Session: 04-09-01: Materials and Structures for Extreme Environments

Paper Number: 77569

Start Time: Monday, 12:15 PM

77569 - Effects of Fluid Thermal Structural Interactions in High-Speed Flows 

The focus of this presentation is to discus and demonstrate a method for determining the effective thermostructural response of spatially tailored metal-ceramic composite monocoque airframe during high-speed flight using finite element analysis (FEA). The typical development of a high-speed airframe is a two step design process. First, the outer mold line (OML) of the structure is established by assuming no deformation of the structure takes place during the aerothermodynamic analysis. During this analysis a worst case scenario (trajectory, operating conditions, etc.), which in many cases may or not be correct, is used. Second, with the OML established, features of the structure are examined. For instance, the thickness and materials of the OML and the thermal protection system (TPS). The benefit of this method is that it greatly reduces the amount of time to complete the design. Finally, similar to many engineering design operations, a factor of safety is applied to the design to account for unknown design variables or loading conditions, which can result in increased weight, reduced operating envelope, etc. However, it has been well established that airframes operating at high-speeds are subjected to complex coupling of fluid, thermal, and structural interactions (FTSI). In FTSI, the interaction of the fluid and temperature fields results in deformation of the structure, which in turn results in changes to the flow field and thus, the temperature of the structure. This unique coupling can result in the creation of different worst case scenarios that can drastically change the design approach.

In this work, aerothermodynamic loads characteristic of a high-speed trajectory are applied to the airframe with consideration of FTSI taking place at these operating conditions. To achieve this unique coupling, a numerical/analytical hybrid coupling approach is used. This hybrid technique uses aerodynamic loads calculated using the computational fluid dynamic (CFD) software OpenFOAM. While outside of the scope of the current work, each CFD simulation is performed at a specific angle of attack, altitude and mach number until a steady-state solution is obtained. A database is constructed for simulations over a wide-range of angles of attack, altitude and mach numbers from which a machine learning model is used to quickly estimate aerodynamic on the structure. The steady-state analysis is used to determine the steady pressure on the structure which in many cases, is orders of magnitude greater than unsteady pressure. Unsteady pressure resulting from maneuvers and structural deformation are determined using a 3rd-order Piston theory. Lastly, thermal loads are computed using Eckert's reference temperature method which uses the total (steady and unsteady) pressure to determine the heat flux applied to the structure.

This hybrid approach has advantages over tight coupling techniques in that the time step of simulations is not dictated by CFD, but instead by FEA which is significantly larger. Second, the amount of required computational resources is reduced since CFD and FEA are not computed simultaneously. Lastly, since unsteady CFD is corrected using an analytical model, the time required to complete each simulation is greatly reduced. This processes is demonstrated using two material configurations. First, a benchmark analysis of a Titanium airframe with a Acusil II TPS is performed. Second, a simulation of a spatially tailored metal-ceramic composite airframe is investigated. The thermostructural response of both material configurations using FTSI coupling are compared and contrasted to results obtained without such coupling.

Presenting Author: Phillip Deierling University of Iowa

Authors:

Phillip Deierling University of Iowa