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At least 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Three-dimensional singular points in aerodynamics

When three-dimensional separation occurs on a body immersed in a flow governed by the incompressible Navier-Stokes equations, the geometrical surfaces formed by the three vector fields (velocity, vorticity and the skin-friction) and a scalar field (pressure) become interrelated through topological maps containing their respective singular points and extremal points. A mathematically consistent description of these singular points becomes inevitable when we want to study the geometry of the separation. A separated stream surface requires, for example, the existence of a saddle-type singular point on the skin-friction surface. This singular point is actually, in the proper language of mathematics, a saddle of index two. The index is a measure of the dimension of the outset (set leaving the singular point). Hence, when a saddle of index two is specified, a two dimensional surface that becomes separated from the osculating plane of the saddle is implied. The three-dimensional singular point is interpreted mathematically and the most common aerodynamical singular points are discussed through this perspective.

Unal, Aynur↗
NASA NTRS1988

Simulation of transonic viscous wing and wing-fuselage flows using zonal methods

The thin-layer Navier-Stokes equations are coupled with a zonal scheme (or domain-decomposition method) to develop the Transonic Navier-Stokes (TNS) wing-alone code. TNS has a total of four zones and is extended to a total of 16 zones for the wing-fuselage version of the code. Results are computed on the Cray X-MP-48 and compared with experimental data.

Flores, Jolen↗
NASA NTRS1989

Absolute and convective instability of a cylinder wake

The stability behavior of a circular cross-section cylinder's wake at Reynolds number values of up to 45 is presently investigated by means of local linear stability theory. The steady-wake profiles computed are Navier-Stokes solutions of a uniform, incompressible viscous flow around a cylinder obtained by a spectral method. An absolutely unstable region is found to begin to form at a Reynolds number of about 20, and grows with incresing Reynolds number. The onset of global instability response must be characterized by a critical length of an absolutely unstable region; a critical Reynolds number criterion and preferred frequency are proposed based on linear stability analysis.

Yang, X.↗
NASA NTRS1989

Experimental and predicted pressure and heating distributions for an Aeroassist Flight Experiment vehicle in air at Mach 10

The Aeroassisted Flight Experiment vehicle for whose scale model pressure and heat-transfer rate distributions have been measured in air at Mach 10 is a 60-deg elliptic cone, raked off at a 73-percent angle, with an ellipsoid nose and a skirt added to the base of the rake plane to reduce heating. The predictions of both an inviscid flow-field code and a Navier-Stokes solver are compared with measured values. Good agreement is obtained in the case of pressure distributions; the effect of Reynolds number on heat-transfer distributions is noted to be small.

Micol, John R.↗
NASA NTRS1989

Computational/experimental study of the flowfield on a body of revolution at incidence

A joint computational-experimental study of the vortical flowfield on a pointed body of revolution is undertaken. The incompressible Navier-Stokes equations are solved numerically for steady flow around an ogive-cylinder at angle of attack. The three-dimensional vortical flow is investigated, with emphasis on the tip and the near-wake region. Comparisons of computational results with results of a companion towing-tank experiment are presented at Re(L) = 1000. Results of a wind tunnel experiment at Re(L) = 820,000, featuring use of a seven-hole pressure probe, also are presented. It was observed that at the low Reynolds number of the towing tank study, the leeside flow was symmetric at the angles of attack considered. For the same configuration at the higher Reynolds number of the wind tunnel study, the flow was asymmetric under equivalenat conditions for some of the angles of attack studied. The topology of the flow is discussed and conclusions are drawn concerning the growth and stability of the primary vortices.

Zilliac, G. G.↗
NASA NTRS1989

A CFD study of tilt rotor flowfields

The download on the wing produced by the rotor wake of a tilt rotor vehicle in hover is of major concern because of its severe impact on payload-carrying capability. In a concerted effort to understand the fundamental fluid dynamics that cause this download, and to help find ways to reduce it, computational fluid dynamics (CFD) is employed to study this problem. The thin-layer Navier-Stokes equations are used to describe the flow, and an implicit, finite difference numerical algorithm is the method of solution. The methodology is developed to analyze the tilt rotor flowfield. Included are discussions of computations of an airfoil and wing in freestream flows at -90 degrees, a rotor alone, and wing/rotor interaction in two and three dimensions. Preliminary results demonstrate the feasibility and great potential of the present approach. Recommendations are made for both near-term and far-term improvements to the method.

