1. The primary obstacle to including separated flows in calculation programs is the difficulty of predicting where the separation occurs: the actual separation line on a helicopter fuselage is 3-dimensional, and may be expected to move around during every rotor cycle. To compute this would require detailed 3-D boundary layer calculations, coupled with a 3-D external potential flow. Thereafter, calculating the separated flow would require Navier-Stokes computations, tremendously resource-demanding.
So, in designing the experiment to understand the flow, we fixed the separation line by providing a sharp backward-facing step on the fuselage. The flow over a backstep is a fundamental test case in fluid dynamics, studied for its importance in combustors, river hydrodynamics, and various other applications.
Our modified test configuration is shown below. We retained the basic hemisphere-cylinder / 2-bladed rotor test configuration, and only removed the aft end of the cylinder, providing the backward-facing step. A set of 4 microphones could be traversed along the narrowed-down part.