Shallow water turbulence
Up to Velocimeters - profiling
I'm currently using the eddy covariance method to measure oxygen flux in estuarine systems and would like to research and understand more the characteristics of turbulence in these shallow tidal areas. When using the eddy covariance system we position the Velocimeter’s measurement zone typically 15 to 50 cm above the sediment or vegetation water interface. As we measure in tidal areas I would like to improve my understanding of what would be the limitations due to water depth of the eddy covariance method in shallow water. We have a couple of vectrinos that I plan to use to further this understanding. I was thinking I could set the vectrinos at set depths and measure through a tidal cycle. I could then from each vectrino, calculate a correlation of the velocity at V1 and V1-T1, then V1-T2 etc to look at the time needed to loose correlation at each depth, this would also give the eddy size. I have had a look for similar research but have not found anything specifically looking at this problem. Any suggestions on methodology and better ways to do this would be greatly appreciated.
Hi Geoffrey,
I'd take a look at a good turbulence book for this type of question. Personally, I prefer Tennekes and Lumley's A First Course in Turbulence and Davidson's Turbulence: An Introduction for Scientists and Engineers. You can also check Pope's Turbulence, I just find the other two much easier to comprehend as they rely more on physical reasoning. This will give you the basic method to look at auto and cross-correlation time scales and lengths from velocity data. You'll probably also want to take a look at something called the Batchelor Length scale and the theory surrounding it.
P.J.
Hello dear administrator.
I would like do one question, maybe it's ridiculous, I don't know, but I hope that you apologize me if you think it.
It's possible to obtain a kz difussivity with the AWAC 1000 hz in a reservoir with 15 depth meters?
I'm new with device and I think that it is the best way to know it. If you can give me some advice, I appreciate it so much.
Best Regards.
Román.
Hi Román,
It might be a little difficult using only an AWAC.
I'm assuming you're looking at estimating the diffusivity coefficient via something like
-<u'w'> = K_z * dU/dz
You will have trouble obtaining the <u'w'> (stress) term from the AWAC data. Using one of the common techniques for estimating these terms from ADCP data is difficult given the head geometry. Given you're measuring in a reservoir I would also expect the turbulence levels will be fairly low. The AWAC isn't the best option for measuring turbulence in this scenario.
It should have no problem resolving the mean velocity gradient, but won't have the best resolution since the cell size is typically 0.5-1 m.
P.J.
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