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cfxmar May 27, 2010 04:44

Good morning from Spain.

Im 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 isnt obvious to calculate acceleration???

Kind regards

ghorrocks May 27, 2010 18:59

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.

cfxmar May 28, 2010 07:44

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.

ghorrocks May 28, 2010 07:53

From a quick read of (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.

cfxmar June 2, 2010 06:45

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 doesnt support that sort of expression, Im 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

ghorrocks June 2, 2010 08:39

If you want an averaged acceleration as I describe above this can be done with a monitor point and some easy post-processing 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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