Vectrino+ in Lab

Hi;

I'm using Vectrino+ in a submerged hydraulic jump in a lab flume.

The correlation are as low as 40% or even less. But if the Vectrino is

moved downstream of the jump were the velocities and turbulence are

lower, the correlations rise for a point at the same height. 

How can I overcome the problem of low correlations in high velocity

(high turbulence) flow?

Velocities are around 2 m/s. 

Thanks

Measuring in a hydraulic jump is going to be very difficult for any instrument because of the entrained air in the jump. This will affect the instrument's ability to make good quality measurements because of the presence of bubbles, etc. You can try to increase the velocity range to the maximum to allow for the high turbulence in the jump and might see some improvement in the data quality metrics, but I would not expect much improvement.

P.J.

In my case the jump is submerged and has no air entrained. However, the correlations are still low.

I tried to change the velocity range and sampling volume size but it didn't help.

Ali

Ali,

This will be a difficult measurement to make unfortunately. I imagine the turbulence and shear are similar to what is seen in a turbulent round jet near the orifice, a notoriously hard flow to measure, particularly with a velocimeter like the Vectrino. The shear is the main problem here.

You have done what I would recommend, namely increase the Nominal Velocity Range and decrease the Sample Volume size. Make sure you don't use the smallest Sample Volume size as it has much higher noise for a given velocity range. Try altering only one parameter at a time as well to keep things simple.

Do you have some expectation of what your measurements should be? You could try adjusting the velocity range and sample volume size to get the best data quality indicators you can and then take a dataset. It might work out despite the low correlations your data is still valid and will provide a good statistical description of the flow. I would pay particular attention to the velocity histograms and develop a good outlier removal scheme to remove the really bad data.

Do you have plots of the data you can post?

P.J.

P.J.;

Please find attached the plots of velocities and histograms generated by ExploreV.

I checked changing configurations, what I experienced is that if everything is identical,

Vectrino gives good quality data only if the velocities (and turbulence) are low! When I move it 

downstream of the jump where the only difference is the lower velocity, the data are ok. 

Ali

Ali,

The Vectrino can easily measure several m/s mean velocities and high turbulence, so it's not the magnitude of mean flow that's causing problems. Rather, it's the intense shearing (caused by very high turbulence) I mentioned before. Vectrinos have been used quite successfully in grid turbulence tanks, under breaking waves and many other highly turbulent flows. It will take some tinkering to find the optimal setup and you will probably have to settle for slightly lower correlation values than typically recommended, but you should be able to get some good data.

Based on the velocity traces and histograms you posted your data doesn't look totally unusable. Play around with the "Editing Time Series" menu items, paying particular attention to the Velocity Threshold filter and the spike filter. These will reduce the variance by removing the outliers from the data set. I would not implement a correlation filter except to remove really low values, an initial limit there might be 20%.

Take a look at how each of these affect the velocity histogram and you should be able to clean this data up to make is usable.

P.J.

Hi Ali,

A colleague just reminded me to also make sure you are sampling fast enough to measure the process of interest, in this case turbulence. Have you examined the velocity spectra for a -5/3 slope indicating the inertial subrange?

Since you can sample at 200 Hz with the + firmware, I would start there and once you understand a little about where the noise floor of the instrument is for this flow and what the time scales of interest are, you can adjust the sample rate appropriately to meet the Nyquist criteria.

P.J.