Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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a – surface flow on the central body of an aerospike nozzle d - EBF visualisation of the flow past a double code model at Mach 10 Fig. 6: Different techniques of flow visualisation Wall measurements b – laser sheet visualisation of the vortices past an elongated body c – interferogram of a transonic shock wave Pressure Sensitive Paint. This method (known as PSP) which allows to determine the complete pressure distribution over a model, is based on the fact that some compounds emit light (luminescence) when excited by a suitable source, the emitted light having a longer wave-length than the excitation light 10,11 . The quantity of light emitted depends on the oxygen diffused into the paint because oxygen quenches and deactivates the excited molecules. The internal concentration of oxygen being a linear function of the external pressure of the same gas, one can measure the pressure acting on the paint by detecting some of its luminescent parameters. A difficulty with PSP’s is their simultaneous response to change in temperature, which could restricts their use in hypersonic flows. The difficulty can be circumvented either by using a paint nearly insensitive to temperature or by making a correction from measurement of the wall temperature. Convincing PSP measurements have been performed at high Mach number on a plug nozzle with a PSP component having a low sensitivity to temperature 12 (see Fig. 7). Skin-friction measurement. In a recent technique, the skin-friction is determined by measuring the rate of deformation of a thin oil film deposited on the model surface 13-15 . If the film of oil is thin compared to its length, its surface takes the shape of a small wedge whose thickness y at any time t can be accurately measured by an interferometric technique (see Fig. 8). Knowing the location x of the measurement point and the time t, the determination of the wall shear stress is in principle straightforward. 6
Heat flux measurements. Over the past 20 years, quantitative infrared thermography has experienced a strong development in a large number of laboratories 16-18 . As it is well known, a body emits a radiative signal whose intensity is a strong function of its temperature. In infrared thermography, the model is observed by an infrared camera containing a detector element sensitive to infrared radiations at a certain wave-length. By processing a series of pictures taken at known time intervals, it is possible to construct the time history of the model temperature and to deduce the surface heat flux distribution by means of the heat equation. The example of infrared image (in false colour) in Fig. 9 shows the heat flux distribution on a hemisphere in a Mach 5 flow. Infrared pictures also provide a convenient way to detect laminar-turbulent transition. Fig. 7: PSP measurements on a plug nozzle central body t = 0 m n 2 m n 4 m n Fig. 9: Infrared measurements of the heat flux distribution over an hemisphere in a Mach 5 flow. Field measurements 6 m n Measurement of field quantities such as velocity, temperature, density, gas species concentration,etc is a difficult task. However, the advent of laser sources in the early 60s has given a dramatic impetus to the development of non intrusive methods allowing an insitu determination of the properties of a gas, including its velocity Laser Doppler Velocimetry Fig. 8 : Skin friction measurement by the thin oil film technique The basic idea underlying LDV, which is now a well known technique, is to measure the velocity of tiny particles transported by the flow 19-21 . If these particles are small enough, their velocity is assumed to be that 7
Page 1 and 2: Integrating CFD and Experiment in A
Page 3 and 4: Experimentalist’s requirements fo
Page 5 and 6: persistent discrepancy may be due t
Page 7: numbers significantly lower than th
Page 11 and 12: The measurements are made on select
Page 13 and 14: advantage that the signal is emitte
Page 15 and 16: 12. Mébarki, Y. and Mérienne, M.
Page 17 and 18: castellated nozzles entrained more
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
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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
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Page 47 and 48: 20 Figure 7: Flow visualisation of
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Cs comparison at V=0.6 m/s Fig. 10.
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Investigation of Flow Turning in a
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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 11: Vortex Shedding cycle fo
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Figure 13: Comparison with experime
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