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. November 11, 2013. For example, if there is a temperature-dependent material property such as: It is quite rare that changing these settings is superior to using a combination of the other techniques in this Knowledgebase, although it is possible to tune these settings to reduce solution time and memory requirements, once a model is already converging. In our previous blog entry, we introduced the Fully Coupled and the Segregated algorithms used for solving steady-state multiphysics problems in COMSOL. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. Solving such models in a stationary sense should simply require solving a single (large) system of linear equations and should always be solvable, but there are cases when the software will fail to find a solution. 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. 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. In particular, choosing an improper initial condition or setting up a problem without a solution will simply cause the nonlinear solver to continue iterating without converging. That is, the material property changes instantaneously from 10W/m/K to 20W/m/K at 400K. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) P&S Comsol Team: Yannik Horst, Manuel Kohli, Xinzhi Zhang. Does anyone know what should cause this problem? If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help. Multiscale Modeling in High-Frequency Electromagnetics. k(T,P) = 10[W/m/K]*((1-P)+P*exp(-(T-293[K])/100[K])) It is sometimes necessary to manually scale the dependent variables. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. Linear solvers. An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: Stationary in the COMSOL Multiphysics Programming Reference Manual Damped Newton Methods The nonlinear solver uses an affine invariant form of the damped Newton method as described in Ref. It may also reveal that the model itself is ill-posed in some way. Convergence can be poor when the initial values do not provide a good starting point for this iterative approach. How can I check before my flight that the cloud separation requirements in VFR flight rules are met? Again, introduce a Global Parameter that gets ramped from exactly zero to one. This approach is known as a Continuation Method with a Constant predictor. If one particular material is missing one property, that material will also be highlighted with a red cross over that material icon in the Model Builder. It is thus always advised to start this procedure with a simplified 2D, or 2D-axisymmetric model. Find centralized, trusted content and collaborate around the technologies you use most. Nonlinearity ramping is an especially useful technique if any of the nonlinear terms in the model are very abrupt. Using a predictor of type Constant will take the solution from the iteration and use it as the initial value for the iteration. 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. It is also possible to manually refine the mesh. However, it is usually not possible to know this ahead of time. 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. Hi Jason, So far, weve learned how to mesh and solve linear and nonlinear single-physics finite element problems, but have not yet considered what happens when there are multiple different interdependent physics being solved within the same domain. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Use either a very fine mesh throughout the simulation domain or use adaptive mesh refinement. If both load ramping and nonlinearity ramping are still leading to slow convergence, refine the mesh. -Detail: NaN or Inf found when solving linear system using SOR. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. listed if standards is not an option). There will also be a red cross over the Materials branch icon. However, load ramping will not work in all cases, or may be inefficient. For example, in Solid Mechanics, if the Poisson Ratio set to 0.5, then the model will not solve, as this value in incompatible with the theory of linear elasticity. By providing your email address, you consent to receive emails from COMSOL AB and its affiliates about the COMSOL Blog, and agree that COMSOL may process your information according to its Privacy Policy. 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. There will always already be either a Segregated or Fully Coupled feature beneath this. Get email updates for new Stationary Engineer jobs in Brea, California, United States. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Could you expand a little bit more why the coupling is impossible? Examine the model and identify all terms that introduce nonlinearities, such as multiphysics couplings, nonlinear materials relationships, and nonlinear boundary conditions. Your email address will not be published. Alternatively, delete and re-create the study. You can unsubscribe from these emails at any time. P&S: COMSOL Design Tool for Photonic Devices. Note: there is no way to couple this . This approach is used by default for most 1D, 2D, and 2D-axisymmetric models. Use this parameter to modify the nonlinearity expressions in the model. If instead the model is linear, see: Knowledgebase 1260: What to do when a linear stationary model is not solving. 