Brent Mitchell

Aerosol Propagation in an Urban Environment

The objective of this study is to demonstrate the capability of an arbitrary mach number algorithm to predict aerosol propagation in an urban environment. A preconditioned approach is applied to an unstructured mesh to determine accurately the highly unsteady turbulent flow field about the urban setting. DES modifications are implemented into a hybrid k − , k − ω turbulence model and evaluated. A scalar transport model is used to release and to advect the aerosol agent through the urban landscape. Comparisons between RANS and DES turbulence modeling are presented for multiple agent release scenarios.

Unstructured Nonlinear Free Surface Flow Solutions: Validation and Verification

A nonlinear free surface solution methodology is incorporated within an existing three-dimensional, unstructured Navier-Stokes solver, . This portable, parallel solver uses a node-based finite volume method to solve the incompressible Reynolds-averaged NavierStokes equations on mixed element high-aspect ratio grids, includes several turbulence models to simulate the affects of turbulence within the boundary layer, and has the capability to simulate flow through rotating propellers accurately. To obtain a nonlinear free surface, the kinematic free surface equation is solved at each time level via a finite element implementation, valid on both triangles and quadrilaterals; after several time steps (approximately 200), the grid is moved to match the free surface elevations while conforming to the geometry. Robust grid movement is achieved by using a three-dimensional extension of Farhat’s torsional spring analogy. Validation studies include a grid refinement for the free surface on a circle based on a prescribed velocity field, and of flow around a submerged NACA0012 hydrofoil. For the NACA0012 hydrofoil, for the viscous and inviscid Wigley hull and for the DTMB Model 5415 series hull, wave profiles along the hull are compared against available experimental results as well as numerical results from a more mature structured nonlinear free surface code UNCLE.

SIMULATION OF SPINNING MISSILE FLOW FIELDS USING U 2 NCLE

Euler and Navier-Stokes solutions were obtained to predict the aerodynamic performance of a spinning missile with dithering canards. Integrated force and moment coefficients were computed, along with helicity contours to track the vortices being generated by the canards. Comparisons were made to determine the effect of grid resolution and viscous effects. The grid resolution study was performed using 3 levels of refinement. The results indicated that the force and moment coefficients generated using the medium refinement grid were in agreement with those using the fine grid. However, the finest grid increased the resolution of the vortices, particularly the canard root vortex. Comparison of the viscous and inviscid force and moment coefficients revealed that the viscous effects were not significant. The largest viscous contribution was to the axial force, which was approximately 10% of the total axial force. The main difference between the inviscid and viscous results is the resolution of the canard root vortex. Overall the viscous effects were minimal, as expected, since the flow is supersonic and at this low angle of attack, the flow remains attached, the vortices do not impinge on the tail fins or missile body, and there are no regions of separated flow where viscous effects are important.

Aerodynamic Simulation of Heavy Trucks with Rotating Wheels

Aerodynamic simulations were carried out for the Ground Transportation System model, a 1/8 th scale tractor-trailer model, that was tested in the NASA Ames 7’x10’ tunnel. The computed forces and pressure coefficients are compared to experiment. Detailed comparisons are also carried out for the wake in the symmetry plane of the model. A DES version of the two equation k-e/k-ω ω ω ω hybrid turbulence model is shown to predict the single vortex structure observed in the experiment. Simulations are also carried out for an isolated rotating wheel and the results are compared to experiment data. A theoretically predicted jet arising at the contact patches was observed computationally with its magnitude matching the theoretical predictions. Representative simulations were also carried out for a tractortrailer model with rotating wheels.

Turbulence Modeling For Highly Separated Flows

The objective of this study is to investigate the Detached Eddy Simulation (DES) modifications to the hybrid k , k ! (kk! ) turbulence model and the Scale Adaptive Simulation (SAS) modifications to the one-equation k (1k ) turbulence model. Additionally, modifications to the damping functions to the baseline 1k model are presented and validated for a variety of test cases. Further, the Stress-! (S!) Reynolds stress model is tested alongside the 1k and kk! models. A blended dissipation equation is implemented into the S! model and validated for a variety of test cases. A surface-mounted cube is chosen as the primary test case to examine specifically the highly separated wake flow. Multiple grid levels are used to identify the grid requirements necessary for accurate computations with the kk! DES and 1k SAS models.

Computation of Vortex-Intensive Incompressible Flow Fields

The objective of this study is to use UNCLE, an unstructured, parallel, Reynolds averaged Navier-Stokes solver, to compute vortex intensive flow fields and compare the results with experimental data. Vortex intensive flow fields arise under a variety of conditions. The ones considered in this study are those arising due to rotating propellers and due to flow past submarine hulls. The examples considered here involve computations at design as well as offdesign conditions for the propellers and the SUBOFF model at various angles of drift. Results are presented for propeller P-4381 (overall thrust and torque coefficients) as well as for propeller P-5168 (velocity profiles downstream of the propeller). Forces and moments are also compared for the SUBOFF hull at various angles of drift to experimental data. Good agreement with experimental data is obtained for the various cases.

Computational Prediction of Forces and Moments for Transport Aircraft

A sequence of computations for the DLR-F6 wing-body configuration with and without the FX2B fairing was carried out using Tenasi , a node-centered, finite volume unstructured flow solver. All simulations were carried out at a Mach number of 0.75 and at two different Reynolds numbers (3x10 6 and 5x10 6 ). Significant sensitivity of the lift and drag coefficients to Reynolds number was observed. Lift, drag and moment coefficients along with pressure distributions were compared to experimental data for the DLR-F6 wing-body configuration (Re = 3x10 6 ) and were in good agreement. A turbulence model as well as a grid refinement study was carried out for the DLR-F6 with FX2B fairing (Re = 5x10 6 ). Lift, drag and moment coefficients are compared to other computations for the DLR-F6 with FX2B fairing.