Francis turbine cfd analysis
Title:
STAR-CCM+ Francis Turbine CFD – Pressure-limited cells increasing despite good mesh. Should I keep iterating?
Body:
Hi everyone,
I’m simulating a Francis turbine in STAR-CCM+ and would appreciate some advice before I spend many more hours running the case.
Turbine (published design point)
Discharge: Q = 2.25 m³/s
Mass flow: 2250 kg/s
Net head: 38.335 m
Speed: 1000 rpm
Hydraulic efficiency: 0.93
Published shaft power: 786.9 kW
CFD setup
Steady-state
MRF (Multiple Reference Frame)
Water (incompressible)
Mass Flow Inlet = 2250 kg/s (Constant)
Pressure Outlet = 0 Pa gauge
MRF = +1000 rpm
Turbulence model: k-ω SST
Initial CFL was reduced (around 30–45) because higher CFL caused instability.
Pressure and torque monitored throughout the run.
Mesh
Total cells: ~7.3 million
Mesh diagnostics:
No negative volume cells.
Mesh is topologically valid.
Minimum face validity ≈ 0.784
99.96% of cells have face validity = 1.
Volume change statistics also look good.
So I don’t think the mesh quality itself is the issue.
Moment report
The moment report is calculated on the runner (cark) boundaries only, excluding the rotating interfaces.
Problem
Around 360 iterations:
Torque (moment) appears to be stabilizing around 2–4 kN·m.
TKE residual is now decreasing.
Outlet flow is becoming steadier.
However, STAR-CCM+ keeps reporting:
“Minimum absolute pressure limited to 1000 Pa on XXX,XXX cells”
The number of pressure-limited cells has actually been increasing over roughly the last 10 iterations.
Things I’ve already checked
Tried both +1000 rpm and −1000 rpm. The torque sign flipped with the rotation sign, so I’m currently using +1000 rpm.
Verified the rotating region contains only the runner.
Verified the interfaces.
Verified the outlet boundary.
Mesh quality appears good.
Using the published operating speed (1000 rpm) and published flow rate (2250 kg/s).
My questions
Is it normal for the pressure-limited cell count to keep increasing while the torque and TKE residual are stabilizing?
Would you continue running for another few hundred iterations, or would you stop and investigate before wasting more computation time?
If a steady solution converges but still reports a large number of pressure-limited cells every iteration, would you trust the resulting torque and efficiency?
What would you check next in this situation (boundary conditions, MRF setup, pressure reference, head calculation, something else)?
Any advice from people with STAR-CCM+ or hydraulic turbine CFD experience would be greatly appreciated.
Thanks!