10/1/2018 | 5 MINUTE READ

Keeping the Multi in Multiphysics Analysis

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Vehicular design can be a computer-aided engineering nightmare because of its domain breadth and multi-domain nature. COMSOL Multiphysics can handle that.

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Not only can each of the modules in COMSOL Multiphysics software stand on its own in analyzing a specific physics domain, the modules can be coupled together to analyze multi-domain problems. That is, solve multiphysics problems. Automotive has a lot of those. “Automotive design is so incredibly complicated. It’s almost like, whatever we do, whatever we improve, there’ll be an application for it somewhere in the automotive sector,” says Bjorn Sjodin, vice president, product management, for COMSOL Inc. (comsol.com).

Here are some improvements in COMSOL Multiphysics, version 5.3a, that automakers might find useful.

Boundary element method

Analyzing with the boundary element method (BEM) only requires meshing the surfaces adjacent to the modeling domain of interest. This greatly reduces the creation of large volumetric meshes. That’s not to say the finite element method (FEM) has gone away. The new BEM interface includes features for combining FEM and BEM modeling for complex multiphysics problems. Engineers can use either method or, by using built-in multiphysics couplings, use both at the same time in the same model.

Here are some situations when BEM should be used, explains Mads Herring Jensen, COMSOL technical product manager, acoustics: Meshing large fluid domains; when the source and receiver/scatterer are widely apart; coupling two FEM domains together; scattering and radiation problems; and for very complex geometries. That said, he says, FEM will generally be faster than BEM for “small” problems that fit into memory because FEM generates sparse matrices while BEM generates dense matrices.

BEM is useful in analyzing many automotive-related physics domains, such as for sound waves, permanent magnets, and magnetic fields (the effect on sensors as a function of distance from the magnetic source).

Batteries

The Battery Designer app provides “a nice, friendly starting point for someone about to get into battery design,” says Sjodin. “It shields the user from the complexity behind the scenes.” The app is suitable for casual and expert battery designers—and COMSOL users.

Designers need only fill in a few text fields to define a lithium-ion (Li-ion) battery model. The app then computes the capacity, energy efficiency, heat generation, capacity losses due to parasitic reactions of a Li-ion battery for a specific load cycle, and operating temperature (based on the generated heat and thermal mass). Various battery-design parameters can be controlled, including the size of the battery canister, the thicknesses of the different components (separator, current collectors, and electrodes), the positive electrode material, and the volume fractions of the different phases of the porous materials. The load cycle is a charge-discharge cycle using a constant current load, which may be different for the charge and discharge stages.

While the app is exclusively for Li-ion batteries, users can recreate it for other types of power sources, including lead-acid, fuel cells, and other electrochemical cells.

Bearings

The Rotordynamics Module has six new rolling element bearing types: deep groove ball bearing, angular contact ball bearing, self-aligning ball bearing, spherical roller bearing, cylindrical roller bearing, tapered roller bearing. For most of these, engineers can select whether it’s a single- or double-row bearing, and then enter clearances and other geometry characteristics. The bearing models include a nonlinear representation of the contact stiffness between rolling elements and the inner and outer races. From this, engineers can analyze the stability of a bearing operating at high RPM over a range of clearance settings.

Also new is the ability to model thrust bearings, including tilted pad, tapered, and user defined. The tilted pad bearing can have either a point or a line pivot, and cavitation of the lubricant can be included in the analysis.

 

Bolt threads

Detailed analysis of bolt threads, specifically the localized stresses around the actual threads, is seldom necessary. The exception is when analyzing the wedging caused by the contact pressure between the internal and external threads. In this case, the geometry of the threads push the bolt hole outwards as the bolt is tightened, often creating unneeded stresses in the material surrounding the bolt. The Bolt Thread Contact feature models this effect by using two cylindrical boundaries as internal and external threads, creating a contact pair that “knows” about the geometry of the thread (such as thread angle) and how the contact forces in a particular direction widen the hole.

Electrodeposition

Electrodeposition, says Sjodin, is “an electrochemical process. Kind of the reverse of corrosion. You build up layers instead of removing layers.” The Thin Electrode Surface feature defines the electrode reactions occurring on a thin electrode fully immersed in electrolyte. This feature is an alternative to drawing the actual electrode domain in the model geometry, thereby significantly reducing meshing and solver time in 3D models. This feature is typically for modeling electrodeposition or corrosion on thin sheets of metal.

Fluids

As fluid flow increases around devices that are large enough, the flow becomes turbulent. Modeling that turbulence, all the eddies and vortices, becomes difficult, to say the least, and tremendously costly (in time and compute resources). Hence the need for turbulence models that approximate turbulent reality. “But no turbulence model approximation is perfect,” admits Sjodin. The latest approximation added to COMSOL is the turbulence k-ε model for realizable turbulent flow. The model’s results in a smoother, more physical approach to limiting turbulence.

The new Inflow boundary condition lets engineers model upstream fluid flow and heat transfer properties with more accurate results and physical models. The boundary condition is used when the inflow of heat comes from a “virtual domain” outside a model that’s been simplified for analysis. Because the upstream temperature is known; the Inflow condition module, consistent with the upstream conditions, will compute the temperature downstream (i.e., where the part being analyzed exists­ in the model).

Lumped mechanical system interface

Abstract models are sometimes needed to replace multiphysics problems modeled in FE because of available computer power, time, cost, or other constraints. The new lumped mechanical system interface provides the proper abstraction by applying electrical circuit concepts to the physics being analyzed. The resulting models do not have nodes, elements, or physical geometry; they’re just a set of differential equations, explains Henrik Sönnerlind, COMSOL technology manager, structural mechanics. “You can model with discrete components like mass, spring, damper, impedance, force, and prescribed displacement, velocity, and acceleration. You can even create subsystems of such components for reuse.” Lumped components can be connected to FE models in any dimension.

The lump interface is suitable for analyzing all sorts of physics problems, from electromechanical systems, such as loudspeakers, to vehicle suspension systems, including transmission, suspension, and passenger seats coupled as a single multibody dynamics model.

 

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