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At least 91 records · Page 5

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NASA NTRS1989

Application of a self-adaptive grid method to complex flows

A directional-split, modular, user-friendly grid point distribution code is applied to several test problems. The code is self-adaptive in the sense that grid point spacing is determined by user-specified constants denoting maximum and minimum grid spacings and constants relating the relative influence of smoothness and orthogonality. Estimates of truncation error, in terms of flow-field gradients and/or geometric features, are used to determine the point distribution. Points are redistributed along grid lines in a specified direction in an elliptic manner over a user-specified subdomain, while orthogonality and smoothness are controlled in a parabolic (marching) manner in the remaining directions. Multidirectional adaption is achieved by sequential application of the method in each coordinate direction. The flow-field solution is redistributed onto the newly distributed grid points after each unidirectional adaption by a simple one-dimensional interpolation scheme. For time-accurate schemes such interpolation is not necessary and time-dependent metrics are carried in the fluid dynamic equations to account for grid movement.

Deiwert, G. S.↗

NASA NTRS1989

Unsteady aerodynamic simulation of multiple bodies in relative motion: A prototype method

A prototype method for time-accurate simulation of multiple aerodynamic bodies in relative motion is presented. The method is general and features unsteady chimera domain decomposition techniques and an implicit approximately factored finite-difference procedure to solve the time-dependent thin-layer Navier-Stokes equations. The method is applied to a set of two- and three- dimensional test problems to establish spatial and temporal accuracy, quantify computational efficiency, and begin to test overall code robustness.

Meakin, Robert L.↗

NASA NTRS1988

Numerical investigations in three-dimensional internal flows

In order to develop an understanding of flow fields that exist in sidewall compression inlet models, an explicit, time-accurate full Navier-Stokes code was used to model them. Both laminar and turbulent boundary layers were numerically simulated. These simulations were examined as they were influenced by Mach number, changes in sidewall compression angle, hypersonic viscous interaction effects, and thick entering boundary layers. Cause and effect relationships were established. The numerical simulations were compared with the limited data available to aid in understanding both the simulations and the experiments.

Rose, William C.↗

NASA NTRS1989

Numerical modeling of enclosure convection

A numerical study on the steady and unsteady natural convection in two-dimensional rectangular enclosures has been performed by a time-accurate ADI finite difference scheme. The study covered a range of Rayleigh numbers between 1000 and 10 to the 7th, aspect ratios between 0.2 and 10.0, and tilt angles between -90 (heating from bottom) and +90 deg (heating from top). Various Prandtl numbers have been studied, but only the results of water (Pr = 7.0) are reported here due to space limitations. The physics revealed, however, includes the convection phenomena and the Rayleigh-Benard stability, as well as the combined mechanism of these two. The onset of secondary cells is determined by using a velocity map, which is simpler and cleaner, instead of a streamline plot. The critical Ra number for the occurrence of these secondary cells is shown to be lower than can be detected by experimental studies. On the Rayleigh-Benard stability part, a second transition from stable single-cell convection to periodic multicellular convection is disclosed.

Duh, J. C.↗

NASA NTRS1989

Time-dependent Navier-Stokes computations for flow-induced vibrations of vanes

Flows over two curved vane configurations were computed using a time-accurate compressible Navier-Stokes flow model. One configuration showed the presence of strong flow-induced vibrations at Strouhal numbers near 0.19 and 0.38 for bending and torsional excitation. In the other configuration, a simple modification reduced both types of response. Laminar flows were analyzed for the effects of flow-induced vibrations, and flow fields were solved for a rigid vane and a vane undergoing forced vibrations at prescribed amplitude and frequency simulating vibration response to a coupled vortex-shedding/elastic motion feedback cycle.

