boundary layer with a viscous sublayer and one without were calculated. For the case of Mars it was only necessary to calculate turbulent boundary layer flow with a laminar sublayer because of the low values of friction Reynolds number; however, it was necessary to include the effects of slip flow on a particle caused by the rarefied Martian atmosphere. In the equations of motion the lift force functions were developed to act on a single particle only in the laminar sublayer or a corresponding small region of high shear near the surface for a fully turbulent boundary layer. The lift force functions were developed from the analytical work by Saffman concerning the lift force acting on a particle in simple shear flow.
Cyclone separator having boundary layer turbulence control
Krishna, C. R.; Milau, J. S.
1985-04-16
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator.
Cyclone separator having boundary layer turbulence control
Krishna, Coimbatore R. (Mt. Sinai, NY); Milau, Julius S. (Port Jefferson, NY)
1985-01-01
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator.
Boundary Layers of Air Adjacent to Cylinders
Nobel, Park S.
1974-01-01
Using existing heat transfer data, a relatively simple expression was developed for estimating the effective thickness of the boundary layer of air surrounding cylinders. For wind velocities from 10 to 1000 cm/second, the calculated boundary-layer thickness agreed with that determined for water vapor diffusion from a moistened cylindrical surface 2 cm in diameter. It correctly predicted the resistance for water vapor movement across the boundary layers adjacent to the (cylindrical) inflorescence stems of Xanthorrhoea australis R. Br. and Scirpus validus Vahl and the leaves of Allium cepa L. The boundary-layer thickness decreased as the turbulence intensity increased. For a turbulence intensity representative of field conditions (0.5) and for ?windd between 200 and 30,000 cm2/second (where ?wind is the mean wind velocity and d is the cylinder diameter), the effective boundary-layer thickness in centimeters was equal to [Formula: see text]. PMID:16658855
LDV measurements of turbulent baroclinic boundary layers
Neuwald, P.; Reichenbach, H.; Kuhl, A.L.
1993-07-01
Described here are shock tube experiments of nonsteady, turbulent boundary layers with large density variations. A dense-gas layer was created by injecting Freon through the porous floor of the shock tube. As the shock front propagated along the layer, vorticity was created at the air-Freon interface by an inviscid, baroclinic mechanism. Shadow-schlieren photography was used to visualize the turbulent mixing in this baroclinic boundary layer. Laser-Doppler-Velocimetry (LDV) was used to measure the streamwise velocity histories at 14 heights. After transition, the boundary layer profiles may be approximated by a power-law function u {approximately} u{sup {alpha}} where {alpha} {approx_equal} 3/8. This value lies between the clean flat plate value ({alpha} = 1/7) and the dusty boundary layer value ({alpha} {approx_equal} 0.7), and is controlled by the gas density near the wall.
The benthic boundary layer under fully-nonlinear internal solitary waves of depression
NASA Astrophysics Data System (ADS)
Lin, Yuncheng; Redekopp, Larry
2010-11-01
Long internal waves are common features on the continental shelf and in lakes, but their dissipation via benthic boundary layer drag is largely unknown, particularly when the wave amplitudes are large and boundary layer corrections based on linear theory are clearly invalid. In general, the wave-induced boundary layer experiences a continuous favorable-to-adverse variation of the pressure gradient, undergoes transition, may reach a strongly turbulent state, and frequently separates near the point of maximum adverse pressure gradient in the lee of the wave. In this study a model for fully-nonlinear solitary waves of depression in a two-layer stratification is employed as the inviscid base state, and a RANS solver with k-? turbulence model is used to compute the stationary boundary layer under the wave. Local friction coefficients and eddy viscosities are computed in the footprint of the wave. Locations of boundary layer separation are computed as well as the integrated frictional drag over the region of attached boundary layer flow. Boundary layer characteristics are presented for a range of environmental conditions, Reynolds numbers, and surface roughness in an attempt to provide a quantitative measure of the frictional drag of long internal waves in realistic, shallow environs.
Determination of general relations for the behavior of turbulent boundary layers
NASA Technical Reports Server (NTRS)
Von Doenhoff, Albert E; Tetervin, Neal
1943-01-01
An analysis has been made of a considerable amount of data for turbulent boundary layers along wings and bodies of various shapes in order to determine the fundamental variables that control the development of turbulent boundary layers. It was found that the type of velocity distribution in the boundary layer could be expressed in terms of a single parameter. This parameter was chosen as the ratio of the displacement thickness to the momentum thickness of the boundary layer. The variables that control the development of the turbulent boundary layer apparently are: (1) the ratio of the nondimensional pressure gradient, expressed in terms of the local dynamic pressure outside the boundary layer and boundary-layer thickness, to the local skin-friction coefficient and (2) the shape of the boundary layer. An empirical equation has been developed in terms of these variables that, when used with the momentum equation and the skin-friction relation, makes it possible to trace the development of the turbulent boundary layer to the separation point.
A global climatology of boundary layer ventilation
NASA Astrophysics Data System (ADS)
McNamara, David; Plant, Robert; Belcher, Stephen
2013-04-01
The general circulation pattern of the Earth's atmosphere is well known, however there has been relatively little effort to quantify the climatological effects of the buffer zone known as the atmospheric boundary layer. Turbulent motions in the atmospheric boundary layer act to mix the layer along with its constituent pollutants, below a temperature inversion which separates it from the free troposphere. Exchanges between the boundary layer and free troposphere can occur through the mechanisms of convection, isentropic uplift, and coastal and orographic venting. In particular the rate at which pollutants are removed from the atmosphere can be different depending on whether or not they are resident within the boundary layer or the free troposphere. Thus the limiting factor on the concentrations of, for example, certain eg NOx, pollutants in the free troposphere will be the rate at which they are vented from the boundary layer. A global climatology (spanning 10 years between 1995 and 2005) of boundary layer venting is presented here using the ERA-interim dataset which has a grid scale resolution of 0.7 degrees x 0.7 degrees. The boundary layer height is first calculated using a bulk Richardson number method and then an associated vertical velocity is found by linearly interpolating between the two model levels either side of the boundary layer height. This value along with the change in height of the boundary layer over a 3 hour period is used to give an estimate of the rate of venting. The climatology of this rate allows us to describe and quantify the areas of the globe that are responsible for boundary layer entrainment and boundary layer venting, which could be used as a basis for further comparisons with other suitable datasets. We will also present results for the climatology of the boundary layer height itself. [possibly? That could be attractive for a BL audience anyway] Furthermore we will present and discuss results from a method designed to isolate the venting due to mid-latitude cyclones, and compare that to venting more generally, and in particular to the venting due to the orography in order to show which is the more significant mechanism.
NASA Astrophysics Data System (ADS)
Knuth, Matthew; Marone, Chris
2007-03-01
We report on laboratory experiments designed to investigate three fundamental deformation mechanisms for frictional shear of granular fault gouge: sliding, rolling, and dilation. Mechanisms were isolated by shearing layers composed of rods in geometric configurations that resulted in one-dimensional, two-dimensional, and rolling-only particle interactions. Results of digital video are presented with measurements of friction and strain to illuminate the distribution of shear and the relationship between particle motions and friction. The double-direct-shear configuration was used with boundary conditions of constant layer normal stress (1 MPa) and controlled shear loading rate (10 ?m/s) with initial layer thickness of 6 mm. Layers were sheared in a servo-hydraulic testing machine at room temperature (22C) and relative humidity (5 to 10%). Three materials were studied: alloy 260 brass, dried semolina pasta, and hardwood dowels, with particle diameters of 1.59 mm, 1.86 mm, and 2.06 mm, respectively. Pasta layers had mean sliding friction coefficients of 0.24, 0.11, and 0.02 in 2-D, 1-D, and rolling configurations, respectively. Layers of brass rods had average friction coefficients of 0.23, 0.15, and 0.01, respectively, in 2-D, 1-D, and rolling configurations; and the wood samples exhibited friction values of 0.18, 0.19, and 0.09, respectively. Evolution of strength during shear correlated strongly with the displacement derivative of layer thickness. SEM images document the role of surface finish on frictional properties. Rapid reorientations of particles correspond to stick-slip stress drops and may be related to the collapse and reformation of granular force chains. We find a systematic relationship between the strength of granular layers and (1) the surface roughness of particles and (2) the number of particle contact dimensions. Our data provide important insights on the mechanics of granular fault gouge and constraints on the fundamental parameters used in numerical models of tectonic faulting.
Planetary Boundary Layer Simulation Using TASS
NASA Technical Reports Server (NTRS)
Schowalter, David G.; DeCroix, David S.; Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael
1996-01-01
Boundary conditions to an existing large-eddy simulation model have been changed in order to simulate turbulence in the atmospheric boundary layer. Several options are now available, including the use of a surface energy balance. In addition, we compare convective boundary layer simulations with the Wangara and Minnesota field experiments as well as with other model results. We find excellent agreement of modelled mean profiles of wind and temperature with observations and good agreement for velocity variances. Neutral boundary simulation results are compared with theory and with previously used models. Agreement with theory is reasonable, while agreement with previous models is excellent.
Computation of the shock-wave boundary layer interaction with flow separation
NASA Technical Reports Server (NTRS)
Ardonceau, P.; Alziary, T.; Aymer, D.
1980-01-01
The boundary layer concept is used to describe the flow near the wall. The external flow is approximated by a pressure displacement relationship (tangent wedge in linearized supersonic flow). The boundary layer equations are solved in finite difference form and the question of the presence and unicity of the solution is considered for the direct problem (assumed pressure) or converse problem (assumed displacement thickness, friction ratio). The coupling algorithm presented implicitly processes the downstream boundary condition necessary to correctly define the interacting boundary layer problem. The algorithm uses a Newton linearization technique to provide a fast convergence.
Falkner-Skan Boundary Layer Flow of a Sisko Fluid
NASA Astrophysics Data System (ADS)
Khan, Masood; Shahzad, Azeem
2012-09-01
In this paper, we investigate the steady boundary layer flow of a non-Newtonian fluid, represented by a Sisko fluid, over a wedge in a moving fluid. The equations of motion are derived for boundary layer flow of an incompressible Sisko fluid using appropriate similarity variables. The governing equations are reduced to a single third-order highly nonlinear ordinary differential equation in the dimensionless stream function, which is then solved analytically using the homotopy analysis method. Some important parameters have been discussed by this study, which include the power law index n, the material parameter A, the wedge shape factor b, and the skin friction coefficient Cf. A comprehensive study is made between the results of the Sisko and the power-law fluids.
Active control of boundary layer transition and turbulence
NASA Astrophysics Data System (ADS)
Maestrello, Lucio
1990-06-01
The invention is a system and method for controlling boundary layer flow such that flow separation can be delayed and skin friction drag can be reduced. The invention consists of heater elements used to trigger turbulent flow and audio speakers used to suppress turbulent oscillations. By inducing turbulent oscillations into the flow in a region of positive pressure gradient, pressure patterns became more regular. The suppression of these patterns can be accomplished by imposing an out-of-phase suppressing wave. This wave is the audio output generated by a feedback amplifier using inputs from a hot-wire anemometer reading downstream turbulence. The novel features of the present invention are the tripping of boundary layer flow in a region of positive pressure gradient and the cooperative use of the feedback means to control turbulence. The result is a significant reduction in drag and separation problems.
Growth Of The Summer Daytime Convective Boundary Layer At Anand
NASA Astrophysics Data System (ADS)
Nagar, S. G.; Tyagi, Ajit; Seetaramayya, P.; et al.
The heights of the daytime convective boundary layer (CBL), computed by a one-dimensional model for a bare soil surface at a semi-arid station,Anand, during the dry and hot summer month of May 1997, are presented. As input, the model requires surface heat flux, friction velocity and air temperature as functions of time. Temperature data at the one-metre level from a tower and sonic anemometer data at 9.5 m collected during the period 13-17 May 1997 in the Land Surface Processes Experiment (LASPEX-97) are used to compute hourly values of surface heat flux, friction velocity and Obukhov length following the operational method suggested by Holtslag and Van Ulden [J. Climate Appl. Meteorol. 22,517-529 (1983)]. The model has been tested with different values for the potential temperature gradient ( ) above the inversion. The model-estimated CBL heights comparefavourably with observed heights obtained from radiosonde ascents.
Turbulent boundary-layer control with plasma spanwise travelling waves
NASA Astrophysics Data System (ADS)
Whalley, Richard D.; Choi, Kwing-So
2014-08-01
Arrays of dielectric-barrier-discharge plasma actuators have been designed to generate spanwise travelling waves in the turbulent boundary layer for possible skin-friction drag reductions. Particle image velocimetry was used to elucidate the modifications to turbulence structures created by the plasma spanwise travelling waves. It has been observed that the plasma spanwise travelling waves amalgamated streamwise vortices, lifting low-speed fluid from the near-wall region up and around the peripheries of their cores to form wide ribbons of low-speed streamwise velocity within the viscous sublayer.
Dynamic Acoustic Detection of Boundary Layer transition
NASA Technical Reports Server (NTRS)
Grohs, Jonathan R.
1995-01-01
The wind tunnel investigation into the acoustic nature of boundary layer transition using miniature microphones. This research is the groundwork for entry into the National Transonic Facility (NTF) at the NASA Langley Research Center (LaRC). Due to the extreme environmental conditions of NTF testing, low temperatures and high pressures, traditional boundary layer detection methods are not available. The emphasis of this project and further studies is acoustical sampling of a typical boundary layer and environmental durability of the miniature microphones. The research was conducted with the 14 by 22 Foot Subsonic Tunnel, concurrent with another wind tunnel test. Using the resources of LaRC, a full inquiry into the feasibility of using Knowles Electronics, Inc. EM-3086 microphones to detect the surface boundary layer, under differing conditions, was completed. This report shall discuss the difficulties encountered, product performance and observations, and future research adaptability of this method.
Hypersonic boundary layers: Transition and turbulence effects
NASA Astrophysics Data System (ADS)
Arnal, D.; Aupoix, B.
1991-07-01
Theoretical and experimental results related to the problem of laminar turbulent transition at high speeds are surveyed. In the case of 'natural' transition, the linear stability theory can describe the development of Tollmien-Schlichting waves, of Goertler vortices (on concave walls) and of stationary vortices generated by cross flow instability (three dimensional flows). The problem of boundary layer tripping by large roughness elements is also studied in two dimensional as well as in three dimensional flows. Hypersonic turbulent boundary layers are considered. The importance of compressibility effects upon turbulence and the differences in turbulence structure between low and high speed boundary layers are discussed. Turbulence modeling problems specific to high speed flows are addressed. Some examples of hypersonic boundary layers computations are presented.
Boundary-layer control for drag reduction
NASA Technical Reports Server (NTRS)
Harvey, William D.
1988-01-01
Although the number of possible applications of boundary-layer control is large, a discussion is given only of those that have received the most attention recently at NASA Langley Research Center to improve airfoil drag characteristics. This research concerns stabilizing the laminar boundary layer through geometric shaping (natural laminar flow, NLF) and active control involving the removal of a portion of the laminar boundary layer (laminar flow control, LFC) either through discrete slots or a perforated surface. At low Reynolds numbers, a combination of shaping and forced transition has been used to achieve the desired run of laminar flow and control of laminar separation. In the design of both natural laminar flow and laminar flow control airfoils and wings, boundary layer stability codes play an important role. A discussion of some recent stability calculations using both incompressible and compressible codes is given.
Boundary-layer stability and airfoil design
NASA Technical Reports Server (NTRS)
Viken, Jeffrey K.
1986-01-01
Several different natural laminar flow (NLF) airfoils have been analyzed for stability of the laminar boundary layer using linear stability codes. The NLF airfoils analyzed come from three different design conditions: incompressible; compressible with no sweep; and compressible with sweep. Some of the design problems are discussed, concentrating on those problems associated with keeping the boundary layer laminar. Also, there is a discussion on how a linear stability analysis was effectively used to improve the design for some of the airfoils.
Evaluation of analytical procedures for prediction of turbulent boundary layers on a porous wall
NASA Technical Reports Server (NTRS)
Towne, C. E.
1974-01-01
An analytical study has been made to determine how well current boundary layer prediction techniques work when there is mass transfer normal to the wall. The data that were considered in this investigation were for two-dimensional, incompressible, turbulent boundary layers with suction and blowing. Some of the bleed data were taken in an adverse pressure gradient. An integral prediction method was used three different porous wall skin friction relations, in addition to a solid-surface relation for the suction cases. A numerical prediction method was also used. Comparisons were made between theoretical and experimental skin friction coefficients, displacement and momentum thicknesses, and velocity profiles. The integral method with one of the porous wall skin friction laws gave very good agreement with data for most of the cases considered. The use of the solid-surface skin friction law caused the integral to overpredict the effectiveness of the bleed. The numerical techniques also worked well for most of the cases.
Turbulence structure and polymer drag reduction in adverse pressure gradient boundary layers
NASA Astrophysics Data System (ADS)
Koskie, John E.; Tiederman, William G.
1991-12-01
The ability of some solutions of long chain polymers to greatly reduce the pressure drop in pipes is well documented and commercially applied. In addition, a number of experiments indicate that large reductions of wall friction are possible by injecting polymer into a boundary layer. Therefore, it is reasonable to expect that drag reducing polymers could significantly improve the performance of submersibles when introduced into the boundary layers on the fore and sides of the vessel. However, once polymer is injected at these locations it is unlikely to leave the boundary layer because these polymers have very low mass diffusivities in water and boundary layers continually entrain fluid rather than eject it. As a result, one can expect that drag reducing polymers will be present in the adverse pressure gradient boundary layers on the aft of a vessel whenever the frictional drag on the fore and sides is reduced with polymers. Direct measurements of adverse pressure gradient turbulent boundary layers in which drag reducing polymers are present are not available. However, at fixed speeds, the net drag on a propeller hydrofoil increases while lift decreases when submerged in polymer solution. This apparent increase in form drag suggests a sudden increase in the displacement thickness of the boundary layers which may indicate separation. Therefore, it is necessary to establish whether conditions exist under which turbulent boundary layers of polymer solutions will not separate when they encounter adverse pressure gradients.
NASA Technical Reports Server (NTRS)
Roach, Robert L.; Nelson, Chris; Sakowski, Barbara; Darling, Douglas; Vandewall, Allan G.
1992-01-01
A finite difference boundary layer algorithm was developed to model viscous effects when an inviscid core flow solution is given. This algorithm solved each boundary layer equation separately, then iterated to find a solution. Solving the boundary layer equations sequentially was 2.4 to 4.0 times faster than solving the boundary layer equations simultaneously. This algorithm used a modified Baldwin-Lomax turbulence model, a weighted average of forward and backward differencing of the pressure gradient, and a backward sweep of the pressure. With these modifications, the boundary layer algorithm was able to model flows with and without separation. The number of grid points used in the boundary layer algorithm affected the stability of the algorithm as well as the accuracy of the predictions of friction coefficients and momentum thicknesses. Results of this boundary layer algorithm compared well with experimental observations of friction coefficients and momentum thicknesses. In addition, when used interactively with an inviscid flow algorithm, this boundary layer algorithm corrected for viscous effects to give a good match with experimental observations for pressures in a supersonic inlet.
Dependence of Boundary Layer Mixing On Lateral Boundary Conditions
NASA Astrophysics Data System (ADS)
Straub, D.
Ocean circulation models often show strong mixing in association with lateral bound- ary layers. Such mixing is generally considered to be artifactual rather than real. Fur- thermore, the severity of the problem is boundary condition dependent. For example, an inconsistency between geostrophy and insulating boundary conditions on tempera- ture and salinity cause many modelers to opt for the no slip, rather than slip boundary condtion on the tangential component of momentum. As modellers increasingly move into the eddy revealing regime, biharmonic, rather than harmonic dissipative operators are likely to become more common. Biharmonic operators, however, require specifi- cation of additional boundary conditions. For example, there are several `natural ex- tensions' to each of the slip and no slip conditions. Here, these various possiblities are considered in the context of a simple model. Particular attention is payed to how mixing (and the associated overturning cell) is affected by the choice of boundary condition.
NASA Astrophysics Data System (ADS)
Sarlak, H.; Srensen, J. N.; Mikkelsen, R.
2012-09-01
Large eddy simulation (LES) of flow in a wind farm is studied in neutral as well as thermally stratified atmospheric boundary layer (ABL). An approach has been practiced to simulate the flow in a fully developed wind farm boundary layer. The approach is based on the Immersed Boundary Method (IBM) and involves implementation of an arbitrary prescribed initial boundary layer (See [1]). A prescribed initial boundary layer profile is enforced through the computational domain using body forces to maintain a desired flow field. The body forces are then stored and applied on the domain through the simulation and the boundary layer shape will be modified due to the interaction of the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [1] while having the advantage of resolving the wall layer with a coarser grid than typically required for such problems.
Ground observations of magnetospheric boundary layer phenomena
McHenry, M.A.; Clauer, C.R. ); Friis-Christensen, E. ); Newell, P.T. ); Kelly, J.D. )
1990-09-01
Several classes of traveling vortices in the dayside ionospheric convection have been detected and tracked using the Greenland magnetometer chain (Friis-Christensen et al., 1988, McHenry et al., 1989). One class observed during quiet times consists of a continuous series of vortices moving generally anti-sunward for several hours at a time. The vortices strength is seen to be approximately steady and neighboring vortices rotate in opposite directions. Sondrestrom radar observations show that the vortices are located at the ionospheric convection reversal boundary. Low altitude DMSP observations indicate the vortices are on field lines which map to the inner edge of the low latitude boundary layer. Because the vortices are conjugate to the boundary layer, repeat in a regular fashion and travel antisunward, the authors argue that this class of vortices is caused by the Kelvin-Helmholtz instability of the inner edge of the magnetospheric boundary layer.
Stability of spatially developing boundary layers
NASA Astrophysics Data System (ADS)
Govindarajan, Rama
1993-07-01
A new formulation of the stability of boundary-layer flows in pressure gradients is presented, taking into account the spatial development of the flow. The formulation assumes that disturbance wavelength and eigenfunction vary downstream no more rapidly than the boundary-layer thickness, and includes all terms of O(1) and O(R(exp -1)) in the boundary-layer Reynolds number R. Although containing the Orr-Sommerfeld operator, the present approach does not yield the Orr-Sommerfeld equation in any rational limit. In Blasius flow, the present stability equation is consistent with that of Bertolotti et al. (1992) to terms of O(R(exp -1)). For the Falkner-Skan similarity solutions neutral boundaries are computed without the necessity of having to march in space. Results show that the effects of spatial growth are striking in flows subjected to adverse pressure gradients.
Analogies between transitional and turbulent boundary layers
NASA Technical Reports Server (NTRS)
Blackwelder, R. F.
1983-01-01
One of the interesting aspects of transitional and turbulent boundary layers is the development of counter-rotating streamwise vortices near the wall. The most regular pattern is found in a boundary layer on a concave wall where the generation mechanism is known to be the Goertler instability. The origin of these vortices in other translational and turbulent boundary layers is presently unknown. Since the counter-rotating vortices are located in a region of strong shear, low-speed fluid is pumped away from the wall which coalesces into regions of low momentum lying between the vortices. As this pumping action continues, localized inflectional velocity profiles become apparent in the transitional and turbulent boundary layers. The oscillations which develop upon these profiles scale with the local thickness and velocity difference in the same manner as the two-dimensional steady free shear layer stability problems. The oscillations grow to large amplitude and break down into new turbulence in both the transitional and turbulent boundary layers.
Some Basic Aspects of Magnetohydrodynamic Boundary-Layer Flows
NASA Technical Reports Server (NTRS)
Hess, Robert V.
1959-01-01
An appraisal is made of existing solutions of magnetohydrodynamic boundary-layer equations for stagnation flow and flat-plate flow, and some new solutions are given. Since an exact solution of the equations of magnetohydrodynamics requires complicated simultaneous treatment of the equations of fluid flow and of electromagnetism, certain simplifying assumptions are generally introduced. The full implications of these assumptions have not been brought out properly in several recent papers. It is shown in the present report that for the particular law of deformation which the magnetic lines are assumed to follow in these papers a magnet situated inside the missile nose would not be able to take up any drag forces; to do so it would have to be placed in the flow away from the nose. It is also shown that for the assumption that potential flow is maintained outside the boundary layer, the deformation of the magnetic lines is restricted to small values. The literature contains serious disagreements with regard to reductions in heat-transfer rates due to magnetic action at the nose of a missile, and these disagreements are shown to be mainly due to different interpretations of reentry conditions rather than more complicated effects. In the present paper the magnetohydrodynamic boundary-layer equation is also expressed in a simple form that is especially convenient for physical interpretation. This is done by adapting methods to magnetic forces which in the past have been used for forces due to gravitational or centrifugal action. The simplified approach is used to develop some new solutions of boundary-layer flow and to reinterpret certain solutions existing in the literature. An asymptotic boundary-layer solution representing a fixed velocity profile and shear is found. Special emphasis is put on estimating skin friction and heat-transfer rates.
The kinematics of turbulent boundary layer structure
NASA Technical Reports Server (NTRS)
Robinson, Stephen Kern
1991-01-01
The long history of research into the internal structure of turbulent boundary layers has not provided a unified picture of the physics responsible for turbulence production and dissipation. The goals of the present research are to: (1) define the current state of boundary layer structure knowledge; and (2) utilize direct numerical simulation results to help close the unresolved issues identified in part A and to unify the fragmented knowledge of various coherent motions into a consistent kinematic model of boundary layer structure. The results of the current study show that all classes of coherent motion in the low Reynolds number turbulent boundary layer may be related to vortical structures, but that no single form of vortex is representative of the wide variety of vortical structures observed. In particular, ejection and sweep motions, as well as entrainment from the free-streem are shown to have strong spatial and temporal relationships with vortical structures. Disturbances of vortex size, location, and intensity show that quasi-streamwise vortices dominate the buffer region, while transverse vortices and vortical arches dominate the wake region. Both types of vortical structure are common in the log region. The interrelationships between the various structures and the population distributions of vortices are combined into a conceptual kinematic model for the boundary layer. Aspects of vortical structure dynamics are also postulated, based on time-sequence animations of the numerically simulated flow.
High enthalpy hypersonic boundary layer flow
NASA Technical Reports Server (NTRS)
Yanow, G.
1972-01-01
A theoretical and experimental study of an ionizing laminar boundary layer formed by a very high enthalpy flow (in excess of 12 eV per atom or 7000 cal/gm) with allowance for the presence of helium driver gas is described. The theoretical investigation has shown that the use of variable transport properties and their respective derivatives is very important in the solution of equilibrium boundary layer equations of high enthalpy flow. The effect of low level helium contamination on the surface heat transfer rate is minimal. The variation of ionization is much smaller in a chemically frozen boundary layer solution than in an equilibrium boundary layer calculation and consequently, the variation of the transport properties in the case of the former was not essential in the integration. The experiments have been conducted in a free piston shock tunnel, and a detailed study of its nozzle operation, including the effects of low levels of helium driver gas contamination has been made. Neither the extreme solutions of an equilibrium nor of a frozen boundary layer will adequately predict surface heat transfer rate in very high enthalpy flows.
Boundary layers of accreting neutron stars
NASA Astrophysics Data System (ADS)
Revnivtsev, Mikhail
I would like to present an overview of observational results of studies of X-ray emission of boundary/spreading layer on neutron stars. The boundary/spreading layer is a part of the accretion flow in X-ray binaries with neutron stars, where the rapidly rotating matter of the accretion disk decelerates and settles to the neutron star surface. It was shown that in spite of complexity of physical conditions in the boundary layer, properties of its emission can be effectively used to put constrains on physical parameters of neutron stars. This ensures a rising level of interest in measurements of the boundary layer emission. In spite of that, during long period of time it was hardly possible to measure its energy spectrum in a model independent way. I will demonstrate that it is possible to do with the help of combined spectral timing information on X-ray emission of neutron stars. It will be shown that the emission of the boundary/spreading layer has virtually constant shape over large variations of its total luminosity which supports existing theoretical ideas of its structue in radiation pressure dominated regime.
Pressure Gradient Boundary Layers With Eventual Separation
NASA Astrophysics Data System (ADS)
Wang, Xia; Castillo, Luciano; George, William K.
2001-11-01
Using the similarity analysis for turbulent boundary layer with pressure gradient by Castillo and George(Castillo, L. and George, W.K.,``Similarity Analysis for Turbulent Boundary Layer with Pressure Gradient: out flow,'' AIAA Journal, Vol.39,2001) it will be shown that the outer part of adverse pressure gradient turbulent boundary layers tends to remain in equilibrium similarity, even near (and sometimes past) separation. Thus such boundary layers are characterized by a single pressure parameter, Λ_θ =fracθ ρ U_∞ ^2dθ /dxfracdP_∞ dx, and its value appears to be the same for all adverse pressure gradient flows; i.e., Λ_θ ≈ 0.22. Using this pressure parameter and the momentum integral boundary layer equation, it is possible to show that the shape factor at separation must have a single value, H_sep ≈ 2.5. Both the conditions for equilibrium similarity and the value of H_sep are shown to be in reasonable agreement with a variety of experimental estimates.
Boundary-Layer-Ingesting Inlet Flow Control
NASA Technical Reports Server (NTRS)
Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.
2008-01-01
An experimental study was conducted to provide the first demonstration of an active flow control system for a flush-mounted inlet with significant boundary-layer-ingestion in transonic flow conditions. The effectiveness of the flow control in reducing the circumferential distortion at the engine fan-face location was assessed using a 2.5%-scale model of a boundary-layer-ingesting offset diffusing inlet. The inlet was flush mounted to the tunnel wall and ingested a large boundary layer with a boundary-layer-to-inlet height ratio of 35%. Different jet distribution patterns and jet mass flow rates were used in the inlet to control distortion. A vane configuration was also tested. Finally a hybrid vane/jet configuration was tested leveraging strengths of both types of devices. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow rates through the duct and the flow control actuators. The distortion and pressure recovery were measured at the aerodynamic interface plane. The data show that control jets and vanes reduce circumferential distortion to acceptable levels. The point-design vane configuration produced higher distortion levels at off-design settings. The hybrid vane/jet flow control configuration reduced the off-design distortion levels to acceptable ones and used less than 0.5% of the inlet mass flow to supply the jets.
Boundary Layer Cloudiness Parameterizations Using ARM Observations
Bruce Albrecht
2004-09-15
This study used DOE ARM data and facilities to: (1) study macroscopic properties of continental stratus clouds at SGP and the factors controlling these properties, (2) develop a scientific basis for understanding the processes responsible for the formation of boundary layer clouds using ARM observations in conjunction with simple parametric models and LES, and (3) evaluate cumulus cloud characteristics retrieved from the MMCR operating at TWP-Nauru. In addition we have used high resolution 94 GHz observations of boundary layer clouds and precipitation to: (1) develop techniques for using high temporal resolution Doppler velocities to study large-eddy circulations and turbulence in boundary layer clouds and estimate the limitations of using current and past MMCR data for boundary layer cloud studies, (2) evaluate the capability and limitations of the current MMCR data for estimating reflectivity, vertical velocities, and spectral under low- signal-to-noise conditions associated with weak no n-precipitating clouds, (3) develop possible sampling modes for the new MMCR processors to allow for adequate sampling of boundary layer clouds, and (4) retrieve updraft and downdraft structures under precipitating conditions.
Stability of separating subsonic boundary layers
NASA Technical Reports Server (NTRS)
Masad, Jamal A.; Nayfeh, Ali H.
1994-01-01
The primary and subharmonic instabilities of separating compressible subsonic two-dimensional boundary layers in the presence of a two-dimensional roughness element on a flat plate are investigated. The roughness elements considered are humps and forward- and backward-facing steps. The use of cooling and suction to control these instabilities is studied. The similarities and differences between the instability characteristics of separating boundary layers and those of the boundary layer over a flat plate with a zero pressure gradient are pointed out and discussed. The theoretical results agree qualitatively and quantitatively with the experimental data of Dovgal and Kozlov. Cooling and suction decrease the growth rates of primary and subharmonic waves in the attached-flow regions but increase them in the separated-flow regions.
Hypersonic Boundary Layer Instability Over a Corner
NASA Technical Reports Server (NTRS)
Balakumar, Ponnampalam; Zhao, Hong-Wu; McClinton, Charles (Technical Monitor)
2001-01-01
A boundary-layer transition study over a compression corner was conducted under a hypersonic flow condition. Due to the discontinuities in boundary layer flow, the full Navier-Stokes equations were solved to simulate the development of disturbance in the boundary layer. A linear stability analysis and PSE method were used to get the initial disturbance for parallel and non-parallel flow respectively. A 2-D code was developed to solve the full Navier-stokes by using WENO(weighted essentially non-oscillating) scheme. The given numerical results show the evolution of the linear disturbance for the most amplified disturbance in supersonic and hypersonic flow over a compression ramp. The nonlinear computations also determined the minimal amplitudes necessary to cause transition at a designed location.
Inverted Vs spanning the cusp boundary layer
NASA Technical Reports Server (NTRS)
Menietti, J. D.; Smith, M. F.
1993-01-01
We present several examples of dayside inverted Vs in which ion convection and energy dispersion appear to span the region of both open and closed field lines. All of these examples occur in the region generally called the low-latitude boundary layer or the cusp boundary layer and suggest a connection between closed field lines of the dayside auroral region and open field lines of the dayside cusp. One explanation is that plasma in the cusp boundary layer on closed field lines convects across field lines in a smooth fashion into the cusp. Such cross-field-line convection may be possible in regions of finite conductivity and strong currents, where the field can diffuse through the plasma. Another explanation for the observations is that they represent the mid-altitude manifestation of merging at the dayside magnetopause and the opening of previously closed field lines.
NASA Technical Reports Server (NTRS)
Mack, L. M.
1967-01-01
The fundamentals of stability theory, its chief results, and the physical mechanisms at work are presented. The stability theory of the laminar boundary determines whether a small disturbance introduced into the boundary layer will amplify or damp. If the disturbance damps, the boundary layer remains laminar. If the disturbance amplifies, and by a sufficient amount, then transition to turbulence eventually takes place. The stability theory establishes those states of the boundary layer which are most likely to lead to transition, identifys those frequencies which are the most dangerous, and indicates how the external parameters can best be changed to avoid transition.
NASA Technical Reports Server (NTRS)
Hoffmann, J. A.; Kassir, S. M.; Larwood, S. M.
1989-01-01
The influence of near isotropic free-stream turbulence on the shape factors and skin friction coefficients of turbulent boundary layers is presented for the cases of zero and mild adverse pressure gradients. With free-stream turbulence, improved fluid mixing occurs in boundary layers with adverse pressure gradients relative to the zero pressure gradient condition, with the same free-stream turbulence intensity and length scale. Stronger boundary layers with lower shape factors occur as a result of a lower ratio of the integral scale of turbulence to the boundary layer thickness, and to vortex stretching of the turbulent eddies in the free-stream, both of which act to improve the transmission of momentum from the free-stream to the boundary layers.
NASA Technical Reports Server (NTRS)
Corke, T. C.; Guezennec, Y.; Nagib, H. M.
1981-01-01
The effects of placing a parallel-plate turbulence manipulator in a boundary layer are documented through flow visualization and hot wire measurements. The boundary layer manipulator was designed to manage the large scale structures of turbulence leading to a reduction in surface drag. The differences in the turbulent structure of the boundary layer are summarized to demonstrate differences in various flow properties. The manipulator inhibited the intermittent large scale structure of the turbulent boundary layer for at least 70 boundary layer thicknesses downstream. With the removal of the large scale, the streamwise turbulence intensity levels near the wall were reduced. The downstream distribution of the skin friction was also altered by the introduction of the manipulator.
Simulation of bubble migration in a turbulent boundary layer
NASA Astrophysics Data System (ADS)
Mattson, M.; Mahesh, K.
2011-04-01
This paper presents the results from a one-way coupled, Euler-Lagrangian, direct numerical simulation of bubbles injected into a turbulent boundary layer. The Reynolds number of the turbulent boundary layer varies from 420
On the helicity estimation in the atmospheric boundary layer
NASA Astrophysics Data System (ADS)
Victorovna Vazaeva, Natalia; Feodosevich Kramar, Valeriy; Dmitrievich Kouznetsov, Rostislav; Sergeevich Lyulyukin, Vasily; Guramovich Chkhetiani, Otto
2015-04-01
Large-scale motions in the atmospheric boundary layer (ABL) are sustained by the cooperative effect of friction and Earth rotation. This flows have a non-zero helicity [1,2]. Consequently, turbulence in the ABL is also characterize by the non-zero helicity [3]. This property has been observed in the atmospheric experiments [4] and the DNS modeling [5]. The role of helicity in the atmospheric and ocean large-scale dynamics and its possible prognostics sense attracts permanent interest. The purpose of the present investigation is to determine the helicity in terms of experimental data and to check the possibility of the ABL helicity estimation. The integral helicity in the Ekman layer is given by 0 ∫ ∞ H = UG2 + VG2 [1], PIC
Thermal instability of forced convection boundary layers
Chen, K.; Chen, M.M.
1981-11-01
Thermal instability of forced convection boundary layers with non-zero steamwise pressure gradient is examined. Such an instability increases as the third power of the layer thickness, hence could be significant in the low Peclet number flows encountered in solar and microelectronic heat transfer as well as in reactor safety considerations associated with pump shutdowns. The analysis is carried out for the family of Falkner-Skan flows, here viewed as the lowest order local similarity approximation of general forced convection boundary layers. Only the streamwise vortex mode, which had previously been shown to be the dominant mode for buoyancy generated instability for fluid layers with shear at low Reynolds numbers are considered. 19 refs.
Kinetic friction and atomistic instabilities in boundary-lubricated systems
NASA Astrophysics Data System (ADS)
Aichele, Martin; Mser, Martin
2003-07-01
The contribution of sliding-induced, atomic-scale instabilities to the kinetic friction force is investigated by molecular dynamics. For this purpose, we derive a relationship between the kinetic friction force Fk and the nonequilibrium velocity distribution P(v) of the lubricant particles. P(v) typically shows exponential tails, which cannot be described in terms of an effective temperature. It is investigated which parameters control the existence of instabilities and how they affect P(v) and hence Fk. The effects of the interfaces dimensionality, lubricant coverage, and internal degrees of freedom of lubricant particles on Fk are studied explicitly. Among other results, we find that the kinetic friction between commensurate surfaces is much more susceptible to changes in (i) lubricant coverage, (ii) sliding velocity, and (iii) bond length of lubricant molecules than incommensurate surfaces.
Boundary-layer theory for blast waves
NASA Technical Reports Server (NTRS)
Kim, K. B.; Berger, S. A.; Kamel, M. M.; Korobeinikov, V. P.; Oppenheim, A. K.
1975-01-01
It is profitable to consider the blast wave as a flow field consisting of two regions: the outer, which retains the properties of the inviscid solution, and the inner, which is governed by flow equations including terms expressing the effects of heat transfer and, concomitantly, viscosity. The latter region thus plays the role of a boundary layer. Reported here is an analytical method developed for the study of such layers, based on the matched asymptotic expansion technique combined with patched solutions.
Calculation methods for compressible turbulent boundary layers, 1976
NASA Technical Reports Server (NTRS)
Bushnell, D. M.; Cary, A. M., Jr.; Harris, J. E.
1977-01-01
Equations and closure methods for compressible turbulent boundary layers are discussed. Flow phenomena peculiar to calculation of these boundary layers were considered, along with calculations of three dimensional compressible turbulent boundary layers. Procedures for ascertaining nonsimilar two and three dimensional compressible turbulent boundary layers were appended, including finite difference, finite element, and mass-weighted residual methods.
Body surface adaptations to boundary-layer dynamics.
Videler, J J
1995-01-01
Evolutionary processes have adapted nektonic animals to interact efficiently with the water that surrounds them. Not all these adaptations serve the same purpose. This paper concentrates on reduction of drag due to friction in the boundary layer close to the body surface. Mucus, compliant skins, scales, riblets and roughness may influence the flow velocity gradient, the type of flow and the thickness of the boundary layer around animals, and may seriously affect their drag in a positive or negative way. The long-chain polymers found in mucus decrease the pressure gradient and considerably reduced drag due to friction. The effect is probably due to channelling of the flow particles in the direction of the main flow, resulting in a reduction of turbulence. Compliant surfaces could probably reduce drag by equalising and distributing pressure pulses. However, the existing evidence that drag reduction actually occurs is not convincing. There is no indication that instantaneous heating, reducing the viscosity in the boundary layer, is used by animals as a drag-reducing technique. Small longitudinal ridges on rows of scales on fish can reduce shear stress in the boundary by a maximum of 10% compared with the shear stress of a smooth surface. The mechanism is based on the impedance of cross flow under well-defined conditions. The effect has been visualized with the use of particle image velocimetry techniques. The function of the swords and spears of several fast, pelagic, predatory fish species is still enigmatic. The surface structure of the sword of a swordfish is shown to be both rough and porous. The height of the roughness elements on the tip of the sword is close to the critical value for the induction of a laminar-to-turbulent flow transition at moderate cruising speeds. A flow tank is described that is designed to visualize the effects of surface imperfections on flow in the boundary layer in direct comparison with a smooth flat wall. The flow in a 1 m long, 10 cm high and 1 cm wide channel is visualized by illuminating the particles in a thin laser light sheet. The first results show that a rough surface increases the shear stress in the boundary layer and makes it thinner. The function of the roughness on the sword of a swordfish is probably to reduce the total drag by generating premature turbulence and by boundary layer thinning, despite an increased friction over the surface of the sword. The function of the porous surface structures on the sword, and of the porous skins of sharks and of the castor oil fish, will probably be discovered soon using new particle image velocimetry techniques applied under strong magnification to visualize the local behaviour of the flow. PMID:8571218
Force microscopy of layering and friction in an ionic liquid.
Hoth, Judith; Hausen, Florian; Mser, Martin H; Bennewitz, Roland
2014-07-16
The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1 1 1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface. PMID:24919549
Force microscopy of layering and friction in an ionic liquid
NASA Astrophysics Data System (ADS)
Hoth, Judith; Hausen, Florian; Mser, Martin H.; Bennewitz, Roland
2014-07-01
The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1?1?1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface.
A Vertically Resolved Planetary Boundary Layer
NASA Technical Reports Server (NTRS)
Helfand, H. M.
1984-01-01
Increase of the vertical resolution of the GLAS Fourth Order General Circulation Model (GCM) near the Earth's surface and installation of a new package of parameterization schemes for subgrid-scale physical processes were sought so that the GLAS Model GCM will predict the resolved vertical structure of the planetary boundary layer (PBL) for all grid points.
Flow visualization of turbulent boundary layer structure
NASA Astrophysics Data System (ADS)
Head, M. R.; Bandyopadhyay, P.
1980-01-01
The results from flow visualization experiments performed using an argon-ion laser to illuminate longitudinal and transverse sections of the smoke filled boundary layer in zero pressure gradient are discussed. Most of the experiments were confined to the range 600 Re sub theta 10,000. Results indicate that the boundary layer consists almost exclusively of vortex loops or hairpins, some of which may extend through the complete boundary layer thickness and all of which are inclined at a more or less constant characteristic angle of approximately 45 deg to the wall. Since the cross-stream dimensions of the hairpins appear to scale roughly with the wall variables U sub tau and nu, while their length is limited only by the boundary layer thickness, there are very large scale effects on the turbulence structure. At high Reynolds numbers (Re sub theta = 10,000) there is little evidence of large-scale coherent motions, other than a slow overturning of random agglomerations of the hairpins just mentioned.
The structure of APG turbulent boundary layers
NASA Astrophysics Data System (ADS)
Gungor, Ayse G.; Maciel, Yvan; Simens, Mark P.; Soria, Julio
2013-11-01
A boundary layer under influence of a strong APG is studied using DNS. Transition to turbulence is triggered using a trip wire which is modelled using the immersed boundary method. The Reynolds number close to the exit of the numerical domain is Re? = 2175 and the shape-factor H = 2 . 5 . Two dimensional two-point spatial correlation functions are obtained in this region and close to the transition region. Cvu with a reference point close to the transition region shows a flow periodicity until Re? ~ 1600 . This periodicity is related to the shear layer instability of the separation bubble created as a result of the APG. The Cvv and Cww correlations obtained far from the transition region at Re? = 2175 and at y / ? = 0 . 4 coincide with results obtained for a ZPG boundary layer. Implying that the structure of the v , w fluctuations is the same as in ZPG. However, Cuu indicates that the structure of the u fluctuation in an APG boundary layer is almost twice as short as the ZPG structures. The APG structures are also less correlated with the flow at the wall. The near wall structure of strong APG flows is different from ZPG flows in that streaks are much shorter or absent. Funded in part by ITU, NSERC of Canada, ARC Discovery Grant, and Multiflow program of the ERC.
Thick diffusion limit boundary layer test problems
Bailey, T. S.; Warsa, J. S.; Chang, J. H.; Adams, M. L.
2013-07-01
We develop two simple test problems that quantify the behavior of computational transport solutions in the presence of boundary layers that are not resolved by the spatial grid. In particular we study the quantitative effects of 'contamination' terms that, according to previous asymptotic analyses, may have a detrimental effect on the solutions obtained by both discontinuous finite element (DFEM) and characteristic-method (CM) spatial discretizations, at least for boundary layers caused by azimuthally asymmetric incident intensities. Few numerical results have illustrated the effects of this contamination, and none have quantified it to our knowledge. Our test problems use leading-order analytic solutions that should be equal to zero in the problem interior, which means the observed interior solution is the error introduced by the contamination terms. Results from DFEM solutions demonstrate that the contamination terms can cause error propagation into the problem interior for both orthogonal and non-orthogonal grids, and that this error is much worse for non-orthogonal grids. This behavior is consistent with the predictions of previous analyses. We conclude that these boundary layer test problems and their variants are useful tools for the study of errors that are introduced by unresolved boundary layers in diffusive transport problems. (authors)
Boundary layer control device for duct silencers
NASA Technical Reports Server (NTRS)
Schmitz, Fredric H. (inventor); Soderman, Paul T. (inventor)
1993-01-01
A boundary layer control device includes a porous cover plate, an acoustic absorber disposed under the porous cover plate, and a porous flow resistive membrane interposed between the porous cover plate and the acoustic absorber. The porous flow resistive membrane has a flow resistance low enough to permit sound to enter the acoustic absorber and high enough to damp unsteady flow oscillations.
Boundary Layer Transition on X-43A
NASA Technical Reports Server (NTRS)
Berry, Scott; Daryabeigi, Kamran; Wurster, Kathryn; Bittner, Robert
2008-01-01
The successful Mach 7 and 10 flights of the first fully integrated scramjet propulsion systems by the Hyper-X (X-43A) program have provided the means with which to verify the original design methodologies and assumptions. As part of Hyper-X s propulsion-airframe integration, the forebody was designed to include a spanwise array of vortex generators to promote boundary layer transition ahead of the engine. Turbulence at the inlet is thought to provide the most reliable engine design and allows direct scaling of flight results to groundbased data. Pre-flight estimations of boundary layer transition, for both Mach 7 and 10 flight conditions, suggested that forebody boundary layer trips were required to ensure fully turbulent conditions upstream of the inlet. This paper presents the results of an analysis of the thermocouple measurements used to infer the dynamics of the transition process during the trajectories for both flights, on both the lower surface (to assess trip performance) and the upper surface (to assess natural transition). The approach used in the analysis of the thermocouple data is outlined, along with a discussion of the calculated local flow properties that correspond to the transition events as identified in the flight data. The present analysis has confirmed that the boundary layer trips performed as expected for both flights, providing turbulent flow ahead of the inlet during critical portions of the trajectory, while the upper surface was laminar as predicted by the pre-flight analysis.
Orbiter Boundary Layer Transition Prediction Tool Enhancements
NASA Technical Reports Server (NTRS)
Berry, Scott A.; King, Rudolph A.; Kegerise, Michael A.; Wood, William A.; McGinley, Catherine B.; Berger, Karen T.; Anderson, Brian P.
2010-01-01
Updates to an analytic tool developed for Shuttle support to predict the onset of boundary layer transition resulting from thermal protection system damage or repair are presented. The boundary layer transition tool is part of a suite of tools that analyze the local aerothermodynamic environment to enable informed disposition of damage for making recommendations to fly as is or to repair. Using mission specific trajectory information and details of each d agmea site or repair, the expected time (and thus Mach number) of transition onset is predicted to help define proper environments for use in subsequent thermal and stress analysis of the thermal protection system and structure. The boundary layer transition criteria utilized within the tool were updated based on new local boundary layer properties obtained from high fidelity computational solutions. Also, new ground-based measurements were obtained to allow for a wider parametric variation with both protuberances and cavities and then the resulting correlations were calibrated against updated flight data. The end result is to provide correlations that allow increased confidence with the resulting transition predictions. Recently, a new approach was adopted to remove conservatism in terms of sustained turbulence along the wing leading edge. Finally, some of the newer flight data are also discussed in terms of how these results reflect back on the updated correlations.
