Coherent structures identification.
I am using cross -Hot wire anemometer for characterizing the flow field of a free round jet. Is there any way to identify the coherent structures from the radial and axial velocity variation data?
This type of analysis is typically done with a two-dimensional measurements system such as PIV instead of the point measurement from a hot-film but there are some methods you can look into.
Looking at the autocorrelation of your time signal would allow you to estimate an energy-weighted length scale which would give the approximate size of the dominant coherent structures.
If you are able to acquire a second signal at a fixed location (a free stream microphone for example) you could use this signal to conditionally average your hot-film signal. Then if you are able to move your hot-film about you could synchronize with the fixed point signal and possibly reconstruct the behavior using conditional or phase averaging.
The paper by Zaman, McKinzie, and Rumsey (J. Fluid Mech. 1989) perform some of these methods in an airfoil wake by making a frozen turbulence assumption. (See Fig. 16 and 17)
Hopefully this helps. If you have any more specifics on your experiment that might be helpful.
Perhaps you'd better use a two-point measurement strategy, like what Stuart suggested. Structure is always a 'spatial' term when we talking about it, so the usual way to identify a structure always involves multipoint-measurement. This would help you to determine the scale of structures and its geometrical shape.
You need to use simultaneous time-resolved measurements in two points. See, e.g. Harper-Bourne (1999) AIAA-99-1838 with hot-wires and Armstrong, Michalke, Fuchs AIAA Journal 15, 1011-1017, using microphones inside the flow.
The coherent structures are a consequence of developing instability waves in the jet shear layer. Shear layer instability is a function of Strouhal number, azimuthal wave number m, shear layer thickness in terms of nozzle radius, Mach number and temperature ratio as shown by Michalke 1971 (English Translation NASA TM 75190) The case with external flow (jet in flight) was treated by Michalke and Hermann, J . Fluid Mech. (1982), vol. 114, pp. 343-359.
When planning tests you should consider all parameters that influence the instability of the jet flow.
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