Horizontal Deployment of Vertical Vector Mid-Column
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Hello, All-
I'm planning a deployment for later this month, and hoping to use a Vector mid- water column to get high resolution velocities. I've been searching around to see if anyone else has done this, but it doesn't seem to be extremely popular (understandably).
I'm going to attach the Vector onto a mooring line; unfortunately, any non-rigid mounting system will result in the line going through the sampling volume and very likely contaminating the data (even if it's not in the volume itself, the wake of the line will certainly come into play).
As you may guess from this description, the Vectors are vertical-mount. I am toying with the idea of mounting them horizontally, instead. From the forums and handbooks, it seems this is a viable option as long as XYZ coordinates are used. However, as these will not be rigidly mounted, they will likely swing around in the horizontal plane as currents move past, so keeping track of that is key. My question is thus: how completely useless is the internal pitch/roll/heading when mounted with the incorrect alignment?
And has anyone tried this type of mid-column setup before?
Thanks!
Hi there,
We actually have an integrated IMU for the Vector now. It's being used primarily by groups looking at marine hydrokinetic power where even rigid frames will vibrate becaue of the high velocities. Please contact your local sales representative for a quote if this is something you're interested in. I think we are also hoping to have a couple of presentations on this in the coming months so there should be more technical information available then.
On to your actual questions. I believe we have a mooring line vane available to allow vertical mounting of a Vector or Aquadopp so that it is always in front of the mooring line. So, you should miss the wake but will probably still see some flow disturbance depending on the flow strength, etc. A horizontal mounting would probably negate the flow obstruction further since the sampling volume is now further away. Unfortunately, Vectors ship with either horizontally or vertically mounted attitude sensors, meaning if you Vector is designed to be mounted with the pressure canister mounted vertically, then when its mounted horizontally the heading, pitch and roll will be useless. So, to do this, you will need an external measurement of the attitude of the Vector to make any sense of the XYZ velocities.
I unfortunately don't read the open ocean turbulence literature to see if anyone has done measurements like this off a mooring line, but I do know we have several customers working from moving platforms with the Vector. Take a look at Jim Thomson's work at UW-APL with the SWIFT float. He's making pretty nice measurement under breaking waves with a wave following buoy. We've also made measurements with an HR Profiler off of several different moored profilers, and honestly the considerations are pretty similar there.
P.J.
We have been using a fin as you can see in the picture. The clamps have been purchased from Nortek, while other hardwareHello, All-
I'm planning a deployment for later this month, and hoping to use a Vector mid- water column to get high resolution velocities. I've been searching around to see if anyone else has done this, but it doesn't seem to be extremely popular (understandably).
I'm going to attach the Vector onto a mooring line; unfortunately, any non-rigid mounting system will result in the line going through the sampling volume and very likely contaminating the data (even if it's not in the volume itself, the wake of the line will certainly come into play).
As you may guess from this description, the Vectors are vertical-mount. I am toying with the idea of mounting them horizontally, instead. From the forums and handbooks, it seems this is a viable option as long as XYZ coordinates are used. However, as these will not be rigidly mounted, they will likely swing around in the horizontal plane as currents move past, so keeping track of that is key. My question is thus: how completely useless is the internal pitch/roll/heading when mounted with the incorrect alignment?
And has anyone tried this type of mid-column setup before?
Thanks!
Many thanks to both of you, Ayal and PJ.
We're on a very tight budget and time constraint, so unfortunately the fin idea we'll have to figure out on our own. It looks feasible given the design Ayal posted (thanks for that).
Ayal, it looks like your mooring cable went between the prongs of the Vector head. Was this the case? If so, did you find any interference from this setup? Were you able to remove it? Hypothetically, the beams shouldn't care that there's an object nearby the prongs, if it's not interfering with the flow- but it seems scatter off the cable might well interfere with measurements. That's just intuition, though.
PJ, do you know if there's any determined maximum reliable measurement angle for the pitch/roll/heading on a vertically datumed Vector? Another option would be to angle the Vector on the mooring line, and offset whatever pitch/roll/heading is measured by the instrument with that set mooring angle. Of course, this setup angle would need to be within the reliable threshold. This would also provide offset from the line, and if attached correctly, could swing the instrument upstream (since the weight is not distributed evenly along the length of the shaft).
Max readable tilt is 25º-30º for the Vector's attitude sensor. I'm not positive on the maximum tilt for reasonable heading information, but the compass should be tilt compensated. You'll get the best results with a level instrument, but what you're proposing would work.
Generally, we're more worried about tilts with separated beams because the range cells are at different levels. For the Vector that's not an issue. I think you should be able to set the Vector up at your expected tilt and perform a calibration to get an error estimate on the heading. Not my area of expertise so I'll see if someone else can chime in who's thought about the compass more…
P.J.
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