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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Inverse solutions for laminar boundary-layer flows with separation and reattachment

Numerical solutions of the laminar, incompressible boundary layer equations are presented for flows involving separation and reattachment. Regular solutions are obtained with an inverse approach in which either the displacement thickness or the skin friction is specified; the pressure is deduced from the solution. A vorticity-stream-function formulation of the boundary layer equations is used to eliminate the unknown pressure. Solutions of the resulting finite difference equations, in which the flow direction is taken into account, are obtained by several global iteration schemes which are stable and have unconditional diagonal dominance. Results are compared with Klineberg and Steger's separated boundary layer calculations, and with Briley's solution of Navier-Stokes equations for a separated region. In addition, an approximate technique is presented in which the streamwise convection of vorticity is set equal to zero in the reversed flow region; such a technique results in a quick forward marching procedure for separated flows.

Carter, J. E.↗
NASA NTRS1991

Visualization of boundary-layer development on turbomachine blades with liquid crystals

This report documents a study of the use of liquid crystals to visualize boundary layer development on a turbomachine blade. A turbine blade model in a linear cascade of blades was used for the tests involved. Details of the boundary layer development on the suction surface of the turbine blade model were known from previous research. Temperature sensitive and shear sensitive liquid crystals were tried as visual agents. The temperature sensitive crystals were very effective in their ability to display the location of boundary layer flow separation and reattachment. Visualization of natural transition from laminar to turbulent boundary layer flow with the temperature sensitive crystals was possible but subtle. The visualization of separated flow reattachment with the shear sensitive crystals was easily accomplished when the crystals were allowed to make a transition from the focal-conic to a Grandjean texture. Visualization of flow reattachment based on the selective reflection properties of shear sensitive crystals was achieved only marginally because of the larger surface shear stress and shear stress gradient levels required for more dramatic color differences.

Vanzante, Dale E.↗
NASA NTRS2000

Separation Control at Flight Reynolds Numbers: Lessons Learned and Future Directions

Active separation control, using periodic excitation, was studied experimentally at high Reynolds numbers. The effects of compressibility, mild sweep, location o excitation slot and steady momentum transfer on the efficacy of the method were identified. Tests conducted at chord Reynolds numbers as high as 40 x 10(exp 6) demonstrated that active control using oscillatory flow excitation can effectively delay flow separation from and reattach separated flow to aerodynamic surfaces at flight conditions. The effective frequencies generate one to four vortices over the controlled region at all times, regardless of the Reynolds number. The vortices are initially amplified by the separated shear-layer, and after initiating reattachment, the strength of the vortices decay as they are convected downstream. Large amplitude, low frequency vortices break down to smaller ones upon introduction at the excitation slot. The effects of steady mass transfer were compared to those of periodic excitation. It was found that steady blowing is significantly inferior to periodic excitation in terms o performance benefits and that the response to steady blowing is abrupt, and therefore undesirable from a control point of view. Steady suction and periodic excitation are comparable in effectiveness and both exhibit a gradual response to changes in the magnitude of the control input. The combination of weak steady suction and periodic excitation is extremely effective while the addition of steady blowing could be detrimental. Compressibility effects are weak as long as separation is not caused by a shock-wave/boundary-layer interaction The undesirable effects of the shock-induced separation could be alleviated by the introduction of periodic excitation upstream of the shock wave, inside the region of supersonic flow. The effects of mild sweep were also studied and periodic excitation was found to be very effective in reattaching three-dimensional separated flow. Scaling laws that correlate 2D and 3D controlled flows were tested and verified. Several performance benefits could be gained by applying the method to existing configurations, but it is expected that the full potential of the method can only be realized through the design of new configurations. A comprehensive, fully turbulent, database was generated in order to guide the development, and enable validation, of candidate unsteady CFD design tools.

Seifert, Avi↗
NASA NTRS2000

Separation Control at Flight Reynolds Numbers: Lessons Learned and Future Directions

Active separation control, using periodic excitation, was studied experimentally at high Reynolds numbers. The effects of compressibility, mild sweep, location of excitation slot and steady momentum transfer on the efficacy of the method were identified. Tests conducted at chord Reynolds numbers as high as 40 x 10(exp 6) demonstrated that active control using oscillatory flow excitation can effectively delay flow separation from, and reattach separated flow to aerodynamic surfaces at flight conditions. The effective frequencies generate one to four vortices over the controlled region at all times, regardless of the Reynolds number. The vortices are initially amplified by the separated shear-layer, and after initiating reattachment, the strength of the vortices decay as they are convected downstream. Large amplitude, low frequency vortices break down to smaller ones upon introduction at the excitation slot. The effects of steady mass transfer were compared to those of periodic excitation. It was found that steady blowing is significantly inferior to periodic excitation in terms of performance benefits and that the response to steady blowing is abrupt, and therefore undesirable from a control point of view. Steady suction and periodic excitation are comparable in effectiveness and both exhibit a gradual response to changes in the magnitude of the control input. The combination of weak steady suction and periodic excitation is extremely effective while the addition of steady blowing could be detrimental. Compressibility effects are weak as long as separation is not caused by a shock-wave/boundary-layer interaction. The undesirable effects of the shock-induced separation could be alleviated by the introduction of periodic excitation upstream of the shock wave, inside the region of supersonic flow. The effects of mild sweep were also studied and periodic excitation was found to be very effective in reattaching three-dimensional separated flow. Scaling laws that correlate 2D and 3D controlled flows were tested and verified. Several performance benefits could be gained by applying the method to existing configurations, but it is expected that the full potential of the method can only be realized through the design of new configurations. A comprehensive, fully turbulent, database was generated in order to guide the development, and enable validation, of candidate unsteady CFD design tools.

