Channel flow Simulation using CONVERGE CFD

Objectives:

• To setup the case in CONVERGE and run the channel flow simulation for three different mesh sizes using Cygwin.
• To post Process the results using Paraview
• To compare the results of three simulation

Description:

The fluid which flows through a channel is in direct contact with the surface, it sticks to the surface and there is no slip. This is known as no slip condition. The fluid property responsible for no slip condition and the development of boundary layer is viscosity. The layer that sticks to the surface slows the adjacent fluid layer because of viscous forces between the fluid layers, which slows the next layer and so on. Therefore, no slip condition is responsible for the development of the parabolic velocity profile. The flow region adjacent to the wall, in which the viscous forces are significant is called boundary layer. Thus, the velocity of the fluid increases from the boundary layer to the central axis in the form of a parabolic profile.

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

Geometry: Channel Geometry is created in CONVERGE Studio

• Double click on CONVERGE Studio
• Click on New project from CONVERGESTUDIO wizard
• From Geometry, select

Create -> shape -> Box -> Provide appropriate dimensions for Geometry creation -> Create

• Channel is created as per the above dimensions

Setting up the Case:

• From Geometry, select

Boundary -> Flag -> 'Click on +' for adding the required boundaries

• Select the 'Cursor pick' and 'triangle' for selecting the faces.
• select the boundary, select the faces and click on Apply
• Right click on the Boundary for assigning the name for the selection.

• Click on Normal Toggle, Perpendiculars to the triangular faces are coming out.

• Because of internal flow the normals direction is to be changed, To change the direction of Normals, Click on Transform from Geometry -> Normal and select any triangular face -> Apply

• To check the errors for 3D Geometry, click on Diagnosis, check for Intersections, open edges, Nonmanifold edges, Normal Orientation, Isolated triangles

• Click on Find to identify the checkbox errors in the 3D Geometry

• From the above image, it is clear there are no errors in the geometry, next click on clear.
• Click on Case Setup -> Being Case Setup -> tick General flow -> Done

• Click on Materials, set the Predefined mixtures as Air, tick the boxes of Gas simulation and species then click on Apply -> Yes(to overwrite gas.dat, mech.dat and therm dat) -> Done.
• Click on Global transport parameters -> Ok

• Click on Physical Models, deselect Turbulence modeling ->Done.

• Click on Simulation Parameters -> Done

• From Simulation Parameters, click on Run parameters, set the solver to Pressure-based steady solver then click on OK -> Yes(to load default tolerance/relaxation -> Yes(load the default of Non-Engine application.

• From Simulation Parameters,Click on Simulation time Parameters, assign the Start time, End time, Initial time-step, Minimum time-step, Maximum time-step

• Click on OK.
• From Simulation Parameters, Click on Solver parameters -> Ok.
• From Boundary conditions, Click on Boundary then
• Click on Inlet for assigning the boundary conditions
• select the Boundary Type as INFLOW
• From Pressure Boundary Conditions, change the total pressure to 100001 Pa.
• From Species Boundary Condition, 'Click on +Air' to add species

• Click on Outlet, select the Boundary Type as OUTFLOW
• From Pressure Boundary Conditions, change the total pressure to 100000 Pa.
• From Species Backflow, 'Click on +Air' to add species

• Click on Top-Bottom-Walls, select the Boundary Type as WALL
• From Velocity Boundary Condition, change the motion to No-slip.

• Click on Front-2D, select the Boundary Type as TWO_D then,

• Click on Back-2D, select the Boundary Type as TWO_D

• From Initial conditions & Events, click on Regions and initialization, click on Add.
• From species, click on +Air, change the O2 mass fraction to 0.77 and N2 mass fraction to 0.23

• Double click on Region Name, change the name to Volumetric_region
• Click on Ok.
• From Boundary conditions, click on Boundary.
• Double click on Region Name, set the region name to volumetric_region, for all the boundaries.

• Right Click on Boundary and Click on validate.
• From Geometry, Click on Options -> Visibility -> tick Geometry bounding box

• From Grid Control, Click on Base grid, change the Base grid size dx, dy, dz to 2e-4m then Click on Ok.
• From Output/ Post Processing, Click on Post Variable selection ->Tick Mass Fraction to add O2 and N2 -> Ok.

• From Output/ Post Processing, Click on Post Variable selection -> Combustion/Turbulence -> untick the boxes -> Ok.

• From Output/ Post Processing, Click on Post Output files -> change the Time interval for writing 3D output data files to 100

• Complete Case Setup is validated, no issues found.

• Click on File -> Export -> Export Input Files to the required folder.

Simulation Results:

Velocity:

Pressure:

Plotting the results:

Velocity:

Pressure:

Mass Flow Rate:

Total Cell Count:

Plot over line for Velocity:

Flow through channel for a mesh of 0.0002:

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

The Geometry and case setup is same as the previous case, but in the Grid Control, base grid has to be changed to 1.5e-4, and also the number of cycles from simulation time parameters has changed to 40000.

Velocity:

Pressure:

Plotting the results:

Velocity:

Pressure:

Mass Flow Rate:

Total Cell Count:

Plot over line for Velocity:

Flow through channel for a mesh of 0.00015:

Case 3: Mesh ( dx = 1.2e-4 m, dy = 1.2e-4 m, dz = 1.2e-4 m)

The Geometry and case setup is same as the previous case, but in the Grid Control, base grid has to be changed to 1.2e-4, and also the number of cycles from simulation time parameters has changed to 60000.

Velocity:

Pressure:

Plotting the results:

Velocity:

Pressure:

Mass Flow Rate:

Total Cell Count:

Plot over line for Velocity:

Flow through channel for a mesh of 0.00012:

Conclusion:

• As the grid size reduces the results accuracy increases.
• Simulation time increases with the increase in number of cycles, as the mesh becomes finer, the number of cycles increases.
• The velocity is zero at the surfaces of the channel and it becomes maximum at the centre of the channel, and the velocity profile is parabolic curve.
• As the mesh becomes finer the velocity plot becomes smoother, where as the velocity increase is too small.
• For all the three cases the Average velocity, Total Pressure, Mass Flow rate becomes stable after 'n' number of cycles increase at inlet and outlet.