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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Spurious Numerical Oscillations in Numerical Simulation of Supersonic Flows Using Shock Capturing Schemes

The numerical simulation of transitional and turbulent processes in hypersonic boundary layers often involves a physical process of a shock-disturbance wave interaction in complex two-dimensional and three-dimensional flow fields. For such simulations, it is required that there be a high order of accuracy in capturing both the shock and the small disturbances. The purpose of this paper is to evaluate the viability of using high order shock capturing schemes to track small disturbances in a multi-dimensional steady hypersonic flow. The numerical methods that are to be studied are the Total Variation Diminishing (TVD) scheme, and Essentially Non-Oscillatory (ENO) scheme. This paper shows that the presence of numerical oscillations in the flow field solution may drastically hinder any attempt at tracking the propagation of any physical disturbances. It has been found that the numerical oscillations that exist for shock capturing methods may be significant enough to pollute a flow field containing small physical disturbances. The effects of the refinement of the grid do not reduce the oscillations, but rather they decrease the wavelength of the oscillations. It is shown that by aligning the shock with the grid, the amplitude of these spurious oscillations may be greatly reduced.

Lee, Theodore K.↗
NASA NTRS2005

Schlieren Measurements of Buoyancy Effects on Flow Transition in Low-Density Gas Jets

The transition from laminar to turbulent flow in helium jets discharged into air was studied using Rainbow Schlieren Deflectometry technique. In particular, the effects of buoyancy on jet oscillations and flow transition length were considered. Experiments to simulate microgravity were conducted in the 2.2s drop tower at NASA Glenn Research Center. The jet Reynolds numbers varied from 800 to1200 and the jet Richardson numbers ranged between 0.01 and 0.004. Schlieren images revealed substantial variations in the flow structure during the drop. Fast Fourier Transform (FFT) analysis of the data obtained in Earth gravity experiments revealed the existence of a discrete oscillating frequency in the transition region, which matched the frequency in the upstream laminar regime. In microgravity, the transition occurred farther downstream indicating laminarization of the jet in the absence of buoyancy. The amplitude of jet oscillations was reduced by up to an order of magnitude in microgravity. Results suggest that jet oscillations were buoyancy induced and that the brief microgravity period may not be sufficient for the oscillations to completely subside.

Pasumarthi, Kasyap S.↗
NASA NTRS1991

Effects of leading-edge flap oscillation on unsteady delta wing flow and rock control

The isolated and interdisciplinary problems of unsteady fluid dynamics and rigid-body dynamics and control of delta wings with and without leading-edge flap oscillation are considered. For the fluid dynamics problem, the unsteady, compressible, thin-layer Navier-Stokes (NS) equations, which are written relative to a moving frame of reference, are solved along with the unsteady, linearized, Navier-displacement (ND) equations. The NS equations are solved for the flowfield using an implicit finite-volume scheme. The ND equations are solved for the grid deformation, if the leading-edge flaps oscillate, using an ADI scheme. For the dynamics and control problem, the Euler equation of rigid-body rolling motion for a wing and its flaps are solved interactively with the fluid dynamics equations for the wing-rock motion and subsequently for its control. A four-stage Runge-Kutta scheme is used to explicitly integrate the dynamics equation.

Kandil, Osama A.↗
NASA NTRS2008

Flow Instabilities in Feather Seals due to Upstream Harmonic Pressure Fluctuations

Feather seals (also called slot seals) typically found in turbine stators limit leakage from the platform into the core cavities and from the shroud to the case. They are of various geometric shapes, yet all are contoured to fit the aerodynamic shape of the stator and placed as close as thermomechanically reasonable the powerstream flow passage. Oscillations engendered in the compressor or combustor alter the steady leakage characteristics of these sealing elements and in some instances generate flow instabilities downstream of the seal interface. In this study, a generic feather seal geometry was studied numerically by imposing an upstream harmonic pressure disturbance on the simulated stator-blade gap. The flow and thermal characteristics were determined; it was found that for high pressure drops, large fluctuations in flows in the downstream blade-stator gap can occur. These leakages and pulsations in themselves are not all that significant, yet if coupled with cavity parameters, they could set up resonance events. Computationally generated time-dependent flow fields are captured in sequence video streaming.

Deng, D.↗
NASA NTRS1956

Finite Span Wings in Compressible Flow

Equations are developed using the source distribution method for the velocity potential function and pressure on thin wings in steady and unsteady motion. Closed form solutions are given for harmonically oscillating wings of general plan form including the effect of the wing wake. Some useful examples are presented in an appendix for arrow, semielliptical, and hexagonal plan form wings.

