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At least 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Effects of Wing Sweep on In-flight Boundary-layer Transition for a Laminar Flow Wing at Mach Numbers from 0.60 to 0.79

The variable sweep transition flight experiment (VSTFE) was conducted on an F-14A variable sweep wing fighter to examine the effect of wing sweep on natural boundary layer transition. Nearly full span upper surface gloves, extending to 60 percent chord, were attached to the F-14 aircraft's wings. The results are presented of the glove 2 flight tests. Glove 2 had an airfoil shape designed for natural laminar flow at a wing sweep of 20 deg. Sample pressure distributions and transition locations are presented with the complete results tabulated in a database. Data were obtained at wing sweeps of 15, 20, 25, 30, and 35 deg, at Mach numbers ranging from 0.60 to 0.79, and at altitudes ranging from 10,000 to 35,000 ft. Results show that a substantial amount of laminar flow was maintained at all the wing sweeps evaluated. The maximum transition Reynolds number obtained was 18.6 x 10(exp 6) at 15 deg of wing sweep, Mach 0.75, and at an altitude of 10,000 ft.

Anderson, Bianca Trujillo↗
NASA NTRS1990

Flow and acoustic properties of low Reynolds number underexpanded supersonic jets

An experimental program to investigate the flow and acoustic properties of model underexpanded supersonic jets was conducted. In particular, the role played by large-scale organized fluctuations in the flow evolution and acoustic production processes was examined in detail. The experimental conditions were chosen as low-Reynolds-number (Re = 8000) Mach 1.4 and 2.1 underexpanded jets exhausting from convergent nozzles. A consequence of performing the experiments at low Reynolds number is that the broadband shock-associated noise is suppressed. The focus of the present study is on the generation of noise by large-scale instabilities in the presence of strong shock cell structures. It is demonstrated that the production of screech is related to the modulation and decay of large-scale turbulence structures.

Hu, Tieh-Feng↗
NASA NTRS1990

Transition to hard turbulence in thermal convection at infinite Prandtl number

Direct numerical simulations of two-dimensional high Rayleigh (Ra) number, base-heated thermal convection in large aspect-ratio boxes are presented for infinite Prandtl number fluids, as applied to the earth's mantle. A transition is characaterized in the flow structures in the neighborhood of Ra between 10 to the 7th and 10 to the 8th. These high Ra flows consist of large-scale cells with strong intermittent, boundary-layer instabilities. For Ra exceeding 10 to the 7th it is found that the heat-transfer mechanism changes from one characterized by mushroom-like plumes to one consisting of disconnected ascending instabilities, which do not carry with them all the thermal anomaly from the bottom boundary layer. Plume-plume collisions become much more prominent in high Ra situations and have a tendency of generating a pulse-like behavior in the fixed plume. This type of instability represents a distinct mode of heat transfer in the hard turbulent regime. Predictions of this model can be used to address certain issues concerning the mode of time-dependent convection in the earth's mantle.

Hansen, Ulrich↗
NASA NTRS1991

Effect of convective Mach number on mixing of coaxial circular and rectangular jets

Rectangular supersonic free and coaxial jets were used to enhance mixing relative to a circular jet in a convective Mach number range of 0.5 to 2.2. The different convective Mach numbers were obtained by changing the central jet gas composition, the temperatures of the inner and outer flows, and the velocity of the coaxial flow. The experimental techniques used were schlieren photography, total pressure, and gas-sampling measurements. For all test conditions the rectangular jets showed substantial improved mixing relative to a circular jet. The free jets showed high mixing in the circumferential region of the jet while the coaxial jet had a high mixing rate inside the central jet.

Gutmark, E.↗
NASA NTRS1991

Effects of turbulence and number density on the drag coefficient of droplets

Droplet drag coefficients for polydispersed drops are determined via the behavior of drops decelerating on the stagnation streamline of a cylinder with an afterbody mounted in a wind tunnel test section. A variety of velocity, turbulence levels, and droplet number densities were studied. A force balance equation technique was used to determine drag coefficient. For the levels of number density, up to 700/cc, and turbulence, up to about 7 percent, no definite effects were seen. However, the smallest drops in the high turbulence case showed some evidence of drop-turbulence and/or drop-drop interactions. The drag results that were developed for this set of measurements agreed well with other empirical relations previously determined.

