The goal of this tutorial example is to demonstrate various meshing capabilities of Flow Simulation allowing you to perform manual adjustment of the computational mesh. Although the automatically generated mesh is usually appropriate, intricate problems with thin and/or small, but important, geometrical and physical features can result in extremely high number of cells, for which the computer memory may be too small. In such cases we recommend you trying the Flow Simulation options allowing you to manually adjust the computational mesh to the solved problem's features to resolve them better. This tutorial teaches you how to do this.
The considered Ejector in Exhaust Hood example aims to:
• Settle the large aspect ratio between the minimum gap size and the model size by adjusting the initial mesh manually.
Problem Statement
The ejector model is shown in the picture below. Note that the ejector orifice’s diameter is more than 1000 times smaller than the characteristic model size determined as the computational domain’s overall dimension.
Baffles
Ejector
Opening Exhaust
Opening the SolidWorks Model
Copy the B4 – Mesh Optimization folder into your working directory and ensure that the files are not read-only since Flow Simulation will save input data to these files. Open the Ejector in Exhaust Hood.SLDASM assembly.
Creating a Flow Simulation Project
Using the Wizard create a new project as follows:
When you enable gravitation, pay attention that the hydrostatic pressure is calculated with respect to the global coordinate system, as follows:
Phydrostatic = ρ(gx*x + gy*y+ gz*z), where ρ − reference density, gi - component of the gravitational acceleration vector and x, y, z - coordinates in the global coordinate system.
Project Configuration Use current
Unit system USA
Analysis type Internal; Exclude cavities without flow conditions
Physical features Gravity; Default gravity (Y component: -32.1850394 ft/s^2)
Fluids substances Air, Chlorine
Wall Conditions Adiabatic wall, default smooth walls
Initial Conditions Initial gas concentration: Air – 1, Chlorine - 0 Result and Geometry Resolution Default result resolution level 3;
Default geometry resolution: automatic minimum gap size and minimum wall thickness, other options by default
Specifying Boundary Conditions
At first, let us specify all the necessary boundary conditions because they influence the automatic initial mesh settings through the automatic minimum gap size, which depends on the characteristic size of the faces on which the boundary conditions are set.
Flow Simulation calculates the default minimum gap size using information about the faces where boundary conditions (as well as sources, fans) and goals are specified. Thus, it is recommended to set all conditions before you start to analyze the mesh.
The first two boundary conditions are imposed on the exhaust hood's inlet and outlet.
Open the Initial Mesh dialog box (click Flow Simulation, Initial Mesh) and select the Manual specification of the minimum gap size option. You will see that the current automatic minimum gap size is 0.5 ft, which is the width of the outlet opening. Click
Cancel to close this dialog box. Inlet
Boundary Condition
Environment Pressure:
Default values (14.6959 lbf/in2, gas substance – Air) of the Environment pressure and Temperature (68.09 °F)
at the box’s Lid for Face Opening;
Outlet Boundary Condition
Outlet Volume Flow: Outlet volume flow rate of
The next inlet volume flow rate condition defines the gas ejected from the bottom of the Ejector component.
If you now look at the automatic minimum gap size value (click Flow Simulation, Initial
Mesh, Manual specification of the minimum gap size), you will notice that it is now
changed to approximately 0.00446 ft, which is close to the orifice diameter.
The Minimum gap size is a parameter governing the computational mesh, so that a certain number of cells per the specified gap should be generated. To satisfy this condition the corresponding parameters governing the mesh are set by Flow
Simulation (number of basic mesh cells, small solid features refinement level, narrow channel resolution, etc.). Note that these parameters are applied to the whole
computational domain, resolving all its features of the same geometric characteristics (not only to a specific gap).
Since the minimum gap size value influences the mesh in the entire computational domain, the large aspect ratio between the model and the minimum gap size value will produce a non-optimal mesh: not only will all small gaps be resolved, but there will also be many small cells in places where they are not necessary. As a result, an extremely large mesh will be produced, which may result in overly large computer memory requirements exceeding the computers' available resources. Moreover, if the aspect ratio between the model and the minimum gap size is more than 1000, Flow Simulation may not adequately resolve such models with the automatically generated mesh anyway.
Finally, let us create the ejector’s porous media and apply it to the ejector’s top and side screens.
The material you are going to create is already defined in the Engineering Database under the Pre-Defined folder. You can skip the definition of the porous material, then when creating the porous condition, select the pre-defined "Screen Material" from the Engineering database.
Inlet Boundary Condition
Inlet Volume Flow: Inlet chlorine (Substance
concentrations: Chlorine – 1; Air – 0) volume flow rate of 0.14 ft3/min at the lid that closes the orifice (make sure that you have selected the upper face of the lid).
To see advantages of using the local mesh and refinement options, let us first try to generate the computational mesh governed by the automatic mesh settings. The resulting mesh will consist of more than 1000000 cells, and may be not processed by some computers due to the computer memory restriction (you may get a warning message about insufficient memory)