Planetary Boundary Layer from AERI and MPL
Sawyer, Virginia
2014-02-13
The distribution and transport of aerosol emitted to the lower troposphere is governed by the height of the planetary boundary layer (PBL), which limits the dilution of pollutants and influences boundary-layer convection. Because radiative heating and cooling of the surface strongly affect the PBL top height, it follows diurnal and seasonal cycles and may vary by hundreds of meters over a 24-hour period. The cap the PBL imposes on low-level aerosol transport makes aerosol concentration an effective proxy for PBL height: the top of the PBL is marked by a rapid transition from polluted, well-mixed boundary-layer air to the cleaner, more stratified free troposphere. Micropulse lidar (MPL) can provide much higher temporal resolution than radiosonde and better vertical resolution than infrared spectrometer (AERI), but PBL heights from all three instruments at the ARM SGP site are compared to one another for validation. If there is agreement among them, the higher-resolution remote sensing-derived PBL heights can accurately fill in the gaps left by the low frequency of radiosonde launches, and thus improve model parameterizations and our understanding of boundary-layer processes.
Numerical methods for hypersonic boundary layer stability
NASA Technical Reports Server (NTRS)
Malik, M. R.
1990-01-01
Four different schemes for solving compressible boundary layer stability equations are developed and compared, considering both the temporal and spatial stability for a global eigenvalue spectrum and a local eigenvalue search. The discretizations considered encompass: (1) a second-order-staggered finite-difference scheme; (2) a fourth-order accurate, two-point compact scheme; (3) a single-domain Chebychev spectral collocation scheme; and (4) a multidomain spectral collocation scheme. As Mach number increases, the performance of the single-domain collocation scheme deteriorates due to the outward movement of the critical layer; a multidomain spectral method is accordingly designed to furnish superior resolution of the critical layer.
Coastal boundary layer transition within tropical cyclones at landfall
NASA Astrophysics Data System (ADS)
Howard, James Robert
Hurricanes pose a great risk to life and property with their high winds, excessive rainfall, wave action, and storm surge. Predicting changes within hurricanes at and near the time of landfall requires an understanding of the dynamics that drive the boundary layer flow. Forecasters predict the timing, duration, and effects of the intense winds associated with a hurricane when it comes ashore, while emergency management officials call for public evacuations based upon these forecasts. One region where understanding the magnitude and structure of the wind is critical is within the surface layer just downstream of the coastline in the onshore flow. Within this region the flow begins to adjust to changes in surface triggered by its passage from the shallow coastal waters to the less homogeneous and rougher land. This adjustment may include a slowing of the mean wind with an increase in turbulence, both resulting from the increased friction of the man-made and natural terrain. Hurricane observing programs consisting of portable and mobile equipment and regional coastal mesoscale observing networks are leading to a better understanding of the processes involved with these flow modifications. The Texas Tech University Wind Engineering Mobile Instrumented Tower Experiment (WEMITE) continues to play a leading role in the observation and analysis of the boundary layer of tropical cyclones at landfall. In order to gain further insight into the characteristics of this coastal transition zone, experiments were planned utilizing portable in-situ and remote measuring devices to be placed within the onshore flow at landfall. Experiment plan designs along with results from these experiments are discussed, including the analysis of a dataset collected by multiple institutions during the landfall of Hurricane Lili (2002) along the south-central Louisiana coast. Investigation reveals the existence of frictionally-induced changes in the boundary layer downwind of the coastline within the right semicircle with respect to Lili's forward motion. In the outer reaches of Lili, these transitions appear similar to internal boundary layers produced by flow moving over an abrupt change in surface. The impact on the magnitude of the wind within this near-shore region is a reduction of 4--10% per 10 km distance from the coast up to 50 km inland for open terrain. Results of the study show this reduction to be an exponential function of distance from the coast, which is dependent upon surface roughness. This rate of wind decay slows with farther progression inland and appears to be much faster than the rate found in some modeling studies. In contrast, near Lili's circulation center, little or no decrease in the magnitude of the mean wind was found for distances of up to 20 km inland.
Supersonic turbulent boundary-layer flows with mass injection through slots and/or porous walls
NASA Technical Reports Server (NTRS)
Murray, A. L.; Lewis, C. H.
1975-01-01
An implicit finite-difference method was used to solve the compressible boundary-layer equations, and to study the effects of mass transfer through porous plates, slots, and a combination of the two. The effects of the external pressure field were also included by using a global pressure interaction scheme. Two different eddy viscosity models were used for the slot and slot-porous combination cases: one was a two-layer model with inner and outer laws, and the other was a multi-layer model with as many as five separate layers. Results of the present method were compared with experimental data at a Mach number of 2.8. Comparisons of the skin friction reduction and Mach number profiles gave good to excellent agreement. Pressure interaction had little effect on the slot injection skin friction but increased the skin friction of the porous and slot-porous combination markedly.
Separation behavior of boundary layers on three-dimensional wings
NASA Technical Reports Server (NTRS)
Stock, H. W.
1981-01-01
An inverse boundary layer procedure for calculating separated, turbulent boundary layers at infinitely long, crabbing wing was developed. The procedure was developed for calculating three dimensional, incompressible turbulent boundary layers was expanded to adiabatic, compressible flows. Example calculations with transsonic wings were made including viscose effects. In this case an approximated calculation method described for areas of separated, turbulent boundary layers, permitting calculation of this displacement thickness. The laminar boundary layer development was calculated with inclined ellipsoids.
INDIVIDUAL TURBULENT CELL INTERACTION: BASIS FOR BOUNDARY LAYER ESTABLISHMENT
Boundary layers are important in determining the forces on objects in flowing fluids, mixing characteristics, and other phenomena. For example, benthic boundary layers are frequently active resuspension layers that determine bottom turbidity and transniissivity. Traditionally, bo...
Accretion disk boundary layers in cataclysmic variables. 1: Optically thick boundary layers
NASA Technical Reports Server (NTRS)
Popham, Robert; Narayan, Ramesh
1995-01-01
We develop numerical models of accretions disks in cataclysmic variables (CVs), including and emphasizing the boundary layer region where the accretion disk meets the accreting white dwarf. We confine ourselves to solutions where the boundary layer region is vertically optically thick, and find that these solutions share several common features. The angular and radial velocities of the accreting material drop rapidly in a dynamical boundary layer, which has a radial width approximately 1%-3% of the white dwarf radius. The energy dissipated in this region diffuses through the inner part of the disk and is radiated from the disk surface in a thermal boundary layer, which has a radial width comparable to the disk thickness, approximately 5%-15% of the white dwarf radius. We examine the dependence of the boundary layer structure on the mass accretion rate, the white dwarf mass and rotation rate, and the viscosity parameter alpha. We delineate the boundary between optically thick and optically thin boundary layer solutions as a function of these parameters and suggest that by means of a careful comparison with observations it may be possible to estimate alpha in CVs. We derive an expression for the total boundary layer luminosities as a function of the parameters and show that it agrees well with the luminosites of our numerical solutions. Finally, we calcuate simple blackbody continuum spectra of the boundary layer and disk emission for our solutions and compare these to soft X-ray, EUV, and He II emission-line observations of CVs. We show that, through such comparisons, it may be possible to determine the rotation rates of the accreting stars in CVs, and perhaps also the white dwarf masses and the accretion rates. The spectra are quite insensitive to alpha, so the uncertainty in this parameter does not affect such comparisons.
NASA Technical Reports Server (NTRS)
Hoffmann, Jon A.
1988-01-01
The influence of near isotropic free-stream turbulence on the shape factors and skin friction coefficients of turbulent bounday layers is presented for the cases of zero and mild adverse pressure gradients. With free-stream turbulence, improved fluid mixing occurs in boundary layers with adverse pressure gradients relative to the zero pressure gradient condition, with the same free-stream turbulence intensity and length scale. Stronger boundary layers with lower shape factors occur as a result of a lower ratio of the integral scale of turbulence to the boundary layer thickness, and to vortex stretching of the turbulent eddies in the free stream, both of which act to improve the transmission of momentum from the free stream to the boundary layers.
Bursting frequency prediction in turbulent boundary layers
LIOU,WILLIAM W.; FANG,YICHUNG
2000-02-01
The frequencies of the bursting events associated with the streamwise coherent structures of spatially developing incompressible turbulent boundary layers were predicted using global numerical solution of the Orr-Sommerfeld and the vertical vorticity equations of hydrodynamic stability problems. The structures were modeled as wavelike disturbances associated with the turbulent mean flow. The global method developed here involves the use of second and fourth order accurate finite difference formula for the differential equations as well as the boundary conditions. An automated prediction tool, BURFIT, was developed. The predicted resonance frequencies were found to agree very well with previous results using a local shooting technique and measured data.
Inflow length and tripping effects in turbulent boundary layers
NASA Astrophysics Data System (ADS)
Örlü, Ramis; Schlatter, Philipp
2011-12-01
A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & Örlü, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. For the present paper the DNS of a zero pressure-gradient turbulent boundary layer flow by Schlatter et al. [Phys. Fluids 21, 051702 (2009)] serving as the baseline simulation, was re-simulated, however with physically different inflow conditions and tripping effects. The downstream evolution of integral and global quantities as well as mean and fluctuation profiles are presented and results indicate that different inflow conditions and tripping effects explain most of the differences observed when comparing available DNS. It is also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Reθ < 300, all data agree well for both inner and outer layer for Reθ > 2000; a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments.
Tribological characteristics of few-layer graphene over Ni grain and interface boundaries.
Tripathi, Manoj; Awaja, Firas; Paolicelli, Guido; Bartali, Ruben; Iacob, Erica; Valeri, Sergio; Ryu, Seunghwa; Signetti, Stefano; Speranza, Giorgio; Pugno, Nicola Maria
2016-03-17
The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10(-5) Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned. PMID:26948836
Burst vortex/boundary layer interaction
NASA Technical Reports Server (NTRS)
Bradshaw, P.; Naaseri, M.
1988-01-01
Several configurations of delta wing vortex generator and boundary layer test plate were tested, and two final ones selected. Sample measurements and flow visualizations in the candidate configurations, together with more detailed measurements in one of the two final arrangements, which were selected so that a pure vortex bursts repeatably and then interacts, in as simple fashion as possible, with a simple turbulent boundary layer, are included. It is concluded that different intensities of bursting or breakdown, like different strengths of shock wave or hydraulic jump, can be produced by minor changes of configuration. The weaker breakdowns do not produce flow reversal. The initial measurements were done with a fairly weak, but repeatable, breakdown. Basic measurements on the second final arrangement, with a stronger breakdown, are in progress.
BOREAS AFM-6 Boundary Layer Height Data
NASA Technical Reports Server (NTRS)
Wilczak, James; Hall, Forrest G. (Editor); Newcomer, Jeffrey A. (Editor); Smith, David E. (Technical Monitor)
2000-01-01
The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) site. This data set provides boundary layer height information over the site. The data were collected from 21 May 1994 to 20 Sep 1994 and are stored in tabular ASCII files. The boundary layer height data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).
Shockwave-boundary layer interference heating analysis.
NASA Technical Reports Server (NTRS)
Hung, F. T.; Barnett, D. O.
1973-01-01
Interference heating correlations have been developed based on existing wind tunnel test data taken with simple configurations such as wedge/flat plate and compression corner models. For turbulent flow, peak interference heating was first correlated with shock strength (pressure ratio across shock wave) and then as a function of Reynolds number. The peak interference Stanton number was found to exhibit the same Reynolds number and Prandtl number characteristics as the Stanton number variation for undisturbed flow over a flat plate. Similar correlations were developed for laminar flow. Results indicated that for laminar flow at higher Reynolds number, the impinging shockwave may act as a boundary layer trip and cause boundary layer transition resulting in high interference heating. The correlations derived in this study can be used to scale wind tunnel model test data to a full-scale space vehicle at supersonic or hypersonic flight conditions.
Turbulence in a Hypersonic Boundary Layer
NASA Astrophysics Data System (ADS)
Williams, Owen; Smits, Alexander
2011-11-01
Turbulent fluctuations in hypersonic boundary layers are conventionally collapsed using Morkovin scaling which has been shown to be broadly applicable up to Mach numbers as high as 5. To validate Morkovin's hypothesis at higher Mach numbers, and help improve our understanding of hypersonic wall-bounded turbulence, we report PIV measurements of two components of velocity fluctuations in a flat plate, turbulent boundary layer at Mach 7.4 in a perfect gas, at a Reynolds number based on momentum thickness of about 3500. Multiple tripping methods were evaluated to establish the sensitivity of the flow to initial conditions. Validation of the PIV results will be discussed and comparisons of various turbulent quantities will be made with DNS under identical flow conditions.
Sound radiation from a turbulent boundary layer
NASA Astrophysics Data System (ADS)
Hu, Zhiwei; Morfey, Christopher L.; Sandham, Neil D.
2006-09-01
Sound radiation due to fluctuating viscous wall shear stresses in a plane turbulent boundary layer is investigated by a two-stage procedure using direct numerical simulation (DNS) databases for incompressible turbulent Poiseuille flow in a plane channel, at Reynolds numbers up to Re?=1440. The power spectral density of radiated pressure and spectra of sound power per unit wall area are calculated in the low Mach number limit by substituting source terms obtained from DNS into a Ffowcs Williams-Hawkings wave equation and using a half-space Green function. The same DNS data are used to predict the spectrum of turbulent boundary layer noise measured in a diffuser downstream of a fully developed channel flow [Greshilov and Mironov, Sov. Phys. Acoust. 29, 275 (1983)]. The measured spectrum is 15dB higher at low frequencies, but converges with the prediction at high frequencies.
Stability of boundary layers along curved surfaces
NASA Astrophysics Data System (ADS)
Le Cunff, Cedric
The stability of a boundary layer along a curved wall was first studied by Gortler (27) in 1940. Further investigations have been conducted since 1980. An important contribution was made by Hall (29), when he first proposed a more rigorous approach to this problem. Gortler had used a simple approximation which allowed him to reduce the problem to a set of ordinary differential equations. However such an assumption ignored the growth of the boundary layer, which Hall showed could not be neglected. Hall then rederived the formulation to obtain a set of partial differential equations. We solved the system of equations, including the nonlinear terms, with a method proposed by Herbert (3), called the Parabolic Stability Equations (PSE). The flow is divided into a basic profile, which satisfies the Prandtl boundary layer equations, and a perturbation. We compared our results for the Blasius profile with those of Bottaro, Klinnmann, and Zebib (8) and found excellent agreement between our calculations and their finite- volume simulations. We then applied our code to the wall jet profile. We were able to capture the growth of steady vortices, located in the inner region of the jet for a concave wall, and the outer region for a convex wall, as predicted by Florian's (20) linear inviscid argument. Our calculations were in good agreement with Matsson's (49) experimental results. We also studied the influence of crossflow on a boundary layer. Crossflow might lead to streamwise vortices along a flat plate, and contrary to Gortler vortices, crossflow vortices are co-rotating instead of counter-rotating. We investigated the interactions between these two types of vortices and compared the results with experimental measurements obtained by Bippes (4). The final part of the thesis is the simulation of the secondary time-dependent instability originating from the shear profiles created by the primary streamwise vortices. The onset of the instability is studied by marching both in space and time. If no forcing is prescribed the time-dependent code predicts a steady solution. Time-dependent boundary conditions are then applied by solving the linear stability problem at some streamwise location to obtain the most dangerous streamwise perturbation velocity and the corresponding frequency. We found that the varicose mode is more amplified in the streamwise direction than the sinuous mode. Similarly, if both modes are included in the initial conditions, the varicose mode is still dominant. Furthermore, as the flow evolves downstream, the unsteady behavior exhibits a more complex time-dependence, which was also observed in the experiments of Swearingen and Blackwealder (66). In our computations, higher harmonics are observed near the wall and propagate into the boundary layer.
NASA Technical Reports Server (NTRS)
Rashidnia, N.; Falco, R. E.
1987-01-01
A specially designed wind tunnel was used to examine the effects of tandemly arranged parallel plate manipulators (TAPPMs) on a turbulent boundary-layer structure and the associated drag. Momentum balances, as well as measurements of the local shear stress from the velocity gradient near the wall, were used to obtain the net drag and local skin friction changes. Two TAPPMs, identical except for the thickness of their plates, were used in the study. Results with .003 inch plates were a maximum net drag reduction of 10 percent at 58 beta sub o (using a momentum balance). At 20 beta sub o, simultaneous laser sheet flow visualization and hot-wire anemometry data showed that the Reynolds stress in the large eddies was significantly reduced, as were the streamwise and normal velocity components. Using space-time correlations the reductions were again identified. Furthermore, quantitative flow visualization showed that the outward normal velocity of the inner region was also significantly decreased in the region around 20 beta sub o. However, throughout the first 130 beta sub o, the measured sublayer thickness with the TAPPMs in place was 15 to 20 percent greater. The data showed that the skin friction, as well as the structure of the turbulence, was strongly modified in the first 35 beta sub o, but that they both significantly relaxed toward unmanipulated boundary layer values by 50 beta sub o.
Shock-boundary-layer interaction in flight
NASA Technical Reports Server (NTRS)
Bertelrud, Arild
1989-01-01
A brief survey is given on the study of transonic shock/boundary layer effects in flight. Then the possibility of alleviating the adverse shock effects through passive shock control is discussed. A Swedish flight experiment on a swept wing attack aircraft is used to demonstrate how it is possible to reduce the extent of separated flow and increase the drag-rise Mach number significantly using a moderate amount of perforation of the surface.
The boundary layer on compressor cascade blades
NASA Technical Reports Server (NTRS)
Deutsch, S.
1981-01-01
Some redesign of the cascade facility was necessary in order to incoporate the requirements of the LDA system into the design. Of particular importance was the intended use of a combination of suction upstream of the blade pack with diverging pack walls, as opposed to blade pack suction alone, for spanwise dimensionality control. An ARL blade was used to redo some tests using this arrangement. Preliminary testing and boundary layer measurements began on the double circular arc blades.
Boundary Layer Control for Hypersonic Airbreathing Vehicles
NASA Technical Reports Server (NTRS)
Berry, Scott A.; Nowak, Robert J.; Horvath, Thomas J.
2004-01-01
Active and passive methods for tripping hypersonic boundary layers have been examined in NASA Langley Research Center wind tunnels using a Hyper-X model. This investigation assessed several concepts for forcing transition, including passive discrete roughness elements and active mass addition (or blowing), in the 20-Inch Mach 6 Air and the 31-Inch Mach 10 Air Tunnels. Heat transfer distributions obtained via phosphor thermography, shock system details, and surface streamline patterns were measured on a 0.333-scale model of the Hyper-X forebody. The comparisons between the active and passive methods for boundary layer control were conducted at test conditions that nearly match the Hyper-X nominal Mach 7 flight test-point of an angle-of-attack of 2-deg and length Reynolds number of 5.6 million. For passive roughness, the primary parametric variation was a range of trip heights within the calculated boundary layer thickness for several trip concepts. The passive roughness study resulted in a swept ramp configuration, scaled to be roughly 0.6 of the calculated boundary layer thickness, being selected for the Mach 7 flight vehicle. For the active blowing study, the manifold pressure was systematically varied (while monitoring the mass flow) for each configuration to determine the jet penetration height, with schlieren, and transition movement, with the phosphor system, for comparison to the passive results. All the blowing concepts tested, which included various rows of sonic orifices (holes), two- and three-dimensional slots, and random porosity, provided transition onset near the trip location with manifold stagnation pressures on the order of 40 times the model surface static pressure, which is adequate to ensure sonic jets. The present results indicate that the jet penetration height for blowing was roughly half the height required with passive roughness elements for an equivalent amount of transition movement.
Boundary Layer Transition Results From STS-114
NASA Technical Reports Server (NTRS)
Berry, Scott A.; Horvath, Thomas J.; Cassady, Amy M.; Kirk, Benjamin S.; Wang, K. C.; Hyatt, Andrew J.
2006-01-01
The tool for predicting the onset of boundary layer transition from damage to and/or repair of the thermal protection system developed in support of Shuttle Return to Flight is compared to the STS-114 flight results. The Boundary Layer Transition (BLT) Tool is part of a suite of tools that analyze the aerothermodynamic environment of the local thermal protection system to allow informed disposition of damage for making recommendations to fly as is or to repair. Using mission specific trajectory information and details of each damage site or repair, the expected time of transition onset is predicted to help determine the proper aerothermodynamic environment to use in the subsequent thermal and stress analysis of the local structure. The boundary layer transition criteria utilized for the tool was developed from ground-based measurements to account for the effect of both protuberances and cavities and has been calibrated against flight data. Computed local boundary layer edge conditions provided the means to correlate the experimental results and then to extrapolate to flight. During STS-114, the BLT Tool was utilized and was part of the decision making process to perform an extravehicular activity to remove the large gap fillers. The role of the BLT Tool during this mission, along with the supporting information that was acquired for the on-orbit analysis, is reviewed. Once the large gap fillers were removed, all remaining damage sites were cleared for reentry as is. Post-flight analysis of the transition onset time revealed excellent agreement with BLT Tool predictions.
Turbulent boundary layers in long computational domains
NASA Astrophysics Data System (ADS)
Schlatter, Philipp; Li, Qiang; Oerlue, Ramis; Brethouwer, Geert; Johansson, Arne V.; Alfredsson, P. Henrik; Henningson, Dan S.
2011-11-01
A new series of numerical simulations of spatially evolving turbulent boundary layers is discussed. The very long computational domain starts at a low Reθ = 180 , where laminar-turbulent transition is initiated, reaching up to the (computationally very) high Reθ = 8500 . In the domain, the boundary layer develops naturally from the tripping location to the higher Reynolds numbers without any re-injection or recycling procedures. In consequence, this computational setup allows us to study, e.g., the mean flow development and the scaling behavior of the fluctuating energy free from pseudo-periodic effects. However, such domains require a large number of grid points; in the present case up to 10 billion for running well-resolved large-eddy simulation. The present results show excellent agreement with wind-tunnel experiments at similar Re and previous (lower- Re) simulations (both direct and large- eddy simulations). The mean velocity profiles closely follow the correlation proposed by Monkewitz et al. (2007), just about reaching the plateau in the log-law diagnostic function. In a second part, three-dimensional visualizations of the evolving turbulent boundary layer are discussed with special focus on the persistence of transitional flow structures towards higher Reynolds numbers, having a highly unordered appearance.
Momentum Transport in the Convective Boundary Layer
NASA Astrophysics Data System (ADS)
Soares, P. M. M.; Miranda, P. M. A.; Martins, J.; Teixeira, J.
2010-09-01
The sub-grid scale transport of momentum in the boundary layer is generally treated as a diffusive process in atmospheric models. However, results for the mean wind are frequently poor in test cases, and it is not clear how important are those fluxes in the performance of the models. Nevertheless, it is clear that convective momentum transport in a key issue in the atmospheric circulation, and in the interactions across multiple space and time scales. In the case of scalar fluxes, such as potential temperature and water vapour, it has been shown that "non-local" transport plays an important role in the turbulent transport, implying that a purely diffusive representation is insufficient. Counter-gradient, mass-flux theories and the combined eddy-diffusivity/mass-flux (EDMF) scheme were built to overcome that problem. The role of non-local effects in momentum is still largely an opened question. In the present study we use a extensive set of results from LES simulations to diagnose vertical profiles of momentum related quantities in different convective boundary layers: the nieuwstadt clear boundary layer, the trade wind cumulus BOMEX case, the shallow cumulus diurnal cycle from the ARM experiment and a LBA deep convection case. In many situations these results show that the momentum transport made by organized structures, as clouds, updraughts and downdraughts contribute significantly to the total turbulent flux, suggesting that they should be included in convective parameterizations.
Entropy production in relativistic jet boundary layers
NASA Astrophysics Data System (ADS)
Kohler, Susanna; Begelman, Mitchell C.
2015-01-01
Hot relativistic jets, passing through a background medium with a pressure gradient p ? r-? where 2 < ? ? 8/3, develop a shocked boundary layer containing a significant fraction of the jet power. In previous work, we developed a self-similar description of the boundary layer assuming isentropic flow, but we found that such models respect global energy conservation only for the special case ? = 8/3. Here, we demonstrate that models with ? < 8/3 can be made self-consistent if we relax the assumption of constant specific entropy. Instead, the entropy must increase with increasing r along the boundary layer, presumably due to multiple shocks driven into the flow as it gradually collimates. The increase in specific entropy slows the acceleration rate of the flow and provides a source of internal energy that could be channelled into radiation. We suggest that this process may be important for determining the radiative characteristics of tidal disruption events and gamma-ray bursts from collapsars.
NASA Technical Reports Server (NTRS)
Schlichting, H.
1949-01-01
Various ways were tried recently to decrease the friction drag of a body in a flow; they all employ influencing the boundary layer. One of them consists in keeping the boundary layer Laminar by suction; promising tests have been carried out. Since for large Reynolds numbers the friction drag of the laminar boundary layer is much lower than that of the turbulent boundary layer, a considerable saving in drag results from keeping the boundary layer laminar, even with the blower power required for suction taken into account. The boundary layer is kept laminar by suction in two ways: first, by reduction of the thickness of the boundary layer and second, by the fact that the suction changes the form of the velocity distribution so that it becomes more stable, in a manner similar to the change by a pressure drop. There by the critical Reynolds number of the boundary layer (USigma*/V) (sub crit) becomes considerably higher than for the case without suction. This latter circumstance takes full effect only if continuous suction is applied which one might visualize realized through a porous wall. Thus the suction quantities required for keeping the boundary layer laminar become so small that the suction must be regarded as a very promising auxiliary means for drag reduction.
NASA Astrophysics Data System (ADS)
Melnick, Blake; Lang, Amy
2008-11-01
With increasing fuel costs, research into reducing drag over solid surfaces in high Reynolds number flows is still an area of interest. There have been many studies examining the boundary layer flow over two-dimensional microgeometries (e.g. riblets), but very few studies involving three dimensional microgeometries. The main objective of this study was to examine how embedded vortices, forming in hexagonal cavities, affect the boundary layer flow over a solid surface. It is believed that stable embedded vortices produce a partial slip condition, which could result in decreasing the skin friction and delaying the transition to turbulence while also acting as a means of separation control. To study the boundary layer flow, a model was constructed using a hexagonal array of cavities embedded into a flat plate. Using a water tunnel, dye visualization and DPIV measurements, the boundary layer flow forming above the cavities was examined. Measurements were also compared when changing the orientation of the hexagonal cavities.
Unsteadiness of Shock Wave / Boundary Layer Interactions
NASA Astrophysics Data System (ADS)
Clemens, Noel
2009-11-01
Shock wave / boundary layer interactions are an important feature of high-speed flows that occur in a wide range of practical configurations including aircraft control surfaces, inlets, missile base flows, nozzles, and rotating machinery. These interactions are often associated with severe boundary layer separation, which is highly unsteady, and exhibits high fluctuating pressure and heat loads. The unsteady motions are characterized by a wide range of frequencies, including low-frequency motions that are about two orders of magnitude lower than those that characterize the upstream boundary layer. It is these low-frequency motions that are of most interest because they have been the most difficult to explain and model. Despite significant work over the past few decades, the source of the low-frequency motions remains a topic of intense debate. Owing to a flurry of activity over the past decade on this single topic we are close to developing a comprehensive understanding of the low-frequency unsteadiness. For example, recent work in our laboratory and others suggests that the driving mechanism is related to low-frequency fluctuations in the upstream boundary layer. However, several recent studies suggest the dominant mechanism is an intrinsic instability of the separated flow. Here we attempt to reconcile these views by arguing that the low-frequency unsteadiness is driven by both upstream and downstream processes, but the relative importance of each mechanism depends on the strength (or length-scale) of separation. In cases where the separation bubble is relatively small, then the flow is intermittently separated, and there exists a strong correlation between upstream velocity fluctuations and the separation bubble dynamics. It appears that superstructures in the upstream boundary layer can play an important role in driving the unsteadiness for this case. It is not clear, however, if the upstream fluctuations directly move the separation point or indirectly couple to a global instability. In cases where the separation is strong (and the bubble large) then the bubble pulsates owing to a global instability, as has been suggested by other researchers. In this case upstream turbulence may serve mainly as a source of broadband fluctuations that seed the large-scale instability of the separated flow.
The role of nonlinear critical layers in boundary layer transition
NASA Technical Reports Server (NTRS)
Goldstein, M.E.
1995-01-01
Asymptotic methods are used to describe the nonlinear self-interaction between pairs of oblique instability modes that eventually develops when initially linear spatially growing instability waves evolve downstream in nominally two-dimensional laminar boundary layers. The first nonlinear reaction takes place locally within a so-called 'critical layer', with the flow outside this layer consisting of a locally parallel mean flow plus a pair of oblique instability waves - which may or may not be accompanied by an associated plane wave. The amplitudes of these waves, which are completely determined by nonlinear effects within the critical layer, satisfy either a single integro-differential equation or a pair of integro-differential equations with quadratic to quartic-type nonlinearities. The physical implications of these equations are discussed.
Numerical Simulations of Hypersonic Boundary Layer Transition
NASA Astrophysics Data System (ADS)
Bartkowicz, Matthew David
Numerical schemes for supersonic flows tend to use large amounts of artificial viscosity for stability. This tends to damp out the small scale structures in the flow. Recently some low-dissipation methods have been proposed which selectively eliminate the artificial viscosity in regions which do not require it. This work builds upon the low-dissipation method of Subbareddy and Candler which uses the flux vector splitting method of Steger and Warming but identifies the dissipation portion to eliminate it. Computing accurate fluxes typically relies on large grid stencils or coupled linear systems that become computationally expensive to solve. Unstructured grids allow for CFD solutions to be obtained on complex geometries, unfortunately, it then becomes difficult to create a large stencil or the coupled linear system. Accurate solutions require grids that quickly become too large to be feasible. In this thesis a method is proposed to obtain more accurate solutions using relatively local data, making it suitable for unstructured grids composed of hexahedral elements. Fluxes are reconstructed using local gradients to extend the range of data used. The method is then validated on several test problems. Simulations of boundary layer transition are then performed. An elliptic cone at Mach 8 is simulated based on an experiment at the Princeton Gasdynamics Laboratory. A simulated acoustic noise boundary condition is imposed to model the noisy conditions of the wind tunnel and the transitioning boundary layer observed. A computation of an isolated roughness element is done based on an experiment in Purdue's Mach 6 quiet wind tunnel. The mechanism for transition is identified as an instability in the upstream separation region and a comparison is made to experimental data. In the CFD a fully turbulent boundary layer is observed downstream.
Boundary Layer Experiment 1996 (BLX96).
NASA Astrophysics Data System (ADS)
Stull, Roland; Santoso, Edi; Berg, Larry; Hacker, Joshua
1997-06-01
The University of Wyoming King Air aircraft was the primary instrument platform for turbulence measurements in the bottom half of the convective boundary layer during 15 July-13 August 1996. A total of 12 successful research flights were made, each of about 4.5-h duration. Crosswind (east-west) flight patterns were flown in Oklahoma and Kansas over three sites of different land use: forest, pasture, and crops.Measurements of mean values, turbulent deviations, and turbulent fluxes of temperature, moisture, and momentum were made to test theories of convective transport, the radix layer, and cumulus potential. Additional portions of each flight included slant soundings and near-surface horizontal flights in order to determine mixed layer (ML) scaling variables such as ML depth zi, Deardorff velocity w and buoyancy velocity wB. While the ML was shallower and the ground wetter than anticipated based on climatology, a high-quality dataset was obtained.
Turbulent Boundary Layer Facility to Investigate Superhydrophobic Drag Reduction
NASA Astrophysics Data System (ADS)
Gose, James W.; Perlin, Marc; Ceccio, Steven L.
2013-11-01
Recent developments in superhydrophobic surfaces have led to potential economic and environmental benefits, perhaps most notably in skin-friction drag reduction. A team from the University of Michigan has developed a recirculating turbulent boundary layer facility to investigate the reduction of drag along engineered superhydrophobic surfaces (SHS). The facility can accommodate both small and large SHS samples in a test section 7 mm (depth)100 mm (span)1000 mm (length). Coupled with an 11.2 kilowatt pump and a 30:1 contraction the facility is capable of producing an average flow velocity of 25 m/s, yielding a Reynolds number of 84,000. Flexure-mounted test samples subjected to shear deflect to a max of 50 microns; movements are measured using a digital microscope composed of a high-resolution camera and a water immersion objective. The setup yields an optical resolution of about one micron whereas sub-micron resolution is achieved by implementing an FFT of two Ronchi rulings. Additional drag measurement methods include pressure drop across the test specimen and PIV measured boundary layers. Additional SHS investigations include the implementation of active gas replenishment, providing an opportunity to replace gas-pockets that would otherwise be disrupted in traditional passive SHS surfaces due to high shear stress and turbulent pressure fluctuations. The authors recognize the support of ONR.
Boundary-layer turbulence characteristics during aeolian saltation
NASA Astrophysics Data System (ADS)
Li, Bailiang; McKenna Neuman, Cheryl
2012-06-01
A great deal of effort has been expended in measuring turbulence phenomena in clean air flows. However, no previous measurements have been successfully made of the vertical distributions of turbulence intensity and Reynolds stress in a fully adjusted boundary-layer flow saturated with saltating particles. The present wind tunnel study addresses this knowledge gap using a custom designed laser-Doppler anemometer (LDA). The amount of turbulence is found to increase with the introduction of saltating particles to the airflow. Over the lowest 15% of boundary layer, vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the Reynolds stress. Relative to clean air, aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst-sweep events that primarily contribute to the total fluid stress. Within several millimeters above the bed surface, all vertical profiles of wind speed converge upon a focal point, as the local fluid stress declines toward the mobile bed.
Boundary-layer turbulence characteristics during aeolian saltation
NASA Astrophysics Data System (ADS)
Li, B.; McKenna Neuman, C. L.
2012-12-01
A great deal of effort has been expended in measuring turbulence phenomena in clean air flows. However, no previous measurements have been successfully made of the vertical distributions of turbulence intensity and Reynolds stress in a fully adjusted boundary-layer flow saturated with saltating particles. The present wind tunnel study addresses this knowledge gap using a custom designed laser-Doppler anemometer (LDA). The amount of turbulence is found to increase with the introduction of saltating particles to the airflow. Over the lowest 15% of boundary layer, vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the Reynolds stress. Relative to clean air, aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst-sweep events that primarily contribute to the total fluid stress. Within several millimeters above the bed surface, all vertical profiles of wind speed converge upon a focal point, as the local fluid stress declines toward the mobile bed.
Planetary Boundary-Layer Modelling and Tall Building Design
NASA Astrophysics Data System (ADS)
Simiu, Emil; Shi, Liang; Yeo, DongHun
2016-04-01
Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both "neutral" flows, and "conventionally neutral" flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5°. This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers.
Planetary Boundary-Layer Modelling and Tall Building Design
NASA Astrophysics Data System (ADS)
Simiu, Emil; Shi, Liang; Yeo, DongHun
2015-11-01
Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both "neutral" flows, and "conventionally neutral" flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5. This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers.
Predetermined control of turbulent boundary layer with a piezoelectric oscillator
NASA Astrophysics Data System (ADS)
Xiao-Bo, Zheng; Nan, Jiang; Hao, Zhang
2016-01-01
With a piezoelectric (PZT) oscillator, the predetermined controls of the turbulent boundary layer (TBL) are effective in reducing the drag force. The stream-wise velocities in the TBL are accurately measured downstream of the oscillator driven by an adjustable power source. The mean velocity profiles in the inner and outer scales are reported and the skin friction stresses with different voltage parameters are compared. Reduction of integral spatial scales in the inner region below y+ of 30 suggests that the oscillator at work breaks up the near-wall stream-wise vortices responsible for high skin friction. For the TBL at Reθ of 2183, the controls with a frequency of 160 Hz are superior among our experiments and a relative drag reduction rate of 26.83% is exciting. Wavelet analyses provide a reason why the controls with this special frequency perform best. Project supported by the National Natural Science Foundation of China (Grant Nos. 11332006, 11272233, and 11411130150) and the National Basic Research Program of China (Grant Nos. 2012CB720101 and 2012CB720103).
NASA Astrophysics Data System (ADS)
Sweeney, James; Hausen, Florian; Hayes, Robert; Webber, Grant B.; Endres, Frank; Rutland, Mark W.; Bennewitz, Roland; Atkin, Rob
2012-10-01
The lubricating properties of an ionic liquid on gold surfaces can be controlled through application of an electric potential to the sliding contact. A nanotribology approach has been used to study the frictional behavior of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4]FAP) confined between silica colloid probes or sharp silica tips and a Au(111) substrate using atomic force microscopy. Friction forces vary with potential because the composition of a confined ion layer between the two surfaces changes from cation-enriched (at negative potentials) to anion-enriched (at positive potentials). This offers a new approach to tuning frictional forces reversibly at the molecular level without changing the substrates, employing a self-replenishing boundary lubricant of low vapor pressure.
Technology Transfer Automated Retrieval System (TEKTRAN)
The frictional behaviors of soybean oil and heat modified soybean oils with different Gardner scale viscosities as additives in hexadecane have been examined in a boundary lubrication test regime (steel contacts) using Langmuir adsorption model. The free energy of adsorption (delta-Gads) of various...
Boundary layers in favourable pressure gradients
NASA Astrophysics Data System (ADS)
Piomelli, Ugo
2012-11-01
Turbulent boundary layers subjected to freestream acceleration due to a favorable pressure gradient (FPG) are common in many engineering applications. For strong acceleration the flow tends to revert to a laminar state; whether it re-laminarizes fully depends on the strength of the acceleration, and on the distance over which the acceleration is maintained. As the pressure gradient is removed, the flow may then return to a turbulent state; the re-transitioning process is strongly affected by the state of the turbulence at the end of the acceleration region. In this talk we present results of simulations of turbulent flow in flat-plate boundary layers subjected to strong acceleration, exceeding the critical Reynolds number for extended distance. Two Reynolds numbers are considered: a low one is studied by direct simulations, a higher one by large-eddy simulations. As the acceleration increases, the logarithmic layer is initially preserved, albeit with a higher value of the von Krmn constant; in the region of high acceleration, however, the velocity profile becomes laminar-like; in the high- Re case, a new logarithmic layer is established shortly after the end of the acceleration, while in the low- Re case re-transition occurs much later. Good agreement of the high- Re LES with the experimental data is observed. The region of maximum acceleration is characterized by significant reorganization of the wall layer, with streaks that remain stable for very long distances. Frozen turbulence advected from upstream is still present, but it does not adjust to the freestream acceleration (i.e., the freestream velocity increases, but the turbulent kinetic energy maintains its upstream value); the residual turbulent fluctuations are large enough that, once the acceleration ends, a bypass-like transition process is triggered.
Marine boundary layer structure and fractional cloudiness
NASA Astrophysics Data System (ADS)
Albrecht, Bruce A.; Jensen, Michael P.; Syrett, William J.
1995-07-01
Radiosonde data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), 1987, the Atlantic Stratocumulus Transition Experiment (ASTEX), 1992, and the Tropical Instability and Waves Experiment (TIWE), 1990, were used to develop four composite thermodynamic soundings of the marine boundary layer for cloud conditions ranging from solid stratocumulus to trade cumulus and sea surface temperatures ranging from 16 to 27C. Average cloud fraction and cloud base height for the composite soundings were estimated using laser ceilometers located at each of the four sites. The average fractional cloudiness varied from 0.83 off the coast of California at San Nicolas Island (33.4N, 119.6W) during FIRE to 0.26 over the central equatorial Pacific during TIWE (0N, 140.0W). During ASTEX, two sites in the vicinity of the Azores generally experienced cloud conditions characterized by cumulus clouds penetrating into a stratocumulus layer. At the more northerly site, the island of Santa Maria (37.0N, 25.2W), the average fractional cloudiness was 0.67 compared with 0.40, 1000 km downstream at the German R/V Valdivia (28.0N, 24.0W). The two composite soundings from ASTEX and the composite sounding from TIWE indicate decoupled boundary layer structures. These three soundings have a cloud layer that is conditionally unstable and show a systematic increase in relative humidity with increasing fractional cloudiness. The effect of cloud top entrainment on fractional cloudiness was evaluated. For the four composite soundings the fractional cloudiness decreases as the slope of the normalized w-? mixing line increases (greater instability), but this decrease is 4 times less than that from previous studies. Fractional cloudiness diagnosed from parameterizations using cloud layer relative humidity compares well with the observed mean cloudiness.
Typhoon kinematic and thermodynamic boundary layer structure from dropsonde composites
NASA Astrophysics Data System (ADS)
Ming, Jie; Zhang, Jun A.; Rogers, Robert F.
2015-04-01
The data from 438 Global Positioning System dropsondes in six typhoons are analyzed to investigate the mean atmospheric boundary layer structure in a composite framework. Following a recent study on boundary layer height in Atlantic hurricanes, we aim to quantify characteristics of boundary layer height scales in Western Pacific typhoons including the inflow layer depth (hinflow), height of the maximum tangential wind speed (hvtmax), and thermodynamic mixed layer depth. In addition, the kinematic and thermodynamic boundary layer structures are compared between the dropsonde composites using data in typhoons and hurricanes. Our results show that similar to the hurricane composite, there is a separation between the kinematic and thermodynamic boundary layer heights in typhoons, with the thermodynamic boundary layer depth being much smaller than hinflow and hvtmax in the typhoon boundary layer. All three boundary layer height scales tend to decrease toward the storm center. Our results confirm that the conceptual model of Zhang et al. (2011a) for boundary layer height variation is applicable to typhoon conditions. The kinematic boundary layer structure is generally similar between the typhoon and hurricane composites, but the typhoon composite shows a deeper inflow layer outside the eyewall than the hurricane composite. The thermodynamic structure of the typhoon boundary layer composite is warmer and moister outside the radius of maximum wind speed than the hurricane composite. This difference is attributed to different environmental conditions associated with typhoons compared to the hurricanes studied here.
Studies of the Martian boundary-layer
NASA Astrophysics Data System (ADS)
Davy, Richard A. J.
2009-12-01
A coupled boundary layer---aeolian dust model of the Martian atmosphere is presented. This model was developed to determine how radiation scattering, absorption and emission by dust affects the boundary layer and, in turn, how this affects the dust distribution in the atmosphere. This was achieved by coupling a planetary boundary layer (PBL) model with a dynamical dust model. The model is applied to the 1977B dust storm optical data of Viking Lander 1 and our analysis indicates a significant improvement over previous 1D studies of dust storm decay. By calibrating the model to observations of temperature and pressure from the Phoenix lander we have simulated the atmospheric ice formation observed by the Phoenix LIDAR and explored the potential influences of such formations on the atmospheric conditions. By simulating dust settling between multiple LIDAR observations we have estimated the effective radius of suspended dust, which is found to be in the range of estimates made by other methods. Thermocouples at three levels on a 1-m mast on the deck of the Phoenix lander provided temperature data throughout the 151 sol Phoenix mission. Air temperatures showed a large diurnal cycle which showed little sol to sol variation, especially over the first 90 sots of the mission. Daytime temperatures at the top (2 m) level typically rose to about 243 K (-30 C) in early afternoon and had large (10 K) turbulent fluctuations. These are analysed and used to estimate heat fluxes which are found to be in the range 2-10 W m-2 .
Boundary-Layer-Ingesting Inlet Flow Control
NASA Technical Reports Server (NTRS)
Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.
2006-01-01
This paper gives an overview of a research study conducted in support of the small-scale demonstration of an active flow control system for a boundary-layer-ingesting (BLI) inlet. The effectiveness of active flow control in reducing engine inlet circumferential distortion was assessed using a 2.5% scale model of a 35% boundary-layer-ingesting flush-mounted, offset, diffusing inlet. This experiment was conducted in the NASA Langley 0.3-meter Transonic Cryogenic Tunnel at flight Mach numbers with a model inlet specifically designed for this type of testing. High mass flow actuators controlled the flow through distributed control jets providing the active flow control. A vortex generator point design configuration was also tested for comparison purposes and to provide a means to examine a hybrid vortex generator and control jets configuration. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow through the duct and the actuators. The distortion and pressure recovery were determined by 40 total pressure measurements on 8 rake arms each separated by 45 degrees and were located at the aerodynamic interface plane. The test matrix was limited to a maximum free-stream Mach number of 0.85 with scaled mass flows through the inlet for that condition. The data show that the flow control jets alone can reduce circumferential distortion (DPCPavg) from 0.055 to about 0.015 using about 2.5% of inlet mass flow. The vortex generators also reduced the circumferential distortion from 0.055 to 0.010 near the inlet mass flow design point. Lower inlet mass flow settings with the vortex generator configuration produced higher distortion levels that were reduced to acceptable levels using a hybrid vortex generator/control jets configuration that required less than 1% of the inlet mass flow.
Boundary-Layer-Ingesting Inlet Flow Control
NASA Technical Reports Server (NTRS)
Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.
2006-01-01
This paper gives an overview of a research study conducted in support of the small-scale demonstration of an active flow control system for a boundary-layer-ingesting (BLI) inlet. The effectiveness of active flow control in reducing engine inlet circumferential distortion was assessed using a 2.5% scale model of a 35% boundary-layer-ingesting flush-mounted, offset, diffusing inlet. This experiment was conducted in the NASA Langley 0.3-meter Transonic Cryogenic Tunnel at flight Mach numbers with a model inlet specifically designed for this type of testing. High mass flow actuators controlled the flow through distributed control jets providing the active flow control. A vortex generator point design configuration was also tested for comparison purposes and to provide a means to examine a hybrid vortex generator and control jets configuration. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow through the duct and the actuators. The distortion and pressure recovery were determined by 40 total pressure measurements on 8 rake arms each separated by 45 degrees and were located at the aerodynamic interface plane. The test matrix was limited to a maximum free-stream Mach number of 0.85 with scaled mass flows through the inlet for that condition. The data show that the flow control jets alone can reduce circumferential distortion (DPCP(sub avg)) from 0.055 to about 0.015 using about 2.5% of inlet mass flow. The vortex generators also reduced the circumferential distortion from 0.055 to 0.010 near the inlet mass flow design point. Lower inlet mass flow settings with the vortex generator configuration produced higher distortion levels that were reduced to acceptable levels using a hybrid vortex generator/control jets configuration that required less than 1% of the inlet mass flow.
NASA Astrophysics Data System (ADS)
Sarif, Norhafizah Md; Salleh, Mohd Zuki; Tahar, Razman Mat; Nazar, Roslinda
2014-06-01
Numerical solution for the steady free convection boundary layer flow near the lower stagnation point of a horizontal circular cylinder subjected to a convective boundary condition, where the heat is supplied to the fluid through a bounding surface with a finite heat capacity are presented in this paper. The governing boundary layer equations are transformed using non-similar variables into non-similar equations and were solved numerically using an implicit finite difference scheme known as the Keller-box method. The solutions are obtained for the skin friction coefficient, the local wall temperature, as well as the velocity and temperature profiles with two the variations of two parameters, namely the conjugate parameter γ and the Prandtl number Pr.
Coherent motions in the turbulent boundary layer
NASA Technical Reports Server (NTRS)
Robinson, Stephen K.
1991-01-01
The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.
Boundary-layer Transition at Supersonic Speeds
NASA Technical Reports Server (NTRS)
Low, George M
1956-01-01
Recent results of the effects of Mach number, stream turbulence, leading-edge geometry, leading-edge sweep, surface temperature, surface finish, pressure gradient, and angle of attack on boundary-layer transition are summarized. Factors that delay transition are nose blunting, surface cooling, and favorable pressure gradient. Leading-edge sweep and excessive surface roughness tend to promote early transition. The effects of leading-edge blunting on two-dimensional surfaces and surface cooling can be predicted adequately by existing theories, at least in the moderate Mach number range.
The minisodar and planetary boundary layer studies
Coulter, R.L.
1996-06-01
The minisodar, in addition to being smaller than conventional sodar, operates at higher frequencies, obtains usable signal returns closer to the surface, and can use smaller range gates. Because the max range is generally limited to the lower 200 m above the surface, the minisodar is not able to interrogate the entire daytime atmospheric Planetary Boundary Layer (PBL); however it can be a very useful tool for understanding the PBL. In concert with other instruments, the minisodar can add significant new insights to our understanding of the PBL. This paper gives examples of past and potential uses of minisodars in such situations.
Calculation of boundary layers of oscillating airfoils
NASA Technical Reports Server (NTRS)
Cebeci, T.; Carr, L. W.
