Sudden pressure change in overset

Ben

Hi,

my original incompressible fractional step NS code is used for simulating moving bodies with c-grid.

after converting to overset grid, ie background cartesian + original c-grid, i found 1 problem.

in the cl,cd graph vs time, there are some spikes in the graphs. on investigation, it is found that the pressure on the surface changes quite a lot before and after the spike occurs e.g. -0.4 to 8. usually from 1 time step to another, the change is around 20 to 100%. Since this is an imcompressible code, only the difference in pressure matters. But this sudden change is still not normal.

i've also used tecpot to calculate the cl,cd and the same spike happens. the connection between c-grid and cartesian grid is done using bilinear or biquadratic interpolation.

i've found another strange thing too. if i fixed the time step to 1e-4 and the spike occurs at time=.522, when i change the time step to 0.9e-4, the spike will not occur at time=.522, but at another time.

the velocity vectors all seems normal and the viscous force calculation has no such problem. changing Re from 100 to 1000 to 10000 did not make a difference. moreover, if the body remains stationary, there's no such problem e.g. vortex shedding of a cylinder. the BC used is dp/dn=0 on surface, while at far field, p is fixed at 0 or dp/dn=0. this has not caused any problem in the past.

anyone has experience this before? i'm still wondering what's the cause....

thanks for reading.