Steady State simulation of flow over a throttle body
AIM:
In this project, a Steady state simulation of flow over a throttle body is simulated using softwares called Converge Studio 3.0, Cygwin64 Terminal and Paraview.
The simulationm is run in three different mesh sizes:
1) Mesh size 1: 2e-3 m
2) Mesh size 2: 1.5e-3 m
3) Mesh size 3: 1e-3 m
OBJECTIVE:
The objectives of the project are:
To simulate the steady state flow over a throttle body, post process the results obtained from the simulation. Then, analyze and compare the following results obtained for the three mesh sizes:
i) Mesh size
ii) Velocity and Pressure Contours
iii) Plots for average velocity of the fluid mixture(air), static pressure, total pressure, mass flow rate and total cell count vs the total no. of cycle simuations.
iv) Velocity and Pressure animations.
THEORY:
A throttle is a mechanism by which the flow of a fluid is regulated by constriction or obstruction. We usually use a circular disc type constriction. In the case of an Internal Combustion engine, throttle mechanism is used to control the engine's power by controlling the amount of fluid (which is usually air) in the engine.
A sketch showing a throttle mechanism is shown below:
A throttle body is the part of the air intake system that controls the amount of air flowing into the engine. In the image above, two cases have been shown for a throttle body under operation. Case 'a' shows the flow of air inside a circular pipe whose cross section is shown. The throttle valve is closed and hence the air has stopped at the throttle body. Case 'b' shows a partially open throttle valve and hence allows restricted movement of air through it.
GEOMETRY:
The geomerty of the throttle body is shown below. The geometry is a made up geometry and the parts of the throttle body which are shown in below is only the circular disc and it does not completely block the flow of the fluid when the tnrottle body is closed.
The image in the left side shows a pipe in the shape of an elbow. We will call it as elbow throughout this project. On the right hand side the image shows the cross section of th elbow and the circular disc that can be seen is the throttle valve which is a circular disc.
This geometry is drawn in any CAD software and is imported as an stl file. This type of file is a file that converts a file into a triangulated form of the CAD file format. This type of file is required to open in Converge studio.
After the geometry is imported, it is checked for any errors in the diagnosis dock.
It checks for the following problems in the geometry:
1) Intersections.
2) Nonmanifold edges.
3) Open edges.
4) Overlapping Tris.
5) Normal orientation.
6) Isolated tris.
The diagnosis dock is shown in the picture below.
We can click the find option marked in the above picture of diagnosis dock to check for the problems in the geometry.
As it can be seen in the above picture that there are no problems in the geometry.
Next step is to set up the case. In this process we input all the necessary parameters for before processing the solution for the simulation.
CASE SETUP:
i) Application type: In the application type we select the time-based type as in our case the simulation is not for an IC engine. In the version prior to converge studio 3.0, the application type can be selected as General flow.
ii) Materials: We check Gas simulation, reaction mechanism. We select Air as the pre-defined mixture.
The values for the global transport parameters are taken as follows:
1) Prandtl Number: 0.9
2) Schmidt Number: 0.78
iii) Simulation parameters:
In simulation parameters we check run parameters, simulation time parameters and solver parameters.
Under run parameters set the simulation type to steady state and run the simulation on full hydro mode. Also in the miscellaneous section, check the steady state monitor. in the steady state monitor section add the quantity which you want to be monitored. In this case, Avg. Velocity is monitored at the inlet boundary.
Under the simulation time parameters the conditions are set as following:
1) start time = 0 cycles
2) End time = 15000 cycles
3) Initial time step = 1e-9 s
4) Minimum time step = 1e-9 s
5) Maximum time step = 1 s
6) Maximum convetion cfl limit = 1.0
In the solver parameters, the maximum convection cfl limit (final stage) is set to 0.5.
Navier-stokes solver scheme: PISO
Initial Conditions:
The following settings are applied as the initial conditions:
Velocity: X component=0; Y Component=0; Z component =0
Temperature: 300 k
Pressure: 101325 Pa
Species: Mass fractions: Oxygen=0.23; Nitrogen=0.77
Boundary Conditions:
The boundary conditions are as follows:
i) Inlet: The boundary type INFLOW is selected.
Total Pressure is set as 150000 Pa
Temperature 300 k
Species: Air with mass fractions of oxygen : 0.23 and Nitrogen: 0.77
ii) Outlet: The boundary type OUTFLOW is selected.
Total Pressure is set as 100000 Pa
Temperature 300 k
Species: Air with mass fractions of oxygen : 0.23 and Nitrogen: 0.77
iii) Elbow: The boundary type WALL is selected
Wall motion type is set to stationary
Surface movement: FIXED
Velocity: Law of wall
Temperature: Law of wall; 300K
iv) Throttle: The boundary type WALL is selected
Wall motion type is set to stationary
Surface movement: FIXED
Velocity: Law of wall
Temperature: Law of wall; 300K
Mesh: The following three mesh sizes are used:
1) Mesh size 1: dx=dy=dz=2e-3 m
2) Mesh size 2: dx=dy=dz=1.5e-3 m
3) Mesh size 3: dx=dy=dz=1e-3 m
In Grid control, enable Fixed embedding.
