Different result with 1st/2nd order implicit schem

CH

hello,

I tried to simulate vortex shedded for a cylinder. The mesh comes from fluent tutorial so there shouldn't be a problem.

The Re=185 and according to literature, vortex shedding 'll occur with cl having an amplitude of around 0.6. when i used 2nd order implicit PISO with time step=0.2, i got the correct answer. however, if 1st order implicit PISO is used, with time step=0.2, i got the vortex shedding but the cl amplitude stablises at around 0.2. with time step=0.1, amplitude is 0.4.

is it that for 1st order, the simulation must use a smaller time step to get correct ans? is there a way to estimate or is it better to use adpative time stepping?

thank you.