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At least 1,207 records · Page 67
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NASA NTRS1992

Flow prediction over a transport multi-element high-lift system and comparison with flight measurements

Accurate prediction of surface-pressure distributions, merging boundary-layers, and separated-flow regions over multi-element high-lift airfoils is required to design advanced high-lift systems for efficient subsonic transport aircraft. The availability of detailed measurements of pressure distributions and both averaged and time-dependent boundary-layer flow parameters at flight Reynolds numbers is critical to evaluate computational methods and to model the turbulence structure for closure of the flow equations. Several detailed wind-tunnel measurements at subscale Reynolds numbers were conducted to obtain detailed flow information including the Reynolds-stress component. As part of a subsonic-transport high-lift research program, flight experiments are conducted using the NASA-Langley B737-100 research aircraft to obtain detailed flow characteristics for support of computational and wind-tunnel efforts. Planned flight measurements include pressure distributions at several spanwise locations, boundary-layer transition and separation locations, surface skin friction, as well as boundary-layer profiles and Reynolds stresses in adverse pressure-gradient flow.

Vijgen, P. M. H. W.↗
NASA NTRS1993

Reynolds number influences in aeronautics

Reynolds number, a measure of the ratio of inertia to viscous forces, is a fundamental similarity parameter for fluid flows and therefore, would be expected to have a major influence in aerodynamics and aeronautics. Reynolds number influences are generally large, but monatomic, for attached laminar (continuum) flow; however, laminar flows are easily separated, inducing even stronger, non-monatomic, Reynolds number sensitivities. Probably the strongest Reynolds number influences occur in connection with transitional flow behavior. Transition can take place over a tremendous Reynolds number range, from the order of 20 x 10(exp 3) for 2-D free shear layers up to the order of 100 x 10(exp 6) for hypersonic boundary layers. This variability in transition behavior is especially important for complex configurations where various vehicle and flow field elements can undergo transition at various Reynolds numbers, causing often surprising changes in aerodynamics characteristics over wide ranges in Reynolds number. This is further compounded by the vast parameterization associated with transition, in that any parameter which influences mean viscous flow development (e.g., pressure gradient, flow curvature, wall temperature, Mach number, sweep, roughness, flow chemistry, shock interactions, etc.), and incident disturbance fields (acoustics, vorticity, particulates, temperature spottiness, even electro static discharges) can alter transition locations to first order. The usual method of dealing with the transition problem is to trip the flow in the generally lower Reynolds number wind tunnel to simulate the flight turbulent behavior. However, this is not wholly satisfactory as it results in incorrectly scaled viscous region thicknesses and cannot be utilized at all for applications such as turbine blades and helicopter rotors, nacelles, leading edge and nose regions, and High Altitude Long Endurance and hypersonic airbreathers where the transitional flow is an innately critical portion of the problem.

Bushnell, Dennis M.↗
NASA NTRS1993

Effect of sound on boundary layer stability

Experiments are conducted in the Arizona State University Unsteady Wind Tunnel with a zero-pressure-gradient flat-plate model that has a 67:1 elliptical leading edge. Boundary-layer measurements are made of the streamwise fluctuating-velocity component in order to identify the amplified T-S waves that are forced by downstream-traveling sound waves. Measurements are taken with circular 3-D roughness elements placed at the Branch 1 neutral stability point for the frequency under consideration, and then with the roughness element downstream of Branch 1. These roughness elements have a principal chord dimension equal to 2 lambda(sub TS)/pi of the T-S waves under study and are 'stacked' in order to resemble a Gaussian height distribution. Measurements taken just downstream of the roughness (with leading-edge T-S waves, surface roughness T-S waves, instrumentation sting vibrations, and the Stokes wave subtracted) show the generation of 3-D T-S waves, but not in the characteristic heart-shaped disturbance field predicted by 3-D asymptotic theory. Maximum disturbance amplitudes are found on the roughness centerline. However, some near-field characteristics predicted by numerical modeling are observed.

