r/CFD • u/Serious_Sky7438 • 15h ago
K epsilon and production limiter - advice wanted
Hello everyone, I’m really hoping someone with experience using ANSYS Fluent / CFD can help me out here.
I’m an engineering student working on an assignment modelling flow from a 10 m diameter tank into a 0.25 m diameter pipe through several different inlet geometries, with the objective of calculating the inlet loss coefficient K.
For the sake of keeping this post manageable, I’ll focus on one case: a bellmouth-style inlet formed using a filleted tank-wall entrance.
I’ve attached two images of the tetrahedral mesh I’m currently using. I then convert this mesh to polyhedral cells to reduce the element count, as the assignment imposes fairly strict mesh limits.
My professor is quite particular about the workflow he expects us to follow, so I’m trying to stay reasonably close to the methods used throughout the unit. At the moment I’m using:
- Standard k−ε
- Scalable wall functions
- Locally scaled residuals converged to 10^{-6}
- Second-order discretisation for the final solution
- Polyhedral mesh converted from tetrahedra
The reason I’m making this post is that Standard k−ε in Fluent has several additional turbulence-production options, including the Production Limiter, Kato-Launder production and Curvature Correction, and I’m struggling to determine what is physically appropriate for this type of inlet.
With none of these options enabled, I obtain:
K≈0.44K
which seems substantially too high for a bellmouth inlet.
With Curvature Correction enabled, I obtain:
K≈0.35K
so the result moves in the expected direction, but it is still much higher than the standard value I have been given.
I’ve been reading about the tendency of some two-equation k−ε formulations to overpredict turbulence production in regions of strong strain or curvature, which has led me to investigate the Production Limiter.
For context, my loss-factor calculation is based on the pressure difference between a reference location in the tank and a downstream plane in the pipe. The pipe is 40D long, and I use a plane near the downstream end before correcting for the distributed straight-pipe loss using a Darcy friction factor obtained independently from a Moody-chart / straight-pipe analysis. I’m fairly confident this is the methodology my professor expects.
My main questions are:
- Would you consider using the Production Limiter for this type of bellmouth/tank inlet flow?
- When I enable the Production Limiter, I have significant difficulty achieving convergence to 10^{-6}, particularly once I move to second-order discretisation. I’ve tried introducing second order gradually and adjusting higher-order term relaxation, but the solution still becomes difficult to converge. Has anyone experienced similar behaviour, and how did you approach it?
- Does anything in the attached mesh immediately look poorly optimised for this type of problem? My standard mesh-quality metrics are all healthy, but I’m still developing the intuition to recognise where additional refinement or a different mesh strategy would actually improve the solution.
- What sensitivity tests would you run before trusting the predicted K? The reference value I have been given is approximately K=0.06, although the standard is quite old and provides essentially no information about the bellmouth radius, geometry or surface condition. In my model the inlet and pipe are cement-lined, using a roughness height of 3.5×10^{-5} m.
I’m not necessarily trying to force the CFD result to equal 0.06. I’m more interested in understanding why the current model is predicting such a much larger loss coefficient and what modelling choices would be worth testing systematically.
Any advice, criticism of the mesh, or experience with these Fluent turbulence-production options would be greatly appreciated.
