Sampling volume location using Vectrino
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How do I know the exact distance of the sampling volume to the bottom using Vectrino? In your manual you say that the distance between the centre transducer and the sampling volume is approximately 5 cm; on the other hand he Vectrino gives me the distance from the centre transducer to the bottom. Since the first distance is approximated, how can I have the real distance of the sampling volume to the bottom?
This is very important for me since I have to calculate the wall shear stress based on the mean velocity gradient in the viscous sublayer (I can't apply the law of the wall because I have a recirculation)
Thanks.
Amy
I am also curious about the answer to this question. The manual says that the sampling diameter is 6mm, 0.05 m away from the probe. But the sampling height is "user selected" between 3-15mm. Assuming the sampling volume is a cylinder, what does the height mean? Total cylinder height? Is the measurement I get back an average over the sampling height?
The reason that I ask is that I am using law of the wall with two Vectrinos and one Vectrino II and need a precise height for both of my Vectrinos. Say I set the height to 3 mm and put the Vectrino touching the bed. Does that mean that my sampling volume is 1.5 mm higher and lower than the center of the probe head? Is the height the center of the head?
The probe head appears to be around 6-8 mm tall. What does it mean if I choose a height of 15mm and put the instrument on the bottom of the bed? Correlation does not appear to be 100, but wouldn't the instrument have bottom interference problems?
Clarification how to calculate a precise (to the mm) height would be useful, even if it's an average velocity measurement for the height.
Thanks,
Kealie
Hi Kealie,
There is a distance reported in the Vectrino header file during data export. This is the distance from the central transmitter to the boundary (same definition as for the Vectrino II distance measurement).
The sample volume height in the instrument configuration is the vertical (instrument z axis) extent of the sample volume, nominally centered at 50 mm from the central transmitter.
This means if the distance reported in the header was 75 mm (I think it's actually in m in the header), the sample volume would be centered at nominally 25 mm from the boundary. With the default sample volume size of 7 mm, the velocities measured would represent a weighted average over the region from 21.5-28.5.
If you'd like to know more precisely where the sample volume is centered, please see the forum thread: http://www.nortekusa.com/en/knowledge-center/forum/velocimeters/270676734
Note the correction is fractions of a mm typically, so I wouldn't worry too much about the last part personally.
P.J.
PJ,
I forgot to mention that I am using a side-looking Vectrino, so in my case the distance is actually in the y-direction. It's reporting a distance of 36 cm, which might be close, but since I start my measurements in air during my flash flood experiments and it seems to be calculated in the beginning of measurement, I'm not inclined to believe it. However, given that I'm using a side-looking Vectrino, this isn't what I'm looking for anyway. Is there any way to get the z-height for law of the wall? The easiest way for me to understand this might be a diagram of the sampling volume in front of a side-looking Vectrino, if you have it.
Kealie
Yes, the measurement is made at the start of data collection for the Vectrino. So, if you are starting data collection in air, the distance measurement won't be valid.
For the side looking head, everything else I mentioned should apply. The sample volume location is set by timing an speed of sound, so you should be able to determine the exact location relative to the central transmitter the same way. The sample volume height in this case is going to be set by the central transmitter diameter, which is the same for both heads I think, so the reported velocity is an average value over an approximately 6 mm vertical extent.
As for z positioning, for a side looking probe you'll need to develop a different way of determine the probe head's vertical location. None of the information available from the instrument can constrain this.
P.J.
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