WINGS, ELLIPTICAL↗
NASA NTRS1989

Oscillating aerodynamics and flutter of an aerodynamically detuned cascade in an incompressible flow

A mathematical model is developed and utilized to demonstrate the enhanced torsion mode stability associated with alternate blade circumferential aerodynamic detuning of a rotor operating in an incompressible flow field. The oscillating cascade aerodynamics, including steady loading effects, are determined by developing a complete first order unsteady aerodynamic analysis. An unsteady aerodynamic influence coefficient technique is then utilized, thereby enabling the stability of both conventional uniformly spaced rotors and detuned nonuniform circumferentially spaced rotors to be determined. To demonstrate the enhanced flutter aeroelastic stability associated with this aerodynamic detuning mechanism, this model is applied to a baseline unstable rotor with a Gostelow flow geometry.

Chiang, Hsiao-Wei D.↗
NASA NTRS1992

Some effects of oscillation waveform and amplitude on unsteady turbulent shear flows

Some physical features of several unsteady separating turbulent boundary layers are presented for practical Reynolds numbers and reduced frequencies such as for helicopter and turbomachinery flows. The effects of unsteadiness amplitude and waveform are examined for flows along the floor of a converging and diverging wind tunnel test section. At the end of the converging portion, the mean skin friction coefficient normalized on the mean dynamic pressure is independent of the waveform and amplitude within low experimental uncertainties. In the detaching and detached portions of the flow, wall values of the fraction of time that the flow moves downstream of gamma sub pu, which is a separated flow state variable, shows that oscillation waveform and amplitude strongly influence the detached flow behavior. Distributions of gamma sub pu during a cycle indicate hysteresis within the detached flow and the effects of the higher harmonics of pressure gradient and velocity.

Agarwal, Naval K.↗
NASA NTRS2014

Investigation of Active Flow Control to Improve Aerodynamic Performance of Oscillating Wings

The objective of this effort is to design a promising active flow control concept on an oscillating airfoil for on-blade alleviation of dynamic stall. The concept must be designed for a range of representative Mach numbers (0.2 to 0.5) and representative reduced frequency characteristics of a full-scale rotorcraft. Specifications for a sweeping-jet actuator to mitigate the detrimental effects of retreating blade stall experienced by edgewise rotors in forward flight has been performed. Wind tunnel modifications have been designed to accommodate a 5x6 test section in the Oran W. Nicks Low Speed Wind Tunnel at Texas A&M University that will allow the tunnel to achieve Mach 0.5. The flow control design is for a two-dimensional oscillating VR-7 blade section with a 15- inch chord at rotor-relevant flow conditions covering the range of reduced frequencies from 0.0 to 0.15 and Mach numbers from 0.2 to 0.5. A Computational Fluid Dynamics (CFD) analysis has been performed to influence the placement of the flow control devices for optimal effectiveness.

Narducci, Robert P.↗
NASA NTRS1976

On the computation of the transonic perturbation flow field around two- and three-dimensional oscillating wings

A finite difference method for solving the unsteady flow about harmonically oscillating wings is investigated. The procedure is based on separating the velocity potential into steady and unsteady parts and linearizing the resulting unsteady differential equation for small disturbances. Solutions are obtained using relaxation procedures. It is determined that there is a limit on reduced frequency, which is a function of Mach number and size of mesh region, above which the relaxation procedures will not converge. It is found that row line relaxation is more efficient than column relaxation and results are presented for a rectangular wing in harmonic pitch.

Weatherill, W. H.↗
NASA NTRS1985

Numerical simulation of self-excited oscillations in a ramjet inlet-diffuser flow

This paper describes numerical simulations of self-excited oscillations in a two-dimensional transonic inlet-diffuser flow by solving the Navier-Stokes equations with a two-equation turbulence model. The calculated amplitudes of oscillations for the terminal shock and the velocity fields compare well with experimental measurements; however, the predicted frequency of oscillations is about 50 percent higher. The formation of a pair of downstream-traveling, counter-rotating vortices at each cycle of velocity fluctuations, as reported experimentally, is vividly revealed by the numerical results.