Liu, B. L.↗

NASA NTRS1990

Numerical investigation of the origin of vortex asymmetry

The flow about a slender body of revolution placed at incidence to an oncoming stream is numerically investigated for angles of attack ranging from 20 to 80 degrees and a Reynolds number of 200,000, based on freestream conditions and maximum body diameter. Navier-Stokes computations are applied to investigate the phenomena governing the onset of vortex asymmetry. Time-accurate solutions are given for an ogive-cylinder body with and without a space-fixed time-invariant disturbance added near the tip. At a 40-degree angle of attack the flow is found to be steady but becomes highly asymmetric when the disturbance is added. The level of asymmetry is dependent upon the size and location of the disturbance. It is suggested that the origin of the asymmetry is a convective-type instability of the originally symmetric flow. For higher angles of attack (60 and 80 degrees) the flow around the cylindrical part becomes unsteady, and vortex shedding is observed. It is suggested that the origin of flow unsteadiness and vortex shedding in the wake is an absolute-type instability of the originally steady flow.

Degani, David↗

NASA NTRS1990

Prediction of steady and unsteady asymmetric vortical flows around cones

Steady and unsteady, supersonic asymmetric vortical flows and their passive control around circular and noncircular cones are considered in this paper. These problems are formulated by using the unsteady, compressible, single and double, thin-layer. Navier-Stokes equations. The equations are solved by using an implicit, upwind, flux-difference splitting, finite-volume scheme, either in a pseudotime stepping or in an accurate-time stepping. An implicit, approximately-factored, central-difference finite-volume scheme has also been used to validate some applications of the upwind scheme. Steady asymmetric vortical flows have been predicted by using random and controlled disturbances for circular and noncircular cones. Unsteady asymmetric vortex-shedding flows have also been predicted, for the first time, using time-accurate solutions, for circular and noncircular cones. Control of flow asymmetry have been demonstrated computationally, for the first time, by inserting a vertical fin the leeward plane of geometric symmetry.

Kandil, Osama A.↗

NASA NTRS1990

Counterrotating prop-fan simulations which feature a relative-motion multiblock grid decomposition enabling arbitrary time-steps

Improvements are presented of a computer algorithm developed for the time-accurate flow analysis of rotating machines. The flow model is a finite volume method utilizing a high-resolution approximate Riemann solver for interface flux definitions. The numerical scheme is a block LU implicit iterative-refinement method which possesses apparent unconditional stability. Multiblock composite gridding is used to orderly partition the field into a specified arrangement of blocks exhibiting varying degrees of similarity. Block-block relative motion is achieved using local grid distortion to reduce grid skewness and accommodate arbitrary time step selection. A general high-order numerical scheme is applied to satisfy the geometric conservation law. An even-blade-count counterrotating unducted fan configuration is chosen for a computational study comparing solutions resulting from altering parameters such as time step size and iteration count. The solutions are compared with measured data.

Janus, J. Mark↗

NASA NTRS1990

Implicit and explicit computations of flows past cavities with and without yaw

Implicit and explicit computations are presented to study the supersonic, turbulent flows past three-dimensional, rectangular cavities at 0 and 45 deg yaw angle. The Reynolds-averaged, unsteady, compressible, complete Navier-Stokes equations are solved time-accurately, including the Reynolds stresses. The number of major vortices inside and the separation characteristics on the cavity floor are different for cavities with length-to-depth ratios of 3.0 and 6.7. Due to the oscillations of the shear layer, which randomly exposes the cavity flow to the external flow, vortices are shed into the mainstream. When the upstream flow approaches a cavity at yaw, the width-to-depth ratio also becomes an important parameter. The dominant rotational planes of the major vortices incline at an angle to the vertical plane due to yaw. The execution of the implicit scheme is about three times faster, but it requires about four times more computer memory.

