Vladimyr M. Gidzak

Development of the US3D Code for Advanced Compressible and Reacting Flow Simulations

Aerothermodynamics and hypersonic flows involve complex multi-disciplinary physics, including finite-rate gas-phase kinetics, finite-rate internal energy relaxation, gas-surface interactions with finite-rate oxidation and sublimation, transition to turbulence, large-scale unsteadiness, shock-boundary layer interactions, fluid-structure interactions, and thermal protection system ablation and thermal response. Many of the flows have a large range of length and time scales, requiring large computational grids, implicit time integration, and large solution run times. The University of Minnesota / NASA US3D code was designed for the simulation of these complex, highly-coupled flows. It has many of the features of the well-established DPLR code, but uses unstructured grids and has many advanced numerical capabilities and physical models for multi-physics problems. The main capabilities of the code are described, the physical modeling approaches are discussed, the different types of numerical flux functions and time integration approaches are outlined, and the parallelization strategy is overviewed. Comparisons between US3D and the NASA DPLR code are presented, and several advanced simulations are presented to illustrate some of novel features of the code.

Estimation of dynamic stability coefficients for aerodynamic decelerators using CFD

A method for performing dynamic simulations of entry vehicles is developed. This capability uses existing infrastructure within the US3D flow solver, developed for doing fluid structure interaction (FSI) simulations, to allow for up to six degree of freedom (6DoF) simulations. Inviscid, free-to-oscillate simulations of the Mars Science Laboratory (MSL) capsule at Mach2.5and3.5are used to evaluate di↵erent data reduction methods. It is found that many simulations may be required to get reliable data. The computed pitch damping coecients show comparable trends to ballistic range data, though they di↵er in magnitude, particularly at high angles of attack. Preliminary viscous simulation results are presented, and show improved agreement with experimental data compared to the inviscid analysis.

Validation of fluid-structure interaction capabilities in US3D

The method used for coupling US3D to a structural solver for the purpose of simulating fluid-structure interactions (FSI) is detailed. An overview of the solver components is provided, with an explanation of the necessary modification to US3D. Component level validation is undertaken against analytic solutions for both the structure to fluid and the fluid to structure parts of the FSI capabilities. The coupled solver is used on two instructive model problems to demonstrate FSI phenomena, and a full scale tension cone is also simulated to show the usefulness of a coupled solver in design evaluation.

Computational fluid-structure interaction methods for simulation of inflatable aerodynamic decelerators

Inflatable aerodynamic decelerators have potential advantages for planetary re-entry in robotic and human exploration missions. It is theorized that volume-mass characteristics of these decelerators are superior to those of common supersonic/subsonic parachutes and after deployment they may suffer no instabilities at high Mach numbers. A high fidelity computational fluid-structure interaction model is employed to investigate the behavior of tension cone inflatable aeroshells at supersonic speeds up to Mach 2.0. The computational framework targets the large displacements regime encountered during the inflation of the decelerator using fast level set techniques to incorporate boundary conditions of the moving structure. The preliminary results indicate large but steady aeroshell displacement with rich dynamics, including buckling of the inflatable torus that maintains the decelerator open under normal operational conditions, owing to interactions with the turbulent wake. Copyright

Numerical simulation of drogue parachute in the wake of the multi-purpose crew vehicle

Simulations of a 10% scale Multi-Purpose Crew Vehicle and its drogue parachute were carried out using US3D, a finite–volume computational fluid dynamics code, with capsule angles of attack at 0, 30 and 50 degrees. A model for the geometric porosity of the parachute is incorporated which locally varies the mass flux through the surface based on the instantaneous flowfield. These simulations were then compared to wind tunnel tests, including mean drag measurements and particle image velocimetry data. It was found that the simulations matched the wind tunnel drag within 4% for α = 0 and 30◦ with the maximum drag deficit due to the capsule being 13.4% at α = 0◦. Pressure data from the simulations was used to deform a structural model of the parachute to the correct inflated shape, as a precursor to future fluid structural interaction work.

Wake-fabric interactions in ADEPT-VITaL

Fluid-Structure simulations are performed on a single gore of a flight-scale ADEPT decelerator configuration to study interactions between turbulent wake and carbon cloth at supersonic Venus entry conditions. Three areas of the geometry are examined; the center of the gore(acreage), the shoulder where the flow is turned, and the trailing edge of the gore. Fully coupled fluid-structure interaction (FSI) simulations are performed at Mach 1.5. Results are provided showing that large scale motion of the acreage is highly damped, that the shoulder of the gore has a tendency to sharpen due to the strong pressure gradient of the turning flow, and that the trailing edge undergoes small amplitude displacements due to pressure variations, accompanied by low amplitude buzz in the range of 70-80 Hz.

An Investigation into a Non-Axisymmetric Tension Shell Decelerator Concept via a Computational Design Procedure

The possibility of constructing non-axisymmetric tension cones is examined with the goal of creating a tension cone that naturally trims to a non-zero angle of attack. The shape derivation for axisymmetric tension cones is used to map a design space from which an initial shape for a non-axisymmetric tension shell can be constructed. A version of the US3D ow solver with uid-structure interaction (FSI) capabilities is used as a tool for both design and evaluation. Variations on the degree of non-symmetry are compared in terms of aerodynamic and structural performance. It is found that simple modi cations to the basic tension cone design can result in an in atable decelerator that trims to an angle of attack of at least 28 , with an L/D of 0:49, while still maintaining a state of pure tension in the tension shell.

Development of Stencil-Based Mesh Partitioning for Parallel Unstructured CFD Solvers

A study evaluating unstructured mesh partitioning for computational uid dynamics (CFD) simulations on parallel computers is presented. Considerations for eectiv e partitioning of computational unstructured meshes for a family of implicit time-integration methods using the line-relaxation algorithm are outlined. Mesh partitioning using the Metis library is evaluated for two- and three-dimensional meshes. Dieren t combinations of computational stencil information provided to the partitioning library are outlined and comparison of the resulted load balance and communication volume measures are presented. An augmentation to the existing mesh partitioning method via a heuristic algorithm is proposed. The heuristic algorithm relies on an initial partitioning of the mesh, which is then rened iteratively. Results using the new method are compared with the standard partitioning methods. This is done both from a theoretical perspective and by running the CFD code. Elapsed time results of the CFD code are presented from simulations on a parallel computer. Results show that the improved partitioning using the stencil information and the heuristic algorithm result in faster turn-around times. The stencil-based partitioning and the proposed heuristic algorithm are simple modications to existing codes and are applicable to general simulations employing domain decomposition.