David L. Whitfield

High-resolution viscous flow simulations at arbitrary Mach number

A characteristic-based unsteady viscous flow solver is developed with preconditioning that is uniformly applicable for Mach numbers ranging from essentially incompressible to supersonic. A preconditioned flux-difference formulation for nondimensional primitive variables is a key element of the present approach. The simple primitive-variable numerical flux is related to Roes flux-difference scheme and preserves contact discontinuities using primitive variables, with or without preconditioning. Preconditioning by a single-parameter diagonal matrix conditions the system eigenvalues in terms of nondimensional local velocity and local temperature. An iterative implicit solution algorithm is given for the preconditioned formulation and is used for several simple test and validation cases. These include an inviscid shock-tube case, flat-plate boundary layer flow at low Mach number, viscous flow past a circular cylinder at low Reynolds number and with different thermal boundary conditions, and validation cases for incompressible and transonic flows.

DOE's Effort to Reduce Truck Aerodynamic Drag Through Joint Experiments and Computations

Class 8 tractor-trailers are responsible for 11-12% of the total US consumption of petroleum. Overcoming aero drag represents 65% of energy expenditure at highway speeds. Most of the drag results from pressure differences and reducing highway speeds is very effective. The goal is to reduce aerodynamic drag by 25% which would translate to 12% improved fuel economy or 4,200 million gal/year. Objectives are: (1) In support of DOEs mission, provide guidance to industry in the reduction of aerodynamic drag; (2) To shorten and improve design process, establish a database of experimental, computational, and conceptual design information; (3) Demonstrate new drag-reduction techniques; and (4) Get devices on the road. Some accomplishments are: (1) Concepts developed/tested that exceeded 25% drag reduction goal; (2) Insight and guidelines for drag reduction provided to industry through computations and experiments; (3) Joined with industry in getting devices on the road and providing design concepts through virtual modeling and testing; and (4) International recognition achieved through open documentation and database.

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.

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.

The role of computers in aerodynamic testing

Abstract Major and rapid improvements are currently being made in the capabilities of experimental facilities by integrating the computational capabilities of large digital computers with the aerodynamic simulation capabilities of ground test facilities. For the foreseeable future, these improvements will have a significant impact on the aeronautical research and development community because the testing facilities will continue to play an essential and principal part in all advanced aircraft and missile development programs. At the present time, the greatest demand on existing facilities is to provide significant increases in testing efficiency, overall improvements in data quality, and improved simulation at transonic speeds. Increases in testing efficiency are needed to offset increased operational costs caused primarily by large increases in electrical power costs and to support energy conservation programs. Improvements in simulation capabilities and increased data quality are needed to meet the critical requirements of the new and highly sophisticated classes of aircraft and missiles and for verification of new “total flow-field prediction” computer codes. This paper describes some of the progress that has been achieved by interfacing the digital computer with the major developmental wind tunnels and engine test units at the USAF Arnold Engineering Development Center. Present trends and future testing needs are identified, and the role of the digital computer in future aerodynamic testing is discussed.

High resolution numerical simulation of the linearized Euler equations in conservation law form

A linearized Euler solver based on a high resolution numerical scheme is presented. The approach is to linearize the flux vector as opposed to carrying through the complete linearization analysis with the dependent variable vector written as a sum of the mean and the perturbed flow. This allows the linearized equations to be maintained in conservation law form. The linearized equations are used to compute unsteady flows in turbomachinery blade rows arising due to blade vibrations. Numerical solutions are compared to theoretical results (where available) and to numerical solutions of the nonlinear Euler equations.

Passive Devices for Reducing Base Pressure Drag in Class 8 Trucks

Full scale CFD simulations of the Generic Conventional Model (GCM), a simplified model of a Class 8 truck, were used to explore passive devices for improving the drag performance of the trailer base. Significant improvements over conventional straight base flaps were achieved using an Extended Bent (EB) flap that stays within the length limits imposed by US federal law. An additional boat tail device for the cab bogie base was also found to yield improvements in the base drag in that region. This device in combination with the EB flap leads to a wind-averaged drag reduction of 21 % over the non-modified GCM model. An under-trailer scoop to generate air for pressurizing the trailer base or for use in active flow control devices was found to add too much drag to be effective.

Parallel FAS Multigrid for Arbitrary Mach Number, High Reynolds Number Unstructured Flow Solvers

A FAS multigrid capability has been developed in conjunction with a fully parallel Reynolds averaged Navier-Stokes solution algorithm, capable of eciently simulating flows with mixed high speed (Mach number > 1) and low speed (Mach number < 0.01) flow regimes. Unique features of this multigrid approach include 1) fully general parallel agglomeration routines that are independent of the parallel subdomain layout, and 2) novel agglomeration procedures designed to take the high aspect ratio control volumes in the boundary layer into account. This multigrid algorithm has been applied to several aerodynamic geometries of interest and validated against experimental results. The performance of the multigrid code has been investigated and is shown to provide significant improvements in solution convergence.

Scalable Parallel Implicit Algorithm for Advanced Turbulence Closures

*† ‡ § Two Reynolds Stress models are implemented in an existing scalable parallel incompressible flow solver. Computations for standard channel and flat plate cases are compared to theoretical and Direct Numerical Simulation results. Simulations of the DARPA SUBOFF body are compared to experimental data. The simulations displayed robust convergence for high Reynolds number flows with sublayer resolved grids. Reynolds stress model based simulations were found to be a third more expensive than two-equation turbulence model computations in terms of runtime. Nomenclature Cp = pressure coefficient Cf = skin friction coefficient j iu u ′ ′ − = Cartesian components of the Reynolds stress tensor K = turbulent kinetic energy ( ) k ku u ′ ′ 2 1 k v = mean flow contravariant velocity components m u = mean flow Cartesian velocity components m x = Cartesian coordinates () ( ) z y x

Computation of Dynamic Stability and Control Derivatives

The objective of this research is to develop a predictive technology to support virtual design and evaluation of underwater vehicles systems, employing a Computational Fluid Dynamics (CFD) based methodology for predicting stability and control derivatives. Computational Fluid Dynamics technology coupled with modeling and control system design will allow vehicle conceptual designs to be evaluated within the context of a realistic mission. The preliminary goal of this effort is to estimate stability and control derivatives of underwater vehicles from CFD data as an evaluation of the potential for this method to replace/reduce expensive experimental (i.e. tow-tank) tests. The need to establish a predictive capability technologies to support virtual design and evaluation of underwater vehicles systems, presented an opportunity to apply a multiblock, structured-grid, incompressible Navier-Stoke flow solver referred to as Tenasi, which have evolved over many years. The solver utilizes a state-of-the-art implicit upwind numerical scheme to solve the time-dependent Navier-Stokes equations in a generalized time-dependent curvilinear coordinate system. The numerical studies were performed for two underwater configurations to validate the accuracy and reliability of the present software tool. The computed results show favorable agreement with experimental data. The results from this study are very encouraging and strongly suggest that further development and application of this tool to more complicated underwater and high-speed supercavitating vehicles is needed. The results of this tool can be used to improve the performance of autonomous underwater vehicles. Methodologies and software tools are developed here that utilize vehicle-maneuvering simulated results to directly estimate standard hydrodynamic model coefficients, which are used for high-fidelity simulations and control system design.