Baysal, Oktay↗

NASA NTRS1990

Physical considerations governing computation of turbulent flows over bodies at large incidence

The physical aspects governing accurate numerical simulation of turbulent flows having large regions of crossflow separation are re-examined. Time-accurate, three-dimensional fine-grid Navier-Stokes solutions were obtained for turbulent subsonic and supersonic flows over a slender ogive-cylinder body of revolution at large angles of attack. These flowfields are complex and contain regions of crossflow separation and an organized leeward-side vortex structure. An algebraic eddy-viscosity turbulence model has been modified to correctly account for the effects of the vortices on the underlying viscous layers. The numerical results show that the vortical flow structure of both subsonic and supersonic high incidence flows are similar, and can be accurately treated by the turbulence model. As the angle of attack is increased, the effectiveness of the model increases since the influence of the vortices on the underlying boundary layer decreases.

Degani, David↗

NASA NTRS1990

Unsteady flow computation of oscillating flexible wings

The problem of unsteady flow around flexible wings is solved using the unsteady, compressible, thin-layer Navier-Stokes equations in conjunction with the unsteady, linearized, Navier-displacement equations. Starting with the initial shape of the wing, the Navier-Stokes equations are solved on an initial structured grid to obtain the steady-flow solution which is used for the initial conditions. The forced deformation motion of the wing boundaries is then applied, and the problem is solved accurately in time. During the time-accurate stepping, the Navier-displacement equations are used to solve for the grid deformation and sequently, the Navier-Stokes equations are used to solve for the flowfield. Two applications are presented; the first is for a pulsating oscillation of a bending-mode airfoil in transonic flow, and the second is for a bending-mode oscillation of a sharp-edged delta wing in supersonic flow.

Kandil, Osama A.↗

NASA NTRS1990

Temporal-adaptive Euler/Navier-Stokes algorithm for unsteady aerodynamic analysis of airfoils using unstructured dynamic meshes

A temporal adaptive algorithm for the time-integration of the two-dimensional Euler or Navier-Stokes equations is presented. The flow solver involves an upwind flux-split spatial discretization for the convective terms and central differencing for the shear-stress and heat flux terms on an unstructured mesh of triangles. The temporal adaptive algorithm is a time-accurate integration procedure which allows flows with high spatial and temporal gradients to be computed efficiently by advancing each grid cell near its maximum allowable time step. Results indicate that an appreciable computational savings can be achieved for both inviscid and viscous unsteady airfoil problems using unstructured meshes without degrading spatial or temporal accuracy.

Kleb, William L.↗

NASA NTRS1990

A computational study of the impingement region of an unsteady subsonic jet

Vertical/Short Take-Off and Landing (V/STOL) aircraft may encounter operating difficulties while hovering near the ground. Lift loss, hot gas and debris ingestion, and landing surface damage are some of the potential problems which can result from the interaction of the propulsive jets with the airframe and ground. The purpose of the present work is to develop a detailed understanding of a flowfield, a jet exiting from a flat plate impacting a ground plane, that contains much of the essential physics of the complex flows encountered by V/STOL aircraft in hover. To achieve this, a high-resolution time-accurate numerical simulation of the impinging jet flow was performed. The numerical simulation is based on the solution of the laminar compressible Navier-Stokes equations using the Beam-Warming algorithm. Complex start-up and periodic jet and jet-induced vortex structures were captured, including the formation of primary and secondary jet-induced vortices and wall-jet separation. The formation and development of these phenomena are supported by experimental measurements and smoke visualization. Results are compared with appropriate experiments.

Jalamani, Zakaria A.↗

NASA NTRS1990

Supercomputer applications in gas turbine flowfield simulation

The numerical simulation of the unsteady three-dimensional viscous flow in a gas turbine stage is considered. Results from a three-dimensional time-accurate Navier-Stokes simulation of rotor-stator interaction in an axial turbine stage are presented. The present study uses a fine grid in the spanwise direction to better resolve the complex three-dimensional flowfield, and complements earlier reported coarse-grid calculations. Several different features of the flowfield are analyzed and compared to earlier calculations and to experimental data whenever possible. Computer animation techniques are used to visualize various unsteady three-dimensional features of the flow. The results demonstrate the capabilities of current computing hardware in obtaining accurate simulations of unsteady flows in turbomachines.

