Variable timestep size for coupled dynamic mesh problem

Coupled dynamic mesh problem adjusts the motion of the moving body/surfaces
based on the computed aerodynamic load and applied external forces at each

Thus, to specify an optimum timestep size that will apply generally for the
duration of the coupled motion is difficult since the aerodynamci load is
not known apriori.

Continual and manual adjustment of the timestep size is required to avoid
using an excessively conservative value of timestep size which will increase
the running time.

How to define an automated adjustment of timestep size for dynamic mesh
application ?

A process to do automated and calculated update of the timestep size is given
below. The idea is to allow the body/surface to translate a certain
translational distance that is specified by the user.

User will need to specify the followings:

- The average cell height (h_avg) of the mesh elements through which
the body is expected to pass through
- The number of steps for one cell passing (Nstep)
- Maximum allowable timestep size (dt_max)

The process will then:
- Compute the maximum allowable translational distance at each timestep
h_max = h_avg/Nstep
- Knowing the current translational velocity (V_n), compute the next
velocity and timestep size based on a quadratic equation obtained
from solving the following two relations:

(1) h_max = V_n+1 * dt
(2) (force/body_mass) = ( V_n+1 - V_n )/dt

The above two relations represent two equations in two unknowns (dt
and V_n+1).
- If the calculated dt is bigger than the maximum allowed by user (dt_max),
then clipped the timestep size an use (2) to compute V_n+1.

A 2D sample problem which uses the above process is provided. It is a 2D
coupled ball diverter case. How to run the sample problem ?

- Go inside the Presentation directory and study the presentation
- Study the valve-motion.c
- Start 2D version of Fluent
- Compile valve-motion.c to get updated library
- Read the case/data files (start-t=0.cas.gz and start-t=0.dat.gz)
- Iterate for 100 or so timesteps and observe from the cortex output
on how the timestep size is changing

The files for this sample problem can be downloaded from:
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This process has also been applied to the 6-DOF UDF. If interested, please contact

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