Hypervelocity flow testing of hot test models in ground test facilities

Hypersonic vehicles experience high thermal loads due to the energy conversion of the enormous kinetic energy from high velocity into internal energy near the boundary layer. The energy conversion results in high heat transfer, elevating surface temperatures. Depending on the temperature range, various flow phenomena can occur, which are relevant to different types of flight vehicles, from shock/boundary layer interactions, ablation, spallation, and gas radiation. Impulse test facilities provide a cost-effective method for creating the ideal aerothermal environment. However, their micro-to-millisecond steady test time does not allow sufficient time for the temperature to rise to observe any differences.

This talk will present a twofold investigation of hypersonic flow fields, focusing on preheating the test models to overcome these difficulties. Firstly, the lower surface temperature range can be relevant to hypersonic impinging shock-boundary layer interactions. Using a DC power supply, the surface temperature of the flat-plate model reached 800 K. Experiments in Mach 7 flight-equivalent flow conditions in the T4 shock tunnel revealed an enlarged separation region in both laminar and turbulent boundary layers at higher wall temperatures, which was developed into a new correlation between separation length and wall temperature.

Secondly, a plasma preheating approach integrates a thermal arc-jet with an impulse facility, such as an expansion tube, to simulate a hypervelocity flow-field. The method is agnostic to material properties and geometry, enabling gas blowing/pyrolysis from ablation, which allows for a more complete simulation of atmospheric entry. This was realized by the development and commissioning of the OPG small-scale thermal arc-jet. Freestream characterization and various high-temperature material testing yielded surface heat fluxes exceeding 5 MW/m² with a surface temperature of 2200 K. The insertion into the CXT expansion tube, at speeds of up to 6 km/s (Mach 20), is currently underway, and preliminary results will be presented in the talk.