Madavan, Nateri K.↗

NASA NTRS1990

Assessment of shock capturing schemes for resonant flows in nonlinear instability analysis

The paper presents computational assessment of advanced numerical schemes for nonlinear acoustic problems related to combustion instabilities in liquid rocket engines. Several time-accurate, shock capturing schemes have been evaluated on a benchmark, closed-end resonant pipe flow problem. It involves the numerical solution of inviscid, compressible gas dynamics equations to predict acoustic wave propagation, wave steepening, formation of shocks, acoustic energy dissipation and wave-wall reflection for several hundred wave cycles. It was demonstrated that high accuracy TVD type schemes can be used for direct, exact nonlinear analysis of combustion instability problems, preserving high harmonic energy content for long periods of time. The selected scheme was then applied to analyze the acoustic responses of resonant pipe-resonator, radial acoustic modes and hub-baffle configurations. Interesting observations of wave shape and damping characteristics have been drawn from presented computational studies.

Przekwas, A. J.↗

NASA NTRS1990

Thermal interaction between an impinging hot jet and a conducting solid surface

Powered-lift aircraft may produce severe high-temperature environments which are potentially damaging to a landing surface or the aircraft. The interaction betweean the high temperature flow field and a nonadiabatic landing surface is analyzed with a coupled computational fluid dynamics/solid thermal conduction computer code, HOTJET. The HOTJET code couples time-accurate, implicit, factored solution schemes for the governing fluid dynamics equations (Reynolds-averaged Navier-Stokes equations) to the unsteady thermal conduction equation, which governs heat flux within a solid. HOTJET is validated against exact solutions to the thermal conduction and Navier-Stokes equations. First-of-a-kind results are included which show the impact of surface material properties on the fluid physics and the coupled fluid/material thermal fields.

Abeloff, P. A.↗

NASA NTRS1989

Prediction of turbine rotor-stator interaction using Navier-Stokes methods

Flows in turbomachinery are generally complex and do not easily lend themselves to numerical computation. The flows are three-dimensional and inherently unsteady. Complicated blade geometries and flow phenomena such as separation and periodic transition from laminar to turbulent flow add to the numerical complexity. Nevertheless, the accurate numerical analysis of such flows is a problem of considerable interest and practical importance to the turbomachinery community. Much of the early work in turbomachinery flow prediction focussed on airfoil cascades. While such analyses of flows in isolated airfoil rows have helped improve understanding of the flow phenomena and have gained widespread acceptance in the industrial community as a design tool, they do not yield any information regarding the unsteady effects arising out of rotor-stator aerodynamic interaction. These interaction effects become increasingly important as the distance between successive stator and rotor rows is decreased. Thus, the need exists for analytical tools that treat the rotor and stator airfoils as a system and provide information regarding the magnitude and the impact of the unsteady effects. The focus a three-dimensional, time-accurate, thin-layer Navier-Stokes code that was recently developed to study rotor-stator interaction problems. A system of patched and overlaid grids that move relative to each other is used to discretize the flow field and the governing equations are integrated using a third-order upwind scheme set in an iterative, implicit framework. The code was used to simulate subsonic flow through an axial turbine configuration for which considerable experimental data exists. Grid refinement studies were also conducted as part of the code validation process. The current status of the research, along with planned future directions, are also discussed.

Madavan, Nateri K.↗

NASA NTRS1989

An unconditionally stable Runge-Kutta method for unsteady rotor-stator interaction

A quasi-three-dimensional analysis has been developed for unsteady rotor-stator interaction in turbomachinery. The analysis solves the unsteady Euler or thin-layer Navier-Stokes equations in a body-fitted coordinate system. It accounts for the effects of rotation, radius change, and stress-surface thickness. The Baldwin-Lomax eddy-viscosity model is used for turbulent flows. The equations are integrated in time using an explicit four-stage Runge-Kutta scheme with a constant time step. Implicit residual smoothing is used to increase the stability limit of the time-accurate computations. The scheme is described, and stability and accuracy analyses are given.

Chima, Rodrick V.↗