Measurements to Understand the Flow Mechanisms Contributing to Tandem-Rotor Outwash

Downwash and outwash characteristics of a model-scale
tandem-rotor system in the presence of the ground were
analyzed by identifying and understanding the physical
mechanisms contributing to the observed flow field behavior.
A building block approach was followed in simplifying
the problem, separating the effects of the fuselage,
effects of one rotor on the other, etc. Flow field velocities
were acquired in a vertical plane at four aircraft
azimuths of a small-scale tandem rotor system using the
particle image velocimetry (PIV) technique for radial distances
up to 4 times the rotor diameter. Results were compared
against full-scale CH-47D measurements. Excellent
correlation was found between the small- and full-scale
mean flow fields (after appropriate normalization using
rotor and wall jet parameters). Following the scalability
analysis, the effect of rotor height on the outwash was also
studied. Close to the aircraft, an increase in rotor height
above ground decreased the outwash velocity at all aircraft
azimuths. However, farther away, the longitudinal
and lateral axes of the aircraft showed increasing and decreasing
outwash velocities, respectively, with increasing
rotor height. Measurements also indicated the presence of
large-scale (of the size of the rotor height) shear-layer vortical
structures along the ground that could be the source
of low-frequency (approximately 1 Hz) flow variation observed
in the full-scale measurements. Flow visualization
studies and PIV measurements were also made on jets of
different sizes to complement the observations made on
rotors wherever possible. Baseline rotor measurements
were made out-of-ground effect to understand the nature
of inflow distribution for realistic rotor configurations and
their modified characteristics in the presence of ground.
Lastly, a feasibility study on applying high-fidelity CFD
simulations for outwash study was conducted using Helios
to model an isolated rotor configuration IGE at full scale Reynolds number. The results were encouraging and
demonstrated the practical challenges associated with predicting
rotor outwash.