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. - 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. 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. This guide applies solely to nonlinear stationary models. The software then computes an initial solution and from there it iteratively re-computes the solution, taking into account how these intermediate solutions affect the nonlinearities. Click the link in the email we sent to to verify your email address and activate your job alert. Despite this, the segregated approach can often converge very robustly, unless there are very strong couplings between the physics in the model. The latter method is known as the Continuation Method with a Linear predictor, and is controlled within the Study Configurations as shown in the screenshot below. If it is not clear that any of the above strategies are working, it is useful to take a more general approach to verifying the general validity of the model. Any trademarks referenced in this document are the property of their respective owners. Linear solvers. Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. Common Mistakes: Not assigning materials to all the domains. Use this parameter to modify the nonlinearity expressions in the model. COMSOL does not assume any legal liability for the accuracy of the data disclosed. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. 140K views 8 years ago COMSOL Multiphysics Tutorial for Beginners Please note that an updated version of the content in this video can be found in the Modeling Workflow video in the COMSOL. 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. They are usually called comp1.u, comp1.v, and comp1.w though. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version An example model that combines the techniques of nonlinearity ramping and adaptive mesh refinement with multiple study steps is: The former approach solves for all unknowns in the problem at once, and considers all coupling terms between all unknowns within a single iteration. Cecilia M. came from the backroom to help me and went out of her way to help me even though I had not called before going into the store. Using this technique systematically, along with the techniques described previously, will usually identify the nonlinearities in the model that are leading to issues. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. Such problems must solved in the time domain. Any trademarks referenced in this document are the property of their respective owners. (Frequency Domain should be the last step) You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Therefore, an initial value of zero is almost always reasonable if a very small load is applied. The Continuation method is enabled by default when using the Auxiliary sweep study extension, as shown below. Note that while COMSOL employees may participate in the discussion forum, COMSOL software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team. Segregated approach and Direct vs. Iterative linear solvers, About the time step setting of the solver, Introducing Goal Seeking into the Segregated Solver. The other low-level default settings within the Stationary Solver are chosen for robustness. Feature: Stationary Solver 1 (sol1/s1) Attempt to evaluate nonintegral power of negative number. The Fully Coupled solution approach, with the Plot While Solving enabled. Dr.S.Ravindran Cite 1 Recommendation Popular answers (1). Starting from zero initial conditions, the nonlinear solver will most likely converge if a sufficiently small load is applied. Then I stored the resulting solution, and used it to to solve the stationary solver again by using the solver manager. Not the answer you're looking for? Extending this logic, if one wants to solve for any arbitrary load on a nonlinear system, it makes sense to solve a sequence of intermediate problems with gradually increasing load values and using the solutions from each previous step as the initial condition for the next step. Such problems must solved in the time domain. Stationary Solver Iterative Direct . 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. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Understanding the Fully Coupled vs. This algorithm was also useful for understanding what happens near a failure load. There are two approaches that can be used when iteratively solving the nonlinear system of equations: a Fully Coupled or a Segregated approach. I highly recommend this flower shop. Feature: Stationary Solver 1 (sol1/s1)" . Right-click on the Stationary Solver node and add either the Segregated or Fully Coupled feature. The stationary solver is used both for Stationary (time-invariant) and Frequency Domain (time-harmonic) study types. Understanding the Fully Coupled vs. Sign in to create your job alert for Stationary Engineer jobs in Brea, California, United States. Repeat this for every nonlinearity of the model. Iterative , Direct . In such cases it will be particularly helpful to ramp the load gradually in time, from consistent initial values. The advantages of the continuation method are two-fold. The algorithm is, generally speaking, a Newton's method approach. - Variable: B1 - Defined as: 1/ ( ( (comp1.cH2 (unit_m_cf^3))/unit_mol_cf)^2.5) Failed to evaluate variable. . This is useful since the software will then return an estimation of the maximum possible loadcase for which the solver can converge. These are some cards & stationery with a large number of reviews in Brea, CA. 