The Department of Defense, specifically the Navy, is seeking innovative solutions through the SBIR program to develop an efficient multiphysics modeling framework for simulating rain-induced damage and aerodynamic effects on hypersonic vehicles. The objective is to create and validate automated computational tools that accurately predict raindrop interactions with hypersonic vehicles, focusing on critical phenomena such as droplet-shock interactions, material damage, and flow-field modifications while minimizing user expertise requirements. This initiative is crucial for enhancing the safety and performance of hypersonic vehicles under adverse weather conditions, addressing significant gaps in current modeling capabilities. The solicitation is set to open on April 23, 2025, with applications due by May 21, 2025, and further details can be found at the official SBIR website.

## Description

Hypersonic vehicles will encounter various weather conditions, including rain. Raindrops can threaten structural integrity and TPS performance, potentially leading to catastrophic failures [Ref 1]. Rain droplet impacts during flight pose substantial risks to the structural integrity and performance of TPS, potentially leading to catastrophic failures [Ref 1]. Interactions between droplet impacts and the surrounding flow field can disrupt vehicle maneuverability, increasing the risk of loss of control. Existing modeling tools, originally developed for blunt cone geometries, are inadequate for addressing the complexities of modern maneuvering hypersonic vehicles [Ref 2]. New tools are required to simulate droplet interactions with hypersonic shock waves and boundary layers, characterize breakup dynamics, and evaluate flow field modifications.High-speed droplet impingement is a challenging problem involving a wide variety of physical mechanisms, including aero-breakup and phase change. Modeling droplet impingement at high Mach numbers involves a wide range of physical mechanisms, including material and flow discontinuities, interfacial instabilities, phase change, and turbulence [Ref 3]. Additional complexities stem from the limited algorithms available that can handle high Mach number flows while maintaining a high-order of accuracy, as the variable reconstruction and flux differentiation steps can lead to unbounded volume fractions, negative speeds of sound, or partial densities, requiring positivity-preserving numerical schemes. Advancements in multiphase computational fluid dynamics (CFD) methods have improved the simulation of rain droplet interactions with hypersonic vehicles. For example, diffuse interface multiphase methods provide a simple, computationally efficient, and numerically robust approach to model droplet breakup and impingement [Ref 4]. Additionally, Adaptive Mesh Refinement (AMR) allows for efficient tracking of important flow features such as gas/liquid interfaces, shocks and wakes and the target geometries. Diffuse interface methods, coupled with AMR, have enabled modeling of droplet-shock interactions and instabilities, such as Rayleigh-Taylor (RT) and Kelvin-Helmholtz (KH) [Ref 3]. Integration of CFD with structural solvers like peridynamics has enhanced predictions of material damage during high-speed impacts [Ref 5]. GPU-accelerated multiphase solvers have demonstrated up to 300x speed-ups while maintaining fidelity for shock-droplet interactions [Ref 6]. Experimentally, the effects of weather encounters are difficult to simulate through ground testing. Historically, most knowledge of weather encounters has been provided by flight testing, which has numerous shortcomings such as expense, limited data throughput, and uncertain characterization of the environment. Developments in high-speed diagnostics and advances in experimental campaigns, including electromagnetic-launcher tests, have validated droplet deformation and aerobreakup under hypersonic conditions [Ref 7]. Despite this progress, gaps remain in material modeling, robustness, and aerodynamic impact analysis, limiting current tools.Challenges persist in accurately modeling advanced composite materials, such as carbon-carbon, including accumulated damage effects like surface roughness evolution and its impact on erosion rates. Debris shielding, where water and material fragments alter subsequent droplet impacts, remains underexplored [Ref 8]. Integrating key physics like phase change and cavitation into robust simulations is critical. Current tools provide limited insight into aerodynamic changes, including forces and moments crucial to vehicle stability. Addressing these gaps requires next-generation tools that leverage GPU acceleration for scalable simulations while fully integrating composite material behavior, shielding effects, and aerodynamic impacts.The target requirements for this SBIR topic include:<ul><li> Robust Multiphysics Integration: Simulate raindrop interactions with hypersonic vehicles, incorporating critical physical phenomena such as droplet-shock interactions, phase change, and arbitrary equations of state, while ensuring computational stability.</li><li> Advanced Material Modeling: Predict the behavior of advanced composite materials, such as carbon-carbon, capturing high-strain-rate responses, crack propagation, and accumulated surface damage. Fully couple the material response with CFD solutions.</li><li> Shielding and Ejecta Tracking: Develop the ability to track ejecta and model shielding effects, including debris impacts on vehicle surfaces and resulting flow field changes.</li><li> Aerodynamic Effects Analysis: Provide predictions of aerodynamic changes, including forces and moments caused by surface deformation, to assess vehicle stability under adverse conditions.</li><li> High Computational Efficiency: Leverage GPU-accelerated architectures and AMR to achieve scalable, high-fidelity simulations for realistic operating conditions.</li><li> Automated Pre- and Post-Processing: Implement workflows for solver setup, grid generation, and data analysis to minimize user intervention.</li></ul>

## Objective

Develop and validate automated multiphysics computational tools to accurately predict raindrop interactions with hypersonic vehicles, including droplet-shock interactions, breakup dynamics, flow-field modifications, and thermal protection system (TPS) material damage, while minimizing user expertise requirements and computational costs.

## Phase I

Develop a prototype multiphysics modeling framework for simulating raindrop impacts on hypersonic vehicles. Incorporate key phenomena such as droplet-shock interactions, breakup, impingement, and material response. Validate using canonical geometries and existing experimental data. Assess accuracy and feasibility. Define a path for platform-independent computation and establish a roadmap for scaling to complex, three-dimensional vehicle geometries. Prepare a Phase II plan.

## Phase II

Develop a fully integrated multiphysics simulation framework for raindrop impacts on hypersonic vehicles. Incorporate advanced material response models, including damage evolution for carbon/carbon. Implement shielding and ejecta modeling, adaptive mesh refinement, and critical physics such as phase change. Validate and demonstrate the framework on realistic hypersonic vehicle geometries using existing experimental data. Ensure efficient operation on emerging computing platforms and implement workflow automation to minimize user intervention, enabling repeatable and high-quality results.

## Phase III: Dual Use Applications

Transition the developed simulation framework for raindrop impacts on hypersonic vehicles to practical applications within the Department of Defense (DoD) and commercial sectors. Validate and optimize the framework for a wide range of hypersonic vehicle configurations, materials, and flight conditions. Collaborate with industry partners and DoD agencies to ensure the framework meets deployment standards and operational requirements. Develop comprehensive training programs, user documentation, and technical support resources to enable widespread adoption and effective use by non-expert users.

Efficient Multiphysics Modeling Framework for Rain-Induced Damage and Aerodynamic Effects on Hypersonic Vehicles

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