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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

Stability of boundary layers with porous suction strips: Experiment and theory

Low turbulence tunnel experiments on the stability and transition of 2 D boundary layers on flat plates with and without suction are described. A number of general suction cases are discussed. Test results showed that the maximum stabilization occurred when the suction was moved toward the Branch I neutral point. An analytical study of the stability of two dimensional, incompressible boundary layer flows over plates with suction through porous strips was performed. The mean flow was calculated using linearized triple deck, closed form solutions. The stability results of the triple deck theory are shown to be in good agreement with those of the interacting boundary layers. An analytical optimization scheme for the suction configuration was developd. Numerical calculations were performed corresponding to the experimental configurations. In each case, the theory correctly predicts the experimental results.

Reynolds, G. A.↗
NASA NTRS1981

Numerical experiments in homogeneous turbulence

The direct simulation methods developed by Orszag and Patternson (1972) for isotropic turbulence were extended to homogeneous turbulence in an incompressible fluid subjected to uniform deformation or rotation. The results of simulations for irrotational strain (plane and axisymmetric), shear, rotation, and relaxation toward isotropy following axisymmetric strain are compared with linear theory and experimental data. Emphasis is placed on the shear flow because of its importance and because of the availability of accurate and detailed experimental data. The computed results are used to assess the accuracy of two popular models used in the closure of the Reynolds-stress equations. Data from a variety of the computed fields and the details of the numerical methods used in the simulation are also presented.

Rogallo, R. S.↗
NASA NTRS1984

Magnetic field line reconnection experiments. VI - Magnetic turbulence

Extensive statistical analysis of the vector components of fluctuating magnetic fields have been performed in a time dependent neutral magnetic sheet. Cross spectral analysis indicates a variety of wave numbers present for each frequency investigated. Comparison of Fourier components of the cross spectral function with dispersion surfaces in k space demonstrates the waves are large amplitude whistlers; this is verified by polarization analysis, which shows the random waves' magnetic fields to be right-hand circular. Ion acoustic and Langmuir turbulence are also observed along with bursts of microwave radiation. Measurements of the electron distribution function f(v, r, t) and its fluctuations in velocity space relate wave and particle activity.

Gekelman, W.↗
NASA NTRS1986

Experiments on supersonic turbulent flow development in a square duct

The nature of supersonic, turbulent, adiabatic-wall flow in a square duct is investigated experimentally over a development length of x/D between 0 and 20 for a uniform flow, Mach 3.9 condition at the duct inlet. Initial discussion centers on the duct configuration itself, which was designed specifically to minimize wave effects and nozzle-induced distortion in the flow. Total pressure contours and local skin friction coefficient distributions are presented which show that the flow develops in a manner similar to that observed for the incompressible case. In particular, undulations exist in total pressure contours within the cross plane and in transverse skin friction coefficient distributions, which are indicative of the presence of a well-defined secondary flow superimposed upon the primary flow. The results are analyzed to show that local law-of-the-wall behavior extends well into the corner region, which implies that wall functions conventionally applied in two-equation type turbulence models, when suitably defined for compressible flow, may also be applied to supersonic streamwise corner flows.

Gessner, F. B.↗
NASA NTRS1987

Further experiments on supersonic turbulent flow development in a square duct

The mean-flow structure of supersonic, turbulent, adiabatic-wall flow in a square duct is investigated experimentally over a development length x/D = 0-50 for a uniform flow, Mach 3.9 condition at the duct inlet. The results show that a secondary flow cell structure develops which is similar to that for the incompressible case. Development of the primary flow is influenced by the combined effects of the secondary flow and the streamwise adverse pressure gradient. Total pressure, axial mean velocity, and Mach number profiles are presented which show that the outer flow is sensitive primarily to the streamwise pressure gradient, while flow in the near-wall region is dominated by the secondary flow. Axial mean-velocity profiles plotted in terms of van Driest-scaled variables show that a well-defined log-law region exists in the near-wall layer. This region exists in the presence of a secondary flow which continuously modifies spanwise wall shear stress behavior along the length of the duct.

Davis, D. O.↗
NASA NTRS2000

High Resolution Experiments of Compressible Turbulence Interacting With Shock Waves

Most of the second year of our research program focused on exploring the potentially favorable the effects of expansion waves on homogeneous and isotropic turbulence, which is formed downstream of a grid. Expansion waves are associated with compressible flows and may reduce the drag over airfoils by suppressing turbulence. In the very few previous investigations of interactions of turbulence with expansion waves the effects due to stabilizing streamline curvature substantially masked the effects of turbulence suppression due to flow expansion though the waves. In the present flow configuration planar expansion waves interact with grid generated turbulence in our high-resolution shock tube research facility. This approach will assess directly the effects of the interaction on turbulence. The first objective of our study was to identify the nature of expansion waves present in our shock tube facility. Our time-dependent numerical simulations of the flow in our facility indicated the existence of two regions of traveling expansion waves. The system of expansion waves utilized in this investigation is generated by the exiting shock wave and the induced flow behind it at the end of the driver. Several new measuring techniques are being developed which are capable of providing velocity-gradient-related quantities in compressible flows for the first time.