Saric, William S.↗
NASA NTRS1992

The dimension of attractors underlying periodic turbulent Poiseuille flow

A lower bound on the Liapunov dimenison, D-lambda, of the attractor underlying turbulent, periodic Poiseuille flow at a pressure-gradient Reynolds number of 3200 is calculated, on the basis of a coarse-grained (16x33x8) numerical solution, to be approximately 352. Comparison of Liapunov exponent spectra from this and a higher-resolution (16x33x16) simulation on the same spatial domain shows these spectra to have a universal shape when properly scaled. On the basis of these scaling properties, and a partial exponent spectrum from a still higher-resolution (32x33x32) simulation, it is argued that the actual dimension of the attractor underlying motion of the given computational domain is approximately 780. It is suggested that this periodic turbulent shear flow is deterministic chaos, and that a strange attractor does underly solutions to the Navier-Stokes equations in such flows.

Keefe, Laurence↗
NASA NTRS1992

The linear stability of a flat plate boundary-layer approaching a cylindrical obstacle

The linear stability of the low-speed three-dimensional flow over a flat plate with an attached cylinder is studied. The region of interest is upstream of the initial separation point and includes the effects of both adverse and favorable pressure gradients, as well as crossflow. The resulting boundary-layer is subject to both the Tollmien-Schlichting (TS) and crossflow instabilities. Linear stability calculations, using N-factor correlations, indicate that the transition process would be dominated by TS instabilities, although for low frequencies crossflow-type disturbances are important.

Spall, R. E.↗
NASA NTRS1992

Does fast magnetic reconnection exist?

The main features of the Priest-Forbes (1986) and Priest-Lee (1990) models of magnetic reconnection in astrophysical plasmas are discussed, and the Priest-Lee model is generalized to include inflow pressure gradients and thus different regimes of reconnection. It is shown that different scaling results can be obtained depending on the boundary conditions. These results are compared to the ones observed in the numerical experiments of Biskamp (1986) and Lee and Fu (1986). It is concluded that numerical experiments with suitably designed boundary conditions are likely to exhibit fast reconnection, and that such reconnection is a common process in astrophysical and space plasmas.

Priest, E. R.↗
NASA NTRS1992

Thermal and dynamical regimes of single- and two-phase magmatic flow in dikes

The coupling between thermal and dynamical regimes of single- and two-phase magmatic flow in dikes, due to temperature-dependent viscosity and dissipation, was investigated using finite element calculations of magma flow in dikelike channels with length-to-width ratios of 1000:1 or more. Solutions of the steady state equations governing magma flow are obtained for a variety of conditions ranging from idealized plane-parallel models to cases involving nonparallel geometry and two-phase flows. The implications of the numerical simulations for the dynamics of flow in a dike-reservoir system and the consequences of dike entrance conditions on magmatic storage are discussed. Consideration is also given to an unmixing/self-lubrication mechanism which may be important for the lubrication of silicic magmas rising to the earth's surface in mixed magma ascent scenarios, which naturally segregates magma mixtures of two components with differing viscosities to minimize the driving pressure gradient.

Carrigan, Charles R.↗
NASA NTRS1991

A general approach for the prediction of localized instability generation in boundary layer flows

The present approach to the prediction of instability generation that is due to the interaction of freestream disturbances with regions of subscale variations in surface boundary conditions can account for the finite Reynolds number effects, while furnishing a framework for the study of receptivity in compressible flow and in 3D boundary layers. The approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer, due to the interaction of a freestream acoustic wave with a localized wall inhomogeneity. Results are presented for the generation of viscous and inviscid instabilities in adverse pressure-gradient boundary layers, supersonic boundary layer instabilities, and cross-flow vortex instabilities.

Choudhari, Meelan↗
NASA NTRS1993

Effect of micron-sized roughness on transition in swept-wing flows

Boundary-layer transition-to-turbulence studies are conducted in the Arizona State University Unsteady Wind Tunnel on a 45-degree swept airfoil. The pressure gradient is designed so that the initial stability characteristics are purely crossflow-dominated. Flow visualization and hot-wire measurements show that the development of the crossflow vortices is influenced by roughness near the attachment-line. Comparisons of transition location are made between a painted surface, a machine-polished surface, and a hand-polished surface. Then, isolated 6 micron roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25 micron rms). These elements amplify a centered stationary crossflow vortex and its neighbors, resulting in localized early transition. The diameter, height, and location of these roughness elements are varied in a systematic manner. Spanwise hot-wire measurements are taken behind the roughness element to document the enhanced vortices. These scans are made at several different chord locations to examine vortex growth.