1984-01-01
A two-point finite difference unsteady laminar and turbulent boundary layer computational method was used to investigate the properties of the flow around an airfoil (NACA 0012) oscillating through angles of attack up to 18 degrees, for reduced frequencies of 0.01 and 0.20. The unsteady potential flow was determined using the unsteady potential flow method of Geissler. The influence of transition location on stal behavior was investigated, using both experimentally determined transition information, and transition located at the pressure peak; the results show the need for viscous-inviscid interaction in future computation of such flows.
Effects of viscous dissipation on MHD boundary layer flow of Sisko fluid over a stretching cylinder
NASA Astrophysics Data System (ADS)
Malik, M. Y.; Hussain, Arif; Salahuddin, T.; Awais, M.
2016-03-01
The present study concentrates on the analysis of magnetohydrodynamic boundary layer flow of Sisko fluid over continuously stretching cylinder. The viscous dissipation effect is assumed in heat equation. To modify the governing equations first boundary layer approximations are applied. After this simultaneous partial differential equations are converted into the ordinary differential equations by applying proper similarity transformations. To find the numerical solution of this system of ordinary differential equations shooting method is utilized. Graphs are plotted to figure out the characteristics of physical parameters on momentum and heat equations. The variations of all physical parameters on skin friction coefficient and local Nusselt number are displayed via figures and tables.
Analysis of hot-wire measurements accuracy in turbulent boundary layer
NASA Astrophysics Data System (ADS)
Drzdz, Artur; Elsner, Witold
2015-09-01
This paper discusses the issue of measuring velocity fluctuations of turbulent boundary layer using hot-wire probes. The study highlights the problem of spatial resolution, which is essential when measuring small-scales in wall-bounded flows. Additionally, attention was paid to the inconsistency in streamwise fluctuation measurements using single- and X-wire probes. To clarify this problem, the energy spectra using wavelet transformation were calculated. The analysis was performed for turbulent boundary layer flow, which was characterized by Reynolds number based on the friction velocity equal Re?? 1000.
Deplacement effect of the laminar boundary layer and the pressure drag
NASA Technical Reports Server (NTRS)
Gortler, H
1951-01-01
The displacement effect of the boundary layer on the outer frictionless flow is discussed for both steady and unsteady flows. The analysis is restricted to cases in which the potential flow pressure distribution remains valid for the boundary-layer calculation. Formulas are given for the dependence of the pressure drag, friction drag, and total drag of circular cylinders on the time from the start of motion for cases in which the velocity varies as a power of the time. Formulas for the locations and for the time for the appearance of the separation point are given for two dimensional bodies of arbitrary shape.
In-flight boundary-layer measurements on a hollow cylinder at a Mach number of 3.0
NASA Technical Reports Server (NTRS)
Quinn, R. D.; Gong, L.
1980-01-01
Skin temperatures, shear forces, surface static pressures, boundary layer pitot pressures, and boundary layer total temperatures were measured on the external surface of a hollow cylinder that was 3.04 meters long and 0.437 meter in diameter and was mounted beneath the fuselage of the YF-12A airplane. The data were obtained at a nominal free stream Mach number of 3.0 (a local Mach number of 2.9) and at wall to recovery temperature ratios of 0.66 to 0.91. The local Reynolds number had a nominal value of 4,300,000 per meter. Heat transfer coefficients and skin friction coefficients were derived from skin temperature time histories and shear force measurements, respectively. In addition, boundary layer velocity profiles were derived from pitot pressure measurements, and a Reynolds analogy factor was obtained from the heat transfer and skin friction measurements. The measured data are compared with several boundary layer prediction methods.
Analysis of a turbulent boundary layer over a moving ground plane
NASA Technical Reports Server (NTRS)
Roper, A. T.; Gentry, G. L., Jr.
1972-01-01
Four methods of predicting the integral and friction parameters for a turbulent boundary layer over a moving ground plane were evaluated by using test information obtained in 76.2- by 50.8-centimeter tunnel. The tunnel was operated in the open sidewall configuration. These methods are (1) relative integral parameter method, (2) modified power law method, (3) relative power law method, and (4) modified law of the wall method. The modified law of the wall method predicts a more rapid decrease in skin friction with an increase in the ratio of belt velocity to free steam velocity than do methods (1) and (3).
Turbulence modeling for sharp-fin-induced shock wave/turbulent boundary-layer interactions
NASA Technical Reports Server (NTRS)
Horstman, C. C.
1990-01-01
Solutions of the Reynolds averaged Navier-Stokes equations are presented and compared with a family of experimental results for the 3-D interaction of a sharp fin induced shock wave with a turbulent boundary layer. Several algebraic and two equation eddy viscosity turbulence models are employed. The computed results are compared with experimental surface pressure, skin friction, and yaw angle data as well as the overall size of the interaction. Although the major feature of the flow fields are correctly predicted, several discrepancies are noted. Namely, the maximum skin friction values are significantly underpredicted for the strongest interaction cases. These and other deficiencies are discussed.
Friction-term response to boundary-condition type in flow models
Schaffranek, R.W.; Lai, C.
1996-01-01
The friction-slope term in the unsteady open-channel flow equations is examined using two numerical models based on different formulations of the governing equations and employing different solution methods. The purposes of the study are to analyze, evaluate, and demonstrate the behavior of the term in a set of controlled numerical experiments using varied types and combinations of boundary conditions. Results of numerical experiments illustrate that a given model can respond inconsistently for the identical resistance-coefficient value under different types and combinations of boundary conditions. Findings also demonstrate that two models employing different dependent variables and solution methods can respond similarly for the identical resistance-coefficient value under similar types and combinations of boundary conditions. Discussion of qualitative considerations and quantitative experimental results provides insight into the proper treatment, evaluation, and significance of the friction-slope term, thereby offering practical guidelines for model implementation and calibration.
NASA Technical Reports Server (NTRS)
Murthy, V. S.; Rose, W. C.
1977-01-01
Detailed measurements of wall shear stress (skin friction) were made with specially developed buried wire gages in the interaction regions of a Mach 2.9 turbulent boundary layer with externally generated shocks. Separation and reattachment points inferred by these measurements support the findings of earlier experiments which used a surface oil flow technique and pitot profile measurements. The measurements further indicate that the boundary layer tends to attain significantly higher skin-friction values downstream of the interaction region as compared to upstream. Comparisons between measured wall shear stress and published results of some theoretical calculation schemes show that the general, but not detailed, behavior is predicted well by such schemes.
Natural by-pass boundary layer transition
NASA Astrophysics Data System (ADS)
Shahinfar, Shahab; Fransson, Jens H. M.
2011-11-01
The present measurement campaign on the free-stream turbulence induced boundary layer transition scenario has provided a unique set of experimental data, with potential to enhance the understanding of the effect of the free-stream turbulence characteristic length scales on the transition location and not only the turbulence intensity, which has been the focus in most previous studies. Recent investigations where the turbulence intensity has been kept essentially constant, while the integral length scale has been changed, show that the transition location is advanced for increasing length scale. However, the present data show that the integral length scale has a relatively small influence on the transition location as compared to the turbulence intensity and data analyses are now directed towards enhanced understanding of how the different parts of the incoming energy spectrum affects the energy growth inside the boundary layer. The Linn Flow Centre is greatly acknowledged for financial support of this work. The APS 2011 attendance is financed through the AFRODITE programme.
Soot profiles in boundary-layer flames
Beier, R.A.; Pagni, P.J.
1981-12-01
Carbon particulate volume fractions and approximate particle size distributions are measured in a free laminar combusting boundary layer for liquid hydrocarbon fuels (n-heptane, iso-octane, cyclohexane, cyclohexene, toluene) and polymethylmethacrylate (PMMA). A multiwavelength laser transmission technique determines a most probable radius and the total particle concentration, which are two parameters in an assumed form for the size distribution. In the combusting boundary layer, a sooting region exists between the pyrolyzing fuel surface and the flame zone. The liquid fuel soot volume fractions, f/sub v/, range from f/sub v/ approx. 10/sup -7/ for n-heptane, a paraffin, to f/sub v approx. 10/sup -5/ for toluene, an aromatic. The PMMA volume fractions, f/sub v/ approx. 5 X 10/sup -7/, are approximately the same as the values previously reported for pool fires. The soot volume fractions increase with height; convection of carbon particles downstream widens the soot region with height. For all fuels tested, the most probable radius is between 20 nm and 50 nm, and it changes only slightly with height and distance from the fuel surface.
Boundary Layer Transition Flight Experiment Overview
NASA Technical Reports Server (NTRS)
Berger, Karen T.; Anderson, Brian P.; Campbell, Charles H.; Garske, Michael T.; Saucedo, Luis A.; Kinder, Gerald R.; Micklos, Ann M.
2011-01-01
In support of the Boundary Layer Transition Flight Experiment (BLT FE) Project, a manufactured protuberance tile was installed on the port wing of Space Shuttle Orbiter Discovery for STS-119, STS-128, STS-131 and STS-133 as well as Space Shuttle Endeavour for STS-134. Additional instrumentation was installed in order to obtain more spatially resolved measurements downstream of the protuberance. This paper provides an overview of the BLT FE Project with emphasis on the STS-131 and STS-133 results. A high-level overview of the in-situ flight data is presented, along with a summary of the comparisons between pre- and post-flight analysis predictions and flight data. Comparisons show that empirically correlated predictions for boundary layer transition onset time closely match the flight data, while predicted surface temperatures were significantly higher than observed flight temperatures. A thermocouple anomaly observed on a number of the missions is discussed as are a number of the mitigation actions that will be taken on the final flight, STS-134, including potential alterations of the flight trajectory and changes to the flight instrumentation.
X-33 Hypersonic Boundary Layer Transition
NASA Technical Reports Server (NTRS)
Berry, Scott A.; Horvath, Thomas J.; Hollis, Brian R.; Thompson, Richard A.; Hamilton, H. Harris, II
1999-01-01
Boundary layer and aeroheating characteristics of several X-33 configurations have been experimentally examined in the Langley 20-Inch Mach 6 Air Tunnel. Global surface heat transfer distributions, surface streamline patterns, and shock shapes were measured on 0.013-scale models at Mach 6 in air. Parametric variations include angles-of-attack of 20-deg, 30-deg, and 40-deg; Reynolds numbers based on model length of 0.9 to 6.6 million; and body-flap deflections of 0, 10 and 20-deg. The effects of discrete and distributed roughness elements on boundary layer transition, which included trip height, size, location, and distribution, both on and off the windward centerline, were investigated. The discrete roughness results on centerline were used to provide a transition correlation for the X-33 flight vehicle that was applicable across the range of reentry angles of attack. The attachment line discrete roughness results were shown to be consistent with the centerline results, as no increased sensitivity to roughness along the attachment line was identified. The effect of bowed panels was qualitatively shown to be less effective than the discrete trips; however, the distributed nature of the bowed panels affected a larger percent of the aft-body windward surface than a single discrete trip.
NASA Technical Reports Server (NTRS)
Hopkins, E. J.
1974-01-01
Two methods for determining the virtual origin of turbulent boundary layers in hypersonic flow are evaluated. The results of the analyses are restricted to wind-tunnel models having sharp-edged surfaces with zero or small pressure gradients. Virtual origin and skin friction estimates from these two methods are compared with values from a base method for which the virtual origin is calculated from the measured momentum thickness at a station downstream of boundary layer transition.
Performance and boundary-layer evaluation of a sonic inlet
NASA Technical Reports Server (NTRS)
Schmidt, J. F.; Ruggeri, R. S.
1976-01-01
Tests were conducted to determine the boundary layer characteristics and aerodynamic performance of a radial vane sonic inlet with a length/diameter ratio of 1 for several vane configurations. The sonic inlet was designed with a slight wavy wall type of diffuser geometry, which permits operation at high inlet Mach numbers (sufficiently high for good noise suppression) without boundary layer flow separation and with good total pressure recovery. A new method for evaluating the turbulent boundary layer was developed to separate the boundary layer from the inviscid core flow, which is characterized by a total pressure variation from hub to tip, and to determine the experimental boundary layer parameters.
Flow Visualization in Supersonic Turbulent Boundary Layers.
NASA Astrophysics Data System (ADS)
Smith, Michael Wayne
This thesis is a collection of novel flow visualizations of two different flat-plate, zero pressure gradient, supersonic, turbulent boundary layers (M = 2.8, Re _theta ~ 82,000, and M = 2.5, Re_ theta ~ 25,000, respectively). The physics of supersonic shear flows has recently drawn increasing attention with the renewed interest in flight at super and hypersonic speeds. This work was driven by the belief that the study of organized, Reynolds -stress producing turbulence structures will lead to improved techniques for the modelling and control of high-speed boundary layers. Although flow-visualization is often thought of as a tool for providing qualitative information about complex flow fields, in this thesis an emphasis is placed on deriving quantitative results from image data whenever possible. Three visualization techniques were applied--'selective cut-off' schlieren, droplet seeding, and Rayleigh scattering. Two experiments employed 'selective cut-off' schlieren. In the first, high-speed movies (40,000 fps) were made of strong density gradient fronts leaning downstream at between 30^circ and 60^ circ and travelling at about 0.9U _infty. In the second experiment, the same fronts were detected with hot-wires and imaged in real time, thus allowing the examination of the density gradient fronts and their associated single-point mass -flux signals. Two experiments employed droplet seeding. In both experiments, the boundary layer was seeded by injecting a stream of acetone through a single point in the wall. The acetone is atomized by the high shear at the wall into a 'fog' of tiny (~3.5mu m) droplets. In the first droplet experiment, the fog was illuminated with copper-vapor laser sheets of various orientations. The copper vapor laser pulses 'froze' the fog motion, revealing a variety of organized turbulence structures, some with characteristic downstream inclinations, others with large-scale roll-up on the scale of delta. In the second droplet experiment, high-speed movies were made of the fog under general illumination, thus providing information about the streamwise evolution of the structures seen in the planar stills. Rayleigh scattering from a laser sheet was used to create instantaneous density cross-sections in the M = 2.5 boundary layer. The Rayleigh scattering experiment represents the first measurement of the instantaneous 2-D field of an intrinsic fluid property in any boundary layer. Imaged by an intensified UV camera, scattering from the Argon-Fluoride laser (193 nm) revealed density structures with sharp interfaces between high and low-density fluid. These pictures were also used to generated quantitative turbulence information. Density pdf profiles, intermittency values, density correlations, and structure shape data were derived with standard digital image-processing techniques.
Exploring Isothermal Layers in the Stable Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Wilkins, Joseph
2011-03-01
Simulating the stable atmospheric boundary-layer presents a significant challenge to numerical models due to the interactions of several processes with widely varying scales. The goal of this project is to more clearly define the cause of isothermal layers observed during the Meteorological Experiment in Arizona's Meteor Crater and to test the National Taiwan University/Purdue University (NTU/P) model in stable environments with complex terrain. The NTU/P model is able to utilize the actual terrain data with minimal smoothing for stability. We have found that isothermal profiles can be generated by the standing wave that develops due to weak wind flowing over the crater. However, the horizontal heterogeneity is greater than observed. Continued effort will explore enhancing horizontal mixing due to turbulence and radiative transfer. Louis Stokes Alliances for Minority Participation Program, Summer Research Opportunities Program.
2014-01-01
The evolution of a friction surface geometry with initially directed microscale grooves on a nanoscale polished surface in ring-on-block sliding contact is studied experimentally. Reduced wear and friction is observed when the orientation of grooves coincides with the direction of sliding. A new compressive-vacuum hypothesis of friction force nature under a condition of boundary lubrication is proposed, which successfully explains the observed phenomena. Grooves supply lubricant into the contact zone and facilitate its devacuumization, which lead to substantial reduction of surface wear. The obtained results enable developing optimized roughness profiles of friction surfaces to create high-performance durable friction units. PMID:24872807
Turbulent boundary layer drag reduction using riblets
NASA Technical Reports Server (NTRS)
Walsh, M. J.
1982-01-01
An experimental study of low-speed turbulent boundary layer flow over longitudinally grooved surfaces (i.e., riblets) is discussed. Results obtained with a highly accurate drag balance indicate that v-groove riblet surfaces can produce consistent net drag reductions as large as 8 percent provided the height and spacing of the grooves in terms of law of the wall variables are less than 25 wall units. Momentum balances confirmed these direct drag measurements. Conditionally sampled data indicate that the burst frequency for riblets is approximately the same as that for a flat plate but turbulence intensity is reduced. Attempts to optimize the net drag reduction by varying riblet cross-sectional geometry and alignment are also discussed.
Digital image filtering in visualized boundary layers
NASA Technical Reports Server (NTRS)
Corke, T. C.
1984-01-01
The application of two-dimensional low-pass matched filtering is presented for use in objective processing of digitized flow visualization images in order to identify instantaneous large-scale organized structures in turbulent boundary layers. The images were digitally acquired simultaneously with the outputs of a two-dimensional rake of hot-wire sensors in the field of view of the digital camera. Two-dimensional low wavenumber analysis brought out patterns in the visualization images which consisted of slender inclined structures having an average streamwise scale of 100-200 v/u (tau) and a length on the order of 1-2 delta. The similarly processed two-dimensional streamwise velocity reconstructions reveal similar features. The ensemble statistics indicate that these inclined features brought out by this processing may be a basic flow module in higher Reynolds number flows which links the so-called wall 'bursting' process and the larger outer scale motions.
Modelling of the Evolving Stable Boundary Layer
NASA Astrophysics Data System (ADS)
Sorbjan, Zbigniew
2014-06-01
A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of "horse-shoe" shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing.
Persistent Structures in the Turbulent Boundary Layer
NASA Technical Reports Server (NTRS)
Palumbo, Dan; Chabalko, Chris
2005-01-01
Persistent structures in the turbulent boundary layer are located and analyzed. The data are taken from flight experiments on large commercial aircraft. An interval correlation technique is introduced which is able to locate the structures. The Morlet continuous wavelet is shown to not only locates persistent structures but has the added benefit that the pressure data are decomposed in time and frequency. To better understand how power is apportioned among these structures, a discrete Coiflet wavelet is used to decompose the pressure data into orthogonal frequency bands. Results indicate that some structures persist a great deal longer in the TBL than would be expected. These structure contain significant power and may be a primary source of vibration energy in the airframe.
Reverse draping of magnetic field lines in the boundary layer
NASA Technical Reports Server (NTRS)
Hones, E. W., Jr.; Bame, S. J.; Sonnerup, B. U. O.; Paschmann, G.; Russell, C. T.
1982-01-01
An unexpected orientation of field lines has been found by ISEE satellite measurements of the earth magnetosphere boundary layer, perhaps indicating a reverse draping. Three-dimensional plasma flow measurements by ISEE 1 and 2 show the tailward flow in the boundary layer to have an equatorward component which suggests high-latitude plasma entry. Simultaneous magnetic field measurements with the two satellites during a boundary crossing sequence yielded data on the boundary layer electric currents and the boundary layer interface with plasma sheet and magnetosheath. The currents are found to be largely field-aligned, with intensities in the 0.03-0.06 microampere/sq m range.
A study of the effects of Lebu devices on turbulent boundary layer drag
NASA Technical Reports Server (NTRS)
Falco, R. E.
1983-01-01
Initial measurements of the changes in local skin friction, velocity profile shape, and turbulence structure which result from the placement of tandem plates parallel to the wall in the outer region of thick turbulent boundary layers were made. Using a tunnel with a .75 m x 1.2 m x 7.3 m test section, which diverged so as to keep the pressure gradient less than 2x1000/ft, on the test wall, a skin friction reduction of approximately 30% was measured at xi/h = 62. This relaxed to a reduction of approximately 16% at xi/h = 124 for h/delta M = .6. The c sub f measurements for both the normal and modified boundary layers were obtained by measuring the slope of the velocity profile within the linear sublayer. Visual results indicated a continued presence of strong large eddy structure downstream of the devices. Local skin friction reduction of 12% at xi/h = 62 was also obtained with the manipulators above the boundary layer at y/delta m = 1.1.
Halogen chemistry in the marine boundary layer
NASA Astrophysics Data System (ADS)
Plane, J. M. C.; Gomez Martin, J. C.; Kumar, R.; Mahajan, A. S.; Oetjen, H.; Saunders, R. W.
2009-04-01
Important atmospheric sources of iodine include the air-sea exchange of biogenic iodocarbons, and the emission of I2 from macro-algae. The major source of bromine is the release of bromide ions from sea-salt aerosol. The subsequent atmospheric chemistry of these halogens (1), changes the oxidizing capacity of the marine boundary layer by destroying ozone and changing the hydroxyl radical concentration; (2), reacts efficiently with dimethyl sulphide and mercury (in the polar regions); and (3), leads to the formation of ultra-fine particles which may contribute to cloud condensation nuclei (CCN) and hence affect climate. This paper will report observations of IO, BrO, OIO and I2 made by the technique of differential optical absorption spectroscopy, in several contrasting marine environments: the equatorial mid-Atlantic (Cape Verde); mid-latitude clean coastal (Mace Head, Ireland); polluted coastal (Roscoff, France); and the polar marine boundary layer (Hudson Bay, Canada). Both IO and BrO are observed in all these locations at significant concentrations (> 1 pptv), and so have a major impact on (1) and (2) above. To complement the field campaigns we have also carried out wide-ranging laboratory investigation. A new study of OIO photochemistry shows that absorption in the visible bands between 490 and 630 nm leads to I atom production with a quantum yield of unity, which now means that iodine is a particularly powerful ozone-depleting agent. We have also studied the formation and growth kinetics of iodine oxide nano-particles, and their uptake of water, sulphuric acid and di-carboxylic organic acids, in order to model their growth to a size where they can act as CCN. Their ice-nucleating properties will also be reported.
Acoustics of laminar boundary layers breakdown
NASA Technical Reports Server (NTRS)
Wang, Meng
1994-01-01
Boundary layer flow transition has long been suggested as a potential noise source in both marine (sonar-dome self noise) and aeronautical (aircraft cabin noise) applications, owing to the highly transient nature of process. The design of effective noise control strategies relies upon a clear understanding of the source mechanisms associated with the unsteady flow dynamics during transition. Due to formidable mathematical difficulties, theoretical predictions either are limited to early linear and weakly nonlinear stages of transition, or employ acoustic analogy theories based on approximate source field data, often in the form of empirical correlation. In the present work, an approach which combines direct numerical simulation of the source field with the Lighthill acoustic analogy is utilized. This approach takes advantage of the recent advancement in computational capabilities to obtain detailed information about the flow-induced acoustic sources. The transitional boundary layer flow is computed by solving the incompressible Navier-Stokes equations without model assumptions, thus allowing a direct evaluation of the pseudosound as well as source functions, including the Lighthill stress tensor and the wall shear stress. The latter are used for calculating the radiated pressure field based on the Curle-Powell solution of the Lighthill equation. This procedure allows a quantitative assessment of noise source mechanisms and the associated radiation characteristics during transition from primary instability up to the laminar breakdown stage. In particular, one is interested in comparing the roles played by the fluctuating volume Reynolds stress and the wall-shear-stresses, and in identifying specific flow processes and structures that are effective noise generators.
Soot and radiation in combusting boundary layers
Beier, R.A.
1981-12-01
In most fires thermal radiation is the dominant mode of heat transfer. Carbon particles within the fire are responsible for most of this emitted radiation and hence warrant quantification. As a first step toward understanding thermal radiation in full scale fires, an experimental and theoretical study is presented for a laminar combusting boundary layer. Carbon particulate volume fraction profiles and approximate particle size distributions are experimentally determined in both free and forced flow for several hydrocarbon fuels and PMMA (polymethylmethacrylate). A multiwavelength laser transmission technique determines a most probable radius and a total particle concentration which are two unknown parameters in an assumed Gauss size distribution. A sooting region is observed on the fuel rich side of the main reaction zone. For free flow, all the flames are in air, but the free stream ambient oxygen mass fraction is a variable in forced flow. To study the effects of radiation heat transfer, a model is developed for a laminar combusting boundary layer over a pyrolyzing fuel surface. An optically thin approximation simplifies the calculation of the radiant energy flux at the fuel surface. For the free flames in air, the liquid fuel soot volume fractions, f/sub v/, range from f/sub v/ approx. 10/sup -7/ for n-heptane, a paraffin, to f/sub v/ approx. 10/sup -7/ for toluene, an aromatic. The PMMA soot volume fractions, f/sub v/ approx. 5 x 10/sup -7/, are approximately the same as the values previously reported for pool fires. Soot volume fraction increases monotonically with ambient oxygen mass fraction in the forced flow flames. For all fuels tested, a most probable radius between 20 nm and 80 nm is obtained which varies only slightly with oxygen mass fraction, streamwise position, or distance normal to the fuel surface. The theoretical analysis yields nine dimensionless parameters, which control the mass flux rate at the pyrolyzing fuel surface.
NASA Astrophysics Data System (ADS)
Matsuo, Yoshihiro; Clarke, Daryl D.; Ozeki, Shinichi
Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various resins such as asphalt or resin combined with pitch. These efforts were further developed by the use of woven asbestos material saturated by either rubber solution or liquid resin binder and functioned as an internal expanding brake, similar to brake lining system. The role of asbestos continued through the use of chopped asbestos saturated by rubber, but none was entirely successful due to the poor rubber heat resistance required for increased speeds and heavy gearing demands of the automobile industry. The use of phenolic resins as binder for asbestos friction materials provided the necessary thermal resistance and performance characteristics. Thus, the utility of asbestos as the main friction component, for over 100 years, has been significantly reduced in friction materials due to asbestos identity as a carcinogen. Steel and other fibrous components have displaced asbestos in disk pads. Currently, non-asbestos organics are the predominate friction material. Phenolic resins continue to be the preferred binder, and increased amounts are necessary to meet the requirements of highly functional asbestos-free disk pads for the automotive industry. With annual automobile production exceeding 70 million vehicles and additional automobile production occurring in developing countries worldwide and increasing yearly, the amount of phenolic resin for friction material is also increasing (Fig. 14.1).
A realistic model of a wall-transpiration actuator for boundary layer control
NASA Astrophysics Data System (ADS)
Tilton, Nils; Cortelezzi, Luca
2008-11-01
Experimental studies of boundary layer control using continuously distributed wall-suction usually implement suction by applying a pressure gradient to a layer of porous material via an underlying plenum chamber. Theoretical studies, however, usually neglect the penetration of fluid into the porous layer and plenum chamber by forcing the base flow and velocity perturbations to vanish at the interface with the porous layer. We present a realistic model of a wall-transpiration actuator which implements suction through a fluid saturated, rigid, homogeneous, isotropic, porous layer stretched over a semi-infinite plenum chamber. We test our model on the asymptotic suction boundary layer (ASBL) and perform a linear stability analysis. We take account of the full coupling between the flow fields in the boundary layer, porous layer, and plenum chamber using boundary conditions derived by Ochoa-Tapia and Whitaker (Int. J. Heat Mass Transfer, Vol. 38, 1995, pp 2635-2646). We illustrate the impact of wall-permeability, porous layer thickness, and the plenum chamber on the critical Reynolds number and the stability of the Tollmien-Schlichting wave. We use our model to find the optimal operating conditions of an ASBL which minimize the skin friction drag and power required to apply the suction.
Internal Friction Evidence on the Formation of Grain Boundary in Al Powder Sintering Process
NASA Astrophysics Data System (ADS)
Hao, Gang-Ling; Wang, Xin-Fu; Li, Xian-Yu
2015-02-01
The temperature dependence of internal friction is first investigated to understand the microstructure transition during the sintering process for the green compact of aluminum powder. An internal friction (IF) peak is observed only during the first heating process while not in the subsequent cooling and repeated heating process. The temperature position of the peak is independent of the measuring frequency and the height decreases with the increasing frequency. The appearance of the peak is closely related to the weak bonding interfaces between deformed aluminum particles and increased dislocation density induced by the pressing. The appearance of the peak well responds to a recrystallization process of deformed particles and thus the formation of the grain boundary which is proven by the appearance of the grain boundary IF peak. The peak temperature position is rationalized with the onset of the recrystallization process during the sintering process.
NASA Astrophysics Data System (ADS)
Mott, R.; Gromke, C.; Grünewald, T.; Lehning, M.
2012-04-01
Once the mountain snow-cover gets patchy in the course of the ablation season two processes are expected to increase in magnitude: the advective heat ransport and the near-surface boundary layer decoupling. These two processes, which have an opposite effect on sensible heat transport onto the snow surface, are, however, not well understood. The aim of this study is to investigate the effects of locally developing atmospheric stratification over snow patches. Especially stable internal boundary layers over cold surfaces can result in a decoupling of the near-surface air from the warmer atmosphere. In this investigation we are particularly interested in the effect of boundary layer decoupling on the net sensible heat flux towards the snow surface. At two experimental sites we applied local eddy flux measurements over snow patches at three different heights above the snow surface. The measurement results suggested wind velocity, turbulence intensity, wind fetch distance and topographical curvature to be driving factors for boundary layer growth above patchy snow covers. These factors also control the efficiency of advective heat transport to contribute to snow ablation. The turbulence data clearly show that boundary layer decoupling inhibits the transfer of additional energy to the snow cover potentially gained from advective heat transport, leading to an upward flux of sensible heat above the stable internal layer. The atmospheric decoupling primarily occurs for shallow stable internal boundary layers, calm winds and low friction velocities. Contrary, the transfer of sensible heat towards the snow cover is promoted by high mechanical turbulence initiated by strong winds. Advective heat transport is shown to be especially effective under these conditions. Thus, strong winds additionally increase the role of advective heat transport by decreasing boundary layer decoupling. Furthermore, concave topographies reduce snow ablation by enhancing the potential of boundary layer decoupling. The atmospheric decoupling is thus shown to be a key mechanism in snow patch survival.
Turbulent dispersion in cloud-topped boundary layers
NASA Astrophysics Data System (ADS)
Verzijlbergh, R. A.; Jonker, H. J. J.; Heus, T.; Vilguerau de Arellano, J.
2009-02-01
Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion - a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.
Turbulent dispersion in cloud-topped boundary layers
NASA Astrophysics Data System (ADS)
Verzijlbergh, R. A.; Jonker, H. J. J.; Heus, T.; Vilguerau de Arellano, J.
2008-11-01
Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.
Boundary layer flow and heat transfer past a moving plate with suction and injection
NASA Astrophysics Data System (ADS)
Ishak, Anuar; Nazar, Roslinda; Pop, Ioan
2014-06-01
The behavior of an incompressible steady boundary layer flow past a permeable semi-infinite flat plate moving in a free stream is discussed in this paper. In addition to the mass transfer from the plate (suction or injection), the viscous dissipation term is also included into the energy equation. The solutions of the transformed ordinary differential equations are obtained numerically using an implicit finite-difference method. The numerical results are given for the velocity and temperature profiles as well as for the skin friction coefficient and the local Nusselt number for various values of the suction/injection parameter ?, ratio of the wall velocity to the free stream velocity parameter ?, Prandtl number Pr and Eckert number Ec. It is found that suction increases the heat transfer by decreasing the thermal boundary layer thickness and the reverse happens for injection. Furthermore, it is also found that the boundary layer equations have non-unique (dual) solutions in some cases.
Wall Cooling Effects on Hypersonic Transitional/Turbulent Boundary Layers at High Reynolds Numbers
NASA Technical Reports Server (NTRS)
Watson, Ralph D.
1975-01-01
A 4 degree wedge was used to produce a thick turbulent boundary layer with an edge Mach number of 11. By using a two-dimensional model, the boundary layer was nearly free from upstream history effects associated with nozzle wall turbulent boundary layers. Heat-transfer distributions were used to define regions of laminar, transitional, and turbulent flow at several values of T(sub w)/T(sub t) for an edge unit Reynolds number of 0.47 x lot per cm. Pitot and total temperature profiles and skin-friction measurements were obtained at selected stations along the model. Turbulence parameters (mixing length/sigma and epsilon) were derived from the fully turbulent profiles and used to more completely define the "low Reynolds number" effect. Turbulent Prandtl number distributions are also presented.
Development of a turbulent boundary layer beneath finite-amplitude continuous freestream turbulence
NASA Astrophysics Data System (ADS)
Wu, Xiaohua; Moin, Parviz
2011-11-01
Following the earlier work of Wu & Moin (JFM 2009, PoF 2010) and Wu (JFM 2010), here we will present our third, most recent, direct numerical simulation of the incompressible, zero-pressure-gradient flat-plate boundary layer. Heat transfer between the constant-temperature plate and the free-stream is also simulated with unit molecular Prandtl number. The freestream of the present boundary layer has continuous isotropic turbulence whose inlet strength is 3% of the mean velocity. Its decay characteristics agree with existing water channel experiments. Despite the finite-level freestream perturbation, the boundary layer is clean in the sense that the deviation of skin-friction from Blasius prior to breakdown is less than 1%. Both the statistics and structures from this simulation will be compared with our previous DNS studies using periodically fed patches of isotropic turbulence. The associated bypass transition process will also be evaluated.
NASA Astrophysics Data System (ADS)
Tee, Boon Tuan; Nickels, Timothy
2008-11-01
Due to the practical importance of turbulent boundary layers in fluid dynamic engineering, there is a need to predict or compute their behaviour. Numerous experimental and numerical studies have been conducted to examine the characteristics of turbulent boundary layers. Low-speed PIV was employed to measure the stream-wise velocities of zero pressure-gradient boundary layers in the turbulence water tunnel research facility at Cambridge University Engineering Department. The measurement position was 4 m downstream of a tripping rod with freestream velocities of 0.53 m/s and 0.64 m/s respectively. The use of low-speed PIV in this experiment enabled measurements of the mean flow fields and the flow statistics. Data is also reported for normalized mean velocity based on friction velocity (uτ) calculated using Clauser chart method.
Turbulent boundary layer over solid and porous surfaces with small roughness
NASA Technical Reports Server (NTRS)
Kong, F. Y.; Schetz, J. A.; Collier, F.
1982-01-01
Skin friction and profiles of mean velocity, axial and normal turbulence intensity, and Reynolds stress in the untripped boundary layer were measured directly on a large diameter, axisymmetric body with: (1) a smooth, solid surface; (2) a sandpaper-roughened, solid surface; (3) a sintered metal, porous surface; (4) a smooth, perforated titanium surface; (5) a rough solid surface made of fine, diffusion bonded screening, and (6) a rough, porous surface of the same screening. Results obtained for each of these surfaces are discussed. It is shown that a rough, porous wall simply does not influence the boundary layer in the same way as a rough solid wall. Therefore, turbulent transport models for boundary layers over porous surfaces either with or without injection or suction, must include both surface roughness and porosity effects.
A conditioned volumetric view of ``superstructure'' events in turbulent boundary layers
NASA Astrophysics Data System (ADS)
Hutchins, Nicholas; Ganapathisubramani, Bharathram; Monty, Jason; Marusic, Ivan; Chong, Min
2008-11-01
A conditionally averaged view of ``superstructure'' type events is presented for the case of zero pressure gradient turbulent boundary layers at friction Reynolds number, Re? 14,00. Detailed boundary layer traverses are acquired above a simultaneously sampled spanwise rake of 10 flush-mounted hot-film sensors, affixed to the tunnel wall with a spanwise spacing of approximately 0.08 boundary layer thicknesses (?). The data from the traversing probes are conditioned on the occurrence of superstructure-type footprints sensed by the wall array. The resulting data give a more complete volumetric view of the large-scale meandering log-region features than has previously been afforded by hot-wire rake and PIV measurements. Such detailed views are used to further investigate the existence of an amplitude modulation effect, in which the footprints of large ?- scaled structures (that typify the logarithmic region) have been observed to modulate the viscous-scaled near-wall cycle.
Stationary flow of non-Newtonian fluid with nonmonotone frictional boundary conditions
NASA Astrophysics Data System (ADS)
Dudek, Sylwia; Kalita, Piotr; Migrski, Stanis?aw
2015-10-01
We study the stationary two-dimensional incompressible flow of non-Newtonian fluid governed by a nonlinear constitutive law and with a multivalued nonmonotone subdifferential frictional boundary condition. We provide an abstract result on existence of solution to an operator inclusion modeling the flow phenomenon. We prove a theorem on existence and, under additional assumptions, also uniqueness of weak solution to the flow problem.
Improved Boundary Layer Depth Retrievals from MPLNET
NASA Technical Reports Server (NTRS)
Lewis, Jasper R.; Welton, Ellsworth J.; Molod, Andrea M.; Joseph, Everette
2013-01-01
Continuous lidar observations of the planetary boundary layer (PBL) depth have been made at the Micropulse Lidar Network (MPLNET) site in Greenbelt, MD since April 2001. However, because of issues with the operational PBL depth algorithm, the data is not reliable for determining seasonal and diurnal trends. Therefore, an improved PBL depth algorithm has been developed which uses a combination of the wavelet technique and image processing. The new algorithm is less susceptible to contamination by clouds and residual layers, and in general, produces lower PBL depths. A 2010 comparison shows the operational algorithm overestimates the daily mean PBL depth when compared to the improved algorithm (1.85 and 1.07 km, respectively). The improved MPLNET PBL depths are validated using radiosonde comparisons which suggests the algorithm performs well to determine the depth of a fully developed PBL. A comparison with the Goddard Earth Observing System-version 5 (GEOS-5) model suggests that the model may underestimate the maximum daytime PBL depth by 410 m during the spring and summer. The best agreement between MPLNET and GEOS-5 occurred during the fall and they diered the most in the winter.
Acoustic sounding in the planetary boundary layer
NASA Technical Reports Server (NTRS)
Kelly, E. H.
1974-01-01
Three case studies are presented involving data from an acoustic radar. The first two cases examine data collected during the passage of a mesoscale cold-air intrusion, probably thunderstorm outflow, and a synoptic-scale cold front. In these studies the radar data are compared to conventional meteorological data obtained from the WKY tower facility for the purpose of radar data interpretation. It is shown that the acoustic radar echoes reveal the boundary between warm and cold air and other areas of turbulent mixing, regions of strong vertical temperature gradients, and areas of weak or no wind shear. The third case study examines the relationship between the nocturnal radiation inversion and the low-level wind maximum or jet in the light of conclusions presented by Blackadar (1957). The low-level jet is seen forming well above the top of the inversion. Sudden rapid growth of the inversion occurs which brings the top of the inversion to a height equal that of the jet. Coincident with the rapid growth of the inversion is a sudden decrease in the intensity of the acoustic radar echoes in the inversion layer. It is suggested that the decrease in echo intensity reveals a decrease in turbulent mixing in the inversion layer as predicted by Blackadar. It is concluded that the acoustic radar can be a valuable tool for study in the lower atmosphere.
Assessment of Turbulent Shock-Boundary Layer Interaction Computations Using the OVERFLOW Code
NASA Technical Reports Server (NTRS)
Oliver, A. B.; Lillard, R. P.; Schwing, A. M.; Blaisdell, G> A.; Lyrintzis, A. S.
2007-01-01
The performance of two popular turbulence models, the Spalart-Allmaras model and Menter s SST model, and one relatively new model, Olsen & Coakley s Lag model, are evaluated using the OVERFLOWcode. Turbulent shock-boundary layer interaction predictions are evaluated with three different experimental datasets: a series of 2D compression ramps at Mach 2.87, a series of 2D compression ramps at Mach 2.94, and an axisymmetric coneflare at Mach 11. The experimental datasets include flows with no separation, moderate separation, and significant separation, and use several different experimental measurement techniques (including laser doppler velocimetry (LDV), pitot-probe measurement, inclined hot-wire probe measurement, preston tube skin friction measurement, and surface pressure measurement). Additionally, the OVERFLOW solutions are compared to the solutions of a second CFD code, DPLR. The predictions for weak shock-boundary layer interactions are in reasonable agreement with the experimental data. For strong shock-boundary layer interactions, all of the turbulence models overpredict the separation size and fail to predict the correct skin friction recovery distribution. In most cases, surface pressure predictions show too much upstream influence, however including the tunnel side-wall boundary layers in the computation improves the separation predictions.
Mixed convection boundary layer flow over a horizontal elliptic cylinder with constant heat flux
NASA Astrophysics Data System (ADS)
Javed, Tariq; Ahmad, Hussain; Ghaffari, Abuzar
2015-09-01
Mixed convection boundary layer flow of a viscous fluid over a horizontal elliptic cylinder with a constant heat flux is investigated numerically. The governing partial differential equations are transformed to non-dimensional form and then are solved by an efficient implicit finite different scheme known as Keller-box method. The solutions are expressed in the form of skin friction and Nusselt number, which are plotted against the eccentric angle. The effect of pertinent parameters such as mixed convection parameter, aspect ratio (ratio of lengths of minor axis to major axis), and Prandtl number on skin friction and Nusselt number are illustrated through graphs for both blunt and slender orientations. The increase in the value of mixed convection parameter results in increase in skin friction coefficient and Nusselt number for blunt as well as slender orientations.
Mixed convection boundary layer flow over a horizontal elliptic cylinder with constant heat flux
NASA Astrophysics Data System (ADS)
Javed, Tariq; Ahmad, Hussain; Ghaffari, Abuzar
2015-12-01
Mixed convection boundary layer flow of a viscous fluid over a horizontal elliptic cylinder with a constant heat flux is investigated numerically. The governing partial differential equations are transformed to non-dimensional form and then are solved by an efficient implicit finite different scheme known as Keller-box method. The solutions are expressed in the form of skin friction and Nusselt number, which are plotted against the eccentric angle. The effect of pertinent parameters such as mixed convection parameter, aspect ratio (ratio of lengths of minor axis to major axis), and Prandtl number on skin friction and Nusselt number are illustrated through graphs for both blunt and slender orientations. The increase in the value of mixed convection parameter results in increase in skin friction coefficient and Nusselt number for blunt as well as slender orientations.
Turbulent Boundary Layer in High Rayleigh Number Convection in Air
NASA Astrophysics Data System (ADS)
du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, Andr; Willert, Christian
2014-03-01
Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bnard experiment are presented for the Rayleigh number Ra =1.41010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re ?200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bnard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.
NASA Astrophysics Data System (ADS)
Nowotarski, Christopher J.
Nearly all previous numerical simulations of supercell thunderstorms have neglected surface uxes of heat, moisture, and momentum as well as horizontal inhomogeneities in the near-storm environment from resulting dry boundary layer convection. This investigation uses coupled radiation and land-surface schemes within an idealized cloud model to identify the effects of organized boundary layer convection in the form of horizontal convective rolls (HCRs) on the strength, structure, and evolution of simulated supercell thunderstorms. The in uence of HCRs and the importance of their orientation relative to storm motion is tested by comparing simulations with a convective boundary layer (CBL) against those with a horizontally homogeneous base state having the same mean environment. The impact of anvil shading on the CBL is tested by comparing simulations with and without the effects of clouds in the radiative transfer scheme. The results of these simulations indicate that HCRs provide a potentially important source of environmental vertical vorticity in the sheared, near-storm boundary layer. These vorticity perturbations are amplified both beneath the main supercell updraft and along the trailing out ow boundary, leading to the formation of occasionally intense misovortices. HCRs perpendicular to storm motion are found to have a detrimental effect on the strength and persistence of the lowlevel mesocyclone, particularly during its initial development. Though the mean environment is less supportive of low-level rotation with a wind profile conducive to HCRs oriented parallel to storm motion, such HCRs are found to often enhance the low-level mesocyclone circulation. When anvil shading is included, stabilization results in generally weaker low-level mesocyclone circulation, regardless of HCR orientation. Moreover, HCRs diminish in the near-storm environment such that the effects of HCRs on the supercell are mitigated. HCRs are also shown to be a necessary condition for the formation of so-called "feeder clouds" and anking line convection in these simulations. These findings suggest potentially important rami fications regarding both non-mesocyclone and mesocyclone tornadoes in supercell thunderstorms in an environment with active boundary layer convection.
Atomic-scale friction modulated by potential corrugation in multi-layered graphene materials
Zhuang, Chunqiang; Liu, Lei
2015-03-21
Friction is an important issue that has to be carefully treated for the fabrication of graphene-based nano-scale devices. So far, the friction mechanism of graphene materials on the atomic scale has not yet been clearly presented. Here, first-principles calculations were employed to unveil the friction behaviors and their atomic-scale mechanism. We found that potential corrugations on sliding surfaces dominate the friction force and the friction anisotropy of graphene materials. Higher friction forces correspond to larger corrugations of potential energy, which are tuned by the number of graphene layers. The friction anisotropy is determined by the regular distributions of potential energy. The sliding along a fold-line path (hollow-atop-hollow) has a relatively small potential energy barrier. Thus, the linear sliding observed in macroscopic friction experiments may probably be attributed to the fold-line sliding mode on the atomic scale. These findings can also be extended to other layer-structure materials, such as molybdenum disulfide (MoS{sub 2}) and graphene-like BN sheets.
Large-Eddy Simulation of the Flat-plate Turbulent Boundary Layer at High Reynolds numbers
NASA Astrophysics Data System (ADS)
Inoue, Michio
The near-wall, subgrid-scale (SGS) model [Chung and Pullin, "Large-eddy simulation and wall-modeling of turbulent channel flow'', J. Fluid Mech. 631, 281--309 (2009)] is used to perform large-eddy simulations (LES) of the incompressible developing, smooth-wall, flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. This is then used to study several aspects of zero- and adverse-pressure gradient turbulent boundary layers. First, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Retheta based on the free-stream velocity and the momentum thickness in the range Retheta = 103-1012. Results include the inverse skin friction coefficient, 2/Cf , velocity profiles, the shape factor H, the Karman "constant", and the Coles wake factor as functions of Re theta. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Retheta = O (106). At extremely large Retheta , the empirical Coles-Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot of itself probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that the LES is suggestive of the asymptotic, infinite Reynolds-number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum Retheta of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger Retheta could be achieved with quad- or higher-precision arithmetic. Second, the time series velocity signals obtained from LES within the logarithmic region of the zero-pressure gradient turbulent boundary layer are used in combination with an empirical, predictive inner--outer wall model [Marusic et al., "Predictive model for wall-bounded turbulent flow'', Science 329, 193 (2010)] to calculate the statistics of the fluctuating streamwise velocity in the inner region of the zero-pressure gradient turbulent boundary layer. Results, including spectra and moments up to fourth order, are compared with equivalent predictions using experimental time series, as well as with direct experimental measurements at Reynolds numbers Retau based on the friction velocity and the boundary layer thickness, Retau = 7,300, 13,600 and 19,000. LES combined with the wall model are then used to extend the inner-layer predictions to Reynolds numbers Retau = 62,000, 100,000 and 200,000 that lie within a gap in log(Retau) space between laboratory measurements and surface-layer, atmospheric experiments. The present results support a log-like increase in the near-wall peak of the streamwise turbulence intensities with Retau and also provide a means of extending LES results at large Reynolds numbers to the near-wall region of wall-bounded turbulent flows. Finally, we apply the wall model to LES of a turbulent boundary layer subject to an adverse pressure gradient. Computed statistics are found to be consistent with recent experiments and some Reynolds number similarity is observed over a range of two orders of magnitude.
Development of a laminar boundary layer behind a suction point
NASA Technical Reports Server (NTRS)
Wuest, Walter
1952-01-01
A theoretical investigation is made of the development of a laminar boundary layer behind a suction slot that is assumed to cut off part of the boundary layer without exerting any sink effect. The development, which is approximate, is based on the heat conduction equation. The heat conduction equation enters the analysis through a linearization of the Prandtl-Mises form of the boundary-layer equation.
Boundary Layer Transition Flight Experiment Implementation on OV-103
NASA Technical Reports Server (NTRS)
Spanos, Theodoros A.
2009-01-01
This slide presentation reviews the boundary layer transition experiment flown on Discovery. The purpose of the boundary layer transition flight experiment was to obtain hypersonic aero-thermodynamic data for the purpose of better understanding the flow transition from a laminar to turbulent boundary layer using a known height protuberance. The preparation of the shuttle is described, with the various groups responsibilities outlined. Views of the shuttle in flight with the experimental results are shown.
Methods and results of boundary layer measurements on a glider
NASA Technical Reports Server (NTRS)
Nes, W. V.
1978-01-01
Boundary layer measurements were carried out on a glider under natural conditions. Two effects are investigated: the effect of inconstancy of the development of static pressure within the boundary layer and the effect of the negative pressure difference in a sublaminar boundary layer. The results obtained by means of an ion probe in parallel connection confirm those results obtained by means of a pressure probe. Additional effects which have occurred during these measurements are briefly dealt with.
Control of the Transitional Boundary Layer
NASA Astrophysics Data System (ADS)
Belson, Brandt A.
