Channel flow simulation using CONVERGE CFD

                                                                 STEADY STATE CHANNEL FLOW SIMULATION USING CONVERGE CFD

                                                                                                       (WEEK-1 CHALLENGE)

1. AIM

Our aim is to setup a steady state channel flow in converge, simulate it using Cygwin terminal and post process in Paraview for three different mesh sizes and compare the results.  

2. THEORY/EQUATIONS/FORMULAE USED

Structure of CONVERGE CFD simulations

The basic steps for a simulation are as follows,

1. Pre-processing [Converge Studio v3.0] – It is a leading CFD package for simulating 3D fluid flow. It is a user friendly interface which provides high productivity and easy-to-use workflows. Its features includes autonomous meshing, state of the art physical models, robust chemistry solver, and the ability easily accommodate complex moving geometries.
2. Solving [Cygwin] – It is a large collection of GNU and Open Source tools which provide functionality similar to a Linux distributionon Windows.
3. Post-processing [ParaView 5.9.0] – It includes analyzing the results by plotting the results as charts, contour, and exporting the data, also validating the results both qualitatively and quantitatively.

Problem – Rectangular Channel Flow

The challenge includes steady state flow simulation through a rectangular channel, Grid Independent study and to check the results.   

Mesh Size

dx = 0.0002 m, dy = 0.0002 m, dz = 0.0002 m

     dx = 0.00015 m, dy = 0.00015 m, dz = 0.00015 m

dx = 0.0001 m, dy = 0.0001 m, dz = 0.0001 m

3. PROCEDURE

• Firstly all the softwares are downloaded and installed in the system.
• Geometry is created in converge studio, boundary is flagged, and boundaries are checked for normals and orientation is done as per the requirement and finally diagnosis check is carried out to check for any errors such as intersections, open edges, non-manifold edges, overlapping triangles etc.
• Case setup is done using setup wizard, then the input files are exported to a specific directory and also converge executable application are also copied into the directory.
• The simulation is run using Cygwin commands and post converted the output files so that it is compatible for post processing in Paraview.

4. NUMERICAL ANALYSIS
   • Geometric Model

The 3D geometry of Rectangular channel flow domain is created in Converge Studio as per the figure given below.   

Origin – (0.05, 0, 0)        Size - dx - 0.1   dy- 0.01 dz - 0.01

3D Geometry – Rectangular Channel

• Mesh

                                                                                                                            Fig: Mesh

• Boundaries

                                                                                                           Fig: Boundaries of the domain

Case SetUp

• In Materials tab, materials – Air, species – O2, N2 and global transport parameters are selected.Case Set-up                                                                                                                                                

• In Simulation Parameters tab, Run parameters such as Solver type, Simulation Mode and Gas flow Solver are set. In Misc. Tab uncheck the shared memory and check the steady state monitor.

• Steady State monitors are created as by specifying the target type as boundaries and target as outlet for the problem.

• Simulation time parameters such as total number of cycles, time step, CFL limits etc are set as shown in the figure below.

• Solver parameters such as the Navier stokes solver type, equations preconditions type, solver controls are set.

• Boundary Conditions and Initial Conditions

• Regions and Initialization – Fluid domain is created and the Species is added to the fluid domain.

• Base Grid – Mesh sizes are entered as per the problem.

• Post Variable Selection - Select all the necessary variables and the location that needs to be checked while post processing.

• Output Files – Output files writing time intervals, restart files are set as per the requirement.

• After the setup is done click on “Validate all” option to check for any issues with the case setup. If everything is correct then green tick will appear for all tabs as shown in the figure below. Once the Setup is done, export the input files.

• With input and executable files, navigate to the specific directory in Cygwin and run the simulation using "exe -n 4 converge.exe restricted logfile &"
• After the run is completed Post convert the output files using “exe -n 4 post_convert.exe” into binary inline vtk format.
• Using the vtm group files, post processing is done in ParaView in order to study the results.

5. RESULTS

• Case 1 [dx = 0.0002 m, dy = 0.0002 m, dz = 0.0002 m]

                                                                                                    Fig: Mesh                                                                                                                                  Fig: Total Cell Count

                                                                                        Fig: Mass Flow Rate Plot                                                                                                                  Fig: Total Pressure Plot

                                                                                     Fig: Average Velocity Plot                                                                                                    Fig: Average Velocity Profile

                                                                                                Fig: Pressure Contour                                                                                            Fig: Velocity Contour

Animation Link

Velocity – https://youtu.be/ODpjmY3G3U4

Pressure – https://youtu.be/F9b5JCTt2ls

• Case 2 [dx = 0.00015 m, dy = 0.00015 m, dz = 0.00015 m]

                                                                                                    Fig: Mesh                                                                                                                              Fig: Total Cell Count

                                                                                               Fig: Mass Flow Rate Plot                                                                                                Fig: Total Pressure Plot

                                                                                                     Fig: Average Velocity Plot                                                                                                   Fig: Average Velocity Profile

                                                                                                         Fig: Pressure Contour                                                                                             Fig: Velocity Contour

Animation Link   

Velocity – https://youtu.be/ZSsgJclu_5Y

Pressure – https://youtu.be/qrTu_UyEol0

• Case 3 [dx = 0.0001 m, dy = 0.0001 m, dz = 0.0001 m]

                                                                                                   Fig: Mesh                                                                                                                       Fig: Total Cell Count

                                                                                                               Fig: Mass Flow Rate Plot                                                                                Fig: Total Pressure Plot

                                                                                                              Fig: Average Velocity Plot                                                                                                 Fig: Average Velocity Profile

                                                                                                                        Fig: Pressure Contour                                                                                                   Fig: Velocity Contour

Animation Link

Velocity – https://youtu.be/-iSqRE8K1fI

Pressure – https://youtu.be/-s8kzEVDqRk

CONCLUSION

• The steady state internal flow simulation of rectangular channel is carried out for three different mesh sizes and run for 15000 cycles.
• Plots of Velocity, Total Mass flow rate, Pressure at the inlet and outlet regions for different cases are compared.
• The total cell count for case 1, 2 and 3 are 25000, 45000, and 100000 respectively. As the mesh gets finer the computational time increases.
• The physical quantity plot shows that initially there are fluctuations, they eventually die down and after about 10000 cycles the steady state convergence in the results is attained.
• Velocity changes slightly, total mass flow rate remains almost same but the pressure remains same, as the boundary conditions are specified in terms of pressure in all 3 cases.
• Contours shows that the pressure at the inlet remains high compared to the outlet and velocity at the wall becomes zero as the flow develops and the laminar velocity profile governed by pressure gradient is parabolic.
• The streamwise velocity and pressure animation on a slice of channel shows the flow behaviour.
• The velocity profile at x = 0.095 becomes much sharper/ flow seems fully developed as the mesh gets finer. Hence we can consider that as the mesh becomes finer the results are much accurate and precise.

 REFERENCES

• https://convergecfd.com/
• https://www.cygwin.com/
• https://physics.stackexchange.com/questions/533371/does-the-flow-rate-of-a-pipe-change-with-its-length