Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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Flap angle is assumed to be fixed rather than freely hinged with flap weight considered negligible. By studying force data across a range of fixed angles it is possible to deduce hinge moments and therefore the steady state angle at which the flap will balance for a given pressure ratio and Mach number. The discharge characteristics for this flap angle can then be determined and therefore the performance of the outlet for given conditions. Meshes were created for flap angles from 15 to 45, in 5 increments. The free-stream Mach number was varied from 0.4 to 0.85 in increments of 0.15. As a result, the ratio of leading edge boundary layer thickness to orifice length varied between 0.095 and 0.110. Figure 2: Computational domain The performance of the duct is measured in terms of Discharge flow ratio (DFR) which is defined as the ratio of mass flow through the effective area of the orifice to the mass flow in the free stream through the same effective area as the orifice. The pressure ratio, defined as the ratio of duct total pressure to free stream total pressure, was varied between 0.64 and 0.97 in order to obtain the range of flow ratios required. Pressure inlet and outlet boundary conditions were applied and the realisable k- turbulence model was used because of its known accuracy when dealing with flows involving jets, separations and secondary flows. A mesh dependence study was performed to ensure that converged solutions were mesh independent. Results Mass flow through the effective outlet area was extracted from the data files and discharge flow rates were calculated for each case, doubled to account for the symmetry plane. These values were plotted against angle for each pressure ratio and Mach number, as shown in figure 3. In each case the value of DFR increases with flap angle up to a maximum before falling off. The angle at which this maximum occurs decreases with increasing pressure ratio. Increasing Mach number also reduces the angle at which maximum discharge occurs. The maximum value of DFR increases with increasing pressure ratio but decreases with increasing Mach number. Next the force on the flap, and load centre were extracted and used to calculate the moment about the hinge point for each case. These were converted to pitching moment coefficients by normalising with free-stream dynamic head, effective outlet area and flap length, with positive moment defined to be closing the flap. These values are then plotted against flap angle, shown in figure 4. Extrapolation of the data shows that for the majority of combinations of pressure ratio and Mach number, the zero pitching moment coefficients occurred in the range of 10 o to 15 o . At lower pressure ratios and Mach numbers this point is lowered below 10 o .
Figure 3 : DFR against Flap angle for a range of pressure ratios (legend) and angles Figure 4 : Hinge moment coefficient against flap angle for a range of pressure ratios (legend) and Mach numbers
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