Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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3 2. Surface flow visualisation. Oil flow visualisation gives an indication of surface streamlines, but only in a time averaged sense. Tufts also give an indication of surface streamlines and can reveal flow separation and reattachment, but are limited with the response time in unsteady flows and can also be intrusive. 3. Off surface flow visualisation (smoke/dye). This can provide useful information on shear layer structures and vortex breakdown, but extra care should be taken in interpreting the streakline patterns in unsteady flows. 4. Multi-hole velocity probes. These can measure three-components of mean velocity, but are intrusive and can cause premature breakdown. 5. Hot-wire anemometry. This can provide unsteady velocity components but can be intrusive. 6. LDV and PIV. These are non-intrusive point and field measurements respectively of velocity vectors in a plane. Seeding of vortical flow near the axis becomes problematic with increasing speed in air flows. 2.2 CFD Techniques It has been well documented that CFD has developed at a rapid pace over the past 30 years. With developments in algorithms and computers it is possible to simulate complex flows on real aircraft using cheap computers. A recent NATO technical organisation (RTO) working group has examined the predictive capability for vortical flows on generic delta wing configurations (AVT 80) [2]. 1. Euler simulations can predict vortex breakdown and vortical interactions when a sharp leading edge is used, fixing the separation point. No secondary separation can be predicted since this is due to boundary layer separation having the effect of shifting the primary vortex closer to the wing leading edge. In addition the strength of the leading edge vortex is strongly dependent on the grid used. However, for sharp leading edges this level of modelling is useful for evaluating qualitative behaviour at a low cost. 2. Unsteady RANS simulations can give good prediction for the secondary separation although the prediction of primary separation and vortex formation for rounded leading edge wings has
4 not received much attention in the literature. A major problem with URANS is the prediction of the levels of turbulence in the vortex itself which can strongly influence the development of breakdown. Ad-hoc treatments [3] can be used to limit production in regions of high vorticity but the turbulence levels after breakdown are still too high, making the simulation of the helical instability questionable. 3. Detached Eddy Simulation (DES) [4] has been used to overcome this problem by simulating the large scale turbulence in the vortex by Large Eddy Simulation (LES). In the wing boundary layer, where the cost of LES would be prohibitive at realistic Reynolds numbers, the RANS model is used. Some promising results for the prediction of vortex breakdown have been published, indicating the promise of the approach. The disadvantage is that the simulations are more costly in terms of the finer grids needed in the vortex and the small time steps that are required. In addition, the DES gives no improvement over URANS in terms of the vortex formation from rounded leading edges and predicting the influence of transition. 4. Finally, LES and Direct Numerical Simulation (DNS) [5] have been used at low Reynolds numbers to indicate fundamental physics. The cost of these calculations is prohibitive at flight Reynolds numbers because of the grid and temporal resolution required. CFD predictions have progressed to the point where a current RTO working group (AVT-113) is evaluating the predictions of the flow on the F-16XL aircraft through comparison with in-flight measurements. There are clearly a number of useful tools in the CFD bag with varying cost and predictive capability. 3 Delta Wing Phenomena 3.1 Shear Layer Instabilities The separated shear layers on a delta wing roll up periodically into discrete vortical substructures as visualised by Gad-el-Hak and Blackwelder [6]. This phenomenon was attributed to a Kelvin- Helmholtz type instability of the shear layer. The origin of these structures has been the subject of controversy as several researchers [7] [8] revealed the existence of stationary small-scale vortices
Page 1 and 2: Integrating CFD and Experiment in A
Page 3 and 4: Experimentalist’s requirements fo
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Page 7 and 8: numbers significantly lower than th
Page 9 and 10: Heat flux measurements. Over the pa
Page 11 and 12: The measurements are made on select
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Page 15 and 16: 12. Mébarki, Y. and Mérienne, M.
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Page 19 and 20: Axisymmetric Regular Convergent Div
Page 21 and 22: Figure 5: Typical instantaneous PIV
Page 23 and 24: Figure 8: Streamwise Velocity Profi
Page 25 and 26: 1.25 CFD (SKW PRESTO) Experimental
Page 27 and 28: 5 Conclusions In this paper we have
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Page 35 and 36: 8 Computational simulations can con
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Page 41 and 42: 14 [8] Mitchell, A.M. and Molton, P
Page 43 and 44: 16 [31] Gursul, I., Proposed Mechan
Page 45 and 46: 18 Figure 3: Spectrum of unsteady f
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Figure 1. Natural Blockage Thrust R
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Figure 7. Post-Exit rake Total Span
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Figure 11. Comparison of Static Pre
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2.2 Data Aquired The Embedded Laser
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First, plots of the turbulent Reyno
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Particles seeding tube 3 m 1 m mode
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Figure 5: Influence of the pseudo t
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Figure 7: Turbulent Reynolds' numbe
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Figure 9: Turbulent Reynolds' numbe
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Figure 11: Vortex Shedding cycle fo
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Figure 13: Comparison with experime
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