Flow measurements close to the bed

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Flow measurements close to the bed

Posted by Franziska Staudt at April 14. 2015

Hi everyone, 

I used the Vectrino Profiler in a laboratory flume to measure the flow velocities above the sediment bed, and I'm currently trying to interpret my results. I would like to focus on the flow velocities right at the bed surface (+/- 5 mm), however, I'm unsure how reliable these measurements are. The correlation and SNR of the measurements in question are usually very high. I installed the instrument according to the Vectrino Profiler User Guide and I applied Goring & Nikora's method to despike the velocity data before any further data processing. 

I know that common ADVs may overestimate the flow velocities close to a boundary or sediment bed. I read some studies about the instrument (e.g. Chanson et al., 2008; Chanson, 2008; Zedel & Hay, 2010) and none of them mentions any major issues with near-bed flow data. The Vectrino Profiler brochure and the information on the Nortek website also suggest that it should be possible to use the instrument for flow measurements close to the bed. Are there any studies/experiences about the reliability of Vectrino Profiler flow measurements close to a boundary?

 

Thanks, 

Franziska

Re: Flow measurements close to the bed

Posted by Robert Craig at April 14. 2015

Hi Franziska:

    Unfortunately, the question "how close to the boundary can you measure" is not one that can be answered definitively.  The problem with measuring close to the boundary is that you end up with interference from the acoustic signal reflecting off the boundary.  This is determined by the overall dimensions of the acoustic pulse (which is typically 6mm in diameter), not just the length.  Our general rule of thumb is that measurements down to 5 mm should be OK.  Anything below that will be highly dependent upon the geometry and the composition of the boundary.  If you have a highly absorbent boundary (e.g. mud), you may be able to get reasonable measurements closer than that.  If you have a highly reflective boundary (e.g. metal), then your good measurement region may be farther away than 5mm.  High correlations / SNR aren't going to be a good indicator of the measurement quality since these will naturally be high as a result of the reflection (in fact, an increase in SNR / correlation as you approach the boundary is likely an indication that you're moving into the interference zone).

   Hopefully someone with a bit more operational experience will chime in here.  I know that some groups have looked at this but I don't have any references handy with results.

     Robert Craig

Re: Flow measurements close to the bed

Posted by Franziska Staudt at April 20. 2015

Thanks for your reply, Robert!

I will continue looking for more information about this issue and also keep an eye on new publications.

 

Franziska

Re: Flow measurements close to the bed

Posted by Kaan Koca at February 07. 2017

Hi Franziska,

This is a late response, but I hope this can still be helpful. We have just published a paper on the performance of the Vectrino Profiler in comparison to particle image velocimetry close to the bed. You can access the paper from http://www.tandfonline.com/eprint/IYesmh57qFARYhnYuunI/full. As Robert Craig also pointed out above, mean velocities are fine down to 5 mm, which is dependent on the bed material (please see Figure 4), and the vertical extent of the interference region can be identified by the local minimum of the mean SNR profile. However, what we observed is that, one should be careful with turbulent quantities. We found out that turbulence characterization will be most reliable within 3 mm around the sweet-spot of the profiler, which was corresponding to bin 12 in our case.

Kaan

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