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.

RESULTS: 

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