Ph.D. (Engg): Studies on Fluid Structure Interactions in Hypersonic Flow
March 7 @ 11:00 AM - 1:00 PM
The global thrust towards the development of hypersonic cruise systems for various applications is leading towards slender configurations with lifting and control surfaces which are thin and complaint and face a hypersonic flow. Hypersonic flows are characterized by large flow kinetic energy and momentum, which manifests into strong shocks, high temperatures, and associated effects that can cause coupling between the flow, structure, and thermal effects. Therefore, understanding Fluid-Structure Interactions (FSI) in hypersonic flow gains significance, and its predictive modelling is necessary to avoid adverse effects in flight. The majority of literature in supersonic and hypersonic FSI consider low-fidelity modelling using piston theory, two-dimensional FSI computations, and a limited number of experiments on mainly fully clamped flat panels subjected to aerodynamic loads, including shock-boundary layer interactions at supersonic Mach numbers. Studies on cantilevered panels, which are template shapes of control surfaces, at hypersonic Mach numbers are few, and there is a significant need to obtain experimental data to aid physical understanding, validate computational tools and methodology and model the hypersonic FSI phenomena.
This motivated the study of three different template flat plate experimental models in the hypersonic shock tunnel HST-2 in the Ludwieg Mode of Operation, which has 35 ms of test time. The freestream Mach number of M=6.6 is incident upon a) a cantilevered panel placed along the direction of the flow, b) a cantilevered panel with an impinging shock, and c) a trapezoidal wing-like shape fixed at the root and placed transverse to the flow. High-speed schlieren imaging and static pressure measurements at specific locations yield information on the flow characteristics. Image tracking methods are used to extract structural deformation, and accelerometers measure the oscillatory structural response in the presence of hypersonic flow. Complementary two-dimensional numerical simulations in a fully coupled format are conducted for a limited number of cases. Parametric studies are conducted by varying the panel thickness, angle of attack, and mass ratio for plain panels and the impinging shock characteristics for the panel with shock impingement. Natural, free vibration experiments using an impact hammer excitation are first carried out to evaluate the natural structural modal frequencies.
Great care is taken in designing all experimental models so that the FSI response can be captured during the short test time. Oscillatory response is captured successfully using the different diagnostic tools. For a plain cantilevered panel placed at the Angle of Attack of 20 degrees, the FSI response is dominantly near the first structural bending mode at a frequency of 89.65 Hz, which is higher in comparison to the natural frequency of 75.82 Hz. Multiple diagnostic tools and Dynamic Mode Decomposition analysis confirm these observations. The angle of attack and mass ratio affect the amplitude of oscillations. Varying thickness changes the structural stiffness, and accordingly, the oscillations occur at higher frequencies. Higher downstream pressures on the top surface of the panel due to forebody shock first cause the panel to bend away from the flow, which leads to the formation of expansion fans, releasing the pressure and causing the elastic restoring force to bring it back. Complementary two-dimensional FSI simulations showed good agreement with the experiments, though the magnitude of amplitude was higher due to the 2D nature of the simulation. Shock Boundary Layer Interaction is significantly affected by the panel’s compliance. There is a 29.6% reduction of the SBLI separation bubble size on a complaint cantilevered panel. The twin effects of a relaxation in pressure gradient and the existence of wall-normal velocities due to a vibrating panel can be attributed to the observed effect. The trapezoidal wing shape exhibited significantly higher magnitudes in the second structural mode.
The studies have laid the foundations for deeper investigations using field imaging techniques like 3D-Digital Image Correlation in the future. The experimental database can be used to develop predictive modelling approaches and data-driven modelling.
Speaker: Ms. Kartika Ahuja
Research Supervisor: Prof. Srisha Rao M V