Steady State Flow Simulation through a Throttle Body

OBJECTIVE

The objective of this study is to set up a steady state flow simulation through a throttle body and understand the dependency of grid size on simulation time and results. Apart from this, the effect of fixed embedding will be analyzed in the study. The project includes the key results for three simulations setups based on the three different grid sizes and fixed embedding parameters as per the table given below

Case	Grid Size, dx (m)	Grid Size, dy (m)	Grid Size, dz (m)
Case 1	4.00E-03	4.00E-03	4.00E-03
Case 2	4.00E-03	4.00E-03	4.00E-03
Fixed Embedding:	Scale-3	Embed Layer-2
Case 3	2.0E-03	2.0E-03	2.0E-03
Fixed Embedding:	Scale-3	Embed Layer-2

Note: No fixed embedding parameters will be used in Case 1

The simulation process includes three major steps:

1. Preprocessing

1.1. Geometry Import and Cleaning

1.2. Diagnosis

1.3. Boundary Flagging

1.4. Case Setup

2. Solving

3. Post Processing

3.1. Mesh

3.2. Plots (Mass Flow, Pressure, Velocity, Iteration Error)

3.3. Velocity Gradient Plot

3.4. Contours (Velocity, Pressure)

PROCEDURE:

The procedure followed for setting up the simulation is provided below

1. Preprocessing

1.1. Geometry Import and Cleaning

a. The geometry provided was imported by dragging and dropping the file in the converge studio window

Fig. 1: Geometry (All dimension are in m)

b. The geometry imported had all the dimensions in the scale of hundreds of meters and had to be converted to mm. This would result in the mesh generation beyond the license limits. To overcome this so the transform command was used to scale down the geometry by 1000. Fig. 1. depicts the scaled down model which was used for further analysis

Fig. 2: Geometry (Section View)

c. A section cut was performed on the geomerty to inspect it again as shown in Fig.2. The geometry shows a thickness layer in the throttle body. For the flow through the body, the outer layer would be irrelevant. So the outer layer was selected and deleted along with the thickness walls at the inlet and outlet. The final geometry generated as depicted below in Fig 3.

d. Checked the normal orientation on surface and made sure the normals are pointing towards the fluid regime. In case it was poiting in the reverse direction, Goto Transform>Normal>Propagate Change from a single triangle>select any any traingle and click on apply

Fig.3. Geometry (Section View-Without Thickness Layer)

1.2. Diagnosis: The diagnosis tool helps in identifying the different problems in the geometry.

The Diagnosis tool looks for the following geometric issues

a. Intersection: Highlights number of triangles with piercing inbetween adjacent two triangles

b. Nonmanifold Vertices: Checks for number vertex which are being shared between two triangles

c. Open Edge: This highlights the number of edges of a triangle which not shared by any other triangle

d. Normal Orientation: This highlights the number of traingles whose normal vector is not pointing towards the interior of fluid domain

e. Isolated Triangles: This checks for the number of triangle surrounded by any neighbouring triangles that have different boundary ID number

Goto Diagnostics Dock>Find>

Since all the problem traingles are green ticked, there were no issues in the geometry

1.3. Boundary Flagging:

a. Goto Boundary>Flag>Create a New Boundary>Create Multiple Boundary>Enter 3 in \"Number of Multiple Boundaries\"

b. Since the inlet and outlet of the throttle body are open, these two features needed to be repaired so as to the assign them to respective boundaries. For this, followed the following step: Repair>Patch>Free Edge Loop>Select the circle at the inlet>Apply. Repeated the same step for the outlet

c. Selected the inlet and oulet traiangles, and applied to the respective boundaries.

d.To select the throttle valve, created a section cut and selected the triangled on the throttle and assigned to the Throttle boundary.

The figure below represent the boundaries flagged

1.4. Case Setup:

a. Clicked on \"Begin case Setup\" and entered the value provided below

Settings	Values to be Entered
Application	General Flow
Material	Select Air (predefined mixture) Gas simulation>No Change Global Transport Paramters>No Change
Simulation Parameters	Run Paramters> Pressure-based steady solver Simulation Time Parameter> Start Time: 0 End time: 15000 cycles Initial time step: 1.0e-9 s Minimum time step: 1.0e-9 s Maximum time step: 1.0 s Solver Parameters> No Change
Boundary Conditions	Inlet> Boundary Type- INFLOW>Pressure Boundary Condition>Specified Value (DI)-150000 Pa> Species Boundary Condition> Air Outlet>Boundary Type-OUTFLOW>Pressure Boundary Condition>Specified Value (DI)-100000 Pa>Species>Backflow>Air elbowWall-Wall>Velocity Boundary Conditions>Wall Type motion>Stationary. Select Law of Wall in the velocity boundary conditions and in the Temperature Boundary Conditions as well Throttle> Followed the same as elbowWall Assign Region 0 to all the boundaries
Initial Conditions & Events	Regions and Initialization Add region>\"Region 0\" Enter Pressure value:101325Pa Enter Temperature: 300K Species: Add Air
Physical Models	Turbulence Modeling-RNG-K Eplison is selected
Grid Controls	Enter the grid values for (dx, dy, dz) as per the case 1 dx = 4.0e-4 m, dy = 4.0e-4 m, dz = 4.0e-4 m similarly for Case 2 & 3 the following values were used sequentially Case 2: dx = 4.0e-4 m; dy = 4.0e-4 m; dz = 4.0e-4 m Case 3: dx = 2.0e-4 m; dy = 2.0e-4 m; dz = 2.0e-4 m Activated Fixed Embedding from the menu for Case 2 and 3 Fixed Embedding-Add Embedding>Entity Type-Boundary Boundary Type- Throttle Mode>PERMANENT Case 2 & 3: Scale> Enter a value of 3 Ember Layer>Enter a value of 2
Output/Post Processing	Post Variable Selection>No change
Output Files	Time Interval for writing 3D output data files: 1000 cyc Time Interval for writing text output: 1.0 cyc Time Interval for restarting output: 1000 cyc