Rudoff, R. C.↗
NASA NTRS1991

Simulating boundary layer transition with low-Reynolds-number k-epsilon turbulence models. I - An evaluation of prediction characteristics. II - An approach to improving the predictions

The capability of two k-epsilon low-Reynolds number (LRN) turbulence models, those of Jones and Launder (1972) and Lam and Bremhorst (1981), to predict transition in external boundary-layer flows subject to free-stream turbulence is analyzed. Both models correctly predict the basic qualitative aspects of boundary-layer transition with free stream turbulence, but for calculations started at low values of certain defined Reynolds numbers, the transition is generally predicted at unrealistically early locations. Also, the methods predict transition lengths significantly shorter than those found experimentally. An approach to overcoming these deficiencies without abandoning the basic LRN k-epsilon framework is developed. This approach limits the production term in the turbulent kinetic energy equation and is based on a simple stability criterion. It is correlated to the free-stream turbulence value. The modification is shown to improve the qualitative and quantitative characteristics of the transition predictions.

Schmidt, R. C.↗
NASA NTRS1990

Design and validation of advanced transonic wings using CFD and very high Reynolds number wind tunnel testing

A study is presented that opens the possibility for further wing aerodynamic technology advances when the test and design environment is at a significantly higher Reynolds number than that used for previous generations of commercial transports. Early generation wings were based primarily on NACA airfoil sections integrated simply into three-dimensional designs. Recently, designs have been developed with a major influence from CFD and have depended less on iterative wind tunnel testing. It is shown that, coupled with improvements in CFD wing modeling and advances in test techniques, additional improvements in wing technology can be realized at significantly higher Reynolds numbers.

Goldhammer, Mark I.↗
NASA NTRS1990

Constructing second-order structural models with specified mass submatrix condition numbers

This paper presents a simple new algorithm for constructing second-order models for any given flexible structure, with these models satisfying the constraint that each leading principal submatrix of the mass matrix must have a specified condition number. Now, the effects of rounding errors in practical computation generally increase with increasing condition number, i.e. as the matrix considered approaches singularity. Thus, the new models should prove useful for the systematic testing of the numerical properties of many algorithms in the dynamics and control of flexible structures.

Williams, T.↗
NASA NTRS1990

Inhibition of mixing and self-focusing in high Mach number turbulence

It is shown that the hypothesis that eddy shocklets dissipate excess kinetic energy in compressible flow and thus limit the effective eddy Mach number to order 1 leads to the inhibition of mixing and self-focusing of turbulent jets at high Mach numbers, in agreement with experiment.

Orszag, Steven A.↗
NASA NTRS1991

Determination of confidence limits for experiments with low numbers of counts

Two different methods, classical and Bayesian, for determining confidence intervals involving Poisson-distributed data are compared. Particular consideration is given to cases where the number of counts observed is small and is comparable to the mean number of background counts. Reasons for preferring the Bayesian over the classical method are given. Tables of confidence limits calculated by the Bayesian method are provided for quick reference.

Kraft, Ralph P.↗
NASA NTRS1991

Turbulent Prandtl number in the near-wall region of a turbulent channel flow

Calculations are presented of the turbulent Prandtl number Pr(T) in the near-wall region of a turbulent channel flow. It is shown that only the first-order terms in the Taylor series expansions for the eddy diffusivity and the Pr(T) are independent of the molecular Prandtl number Pr, at least to a first approximation. Also presented are calculations of the near-wall behavior of the correlations between temperature fluctuations and velocity fluctuations, as well as for their dependence on Pr.

Antonia, R. A.↗
NASA NTRS1991

Winglet effectiveness on low aspect ratio wings at supersonic Mach numbers

A computational study has been conducted on two wings of aspect ratios 1.244 and 1.865, each having 65-deg leading edge sweep angles, to determine the effects of nonplanar winglets at supersonic Mach numbers. A design Mach number of 1.62 was selected. The winglets studied were parametrically varied in alignment, length, sweep, camber, and thickness to determine the effects of winglet geometry on predicted performance. For the computational analysis, an existing Euler code that employed a marching technique was used. The results indicated that the possibility existed for wing-winglet geometries to equal the performance of wing-alone bodies in supersonic flows with both bodies having the same semispan length. The performance parameters of main interest were the lift-to-pressure drag ratio and the pressure drag coefficient as functions of lift coefficient. The lift coefficient range for this study was from -0.20 to 0.70 with emphasis on the range of 0.10 to 0.22.

