Week 4: Project - Transient simulation of flow over a throttle body

Aim: Transient simulation for flow through an elbow joint containing a throttle valve.

Introduction and Theory: A throttle is a mechanism by which fluid flow is managed by constriction or obstruction. In a traditional IC engine, the throttle valve is part of the air-intake system, which controls the amount of air sent to the combustion chamber. The throttle valve is connected to a shaft which is connected to the accelerator. So as the driver presses the accelerator pedal the air intake increases, and the engine RPM increases thereby increasing vehicle speed. When the driver releases the accelerator pedal, the air gets essentially choked off, and the engine RPM decreases, thereby slowing the vehicle down.

Importing Geometry:

The following geometry was imported into Converge:

Run a diagnosis check, to make sure the geometry doesn't have any errors.

The elbow initially had a thickness, so the thickness was removed because we are simulating to find out the fluid flow characteristics, not heat transfer.

After importing make sure to change the dimensions to mm.

The open inlet and outlet were repaired using the patch tool.

Boundary Flagging:

Case Setup:

1. Application Type: Time-based.

2. Materials

2.1. Gas simulation

2.2. Global Transport parameters

2.3. Species

3. Simulation Parameters

3.1. Run parameters

Shared memory was switched off.

3.2. Simulation time parameters

Flow-through time calculation:

From the bounding box, we can see that the elbow joint has a length of 0.1 m in the x-direction and 0.12 m in the y-direction.

Hence we can approximate that the length of the pipe will be less than 0.2 m.

From the steady-state analysis, we know that the average velocity at the inlet is 200 m/s.

So, the flow-through time = length/velocity = 0.2/200 = 0.001 s

Hence, it takes 0.001 s for one complete flow.

To capture 10 such flows = 0.001*10 = 0.01 s

Therefore, we ran the simulation for 0.01 s.

3.3. Solver parameters

4. Boundary Conditions

4.1.Elbow wall

4.2. Inlet

Inlet pressure of 150 KPa.

4.3 Outlet

Outlet pressure of 100KPa.

4.4. Throttle

The rotation center and rotation axis(about) is found out using Arc normal function under measure direction. Using arc normal function we have to select three vertices of an arc, which then gives us the arc center and arc normal vectors. (Note: In the figure, I have only shown the two cylindrical protrusions on the sides of the throttle valve.)

4.4.1 Rotation rate file

From 0 to 2 ms - the valve will rotate by 25 degrees.

From 2 to 4 ms - the valve will stay at 25 degrees.

From 4 to 8 ms - the valve will rotate back to 0 degrees.

5. Region Initialization

6. Turbulence modeling:

7. Grid Control

7.1. Base Grid

7.2. Fixed embedding

Scale = `dx/2^n` , where n is the scale.

Embed layers decide how far from the body will the refined mesh propagate.

8. Output variables

9. Output files

The input files were exported to a file and using Cygwin command line, we used converge-intelmpi solver to run a parallel simulation with 4 processors.

Using the post-processing tools, the output files obtained were converted into Ensight file type which can be then opened by ParaView.

Mesh:

Using fixed embedding at the throttle body, we get a refined mesh in the boundary of the throttle body, allowing us to finely capture the physics around it, which is of importance.

The mesh gets updated continuously as the valve rotates.

Results:

Contours:

1. Velocity Contour

From the velocity contour, we can see that velocity is highest at the top and bottom of the valve, because of the reduction in the area the velocity will increase according to Bernoulli's principle. Also, as the valve turns the flow behind the valve(downstream) is high because of Coanda effect, since fluids tend to follow the curvature of the surface in its path. Since it causes blockage at 25 degrees, the velocity decreases upstream. In its default, open position we get the same results as the steady-state analysis.

2. Pressure Contour

By observing the pressure contour, we can observe that the pressure is highest upstream of the valve at 25 degrees since the valve causes some obstruction to the flow. The pressure downstream(just behind the valve) is low, even lower than the default 0 degrees position, it is due to the fact that the flow area has been decreased, and the velocity increases further as it passes by the valve.

Plots:

1. Velocity plot

The average velocity at the outlet is 226.227 m/s and at the inlet is -195.196 m/s. The values are getting changed only during the rotation otherwise they are constant when the valve is not in rotation.

2. Pressure plot

The total pressure at the outlet is 137,145 Pa and static pressure at the inlet is 119,476 Pa. The boundary condition values remain the same throughout.

3. Mass flow rate plot

The mass flow rate at the inlet is -0.0665591 kg/s and at the outlet is 0.0665728 kg/s.

4. Cell count

The cell count is not constant, since we are running a transient simulation with valve rotation. The mesh gets continuously refined when the valve is rotating, which can be observed during 0-2 ms and 4-8 ms when the valve is rotating. When the valve is stationary at 25 degrees, during 2-4 ms we can see that the cell count remains constant.

Streamlines:

Different views of the fluid streamlines through the elbow joint:

The streamlines and throttle valve are colored with velocity.

Conclusion:

1. Transient flow through the elbow joint with valve rotation was simulated.

2. The pressure and velocity values close to the moving throttle valve was captured accurately.

3. From the mass flow rate plot, we can confirm that the mass is conserved, which is in keeping with the law of conservation of mass.