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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

Broadband Shock Noise in Internally-Mixed Dual-Stream Jets

Broadband shock noise (BBSN) has been studied in some detail in single-flow jets and recently in dual-stream jets with separate flow exhaust systems. Shock noise is of great concern in these latter cases because of the noise created for the aircraft cabin by the underexpanded nozzle flow at cruise. Another case where shock noise is of concern is in the case of future supersonic aircraft that are expected to have bypass ratios small enough to justify internally mixed exhaust systems, and whose mission will push cycles to the point of imperfectly expanded flows. Dual-stream jets with internally mixed plume have some simplifying aspects relative to the separate flow jets, having a single shock structure given by the common nozzle pressure. This is used to separate the contribution of the turbulent shear layer to the broadband shock noise. Shock structure is held constant while the geometry and strength of the inner and merged shear layers are varying by changing splitter area ratio and core stream temperature. Flow and noise measurements are presented which document the efforts at separating the contribution of the inner shear layer to the broadband shock noise.

Bridges, James E.↗
NASA NTRS1989

Coherent structures in a boundary layer and shear layer of a turbulent backward-facing step flow

A wind tunnel experiment has been carried out at the NASA Ames Research Center to analyze the evolution of coherent structures from a boundary layer to a shear layer in a turbulent, backward-facing, step flow. A miniature X-wire/cold-wire probe has been used in conjunction with two arrays of cold wires, one aligned in the plane of main shear and the other in the spanwise direction of the flow, to detect and characterize delta-scale organized structures in the outer regions of the flow and to provide detailed information concerning these structures. Kinematic features of the events associated with the large scale structures were analyzed and topological pictures of the evolving flow, as well as the contributions to the Reynolds shear stress components are presented.

Jovic, Srba↗
NASA NTRS1993

Turbulence modeling of free shear layers for high-performance aircraft

The High Performance Aircraft (HPA) Grand Challenge of the High Performance Computing and Communications (HPCC) program involves the computation of the flow over a high performance aircraft. A variety of free shear layers, including mixing layers over cavities, impinging jets, blown flaps, and exhaust plumes, may be encountered in such flowfields. Since these free shear layers are usually turbulent, appropriate turbulence models must be utilized in computations in order to accurately simulate these flow features. The HPCC program is relying heavily on parallel computers. A Navier-Stokes solver (POVERFLOW) utilizing the Baldwin-Lomax algebraic turbulence model was developed and tested on a 128-node Intel iPSC/860. Algebraic turbulence models run very fast, and give good results for many flowfields. For complex flowfields such as those mentioned above, however, they are often inadequate. It was therefore deemed that a two-equation turbulence model will be required for the HPA computations. The k-epsilon two-equation turbulence model was implemented on the Intel iPSC/860. Both the Chien low-Reynolds-number model and a generalized wall-function formulation were included.

Sondak, Douglas L.↗
NASA NTRS1950

Investigations of the wall-shearing stress in turbulent boundary layers

Because of the unsatisfactory state of knowledge concerning the surface shearing stress of boundary layers with pressure gradients, the problem is re-examined. It is found that for general turbulent boundary layers in wall proximity, that is, in the laminar sublayer, in the transition zone and in the part of the completely turbulent zone near the wall, the same universal law applies as for the plate flow.

BOUNDARY LAYER, TURBULENT - WALLS↗
NASA NTRS1987

Shear-layer structures in near-wall turbulence

The structure of internal shear layer observed in the near-wall region of turbulent flows is investigated by analyzing flow fields obtained from numerical simulations of channel and boundary-layer flows. It is found that the shear layer is an important contributor to the turbulence production. The conditionally averaged production at the center of the structure was almost twice as large as the long-time mean value. The shear-layer structure is also found to retain its coherence over streamwise distances on the order of a thousand viscous length units, and propagates with a constant velocity of about 10.6 u sub rho throughout the near wall region.

Johansson, A. V.↗
NASA NTRS1979

Turbulent shear stresses in compressible boundary layers

Hot-wire anemometer measurements of turbulent shear stresses in a Mach 3 compressible boundary layer were performed in order to investigate the effects of heat transfer on turbulence. Measurements were obtained by an x-probe in a flat plate, zero pressure gradient, two dimensional boundary layer in a wind tunnel with wall to freestream temperature ratios of 0.94 and 0.71. The measured shear stress distributions are found to be in good agreement with previous results, supporting the contention that the shear stress distribution is essentially independent of Mach number and heat transfer for Mach numbers from incompressible to hypersonic and wall to freestream temperature ratios of 0.4 to 1.0. It is also found that corrections for frequency response limitations of the electronic equipment are necessary to determine the correct shear stress distribution, particularly at the walls.

Laderman, A. J.↗
NASA NTRS1979

Experimental studies of combustion in a two dimensional free shear layer

The effect of combustion on the turbulent free shear layer formed at a rearward facing step has been studied. Schlieren movies confirm the importance of large scale vortices in determining entrainment and mixing behavior. The movies, long exposure schlieren photographs, and laser anemometry velocity profiles are used to observe the spreading rate of the layer and to study the vortex formation process. It is concluded that to first order, the primary effect of combustion is felt through the change in density ratio across the layer and acceleration of the flow due to volumetric expansion of the fluid in a confined duct.

Pitz, R. W.↗
NASA NTRS1989

Calculation of reattaching shear layers in divergent channel with a multiple-time-scale turbulence model

Numerical calculations of turbulent reattaching shear layers in a divergent channel are presented. The turbulence is described by a multiple-time-scale turbulence model. The turbulent flow equations are solved by a control-volume based finite difference method. The computational results are compared with those obtained using k-epsilon turbulence models and algebraic Reynolds stress turbulence models. It is shown that the multiple-time-scale turbulence model yields significantly improved computational results than the other turbulence models in the region where the turbulence is in a strongly inequilibrium state.

