Calculate forces without hydrostatic pressure

geir_oye · September 30, 2009, 10:40

Hi all.
I need to find forces affecting different appendages on a hull. My simulation is a 2 phase VOF with OpenChannels BC including gravity.

Fluent report the forces in two components: pressure and viscous forces.
I want to compare the hydrodynamic forces for different parts/wall on my hull. The pressure forces on different walls gives no sense since the effect of the hydrostatic pressure/gravity is included in the pressure force dump. This is because the appendages/walls are not solid surfaces and the hydrostatic pressure generates nonzeros forces on that surface.

How can I find the drag on specific walls without the effect of hydrostatic pressure/gravity?

Gratefull for all comments.

geir_oye · November 6, 2009, 05:36

Anyone having any idea?

On possible solution would be to hook of the VOF equation and do some iterations without the gravity included. I've tried this but FLUENT crashes most of the time.

srjp · November 9, 2009, 06:32

You can create a custom field (your own variable) in Fluent with the existing variables.
Go to DEFINE > CUSTOM FIELD FUNCTIONS

Here you can define the equation for your drag subtracting your static forces.

geir_oye · November 12, 2009, 09:17

Hello again.
In the custom field function then we can define or approximate the hydrostatic pressure. Approximate because this pressure is dependent on the free-surface level, which are varying along the hull.

If we have the hydrostatic pressure we can integrate this along projected surface of each appendages in each direction to find the hydrostatic forces.

Is it this approach you suggest?. Could you be more spesific?

srjp · November 12, 2009, 10:12

Why would the hydrostatic pressure be approximate? Wouldn't it be exact since it will account for the height of the free surface level?
You can then find the area weighted average of this function on each area of interest.

September 30, 2009, 10:40	Calculate forces without hydrostatic pressure	#1
geir_oye New Member Geir Age Oye Join Date: Mar 2009 Posts: 4 Rep Power: 17	Hi all. I need to find forces affecting different appendages on a hull. My simulation is a 2 phase VOF with OpenChannels BC including gravity. Fluent report the forces in two components: pressure and viscous forces. I want to compare the hydrodynamic forces for different parts/wall on my hull. The pressure forces on different walls gives no sense since the effect of the hydrostatic pressure/gravity is included in the pressure force dump. This is because the appendages/walls are not solid surfaces and the hydrostatic pressure generates nonzeros forces on that surface. How can I find the drag on specific walls without the effect of hydrostatic pressure/gravity? Gratefull for all comments.

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November 6, 2009, 05:36		#2
geir_oye New Member Geir Age Oye Join Date: Mar 2009 Posts: 4 Rep Power: 17	Anyone having any idea? On possible solution would be to hook of the VOF equation and do some iterations without the gravity included. I've tried this but FLUENT crashes most of the time.

November 9, 2009, 06:32		#3
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	You can create a custom field (your own variable) in Fluent with the existing variables. Go to DEFINE > CUSTOM FIELD FUNCTIONS Here you can define the equation for your drag subtracting your static forces.

November 12, 2009, 09:17		#4
geir_oye New Member Geir Age Oye Join Date: Mar 2009 Posts: 4 Rep Power: 17	Hello again. In the custom field function then we can define or approximate the hydrostatic pressure. Approximate because this pressure is dependent on the free-surface level, which are varying along the hull. If we have the hydrostatic pressure we can integrate this along projected surface of each appendages in each direction to find the hydrostatic forces. Is it this approach you suggest?. Could you be more spesific?

November 12, 2009, 10:12		#5
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	Why would the hydrostatic pressure be approximate? Wouldn't it be exact since it will account for the height of the free surface level? You can then find the area weighted average of this function on each area of interest.