If you are using a Floating Network License you can also solve on a cluster. . This post was originally published in 2013. That is, the metric can be defined at one location in the model or as the integral of the fields over the entire model space. Referring back to the figure at the top of this blog post, we can observe that for the flat plate (and for most flow problems), the velocity field changes quite slowly in the direction tangential to the wall, but quite rapidly in the normal direction, especially if we consider the buffer layer region. In this case, it is necessary to solve the time-dependent Navier-Stokes equations, and the mesh used must be fine enough to resolve the size of the smallest eddies in the flow. Here, the Schmidt number is the ratio of observed momentum diffusivity (viscosity) to mass diffusivity. Another way to check the Minimum element quality is in the Statistics window. Fluid Flow, Heat Transfer, and Mass Transport, Fluid Flow: Conservation of Momentum, Mass, and Energy. Local adaptive mesh refinement differs from global adaptive mesh refinement in that the error is evaluated only over some subset of the entire model space, with respect to a specific metric. That said, the steps for setting up a model are the same no matter what physics you are modeling. All turbulence models in COMSOL Multiphysics, except the k- model, support automatic wall treatment. Consult your product manuals for complete trademark details. Finite element method Geometry: 1 Links below the video will allow you to jump back to certain modeling steps in order to review them again. It is lower in regions where the element growth rate increases from one element to the next. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Or, new boundary conditions have to be computed using wall functions. Browse the Application Gallery and download tutorial models with instructions and view example apps. An alternative way is to use the automatic wall treatment and start with a coarse boundary layer mesh to get wall functions and then refine the boundary layer at the interesting walls to get the low Reynolds number models. The results are shown to compare well with experimental data. The button Remove Clipping removes the Clip Sphere to again see the full mesh. Your internet explorer is in compatibility mode and may not be displaying the website correctly. This allows analysts to study many model and mesh variations in much less time, giving them the ability to quickly address all of the uncertainties. Time-adaptive mesh refinement remeshes the model at distinct time intervals and considers an error estimate of the solution at each interval as the metric by which to remesh the model. I am using COMSOL version 5.4. A screenshot of the COMSOL Multiphysics user interface. Any trademarks referenced in this document are the property of their respective owners. But the standard k- model and other commonly encountered k- models are not low Reynolds number models. The numerical solution of the steady-state NS (the time-dependent derivative in (1) is set to zero) and continuity equations in the laminar regime and for constant boundary conditions is as follows: Depending on the flow regime of interest, it is often possible to simplify these equations. In general, elements with a quality below 0.1 are considered poor quality for many applications. Low-quality elements can lead to difficulties for the solver to converge or make the solution sensitive to small changes in the mesh. The Reynolds number, Re=UL/, corresponds to the ratio of inertial forces (1) to viscous forces (3). Volume versus circumradius is based on a quotient of the element volume and the radius of the circumscribed sphere (or circle) of the element. Lets start by considering the fluid flow over a flat plate, as shown in the figure below. Spatial and Temporal Integration for a Heat Transfer Example Model. The volume elements for the expression qualskewness below 0.04 are displayed for the Shell-and-Tube Heat Exchanger model. Thermo-Structural Effects on a Cavity Filter, Two-Phase Flow with Fluid-Structure Interaction. Salmon, T.J. Bogar, and M. Sajben, Laser Doppler Velocimetry in Unsteady, Separated, Transonic Flow, AIAA Journal, vol. Thank to Comsol for updating the turbulent models, the new models in v.5.0 are very stable and fast to solve time-dependent response. For instance, for a coarse boundary layer mesh, the feature will utilize a robust wall function formulation. The domain of interest covers 640 m by 320 m. Low Reynolds number models often give a very accurate description of the boundary layer. Depending on the quality measure used, the Minimum element quality, Average element quality, and the Element Quality Histogram sections will change accordingly. To do so for a volume mesh, we set Level to Volume and select Type from the Element Color list. The software blends the two formulations in the boundary elements. Heat Exchangers You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version Ideally, you would have a very fast computer with many gigabytes of RAM to solve such problems, but simulations can still take hours or days for larger 3D models. In the image below, you see the boundary layer mesh around a sports car. These equations approximately represent the governing equation of interest via a set of polynomial functions defined over each element. The different terms correspond to the inertial forces (1), pressure forces (2), viscous forces (3), and the external forces applied to the fluid (4). The combined formulations are then used for the boundary conditions. Variable: mod1.epx If you are interested in using the COMSOL software for your CFD and multiphysics simulations, or if you have a question that isnt addressed here, please contact us. COMSOLMultiphysics . Monitor the memory requirements and DOFs for these smaller models. al. These time-averaged equations can then be computed in a stationary way on a relatively coarse mesh, thus drastically reducing the computing power and time required for such simulations (typically a few minutes for two-dimensional flow and a few minutes to a few days for three-dimensional flow). This will give you an idea of which mesh element size is needed to resolve the physics you are solving. This is a very useful approach for many practical engineering applications. Consider the example of a wheel rim, shown below. However, since the thickness of the buffer layer is so small, it can be advantageous to use an approximation in this region. COMSOL The uniform velocity profile hits the leading edge of the flat plate, and a laminar boundary layer begins to develop. This nano-design, combined with a systems level modelling, can enable substantially enhanced air It can sometimes be useful to use the k- model to first compute a good initial condition for solving the low Reynolds number k- model. I get an error message, that contains the words Out of memory, for example: Before resolving this issue, you should go through the following steps: First, check how much installed memory (RAM) is in your computer. Wall functions ignore the flow field in the buffer region and analytically compute a nonzero fluid velocity at the wall. The creeping flow example showing water flowing at a low speed through the porous media is a good example of incompressible flow. Resources and documents are provided for your information only, and COMSOL makes no explicit or implied claims to their validity. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Thank you, this post helped me a lot to understand the nature of my system. It is possible to use a RANS model to compute the flow field in all four of these regions. It is a low Reynolds number model and kind of the go to model for industrial applications. This involves investigating the relationship between heat transfer and structural mechanics, focusing on the materials of the structure as well as the displacement fields. The mesher will automatically inform the user if the quality is poor and warn the user if the quality is very poor, as elements of very poor quality should be avoided in most cases. Turbulence dissipation usually dominates over viscous dissipation everywhere, except for in the viscous sublayer close to solid walls. Get started using the COMSOL Multiphysics software. A good example of internal flow is flow through a pipe bend. You will always follow the same workflow when using COMSOL Multiphysics: 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. When the geometry does lead to a poor-quality mesh, the mesher returns the poor-quality mesh for examination, rather than no mesh at all. Meshing Example 1: Wheel Rim. Use the Measure button, available from the toolbar or the Graphics context menu for selected entities, to get the length of edges or distance between points. In that case, the density is assumed to be constant and the continuity equation reduces to u=0. 16901697, 1983. We sometimes encounter information, warning, and error messages while building a mesh. It has similar resolution requirements to the k- model and the low Reynolds number k- model, but its formulation eliminates some weaknesses displayed by pure k- and k- models. This information enables the prediction of real-world behavior, often with very high levels of accuracy. The v2-f turbulence model describes the anisotropy of the turbulence intensity in the turbulent boundary layer using two new equations, in addition to the two equations for turbulence kinetic energy (k) and dissipation rate (). When this happens, we should inspect the entities listed in the accompanied Information, Warning, and Error nodes. Knowing the size of the models that you want to solve is important. Thank you! If you anticipate running many models of similar size, then it is reasonable to consider a hardware upgrade. As we already mentioned, it can be important to understand where the elements of poor quality are located. The first equation describes the transport of turbulent velocity fluctuations normal to the streamlines. With our measurements and information about the entities, we can then set up Remove Details, Virtual Operations, or CAD Defeaturing, to eliminate the small geometric entities or reduce the mesh size if the features are important for the simulation. To find these geometric entities, we can use the Zoom to Selection button in the Settings window, next to the Selection list. There is a button, Center at Coordinates, that enables us to zoom in on a selected location. However, there are additional values you need to check when modeling turbulence. We do this either by clicking the Plot button in the Mesh ribbon or by right-clicking the Mesh head node of the mesh we would like to plot and selecting Plot. Your internet explorer is in compatibility mode and may not be displaying the website correctly. Space distributions of several modes for a rib waveguide. Some changes are underway that will ease these challenges. To understand where low-quality elements are positioned and which mesh size parameters to change, it can be a good idea to perform a plot of the mesh. The computational resources required to solve an FEA model are directly related to the number of DOF. Including initial strain It has also shown to predict separation and reattachment quite well for a number of cases. For this, we use the Geometric Entity Level drop-down menu at the top of the window. One way to quickly get an overview of the created mesh is to have a look at the statistics in the Mesh Statistics window, which we open by right-clicking the Mesh head node. In the laminar regime, the fluid flow can be completely predicted by solving Navier-Stokes equations, which gives the velocity and the pressure fields. You can fix this by pressing 'F12' on your keyboard, Selecting 'Document Mode' and choosing 'standards' (or the latest version The use of FEA software begins with a computer-aided design (CAD) model that represents the physical parts being simulated as well as knowledge of the material properties and the applied loads and constraints. After comparing a minimum of three successive solutions, an asymptotic behavior of the solution starts to emerge, and the changes in the solution between meshes become smaller. The disadvantage to this technique is that the computational requirements increase faster than with other mesh refinement techniques. Thank you for your help and time. The NS equations are valid for Kn<0.01. Additionally, the solvers are becoming more efficient, with the ability to solve huge models on cluster computers. The viscous and buffer layers are very thin and if the distance to the end of the buffer layer is \delta, then the log-law region will extend about 100\delta away from the wall. The low Reynolds number k- model can compute lift and drag forces and heat fluxes can be modeled with higher accuracy compared to the k- model. In this blog post, we discuss how to check entities reported in mesh Information, Warning, and Error nodes; how to view the statistics of a mesh or parts of a mesh; and how to plot a mesh and