[RobbieAllen]

Hi all,

I'm running some Fluent simulations on a centrifugal pump I've designed. I've added some images of the pump design for completeness, as it's somewhat unconventional. For context, the impeller has a 64mm diameter and spins ~30,000 rpm, so the flow mechanics are probably somewhat complicated.

I've been running the simulations as steady state, and I am not overly happy with the residual performance. From what I've seen, residuals vary from case to case. Some sources suggest 10^-4 is loosely converged, and others suggest achieving 10^-3 can be a challenge. My residuals tend to fall just short of 10^-3, although the continuity from the attached image did not even reach 10^-2 (I'm looking to see the impacts of reducing blade number on performance, so this simulation had less blades).
I've been using monitoring points to assess convergence, as attached, and they consistently converge. I've also been looking at mass flow flux between inlet and outlet, and this simulation had 6e-6, which seems reasonable to me?

Boundary conditions are pressure inlet and mass flow outlet, both of which are appropriate for pump CFD.

As far as I am aware, the mesh quality is good. Orthogonality and skewness are more than acceptable as per metrics I found online, and 95% y+<1. Reducing mesh size seems to make convergence worse.
The simulations are run with k-w SST turbulence model using the coupled solver, with turbulence kinetic energy and specific dissipation rate relaxation factors set to 0.5, as I've read that can help convergence.

I've read that poor steady state convergence could indicate transient flow, but I am struggling with setting up and running a transient simulation.


My question is how much should I be concerned about the residual performance, and is there anything I can try to improve it?
Is it simply a case of complex flow mechanics making them poor, or am I approaching it wrong?


Thanks in advance for any replies, should you need any information please let me know.
Robbie

p.s. I've attached a CAD image and an image from an older simulation so you can see the flow mechanics in the pump, if it is any help. Flow essentially gets trapped in a vortex, and some is bled off and expanded.