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At least 145 records · Page 8

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NASA NTRS1989

Diverging boundary layers with zero streamwise pressure gradient

The effects of spanwise divergence on the boundary layer forming between a pair of embedded streamwise vortices with the common flow between them directed toward the wall was studied. Measurements indicate that divergence controls the rate of development of the boundary layer and that large divergence significantly retards boundary layer growth and enhances skin friction. For strongly diverging boundary layers, divergence accounts for nearly all of the local skin friction. Even with divergence, however, the local similarity relationships for two-dimensional boundary layers are satisfactory. Although divergence modifies the mean development of the boundary layer, it does not significantly modify the turbulence structure. In the present experiments with a zero streamwise pressure gradient, it was found that spanwise divergence dit not significantly affect the Reynolds stress and the turbulent triple product distributions.

Pauley, Wayne R.↗

NASA NTRS1993

Observations of a quasi-static plasma sheet boundary

No high-speed flows or discernible counterstreaming ion beams were observed during a series of plasma sheet boundary encounters resulting from solar wind-driven plasma sheet motions. We conclude that the boundary may be active primarily during plasma sheet 'recovery'. A temporal onset of flows in the inner plasma sheet (IPS) was associated with the appearance of counterstreaming beams embedded in an already isotropic plasma sheet boundary and close to the neutral sheet may have a common generation mechanism.

Angelopoulos, V.↗

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.↗

DOE OSTI2020

A reduced-order shifted boundary method for parametrized incompressible Navier–Stokes equations

Here, we investigate a projection-based reduced order model of the steady incompressible Navier–Stokes equations for moderate Reynolds numbers. In particular, we construct an “embedded” reduced basis space, by applying proper orthogonal decomposition to the Shifted Boundary Method, a high-fidelity embedded method recently developed. We focus on the geometrical parametrization through level-set geometries, using a fixed Cartesian background geometry and the associated mesh. This approach avoids both remeshing and the development of a reference domain formulation, as typically done in fitted mesh finite element formulations. Two-dimensional computational examples for one and three parameter dimensions are presented to validate the convergence and the efficacy of the proposed approach.

"embedded" basis↗

NASA NTRS1980

Computation of three-dimensional viscous supersonic flow in inlets

A new approach has been developed for the computation of the three-dimensional viscous supersonic flow with embedded subsonic regions adjacent to solid boundaries and is applied to a mixed-compression supersonic inlet typical of current designs. The approach uses a reduced form of the three-dimensional Navier-Stokes equations so that the resultant equations can be treated as an initial boundary value problem and thus be solved by non-iterative forward marching in space. The numerical procedure utilizes an efficient consistently-split linearized block implicit technique to solve the finite difference analogues to the set of governing partial differential equations.

Buggeln, R. C.↗

NASA NTRS1982

Computation of nonlinear supersonic potential flow over three dimensional surfaces

A fully implicit three dimensional marching technique is presented for the computation of inviscid supersonic flows. The nonconservative full potential equation is solved on spherical cross flow planes using standard transonic SLOR techniques. The three dimensional bow shock is fitted as a boundary with isentropic jump conditions and embedded shocks are captured. Applications of these techniques are presented for three dimensional bodies, wings, and wing-body configurations and the computed solutions are compared to experimental pressure data. Zero lift wave drag calculations are also presented and compared with measurements to evaluate the accuracy of the present techniques.

Siclari, M. J.↗

NASA NTRS1986

An embedding method for the steady Euler equations

Certain difficulties arise in connection with the numerical solution of a direct finite difference representation of the steady Euler equations. Johnson (1979, 1981, 1982) has, therefore, proposed a surrogate-equation technique, in which the first-order steady Euler equations are embedded in a certain second-order system of equations. The present paper is concerned with the theoretical justification for such an embedding approach. For the numerical solution of the two-dimensional steady Euler equations, it is shown that, under a continuity restriction, it is possible to solve a second-order embedded system together with appropriate additional boundary conditions. The result indicates that a more direct and potentially more efficient approach to the steady solutions exists than the alternative of solving the unsteady equations.