After this all the input files was imported to a seperate folder. Converge creates .txt files which contains all the values entered while setting up the case for respective settings done above. These input files will be processed by converge using Message Passing Interface (MPI) standard

2. Solving:

For running the simulation, Cygwin tool was used instead of converge studio interface. Cygwin is a POSIX-compatible API which is based on command prompt is used to run the simulation

Steps:

a. Navigated to the folder location containing the input files

b. Entered the command> mpiexec.exe -n 2 converge.exe </dev/null> logfile & : This allows converge to run two processors (selection on number of processor cores present as per the system configuration, can use multiple cores if available) for the parallel processing and load balancing

c. Entered the command> taif -f logfile

d. After the simulation is over. Went back to converge studio>Post-Processing 3D

e. Enter Case Name>\"Test\", Change the File Type to Paraview VTK in-line binary format

d. Enter the address for the output files>Select all files>Select all Cell Variables>click on Convert

e. After the conversion of the post files to the binary output, open the Test.vtm (group file) file in Paraview

3. Post-Processing:

After opening the group file in paraview, clicked on apply to load the results. Created slice, glyphs and other plots as required

3.1. Meshing:

Fig: Cell Count (Case 1, 2 & 3)

Fig: Case 1: Mesh View (Output2)

Fig: Case 2: Mesh View

Fig: Case 3: Mesh View

Fig: Case 3: Mesh View (Near Throttle)

3.2. Plots

Iteration Error Plots

Fig: Case 1: Iteration Error Plots

Fig: Case 2: Iteration Error Plots

Fig: Case 3: Iteration Error Plots

Inference: The above graphs depict the error values in the calculation of Momentum, Energy and Pressure. The smoothening of the graph towards the end of cycle time represent that the solutions are converged

Mass Flow Rate Plots

Fig: Case 1: Mass Flow Rate Plots

Fig: Case 2: Mass Flow Rate Plots

Fig: Case 3: Mass Flow Rate Plots

Inference: From the above graphs it can be conclude that the mass is conserved, i.e., the magnitude of inlet mass flow rates and outlet mass flow rates are alternate in nature. Also, with the decrease in the grid size, the velocity curves were observed to be more smoothened as the iteration error decreased with increase in cell count

Velocity Plots

Fig: Case 1: Velocity Plots

Fig: Case 2: Velocity Plots

Fig: Case 3: Velocity Plots

Inference: The graphs above represent the velocities at inlet and outlet. The positive inlet velocity vector and negative outlet velocity vector showcases the law of conservation of mass.

Pressure Plots

Fig: Case 1: Pressure Plots

Fig: Case 2: Pressure Plots

Fig: Case 3: Pressure Plots

Inference: The pressure at inlet is constant thorughout the iteration cycle, however, at the outlet the magnitude of pressure has been observed to gradually increase from 100,000 Pa to approximately in the range of 130,000 Pa around 4000 cycles and then remains constant by the end of 15000

3.3. Contours:

Fig: Velocity Contour (Case 1) Fig: Pressure Contour (Case 1)

Fig: Velocity Contour (Case 2: Steady State with AMR)

Fig:Velocity Contour (Case 2: Steady State with AMR:Magnified View Near Throttle)

Fig:Pressure and Velocity Contour With Glyphs (Case 2: Steady State with AMR)

Fig:Pressure and Velocity Contour Near Throttle with Glyphs (Case 2: Steady State with AMR )

Fig:3D View-Pressure and Velocity Contour Near Throttle with Glyphs (Case 2: Steady State with AMR )

Fig: Velocity Contour(Case 3: Steady State with AMR)

Fig: Velocity Contour (Case 3: Steady State with AMR:Magnified View Near Throttle)

Fig:Pressure and Velocity Contour With Glyphs (Case 3: Steady State with AMR)

Fig:Pressure and Velocity Contour Near Throttle with Glyphs (Case 3: Steady State with AMR)

Fig:3D View-Pressure and Velocity Contour Near Throttle with Glyphs (Case 3: Steady State with AMR)

Inference: As the flow establishes, an increase in velocity has been observed at the edges of the throttle and the body wall due to the law of continuity. The pressure on the throttle body faces,oriented towards the flow has also been observed to witness high pressure and other side facing away from the flow direction to witness low pressure due to flow separation.

Animation Link:

Animation 1: Case 3 (Velocity Contour)

Animation 2: Case 3 (Velocity Contour Near Throttle)

Key Conclusions

1.With the decrease in grid size the total cell increases and the flow profile becomes more refined as more number of the equations are solved

2. The inclusion of adaptive mesh refinement results to the generation of more number of cells near the throttle body which further adds up towards higher accuracy capture of the physics underlying the flow in that region. On further reducion of base grid size and the use of AMR settings in combination, refines the results to further extent

3. High velocities were observed at the top and bottom regions of the throttle body and this occurs due to law continuity

4. The cell count increased significantly when the AMR settings were used