Flow control by energy deposition in hypersonic flow - some fundamental considerations

Kurzfassung

This paper discusses some general properties of energy deposition in hypersonics. Extending the flight envelope of passenger aircraft to high Mach numbers comes with problems unsolved before in civil flight, such as sonic boom, or excessive thermal and mechanical loads. Energy deposition is regarded as a possible remedy to a multitude of problems occurring in this flight regime and thus studied here. A new energy source term has been implemented into the DLR TAU CFD code which models a physical distributed heat addition. In a first step, energy deposition in free flow is investigated using this new term. Four flow topologies, dependent on the deposition energy, have been identified, ranging from negligible induced cross flow component, to a massive detached bow shock ahead of the deposition region. In a second step, ramp flow, as a precursor to flow about an airfoil, is studied under the influence of an upstream energy deposition region. An undesired low pressure region could be explained with a simplified model of wave refraction at thermally stratified ramp flow. Key of this model is a flow deflection above the ramp which is needed to balance pressure differences. This model is discussed in detail to fully understand the physical nature of the flow structure. It predicts a way of increasing surface pressure compared to regular ramp flow at reduced total pressure loss. It furthermore predicts the occurrence of premature shock separation and oscillating shock patterns. Analyzing this model, a simple yet accurate functional formula is found for the additional deflection angle due to refraction which simplifies the calculation of the surface pressure distribution as no pressure balance iteration is needed anymore. These studies are currently carried out within the European ATLLAS project which is concerned with the development of hypersonic passenger aircraft. Computations are performed using the DLR TAU code, a finite volume, second order accuracy, compressible flow solver.