Predicting calm water resistance accurately is crucial for vessel performance optimization. In this blog we use Simcenter STAR-CCM+ to evaluate different approaches for solving the Volume of Fluid (VOF) interface, to find an answer to the question whether we can solve a standard calm water resistance simulation faster with equivalent accuracy. Simcenter STAR-CCM+ has obtained several new tools to improve accuracy and simulation time over the last years. Now it is time to evaluate these on the calm water resistance calculations. The KRISO Container Ship (KCS) served as our benchmark to systematically compare interface handling methods. Our focus was on testing variations of the HRIC scheme, implicit multi-stepping, and adaptive time-stepping to assess their impact on computational efficiency and accuracy.
HRIC and Modified HRIC (MHRIC) Schemes
The High-Resolution Interface Capturing (HRIC) scheme is commonly used for free-surface capturing in CFD. It provides a balance between accuracy and numerical stability. The Modified HRIC (MHRIC) approach improves upon HRIC by producing a slightly thicker interface (typically three cells thick), reducing the need for excessive resolution of small droplets and bubbles. This enhances robustness and mass balance while reducing solution time. MHRIC also mitigates artifacts at the free surface, making it more reliable than an under-resolved HRIC setup and faster than an over-resolved HRIC case.
MHRIC is particularly beneficial in scenarios such as tank sloshing and ship resistance analyses with breaking waves, where it maintains the relevant physics while offering significant computational speedup. More details on HRIC and MHRIC can be found here.
Implicit Multi-Stepping Approach
VOF simulations often require small timesteps to properly resolve the free surface, constrained by the CFL number. This issue is more pronounced when simultaneously resolving large waves and small droplets in the same simulation. The small timestep is required to track the free surface in the volume fraction field, but the overall flow field can often tolerate much larger timesteps.
Implicit VOF multi-stepping addresses this by sub-stepping the volume fraction multiple times within each main flow timestep. This allows the effective timestep for the volume fraction to be smaller while keeping the overall flow timestep larger, still adhering to CFL constraints. Since volume fraction sub-steps are computationally cheaper than full flow timesteps, this method typically achieves a speedup of more than 3x compared to traditional approaches.
When combined with adaptive time-stepping, it enables better control over numerical accuracy while maintaining efficiency. More details on implicit multi-stepping are available in the STAR-CCM+ documentation: link.
Simulation Approach
The primary focus of this study was to assess different approaches to handling the VOF interface, particularly in calm water resistance Template (The VTT Template) of STAR-CCM+:
- Standard HRIC (High-Resolution Interface Capturing) – The default free-surface capturing method.
- Modified HRIC (MHRIC) – An adjusted version of HRIC aimed at improving stability.
- HRIC with Implicit Multi-Stepping – A method combining HRIC with an implicit multi-step solver.
- Multi-Stepping with Adaptive Timestep – Limits the time-step size to allow for a sharp interface when using VOF Implicit Multi-Stepping.
The default settings of VTT overestimate the resistance significantly. That is why ee primarily tested the VTT Template version from 2023, as it appears to be the most stable for recent STAR-CCM+ versions and showed good agreement of the predicted resistance (as discussed here). However, for comparison, we eventually included the latest Template version 2024 with adjustments of the added Chevron Threshold Angle and Aspect Ratio Tolerance and multi-stepping to the VOF solver as this showed good performance with the VTT 2023 version. The combination of the adjustments made this version not only compatible again but showing also the smallest error compared to experiments. The results were compared with a published paper on total resistance coefficients of the full-scale KCS (source). All results were within 1.5% of the extrapolated experimental results.
One notable observation was that MHRIC solved each individual timestep faster than standard HRIC but required more timesteps to converge, increasing overall computational cost. This trade-off highlights the importance of selecting an appropriate method based on the specific goals of a simulation—whether prioritizing per-step efficiency or overall convergence speed.
Among the tested methods:
- The 2024 Template with multi-stepping was the fastest.
- The multi-stepping and adaptive time-stepping approach had the longest solver time.
- Most approaches underestimate the total resistance. The two approaches that overestimate predict smaller wave pattern in the wake.
The results of this study underscore the impact of interface capturing methods on the efficiency and accuracy of calm water resistance simulations. While MHRIC offers faster computations per step, its requirement for additional time steps suggests that its advantage may be limited depending on the scenario. The Template 2024 version with multi-stepping proved to be the fastest and most stable, achieving the closest agreement with experimental data. Future work could focus on refining adaptive timestep strategies to better optimize the trade-off between speed and accuracy.
This test shows that utilizing the advanced capabilities in STAR-CCM+, we can still fine-tune the simulations and achieve more reliable performance predictions faster. Let´s discuss how we can fine-tune your simulation support@volupe.com
The Author
Florian Vesting, PhD
Contact: support@volupe.com
+46 768 51 23 46
