Lewis B. Schiff

Computation of turbulent supersonic flows around pointed bodies having crossflow separation

Abstract A recently reported thin-layer parabolized Navier-Stokes method has been used to compute turbulent supersonic flows around pointed bodies at large incidence. These flow fields are complex and contain extensive regions of crossflow separation. Extensive investigations were carried out to assess the effects of grid resolution in the viscous region and inviscid region of the leeward side vortices, and the effects of the algebraic eddy-viscosity turbulence model. Comparisons between computed and experimentally measured flow fields of several pointed bodies show significant improvement in the computed flow fields obtained using a properly modified turbulence model in the crossflow separation region and adequate spatial grid resolution. The effect of adding the circumferential and cross viscous terms was found to be insignificant for the present cases.

Numerical Simulation of Steady Supersonic Viscous Flow

A noniterative, implicit, space-marching, finite-difference algorithm is developed for the steady thin-layer Navier-Stokes equations in conservation-law-form. The numerical algorithm is applicable to steady supersonic viscous flow over bodies of arbitrary shape. In addition, the same code can be used to compute supersonic inviscid flow or three-dimensional boundary layers. Computed results from two-dimensional and three-dimensional versions of the numerical algorithm are in good agreement with those obtained from more costly time-marching techniques.

Numerical simulation of the effect of spatial disturbances on vortex asymmetry

The steady asymmetric vortex pattern observed on slender bodies of revolution at large angle of attack was investigated using fine-grid, thin-layer Navier-Stokes computation. The results demonstrate the marked asymmetry that has been observed in experiments. To obtain asymmetry, it was essential to introduce a space-fixed, time-invariant perturbation into the computation. If the perturbation was removed, the asymmetric flow returned toward symmetry. The perturbations were found to be more effective when located close to the nose. Taken together with the experimental observations, the computational results suggest that vortex asymmetry is forced by amplification of small disturbances, such as those due to surface roughness, occurring within the body viscous boundary layer.

A numerical study of three-dimensional separated flow past a hemisphere cylinder

Separated and vortical flow about a hemisphere-cylinder body has been investigated. An algorithm featuring two implicit factors, and partial flux splitting has been used to solve the thin-layer Navier-Stokes equations. In analyzing the complex flow patterns, experimental data and topological concepts are used to complement the numerical results in interpreting the surface-flow patterns as well as the flowfield structures. Basic issues concerning the three-dimensional separation characteristics and the leeward vortical structures are examined.

Nonlinear Aerodynamics of Bodies in Coning Motion

A numerical method for computing the nonlinear inviscid flowfield surrounding a body performing coning motion is described. The method permits accurate computation of the aerodynamic moment due to one of the four motions characterizing an arbitrary nonplanar motion. Results of computations for a slender circular cone in coning motion are presented, and show good agreement with experiment for angles of attack up to twice the cone half-angle. The computational results display significant departure of the side moment from the linear theory value with increasing angle of attack, but agree well with experimental measurements. This indicates that the initial nonlinear behavior of the aerodynamic moment is determined primarily by the inviscid flow.

Pneumatic vortical flow control at high angles of attack

The injection of thin, high-momentum jets of air into the fuselage forebody boundary layers of the F-18 aircraft is explored numerically as a means of controlling the onset of fuselage vortices and of generating yaw control forces. The study was carried out for an angle of attack of 30 deg with symmetrical and asymmetrical blowing configurations. One-sided blowing results in a strongly asymmetrical flow pattern in the fore portion of the fuselage, leading to a net lateral force.

Turbulence model effects on separated flow about a prolate spheroid

The three-dimensional separated flow about a prolate spheroid at high incidence is numerically investigated using the F3D thin-layer Navier-Stokes code. The effect of different turbulence models on the flowfield solution and the characteristics of the predicted flow are analyzed. The models used in this study are the Baldvvin-Lomax algebraic model, the Baldvvin-Lomax model as modified for crossflow separation by Degani and Schiff, and a modified version of the Johnson-King model with and without the Degani-Schiff crossflow modifications applied. The Johnson-King model is applied to assess the importance of modeling nonequilibrium effects in predicting flow about a slender body at high incidence. The computations are made for steady-state, fully turbulent flow. The results are compared with experimental pressure data and with computational results obtained by Panaras and Steger, using the identical code and grid, but with their own modifications to the Baldwin-Lomax model. In addition, the computed solutions are analyzed using surface flow patterns, helicity density contours, and turbulent eddy-viscosity profiles. The results of this analysis provide insight into the effects of the turbulence models on flow characteristics and demonstrate the effect of the models on the accurate prediction of highly separated and vortical flows about a slender body.

Numerical simulation of high-incidence flow over the F-18 fuselage forebody

As part of the NASA High Alpha Technology Program, fine-grid Navier-Stokes solutions have been obtained for flow over the fuselage forebody and wing leading-edge extension of the F/A-18 High Alpha Research Vehicle at large incidence. The resulting flows are complex and exhibit cross-flow separation from the sides of the forebody and from the leading-edge extension. A well-defined vortex pattern is observed in the leeward-side flow. Results obtained for laminar flow show good agreement with flow visualizations obtained in ground-based experiments. Further, turbulent flows computed at high-Reynolds-number flight-test conditions show good agreement with surface and off-surface visualizations obtained in flight.

Numerical simulation of forced and free-to-roll delta-wing motions

The three-dimensional, Reynolds-averaged, Navier-Stokes (RANS) equations are used to numerically simulate nonsteady vortical flow about a 65-deg sweep delta wing at 30-deg angle of attack. Two large-amplitude, high-rate, forced-roll motions, and a damped free-to-roll motion are presented. The free-to-roll motion is computed by coupling the time-dependent RANS equations to the flight dynamic equation of motion. The computed results are in good agreement with the forces, moments, and roll-angle time histories. Vortex breakdown is present in each case. Significant time lags in the vortex breakdown motions relative to the body motions strongly influence the dynamic forces and moments.

Numerical prediction of subsonic turbulent flows over slender bodies at high 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 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