Radeztsky, Ronald H., Jr.↗
NASA NTRS1992

Dynamical polar wind and its response to kinetic ion heating

The effect of the kinetic ion heating of the polar wind on the dynamic expansion of the polar wind outflow is investigated by incorporating the kinetic ion heating effect into a time-dependent hydrodynamic polar wind model in which the polar wind O(+) and H(+) continuity and momentum equations are solved in a flux tube from ionospheric to magnetospheric altitudes. Two limiting cases were considered: (1) preferential O(+) escape and (2) preferential H(+) heating. It was found that ion heating does not have much effect on the escape of H(+) ions because of the limiting nature of the H(+) escape fluxes. However, O(+) heating can substantially increase the upward O(+) pressure gradient in the heating region, indicating that O(+) heating is a viable explanation for the unexpectedly large O(+) fluxes frequently observed in the polar magnetosphere.

Chen, Margaret W.↗
NASA NTRS1993

An evaluation of nozzle relaminarization using Low Reynolds Number k-epsilon turbulence models

Two nozzle flowfields are evaluated numerically using a Low Reynolds k-epsilon turbulence model to examine possible boundary layer relaminarization. Flow enters the scramjet-like nozzles at a Mach Number of approximately 4.5 and is expanded around smooth shoulders to an exit Mach number of about 6.5. The strong streamwise acceleration and large favorable pressure gradients act with the convex curvature to form small regions of laminar-like boundary layer behavior along the expansion surface. Two relaminarization criteria are evaluated and they both predict relaminarization to be occurring, with the trends accurately modeled by the Low-Reynolds Number turbulence model that is used. The Baldwin-Lomax turbulence model cannot model the near-wall changes that take place in the rapidly accelerating boundary layer, but it proves to be useful in providing the acceleration parameters used in predicting the potential for relaminarization.

Jentink, Thomas N.↗
NASA NTRS1992

A two satellite study of nightside flux transfer events in the plasma sheet

The short-term (about 1 min) events during two consecutive substorms on March 23 1979 have been studied using high time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X about -21 R(e). Data obtained make it possible to describe the localized magnetic structures transported or propagated within the plasma sheet and to identify these events as plasma sheet nightside magnetic flux transfer events (NFTEs). The best examples of the NFTEs found so far are all observed in a thin portion of plasma sheet where the contraction of the reconnected field lines dominates over the oppositely directed pressure gradient force. Systematic differences in the magnetic variations observed at ISEE-1 and 2 indicate current concentration at the outer plasma sheet boundary during the passage of a NFTE.

Sergeev, V. A.↗
NASA NTRS1993

Diffusion effects on the helium abundance of the solar transition region and corona

The diffusion of helium in the solar transition region is studied by solving the mass and momentum conservation equations for a hydrogen-helium plasma given a representative temperature profile. Steady state solutions show that two distinct atmospheres may result. In cases where the thermal force on alpha-particles is balanced by the partial pressure gradient force, helium is the dominant coronal species. On the other hand, if it is the frictional force between protons and alpha-particles which balances the thermal force on alpha-particles then hydrogen is the major coronal component. In order to explore which of these solutions are attainable within reasonable time scales, the time-dependent equations are solved, starting from an initial state with a uniform helium abundance of 10 percent. The atmosphere as a whole is close to hydrostatic equilibrium, but due the thermal forces the individual elements are not. This force inbalance leads to a differential flow between species. It is found that this differential flow leads to a significant enhancement of the coronal helium abundance. Even for the relatively shallow temperature gradient used the helium abundance in the lower corona increases to 30 percent over a 24 hr period.

Hansteen, Viggo H.↗
NASA NTRS1992

The effect of entrance radius and film injection on wall heating in scramjet nozzles

An experimental study has been performed with a scramjet combustor and nozzle model to determine the effect of nozzle radii on relaminarization, and the effectiveness of film injection in reducing wall heat flux. The nozzle model was installed downstream of a swept-ramp scramjet combustor model, operated at Mach 2.7 with vitiated air, simulating Mach 6-7 flight enthalpies. Results from tests conducted with three nozzle radii, varying in curvature from a sharp Prandtl-Meyer expansion to a radius of two scramjet combustor exit heights, showed little or no effect on the downstream wall heat flux, which is not in agreement with both subsonic and supersonic relaminarization correlations in the available literature. In contrast, film injection upstream of all three radii produced reductions of 70 percent in the entire nozzle model heating rate, for the nominal film-to-free stream pressure matched condition. Results from film injection, with and without pressure gradient, but without wall curvature, show that the gradients appear to stabilize the film, with the resulting reductions in wall heat transfer rates apparent far downstream of the point of injection.

