Flow over a backward facing step using Converge CFD

Aim:

The major objective of the project was to set up CFD simulation of the flow over a backward-facing step for 3 progressively refining mesh setting using converge Studio and result visualization using ParaView. To generate and analyze plot and contours for pressure, velocity, mass flow rate, and cell count.

Introduction:

The topic "Flow over a backward-facing step" is very important for those who want to check and validate their newly written cfd code so a lot amount of physical validation is available on this problem in order to validate the code. The report contains a detailed description of the simulation of Flow over a backward-facing step using converge CFD software, solving it using CYGWIN, and post-processing the result using ParaView. Various steps are discussed in this report while making a case set up for each case study. the report contains a detailed description of 3 case study with different grid sizing, line plots, and contours.

The three different mesh grids size are:

S.No.	Dx(m)	Dy(m)	Dz(m)
1	2.0e-3	2.0e-3	2.0e-3
2	1.5e-3	1.5e-3	1.5e-3
3	1.0e-3	1.0e-3	1.0e-3

1.Geometry Creation

First of all, the backward step geometry was setup using converge CFD software. The geometry was separately prepared using different CAD package ie Solidworks, then .stl file was imported to Converge Studio to set up the case study.

Image 1: Box geometry creation using Converge CFD.

*Each face of the step was split into triangles.

The Box Dimension is :

Dx(m)	Dy(m)	Dz(m)
0.272009	0.0199757	0.025

The Geometry can be downloaded from this link: Geometry

To create the geometry, the geometry was uploaded from the different CAD software package. All the normal vector were pointing out for this geometry, so we wanted it to point inside and normal to each surface. Normals point the direction of the fluid region generally. The command "Transform" was used to make the normal inside pointing the direction of the fluid region.

2.Boundary Creation

After creating the box Geometry, The Boundry tag was created along with different boundary id and coloring. A total of 5 different boundaries tag was created for 20 different split triangles of the box with each face split into two triangles. To create the boundaries naming, the "Boundary" command in the "Geometry" dock was used, and then the faces were appointed to appropriate tags along with id.

Image 2: Boundary creation representing the different id and color.

The five different boundaries were Inlet, Outlet, Top-Bottom, Front, and Back.

3.Case-Study Setup

The various parameter was considered while setup this case study. The case study setup dock was used to set up the problem which appears on the right-hand side of the converge studio.

Image 3: Case Setup Dock.

3.1 Application Type:

In order to run a steady-state simulation, firstly, under the Application type in the setup tree, a "Time-based" simulation must be selected.

3.2 Materials:

Under the material tab "Gas simulation" was selected and "Air" as a predefined mixture.

Gas: Air

Global Transport Parameters: Fixed value

by default, the following value is considered.

Prandtl Number : 0.9

Schmidt Number : 0.78

Species: Oxygen-O2 & Nitrogen-N2

Reaction Mechanism (mech.dat) was selected for this problem.

3.3 Simulation Parameters

The Steady-state solver was selected for running this case study along with a "Pressure based" solver.

STEADY STATE SIMULATION:

These are the simulation we run if are just interested in the end result and not in how those results are getting formed with time. The time is in CYCLES for STEADY STATE SIMULATION.

The Following Simulation parameters were selected for running each simulation.

3.4: Boundary Conditions

In this tab, The volumetric region and physics for each boundary were defined.

Boundary_Name	Boundary_Type	Paramter_Defined	Boundary ID
1.Inlet	Inflow	110325.0 Pa	1
2.Outlet	OutFlow	101325.0 Pa	2
3.Top-Bottom	Wall	Specified Value(300k) Law of wall	3
4.Front	Two_D		4
5.Back	Two_D		5

3.5: Initial Conditions and Events

Volumetric Region was created in order to assign this to all the boundary tag.

Species:- O2 (0.23) & N2 (0.77)

Pressure :101325Pa

Temperature:300K

3.6: Physical Models

A turbulent model was selected out of so many models.

3.7: Grid Control

The case study was run using 3 different mesh grid size. So different case study was set up in order to study the effect of value when the mesh is more refined.

3 different base mesh sizes are,

S.No.	Dx(m)	Dy(m)	Dz(m)
1	2.0e-3	2.0e-3	2.0e-3
2	1.5e-3	1.5e-3	1.5e-3
3	1.0e-3	1.0e-3	1.0e-3

3.7 Output/Post-Processing

Post-Variable Selection- Existing value

Output Files- Time interval for writing 3D output files-100.0

The time interval for writing restart files -1000.0

The case study was solved using CYGWIN and the post-processing was done in ParaView.

SIMULATION RESULTS

Mesh Size

Case Study 1: dx=dy=dz=2e-3 m

Case Study 2:dx=dy=dz=1.5e-3 m

Case Study 3:dx=dy=dz=1.0e-3 m

Line Plots & Contours

Case Study 1: dx=dy=dz=2e-3 m

Case Study 2:dx=dy=dz=1.5e-3 m

Case Study 3:dx=dy=dz=1.0e-3 m

Conclusion

Graph for the average mass flow rate through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow and inflow mass rate are equal to 1.5 kg/s approximately.

Graph for the average velocity through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow average flow velocity equal to 60m/s approximately.

Graph for the Total pressure through both inlet and outlet for all the three cases is compared below:- The mass flow rate for all the 3 cases are the same after 2500 cycles approx. the outflow total pressure equal to 104000 Pa approximately.

Image showing Different Zones of Turbulence

The case study can be downloaded from this link: https://drive.google.com/drive/folders/1CD8L366P30iM5zYtg7xllTeqviU3nOEl?usp=sharing