Chang, S.-H.↗

NASA NTRS1992

Control of low-speed turbulent separated flow over a backward-facing ramp

The relative performance and flow phenomena associated with several devices for controlling turbulent separated flow were investigated at low speeds. Relative performance of the devices was examined for flow over a curved, backward-facing ramp in a wind tunnel, and the flow phenomena were examined in a water tunnel using dye-flow visualization. Surface static pressure measurements and oil-flow visualization results from the wind tunnel tests indicated that transverse grooves, longitudinal grooves, submerged vortex generators, vortex generator jets (VGJ's), Viets' fluidic flappers, elongated arches at positive angle of attack, and large-eddy breakup devices (LEBU's) at positive angle of attack placed near the baseline separation location reduce flow separation and increase pressure recovery. Spanwise cylinders reduce flow separation but decrease pressure recovery downstream. Riblets, passive porous surfaces, swept grooves, Helmholtz resonators, and arches and LEBU's with angle of attack less than or = 0 degrees had no significant effect in reducing the extent of the separation region. Wall-cooling computations indicated that separation delay on a partially-cooled ramp is nearly the same as on a fully-cooled ramp, while minimizing the frictional drag increase associated with the wall cooling process. Dry-flow visualization tests in the water tunnel indicated that wishbone vortex generators in the forward orientation shed horseshoe vortices; wishbone vortex generators oriented in the reverse direction and doublet vortex generators shed streamwise counterrotating vortices; a spanewise cylinder located near the wall and LEBU's at angle of attack = -10 degrees produced eddies or transverse vortices which rotated with the same sign as the mean vorticity in a turbulent boundary layer; and the most effective VGJ's produced streamwise co-rotating vortices. Comparative wind-tunnel test results indicated that transferring momentum from the outer region of a turbulent boundary layer through the action of embedded streamwise vortices is more effective than by transverse vortices for the separation control application studied herein.

Lin, John C.↗

NASA NTRS2019

Adaptive Embedded LES of the NASA Hump

A scheme for adaptive embedded LES is proposed which automatically determines boundaries for LES regions in a hybrid LES-RANS computation, with the goal of minimizing the LES part of the computation for maximum accuracy with minimum cost. The model-invariant hybrid formulation enables this scheme through greater flexibility in the placement of RANS-LES transitions. An adaptive embedded large-eddy simulation is carried out for the NASA hump test case and adaptive meshing is added to show how additional adaptive features may be controlled by the adaptive hybrid scheme.

Woodruff, Stephen↗

NASA NTRS1983

Embedding methods for the steady Euler equations

An approach to the numerical solution of the steady Euler equations is to embed the first-order Euler system in a second-order system and then to recapture the original solution by imposing additional boundary conditions. Initial development of this approach and computational experimentation with it were previously based on heuristic physical reasoning. This has led to the construction of a relaxation procedure for the solution of two-dimensional steady flow problems. The theoretical justification for the embedding approach is addressed. It is proven that, with the appropriate choice of embedding operator and additional boundary conditions, the solution to the embedded system is exactly the one to the original Euler equations. Hence, solving the embedded version of the Euler equations will not produce extraneous solutions.

Chang, S. H.↗

NASA NTRS1988

Passive control plate for shock-boundary layer interactions

Computational and experimental results are presented for a passive method capable of reducing or completely eliminating the separation region in shock-boundary layer interactions of the type that can result in severe performance degradation due to flow separation in supersonic and hypersonic inlets. The method involves the placement of an embedded plate in the outer portion of the boundary layer, parallel to the wall, so that the incident shock will impinge upon, and be reflected from, its surface. This will lower the pressure gradient in the low-momentum, near-wall region. A porous control plate's effect has also been investigated.

