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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Calculation of pressure statistics in turbulent free shear flows by direct numerical simulation

Accurate numerical simulations of turbulent free shear flows were performed in order to study the behavior of certain critical turbulence quantities which have been difficult to measure in the laboratory. The three-dimensional time-dependent nonlinear Navier-Stokes equations are solved by the direct numerical simulation technique without turbulence modeling or ad hoc closure assumptions. The pressure interaction terms were factored into diffusion and return-to-isotropy components according to two models. Simulations of the evolution of a turbulent wake with and without swirl present were performed at sufficiently high Reynolds numbers to insure that the flows were strongly nonlinear and turbulent, but low enough so that all scales of motion containing significant energy are adequately resolved. It is noted that the presence of swirl tends to enhance the amplitude of the rms pressure levels.

Metcalfe, R. W.↗
NASA NTRS1980

Turbulence suppression in free shear flows by controlled excitation

The mechanism of turbulence suppression in the near field of a free shear flow under controlled acoustic excitation is investigated. The near fields of four circular air jets, a plane air jet and a large single-stream plane free air mixing layer with laminar efflux boundary layers were monitored with hot-wire anemometers. The suppression effect is found to occur over the entire frequency range in all the flows studied, with maximal suppression (as much as 80%) at about 400 initial shear layer momentum thicknesses downstream from the exit and for excitation at a Strouhal number of 0.017. Suppression is shown to be a consequence of excitation-induced modification of the shear layer instability mode and the resulting structure, and to occur at the excitation frequency corresponding to the maximally unstable disturbance frequency of the initial free shear layer. Flow visualization reveals the presence of strong, large-scale vortical structures in the unexcited flow, which are weakened and diminished by excitation. It is concluded that excitation produces rapid roll-up and early breakdown of the shear layer, thus inhibiting the formation of energetic large-scale vortices and reducing the large fluctuation intensities caused by the passage and interaction of these vortices.

Zaman, K. B. M. Q.↗
NASA NTRS1971

Heat transfer and forces on concave surfaces in free molecule flow.

A Monte Carlo modeling technique is described for mathematically simulating free molecular flows over a concave spherical surface and a concave cylindrical surface of finite length. The half-angle of the surfaces may vary from 0 to 90 degrees, and the incident flow may have an arbitrary speed ratio and an arbitrary angle of attack. Partial diffuse reflection and imperfect energy accommodation for molecules colliding with the surfaces are also considered. Results of heat transfer, drag and lift coefficients are presented for a variety of flow conditions. The present Monte Carlo results are shown to be in very good agreement with certain available theoretical solutions.

Fan, C.↗
NASA NTRS1981

Turbulence suppression in free shear flows by controlled excitation

Turbulence suppression in the near field of a free shear flow under controlled excitation is investigated in four circular jets, a plane jet, and a plane mixing layer. The suppression is a consequence of an excitation-induced modification of the shear layer structure and occurs at the excitation frequency corresponding to the maximally unstable disturbance frequency of the initial free shear layer. The most pronounced suppression occurs when the shear layer is excited at a frequency 40% higher than the natural roll-up frequency. Excitation at a Strouhal number of about 0.017 produces a rapid roll-up and early breakdown of the shear layer, and thus inhibits the formation of the energetic large-scale vortices which otherwise survive farther downstream, grow to larger sizes, and undergo successive pairings in the corresponding unexcited flow.

Zaman, K. B. M. Q.↗
NASA NTRS1981

Higher level simulations of turbulent flows

The fundamentals of large eddy simulation are considered and various approaches to this simulation are compared. The subgrid model required by large eddy simulation is discussed as well as the use of this type of simulation in the development of models for the Reynolds-averaged equations and the application of direct simulation to the testing of both subgrid scale and Reynolds-averaged models. Numerical methods used in large eddy and direct simulation are described with emphasis on special purpose methods. Topics covered include the simulation of homogeneous flow, free shear flows, the mixing layer, wakes, and wall-bounded flows including channel flow and the boundary layer. Applications of large eddy simulation in the laboratory as in meteorological and other environmental flows are examined. Directions in which the work is proceeding and what can be expected from higher levels simulated on are examined.

Ferziger, J. H.↗
NASA NTRS1994

A vorticity dynamics based model for the turbulent dissipation: Model development and validation

A new model dissipation rate equation is proposed based on the dynamic equation of the mean-square vorticity fluctuation for large Reynolds number turbulence. The advantage of working with the vorticity fluctuation equation is that the physical meanings of the terms in this equation are more clear than those in the dissipation rate equation. Hence, the model development based on the vorticity fluctuation equation is more straightforward. The resulting form of the model equation is consistent with the spectral energy cascade analysis introduced by Lumley. The proposed model dissipation rate equation is numerically well behaved and can be applied to any level of turbulence modeling. It is applied to a realizable eddy viscosity model. Flows that are examined include: rotating homogeneous shear flows; free shear flows; a channel flow and flat plate boundary layers with and without pressure gradients; and backward facing step separated flows. In most cases, the present model predictions show considerable improvement over the standard kappa-epsilon model.

