Beam Velocity
Up to Velocimeters - single point
Dear Nortek Team,
I'm working with a Vectrino to measure the velocity field of modell propellers inside a water channel.
I would like understand the following statement of the Vectrino Guide (Oct. 2004, Rev. C, Chapter 3, P. 18):
"since the receive beams are slanted at 30°, all receivers measure the velocity that is slanted about 15° from the transmit beam".
I always thought that the receivers measure the velocity components normal to them, i.e. slanted at 30° from the transmit beam. Why is it, that the velocity component slanted at 15° is actually measured??
Is there maybe a Transformation Matrix like the one found in Reference N4000-702 (Eq. 1, Page 345) but for the Vectrino, so I could understand the angles that are ment? I don't mean the Transformation Matrix for the probe. I already got that one from the *.hdr File; I'm more interested in the angles. As I said, I would like to get some insight into the physical reasons, as for why velocity component at 15° and not at 30° is measured.
Kind regards from Erlangen, Germany
David Botello-Payró
IFor mono-static systems such as the Nortek Aquadopp, it is true relative motion along the transducer's normal vector is measured. For a bi-static system like the Nortek Vectrino, the transmitter and receiver are physically separated. This results in motion along two axes, corresponding to each transducer's normal vector, contributing to the motion, with the result that motion along the angle bisector is what is measured.
Equation 1 from the reference you mention is the transformation matrix from the header file. It's modified to account for the fourth receiver arm on the Vectrino. Derivation is covered in several references (search for "coordinate transform" on the forums) to see how these equations are derived.
Dear Peter,
Thank you very much for the prompt answer to my question. Before posting on the forum, I did go through the threads on “coordinate transformation” but wasn’t really successful in finding the information I was looking for.
I do understand that in the case of bi-static systems, receiver and transmitter are physically separated. Unfortunately it still escapes my mind, why would this fact have an influence on the detection direction of the single receivers.
I’ve attached a sketch (.pdf) of how I thought that particle velocity is being decomposed into velocity components along the detection lines of Receivers 1 and 3. But with this sketch, I can’t really explain why velocity measurement along the angle bisector takes place. Maybe you can help me out with this one. If you’re aware of any reference that explains this, I would be very grateful to learn about it.
At the moment, I could only explain the measurement along the angle bisector, if the sensors inside of the Vectrino’s arms (receivers) had an additional tilt, which can’t be seen from the outside.
I’m really looking forward to further comments.
Best regards,
David
Hi David,
Please see the following reference. It discusses the differences between mono-static and bi-static system operation.
Zedel, L. (2002). A three-component bistatic coherent Doppler velocity profiler: error sensitivity and system accuracy. Oceanic Engineering.
As you will find, bi-static systems generate quite a few unusual operational conditions.
P.J.
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