This work makes advances in the delay of boundary layer transition from laminar to turbulent flow via feedback control. The applications include the reduction of drag over streamline bodies (e.g., airplane wings) and the decrease of mixing and heat transfer (e.g., over turbine blades in jet engines). A difficulty in many fields is designing feedback controllers for high-dimensional systems, be they experiments or high-fidelity simulations, because the required time and resources are too large. A cheaper alternative is to approximate the high-dimensional system with a reduced-order model and design a controller for the model. We implement several model reduction algorithms in "modred", an open source and publicly available library that is applicable to a wide range of problems. We use this library to study the role of sensors and actuators in feedback control of transition in the 2D boundary layer. Previous work uses a feedforward configuration in which the sensor is upstream of the actuator, but we show that the actuator-sensor pair is unsuitable for feedback control due to an inability to sense the exponentially-growing Tollmien-Schlichting waves. A new actuator-sensor pair is chosen that more directly affects and measures the TS waves, and as a result it is effective in a feedback configuration. Lastly, the feedback controller is shown to outperform feedforward controllers in the presence of unmodeled disturbances. Next, we focus on a specific type of actuator, the single dielectric barrier discharge (SDBD) plasma actuator. An array of these plasma actuators is oriented to produce stream-wise vorticity and thus directly cancel the structures with the largest transient growth (so-called stream-wise streaks). We design a feedback controller using only experimental data by first developing an empirical input-output quasi-steady model. Then, we design feedback controllers for the model such that the controllers perform well when applied to the experiment. Lastly, we also simulate the plasma actuators and determine a suitable numerical model for the forces they create by comparing with experimental results. This physical force model is essential to future numerical studies on delaying bypass transition via feedback control and plasma actuation.
Boundary layer features observed during NAME 2004
NASA Astrophysics Data System (ADS)
Stuckmeyer, Elizabeth A.
2011-12-01
S-Pol radar data from the North American Monsoon Experiment (NAME) are examined to investigate the characteristics of sea breezes that occurred during the North American Monsoon in the late summer of 2004, as well as their role in modulating monsoon convection. Zero degree plan position indicated (PPI) scans were examined to determine the presence of a sea breeze fine line in the S-Pol radar data. Sea breeze fine lines were typically observed over land very near the coast of the Gulf of California (GoC), and usually moved onshore around 1700--1800 UTC (11:00 AM--12:00 PM local time), and then continued to move slowly inland on the coastal plain. The sea breezes typically moved on land and dissipated before any significant interactions with Sierra Madre Occidental (SMO) convection could occur. Fine lines varied in reflectivity strength, but were typically around 10 to 20 dBZ. Surface winds from the Estacion Obispo (ETO) supersite were analyzed to confirm the presence of a shift in wind direction on days in which a fine line had been identified. Typically winds changed from light and variable to consistently out of the west or southwest. Vertical plots of S-Pol reflectivity were created to examine sea breeze structure in the vertical, but these were not found to be useful as the sea breeze signature was nearly impossible to distinguish from other boundary layer features. Horizontal structure was further investigated using wind profiler relative reflectivity, vertical velocity, and horizontal winds from the profiler located at ETO. Relative reflectivity and vertical velocity fields revealed a complex boundary layer structure on some days of repeating updrafts and downdrafts. Further examination of S-Pol PPI data revealed that these vertical motions are likely due to the presence of horizontal convective rolls. Profiler horizontal winds revealed that the depth and vertical structure of the sea breezes varied significantly from day to day, but that the height of the sea breeze is around 1 km above the ground. Sea breezes observed during NAME almost never initiated convection on their own. It is hypothesized that a weak thermal contrast between the GoC and the land leads to comparatively weak sea breezes, which don't have enough lift to trigger convection.
The azimuthally averaged boundary layer structure of a numerically simulated major hurricane
NASA Astrophysics Data System (ADS)
Abarca, Sergio F.; Montgomery, Michael T.; McWilliams, James C.
2015-09-01
This work examines the azimuthally averaged boundary layer structure of a numerically simulated hurricane. We nominally define the hurricane boundary layer as the layer in which the effects of surface friction are associated with significant departures from gradient wind balance. The boundary layer in the intensifying primary and forming secondary eyewalls is found to be nonlinear. At large radii, exterior to the eyewalls, Ekman-like balance as traditionally defined, is found to hold true. Where significant departures from Ekman-like balance are found, the departures are characterized by large vertical advection of horizontal velocity through the depth of the boundary layer. Shock-like structures are not found to be prominent in the azimuthally averaged view of the vortex boundary layer, with the largest azimuthally averaged radial gradients of the radial and tangential velocities being on the order of only a few meters per second per kilometer. Also, in the radial regions of the eyewalls, at the height where the averaged tangential wind is a maximum, the radial advection of radial velocity is an order of magnitude smaller than the agradient force per unit mass. Some physical implications of these findings are discussed.
Observations of the magnetospheric boundary layers. [International Magnetospheric Study
NASA Technical Reports Server (NTRS)
Eastman, T. E.
1984-01-01
Results on magnetospheric boundary layers are reviewed, emphasizing their dynamical importance based on hot plasma observations, energetic particle signatures, heavy ion contributions and the effects of wave-particle interactions. Satellite plasma observations show that 1% to 2% of the oncoming solar wind plasma enters the magnetosphere and is initially transported within the magnetospheric boundary layer. Some of this boundary layer plasma is entrained within the Earth's magnetotail where it can be accelerated. Tests are needed to determine the relative contributions of the primary acceleration processes whose effects are especially evident in the plasma sheet boundary layer.
NASA Astrophysics Data System (ADS)
Marsham, J. H.; Parker, D. J.; Grams, C. M.; Grey, W. M. F.; Johnson, B. T.
2008-05-01
Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations) induced mesoscale circulations, and that mesoscale and boundary-layer circulations affected dust uplift and transport. These processes are unrepresented in many climate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parameterise. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Air Layer (SAL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary-layer. In a region of local uplift, with strong along-track winds, boundary-layer rolls are shown to lead to warm moist dusty updraughts in the boundary layer. Large eddy model (LEM) simulations suggest that these rolls increased uplift by approximately 30%. The modelled effects of boundary-layer convection on uplift is shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur.
Three-dimensional boundary layer stability and transition
NASA Technical Reports Server (NTRS)
Malik, M. R.; Li, F.
1992-01-01
Nonparallel and nonlinear stability of a three-dimensional boundary layer, subject to crossflow instability, is investigated using parabolized stability equations (PSEs). Both traveling and stationary disturbances are considered and nonparallel effect on crossflow instability is found to be destabilizing. Our linear PSE results for stationary disturbances agree well with the results from direct solution of Navier-Stokes equations obtained by Spalart (1989). Nonlinear calculations have been carried out for stationary vortices and the computed wall vorticity pattern results in streamwise streaks which resemble remarkably well with the surface oil-flow visualizations in swept-wing experiments. Other features of the stationary vortex development (half-mushroom structure, inflected velocity profiles, vortex doubling, etc.) are also captured in our nonlinear calculations. Nonlinear interaction of the stationary amplitude of the stationary vortex is large as compared to the traveling mode, and the stationary vortex dominates most of the downstream development. When the two modes have the same initial amplitude, the traveling mode dominates the downstream development owing to its higher growth rate, and there is a tendency for the stationary mode to be suppressed. The effect of nonlinear wave development on the skin-friction coefficient is also computed.
Influence of transverse surface waves on turbulent boundary layers
NASA Technical Reports Server (NTRS)
Balasubramanian, R.; Ash, R. L.; Cary, A. M., Jr.; Bushnell, D. M.
1977-01-01
Wavy wall experiments using solid waves and progressive waves have been reported. For this paper, the major effects of waviness of the wall on the flow are identified as due to oscillatory curvature (convex-concavity) and oscillatory acceleration/deceleration of the flow, which imposes a highly nonequilibrium influence upon the turbulence structure. The theoretical analysis in this presentation takes into account proper turbulence modeling (including the nonequilibrium effects) for the wavy wall problem. The analysis proceeds in three stages: (1) inviscid solution for induced pressure due to the physical wall, (2) solution of a turbulent boundary layer with pressure gradients and curvature effects in the modeling from which the profile correction is computed, and (3) induced pressure computations for the corrected profile. The phase shift of pressure perturbations with respect to the physical wall can be predicted, and pressure drag and skin friction drag can be estimated, with nonlinear viscous effects included. Comparison of the theoretical estimates with experimental data are also presented.
On the extension of the wind profile over homogeneous terrain beyond the surface boundary layer
NASA Astrophysics Data System (ADS)
Gryning, Sven-Erik; Batchvarova, Ekaterina; Brmmer, Burghard; Jrgensen, Hans; Larsen, Sren
2007-08-01
Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Hvsre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50 80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200 300 m and considering only wind speeds above 3 m s-1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale ( L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.
Effect of sound on boundary layer stability
NASA Technical Reports Server (NTRS)
Saric, William S.; Spencer, Shelly Anne
1993-01-01
Experiments are conducted in the Arizona State University Unsteady Wind Tunnel with a zero-pressure-gradient flat-plate model that has a 67:1 elliptical leading edge. Boundary-layer measurements are made of the streamwise fluctuating-velocity component in order to identify the amplified T-S waves that are forced by downstream-traveling sound waves. Measurements are taken with circular 3-D roughness elements placed at the Branch 1 neutral stability point for the frequency under consideration, and then with the roughness element downstream of Branch 1. These roughness elements have a principal chord dimension equal to 2 lambda(sub TS)/pi of the T-S waves under study and are 'stacked' in order to resemble a Gaussian height distribution. Measurements taken just downstream of the roughness (with leading-edge T-S waves, surface roughness T-S waves, instrumentation sting vibrations, and the Stokes wave subtracted) show the generation of 3-D T-S waves, but not in the characteristic heart-shaped disturbance field predicted by 3-D asymptotic theory. Maximum disturbance amplitudes are found on the roughness centerline. However, some near-field characteristics predicted by numerical modeling are observed.
Effect of sound on boundary layer stability
NASA Technical Reports Server (NTRS)
Saric, William S. (Principal Investigator); Spencer, Shelly Anne
1993-01-01
Experiments are conducted in the Arizona State University Unsteady Wind Tunnel with a zero-pressure-gradient flat-plate model that has a 67:1 elliptical leading edge. Boundary-layer measurements are made of the streamwise fluctuating-velocity component in order to identify the amplified T-S waves that are forced by downstream-travelling, sound waves. Measurements are taken with circular 3-D roughness elements placed at the Branch 1 neutral stability point for the frequency under consideration, and then with the roughness element downstream of Branch 1. These roughness elements have a principal chord dimension equal to 2(lambda)(sub TS)/pi, of the T-S waves under study and are 'stacked' in order to resemble a Gaussian height distribution. Measurements taken just downstream of the roughness (with leading-edge T-S waves, surface roughness T-S waves, instrumentation sting vibrations and the Stokes wave subtracted) show the generation of 3-D-T-S waves, but not in the characteristic heart-shaped disturbance field predicted by 3-D asymptotic theory. Maximum disturbance amplitudes are found on the roughness centerline. However, some near-field characteristics predicted by numerical modelling are observed.
Nonisotropic turbulence: A turbulent boundary layer
NASA Astrophysics Data System (ADS)
Liu, Kunlun
2005-11-01
The probability density function (PDF) and the two-point correlations of a flat-plate turbulent boundary layer subjected to the zero pressure gradient have been calculated by the direct numerical simulation. It is known that the strong shear force near the wall will deform the vortices and develop some stretched coherent structures like streaks and hairpins, which eventually cause the nonisotropy of wall shear flows. The PDF and the two-point correlations of isotropic flows have been studied for a long time. However, our knowledge about the influence of shear force on the PDF and two-point correlations is still very limited. This study is intended to investigate such influence by using a numerical simulation. Results are presented for a case having a Mach number of M=0.1 and a Reynolds number 2000, based on displacement thickness. The results indicate that the PDF of the streamwise velocity is Lognormal, the PDF of normal velocity is approximately Cauchy, and the PDF of the spanwise velocity is nearly Gaussian. The mean and variance of those PDFs vary according to the distance from the wall. And the two-point correlations are homogenous in the spanwise direction, have a slightly variation in the streamwise direction, but change a lot in the normal direction. Rww or Rvv can be represented as elliptic balls. And the well-chosen normalized system can enable Rww and Rvv to be self-similar.
Turbulent Boundary Layers: An Energy Harvesting Perspective
NASA Astrophysics Data System (ADS)
Lemaire, Pierre; Dogus Akaydin, Huseyin; Elvin, Niell; Anreopoulos, Yiannis
2011-11-01
A turbulent boundary layer (TBL) carries mechanical energy distributed over a range of temporal and spatial scales. The inherent unsteadiness in the TBL induces a strain field on a solid body immersed in it. The induced strain can be converted to electrical energy using a solid body of piezoelectric material. This energy harvesting method can be used for developing self-powered flow sensors. In the present work, we experimentally investigate the interaction of a TBL with a thin flexible beam. The vibration frequency and amplitude of the beam is measured using strain gages. Three relevant parameters are the length of the beam (l) , the distance of the beam from the wall (h) and the free stream speed (V?) . While V? changes the TBL characteristics, h and l primarily affect the fluid-structure interaction. In our wind tunnel tests we traversed the piezoelectric beam across the TBL by varying these three parameters for the purpose of finding values maximizing the vibrations. We present a ``power map'' of the TBL indicating the optimal h and V? values for a given value of l. We also discuss the effect of l in flow-induced vibrations by presenting spectrum analysis of strain signals at various h and V?. Sponsored by NSF Grant: CBET #1033117.
Helical circulations in the typhoon boundary layer
NASA Astrophysics Data System (ADS)
Ellis, Ryan; Businger, Steven
2010-03-01
Low-level wind data from the WSR-88D in Guam obtained in Typhoon Dale (1996) and Typhoon Keith (1997) are analyzed for coherent structures. Consistent with the results of previous studies of Atlantic hurricanes, velocity anomalies associated with coherent structures were found in the boundary layer of both storms. A total of 99 cases of coherent structures, also known as roll vortices, were documented during a 6 h evaluation period for each storm. Storm-relative roll location, roll vorticity, asymmetries in the upward and downward momentum fluxes, and signatures of circulations transverse to the mean flow associated with roll circulations were explored. The effects of terrain and convective precipitation systems, such as rainbands, on the occurrence of rolls were investigated. The results support and extend prior findings of roll observations, and can be used to help validate theoretical and numerical models of coherent structures within tropical cyclones. Moreover, the wind variations documented in this study may have application for wave runup and wind damage potential in tropical cyclones.
Study of boundary-layer transition using transonic cone Preston tube data
NASA Technical Reports Server (NTRS)
Reed, T. D.; Abu-Mostafa, A.
1982-01-01
Laminar layer Preston tube data on a sharp nose, ten degree cone obtained in the Ames 11 ft TWT and in flight tests are analyzed. During analyses of the laminar-boundary layer data, errors were discovered in both the wind tunnel and the flight data. A correction procedure for errors in the flight data is recommended which forces the flight data to exhibit some of the orderly characteristics of the wind tunnel data. From corrected wind tunnel data, a correlation is developed between Preston tube pressures and the corresponding values of theoretical laminar skin friction. Because of the uncertainty in correcting the flight data, a correlation for the unmodified data is developed, and, in addition, three other correlations are developed based on different correction procedures. Each of these correlations are used in conjunction with the wind tunnel correlation to define effective freestream unit Reynolds numbers for the 11 ft TWT over a Mach number range of 0.30 to 0.95. The maximum effective Reynolds numbers are approximately 6.5% higher than the normal values. These maximum values occur between freestream Mach numbers of 0.60 and 0.80. Smaller values are found outside this Mach number range. These results indicate wind tunnel noise affects the average laminar skin friction much less than it affects boundary layer transition. Data on the onset, extent, and end of boundary layer transition are summarized. Application of a procedure for studying the relative effects of varying nose radius on a ten degree cone at supercritical speeds indicates that increasing nose radius promotes boundary layer transition and separation of laminar boundary layers.
Is the boundary layer of an ionic liquid equally lubricating at higher temperature?
Hjalmarsson, Nicklas; Atkin, Rob; Rutland, Mark W
2016-03-23
Atomic force microscopy has been used to study the effect of temperature on normal forces and friction for the room temperature ionic liquid (IL) ethylammonium nitrate (EAN), confined between mica and a silica colloid probe at 25 °C, 50 °C, and 80 °C. Force curves revealed a strong fluid dynamic influence at room temperature, which was greatly reduced at elevated temperatures due to the reduced liquid viscosity. A fluid dynamic analysis reveals that bulk viscosity is manifested at large separation but that EAN displays a nonzero slip, indicating a region of different viscosity near the surface. At high temperatures, the reduction in fluid dynamic force reveals step-like force curves, similar to those found at room temperature using much lower scan rates. The ionic liquid boundary layer remains adsorbed to the solid surface even at high temperature, which provides a mechanism for lubrication when fluid dynamic lubrication is strongly reduced. The friction data reveals a decrease in absolute friction force with increasing temperature, which is associated with increased thermal motion and reduced viscosity of the near surface layers but, consistent with the normal force data, boundary layer lubrication was unaffected. The implications for ILs as lubricants are discussed in terms of the behaviour of this well characterised system. PMID:26976694
Green House Gases Flux Model in Boundary Layer
NASA Astrophysics Data System (ADS)
Nurgaliev, Ildus
Analytical dynamic model of the turbulent flux in the three-layer boundary system is presented. Turbulence is described as a presence of the non-zero vorticity. The generalized advection-diffusion-reaction equation is derived for an arbitrary number of components in the flux. The fluxes in the layers are objects for matching requirements on the boundaries between the layers. Different types of transport mechanisms are dominant on the different levels of the layers.
Evidence for the influence of wave-current interaction in a tidal boundary layer
Green, M.O. ); Rees, J.M.; Pearson, N.D. )
1990-06-15
Near-bed velocity profiles were measured in 24-m water depth off the northeast coast of England. Superimposed on the tidal mean flow were progressively decaying wave-orbital motions; the data span a wide range of relative wave and current energies and offer an ideal opportunity to test wave-current boundary layer theory. The magnitude of the tide-modulated friction velocity appeared to decrease concurrently with the near-bed wave energy. Also, the discrepancy between roughness inferred from the measured velocity profiles and the expected roughness was greatest when the waves were most energetic. Both are consistent with wave-current theory. The best evidence for a dynamic effect of the waves on the mean flow above the wave boundary layer was the correlation of the roughness discrepancy with the regular tidal variation in the strength of the wave-orbital velocity relative to the mean flow. A model of the wave-current boundary layer was used to predict the time-averaged friction velocity, and the model predictions compared well with the observations with exceptions that formed two groups. The first group comprised observations from the time of peak observed bed shear stress. The second group comprised observations from the times of minimum observed bed shear stress. Since laboratory measurements imply wave-current interaction does not occur in the smooth-turbulent combined flow boundary layer, the predictions of time-averaged friction velocity were repeated using a smooth-turbulent pure-current model. The predictions were significantly improved, thus supporting that condition.
NASA Astrophysics Data System (ADS)
Attili, Basem S.
2011-09-01
A boundary value method for solving a class of nonlinear singularly perturbed two point boundary value problems with a boundary layer at one end is proposed. Using singular perturbation analysis the method consists of solving two problems; namely, a reduced problem and a boundary layer correction problem. We use Pade' approximation to obtain the solution of the latter problem and to satisfy the condition at infinity. Numerical examples will be given to illustrate the method.
MHD boundary layer flow of a power-law nanofluid with new mass flux condition
NASA Astrophysics Data System (ADS)
Khan, Masood; Khan, Waqar Azeem
2016-02-01
An analysis is carried out to study the magnetohydrodynamic (" separators=" MHD ) boundary layer flow of power-law nanofluid over a non-linear stretching sheet. In the presence of a transverse magnetic field, the flow is generated due to non-linear stretching sheet. By using similarity transformations, the governing boundary layer equations are reduced into a system of ordinary differential equations. A recently proposed boundary condition requiring zero nanoparticle mass flux is employed in the flow analysis of power-law fluid. The reduced coupled differential equations are then solved numerically by the shooting method. The variations of dimensionless temperature and nanoparticle concentration with various parameters are graphed and discussed in detail. Numerical values of physical quantities such as the skin-friction coefficient and the reduced local Nusselt number are computed in tabular form.
Drag of a turbulent boundary layer with transverse 2D circular rods on the wall
NASA Astrophysics Data System (ADS)
Kamruzzaman, Md; Djenidi, L.; Antonia, R. A.; Talluru, K. M.
2015-06-01
In this paper, we present the results of a turbulent boundary layer developing over a rod-roughened wall with a spacing of ( is the spacing between two adjacent roughness elements, and is the rod diameter). Static pressure measurements are taken around a single roughness element to accurately determine the friction velocity, and the error in the origin, , which are the two prominent issues that surround rough-wall boundary layers. In addition, velocity measurements are taken at several streamwise locations using hot-wire anemometry to obtain from the momentum integral equation. Results showed that both methods give consistent values for , indicating that the contribution of the viscous drag over this rough wall is negligible. This supports the results of Perry et al. (J Fluid Mech 177:437-466, 1969) and Antonia and Luxton (J Fluid Mech 48(04):721-761, 1971) in a boundary layer and of Leonardi et al. (2003) in a channel flow but does not agree with those of Furuya et al. (J Fluids Eng 98(4):635-643, 1976). The results show that both and can be unambiguously measured on this particular rough wall. This paves the way for a proper comparison between the boundary layer developing over this wall and the smooth-wall turbulent boundary layer.
NASA Technical Reports Server (NTRS)
Anand, A.; Gorton, C.; Lakshminarayana, B.; Yamaoka, H.
1973-01-01
A study of the boundary layer and turbulence characteristics inside the passages of an axial flow inducer is reported. The first part deals with the analytical and experimental investigation of the boundary layer characteristics in a four bladed flat plate inducer passage operated with no throttle. An approximate analysis for the prediction of radial and chordwise velocity profiles across the passage is carried out. The momentum integral technique is used to predict the gross properties of the boundary layer. Equations are given for the exact analysis of the turbulent boundary layer characteristics using the turbulent field method. Detailed measurement of boundary layer profiles, limiting streamline angle and skin friction stress on the rotating blade is also reported. Part two of this report deals with the prediction of the flow as well as blade static pressure measurements in a three bladed inducer with cambered blades operated at a flow coefficient of 0.065. In addition, the mean velocity and turbulence measurements carried out inside the passage using a rotating triaxial probe is reported.
Interaction between surface and atmosphere in a convective boundary layer
NASA Astrophysics Data System (ADS)
Garai, Anirban
Solar heating of the surface causes the near surface air to warm up and with sufficient buoyancy it ascends through the atmosphere as surface-layer plumes and thermals. The cold fluid from the upper part of the boundary layer descends as downdrafts. The downdrafts and thermals form streamwise roll vortices. All these turbulent coherent structures are important because they contribute most of the momentum and heat transport. While these structures have been studied in depth, their imprint on the surface through energy budget in a convective atmospheric boundary layer has received little attention. The main objective of the present study is to examine the turbulence-induced surface temperature fluctuations for different surface properties and stratification. Experiments were performed to measure atmospheric turbulence using sonic anemometers, fine wire thermocouples and LIDAR; and surface temperature using an infra-red camera over grass and artificial turf fields. The surface temperature fluctuations were found to be highly correlated to the turbulent coherent structures and follow the processes postulated in the surface renewal theory. The spatio-temporal scales and advection speed of the surface temperature fluctuation were found to match with those of turbulent coherent structures. A parametric direct numerical simulation (DNS) study was then performed by solving the solid-fluid heat transport mechanism numerically for varying solid thermal properties, solid thickness and strength of stratification. Even though there were large differences in the friction Reynolds and Richardson numbers between the experiments and numerical simulations, similar turbulent characteristics were observed. The ejection (sweep) events tend to be aligned with the streamwise direction to form roll vortices with unstable stratification. The solid-fluid interfacial temperature fluctuations increase with the decreases in solid thermal inertia; and with the increase in solid thickness to attain a constant value for a sufficiently thick solid. The temperature fluctuation changes from a Gaussian distribution near the wall to a positively skewed distribution away from the wall. The turbulent temperature fluctuations influence the solid interfacial temperature by thermal conduction only. These studies provided unique insights into the solid-fluid coupled heat transport in low and high Reynolds number flows. This turbulence induced surface temperature fluctuation can influence the performances of several satellite remote sensing models.
Frictional sliding in layered rock: laboratory-scale experiments
Buescher, B.J.; Perry, K.E. Jr.; Epstein, J.S.
1996-09-01
The work is part of the rock mechanics effort for the Yucca Mountain Site Characterization Program. The laboratory-scale experiments are intended to provide high quality data on the mechanical behavior of jointed structures that can be used to validate complex numerical models for rock-mass behavior. Frictional sliding between simulated rock joints was studied using phase shifting moire interferometry. A model, constructed from stacks of machined and sandblasted granite plates, contained a central hole bore normal to the place so that frictional slip would be induced between the plates near the hole under compressive loading. Results show a clear evolution of slip with increasing load. Since the rock was not cycled through loading- unloading, the quantitative differences between the three data sets are probably due to a ``wearing-in`` effect. The highly variable spatial frequency of the data is probably due to the large grain size of the granite and the stochastic frictional processes. An unusual feature of the evolution of slip with increasing load is that as the load gets larger, some plates seem to return to a null position. Figs, 6 refs.
Structure and friction-reducing property of the sulfide layer produced by ion sulfuration
Ning, Z.; Da-Ming, Z.; Yan-Hua, W.; Jia-Jun, L.; Xiao-Dong, F.; Ming-Xi, G.
2000-04-01
Sulfide layers with a certain thickness were made on the surface of 1045 and 52100 steels by means of the low-temperature ion sulfuration technique. Metallography, scanning electron microscope (SEM) + energy-dispersive x-ray analysis (EDX), and x-ray diffraction (XRD) were adopted to analyze the structure of sulfide layers; the tribological properties of the layers lubricated by paraffin oil were also investigated on a reciprocating tester. The results showed that sulfide layer is porous, and its structure is mainly composed of FeS, FeS{sub 2}, and substrate phases. The sulfide layer possessed a remarkable friction-reducing effect; its friction coefficient was lower on average, by about 50%, than that of the surface without layer. With the increase of layer thickness, its friction coefficient was unchanged, and under low load conditions, its operational period was prolonged. Under the same experimental conditions, the operational period of sulfide layer on 52100 steel was longer than that on 1045 steel, and its friction coefficient was lower as well.
A study of methods to investigate nozzle boundary layer transition
NASA Technical Reports Server (NTRS)
Pauley, Laura L.
1991-01-01
To further investigate nozzle flow, numerical computations are employed. The computations produce complete flow velocity and temperature fields within the nozzle. As a check, these results can be compared with experimental data at the wall. Once an accurate numerical scheme has been validated, it can be used as a design tool to predict the performance of other nozzle designs without the cost of experimental testing. Typically, the numerical analysis assumes either a laminar boundary layer or a fully turbulent boundary layer which is steady and two-dimensional. Boundary layer transition is not considered. Computing both the completely laminar boundary layer and the completely turbulent boundary layer conditions gives the minimum and maximum wall heat flux possible for a specified geometry. When the experimental heat flux measurements lie between these two values, the nature of the boundary layer is unknown. The boundary layer may have transitioned from laminar to turbulent; three-dimensional structures may be present in the boundary layer, or the inlet flow conditions may not be correctly specified in the computation.
Symmetries in Turbulent Boundary Layer Flows
NASA Technical Reports Server (NTRS)
Oberlack, M.
1996-01-01
The objective is the development of a new theory which enables the algorithmic computation of all self-similar mean velocity profiles. The theory is based on Liegroup analysis and unifies a large set of self-similar solutions for the mean velocity of stationary parallel turbulent shear flows. The results include the logarithmic law of the wall, an algebraic law, the viscous sublayer, the linear region in the middle of a Couette flow and in the middle of a rotating channel flow, and a new exponential mean velocity profile not previously reported. Experimental results taken in the outer parts of a high Reynolds number flat-plate boundary layer, strongly support the exponential profile. From experimental as well as from DNS data of a turbulent channel flow the algebraic scaling law could be confirmed in both the center region and in the near wall region. In the case of the logarithmic law of the wall, the scaling with the wall distance arises as a result of the analysis and has not been assumed in the derivation. The crucial part of the derivation of all the different mean velocity profiles is to consider the invariance of the equation for the velocity fluctuations at the same time as the invariance of the equation for the velocity product equations. The latter is the dyad product of the velocity fluctuations with the equation for the velocity fluctuations. It has been proven that all the invariant solutions are also consistent with similarity of all velocity moment equations up to any arbitrary order.
A New View on Origin, Role and Manipulation of Large Scales in Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Corke, T. C.; Nagib, H. M.; Guezennec, Y. G.
1982-01-01
The potential of passive 'manipulators' for altering the large scale turbulent structures in boundary layers was investigated. Utilizing smoke wire visualization and multisensor probes, the experiment verified that the outer scales could be suppressed by simple arrangements of parallel plates. As a result of suppressing the outer scales in turbulent layers, a decrease in the streamwise growth of the boundary layer thickness was achieved and was coupled with a 30 percent decrease in the local wall friction coefficient. After accounting for the drag on the manipulator plates, the net drag reduction reached a value of 20 percent within 55 boundary layer thicknesses downstream of the device. No evidence for the reoccurrence of the outer scales was present at this streamwise distance thereby suggesting that further reductions in the net drag are attainable. The frequency of occurrence of the wall events is simultaneously dependent on the two parameters, Re2 delta sub 2 and Re sub x. As a result of being able to independently control the inner and outer boundary layer characteristics with these manipulators, a different view of these layers emerged.
Destiny of earthward streaming plasma in the plasmasheet boundary layer
NASA Technical Reports Server (NTRS)
Green, J. L.; Horwitz, J. L.
1986-01-01
The dynamics of the earth's magnetotail have been investigated, and it has become clear that the plasmasheet boundary layer field lines map into the Region I Field-Aligned Currents (FAC) of the auroral zone. It is pointed out that the role of earthward streaming ions in the plasmasheet boundary layer may be of fundamental importance in the understanding of magnetotail dynamics, auroral zone physics, and especially for ionospheric-magnetospheric interactions. The present paper has the objective to evaluate propagation characteristics for the earthward streaming ions observed in the plasmasheet boundary layer. An investigation is conducted of the propagation characteristics of protons in the plasmasheet boundary layer using independent single particle dynamics, and conclusions are discussed. The density of earthward streaming ions found in the plasmasheet boundary layer should include the ring current as well as the auroral zone precipitaiton and inner plasmasheet regions of the magnetosphere.
Dynamic behavior of an unsteady trubulent boundary layer
NASA Technical Reports Server (NTRS)
Parikh, P. G.; Reynolds, W. C.; Jayaramen, R.; Carr, L. W.
1981-01-01
Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle.
Computer simulation of shock wave boundary layer interaction
NASA Technical Reports Server (NTRS)
Bauer, F.; Garabedian, P.
1973-01-01
Development of a boundary layer correction for an analysis program previously used by Bauer et al. (1972) to study two-dimensional flow past transonic airfoils at off-design conditions where shocks which interact with the boundary layer appear. Using a method which combines conformal mapping with a finite-difference scheme due to Murman and Cole (1971), an analysis is made of the interaction between shock waves on a supercritical wing section and a turbulent boundary layer, assuming that the shocks are weak and that separation is insignificant. It is shown that relatively simple shock wave/boundary layer interactions of the kind envisioned in this study can be treated by determining the displacement thickness iteratively together with the flow and altering the profile accordingly. It is concluded that the proposed boundary layer correction appears to furnish an essential improvement in cases where both pronounced aft loading and a large enclosed supersonic region of flow are present.
Flight experience with a pivoting traversing boundary-layer probe
NASA Technical Reports Server (NTRS)
Montoya, L. C.; Brauns, D. A.; Cissell, R. E.
1974-01-01
A pivoting traversing boundary layer probe was evaluated in flight on an F-104 airplane. The evaluation was performed at free stream Mach numbers from 0.8 to 2.0. The unit is described, and operating problems and their solutions are discussed. Conventional boundary layer profiles containing variations in flow angle within the viscous layer are shown for free stream Mach numbers of 0.8, 1.6, and 2.0. Although the unit was not optimized for size and weight, it successfully measured simultaneously flow angularity, probe height, and pitot pressure through the boundary layer.
Structure of the low-latitude boundary layer. [in magnetopause
NASA Technical Reports Server (NTRS)
Sckopke, N.; Paschmann, G.; Haerendel, G.; Sonnerup, B. U. OE.; Bame, S. J.; Forbes, T. G.; Hones, E. W., Jr.; Russell, C. T.
1981-01-01
High temporal resolution observations of the frontside magnetopause and plasma boundary layer made with the fast plasma analyzer aboard the ISEE 1 and 2 spacecraft are reported. The data are found to be compatible with a boundary layer that is always attached to the magnetopause but where the layer thickness has a large-scale spatial modulation pattern which travels tailward past the spacecraft. Periods are included when the thickness is essentially zero and others when it is of the order of 1 earth radius. The duration of these periods is highly variable but is typically in the range of 2-5 min corresponding to a distance along the magnetopuase of approximately 3-8 earth radii. The observed boundary layer features include a steep density gradient at the magnetopause with an approximately constant boundary layer plasma density amounting to about 25% of the magnetosheath density, and a second abrupt density decrease at the inner edge of the layer.
Boundary Layer Rolls Observed Above and Below a Jet in a Marine Boundary Layer
NASA Astrophysics Data System (ADS)
Foster, R. C.; Emmitt, G. D.; Godwin, K.; Greco, S.
2013-12-01
We have flown a coherent Doppler wind lidar (DWL) on the Cirpas Twin Otter off the California coast near Monterey since 2003. One scientific purpose of these flights is to understand the relationship between the turbulent fluxes measured on the aircraft or on other platforms and the observed structure of the marine boundary layer (MBL). Two common features are found in the MBL flow: (1) a strong jet at approximately 200 m above the sea surface; and (2) organized large eddies (OLE) in the form of roll vortices that are approximately aligned along the mean wind direction. On two flights (April 13, 2007 and September 30, 2012), the DWL data indicated that roll OLE existed simultaneously both above and below the jet. The DWL winds suggest that the OLE in these layers are sometimes independent and sometimes connected. Standard flux data are obtained on the Twin Otter at flight level, which is nominally 300 m. The 10 Hz wind and temperature data exhibit variability at spatial scales corresponding to the OLE wavelength. We have constructed a nonlinear theoretical model that includes triad wave-wave interactions to test the hypothesis that rolls could form both above and below the jet. This model shows that this is possible and that the rolls in the two layers could have unique characteristics compared to standard boundary layer rolls. The model further shows that the rolls above and below the jet are due to separate instabilities that interact. This is consistent with the observations of both connected and independent OLE above and below the jet. Contrast-enhanced DWL line-of-sight winds. Jet maximum 200 m below aircraft. Typical resonant triad solution for rolls above and below a PBL jet.
On Reflection of Shock Waves from Boundary Layers
NASA Technical Reports Server (NTRS)
Liepmann, H W; Roshko, A; Dhawan, S
1952-01-01
Measurements are presented at Mach numbers from about 1.3 to 1.5 of reflection characteristics and the relative upstream influence of shock waves impinging on a flat surface with both laminar and turbulent boundary layers. The difference between impulse and step waves is discussed and their interaction with the boundary layer is compared. General considerations on the experimental production of shock waves from wedges and cones and examples of reflection of shock waves from supersonic shear layers are also presented.
Friction phenomena in the overdamped three-layer model.
Jia, Li-Ping; Teki?, Jasmina; Yang, Yang; Wang, Cang-Long; Duan, Wen-Shan; Yang, Lei
2015-02-01
An overdamped three-layer model consisting of two harmonic chains of interacting particles, representing the upper and the middle layers, which move over the substrate potential, is studied in the present paper. A dc+ac force is applied only on the upper harmonic chain, and dynamics of both layers are investigated. The results show that the dynamical mode locking and Shapiro steps appear not only in the upper layer but also in the middle one. It is noted that the motion of particles in the upper layer corresponds to the standard Frenkel-Kontorova model. The dependence of the Shapiro steps of the middle layer on the system parameters are determined. It is shown that the height of the first Shapiro step of the upper layer is unrelated to the interaction parameters of the particles of both the upper and the middle layers, while the height of the first Shapiro step of the middle layer depend only on the interaction parameters of the particles of the middle layers. Two critical forces which transfer from locked state to the sliding one of both the upper and the middle layers are also studied. They depend on the amplitude and the frequency of the external ac driving force. PMID:25768573
On the theory of laminar boundary layers involving separation
NASA Technical Reports Server (NTRS)
Von Karman, TH; Millikan, C
1934-01-01
This paper presents a mathematical discussion of the laminar boundary layer, which was developed with a view of facilitating the investigation of those boundary layers in particular for which the phenomenon of separation occurs. The treatment starts with a slight modification of the form of the boundary layer equation first published by Von Mises. Two approximate solutions of this equation are found, one of which is exact at the outer edge of the boundary layer while the other is exact at the wall. The final solution is obtained by joining these two solutions at the inflection points of the velocity profiles. The final solution is given in terms of a series of universal functions for a fairly broad class of potential velocity distributions outside of the boundary layer. Detailed calculations of the boundary layer characteristics are worked out for the case in which the potential velocity is a linear function of the distance from the upstream stagnation point. Finally, the complete separation point characteristics are determined for the boundary layer associated with a potential velocity distribution made up of two linear functions of the distance from the stagnation point. It appears that extensions of the detailed calculations to more complex potential flows can be fairly easily carried out by using the explicit formulae given in the paper. (author)
Study of stirred layers on 316L steel created by friction stir processing
NASA Astrophysics Data System (ADS)
Langlade, C.; Roman, A.; Schlegel, D.; Gete, E.; Folea, M.
2014-08-01
Nanostructured materials are known to exhibit attractive properties, especially in the mechanical field where high hardness is of great interest. The friction stir process (FSP) is a recent surface engineering technique derived from the friction stir welding method (FSW). In this study, the FSP of an 316L austenitic stainless steel has been evaluated. The treated layers have been characterized in terms of hardness and microstructure and these results have been related to the FSP operational parameters. The process has been analysed using a Response Surface Method (RSM) to enable the stirred layer thickness prediction.
NASA Astrophysics Data System (ADS)
Ikari, M.; Trtner, S.; Toy, V. G.; Carpenter, B. M.; Kopf, A.
2014-12-01
The Alpine Fault is a major plate-boundary fault zone that poses a significant seismic hazard in southern New Zealand, with the next major earthquake expected to be imminent. Core samples from the Alpine Fault were recovered from two Deep Fault Drilling Project pilot boreholes that penetrated the principal slip zone (PSZ). We show here that at room temperature and low effective stress (30 MPa), materials from within and very near the PSZ are weaker than the surrounding cataclasites (? = 0.45), exhibit velocity-strengthening friction, and also tend to restrengthen (heal) rapidly. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 C, fluid-saturated), a cataclasite/gouge sample located very near to the PSZ exhibits ? = 0.67, which is high compared to measurements performed at lower pressures and temperatures for the Alpine Fault and other major fault zones sampled by scientific drilling. Every major lithological unit tested under elevated P-T conditions exhibits both positive and negative values of friction velocity-dependence suggesting that they are all capable of earthquake nucleation. Using representative values of the friction velocity-dependent parameter a-b, the critical slip distance Dc, and previously documented elastic properties of the wall rock, estimated critical nucleation patch lengths may be as low as ~3 m. This small value is consistent with a seismic moment Mo = ~4x1010 or a Mw = ~1, which suggests that events of this size or larger are expected to occur as normal earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3-7 km. In conjunction with previous geodetic and seismologic observations, our results indicate that the Alpine Fault has a high potential for frictional instability throughout the brittle crust, in contrast with other major fault zones on which the uppermost portion is relatively stable.
High Reynolds number rough wall turbulent boundary layer experiments using Braille surfaces
NASA Astrophysics Data System (ADS)
Harris, Michael; Monty, Jason; Nova, Todd; Allen, James; Chong, Min
2007-11-01
This paper details smooth, transitional and fully rough turbulent boundary layer experiments in the New Mexico State high Reynolds number rough wall wind tunnel. The initial surface tested was generated with a Braille printer and consisted of an uniform array of Braille points. The average point height being 0.5mm, the spacing between the points in the span was 0.5mm and the surface consisted of span wise rows separated by 4mm. The wavelength to peak ratio was 8:1. The boundary layer thickness at the measurement location was 190mm giving a large separation of roughness height to layer thickness. The maximum friction velocity was u?=1.5m/s at Rex=3.8 x10^7. Results for the skin friction co-efficient show that this surface follows a Nikuradse type inflectional curve and that Townsends outer layer similarity hypothesis is valid for rough wall flows with a large separation of scales. Mean flow and turbulence statistics will be presented.
Evaluation of boundary lubricants using steady-state wear and friction
NASA Technical Reports Server (NTRS)
Loomis, W. R.; Jones, W. R., Jr.
1981-01-01
A friction and wear study was made at 20 C to establish operating limits and procedures for obtaining improved reproducibility and reliability in boundary lubrication testing. Ester base and C-other base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk in a pin-on-disk apparatus. Results of a parametric study with varying loads and speeds slowed that satisfactory test conditions for studying the direction and wear characteristics in the boundary lubrication regime with this test device were found to be 1 kilogram load; 7 to 9 meters-per-minute (50 rpm) surface speed; dry air test atmosphere (less than 100 ppm H2O); and use of a time stepwise procedure for measuring wear. Highly reproducible steady-state wear rates resulted from the two fluid studies which had a linearity of about 99 percent after initially higher wear rates and friction coefficients during run-in periods of 20 to 40 minutes.
NASA Astrophysics Data System (ADS)
Williams, Peter T.
2016-01-01
Twenty-five years ago, Pringle suggested a boundary-layer origin for jets from YSOs. The jets were driven by a toroidal magnetic field generated by strong shear in the accretion boundary layer. Such a mechanism is clearly non-magnetocentrifugal in nature.Nearly fifteen years ago, we suggested a cartoon of the jet-launching mechanism in protostars in which shear, acting upon MHD turbulence generated by the magnetorotational instability (MRI), generated a tangled, toroidal magnetic field capable of driving a jet. This picture, which is also manifestly non-magnetocentrifugal in nature, relied upon a novel model for MRI-driven MHD turbulence based on a viscoelastic, rather than a viscous, prescription for the turbulent stress. Our hypothesis has some clear similarities to Pringle's mechanism, but it relied upon a large envelope surrounding the central star.An accretion boundary layer has long been recognized as a promising source for protostellar jets in good part because in a standard thin disk, matter loses circa half of all its accretion energy in this layer, but it is problematic to drive a well-collimated outflow from a boundary layer in a thin disk. In this presentation, we argue paradoxically that the "boundary layer" can drive jets when a true boundary layer, like the thin disk, does not exist. This changes the inner boundary condition for viscous angular momentum flux in the disk.The standard argument for a thin boundary layer is, we argue, circular. In high accretion-rate systems, or when the gas cannot cool efficiently, there is no reason to suspect the turbulent viscosity in this boundary layer to be small, and therefore neither is the boundary layer. When the boundary layer becomes larger than the central accretor itself, it is arguably no longer a boundary layer, but rather an envelope. It is still, however, a substantial source of power and toroidal MRI-driven magnetic fields.It is, again, only in relatively hot or high-accretion rate systems in which the boundary layer would be expected to inflate and so disappear. Not coincidentally, it is in such systems, such as Class 0 and Class I protostars, in which we have the strongest evidence for powerful, well-collimated jet outflows.
Structure of turbulence in three-dimensional boundary layers
NASA Technical Reports Server (NTRS)
Subramanian, Chelakara S.
1993-01-01
This report provides an overview of the three dimensional turbulent boundary layer concepts and of the currently available experimental information for their turbulence modeling. It is found that more reliable turbulence data, especially of the Reynolds stress transport terms, is needed to improve the existing modeling capabilities. An experiment is proposed to study the three dimensional boundary layer formed by a 'sink flow' in a fully developed two dimensional turbulent boundary layer. Also, the mean and turbulence field measurement procedure using a three component laser Doppler velocimeter is described.
Formation of pre-sheath boundary layers in electronegative plasmas
Vitello, P., LLNL
1998-05-01
In electronegative plasmas Coulomb scattering between positive and negative ions can lead to the formation of a pre-sheath boundary layer containing the bulk of the negative ions. The negative ion boundary layer forms when momentum transfer from positive to negative ions dominates the negative ion acceleration from the electric field. This condition is met in Inductively Coupled Plasma reactors that operate at low pressure and high plasma density. Simulations of the GEC reactor for Chlorine and Oxygen chemistries using the INDUCT95 2D model are presented showing the pre-sheath boundary layer structure as a function of applied power and neutral pressure.
Grain-boundary motion in layered phases
NASA Astrophysics Data System (ADS)
Boyer, Denis; Vials, Jorge
2001-06-01
We study the motion of a grain boundary that separates two sets of mutually perpendicular rolls in Rayleigh-Bnard convection above onset. The problem is treated either analytically from the corresponding amplitude equations, or numerically by solving the Swift-Hohenberg equation. We find that if the rolls are curved by a slow transversal modulation, a net translation of the boundary follows. We show analytically that although this motion is a nonlinear effect, it occurs in a time scale much shorter than that of the linear relaxation of the curved rolls. The total distance traveled by the boundary scales as ?-1/2, where ? is the reduced Rayleigh number. We obtain analytical expressions for the relaxation rate of the modulation and for the time-dependent traveling velocity of the boundary, and especially their dependence on wave number. The results agree well with direct numerical solutions of the Swift-Hohenberg equation. We finally discuss the implications of our results on the coarsening rate of an ensemble of differently oriented domains in which grain-boundary motion through curved rolls is the dominant coarsening mechanism.
Implementation of wall boundary conditions for transpiration in F3D thin-layer Navier-Stokes code
NASA Technical Reports Server (NTRS)
Kandula, M.; Martin, F. W., Jr.
1991-01-01
Numerical boundary conditions for mass injection/suction at the wall are incorporated in the thin-layer Navier-Stokes code, F3D. The accuracy of the boundary conditions and the code is assessed by a detailed comparison of the predictions of velocity distributions and skin-friction coefficients with exact similarity solutions for laminar flow over a flat plate with variable blowing/suction, and measurements for turbulent flow past a flat plate with uniform blowing. In laminar flow, F3D predictions for friction coefficient compare well with exact similarity solution with and without suction, but produces large errors at moderate-to-large values of blowing. A slight Mach number dependence of skin-friction coefficient due to blowing in turbulent flow is computed by F3D code. Predicted surface pressures for turbulent flow past an airfoil with mass injection are in qualitative agreement with measurements for a flat plate.
NASA Astrophysics Data System (ADS)
John, Volker
2002-10-01
We consider slip with friction and penetration with resistance boundary conditions in the steady state Navier-Stokes equations. This paper describes some aspects of the implementation of these boundary conditions for finite element discretizations. Numerical tests on two- and three-dimensional channel flows across a step using the slip with friction boundary condition study the influence of the friction parameter on the position of the reattachment point and the reattachment line of the recirculating vortex, respectively.
The Compressible Laminar Boundary Layer with Heat Transfer and Arbitrary Pressure Gradient
NASA Technical Reports Server (NTRS)
Cohen, Clarence B; Reshotko, Eli
1956-01-01
An approximate method for the calculation of the compressible laminar boundary layer with heat transfer and arbitrary pressure gradient, based on Thwaites' correlation concept, is presented. With the definition of dimensionless shear and heat-transfer parameters and an assumed correlation of these parameters in terms of a momentum parameter, a complete system of relations for calculating skin friction and heat transfer results. Knowledge of velocity or temperature profiles is not necessary in using this calculation method. When the method is applied to a convergent-divergent, axially symmetric rocket nozzle, it shows that high rates of heat transfer are obtained at the initial stagnation point and at the throat of the nozzle. Also indicated are negative displacement thicknesses in the convergent portion of the nozzle; these occur because of the high density within the lower portions of the cooled boundary layer. (author)
Investigation of blown boundary layers with an improved wall jet system
NASA Technical Reports Server (NTRS)
Saripalli, K. R.; Simpson, R. L.
1980-01-01
Measurements were made in a two dimensional incompressible wall jet submerged under a thick upstream boundary layer with a zero pressure gradient and an adverse pressure gradient. The measurements included mean velocity and Reynolds stresses profiles, skin friction, and turbulence spectra. The measurements were confined to practical ratios (less than 2) of the jet velocity to the free stream velocity. The wall jet used in the experiments had an asymmetric velocity profile with a relatively higher concentration of momentum away from the wall. An asymmetric jet velocity profile has distinct advantages over a uniform jet velocity profile, especially in the control of separation. Predictions were made using Irwin's (1974) method for blown boundary layers. The predictions clearly show the difference in flow development between an asymmetric jet velocity profile and a uniform jet velocity profile.
