The subsonic near wake of bluff bodies

• Die subsonische Nachlaufströmung stumpfer Körper

Wolf, Claus Christian; Stumpf, Eike (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2014)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2013

Abstract

The near-wake aerodynamics of space launch vehicles is characterized by complex interactions between external flow, separated recirculation flow, and jet flow. The implications may seriously affect the overall performance and safety. An experimental wind tunnel investigation of subscale models in subsonic freestream conditions is presented, focusing on the spatio-temporal behavior of the wake. Three base configurations with different levels of abstraction were contemplated, namely truncated bluff base', dummy nozzle', and supersonic overexpanded jet'. The experiments combined several flow field and surface pressure measurement techniques. The recirculation system of the bluff base geometry exhibits a pronounced sensitivity towards small freestream angles. A spectral and modal analysis of the time-dependent fluctuations reveals the occurrence of a discontinuous and arbitrarily oriented vortex shedding mode, whose relative energy is comparably low. The dummy nozzle extension further suppresses this mode and the angular sensitivity through geometric blockage. A reattachment of the external flow shortly upstream of the nozzle exit plane was observed, resulting in high-frequency fluctuations connected to the main body's shear layer. The entrainment caused by the supersonic jet evokes a relocation of both primary vortex system and reattachment. In contrast to the velocity fluctuations, the surface pressure fluctuations are strongly enhanced as a consequence of jet noise. Distinct acoustic modes were identified and attributed to the geometry of the shock cell system. In summary, this thesis offers two main conclusions. Firstly, the angular sensitivity strongly impacts bluff bodies with truncated base, resulting in a local minimum of the base drag for a symmetric wake layout. When comparing different studies (i.e., experimental and numerical), the respective freestream conditions have to be monitored carefully to draw correct conclusions. Secondly, the jet configuration revealed that at least for subsonic freestream conditions, the fluctuation environment consists of a multitude of modes (jet noise, reattaching shear layer, etc.) rather than a single phenomenon (i.e., vortex shedding). Hence, future launcher designs or measures of flow control have to adapt to a rather broad-banded environment.