Date of Award

Winter 2002

Document Type


Degree Name

Doctor of Philosophy (PhD)


Aerospace Engineering

Committee Director

Osama A. Kandil

Committee Member

Oktay Baysal

Committee Member

Brett A. Newman

Committee Member

Gene Hou


Modern fighter aircraft with dual vertical tails are operated at high angles of attack. The vortex generated by leading edge extension (LEX) breaks down before reaching the two vertical tails. The wake of highly unsteady, turbulent flow causes unbalanced broadband aerodynamic loading on the tails and may produce severe buffet on the tails and lead to tail fatigue failure.

Flow suction along the vortex cores (FSVC) is investigated as an active control method for tail-buffet alleviation. Suction tubes have been tilted at different angles to study the control effectiveness of suction tubes orientation. Flow field response, aerodynamic loading and aeroelastic results are compared with the no-control case. These flow modifications produce lower tip bending and rotation angle deflections and accelerations. Moreover, the root bending and twisting moments are reduced in comparison with the no-control case. However, there was no shift in the frequencies at which the peaks of the power spectral density (PSD) responses occurred. The primary effect of the FSVC methods is the amplitude reduction of the aeroelastic responses up to 30%. A parametric investigation is conducted and the best control effectiveness is obtained with the suction tubes tilted at −10°. Next, the twin-tail buffet alleviation is addressed by using adaptive flow control, and an adaptive active control method is developed. Control ports, whose locations are determined according to the locations of a range of high-pressure difference, are placed within a small area on the tail surfaces. Flow suction and blowing are applied through these control ports in order to equalize the pressures on the two surfaces of the tail. Mass flow rate through each port is proportional to the pressure difference across the tail at the location of this port. Comparing the flow field and aeroelastic response with the no-control case, the normal-force and twisting-moment distributions are substantially decreased along with the damping of their amplitudes of variation. The bending-deflection and rotation-angle responses have not changed their sign. The PSD of the root bending moment and root twisting moment have shown substantial decreases of more than 70%. The tail tip acceleration responses have shown similar decreases too.

Next, a parallel high-order compact-scheme code (PHCC) is developed to investigate flow control more accurately and more efficiently. The validation cases are presented and compared with theoretical results, experimental results and other computational results. The PHCC results show good accuracy and high efficiency. Flow computational simulations of Jet and Vortex Actuator (JaVA) or synthetic jet have been investigated. The computational results show good agreement with the experimental data and other computational results. Simplified 2D models, which include an airfoil under the effect of JaVAs and synthetic jet actuators, are developed and investigated for control effectiveness. Simulation results show: with properly selected parameters, the oscillating amplitude of pressure difference and normal force acting on airfoil can be reduced, the peak of the normal force PSD can be reduced and the frequencies at which the peaks of the pressure difference PSD responses occurred can be shifted to higher frequency levels. Too low or too high exciting frequencies have no effect or adverse effect. Low exciting velocity may not produce enough disturbances to suppress the pressure oscillation.