# Unsteady hydrodynamic pressures water wave

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 December 6, 2021, 05:31 Unsteady hydrodynamic pressures water wave #1 New Member   suresh Join Date: Dec 2021 Posts: 11 Rep Power: 3 Hi, i am working with numerical wave tank modelling. 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

 December 6, 2021, 09:48 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,149 Rep Power: 61 99% of the time, dynamic pressure in fluid dynamics does indeed refer to the the half rho U^2 thingy for incompressible flows and the corresponding difference between static and total pressure for compressible flows. If you are looking for the time derivative of pressure, don't look for dynamic pressure and definitely don't call it dynamic pressure or you will confuse the 99%. Off the top of my head, I can't remember if there is an expert trigger that enables retrieving the time derivative of pressure easily. I don't think this variable is ever calculated so it would make sense that it doesn't even exist. You have anyway static pressure at time-levels n and n-1 in your simulation. You could define custom field functions using these to calculate the temporal derivative but you have to take care of the discretization manually.

December 6, 2021, 10:16
#3
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suresh
Join Date: Dec 2021
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Quote:
 Originally Posted by LuckyTran 99% of the time, dynamic pressure in fluid dynamics does indeed refer to the the half rho U^2 thingy for incompressible flows and the corresponding difference between static and total pressure for compressible flows. If you are looking for the time derivative of pressure, don't look for dynamic pressure and definitely don't call it dynamic pressure or you will confuse the 99%. Off the top of my head, I can't remember if there is an expert trigger that enables retrieving the time derivative of pressure easily. I don't think this variable is ever calculated so it would make sense that it doesn't even exist. You have anyway static pressure at time-levels n and n-1 in your simulation. You could define custom field functions using these to calculate the temporal derivative but you have to take care of the discretization manually.
Thank u tran, for the explanation.
But, linear (Airy) wave theory completely ignores half rho U^2, due to it being non linear, still we have hydrodynamic pressure which turns out to be :
p= rho *g*wave height/ 2* (cosh(k(h+y) /cosh(kh) )
This infact is the static head on the surface due wave height which decays exponentially with waterdepth.
This is significant for long waves.
So, I am unable to get convinced that it doesn't exist..
Can u pls once again look into.

 December 6, 2021, 13:03 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,149 Rep Power: 61 I need you to level with me here for a second. The two meanings of dynamic pressure are completely different. Static pressure is the mechanical pressure of the fluid at rest. If the fluid moves (i.e. if it flows), it has a mechanical pressure higher than the static pressure. The additional contribution due to the bulk motion we call dynamic pressure and for incompressible stuff is half rho U squared. This has nothing to do with wave theory. It's not that it's being ignored in wave theory due to being non-linear, it's a completely different creature. Wave motion is a different animal than bulk motion of the fluid. But still, don't argue with what the rest of the world refers to as dynamic pressure. It's not related to what you are looking for anyway. This part of the conversation is over. The other meaning is for any signal that varies in time, these signals are dynamic (as opposed to stationary). A dynamic pressure signal is (for the 1%) a pressure signal that changes in time. I know English can be complicated but that's what it is. You could also have a dynamic temperature and so on. Pressure itself doesn't have its own independent equation, leading to the well-known pressure-velocity coupling problem. The time derivative of pressure is also balanced by other terms in the momentum equation due to mechanical energy balance. That's why I think this term (the time derivative of pressure) doesn't exist in the solver. Obviously the time derivative of any differentiable function exists. So... just completely ignore dynamic pressure in Fluent. It's not what you want and go calculate the thing you want.

