Week 4: Project - Transient simulation of flow over a throttle body
Aim: Transient simulation of flow over a throttle body
Objective:
- To set up and run the steady-state simulation for flow over the throttle body.
- To post-process the results and show pressure and velocity contours.
- To show the mesh (i.e surface with edges)
- To show the plots for pressure, velocity, mass flow rate and total cell count.
- To create an animation in which its throttle movement should be visible.
- From 0 to 2 ms the valve should rotate by 25 degrees.
- From 2 to 4ms it should stay at 25 degrees.
- After which by 8ms the valve should get back to its original position.
Introduction
The throttle is a mechanism by which airflow is managed by construction or obstruction in the intake manifold of an SI engine. The throttle valve in the intake system controls the airflow into the engine and thereby cylinder charge, which determines the engine power and torque output. Variation in throttle angle governs the engine speed and torque output by adjusting the airflow into the engine. In a typical SI engine, a throttle valve in the form of circular plate is located in the airflow passage of intake manifold.
Theory:
Throttle
In fuel-injected engines, the throttle body is the part of the air intake system that controls the amount of air flowing into the engine, in response to driver accelerator pedal input in the main. The throttle body is usually located between the air filter box and the intake manifold, and it is usually attached to, or near, the mass airflow sensor. Often, an engine coolant line also runs through it for the engine to draw intake air at a certain temperature and therefore with a known density. The largest piece inside the throttle body is the throttle plate, which is a butterfly valve that regulates the airflow.
On many cars, the accelerator pedal motion is communicated via the throttle cable, which is mechanically connected to the throttle linkages, which, in turn, rotate the throttle plate. In cars with electronic throttle control (also known as "drive-by-wire"), an electric actuator controls the throttle linkages and the accelerator pedal connects not to the throttle body, but to a sensor, which outputs a signal proportional to the current pedal position and sends it to the ECU. The ECU then determines the throttle opening based on the accelerator pedal's position and inputs from other engine sensors such as the engine coolant temperature sensor. Throttle body showing throttle position sensor. The throttle cable attaches to the curved, black portion on the left. The copper-coloured coil visible next to this returns the throttle to its idle (closed) position when the pedal is released. When the driver presses on the accelerator pedal, the throttle plate rotates within the throttle body, opening the throttle passage to allow more air into the intake manifold, immediately drawn inside by its vacuum. Usually, a mass airflow sensor measures this change and communicates it to the ECU. The ECU then increases the amount of fuel injected by the injectors to obtain the required air-fuel ratio.
Transient State:
A system is said to be in a transient state when a process variable or variables have been changed and the system has not yet reached a state. The time taken for the circuit to change from one steady-state to another steady state is called the transient time.
Transient Flow Over the throttle Body
Geometry Setup:
1. Import Geometry:
Imported the .stl file geometry.
2. Scaling down the geometry
Converge assumes the dimensions in metres i.e in below-shown image l_x=107.317m, l_y=121.006m, l_z=19.1742m which is too large to handle. So, need to scale down.
Steps to scale down the geometry:
Geometry>Transform>Scale>Entity Type-Entire Surface>Scale Factor-Uniform=0.001>Apply
Desired Scaled-down Geometry:
3. Creating a Boundary Fence:
To automatically create a boundary fence-
Geometry>Boundary>Find/Clean>Find
4. Deleting Thickness:
The flow is to be simulated inside the elbow so the thickness of the elbow must be converted to the flow region.
The imported elbow geometry has a particular thickness which is of not much importance.
`therefore` deleting the thickness:
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>selecting the unwanted thickness>with hot press key enabled>press 'D'
Thickness is deleted.
Deleting the thickness of an elbow leads to open edges. Need to close the open edges.
5. Closing open edges
Geometry>Repair>Patch>Patch options-Free Edge Loop>Entity Edge-By Open Edge>Select the edge>Apply and Selecting another open edge>Apply
The elbow edges are closed and that has triangles on its face.
6. Creating Boundaries:
Geometry>Boundary>Flag> '+' create a new boundary>Create Multiple boundaries>OK
Boundaries are Created:
7. Assigning Boundaries:
In converge every part of geometry is assumed to be a triangle. Selecting triangles and grouped to assign to specific boundaries.
i) Elbow Wall:
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>Assigning them to Elbow wall Boundary> Apply
ii) Throttle:
The throttle is located inside the elbow wall, to assign that to a particular boundary- hide the elbow wall.
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>right click and>Hide selection.
`triangle` as an entity selection-By Surface>using cursor pick option>Selecting the triangles as shown in below image>Assigning them to Throttle Boundary> Apply
Need to select the entire throttle plate because it is not connected to elbow pipe anywhere, it is a separate close surface.
iii) Inlet:
Using right-click and selecting the redisplay option, will display the earlier hidden portion.
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>Assigning them to Inlet Boundary> Apply
iv) Outlet:
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>Assigning them to Outlet Boundary> Apply
The geometry with assigned boundaries:
8. Normal Orientation:
To check the normals first need to hide the half portion of the elbow wall.