Shih, Tsan-Hsing↗
NASA NTRS1975

Mach reflection associated with over-expanded nozzle free jet flows

The occurrence of Mach reflection within the over-expanded nozzle free jet flow has been examined. A flow model emphasizing the interaction between the outer and central core streams has been developed to deal with flow situations where detailed inviscid calculations of the flowfield with Mach reflexion are not possible. The results obtained show reasonably good agreement with the available experimental data. This method has also produced comparable results where detailed calculations of the flowfield are possible.

Chow, W. L.↗
NASA NTRS1989

Direct numerical simulation of compressible free shear flows

Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.

Lele, Sanjiva K.↗
NASA NTRS1989

Direct numerical simulation of compressible free shear flows

Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. A simple explanation of the stabilizing effect of compressibility is offered. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.

Lele, Sanjiva K.↗
NASA NTRS1988

Direct numerical simulation of compressible free shear flows

Direct numerical simulations of compressible free shear layers in open domains are conducted. Compact finite-difference schemes of spectral-like accuracy are used for the simulations. Both temporally-growing and spatially-growing mixing layers are studied. The effect of intrinsic compressibility on the evolution of vortices is studied. The use of convective Mach number is validated. Details of vortex roll up and pairing are studied. A simple explanation of the stabilizing effect of compressibility is offered. Acoustic radiation from vortex roll up, pairing and shape oscillations is studied and quantified.

Lele, Sanjiva K.↗
NASA NTRS1989

Coherent structures in transitional and turbulent free shear flows

The development of a quantitative understanding of large-scale coherent structures in shear flows has led to the recognition of their evolutionary features and 'rules' of nonlinear interaction among each other. These determinations, in conjunction with the study of fine-grained turbulence and mean motion, constitute a general view of hydrodynamical instabilities that has resulted in useful concepts for the achievement of shear-flow control. The dynamical role of longitudinal structures in free shear flows is presently exemplified by the axisymmetric and helical modes in a round jet, which resemble the two-dimensional and spanwise-periodic three-dimensional modes in a two-dimensional mean flow.

Liu, J. T. C.↗
NASA NTRS2011

Wedge and Conical Probes for the Instantaneous Measurement of Free-Stream Flow Quantities at Supersonic Speeds

Wedge and conical shaped probes for the measurement of free-stream flow quantities at supersonic speeds have been tested in both wind tunnel and flight. These probes have improved capabilities over similar ones used in the past. Through the use of miniature pressure sensors, that are located inside the probes, they are able to provide instantaneous measurements of a time-varying environment. Detailed herein are the results of the tests in NASA Langley Researcher Center s Unitary Plan Wind Tunnel (UPWT) at Mach numbers of 1.6, 1.8 and 2.0, as well as flight tests carried out at the NASA Dryden Flight Research Center (DFRC) on its F-15 aircraft up to Mach numbers of 1.9. In the flight tests the probes were attached to a fixture on the underside of the F-15 fuselage. Problems controlling the velocity of the flow through the conical probe, required for accurate temperature measurements, are noted, as well as some calibration problems of the miniature pressure sensors that impact the accuracy of the measurements.

Bobbitt, Percy J.↗
NASA NTRS1991

Investigation of supersonic modes and three-dimensionality in bounded, free shear flows

It is found through two-dimensional temporal simulations of high-speed free shear layers that mean flow distortion is significantly increased when supersonic disturbances are introduced as initial conditions. The shear layer exhibits no subharmonic growth or roll-up, but rather a spectral broadening as energy is distributed into higher harmonics. Increasing the velocity of one side of the mixing layer (u2) to roughly 1/5 the speed of the high speed side (u1), allows a slight subharmonic growth at a very slow rate for two-dimensional modes. A first look at three-dimensional free shear flows is also presented for M = 2. No effect is seen for incompressible flow; however, stabilization is seen with respect to maximum temporal growth rates as the transverse velocity is increased. A much stronger, but similar effect is seen if u2 is increased. The wave direction of maximum growth for u2 is greater than 0.4 is found to be the direction of the faster stream (u1) over a broad range of transverse velocities.

Macaraeg, Michele G.↗
NASA NTRS1986

Large-scale coherent structures in free turbulent flows and their aerodynamic sound

After interpreting the observed physical features of large-scale coherent structures in free shear flows on the basis of conservation principles, the role of such structures in sources of turbulent jet sound is discussed. It is found that the lower-frequency sound, which comes from lower-frequency coherent structures peaking further downstream, radiates preferentially nearer the jet axis; the peak radiation moves away from the jet axis as the frequency increases.

Liu, J. T. C.↗