Goodman, Wesley L.↗

DOE OSTI2022

Finite boundary effects on the spherical Rayleigh–Taylor instability between viscous fluids

For the Rayleigh–Taylor unstable arrangement of a viscous fluid sphere embedded in a finite viscous fluid spherical shell with a rigid boundary and a radially directed acceleration, a dispersion relation is developed from a linear stability analysis using the method of normal modes. [Formula: see text] is the radially directed acceleration at the interface. ρ i denotes the density, μ i is the viscosity, and R i is the radius, where i = 1 is the inner sphere and i = 2 is the outer sphere. The dispersion relation is a function of the following dimensionless variables: viscosity ratio [Formula: see text], density ratio [Formula: see text], spherical harmonic mode n, [Formula: see text], [Formula: see text], and the dimensionless growth rate [Formula: see text], where σ is the exponential growth rate. We show that the boundedness provided by the outer spherical shell has a strong influence on the instability behavior, which is reflected not only in the modulation of the growth rate but also in the selection of the most unstable modes that are physically possible. This outer boundary effect is quantified by the relative magnitude of the radius ratio H. We find that when H is close to unity, lower order harmonics are excluded from becoming the most unstable within a vast region of the parameter space. In other words, the effect of H has precedence over the other controlling parameters d, B, and a wide range of s in establishing what the lowest most unstable mode can be. When H ~ 1, low order harmonics can become the most unstable only for s >> 1. However, in the limit when s → ∞, we show that the most unstable mode is n = 1 and derive the dispersion relation in this limit. The exclusion of most unstable low order harmonics caused by a finite outer boundary is not realized when the outer boundary extends beyond a certain threshold length-scale in which case all modes are equally possible depending on the value of B.

74 ATOMIC AND MOLECULAR PHYSICS↗

NASA NTRS2001

Thermocouple Rakes for Measuring Boundary Layer Flows Extremely Close to Surface

Of vital interest to aerodynamic researchers is precise knowledge of the flow velocity profile next to the surface. This information is needed for turbulence model development and the calculation of viscous shear force. Though many instruments can determine the flow velocity profile near the surface, none of them can make measurements closer than approximately 0.01 in. from the surface. The thermocouple boundary-layer rake can measure much closer to the surface than conventional instruments can, such as a total pressure boundary layer rake, hot wire, or hot film. By embedding the sensors (thermocouples) in the region where the velocity is equivalent to the velocity ahead of a constant thickness strut, the boundary-layer flow profile can be obtained. The present device fabricated at the NASA Glenn Research Center microsystem clean room has a heater made of platinum and thermocouples made of platinum and gold. Equal numbers of thermocouples are placed both upstream and downstream of the heater, so that the voltage generated by each pair at the same distance from the surface is indicative of the difference in temperature between the upstream and downstream thermocouple locations. This voltage differential is a function of the flow velocity, and like the conventional total pressure rake, it can provide the velocity profile. In order to measure flow extremely close to the surface, the strut is made of fused quartz with extremely low heat conductivity. A large size thermocouple boundary layer rake is shown in the following photo. The latest medium size sensors already provide smooth velocity profiles well into the boundary layer, as close as 0.0025 in. from the surface. This is about 4 times closer to the surface than the previously used total pressure rakes. This device also has the advantage of providing the flow profile of separated flow and also it is possible to measure simultaneous turbulence levels within the boundary layer.

Hwang, Danny P.↗

NASA NTRS1997

The Path Resistance Method for Bounding the Smallest Nontrivial Eigenvalue of a Laplacian

We introduce the path resistance method for lower bounds on the smallest nontrivial eigenvalue of the Laplacian matrix of a graph. The method is based on viewing the graph in terms of electrical circuits; it uses clique embeddings to produce lower bounds on lambda(sub 2) and star embeddings to produce lower bounds on the smallest Rayleigh quotient when there is a zero Dirichlet boundary condition. The method assigns priorities to the paths in the embedding; we show that, for an unweighted tree T, using uniform priorities for a clique embedding produces a lower bound on lambda(sub 2) that is off by at most an 0(log diameter(T)) factor. We show that the best bounds this method can produce for clique embeddings are the same as for a related method that uses clique embeddings and edge lengths to produce bounds.

