researching bbl with vectrino

Hi...

I'm presently researching the bbl in a flume channel with vectrino. The velocities look good, except in height of between 5 and 7cm above bottom of the channel. (no improves by changing sampling vol.&rate, power level, velocity range, water level and freestream velocity)

How can I solve that problem???

Attached in Jpeg format are screen-shots of the bbl-profile&korrelation and probechecks in height of 5, 6 and 7cm.

Matthias

Hi Matthias,

What type of Vectrino head do you have (the standard one, right angle, or field)? From the probe checks you attached it looks like the wall is intruding on your sample volume at the lowest elevations and you are using a field probe.

Measuring near the wall is not the easiest thing to do with an instrument like the Vectrino unfortunately. I would suggest ensuring you have sufficient scatterers in the water first, your SNR is a little low when the sample volume is finally out of the wall, about 10 dB. Then, as you approach the wall, you'll want to reduce the sample volume size and the transmit length to shrink the overall sample volume size. This will help you get the sample volume close to the wall.

Even with the above however, the near wall region experiences high sheer, which will generally lower correlations and result in less than ideal data.

Is there a reason you need to measure so close to the wall? Most boundary layer measurements can easily be made slightly further from the wall to capture the log region, with various scaling arguments used to describe the viscous sublayer. The log region should be very easy to measure with Vectrino.

P.J.

Thank you for your quick answer...

I use a field head and I don`t have to measure very close to the wall...My problem is the height of between 5 and 7cm above the bottom...

Why does the speed (u) go to zero in that layer???The probechecks show that the first and second peak overlap there (at range of 100mm)...

Why is the first peak at 100mm???Shouldn`t it be at 50mm???

Matthias

Hi Matthias,

The center of the sample volume on the Vectrino field probe is located 10 cm from the central transducer, not 5 cm as with the standard head. I think you are trying to take measurements with the sample volume located below the bed.

For you Vectrino, if you want to measure with the sample volume 5 cm from the wall, you will need the distance reading to be 15 cm as shown in the right hand status area of the main Vectrino software window or when looking at a probe check output.

This should make your measurements look much better, but I would still recommend adding some additional scatterers to the water based on the lowest probe check from your original post.

P.J.

oh, I didn`t know that the center of the sample volume is located 10 cm from the central transducer...That explains everything...

Thank you very very much!!!

Matthias