Pack, LaTunia G.↗
NASA NTRS1972

Heat transfer at reattachment of a compressible flow over a backward facing step with a suction slot.

A simplified model is suggested for engineering estimates of heat transfer in the vicinity of reattachment for the case of supersonic flow over a backward facing step with a suction slot. The presented approach gives a closed-form approximate solution which is shown to be in satisfactory agreement with experimental results for the case of no mass transfer, and is expected to predict the effect of mass transfer in the case of suction.

Telionis, D. P.↗
NASA NTRS1981

Application of a laser interferometer skin-friction meter in complex flows

The application of a nonintrusive laser-interferometer skin-friction meter, which measures skin friction with a remotely located laser interferometer that monitors the thickness change of a thin oil film, is extended both experimentally and theoretically to several complex wind-tunnel flows. These include two-dimensional seperated and reattached subsonic flows with large pressure and shear gradients, and two and three-dimensional supersonic flows at high Reynolds number, which include variable wall temperatures and cross-flows. In addition, it is found that the instrument can provide an accurate location of the mean reattachment length for separated flows. Results show that levels up to 120 N/sq m, or 40 times higher than previous tests, can be obtained, despite encountering some limits to the method for very high skin-friction levels. It is concluded that these results establish the utility of this instrument for measuring skin friction in a wide variety of flows of interest in aerodynamic testing.

Monson, D. J.↗
NASA NTRS1979

On the laminar separation, transition, and turbulent reattachment of low Reynolds number flows near the leading edge of airfoils

The laminar separation, transition, and turbulent reattachment near the leading edge of a cylindrical nose-constant thickness airfoil model were investigated using a low turbulence, low speed smoke wind tunnel. The locations of separation, transition, and reattachment were obtained from smoke flow photographs and surface oil flow techniques for chord Reynolds numbers from about 150,000 to 470,000. These visual data combined with static pressure distributions and lift and drag measurements delineate the effects of angle of attack, flap deflection angle, and chord Reynolds number on the separation bubble characteristics. The data concerning the length of the laminar and turbulent portions of the bubble agree with the empirical prediction methods for short bubbles.

Arena, A. V.↗
NASA NTRS1983

Numerical modeling of turbulent flow in a channel

Two-dimensional incompressible turbulent flow in a channel with a backward-facing step was studied numerically by Chorin's Random Vortex Method (RVM), an algorithm capable of tracing the action of elementary turbulent eddies and their cumulative effects without imposing any restrictions upon their motions. The step occurs in one side of a channel with otherwise flat, parallel walls; its height equals 1/3, 1/4 or 1/5 the width of the channel downstream. The main objective was to investigate the behavior of the large-scale turbulent eddies in a flow and the flow characteristics in the separated shear layer, the reattached zone, and the rebuilding boundary layer after reattachment. The unsteady vorticity field and the distribution of time-averaged turbulent statistics were obtained. The effects of expansion step height and initial boundary layer state were also studied. Comparisons were made with the available experimental results. The agreement is satisfactory in the velocity profiles and in the reattachment length, and fairly good in the turbulence profiles. Also a mechanism of the development of the reattaching turbulent flow was suggested by the numerical results.

Dai, Y. W.↗
NASA NTRS1988

A closure model of diffusion transport of the Reynolds-stress equations and its application to a turbulent step flow

A study is made for the development and computations of the separating and reattaching shear flows. The highlight of the study is an attempt to predict the third-moments of turbulent velocity which is responsible for the diffusion transport of the Reynolds stresses. The present computations show that the third-moments obtained by employing the low-Reynolds number model of transport equations improve the prediction of the third-moments.

Amano, R. S.↗
NASA NTRS1989

Evolution of coherent structures in the reattachment region of a separated flow

An experimental investigation of the evolution of coherent structures in a turbulent flow over a backward facing step has been made. Downstream of reattachment the large scale structures do not return rapidly to equilibrium boundary layer conditions and their coherent contribution to the Reynolds stresses continue to increase for some distance.

