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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Cell volume control at a surface for three-dimensional grid generation packages

An alternate method of calculating the cell size for orthogonality control in the solution of Poisson's 3D space equations is presented. The method provides the capability to enforce a better initial guess for the grid distribution required for boundary layer resolution. This grid point distribution is accomplished by enforcing grid spacing from a grid block boundary where orthogonality is required. The actual grid spacing or cell size for that boundary is determined by the two or four adjacent boundaries in the grid block definition, which are two dimensional grids. These two dimensional grids are in turn defined by the user using insight into the flow field and boundary layer characteristics. The adjoining boundaries are extended using a multifunctional blending scheme, with user control of the blending and interpolating functions to be used. This grid generation procedure results in an enhanced computational fluid dynamics calculation by allowing a quicker resolution of the configuration's boundary layer and flow field and by limiting the number of grid re-adaptations. The cell size specification calculation was applied to a variety of configurations ranging from axisymmetric to complex three-dimensional configurations. Representative grids are shown for the Space Shuttle and the Langley Lifting Body (HL-20).

Alter, Stephen J.↗
NASA NTRS1979

Effect of viscosity on wind-tunnel wall interference for airfoils at high lift

The effect of the walls of a wind tunnel on the subsonic, two-dimensional flow past airfoils at high angles of attack is studied theoretically and experimentally. The computerized analysis, which is based on iteratively coupled potential-flow, boundary-layer, and separated-flow analyses, includes determining the effect of viscosity and flow separation on the airfoil/wall interaction. Predictions of the effects of wind-tunnel wall on the lift of airfoils are compared with wall corrections based on inviscid image analyses, and with experimental data. These comparisons are made for airfoils that are large relative to the size of the test section of the wind tunnel. It is shown that the inviscid image modeling of the wind-tunnel interaction becomes inaccurate at lift coefficients near maximum lift or when the airfoil/wall interaction is particularly strong. It is also shown that the present method of analysis (which includes boundary-layer and flow-separation effects) will provide accurate wind-tunnel wall corrections for lift coefficients up to maximum lift.

Olson, L. E.↗
NASA NTRS1993

Simultaneous measurement of velocity and temperature fluctuations in the boundary layer of a supersonic flow

The instantaneous velocity and temperature behavior of a Mach 3 turbulent boundary layer was studied using two closely spaced constant temperature hot wires. By operating the wires at different temperatures, it was possible to obtain the instantaneous velocity and temperature fluctuations and to evaluate the strong Reynolds analogy. Our analysis revealed that the strong Reynolds analogy is a good approximation for describing the instantaneous velocity and temperature behavior for the inner 70 percent of the boundary layer.

Smith, Douglas R.↗
NASA NTRS1982

Near-wall similarity in three-dimensional turbulent boundary layers. I - Model review. II - Pressure-driven flow results

Eleven proposed models for near-wall similarity for three-dimensional turbulent boundary layer flows are reviewed. Six of these models are comparatively simple scalar models and five are more complex and/or two-component vector models. Ten of the models can be tested as to their validity or predictive capability with the aid of measured mean velocity field, wall pressure field, and direct wall shear stress field (magnitude and direction) data. One of the models cannot be tested owing to its dependence on two parameters that are at present extremely difficult (if not impossible) to measure. Ten three-dimensional near-wall similarity models are then evaluated with direct wall shear, velocity field, and pressure gradient data from a three-dimensional pressure-driven boundary layer flow. In a primary focus of the interval where y+ is between 50 and 300, graphical results suggest that six simpler models and the freestream component of one complex model are adequate for profiles with monotone increasing skew up to about 15 deg.

Pierce, F. J.↗
NASA NTRS1986

Mean flow in turbulent boundary layers disturbed to alter skin friction

A comparative evaluation is made of recent developments in methods for the reduction of boundary layer drag, encompassing longitudinal surface riblets, 'outer layer' devices, (OLDs) and longitudinal convex surface curvature. The boundary layer of a surface with a longitudinal concave curvature is also studied, to complement the convex case results. The net drag reductions achievable by both riblets and OLDs are noted to be a rather modest 10 percent. Boundary layers exhibit asymmetric response to streamwise surface curvatures, with the response being slower for the case of a concave than a convex curvature.

Bandyopadhyay, P. R.↗
NASA NTRS1989

Physical analysis and second-order modelling of an unsteady turbulent flow - The oscillating boundary layer on a flat plate

The turbulent boundary layer under a freestream whose velocity varies sinusoidally in time around a zero mean is computed using two second order turbulence closure models. The time or phase dependent behavior of the Reynolds stresses are analyzed and results are compared to those of a previous SPALART-BALDWIN direct simulation. Comparisons show that the second order modeling is quite satisfactory for almost all phase angles, except in the relaminarization period where the computations lead to a relatively high wall shear stress.