Baker, N. R.↗
NASA NTRS1993

An algebraic turbulence model for three-dimensional viscous flows

An algebraic turbulence model is proposed for use with three-dimensional Navier-Stokes analyses. It incorporates features of both the Baldwin-Lomax and Cebeci-Smith models. The Baldwin-Lomax model uses the maximum of a function f(v) to determine length and velocity scales. An analysis of the Baldwin-Lomax model shows that f(v) can have a spurious maximum close to the wall, causing numerical problems and non-physical results. The proposed model uses integral relations to determine delta(*) u(sub e) and delta used in the Cebeci-Smith mode. It eliminates a constant in the Baldwin-Lomax model and determines the two remaining constants by comparison to the Cebeci-Smith formulation. Pressure gradient effects, a new wake model, and the implementation of these features in a three-dimensional Navier-Stokes code are also described. Results are shown for a flat plate boundary layer, an annular turbine cascade, and endwall heat transfer in a linear turbine cascade. The heat transfer results agree well with experimental data which shows large variations in endwall Stanton number contours with Reynolds number.

Chima, R. V.↗
NASA NTRS1993

A simplified Reynolds stress model for unsteady turbulent boundary layers

A simplified Reynolds stress model has been developed for the prediction of unsteady turbulent boundary layers. By assuming that the net transport of Reynolds stresses is locally proportional to the net transport of the turbulent kinetic energy, the time dependent full Reynolds stress model is reduced to a set of ordinary differential equations. These equations contain only time derivatives and can be readily integrated in a time dependent boundary layer or Navier-Stokes code. The turbulent kinetic energy and dissipation rate needed for the model are obtained by solving the k-epsilon equations. This simplified Reynolds stress turbulence model (SRSM) does not use the eddy viscosity assumption, which may not be valid for unsteady turbulent flows. The anisotropy of both the steady and the unsteady turbulent normal stresses can be captured by the SRSM model. Through proper damping of the shear stresses, the present model can be used in the near wall region of turbulent boundary layers. This model has been validated against data for steady and unsteady turbulent boundary layers, including periodic turbulent boundary layers subjected to a mean adverse pressure gradient. For the cases tested, the predicted unsteady velocity and turbulent stress components agree well with the experimental data. Comparison between the predictions from the SRSM model and a k-epsilon model is also presented.

Fan, Sixin↗
NASA NTRS1993

The computation of the post-stall behavior of a circulation controlled airfoil

The physics of the circulation controlled airfoil is complex and poorly understood, particularly with regards to jet stall, which is the eventual breakdown of lift augmentation by the jet at some sufficiently high blowing rate. The present paper describes the numerical simulation of stalled and unstalled flows over a two-dimensional circulation controlled airfoil using a fully implicit Navier-Stokes code, and the comparison with experimental results. Mach numbers of 0.3 and 0.5 and jet total to freestream pressure ratios of 1.4 and 1.8 are investigated. The Baldwin-Lomax and k-epsilon turbulence models are used, each modified to include the effect of strong streamline curvature. The numerical solutions of the post-stall circulation controlled airfoil show a highly regular unsteady periodic flowfield. This is the result of an alternation between adverse pressure gradient and shock induced separation of the boundary layer on the airfoil trailing edge.

Linton, Samuel W.↗
NASA NTRS1993

Juncture flow improvement for wing/pylon configurations by using CFD methodology

Transonic flow field around a fighter wing/pylon configuration was simulated by using an implicit upwinding Navier-Stokes flow solver (F3D) and overset grid technology (Chimera). Flow separation and local shocks near the wing/pylon junction were observed in flight and predicted by numerical calculations. A new pylon/fairing shape was proposed to improve the flow quality. Based on numerical results, the size of separation area is significantly reduced and the onset of separation is delayed farther downstream. A smoother pressure gradient is also obtained near the junction area. This paper demonstrates that computational fluid dynamics (CFD) methodology can be used as a practical tool for aircraft design.

Gea, Lie-Mine↗