Engineering topics

Yen, Guan-Wei

Publications and source records attributed to Yen, Guan-Wei.

7 records

NASA NTRS1993

Dynamic-overlapped-grid simulation of aerodynamically determined relative motion

Currently, there is a need to develop a means of analyzing and studying unsteady flowfields which involve multiple component configurations with at least one of the components in relative motion with respect to the others. Two of the important phenomena that such analyses can help to understand are the unsteady aerodynamic interference and the boundary-induced component of the flowfield. With this motivation, a computational method is developed which couples the governing equations of the unsteady flowfield and the rigid-body dynamics in six degrees-of-freedom. These equations are solved on composite meshes of overlapped subdomain grids which can move with respect to each other. Initially, several measures that reduce the numerical error are studied and compared with the exact solution of a moving normal shock in a tube. It is concluded that a second-order accurate method, for spatial and temporal discretizations as well as for the moving subdomain interpolations, is needed as a minimum measure. Furthermore, the CFL numbers should be restricted to unity. Then, the method is used to simulate the flowfield history and predict the aerodynamically determined trajectory of a store dropped from its initial position under a wing.

Yen, Guan-Wei↗

NASA NTRS1992

Computing high-speed flows past an oscillating cylinder near a vertical wall

A computational method to simulate unsteady flows involving moving rigid boundaries and interference has been developed. The method is used to solve inviscid equations governing the fluid flow and the dynamic equations governing the motion of rigid bodies. A second-order accurate, upwind-biased, and alterating-direction-implicit method is used to solve the governing equations of the flow. A kinematic domain decomposition (KDD) procedure is extended to treat 3D problems with a high degree of accuracy and generality. The method under consideration is applied to both transonic and supertransonic flows. Both cases involve flow past a cylinder which is forced to pitch sinusoidally near a vertical wall. Benefits of the proposed approach include accurate calculation of the flow around 3D moving multiple bodies with interference; reduction of a numerical error; in particular, the dispersion error which strongly affects wave propagation; and minimization of the phase error which is accumulated according to the time advance procedure.

Yen, Guan-Wei↗

NASA NTRS1992

Navier-Stokes computations of cavity aeroacoustics with suppression devices

Effectiveness of two devices to suppress the cavity acoustics was computationally investigated. Two dimensional, computational simulations were performed for the transonic, turbulent flows past a cavity, which was first equipped with a rear face ramp and then with a spoiler. The Reynolds-averaged, unsteady, compressible, full Navier-Stokes equations were solved time accurately by a second-order accurate, implicit, upwind, finite-volume method. The effect of turbulence was included through a Baldwin-Lomax model with modifications for the multiple-wall effects and for the highly vortical flow with a shear layer. The results included instantaneous and time-averaged flow properties, and time-series analyses of the pressure inside the cavity, which compared favorably with the available experimental data. These results were also contrasted with the computed aeroacoustics of the same cavity (length-to-depth ratio of 4.5), but without a device, to demonstrate the suppression effectiveness.

Baysal, Oktay↗

NASA NTRS1991

Kinematic domain decomposition for boundary-motion-induced flow simulations

A method is developed to solve the unsteady Navier-Stokes equations on a composite grid, which consists of subdomain grids moving with respect to each other. These subdomains are structured grids with different topologies. This method eliminates assuming the moving components to be instantaneously stationary, where deciding on the particular frozen instants is difficult and affects the solution adversely. Moreover, this method captures the boundary-motion-induced flow component. The method is demonstrated through a transonic flow past an airfoil, which experiences a combined motion of pitching and plunging. An O-grid around the airfoil is overlapped on a fine Cartesian grid, which is zonally embedded in a coarse Cartesian grid. The coarse grid is stationary but the other two grids are plunging. Only the O-grid is also sinusoidally pitching. The results are compared successfully with the experimental data.

Baysal, Oktay↗

NASA NTRS1991

Kinematic domain decomposition to simulate flows past moving objects

A new method is developed to solve the unsteady Navier-Stokes equations on a composite grid which consists of subdomain grids moving with respect to each other. These subdomains are structured grids with different topologies. However, their interfaces are unstructured in the sense that they may or may not arbitrarily overlap with each other. This method captures the boundary-motion-induced flow component. The method is demonstrated for a transonic flow past an airfoil which experiences a combined motion of pitching and plunging.

Baysal, Oktay↗

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 NTRS1989

Explicit and implicit calculations of turbulent cavity flows with and without yaw angle

Computations were performed to simulate turbulent supersonic flows past three-dimensional deep cavities with and without yaw. Simulation of these self-sustained oscillatory flows were generated through time accurate solutions of the Reynolds averaged complete Navier-Stokes equations using two different schemes: (1) MacCormack, finite-difference; and (2) implicit, upwind, finite-volume schemes. The second scheme, which is approximately 30 percent faster, is found to produce better time accurate results. The Reynolds stresses were modeled, using the Baldwin-Lomax algebraic turbulence model with certain modifications. The computational results include instantaneous and time averaged flow properties everywhere in the computational domain. Time series analyses were performed for the instantaneous pressure values on the cavity floor. The time averaged computational results show good agreement with the experimental data along the cavity floor and walls. When the yaw angle is nonzero, there is no longer a single length scale (length-to-depth ratio) for the flow, as is the case for zero yaw angle flow. The dominant directions and inclinations of the vortices are dramatically different for this nonsymmetric flow. The vortex shedding from the cavity into the mainstream flow is captured computationally. This phenomenon, which is due to the oscillation of the shear layer, is confirmed by the solutions of both schemes.

Yen, Guan-Wei↗