Effects of boundary layer and liquid viscosity and compressible air on sloshing characteristics
NASA Astrophysics Data System (ADS)
Zou, Chang-Fang; Wang, De-Yu; Cai, Zhong-Hua
2015-07-01
In this paper, numerical investigations for tank sloshing, based on commercial CFD package FLUENT, are performed to study effects of boundary layer grid, liquid viscosity and compressible air on sloshing pressure, wave height and rising time of impact pressure. Also, sloshing experiments for liquids of different viscosity are carried out to validate the numerical results. Through comparison of numerical and experimental results, a computational model including boundary layer grid can predict the sloshing pressure more accurately. Energy dissipation due to viscous friction leads to reduction of sloshing pressure and wave elevation. Sloshing pressure is also reduced because of cushion effect of compressible air. Due to high viscosity damping effect and compressible air effect, the rising time of impact pressure becomes longer. It is also found that liquid viscosity and compressible air influence distribution of dynamic pressure along the vertical tank wall.
Boundary-layer measurements on a high Reynolds number three-element airfoil
NASA Technical Reports Server (NTRS)
Selby, Gregory V.
1992-01-01
An experimental investigation is being conducted to evaluate the boundary layer associated with a two-dimensional three-element single-flap airfoil at high Reynolds numbers. The present measurements are being made in the Langley Low-Turbulence (centerline turbulence intensity level is 0.034 percent at a Mach number of 0.2 and a total pressure of 60 psia) Pressure Tunnel (LTPT). The LTPT is a closed-circuit wind tunnel with a test section which is 3 ft wide, 7.5 ft high, and 7.5 ft long. Operating total pressure for the LTPT varies from 10 atmospheres to near-vacuum conditions. Tests are being conducted at a Mach number of 0.2 and Reynolds numbers (based on chord length) of 5, 9, and 16 million. Measurements include boundary-layer velocity surveys at several chordwise locations and surface skin-friction measurements using Preston tubes.
Turbulent boundary layer on the surface of a sea geophysical antenna
NASA Astrophysics Data System (ADS)
Smol'Yakov, A. V.
2010-11-01
A theory is constructed that makes it possible to calculate the initial parameters necessary for calculating the hydrodynamic (turbulent) noise, which is a handicap to the operation of sea geophysical antennas. Algorithms are created for calculating the profile and defect of the average speed, displacement thickness, momentum thickness, and friction resistance in a turbulent boundary layer on a cylinder in its axial flow. Results of calculations using the developed theory are compared to experimental data. As the diameter of the cylinder tends to infinity, all relations of the theory pass to known relations for the boundary layer on a flat plate. The developed theory represents the initial stage of creating a method to calculate hydrodynamic noise, which is handicap to the operation of sea geophysical antennas.
NASA Technical Reports Server (NTRS)
Eastman, Timothy E.
1995-01-01
Evidence for the probable existence of magnetospheric boundary layers was first presented by Hones, et al. (1972), based on VELA satellite plasma observations (no magnetic field measurements were obtained). This magnetotail boundary layer is now known to be the tailward extension of the high-latitude boundary layer or plasma mantle (first uniquely identified using HEOS 2 plasma and field observations by Rosenbauer et al., 1975) and the low-latitude boundary layer (first uniquely identified using IMP 6 plasma and field observations by Eastman et al., 1976). The magnetospheric boundary layer is the region of magnetosheath-like plasma located Earthward of, but generally contiguous with the magnetopause. This boundary layer is typically identified by comparing low-energy (less than 10 keV) ion spectra across the magnetopause. Low-energy electron measurements are also useful for identifying the boundary layer because the shocked solar wind or magnetosheath has a characteristic spectral signature for electrons as well. However, there are magnetopause crossings where low-energy electrons might suggest a depletion layer outside the magnetopause even though the traditional field-rotation signature indicates that this same region is a boundary layer Earthward of the current layer. Our analyses avoided crossings which exhibit such ambiguities. Pristine magnetopause crossings are magnetopause crossings for which the current layer is well defined and for which there is no adjoining magnetospheric boundary layer as defined above. Although most magnetopause models to date apply to such crossings, few comparisons between such theory and observations of pristine magnetopause crossings have been made because most crossings have an associated magnetospheric boundary layer which significantly affects the applicable boundary conditions for the magnetopause current layer. Furthermore, almost no observational studies of magnetopause microstructure have been done even though key theoretical issues have been discussed for over two decades. This is because plasma instruments deployed prior to the ISEE and AMPTE missions did not have the required time resolution and most ISEE investigations to-date have focused on tests of MHD plasma models, especially reconnection. More recently, many phenomenological and theoretical models have been developed to explain the existence and characteristics of the magnetospheric boundary layers with only limited success to date. The cases with no boundary layer treated in this study provide a contrary set of conditions to those observed with a boundary layer. For the measured parameters of such cases, a successful boundary layer model should predict no plasma penetration across the magnetopause. Thus, this research project provides the first direct observational tests of magnetopause models using pristine magnetopause crossings and provides important new results on magnetopause microstructure and associated kinetic processes.
Boundary layer flow and heat transfer past a shrinking sheet in a copper-water nanofluid
NASA Astrophysics Data System (ADS)
Aleng, Nur Liyana; Bachok, Norfifah; Arifin, Norihan Md.; Ishak, Anuar
2014-06-01
The problem of laminar fluid flow which results from the shrinking of a flat surface in a water-based copper (Cu) nanofluid is considered in this study. The model used for the nanofluid incorporates the effect of the nanoparticles volume fraction. The governing partial differential equations are transformed into ordinary differential equations by similarity transformations. The transformed equations are solved numerically by using a shooting method. Results for the skin friction coefficient, local Nusselt number, velocity profiles and temperature profiles are presented for different values of the governing parameters. The analysis reveals the conditions for the existence of the steady boundary layer flow due to shrinking of the sheet and it is found that when the mass suction parameter exceeds a certain critical value, steady flow is possible. Dual solutions for the velocity and temperature distributions are obtained. With increasing values of the nanoparticles volume fraction, the skin friction and the heat transfer coefficient increase.
NASA Technical Reports Server (NTRS)
Keener, E. R.; Polek, T. E.
1972-01-01
The purpose of the described experiment was to obtain simultaneous measurements of skin friction and heat transfer at hypersonic Mach numbers, especially with conditions of considerable heat transfer. The experiment was conducted in air in the Ames Hypersonic Wind Tunnel, in which cold air was passed through an alumina storage heater system and heated to total temperatures ranging from about 670 to 1170 K. The nozzle was contoured to produce a flow at Mach 7.4. Thin-skin heat-transfer gauges were placed at equal intervals along the centerline of a sharp-edge flat plate, which was used as the model. A skin-friction balance and a boundary-layer pitot-pressure rake were mounted on each side of the centerline at a certain distance from the leading edge.
An experimental investigation of turbulent boundary layers at high Mach number and Reynolds numbers
NASA Technical Reports Server (NTRS)
Holden, M. S.
1972-01-01
Skin friction, heat transfer and pressure measurements were obtained in laminar, transitional and turbulent boundary layers on flat plates at Mach numbers from 7 to 13 at wall-to-free stream stagnation temperature ratios from 0.1 to 0.3. Measurements in laminar flows were in excellent agreement with the theory of Cheng. Correlations of the transition measurements with measurements on flight vehicles and in ballistic ranges show good agreement. Our transition measurements do not correlate well with those of Pate and Schueler. Comparisons have been made between the skin friction and heat transfer measurements and the theories of Van Driest, Eckert and Spalding and Chi. These comparisons reveal in general that at the high end of our Mach number range (10-13) the theory of Van Driest is in best agreement with the data, whereas at lower Mach numbers (6.5-10) the Spalding Chi theory is in better agreement with the measurements.
A boundary-layer model for Mars - Comparison with Viking lander and entry data
NASA Astrophysics Data System (ADS)
Haberle, R. M.; Houben, H. C.; Hertenstein, R.; Herdtle, T.
1993-06-01
A 1D boundary-layer model of Mars based on a momentum equation that describes friction, pressure gradient, and Coriolis forces is presented. Frictional forces and convective heating are computed using the level-2 turbulence closure theory of Mellor and Yamada (1974). The model takes into account the radiative effects of CO2 gas and suspended dust particles. Both radiation and convection depend on surface temperatures which are computed from a surface heat budget. Model predictions are compared with available observations from Viking landers. It is concluded that, in general, the model reproduces the basic features of the temperature data. The agreement is particularly good at entry time for the V L-2 site, where the model and observations are within several degrees at all levels for which data are available.
Estimating the Monin-Obukhov length in the stable boundary layer for dispersion calculations
NASA Astrophysics Data System (ADS)
Venkatram, A.
1980-12-01
Analysis of data collected during the Prairie Grass, Kansas and Minnesota experiments reveals the following empirical relationship between the Monin-Obukhov length L and the friction velocity u *: L = Au * 2, A = 1.1 103s2m-1. This result combined with the formulation for the height of the stable boundary layer h suggested by Zilitinkevich (1972) leads to h ? u * 3/2 f-1/2 where f is the Coriolis parameter. Data from the Minnesota study (Caughey et al., 1979) provide ample support for this expression. These empirical equations for L and h are useful for routine dispersion estimates during stable conditions.
Control and Identification of Turbulent Boundary Layer Separation
NASA Technical Reports Server (NTRS)
Seifert, Avi; Pack-Melton, La Tunia
2004-01-01
Effective delay of turbulent boundary layer separation could be achieved via closed-loop control. Constructing such a system requires that sensor data be processed, real-time, and fed into the controller to determine the output. Current methods for detection of turbulent boundary layer separation are lacking the capability of localized, fast and reliable identification of the boundary layer state. A method is proposed for short-time FFT processing of time series, measured by hot-film sensors, with the purpose of identifying the alternation of the balance between small and large scales as the boundary layer separates, favoring the large scales. The method has been validated by comparison to other criteria of separation detection and over a range of baseline and controlled flow conditions on a simplified high-lift system, incorporating active flow control.
ATMOSPHERIC DISPERSION MODELING BASED UPON BOUNDARY LAYER PARAMETERIZATION
Characteristic scaling parameters in the planetary boundary layer have been applied to estimate the dispersion of nonbuoyant gaseous pollutants. Vertical and lateral spread are treated separately, and the choice of parameters for the dispersion models depends upon the actual stat...
Calculations of unsteady turbulent boundary layers with flow reversal
NASA Technical Reports Server (NTRS)
Nash, J. F.; Patel, V. C.
1975-01-01
The results are presented of a series of computational experiments aimed at studying the characteristics of time-dependent turbulent boundary layers with embedded reversed-flow regions. A calculation method developed earlier was extended to boundary layers with reversed flows for this purpose. The calculations were performed for an idealized family of external velocity distributions, and covered a range of degrees of unsteadiness. The results confirmed those of previous studies in demonstrating that the point of flow reversal is nonsingular in a time-dependent boundary layer. A singularity was observed to develop downstream of reversal, under certain conditions, accompanied by the breakdown of the boundary-layer approximations. A tentative hypothesis was advanced in an attempt to predict the appearance of the singularity, and is shown to be consistent with the calculated results.
Investigation of the Stable Atmospheric Boundary Layer at Halley Antarctica
NASA Astrophysics Data System (ADS)
Rodrigo, Javier Sanz; Anderson, Philip S.
2013-09-01
Boundary-layer measurements from the Brunt Ice Shelf, Antarctica are analyzed to determine flux-profile relationships. Dimensionless quantities are derived in the standard approach from estimates of wind shear, potential temperature gradient, Richardson number, eddy diffusivities for momentum and heat, Prandtl number, mixing length and turbulent kinetic energy. Nieuwstadt local scaling theory for the stable atmospheric boundary-layer appears to work well departing only slightly from expressions found in mid-latitudes. An - single-column model of the stable boundary layer is implemented based on local scaling arguments. Simulations based on the first GEWEX Atmospheric Boundary-Layer Study case study are validated against ensemble-averaged profiles for various stability classes. A stability-dependent function of the dimensionless turbulent kinetic energy allows a better fit to the ensemble profiles.
Boundary-layer transition effects on airplane stability and control
NASA Technical Reports Server (NTRS)
Van Dam, C. P.; Holmes, B. J.
1986-01-01
Surface contamination of laminar-flow airfoils can significantly modify the location of transition from laminar-to-turbulent boundary-layer flow. The contamination can be the result of insect debris, environmental effects such as ice crystals and moisture due to mist or rain, surface damage, or other contamination adhering to the surface. Location and mode of transition have a dominant effect on the lift-and-drag characteristics of a lifting surface. The influences of laminar boundary-layer flow behavior on airplane stability and control are examined through theoretical results and experimental (wind-tunnel and free-flight) data. For certain airfoils with a relatively steep pressure recovery it is shown that loss of laminar flow near the leading edge can result in premature separation of the turbulent boundary layer and, consequently, in loss of lift and control effectiveness. Aerodynamic modifications which minimize boundary-layer transition effects on airplane stability and control are also discussed.
The current structure of stratified tidal planetary boundary layer flow
Myrhaug, D.; Slaattelid, O.H.
1995-12-31
The paper presents the bottom shear stress and velocity profiles in stratified tidal planetary boundary layer flow by using similarity theory. For a given seabed roughness length, free stream current velocity components, frequency of tidal oscillation, Coriolis parameter and stratification parameter the maximum bottom shear stress is determined for flow conditions in the rough, smooth and transitional smooth-to-rough turbulent regime. Further, the direction of the bottom shear stress and the velocity profiles are given. Comparison is made with data from field measurements of time-independent as well as tidal planetary boundary layer flow for neutral conditions, and the agreement between the predictions and the data is generally good. Further, an example of application for stable stratification is given, and qualitatively the predictions show, as expected, that the bottom shear stress and the thickness of the boundary layer become smaller for stable than for neutral stratification. Other features of the tidal planetary boundary layer flow are also discussed.
Interacting turbulent boundary layer over a wavy wall
NASA Technical Reports Server (NTRS)
Polak, A.; Werle, M. J.
1977-01-01
The two dimensional supersonic flow of a thick turbulent boundary layer over a train of relatively small wave-like protuberances is considered. The flow conditions and the geometry are such that there exists a strong interaction between the viscous and inviscid flow. The problem cannot be solved without inclusion of interaction effects due to the occurrence of the separation singularity in classical boundary layer methods. The interacting boundary layer equations are solved numerically using a time-like relaxation method with turbulence effects represented by the inclusion of the eddy viscosity model. Results are presented for flow over a train of up to six waves for Mach numbers of 10 and 32 million/meter, and wall temperature rations (T sub w/T sub 0) of 0.4 and 0.8. Limited comparisons with independent experimental and analytical results are also given. Detailed results on the influence of small protuberances on surface heating by boundary layers are presented.
Energy efficient engine, low-pressure turbine boundary layer program
NASA Technical Reports Server (NTRS)
Gardner, W. B.
1981-01-01
A study was conducted to investigate development of boundary layers under the influence of velocity distributions simulating the suction side of two state-of-the-art turbine airfoils: a forward loaded airfoil (squared-off design) and an aft loaded airfoil (aft-loaded design). These velocity distributions were simulated in a boundary layer wind tunnel. Detailed measurements of boundary layer mean velocity and turbulence intensity profiles were obtained for an inlet turbulence level of 2.4 percent and an exit Reynolds number of 800,000. Flush-mounted hot film probes identified the boundary layer transition regimes in the adverse pressure gradient regions for both velocity distributions. Wall intermittency data showed good agreement with the correlations of Dhawan and Narasimha for the intermittency factor distribution in transitional flow regimes.
Further studies of unsteady boundary layers with flow reversal
NASA Technical Reports Server (NTRS)
Nash, J. F.
1976-01-01
One set of calculations was performed using the first order, time dependent turbulent boundary layer equations, and extended earlier work by Nash and Patel to a wider range of flows. Another set of calculations was performed for laminar flow using the time dependent Navier-Stokes equations. The results of the calculations confirm previous conclusions concerning the existence of a regime of unseparated flow, containing an embedded region of reversal, which is accessible to first order boundary layer theory. However, certain doubts are cast on the precise nature of the events which accompany the eventual breakdown of the theory due to singularity onset. The earlier view that the singularity appears as the final event in a sequence involving rapid thickening of the boundary layer and the formation of a localized region of steep gradients is called into question by the present results. It appears that singularity onset is not necessarily preceded by rapid boundary layer thickening, or even necessarily produces immediate thickening.
NASA Astrophysics Data System (ADS)
nal, U?ur Oral; nal, Burcu; Atlar, Mehmet
2012-06-01
Whilst recent developments of nanotechnology are being exploited by chemists and marine biologists to understand how the completely environmentally friendly foul release coatings can control marine biofouling and how they can be developed further, the understanding of the hydrodynamic performances of these new generation coatings is being overlooked. This paper aims to investigate the relative boundary layer, roughness and drag characteristics of some novel nanostructured coatings, which were developed through a multi-European and multi-disciplined collaborative research project AMBIO (2010), within the framework of turbulent flows over rough surfaces. Zero-pressure-gradient, turbulent boundary layer flow measurements were conducted over flat surfaces coated with several newly developed nanostructured antifouling paints, along with some classic reference surfaces and a state-of-the-art commercial coating, in the Emerson Cavitation Tunnel (ECT) of Newcastle University. A large flat plane test bed that included interchangeable flat test sections was used for the experiments. The boundary layer data were collected with the aid of a two-dimensional DANTEC Laser Doppler Velocimetry (LDV) system. These measurements provided the main hydrodynamic properties of the newly developed nanostructured coatings including local skin friction coefficients, roughness functions and Reynolds stresses. The tests and subsequent analysis indicated the exceptionally good frictional properties of all coatings tested, in particular, the drag benefit of some new nanostructured coatings in the Reynolds number range investigated. The rapidly decreasing roughness function trends of AKZO19 and AKZO20 as the ks^{ + } increases were remarkable along with the dissimilar roughness function character of all tested coatings to the well-known correlation curves warranting further research at higher Reynolds numbers. The wall similarity concept for the Reynolds stresses was only validated for the transitionally rough surfaces from (y + \\varepsilon)^{ + } ? 100 up to the end of the boundary layer.
Pollutant Plume Dispersion in the Atmospheric Boundary Layer over Idealized Urban Roughness
NASA Astrophysics Data System (ADS)
Wong, Colman C. C.; Liu, Chun-Ho
2013-05-01
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness.
On stability of free laminar boundary layer between parallel streams
NASA Technical Reports Server (NTRS)
Lessen, Martin
1950-01-01
An analysis and calculations on the stability of the free laminar boundary layer between parallel streams were made for an incompressible fluid using the Tollmien-Schlichting theory of small disturbances. Because the boundary conditions are at infinity, two solutions of the Orr-Sommerfeld stability equations need not be considered, and the remaining two solutions are exponential in character at the infinite boundaries. The calculations show that the flow is unstable except for very low Reynolds numbers.
Lecture Series "Boundary Layer Theory". Part I - Laminar Flows. Part 1; Laminar Flows
NASA Technical Reports Server (NTRS)
Schlichting, H.
1949-01-01
In the lecture series starting today author want to give a survey of a field of aerodynamics which has for a number of years been attracting an ever growing interest. The subject is the theory of flows with friction, and, within that field, particularly the theory of friction layers, or boundary layers. A great many considerations of aerodynamics are based on the ideal fluid, that is the frictionless incompressibility and fluid. By neglect of compressibility and friction the extensive mathematical theory of the ideal fluid, (potential theory) has been made possible. Actual liquids and gases satisfy the condition of incomressibility rather well if the velocities are not extremely high or, more accurately, if they are small in comparison with sonic velocity. For air, for instance, the change in volume due to compressibility amounts to about 1 percent for a velocity of 60 meters per second. The hypothesis of absence of friction is not satisfied by any actual fluid; however, it is true that most technically important fluids, for instance air and water, have a very small friction coefficient and therefore behave in many cases almost like the ideal frictionless fluid. Many flow phenomena, in particular most cases of lift, can be treated satisfactorily, - that is, the calculations are in good agreement with the test results, -under the assumption of frictionless fluid. However, the calculations with frictionless flow show a very serious deficiency; namely, the fact, known as d'Alembert's paradox, that in frictionless flow each body has zero drag whereas in actual flow each body experiences a drag of greater or smaller magnitude. For a long time the theory has been unable to bridge this gap between the theory of frictionless flow and the experimental findings about actual flow. The cause of this fundamental discrepancy is the viscosity which is neglected in the theory of ideal fluid; however, in spite of its extraordinary smallness it is decisive for the course of the flow phenomena.
Report of secondary flows, boundary layers, turbulence and wave team
NASA Technical Reports Server (NTRS)
Doviak, R.
1980-01-01
Correspondence concerning the comparison of horizontal wind fields, two dimensional spectra, heat flux, mesoscale divergence and deformation in the prestorm environment, and thunderstorm gust front winds is presented. Other subjects include the use of radar to determine heating rate and evaporation near the Earth's surface for an unstable boundary layer and statistical considerations in the estimation of wind fields from single Doppler radar and application to prestorm boundary layer observations.
Tropical boundary layer equilibrium in the last ice age
NASA Technical Reports Server (NTRS)
Betts, Alan K.; Ridgway, W.
1992-01-01
A radiative-convective boundary layer model is used to assess the effect of changing sea surface temperature, pressure, wind speed, and the energy export from the tropics on the boundary layer equilibrium equivalent potential temperature. It remains difficult to reconcile the observations that during the last glacial maximum (18,000 yr BP) the snowline on the tropical mountains fell 950 m, while the tropical sea surface temperatures fell only 1-2 K.
Shock wave oscillation driven by turbulent boundary layer fluctuations
NASA Technical Reports Server (NTRS)
Plotkin, K. J.
1972-01-01
Pressure fluctuations due to the interaction of a shock wave with a turbulent boundary layer were investigated. A simple model is proposed in which the shock wave is convected from its mean position by velocity fluctuations in the turbulent boundary layer. Displacement of the shock is assumed limited by a linear restoring mechanism. Predictions of peak root mean square pressure fluctuation and spectral density are in excellent agreement with available experimental data.
Acoustic sources in the low Mach number turbulent boundary layer
NASA Technical Reports Server (NTRS)
Hardin, Jay C.
1991-01-01
The sources of sound production in a low Mach number turbulent boundary layer are examined. The sources are shown to be quadrupole in nature and to result from supersonically convecting wave-number components of the fluctuating Reynolds' normal stresses. The primary Tollmien-Schlichting instability of the boundary layer is found to radiate no sound. Analysis of various vortical phenomena suggests that the primary source is the process of formation of horseshoe vortices, with viscous sublayer bursts a possible secondary source.
Separating and turbulent boundary layer calculations using polynomial interpretation
NASA Technical Reports Server (NTRS)
Rubin, S. G.; Rivera, S.
1977-01-01
Higher order numerical methods derived from polynomial spline interpolation or Hermitian differencing are applied to a separating laminar boundary layer, i.e., the Howarth problem, and the turbulent flat plate boundary layer flow. Preliminary results are presented. It is found that accuracy equal to that of conventional second order accurate finite difference methods is achieved with many fewer mesh points and with reduced computer storage and time requirements.
Nanoindentation of the surface layer of Hadfield's steel after sliding friction
NASA Astrophysics Data System (ADS)
Kolubaev, A. V.; Kolubaev, E. A.; Sizova, O. V.
2007-12-01
We have measured the nanohardness of a deformed near-surface layer of Hadfields steel upon friction testing. The phenomenon of shape recovery upon indentation has been observed, which is retained for several days after tribological tests. It s suggested that the strained material exhibits behavior analogous to nonlinear elasticity.
Integral-matrix procedure for boundary-layer problems
NASA Technical Reports Server (NTRS)
Gross, K. W.; Evans, R. M.
1977-01-01
Program, BLIMP, provides fast, highly accurate solution to general class of gas-phase boundary layer flow problems encompassing broad range of boundary conditions. Program is capable of obtaining accurate and economical solutions to governing differential equations of momentum, energy, and species.
NASA Astrophysics Data System (ADS)
Kornilov, V. I.; Boiko, A. V.; Kavun, I. N.
2015-11-01
The characteristics of an incompressible turbulent boundary layer on a flat plate with air blown in though a finely perforated surface from an external confined flow through an input device, located on the "idle" side of the plate, have been investigated experimentally and numerically. A stable decrease in the local values of the coefficient of surface friction along the plate length that attains 85% at the end of the perforated portion is shown. The experimental and calculated data obtained point to the possibility of modeling, under earth conditions, the process of controlling a turbulent boundary layer with air injection by using the resources of an external confined flow.
Self-similar analysis of fluid flow and heat-mass transfer of nanofluids in boundary layer
NASA Astrophysics Data System (ADS)
Avramenko, A. A.; Blinov, D. G.; Shevchuk, I. V.
2011-08-01
Processes of heat, momentum, and concentration transport in a boundary layer of a nanofluid near a flat wall were studied. The study was performed by means of numerical analysis of boundary layer equations in a self-similar form. Self-similar forms of these equations were obtained based on symmetry properties (Lie groups). In doing so, dependence of physical properties (viscosity, thermal conductivity, and diffusion coefficient) on concentration of nanofluids and temperature were taken into account. Effects of concentration of the nano-particles on velocity and temperature profiles, as well as on the relative Nusselt numbers and skin-friction coefficients, were elucidated.
Turbulent boundary layer in high Rayleigh number convection in air.
du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, Andr; Willert, Christian
2014-03-28
Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bnard experiment are presented for the Rayleigh number Ra=1.41010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re?200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bnard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal. PMID:24724653
Development of instrumentation for boundary layer transition detection
NASA Astrophysics Data System (ADS)
Harrison, Steven B.
A steady state heat transfer technique is developed and evaluated for detecting boundary layer transition on a flat plate in incompressible flow. The method involves adhering encapsulated temperature sensitive liquid crystals to a constant heat flux surface. A heater composed of unidirectional carbon fibers is developed and tested with the aim of in-flight boundary layer transition detection on a natural laminar flow nacelle. The individual and combined influences of surface heating and favorable pressure gradient on boundary layer transition are considered. Heating is found to be a destabilizing influence on the boundary layer, while a favorable pressure gradient is a stabilizing influence. A Stanton number correlates the movement of transition with heating for zero pressure gradient flat plate boundary layers. No similar correlation is found when a favorable pressure gradient accompanies the heating. Heating is more destabilizing in a favorable pressure gradient than in zero pressure gradient. Shear sensitive liquid crystal is used to detect transition and to obtain, for the first time, quantitative measurements of surface shear stress in a transitional boundary layer. This involves calibrating the time required for the liquid crystal film to experience a texture change when subjected to a shearing force. Using this technique, shear sensitive liquid crystal shows promise for full coverage measurement of surface shear stress in low speed flows.
Orbiter Boundary Layer Transition Stability Modeling at Flight Entry Conditions
NASA Technical Reports Server (NTRS)
Bartkowicz, Matt; Johnson, Heath; Candler, Graham; Campbell, Charles H.
2009-01-01
State of the art boundary layer stability modeling capabilities are increasingly seeing application to entry flight vehicles. With the advent of user friendly and robust implementations of two-dimensional chemical nonequilibrium stability modeling with the STABL/PSE-CHEM software, the need for flight data to calibrate such analyses capabilities becomes more critical. Recent efforts to perform entry flight testing with the Orbiter geometry related to entry aerothermodynamics and boundary layer transition is allowing for a heightened focus on the Orbiter configuration. A significant advancement in the state of the art can likely be achieved by establishing a basis of understanding for the occurrence of boundary layer transition on the Orbiter due to discrete protruding gap fillers and the nominal distributed roughness of the actual thermal protection system. Recent success in demonstrating centerline two-dimensional stability modeling on the centerline of the Orbiter at flight entry conditions provides a starting point for additional investigations. The more detailed paper will include smooth Orbiter configuration boundary layer stability results for several typical orbiter entry conditions. In addition, the numerical modeling approach for establishing the mean laminar flow will be reviewed and the method for determining boundary layer disturbance growth will be overviewed. In addition, if actual Orbiter TPS surface data obtained via digital surface scans become available, it may be possible to investigate the effects of an as-flown flight configuration on boundary layer transition compared to a smooth CAD reference.
Dense gas boundary layer experiments: Visualization, pressure measurements, concentration evaluation
Reichenbach, H.; Neuwald, P.; Kuhl, A.L.
1992-11-01
This technical report describes methods that were applied to investigate turbulent boundary layers generated by inviscid, baroclinic effects. The Cranz-Schardin 24-sparks camera was used to visualize the interactions of a planar shock wave with a Freon R12-layer. The shock propagates more slowly in the Freon layer than in air because of its smaller sound speed. This causes the shock front to be curved and to be reflected between the wall and the layer interface. As a consequence of the reflection process, a series of compression and expansion waves radiate from the layer. Large fluctuations in the streamwise velocity and in pressure develop for about 1 ms. These waves strongly perturb the interface shear layer, which rapidly transitions to a turbulent boundary flow. Pressure measurements showed that the fluctuations in the Freon layer reach a peak pressure 4 times higher than in the turbulent boundary flow. To characterize the preshock Freon boundary layer, concentration measurements were performed with a differential interferometry technique. The refraction index of Freon R12 is so high that Mach-Zehnder interferometry was not successful in these experiments. The evaluation of the concentration profile is described here in detail. Method and results of corresponding LDV measurements under the same conditions are presented in a different report, EMI Report T 9/92. The authors plan to continue the dense gas layer investigations with the gas combination helium/Freon.
Analysis and Modeling of Boundary Layer Separation Method (BLSM).
Peth?, Dra; Horvth, Gza; Liszi, Jnos; Tth, Imre; Paor, Dvid
2010-09-01
Nowadays rules of environmental protection strictly regulate pollution material emission into environment. To keep the environmental protection laws recycling is one of the useful methods of waste material treatment. We have developed a new method for the treatment of industrial waste water and named it boundary layer separation method (BLSM). We apply the phenomena that ions can be enriched in the boundary layer of the electrically charged electrode surface compared to the bulk liquid phase. The main point of the method is that the boundary layer at correctly chosen movement velocity can be taken out of the waste water without being damaged, and the ion-enriched boundary layer can be recycled. Electrosorption is a surface phenomenon. It can be used with high efficiency in case of large electrochemically active surface of electrodes. During our research work two high surface area nickel electrodes have been prepared. The value of electrochemically active surface area of electrodes has been estimated. The existence of diffusion part of the double layer has been experimentally approved. The electrical double layer capacity has been determined. Ion transport by boundary layer separation has been introduced. Finally we have tried to estimate the relative significance of physical adsorption and electrosorption. PMID:24061827
NASA Technical Reports Server (NTRS)
Oliver, A. B.; Lillard, R. P.; Blaisdell, G. A.; Lyrintizis, A. S.
2006-01-01
The capability of the OVERFLOW code to accurately compute high-speed turbulent boundary layers and turbulent shock-boundary layer interactions is being evaluated. Configurations being investigated include a Mach 2.87 flat plate to compare experimental velocity profiles and boundary layer growth, a Mach 6 flat plate to compare experimental surface heat transfer,a direct numerical simulation (DNS) at Mach 2.25 for turbulent quantities, and several Mach 3 compression ramps to compare computations of shock-boundary layer interactions to experimental laser doppler velocimetry (LDV) data and hot-wire data. The present paper describes outlines the study and presents preliminary results for two of the flat plate cases and two small-angle compression corner test cases.
NASA Technical Reports Server (NTRS)
Anderson, P. S.; Kays, W. M.; Moffat, R. J.
1972-01-01
An experimental investigation of transpired turbulent boundary layers in zero and adverse pressure gradients has been carried out. Profiles of: (1) the mean velocity, (2) the three intensities of the turbulent fluctuations, and (3) the Reynolds stress were obtained by hot-wire anemometry. The friction coefficients were measured by using an integrated form of the boundary layer equation to extrapolate the measured shear stress profiles to the wall.
NASA Technical Reports Server (NTRS)
Stallings, R. L., Jr.; Lamb, M.
1980-01-01
The wind tunnel tests were conducted both with and without boundary layer trips at Mach 3 and nominal free stream Reynolds numbers per meter ranging from 3.3 x 10 the 6th power. Instrumentation consisted of pressure orifices, thermocouples, a boundary layer pitot pressure rake, and a floating element skin friction balance. Measurements from both wind tunnel and flight were compared with existing engineering prediction methods.
Wet but not slippery: boundary friction in tree frog adhesive toe pads
Federle, W; Barnes, W.J.P; Baumgartner, W; Drechsler, P; Smith, J.M
2006-01-01
Tree frogs are remarkable for their capacity to cling to smooth surfaces using large toe pads. The adhesive skin of tree frog toe pads is characterized by peg-studded hexagonal cells separated by deep channels into which mucus glands open. The pads are completely wetted with watery mucus, which led previous authors to suggest that attachment is solely due to capillary and viscous forces generated by the fluid-filled joint between the pad and the substrate. Here, we present evidence from single-toe force measurements, laser tweezer microrheometry of pad mucus and interference reflection microscopy of the contact zone in Litoria caerulea, that tree frog attachment forces are significantly enhanced by close contacts and boundary friction between the pad epidermis and the substrate, facilitated by the highly regular pad microstructure. PMID:16971337
ON AERODYNAMIC AND BOUNDARY LAYER RESISTANCES WITHIN DRY DEPOSITION MODELS
There have been many empirical parameterizations for the aerodynamic and boundary layer resistances proposed in the literature, e.g. those of the Meyers Multi-Layer Deposition Model (MLM) used with the nation-wide dry deposition network. Many include arbitrary constants or par...
Direct Numerical Simulation of Supersonic Turbulent Boundary Layers
NASA Astrophysics Data System (ADS)
Guarini, Stephen; Moser, Robert; Shariff, Karim; Wray, Alan
1997-11-01
Initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers will be presented. The spatially developing boundary layer is first transformed to a parallel shear layer using a transformation similar to that used by Spalart for an incompressible boundary layer. This allows us to avoid inflow and outflow boundary conditions, and to apply periodic boundary conditions in the streamwise and spanwise directions. The resulting equations are then solved using a mixed Fourier B-spline Galerkin method. One challenge to these highly accurate and non-dissipative numerics has been the occurrence of sharp density gradients, which require significantly more resolution than the incompressible case, especially during transients. The first simulation is at Mach 2.5 with a momentum thickness Reynolds number based on wall viscosity of R_?'=825. The simulations are used to examine the physics of the compressible boundary layer and to compute turbulence statistics and terms in the budget equations. The turbulence statistics include: rms and mean profiles, energy spectra, and two-point correlations.
Numerical Studies of Boundary-Layer Receptivity
NASA Technical Reports Server (NTRS)
Reed, Helen L.
1995-01-01
Direct numerical simulations (DNS) of the acoustic receptivity process on a semi-infinite flat plate with a modified-super-elliptic (MSE) leading edge are performed. The incompressible Navier-Stokes equations are solved in stream-function/vorticity form in a general curvilinear coordinate system. The steady basic-state solution is found by solving the governing equations using an alternating direction implicit (ADI) procedure which takes advantage of the parallelism present in line-splitting techniques. Time-harmonic oscillations of the farfield velocity are applied as unsteady boundary conditions to the unsteady disturbance equations. An efficient time-harmonic scheme is used to produce the disturbance solutions. Buffer-zone techniques have been applied to eliminate wave reflection from the outflow boundary. The spatial evolution of Tollmien-Schlichting (T-S) waves is analyzed and compared with experiment and theory. The effects of nose-radius, frequency, Reynolds number, angle of attack, and amplitude of the acoustic wave are investigated. This work is being performed in conjunction with the experiments at the Arizona State University Unsteady Wind Tunnel under the direction of Professor William Saric. The simulations are of the same configuration and parameters used in the wind-tunnel experiments.
Feasibility study of optical boundary layer transition detection method
NASA Technical Reports Server (NTRS)
Azzazy, M.; Modarress, D.; Trolinger, J. D.
1986-01-01
A high sensitivity differential interferometer was developed to locate the region where the boundary layer flow undergoes transition from laminar to turbulent. Two laboratory experimental configurations were used to evaluate the performance of the interferometer: open shear layer, and low speed wind tunnel turbulent spot configuration. In each experiment, small temperature fluctuations were introduced as the signal source. Simultaneous cold wire measurements were compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations in the order of 0.001 the laser wavelength. An attempt to detect boundary layer transition over a flat plate at NASA-Langley Unitary Supersonic Wind Tunnel using the interferometer system was performed. The phase variations during boundary layer transition in the supersonic wind tunnel were beyond the minimum signal-to-noise level of the instrument.
Vortex Generators to Control Boundary Layer Interactions
NASA Technical Reports Server (NTRS)
Babinsky, Holger (Inventor); Loth, Eric (Inventor); Lee, Sang (Inventor)
2014-01-01
Devices for generating streamwise vorticity in a boundary includes various forms of vortex generators. One form of a split-ramp vortex generator includes a first ramp element and a second ramp element with front ends and back ends, ramp surfaces extending between the front ends and the back ends, and vertical surfaces extending between the front ends and the back ends adjacent the ramp surfaces. A flow channel is between the first ramp element and the second ramp element. The back ends of the ramp elements have a height greater than a height of the front ends, and the front ends of the ramp elements have a width greater than a width of the back ends.
NASA Technical Reports Server (NTRS)
Rumsey, C. L.
2009-01-01
The ability of kappa-omega models to predict compressible turbulent skin friction in hypersonic boundary layers is investigated. Although uncorrected two-equation models can agree well with correlations for hot-wall cases, they tend to perform progressively worse - particularly for cold walls - as the Mach number is increased in the hypersonic regime. Simple algebraic models such as Baldwin-Lomax perform better compared to experiments and correlations in these circumstances. Many of the compressibility corrections described in the literature are summarized here. These include corrections that have only a small influence for kappa-omega models, or that apply only in specific circumstances. The most widely-used general corrections were designed for use with jet or mixing-layer free shear flows. A less well-known dilatation-dissipation correction intended for boundary layer flows is also tested, and is shown to agree reasonably well with the Baldwin-Lomax model at cold-wall conditions. It exhibits a less dramatic influence than the free shear type of correction. There is clearly a need for improved understanding and better overall physical modeling for turbulence models applied to hypersonic boundary layer flows.
Turbulent boundary layers: Inflow effects and cross-validation of simulation and experiment
NASA Astrophysics Data System (ADS)
Oerlue, Ramis; Schlatter, Philipp
2011-11-01
A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & Örlü, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. Several DNS of a zero pressure-gradient (ZPG) turbulent boundary layer (TBL) à la Schlatter et al. [Phys. Fluids 21, 051702 (2009)] with physically different inflow conditions and tripping effects were performed. Most of the differences observed when comparing available DNS could thereby be traced back to different initial conditions. It was also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Reθ < 300, all data agree well for both inner and outer layer for Reθ > 2000 a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments. Based on these results a detailed comparison between DNS and experiment of a ZPG TBL flow at Reθ = 2500 and 4000 is presented. Good agreement is obtained for integral quantities, mean and fluctuating streamwise velocity profiles, but also for the probability distribution and spectral map throughout the boundary layer.
NASA Astrophysics Data System (ADS)
Gupta, S.; Heintzman, E.; Jasinski, J.
2014-09-01
We report on the surface properties (friction and work function) of micromechanically cleaved graphene layers placed on thermally gown thick insulating (295 nm of SiO2) films on commercial Si (001) substrates. By employing atomic force microscopy (AFM) and scanning electron microscopy with varying primary-electron acceleration voltage ( V acc) in secondary electron imaging (SEI) mode, we determined the coefficient of friction ( ?) and electronic work function ( ?), respectively, as functions of the number of graphene layers ( n). The friction coefficient was deduced from line scans of friction maps obtained simultaneously while measuring AFM topography. The findings show that supported mono-, bi-, and trilayer graphene all yield similar results (0.03), in contrast to multilayer (0.027) and thicker graphite (0.015) flakes. From the SEI contrast variation, we obtained a reproducible discrete distribution of SE intensity stemming from atomically thick graphene layers on a thick insulating substrate. We were able to determine the number of graphene layers (i.e., n) from the SE intensity contrast or the SE intensity itself. Moreover, we found a distinct linear relationship between the relative SE intensity from the graphene layers and their number, provided a relatively lower V acc was used. The different contrast in SEI micrographs at lower V acc is attributed to the fact that the generation of secondary electrons emitted from the graphene was affected by the different work functions corresponding to different n values (or thickness contrast, C). This simple and facile method is superior to the conventional optical method in its capability to characterize graphene over sub-1- ?m2 areas on various insulating substrates. These results are supplemented by optical microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy and Raman mapping that yield the structural quality (or disorder) of the graphene layers, albeit semiquantitatively.
Nature, theory and modelling of geophysical convective planetary boundary layers
NASA Astrophysics Data System (ADS)
Zilitinkevich, Sergej
2015-04-01
Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in horizontal branches of organised structures. This mechanism (Zilitinkevich et al., 2006), was overlooked in conventional local theories, such as the Monin-Obukhov similarity theory, and convective heat/mass transfer law: Nu~Ra1/3, where Nu and Ra are the Nusselt number and Raleigh numbers. References Hellsten A., Zilitinkevich S., 2013: Role of convective structures and background turbulence in the dry convective boundary layer. Boundary-Layer Meteorol. 149, 323-353. Zilitinkevich, S.S., 1973: Shear convection. Boundary-Layer Meteorol. 3, 416-423. Zilitinkevich, S.S., 1991: Turbulent Penetrative Convection, Avebury Technical, Aldershot, 180 pp. Zilitinkevich S.S., 2012: The Height of the Atmospheric Planetary Boundary layer: State of the Art and New Development - Chapter 13 in 'National Security and Human Health Implications of Climate Change', edited by H.J.S. Fernando, Z. Klaić, J.L. McKulley, NATO Science for Peace and Security Series - C: Environmental Security (ISBN 978-94-007-2429-7), Springer, 147-161. Zilitinkevich S.S., 2013: Atmospheric Turbulence and Planetary Boundary Layers. Fizmatlit, Moscow, 248 pp. Zilitinkevich, S.S., Hunt, J.C.R., Grachev, A.A., Esau, I.N., Lalas, D.P., Akylas, E., Tombrou, M., Fairall, C.W., Fernando, H.J.S., Baklanov, and A., Joffre, S.M., 2006: The influence of large convective eddies on the surface layer turbulence. Quart. J. Roy. Met. Soc. 132, 1423-1456. Zilitinkevich S.S., Tyuryakov S.A., Troitskaya Yu. I., Mareev E., 2012: Theoretical models of the height of the atmospheric planetary boundary layer and turbulent entrainment at its upper boundary. Izvestija RAN, FAO, 48, No.1, 150-160 Zilitinkevich, S.S., Elperin, T., Kleeorin, N., Rogachevskii, I., Esau, I.N., 2013: A hierarchy of energy- and flux-budget (EFB) turbulence closure models for stably stratified geophysical flows. Boundary-Layer Meteorol. 146, 341-373.
The Boundary Layer Late Afternoon and Sunset Turbulence Project
NASA Astrophysics Data System (ADS)
Lothon, Marie; Lohou, Fabienne; Darbieu, Clara; Couvreux, Fleur; Pino, David; Blay, Estel; Vila-Guerau de Arellano, Jordi; Pietersen, Henk; Hartogensis, Oscar; Pardyjak, Eric; Alexander, Daniel; Reuder, Joachim; Baaserud, Line; Nilsson, Erik; Jimenez, Maria Antonia; Faloona, Ian; Sastre-Marugan, Mariano; Angevine, Wayne M.; Canut, Guylaine; Bazile, Eric
2014-05-01
The BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) project aims at better understanding the turbulence processes which occur during the transition from a well-mixed convective boundary layer to a residual layer overlying a stabilized nocturnal layer. This phase of the diurnal cycle is challenging from both modeling and observational perspectives: it is transitory, most of the forcings are small or null during the transition and the turbulence regime changes from the fully convective regime of turbulence, close to homogeneous and isotropic, toward more heterogeneous and intermittent turbulence during its decay. Those issues motivated a field campaign that was conducted from 14 June to 8 July 2011 in southern France in complex terrain and consisted of a range of integrated instrument platforms including: full-size aircraft, Remotely Piloted Airplane Systems (RPAS), remote sensing instruments, radiosoundings, tethered balloons, surface flux stations, and various meteorological towers deployed over different surface covers. The boundary layer, from the earth's surface to free troposphere was densely probed during the entire day, with a focus and intense observations from midday until sunset. The field dataset now forms the base of a set of studies utilizing the observations and several types of models including: Large Eddy Simulation, Mesoscale models, forecast models. The presentation will expose an overview of this experiment and of the current observational and modeling studies, with the focus on: the turbulence decay process within the entire boundary layer from surface to the top, the mesoscale forcings of importance during BLLAST, the ability of the forecast models to represent the diurnal cycle, the relevance of the Monin Obukhov similarity theory, and shallow drainage flows. Reference: Lothon M. et al., 2012. The Boundary-Layer Late Afternoon and Sunset Turbulence field experiment, Proc. of the 20th Symposium on Boundary-Layers and Turbulence, 7-13 July, Boston, MA, USA.
Diamagnetic boundary layers - A kinetic theory. [for collisionless magnetized plasmas
NASA Technical Reports Server (NTRS)
Lemaire, J.; Burlaga, L. F.
1976-01-01
A kinetic theory is presented for boundary layers associated with MHD tangential 'discontinuities' in a collisionless magnetized plasma, such as those observed in the solar wind. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found to have a thickness of the order of a few electron gyroradii, but the drift speed of the current-carrying electrons is found to exceed the Alfven speed, and accordingly such layers are not stable. Several types of layers in which the current is carried by protons are discussed; in particular, cases are considered in which the magnetic-field intensity, direction, or both, changed across the layer. In every case, the thickness was of the order of a few proton gyroradii, and the field changed smoothly, although the characteristics depended somewhat on the boundary conditions. The drift speed was always less than the Alfven speed, consistent with stability of such structures. These results are consistent with observations of boundary layers in the solar wind near 1 AU.
NASA Technical Reports Server (NTRS)
Boyle, R. J.; Haas, J. E.; Katsanis, T.
1984-01-01
A method for calculating turbine stage performance is described. The usefulness of the method is demonstrated by comparing measured and predicted efficiencies for nine different stages. Comparisons are made over a range of turbine pressure ratios and rotor speeds. A quasi-3D flow analysis is used to account for complex passage geometries. Boundary layer analyses are done to account for losses due to friction. Empirical loss models are used to account for incidence, secondary flow, disc windage, and clearance losses.
NASA Technical Reports Server (NTRS)
Sohn, Ki-Hyeon; Reshotko, Eli
1991-01-01
A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were acquired in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities (TI) from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for R(sub theta) is less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length. The measured laminar value of Reynolds analogy factor was as much as 53 percent higher than the Pr(sup -2/3). A small dependence of turbulent results on TI was observed. Conditional sampling performed in the transitional boundary layer indicated the existence of a near-wall drop in intermittency, pronounced at certain low intermittencies, which is consistent with the cross-sectional shape of turbulent spots observed by others. Non-turbulent intervals were observed to possess large magnitudes of near-wall unsteadiness and turbulent intervals had peak values as much as 50 percent higher than were measured at fully turbulent stations. Non-turbulent and turbulent profiles in transitional boundary layers cannot be simply treated as Blasius and fully turbulent profiles, respectively. The boundary layer spectra indicate predicted selective amplification of T-S waves for TI is approximately 0.4 percent. However, for TI is approximately 0.8 and 1.1 percent, T-S waves are localized very near the wall and do not play a dominant role in transition process.
NASA Astrophysics Data System (ADS)
Alkasasbeh, Hamzeh Taha; Salleh, Mohd Zuki; Tahar, Razman Mat; Nazar, Roslinda; Pop, Ioan
2014-06-01
In this paper, the mathematical model for free convection boundary layer flow in a micropolar fluid near the lower stagnation point of a solid sphere with convective boundary conditions, in which the heat is supplied through a bounding surface of finite thickness and finite heat capacity, is considered. The transformed and reduced boundary layer equations in the form of ordinary differential equations are solved numerically using an implicit finite difference scheme known as the Keller-box method. Numerical solutions are obtained for the local wall temperature and the local skin friction coefficient, as well as the velocity, angular velocity and temperature profiles. The features of the flow and heat transfer characteristics for different values of the material or micropolar parameter K, the Prandtl number Prand the conjugate parameter γare analyzed and discussed.