Fejtek, Ian↗
NASA NTRS1990

Upper semicontinuous global attractors for viscous flow

A particular form of regularization of the Navier-Stokes equations is studied. It is shown that as the regularization parameter goes to zero, the global attractor for the regularized system converges to the global attractor for the conventional Navier-Stokes system.

Ou, Yuh-Roung↗
NASA NTRS1990

Evaluation of an adaptive unstructured remeshing technique for integrated fluid-thermal-structural analysis

An adaptive unstructured remeshing technique is evaluated for integrated fluid-thermal-structural analysis. The technique is combined with the finite element method to solve: (1) the Navier-Stokes equations for high-speed compressible flow; (2) the energy equation for the structural-thermal response; and (3) the quasi-static equilibrium equations for the structural response. The remeshing technique and the analysis solution procedure are described. The effectiveness of the approach is evaluated with two application studies. The flow analysis of Mach 8 shock-shock interference on a three-inch-diameter cylinder is used as the first application study to demonstrate the capability of the remeshing technique and to examine proper remeshing indicators for the inviscid and boundary layer regions. The applicability of the approach for the thermal and structural analyses of the structure is evaluated in the second application study of a 0.25-inch-diameter convectively cooled leading edge subjected to intense aerodynamic heating. Issues associated with remeshing indicators for thermal stress problems are identified.

Dechaumphai, Pramote↗
NASA NTRS1990

Computational modeling and validation for hypersonic inlets

Hypersonic inlet research activity at NASA is reviewed. The basis is the experimental tests performed with three inlets: the NASA Lewis Research Center Mach 5, the McDonnell Douglas Mach 12, and the NASA Langley Mach 18. Both three-dimensional parabolized Navier-Stokes and Navier-Stokes codes were used to compute the flow within the three inlets. Modeling assumptions in the codes involve the turbulence model, the nature of the boundary layer, shock wave boundary layer interaction, and the flow spilled to the outside of the inlet. Use of the codes in conjunction with the experimental data are helping to develop a clearer understanding of the inlet flow physics and to focus on the modeling improvements required in order to arrive at validated codes.

Povinelli, Louis A.↗
NASA NTRS1990

Experimental aerothermodynamic research of hypersonic aircraft

Wind tunnel tests were conducted to establish a benchmark experimental data base for a genetic hypersonic aircraft vehicle. Comprehensive measurements were made at Mach 7 to give flow visualization, surface pressure, surface convective heat transfer, and flow field Pitot pressure for a delta platform all-body vehicle. The tests were conducted in the NASA/Ames 3.5-Foot Hypersonic Wind Tunnel at Reynolds numbers sufficient to give turbulent flow. Comparisons are made of the experimental results with computational solutions of the flow by an upwind parabolized Navier-Stokes code developed at Ames. Good agreement of experiment with solutions by the code is demonstrated.

Cleary, Joseph W.↗
NASA NTRS1990

Evaluation of an adaptive unstructured remeshing technique for integrated fluid-thermal-structural analysis

An adaptive unstructured remeshing technique is evaluated for integrated fluid-thermal-structural analysis. The technique is combined with the finite element method to solve: (1) the Navier-Stokes equations for high-speed compressible flow, (2) the energy equation for the structural-thermal response, and (3) the quasi-static equilibrium equations for the structural response. The remeshing technique and the analysis solution procedure are described. The effectiveness of the approach is evaluated with two applications studies. The flow analysis of Mach 8 shock-shock interference on a three-inch-diameter cylinder is used as the first application study to demonstrate the capability of the remeshing technique and to examine proper remeshing indicators for the inviscid and boundary layer regions. The applicability of the approach for the thermal and structural analyses of the structure is evaluated in the second application study of a 0.25-inch-diameter convectively cooled leading edge subjected to intense aerodynamic heating. Issues associated with remeshing indicators for thermal stress problems are identified.

Dechaumphai, Pramote↗
NASA NTRS1990

Aerobraking in a dusty Martian atmosphere

The effects of dust particle impacts on the erosion of the heat shield were estimated for a 26 m diameter aerobraking vehicle entering the Mars atmosphere at 8.6 km/sec. An explicit Navier-Stokes code was used to compute the flow field about the vehicle for the actual Martian atmospheric composition at a speed of 7.5 km/sec. The deceleration and melting of the dust particles within the forebody shock-layer was computed for dust spherules having initial diameters from 3 to 10 microns. Two heat shield materials were considered: Shuttle tiles with glassy surfaces and a low-density ablator known as Avcoat.

