STAR CCM+ - BACKWARD FACING STEP
Aim
To model air flow through a backward facing step & to run simulations for Reynolds numbers of 100 , 1e5, 1e7 & use laminar, Realizable k –epsilon and Realizable k – omega models.
Procedure
Stage 1 Modelling : Backward facing step
We model the geometry using the 3D CAD modeler in STAR CCM+
1) Open the simulation, Expand Geometry, right click on the 3D CAD Modeler and select New
2) The modeling window will open, select the XY plane and select Create Sketch.
3) Draw the backward facing step using line command & give dimensions according to the diagram.
2D sketch of the backward facing step
4) Right click on the 3D model which is on the CAD tree and select Extrude and give the thickness as 1 cm
Extruded view (backward facing step)
5) Close the 3D cad modeller and save.
6) Expand geometry >3D CAD model > Right click on the 3D model created and select New Geometry Part.
Stage 2 Surface Repair
Checked for surface errors & face quality & face proximity errors were found, this can be rectified by remeshesing the surface.
Stage 3 Surface Wrapper
Since there were no surface errors, a surface wrap wasn't needed.
Assigning to regions
Right click on the backward facing step and click on the Assign to regions, a dialog box will appear in that change the boundary setting as Create a boundary each part surface, & click ok.
Applying BC
1) Expand Regions > Regions > Boundaries.
2) The boundaries are listed for inlet select Velocity Inlet, for outlet select Pressure outlet & for the rest select wall.
Stage 4 Surface remecher and Volume Mesher.
1) Select Operations > New > Mesh > Automated Mesh.
2) A dialog box will appear Select the following
- Surface remesher
- Automatic surface repair
- Polyhedral Mesher (Core mesher)
- Prism layer Mesher
and click ok
3) Expand Default Controls, and select Base size and give the value as 1 mm(less than the length of the inlet 1 cm).
4) Right click Custom control > New > Surface control and select the walls of the backward facing step and increase the target & minimum surface size by 4 times the base value(1 mm) and click ok and rename it as wall.
6) Select wall & expand controls > Prism layers > Customize > Customize Total Thickness & check the box & Give 200% in values.
Mesh Scene
7) To see the quality of the volume mesh a cut section of the modeled can be viewed, for that right click on derived parts > New parts > Section > Plane.
8) A new window will open and orient the plane in such a way it is perpendicular to X axis, and click ok.
9) To view the cut section, go to Scenes and select the Mesh Scene 1, then go to Scenes/Plots window, and Expand Displayers > Mesh 1 > Parts > backward facing step.
10) Right click on backward facing step and select Hide and the plane section can be viewed
Cross-sectional view of the volume mesh
For setting BC's for inlet & outlet,
For Reynolds number(Re) = 100 (Laminar)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 100
`= 100 x 1.51e-5 / 0.03 = 0.050 m/s`
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Laminar >& click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residuals Plot
From the above image the equations converges within 200 iterations
For setting BC's for inlet & outlet,
For Reynolds number(Re) = 100 (Turbulent)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 100
`= 100 x 1.51e-5 / 0.03 = 0.050 m/s`
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Laminar >& click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
For setting BC's for inlet & outlet,
For Reynolds number(Re) = 100 (Laminar)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 100
v = 100 x 1.51e-5 / 0.03 = 0.050 m/s
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Laminar >& click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residuals Plots
For Reynolds number(Re) = 1e5 (K-epsilon)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 1e5
v = 1e5 x 1.51e-5 / o.o3 = 50.33 m/s
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Turbulent >K-Epsilon Turbulence & click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residual Plots
For Reynolds number(Re) = 1e5 (K-omega)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 1e5
v = 1e5 x 1.51e-5 / o.o3 = 50.33 m/s
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Turbulent >K-Epsilon Turbulence & click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residuals Plots
For Reynolds number(Re) = 1e7(K-omega)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 1e7
v = 1e5 x 1.51e-5 / o.o3 = 5033 m/s
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Turbulent >K-Epsilon Turbulence & click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residuals Plots
For Reynolds number(Re) = 1e7(K-epsilon)
To calculate velocity we use the formula Re = `(rho*v*D)/mu`
`nu=mu/rho`
where
rho = density
mu = dynamic viscosity
nu = kinematic viscosity
v = velocity
D = diameter of inlet
to find v
D = 0.03
mu = 1.51e-5 (Air at temp 20 C )
Re = 1e7
v = 1e5 x 1.51e-5 / o.o3 = 5033 m/s
1) Goto Continua > Physics 1 > Select Models.
2) A dialog box will appear and select > Steady > Gas > Segregated flow > Constant Density >Turbulent >K-Epsilon Turbulence & click close.
3) Go to Regions & expand it and click Regions > Boundaries.
4) Select inlet >Physics Values > Velocity Magnitude in that give the value of v
5) Select outlet and check whether the value is 0 pa.
Selected Models for simulation
Residuals Plots
After giving the inputs Open a scalar scene by
1) Clicking Create scene > Scalar
2) Go to the scene hide everything & show only the plane.
3) In the scalar field properties window select velocity magnitude.
The Y+ used for this simulation is All y+ because when it is difficult to achieve a pure high- y+ or low-y+ mesh because of varying geometrical and velocity.
All y+ values = Wall Y+ < 1 or Wall Y+ > 30
The Prism Layers are used to analyse the boundary layers near the wall and it is used because we can get high quality result with less number of cell than compared to the core meshers near to the wall.