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Christoph Brehm Awarded Grants from NASA, Air Force Academy

July 15, 2019

Sean Bailey and Alexandre Martin, associate professors of mechanical engineering at UK, will serve as Science-Co-Investigators along with Jens Hannemann from Kentucky State University on the NASA project.

Christoph Brehm, assistant professor in the Department of Mechanical Engineering, has received awards from NASA and the United States Air Force Academy.

The NASA Kentucky EPSCoR award for the project “Modeling of High-Speed Transitional and Turbulent Flows over Ablative Surfaces” is for $1,050,000 with $750,000 coming directly from NASA and $300,000 in matching funds from the state. The Air Force Academy award for the project “Development of a RANS-Based Wall-Modeled LES Approach for Hypersonic Flows” is for approximately $550,000.

Sean Bailey and Alexandre Martin, associate professors of mechanical engineering at UK, will serve as Science-Co-Investigators along with Jens Hannemann from Kentucky State University on the NASA project.

Brehm joined the mechanical engineering faculty in 2016. He received a Ph.D. in aerospace engineering from the University of Arizona in 2011 and worked as a senior research scientist at NASA Ames Research Center from 2012-2016.

Abstracts for both projects are below. 

Modeling of High-Speed Transitional and Turbulent Flows over Ablative Surfaces

Due to its significant impact on heat transfer, skin friction and aerodynamic forces, the laminar-turbulent transition process of high-speed boundary-layers will play an important role in design of NASA’s next generation hypersonic vehicles. The transition process is complex, following different paths depending on mean flow properties and the disturbance environment. This complexity is compounded by its dependence on the characteristics of the surface over which the boundary layer forms. This research proposal takes on this challenge by addressing the numerical modeling of transitional and turbulent flows over ablative surfaces in hypersonic flight regimes, an important complement to Kentucky’s established research capability for modeling and simulating Thermal Protection Systems (TPS) during high-speed atmospheric entry.

Interactions between transitional/turbulent flows and surface ablation have first-order effects on the aerothermodynamic characteristics of aerospace vehicles. However, no existing simulation capability truly captures the relevant physical mechanisms involved in fully-coupled Fluid-Ablation Interactions (FAI), particularly under realistic flight conditions. 

Thus, transition behavior for a wide range of NASA applications, involving both external and internal flows, are currently poorly understood. The main objective of this work is to develop a robust, efficient, and accurate simulation approach that can be used to improve hypersonic aerothermodynamic prediction capabilities, and simultaneously enhance our fundamental understanding of the coupled interactions between transitional and turbulent flows with surface ablation. To obtain accurate and efficient FAI simulation capabilities on relevant temporal and spatial scales, the proposed numerical scheme consists of five key components: (1) a nonlinear disturbance flow formulation, (2) a dual-mesh overset approach to exchange information between the baseflow and the disturbance flow solutions, (3) dynamic adaptive-mesh refinement, (4) a higher-order accurate immersed boundary method, and (5) a dynamic solid surface response model. These methods will be combined and used to simulate high-speed transitional and turbulent flows interacting with ablative surfaces, particularly the production of macroscopic distributed and discrete roughness patterns formed in the presence of transitional/turbulent flows, and to couple the influence of these surfaces, through roughness patterns, outgassing, etc., back onto the fluid flow behavior. In order to develop an efficient simulation approach the computational performance of the solver framework will be thoroughly analyzed and optimized on modern HPC systems employing vectorization, inter-procedural optimization, multi-threading, etc. The numerical method development will be supported by high-fidelity experiments on transitional and turbulent flow over roughened surfaces employing an advanced spatio-temporal high-resolution wave-packet tracking approach. The proposed research has the potential to be truly transformative and advance the state-of-the-art in predicting high-speed transitional and turbulent flows in the presence of ablative surfaces, as well as increase our understanding of the highly complex physics involved. 

This research provides the opportunity to build novel research expertise in high-speed laminar-turbulent transition at the University of Kentucky and combine it with recognized leading research in heat shield modeling. For the design of the next generation of critical TPS, reliable modeling capabilities for FAI are essential—an enabling technology for NASA’s ambitious plans for humans to go and return from the Moon, as well as to conduct missions to Mars and beyond.
Development of a RANS-Based Wall-Modeled LES Approach for Hypersonic Flows

Hypersonic flight vehicles are among the most sophisticated devices ever envisioned. The ability to effectively operate vehicles at hypersonic speeds substantially reduces flight times and increases range which ultimately provides the DoD with a long-range rapid response capability. The flow field around hypersonic cruise hardware is highly complex and contains a wide range of intricate physical phenomena, such as transitional and turbulent flows, steady and unsteady shocks, chemical reactions and particulate laden flows.

The ability to accurately predict the complex flow field around hypersonic vehicles allows a comprehensive understanding of the relevant physics which is essential to reduce design margins and systems uncertainties and, ultimately, guide the development of novel innovative designs.

The proposed research addresses key challenges in the prediction of turbulent flows around complex hypersonic vehicles which is closely aligned with NASA’s “CFD Vision 2030 Study: A Path to Revolutionary Computational Aerosciences.” Accurate prediction of turbulent flows is essential for the design of the next generation of hypersonic vehicles. 

There is a growing consensus within the Computational Fluid Dynamics (CFD) community that in the next few years there will be a substantial increase in the use of hybrid RANSLES and wall-modeled LES (WMLES) methods given that RANS models, when used on their own, are unable to accurately capture unsteady separated flows. While these methods have mainly been developed and tested for incompressible or weakly compressible flows they lack maturity for hypersonic flow applications. A well-integrated wall-modeling approach is an essential ingredient in order to attain computationally efficient solution approaches for highly complex high-speed WMLES flow simulations. In this research we will be testing and further developing an advanced ODE-based wall model (including energy equations, pressure gradient and convective terms) for WMLES of hypersonic flows around complex geometries integrated inside body-fitted and Cartesian grid based Navier-Stokes solvers. The proposed research project will closely involve Cadets and USAFA faculty in the research while answering critical research needs for the government and commercial applications of hypersonic vehicles. We also propose to work closely with the Kestrel CFD development team to ensure that the methods developed this project can be used by the wider DoD CFD community.

These wall models will be integrated and tested in our body-fitted CFD solver, as well as Cartesian grid based solver with Adaptive Mesh Refinement (AMR), which provides a fully-automated mesh generation independent of the complexity of the geometry. For validating these methods we will use available experimental and simulation data from a range of fundamental and complex test cases as well as our own Direct Numerical Simulations (DNS). Long term, this research has the potential of being truly transformative because the proposed methods will significantly improve the accuracy of CFD methods enabling greater trust for CFD within the hypersonic vehicle design process.