Week 4.2: Project - Transient simulation of flow over a throttle body

TRANSIENT-STATE SIMULATION OF FLOW OVER A THROTTLE BODY USING CONVERGE CFD

l. INTRODUCTION

A throttle is a mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal.

ll. OBJECTIVE

1. Simulate fluid flow over a throttle body using a transient state solver.
2. Plot and analyze Velocity and Pressure Contours 
3. Plot and analyze Velocity, Pressure, Mass flow rate, total cell count.

lll. GEOMETRY

The geometry consists of 4 basic components:

• Elbow Wall
• Inlet 
• Outlet
• Throttle

1. Geometry Setup

Geometry setup in converge studio.

• Scaling the geometry 
• Removing all the open edges

2. Diagnosis Check

3. Boundary Flagging

lV. Case Setup

1. Fluid: Air

• Mass Fraction of Nitrogen: 0.77
• Mass Fraction of Oxygen: 0.23

2. Solver:

A transient solver is used to simulate the flow.

3. Simulation Parameters:

The simulation end time can be approximated as follow:

The length of the elbow can be approximated using the characteristic length in the X and Y directions.

Length of the Elbow =`sqrt((0.106^2)+(0.12^2)) = 0.16 approx0.2` m

Velocity at the Inlet (as obtained in the steady-state simulation) = 190 m/s

Flow time = 0.2/190 = 0.001052 `approx` 0.002 s

The fluid should flow at least three to four times through the entire elbow.

Total Flow time =0.002 x 4 =0.008 s

The end time is taken slightly greater than the approximated time.

End Time = 0.01 s

Simulation Time Parameters:

• Start Time: 0 s
• End Time: 0.01 s
• Initial time-step: 1e-9 s
• Minimum Time-step: 1e-9 s 
• Maximum time-step: 1 s

4. Boundary Conditions:

• Elbow Wall: Boundary Type - Wall | Velocity Boundary Condition - Law of Wall
• Inlet: Boundary Type - INFLOW | Total Pressure = 150000 Pa | Temperature = 300 K | Species = Air
• Outlet: Boundary Type - OUTFLOW | Static Pressure = 100000 Pa | Temperature = 300 | Species = Air
• Throttle: Boundary Type - WALL | Wall Motion Type - Rotating | Surface Movement - Moving | Velocity Boundary Condition - Law of Wall
  • Rotation Rate File:
    • From 0 to 2 ms the valve should rotate by 25 degrees.
    • From 2 to 4 ms ut should stay at 25 degrees.
    • After which by 8 ms the valve should get back to its original position

5. Regions and Initialization:

• Flow Region: Volumetric Flow Region
• Total Pressure: 101325 Pa
• Temperature: 300 K
• Species: Air

6. Turbulence Model: RNG `k-epsi`

7. Grid Control:

• Base Grid:
  
  • dx=dy=dz= 2e-3 m
• Fixed Embedding:
  
  • Entity Type: Boundary
  • Boundary ID: Throttle
  • Mode: Permanent
  • Scale: 3
  • Embed Layers: 2

8. Output Files:

The time interval for writing 3-D output files will depend on the number of the outputs desired by the user. In this case, we will take 80 outputs.

Number of Outputs = 80

Time interval for writing 3-D output files= Simulation Time/ Number of Outputs = 0.01/80 = 0.000125 s

Writing Time Intervals

• The time interval for writing 3-D output files: 0.000125 s
• The time interval for writing text output: 1e-6 s
• The time interval for writing restarting output: 0.001 s

V. RESULTS

1. Mesh

Decreasing the mesh size around the throttle helps capture the results at the throttle boundary more accurately and results in the smoothening of the contours around the throttle. However, this also increases the computation time due to the increased number of cells.

Figure 1.1 - Mesh

Figure 1.2 - Mesh around the Throttle

 The mesh also adapts itself to the movement of the throttle at different time steps. This can be observed in the animation below - 

Animation 1 - Transient Simulation - Mesh

2. Velocity Contours and Pressure Contours

The velocity of the fluid at different regions in the elbow under different conditions can be analyzed with the help of the velocity contour. When the valve begins to close, the velocity of the fluid decreases. This also results in a sudden increase in the pressure in the region behind the throttle as the kinetic energy gets converted into pressure energy.

Figure 2.1 - Velocity Contour

Figure 2.2 - Pressure Contour

Animation 2.1 - Velocity Contour

Animation 2.2 - Pressure Contour

3. Velocity Vectors

The velocity contour can be understood better by observing the direction of flow using velocity vectors.

Figure 3.1 - Velocity Vectors

Figure 3.2 - Velocity Vectors around the Throttle

Animation 3- Velocity Vector

4.Velocity Streamlines

The velocity streamlines represent the direction in which a massless fluid element will travel at any point in time. During the small rotation of the valve, a lot of disturbances are created which can be analyzed using the velocity streamlines.

Animation 4- Velocity Streamlines around the Throttle

5. Variation of Average Velocity w.r.t. Time

6. Variation of Pressure w.r.t. Time

7. Variation of Mass Flow Rate w.r.t. Time

The mass flow rate at the inlet is equal to the negative of the mass flow rate at the outlet at the end of the simulation. This shows that the mass flow is conserved.

8. Variation of Cells Count w.r.t. Time

The cells are unevenly distributed among the four processors to achieve a good load balance. In this case, since the simulation is simple, the number of cells is distributed almost equally.

Also, since the throttle is moving, the number of cells near the throttle changes to adapt to the movement of the throttle. It can be seen that when the throttle is stationary i.e. from 2 ms to 4 ms and 8 ms to 10 ms, the number of cells remains constant as no movement of the throttle takes place. 

Vl. CONCLUSION

Analyzing the behavior of the fluid helps us to understand and control the flow and the throttle movement such that we get the required output without any disturbances or irregularities. This helps to control the flow behavior and output depending on the requirement or application of the model.