Jovic, S.↗
NASA NTRS1997

A Near-Wall Reynolds-Stress Closure without Wall Normals

With the aid of near-wall asymptotic analysis and results of direct numerical simulation, a new near-wall Reynolds stress model (NNWRS) is formulated based on the SSG high-Reynolds-stress model with wall-independent near-wall corrections. Only one damping function is used for flows with a wide range of Reynolds numbers to ensure that the near-wall modifications diminish away from the walls. The model is able to reproduce complicated flow phenomena induced by complex geometry, such as flow recirculation, reattachment and boundary-layer redevelopment in backward-facing step flow and secondary flow in three-dimensional square duct flow. In simple flows, including fully developed channel/pipe flow, Couette flow and boundary-layer flow, the wall effects are dominant, and the NNWRS model predicts less degree of turbulent anisotropy in the near-wall region compared with a wall-dependent near-wall Reynolds Stress model (NWRS) developed by So and colleagues. The comparison of the predictions given by the two models rectifies the misconception that the overshooting of skin friction coefficient in backward-facing step flow prevalent in those near-wall, models with wall normal is caused by he use of wall normal.

Yuan, S. P.↗
NASA NTRS1985

A study of Reynolds-stress closure model

A hybrid model of the Reynolds stress closure was developed. This model was tested for various sizes of step flow, and the computed Reynolds stress behavior was compared with experimental data. The third order closure model was reviewed. Transport equations for the triple velocity correlation were developed and implemented in a numerical code to evaluate the behavior of the triple velocity products in various regions of the flow field including recirculating, reattaching, and redeveloping flow domains.

Amano, R. S.↗
NASA NTRS1985

Low-speed stability and control wind-tunnel investigations of effects of spanwise blowing on fighter flight characteristics at high angles of attack

The effects of spanwise blowing on two configurations representative of current fighter airplanes were investigated. The two configurations differed only in wing planform, with one incorporating a trapezoidal wing and the other a 60 delta wing. Emphasis was on determining the lateral-directional characteristics, particularly in the stall/departure angle-of-attack range; however, the effects of spanwise blowing on the longitudinal aerodynamics were also determined. The-tunnel tests included measurement of static force and forced-oscillation aerodynamic data, visualization of the airflow changes created by the spanwise blowing, and free-flight model tests. The effects of blowing rate, chordwise location of the blowing ports, asymmetric blowing, and blowing on the conventional aerodynamic control characteristics were investigated. In the angle-of-attack regions in which the spanwise blowing substantially improved the wing upper-surface flow field (i.e., provided reattachment of the flow aft of the leading-edge vortex), improvements in both static and dynamic lateral-directional stability were observed. Blowing effects on stability could be proverse or adverse depending on blowing rate, blowing port loaction, and wing planform. Free-flight model tests of the trapezoidal wing confirmed the beneficial effects of spanwise blowing measured in the static and dynamic force tests.

Satran, D. R.↗
NASA NTRS1987

Investigation of third-order closure model of turbulence for the computation of incompressible flows in a channel with a backward-facing step

To predict the diffusion process of the Reynolds stresses in reattaching shear flows, the transport model for the triple-velocity products has been developed and tested for the computation of the flow in a channel with a backward-facing step. Upon comparison of the results of uuv, uvv, and vvv with those obtained by using existing algebraic correlations, it was shown that the present model improved the prediction of the triple-velocity products.

Amano, R. S.↗
NASA NTRS1993

Wall pressure fluctuations in the reattachment region of a supersonic free shear layer

A study was made of the wall pressure fluctuations in the reattachment region of a supersonic free shear layer. The free shear layer was formed by the separation of a Mach 2.9 turbulent boundary layer from a backward facing step. Reattachment occurred on a 20-deg ramp. By adjusting the position of the ramp, the base pressure at the step was set equal to the freestream pressure, and the free shear layer formed in the absence of any turning. An array of flush-mounted, miniature, high-frequency pressure transducers was used in the vicinity of the reattachment region to make multichannel measurements of the fluctuating wall pressure. Contrary to previous observations of this flow, the reattachment region was found to be highly unsteady, and the pressure fluctuations were found to be large. The overall behavior of the wall pressure loading is similar in scale and magnitude to the unsteadiness of the wall pressure field in compression ramp flows at the same Mach number.

Shen, Z.-H.↗
NASA NTRS1980

A comprehensive comparison between experiment and prediction for a transonic turbulent separated flow

Attempts to predict surface pressure distributions on lifting surfaces have been relatively unsuccessful in the transonic regime when the shock wave is of sufficient strength to produce an extensive region of turbulent separated flow. For these conditions, the viscous flow behavior must be accurately described even to obtain reasonable predictions of surface pressure. The present paper addresses this problem. Detailed comparisons between prediction and experiment are made for a transonic, turbulent boundary-layer separation (freestream Mach number = 0.875) for which the turbulent flow properties (including the turbulent Reynolds stress) had been measured by the laser velocimeter technique from upstream of the separated region through reattachment. The flow was generated on an axisymmetric 'bump' model designed to simulate the flow on an airfoil at transonic conditions. The numerical methods used in the comparisons include the solution of the time-dependent, mass-averaged Navier-Stokes equations, and the solution of the compressible boundary-layer equations by the inverse method. Solutions were obtained for the well established Cebeci-Smith algebraic turbulence model and the more recently developed Wilcox-Rubesin two-equation turbulence model.

Johnson, D. A.↗