Guattery, Stephen↗

NASA NTRS1987

Full potential integral solution for transonic flows with and without embedded Euler domains

Two methods are presented to solve for the transonic airfoil flow problems. The first method is based on the integral equation solution of the full-potential equation in terms of the velocity field, and a Shock Capturing-Shock Fitting (SCSF) scheme has been developed. The SCSF-scheme consists of a shock-capturing part and a shock-fitting part in which shock panels are introduced at the shock location. The sock panels are fitted and crossed by using the Rankine-Hugoniot relations. The second method is based on coupling the integral equation of the full-potential equation with the pseudo time integration of Euler equations in a small embedded region around the shock. The integral solution provides the initial and boundary conditions for the Euler domain. This scheme is named as the Integral Equation-Embedded Euler (IEEE) scheme. The two methods are applied to NACA 0012 and NACA 64A010A over a wide range of Mach numbers, and the reults are in good agreement with the experimental data and other computational results. The schemes converge within a number of iterations which is one-order of magnitude less than the finite-difference schemes.

Kandil, Osama A.↗

NASA NTRS1985

Overset grids in compressible flow

Numerical experiments have been performed to investigate the importance of boundary data handling with overset grids in computational fluid dynamics. Experience in using embedded grid techniques in compressible flow has shown that shock waves which cross grid boundaries become ill defined and convergence is generally degraded. Numerical boundary schemes were studied to investigate the cause of these problems and a viable solution was generated using the method of characteristics to define a boundary scheme. The model test problem investigated consisted of a detached shock wave on a 2-dimensional Mach 2 blunt, cylindrical body.

Eberhardt, S.↗

NASA NTRS1994

Radiation boundary condition and anisotropy correction for finite difference solutions of the Helmholtz equation

In this paper finite-difference solutions of the Helmholtz equation in an open domain are considered. By using a second-order central difference scheme and the Bayliss-Turkel radiation boundary condition, reasonably accurate solutions can be obtained when the number of grid points per acoustic wavelength used is large. However, when a smaller number of grid points per wavelength is used excessive reflections occur which tend to overwhelm the computed solutions. Excessive reflections are due to the incompability between the governing finite difference equation and the Bayliss-Turkel radiation boundary condition. The Bayliss-Turkel radiation boundary condition was developed from the asymptotic solution of the partial differential equation. To obtain compatibility, the radiation boundary condition should be constructed from the asymptotic solution of the finite difference equation instead. Examples are provided using the improved radiation boundary condition based on the asymptotic solution of the governing finite difference equation. The computed results are free of reflections even when only five grid points per wavelength are used. The improved radiation boundary condition has also been tested for problems with complex acoustic sources and sources embedded in a uniform mean flow. The present method of developing a radiation boundary condition is also applicable to higher order finite difference schemes. In all these cases no reflected waves could be detected. The use of finite difference approximation inevita bly introduces anisotropy into the governing field equation. The effect of anisotropy is to distort the directional distribution of the amplitude and phase of the computed solution. It can be quite large when the number of grid points per wavelength used in the computation is small. A way to correct this effect is proposed. The correction factor developed from the asymptotic solutions is source independent and, hence, can be determined once and for all. The effectiveness of the correction factor in providing improvements to the computed solution is demonstrated in this paper.

Tam, Christopher K. W.↗

NASA NTRS1989

Consequences of a conductive boundary on the local cloud. I - No dust

Using boundary conditions appropriate to the Local Cloud embedded in the Local Bubble, the evaporation of the cloud is modeled assuming steady flow. Included in the model are the effects of radiative cooling and nonequilibrium ionization structure in the interface, and pressure support and heat conduction inhibition due to the magnetic field. The soft X-ray and EUV emission from the interface are calculated and compared with observations. It is found that the emission in the softest X-ray band, the Be band, is harder than that calculated for an equilibrium, normal abundance, 10 to the 6th K plasma. Column densities of C IV, N V, O VI, Si IV, and Si III in the interface are calculated. The column density for O VI is found to be very close to observed values for nearby stars, while all others are too small to contribute significantly to observed values. The ionization of the Local Cloud due to radiation from the interface is calculated. It is found that a significant amount of ionizing radiation is produced, dominating all other known sources. The hardness of the emission, however, results in ionization of helium as well as hydrogen. Thus, the type of boundary modeled cannot explain the ionization of the very local interstellar medium inferred from solar backscatter measurements.

Slavin, Jonathan D.↗