Changing and moving phase interfaces can only be described with great difficulty using classical mesh-based methods. Therefore, mesh-free simulation methods have been investigated at the ICVT for many years. With the program package SiPER developed at the institute various problems can be investigated with the Smoothed Particle Hydrodynamics method.
The Smoothed Particle Hydrodynamics method is a Lagrangian mesh-free discretization method for partial differential equations. Compared to grid-based discretization methods such as the finite-element method or the finite-volume method, the interpolation points (the so-called "particles") can move freely in space and, thus, enable simulations in complex geometries and systems with free surfaces without extensive remeshing and computational effort. An accurate movement and change of interfaces is possible without any additional effort, since the phase boundary between discretization points is defined and moved with them.
SiPER is a highly parallelized program package for the direct numerical simulation of morphology evolution and multiphase transport in porous media. One focus is on interface interaction, wetting properties and the change of porous systems through heat and mass transport processes. An extension to nanoscale transport processes, where thermal fluctuations are relevant, enables the simulation of problems across many length scales.
Past and current application examples for SiPER are shown below.
Modelling of the electrolyte distribution in representative volumes
The performance of gas diffusion electrodes (GDEs) plays a decisive role for a number of technical processes such as chlor-alkali electrolysis or electrochemical CO2 reduction. Within the GDEs, contact is established between the gaseous reactants, the solid catalyst and the electrolyte. By adding non-wetting components such as polytetraflourethylene (PTFE), the penetration behaviour of the electrolyte can be influenced, which is a key component for improving and producing these GDE.
Using the SPH method, the penetration behaviour into the porous structure is carried out on the basis of image recordings of the pore space by means of direct numerical simulation. The transport phenomena on the continuum scale due to the surface tension and the heterogeneous wettability are included physically consistent.
Wetting behavior in porous media
Multiphase flows in porous structures are dominated mainly by capillary and surface forces. Different wetting properties can lead to very different flow behavior. Research at the ICVT covers the investigation of dynamic wetting processes in single capillaries and pore networks and the description of the wetting behavior of fluid drops on different surfaces.
Structure formation processes are usually based on transport processes on different time and length scales. An important scale is the mesoscopic scale, which is the range of nanometers and micrometers. Using the example of the precipitation process of porous polymer membranes, the pore-forming process during phase separation was investigated in order to obtain conclusions about the transport properties on the structure of polymer membranes. The Smoothed Particle Hydrodynamics method has been extended to models for the kinetic modeling of the phase separation as well as for multicomponent transport. By comparison with experiments, the numerical results were qualitatively validated.
Polymer solutions in wall-bounded shear flow
Fouling in technical reactors for polymerization reactions represents a major challenge in the development of new processes. In addition to the reaction mechanism, the properties of the reactor surfaces and the flow field also play an important role. By extending the Smoothed Dissipative Particle Dynamics method with a "bead-spring" model for polymers, the adsorption behavior of macromolecules in near-wall shear flows can be simulated. The aim is to improve the mechanistic understanding of the initial states of fouling in order to support the process development with regard to the avoid fouling.
Further information about the SiPER program package
Compared to other software packages, SiPER specializes in incompressible flow, but also offers common compressible approaches. With the so-called "truly incompressible smoothed particle hydrodynamics" (ISPH) method, based on the established projection method of grid-based CFD, flow problems can be solved efficiently and with high accuracy. For example, pressures of the incompressible flow that are central to multiphase flow in porous media are calculated accurately.
The Smoothed Particle Hydrodynamics method, established for macroscopic and mesoscopic problems, offers an algorithmically simple extension to microscopic problems where thermal fluctuations are relevant due to their molecular nature. With the Smoothed Dissipative Particle Dynamics (SDPD) method, which is an extension of the Smoothed Particle Hydrodynamics method including thermal fluctuations, it is possible to study problems at a coarse-grained level using the Continuum Approach.
The following physical and methodical models / approaches are available in SiPER:
Parallelization with MPI
Different approaches for incompressible flow (ISPH, TWO STEP, implicitly ISPH, Stokes flow)
Different Approaches to Compressible Flow (WCSPH)
Extension to mesoscopic problems (SDPD)
Boundary conditions: periodic, symmetric, solid wall, inflow / outflow
Surface Tension Models (CSF, Particle-Particle WW, Tensor (Adami))
Contact Lines (CLF, Tensor (Breinlinger), Tensor (Adami))
Phase decay with Cahn-Hilliard equation
Rheology (Newton, viscoplastic, viscoelastic, elastic)
Multi-component transport (Maxwell-Stefan, Generalized Fick)
Interface for fluid-fluid coupling with grid-based methods
Stick-slip for rough surfaces
The SiPER program package is freely available on request. If you are interested, contact Manuel Hopp.