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May 27, 2010, 04:44 
Acceleration

#1 
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Good morning from Spain.
I´m simulating a wave tank with a pile inside it and trying to find how to obtain the acceleration of the fluid over the pile. As there is the option to obtain the fluid velocity through an expression: "Water.velocity u"... Why isn´t obvious to calculate acceleration??? Kind regards 

May 27, 2010, 18:59 

#2 
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Glenn Horrocks
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The acceleration of the fluid is not used in the solver so is not calculated. It will be tricky to get it, time derivatives are not easy to calculate with CFX.
Why do you want to see the acceleration of the fluid? Is it not a variable of interest most of the time. 

May 28, 2010, 07:44 

#3 
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Thank you, Glenn.
I want velocity and acceleration components in the x direction near the pile (over the pile are zero) to introduce these values in the Morison equation. Like this I can calculate coefficients cd and cm as I can also obtain the total force calculated with CFX. 

May 28, 2010, 07:53 

#4 
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Glenn Horrocks
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From a quick read of http://en.wikipedia.org/wiki/Morison_equation (I had no idea what the Morison Equation was) it looks like it is an empirical formula which requires a representative velocity and acceleration for the fluid moving past the body. To calculate this you need to do something like work out the volume averaged velocity in a box around the pile and do a time derivative to get the acceleration.
I don't think velocity and acceleration of the flow field is what you are looking for  please correct me if I am wrong. 

June 2, 2010, 06:45 

#5 
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Thank you glenn.
I need it, I have calculated Water.Velocity u and I need the time derivative of it as the acceleration. As CFX doesn´t support that sort of expression, I´m thinking about what to do. Something like = Water.Velocity u in the instant "t + time step" Water.Velocity u in the instant "t" because Water.Velocity u. Gradient is a spacial derivative. Thanks again 

June 2, 2010, 08:39 

#6 
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Glenn Horrocks
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If you want an averaged acceleration as I describe above this can be done with a monitor point and some easy postprocessing of the monitor point output. From what I have seen of the Morison equation this is what you want.
If you want the acceleration vector field (the time derivative of the velocity field) then you will need user fortran I think. But I don't think this is what the Morison equation requires so you don't need to do this. 

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