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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 NTRS1988

Turbulence characteristics inside a turbulent spot in plane Poiseuille flow

In wall-bounded shear flows the transition to turbulence through localized disturbances goes through a pattern starting with a development of shear layers. The localized normal velocity fluctuations induce normal vorticity through the lift-up effect. These shear layers become unstable to secondary disturbances, and if the amplitudes of the disturbances are large enough, a turbulent spot develops. Investigations of the spot in boundary layers has shown that the turbulent part of the spot is very similar to a fully developed boundary layer. Wygnanski et al. (1976) showed that the mean profile at the center-symmetry plane has a logarithmic region and Johansson et al. (1987) showed that both the higher-order statistics and flow structures in the spot were the same as in the corresponding fully developed flow. In what respects the turbulence inside the Poiseuille spot is similar to fully developed turbulent channel flow is studied. The numerically simulated spot is used, where the characteristics inside the spot are compared to those of the wave packet in the wingtip area. A recent experimental investigation of the velocity field associated with the Poiseuille spot by Klingmann et al. is used for comparison.

Henningson, D. S.↗
NASA NTRS2000

The Surface Pressure Response of a NACA 0015 Airfoil Immersed in Grid Turbulence: Characteristics of the Turbulence - Volume 1

Two grids have been developed for the Virginia Tech 6 ft x 6 ft Stability wind tunnel for the purpose of generating homogeneous isotropic turbulent flows for the study of unsteady airfoil response. The first, a square bi-planar grid with a 12" mesh size and an open area ratio of 69.4%, was mounted in the wind tunnel contraction. The second grid, a metal weave with a 1.2 in. mesh size and an open area ratio of 68.2% was mounted in the tunnel test section. Detailed statistical and spectral measurements of the turbulence generated by the two grids are presented for wind tunnel free stream speeds of 10, 20, 30 and 40 m/s. These measurements show the flows to be closely homogeneous and isotropic. Both grids produce flows with a turbulence intensity of about 4% at the location planned for the airfoil leading edge. Turbulence produced by the large grid has an integral scale of some 3.2 inches here. Turbulence produced by the small grid is an order of magnitude smaller. For wavenumbers below the upper limit of the inertial subrange, the spectra and correlations measured with both grids at all speeds can be represented using the von Karman interpolation formula with a single velocity and length scale. The spectra maybe accurately represented over the entire wavenumber range by a modification of the von Karman interpolation formula that includes the effects of dissipation. These models are most accurate at the higher speeds (30 and 40 m/s).

Bereketab, Semere↗
NASA NTRS1971

A method for determining the response of space shuttle to atmospheric turbulence. Volume 1: Space shuttle turbulence response

A computer program has been developed and demonstrated that can analyze the response of space shuttle to atmospheric turbulence. The method developed accounts for propellant slosh, gimballed engine, and stability augmentation system (SAS) coupling with the elastic vehicle. Statistical outputs are generated that relate vehicle loads and accelerations to level of random turbulence. For discrete turbulence descriptions, response time-histories are computed. Preliminary turbulence response analyses of space shuttle were conducted for one ascent and one booster flyback subsonic flight condition. The ascent case considered both symmetric and antisymmetric boundary conditions, while only the symmetric analysis was conducted for booster flyback. The three conditions were analyzed with SAS active and inactive, but apparent instabilities with SAS active (arising from improper gains) invalidated the results. Load and acceleration responses in all cases, however, were well within vehicle design limits.

Source record↗
NASA NTRS1978

An experimental study of three-dimensional turbulent boundary layer and turbulence characteristics inside a turbomachinery rotor passage

Three-dimensional boundary layer and turbulence measurements of flow inside a rotating helical channel of a turbomachinery rotor are described. The rotor is a four-bladed axial flow inducer operated at large axial pressure gradient. The mean velocity profiles, turbulence intensities and shear stresses, and limiting stream-line angles are measured at various radial and chordwise locations, using rotating triaxial hot-wire and conventional probes. The radial flows in the rotor channel are found to be higher compared to those at zero or small axial pressure gradient. The radial component of turbulence intensity is found to be higher than the streamwise component due to the effect of rotation. Flow near the annulus wall is found to be highly complex due to the interaction of the blade boundary layers and the annulus wall resulting in an appreciable radial inward flow, and a large defect in the mainstream velocity. Increased level of turbulence intensity and shear stresses near the midpassage are also observed near this radial location.

Anand, A. K.↗
NASA NTRS1984

Relationship of isolated turbulent regions to the general turbulent background, part 4.4A

The atmosphere below 100 km is often referred to as the homosphere; the region above, the heterosphere. In the homosphere, constitutents are mixed, and average molecular weight is constant. In the heterosphere, diffusive separation occurs, and species tend to fractionate; average molecular weight decreases with height. Mixing in the homosphere has its source dynamical (wind) energy. Against this background turbulence, intermittent enhancements of turbulent intensity are observed. Some of these enhancements are long lived -- for example, a turbulent layer at approximately 86 km altitude seems to be almost a permanent feature of the (global) daytime D region, as oserved by partial reflection. This may well be a delineation of the mesopause level, since long-term radio meteor studies have shown approx. 85 km to be the breakpoint between thermospheric circulation above, and mesospheric circulation below. The use of stratosphere-troposphere (ST) radars in the lower atmosphere, and mesosphere-stratosphere-troposphere (MST) and partial-reflection radars in the mesosphere for height/time profiles of turbulent intensity is discussed.

