Week 1: Channel flow simulation using CONVERGE CFD

The aim to simulte 2D channel flow in Converge CFD software using 3 different mesh sizes & post processes the results in

Paraview.

Creating 2D Box

Usiung create geometry option, bos shape has been developed using below dimensions. As per dimensions, box will be

created in 3D shape by software but the 2d planar surface passing hrough XY plane will be used for simulation.

Creating Boundaries

The boundaries wil be defined to give the boundary conditions during the case setup. Boundaries are created using Boundary

> Flag option & respective surfaces were assigned to them.

 The normals has been transform to inner side of the box, as the flow will be paases through inside of the box. 

After setting up geometry, the diagnosis has been perform to see any error in the geometry. 

 

Case Setup

Simulaiton Type

The Time-based simulation has been selected for current simulation.

Material

Air is defined as the mixtures. 

Reaction mechanism has been turned off, as air just flowing from inlet to outlet & it is not combustion simulation.

Gas Simualtion

Default aprameters. 

  

Species

The two species has been defined which are O2 & N2.

Gloabal Transport parameters

Default parameters

Simulation Parameters

To configure simulation run parameters, first steady state solver has been assigned as this simulation is required to perform

under steady state conditions. In the next tab called Misc., steady state monitor has been turned on. 

Steady State Monitor

Now to configure steady state monitor, folowing options have been entered.

The solver will run the simulation for minimum 500 cycles.

Average velocity will be measured at the outlet.

The monitoring of the average velocity will be started after 200 cycles.

The average of every 100 cycles will be compared with the previous 100 cylces to check the convergence criteria (Tolerance)

has been met or not. The toleranc value has been set as 0.01.

SImulation time parameters

The simulation will be run for maximum 15000 cycles. 

The initial & minimum time step has been entered as 1e-9 which is the smallest time step possible in Converge. 

The CFL limit for maximum convection has been set as 1 to control the simulation stability. 

Solver parameters

In next step, navier-Stokes solver type solver has been selected as Density-based & the rest of the parameters are shown in

the dialog box is set up by converge. So there is no need to change them. 

The other change is made into the Eqautions tab in the same dialog box. The preconditioner for Pressure has been set as

"None" otherwise the solver will throw the error called "Invalid Preconditioners are selected." So always kept it as "None".

 

The Maximum Convection CFL limit is set as 0.5. For steady state solver, this value is always below 1.

Boundary Conditions

- Inlet

The total pressure has been set as 100001 Pa for Pressure boundary condition & Zero Normal Gradient has been set as

Velocity boundary condition. The temperaature has been set as 300 K. The O2 & N2 as defined as species with the mass

fraction values 0.23 & 0.77 respectively. 

- Outlet

- Top- Bottom walls

No-slip walls with stationary motion type. 

- Front 2D

For 2D simulation, the front & back sides has been defined as TWO_D which is used to convert 3D simulation to 2D. 

- Back 2D

Turbulence Model

The turbulence model has been turned off. 

 

Regions & Initialization

In this dialog box, the proportion of the oxygen & nitrogen in the air has been reversed. The purpose of this implementation

is to prove that the effects of the initial bnoundary conditions are limited to initial some cycles. 

 

 

Output/ Post Processing

In Post variable selection, the chnages have been made only in the Species tab. The mass fraction of N2 & O2 have been

activated.

 

In Output files, the time interval for writing 3D output files & writing restarting output files has been decreased from 1000 to

150 to maintain the 100 output files to post process the simulation results.

Exporting Input Files

File>Export>Export Input Files

These files will be used in the Cygwin to perform the simualtion as Converge CFD doest have solver in built. 

 

Part 1

Grid Control

To create grid, first Option> Geometric Bounding Box option has been turned on.

In Base grid dialog box, the values of dx & dy have been entered which is the distance between two nodes in mesh.

 For first part 2e-4m is used as the grid size for all three directions.

After navigating the Cygwin to the directory where simulation input files are located, following commnad, without double

inverted comma, has been used to perform simulation.

"mpiexec.exe -n 2 converge.exe restricted"

In above command, mpiexec.exe is used for parallel computing.

The "n" is used for number of processor will be used for simulation.

The converge.exe is solver file. 

The "restricted" indicates that it is student version & the maximum number of nodes & other restrictions are applicable. 

 

The convergence was achieved in 6140 cycles & time taken to solve this simulation is 2084 seconds.

 

Part 2

Grid size = 1.5e-4m

For this grid size, the solver took 9325 cycles & 3878 seconds which is almost double than the previous case. 

Part 3

Grid size = 1e-4m

The solver took 7859 cycles & 12043 seconds to solve this simulation. The time taken is more than 3 times than the Part 2 &

6 times more than Part 1. 

Post conversion

For Part 1

There are two ways files can be converted for post processing which are using Cygwin & Using Converge software.

Using Cygwin

The following commandd has been used to convrt the files for post processing. 

mpiexec.exe -n 2 post_convert.exe

The rest of the procedure is shown below in images. 

Using Converge

First post processing 3D tab needs to be selected. 

The respective directory is required to assign with case name & type of post processing software for which files are being

converted. 

Paraview - Post Processing

The case has been opened fo post processing. 

Imported geometry in paraview

Slice Option

Z- normal Plane has been created to make a slice of the geometry.

Surface with edges option to display mesh

Above mentioned steps to carry out the simulation using Cygwin & post conversion simulation data using both options,

Cygwin & Converge software will remain same for remaining two simulations. The steps shown in Paraview also remains

same for remaining two cases.

Results:

Velocity Contour

The velocity has been selected as property & last frame has been set for visualization. The scale of legend has been rescale

to make it according to the current results.

Pressure Contour

Plots

Area Avg flow output file is used to plot the varialbes against the cycles. Plots has been plotted from the inlet boundary side.

Mass flow rate

Avg. Velocity

The speed of the air is decreasing from inlet to outlet & the initial fluctuations has been smoothed out as the flow is passing

from inlet to outlet. 

 

Total & Static Pressure

 

Cell Count

 

For remaining two cases, post conversion of simulation files will be carried out in Converge software only, but post processing

will be done in Paraview.

 

Part 2

Mesh

Velocity Contour

Pressure Contour

 

Plots

Mass Flow Rate

Velocity

Pressure

Total Cell Count

 

 

Part 3

Mesh

Velocity

Pressure

Mass Flow Rate

Velocity

Pressure

Total Cells