Numerical investigation of the near wake of generic space launcher systems at transonic and supersonic flows

Abstract

Numerical simulations of the near wake of generic rocket configurations are performed at transonic and supersonic freestream conditions to improve the understanding of the highly intricate near wake structures. The Reynolds number in both flow regimes is 10 7 based on the main body diameter, i.e., specific freestream conditions of ESA’s Ariane launcher trajectory. The geometry matches models used in experiments in the framework of the German Transregional Collaborative Research Center TRR40. Both axisymmetric wind tunnel models possess cylindrical sting supports, representing a nozzle to allow investigations of a less disturbed wake flow. A zonal approach consisting of a Reynolds averaged Navier-Stokes (RANS) and a large-eddy simulation (LES) is applied. It is shown that the highly unsteady transonic wake flow at Ma1 = 0.7 is characterized by the expanding separated shear layer, while the Mach 6.0 wake is defined by a shock, expansion waves, and a recompression region. In both cases, an instantaneous view on the base characteristics reveals complex azimuthal flow structures even for axisymmetric geometries. The flow regimes are discussed by comparing the aerodynamic characteristics, such as the size of the recirculation region and the turbulent kinetic energy. The development of future launcher systems is among other issues, constrained due to highly intricate and barely understood base flow physics. The dominating aerodynamic characteristics can lead to low-frequency pressure fluctuations in the wake, which are predominantly induced by fluctuations of a separated shear layer. Rollstin [10] determines the base drag to be up to 35% of the overall drag for supersonic freestream conditions, which could be even higher in the case of blunt rockets. The fluctuations may also exceed critical amplitudes and can lead to structural failure. Especially during the transonic launch phase, the nozzle is ovalized due to the well-known bu eting problem. Therefore, the flow around the geometry is compared for Mach numbers of Ma1 = 0.7 and 6.0, corresponding to selected stages of a real rocket launch. In spite of simplified base geometries, the flow field remains highly complex, covering the interaction of the recirculation region with the separated shear layer, expansion waves and a recompression region. Since it is impossible to measure all the important flow features simultaneously, there is a basic demand for numerical simulations. The present paper shows the results of a new zonal approach for high Reynolds number numerical investigations and demonstrates the flow behavior in di erent flow regimes with an axisymmetric sting support at the base shoulder. Reynolds averaged Navier-Stokes (RANS) models are not capable to predict unsteady data and also fail to provide accurate results concerning the low pressure recirculation area at the base, while the predictions of the attached flow around the main body are quite satisfactory. Direct numerical simulations (DNS) are restricted to small Reynolds numbers and a small integration domain. Therefore, a zonal RANS/LES approach is applied. RANS simulations are used to predict the attached main body flow field while LES computations are applied to the unsteady wake flow using the RANS results as inflow conditions. The averaged turbulent viscosity of the RANS model is used to generate physical turbulent fluctuations at the inlet of the LES domain.