Journal of Aerospace Science and Technology
Volume:2 Issue: 4, Winter 2005

Details of pressure distributions, on a two dimensional airfoil oscillating in pitch through stall, in a 0.8 0.8 m low-speed wind tunnel are presented. Pitching occurred about the airfoils quarter-chord axis. Pitch rate, Reynolds number, and oscillation amplitudes were varied to determine the effects on pressure and lift distributions. It was found that mean angle of attack and pitching amplitude had strong effects on the flow field hence pressure distribution in the immediate vicinity of the airfoil leading edge,. For pressure ports located at, the aforementioned effects were not strong and seemed that during the oscillatory motions the flow was mostly separated. This investigation shows weak unsteady effects when the maximum dynamic angle of attack was below that of the static stall; i.e.. For higher angles of attack, strong unsteady effects appear that depend on the mean angle of attack, frequency and amplitude of the oscillation. Dynamic stall and dynamic reattachment contribute to a favorable effect of unsteadiness on the surface pressure signature and hence the mean lift coefficient which increases as compared to the steady state one.

In this paper, slewing maneuver of a flexible spacecraft with large angle of rotation is considered and assuming structural frequency uncertainties a robust minimum-time optimal control law is developed. Considering typical bang-bang control commands with multiple symmetrical switches, a parameter optimization procedure is introduced to find the control forces/torques. The constrained minimization problem is augmented with the robustness constraints, which in turn increases the number of switches in the bang-bang control input to match the total number of the constraint equations. The steps of the solution algorithm to obtain the time optimal control input are discussed next. The developed control law is applied on a given satellite during a slewing maneuver, and the simulation results show that the robust control input with just few switching times can significantly lessen the vibrating motion of the flexible appendage in the presence of structural frequency uncertainties, which reveals the merits of the developed control law.

In this paper, two-dimensional and axisymmetric, time dependent transonic and supersonic flows over a projectile overtaking a moving shock wave are considered. The flow is simulated numerically by solving full time averaged Navier-Stokes equations. The equations are linearized by Newton approach. The roe’s flux splitting method, second order central difference scheme for the diffusion terms, and the second order approximation for time derivatives are used. For the turbulence terms, the Baldwin-Lomax and mixing length turbulence models are used. The present algorithm captures all the complicated features of flow including moving shock waves, expansion waves, boundary layers and wakes and their interactions. The results show that as the projectile passes through the moving shock wave, it changes the flow field features and pressure distribution dramatically. The drag force decreases and even becomes negative while the projectile takes over the shock wave. The flow features and the aerodynamic forces in transonic flow changes much more than those in the supersonic flow, as the projectile passes through the shock wave. The results show that when the shock wave passes though the projectile, the flow field structures and the aerodynamic forces change abruptly. The drag force reduces and the shock wave passes through the projectile. Such variations are more pronounced for transonic flows compared to the supersonic flows. For transonic flows, the drag force changes sign and accelerate of the projectile. Such behavior is important when the stability and control of the projectile is studied. Also such changes in pressure around the projectile and in the wake region change the projectile trajectory.

Linear stability analysis of the three dimensional plane wake flow is performed using a mapped finite di?erence scheme in a domain which is doubly infinite in the cross–stream direction of wake flow. The physical domain in cross–stream direction is mapped to the computational domain using a cotangent mapping of the form y =? cot(??). The Squire transformation [2], proposed by Squire, is also used to relate the three–dimensional disturbances to the equivalent two– dimensional disturbances. The compact finite di?erence scheme of Lele [3] and the chain rule of di?erentiation are used to solve the Orr Sommerfeld equation. The results of linear stability analysis indicates that streamwise and the span- wise component of velocity eigenmodes are antisymmetric and the cross stream velocity eigenmode is symmetric. This is consistent with the DNS requirement of plane wake flow pertaining to solvability conditions[5]