Modeling large non-axisymmetric aerodynamic forces on a space launcher’s afterbody

Aerodynamic flow control involves manipulating a flow field with a device to enhance aerodynamic performance — for instance, to reduce drag, augment lift or mitigate noise. Examples of flow control devices include vortex generators on a wing of an aircraft and actuators that add momentum to the flow in a regulated manner.

A new article investigates the pressure fluctuations that lead to large non-axisymmetric aerodynamic forces on a space launcher’s afterbody, with the ultimate goal of designing flow control devices for the vehicle. In particular, the researchers wanted to better understand the origin of fluctuating side loads on an axisymmetric backward-facing step flow in the transonic regime. They used the same geometrical ratios as a real space launcher afterbody to preserve the similarities between the study and a real-life case.

The advanced numerical modeling study employed a Zonal Detached Eddy Simulation (ZDES), a multi-resolution approach that aims to treat all classes of flow problems in a single model. They also performed advanced Fourier analyses and a dynamic mode decomposition to observe the space-time evolution of the helical pattern characteristic of the azimuthal mode that leads the flow dynamics.

The researchers visualized the statistical signature of a helical structure related to the anti-symmetric mode at a shedding-type frequency around an axisymmetric backward-facing step at a high Reynolds number. They identified the main hydrodynamic mechanisms through the spatial distribution of the most energetic frequencies, which correspond to the vortex shedding and Kelvin-Helmholtz instability phenomena. The dynamics related to the shedding frequency were dominant in a band centered around the middle of the extension of the axisymmetric backward-facing step, starting at the wall and expanding until the free stream.

Source: “Large scale dynamics of a high Reynolds number axisymmetric separating/reattaching flow,” by R. Pain, P.-E. Weiss, S. Deck, and J.-C. Robinet, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5121587 .