The role of adsorbed water on the friction of a layer of submicron particles
Sammis, Charles G.; Lockner, David A.; Reches, Ze’ev
2011-01-01
Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near μ = 0.6 at low slip speeds to a coefficient near μ = 0.3 at higher slip speeds is attributed to competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer enables the otherwise rough particles to rotate, the coefficient of friction will drop to μ = 0.3, appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100°C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to μ = 0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1 nm thick layer of adsorbed water, with a minimum particle radius of approximately 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and the range of velocities over which it occurs decrease with increasing normal stress, as predicted by the model. Moreover, the analysis predicts that this high-speed weakening mechanism should operate only for particles with radii smaller than approximately 1 μm. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.
Low-Friction Adsorbed Layers of a Triblock Copolymer Additive in Oil-Based Lubrication.
Yamada, Shinji; Fujihara, Ami; Yusa, Shin-ichi; Tanabe, Tadao; Kurihara, Kazue
2015-11-10
The tribological properties of the dilute solution of an ABA triblock copolymer, poly(11-acrylamidoundecanoic acid)-block-poly(stearyl methacrylate)-block-poly(11-acrylamidoundecanoic acid (A5S992A5), in poly(?-olefin) (PAO) confined between mica surfaces were investigated using the surface forces apparatus (SFA). Friction force was measured as a function of applied load and sliding velocity, and the film thickness and contact geometry during sliding were analyzed using the fringes of equal chromatic order (FECO) in the SFA. The results were contrasted with those of confined PAO films; the effects of the addition of A5S992A5 on the tribological properties were discussed. The thickness of the A5S992A5/PAO system varied with time after surface preparation and with repetitive sliding motions. The thickness was within the range from 40 to 70 nm 1 day after preparation (the Day1 film), and was about 20 nm on the following day (the Day2 film). The thickness of the confined PAO film was thinner than 1.4 nm, indicating that the A5S992A5/PAO system formed thick adsorbed layers on mica surfaces. The friction coefficient was about 0.03 to 0.04 for the Day1 film and well below 0.01 for the Day2 film, which were 1 or 2 orders of magnitude lower than the values for the confined PAO films. The time dependent changes of the adsorbed layer thickness and friction properties should be caused by the relatively low solubility of A5S992A5 in PAO. The detailed analysis of the contact geometry and friction behaviors implies that the particularly low friction of the Day2 film originates from the following factors: (i) shrinkage of the A5S992A5 molecules (mainly the poly(stearyl methacrylate) blocks) that leads to a viscoelastic properties of the adsorbed layers; and (ii) the intervening PAO layer between the adsorbed polymer layers that constitutes a high-fluidity sliding interface. Our results suggest that the block copolymer having relatively low solubility in a lubricant base oil is effective at forming low-friction adsorbed layers in oil-based lubrication. PMID:26479685
Spectral Gap Energy Transfer in Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Bhushan, S.; Walters, K.; Barros, A. P.; Nogueira, M.
2012-12-01
Experimental measurements of atmospheric turbulence energy spectra show E(k) ~ k-3 slopes at synoptic scales (~ 600 km - 2000 km) and k-5/3 slopes at the mesoscales (< 400 km). The -5/3 spectra is presumably related to 3D turbulence which is dominated by the classical Kolmogrov energy cascade. The -3 spectra is related to 2D turbulence, which is dominated by strong forward scatter of enstrophy and weak forward scatter of energy. In classical 2D turbulence theory, it is expected that a strong backward energy cascade would develop at the synoptic scale, and that circulation would grow infinitely. To limit this backward transfer, energy arrest at macroscales must be introduced. The most commonly used turbulence models developed to mimic the above energy transfer include the energy backscatter model for 2D turbulence in the horizontal plane via Large Eddy Simulation (LES) models, dissipative URANS models in the vertical plane, and Ekman friction for the energy arrest. One of the controversial issues surrounding the atmospheric turbulence spectra is the explanation of the generation of the 2D and 3D spectra and transition between them, for energy injection at the synoptic scales. Lilly (1989) proposed that the existence of 2D and 3D spectra can only be explained by the presence of an additional energy injection in the meso-scale region. A second issue is related to the observations of dual peak spectra with small variance in meso-scale, suggesting that the energy transfer occurs across a spectral gap (Van Der Hoven, 1957). Several studies have confirmed the spectral gap for the meso-scale circulations, and have suggested that they are enhanced by smaller scale vertical convection rather than by the synoptic scales. Further, the widely accepted energy arrest mechanism by boundary layer friction is closely related to the spectral gap transfer. This study proposes an energy transfer mechanism for atmospheric turbulence with synoptic scale injection, wherein the generation of 2D and 3D spectra is explained using spectral gap energy transfer. The existence of the spectral gap energy transfer is validated by performing LES for the interaction of large scale circulation with a wall, and studying the evolution of the energy spectra both near to and far from the wall. Simulations are also performed using the Advanced Weather and Research Forecasting (WRF-ARW) for moist zonal flow over Gaussian ridge, and the energy spectra close and away from the ground are studied. The energy spectra predicted by WRF-ARW are qualitatively compared with LES results to emphasize the limitations of the currently used turbulence parameterizations. Ongoing validation efforts include: (1) extending the interaction of large scale circulation with wall simulations to finer grids to capture a wider range of wavenumbers; and (2) a coupled 2D-3D simulation is planned to predict the entire atmospheric turbulence spectra at a very low computational expense. The overarching objective of this study to develop turbulence modeling capability based on the energy transfer mechanisms proposed in this study. Such a model will be implemented in WRF-ARW, and applied to atmospheric simulations, for example the prediction of moisture convergence patterns at the meso-scale in the southeast United States (Tao & Barros, 2008).
The inner core thermodynamics of the tropical cyclone boundary layer
NASA Astrophysics Data System (ADS)
Williams, Gabriel J.
2016-02-01
Although considerable progress has been made in understanding the inner-core dynamics of the tropical cyclone boundary layer (TCBL), our knowledge of the inner-core thermodynamics of the TCBL remains limited. In this study, the inner-core budgets of potential temperature (θ ), specific humidity (q), and reversible equivalent potential temperature (θ _e ) are examined using a high-resolution multilevel boundary layer model. The potential temperature budgets show that the heat energy is dominated by latent heat release in the eyewall, evaporative cooling along the outer edge of the eyewall, and upward surface fluxes of sensible and latent heat from the underlying warm ocean. It is shown that the vertical θ advection overcompensates the sum of radial advective warming from the boundary layer outflow jet and latent heating for the development of cooling in the eyewall within the TCBL. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts, partly by the boundary-layer outflow jet from the bottom eye region, so that the eyewall remains nearly saturated. The θ _e budgets reveal that the TCBL is maintained thermodynamically by the upward surface flux of higher-θ _e air from the underlying warm ocean, the radial transport of low-θ _e air from the outer regions of the TCBL, and the dry adiabatic cooling associated by eyewall updrafts. These results underscore the significance of vertical motion and the location of the boundary layer outflow jet in maintaining the inner core thermal structure of the TCBL.
Highly buoyant bent-over plumes in a boundary layer
NASA Astrophysics Data System (ADS)
Tohidi, Ali; Kaye, Nigel B.
2016-04-01
Highly buoyant plumes, such as wildfire plumes, in low to moderate wind speeds have initial trajectories that are steeper than many industrial waste plumes. They will rise further into the atmosphere before bending significantly. In such cases the plume's trajectory will be influenced by the vertical variation in horizontal velocity of the atmospheric boundary layer. This paper examined the behavior of a plume in an unstratified environment with a power-law ambient velocity profile. Examination of previously published experimental measurements of plume trajectory show that inclusion of the boundary layer velocity profile in the plume model often provides better predictions of the plume trajectory compared to algebraic expressions developed for uniform flow plumes. However, there are many cases in which uniform velocity profile algebraic expressions are as good as boundary layer models. It is shown that it is only important to model the role of the atmospheric boundary layer velocity profile in cases where either the momentum length (square root of source momentum flux divided by the reference wind speed) or buoyancy length (buoyancy flux divided by the reference wind speed cubed) is significantly greater than the plume release height within the boundary layer. This criteria is rarely met with industrial waste plumes, but it is important in modeling wildfire plumes.
Bending Boundary Layers in Laminated-Composite Circular Cylindrical Shells
NASA Technical Reports Server (NTRS)
Nemeth, Michael P.; Smeltzer, Stanley S., III
2000-01-01
An analytical, parametric study of the attenuation of bending boundary layers or edge effects in balanced and unbalanced, symmetrically and unsymmetrically laminated thin cylindrical shells is presented for nine contemporary material systems. The analysis is based on the linear Sanders-Koiter shell equations and specializations to the Love-Kirchhoff shell equations and Donnell's equations are included. Two nondimensional parameters are identified that characterize and quantify the effects of laminate orthotropy and laminate anisotropy on the bending boundary-layer decay length in a very general and encompassing manner. A substantial number of structural design technology results are presented for a wide range of laminated-composite cylinders. For all the laminate constructions considered, the results show that the differences between results that were obtained with the Sanders-Koiter shell equations, the Love-Kirchhoff shell equations, and Donnell's equations are negligible. The results also show that the effect of anisotropy in the form of coupling between pure bending and twisting has a negligible effect on the size of the bending boundary-layer decay length of the balanced, symmetrically laminated cylinders considered. Moreover, the results show that coupling between the various types of shell anisotropies has a negligible effect on the calculation of the bending boundary-layer decay length in most cases. The results also show that in some cases neglecting the shell anisotropy results in underestimating the bending boundary-layer decay length and in other cases it results in an overestimation.
Bending Boundary Layers in Laminated-Composite Circular Cylindrical Shells
NASA Technical Reports Server (NTRS)
Nemeth, Michael P.; Smeltzer, Stanley S., III
2000-01-01
A study of the attenuation of bending boundary layers in balanced and unbalanced, symmetrically and unsymmetrically laminated cylindrical shells is presented for nine contemporary material systems. The analysis is based on the linear Sanders-Koiter shell equations and specializations to the Love-Kirchhoff shell equations and Donnell's equations are included. Two nondimensional parameters are identified that characterize the effects of laminate orthotropy and anisotropy on the bending boundary-layer decay length in a very general manner. A substantial number of structural design technology results are presented for a wide range of laminated-composite cylinders. For all laminates considered, the results show that the differences between results obtained with the Sanders-Koiter shell equations, the Love-Kirchhoff shell equations, and Donnell's equations are negligible. The results also show that the effect of anisotropy in the form of coupling between pure bending and twisting has a negligible effect on the size of the bending boundary-layer decay length of the balanced, symmetrically laminated cylinders considered. Moreover, the results show that coupling between the various types of shell anisotropies has a negligible effect on the calculation of the bending boundary-layer decay length in most cases. The results also show that, in some cases, neglecting the shell anisotropy results in underestimating the bending boundary-layer decay length and, in other cases, results in an overestimation.
Improved methods for thin, surface boundary layer investigations
NASA Astrophysics Data System (ADS)
Lin, H. J.; Perlin, M.
New techniques are developed to improve the velocity flow-field measurement capability within a free-surface boundary layer region on which progressive capillary-gravity waves are present. Due to the extremely thin but rather vortical characteristics of the aforementioned boundary layer, conventional particle image velocimetry (PIV) methods fail to estimate velocity (and vorticity) vectors at an acceptable detection rate. This failure is a direct consequence of optimal PIV parameters that are difficult to achieve in practice for such flow situations. A new technique, Sub-pattern PIV, is developed. This method has features similar to both the super-resolution PIV (Keane et al. 1995) and the particle image distortion (PID) technique (Huang et al. 1993), but is predicated upon a very differential philosophy. Another difficulty that arises in experiments to investigate surface boundary layer flows is that the oscillating and deforming air-water interface has a mirror-like behavior that affects the images, and generates very noisy data. An alternative experimental setup that utilizes the Brewster angle phenomenon is adopted and the specular effects of the free-surface are removed successfully. This Brewster angle imaging, along with the Sub-pattern PIV technique, is used for the target application - a free-surface boundary layer investigation. It proved to be very effective. The methodology of both techniques is discussed, and the modified PIV procedure is validated by numerical probabilistic simulations. Application to the capillary-gravity wave boundary layer is presented in a subsequent paper.
Linear and nonlinear PSE for compressible boundary layers
NASA Technical Reports Server (NTRS)
Chang, Chau-Lyan; Malik, Mujeeb R.; Erlebacher, Gordon; Hussaini, M. Yousuff
1993-01-01
Compressible stability of growing boundary layers is studied by numerically solving the partial differential equations under a parabolizing approximation. The resulting parabolized stability equations (PSE) account for nonparallel as well as nonlinear effects. Evolution of disturbances in compressible flat-plate boundary layers are studied for freestream Mach numbers ranging from 0 to 4.5. Results indicate that the effect of boundary-layer growth is important for linear disturbances. Nonlinear calculations are performed for various Mach numbers. Two-dimensional nonlinear results using the PSE approach agree well with those from direct numerical simulations using the full Navier-Stokes equations while the required computational time is less by an order of magnitude. Spatial simulation using PSE were carried out for both the fundamental and subharmonic type breakdown for a Mach 1.6 boundary layer. The promising results obtained show that the PSE method is a powerful tool for studying boundary-layer instabilities and for predicting transition over a wide range of Mach numbers.
NASA Astrophysics Data System (ADS)
Koge, Hiroaki; Fujiwara, Toshiya; Kodaira, Shuichi; Sasaki, Tomoyuki; Kameda, Jun; Kitamura, Yujin; Hamahashi, Mari; Fukuchi, Rina; Yamaguchi, Asuka; Hamada, Yohei; Ashi, Juichiro; Kimura, Gaku
2014-12-01
The 2011 Tohoku-Oki earthquake (Mw 9.0) produced a fault rupture that extended to the toe of the Japan Trench. The deformation and frictional properties beneath the forearc are keys that can help to elucidate this unusual event. In the present study, to investigate the frictional properties of the shallow part of the plate boundary, we applied the critically tapered Coulomb wedge theory to the Japan Trench and obtained the effective coefficient of basal friction and Hubbert-Rubey pore fluid pressure ratio (?) of the wedge beneath the lower slope. We extracted the surface slope angle and dcollement dip angle (which are the necessary topographic parameters for applying the critical taper theory) from seismic reflection and refraction survey data at 12 sites in the frontal wedges of the Japan Trench. We found that the angle between the dcollement and back-stop interface generally decreases toward the north. The measured taper angle and inferred effective friction coefficient were remarkably high at three locations. The southernmost area, which had the highest coefficient of basal friction, coincides with the area where the seamount is colliding offshore of Fukushima. The second area with a high effective coefficient of basal friction coincides with the maximum slip location during the 2011 Tohoku-Oki earthquake. The area of the 2011 earthquake rupture was topographically unique from other forearc regions in the Japan Trench. The strain energy accumulation near the trench axis may have proceeded because of the relatively high friction, and later this caused a large slip and collapse of the wedge. The location off Sanriku, where there are neither seamount collisions nor rupture propagation, also has a high coefficient of basal friction. The characteristics of the taper angle, effective coefficient of basal friction, and pore fluid pressure ratio along the Japan Trench presented herein may contribute to the understanding of the relationship between the geometry of the prism and the potential for generating seismo-tsunamigenic slips.
Interferometric data for a shock-wave/boundary-layer interaction
NASA Technical Reports Server (NTRS)
Dunagan, Stephen E.; Brown, James L.; Miles, John B.
1986-01-01
An experimental study of the axisymmetric shock-wave / boundary-layer strong interaction flow generated in the vicinity of a cylinder-cone intersection was conducted. The study data are useful in the documentation and understanding of compressible turbulent strong interaction flows, and are part of a more general effort to improve turbulence modeling for compressible two- and three-dimensional strong viscous/inviscid interactions. The nominal free stream Mach number was 2.85. Tunnel total pressures of 1.7 and 3.4 atm provided Reynolds number values of 18 x 10(6) and 36 x 10(6) based on model length. Three cone angles were studied giving negligible, incipient, and large scale flow separation. The initial cylinder boundary layer upstream of the interaction had a thickness of 1.0 cm. The subsonic layer of the cylinder boundary layer was quite thin, and in all cases, the shock wave penetrated a significant portion of the boundary layer. Owing to the thickness of the cylinder boundary layer, considerable structural detail was resolved for the three shock-wave / boundary-layer interaction cases considered. The primary emphasis was on the application of the holographic interferometry technique. The density field was deduced from an interferometric analysis based on the Able transform. Supporting data were obtained using a 2-D laser velocimeter, as well as mean wall pressure and oil flow measurements. The attached flow case was observed to be steady, while the separated cases exhibited shock unsteadiness. Comparisons with Navier-Stokes computations using a two-equation turbulence model are presented.
Yeh, Li-Hsien; Fang, Kuo-Ying; Hsu, Jyh-Ping; Tseng, Shiojenn
2011-12-01
The electrophoresis of a soft particle comprising a rigid core and a charged porous membrane layer in a narrow space is modeled. This simulates, for example, the capillary electrophoresis of biocolloids such as cells and microorganisms, and biosensor types of device. We show that, in addition to the boundary effect, the effects of double-layer polarization (DLP) and the electroosmotic retardation flow can be significant, yielding interesting electrophoretic behaviors. For example, if the friction coefficient of the membrane layer and/or the boundary is large, then the DLP effect can be offset by the electroosmotic retardation flow, making the particle mobility to decrease with increasing double layer thickness, which is qualitatively consistent with many experimental observations in the literature, but has not been explained clearly in previous analyses. In addition, depending upon the thickness of double layer, the friction of the membrane layer of a particle can either retard or accelerate its movement, an interesting result which has not been reported previously. This work is the first attempt to show solid evidence for the influence of a boundary on the effect of DLP and the electrophoretic behavior of soft particles. The model proposed is verified by the experimental data in the literature. The results of numerical simulation provide valuable information for the design of bio-analytical apparatus such as nanopore-based sensing applications and for the interpretation of relevant experimental data. PMID:21840182
Use of Boundary Layer Transition Detection to Validate Full-Scale Flight Performance Predictions
NASA Technical Reports Server (NTRS)
Hamner, Marvine; Owens, L. R., Jr.; Wahls, R. A.; Yeh, David
1999-01-01
Full-scale flight performance predictions can be made using CFD or a combination of CFD and analytical skin-friction predictions. However, no matter what method is used to obtain full-scale flight performance predictions knowledge of the boundary layer state is critical. The implementation of CFD codes solving the Navier-Stokes equations to obtain these predictions is still a time consuming, expensive process. In addition, to ultimately obtain accurate performance predictions the transition location must be fixed in the CFD model. An example, using the M2.4-7A geometry, of the change in Navier-Stokes solution with changes in transition and in turbulence model will be shown. Oil flow visualization using the M2.4-7A 4.0% scale model in the 14'x22' wind tunnel shows that fixing transition at 10% x/c in the CFD model best captures the flow physics of the wing flow field. A less costly method of obtaining full-scale performance predictions is the use of non-linear Euler codes or linear CFD codes, such as panel methods, combined with analytical skin-friction predictions. Again, knowledge of the boundary layer state is critical to the accurate determination of full-scale flight performance. Boundary layer transition detection has been performed at 0.3 and 0.9 Mach numbers over an extensive Reynolds number range using the 2.2% scale Reference H model in the NTF. A temperature sensitive paint system was used to determine the boundary layer state for these conditions. Data was obtained for three configurations: the baseline, undeflected flaps configuration; the transonic cruise configuration; and, the high-lift configuration. It was determined that at low Reynolds number conditions, in the 8 to 10 million Reynolds number range, the baseline configuration has extensive regions of laminar flow, in fact significantly more than analytical skin-friction methods predict. This configuration is fully turbulent at about 30 million Reynolds number for both 0.3 and 0.9, Mach numbers. Both the transonic cruise and the high-lift configurations were fully turbulent aft of the leading-edge flap hingeline at all Reynolds numbers.
NASA Technical Reports Server (NTRS)
Vanness, W.
1978-01-01
A summary report of boundary layer studies is presented. Preliminary results of experimental measurements show that: (1) A very thin layer (approximately 0.4 mm) of the boundary layer seems to be accelerated; (2) the static pressure of the outer flow does not remain exactly constant through the boundary layer; and (3) an oncoming boundary layer which is already turbulent at the suction point can again become laminar behind this point without being completely sucked off.
NASA Astrophysics Data System (ADS)
Korshunov, L. G.; Pushin, V. G.; Chernenko, N. L.
2011-09-01
The effect of frictional heating (whose intensity was varied at the expense of changes in the sliding velocity from 0.35 to 9.00 m/s) on the rate of wear, friction coefficient, friction thermopower, structure, and microhardness of the Ti49.4Ni50.6 alloy in a microcrystalline (MC) state with grains 20-30 ?m in size and in a submicrocrystalline (SMC) state with grains 300 nm in size has been investigated. The tribological tests were conducted under the conditions of dry sliding friction in air using the finger-disk (made of steel Kh12M, hardness HRC = 63) scheme at a normal load of 98 N. Due to the frictional heating, the temperature in the surface layer 0.5 mm thick of the samples changed from 150-200 (at a sliding velocity of 0.35 m/s) to 1100C (at a velocity of 9 m/s). The alloy structure has been studied with the help of metallographic and electronmicroscopic (scanning and transmission microscopy) methods. It has been shown that the rate of wear of the titanium nickelide in the MC and SMC structural states is more than an order of magnitude lower than in the 12Kh18N9 steel and several times less than in the 40Kh13 steel. The fracture of the friction surface of the titanium nickelide occurs predominantly by the fatigue or oxidation-fatigue mechanisms, which are characterized by a relatively low wear rate, whereas the 40Kh13 and 12Kh18N9 steels show a tendency to intense thermal adhesive wear (seizure) at velocities higher than 0.35 m/s. It has been shown by the electron-microscopic investigation that nanocrystalline structures consisting of crystals of the B2 phase, oxides of the TiO2 type, and some amount of martensite B19' are formed in the process of friction in the surface layer of the titanium nickelide. It has been concluded that an enhanced wear resistance of the titanium nickelide is caused by the high heat resistance (strength) and high fracture toughness of the nanocrystalline B2 phase and by the presence of high-strength thermostable oxides of the TiO2 type formed upon friction.
Effect of Far-Field Boundary Conditions on Boundary-Layer Transition
NASA Technical Reports Server (NTRS)
Bertolotti, Fabio P.; Joslin, Ronald D.
1994-01-01
The effect of far-field boundary conditions on the evolution of a finite-amplitude two-dimensional wave in the Blasius boundary layer is assessed. With the use of the parabolized stability equations (PSE) theory for the numerical computations, either asymptotic, Dirichlet, Neumann or mixed boundary conditions are imposed at various distances from the wall. The results indicate that asymptotic and mixed boundary conditions yield the most accurate mean-flow distortion and unsteady instability modes in comparison with the results obtained with either Dirichlet or Neumann conditions.
Effect of Far-Field Boundary Conditions on Boundary-Layer Transition
NASA Technical Reports Server (NTRS)
Bertolotti, Fabio P.; Joslin, Ronald D.
1995-01-01
The effect of far-field boundary conditions on the evolution of a finite-amplitude two-dimensional wave in the Blasius boundary layer is assessed. With the use of the parabolized stability equations (PSE) theory for the numerical computations, either asymptotic, Dirichlet, Neumann or mixed boundary conditions are imposed at various distances from the wall. The results indicate that asymptotic and mixed boundary conditions yield the most accurate mean-flow distortion and unsteady instability modes in comparison with the results obtained with either Dirichlet or Neumann conditions.
A compilation of unsteady turbulent boundary-layer experimental data
NASA Technical Reports Server (NTRS)
Carr, L. W.
1981-01-01
An extensive literature search was conducted and those experiments related to unsteady boundary layer behavior were cataloged. In addition, an international survey of industrial, university, and governmental research laboratories was made in which new and ongoing experimental programs associated with unsteady turbulent boundary layer research were identified. Pertinent references were reviewed and classified based on the technical emphasis of the various experiments. Experiments that include instantaneous or ensemble averaged profiles of boundary layer variables are stressed. The experimental apparatus and flow conditions are described and summaries of acquired data and significant conclusions are summarized. Measurements obtained from the experiments which exist in digital form were stored on magnetic tape. Instructions are given for accessing these data sets for further analysis.
DNS of Turbulent Boundary Layers under Highenthalpy Conditions
NASA Astrophysics Data System (ADS)
Duan, Lian; Martn, Pino
2010-11-01
To study real-gas effects and turbulence-chemistry interaction, direct numerical simulations (DNS) of hypersonic boundary layers are conducted under typical hypersonic conditions. We consider the boundary layer on a lifting-body consisting of a flat plate at an angle of attack, which flies at altitude 30km with a Mach number 21. Two different inclined angles, 35^o and 8^o, are considered,representing blunt and slender bodies. Both noncatalytic and supercatalytic wall conditions are considered. The DNS data are studied to assess the validity of Morkovin's hypothesis, the strong Reynolds analogy, as well as the behaviors of turbulence structures under high-enthalpy conditions.Relative to low-enthalpy conditions [1], significant differences in typical scalings are observed. [4pt] [1] L. Duan and I. Beekman and M. P. Mart'in, Direct numerical simulation of hypersonic turbulent boundary layers. Part 2: Effect of temperature, J. Fluid Mech. 655 (2010), 419-445.
Wind Tunnel Simulation of the Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Hohman, Tristen; Smits, Alexander; Martinelli, Luigi
2013-11-01
To simulate the interaction of large Vertical Axis Wind Turbines (VAWT) with the Atmospheric Boundary Layer (ABL) in the laboratory, we implement a variant of Counihan's technique [Counihan 1969] in which a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements is used to create an artificial ABL profile in a standard closed loop wind tunnel. To examine the development and formation of the artificial ABL hotwire and SPIV measurements were taken at various downstream locations with changes in wall roughness, wall type, and vortex generator arrangements. It was found possible to generate a boundary layer at Reθ ~106 , with a mean velocity that followed the 1/7 power law of a neutral ABL over rural terrain and longitudinal turbulence intensities and power spectra that compare well with the data obtained for high Reynolds number flat plate turbulent boundary layers [Hultmark et al. 2010]. Supported by Hopewell Wind Power Ltd., and the Princeton Grand Challenges Program.
Effect of Blowing on Boundary Layer of Scarf Inlet
NASA Technical Reports Server (NTRS)
Gerhold, Carl H.; Clark, Lorenzo R.
2004-01-01
When aircraft operate in stationary or low speed conditions, airflow into the engine accelerates around the inlet lip and pockets of turbulence that cause noise and vibration can be ingested. This problem has been encountered with engines equipped with the scarf inlet, both in full scale and in model tests, where the noise produced during the static test makes it difficult to assess the noise reduction performance of the scarf inlet. NASA Langley researchers have implemented boundary layer control in an attempt to reduce the influence of the flow nonuniformity in a 12-in. diameter model of a high bypass fan engine mounted in an anechoic chamber. Static pressures and boundary layer profiles were measured in the inlet and far field acoustic measurements were made to assess the effectiveness of the blowing treatment. The blowing system was found to lack the authority to overcome the inlet distortions. Methods to improve the implementation of boundary layer control to reduce inlet distortion are discussed.
The Turbulent Boundary Layer on a Rough Curvilinear Surface
NASA Technical Reports Server (NTRS)
Droblenkov, V. F.
1958-01-01
A number of semiempirical approximate methods exist for determining the characteristics of the turbulent boundary layer on a curvilinear surface. At present, among these methods, the one proposed by L. G. Loitsianskii is given frequent practical application. This method is sufficiently effective and permits, in the case of wing profiles with technically smooth surfaces, calculating the basic characteristics of the boundary layer and the values of the overall drag with an accuracy which suffices for practical purposes. The idea of making use of the basic integral momentum equation ((d delta(sup xx))/dx) + ((V' delta(sup xx))/V) (2 + H) = (tau(sub 0))/(rho V(exp 2)) proves to be fruitful also for the solution of the problems in the determination of the characteristics of the turbulent boundary layer on a rough surface.
Defects and boundary layers in non-Euclidean plates
NASA Astrophysics Data System (ADS)
Gemmer, J. A.; Venkataramani, S. C.
2012-12-01
We investigate the behaviour of non-Euclidean plates with constant negative Gaussian curvature using the Fppl-von Krmn reduced theory of elasticity. Motivated by recent experimental results, we focus on annuli with a periodic profile. We prove rigorous upper and lower bounds for the elastic energy that scales like the thickness squared. In particular we show that are only two types of global minimizersdeformations that remain flat and saddle shaped deformations with isolated regions of stretching near the edge of the annulus. We also show that there exist local minimizers with a periodic profile that have additional boundary layers near their lines of inflection. These additional boundary layers are a new phenomenon in thin elastic sheets and are necessary to regularize jump discontinuities in the azimuthal curvature across lines of inflection. We rigorously derive scaling laws for the width of these boundary layers as a function of the thickness of the sheet.
Finite volume solution of the compressible boundary-layer equations
NASA Technical Reports Server (NTRS)
Loyd, B.; Murman, E. M.
1986-01-01
A box-type finite volume discretization is applied to the integral form of the compressible boundary layer equations. Boundary layer scaling is introduced through the grid construction: streamwise grid lines follow eta = y/h = const., where y is the normal coordinate and h(x) is a scale factor proportional to the boundary layer thickness. With this grid, similarity can be applied explicity to calculate initial conditions. The finite volume method preserves the physical transparency of the integral equations in the discrete approximation. The resulting scheme is accurate, efficient, and conceptually simple. Computations for similar and non-similar flows show excellent agreement with tabulated results, solutions computed with Keller's Box scheme, and experimental data.
Boundary layer integral matrix procedure: Verification of models
NASA Technical Reports Server (NTRS)
Bonnett, W. S.; Evans, R. M.
1977-01-01
The three turbulent models currently available in the JANNAF version of the Aerotherm Boundary Layer Integral Matrix Procedure (BLIMP-J) code were studied. The BLIMP-J program is the standard prediction method for boundary layer effects in liquid rocket engine thrust chambers. Experimental data from flow fields with large edge-to-wall temperature ratios are compared to the predictions of the three turbulence models contained in BLIMP-J. In addition, test conditions necessary to generate additional data on a flat plate or in a nozzle are given. It is concluded that the Cebeci-Smith turbulence model be the recommended model for the prediction of boundary layer effects in liquid rocket engines. In addition, the effects of homogeneous chemical reaction kinetics were examined for a hydrogen/oxygen system. Results show that for most flows, kinetics are probably only significant for stoichiometric mixture ratios.
LES Study of Shock Wave and Turbulent Boundary Layer Interaction
NASA Astrophysics Data System (ADS)
Li, Justine; Priebe, Stephan; Martin, Pino
2012-11-01
The large eddy simulation (LES) of a 24 compression ramp shock wave and turbulent boundary layer interaction (STBLI) is presented. This work builds on previous work on the direct numerical simulation (DNS) of STBLI with similar incoming boundary layer flow conditions (Priebe and Martn, JFM 2012). The fully-turbulent inflow boundary layer is at Mach 2.9 and the Reynolds number based on momentum thickness is Re? = 2900. The LES data cover a sufficiently long time to statistically resolve the low-frequency aperiodic cycle characteristic of supersonic STBLI. We present the characterization of the dynamics in the downstream separated flow. This research is supported by AFOSR Grant Number AF/9550-10-1-0164 and by the Department of Defense through the NDSEG Fellowship Program.
Blow-up and control of marginally separated boundary layers.
Braun, Stefan; Kluwick, Alfred
2005-05-15
Interactive solutions for steady two-dimensional laminar marginally separated boundary layers are known to exist up to a critical value Gamma(c) of the controlling parameter (e.g. the angle of attack of a slender airfoil) Gamma only. Here, we investigate three-dimensional unsteady perturbations of such boundary layers, assuming that the basic flow is almost critical, i.e. in the limit Gamma(c)-Gamma-->0. It is then shown that the interactive equations governing such perturbations simplify significantly, allowing, among others, a systematic study of the blow-up phenomenon observed in earlier investigations and the optimization of devices used in boundary-layer control. PMID:16105768
Measurement in a Hypersonic Turbulent Boundary Layer Using PIV
NASA Astrophysics Data System (ADS)
Williams, Owen; Smits, Alexander
2012-11-01
Experiments are reported on measuring turbulence in a flat plate boundary layer at Mach 7.4 using planar PIV in order to examine Morkovin's hypothesis and scaling at Mach numbers greater than 5. PIV measurements in hypersonic flow are hampered by high dynamic range requirements and low flow density, which leads to stringent particle sizing requirements to avoid particle lag. In addition, high shear can lead to a bias in many cross-correlation algorithms. Experiments to determine the frequency response of a range of titanium dioxide particles using the response across a shock will be detailed. Additionally, the conditions for the appropriate initial conditions for boundary layer development, such as the selection of size and type of tripping device and appropriate development length for the establishment of a fully turbulent boundary layer will be examined.
Turbulence in the convective boundary layer observed by microwave interferometry
Shao, X.M.; Carlos, R.C.; Kirkland, M.W.
1997-12-01
A 9-antenna, 400 meter microwave interferometer was utilized in SALSA MEX on the San Pedro River area in July and August, 1997, to measure the turbulence in the Convective Boundary Layer. Water vapor has an appreciable index of refraction at radio frequencies around 10 GHz, and acts as a passive tracer of the magnitude and motion of turbulence. The relative phase changes of a signal from a satellite were tracked by an array of 9 antennas, and the phase differences between antennas were then used to derive the turbulence properties of the boundary layer. Preliminary analysis shows clearly different characteristics for the convection activity of the boundary layer between day and night. From the structure function analysis they can see that the turbulence structure starts to decorrelate at scale sizes of 200 meters for a temporal passband around 100 seconds. Derivation of average wind fields is currently in process.
Effects of forebody geometry on subsonic boundary-layer stability
NASA Technical Reports Server (NTRS)
Dodbele, Simha S.
1990-01-01
As part of an effort to develop computational techniques for design of natural laminar flow fuselages, a computational study was made of the effect of forebody geometry on laminar boundary layer stability on axisymmetric body shapes. The effects of nose radius on the stability of the incompressible laminar boundary layer was computationally investigated using linear stability theory for body length Reynolds numbers representative of small and medium-sized airplanes. The steepness of the pressure gradient and the value of the minimum pressure (both functions of fineness ratio) govern the stability of laminar flow possible on an axisymmetric body at a given Reynolds number. It was found that to keep the laminar boundary layer stable for extended lengths, it is important to have a small nose radius. However, nose shapes with extremely small nose radii produce large pressure peaks at off-design angles of attack and can produce vortices which would adversely affect transition.
Atmospheric boundary layer processes during a total solar eclipse
SethuRaman, S.; Prabhu, A.; Narahari Rao, K.; Narasimha, R.
1980-01-01
The total solar eclipse that occurred over the southern part of India on February 16, 1980, gave a unique opportunity to study the earth's atmospheric boundary layer. The meteorological experiments during the 1980 solar eclipse were conducted at Raichur, India (16/sup 0/12'N, 77/sup 0/21'E) located in the state of Karnataka, approximately 400-m above sea level. The main objective was to determine the changes in the earth's atmosphere during and immediately after the eclipse. The goal was to study the changes in the momentum and heat fluxes in the boundary layer due to the eclipse. Measurements were made for 2 days prior to and 1 day after the day of the eclipse to determine background characteristics of the boundary layer which might be site-dependent.
Numerical Simulations of Wake/Boundary Layer Interactions
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Choudhari, Meelan M.; Ovchinnikov, Victor; Balaras, Elias
2003-01-01
Direct and large-eddy simulations of the interaction between the wake of a circular cylinder and a flat-plate boundary layer are conducted. Two Reynolds numbers are examined. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady Karman vortex street shed by the cylinder. The fluctuations, however, cannot be self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer fluctuations persist after the wake has decayed (due, in part, to the higher values of the local Reynolds number Re(sub theta) achieved in this case); some evidence could be observed that a self-sustaining turbulence generation cycle was beginning to be established.
Roughness Induced Transition in a Supersonic Boundary Layer
NASA Technical Reports Server (NTRS)
Balakumar, Ponnampalam; Kergerise, Michael A.
2013-01-01
Direct numerical simulation is used to investigate the transition induced by threedimensional isolated roughness elements in a supersonic boundary layer at a free stream Mach number of 3.5. Simulations are performed for two different configurations: one is a square planform roughness and the other is a diamond planform roughness. The mean-flow calculations show that the roughness induces counter rotating streamwise vortices downstream of the roughness. These vortices persist for a long distance downstream and lift the low momentum fluid from the near wall region and place it near the outer part of the boundary layer. This forms highly inflectional boundary layer profiles. These observations agree with recent experimental observations. The receptivity calculations showed that the amplitudes of the mass-flux fluctuations near the neutral point for the diamond shape roughness are the same as the amplitude of the acoustic disturbances. They are three times smaller for the square shape roughness.
Shock Wave--Boundary Layer Interaction in Reflecting Detonations
NASA Astrophysics Data System (ADS)
Damazo, J.; Zeigler, J.; Karnesky, J.; Shepherd, J. E.
2010-11-01
The interaction of a reflecting shock wave with the boundary layer induced by the incident shock wave results in a unique flow field that has been examined in shock tubes. Our recent experiments studying reflecting detonations examine an incident detonation impinging on a normal, planar wall to create a reflected shock wave. We have observed that the pressure records taken near the location of reflection show that the measured speed of the reflected shock wave is inconsistent with the measured wall pressures. We present new experimental results of high-speed video taken of the reflecting detonation and highly-resolved two-dimensional numerical simulations of compressible viscous flow. These results show that the interaction of the reflected shock wave with the boundary layer can result in a three-dimensional shock front structure with an oblique front in the boundary layer similar to that observed in non-reacting shock tubes.
Asymptotic analysis: Working note {number_sign}3, boundary layers
Garbey, M.; Kaper, H.G.
1993-09-01
In this chapter the authors discuss the asymptotic approximation of functions that display boundary-layer behavior. The purpose here is to introduce the basic concepts underlying the phenomenon, to illustrate its importance, and to describe some of the fundamental tools available for its analysis. To achieve their purpose in the clearest way possible, the authors will work with functions that are assumed to be given explicitly -- that is, functions f : (0,{epsilon}{sub 0}) {yields} X whose expressions are known, at least in principle. Only in the following chapter will they begin the study of functions that are given implicitly as solutions of boundary value problems -- the real stuff of which singular perturbation theory is made. Boundary-layer behavior is associated with asymptotic expansions that are regular {open_quotes}almost everywhere{close_quotes} -- that is, expansions that are regular on every compact subset of the domain of definition, but not near the boundary. These regular asymptotic expansions can be continued in a certain sense all the way up to the boundary, but a separate analysis is still necessary in the boundary layer. The boundary-layer analysis is purely local and aims at constructing local approximations in the neighborhood of each point of the singular part of the boundary. The problem of finding an asymptotic approximation is thus reduced to matching the various local approximations to the existing regular expansion valid in the interior of the domain. The authors are thinking, for example, of fluid flow (viscosity), combustion (Lewis number), and superconductivity (Ginzburg-Landau parameter) problems. Their solution may remain smooth over a wide range of parameter values, but as the parameters approach critical values, complicated patterns may emerge.
Direct Numerical Simulation of Supersonic Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Guarini, Stephen; Moser, R.; Shariff, K.; Wray, A.; Merriam, Marshal (Technical Monitor)
1997-01-01
The talk will present some initial results from the direct numerical simulation (DNS) of compressible turbulent boundary layers. We solve numerically the compressible Navier-Stokes equations using a method based on Spalart's transformation for the incompressible turbulent boundary layer. This allows the spatially developing boundary layer to be transformed to a calculation with periodic boundary conditions in the streamwise and spanwise directions. The equations are solved using Fourier expansions in the horizontal directions and B-splines in the wall-normal direction. The first simulation is at Mach 2.5 with a momentum thickness Reynolds number based on wall viscosity of R(sub theta(sup 1)) = 825. We are examining the physics of the compressible boundary layer using turbulence statistics and budget equations. The turbulence statistics include: rms (root mean square) and mean profiles, energy spectra, and two-point correlations. It is found that there are large density gradients which require significantly more resolution than the incompressible case.
Receptivity of Hypersonic Boundary Layers over Straight and Flared Cones
NASA Technical Reports Server (NTRS)
Balakumar, Ponnampalam; Kegerise, Michael A.
2010-01-01
The effects of adverse pressure gradients on the receptivity and stability of hypersonic boundary layers were numerically investigated. Simulations were performed for boundary layer flows over a straight cone and two flared cones. The steady and the unsteady flow fields were obtained by solving the two-dimensional Navier-Stokes equations in axi-symmetric coordinates using the 5th order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. The mean boundary layer profiles were analyzed using local stability and non-local parabolized stability equations (PSE) methods. After the most amplified disturbances were identified, two-dimensional plane acoustic waves were introduced at the outer boundary of the computational domain and time accurate simulations were performed. The adverse pressure gradient was found to affect the boundary layer stability in two important ways. Firstly, the frequency of the most amplified second-mode disturbance was increased relative to the zero pressure gradient case. Secondly, the amplification of first- and second-mode disturbances was increased. Although an adverse pressure gradient enhances instability wave growth rates, small nose-tip bluntness was found to delay transition due to the low receptivity coefficient and the resulting weak initial amplitude of the instability waves. The computed and measured amplitude-frequency spectrums in all three cases agree very well in terms of frequency and the shape except for the amplitude.
Numerical Modeling of the Evolving Stable Boundary Layer
NASA Astrophysics Data System (ADS)
Sorbjan, Z.
2013-12-01
A single-column model of the evolving stable boundary layer is tested for the consistency of turbulence parameterization, self-similar properties of the flow, and effects of ambient forcing. The turbulence closure of the model is based on the K-theory approach, with stability functions based on empirical data, and a semi-empirical form of the mixing length. The model has one internal, governing stability parameter, the Richardson number Ri, which dynamically adjusts to the boundary conditions and to external forcing. Model results, expressed in terms of local similarity scales, are universal functions of the Richardson number, i.e. they are satisfied in the entire stable boundary layer, for all instants of time, and all kinds of external forcing. Based on similarity expression, a realizability condition is derived for the minimum turbulent heat flux in the stable boundary layer. Numerical experiments show that the development of 'horse-shoe' shaped, 'fixed-elevation' wind hodographs in the interior of the stable boundary layer are solely caused by effects imposed by surface thermal forcing, and are not related to the inertial oscillation mechanism.
Axisymmetric fundamental solutions for a finite layer with impeded boundaries.
Cheng, Ze-Hai; Chen, Yun-Min; Ling, Dao-Sheng; Tang, Xiao-Wu
2003-01-01
Axisymmetric fundamental solutions that are applied in the consolidation calculations of a finite clay layer with impeded boundaries were derived. Laplace and Hankel integral transforms were utilized with respect to time and radial coordinates, respectively in the analysis. The derivation of fundamental solutions considers two-boundary value problems involving unit point loading and ring loading in the vertical. The solutions are extended to circular distributed and strip distributed normal load. The computation and analysis of settlements, vertical total stress and excess pore pressure in the consolidation layer subject to circular loading are presented. PMID:12861613
Numerical Study of Boundary-Layer in Aerodynamics
NASA Technical Reports Server (NTRS)
Shih, Tom I-P.
1997-01-01
The accomplishments made in the following three tasks are described: (1) The first task was to study shock-wave boundary-layer interactions with bleed - this study is relevant to boundary-layer control in external and mixed-compression inlets of supersonic aircraft; (2) The second task was to test RAAKE, a code developed for computing turbulence quantities; and (3) The third task was to compute flow around the Ames ER-2 aircraft that has been retrofitted with containers over its wings and fuselage. The appendices include two reports submitted to AIAA for publication.
Simulation of glancing shock wave and boundary layer interaction
NASA Technical Reports Server (NTRS)
Hung, Ching-Mao
1989-01-01
Shock waves generated by sharp fins, glancing across a laminar boundary layer growing over a flat plate, are simulated numerically. Several basic issues concerning the resultant three-dimensional flow separation are studied. Using the same number of grid points, different grid spacings are employed to investigate the effects of grid resolution on the origin of the line of separation. Various shock strengths (generated by different fin angles) are used to study the so-called separated and unseparated boundary layer and to establish the existence or absence of the secondary separation. The usual interpretations of the flow field from previous studies and new interpretations arising from the present simulation are discussed.
Velocity measurements in a turbulent natural convection boundary layer
NASA Astrophysics Data System (ADS)
Cheesewright, R.; Ierokiopitis, E.
The initial results from an investigation of the velocity and temperature profiles for a turbulent natural convective boundary layer using laser Doppler anemometry (LDA) are reported. A DISA type seeding generator spewed corn oil droplets over a flat plate immersed in a flow in order to obtain velocity fluctuations, and thermocouples provided data for the temperature profiles. The plate was maintained at a surface temperature of 80 C. Sufficient measurements were made to characterize the laminar, transition, and the turbulent regions. The measurements taken in the boundary layer satisfied an integral energy balance, with residence time weighting of the values yielding corrections that amounted to only 2-3 percent.
Plasma behavior in the boundary layer near a railgun surface
Kang, Sang-Wook; McCallen, R.E.
1988-03-01
Viscous flow and thermal characteristics are theoretically analyzed for the plasma behind a moving projectile inside a railgun. When only convective effects are included in the turbulent boundary layer analysis, the results suggest a temperature maximum in the wall region for very high velocity flows. The case of radiative as well as convective transport has also been investigated for an optically-thick boundary layer flow by application of an approximate method. Results show a sizable effect of radiation on the flow characteristics, especially on the heat-transfer rate to the railgun surface. 7 refs., 2 figs.
Boundary layer transition: Prediction and wind tunnel simulation
NASA Astrophysics Data System (ADS)
Arnal, D.
1993-02-01
This paper gives a survey of theoretical and experimental results related to the problem of boundary layer transition; emphasis is given on applications of practical prediction methods. In the first part of the paper, it is shown that the linear stability theory can provide a good estimate of the transition location if the free stream disturbance level is low enough; the difficulties to properly simulate free flight conditions in ground facilities is underlined. The second part of the paper is devoted to the problem of boundary layer tripping in the presence of large external disturbances; in this case, the linear theory no longer applies and empirical criteria need to be developed.
Mixing line analysis of clouds and cloudy boundary layers
NASA Technical Reports Server (NTRS)
Betts, A. K.
1985-01-01
The diagnostic study of the thermodynamic structure of nonprecipitating clouds and cloudy boundary layers is formulated using a mixing line and saturation point approach. A parametric model for the mean structure is developed as a tool for diagnostic and prognostic modeling. Cloud-scale mixing processes are analyzed in the same framework, together with the energies of the evaporative instability in cumulus clouds. A velocity scale emerges for this evaporative instability. The statistical study of saturation level distribution in partially cloudy boundary layers is proposed to related cloud fraction to the mean thermodynamic mixing processes.
Characteristics of turbulence in boundary layer with zero pressure gradient
NASA Technical Reports Server (NTRS)
Klebanoff, P S
1955-01-01
The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed.
Interactive-Boundary-Layer Computations For Oscillating Airfoil
NASA Technical Reports Server (NTRS)
Carr, L. W.; Cebeci, T.; Jang, Hong-Ming
1993-01-01
Interactive-boundary-layer method developed for computations of steady flow, extended under assumption of quasi-steady flow, to computations of evolution of two-dimensional flow about oscillating airfoil under light-dynamic-stall conditions. Represents advance toward ability to compute unsteady flows at even greater angles of attack with solutions of equations normally used for description of boundary-layer flows on airfoils prior to stall. Important in practical studies of flow on blades of helicopter rotors, axial compressors, and turbines.
Lower Atmospheric Boundary Layer Experiment (LABLE) Final Campaign Report
Klein, P; Bonin, TA; Newman, JF; Turner, DD; Chilson, P; Blumberg, WG; Mishra, S; Wainwright, CE; Carney, M; Jacobsen, EP; Wharton, S
2015-11-01
The Lower Atmospheric Boundary Layer Experiment (LABLE) included two measurement campaigns conducted at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site in Oklahoma during 2012 and 2013. LABLE was designed as a multi-phase, low-cost collaboration among the University of Oklahoma, the National Severe Storms Laboratory, Lawrence Livermore National Laboratory, and the ARM program. A unique aspect was the role of graduate students in LABLE. They served as principal investigators and took the lead in designing and conducting experiments using different sampling strategies to best resolve boundary-layer phenomena.
Carbon vaporization into a nonequilibrium, stagnation-point boundary layer
NASA Technical Reports Server (NTRS)
Suzuki, T.