`triangle` as an entity selection-By Any>using box select option>box selection from left to right shown in below image>triangles will get selected.
On the selected triangles using right-click hiding the selection.
Now with the hidden portion, normal orientation can be viewed.
Every geometry has a normal vector, it points perpendicular to it.
Normal Toggle - Enables Normal
To run the CFD simulation, the normal should point towards the fluid.
Normals are pointing towards the fluid.
9. Diagnosis:
Run a diagnosis check to ensure that there are no errors. Enable "Check that all the normals are pointing to the interior" then click on find.
All green ticks under problem triangles show that geometry has no errors.
Case Setup:
1. Application Type
Application Type - Time-Based
2. Materials
Selecting Air in the drop-down menu of predefined mixtures and keeping Gas Simulation enabled.
Confirming the permission to overwrite the gas.dat, mech.dat and therm.dat by loading predefined mixture Air.
i) Gas Simulation
Keeping the default values for gas simulation > OK
ii) Global Transport Properties
Keeping all the default values for global transport properties > OK
3. Simulation Parameters
i) Run Parameters
Under Solver tab change the solver to Transient solver.
Simulation mode is Full hydrodynamic and Gas flow solver is Compressible.
The transient solver is used because the throttle body is moving, and this is a case of transient flow over a throttle body.
a) Misc. Tab:
Under Misc. tab disabling use shared memory option > OK.
ii) Simulation Time Parameters
End Time Calculation
The dimensions in x and y directions are roughly 0.1 metres. Length of pipe will not be more than 0.1*2 = 0.2 metres.
The inlet velocity was 200 m/s in the steady-state simulation.
`"Flow through time" = "Characteristic Length" / "Average Velocity"`
`"Flow through Time" = 0.2 / 200 = 1e-3`
Simulating the flow till roughly 10 times the flow-through time 1e-3*10 = 0.01 seconds
Rounding off the end time to 10 milliseconds.
Set the start time as 0 seconds, end time as 0.01 seconds, Initial and minimum time step as 1e-9 and Maximum time step as 1 second.
Keep all the other CFL values default > OK.
Using initial and maximum time-step as 1e-9 because converge uses a dynamic time-step algorithm. At every time-step converge tries to see if the time-step can be increased for next coming time-step.
So, if initial conditions and mesh conditions are good then time is going to increase automatically.
iii) Solver Parameters
Changing the Navier-stokes solver type as Pressure-based.
Keeping all other settings as default > OK.
4. Regions and Initializations
Grouping the boundaries to form volumetric zones called region. This defines a volumetric region with initial conditions like initial pressure, initial temperature, velocity and species concentration
Selecting Add to create a new region, then selecting Air for Species initial conditions and keep the other values default > OK.
5. Boundary
i) Elbow Wall:
Setting boundary type - WALL
Assigning Region0 under region name.
ii) Throttle:
Selecting the boundary type - WALL
Wall motion type - Rotating
Surface movement - Moving
Assigning Region0 under region name.
Now to select Rotation Center and Rotation about:
The throttle is located inside the elbow wall, to assign that to find rotation centre and rotation about of a throttle - hide the elbow wall.
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>right click and>Hide selection.
Again,
`triangle` as an entity selection-By Angle `20^0`>using cursor pick option>Selecting the triangles as shown in below image>right click and>Hide selection.
After hiding the above-shown triangles, triangles displayed are:
Now, the arc centre and arc normal can be found:
Geometry > Measure > Direction > Direction options - Arc normal > vertex as an entity selection-By Any > using cursor pick option>Selecting the three vertices on the arc as shown in below image> Apply
Under measure log, the values of arc centre, arc diameter and arc normal are obtained.
Using the values of the arc centre and inserting that values under rotation centre option and similarly using the arc normal values under rotate about option.
Rotation rate - check use file option
Continuous rotation rate can be given, which is not required in this case.
The requirement is as follows:
- From 0 to 2 ms the valve should rotate by 25 degrees.
- From 2 to 4ms it should stay at 25 degrees.
- After which by 8ms the valve should get back to its original position.
iii) Inlet:
Boundary Type - INFLOW
Pressure boundary condition as the specified value of 1.5e5 Pa i.e 1.5 Bar
keeping the temperature boundary condition as default
Selecting air under the species boundary condition.
iv) Outlet:
For Outlet set the Pressure Boundary condition as Specified Value of 1e5
Boundary Type - OUTFLOW
Pressure boundary condition as the specified value of 1e5 Pa
Keeping the temperature boundary condition as default
Selecting Air for species backflow
5. Turbulence Modeling:
Keep the Turbulence model as RNG k-epsilon
Keep all the settings default > OK.
6. Grid Control
Enabling Fixed Embedding option.
i) Base Grid
Set the mesh size to 2e-3
ii) Fixed Embedding
We want finer mesh around the throttle and it can be achieved using Fixed Embedding.
Selecting Add then setting Entity type - BOUNDARY,
Boundary ID - Throttle
Scale - 3 and Embed Layers - 3.
