Why VOF Destabilizes Your Supersonic Gas Simulation: A STAR-CCM+ Deep Dive

 

This is a classic headache in STAR-CCM+ (and CFD in general). The behavior you are seeing—where the "Gas Only" model is rock-solid but the "VOF (even with 100% Gas)" model falls apart—is due to how the VOF solver handles numerical dissipation and pressure-velocity coupling compared to a standard single-phase solver.

Here is the breakdown of why VOF is killing your stability and how to fix it:

1. The Interface Compression Term (The "Ghost" Interface)

When you enable VOF, even if the domain is 100% air, the solver is actively looking for an interface. It uses specialized convection schemes (like HRIC or WENO) designed to keep interfaces sharp.

• The Problem: These schemes are much less dissipative than the standard Second-Order Upwind schemes used in single-phase flow. In your supersonic jet, the VOF solver is "sharpening" numerical noise or pressure waves that the single-phase solver would simply smooth out.

• The Result: Those pressure waves you see in the jet are being numerically amplified rather than damped, leading to the divergence 50-100 steps later.

2. Free Surface vs. Compressible Flow Coupling

In a standard compressible gas run, the solver couples Pressure and Density tightly. In VOF, the solver has to manage the Volume Fraction (Phase 1 vs. Phase 2).

• Even if there is no liquid yet, the VOF algorithm introduces a "Surface Tension" and "Interface" calculation loop.

• In supersonic regions, the local Mach number creates high-pressure gradients. VOF solvers are notoriously sensitive to these gradients because they can be misinterpreted as "density jumps" across an interface, causing the solver to over-correct the velocity field.

3. The PISO/SIMPLE Convergence Loop

In STAR-CCM+, enabling VOF often changes the default iteration strategy. VOF is usually solved using a Segregated Solver approach with specific pressure-correction steps to ensure mass conservation across the interface.

• Supersonic flow requires very tight coupling between the energy equation and the momentum equation.

• The VOF pressure-correction loop adds an extra layer of "stiffness" to the matrix. If your mesh isn't perfectly aligned with the flow, the VOF scheme creates tiny oscillations that, at supersonic speeds, propagate faster than the solver can converge them.

---

How to Fix it (Without the "Ridiculous" Timestep)

To get your VOF model to behave like your gas-only model, try these specific STAR-CCM+ adjustments:

1. Switch the VOF Scheme:

   Under Physics > VOF > Models, check your Convection Scheme. If it's set to HRIC, try switching to a more dissipative scheme (like Second-Order Upwind) for the initial startup, or adjust the Angle Factor in the HRIC settings to be less aggressive.

2. Adjust the VOF Undershoot/Overshoot:

   Go to Solvers > VOF Solver. Ensure you have Limits enabled. Sometimes the solver calculates a volume fraction of $1.000001$ or $-0.000001$ in the supersonic shock region, which causes the pressure solver to freak out.

3. Use the "Implicit Multi-Step" for VOF:

   In the VOF Solver settings, you can enable Multi-Step. This allows the VOF interface to be calculated at a smaller sub-timestep than the rest of the physics (like Energy and Momentum). It’s much more efficient than lowering the global timestep.

4. CFL-Based Timestep (Adaptive):

   Since you have a supersonic jet, your cell residence time is tiny. Use an Adaptive Time Step based on a Free Surface Courant Number. Set the maximum VOF Courant number to 0.5 - 1.0.

5. Initial Field:

   Don't start the VOF model from a "dead stop" with high pressure. Initialize the field using your successful Gas-Only solution. Map that solution onto the VOF model so the supersonic jet is already established before the VOF solver starts looking for liquid.

Summary

The instability isn't coming from "liquid"—it's coming from the VOF convection schemes being too "sharp" for the shock waves in your supersonic jet. The solver is trying to treat a pressure shock like a liquid-gas interface.

Recommended "Fixes" to Include in Your Post:

• Switch Convection Schemes: Suggest moving from HRIC to a more dissipative scheme during the "startup" phase.

• Implicit Multi-Stepping: Advise users to use a smaller sub-step for the VOF equation specifically.

• Courant Number Control: Highlight that VOF requires a much lower $CFL$ (ideally $< 0.5$ near the interface/shocks) than single-phase gas models.

"Discover why enabling VOF in STAR-CCM+ causes supersonic gas models to diverge. Learn about interface compression errors, shock wave instability, and how to stabilize your multiphase simulation."