Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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7 distributions, and load and moment data for dynamic cases. Flowfield measurements of the vortices would also be required to compare core properties and locations. To obtain results with various model to tunnel ratios, ideally the tunnel geometry should be altered (with artificial walls), as opposed to changing model size. In this way support structure interference would be consistent. If this is not possible and the wing size must vary, the size of the support structure should be adjusted accordingly (for example sting diameter). A useful experimental study would be as follows: • Select for example a square cross section tunnel. • Perform measurements for various angles of attack (upstream and downstream pressure and velocity profiles, tunnel boundary layers, wall pressures at selected locations, surface pressure data and flowfield measurements). • Measure loads and moments for dynamic cases (for example pitching motion). • Add artificial walls to bring side walls closer. • Repeat measurements for static and dynamic cases. • Add artificial walls to bring roof and floor closer • Repeat measurements for static and dynamic cases. Such experimental results could be used to validate a similar CFD study. These tests could also be conducted with and without supports for further validation. There has been less emphasis on the unsteady aspects of vortex breakdown which have an impact on aircraft stability and control, and wing/fin buffeting. The flow downstream of vortex breakdown exhibits a well-documented hydrodynamic instability, called the helical mode instability [20]. Experimentally observed periodic velocity/pressure oscillations correspond to the most unstable normal modes of the time-averaged velocity profiles of the vortex (downstream of breakdown) based on the linearised, inviscid stability analysis. Unsteady flow phenomena relevant to vortical flows over delta wings have been studied in several previous investigations [20] [21] [22] However current knowledge of the unsteady aspects of breakdown is limited to slender wings [23].
8 Computational simulations can contribute to understanding these flows better. Time-accurate CFD simulations of the helical mode instability can predict buffet frequencies for a range of static and manoeuvring cases. Coupled CFD and structural modelling could also be used to predict whether new aircraft designs would undergo wing / tail buffet, and any possible coupling of fluid / structural instabilities. The prediction of core properties is likely to be crucial however and detailed experimental data is needed to improve the simulations in this respect. 3.3 Vortex Interactions It was observed in several experiments that the vortex breakdown location over stationary delta wings is not steady and exhibits fluctuations along the axis of the vortices. Subsequently it was discovered that these oscillations are in the form of an asymmetric motion of breakdown locations for left and right vortices [15]. This is demonstrated by plotting the difference and average of left and right breakdowns in Figure 5. The two breakdowns, which are almost mirror images, oscillate in an asymmetric motion. The amplitude of these fluctuations can be a significant fraction of the chord length. These oscillations may be very important for the stability and control of highly manoeuvrable aircraft, and also have important consequences for wing and tail buffeting. It was also reported [15] that the oscillations of breakdown locations are quasi-periodic. Both flow visualization and pressure measurements at high Reynolds numbers confirmed the existence of vortex interactions. The exact mechanism of this interaction and whether vortex breakdown is an essential part of it remains little understood due to the difficulties of temporal resolution using PIV or spatial resolution with LDA. It was found that the oscillations become larger and more coherent as the time-averaged breakdown locations get closer to each other when the angle of attack or sweep angle is increased. Asymmetric oscillations of breakdown location have been observed computationally with symmetric computational domains. Oscillations have been seen both with Euler simulations [2] and higher fidelity DES simulations [2] and potentially such simulations can provide a great deal of understanding of these interactions. For example, studying cases without vortex breakdown may highlight if breakdown plays an important role in vortex interactions. Careful examination of the apex region and the mid plane of the computational domain may also provide insight into where the interactions start
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