Simulating a Dam Collapse with Large-Scale Interface to Improve Free Surface Resolution
Overview
This article describes how to model the flow after a dam collapse using Large-Scale Interface (LSI) with Mixture Multiphase (MMP) and Eulerian Multiphase (EMP) in Simcenter STAR-CCM+. The resolutions between the LSI modeled free surface and the Volume of Fluid (VOF) modeled free surface are compared within the article. MMP and EMP are known to be dissipative at the free surface, but the LSI model enables the sharpening at the free surface. Hence, this article aims to provide a general guideline for activating and visualizing the effect of the LSI model.
Computational Domain and Boundary Conditions
For the current study, a 2D mesh domain is chosen for simplicity to help visualize the free surface, as shown in Figure 1. There are only two types of boundary conditions applied in this study: no-slip wall and outlet. The initial location of the water stored in the dam is defined and initialized by the user. Users can utilize a user-defined field function to assign desirable volume fraction of water to the designated region., as shown in Figure 2.
The process of applying the field function varies based on the chosen multiphase approach. For the VOF and MMP methods, the volume fraction of water is set at the Initial Conditions node under the Physics node, as shown in Figure 3. For the EMP approach, the initial conditions for the volume fraction of water are under the Multiphase node, as shown in Figure 4.
Methodology
The general physics setup for VOF, MMP, and EMP are typical and can be commonly found in the tutorials of STAR-CCM+. Therefore, this section focuses on the implementation of the LSI model with the MMP and EMP approaches and advanced settings to improve free surface modeling.
MMP-LSI
Activation of the LSI model is different between the MMP and EMP approaches. For the MMP approach, no specific type of phase interaction is required to enable the LSI model and user can simply activate LSI detection by changing the convection scheme to Adaptive Interface Sharpening (ADIS) under the Mixture Multiphase (MMP) node, as shown in Figure 5. Step-by-step instructions for setting up MMP-LSI are shown below.
Step (1): Change convection scheme under the MMP node.
Step (2): Two sub-nodes will appear under the Mixture Multiphase (MMP) node and advanced LSI settings can be applied within those two sub-nodes, as shown in Figure 6.
EMP-LSI
To activate the LSI model with EMP, a Multiple Flow Regime phase interaction must be created before selecting the LSI model. Step-by-step instructions for setting up EMP-LSI are shown below.
Step (1): Create a Multiple Flow Regime phase interaction, as shown in Figure 7.
Step (2): Select the Large-Scale Interface Detection within the new phase interaction node, as shown in Figure 8.
Advanced LSI Settings
To obtain higher fidelity simulation results, advanced settings such as adaptive mesh refinement and adaptive time stepping can be leveraged. For the EMP-LSI approach, STAR-CCM+ offers specialized options on those advanced settings. Like the MMP approach, the EMP approach can also employ the ADIS scheme but only on the volume fraction convection. The ADIS scheme allows LSI specific Adaptive Mesh Criteria and Time-Step Providers to improve the free surface capturing. Step-by-step instructions to activate LSI specific advanced settings for EMP-LSI are shown below.
Step (1): Change the Volume Fraction Convection scheme to Adaptive Interface Sharpening (ADIS) under the Eulerian Multiphase (EMP) node as shown in Figure 9.
Step (2): Select the LSI Mesh Refinement option under the Adaptive Mesh Criteria sub-node and LSI Smoothed CFL option under the Time-Step Providers sub-node as shown in Figure 10.
Conversely, the MMP-LSI approach does not have LSI specific Adaptive Mesh Criteria and Time-Step Providers, but the general options are sufficient for capturing the free surface. For Time-Step Providers, the Free Surface CFL option is the most suitable one among the other general options. However, a User-Defined Mesh Adaption is required to manually define the criteria that govern the mesh refinement around the free surface. With the User-Defined Mesh Adaption, the Adaption Request can be set to refine a range of volume fraction of water, which detects the volume fraction gradient across the free surface. Step-by-step instructions to implement advanced settings for MMP-LSI are shown below.
Step (1): Select the User-Defined Mesh Adaption option under the Adaptive Mesh Criteria sub-node and Free Surface CFL option under the Time-Step Providers sub-node as shown in Figure 11.
Step (2): Define the Adaption Request under User-Defined Mesh Adaption using volume fraction of water as scalar function as shown in Figure 12.
Results
Although MMP and EMP are naturally dissipative at the free surface of stratified flows, coupling the LSI model with MMP and EMP demonstrates sharpened free surface at the volume fraction of water scalar fields in Figure 13. All the multiphase approaches are inherently different based on their formulations and assumptions so a small disparity in subtle flow details among the scalar fields is inevitable. Nevertheless, this article concentrates on highlighting the capability of the LSI model on sharpening free surface, instead of verifying simulation results across different numerical methods. Shown in Figure 13, the free surface is well captured for all three approaches. Especially, VOF is known for its capability on modeling high resolution free surfaces with the requirement of high mesh density near the free surface. On the other hand, MMP and EMP without the LSI model would still model a dissipative free surface even with a high mesh resolution near free surface due to their nature. The LSI model lowers the dissipation at the free surface for the MMP and EMP approaches and captures a sharp free surface like VOF. To ensure proper activation of the LSI model, the large interface marker field function can be used to trace the region where LSI is activated, and the marker also helps identify the dispersed region of the stratified flow when the large interface markers are clustered together.
Figure 13. Volume fraction of water and large interface marker scalar fields for different multiphase approaches.
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