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muhsin June 25, 2012 05:19

DFBI residuals and Initial conditions!
Hi All
I am working with VOF waves and dfbi with waves in +ve Z direction and rotation of body in X direction only.
I have chosen proper time step ( implicit unsteady) and number of iterations.
1. Residuals after every timestep seems to be jumping too high especially X and Y momentum and that of Water. Is there anything that can be done?
2. Reverse flow at pressure outlet though my pressure outlet is far off.
3. Do I need to calculate turbulent properties for initialisation or use default values?
I am using Overset mesh for this purpose...


abdul099 July 6, 2012 18:40

What does "residuals jump TOO HIGH after every time step" mean? What is too high?
It is nothing special that residuals are jumping up after a time step. And it's no issue as long as they drop in the next time step.

Usually reversed flow doesn't harm. You've got a movement of your 6DOF body, pushing fluid out of your domain. Therefore some fluid needs to re-enter the domain. And fluid entering the domain through a pressure OUTLET is ending in the "reversed flow" message. That's all.

muhsin July 6, 2012 21:29

Thanks for your reply sir,
Jumping to to the values of say 5-10. my Sdr and Tke residuals converge well near or below .001, rest all fine.
But when i change time step discretisation to second order my Sdr and Tke residual are too high say 100-500 and dont converge to next time step, but the run goes on.
Also, when i change Turbulent specification to K+omega, free surface distorts and solution diverges soon after that.

abdul099 July 14, 2012 03:45

Don't worry about the level of the residuals since they are normalized in ccm+. It's just also nothing special that the residuals jump up every time step, it's just important that they drop again during the time step.
For your second issue, I don't have an idea without having more information about your setup. But I would check the usual suspects: Time step, mesh resolution and of course, did you specify proper values for k and omega...

muhsin July 14, 2012 05:37

thanks! 1. so i need not to worry about residuals if they converge well every timestep.
2. i can send you my .sim file if u say... time step is .01s for a max velocity of .3m/s and min cell thickess of .005m.
i used both default values and calculated values of K and omega for my set up.
for calculation i used the formulation from fluent manual.

abdul099 July 16, 2012 17:09

The k-omega model is very sensitive to your turbulence specification. I would enter the values for turbulence intensity and turbulent viscosity ratio. Then Star-CCM+ will calculate proper values for k and omega.
Anyway, when the default values don't work, I would check for a proper mesh resolution. Maybe you can post a picture of a section through your domain...

muhsin July 17, 2012 00:05

https://mail-attachment.googleuserco...hPLd4&sadssc=1https://mail-attachment.googleuserco...dZzMs&sadssc=1I am posting picture of my domain. origin is at the base of the body. domain extends from -6m to 20m.

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