moving cylinder

Dear all
my thesis is about moving cylinder in channel .
I supposed the acceleration of cylinder is constant and the mean velocity in channel is .02 LL/LT and the number of grid in channel height is 150.
but i have encountered whit a problem.
the velocity of cylinder increase after 40 or 50 iteration for example the velocity of cylinder become 200 LL/LT .
what do i do?

best wishes.

Dear all
I need a code about moving boundary in non-newtonian (power-law model) flow.
please help me
it is better that your source code written in MATLAB .
thanks in advance

my code is about moving boundary in non-newtonian flow
i have a problem when i want to move cylinder in duct.
i have supposed the cylinder is moving whit constant acceleration and after almost 30 iterate the acceleration increase too much.
please help me