Chang, I-Dee↗
NASA NTRS1990

A numerical study of the effects of reverse sweep on a scramjet inlet performance

A comparative numerical study of performance parameters of a similar and an opposite sweep sidewall compression inlet is made. The focus of the study is the investigation of the impact of alternate backward-forward sweep on the compression sidewalls as opposed to back-ward sweep on all the sidewalls. Two equivalent scramjet inlet configurations are designed for this purpose. These inlets have the same wetted areas of compression and expansion and same height and width; but in one inlet all the compression surface are swept back (similar sweep inlet) whereas in the other inlet, alternate surfaces are swept backward and forward (opposite sweep inlet). The cowl closure in both cases begins at the start of the throat region. A three-dimensional Navier-Stokes code is used to calculate the flow through these inlets. Results of these calculations are used to compare the two designs for their performance and flow quality. Effects of boundary- layer ingestion on the performance and overall flow features are also investigated.

Kumar, Ajay↗
NASA NTRS1990

Computational modeling and validation for hypersonic inlets

Hypersonic inlet research activity at NASA is reviewed. The basis is the experimental tests performed with three inlets: the NASA-Lewis Mach 5, the McDonnell Douglas Mach 12, and the NASA-Langley Mach 18. Both 3-D parabolized Navier-Stokes and Navier-Stokes codes were used to compute the flow within the three inlets. Modeling assumptions in the codes involve the turbulence model, the nature of the boundary layer, shock wave boundary layer interaction, and the flow spilled to the outside of the inlet. Use of the codes in conjunction with the experimental data are helping to develop a clearer knowledge of the inlet flow physics and to focus on the modeling improvements required in order to arrive at validated codes.

Povinelli, Louis A.↗
NASA NTRS1991

Radiative interactions in a hydrogen-fueled supersonic combustor

The two-dimensional, elliptic Navier-Stokes equations are used to investigate supersonic flows with finite-rate chemistry and radiation, for hydrogen-air systems. The chemistry source term in the species equation is treated implicitly to alleviate the stiffness associated with fast reactions. The explicit, unsplit MacCormack finite-difference scheme is used to advance the governing equations in time, until convergence is achieved. The specific problem considered is the premixed flow in a channel with a 10-deg compression ramp. Three different chemistry models are used, accounting for increasing number of reactions and participating species. Two chemistry models assume nitrogen as inert, while the third model accounts for nitrogen reactions and NO(x) formation. The tangent slab approximation is used in the radiative flux formulation. A pseudogray model is used to represent the absorption-emission characteristics of the participating species. Results otained for specific conditions indicate that the radiative interactions vary substantially, depending on reactions involving HO2 and NO species, and that this can have a significant influence on the flowfield.

Chandrasekhar, R.↗
NASA NTRS1991

Application of a Reynolds Stress turbulence model to a supersonic hydrogen-air diffusion flame

A second-order differential Reynolds Stress turbulence model has been applied to the Favre-averaged Navier-Stokes equations for the study of supersonic flows undergoing hydrogen-air chemical reactions. An assumed Beta Probability Density Function is applied to account for the chemical source terms in the conservation equations. An algebraic Reynolds Flux model is used for the fluctuating density-velocity as well as the species mass fraction-velocity correlations. The variances of temperature and species fluctuations are also modelled using an algebraic flux technique. A seven-species, seven-reaction finite rate chemistry mechanism is used to simulate the combustion processes. The resulting formulation is validated by comparison with experimental data on reacting supersonic axisymmetric jets. Results obtained for specific conditions indicate that the effect of chemical reaction on the turbulence is significant.

Chandrasekhar, R.↗
NASA NTRS1991

A hybrid numerical method for three-dimensional spatially-developing free-shear flows

The present novel algorithm for 3D incompressible Navier-Stokes equations treats a domain which is (1) infinite in the vertical direction, using a mapped spectral method; (2) finite in the streamwise direction, using a classical Fourier method; and (3) homogeneous in the spanwise direction, using high-order compact finite differencing. A projection method is employed which ensures the exact conservation of mass, as well as the satisfaction of the boundary conditions at infinity. The novel aspects of these methods are noted, and the code they constitute is validated in light of several test cases. Results are presented for two- and three-dimensional mixing layers.

Buell, Jeffrey C.↗