Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 5
RPM with a standard deviation of only 0.4%. The value and the independence from
RPM are reasonable considering the theoretical bounds just described.
5.5 Three-Dimensional Analysis using
OVERFLOW-D
The Reynolds-averaged Navier-Stokes flow solver used to validate the results from the
rapid design method is based on a version of the OVERFLOW code developed by
Buning et al [33]. OVERFLOW is a general-purpose Navier-Stokes code designed for
overset-grid computations on static grids. Meakin [34, 35] has adapted this code to
accommodate arbitrary relative motion between vehicle components and to facilitate off-
body solution adaption. The modified code automatically organizes grid components
into groups of approximately equal size, facilitating efficient parallel computations of
multi-body problems on scalable computer platforms. On parallel machines, each
processor is assigned a group of grids for computation, with inter-group communications
performed using the Message Passing Interface (MPI) protocol. This code is known as
OVERFLOW-D. The solution of hovering-rotor problems requires a number of
modifications to the OVERFLOW-D flow solver. Strawn and Djomehri [36] describes
these modifications in detail. Post-processing of the sectional and global rotor forces
uses the FOMOCO force integration code [37].
This modified version of OVERFLOW-D has been validated with experimental data for
a model UH-60A rotor by Strawn and Djomehri. Figure 5.11 compares the experimental
and computational global thrust and torque coefficients. Figure 5.12 compares the
computed and sectional thrust distribution for the same model. The over prediction of
thrust visible near the tip is to some extent an artifact of the OVERFLOW-D analysis and
grid density issues [36]. Grid related vortex diffusion exacerbates the blade vortex
interaction (BVI), resulting in a local exchange in circulation between the BVI and the
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