A few weeks back we looked at the Eulerian multiphase news for Simcenter STAR-CCM+ version 2206 [Eulerian multiphase news Simcenter STAR-CCM+ version 2206 (volupe.se)]. This week we continue to the Lagrangian multiphase news. In the Lagrangian framework we follow what is referred to a Lagrangian particle or parcel and we look at what happens to the particle, how it is affected by and in turn affects the continuous flow (two-way coupling). The Lagrangian method is a sub-grid method, but the particles are coupled to the continuous flow in either a one-way (particles are affected by the surrounding flow) or two-way fashion (the continuous flow is also affected by the particles). For the sake of Simcenter STAR-CCM+ the LMP (Lagrangian multiphase) and DEM (Discrete element method) are considered methods in the Lagrangian framework. The DEM-particles typically also host a coupling between them internally, providing what is referred to as a 4-way coupling. It simply means that particle-particle (and particle-wall) interactions cannot be neglected.
UNIFAC evaporation model for liquid film and Lagrangian droplets
This Simcenter STAR-CCM+ version 2206 update provides a new evaporation law within Simcenter STAR-CCM+. It is useful when we have components with dissimilar molecular structures like those found in multi-component fuels. The target application here is typically internal combustion engines with an ethanol component in the fuel. And the new model helps in the transition from STAR-CD to Simcenter STAR-CCM+ in-cylinder solution. A few releases back we got the REA model for Droplet drying using the Reaction engineering approach [REA Spray Drying Evaporation – VOLUPE Software]. That model is mainly for spray drying while this new UNIFAC evaporation model is for both fluid film and Lagrangian droplets. The theory section on the UNIFAC model is found in the Simcenter STAR-CCM+ documentation under the theory section of the Fluid film model. The model is a complement to Raoult’s law, in which the activity coefficient is approximated as 1 for all mixtures, in the calculation of the vapor pressure for a component. The activity coefficient accounts for interactions between the different components in the mixture and differs from 1 when the molecules in the liquid fuel are not homogenous in size.
The modified UNIFAC is an adapted version of the UNIFAC (UNIQUAC Functional-group Activity Coefficients). The UNIFAC database contains two tables namely the surface area and volume contributions listed by structural groups and the energy interaction parameters between different groups. The modified UNIFAC model regards a molecule as an aggregate of functional groups and assumes that certain thermodynamic properties can be calculated by summing the group contributions. Essentially it splits the calculation of the molecular activity coefficient into two parts; a combinatorial term for contributions due to differences in molecular size and one residual term representing contributions due to differences in the molecular interactions. The mathematical details can be found in the documentation.
The video below shows the difference between Raoult’s law and the modified UNIFAC for ethanol mass fraction in an example of multicomponent fuel spray simulation.
Meshfree DEM compatibility with Dynamic Fluid-Body Interaction (DFBI)
This news was presented in the overall news from a few weeks back. The meshfree DEM speed and ease of use benefits are available for new applications that require Dynamic Fluid-Body Interaction (DFBI) technology. This new compatibility is expected to be useful in the following applications:
- Automotive: Tire-soil interaction modelling, Shown below in the video
- Material testing: Compression and shear tests
- Heavy equipment, essentially the case seen previously, an excavator digging through soil.
- Projectile penetrations in sand beds, Brazilian nut effect (shaking for instance a mix of nuts in a jar and seeing the largest Brazilian nuts end up on the top), flap valve
The flexible fiber model was introduced in Simcenter STAR-CCM+ version 2021.3 and has been around for a couple of releases. What is new is that the flexible fibers are now possible to combine with tow breakage models. Before you had to use the particle clump models when simulating grass cutting or fiber processing as that was the model compatible with the breakage models. The two models available now with the flexible fibers are the simple failure model and the constant rate damage model.
The simple failure model breaks a bond in the simulation if tensile stress or shear stress on the bond exceed one of the user specified maximum values for tensile stress or shear stress.
The constant rate damage model simulates the weakening and breaking of bonds between particles in a clump due to collisions.
Another addition to this compatibility is two field functions, Particle bonded components and Particle bonded component size that can now be applied to flexible fibers.
Cohesion model for coarse grain particles
The coarse grain modelling approach for DEM has now been made compatible with Linear cohesion. An addition that allows for cohesive simulations of fluidized beds. The video below shows a comparison of 608 million particles of size 100 microns represented by 76000 coarse grain parcels. Both the Coarse grain Particle model and the Parcel contact coarse grain model can be run with linear cohesion.
A lot of exciting news for the multiphase capabilities lately. I hope this has been useful to you and as usual reach out to firstname.lastname@example.org if you have any questions.