receiver arm distance to boundary

Hi,

I'm currently using a side-looking probe to measure velocity past a cylinder. Ideally I would like to be measuring as close to the intersection of the (12 cm diameter) cylinder and porous bed as possible. I'm considering purchasing a down-looking probe and have found plenty of info on advantages/disadvantages between the two configurations. I like that I can get a little closer to the bed with the down looking probe but I'm afraid I will be moved much further away from the cylinder. I realize there are limitations to how close the sample volume can be to this boundary but what about the receiver arm? Is there a practical distance this should remain from the boundary?

Thanks,

Tim

Hi Tim,

if I understand your question correctly, I think you can get pretty close to the cylinder with the down looking probe before you will see much interference. The best way to gauge this would be to start a probe check with the head far enough away so everything looks good and move it closer gradually and watch for changes. Your limiting factor might be more physical constraints of instrument mounting rather than the instrument performance. If you do get it really close though, there might be a bit of interference because of the beam spread as it moves through the water, especially if your water depth is fairly large.

In terms of getting closer to the bottom, I think the side looking probe is probably going to be better for this. Typically, okay measurements can be made with a down-looking probe between 0.5-1 cm from the bottom. Above this things are good, below this and high shear, strong echos, etc. tend to reduce data quality significantly. The side looking probe negates a lot of these problems by moving the transmit axis parallel to the bottom.

P.J.

Thanks P.J. that was very helpful.