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. Once a simplified solvable version of the model has been found, gradually increase the model complexity again, re-introducing nonlinearities and multiphysics couplings. This parameter is used within the physics interfaces to multiply one, some, or all of the applied loads. The Auxiliary Sweep can be used to implement ramping of any Global Parameter. The Fully Coupled solution approach, with the Plot While Solving enabled. That is, start by first solving a model with a small, but non-zero, load. If so, see: Knowledgebase 1030: Error: "Out of memory". Get notified about new Stationary Engineer jobs in Brea, California, United States. I have searched all over comsol forum to fix this stationary solver configuration and still doesn't work because I don't know the logic behind the solver system. Therefore, an initial value of zero is almost always reasonable if a very small load is applied. If these settings have been manually changed, you will see a small star symbol on the Solution feature, as shown in the screenshot below. For example, in an Electric Currents problem, you may want to consider a system of materials including a good conductor such as copper (with an electric conductivity of ~6e7 S/m) and an insulative material such as glass (which can have electric conductivity of ~1e-14 S/m.) That is: Even if the forces on a part are opposite and equal, this is not sufficient information to say where the part is, so you must add some other condition, such as as Fixed Constraint to constrain displacement. COMSOL does not assume any legal liability for the accuracy of the data disclosed. To start a new discussion with a link back to this one, click here. Such a case would be better to address instead with the Shell physics interface, which is specially formulated for handling thin-walled structural parts. - Feature: Stationary Solver 1 (sol1/s1) Mesh Each physics is thus solved as a standalone problem, using the solution from any previously computed steps as initial values and linearization points. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. 3 Replies, Please login with a confirmed email address before reporting spam. To start a new discussion with a link back to this one, click here. That is, within each outer Newton-type iteration, the segregated approach solves for each segregated group sequentially. Why do many companies reject expired SSL certificates as bugs in bug bounties? Reviews on Stationary Stores in Brea, CA - Typo, Mievic, Posh Paper, Morning Glory, Select Graphics & Printing, Daiso Japan, Urban Outfitters, Office Depot, Barnes & Noble (Frequency Domain should be the last step) Leverage your professional network, and get hired. A Global Parameter has to be introduced (in the above screenshot, P) and is ramped from a value nearly zero up to one. The technique of load ramping is not always reasonable for all problems. The default Initial Values for the unknowns in most physics interfaces are zero. k(T) = 10[W/m/K]+10[W/m/K]*(T>400[K]) Second, the continuation method will automatically take smaller load increments if a solution cannot be found. The objective here is to simplify the model to a state where the model will solve, with linear approximations. thanks for reply It can be useful while solving sequences of linear systems arising from, for example, nonlinear problems. About the Stationary Solver The following background information about the Stationary Solver discusses these topics: Damped Newton Methods, Termination Criterion for the Fully Coupled and Segregated Attribute Nodes, Linear Solvers versus Nonlinear Solvers, and Pseudo Time Stepping. I use comsol multiphysics 5.2a and . These can be used alone, or in combination with other interfaces. See Knowledge Base 1240: Manually Setting the Scaling of Variables. An example would be a linear static structural model of an aluminum beverage can. With respect to multiphysics couplings, rather than solving the problem using a fully coupled approach (the default) solve the problem sequentially, with one physics being solved after another. Changes to these low-level settings from the defaults will usually be quite model- and case-specific. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Tutti i diritti sono riservati. Stationary (time-invariant) models with nonlinearities may converge very slowly. The exceptions are the Heat Transfer interfaces, which have a default Initial Value of 293.15K, or 20C, for the temperature fields. The fully coupled and segregated approaches are discussed below. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. The fully coupled and segregated approaches are discussed below. Any trademarks referenced in this document are the property of their respective owners. This involves a systematic reduction in the model complexity. Posted Sep 9, 2020, 1:44 p.m. EDT Contact COMSOL at Bangalore on their telephone: +9180 25323003. This case is generally difficult, or impossible, to solve since this material property is non-smooth.

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