December 6, 2021, 13:44
#5
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suresh
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Quote:
 Originally Posted by LuckyTran I need you to level with me here for a second. The two meanings of dynamic pressure are completely different. Static pressure is the mechanical pressure of the fluid at rest. If the fluid moves (i.e. if it flows), it has a mechanical pressure higher than the static pressure. The additional contribution due to the bulk motion we call dynamic pressure and for incompressible stuff is half rho U squared. This has nothing to do with wave theory. It's not that it's being ignored in wave theory due to being non-linear, it's a completely different creature. Wave motion is a different animal than bulk motion of the fluid. But still, don't argue with what the rest of the world refers to as dynamic pressure. It's not related to what you are looking for anyway. This part of the conversation is over. The other meaning is for any signal that varies in time, these signals are dynamic (as opposed to stationary). A dynamic pressure signal is (for the 1%) a pressure signal that changes in time. I know English can be complicated but that's what it is. You could also have a dynamic temperature and so on. Pressure itself doesn't have its own independent equation, leading to the well-known pressure-velocity coupling problem. The time derivative of pressure is also balanced by other terms in the momentum equation due to mechanical energy balance. That's why I think this term (the time derivative of pressure) doesn't exist in the solver. Obviously the time derivative of any differentiable function exists. So... just completely ignore dynamic pressure in Fluent. It's not what you want and go calculate the thing you want.
Dear Tran thanks again, for the explanation on static pressure and insight into time variant pressure.

Suppose a floting body is close to a trough, hence experience s a negative pressure, equal to rho*g*waveheight/2
So how can this be retrieved in fluent?
It shows zero static pressure, with some negligible half*rho*v2.
But not rho*g*waveheight/2.
So what may I infer.

 December 6, 2021, 14:32 #6 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,149 Rep Power: 61 Okay now this question I can help you with. Fluent uses gauge pressures everywhere. When you plot pressure in Fluent, it (usually) does not contain the barotropic component (the rho*g*height) contribution at all. This is true whether you plot pressure, total pressure, and even absolute pressure. You must add the barotropic contribution to "pressure" in order to get what you need. This is well covered in the manual and even the topic of one of the earlier tutorials in Fluent. There are a few models where the gravity component is included (they all involve linearizations about the operating density). Btw, this is also the reason why the field is labeled "pressure" and not "static pressure" in Fluent. They knew when they coded this software that it is not always the static pressure. What you need to do is define a custom field function equal to pressure + rho*g*z-coordinate. Now, remember that pressure is still a gauge pressure so this is also still a gauge hydrostatic pressure. And then you still need to independently get the time-derivative of this field.

December 7, 2021, 04:26
#7
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suresh
Join Date: Dec 2021
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Quote:
 Originally Posted by LuckyTran Okay now this question I can help you with. Fluent uses gauge pressures everywhere. When you plot pressure in Fluent, it (usually) does not contain the barotropic component (the rho*g*height) contribution at all. This is true whether you plot pressure, total pressure, and even absolute pressure. You must add the barotropic contribution to "pressure" in order to get what you need. This is well covered in the manual and even the topic of one of the earlier tutorials in Fluent. There are a few models where the gravity component is included (they all involve linearizations about the operating density). Btw, this is also the reason why the field is labeled "pressure" and not "static pressure" in Fluent. They knew when they coded this software that it is not always the static pressure. What you need to do is define a custom field function equal to pressure + rho*g*z-coordinate. Now, remember that pressure is still a gauge pressure so this is also still a gauge hydrostatic pressure. And then you still need to independently get the time-derivative of this field.
Thanks Tran,
Thank u again and may I have some link for looking into the barotropic component?
And one thing I got to understand after applying some thought is this pressure component does not seem to depend on time.
The pressure on a trough experienced by pt absorber at trough is -ro*g*H/2, irrespective of speed of wave. Even a long slow wave and a quick short wave.
So , may I know if theory of barometric thing would solve me retrieving this?

 December 7, 2021, 09:34 #8 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,149 Rep Power: 61 Fluent tutorial 3 the mixing elbow shows you how to include the hydrostatic pressure in the pressure by manipulating the operating density. See the Fluent user's guide for Custom Field Functions if that's the route you want to take. There's even youtube videos. I don't follow what theory about gravity you would need in order to show that hydrostatic pressure is rho*g*h other than the theory about gravity.

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