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-   -   Pressure for wave height in an Water Wave Tank (https://www.cfd-online.com/Forums/cfx/104129-pressure-wave-height-water-wave-tank.html)

 Cluain July 3, 2012 14:01

Pressure for wave height in an Water Wave Tank

I am modelling a water wave tank with a floating body
In the geometry I set up a plane perpendicular to the wave propagation, and in CFX-Pre I set up an expression to monitor the wave height by calculating the area of water on the plane – as is regularly explained on the forum here.
But the mesh motion required for the rigid body solver changes the length of the plane.

To solve this I could set up a plane parallel to the gravity vector in CFX-Pre as described in another thread, but I would rather not because It will take time to figure out when I could figure out how to measure wave height by measuring pressure. In addition pressure gauges are often used to measure ocean waves.

The formula for Pressure on the base of the tank in the deep-water condition is:

P = p*g*H*e^(2*pi*z/L)-p*g*z

Where P is pressure
p is density
H is wave height at a particular time
pi is 3.14
z is mean water depth measured from the free surface
g is acceleration due to gravity

If H is 0m, which it is at time = 0s then the formula simplifies to Hydrostatic pressure p*g*z

But the value for pressure I am reading on the plot monitors in the solver is not equal to that. I am recording both absolute pressure and Pressure, for different locations, the options do not seem to give any other pressure options.
I have looked at the documentation but could not discern exactly what was meant by absolute pressure

Seems simple, yet I have not seen it recommended. Is there something I am missing. I would really like to measure wave height this way.

What pressure measurement do I need to record?

Note: The mesh motion is ok according to the cfx-help desk

 ghorrocks July 3, 2012 19:16

Pressure in a buoyant simulation has the hydrostatic component removed. See the documentation for details. Absolute pressure contains both the hydrostatic and reference pressure components, so would be equivalent to the pressure measured by an absolute perssure gauge in an experiment.

 Cluain July 4, 2012 05:05

Cheers Ghorrocks ,
Absolute Pressure is the most relevant
If I update the formula above to include Atmospheric pressure, it works perfectly
P = p*g*H*e^(2*pi*z/L)-p*g*z - Patm

Currently the output is Pressure and I use an excel spreadsheet to calculate wave height.
The reason I do this is that when writing an expression for a monitor point it does not recognise another monitor point, is there any way around this?

 ghorrocks July 4, 2012 07:34

No, you cannot get one monitor point to refer to another. But you might be able to use the probe() function as a work-around.

 Cluain July 10, 2012 06:48

The Pressure output does not match the wave height output using a plane, for the simple case. it is probably a direct result of the physics I have set.

I am considering setting up a plane in CFX-Pre that will not move with mesh motion. I will then be able to calculate wave height based on the area of water on the plane.

So the question is, how do I set up a plane in CFX-Pre that will not move when the mesh moves?

 Cluain July 11, 2012 09:50

As the pressure formula is based on linear wave theory, discrepancies will appear in the relationship between wave height and pressure.
If I were to fix my plane so that it is not affected by mesh motion, how would I do that?

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