http://hdl.handle.net/1721.1/50867 Fri, 24 May 2013 05:06:38 GMT 2013-05-24T05:06:38Z http://hdl.handle.net/1721.1/70578 Investigation of the F117A Vortical Flow Characteristics Preliminary Results Vermeersch, Sabine The investigation of the vortical flow around the F117A Stealth Fighter is presented in order to demonstrate the capability to resolve leading edge vortices with an adaptive finite element solver for the Euler equations. The major goal is to capture vortex breakdown at high angles of attack. This work presents the five main steps involved in a typical study of the flow characteristics of a complete aircraft : the definition of the model geometry, the realization of a suitable grid around the discretized model, the implementation of a flow solver, the subsequent analysis of the flow field and the comparison to experimental data sets. The computational data are compared to the lift curves of the aircraft obtained in a subsonic 5' x 7' wind tunnel. The occurance and location of vortex breakdown is determined by performing flow visualization in the tunnel. Five cases are computed for this work. Each case is studied at Mach 0.3 and angles of attack range between 7 and 30 degrees. Thu, 01 Oct 1992 00:00:00 GMT http://hdl.handle.net/1721.1/70578 1992-10-01T00:00:00Z http://hdl.handle.net/1721.1/70577 Adaptive Mesh Euler Equation Computations of Vortex Breakdown in Delta Wing Flow Modiano, David A solution method for the three-dimensional Euler equations is formulated and implemented. The solver uses an unstructured mesh of tetrahedral cells and performs adaptive refinement by mesh-point embedding to increase mesh resolution in regions of interesting flow features. The fourth-difference artificial dissipation is increased to a higher order of accuracy using the method of Holmes and Connell. A new method of temporal integration is developed to accelerate the explicit computation of unsteady flows. The solver is applied to the solution of the flow around a sharp edged delta wing, with emphasis on the behavior of the leading edge vortex above the leeside of the wing at high angle of attack, under which conditions the vortex suffers from vortex breakdown. Large deviations in entropy, which indicate vortical regions of the flow, specify the region in which adaptation is performed. Adaptive flow calculations are performed at ten different angles of attack, at seven of which vortex breakdown occurs. The aerodynamic normal force coefficients show excellent agreement with wind tunnel data measured by Jarrah, which demonstrates the importance of adaptation in obtaining an accurate solution. The pitching moment coefficient and the location of vortex breakdown are compared with experimental data measured by Hummel and Srinivasan, with which fairly good agreement is seen in cases in which the location of breakdown is over the wing. A series of unsteady calculations involving a pitching delta wing were performed. The use of the acceleration technique is validated. A hysteresis in the normal force is observed, as in experiments, and a lag in the breakdown position is demonstrated. Fri, 01 Jan 1993 00:00:00 GMT http://hdl.handle.net/1721.1/70577 1993-01-01T00:00:00Z http://hdl.handle.net/1721.1/70576 System Identification and Active Control of a Turbulent Boundary Layer Rathnasingham, Ruben An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer using a linear control scheme. System identification in the boundary layer provides optimal transfer functions that predict the downstream of characteristics of the streamwise velocity and wall pressure fluctuation using an array of upstream flush-mounted sensors that are sensitive to spanwise shear. Enhanced direction techniques isolated the large scale turbulent motion and improved the downstream correlations resulting in greater controllability. The techniques were based on the conditioned spectral analysis between adjacent sensors to extract the most correlated flow structures that span the distance between them. The control is applied using a spanwise array of resonant actuators that introduce a pair of streamwise vortices into the flow. Control experiments were carried out for a single and multiple input/output configurations. The single output results show that a maximum reduction of 34% is achieved in the streamwise velocity fluctuation. This reduction is greatest at the point of optimization but spans over a few hundred viscous lengths downstream of the actuator and about 50 viscous lengths in the spanwise and wall-normal directions. The wall pressure fluctuation and the mean wall shear stress (measured approximately using mean velocity profiles near the wall) was reduced by 17% and 7% respectively. The multiple-input/single-output configuration resulted in a wider spatial influence of the control while maintaining the maximum reductions in the fluctuations. The multiple-input/multiple-output configuration showed a marked increase in the spatial extent of the control (primarily in the spanwise direction), at the expense of a lower reduction in the fluctuations (maximum of 30% and 15% for the streamwise velocity and wall pressure respectively). The bursting frequency was computed from a VITA algorithm applied to the streamwise velocity fluctuation. The bursting frequency was reduced at all threshold levels examined but the maximum reduction of 23% occurred at a threshold level of 3. The spanwise spatial correlation was measured at different streamwise locations downstream of the actuator array. This result suggests that the reduction in turbulent fluctuations obtained using the current control scheme was achieved by reducing the strength of the most coherent flow structures and to inhibit their ability to interact with each other by increasing their average spanwise separation by more than 25% (from ~ 90l* to 120l*). Sun, 01 Jun 1997 00:00:00 GMT http://hdl.handle.net/1721.1/70576 1997-06-01T00:00:00Z http://hdl.handle.net/1721.1/70569 A Comparison of Numerical Schemes on Triangular and Quadrilateral Models Lindquist, Dana Rae Sun, 01 May 1988 00:00:00 GMT http://hdl.handle.net/1721.1/70569 1988-05-01T00:00:00Z