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*BREAKING DOWN THE PROBLEM OF FLOW SEPARATION*

*From the PhD Thesis of Jaimoo Kim*

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.

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