The Subsonic Cartesian analysis type offers a robust meshing strategy producing hexahedral cells suitable for the underlying solver thereby reducing mesh generation times by huge margins. Because the mesh is high quality, it requires fewer cells to achieve the same level of accuracy. It offers faster convergence at the cost of a reduced feature set. Some highlights of the mesher are:
Subsonic Cartesian is a Finite Volume based CFD solver with segregated pressure-velocity coupling using a proprietary variant of the SIMPLE\(^1\) algorithm. It provides the possibility to simulate both incompressible and compressible, laminar or turbulent flow with or without heat transfer in a single framework.
Within SimScale, one can effortlessly set up a Subsonic Cartesian simulation which involves the following steps:
To create a Subsonic Cartesian analysis, first, select the desired geometry and click on ‘Create Simulation’:
Next, a window with a list of several analysis types supported in SimScale will be displayed:
Choose Subsonic Cartesian analysis type and click on ‘Create Simulation’. This will lead to the SimScale Workbench with the following simulation tree and the respective settings:
To access the global settings, click on ‘Subsonic Cartesian’ in the simulation tree. At present, the user can set a steady-state analysis and the following parameters are available to define the fluid simulation:
The Geometry section allows you to view and select the CAD model required for the simulation. In general, having a clean CAD model always helps to avoid any meshing or simulation related errors. However, the mesher is pretty robust and if the CAD is genuinely dirty then a message of mesh failure and further instructions on CAD cleaning or mesh refinement should appear.
Find more details on CAD preparation and upload here.
Under Materials, the appropriate fluid for the simulation can be chosen from the materials library. The user has the option to change certain fluid properties. For compressible simulations, since the temperature is involved the user needs to specify the corresponding thermophysical properties for the fluid in consideration.
For more information, please visit the relevant documentation page for materials.
Boundary conditions help to add a closure to the problem at hand by defining how a system interacts with the environment. In an incompressible simulation, the computational domain will be solved for two fields: pressure \((P)\), velocity \((U)\), while for a compressible simulation the temperature \((T)\) field is also involved. Additional turbulent transport quantities may be included based on the turbulence model selected.
The user gets to choose from velocity, pressure, and wall boundary conditions to be assigned.
Important
In case no boundary conditions are assigned to a face, by default it will receive a no-slip wall boundary condition with wall function for turbulence resolution along with a zero-gradient condition for temperature.
The Simulation control settings define the general controls over the simulation. For a Subsonic Cartesian analysis the following series of variables can be set:
For a complete overview of these parameters and their meaning, check out this page.
Meshing is the process of discretizing the simulation domain into a large set of finite volumes adhering to the CAD geometry.
For this analysis type, the mesh generated is a Cartesian mesh which means that each cell represents a cube with edges parallel to the XYZ Cartesian axes making the mesh highly orthogonal.
The meshing algorithm follows a top-down approach which works by creating a binary tree structure. The bounding box (an imaginary box encompassing the dimensions of the flow domain) is refined anisotropically by cutting cells in the first layer in the x-direction, second layer in the y-direction, and third layer in the z-direction.
Most features on an input tessellated surface smaller than the local cell size are ignored. More importantly, the algorithm does not generate boundary layers. Pyramidal cells are also used to capture the features and walls by projecting the mesh to the object’s surface.
For the Subsonic Cartesian mesh, the settings can be set to automatic or manual. The parameters under each mode are described below:
The user gets to define a fineness level for the mesh cells varying from Coarse (1) to Fine (10).
The user gets to define the following parameters:
Note
The meshing log and the mesh are only visible once the simulation is finished. However, the time per iteration and the time to convergence are a lot lower compared to other solvers.
Your core hours will be consumed only after a successful
simulation run.
The Result Control section allows users to define additional simulation result outputs. It controls how the results will be written meaning the write frequency, location, statistics of the output data, etc. The following control items are supported:
Did you know?
An advantage of using Subsonic Cartesian analysis involving rotating flows is that power about the axis of rotation is directly output.
Under Advanced concepts, you will find additional setup option for rotating zones. Rotating zones can be used to model rotating systems such as turbines, fans, ventilators, and similar systems. Currently, for the steady-state analysis type, the MRF (Multiple Reference Frame) type is supported.
We recommend you to visit this dedicated page on how to set up rotating zones for more details.
Once all the settings are defined, click on ‘Simulation Runs’ to begin the simulation.
Last updated: July 20th, 2021
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