Agui, Juan↗
NASA NTRS1992

Model experiment to study the effect of turbulence on risetime and waveform of N waves

Typical measured sonic boom rise times are two to five times longer than rise times calculated using molecular relaxation theory. The difference may be due to atmospheric turbulence. A model experiment was set up to study the influence of turbulence on waveform and rise time of spark-produced N waves. The N waves propagate through turbulence generated by a plane jet. The model turbulence is scaled down from atmospheric turbulence by approximately the same factor as the model N wave is scaled down from the sonic boom. Our experiments show that passage through the turbulence produces a wide variety of changes in the N waveform. Spiked and rounded N waves are observed, and average rise time is increased by a factor of about 2. A tentative observation based on data obtained so far is that rise time is always increased, never decreased, by turbulence.

Lipkens, Bart↗
NASA NTRS2007

The Evolution of Modulated Wavetrains Into Turbulent Spots

Experiment are being carried out to study the process by which th almost periodic disturbance waves generated naturally by the freestream evolve into turbulence. The boundary layer on a flat plate has been used for this study. The novelty of the approach is in the form of artificial excitation that is used. In this work the flow is excited artificially by deterministic white noise. The weak T-S wave created develops down stream, becomes nonlinear and blows up locally onto a highly distorted flow. These large local distortions of the mean flow allow very high frequency disturbances to grow and form into small turbulent spots. The spots arise from the excitation, and if the same noise sequence is repeated a spot will form at the same position and time instant relative to the excitation.

Gaster, M.↗
NASA NTRS1972

Effects of spanwise rotation on the structure of two-dimensional fully developed turbulent channel flow.

Experiments on fully developed turbulent flow in a channel which is rotating at a steady rate about a spanwise axis are described. The Coriolis force components in the region of two-dimensional mean flow affect both local and global stability. Three stability-related phenomena were observed or inferred: (1) the reduction (increase) of the rate of wall-layer streak bursting in locally stabilized (destabilized) wall layers; (2) the total suppression of transition to turbulence in a stabilized layer; (3) the development of large-scale roll cells on the destabilized side of the channel by growth of a Taylor-Gortler vortex instability. Local effects of rotational stabilization, such as reduction of the turbulent stress in wall layers, can be related to the local Richardson number in a simple way. This paper not only investigates this effect, but also, by methods of flow visualization, exposes some of the underlying structure changes caused by rotation.-

Johnston, J. P.↗
NASA NTRS1981

Dynamic behavior of an unsteady turbulent boundary layer

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 percent 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. Previously announced in STAR as N81-28392

Parikh, P. G.↗
NASA NTRS1985

Unsteady turbulent boundary-layer experiments with rapidly changing free-stream conditions

Selected data sets produced by the Stanford Unsteady Turbulent Boundary Layer Program are compared to analytic solutions of the Stokes equations for different kinds of imposed, organized unsteadiness. Good agreement is found, qualifying the facility as one that faithfully produces flows of different prescribed unsteady conditions. Data sets from the facility are therefore considered especially suitable for comparison with predictions of turbulence models for unsteady flows. New data describing the development of an unsteady separation bubble are presented.

Brereton, G. J.↗
NASA NTRS1995

Numerical Simulation of Flow in a Whirling Annular Seal and Comparison with Experiments

The turbulent flow field in a simulated annular seal with a large clearance/radius ratio (0.015) and a whirling rotor was simulated using an advanced 3D CFD code SCISEAL. A circular whirl orbit with synchronous whirl was imposed on the rotor center. The flow field was rendered quasi-steady by making a transformation to a totaling frame. Standard k-epsilon model with wall functions was used to treat the turbulence. Experimentally measured values of flow parameters were used to specify the seal inlet and exit boundary conditions. The computed flow-field in terms of the velocity and pressure is compared with the experimental measurements inside the seal. The agreement between the numerical results and experimental data with correction is fair to good. The capability of current advanced CFD methodology to analyze this complex flow field is demonstrated. The methodology can also be extended to other whirl frequencies. Half- (or sub-) synchronous (fluid film unstable motion) and synchronous (rotor centrifugal force unbalance) whirls are the most unstable whirl modes in turbomachinery seals, and the flow code capability of simulating the flows in steady as well as whirling seals will prove to be extremely useful in the design, analyses, and performance predictions of annular as well as other types of seals.

Athavale, M. M.↗