1978-01-01
The heat transfer to the stagnation point of an ablating carbonaceous heat shield, where both the gas-phase boundary layer and the heterogeneous surface reactions are not in chemical equilibrium, is examined. Specifically, the nonequilibrium changes in the mass fraction profiles of carbon species calculated for frozen flow are studied. A set of equations describing the steady-state, nonequilibrium laminar boundary layer in the axisymmetric stagnation region, over an ablating graphite surface, is solved, with allowance for the effects of finite rate of carbon vaporization.
New concepts on the interfacial friction behavior between flat steel ribbon layers
Zheng, J.; Zhu, G.
1995-11-01
Flat steel ribbon wound pressure vessels are widely used in chemical, petrochemical, and other industries. However, no satisfactory theoretical formulae are available to estimate the additional strengthening induced by the friction between the layers. Effective normal stress in the ribbon wide direction and shear stress are new concepts for describing such strengthening effect. These concepts are analyzed further to obtain expressions for both axial and circumferential bursting pressure, and stresses of the vessel. Comparison with one set of experimental results shows excellent agreement.
Boundary Layer Regimes Conducive to Formation of Dust Devils on Mars
NASA Astrophysics Data System (ADS)
Williams, B.; Nair, U. S.
2014-12-01
Dust devils on Mars contribute to maintenance of background atmospheric aerosol loading and thus dust radiative forcing, which is an important modulator of Martian climate. Dust devils also cause surface erosion and change in surface albedo which impacts radiative energy budget. Thus there is a need for parameterizing dust devil impacts in Martian climate models. In this context it is important to understand environmental conditions that are favorable for formation of dust devils on Mars and associated implications for diurnal, seasonal, and geographical variation of dust devil occurrence. On earth, prior studies show that thresholds of ratio of convective and friction scale velocities may be used to identify boundary layer regimes that are conducive to formation of dust devils. On earth, a w*/u* ratio in excess of 5 is found to be conducive for formation of dust devils. In this study, meteorological observations collected during the Viking Lander mission are used to constrain Martian boundary layer model simulations, which is then used to estimate w*/u* ratio. The w*/u* ratio is computed for several case days during which dust devil occurrence was detected. A majority of dust devils occurred in convective boundary layer regimes characterized by w*/u* ratios exceeding 10. The above described analysis is being extended to other mars mission landing sites and results from the extended analysis will also be presented.
Feasibility of generating an artificial burst in a turbulent boundary layer, phase 2
NASA Technical Reports Server (NTRS)
Gad-El-hak, Mohamed
1989-01-01
Various drag accounts for about half of the total drag on commercial aircraft at subsonic cruise conditions. Two avenues are available to achieve drag reduction: either laminar flow control or turbulence manipulation. The present research deals with the latter approach. The primary objective of Phase 2 research was to investigate experimentally the feasibility of substantially reducing the skin-friction drag in a turbulent boundary layer. The method combines the beneficial effects of suction and a longitudinally ribbed surface. At a sufficiently large spanwise separation, the streamwise grooves act as a nucleation site causing a focusing of low-speed streaks over the peaks. Suction is then applied intermittently through longitudinal slots located at selected locations along those peaks to obliterate the low-speed regions and to prevent bursting. Phase 2 research was divided into two tasks. In the first, selective suction from a single streamwise slot was used to eliminate either a single burst-like event or a periodic train of artificially generated bursts in laminar and turbulent boundary layers that develop on a flat plate towed in a water channel. The results indicate that equivalent values of the suction coefficient as low as 0.0006 were sufficient to eliminate the artificially generated bursts in a laminar boundary layer.
Low-Reynolds-number k-epsilon model for unsteady turbulent boundary-layer flows
NASA Technical Reports Server (NTRS)
Fan, Sixin; Lakshminarayana, Budugur; Barnett, Mark
1993-01-01
An assessment of the near-wall and low-Reynolds-number functions used in low-Reynolds-number k-epsilon models suggests that they are not suitable for the near-wall region of unsteady turbulent boundary layers, where the flow is characterized by rapid changes in phase. An improved low-Reynolds-number k-epsilon model is developed in this paper. The near-wall and low-Reynolds-number functions in this model are formulated as functions of the local turbulent Reynolds numbers instead of the inner variable y(+). The present model also has the correct asymptotic behavior in the near-wall region. The turbulence model has been incorporated in an unsteady boundary-layer code and validated for unsteady turbulent boundary layers with and without adverse pressure gradients. The predictions agree well with the experimental data and the theoretical analysis. For the cases tested, the present model correctly predicts the unsteady near-wall flow and the unsteady shin friction at various frequencies.
Turbulence structure of the marine stable boundary layer over the Baltic Sea
Smedman, A.S.; Hoegstroem, U.
1994-12-31
For more than half of the year the land surfaces surrounding the Baltic Sea is warmer than the sea surface, and the marine boundary layer over the Baltic is stable. Observations, at various sites in the Baltic Sea area during the last decade. also indicate frequent occurrence of low-level jets at the top of the stable boundary layer. In many cases the marine jet can be considered as an analogy in space to the evolution of the nocturnal jet with time. The frictional decoupling occurs when warm air over the land is flowing out over the sea. Data from two areas together with model simulations are used in this study to characterize turbulence structure in the marine boundary layer. The measurements include profiles of wind and temperature on towers situated at two isolated islands, together with turbulence recordings and aircraft measurements. Also wave height and water surface temperature have been measured. The model simulations are performed with a second-order closure model.
Hot-Film and Hot-Wire Anemometry for a Boundary Layer Active Flow Control Test
NASA Technical Reports Server (NTRS)
Lenahan, Keven C.; Schatzman, David M.; Wilson, Jacob Samuel
2013-01-01
Unsteady active flow control (AFC) has been used experimentally for many years to minimize bluff-body drag. This technology could significantly improve performance of rotorcraft by cleaning up flow separation. It is important, then, that new actuator technologies be studied for application to future vehicles. A boundary layer wind tunnel was constructed with a 1ft-x-3ft test section and unsteady measurement instrumentation to study how AFC manipulates the boundary layer to overcome adverse pressure gradients and flow separation. This unsteady flow control research requires unsteady measurement methods. In order to measure the boundary layer characteristics, both hot-wire and hot-film Constant Temperature Anemometry is used. A hot-wire probe is mounted in the flow to measure velocity while a hot-film array lays on the test surface to measure skin friction. Hot-film sensors are connected to an anemometer, a Wheatstone bridge circuit with an output that corresponds to the dynamic flow response. From this output, the time varying flow field, turbulence, and flow reversal can be characterized. Tuning the anemometers requires a fan test on the hot-film sensors to adjust each output. This is a delicate process as several variables drastically affect the data, including control resistance, signal input, trim, and gain settings.
Diffusion of drag-reducing polymer solutions within a rough-walled turbulent boundary layer
NASA Astrophysics Data System (ADS)
Elbing, Brian R.; Dowling, David R.; Perlin, Marc; Ceccio, Steven L.
2010-04-01
The influence of surface roughness on diffusion of wall-injected, drag-reducing polymer solutions within a turbulent boundary layer was studied with a 0.94 m long flat-plate test model at speeds of up to 10.6 m s-1 and Reynolds numbers of up to 9106. The surface was hydraulically smooth, transitionally rough, or fully rough. Mean concentration profiles were acquired with planar laser induced fluorescence, which was the primary flow diagnostic. Polymer concentration profiles with high injection concentrations (?1000 wppm) had the peak concentration shifted away from the wall, which was partially attributed to a lifting phenomenon. The diffusion process was divided into three zonesinitial, intermediate, and final. Studies of polymer injection into a polymer ocean at concentrations sufficient for maximum drag reduction indicated that the maximum initial zone length is of the order of 100 boundary layer thicknesses. The intermediate zone results indicate that friction velocity and roughness height are important scaling parameters in addition to flow and injection conditions. Lastly, the current results were combined with those in Petrie et al. ["Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow," Exp. Fluids 35, 8 (2003)] to demonstrate that the influence of polymer degradation increases with increased surface roughness.
NASA Astrophysics Data System (ADS)
Santoso, Edi; Stull, Roland
2001-06-01
In the middle of the convective boundary layer, also known as the mixed layer, is a relatively thick region where wind speed and potential temperature are nearly uniform with height. Below this uniform layer (UL), wind speed decreases to zero at the ground, and potential temperature increases to the surface skin value. This whole region below the UL is called the radix layer (RxL), and is of order hundreds of meters thick. Within the bottom of the RxL lies the classical surface layer (order of tens of meters thick) that obeys traditional Monin-Obukhov similarity theory.The RxL depth is shown to depend on friction velocity, Deardorff velocity, and boundary layer depth. The wind RxL is usually thicker than the temperature RxL. Using RxL depth, UL wind speed, and UL potential temperature as length, velocity, and temperature scales, respectively, one can form dimensionless heights, velocities, and temperatures. When observations obtained within the RxL are plotted in this dimensionless framework, the data collapse into similarity curves. This data collapse is tightly packed for data collected over single-location homogeneous surfaces, and shows more scatter for data collected along 72-km flight tracks over heterogeneous surfaces. Empirical profile equations are proposed to describe this RxL similarity. When these profile equations are combined with the flux equations from convective transport theory, the results are new flux-profile equations for a deep region within the bottom of the convective boundary layer.These RxL profile similarity equations are calibrated using data from four sites with different roughnesses: Minnesota, BLX96-Lamont, BLX96-Meeker, and BLX96-Winfield. The empirical parameters are found to be invariant from site to site, except for the profile shape parameter for wind speed. This parameter is found to depend on standard deviation of terrain elevation, rather than on the aerodynamic roughness length. The resulting parameter values are compared with independent data from a forested fifth site, Koorin, and it is found that displacement height must be subtracted from all the heights in the RxL profile equations. The resulting profile equations could be useful for calculating wind loading on bridges, wind turbine power estimation, air pollutant transport, or other applications where wind speeds or temperatures are needed over the bottom hundreds of meters of the convective boundary layer.
Air Flow in the Boundary Layer of an Elliptic Cylinder
NASA Technical Reports Server (NTRS)
Schubauer, G B
1939-01-01
The boundary layer of an elliptic cylinder of major and minor axis 11.78 and 3.98 inches, respectively, was investigated in air stream in which the turbulence could be varied. Conditions were arranged so that the flow was two-dimensional with the major axis of the ellipse parallel to the undisturbed stream. Speed distributions across the boundary layer were determined with a hot-wire anemometer at a number of positions about the surface for the lowest and highest intensities of turbulence, with the air speed in both cases sufficiently high to produce a turbulent boundary layer over the downstream part of the surface. The magnitude and the frequency of the speed fluctuations in the boundary layer were also measured by the use of the conventional type of hot-wire turbulence apparatus. Stream turbulence was found to affect both the nature of transition from laminar to turbulent flow in the layer and the position on the surface at which transition occurred. Transition was then investigated in detail with stream turbulence of several different scales and intensities.
Boundary Layer Clouds and Vegetation-Atmosphere Feedbacks.
NASA Astrophysics Data System (ADS)
Freedman, Jeffrey M.; Fitzjarrald, David R.; Moore, Kathleen E.; Sakai, Ricardo K.
2001-01-01
An analysis of boundary layer cumulus clouds and their impact on land surface-atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995-98 but reveals large variations in monthly frequency due to periods of drought or excessive wetness. Light-cloud cover regression relationships developed from ASOS ceilometer reports for Orange, Massachusetts, and Harvard Forest insolation data show a good linear fit (r2 = 0.83) for overall cloud cover versus insolation, and a reasonable quadratic fit (r2 = 0.48) for cloud cover versus the standard deviation of insolation, which is an indicator of sky type. Diffuse fraction (the ratio of diffuse to global insolation) shows a very good correlation (r2 = 0.79) with cloud cover. The sky type-insolation relationships are then used to analyze the impact that boundary layer clouds have on the forest ecosystem, specifically net carbon uptake (FCO2), evapotranspiration (ET), and water use efficiency (WUE). During 1995, afternoon FCO2 was 52% greater on days with boundary layer cumulus clouds than on clear days, although ET was the same, indicating greater light use efficiency and WUE on partly cloudy days. For 1996-98, afternoon FCO2 was also enhanced, especially during dry periods. Further analysis indicates that the vapor pressure deficit (VPD) was significantly greater (>8 hPa) during 1995 and parts of 1996-98 on clear days as compared with partly cloudy days. A long-term drought combined with abnormally warm weather likely contributed to the high VPDs, reduced FCO2, ET, and the dearth of clouds observed during 1995. In general, the presence of boundary layer cumulus clouds enhances net carbon uptake, as compared with clear days.
Heat transfer and fluid mechanics measurements in transitional boundary layer flows
NASA Technical Reports Server (NTRS)
Wang, T.; Simon, T. W.; Buddhavarapu, J.
1985-01-01
Experimental results are presented to document hydrodynamic and thermal development of flat-plate boundary layers undergoing natural transition. Local heat transfer coefficients, skin friction coefficients and profiles of velocity, temperature and Reynolds normal and shear stresses are presented. A case with no transition and transitional cases with 0.68 percent and 2.0 percent free-stream disturbance intensities were investigated. The locations of transition are consistent with earlier data. A late-laminar state with significant levels of turbulence is documented. In late-transitional and early-turbulent flows, turbulent Prandtl number and conduction layer thickness values exceed, and the Reynolds analogy factor is less than, values previously measured in fully turbulent flows.
Drag reduction for external and internal boundary layers using riblets and polymers
NASA Astrophysics Data System (ADS)
Reidy, Laurel W.; Anderson, Greg W.
1988-01-01
The efficiency of riblets and a drag-reducing polymer solution (a polyacrylamide slurry) in high-speed water tunnels for reducing drag in turbulent boundary layers was investigated in two experiments. One was an external flow experiment, in which riblets were applied to a flat plate in a high-speed water tunnel and the skin friction drag was calculated from velocity profile data. The second was an internal flow experiment, in which riblets were applied to the inside of a 6-in diameter pipe and the friction factor was calculated from mass flow rate and pressure drop measurements. Both experiments used adhesive-backed vinyl riblet film with 0.003-in height and spacing of the symmetric V-grooves. For the flat plate test, free stream velocity and Re data indicated a maximum drag reduction of about 8.1 percent. With riblets in the pipe, however, there was about three times as much friction reduction. When the polymer slurry was used in conjunction with riblets in the pipe flow, the total drag reduction was approximately equal to the sum of the drag reductions of the two techniques used separately, with some dependence on Reynolds number.
Large mixed Ekman Hartmann boundary layers in magnetohydrodynamics
NASA Astrophysics Data System (ADS)
Rousset, F.
2004-03-01
In this paper, we study the nonlinear stability of Ekman-Hartmann type boundary layers in a rotating magnetohydrodynamics flow under a sharp spectral assumption. This generalizes the result of Desjardins et al (1999 Nonlinearity 12 181-99) obtained under a smallness assumption on a Reynolds number and the result of Rousset (2003 Arch. Rat. Mech. Anal. in press) about the stability of Ekman layers.
Magnetic field maxima in the low latitude boundary layer
NASA Technical Reports Server (NTRS)
Sonnerup, B.; Paschmann, G.; Phan, T.-D.; Luehr, H.
1992-01-01
The magnetic field often exhibits a maximum in the earth's low-latitude boundary layer. Examples of this behavior are shown using data from the AMPTE/IRM spacecraft, and it is argued that two fundamentally distinct causes exist for the excess field: (1) a depression, within the layer, of the population of medium-energy ions of magnetospheric origin and (2) field curvature effects associated with undulations of the magnetopause itself.
NASA Astrophysics Data System (ADS)
Loyer, A.; Sinou, J.-J.; Chiello, O.; Lorang, X.
2012-02-01
As noise reduction tends to be part of environmental directives, predicting squeal noise generated by disc brakes is an important industrial issue. It involves both the transient and stationary nonlinear dynamics of self-excited systems with frictional contact. Time simulation of the phenomenon is an attractive option for reducing experiment costs. However, since such computations using full finite element models of industrial disc brake systems is time-consuming, model reduction has to be performed. In this paper, both the transient and stationary nonlinear behaviors of the friction destabilized system and the effect of dynamical reduction on the nonlinear response of a simple friction destabilized system are carried out. The first part provides a description of the general modeling retained for friction destabilized systems. Then, discretization and solving processes for the stability analysis and the temporal evolution are presented. The third part presents an analysis of a sliding elastic layer for different operating conditions, in order to better understand the nonlinear behavior of such systems. Finally, spatial model reduction is performed with different kinds of reduction bases in order to analyze the different effects of modal reductions. This clearly shows the necessity of including static modes in the reduction basis and that nonlinear interactions between unstable modes are very difficult to represent with reduced bases. Finally, the proposed model and the associated studies are intended to be the benchmark cases for future comparison.
Multiple pass and multiple layer friction stir welding and material enhancement processes
Feng, Zhili [Knoxville, TN; David, Stan A. [Knoxville, TN; Frederick, David Alan [Harriman, TN
2010-07-27
Processes for friction stir welding, typically for comparatively thick plate materials using multiple passes and multiple layers of a friction stir welding tool. In some embodiments a first portion of a fabrication preform and a second portion of the fabrication preform are placed adjacent to each other to form a joint, and there may be a groove adjacent the joint. The joint is welded and then, where a groove exists, a filler may be disposed in the groove, and the seams between the filler and the first and second portions of the fabrication preform may be friction stir welded. In some embodiments two portions of a fabrication preform are abutted to form a joint, where the joint may, for example, be a lap joint, a bevel joint or a butt joint. In some embodiments a plurality of passes of a friction stir welding tool may be used, with some passes welding from one side of a fabrication preform and other passes welding from the other side of the fabrication preform.
Simulating Dispersion in the Evening-Transition Boundary Layer
NASA Astrophysics Data System (ADS)
Taylor, Alexander C.; Beare, Robert J.; Thomson, David J.
2014-12-01
We investigate dispersion in the evening-transition boundary layer using large-eddy simulation (LES). In the LES, a particle model traces pollutant paths using a combination of the resolved flow velocities and a random displacement model to represent subgrid-scale motions. The LES is forced with both a sudden switch-off of the surface heat flux and also a more gradual observed evolution. The LES shows `lofting' of plumes from near-surface releases in the pre-transition convective boundary layer; it also shows the subsequent `trapping' of releases in the post-transition near-surface stable boundary layer and residual layer above. Given the paucity of observations for pollution dispersion in evening transitions, the LES proves a useful reference. We then use the LES to test and improve a one-dimensional Lagrangian Stochastic Model (LSM) such as is often used in practical dispersion studies. The LSM used here includes both time-varying and skewed turbulence statistics. It is forced with the vertical velocity variance, skewness and dissipation from the LES for particle releases at various heights and times in the evening transition. The LSM plume spreads are significantly larger than those from the LES in the post-transition stable boundary-layer trapping regime. The forcing from the LES was thus insufficient to constrain the plume evolution, and inclusion of the significant stratification effects was required. In the so-called modified LSM, a correction to the vertical velocity variance was included to represent the effect of stable stratification and the consequent presence of wave-like motions. The modified LSM shows improved trapping of particles in the post-transition stable boundary layer.
A model of wind shear and turbulence in the surface boundary layer
NASA Technical Reports Server (NTRS)
Luers, J. K.
1973-01-01
A model of wind and turbulence has been described for the surface boundary layer. The wind structure in the surface layer is considered to be a function of the surface parameters, stability, and height. The surface parameters considered are: (1) the surface roughness length; (2) the surface friction velocity; and (3) the zero plane displacement height. The stability parameter, Z/L, where L is the Monin-Obukov stability length, describes the thermal effect on the wind profile. The logarithmic wind profile is used to describe the mean wind field in the neutral boundary layer, and a logarithmic profile with a stability defect is used to describe the stable and unstable atmospheric conditions. For the very stable conditions, the logarithmic wind law does not hold. Under this condition, the layers of the atmosphere become disconnected and large scale frontal motions are the predominate factor in defining the wind profile. Figures are presented which represent some typical wind profiles in the very stable condition.
A scaling analysis of the turbulent boundary-layer in a shallow urban lake
NASA Astrophysics Data System (ADS)
Mezemate, Yacine; Fitton, George; Tchiguirinskaia, Ioulia; Schertzer, Daniel; Bonhomme, Cline; Soulignac, Frdric; Lemaire, Bruno; Vinon Leite, Brigitte
2014-05-01
The turbulent boundary-layer (TBL) has been the focus of countless experimental and numerical studies. Due to its complex nature the dynamics of the TBL are still far from being understood. Thus, to study, in particular the scaling properties of a TBL, we use a three-dimensional velocity time-series measured from an Acoustic Doppler Current Profiler(ADCP). The ADCP is particularly useful for analysing the TBL as it is able to measure the 3D velocity in the vertical, 127 cells over 3 meters. The ADCP is positioned next to a storm water discharge point at the bottom of a shallow urban lake in Crteil, a region in Paris. The positioning of the ADCP, in a stable, stratified lake, with a strong turbulent flow occurring close to the surface has given us a unique situation in which a turbulent bounded-layer can be analysed. Vertical profiles measured in the atmospheric boundary-layer are typically intrusive due to the requirement of masts and other complex measuring structures. Moreover atmospheric profilers are normally coarsely spaced in the vertical. In order to analyse the scaling properties of the velocity we compute its energy spectrum. In a log- log plot, if the velocity is scaling, the spectral exponent is its slope. It frequently that in the presence of a boundary-layer, a -1 spectral exponent is observed. Dimensional arguments suggest a -1 spectral exponent when the energy flux becomes dependent on the friction velocity instead of the length-scale. Due to the fine vertical spacing of the measurements we are not only able to observe a -1 spectral exponent, but observe a smooth transition from a free-stream turbulent regime (spectral exponent close to -5/3) to a boundary-layer -1 exponent. Because the transition shows such a strong a depth dependence we are able to propose a general model based on dynamical equations for the scaling exponent as a function of height. This generalised scaling boundary-layer model allows one to easily reproduce the turbulent statistics in the boundary-layer.
Boundary layer measurements using hot-film sensors
NASA Technical Reports Server (NTRS)
Holmes, Harlan K.; Carraway, Debra L.
1986-01-01
Measurements in the aerodynamic boundary layer using heat transfer, hot-film sensors are receiving a significant amount of effort at the Langley Research Center. A description of the basic sensor, the signal conditioning employed, and several manifestations of the sensor are given. Results of a flow reversal sensor development are presented, and future work areas are outlined.
Atmospheric boundary layer evening transitions over West Texas
Technology Transfer Automated Retrieval System (TEKTRAN)
A systemic analysis of the atmospheric boundary layer behavior during some evening transitions over West Texas was done using the data from an extensive array of instruments which included small and large aperture scintillometers, net radiometers, and meteorological stations. The analysis also comp...
Secondary three-dimensional instability in compressible boundary layers
NASA Technical Reports Server (NTRS)
El-Hady, Nabil M.
1989-01-01
Three dimensional linear secondary instability theory is extended for compressible boundary layers on a flat plate in the presence of finite amplitude Tollmien-Schlichting waves. The focus is on principal parametric resonance responsible for strong growth of subharmonics in low disturbance environment.
Three-Dimensional Turbulent Boundary Layer With Adverse Pressure Gradient
NASA Technical Reports Server (NTRS)
Driver, David M.; Hebbar, Sheshagiri K.
1992-01-01
Report describes experiment to measure effects of adverse pressure gradient on three-dimensional turbulent boundary-layer flow; effect of streamwise gradient of pressure on crossflow of particular interest. Production of turbulent kinetic energy grows rapidly in vicinity of step as result of steep mean-flow velocity gradients. Dissipation grows less quickly than production; leading to net growth with distance along streamline.
Boundary-Layer Receptivity and Integrated Transition Prediction
NASA Technical Reports Server (NTRS)
Chang, Chau-Lyan; Choudhari, Meelan
2005-01-01
The adjoint parabold stability equations (PSE) formulation is used to calculate the boundary layer receptivity to localized surface roughness and suction for compressible boundary layers. Receptivity efficiency functions predicted by the adjoint PSE approach agree well with results based on other nonparallel methods including linearized Navier-Stokes equations for both Tollmien-Schlichting waves and crossflow instability in swept wing boundary layers. The receptivity efficiency function can be regarded as the Green's function to the disturbance amplitude evolution in a nonparallel (growing) boundary layer. Given the Fourier transformed geometry factor distribution along the chordwise direction, the linear disturbance amplitude evolution for a finite size, distributed nonuniformity can be computed by evaluating the integral effects of both disturbance generation and linear amplification. The synergistic approach via the linear adjoint PSE for receptivity and nonlinear PSE for disturbance evolution downstream of the leading edge forms the basis for an integrated transition prediction tool. Eventually, such physics-based, high fidelity prediction methods could simulate the transition process from the disturbance generation through the nonlinear breakdown in a holistic manner.
Stability of hypersonic boundary-layer flows with chemistry
NASA Technical Reports Server (NTRS)
Reed, Helen L.; Stuckert, Gregory K.; Haynes, Timothy S.
1993-01-01
The effects of nonequilibrium chemistry and three dimensionality on the stability characteristics of hypersonic flows are discussed. In two-dimensional (2-D) and axisymmetric flows, the inclusion of chemistry causes a shift of the second mode of Mack to lower frequencies. This is found to be due to the increase in size of the region of relative supersonic flow because of the lower speeds of sound in the relatively cooler boundary layers. Although this shift in frequency is present in both the equilibrium and nonequilibrium air results, the equilibrium approximation predicts modes which are not observed in the nonequilibrium calculations (for the flight conditions considered). These modes are superpositions of incoming and outgoing unstable disturbances which travel supersonically relative to the boundary-layer edge velocity. Such solutions are possible because of the finite shock stand-off distance. Their corresponding wall-normal profiles exhibit an oscillatory behavior in the inviscid region between the boundary-layer edge and the bow shock. For the examination of three-dimensional (3-D) effects, a rotating cone is used as a model of a swept wing. An increase of stagnation temperature is found to be only slightly stabilizing. The correlation of transition location (N = 9) with parameters describing the crossflow profile is discussed. Transition location does not correlate with the traditional crossflow Reynolds number. A new parameter that appears to correlate for boundary-layer flow was found. A verification with experiments on a yawed cone is provided.
Determination of Stability and Translation in a Boundary Layer
NASA Technical Reports Server (NTRS)
Crepeau, John; Tobak, Murray
1996-01-01
Reducing the infinite degrees of freedom inherent in fluid motion into a manageable number of modes to analyze fluid motion is presented. The concepts behind the center manifold technique are used. Study of the Blasius boundary layer and a precise description of stability within the flow field are discussed.
ATMOSPHERIC DISPERSION IN THE ARCTIC: WINTERTIME BOUNDARY-LAYER MEASUREMENTS
The wintertime arctic atmospheric boundary layer was investigated with micro-meteorological and SF6 tracer measurements collected in Prudhoe Bay, AK. he flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. he relatively warm maritime ...
ON HYDROMAGNETIC STRESSES IN ACCRETION DISK BOUNDARY LAYERS
Pessah, Martin E.; Chan, Chi-kwan E-mail: ckch@nordita.org
2012-05-20
Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. In order to shed light on physically viable mechanisms for angular momentum transport in this inner disk region, we examine the generation of hydromagnetic stresses and energy density in differentially rotating backgrounds with angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption of efficient angular momentum transport in the inner disk regions. This suggests that the detailed structure of turbulent MHD accretion disk boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.
An experimental investigation of an accelerated turbulent boundary layer
NASA Astrophysics Data System (ADS)
Bourassa, Corey
The experimentally determined response of the time-averaged turbulence of a Retheta = 4590 boundary layer exposed to a large favorable pressure gradient is presented. The favorable pressure gradient is generated using a linear contraction with a contraction ratio of 9.5 : 1 and is characterized by a peak acceleration parameter, Kmax = 4.4 x 10-6. Three components of velocity are measured using multiple X-wires and the wall shear stress is directly measured using oil film interferometry. The accelerated boundary layer is found to exhibit behavior typical of relaminarization for values of K ? 3.0 x 10-6 . The onset of reverse transition is believed to coincide with the modification of the near-wall structure and its eventual propagation into the downstream boundary layer. Measurements of the time-averaged turbulence flow field support this. Finally, a conceptual model of the motions of the coherent structures in the accelerated turbulent boundary layer is shown to be consistent with the measured data.
On the Effects of Surface Roughness on Boundary Layer Transition
NASA Technical Reports Server (NTRS)
Choudhari, Meelan M.; Li, Fei; Chang, Chau-Lyan; Edwards, Jack
2009-01-01
Surface roughness can influence laminar-turbulent transition in many different ways. This paper outlines selected analyses performed at the NASA Langley Research Center, ranging in speed from subsonic to hypersonic Mach numbers and highlighting the beneficial as well as adverse roles of the surface roughness in technological applications. The first theme pertains to boundary-layer tripping on the forebody of a hypersonic airbreathing configuration via a spanwise periodic array of trip elements, with the goal of understanding the physical mechanisms underlying roughness-induced transition in a high-speed boundary layer. The effect of an isolated, finite amplitude roughness element on a supersonic boundary layer is considered next. The other set of flow configurations examined herein corresponds to roughness based laminar flow control in subsonic and supersonic swept wing boundary layers. A common theme to all of the above configurations is the need to apply higher fidelity, physics based techniques to develop reliable predictions of roughness effects on laminar-turbulent transition.
Measurements Of Shock-Separated Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Brown, J. D.; Brown, J. L.; Kussoy, M. I.
1990-01-01
Report documents surface and flow-field measurements of two- and three-dimensional, shock-separated, turbulent boundary layers. Data tabulated to facilitate comparison with other measurements and computations. Shows shock-interaction shadowgraph and oil-flow pattern for each angle. Plotted profiles given for pressures, velocities, Reynolds stresses, and turbulent kinetic energies.
Hair receptor sensitivity to changes in laminar boundary layer shape.
Dickinson, B T
2010-03-01
Biologists have shown that bat wings contain distributed arrays of flow-sensitive hair receptors. The hair receptors are hypothesized to feedback information on airflows over the bat wing for enhanced stability or maneuverability during flight. Here, we study the geometric specialization of hair-like structures for the detection of changes in boundary layer velocity profiles (shapes). A quasi-steady model that relates the flow velocity profile incident on the longitudinal axis of a hair to the resultant moment and shear force at the hair base is developed. The hair length relative to the boundary layer momentum thickness that maximizes the resultant moment and shear-force sensitivity to changes in boundary layer shape is determined. The sensitivity of the resultant moment and shear force is shown to be highly dependent on hair length. Hairs that linearly taper to a point are shown to provide greater output sensitivity than hairs of uniform cross-section. On an order of magnitude basis, the computed optimal hair lengths are in agreement with the range of hair receptor lengths measured on individual bat species. These results support the hypothesis that bats use hair receptors for detecting changes in boundary layer shape and provide geometric guidelines for artificial hair sensor design and application. PMID:20157224
ANALYTICAL PARAMETERIZATIONS OF DIFFUSION: THE CONVECTIVE BOUNDARY LAYER
A brief review is made of data bases which have been used for developing diffusion parameterizations for the convective boundary layer (CBL). A variety of parameterizations for lateral and vertical dispersion, (sigma sub) and (sigma sub z), are surveyed; some of these include mec...
Response of the Tropical Boundary Layer to Weak Surface Forcing
NASA Technical Reports Server (NTRS)
Hagan, D.; Rogers, D.
1995-01-01
During the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE), a series of airborne thermal infrared observations and in situ atmospheric measurements were made near the sea surface through heights exceeding 4 km. Air movements associated with the sea surface temperature and the marine atmospheric boundary layer were studied.
FLUID MODELING OF ATMOSPHERIC DISPERSION IN THE CONVECTIVE BOUNDARY LAYER
Study of convective boundary layer (CBL) processes has depended largely upon laboratory analogs for many years. The pioneering work of Willis and Deardorff (1974) and some 35 subsequent papers by the same authors showed that much useful research could be accomplished with a re...
Carbon transport in the bottom boundary layer. Final report
Agrawal, Y.C.
1998-10-05
This report summarizes the activities and findings from a field experiment devised to estimate the rates and mechanisms of transport of carbon across the continental shelves. The specific site chosen for the experiment was the mid-Atlantic Bight, a region off the North Carolina coast. The experiment involved a large contingent of scientists from many institutions. The specific component of the program was the transport of carbon in the bottom boundary layer. The postulate mechanisms of transport of carbon in the bottom boundary layer are: resuspension and advection, downward deposition, and accumulation. The high turbulence levels in the bottom boundary layer require the understanding of the coupling between turbulence and bottom sediments. The specific issues addressed in the work reported here were: (a) What is the sediment response to forcing by currents and waves? (b) What is the turbulence climate in the bottom boundary layer at this site? and (c) What is the rate at which settling leads to carbon sequestering in bottom sediments at offshore sites?
Stability of Supersonic Boundary Layers Over Blunt Wedges
NASA Technical Reports Server (NTRS)
Balakumar, Ponnampalam
2006-01-01
Receptivity and stability of supersonic boundary layers over blunt flat plates and wedges are numerically investigated at a free stream Mach number of 3.5 and at a high Reynolds number of 10(exp 6)/inch. Both the steady and unsteady solutions are obtained by solving the full Navier-Stokes equations using the 5th-order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. Computations are performed for a flat plate with leading edge thicknesses of 0.0001, 0.001, 0.005 and 0.01 inches that give Reynolds numbers based on the leading edge thickness ranging from 1000 to 10000. Calculations are also performed for a wedge of 10 degrees half angle with different leading edge radii 0.001 and 0.01 inches. The linear stability results showed that the bluntness has a strong stabilizing effect on the stability of two-dimensional boundary layers. The transition Reynolds number for a flat plate with a leading edge thickness of 0.01 inches is about 3.5 times larger than it is for the Blasius boundary layer. It was also revealed that boundary layers on blunt wedges are far more stable than on blunt flat plates.
Inertial particle accelerations in a turbulent boundary layer
NASA Astrophysics Data System (ADS)
Warhaft, Zellman; Gerashchenko, Sergiy; Sharp, Nicole; Neuscamman, Stephanie
2008-11-01
Two dimensional Lagrangian acceleration statistics of inertial particles in a turbulent boundary layer with free stream turbulence are determined by means of a high speed particle tracking technique (Ayyalasomayajula et al. PRL, 95, 144507, 2006). The boundary layer is formed above a flat plate, and water droplets are fed into the flow from sprays placed down-stream from an active grid, and from tubes fed into the boundary layer from humidifiers. The free stream Stokes number is varied from 0.035 to 0.47. As the boundary layer plate is approached, the tails of the pdfs narrow, become negatively skewed, and their peak occurs at negative accelerations (decelerations in the stream-wise direction). The mean deceleration and its r.m.s. increase to large values close to the plate and are more pronounced with increasing Stokes number, in marked contrast to what is found in isotropic turbulence where the acceleration r.m.s. decreases with increasing Stokes number. A model shows the significance of the combined effects of shear and gravity on the acceleration statistics. The work is funded by the US NSF.
Autumnal Mixed-Phase Cloudy Boundary Layers in the Arctic.
NASA Astrophysics Data System (ADS)
Pinto, James O.
1998-06-01
Two mixed-phase cloudy boundary layer events observed over the Arctic ice pack in autumn are extensively analyzed. The local dynamic and thermodynamic structure of the boundary layers is determined from aircraft measurements including analysis of turbulence, longwave radiative transfer, and cloud microphysics. The large-scale forcing is determined from the National Centers for Environmental Prediction reanalysis fields while mesoscale forcing is estimated from 40-km aircraft box patterns. The two cases differed somewhat in their local static stability, surface characteristics, and large-scale forcing. One case was characterized by a stably stratified cloudy boundary layer over a heterogeneous surface containing numerous open leads. The other case occurred over a fairly homogenous surface of multiyear ice and consisted of a surface-based stable layer surmounted by a low-level jet and a cloud-topped mixed layer. An important large-scale factor in the development of low clouds appears to have been water vapor advection. Low clouds formed irrespective of the sign of the large-scale vertical velocity. Observed flux profiles indicate that both cloudy boundary layers are cooled through turbulent eddies except at cloud top where entrainment of warm moist air aloft occurs. Maximum turbulent kinetic energy occurs near cloud top where turbulent motions are driven by strong radiative cooling (>70 K day1) and in the vicinity of the low-level jet where turbulence is shear induced. The presence of both liquid and ice in the cloud layers appears to be a nearly steady-state feature at temperatures between 13 and 20C. Results of a simple condensed water budget indicate that these colloidally unstable mixed-phase clouds may be maintained through strong cloud-top radiative cooling. The isobaric cooling rate required to maintain the presence of both liquid and ice in a stratiform cloud is quite sensitive to variations in the highly uncertain concentration of ice-forming nuclei.
Linear and nonlinear stability of the Blasius boundary layer
NASA Technical Reports Server (NTRS)
Bertolotti, F. P.; Herbert, TH.; Spalart, P. R.
1992-01-01
Two new techniques for the study of the linear and nonlinear instability in growing boundary layers are presented. The first technique employs partial differential equations of parabolic type exploiting the slow change of the mean flow, disturbance velocity profiles, wavelengths, and growth rates in the streamwise direction. The second technique solves the Navier-Stokes equation for spatially evolving disturbances using buffer zones adjacent to the inflow and outflow boundaries. Results of both techniques are in excellent agreement. The linear and nonlinear development of Tollmien-Schlichting (TS) waves in the Blasius boundary layer is investigated with both techniques and with a local procedure based on a system of ordinary differential equations. The results are compared with previous work and the effects of non-parallelism and nonlinearity are clarified. The effect of nonparallelism is confirmed to be weak and, consequently, not responsible for the discrepancies between measurements and theoretical results for parallel flow.
On Supersonic-Inlet Boundary-Layer Bleed Flow
NASA Technical Reports Server (NTRS)
Harloff, Gary J.; Smith, Gregory E.
1995-01-01
Boundary-layer bleed in supersonic inlets is typically used to avoid separation from adverse shock-wave/boundary-layer interactions and subsequent total pressure losses in the subsonic diffuser and to improve normal shock stability. Methodologies used to determine bleed requirements are reviewed. Empirical sonic flow coefficients are currently used to determine the bleed hole pattern. These coefficients depend on local Mach number, pressure ratio, hole geometry, etc. A new analytical bleed method is presented to compute sonic flow coefficients for holes and narrow slots and predictions are compared with published data to illustrate the accuracy of the model. The model can be used by inlet designers and as a bleed boundary condition for computational fluid dynamic studies.
Modeling Disturbance Dynamics in Transitional and Turbulent Boundary Layers
NASA Technical Reports Server (NTRS)
Grosch, C. E.; Gatski, T. B. (Technical Monitor)
2002-01-01
The dynamics of an ensemble of linear disturbances in boundary-layer flows at various Reynolds numbers is studied through an analysis of the transport equations for the mean disturbance kinetic energy and energy dissipation rate. Effects of adverse and favorable pressure-gradients on the disturbance dynamics are also included in the analysis. Unlike the fully turbulent regime where nonlinear phase scrambling of the fluctuations affects the flow field even in proximity to the wall, the early stage transition regime fluctuations studied here are influenced across the boundary layer by the solid boundary. In addition, the dominating dynamics in the disturbance kinetic energy equation is governed by the energy production, pressure-transport and viscous diffusion - also in contrast to the fully turbulent regime. For the disturbance dissipation rate, a dynamic balance exists between the destruction and diffusion of dissipation.
On the theory of the turbulent boundary layer
NASA Technical Reports Server (NTRS)
Rotta, J
1953-01-01
As a rule, a division of the turbulent boundary layer is admissible: a division into a part near the wall, where the flow is governed only by the wall effects, and into an outer part, where the wall roughness and the viscosity of the flow medium affects only the wall shearing stress occurring as boundary condition but does not exert any other influence on the flow. Both parts may be investigated to a large extent independently. Under certain presuppositions there result for the outer part "similar" solutions. The theoretical considerations give a cue how to set up, by appropriate experiments and their evaluation, generally valid connections which are required for the approximate calculation of the turbulent boundary layer according to the momentum and energy theorem.
Effects of mesoscale surface inhomogeneities on atmospheric boundary layer transfer
Shaw, W.J.; Doran, J.C.; Hubbe, J.M.
1992-09-01
Defining the nature of turbulent transfer over horizontally inhomogeneous surfaces remains one of the challenges in meteorology. Because the transfer of energy and momentum through the atmospheric boundary layer forms part of the lower boundary condition for global climate models (GCMs), the problem is important. Over the last two decades, advances in sensor and computer technology wave made good point measurements of turbulent fluxes fairly routine. A fundamental question with respect to climate models, however, is how such point measurements are related to average fluxes over the area of a GCM grid box. In this paper we will use data from the field program to depict the evolution of the boundary layer over adjacent, sharply contrasting surface types on two separate occasions. We will then use simple scaling based on the observations to argue that sub-gridscale motions would often be likely to significantly alter the estimates and resulting parameterizations of GCM-scale surface fluxes in the region.
Anomalous plasma diffusion and the magnetopause boundary layer
NASA Technical Reports Server (NTRS)
Treumann, Rudolf A.; Labelle, James; Haerendel, Gerhard; Pottelette, Raymond
1992-01-01
An overview of the current state of anomalous diffusion research at the magnetopause and its role in the formation of the magnetopause boundary layer is presented. Plasma wave measurements in the boundary layer indicate that most of the relevant unstable wave modes contribute negligibly to the diffusion process at the magnetopause under magnetically undisturbed northward IMF conditions. The most promising instability is the lower hybrid drift instability, which may yield diffusion coefficients of the right order if the highest measured wave intensities are assumed. It is concluded that global stationary diffusion due to wave-particle interactions does not take place at the magnetopause. Microscopic wave-particle interaction and anomalous diffusion may contribute to locally break the MD frozen-in conditions and help in transporting large amounts of magnetosheath plasma across the magnetospheric boundary.
Accurate velocity measurements of boundary-layer flows using Doppler optical coherence tomography
NASA Astrophysics Data System (ADS)
Haavisto, Sanna; Salmela, Juha; Koponen, Antti
2015-05-01
Pulsed ultrasound Doppler velocimetry and nuclear magnetic resonance imaging are popular non-invasive measurement methods for flows of opaque fluids. The spatial and temporal resolution of these methods, however, is quite limited, and they lack accuracy, especially close to solid boundaries. In this paper, we show that solution to these problems is achieved by using Doppler optical coherence tomography (DOCT). DOCT provides simultaneous information about the fluid structure and velocity with very high spatial and temporal resolution. For benchmarking of the method we use water as the reference fluid. We show how DOCT gives a very good agreement with theory for the velocity profile, skin friction and viscosity directly from the measurement signal. The velocity profile extends from the turbulent region to viscous sublayer, and viscosity of the fluid can be calculated also from a turbulent flow with a good accuracy. Overall, DOCT is seen to be very well suited for providing new insight into boundary-layer flows, rheology and skin friction.
Evaluation of a surface panel method coupled with several boundary layer analyses
NASA Technical Reports Server (NTRS)
Kjelgaard, S. O.
1983-01-01
Three boundary layer models are interacted with an inviscid panel method to evaluate their capabilities to predict aerodynamic performance characteristics. These boundary layer models include two two-dimensional boundary layer models and one three-dimensional boundary layer model. Previously reported discrepancies between the transpiration and surface displacement inviscid boundary layer simulations are discussed and resolved. Results from each of the models are compared with experimental data.
Asymmetric turbulent boundary layers along long thin circular cylinders at low-Re
NASA Astrophysics Data System (ADS)
Jordan, Stephen A.
2015-09-01
Notable deviations of the asymmetric turbulent boundary layer (TBL) statistics from their axisymmetric counterpart along long thin circular cylinders are vitally important to the naval and oceanographic jurisdictions. Although the available experimental evidence backs their concern, the realm of parametric variability (both geometric and kinematic) is extremely limited to draw solid conclusions. We know that only small misalignments which quantify less than one degree of incidence between the freestream and the straight cylinder axis can substantially alter the boundary layer thicknesses, mean axial velocity, and Reynolds stresses. But the statistical database is plainly inadequate to justify modifying the design tools that were founded solely for axisymmetric flow conditions. Herein, we begin rectifying this drawback by numerical means. The investigation centers on low turbulent Reynolds numbers (500 ? Rea ? 2500) and small angles-of-incidence (0 < ? < 9) to validate and complement the lions-share of the present database (Rea = aUo/?, where a, Uo, and ? are the cylinder radius, freestream velocity, and kinematic viscosity, respectively). In particular, we numerically resolved the statistical responses of the TBL, mean axial velocity, Reynolds stresses, and skin friction under angles-of-incidence up to the earliest signs of Strouhal-type shedding. Clearly, the first prominent response was the thinning and thickening of the TBL along the respective windward and leeward sides to only a minor misalignment. Tilting the straight cylinder to slightly higher yaw angles transformed the TBL to a transitional boundary layer along the windward side for all simulated Reynolds numbers. For yaw angles ? > 2, all turbulent statistics of the asymmetric boundary layer were measurably dissimilar to those of the axisymmetric state.
Carrillo, Jan-Michael; Brown, W Michael; Dobrynin, Andrey
2012-01-01
We study friction between charged and neutral brush layers of bottle-brush macromolecules using molecular dynamics simulations. In our simulations the solvent molecules were treated explicitly. The deformation of the bottle-brush macromolecules under the shear were studied as a function of the substrate separation and shear stress. For charged bottle-brush layers we study effect of the added salt on the brush lubricating properties to elucidate factors responsible for energy dissipation in charged and neutral brush systems. Our simulations have shown that for both charged and neutral brush systems the main deformation mode of the bottle-brush macromolecule is associated with the backbone deformation. This deformation mode manifests itself in the backbone deformation ratio, , and shear viscosity, , to be universal functions of the Weissenberg number W. The value of the friction coefficient, , and viscosity, , are larger for the charged bottle-brush coatings in comparison with those for neutral brushes at the same separation distance, D, between substrates. The additional energy dissipation generated by brush sliding in charged bottle-brush systems is due to electrostatic coupling between bottle-brush and counterion motion. This coupling weakens as salt concentration, cs, increases resulting in values of the viscosity, , and friction coefficient, , approaching corresponding values obtained for neutral brush systems.
Characterization and friction performance of Zn/Mg/Al-CO3 layered double hydroxides
NASA Astrophysics Data System (ADS)
Li, Shuo; Bai, Zhimin; Zhao, Dong
2013-11-01
Zn/Mg/Al-CO3 layered double hydroxides (LDHs) were synthesized by coprecipitation method and the products were surface modified by oleic acid. The materials were characterized by X-ray diffraction, scanning electron microscopy, wavelength dispersive X-ray fluorescence, Fourier transform infrared spectroscopy, thermogravimetry and differential scanning calorimetry. The tribological property of LDHs in base oil was studied by using a four-ball friction test machine. The results showed that Zn/Mg/Al-CO32--LDHs had high crystallinity and hexagonal lamellar structure with average disk diameter of about 150 nm and the chemical formula was Zn0.40Mg0.28Al0.32(OH)2(CO3)0.160.28H2O. The interaction between LDHs and oleic acid molecules was based on chemisorption with a monomolecular layer on the surface of laminate. The friction test results indicated that base oil with 0.5 wt% LDHs performed optimal antifriction property and the friction coefficient and wear scar diameter reduced by 68.6% and 24.6% respectively.
The influence of bulges on boundary-layer instability
NASA Technical Reports Server (NTRS)
Elli, S.; Vandam, C. P.
1992-01-01
Local disturbances caused by a spanwise surface corrugation affect the position of the boundary-layer transition, and so the drag, of an object. This premature transition from laminar to turbulent flow is often associated with a separation of the laminar boundary-layer from its surface. Also the roughness-induced separation bubble provides an important link between the pressure and velocity fluctuations in the environment and the development of the disturbance in the laminar boundary-layer, i.e., the receptivity problem. To investigate the influence of a laminar separation bubble on boundary-layer instability, a separated flow generated by a velocity gradient over a flat plate was analyzed by direct numerical simulation using finite-difference solutions of the Navier-Stokes equations. The bubble acts as a strong amplifier of the instability waves and a highly nonlinear flow field is shown to develop downstream of the bubble. Consequently, the results of the direct numerical simulation differ noticeably from those of the classical linear stability theory proving the fact that the nonparallel effects together with the nonlinear interactions are crucial to this flow development. In the present paper, the effect of physical perturbations such as humps and hollows on boundary-layer instability is analyzed. This problem has been considered theoretically by several researchers (e.g., Nayfeh et al., 1987 and 1990; Cebeci et al., 1988). They used linear stability theory in their approach which does not include the nonparallel nor the nonlinear effects. Therefore, to account for these important effects in studying flow over humps and hollows the direct simulation technique is being implemented in generalized coordinates.
