Boundary Conditions for External Aerodynamics Simulations

This project was done as a part of the course - External Aerodynamics Simulations using STARCCM+

1. BCs used in External Aerodynamics:

Boundary Conditions are the set of constraints to boundary value problems in computational fluid dynamics. These boundary conditions include inlet boundary conditions, outlet boundary conditions, wall boundary conditions, constant pressure boundary conditions, axisymmetric boundary conditions, symmetric boundary conditions, and periodic or cyclic boundary conditions.

The specification of boundary conditions should be geared as close as possible to the measurement conditions in the wind tunnel or eal-world experimentation set-up. In the majority of cases, flow velocity and turbulent intensity of the windtunnel are known. Therefore a velocity-inlet boundary condition is used to model the incoming flow. Effects like rotating components can be modeled using the Moving/Rotating Wall Boundary
Condition. This adds tangential velocity to the selected walls.

2. Value of Pressure at Outlet exposed to atmosphere:

The value of pressure at the outlet is zero (or very close to zero). This is explained as below:

When a fluid is moving through a pipe, the flow static pressure need not be the same as that of the ambient pressure. In fact, the higher or lower value of pressure inside the tube makes the fluid flow through the tube possibe. Thus, in a flow through pipe, the pressure at the inlet of the pipe is higher than the ambient pressure which forces the fluid through the pipe. Whereas at the end of the pipe, the outlet is open to atmosphere which exerts the atmospheric pressure at the outlet. Thus, the ambient pressure acts on the fluid and makes both these pressures equal to the atmospheric pressure. This is possible only if the fluid is incompressible and the flow subsonic. In case of supersonic flows, the expansion waves cannot go upstream and we end up having higher pressure at the outlet than the atmospheric pressure.

3. Modelling Pipe Geometry, applying BCs and creating Volume Mesh:

STEP - 1 : Modelling Geometry

To create the geometry, right click on 3D-CAD Models --> New. Here, select the Y-Z plane, right click and select Create Sketch.

In the sketch window, draw a circle with radius 10mm and exit the sketch. This sketch will be saved as Sketch 1 by default.

Then, right click on the X-Y plane and create a new sketch plane. Here, using the line functionality, sketch the profile of the pipe with each side 50mm.

After creating the profile, exit this sketch. This sketch will be saved as Sketch 2 by default.

Now, select both the sketches and right click --> Sweep. This will enable the sweep functionality. Here, make sure that the Body Type is solid and click OK. A pipe geometry will then be created.

                                                 Sweep Function

                                                 Final Geometry

STEP -2: Surface Repair:

Once the geometry is completed, select the geometry under 3D CAD Models, right click and select New Geometry Part. This will set the body as a new part. 

Now go in parts, right click on the geometry and select 'Surface repair'. Once inside the surface repair window, make sure all the 6 errors are selected and select 'Execute All'. Here, only 4 Face Quality errors were detected and all other errors were 0. The face quality errors would be taken care of during the surface remeshing operation.

STEP -3: Defining Boundary Conditions (BCs):

First we need to assign the part to regions. To do this, right click on the geometry under parts --> Assign Parts to Regions. A dialog box will appear. In that, select 'Create a region for each part surface' and click OK. 

Defining BCs:

To define the boundary conditions, go to Regions --> Boundaries. Here, we can find the surfaces and can set boundaries for them. Select the following boundaries:

Inlet - Velocity Inlet

Outlet - Pressure Outlet

Pipe - Wall

STEP -4: Surface Remesher:

As there were only 4 Face Quality errors and no other errors, we can skip the surface wrapper step. For remeshing, right click on Operations --> New --> Mesh --> Automated Mesh. This will pop-up a dialog box. In the dialog box, select the following:

Surface Remesher

Automatic Surface Repair

Polyhedral Mesher (Since it is mainly used for internal flow simulations)

Prism Layer Mesher (For the pipe boundaries)

Under Default Controls, mesh parameters can be defined. The Base Size was defined as 1mm. Other parameters were kept default.

The Number of Prism layers were set to 4 and the Total Prism Layer Thickness was set to 50% of base size.

Once the parameters are set, right click on Automated Mesh and select Execute. The volume Mesh will be created which can be viewed by going into the Mesh Scene.

                  Polyhedral Mesh (The line in the middle is the derived plane)

To view a cut section of the pipe, right click on Derived Parts --> New Part --> Section --> Plane. Select the plane parallel to the Z-axis and click OK.

To view the plane, go to the Mesh Scene and toggle visibility of the Geometry. This will show only the Plane. Select the 'Show Mesh' option to view the mesh. 

The mesh quality was found to be good and hence, no changes were needed.

                                         Plane Section showing the Mesh

                                         Detailed view of the Prism Layers

STEP -5: Setting up of Boundary Conditions

To set-up the BCs, go to Continua --> Physics 1 --> Select Models.

In the dialog box that appears, select the following: Steady, Gas, Segregated Flow, Constant Density and Laminar Flow and click Close.

Go to Regions --> Boundaries. Select Pipe Inlet Physics Values--> Velocity Magnitude and give velocity value 20m/s.

Select pipe outlet and check whether the Pressure value is 0 (Pa).

STEP -6: Confirmation through simulation

Another way to confirm the outlet pressure value is by running the simulation. We have already defined the required geometry and physics conditions. To run the simulation, we first need to give the stopping criteria. To do so, go to Stopping Criteria --> Maximum Steps. In the dialog box, select maximum steps value as 250. Then to initiate the simulation, click on the Green Flag in the toolbar and then click the Run button to run the simulation.

The completed simulation can be viewed by setting up a new Scalar View. In that, select the Pressure value in the indicator below. This will display the pressure distribution inside the pipe.

                                   Pressure Distribution inside the pipe

This shows that the value at the exit of the pipe is 0 Pa. Further post-processing was also done by plotting a Pressure plot.

For this, we need to create a line probe. This can be done by right clicking on Derived Parts --> New Part --> Probe --> Line. Select the co-ordinates of the line such that the line is drawn at the centre of the outlet section of the pipe as shown below.

To create a plot, go into Plots, right click --> New Plot --> XY plot.

Select Plot 1 --> Parts --> Line Probe. This will plot the graph along the line probe we just created.

Expand the plot 1 and go to Y Type --> Y Type 1 --> Scalar Function. Under scalar function, set the Field Function as 'pressure'. 

Under the 'Line Probe', properties of the line can be selected as per the user's choice.

As can be seen from the line plot, we can confirm that the Pressure outlet is 0 gauge (or 101325 Pa). The reason for this is explained in section [2].