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-   -   Calculate forces without hydrostatic pressure (https://www.cfd-online.com/Forums/fluent/68733-calculate-forces-without-hydrostatic-pressure.html)

 geir_oye September 30, 2009 09:40

Calculate forces without hydrostatic pressure

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?

 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.

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