[sai_ram]

I am working on transient analysis of multiphase water entry body (3-Dimensional) using VOF, k-omega SST turbulence model, with velocity inlet and pressure outlet boundary conditions, overset mesh, dynamic mesh zone conditions, while running calculations for a time step size of 0.01, number of time steps 100, max iterations as 90 after completing max iterations my continuity residual gets a straight line from 15 iterations, rest of all velocities and k, omega residuals gets converged which i have taken convergence criterion as 1e-05. what could be the reason for the continuity residual being a straight line?

[CFDKareem]

Depending on the model the time step of .01 seconds is pretty big for a VOF model. I would check your CFL number. You should try to keep your CFL number under 1 by reducing the timestep. This should also help your solution converge.

A general rule of thumb is that it is faster to use a small timestep that converges in less iterations then a larger step. You should aim for each timestep to converge in 10-20 iterations.