7. Output/Post-Processing
i) Post-Variable Selection
Using default settings
ii) Output Files
`"Time interval to write 3D output files" = "Simulation run time"/"No. of output files required"`
`t= 0.01/100`
t= 0.0001 sec
8. Export all the input files
Export all the input files into a separate folder
Files>Export>Export Input Files>setting up the desired location>OK
9. Copy and paste the mpiexec.exe file to the input files folder
Run Simulation
Cygwin - Cygwin is a collection of GNU and open-source tools that provide functionality similar to Linux distribution on windows.
To run the simulation, open Cygwin and navigate to the folder in which the input files are exported.
To run the simulation command with executable to be entered
$ mpiexec.exe -n 2 converge-intelmpi.exe restricted </dev/null> logfile.txt &
Taking output mpiexec.exe and sending it to </dev/null> further Taking data from </dev/null> and sending to logfile.txt
Post convert all the output files
Enter the case name, select the file type as EnSight format, select boundary surface output to yes, navigate to the directory where the output files are located.
Then for files select all, for cell variables select all and hit Convert. The file will be with case name in .case format
Post-processing
Launch Paraview from ...ParaView-5.4.1-Qt5-OpenGL2-Windows-64bitbin folder and open the .case file and press apply.
Computational Time:
Simulation Results:
Mesh
The 'fixed embedding' created a fine mesh near the 'Throttle body'. The scale parameter (n) determines the final size of elements as `dx/(2^n)` and layers determines the number of layers of fine mesh along the boundary. The software creates the gradual fine mesh by dividing neighbouring cells of boundary into half the size of the previous cell, till the finest size becomes as decided by the scale parameter.
It is clearly visible that mesh around the throttle keeps on changing as throttle moves along the rotation axis, which helps in providing the accurate results around the throttle valve.
Pressure and Velocity Contours
Pressure Contour
.
Initially, when the throttle is at 25 degrees there is a maximum obstruction to the fluid flow so the pressure increases to its highest point. The total inlet pressure is observed equal to 150000 Pa exactly equal to the value used while setting up the case.
Pressure Contour at `25^0` Throttle angle i.e at 2ms and 4ms:
Pressure Contour at 8ms:
Pressure Animation:
Pressure Plot:
The static pressure drops at the inlet with a significant dip at around 0.002 where the throttle rotates and gets into normal curve after that as the throttle gets back to its original position.
At the Outlet, the total pressure increases and the static pressure remains the same as it is set ie 100000 Pa.
Velocity Contour Plot
Velocity Contour at `25^0` Throttle angle i.e at 2ms and 4ms:
Velocity Contour at 8ms:
The throttle plate and the throttle shaft provide restriction to the flow of intake air. Lesser is the restriction across the flow, lower is the loss of energy. This increases the velocity at the outlet of the throttle body. Thus it can be seen from the above contours at different time step showing velocity changes across the flow.
At 0.002 seconds the throttle position is at 25 degrees. thus due to the sudden restriction in the flow velocity decreases. As the flow area is minimized the flow velocity decreases but after the flow passes over the throttle the velocity reaches its highest point due to the sudden expansion of the area. The velocity fluctuates at the beginning due to the obstruction in the flow after that it increases constantly.
Velocity Animation:
Velocity Vector Propagation:
Velocity Plot:
Mass Flow Rate Plot:
The above plot shows that the mass flow rate initially takes a little turbulence and finally converges as expected. The law of conservation of the mass is followed as the inflow mass flow rate is balanced by the outflow mass flow rate. Initially due to the movement of throttle mass flow rate fluctuations at that time period are high. From 0.002 to 0.004 the fluctuations decreases and also approximately after 8 milliseconds the mass flow rate converges.
Total Cell Count Plot
The total cell count is not constant because the mesh is being continuously refined as the Throttle valve moves. The cell count is varying exactly on the time of flow when the valve is rotating and becomes constant when it's not rotating. The maximum flow time is 0.01.
Cell Count:
Conclusions:
1. The mass flow rate initially takes a little turbulence and finally converges as expected. It can be seen from the graph that the law of conservation of the mass is followed as the inflow mass flow rate is balanced by the outflow mass flow rate. It can be seen that initially due to the movement of throttle mass flow rate fluctuations at that time period are high. From 0.002 to 0.004 the fluctuations decreases and also approximately after 8 milliseconds the mass flow rate converges.
2. The throttle plate and the throttle shaft provide restriction to the flow of intake air. Lesser is the restriction across the flow, lower is the loss of energy. This increases the velocity at the outlet of the throttle body. As the flow area is minimized the flow velocity decreases but after the flow passes over the throttle the velocity reaches its highest point due to the sudden expansion of the area.
3. The total inlet pressure is observed equal to 150000 Pa exactly equal to the value used while setting up the case. The static pressure drops at the inlet with a significant dip at around 0.002 where the throttle rotates and gets into normal curve after that as the throttle gets back to its original position.
4. At the Outlet, the total pressure increases and the static pressure remains the same as set in the case set up ie 100000 Pa.
5. Simulation time is much more as compared to the steady State Simulation.