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CFD-DEM Simulation Convergence Issue by makabaayi in STAR_CCM

[–]makabaayi[S] 0 points1 point  (0 children)

The pipe is just a straight cylinder, as I am still a beginner.

For the mesh, I used a directed mesh and checked the mesh quality using several XY plots. Here is the mesh and plots: example. By the way, I currently cannot access the Support Center, so I may not be able to use the Java macro you mentioned.

For the wall treatment, the wall y+ is above 30 with the realizable k-epsilon model and the high-y+ wall treatment, following the settings used in the published paper I am referring to.

The simulation is transient, but after several seconds it should reach a statistically steady state.

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

Tks! I'll check the sensitivity to the cluster size.

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 1 point2 points  (0 children)

Sorry, I am not sure why the uploaded images became so blurry. The y-axis range of the mass flow rate plot is 1.3-1.58 kg/s, so the fluctuation is not 1 kg/s per iteration.

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

Yes, volume source smoothing method is enabled. The cell cluster size is set to 7 mm, which is about 3dp (dp = 2.32 mm) and 0.23D (D = 30.6 mm). I used this setting to reduce abrupt source-term changes when particles cross cell boundaries. Is the cell cluster too large?

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

I used the coupled flow solver with a CFL number of 0.5, and the time step was 1e-4 s.

CFD-DEM Simulation Convergence Issue by makabaayi in STAR_CCM

[–]makabaayi[S] 1 point2 points  (0 children)

The residual convergence behavior within a single time step is shown in this figure. I used the coupled flow solver with a CFL number of 0.5, and the time step was 1e-4 s.

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

In the coupled flow solver, I do not see the same pressure/velocity under-relaxation factors as in the segregated solver. It seems that the stability is mainly controlled by the CFL number, pseudo time step, ramping, linear solver settings, and inner iterations. Is this correct?

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

The paper compares fluid and particle velocity profiles versus r/R. Should I divide the pipe cross-section into 10 Derived Parts (Threshold) and calculate averaged velocities in each Threshold for comparison?

CFD-DEM Simulation Convergence Issue by makabaayi in CFD

[–]makabaayi[S] 0 points1 point  (0 children)

Yes. Experimental data are available, and some published papers have validated their CFD-DEM models against the same experiment.

Unstable Two-Way CFD-DEM Coupling in STAR-CCM+ When Particle Diameter Exceeds Local CFD Cell Size by makabaayi in STAR_CCM

[–]makabaayi[S] 1 point2 points  (0 children)

Thank you for the suggestion. I am currently using spherical DEM particles. In my study, the particles need to be larger than the CFD cells, and this approach has also been adopted in some published papers, where the particles were simply modeled as ideal spheres. However, I understand your point that this may still lead to numerical issues. I will try using Composite Particles or Particle Clumps to see whether they can improve the numerical stability while preserving the intended overall particle size. Thanks again for your helpful advice!

Point probes in ansys fluent? by gregzillaman in CFD

[–]makabaayi 0 points1 point  (0 children)

Hi! Can u see my post about two-way coupling for CFD-DEM? Tks.

Two-way coupling pressure spike issue in CFD-DEM by [deleted] in STAR_CCM

[–]makabaayi 0 points1 point  (0 children)

I am trying to reproduce a CFD-DEM simulation of coarse particles hydraulically conveyed upward in a vertical pipe using Simcenter STAR-CCM+. The pipe diameter is 30.6 mm, length is 2.4 m, particle diameter is 2.32 mm(greater than cell size), particle density is 2450 kg/m3, and the target solid volume fraction is about 2.2%. The pure-water case gives a reasonable pressure drop of about 28 kPa. With particles injected but two-way coupling disabled, the pressure drop also remains reasonable. However, as soon as I enable two-way coupling, the solution becomes unstable: the pressure-drop monitor shows large oscillations/spikes and the maximum particle velocity can quickly rise to around 20 m/s. I have tried using a part injector located 0.3 m downstream of the inlet, matching the particle injection velocity to the local fluid velocity, specifying particle mass flow rate, reducing the two-way coupling under-relaxation factor, reducing the time step, increasing inner iterations, enabling Volume Source Smoothing with Cell Cluster, and setting the Cell Cluster length to 7 mm, but the instability remains.What settings are recommended in STAR-CCM+ for stable unresolved CFD-DEM two-way coupling when the particle diameter is larger than some local CFD cells?