# Pressure for wave height in an Water Wave Tank

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 July 3, 2012, 14:01 Pressure for wave height in an Water Wave Tank #1 Member   Join Date: Oct 2011 Posts: 32 Rep Power: 14 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

 July 3, 2012, 19:16 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,750 Rep Power: 143 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.

 July 4, 2012, 05:05 #3 Member   Join Date: Oct 2011 Posts: 32 Rep Power: 14 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?

 July 4, 2012, 07:34 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,750 Rep Power: 143 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.

 July 10, 2012, 06:48 #5 Member   Join Date: Oct 2011 Posts: 32 Rep Power: 14 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? Last edited by Cluain; July 11, 2012 at 09:45.

 July 11, 2012, 09:50 #6 Member   Join Date: Oct 2011 Posts: 32 Rep Power: 14 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?

 December 6, 2021, 01:18 Unsteady dynamic pressure #7 New Member   suresh Join Date: Dec 2021 Posts: 11 Rep Power: 4 Hi Cluian. May I know if u could solve the problem of finding the wave height as per the query floated in 2011..?

 December 6, 2021, 03:24 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,750 Rep Power: 143 Normally the best way to get wave height is to define a thin surface which goes from deep underwater to above the water, and then calculate the wet area with something like areaInt(Water.vf)@surface. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

December 6, 2021, 03:58
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suresh
Join Date: Dec 2021
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Quote:
 Originally Posted by ghorrocks Normally the best way to get wave height is to define a thin surface which goes from deep underwater to above the water, and then calculate the wet area with something like areaInt(Water.vf)@surface.
Hi ghorrocks, my actual query ,is
i have modelled a simple water wave in a tank.
I need to know the hydrodynamic pressures in the wave field.
But i find that the dynamic pressures in fluent only gives
=0.5*rho*v^2

But this dynamic pressure as i understand is not the complete,
For instance, from linear wave theory, p=rho*d(phi)/dt= rho*omega*phii (amplitude). so this unsteady term is not accounted in fluent

So, I would like to know how i may retrieve this unsteady pressure ??

Last edited by dskvsp; December 6, 2021 at 05:20.

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