Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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11 aircraft. For a pitching delta wing, both the formation of leading-edge vortices [28] and vortex breakdown [29] [30] show hysteresis and time lag compared with respect the quasi-steady case. This time lag, which is important for the stability and control of aircraft, has also been observed for other types of wing motion, such as plunging and rolling. The time lag of vortex breakdown is much larger than that of vortex formation. Although it is common to all unsteady flows regardless of the type of unsteady motion [31], the mechanism of hysteresis and time lag is not well understood. The dynamic response of vortex breakdown is strongly linked to the adverse pressure gradient along the vortex axis [30], which cannot be measured experimentally and which as previously mentioned, is hard to obtain with CFD. As CFD simulations has become more realistic the opportunity to couple CFD and flight mechanics has been exploited. A great deal of experimental data is available for 1 Degree of Freedom (DOF) motion around the roll axis of a delta wing, when a highly swept delta wing exhibits wing rock (see for example figure 11). CFD has been able to predict the wing rock phenomenon of highly swept wings with Euler, laminar, and RANS models of the flow. For a Λ = 65 o delta wing rolling about its x- (body)axis, RANS simulations have been performed [32]. In this case the experimental results were contaminated by mechanical friction between the sting and the support structure. As such, instead of the experimental results exhibiting an aerodynamically damped oscillation, the model stopped at nonzero roll angles for various initial roll angles. CFD simulations were able to reproduce such behaviour if mechanical friction was added, though the choice of mechanical friction model was governed by comparison with experiment. As an extension to the 1 DOF roll cases discussed it is also currently feasible to perform multiple degree-of-freedom (rigid body motion) simulations with CFD. Multiple degree of freedom experimental studies are uncommon and problematic due to the support structures required to move freely (though as discussed mechanical friction remains a problem) and in any direction. Coupling CFD and flight mechanics in such a way will allow virtual studies of new aircraft configurations in regimes which are usually avoided due to highly non-linear aerodynamics. However, experiments with simplified free response cases are required to allow evaluation of the influence of modelling induced effects on the rigid body dynamics.
12 3.7 Vortex / flexible wing interaction Because of unusual designs and high rate motions for future aircraft, wing flexibility could become an issue. Coupling of unsteady, separated and vortical flows with flexible wings may result in limitcycle-oscillations or control problems. For flexible delta wings, vortex/wing interaction (see Figure 6) may lead to limit cycle oscillations, where the vortex acts like an aerodynamic spring [33]. Unsteady flow phenomena may interact and couple with structural vibrations. As it is very difficult to simulate aeroelastic phenomena experimentally due to model scaling requirements, validated computational simulations may be very useful for this kind of multidisciplinary and challenging engineering problem. CFD simulations have the advantage of being able make predictions at real flight conditions with structural models representing the full aircraft behaviour. 4 Conclusions For experimentalists, with the current capabilities of CFD and the assumptions it employs, CFD should be primarily used as a tool to build on measurement opportunities. Ideally an iterative process should be used, using CFD to highlight areas of interest either before or after experiments. As a greater understanding is gained of the flowfield, further experiments or CFD simulations could be done which would provide a much more detailed picture of the flowfield. Due to the temporal limitations of PIV and the spatial restrictions of LDA, using CFD to focus (and also understand) the measurements is seen as particularly advantageous. Since delta wing flows are particularly susceptible to facility interference an accurate tool for predicting tunnel interference is required. A suitably validated CFD method would be able to provide details of combined tunnel wall, tunnel boundary layer, and support structure interference effects. The tool would also be applicable to all facilities and all tests. For the CFD practitioners more detailed high quality data is required, especially in boundary layers. There is little insight to be gained from validating an expensive DES simulation with force and moment data. Instead to validate models high quality flowfield data is required, especially in vortical flows where the understanding of off surface flow features is of vital importance. Similarly as the effects of facility interference often contaminate experimental results, modelling the entire
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Page 9 and 10: Heat flux measurements. Over the pa
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Figure 11: Vortex Shedding cycle fo
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Figure 13: Comparison with experime
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