Fluid mechanics -- Mathematical models

This project is intended to demonstrate the current state of knowledge in the prediction of the tonal and broadband noise radiation from a Sevik rotor. The rotor measurements were made at the Virginia Tech Stability Wind Tunnel. Details of the rotor noise and flow measurements were presented by Wisda et al(2014) and Murray et al(2015) respectively. This study presents predictions based on an approach detailed by Glegg et al(2015) for the broadband noise generated by a rotor in an inhomogeneous flow, and compares them to measured noise radiated from the rotor at prescribed observer locations. Discrepancies between the measurements and predictions led to comprehensive study of the flow in the wind tunnel and the discovery of a vortex upstream of the rotor at low advance ratios. The study presents results of RANS simulations. The static pressure and velocity profile in the domain near the rotor's tip gap region were compared to measurements obtained from a pressure port array and a PIV visualization of the rotor in the wind tunnel.

The numerical method presented in this study attempts to predict the mean, non-uniform flow field upstream of a propeller partially immersed in a thick turbulent boundary layer with an actuator disk using CFD based on RANS in ANSYS FLUENT. Three different configurations, involving an infinitely thin actuator disk in the freestream (Configuration 1), an actuator disk near a wall with a turbulent boundary layer (Configuration 2), and an actuator disk with a hub near a wall with a turbulent boundary layer (Configuration 3), were analyzed for a variety of advance ratios ranging from J = 0.48 to J =1.44. CFD results are shown to be in agreement with previous works and validated with experimental data of reverse flow occurring within the boundary layer above the flat plate upstream of a rotor in the Virginia Tech’s Stability Wind Tunnel facility. Results from Configuration 3 will be used in future aero-acoustic computations.

A motion compensated ADV system was evaluated to determine its ability to
make measurements necessary for characterizing the variability of the ambient current in
the Gulf Stream. The impact of IMU error relative to predicted turbulence spectra was
quantified, as well as and the ability of the motion compensation approach to remove
sensor motion from the ADV measurements. The presented data processing techniques
are shown to allow the evaluated ADV to be effectively utilized for quantifying ambient
current fluctuations from 0.02 to 1 Hz (50 to 1 seconds) for dissipation rates as low as
3x10-7. This measurement range is limited on the low frequency end by IMU error,
primarily by the calculated transformation matrix, and on the high end by Doppler noise.
Inshore testing has revealed a 0.37 Hz oscillation inherent in the towfish designed and
manufactured as part of this project, which can nearly be removed using the IMU.

For aerospace and naval applications where low radiated noise levels are a
requirement, rotor noise generated by inflow turbulence is of great interest. Inflow
turbulence is stretched and distorted as it is ingested into a thrusting rotor which can have
a significant impact on the noise source levels. This thesis studies the distortion of
subsonic, high Reynolds number turbulent flow, with viscous effects ignored, that occur
when a rotor is embedded in a turbulent boundary layer. The analysis is based on Rapid
Distortion Theory (RDT), which describes the linear evolution of turbulent eddies as they
are stretched by a mean flow distortion. Providing that the gust does not distort the mean
flow streamlines the solution for a mean flow with shear is found to be the same as the
solution for a mean potential flow with the addition of a potential flow gust. By
investigating the inflow distortion of small-scale turbulence for various simple flows and
rotor inflows with weak shear, it is shown that RDT can be applied to incompressible
shear flows to determine the flow distortion. It is also shown that RDT can be applied to more complex flows modeled by the Reynolds Averaged Navier Stokes (RANS)
equations.