Roper, R. G.↗
NASA NTRS1986

Atmospheric waves and the nature of buoyancy turbulence in the context of the waves VS 2D-turbulence debate

The problem of how to empirically distinguish between velocity fluctuations due to turbulence and those due to atmospheric waves is addressed. The physical differences between waves and turbulence are reviewed. New theoretical ideas on the subject of bouyancy range turbulence are presented. A unique scale K sub B is given that allows one to differentiate between waves and turbulence for the special case of theta = 0 (i.e., horizontal propagating waves).

Dewan, E. M.↗
NASA NTRS1990

Effects of organized turbulence structures on the phase distortion in a coherent optical beam propagating through a turbulent shear flow

Phase distortion in a coherent optical beam propagating through a turbulent shear flow is studied. The instantaneous distribution of the index refraction is represented by a passive-scalar field in a computed homogeneous shear flow. The flow contains organized vortical structures (hairpin eddies), which are characteristic of turbulent shear flows. The phase distortion induced by turbulent fluctuations is calculated from the optical path difference through the flow. A conceptual model is proposed for the distribution of scalar fluctuations produced by the hairpin vortices in the shear flow. It is shown that the phase distortion of an optical beam can be minimized by propagating the beam at an angle approximately normal to the organized vortical structures in a turbulent shear flow.

Truman, C. Randall↗
NASA NTRS1989

Detachment of turbulent boundary layers with varying free-stream turbulence and lower Reynolds numbers

Experiments conducted to determine if free-stream turbulence scale affects detachment of turbulent boundary layers are described. Hot-wire anemometry, liquid-film visualization, a Preston tube, and static pressure measurements were used to study flow over a two-dimensional airfoil in a subsonic wind tunnel. The free-stream turbulence intensity and scale were found to spread to the entire turbulent boundary layer. However, the effect decreased upon approaching the airfoil surface.

Potter, J. Leith↗
NASA NTRS1990

Modeling the turbulent kinetic energy equation for compressible, homogeneous turbulence

The turbulent kinetic energy transport equation, which is the basis of turbulence models, is investigated for homogeneous, compressible turbulence using direct numerical simulations performed at CTR. It is shown that the partition between dilatational and solenoidal modes is very sensitive to initial conditions for isotropic decaying turbulence but not for sheared flows. The importance of the dilatational dissipation and of the pressure-dilatation term is evidenced from simulations and a transport equation is proposed to evaluate the pressure-dilatation term evolution. This transport equation seems to work well for sheared flows but does not account for initial condition sensitivity in isotropic decay. An improved model is proposed.

Aupoix, B.↗
NASA NTRS1992

Reynolds number dependence of the freestream turbulence effects on turbulent boundary layers

Published experimental data on the influence of freestream turbulence on turbulent boundary layers were examined to determine the effect of Reynolds number on such influence. Two manifestations of the effect of low Reynolds numbers on the outer layer were observed: (1) the dependence of Clauser's shape parameter G on Reynolds number at very low Reynolds numbers and (2) the reduction in the wake component due to freestream turbulence undergoing a reversal in the Reynolds number dependence. These observations were used to modify Hancock's (1980) freestream turbulence parameter.

Bandyopadhyay, Promode R.↗
NASA NTRS1992

Exploiting similarity in turbulent shear flows for turbulence modeling

It is well known that current k-epsilon models cannot predict the flow over a flat plate and its wake. In an effort to address this issue and other issues associated with turbulence closure, a new approach for turbulence modeling is proposed which exploits similarities in the flow field. Thus, if we consider the flow over a flat plate and its wake, then in addition to taking advantage of the log-law region, we can exploit the fact that the flow becomes self-similar in the far wake. This latter behavior makes it possible to cast the governing equations as a set of total differential equations. Solutions of this set and comparison with measured shear stress and velocity profiles yields the desired set of model constants. Such a set is, in general, different from other sets of model constants. The rational for such an approach is that if we can correctly model the flow over a flat plate and its far wake, then we can have a better chance of predicting the behavior in between. It is to be noted that the approach does not appeal, in any way, to the decay of homogeneous turbulence. This is because the asymptotic behavior of the flow under consideration is not representative of the decay of homogeneous turbulence.

Robinson, David F.↗
NASA NTRS1992

Multigrid acceleration and turbulence models for computations of 3D turbulent jets in crossflow

A multigrid method is presented for the calculation of three-dimensional turbulent jets in crossflow. Turbulence closure is achieved with either the standard k-epsilon model or a Reynolds stress model (RSM). Multigrid acceleration enables convergence rates which are far superior to that for a single grid method to be obtained with both turbulence models. With the k-epsilon model the rate approaches that for laminar flow, but with RSM it is somewhat slower. The increased stiffness of the system of equation in the latter may be responsible. Computed results with both turbulence models are compared to experimental data for a pair of opposed jets in crossflow. Both models yield reasonable agreement for the mean flow velocity, but RSM yields better predictions of the Reynolds stresses.

