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The mean age of air in a room validation case belongs to fluid dynamics. This case aims to validate the following parameter:
In this project, the local mean age of air (LMA) within a room is calculated. The SimScale results are compared to the experimental results reported by Martin et. al
The geometry consists of a rectangular room with one inlet and one outlet, as in Figure 1:
The center points of the inlet and outlet are placed at y = 1.8 meters. Details of the room dimensions are provided in Table 1:
Part | Length in x-direction | Length in y-direction | Length in z-direction |
Inlet | 1.92 | 0.30 | 0.20 |
Outlet | 0.20 | 0.30 | 1.92 |
Room | 4.20 | 3.60 | 3.00 |
In the study by Martin et. Al
Note
By extruding the inlet and outlet sufficiently, we can allow the flow field to develop in these sections. In this case study, the inlet and outlet are extruded by a length equal to 8 times their hydraulic diameter
In the formula above,
Tool Type: OpenFOAM®
Analysis Type: Incompressible
Turbulence Model: k-omega SST
Mesh and Element Types: This validation case uses a total of 3 meshes to perform a mesh independence study. All meshes were created in SimScale with the Standard mesher algorithm. In Table 2, an overview of them is presented:
Mesh | Mesh Type | Cells | Element Type |
Coarse | Standard | 192199 | 3D tetrahedral/hexahedral |
Moderate | Standard | 538610 | 3D tetrahedral/hexahedral |
Fine | Standard | 2009591 | 3D tetrahedral/hexahedral |
Figure 3 highlights the discretization of the inlet, obtained with the fine standard mesh.
Material:
Boundary Conditions:
Figure 4 shows the assignment of the boundary conditions and the corresponding surfaces:
The exact configuration of the boundary conditions is given in the table below:
Boundary | Boundary Type | Velocity | Pressure |
Inlet | Velocity Inlet | 1.68 in the x-direction | Zero gradient |
Outlet | Pressure Outlet | Zero Gradient | Fixed at 0 |
Walls | Automatic Wall | No-Slip | Zero gradient |
Model:
Note
The diffusion coefficient is purposedly set to a small number. The objective is to prevent the scalar from spreading in the domain via diffusion effects, which would reduce the accuracy of the local mean age of air.
Mean age of air calculation
To track the LMA within the room the in-built SimScale field function is used. To activate this feature extend Result control and create a new ‘Mean age of fluid’ field function by clicking on the plus icon. The diffusion coefficients and the turbulent Schmidt number are set as described before.
Mean age of Fluid Field Function
You can find out moe information about the Mean age of Fluid Field Function here.
In the experimental tests, Martin et. al
A series of gas monitors are used to measure how the concentration of the tracer gas evolves with time. The resulting LMA is obtained by calculating the area under the concentration versus time curve.
Note
The theoretical value for the mean age of fluid at the outlet is given by Equation 2:
Where
The numerical simulation results for the LMA are compared with experimental data presented by Martin et. al
Where:
In addition, the results by Martin et. al
To perform a mesh independence study, the results from the three meshes created in SimScale were compared. The results for all three meshes were found to be mesh independent. Figure 7 shows the results over the line located 1.13 meters away from the inlet.
In the remaining figures, you will find the comparison between the experimental data and the fine mesh results:
In all cases, the SimScale results show the same trends and range as the experimental values obtained by Martin et. al
The figure below shows the mean age of fluid on the symmetry plane of the geometry. The fresh air coming from the inlet quickly mixes with the old air in the room. The mean age of air at the outlet for the fine mesh was found to be 451.8 seconds.
Last updated: January 3rd, 2023
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