The mesh in that region is relatively coarse comparing with that around the inner tubes. The minimum orthogonal quality is 0.17, and there are 18 million cells in the whole 50 mm long domain. I do get back flow warnings all the time, but the back flow area are becoming smaller and smaller with time. There are reversed flow on 10.4% area at 0.603 s, 4.7% area at 1.2545 s, and 1.2% area at 1.2926204 s. The residuals reach the convergence stardard I set every time step before diverging, and I also tried decreasing the convergence standard. Does the reversed flow information look strange? Thanks.nnn