It has become bit of a tradition for me personally to make a blog post on the news related to multiphase flow, each time a new version of Simcenter STAR-CCM+ is released. And this holds true this time as well. This week, Simcenter STAR-CCM+ version 2302 was released. This post will look at both the Eulerian and Lagrangian multiphase news this time and we will see how we can improve our simulations in the ever-fickle world of more than one phase.
Implicit Multistep for MMP-LSI
A couple of updates ago the implicit multistep for VOF was introduced in order to speed up simulations where the interface, or free surface, is tracked. This release, the same thing is released for Mixture multiphase – Large scale interface detection (MMP-LSI) also. Implicit Multi-Step allows a larger timestep to be used for the flow (excluding volume fraction) by sub-stepping for the volume fraction multiple times within the flow timestep. This decouples the choice of flow timestep from that needed to fulfil the small timestep required for the volume fraction due to CFL constraints. Except the possibility of speeding up our simulations, this can also be used to increase fidelity of our simulation with very little additional computational cost, by adding additional sub-steps withing the flow timestep. In the case of VOF, where the solver is already fairly accurate, hence the implementation of Implicit VOF Multistep targets more toward a general speed up. Going from 1 to 4 sub steps gives only a 10% overhead on the calculation time but a visible accuracy improvement. The picture below shows the increase in fidelity going from 1 to 16 sub steps in a gear box simulation, together with a speedup of 3.8.
Since the introduction of Implicit VOF multistep the general recommendation for VOF is to use the method, and similarly for Implicit MMP-LSI the recommendation is now to use the sub stepping method.
The most obvious and relevant application for the new feature is E-machine cooling since you break up droplets in smaller and smaller size until you model the droplet as MMP and no longer as a Lagrangian particle. Generally, the Implicit Multi-step for MMP-LSI is recommended anywhere you would have VOF, but that is too expensive, and where mixture is present. The two methods (VOF and MMP-LSI) are interchangeable, meaning you can switch back and forth between them. Investigation has also shown that compared to VOF, the MMP-LSI is more reliable.
Volumetric source timestep provider
This timestep provider is available for VOF, MMP and for Two phase thermodynamic equilibrium and is designed to help accelerate simulation where phase change is occurring. The volumetric source timestep provider sets the timestep based on the interphase mass transfer sources. This is important and necessary in cases involving boiling, evaporation, condensation, and cavitation for instance. Because in cases like that, if the timestep is set too large, the sudden jump in mass transfer between phases can make the timescale very nonlinear, and not taking this into account can lead to instabilities and divergence. The time step provider helps out by setting a very small time step initially, to not overwhelm small cells with the mass being transferred, and as the process reaches some sort of quasi steady state, usually the timestep can be increased. The time step provider dynamically adjusts the time step, based both on the mass being transferred and the cell size receiving the mass.
The picture below shows boiling in a PWR (pressurized water reactor) cooling simulation. The picture shows the vapor fraction along the length of the bundle section. The time step size is shown at the bottom. In the beginning of the boiling process a small time step is needed, but after some time the timestep can be increased.
Force and moment reports for Eulerian multiphase
To better be able to utilize the results of your EMP simulation and to better be able to understand the performance of your multiphase design tanks, force and moment reports have been added for EMP. This helps evaluating important design quantities such as torque on impellers. These are already available in single phase simulations and other multiphase applications such as MMP and VOF. The picture below shows a mixing tank and torque on one if its impellers with time.
Cone angle sample distribution in cone injectors
This new feature in Lagrangian Multiphase allows an additional control over the spray nozzle output. This feature is especially useful for fuel spray applications, including Internal Combustion Engine sprays. Previously, Simcenter STAR-CCM+ cone injectors provided the uniform distribution of parcels in cross-section of the spray perpendicular to injection direction, see the image below. The distribution of parcels in a spray modeled with STAR-CD is shown on the left. Here we see more parcels in the center of the spray compared to the area near spray edges. The same image shows the comparison of spray shape and spray penetration length for these two cases, showing the non-uniform distribution results in larger penetration of the spray. In version 2302 user can control the distribution of parcels as a function of the angle with injector axis for both solid cone injector and hollow cone injectors. Users can set the level of non-uniformity to reproduce experimental in-spray mass distribution data, if they are available, or tune the non-uniformity to match the measured data for spray penetration. It is expected that this feature is especially useful for sprays with higher injection velocities where the non-uniformity of droplet distribution inside the spray is often higher. A higher non uniformity means here that higher velocity tend to give a more biased distribution similar to what the figure below shown.
Liquid-solid-gas material option for DEM particles
Version 2022.1 introduced the Liquid-solid-Gas material option for Lagrangian particles (not DEM) to allow for evaporation modeling of droplets that contain a solid material, like milk droplets in spray drying. In this version the idea has been incorporated also in the DEM framework, allowing for a lot more applications and not only limited to droplet evaporations. This is expected to be potentially useful in the chemical process industry, mining industry, food industry, steel industry, battery manufacturing – basically all applications that potentially involve drying of wet solids. The example below shows the drying of electrode slurry in an industrial convection oven, Parcels here represent wet particles, with a 20% liquid component.
Particle-Wall link model for end segments of flexible fibers
Flexible fibers were introduced in version 2021.3 and have gotten some updates since that version. In this version another update is presented. And that is the particle-wall link model for end segments of the flexible fibers. What this means is that when previously activating the particle-wall link for flexible fibers, it implied that every segment of the fibers became permanently “glued” to the wall upon contact. Now, this has been remedied. You can now change the “Link mode” of fibers from “All segments” to “Single end”, and in cases like the grass cutting example, the behavior of the cut grass, will be more physical, and a cut straw that leaves the lawn mower will not instantly stick to the ground. See the animation below for visualization of the feature.
I hope you find the latest multiphase news interesting and applicable to your own simulations. And as usual, do not hesitate to reach out if you have any questions to email@example.com.