Kim, S.-W.↗
NASA NTRS1990

Calculation of reattaching shear layers in divergent channel with a multiple-time-scale turbulence model

Numerical calculations of turbulent reattaching shear layers in a divergent channel are presented. The turbulence is described by a multiple-time-scale turbulence model. The turbulent flow equations are solved by a control-volume based finite difference method. The computational results are compared with those obtained using k-epsilon turbulence models and algebraic Reynolds stress turbulence models. It is shown that the multiple-time-scale turbulence model yields significantly improved computational results than the other turbulence models in the region where the turbulence is in a strongly inequilibrium state.

Kim, S.-W.↗
NASA NTRS1990

Experimental Study of a Three-Dimensional Shear-Driven Turbulent Boundary Layer with Streamwise Adverse Pressure Gradient

The effects of a strong adverse pressure gradient on a three-dimensional turbulent boundary layer are studied in an axisymmetric spinning cylinder geometry. Velocity measurements made with a three-component laser Doppler velocimeter include all three mean flow components, all six Reynolds stress components, and all ten triple-product correlations. Reynolds stress diminishes as the flow becomes three-dimensional. Lower levels of shear stress were seen to persist under adverse pressure gradient conditions. This low level of stress was seen to roughly correlate with the magnitude of cross-flow (relative to free stream flow) for this experiment as well as most of the other experiments in the literature. Variations in pressure gradient do not appear to alter this correlation. For this reason, it is hypothesized that a three-dimensional boundary layer is more prone to separate than a two-dimensional boundary layer, although it could not be directly shown here. None of the computations performed with either a Prandtl mixing length, k-epsilon, or a Launder-Reece-Rodi full Reynolds-stress model were able to predict the reduction in Reynolds stress.

Driver, David M.↗
NASA NTRS1992

The dynamics and control of fluctuating pressure loads in the reattachment region of a supersonic free shear layer

An experimental program to investigate the control of a turbulent, reattaching shear layer at Mach 2.9 is described. In preliminary experiments, schlieren photography and Rayleigh scattering were used to visualize the effects of air injection normal to the plane of the shear layer. Localized blowing was qualitatively found to increase the growth rate of the shear layer, the intensity of the turbulence, and the unsteadiness of the reattachment shock. However, uniformly distributed air injection did not appear to affect the flow strongly. These results suggest that three-dimensional disturbances are more effective for flow control than two-dimensional disturbances, and that there is a strong connection between the incoming turbulence and the shock motion in this flow.

Poggie, J.↗
NASA NTRS1981

On the structure of turbulent wakes and merging shear layers of multielement airfoils

The structure of the attached turbulent flow in the vicinity of a NACA 4412 airfoil equipped with a single-slotted flap was studied. The airfoil/flap configuration was tested at a Mach number of 0.06 and a Reynolds number of 1.3 x 10 to the 6th in a 7- by 10-Foot Wind Tunnel. Surface-pressure measurements were made on the main airfoil and on the flap. Detailed measurements, obtained using a high-spatial-resolution laser Doppler anemometer, were made of the mean velocity flow field and of the second-order statistical quantities (Reynolds stresses) in the boundary layers, wakes, and merging shear layers. The experimental observations are compared with theoretical predictions of pressure, mean velocity, and Reynolds stress.

Olson, L. E.↗
NASA NTRS1973

Mean flow and turbulence measurements in a Mach 5 free shear layer

A study of the time-averaged mean flow and the turbulence in a Mach 5 free turbulent shear layer has been performed. When the experimental data were reduced with the assumption of constant static pressure through the shear layer, the mean-velocity profile in similarity coordinates was in good agreement with the low speed velocity profile. The intensities of the velocity fluctuations were about a factor of 2 lower than the low speed measurements but with the maximum velocity fluctuations occurring in the same regions of the supersonic and low speed shear layers. A large density fluctuation was observed in the outer part of the shear layer near the boundary of the shear layer and the potential core.

Wagner, R. D.↗
NASA NTRS1993

Turbulence modeling of free shear layers for high performance aircraft

In many flowfield computations, accuracy of the turbulence model employed is frequently a limiting factor in the overall accuracy of the computation. This is particularly true for complex flowfields such as those around full aircraft configurations. Free shear layers such as wakes, impinging jets (in V/STOL applications), and mixing layers over cavities are often part of these flowfields. Although flowfields have been computed for full aircraft, the memory and CPU requirements for these computations are often excessive. Additional computer power is required for multidisciplinary computations such as coupled fluid dynamics and conduction heat transfer analysis. Massively parallel computers show promise in alleviating this situation, and the purpose of this effort was to adapt and optimize CFD codes to these new machines. The objective of this research effort was to compute the flowfield and heat transfer for a two-dimensional jet impinging normally on a cool plate. The results of this research effort were summarized in an AIAA paper titled 'Parallel Implementation of the k-epsilon Turbulence Model'. Appendix A contains the full paper.

Sondak, Douglas↗
NASA NTRS1981

An experimental study of combustion: The turbulent structure of a reacting shear layer formed at a rearward-facing step

A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward-facing step. The mean and rms averages of the turbulent velocity flow field were determined by LDV for both reacting and non-reacting flows. The reaching flow was visualized by high speed schlieren photography. Large scale structures dominate the reacting shear layer. The growth of the large scale structures is tied to the propagation of the flame. The linear growth rate of the reacting shear layer defined by the mean velocity profiles is unchanged by combustion but the virtual origin is shifted downstream. The reacting shear layer based on the mean velocity profiles is shifted toward the recirculation zone and the reattachments lengths are shortened by 30%.

Pitz, R. W.↗