Turbulent boundary layer on a convex, curved surface
NASA Technical Reports Server (NTRS)
Gillis, J. C.; Johnston, J. P.; Kays, W. M.; Moffat, R. J.
1980-01-01
The effects of strong convex curvature on boundary layer turbulence were investigated. The data gathered on the behavior of Reynolds stress suggested the formulation of a simple turbulence model. Three sets of data were taken on two separate facilities. Both rigs had flow from a flat surface, over a convex surface with 90 deg of turning, and then onto a flat recovery surface. The geometry was adjusted so that, for both rigs, the pressure gradient along the test surface was zero - thus avoiding any effects of streamwise acceleration on the wall layers. Results show that after a sudden introduction of curvature, the shear stress in the outer part of the boundary layer is sharply diminished and is even slightly negative near the edge. The wall shear also drops off quickly downstream. In contrast, when the surface suddenly becomes flat again, the wall shear and shear stress profiles recover very slowly towards flat wall conditions.
Hypersonic Turbulent Boundary-Layer and Free Sheer Database Datasets
NASA Technical Reports Server (NTRS)
Settles, Gary S.; Dodson, Lori J.
1993-01-01
A critical assessment and compilation of data are presented on attached hypersonic turbulent boundary layers in pressure gradients and compressible turbulent mixing layers. Extensive searches were conducted to identify candidate experiments, which were subjected to a rigorous set of acceptance criteria. Accepted datasets are both tabulated and provided in machine-readable form. The purpose of this database effort is to make existing high quality data available in detailed form for the turbulence-modeling and computational fluid dynamics communities. While significant recent data were found on the subject of compressible turbulent mixing, the available boundary-layer/pressure-gradient experiments are all older ones of which no acceptable data were found at hypersonic Mach numbers.
Lateral straining of turbulent boundary layers. I - Streamline divergence
NASA Technical Reports Server (NTRS)
Saddoughi, Seyed G.; Joubert, Peter N.
1991-01-01
The effect of prolonged streamline divergence on developing turbulent boundary layers is investigated using an experimental approximation of the source flow over a flat plate to achieve a simple divergence. Results are presented of hot-wire measurements for the planes of symmetry of two layers which had the same (low) Reynolds number and were developed in the presence of the same amount of simple divergence with a maximum divergence parameter of about 0.075 but with different (by a factor of 2) pressure-gradient parameters. It was found that there were two overlapping stages of development. In the initial stage, which covered a distance of about 20 initial boundary-layer thicknesses from the start of divergence, the coupled effects of both the pressure gradient and divergence were present. In the second region, which lasts nearly to the end of the diverging section, the pressure-gradient effects were negligible.
Provenance of the K/T boundary layers
NASA Technical Reports Server (NTRS)
Hildebrand, A. R.; Boynton, W. V.
1988-01-01
An array of chemical, physical and isotopic evidence indicates that an impact into oceanic crust terminated the Cretaceous Period. Approximately 1500 cu km of debris, dispersed by the impact fireball, fell out globally in marine and nonmarine environments producing a 2 to 4 mm thick layer (fireball layer). In North American locales, the fireball layer overlies a 15 to 25 mm thick layer of similar but distinct composition. This 15 to 25 mm layer (ejecta layer) may represent approximately 1000 cu km of lower energy ejecta from a nearby impact site. Isotopic and chemical evidence supports a mantle provenance for the bulk of the layers. The extraordinary REE pattern of the boundary clays was modelled as a mixture of oceanic crust, mantle, and approximately 10 percent continental material. The results are presented. If the siderophiles of the ejecta layer were derived solely from the mantle, a test may be available to see if the siderophile element anomaly of the fireball layer had an extraterrestrial origin. Radiogenic Os-187 is depleted in the mantle relative to an undifferentiated chondritic source. Os-187/Os-186 ratios of 1.049 and 1.108 were calculated for the ejecta and fireball layers, respectively.
Role of interfacial friction for flow instabilities in a thin polar-ordered active fluid layer
NASA Astrophysics Data System (ADS)
Sarkar, Niladri; Basu, Abhik
2015-11-01
We construct a generic coarse-grained dynamics of a thin inflexible planar layer of polar-ordered suspension of active particles that is frictionally coupled to an embedding isotropic passive fluid medium with a friction coefficient Γ . Being controlled by Γ , our model provides a unified framework to describe the long-wavelength behavior of a variety of thin polar-ordered systems, ranging from wet to dry active matter and free-standing active films. Investigations of the linear instabilities around a chosen orientationally ordered uniform reference state reveal generic moving and static instabilities in the system that can depend sensitively on Γ . Based on our results, we discuss estimation of bounds on Γ in experimentally accessible systems.
On buffer layers as non-reflecting computational boundaries
NASA Technical Reports Server (NTRS)
Hayder, M. Ehtesham; Turkel, Eli L.
1996-01-01
We examine an absorbing buffer layer technique for use as a non-reflecting boundary condition in the numerical simulation of flows. One such formulation was by Ta'asan and Nark for the linearized Euler equations. They modified the flow inside the buffer zone to artificially make it supersonic in the layer. We examine how this approach can be extended to the nonlinear Euler equations. We consider both a conservative and a non-conservative form modifying the governing equations in the buffer layer. We compare this with the case that the governing equations in the layer are the same as in the interior domain. We test the effectiveness of these buffer layers by a simulation of an excited axisymmetric jet based on a nonlinear compressible Navier-Stokes equations.
Investigations on entropy layer along hypersonic hyperboloids using a defect boundary layer
NASA Technical Reports Server (NTRS)
Brazier, J. P.; Aupoix, B.; Cousteix, J.
1992-01-01
A defect approach coupled with matched asymptotic expansions is used to derive a new set of boundary layer equations. This method ensures a smooth matching of the boundary layer with the inviscid solution. These equations are solved to calculate boundary layers over hypersonic blunt bodies involving the entropy gradient effect. Systematic comparisons are made for both axisymmetric and plane flows in several cases with different Mach and Reynolds numbers. After a brief survey of the entropy layer characteristics, the defect boundary layer results are compared with standard boundary layer and full Navier-Stokes solutions. The entropy gradient effects are found to be more important in the axisymmetric case than in the plane one. The wall temperature has a great influence on the results through the displacement effect. Good predictions can be obtained with the defect approach over a cold wall in the nose region, with a first order solution. However, the defect approach gives less accurate results far from the nose on axisymmetric bodies because of the thinning of the entropy layer.
Constrained Large-eddy Simulation of Supersonic Turbulent Boundary Layer over a Compression Ramp
NASA Astrophysics Data System (ADS)
Chen, Liang; Xiao, Zuoli; Shi, Yipeng; Chen, Shiyi
2013-11-01
The mean and statistical quantities as well as the flow patterns of a supersonic turbulent boundary layer over a compression ramp are numerically investigated using the constrained large-eddy simulation (CLES) method. The compression ramp is characterized by a deflection angle of 24. The free-stream Mach number is Ma = 2.9, and the Reynolds number based on the inlet boundary layer thickness is Re ? = 2300, in accordance with the reference experiment. A rescaling recycling technique is utilized for imposing the inflow boundary. Both the spatial average and the time average methods are employed in the constraint conditions for the Reynolds stresses and heat flux in the near-wall region. The results from CLES are well compared with those from detached-eddy simulation (DES), Reynolds-averaged Navier-Stokes (RANS) simulation, traditional large-eddy simulation (TLES), the experimental and DNS data. It is found that the wall-friction distribution, the wall-pressure distribution, the size of separation bubble, etc., predicted by CLES are in good agreement with the experimental and/or DNS data. Meanwhile, CLES proves to be able to predict the locations of separation and reattachment more accurately than DES, RANS and TLES.
Linear Stabilty of a Laminar Boundary Layer with Shock Boundary Layer Interaction at Ma=4.8
NASA Astrophysics Data System (ADS)
Pagella, Alessandro; Rist, Ulrich; Wagner, Siegfried
2001-11-01
The stability behavior of a laminar boundary layer at Ma=4.8 with shock boundary layer interaction and small amplitude disturbances is investigated by linear stability theory for compressible flows (Mack 1969) and direct numerical simulation. The effect of the shock strength is assessed. The numerical scheme is based on the unsteady, compressible, three-dimensional Navier-Stokes equations. In streamwise direction, high order split type compact finite differences are used, while in wall normal direction central differences for viscous and alternating one-sided finite differences for convective terms, in spanwise direction, a spectral Fourier Series expansion are applied. Numerical oscillations, caused by high gradients of the flow variables at the shock, are damped by an implicit filter of high order in streamwise direction. For the results obtained by the simulation without impinging shock wave, non-parallel effects could be identified and quantified. Taking these non-parallel effects into account, linear stability theory could represent stability behavior of wall distant disturbance amplitude maxima with small obliqueness angles of the disturbances for the investigated cases with shock. The impinging shock wave locally influences stability behavior of the boundary layer, which is dependent on its shock-strength, applied disturbance frequency and disturbance propagation angle. A separation bubble locally displaces the boundary layer in wall normal direction. Hence, viscous instability becomes weaker and the inviscid instability picks up.
Hussanan, Abid; Zuki Salleh, Mohd; Tahar, Razman Mat; Khan, Ilyas
2014-01-01
In this paper, the heat transfer effect on the unsteady boundary layer flow of a Casson fluid past an infinite oscillating vertical plate with Newtonian heating is investigated. The governing equations are transformed to a systems of linear partial differential equations using appropriate non-dimensional variables. The resulting equations are solved analytically by using the Laplace transform method and the expressions for velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions and reduce to some well-known solutions for Newtonian fluids. Numerical results for velocity, temperature, skin friction and Nusselt number are shown in various graphs and discussed for embedded flow parameters. It is found that velocity decreases as Casson parameters increases and thermal boundary layer thickness increases with increasing Newtonian heating parameter. PMID:25302782
Hussanan, Abid; Zuki Salleh, Mohd; Tahar, Razman Mat; Khan, Ilyas
2014-01-01
In this paper, the heat transfer effect on the unsteady boundary layer flow of a Casson fluid past an infinite oscillating vertical plate with Newtonian heating is investigated. The governing equations are transformed to a systems of linear partial differential equations using appropriate non-dimensional variables. The resulting equations are solved analytically by using the Laplace transform method and the expressions for velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions and reduce to some well-known solutions for Newtonian fluids. Numerical results for velocity, temperature, skin friction and Nusselt number are shown in various graphs and discussed for embedded flow parameters. It is found that velocity decreases as Casson parameters increases and thermal boundary layer thickness increases with increasing Newtonian heating parameter. PMID:25302782
NASA Technical Reports Server (NTRS)
Omori, S.
1973-01-01
The turbulent kinetic energy equation is coupled with boundary layer equations to solve the characteristics of compressible turbulent boundary layers with mass injection and combustion. The Reynolds stress is related to the turbulent kinetic energy using the Prandtl-Wieghardt formulation. When a lean mixture of hydrogen and nitrogen is injected through a porous plate into the subsonic turbulent boundary layer of air flow and ignited by external means, the turbulent kinetic energy increases twice as much as that of noncombusting flow with the same mass injection rate of nitrogen. The magnitudes of eddy viscosity between combusting and noncombusting flows with injection, however, are almost the same due to temperature effects, while the distributions are different. The velocity profiles are significantly affected by combustion; that is, combustion alters the velocity profile as if the mass injection rate is increased, reducing the skin-friction as a result of a smaller velocity gradient at the wall. If pure hydrogen as a transpiration coolant is injected into a rocket nozzle boundary layer flow of combustion products, the temperature drops significantly across the boundary layer due to the high heat capacity of hydrogen. At a certain distance from the wall, hydrogen reacts with the combustion products, liberating an extensive amount of heat. The resulting large increase in temperature reduces the eddy viscosity in this region.
BLSTA: A boundary layer code for stability analysis
NASA Technical Reports Server (NTRS)
Wie, Yong-Sun
1992-01-01
A computer program is developed to solve the compressible, laminar boundary-layer equations for two-dimensional flow, axisymmetric flow, and quasi-three-dimensional flows including the flow along the plane of symmetry, flow along the leading-edge attachment line, and swept-wing flows with a conical flow approximation. The finite-difference numerical procedure used to solve the governing equations is second-order accurate. The flow over a wide range of speed, from subsonic to hypersonic speed with perfect gas assumption, can be calculated. Various wall boundary conditions, such as wall suction or blowing and hot or cold walls, can be applied. The results indicate that this boundary-layer code gives velocity and temperature profiles which are accurate, smooth, and continuous through the first and second normal derivatives. The code presented herein can be coupled with a stability analysis code and used to predict the onset of the boundary-layer transition which enables the assessment of the laminar flow control techniques. A user's manual is also included.
NASA Technical Reports Server (NTRS)
Tetervin, Neal; Lin, Chia Chiao
1951-01-01
A general integral form of the boundary-layer equation, valid for either laminar or turbulent incompressible boundary-layer flow, is derived. By using the experimental finding that all velocity profiles of the turbulent boundary layer form essentially a single-parameter family, the general equation is changed to an equation for the space rate of change of the velocity-profile shape parameter. The lack of precise knowledge concerning the surface shear and the distribution of the shearing stress across turbulent boundary layers prevented the attainment of a reliable method for calculating the behavior of turbulent boundary layers.
NASA Astrophysics Data System (ADS)
Wood, Robert; Bretherton, Christopher S.
2004-09-01
Estimates of marine boundary layer (MBL) depth and degree of decoupling for two regions of the subtropical and tropical east Pacific are presented using satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These data are combined in a novel way with NCEP reanalysis data and a mixing line parameterization to estimate the mean entrainment rate we over the two regions. Mean entrainment rates vary geographically and have maxima just downwind of the Californian coast (e 4 5 mm s-1), and also in the core of the equatorial east Pacific cold tongue where mean we exceeds 6 mm s-1. Entrainment exceeds subsidence by 30% or less in the subtropical stratocumulus regions. North of the equatorial cold tongue entrainment greatly exceeds subsidence, producing a rapid deepening of the MBL as air flows over a marked SST gradient.Shallow MBLs (zi < 500 700 m) are found to be well mixed in general. The decoupling increases markedly for deeper boundary layers and is well parameterized as a function of the thickness of the layer extending from the top of the surface mixed layer to the MBL inversion. This study demonstrates new ways in which large-scale observational and reanalysis datasets may be used to aid understanding of MBL boundary layer and cloud systems.
Edge Plasma Boundary Layer Generated By Kink Modes in Tokamaks
L.E. Zakharov
2010-11-22
This paper describes the structure of the electric current generated by external kink modes at the plasma edge using the ideally conducting plasma model. It is found that the edge current layer is created by both wall touching and free boundary kink modes. Near marginal stability, the total edge current has a universal expression as a result of partial compensation of the δ-functional surface current by the bulk current at the edge. The resolution of an apparent paradox with the pressure balance across the plasma boundary in the presence of the surface currents is provided.
Numerical Simulation of a Spatially Evolving Supersonic Turbulent Boundary Layer
NASA Technical Reports Server (NTRS)
Gatski, T. B.; Erlebacher, G.
2002-01-01
The results from direct numerical simulations of a spatially evolving, supersonic, flat-plate turbulent boundary-layer flow, with free-stream Mach number of 2.25 are presented. The simulated flow field extends from a transition region, initiated by wall suction and blowing near the inflow boundary, into the fully turbulent regime. Distributions of mean and turbulent flow quantities are obtained and an analysis of these quantities is performed at a downstream station corresponding to Re(sub x)= 5.548 x10(exp 6) based on distance from the leading edge.
A modeling study of marine boundary layer clouds
NASA Technical Reports Server (NTRS)
Wang, Shouping; Fitzjarrald, Daniel E.
1993-01-01
Marine boundary layer (MBL) clouds are important components of the earth's climate system. These clouds drastically reduce the amount of solar radiation absorbed by the earth, but have little effect on the emitted infrared radiation on top of the atmosphere. In addition, these clouds are intimately involved in regulating boundary layer turbulent fluxes. For these reasons, it is important that general circulation models used for climate studies must realistically simulate the global distribution of the MBL. While the importance of these cloud systems is well recognized, many physical processes involved in these clouds are poorly understood and their representation in large-scale models remains an unresolved problem. The present research aims at the development and improvement of the parameterization of these cloud systems and an understanding of physical processes involved. This goal is addressed in two ways. One is to use regional modeling approach to validate and evaluate two-layer marine boundary layer models using satellite and ground-truth observations; the other is to combine this simple model with a high-order turbulence closure model to study the transition processes from stratocumulus to shallow cumulus clouds. Progress made in this effort is presented.
Steady and unsteady 3D interactive boundary layers
NASA Astrophysics Data System (ADS)
Smith, F. T.
The paper describes theoretical and computational research on 3D steady and unsteady flows at medium-to-high Reynolds numbers (Re), aimed at increasing understanding of 3D separation and boundary-layer transition. Concerning steady 3D flows first, an interactive-boundary-layer (IBL) formulation for 3D laminar flow of an incompressible fluid over a surface-mounted obstacle is addressed computationally and compared with other methods at various Re. The computational approach is designed deliberately to capture the extra ellipicity present due to the three-dimensionality, making use of skewed shears in linear quasi-planar sweeps of the boundary layer and local updating in the 3D interaction law. Results including separation are presented for a range of Re and obstacle heights, together with grid-effect studies, and comparisons are made, first with triple-deck predictions for high Re and, second, with an alternative IBL approach presented in a companion work. The latter and the current work together yield a broad agreement on predictions for the 3D flow, stretching from the triple-deck through the IBL to thin-layer Navier-Stokes predictions, over a wide range of Re. Second, the computational approach is extended to unsteady 3D flows, for the triple-deck limit including linear and nonlinear Tollmien-Schlichting waves. Results for small and nonsmall disturbances and comparisons are presented, showing fairly encouraging agreement between theory, computations and experiments.
A numerical study of compressible turbulent boundary layers
NASA Astrophysics Data System (ADS)
Lagha, Maher; Kim, John; Eldredge, Jeff; Zhong, Xiaolin
2010-11-01
Compressible turbulent boundary layers with free-stream Mach number ranging from 2.5 up to 20 are analyzed by means of direct numerical simulation of the Navier--Stokes equations. The simulation generates its inflow condition using the rescaling-recycling method. The main objective is to study the effect of Mach number on turbulence statistics and near-wall turbulence structures. The present study shows that the main turbulence statistics can be correctly described as variable-density extensions of incompressible results. We show that the apparent increase in the magnitude of the fluctuating Mach number with increasing free-stream Mach number is a variable-property effect. Using the mean density to scale the fluctuating Mach number collapses results for different freestream Mach number. The increase in the pdf tails of the dilatation is also shown to be a variable-property effect. Compressible boundary layers are shown to be similar to incompressible boundary layers in that, without the linear coupling term, the turbulence cannot be sustained. The linear coupling term is necessary to generate the wall-layer streaks. For an adiabatic wall, the near-wall structure exhibits the same characteristics as in incompressible turbulent flow in terms of the spanwise spacing of the streaks ( 100^+). For isothermal walls, near-wall turbulence structures show their dependence on the surface heat flux.
Mean-Velocity Profile of Turbulent Boundary Layers Approaching Separation
NASA Astrophysics Data System (ADS)
Indinger, Thomas; Buschmann, Matthias H.; Gad-El-Hak, Mohamed
2005-11-01
Turbulent boundary layers approaching separation are a common flow situation in many technical applications. Numerous theoretical, experimental and numerical attempts have been made to find the proper scaling for the mean-velocity profile of this type of wall-bounded flow. However, none of these approaches seems to be completely satisfactory, and controversy still persists regarding the behavior of the mean velocity profile of turbulent boundary layers approaching separation. In this talk, we present new water-tunnel experiments of adverse-pressure-gradient turbulent boundary layers that clearly show the breakdown of the logarithmic law. Using these data and experimental results from several independent research groups, we analyze the classical scaling for ZPG TBL and the scaling by George & Castillo and Zagarola & Smits for APG TBL. Only the latter can be applied successfully for the outer region of the mean-velocity profile close to separation. It is shown that Zagarola & Smits' scaling is consistent with the classical two-layer approach, and can be applied to collapse the different data. Analyzing the Reynolds shear stress, the George & Castillo's scaling shows a reasonably good collapse of the data in the outer region.
Aero-optic characteristics of turbulent compressible boundary layers
NASA Astrophysics Data System (ADS)
Wyckham, Christopher Mark
This dissertation presents a detailed study of the aberrating effect on a plane incident wavefront of light due to its passage through a turbulent, compressible boundary layer. This aberration has important implications for the design of airborne optical systems for imaging, communications, or projection. A Shack-Hartmann sensor and associated data analysis software suite were developed and validated for the high resolution measurement of two dimensional wavefront phase. Significant improvements in wavefront reconstruction were achieved by using the calculated centroid uncertainties to weight the least squares fitting of the phase surface. Using the Shack-Hartmann sensor in a high speed, one dimensional mode, individual structures are observed propagating past the sensor in a transonic flow. The uncertainties on the reconstructed phase in this mode are very high, however. In a two dimensional mode the uncertainties are greatly reduced and a large database of individual, uncorrelated wavefronts was collected, allowing statistics to be calculated such as the rms wavefront height and the Strehl ratio. Data were collected at transonic and hypersonic speeds and with no injection or with helium or nitrogen injection into the boundary layer. In all cases except the hypersonic helium injection case, the time averaged wavefronts reveal no features in the boundary layer which are steady in time. In the hypersonic helium injection case, however, steady, longitudinal features are observed, in agreement with previous observations. When helium is injected for window cooling at high speeds, the results show there may be an opportunity to reduce the resulting distortion by taking advantage of the stable structures that form in the boundary layer by using a low bandwidth adaptive optic system. A new scaling argument is also presented to allow the prediction and comparison of wavefront data for different compressible boundary layer flow conditions. The proposed formula gives promising results over a very wide range of Mach numbers and conditions when used to compare the current work as well as previous work by others, and may prove to be a crucial tool in the study of boundary layer aero-optic behavior.
NASA Astrophysics Data System (ADS)
Lee, Hyunsoo; Son, Narae; Jeong, Hu Young; Kim, Tae Gun; Bang, Gyeong Sook; Kim, Jong Yun; Shim, Gi Woong; Goddeti, Kalyan C.; Kim, Jong Hun; Kim, Namdong; Shin, Hyun-Joon; Kim, Wondong; Kim, Sehun; Choi, Sung-Yool; Park, Jeong Young
2016-02-01
We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp2-rich phase and high friction (low conductance) in the sp3-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp2/sp3 carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp2-rich phase and high friction (low conductance) in the sp3-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp2/sp3 carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr06469d
Simulation and optimal control of wind-farm boundary layers
NASA Astrophysics Data System (ADS)
Meyers, Johan; Goit, Jay
2014-05-01
In large wind farms, the effect of turbine wakes, and their interaction leads to a reduction in farm efficiency, with power generated by turbines in a farm being lower than that of a lone-standing turbine by up to 50%. In very large wind farms or `deep arrays', this efficiency loss is related to interaction of the wind farms with the planetary boundary layer, leading to lower wind speeds at turbine level. Moreover, for these cases it has been demonstrated both in simulations and wind-tunnel experiments that the wind-farm energy extraction is dominated by the vertical turbulent transport of kinetic energy from higher regions in the boundary layer towards the turbine level. In the current study, we investigate the use of optimal control techniques combined with Large-Eddy Simulations (LES) of wind-farm boundary layer interaction for the increase of total energy extraction in very large `infinite' wind farms. We consider the individual wind turbines as flow actuators, whose energy extraction can be dynamically regulated in time so as to optimally influence the turbulent flow field, maximizing the wind farm power. For the simulation of wind-farm boundary layers we use large-eddy simulations in combination with actuator-disk and actuator-line representations of wind turbines. Simulations are performed in our in-house pseudo-spectral code SP-Wind that combines Fourier-spectral discretization in horizontal directions with a fourth-order finite-volume approach in the vertical direction. For the optimal control study, we consider the dynamic control of turbine-thrust coefficients in an actuator-disk model. They represent the effect of turbine blades that can actively pitch in time, changing the lift- and drag coefficients of the turbine blades. Optimal model-predictive control (or optimal receding horizon control) is used, where the model simply consists of the full LES equations, and the time horizon is approximately 280 seconds. The optimization is performed using a nonlinear conjugate gradient method, and the gradients are calculated by solving the adjoint LES equations. We find that the extracted farm power increases by approximately 20% when using optimal model-predictive control. However, the increased power output is also responsible for an increase in turbulent dissipation, and a deceleration of the boundary layer. Further investigating the energy balances in the boundary layer, it is observed that this deceleration is mainly occurring in the outer layer as a result of higher turbulent energy fluxes towards the turbines. In a second optimization case, we penalize boundary-layer deceleration, and find an increase of energy extraction of approximately 10%. In this case, increased energy extraction is balanced by a reduction in of turbulent dissipation in the boundary layer. J.M. acknowledges support from the European Research Council (FP7-Ideas, grant no. 306471). Simulations were performed on the computing infrastructure of the VSC Flemish Supercomputer Center, funded by the Hercules Foundation and the Flemish Government.
Boundary layer ozone - An airborne survey above the Amazon Basin
NASA Technical Reports Server (NTRS)
Gregory, Gerald L.; Browell, Edward V.; Warren, Linda S.
1988-01-01
Ozone data obtained over the forest canopy of the Amazon Basin during July and August 1985 in the course of NASA's Amazon Boundary Layer Experiment 2A are discussed, and ozone profiles obtained during flights from Belem to Tabatinga, Brazil, are analyzed to determine any cross-basin effects. The analyses of ozone data indicate that the mixed layer of the Amazon Basin, for the conditions of undisturbed meteorology and in the absence of biomass burning, is a significant sink for tropospheric ozone. As the coast is approached, marine influences are noted at about 300 km inland, and a transition from a forest-controlled mixed layer to a marine-controlled mixed layer is noted.
NASA Astrophysics Data System (ADS)
Ferrill, David A.; Morris, Alan P.; McGinnis, Ronald N.
2012-11-01
"Fault drag" (deflection of beds or other markers into folds that are convex in the direction of relative slip) is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of displacement along a fault. We analyze two sets of normal faults, with throws ranging from 0.5 m to 5 m, and associated fault-related folds in mechanically layered upper Cretaceous carbonate, anhydrite, and shale in central Texas. For each fault set, we interpret the fault displacement and fault-related folds exposed in outcrop to represent different stages in the developmental sequence. In both fault sets, faults in dolostone or limestone lose displacement and tip into less competent anhydrite or shale where deformation is accommodated by folding and smaller scale deformation. Fold wavelength is established early and at small displacement (< 1 m throw). With increasing displacement the monoclinal fold limb steepens and is extended parallel to bedding, locally producing boudinage in the most competent bed between incompetent beds. Clay smear is well developed where a 0.35 m thick clay shale is locally thinned to 0.1 m associated with fault throws of 0.5 to 5 m. Bed tilting and the development of apparent drag is not the product of frictional sliding but instead folding at the tip of an arrested, in this case upwardly, propagating normal fault. We conclude that synthetic dip associated with steep normal faults (i.e., fault drag) should not be assumed to be the product of frictional drag, but must be considered in the context of the mechanical stratigraphy. Instead, fault-tip folding in mechanically layered rocks produces synthetic dip (drag) early in the fault development history prior to propagation of the fault tip through the folded layer.
Direct numerical simulation of supersonic turbulent boundary layers
NASA Astrophysics Data System (ADS)
Guarini, Stephen
The objectives of this research were to develop a method by which the spatially developing compressible turbulent boundary layer could be simulated using a temporally developing numerical simulation and to study the physics of the compressible turbulent boundary layer. We take advantage of the technique developed by Spalart (1987, 1988) for the incompressible case. In this technique, it is recognized that the boundary layer exhibits slow growth in the streamwise direction, so the turbulence can be treated as approximately homogeneous in this direction. The slow growth is accounted for with a coordinate transformation and a multiple scale analysis. The result is a modified system of equations (Navier-Stokes plus some extra terms, which we call "slow growth terms") that are homogeneous in both the streamwise and spanwise directions and represent the state of the boundary layer at a given streamwise location (or, equivalently, a given thickness). The compressible Navier-Stokes equations are solved using a mixed Fourier and B-spline "spectral" method. The dependent variables are expanded in terms of a Fourier representation in the horizontal directions and a B-spline representation in the wall-normal direction. In the wall-normal direction non-reflecting boundary conditions are used at the freestream boundary, and zero-heat-flux no-slip boundary conditions are used at the wall. This combination of splines and Fourier methods produces a very accurate numerical method. Mixed implicit/explicit time discretization is used. Results are presented for a case with a Mach number of 2.5, and a Reynolds number, based on momentum integral thickness and wall viscosity, of Rsb{thetasp'} = 840. The results show that the van Driest transformed velocity satisfies the incompressible scalings and a narrow logarithmic region is obtained. The results for the turbulence intensities compare well with the incompressible simulations of Spalart. Pressure fluctuations are found to be higher than in incompressible flow. Morkovin's strong Reynolds analogy does not agree with the results of the simulation, however, an analogy is found between the rate of turbulent heat transfer and the rate of turbulent momentum transfer. Reynolds stress and turbulent kinetic energy budgets are computed and compared with the budgets from Spalart's incompressible simulations.
Bandgap tunability at single-layer molybdenum disulphide grain boundaries.
Huang, Yu Li; Chen, Yifeng; Zhang, Wenjing; Quek, Su Ying; Chen, Chang-Hsiao; Li, Lain-Jong; Hsu, Wei-Ting; Chang, Wen-Hao; Zheng, Yu Jie; Chen, Wei; Wee, Andrew T S
2015-01-01
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 0.05 eV for single-layer, 2.10 0.05 eV for bilayer and 1.75 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering. PMID:25687991
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
NASA Astrophysics Data System (ADS)
Huang, Yu Li; Chen, Yifeng; Zhang, Wenjing; Quek, Su Ying; Chen, Chang-Hsiao; Li, Lain-Jong; Hsu, Wei-Ting; Chang, Wen-Hao; Zheng, Yu Jie; Chen, Wei; Wee, Andrew T. S.
2015-02-01
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.400.05?eV for single-layer, 2.100.05?eV for bilayer and 1.750.05?eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.850.05?eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
Air Flow in a Separating Laminar Boundary Layer
NASA Technical Reports Server (NTRS)
Schubauer, G B
1936-01-01
The speed distribution in a laminar boundary layer on the surface of an elliptic cylinder, of major and minor axes 11.78 and 3.98 inches, respectively, has been determined by means of a hot-wire anemometer. The direction of the impinging air stream was parallel to the major axis. Special attention was given to the region of separation and to the exact location of the point of separation. An approximate method, developed by K. Pohlhausen for computing the speed distribution, the thickness of the layer, and the point of separation, is described in detail; and speed-distribution curves calculated by this method are presented for comparison with experiment.
Transport of contaminants in the planetary boundary layer
NASA Technical Reports Server (NTRS)
Lee, I. Y.; Swan, P. R.
1978-01-01
A planetary boundary layer model is described and used to simulate PBL phenomena including cloud formation and pollution transport in the San Francisco Bay Area. The effect of events in the PBL on air pollution is considered, and governing equations for the average momentum, potential temperature, water vapor mixing ratio, and air contaminants are presented. These equations are derived by integrating the basic equations vertically through the mixed layer. Characteristics of the day selected for simulation are reported, and the results suggest that the diurnally cyclic features of the mesoscale motion, including clouds and air pollution, can be simulated in a readily interpretable way with the model.
Finite-element numerical modeling of atmospheric turbulent boundary layer
NASA Technical Reports Server (NTRS)
Lee, H. N.; Kao, S. K.
1979-01-01
A dynamic turbulent boundary-layer model in the neutral atmosphere is constructed, using a dynamic turbulent equation of the eddy viscosity coefficient for momentum derived from the relationship among the turbulent dissipation rate, the turbulent kinetic energy and the eddy viscosity coefficient, with aid of the turbulent second-order closure scheme. A finite-element technique was used for the numerical integration. In preliminary results, the behavior of the neutral planetary boundary layer agrees well with the available data and with the existing elaborate turbulent models, using a finite-difference scheme. The proposed dynamic formulation of the eddy viscosity coefficient for momentum is particularly attractive and can provide a viable alternative approach to study atmospheric turbulence, diffusion and air pollution.
Atmospheric surface and boundary layers of the Amazon Basin
NASA Technical Reports Server (NTRS)
Garstang, Michael
1987-01-01
Three phases of work were performed: design of and preparation for the Amazon Boundary Layer Experiment (ABLE 2-A); execution of the ABLE 2-A field program; and analysis of the ABLE 2-A data. Three areas of experiment design were dealt with: surface based meteorological measurements; aircraft missions; and project meteorological support. The primary goal was to obtain a good description of the structure of the atmosphere immediately above the rain forest canopy (top of canopy to a few thousand meters), to describe this region during the growing daytime phase of the boundary layer; and to examine the nighttime stratified state. A secondary objective was to examine the role that deep convective storms play in the vertical transport of heat, water vapor, and other trace gases. While significant progress was made, much of the analysis remains to be done.
A portable imaging lidar for lower boundary layer atmospheric measurement
NASA Astrophysics Data System (ADS)
Liu, Xiaoqin; Hou, Zaihong; Qin, Laian; Weng, Ningquan
2015-10-01
A portable imaging lidar using continuous wave(CW) laser is built for the remote sensing of aerosol in lower boundary layer. The output beam from a simple, stable powered CW laser no modulated is transmitted into the atmosphere, and backscattered light from along the visible beam path is imaged onto a charge-coupled-device (CCD) camera. It can be used to scan atmosphere from different angles. The horizontal measurements are obtained and compared with those obtained by the America Belfort model 6230A visibility meter. The horizontal results show that the average relative error is below 20%. The temporal-spatial variations of aerosol profiles in low boundary layer are presented and discuss.
Clues and modelling for missing boundary layer in cataclysmic variables
Huang, M.; Sion, E.M.; Sparks, W.M.
1996-12-31
Recently, it has become observationally evident that during a dwarf nova (DN) outburst, a significant portion of the gravitational energy accreted onto the white dwarf (WD) may not be radiated away instantly from a narrow boundary layer as predicted by the standard disk theory. Instead, it may be stored in the WD through various mechanisms and the radiative area may be much larger; thus the long-puzzling {ital missing boundary layer} may be accounted for when the response of the WD to the accretion is considered. The results from our group and collaborators on this aspect are outlined in the first part. A progress report on the development, of a new numerical model forms the second.
Eddy mixing in planetary boundary layer and free atmosphere
NASA Astrophysics Data System (ADS)
Kurbatskiy, A.; Kurbatskaya, L.
2014-11-01
The results obtained from both atmospheric and laboratory experiment and from LES data show that, in the stably stratified flows of the atmospheric boundary layer, turbulent mixing occurs at gradient Richardson number that significantly exceed one: the inverse turbulent Prandtl number decreases with an increase in the thermal stability. The decreasing trend of the inverse turbulent Prandtl number is reproduced in a stably stratified planetary boundary layer in agreement with measurement data with aid of the high closure RANS turbulence scheme, which takes into account the influence of internal gravity waves on the eddy mixing of momentum and heat. Applicability of such RANS turbulence approach for the estimate of eddy diffusivities of momentum and heat in the upper troposphere and lower stratosphere also examined. It is concluded that the high closure RANS turbulence scheme shows the good agreement with the direct measurement data of eddy diffusivities for momentum and heat in the upper troposphere and lower stratosphere during clear-air conditions.
Temporally optimized spanwise vorticity sensor measurements in turbulent boundary layers
NASA Astrophysics Data System (ADS)
Morrill-Winter, C.; Klewicki, J.; Baidya, R.; Marusic, I.
2015-12-01
Multi-element hot-wire anemometry was used to measure spanwise vorticity fluctuations in turbulent boundary layers. Smooth wall boundary layer profiles, with very good spatial and temporal resolution, were acquired over a Krmn number range of 2000-12,700 at the Melbourne Wind Tunnel at the University of Melbourne and the University of New Hampshire's Flow Physics Facility. A custom hot-wire probe was necessary to simultaneously obtain velocity and spanwise vorticity measurements centered at a fixed point in space. A custom calibration/processing scheme was developed to utilize single-wall-parallel wires to optimize the accuracy of the measured wall-normal velocity fluctuations derived from the sensor's -array.
Effects of surface wave breaking on the oceanic boundary layer
NASA Astrophysics Data System (ADS)
He, Hailun; Chen, Dake
2011-04-01
Existing laboratory studies suggest that surface wave breaking may exert a significant impact on the formation and evolution of oceanic surface boundary layer, which plays an important role in the ocean-atmosphere coupled system. However, present climate models either neglect the effects of wave breaking or treat them implicitly through some crude parameterization. Here we use a one-dimensional ocean model (General Ocean Turbulence Model, GOTM) to investigate the effects of wave breaking on the oceanic boundary layer on diurnal to seasonal time scales. First a set of idealized experiments are carried out to demonstrate the basic physics and the necessity to include wave breaking. Then the model is applied to simulating observations at the northern North Sea and the Ocean Weather Station Papa, which shows that properly accounting for wave breaking effects can improve model performance and help it to successfully capture the observed upper ocean variability.
3D LDV Measurements in Oscillatory Boundary Layers
NASA Astrophysics Data System (ADS)
Mier, J. M.; Garcia, M. H.
2012-12-01
The oscillatory boundary layer represents a particular case of unsteady wall-bounded flows in which fluid particles follow a periodic sinusoidal motion. Unlike steady boundary layer flows, the oscillatory flow regime and bed roughness character change in time along the period for every cycle, a characteristic that introduces a high degree of complexity in the analysis of these flows. Governing equations can be derived from the general Navier-Stokes equations for the motion of fluids, from which the exact solution for the laminar oscillatory boundary layer is obtained (also known as the 2nd Stokes problem). No exact solution exists for the turbulent case, thus, understanding of the main flow characteristics comes from experimental work. Several researchers have reported experimental work in oscillatory boundary layers since the 1960's; however, larger scale facilities and the development of newer measurement techniques with improved temporal and spatial resolution in recent years provides a unique opportunity to achieve a better understanding about this type of flows. Several experiments were performed in the Large Oscillatory Water and Sediment Tunnel (LOWST) facility at the Ven Te Chow Hydrosystems Laboratory, for a range of Reynolds wave numbers between 6x10^4 < Rew < 6x10^6 over a flat and smooth bottom. A 3D Laser Doppler Velocimetry (LDV) system was used to measure instantaneous flow velocities with a temporal resolution up to ~ 1,000 Hz. It was mounted on a 3-axis traverse with a spatial resolution of 0.01 mm in all three directions. The closest point to the bottom was measured at z = 0.2 mm (z+ ? 4), which allowed to capture boundary layer features with great detail. In order to achieve true 3D measurements, 2 probes were used on a perpendicular configuration, such that u and w components were measured from a probe on the side of the flume and v component was measured from a probe pointing down through and access window on top of the flume. The top probe was submerged in a water container, such that the focal length remained constant and coincidence in the measurement volume for all 3 components was maintained when traversing the probes along the measurement profiles. Results show the existence of high turbulence levels inside the boundary layer up to about 30 mm away from the bottom. The streamwise component u shows greater intensities closer to the bottom and ahead of the freestream velocity maximum. On the contrary, the vertical component w shows smaller values of turbulent intensity, located higher up in the profile and lagging with respect to the freestream velocity maximum. Meanwhile, the spanwise component v shows similar intensities than w, happening in phase with it, but distributed all along the boundary layer, overlapping the areas of greater intensity of u and w. In addition, wall shear stress and other turbulent magnitudes related to the boundary layer were analyzed from the experimental results obtained through this research.
Thermal Separation Effect in Swirling Axisymmetric Boundary Layers
NASA Astrophysics Data System (ADS)
Herrada, M. A.; Prez-Saborid, M.; Barrero, A.
1999-11-01
Total temperature reduction in axisymmetric vortex cores is a well known phenomenon after the works by Ranque and later by Hilsch. This phenomenon has been modelled here by considering a quasi-incompressible, self-similar, near-axis, thermal, swirling boundary layer matching to a class of inviscid vortex flows which satisfy Euler equations. The analysis shows that the flow field depends on the Prandtl number and other two dimensionless parameters characterizing the inviscid vortices: a swirl parameter and a parameter accounting for the total temperature variation across the streamsurfaces. The results of the boundary layer analysis permit us to get insight about the influence of the above parameters on the thermal separation effect. Asymptotic results for large and small values of the Prandtl number are also given.
Possibilities for drag reduction by boundary layer control
NASA Technical Reports Server (NTRS)
Naiman, I.
1946-01-01
The mechanics of laminar boundary layer transition are reviewed. Drag possibilities for boundary layer control are analyzed using assumed conditions of transition Reynolds number, inlet loss, number of slots, blower efficiency, and duct losses. Although the results of such analysis are highly favorable, those obtained by experimental investigations yield conflicting results, showing only small gains, and sometimes losses. Reduction of this data indicates that there is a lower limit to the quantity of air which must be removed at the slot in order to stabilize the laminar flow. The removal of insufficient air permits transition to occur while the removal of excessive amounts of air results in high power costs, with a net drag increases. With the estimated value of flow coefficient and duct losses equal to half the dynamic pressure, drag reductions of 50% may be obtained; with twice this flow coefficient, the drag saving is reduced to 25%.
Benthic boundary layer processes in the Lower Florida Keys
Lavoie, D.L.; Richardson, M.D.; Holmes, C.
1997-01-01
This special issue of Geo-Marine Letters, "Benthic Boundary Layer Processes in the Lower Florida Keys," includes 12 papers that present preliminary results from the Key West Campaign. The Dry Tortugas and Marquesas Keys test sites were selected by a group of 115 scientists and technicians to study benthic boundary layer processes in a carbonate environment controlled by bioturbation and biogeochemical processes. Major activities included remote sediment classification; high-frequency acoustic scattering experiments; sediment sampling for radiological, geotechnical, biological, biogeochemical, physical, and geoacoustic studies; and hydrodynamic studies using an instrumented tetrapod. All these data are being used to improve our understanding of the effects of environmental processes on sediment structure and behavior.
Boundary layer integral matrix procedure code modifications and verifications
NASA Technical Reports Server (NTRS)
Evans, R. M.; Morse, H. L.
1974-01-01
A summary of modifications to Aerotherm's Boundary Layer Integral Matrix Procedure (BLIMP) code is presented. These modifications represent a preliminary effort to make BLIMP compatible with other JANNAF codes and to adjust the code for specific application to rocket nozzle flows. Results of the initial verification of the code for prediction of rocket nozzle type flows are discussed. For those cases in which measured free stream flow conditions were used as input to the code, the boundary layer predictions and measurements are in excellent agreement. In two cases, with free stream flow conditions calculated by another JANNAF code (TDK) for use as input to BLIMP, the predictions and the data were in fair agreement for one case and in poor agreement for the other case. The poor agreement is believed to result from failure of the turbulent model in BLIMP to account for laminarization of a turbulent flow. Recommendations for further code modifications and improvements are also presented.
Works on theory of flapping wing. [considering boundary layer
NASA Technical Reports Server (NTRS)
Golubev, V. V.
1980-01-01
It is shown mathematically that taking account of the boundary layer is the only way to develop a theory of flapping wings without violating the basic observations and mathematics of hydromechanics. A theory of thrust generation by flapping wings can be developed if the conventional downstream velocity discontinuity surface is replaced with the observed Karman type vortex streets behind a flapping wing. Experiments show that the direction of such vortices is the reverse of that of conventional Karman streets. The streets form by breakdown of the boundary layer. Detailed analysis of the movements of certain birds and insects during flight 'in place' is fully consistent with this theory of the lift, thrust and drag of flapping wings. Further directions for research into flight with flapping wings are indicated.
Excitation of Crossflow Instabilities in a Swept Wing Boundary Layer
NASA Technical Reports Server (NTRS)
Carpenter, Mark H.; Choudhari, Meelan; Li, Fei; Streett, Craig L.; Chang, Chau-Lyan
2010-01-01
The problem of crossflow receptivity is considered in the context of a canonical 3D boundary layer (viz., the swept Hiemenz boundary layer) and a swept airfoil used recently in the SWIFT flight experiment performed at Texas A&M University. First, Hiemenz flow is used to analyze localized receptivity due to a spanwise periodic array of small amplitude roughness elements, with the goal of quantifying the effects of array size and location. Excitation of crossflow modes via nonlocalized but deterministic distribution of surface nonuniformity is also considered and contrasted with roughness induced acoustic excitation of Tollmien-Schlichting waves. Finally, roughness measurements on the SWIFT model are used to model the effects of random, spatially distributed roughness of sufficiently small amplitude with the eventual goal of enabling predictions of initial crossflow disturbance amplitudes as functions of surface roughness parameters.
Application of boundary layer control to HSCT low speed configuration
NASA Technical Reports Server (NTRS)
Parikh, P. G.; Chen, A. W.; Yu, N. J.; Wyatt, G. H.; Timar, T.
1990-01-01
The feasibility of using boundary layer control (BLC) on a high speed civil transport (HSCT) high lift configuration for low speed performance improvement is studied. The possibility of using a part of the suction system previously designed for cruise laminar flow control (LFC) for low speed BLC mode was explored. A suction scheme for BLC was devised for the HSCT high lift configuration. The extent of the suction regions and levels were determined via panel method based inviscid flow analysis coupled with three-dimensional boundary layer analyses. The compatibility of the BLC suction requirements was assessed, and the modifications necessary to operate the system in both modes of operation, were determined. An assessment of the potential aerodynamic performance improvement resulting from an implementation of the BLC concept compared to an optimized simple flap configuration without BLC was made by an Euler code analysis of the simple flap configuration.
Optimal disturbances in boundary layers subject to streamwise pressure gradient
NASA Astrophysics Data System (ADS)
Ashpis, David; Tumin, Anatoli
2002-11-01
An analysis of the optimal non-modal growth of perturbations in a boundary layer in the presence of a streamwise pressure gradient is presented. The analysis is based on PSE equations for an incompressible fluid. Examples with Falkner-Scan profiles indicate that a favorable pressure gradient decreases the non-modal growth, while an unfavorable pressure gradient leads to an increase of the amplification. It is suggested that the transient growth mechanism be utilized to choose optimal parameters of tripping elements on a low-pressure turbine (LPT) airfoil. As an example, a boundary layer flow with a streamwise pressure gradient corresponding to the pressure distribution over a LPT airfoil is considered. It is shown that there is an optimal spacing of the tripping elements and that the transient growth effect depends on the starting point.
Optimal disturbances in boundary layers subject to streamwise pressure gradient
NASA Technical Reports Server (NTRS)
Ashpis, David E.; Tumin, Anatoli
2003-01-01
An analysis of the optimal non-modal growth of perturbations in a boundary layer in the presence of a streamwise pressure gradient is presented. The analysis is based on PSE equations for an incompressible fluid. Examples with Falkner-Scan profiles indicate that a favorable pressure gradient decreases the non-modal growth, while an unfavorable pressure gradient leads to an increase of the amplification. It is suggested that the transient growth mechanism be utilized to choose optimal parameters of tripping elements on a low-pressure turbine (LPT) airfoil. As an example, a boundary layer flow with a streamwise pressure gradient corresponding to the pressure distribution over a LPT airfoil is considered. It is shown that there is an optimal spacing of the tripping elements and that the transient growth effect depends on the starting point.
Characteristics of Mach 10 transitional and turbulent boundary layers
NASA Technical Reports Server (NTRS)
Watson, R. D.
1978-01-01
Measurements of the mean flow properties of transitional and turbulent boundary layers in helium on 4 deg and 5 deg wedges were made for flows with edge Mach numbers from 9.5 to 11.3, ratios of wall temperature to total temperature of 0.4 to 0.95, and maximum length Reynolds numbers of one hundred million. The data include pitot and total temperature surveys and measurements of heat transfer and surface shear. In addition, with the assumption of local similarity, turbulence quantities such as the mixing length were derived from the mean flow profiles. Low Reynolds number and precursor transition effects were significant factors at these test conditions and were included in finite difference boundary layer predictions.
Fluctuating pressure loads under high speed boundary layers
NASA Technical Reports Server (NTRS)
Zorumski, William E.
1987-01-01
Aeroacoustic fatigue is anticipated to control the design of significant portions of the structures of high-speed vehicles. This is due to contemplated long-duration flights at high dynamic pressures and Mach numbers with related high skin temperatures. Fluctuating pressure loads are comparatively small beneath attached turbulent boundary layers, but become important in regions of flow separation such as compression and expansion corners on elevons and rudders. The most intense loads are due to shock/boundary-layer interaction. These flows may occur in the engine-exhaust wall jet and in flows over control surfaces. A brief review is given of available research in these areas with a description of work under way at Langley Research Center.