Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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Increasing Mach number increases the hinge moments and curve gradients considerably. However for every pressure ratio and Mach number the curves intersect at a flap angle of 25 o . For angles above this, increasing pressure ratio increases the pitching moment. Angles under 25 o show the opposite trend. There appears to be a maximum value for the moment as the curves begin to flatten at 45 0 , however more data at higher angles is required to verify this prediction. To investigate the accuracy and validity of the CFD model, DFR and pressure ratio data were extracted from the plots in figure 3 for a single angle and plotted in figure 5 (dashed line) with the corresponding data from the NACA paper (solid line) across a range of Mach numbers. The curves through the CFD data points match the trend of the curves for the experimental data. However for a given pressure ratio the CFD result appears to under predict the DFR. This discrepancy increases with increasing pressure ratio with the effect becoming more severe with increasing freestream Mach number. The maximum error at each Mach number increases from 5% at M=0.4 to 20% at M=0.85. Figure 5 : DFR against pressure ratio, for flap angle=25 o , comparing NACA TN4007 and CFD results The thrust generated by the outlet, defined positive in the stream-wise direction, was measured in the NACA paper through the use of a force gauge (see figure 1) and non-dimensionalised with free-stream dynamic head and effective outlet area. For comparison, the same thrust coefficient was calculated using the CFD results and taking into account the force on both the curved duct and flap. The envelope of thrust data from the NACA experiments is plotted in figure 6, along with the corresponding CFD data points, for a given flap angle. At lower values of DFR the predicted values of thrust fall within the envelope from the experimental data. At the larger values of DFR the CFD results over predict the generated thrust with the points lying just outside the envelope, with an error between 5% and 10%. Discussion From the plots in figure 3 it can be clearly seen that flap angle has a pronounced effect on the discharge performance of the outlet. Previous studies 1,2 had indicated that flaps, or other protrusions, generated areas of low pressure over the outlet which increased discharge through suction. The mechanism behind this is the formation of a pair of longitudinal vortices, shed from the edges of the flap (c.f. a delta wing). As the flap angle increases the strength of the vortices increases until a maximum angle is reached. Beyond that angle the flap could be said to “stall” and behaves like a bluff body.
Figure 7 shows a series of plots of total pressure contours in the Y-Z plane of the computational domain, downstream of the outlet, for a flap angle of 15 0 , free-stream Mach number of 0.7 and pressure ratio of 0.8. Note that the symmetry plane has been plotted to illustrate the pair of vortices. The structure of the flow can be seen to develop downstream of the outlet as the vortices interact with the exhaust jet and shear layer shed from the trailing edge of the flap. Figure 6 : Coefficient of thrust against DFR, for flap angle=25 o , comparing NACA TN4007 and CFD results Figure 7 : Contours of total pressure, flap angle = 15 o , M=0.7, pressure ratio = 0.8
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