(I am begginer in comsol) Thank you. At low flow speeds the flow solution will be time invariant, but at higher flow rates there will be vortex shedding, a time-varying change in the flow field behind the cylinder. The unknowns are segregated into groups, usually according the physics that they represent, and these groups are solved one after another. - the incident has nothing to do with me; can I use this this way? In that case, the continuation method will automatically backtrack and try to solve for intermediate values in the range of 0.6 through 0.8. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. That is, when solving, the software starts with the user-specified initial values to evaluate all solution-dependent terms. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Under Initial values of variables solved for, the default value of the Settingslist is Physics controlled. . What version of COMSOL are you using? Repeat this for every nonlinearity of the model. Not the answer you're looking for? At low flow speeds the flow solution will be time invariant, but at higher flow rates there will be vortex shedding, a time-varying change in the flow field behind the cylinder. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. One can say that, in general, if the loads on a nonlinear system are zero, the system will be at rest; that is, the solution will be zero. Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. Save my name, email, and website in this browser for the next time I comment. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? there is no defined multiphysics for it as I know, I have a standing accoustic wave and a flow in the background but I don't see their connection. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. For more details, see: Performing a Mesh Refinement Study, Mesh refinement may often need to be combined with load or nonlinearity ramping and may require a set of studies, first starting with a relatively coarse mesh for nonlinearity ramping, refining the mesh, and the ramping further on the refined mesh. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. That is: It is also possible to compute the derivative of the solution with respect to the continuation parameter and use that derivative (evaluated at the iteration) to compute a new initial value: where is the stepsize of the continuation parameter. $130,000.00. Cooling and Solidification of Metal. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Tutti i diritti sono riservati. thanks for reply Alternatively, delete and re-create the study. Second, the continuation method will automatically take smaller load increments if a solution cannot be found. This is for COMSOL 5.2, but should be similar for 4.2: Create the stationary study. 3 Replies, Please login with a confirmed email address before reporting spam. listed if standards is not an option). Within either of these features, it can also be helpful to enable the Results While Solving option, as shown in the screenshot below, to visualize the iterations being taken during the solution. This can arise as a consequence of extreme variations in the material properties, or high aspect ratio geometry. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) Does ZnSO4 + H2 at high pressure reverses to Zn + H2SO4? That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. Load ramping and nonlinearity ramping can be used in combination, but start with only one or a few of the loads or nonlinearities being ramped. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. This will use the initial conditions you specified in your physics setting (usually 0 is used in the physics settings). Stationary (time-invariant) models with nonlinearities may converge very slowly. Your Discussion has gone 30 days without a reply. A classic example of this is fluid flow around a cylinder with high, but constant, flow rates. From there, if an additional small load increment is applied, the previously computed solution is a reasonable initial condition. The continuation method will again backtrack and try intermediate values of the ramping parameter, thus giving you the nearest approximation to the abrupt transition that is solvable. As we saw previously in the blog entry on Solving Nonlinear Static Finite Element Problems, not all nonlinear problems will be solvable via the damped Newton-Raphson method. Wrong ordering of study steps. Repeat this for every nonlinearity of the model. Any trademarks referenced in this document are the property of their respective owners. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version What are some highly rated cards & stationery in Brea, CA? If this was solved using the Solid Mechanics physics interface the thin walls of the container would need to be explicitly modeled, but the wall thickness is much smaller than the overall can dimensions. COMSOL does not assume any legal liability for the accuracy of the data disclosed. $140,000.00, $120,000.00 Instead, use a nonlinear material property expression that ramps from a very smooth function to a very nearly discontinuous one. Posted Sep 9, 2020, 1:44 p.m. EDT We have also introduced meshing considerations for linear static problems, as well as how to identify singularities and what to do about them when meshing. The advantages of the continuation method are two-fold. Find centralized, trusted content and collaborate around the technologies you use most. Understanding the Fully Coupled vs. (Frequency Domain should be the last step). I'm trying to model a solid that's moving through a steady background field in a background flow, I want to take into account the effect of movement of the solid after each time step so I have to use stationary solver after each time step in order to see how field has changed after solid moved. Stationary Solver Use the Stationary Solver () to find the solution to linear and nonlinear stationary problems (also called static or steady-state problems). COMSOL does not assume any legal liability for the accuracy of the data disclosed. Not entering required material parameters. Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Assuming a well-posed problem, the solver may converge slowly (or not at all) if the initial values are poor, if the nonlinear solver is not able to approach the solution via repeated iterations, or if the mesh is not fine enough to resolve the spatial variations in the solution. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version This case is generally difficult, or impossible, to solve since this material property is non-smooth. Your internet explorer is in compatibility mode and may not be displaying the website correctly. As P is ramped up, the continuation method uses the previous solutions to compute initial conditions for the more nonlinear cases. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. To switch between these solver types, go to the Stationary Solver node within the Study sequence. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Therefore, it is recommended to use Adaptive Mesh Refinement which will automatically refine the mesh only in regions where it is needed, and coarsen the mesh elsewhere. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Numerically ill-conditioned means that the system matrix is nearly singular and that it will be difficult to solve on a finite-precision computer. This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. When the difference in the computed solutions between successive iterations is sufficiently small, or when the residual is sufficiently small, the problem is considered converged to within the specified tolerance. Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. By default, the COMSOL Multiphysics software determines these values heuristically depending on the physics as, for example, the specified initial values or a solution from an earlier study step. To switch between these solver types, go to the Stationary Solver node within the Study sequence. Such problems must solved in the time domain. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. Most multiphysics problems are nonlinear. Linear solvers. The fully coupled and segregated approaches are discussed below. Hi Jason, Any trademarks referenced in this document are the property of their respective owners. Name: actdep_int1, Your email address will not be published. The technique of load ramping is not always reasonable for all problems. With sufficient simplification, a model can be reduced to a linear problem, and if this simplified model does not converge, see: What to do when a linear stationary model is not solving. The Fully Coupled solution approach, with the Plot While Solving enabled. The coupling terms between the different groups are thus neglected. If it does so, use a finer increment in that range. The algorithm is, generally speaking, a Newton's method approach. Stationary (time-invariant) models with nonlinearities may converge very slowly. If the material properties entered are incorrect for the governing equation, the model will generate an error at runtime, usually a Singular Matrix error. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. My comment is perhaps a bit nave but it seems to me that you could simply deactivate the \frac{\partial \cdot}{\partial t} term of the background field equation but keep its connexion to the solid to get what you want. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. Ideally, one would use small elements in regions where the solution varies quickly in space, and larger elements elsewhere.
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