Ha Minh, H.↗
NASA NTRS1948

Maintaining Laminar Flow in the Boundary Layer using a Swept-Back Wing

The positions of boundary-layer transition were ascertained experimentally for a swept-back wing and a wing without sweepback which were alike in all other respects and were compared for the same angle of attack (R(sub e) = 5.6 x 10(exp 5)). The swept-back wing in a definite range of angle of attack resulted in a backward shift of the transition point on the suction side of the wing. The favorable effect of sweepback on the position of the transition point is confirmed, consequently. In addition to decreasing the drag at high Mach numbers, the swept-back wing is acknowledged to have additional advantages. These are: (1) Decrease of the pressure drag. The reduction factor is approximately equal to the cosine of the angle of sweepback. (2) Backward shift of the transition point. There are no known experiments which establish experimentally the advantage anticipated. It appeared justifiable, therefore, to carry out some fundamental experiments which might furnish some idea of the magnitude of the advantage expected. Such an experiment is reported in what follows; the advantage of the sweepback appears clearly.

Brennecke↗
NASA NTRS1949

Boundary Layer Theory: Turbulent Flows - Part 2

The flow laws of the actual flows at high Reynolds numbers differ considerably from those of the laminar flows treated in the preceding part. These actual flows show a special characteristic, denoted as turbulence. The character of a turbulent flow is most easily understood the case of the pipe flow. Consider the flow through a straight pipe of circular cross section and with a smooth wall. For laminar flow each fluid particle moves with uniform velocity along a rectilinear path. Because of viscosity, the velocity of the particles near the wall is smaller than that of the particles at the center. i% order to maintain the motion, a pressure decrease is required which, for laminar flow, is proportional to the first power of the mean flow velocity. Actually, however, one ob~erves that, for larger Reynolds numbers, the pressure drop increases almost with the square of the velocity and is very much larger then that given by the Hagen Poiseuille law. One may conclude that the actual flow is very different from that of the Poiseuille flow.

Schlichting, H.↗
NASA NTRS2011

Experimental Measurement of Transonic Fan Wake Response to Uniform and Simulated Boundary Layer Ingesting Inlet Flows

BWB Aircraft with embedded engines and BLI inlets offer attractive advantages in terms of reduced noise from engines and increased range and fuel economy. The BLI inlet produces inlet distortion patterns that can reduce fan performance and stall margin, and can produce undesirable forced responses. Knowledge of the dynamic response of fan flow when subjected to flow distortions of the type produced by BLI inlets is important for the design of distortion tolerant fans. This project is investigating fan response to flow distortion by measuring the response of the fan of a JT15D engine to a flow pattern following the results of the NASA Inlet A BLI wind tunnel tests.

O'Brien, Walter F.↗
NASA NTRS1992

Hypersonic flow separation in shock wave boundary layer interactions

An assessment is presented for the experimental data on separated flow in shock wave turbulent boundary layer interactions at hypersonic and supersonic speeds. The data base consists mainly of two dimensional and axisymmetric interactions in compression corners or cylinder-flares, and externally generated oblique shock interactions with boundary layers over flat plates or cylindrical surfaces. The conditions leading to flow separation and the subsequent changes in the flow empirical correlations for incipient separation are reviewed. The effects of the Mach number, Reynolds number, surface cooling and the methods of detecting separation are discussed. The pertinent experimental data for the separated flow characteristics in separated turbulent boundary layer shock interaction are also presented and discussed.

Hamed, A.↗
NASA NTRS1982

Near-wall similarity in the three-dimensional turbulent boundary layers. III - Shear-driven flow results

Ten of eleven proposed three-dimensional similarity models identified in the literature are evaluated with direct wall shear, velocity field, and pressure gradient data from a three-dimensional shear-driven boundary layer flow. Results define an upper limit on velocity vector skewing for each model's predictive ability. When combined with earlier results for pressure-driven flows, each model's predictive ability with and without pressure gradients is summarized. The utility of some two-dimensional type indirect wall shear measurement methods and wall shear inference methods from near-wall velocity measurements for three-dimensional flows is also discussed.

Pierce, F. J.↗
NASA NTRS1984

Boundary layer transition effects on flow separation around V/STOL engine inlets at high incidence

Numerical methods for calculating laminar and turbulent boundary layer development around vertical-short take off and landing engine inlets at high incidence angles are investigated. Various transition models were compared and evaluated in calculations off flow separation bound inside the inlet. Results of the transition effects on the boundary layer characteristics at onset of separation for two types of engine inlet geometries are presented. Some of the numerical results are compared with existing wind-tunnel test data for scaled inlet models to demonstrate the effects of transition models in the numerical scheme. The effects of transition modeling on the boundary layer development are illustrated for typical engine operating conditions.

Chou, D. C.↗