Week 1: Channel flow simulation using CONVERGE CFD

In this report, I will be simulating a channel flow in a domain of the dimensions: 0.1m long, 0.1m tall and the wideness of the domain is infinity (the flow does not vary across the z axis) for 3 different mesh sizes.

Geometry Creation

1. Create Tool - using the create tool, a cuboidal geometry is created of l_x = 0.1m, l_y = 0.01m and l_z = 0.01m

2. Boundaries - using the select triangles tool, multiple triangles are selected and given user defined names

3. Normals were made visible, the normals should be pointing into the flow/domain

4. Transform - this tool was used modify normal options

5. Diagnosis - a final diagnosis was done to ensure no errors are presented in the geometry

 

This is the final geometry seen in converge

Case Setup

Now, on to the case setup tab:

Application Type

Materials: Gas Simulation and Global Transport Parameters were set to default. In species, O2 and N2 were added.

Simulation Parameters

Boundary Conditions

Inlet: 10001 Pa

Outlet: 10000 Pa

Top and Bottom walls: No-Slip, Temperature = 300K

Front and Back: TWO_D

Initial Conditions

Physical Models

Turbulence was unchecked as pressure in this case is too small to cause turbulent flow.

Grid Control

This is the step were sizes of each element is provided. 3 sizes were used in this analysis, they are:

 

•  dx = 2e-4m,              dy = 2e-4m,       dz = 2e-4m
•  dx = 1.5e-4m,           dy = 1.5e-4m,    dz = 1.5e-4m
•  dx =  1.0e-4m,          dy = 1.0e-4m,    dz = 1.0e-4m

 

Output/Post Processing

Again, since flow will be laminar - all turbulent variables were unchecked.

Now, our case setup is complete. The files will be exported into a folder using the Files Export tool (File -> Export->Export input files)

In total 10 files were exported, these are:

These files contain all the necessary information for the simulation.

Running the Simulation

1. Open cygwin

2. Navigate to directory where case files were exported

3. Run the following command

mpiexec.exe -n 4 "C:\Program Files\Convergent_Science\CONVERGE\3.0.16\bin\intelmpi\converge.exe" restricted </dev/null> logfile.txt &

This will take a while, you can view the progress in task manager. CPU usage is usually maxed out.

Once CPU usage drops from 100%, the output files are generated. To view them in paraview, we must export them to a format which is supported by paraview.

Go to 3D-post processing in converge,

Post-Processing

In Paraview

Import these files into paraview

The required plots/animation are generated

In converge

Go to Line plotting, select the case folder and plots can be viewed

 

Outputs and Plots for each Case

Case 1: dx = 2e-4m, dy = 2e-4m, dz = 2e-4m

This is the Grid Control values used for this case

1. Velocity and Pressure Contours

These were taken in paraview.

Inlet Pressure is higher as expected (and from the boundary condition) and pressure gradually decreases across the domain as the flow stabilises.

Velocity plot is as expected for a developing laminar flow towards the left of the domain and eventually becomes laminar flow. The Velocity plot at the end of the domain across the section shows the distinct velocity plot expected for laminar flow. The flow is the lowest near the walls (due to boundary layer effect) and increases gradually, the maximum value of velocity is seen at the center of the domain (around 3.3 m/s).

2. Mesh

This was taken in paraview.

The mesh is less fine as grid spacing is relatively high for this case.

3. Velocity, Pressure, MassFlowRate and Cell Count Plots

This was taken in converge -> Line plotting

Velocity at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both velocities' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed (since density is constant). The velocities initially start from 0 and slowly approach the true value as the solution nears the true value.

Pressure at the inlet stays constant because of the boundary condition, and outlet pressure actually increases slightly to compensate for mass conservation.

Mass flow rate at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both masses' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed. The masses initially start from 0 and slowly approach the true value as the solution nears the true value.

Cellcount, all values remain constant as expected, the total cell count is around 25000, and number of cells solved by each core is also seen.

5. Animation

https://youtu.be/C6k2Fi_kJVg

In this animation, we can see flow developing and eventually stabilising

Case 2: dx = 1.5e-4m, dy = 1.5e-4m, dz = 1.5e-4m

This is the Grid Control values used for this case

1. Velocity and Pressure Contours

These were taken in paraview.

Inlet Pressure is higher as expected (and from the boundary condition) and pressure gradually decreases across the domain as the flow stabilises.

 

Velocity plot is as expected for a developing laminar flow towards the left of the domain and eventually becomes laminar flow. The Velocity plot at the end of the domain across the section shows the distinct velocity plot expected for laminar flow. The flow is the lowest near the walls (due to boundary layer effect) and increases gradually, the maximum value of velocity is seen at the center of the domain (around 3.1 m/s).

2. Mesh

This was taken in paraview.

The mesh is more fine as grid spacing is relatively medium for this case.

3. Velocity, Pressure, MassFlowRate and Cell Count Plots

This was taken in converge -> Line plotting

Velocity at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both velocities' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed (since density is constant). The velocities initially start from 0 and slowly approach the true value as the solution nears the true value.

Pressure at the inlet stays constant because of the boundary condition, and outlet pressure actually increases slightly to compensate for mass conservation.

 

Mass flow rate at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both masses' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed. The masses initially start from 0 and slowly approach the true value as the solution nears the true value.

 

Cellcount, all values remain constant as expected, the total cell count is around 45000, and number of cells solved by each core is also seen.

5. Animation

https://youtu.be/1v2pBxTHzcg

In this animation, we can see flow developing and eventually stabilising

Case 3: dx = 1e-4m, dy = 1e-4m, dz = 1e-4m

This is the Grid Control values used for this case

1. Velocity and Pressure Contours

These were taken in paraview.

Inlet Pressure is higher as expected (and from the boundary condition) and pressure gradually decreases across the domain as the flow stabilises.

Velocity plot is as expected for a developing laminar flow towards the left of the domain and eventually becomes laminar flow. The Velocity plot at the end of the domain across the section shows the distinct velocity plot expected for laminar flow. The flow is the lowest near the walls (due to boundary layer effect) and increases gradually, the maximum value of velocity is seen at the center of the domain (around 3.05 m/s).

2. Mesh

This was taken in paraview.

The mesh is most fine as grid spacing is relatively low for this case.

3. Velocity, Pressure, MassFlowRate and Cell Count Plots

This was taken in converge -> Line plotting

Velocity at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both velocities' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed (since density is constant). The velocities initially start from 0 and slowly approach the true value as the solution nears the true value.

Pressure at the inlet stays constant because of the boundary condition, and outlet pressure actually increases slightly to ensure mass conservation.

Mass flow rate at the inlet is negative since flow enters the domain is considered negative, while velocity at the outlet is positive. Both masses' abosolute values are roughly the same throughout the simulation showing that mass conservation is followed. The masses initially start from 0 and slowly approach the true value as the solution nears the true value.

Cellcount, all values remain constant as expected, the total cell count is around 100000, and number of cells solved by each core is also seen.

5. Animation

https://youtu.be/z5ZvyrFkjwQ

In this animation, we can see flow developing and eventually stabilising.