Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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Figure 8 shows a similar plot for a flap angle of 40 o but with the same pressure ratio and free-stream Mach number, as shown in figure 8. A marked difference in the flow field can be seen, with a much stronger initial vortex pair leading to a larger flow structure further downstream. For the smaller flap angle, the flow structure impinges on the surface downstream which appears to have a thinning effect on the boundary layer in the downstream area outboard of the vortices. Figure 8 however shows that for a large flap angle the structure is lifted away from the surface with the result that the boundary layer thinning does not occur, in fact, between the vortices the boundary layer is substantially thickened. Variations in pressure ratio will change the nature of the discharge jet from the outlet, which will then in turn effect the manner in which the vortices, jet and shear layer interact. Free-stream Mach number will also determine the properties of the shed vortices and therefore the resulting flow structure. In cases of high Mach number and pressure ratio, flow in the outlet becomes choked as flow velocity exceeds sonic conditions and a normal shock is formed, as illustrated in figure 9. The position of this shock moves forward as the flap angle decreases, due to a repositioning of the throat of the outlet as effective area decreases. Figure 8 : Contours of total pressure, flap angle = 40 o , M=0.7, pressure ratio = 0.8 Conclusions Generally good agreement was achieved between the previously published data and the CFD prediction of integral properties such as DFR and thrust coefficient. Confidence has been gained that the numerical model, to a degree of accuracy, predicts the flow physics involved. A large amount of information is now available to be studied in an attempt to understand fluid mechanics involved. The numerical model can also be adapted to aid the design of a new experimental test rig, including the sizing of the required plenum chamber. It is also noted that the information obtained may be of use in areas other than PRD design. A large number of engineering applications involve longitudinal vortices impinging on a surface, either from vortex generators or jets emerging into a cross flow. Such applications include boundary layer control, prevention of shock induced separation, cooling and chemical mixing. Through studying the flow structures around PRDs some insight into the interaction of longitudinal vortices and boundary layers may be obtained and applied to the applications mentioned above.
Figure 9 : Contours of Mach number, flap angle = 40 o , free-stream M=0.85, pressure ratio = 0.85 Bibliography 1. Rogallo, F.M., Internal-flow systems for Aircraft, NACA, Report 713, 1941. 2. Drag of Auxiliary Inlets and Outlets, AGARD, 264. 3. Vick, A.R., An Investigation of Discharge and Thrust Characteristics of Flapped Outlets for Stream Mach Numbers from 0.4 to 1.3, NACA TN 4007, 1957. 4. Dewey, P., A Preliminary Investigation of Aerodynamic Characteristics of Small Inclined Air Outlets at Transonic Mach Numbers, NACA TN 3442, 1955. 5. Dewey, P. and Vick, A.R., An Investigation of the Discharge and Drag Characteristics of Auxiliary Air Outlets Discharging into a Transonic Stream, NACA TN 3466, 1955.
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