A method for the estimation of transient aerodynamic data from dynamic wind tunnel
tests has been developed and employed in the study of the unsteady response of
simple automotive type bodies. The experimental setup consists of the test model
mounted to the oscillating model facility such that it is constrained to oscillate with a
single degree of freedom of pure yawing motion. The yaw position is recorded from a
potentiometer and the time response provides the primary measurement. Analysis of
the wind-off and wind-on response allows the transient aerodynamic loads to be
estimated. The frequency of oscillation, (synonymous with the frequency of
disturbing wind input) is modified by altering the mechanical stiffness of the facility.
The effects of Reynolds number and oscillation frequency are considered and the
model is shown to exhibit damped, self-sustained and self-excited behaviour. The
transient results are compared with a quasi-steady prediction based on conventional
tunnel balance data and presented in the form of aerodynamic magnification factor.
The facility and analysis techniques employed are presented and the results of a
parametric study of model rear slant angle and of the influence of C-pillar strakes is
reported. The results are strongly dependent on shape but for almost all rear slant
angles tested the results show that the transient response exceeds that predicted from
steady state data. The level of unsteadiness is also significantly influenced by the rear
slant angles. The addition of C-pillar strakes is shown to stabilise the flow with even
small strakes yielding responses below that of steady state.
From the simulation results the self-sustained oscillation is shown to occur when the
aerodynamic damping cancels the mechanical damping. The unsteadiness in the
oscillation can be simulated by adding band-limited white noise with an intensity
close to that of the turbulence intensity found in the wake. From vehicle crosswind
simulation results the aerodynamic yaw moment derivative and its magnification
factor are shown to be the important parameters influencing the crosswind sensitivity
and path deviation.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.