Hsieh, T.↗
NASA NTRS1988

Investigation of the validity of Reynolds averaged turbulence models at the frequencies that occur in turbomachinery

Turbulent flows subjected to various kinds of unsteady disturbances were simulated using a large-eddy-simulation computer code for flow in a channel. The disturbances were: a normal velocity expressed as a traveling wave on one wall of the channel; staggered blowing and suction distributions on the opposite walls of the channel; and oscillations of the mean flow through the channel. The wall boundary conditions were designed to simulate the effects of wakes of a stator stage passing through a rotor channel in a turbine. The oscillating flow simulated the effects of a pressure pulse moving over the rotor blade boundary layer. The objective of the simulations was to provide better understanding of the effects of time-dependent disturbances on the turbulence of a boundary layer and of the underlying physical phenomena regarding the basic interaction between the turbulence and external disturbances of the type found in turbomachinery. Results showed that turbulence is sensitive to certain ranges of frequencies of disturbances. However, no direct connection was found between the frequency of imposed disturbances and characteristic burst frequency of turbulence. New insight into the nature of turbulence at high frequencies was found. The viscous phenomena near solid walls was found to be the dominant influence for high frequency perturbations. At high frequencies, the turbulence was found to be undisturbed, remaining the same as for the steady mean flow. A transition range exists between the high frequency range and the low, or quasi-steady, range in which the turbulence is not predictable by either quasi-steady models or the steady flow model. The limiting lowest frequency for use of the steady flow turbulence model is that for which the viscous Stokes layer based on the blade passing frequency is thicker than the laminar sublayer.

Kuhn, Gary D.↗
NASA NTRS1988

An upwind differencing scheme for the time-accurate incompressible Navier-Stokes equations

The two-dimensional incompressible Navier-Stokes equations are solved in a time-accurate manner in using the method of pseudocompressibility. Using this method, subiterations in pseudotime are required to satisfy the continuity equation at each time step. An upwind differencing scheme based on flux-difference splitting is used to compute the convective terms. The upwind differencing is biased based on the sign of the local eigenvalue of the Jacobian matrix. Third-order or fifth-order spatial accuracy is maintained throughout the interior grid points. The equations are solved using an implicit line-relaxation scheme. This solution scheme is stable and is capable of running at large time steps in pseudotime, leading to fast convergence for each physical time step. A variety of computed results are presented to validate the present scheme. Results for the flow over an oscillating plate are compared with the exact analytic solution, good agreement is seen. Excellent comparison is obtained between the computed solution and the analytical results for inviscid channel flow with an oscillating back pressure. Flow solutions over a circular cylinder with vortex shedding are also presented. Finally, the flow past an airfoil at -90 deg angle-of-attack is also computed.

Rogers, Stuart E.↗
NASA NTRS1990

Computation of vortex-dominated flow for a delta wing undergoing pitching oscillation

The conservative, unsteady Euler equations for the flow relative to a moving frame of reference are used to solve for the three-dimensional steady and unsteady flows around a sharp-edged delta wing. The resulting equations are solved by using an implicit, approximately factored, finite-volume scheme. Implicit second-order and explicit second- and fourth-order dissipations are added to the scheme. The boundary conditions are explicitly satisfied. The grid is generated by locally using a modified Joukowski transformation in crossflow planes at the grid-chord stations. The computational applications cover a steady flow around a delta wing, whose results serve as the initial conditions for the unsteady flow around a pitching delta wing at a large mean angle of attack. The steady results are compared with the experimental data, and the unsteady results are compared with results of a flux-difference splitting scheme.

Kandil, Osama A.↗
NASA NTRS1987

Computation of steady and unsteady vortex dominated flows

The unsteady Euler equations have been written in the conservation form for the relative motion in a rotating frame of reference. The resulting equations are solved by using a central-difference finite-volume scheme with four-stage Runge-Kutta time stepping. For steady flow problems, local time stepping is used and for unsteady flow problems, the minimum global time stepping is used. A three-dimensional fully vectorized computer program has been developed and applied to steady and unsteady maneuvering delta wing. The capability of the three-dimensional program has been demonstrated for a rigid sharp-edged delta wing undergoing uniform rolling in a conical flow and rolling oscillations in a locally conical flow.

Kandil, Osama A.↗
NASA NTRS1988

Computation of steady and unsteady vortex-dominated flows with shock waves

The unsteady Euler equations have been derived in the conservation form for the flow relative motion with respect to a rotating frame of reference. The resulting equations are solved by using a central-difference finite-volume scheme with four-state Runge-Kutta time stepping. For steady flow applications local time stepping is used, and for unsteady applications the minimum global time stepping is used. A three-dimensional fully vectorized computer program has been developed and applied to steady and unsteady maneuvering delta wings. The capability of the three-dimensional program has been demonstrated for a rigid sharp-edged delta wing undergoing uniform rolling in a conical flow and rolling oscillations in a locally conical flow.

Kandil, Osama A.↗