The "Negative Cell Volume Detected" error in your multiphase CFD simulation likely stems from mesh distortions, particularly in complex regions involving moving boundaries. To resolve this, ensure high mesh quality by refining critical areas, maintaining low skewness, and smoothing mesh transitions. Look for some critical mesh quality metrics such as aspect ratio, and skewness. Don't forget to calculate your y+ for a simpler case.

To improve stability, use smaller time steps and adjust the Courant number.

The "Negative Cell Volume Detected" error in ANSYS Fluent is commonly encountered during multiphase flow simulations, especially when dealing with complex geometries or meshes. This error typically indicates issues with the mesh quality or numerical instability due to incorrect solver settings, leading to inverted or distorted cells. To resolve this error, consider the following approaches:1. Check and Improve Mesh Quality: Examine the Mesh: Use the Mesh Metrics in Fluent or the Meshing tool to check for skewness, aspect ratio, and orthogonal quality. Ideally, ensure that:Maximum skewness < 0.85 Minimum orthogonal quality > 0.1 Aspect ratio < 10 (for most cases) Fix High Skewness Cells: If there are cells with high skewness, use Local Mesh Refinement or Smoothing techniques to improve the mesh. Pay special attention to regions with complex geometry, sharp corners, or narrow gaps. Refine the Mesh Near Interfaces: For multiphase simulations, refine the mesh at phase boundaries or regions with high gradients to capture phase interactions more accurately. 2. Ensure Proper Mesh Connectivity: Check for any disconnected regions, hanging nodes, or unconnected faces in the mesh. If using interfaces (e.g., for connecting different mesh regions), make sure the interfaces are properly defined and connected. 3. Use Smoother Initialization: Choose a smoother initialization method like Hybrid Initialization instead of Standard Initialization to start the simulation with a more balanced initial condition. Alternatively, use Patch to set initial values close to expected results for velocity, pressure, and volume fraction in different regions. 4. Modify Solver and Discretization Settings: Time Step Size:Reduce the time step size significantly, especially if you are running a transient simulation. Start with a small value, and gradually increase it as the solution progresses. Under-Relaxation Factors:Lower the under-relaxation factors for pressure, momentum, and other parameters to improve stability. Pressure-Velocity Coupling:Use a robust scheme like Coupled or SIMPLE. If using the Coupled solver, ensure that the Courant number is kept low (e.g., < 1). Multiphase Model Settings:For Volume of Fluid (VOF) or Eulerian multiphase models, use the Geometric Reconstruction scheme for interface tracking. Enable Body Force Weighted for the pressure interpolation scheme if gravity or body forces are involved. 5. Adjust Boundary and Operating Conditions: Ensure boundary conditions are set correctly and are physically realistic. For instance:Avoid specifying extremely high velocities or pressures that might cause instability. For inlets, specify flow rates or velocities based on realistic physical scenarios. Set the Operating Pressure correctly, especially for compressible flow scenarios or if buoyancy effects are included. 6. Enable Mesh Smoothing or Remeshing: If using dynamic mesh or overset mesh, enable Mesh Smoothing or Local Remeshing options. This will help adjust mesh nodes in regions where high deformation or movement occurs. For multiphase flows involving phase change or high deformation, consider enabling the Remeshing option with suitable remeshing parameters. 7. Use a More Stable Solver Setup: For multiphase flows, especially when sharp phase boundaries are present, try switching to the Coupled Implicit Solver. If using the Pressure-Based solver, enable PISO for better pressure-velocity coupling, or switch to the Density-Based Solver if applicable to your flow conditions. 8. Check for Problematic Geometrical Features: Look for small features, sharp edges, or tight gaps in the geometry that might be causing poor mesh quality. Consider simplifying or refining these regions. Use Virtual Topology in ANSYS Meshing to combine small edges and faces that are not crucial for the flow solution. 9. Use Mesh Motion Options (for Dynamic Mesh): If the problem involves mesh motion or deformation (e.g., moving boundaries or free surface flow), ensure that the Mesh Motion settings (like smoothing and remeshing) are properly configured to prevent negative volumes. Use Layering or Re-meshing options to maintain mesh quality during dynamic movements. 10. Monitor Solution and Intermediate Steps: Monitor the volume of each phase, residuals, and cell volumes during the simulation.

Hello Mohammed Mansour Meky ,

The negative cell volume in ANSYS Fluent is related to dynamic mesh whether the flow is single phase or multiphase. You have to double-check the dynamic mesh model and settings and try to reduce the time step size. If there is a boundary layer adjacent to the moving wall (object) you have to check the option that excludes the boundary layer from remeshing.

Kind regards,

Ashkan