Keenan, J. A.↗
NASA NTRS1991

Acoustic sources in the low Mach number turbulent boundary layer

The sources of sound production in a low Mach number turbulent boundary layer are examined. The sources are shown to be quadrupole in nature and to result from supersonically convecting wave-number components of the fluctuating Reynolds' normal stresses. The primary Tollmien-Schlichting instability of the boundary layer is found to radiate no sound. Analysis of various vortical phenomena suggests that the primary source is the process of formation of horseshoe vortices, with viscous sublayer bursts a possible secondary source.

Hardin, Jay C.↗
NASA NTRS1991

The wave numbers of supercritical surface tension driven Benard convection

The cell size or the wave numbers of supercritical hexagonal convection cells in primarily surface tension driven convection on a uniformly heated plate was studied experimentally in thermal equilibrium in thin layers of silicone oil of large aspect ratio. It was found that the cell size decreases with increased temperature difference in the slightly supercritical range, and that the cell size is unique within the experimental error. It was also observed that the cell size reaches a minimum and begins to increase at larger temperature differences. This reversal of the rate of change of the wave number with temperature difference is attributed to influences of buoyancy on the fluid motion. The consequences of buoyancy were tested with three fluid layers of different depth.

Koschmieder, E. L.↗
NASA NTRS1992

High-alpha vortex decoupling investigations on a chine forebody/Delta wing configuration at transonic Mach numbers

A test program conducted in the LaRC 8-foot Transonic Pressure Tunnel on a blended chine/delta wing model to verify the concept of controlled vortex decoupling via inboard leading-edge flaps for improved high-alpha lateral/directional characteristics is discussed. This test extended the Mach number range of a data base previously generated in a LaRC 7 ft. by 10 ft High Speed Tunnel investigation. Six-component force/moment, forebody surface pressures, and central and twin tall static and dynamic loads were measured at Mach numbers of 0.4 to 1.2; laser light-sheet visualizations were also performed. Selected data are analyzed and discussed, emphasizing lateral/directional improvements and tail environment enhanced attainable by leading-edge flaps in the maximum lift region.

Rao, Dhanvada M.↗
NASA NTRS1990

Low Reynolds number kappa-epsilon modeling with the aid of direct simulation data

The constant C(sub mu) and the near-wall damping function f(sub mu) in the eddy-viscosity relation of the kappa-epsilon model are evaluated from direct numerical simulation (DNS) data for developed channel and boundary layer flow at two Reynolds numbers each. Various existing f(sub mu) model functions are compared with the DNS data, and a new function is fitted to the high-Reynolds-number channel flow data. The epsilon-budget is computed for the fully developed channel flow. The relative magnitude of the terms in the epsilon-equation is analyzed with the aid of scaling arguments, and the parameter governing this magnitude is established. Models for the sum of all source and sink terms in the epsilon-equation are tested against the DNS data, and an improved model is proposed.

Rodi, W.↗
NASA NTRS1992

Analytical estimates of the PP-algorithm at low number of Doppler periods per pulse length

When discussing the Doppler velocity estimators, it is of significant interest to analyze their behavior at a low number of Doppler periods n(sub D) = 2v(sub r)t(sub s)/lambda is approximately equal to 1 within the resolution cell t(sub s) (v(sub 4) is the radial velocity, lambda is the wavelength). Obviously, at n(sub D) is approximately less than 1 the velocity error is essentially increased. The problem of low n(sub D) arises in the planetary boundary layer (PBL), where higher resolutions are usually required but the signal-to-noise ratio (SNR) is relatively high. In this work analytical expression for the relative root mean square (RMS) error of the PP Doppler estimator at low number of periods for a narrowband Doppler signal and arbitrary model of the noise correlation function is obtained. The results are correct at relatively high SNR. The analysis is supported by computer simulations at various SNR's.

Angelova, M. D.↗