Thesis (M.S.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains viii, 59 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 51-52).

M.Ing. / The aim of this dissertation is to apply numerical aerodynamic principles to the characterization of an alternative stepped aerofoil concept. The accurate and efficient determination of the aerodynamic forces caused by the relative fluid motion and the consequent lift and drag coefficients are essential for the characterization of new aerofoils. The numerical method used is in the form of a Computational Fluid Dynamics code, which integrates the Navier-Stokes equations through finite-volume dictretization principals. A two-dimensional approximate analysis procedure is used together with a two-equation turbulence approximation in the form of the "standard" k-c turbulence model. Available software is used and adapted where applicable. A suitable method for comparing wing section characteristics as a function of profile geometry and attitude is developed in this thesis. This is achieved by first refining a numerical test case and quantifying the influences of model parameters such as grid design, boundary conditions and solution variables. Alternative geometrical aerofoil concepts can then be characterized by employing the same principles. This thesis contains selected results of hundreds such numerical simulations, all of which were necessary to refine the test case and eventually characterize the aerofoils. The proposed wing section geometry, incorporating a rearward-facing step shows some improvement in aerodynamic performance over a standard reference case. Geometrical variations of the step concept are also investigated and can later be used in an optimization procedure. A transient simulation approach is employed for unsteady cases and flow visualization is done in order to learn more about the unique aerodynamic action of the proposed concept. Experimental results obtained in a wind tunnel for the pressure around the investigated aerofoils are used to verify numerical results. Further development in the numerical approach may include the use of additional, more advanced turbulence models. This may allow the research of more complex phenomena such as stall and also broader ranges of Reynolds numbers in more detail. To complete the characterization process, the moment coefficients should also be included.

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The effect of humps on the stability of subsonic boundary layers over an airfoil is investigated. The mean flow is calculated by using an interacting boundary-layer solver which accounts for strong viscous/inviscid interaction and separation bubbles. The code is capable of solving compressible as well as incompressible flows. Then, the two-dimensional mean flow is fed into a stability program which is capable of doing two-and three-dimensional analysis. The output of this stability program is the growth rates which are integrated along a prescribed path to yield the amplification factor (i.e., N-factor), which is used to predict transition from laminar to turbulent flow. The analysis is performed for different heights and locations of the hump and for different Mach numbers. The results show that compressibility stabilizes the flow and that the most dangerous frequency decreases as the Mach number increases for a fixed location of the hump. Also this most dangerous frequency decreases as the hump is moved downstream. Moreover, the amplification factor increases as the hump height increases and as the hump is moved downstream. The influence of suction and heat-transfer strips on controlling the destabilizing influence of the hump is investigated. The results show that cooling and suction strips stabilize the flow and therefore delay transition from laminar to turbulent flow. Moreover, a heating strip destabilizes the flow in the presence of a hump. Applying suction through multiple strips can be as effective as continuous suction. Also the total flow rate required using multiple strips is less than that required using a single strip. We optimize the locations of these strips for a certain hump location. Moreover, cooling through multiple strips is as effective as cooling through a single strip. We optimize the locations and levels of these cooling strips for a certain hump location. / Master of Science

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The behavior of a junction vortex formed around an obstacle in a boundary layer flow was studied experimentally in a water tunnel for two low speed cases. A wing consisting of a 3 : 2 elliptical nose and an NACA 0020 tail was used to simulate the junction vortex. A visual study using a hydrogen bubble technique was extensively conducted to investigate the flow structures in the stagnation region of the wing. It was observed that a multiple vortex system exists in this region and shows an acyclic flow pattern. LDV measurements were performed in the plane of symmetry upstream of the wing. The general behavior of the flow agrees with an earlier wind tunnel test of Devenport and Simpson which was conducted at higher speed. A low frequency, bistable flow structure was observed as in the wind tunnel measurements. The switching between two flow modes (a backfiow mode and a zero flow mode) was analyzed using LDV signals in the zone of a bimodal structure. A dimensionless frequency group (StT) was found to represent the average frequency of successive switches from a given mode to the other. The visual evidence of acyclic flow pattern was consistent with the LDV measurements, and revealed that aperiodic stretching of the junction vortex appears responsible for the bimodal (double-peaked) structure in the velocity histograms. An attempt to measure the three-dimensional instantaneous velocity field in this region was made. A unique PIDV (particle image displacement velocimetry) technique was developed using a multiple wire hydrogen bubble method and a high speed video system. A stereo vision approach was implemented to capture two orthogonal views simultaneously for the three-dimensional motion analysis. / Ph. D.

Laser Doppler velocimetry, a non-intrusive flow measurement technique, was employed to experimentally investigate two-dimensional airfoil-vortex interaction. Vortices were generated by sinusoidally oscillating a NACA 0012 airfoil about its quarter-chord at a reduced frequency of k = 2.05 and an amplitude of ±10° angle of attack. The target airfoil, a NACA 63₂A015, was immersed in the wake, two chord lengths downstream of the vortex generators trailing edge. Phase-averaged velocity measurements of the flow around the target airfoil were made with the airfoil at angles of attack of α = 0° and α = 10°. A close encounter with a counterclockwise rotating vortex was observed for both angles of attack, and a head-on collision, which split the counterclockwise rotating vortex in two, was observed for α = 10°. Vorticity fields were constructed from the velocity measurements and the circulation of the vortex was evaluated throughout the interaction. The surface pressure fluctuations on the airfoil were determined by substituting the measured velocities into the Navier-Stokes equations and numerically integrating the resulting pressure gradients. Furthermore, an extensive investigation of the undisturbed wake of the oscillating airfoil was performed in order to determine the effect of oscillation frequency and amplitude on the wake development. / Ph. D.

It was found in this investigation that the testing of supersonic airfoils at subsonic speeds to obtain pressure distributions will result in considerable error if the models are tested at very low Reynolds Numbers. The thickness of the boundary layer is critical at low Reynolds Numbers and causes a decrease in the overpressure region. This, coupled with the fact that the overpressure region increases more with Mach Number than predicted by the theory, could lead to erroneous calculations of the drag. / Master of Science