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Errors in simulation of water entry of cylinder or wrong physics when done.

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Old   April 22, 2021, 10:17
Default Errors in simulation of water entry of cylinder or wrong physics when done.
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Shubham Sharma
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I have been trying to simulate the water entry of cylinders problems on Ansys fluent. Every single time I either get the negative mesh error of the solution diverges due to courant number exceeding 250 in a single time step. The problem statement is a 2D circular cylinder with its center 1.25 m above the free surface is dropped at the velocity of 1 m/s in a 10 m x 8 m tank with an outlet on the top wall and the rest three as walls. Gravity in the Y direction is present. VOF model is used for 2 phases. I have tried with surface tension force interaction present between the two fluids (water and air, at 0.072 N/m) and without it too. Dynamic mesh with smoothing (spring-based with spring constant at 0.5) and remeshing (with mesh scale info pasted). For 6-DOF, I have tried both UDF and creating 6-DOF in the solution itself with mass varying from 1-1000 kg and the values of mass moment of inertia varying from 0.01-100 in both cases. The method is left at default. I have tried the simulation with both coarse and refine mesh, but I either get negative mesh error or the solution diverges with courant number exceeding 250. This too happening when the time step size is 1e-5. I also tried with variable time step size with a minimum of 1e-7 to a maximum of 1e-4, even then the solution diverged due to courant number exceeding and time step size reaching as low as 1e-8. Even if the simulation somehow got completed, the cylinder either bounced of the free surface (at lower mass) or the water sticks to the cylinder surface instead of creating a splash (at higher masses).
I have also tried using adaptive mesh (gradient, scaled by global maximum, cells more than 0.06) but the same problem as above
Any help would be great so please tell me what am I missing and/or where am I going wrong.
Thank you.
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dynamic mesh, fluent, two phase model, vof, water entry

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