mesh-specific quantities and delimit these plots to selected regions. COMSOL For an example of this technique, please see Virtual Operation on a Wheel Rim Geometry, Submodeling is the process of solving a sequence of models with different levels of details and different meshes. Do not be deterred if this is not your application area; the process for creating a model is the same no matter the physics being modeled. Utilizing the unparalleled capabilities in COMSOL Multiphysics, this interface can be seamlessly coupled to other interfaces that describe fluid flow and heat transfer. The characteristic length of your model should therefore be larger than 6.8 m for the NS equations to be valid. The flow in the pores does not penetrate the solid part (gray area in the figure above). Most operating systems will also be using the hard drive (virtual memory) to store data. COMSOL A finite element solution must be verified by repeating the solution on different sized meshes. MPH, , COMSOL COMSOL . To gain confidence in the accuracy of your result, it is recommended to perform a mesh refinement study. Now that we know how to find out where the low-quality mesh elements are, we can either manually adjust the mesh in these regions or address the issues with the underlying CAD geometry itself. Error: "Out of memory Then, the software calculates the wall distance of the boundary elements grid points (this is in viscous units given by a liftoff). Remove cosmetic features. Note that the flow is unsteady, but still laminar in this model. When solving a model using low Reynolds number wall treatment, check the dimensionless distance to cell center (also generated by default). This means that the low Reynolds number models can be used for industrial applications and that their low Reynolds number modeling capability is only invoked when the mesh is fine enough. In the case of a compressible Newtonian fluid, this yields. I want to simulate lightning induced voltages on power systems by Comsol can you guide me that how can i do that? Inspecting the curved elements will be discussed at the end of the blog post. The k- model solves for two variables: k, the turbulence kinetic energy; and (epsilon), the rate of dissipation of turbulence kinetic energy. In the field of fluid dynamics, the different flow regimes are categorized using a nondimensional number, such as the Reynolds number and the Mach number. As the flow begins to transition to turbulence, oscillations appear in the flow, despite the fact that the inlet flow rate does not vary with time. , This example of turbulent flow in a partially baffled turbulent mixer shows how to set up The Rotating Machinery, Turbulent Flow interfaces from the Mixer Module with free surface and stationary free surface features. In the flow past a backstep example, Re = 100 and M = 0.001, which means that the flow is laminar and nearly incompressible. If the elements are located inside of the domain, add a Filter node below the Volume node and enter linearizedelem==1 in the Expression field. These models solve for the flow everywhere and are the most robust and least computationally intensive of the eight turbulence models. Toggling off the Mesh Rendering button and toggling on the Wireframe button for 3D meshes lets us easily see the entities reported inside the geometry, as shown below. On the other hand, if the applied excitation is of a single frequency or a range of known frequencies and the material properties are linear, then it is preferred for the modeling to take place in the frequency domain. I believe that what youre asking about here is a mesh refinement study: Switch to a segregated approach to solving the multiphysics problem. As a general rule of thumb, I would go with the default Discretization setting. COMSOL requests memory from the operating system and will always use both the physical RAM and virtual memory available on your system. 2) Check the size of your COMSOL model. The flow in this region is very predictable. By comparing these scalar quantities, it is possible to judge the convergence of the solution with respect to mesh refinement. It is important to keep this in mind during the modeling process, as there is no benefit in trying to resolve a model to greater accuracy than the input data admits. After computing the solution on the coarse mesh, the process of mesh refinement begins. When a message indicates that one or more low-quality elements has been generated, it might require some special attention. Solving for any kind of fluid flow problem laminar or turbulent is computationally intensive. to Make Boundary Conditions Conditional in Your Simulation Boundary layer meshes (which are the default mesh type on walls when using our physics-based meshing) insert thin rectangles in 2D or triangular prisms in 3D at the walls. This tutorial model represents a simple case of cavity flow noise in a ducted system. This gives us a Mesh dataset, available under Results > Datasets, under which we can add Selections to narrow down the amount of entities shown in the plot. The model is set up based on an example from Lafon et al. See if you have access to a machine with the needed amount of memory. They can be useful on any geometry to quickly ignore details which are not critical to the analysis. best personal regards, The model shown includes an electrical-thermal-structural coupling, with Joule heating and thermal expansion. An experienced user of FEA software should be familiar with each of these techniques and the tradeoffs between them. The bracket models are used as an introduction to structural mechanics modeling with the Structural Mechanics Module. , , , , COMSOLMultiphysics . Your internet explorer is in compatibility mode and may not be displaying the website correctly. This is why alternative methods to model the flow close to walls are often employed for industrial applications. The most labor intensive approach is for the analyst to manually create a series of different finite element meshes based upon the physics of the particular problem and an intuition as to where finer elements may be needed. Here, the aim is to improve mass transfer coefficients while simultaneously trying not to enhance heat transfer as much, thus improving the process at a systems-level. Let us first assume that the velocity field does not vary with time. Nicolas Huc November 18, 2019 COMSOL Employee. If you have further questions, feel free to send the model MPH file over to support@comsol.com and we will continue the discussion there.