Demuren, A. O.↗
NASA NTRS1972

Laminarization model for turbulent eddy transport in highly accelerated nozzle turbulent boundary layers

A laminarization model which consists of a completely laminar sublayer region near the wall and a turbulent wake region is developed for the turbulent eddy transport in accelerated turbulent boundary layers. This laminarization model is used in a differential boundary layer calculation which was applied to nozzle flows. The resulting theoretical velocity profiles are in good agreement with the experimental nozzle data in the convergent region.

Schmidt, J. F.↗
NASA NTRS1974

Turbulent vortex streets and the entrainment mechanism of the turbulent wake

The results of an experimental investigation of a turbulent vortex street in the range from 1000 to 20,000 are presented. The vortex street was created by the motion of a circular cylinder in a motionless fluid (mercury). Photographs obtained showed that the turbulent street, created by the vortex shedding behind the cylinder, persisted at longer downstream distances and higher Reynolds numbers than previously reported in the literature. A theory was developed to account for the experimental measurements pertaining to the change of the geometrical characteristics, (the distance between the two rows of vortices and the longitudinal distance between two consecutive vortices on the same row), of the street in the downstream direction. The implications of the structure of the vortex street on the entrainment mechanism of the turbulent wake are discussed.

Papailiou, D. D.↗
NASA NTRS1975

Effort of spanwise variation of turbulence on the normal acceleration of airplanes with small span relative to turbulence scale

A rigid airplane with an unswept wing is analyzed. The results show that the power spectrum, relative to that for a one-dimensional turbulence field, is significantly attenuated at the higher frequencies even for airplanes with arbitrarily small ratios of span to scale of turbulence. This attenuation is described by a simple weighting function of frequency that depends only on aspect ratio. The weighting function, together with the attenuation due to the unsteady flow of gust penetration, allows the determination of the average rate of zero crossings for airplanes having very small spans without recourse to an integral truncation which is often required in calculations based on a one-dimensional turbulence field.

Pratt, K. G.↗
NASA NTRS1977

Evaluation of subgrid-scale turbulence models using a fully simulated turbulent flow

An exact turbulent flow field was calculated on a three-dimensional grid with 64 points on a side. The flow simulates grid-generated turbulence from wind tunnel experiments. In this simulation, the grid spacing is small enough to include essentially all of the viscous energy dissipation, and the box is large enough to contain the largest eddy in the flow. The method is limited to low-turbulence Reynolds numbers, in our case R sub lambda = 36.6. To complete the calculation using a reasonable amount of computer time with reasonable accuracy, a third-order time-integration scheme was developed which runs at about the same speed as a simple first-order scheme. It obtains this accuracy by saving the velocity field and its first-time derivative at each time step. Fourth-order accurate space-differencing is used.

Clark, R. A.↗
NASA NTRS1977

Mean velocity, turbulence intensity, and scale in a subsonic turbulent jet impinging normal to a large flat plate

To explain the increase in noise when a jet impinges on a large flat plate, mean velocity, turbulence intensity, and scale were measured at nominal nozzle-exit velocities of 61, 138, and 192 meters per second with the plate located 7.1 nozzle-exit diameters from the nozzle. The maximum turbulence intensities in free and impinging jets were about the same; however, the integral length scale near the plate surface was only about one-half the free jet scale. The measured intensities and length scales, in conjunction with a contemporary theory of aerodynamic noise, provided a good explanation for the observed increase in noise associated with the impinging jet. An increase in the volume of highly turbulent flow could be the principal reason for the increase in noise.

Boldman, D. R.↗
NASA NTRS1977

Experimental and theoretical investigation of three-dimensional turbulent boundary layers and turbulence characteristics inside an axial flow inducer passage

Analytical and experimental investigations of the characteristics of three dimensional turbulent boundary layers in a rotating helical passage of an inducer rotor are reported. Expressions are developed for the velocity profiles in the inner layer, where the viscous effects dominate, in the outer layer, where the viscous effects are small, and in the interference layer, where the end walls influence the flow. The prediction of boundary layer growth is based on the momentum integral technique. The equations derived are general enough to be valid for all turbomachinery rotors with arbitrary pressure gradients. The experimental investigations are carried out in a flat plate inducer 3 feet in diameter. The mean velocity profiles, turbulence intensities and shear stresses, wall shear stress, and limiting streamline angles are measured at various radial and chordwise locations by using rotating probes. The measurements are in general agreement with the predictions. The radial flows are well represented by an expression which includes the effect of stagger angle and radial pressure gradient. The radial flows in the rotor channel are higher than those on a single blade. The collateral region exists only very near the blade surface. The radial component of turbulence intensity is higher than the streamwise component because of the effect of rotation.

Anand, A. K.↗