Channel flow simulation using CONVERGE CFD

OBJECTIVE:

To perform a 2-D channel flow simulation using CONVERGE CFD by applying three different mesh sizes and comparing the results of resulting three simulations.

The different mesh sizes 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

INTRODUCTION:

Any given CFD simulation can be classified into 3 stages:

• Pre-processing
• Simulation
• Post-processing

Before entering briefly into these stages it is important to know the applications and solvers used for these simulations.

Pre-processing:

Application:- Converge Studio

It provides an easy interface for navigation. Even though it is not a go to application for geometry creation, it provides a decent scope for basic geometry creation.

During this stage, all the required physical dimensions (Fig:1) and properties of the setup are provided.

Dimensions: x-axis= 0.1 m, y-axis=0.01 m, z-axis= 0.01 m

Physical parameters have to be given with great accuracy in order for the results to be perfect. The physical properties include:

• Composition of the fluid inside the geometry (i.e Air- O2 and N2)
• Simulation time parameters
• Boundary conditions
• Volumetric regions
• Mesh size
• Time period for the simulation to run

Along with these the conditions to generate the output data should also be provided.

Fig: 1

NOTE:

Ip >= Vp >= Op. Since the flow starts from the Inlet, through the Volumetric regionand then to the Outlet, the pressure of Inlet cannot be less than that of Volumetric region as fluids flow from high pressure to low pressure regions.

Ip- Inlet Pressure, Vp- Volumetric Region Pressure, Op- Output Pressure

Simulation:

Command Application: CYGWIN

Solver:- convergempi.exe for serial run and mpiexec.exe + convergempi.exe for parallel run.

These solvers help to solve the PDE for mass and momentum equations. By entering the appropriate command in CYGWIN, the simulation begins and the output results are made to dump in a logfile for data and simulation preservence.

Post-processing:

Application:- PARAVIEW. It is an open source application which can be used to post-process the output files into 3-D results.

VIDEO LINK:

Case 1: https://youtu.be/d3AHAOjd9z8

Case 2: https://youtu.be/wmb4oOxJ0RM

Case 3: https://youtu.be/-_2nxXH1EcY

RESULTS:

Mesh Size: Mesh for each cases are zoomed to the same extent.

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

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

Case: 3(dx =1.0e-4m, dy = 1.0e-4m, dz = 1.0e-4m)

COMPUTATIONAL TIME:

As the mesh size becomes denser, the time taken to complete the simulation increases drastically.

Case	Mesh Size	Time Taken
1	dx=2e-4m,dy=2e-4m,dz=2e-4m	960.100047 seconds
2	dx=1.5e-4m ,dy=1.5e-4m, dz=1.5e-4m	1780.148035 seconds
3	dx=1e-4m, dy=1e-4m, dz=1e-4m	3900.878263 seconds

TOTAL CELL SIZE:

As the mesh size increases, the total number of cell also increases.

Case	Mesh Size	Cell Count
1	dx=2e-4m,dy=2e-4m,dz=2e-4m	25000
2	dx=1.5e-4m ,dy=1.5e-4m, dz=1.5e-4m	45420
3	dx=1e-4m, dy=1e-4m, dz=1e-4m	100000

Case: 1

Case: 2

Case: 3

PRESSURE PLOT:

As the mesh size increases, it takes more cycles to attain covergence.

Case: 1

Case: 2

Case: 3

MASS FLOW RATE:

Case: 1

Case: 2

Case: 3

AVERAGE VELOCITY PLOT:

Case: 1

Case: 2

Case: 3

CONCLUSION:

1. By increasing the mesh size the number of cells and simulation time increases.
2. Very fine mesh sizes require large cycles to converge.
3. Initial conditions in a steady state simulation gets washed away eventually.