Re: Can I use AWAC data to calculate TKE

Hi Nortek AS,

Is it possible that the data from AWAC can be used to calculate the Reynolds stresses or TKE? 

When subtracting the mean from the raw, I will sure get some velocity fluctuations. However, I'm confused with the fluctuations below the velocity precision. Should I neglect them or I'm totally on the wrong track?  (I know that the velocity precision tells me if the velocity measured is 20cm/s, it is unlikely that the real velocity value is outside 20+/- the precision. But, when it comes to the velocity fluctuations, I have no idea how to relate it to the precision.) 

Thank you,

TC

Hi TC,

  Yes, you can use an AWAC to measure turbulence, but there are a lot of caveats to this.

You'll only be able to look at the radial (beam) velocities, not XYZ or ENU velocities to calculate turbulent statistics.

Because of the head geometry of the AWAC, the beam velocities will be predominantly measuring the vertical fluctuations. So, there won't be a way to estimate a Reynolds shear stress (what I'm assuming you mean by Reynolds stress). The only term (somewhat) directly measured is the vertical fluctuation. So, TKE would require assumptions on what the two horizontal terms should be in this quantity.

Also, note that in your attached configuration the average interval is 240 seconds. This will suppress variance in the current measurement cells, making only the wave cells useful for estimating turbulence.

The precision can be thought of as a standard deviation of the signal. Fluctuations less than the precision will simply be lost in the noise. To get a reasonable estimate of the noise for a given wave cell size, set the average interval to 1 Hz and the cell size to the wave cell size and look at the vertical velocity precision. I think the wave cells are sampled at 2 Hz so this estimate will be a little less than in practice however. 

P.J.

Previously P.J. Rusello wrote:

Hi TC,

  Yes, you can use an AWAC to measure turbulence, but there are a lot of caveats to this.

You'll only be able to look at the radial (beam) velocities, not XYZ or ENU velocities to calculate turbulent statistics.

Because of the head geometry of the AWAC, the beam velocities will be predominantly measuring the vertical fluctuations. So, there won't be a way to estimate a Reynolds shear stress (what I'm assuming you mean by Reynolds stress). The only term (somewhat) directly measured is the vertical fluctuation. So, TKE would require assumptions on what the two horizontal terms should be in this quantity.

Also, note that in your attached configuration the average interval is 240 seconds. This will suppress variance in the current measurement cells, making only the wave cells useful for estimating turbulence.

The precision can be thought of as a standard deviation of the signal. Fluctuations less than the precision will simply be lost in the noise. To get a reasonable estimate of the noise for a given wave cell size, set the average interval to 1 Hz and the cell size to the wave cell size and look at the vertical velocity precision. I think the wave cells are sampled at 2 Hz so this estimate will be a little less than in practice however. 

P.J.

how long it can last a AWAC taking data at the bottom of the sea?
wich awac is recommended to develop desmbocaduras work in rivers and the rivers?