First Thank you for your response .
The problem is the following :
I have a 2D model , I want to do a FSI analsysi ( which can't be done in 2D in ansys ) so i extrude the geometry to a 1mm thickness and i have a done a one element through thickness mesh , and I have done the fsi analsys .
After that , I want to check if method is good , so i have make an extrude about 10 mm , then when i have run the analysis the results of stress and deformation was changed .
The stress in the wall caused by the fluid has different value when i changed the thickness .
So the result have differ .
If there is any other solution to do the 2d fsi ?

Thank you .