Under the fixed embedding sst the following as mentioned below:
Scale: 3
Layers: 2
Now, the input files are exported to a folder and are processed in the software Cygwin terminal 64.Here the solver solves the steady state Navier stokes equations and the results thus obtained are post processed in the Software called Paraview.
RESULTS AND DISCUSSION:
The mesh size for all the three meshes is shown below.
The mesh size 1 which is dx=dy=dz=2e-3:
The mesh size 2 which is dx=dy=dz=1.5e-3:
The mesh size 3 which is dx=dy=dz=1e-3:
In the above images for the three mesh sizes, one image which is on the left side for each mesh shows the mesh for the elbow and the image on the right side for each of the mesh sizes shows the mesh size around the throttle body.
The mesh size has been refined to much finer size by using fixed embedding. This is done to get accurate results around the throttle body.
The mesh size near the throttle body can be calculated as,
Mesh size around the throttle body= (grid size / 2^(embed level)).
For mesh size 2 mm, the value for the mesh size around the throttle body= (2mm/2^2) = 0.5 mm
Similarly, for mesh sizes 1.5 mm and 1 mm, the value for the mesh size around the throttle body is 0.375 mm and 0.25 mm respectively.
The Velocity contours for all the three mesh sizes are shown below:
Velocity contour for mesh size 1 which is dx=dy=dz=2e-3 m:
In the above velocity contour plot, the variation of velocity of air inside the elbow and also over the throttle body can be seen. The above velocity contour shown, is observed at the last time step of the simulation. The maximum velocity is observed at the region between the throttle body and the elbow wall, with a value of 290 m/s. This is because the area through which the air mixture moves has suddenly decreased but still there is a pressure difference at the inlet and outlet of he elbow which is driving the air. Also, the flow is continuous. So, when there is a decrease in area of cross section, then at that cross section area the velocity increases. The minimum velocity is observed to be 170 m/s.
In the above velocity contour plot, the variation of velocity of the air over the throttle body is shown. The maximum velocity observed is 310 m/s. The minimum velocity observed is 2.3 m/s.
Velocity contour for mesh size 2 which is dx=dy=dz=1.5e-3 m:
In the above velocity contour plot, the variation of velocity of air inside the elbow and also over the throttle body can be seen. The above velocity contour shown, is observed at the last time step of the simulation. The maximum velocity is observed at the region between the throttle body and the elbow wall, with a value of 310 m/s. This is because the area through which the air mixture moves has suddenly decreased but still there is a pressure difference at the inlet and outlet of he elbow which is driving the air. Also, the flow is continuous. So, when there is a decrease in area of cross section, then at that cross section area the velocity increases. The minimum velocity is observed to be 150 m/s.
In the above velocity contour plot, the variation of velocity of the air over the throttle body is shown. The maximum velocity observed is 330 m/s. The minimum velocity observed is 6.3 m/s.
Velocity contour for mesh size 3 which is dx=dy=dz= 1e-3 m:
In the above velocity contour plot, the variation of velocity of air inside the elbow and also over the throttle body can be seen. The above velocity contour shown, is observed at the last time step of the simulation. The maximum velocity is observed at the region between the throttle body and the elbow wall, with a value of 310 m/s. This is because the area through which the air mixture moves has suddenly decreased but still there is a pressure difference at the inlet and outlet of he elbow which is driving the air. Also, the flow is continuous. So, when there is a decrease in area of cross section, then at that cross section area the velocity increases. The minimum velocity is observed to be 94 m/s.
In the above velocity contour plot, the variation of velocity of the air over the throttle body is shown. The maximum velocity observed is 310 m/s. The minimum velocity observed is 94 m/s.
Pressure Contours for all the three mesh sizes are shown below:
Pressure contour for the mesh size 1 which is dx=dy=dz=2e-3 m:
In the pressure contour shown above, the pressure varies from a maximum of 120000 pa to a minimum of 72000 pa. The pressure varies from inlet to outlet because we have provided a pressure difference of around 50000 pa initially. As we can see, before and after the throttle, there is a sudden decrease in pressure as the throttle body is acting as an obstacle and allowing a very narrow cross section of the elbow for the air to pass through it.
The pressure variation over the throttle is shown in the above image. The maximum pressuure is found at that side of the throttle body which is nearest to the inlet of the elbow which is equal to 150000 pa. The minimum pressure found at the side of the throttle body which is along the cylindrical axis of rotation. The value is 60000 pa.
Pressure contour for the mesh size 2 which is dx=dy=dz=1.5e-3 m:
In the pressure contour shown above, the pressure varies from a maximum of 130000 pa to a minimum of 70000 pa. The pressure varies from inlet to outlet because we have provided a pressure difference of around 50000 pa initially. As we can see, before and after the throttle, there is a sudden decrease in pressure as the throttle body is acting as an obstacle and allowing a very narrow cross section of the elbow for the air to pass through it.
The pressur variation over the throttle is shown in the above image. The maximum pressuure is found at that side of the throttle body which is nearest to the inlet of the elbow which is equal to 130000 pa. The minimum pressure found at the side of the throttle body which is along the cylindrical axis of rotation. The value is 70000 pa.
Pressure contour for the mesh size 3 which is dx=dy=dz=1e-3 m:
In the pressure contour shown above, the pressure varies from a maximum of 120000 pa to a minimum of 71000 pa. The pressure varies from inlet to outlet because we have provided a pressure difference of around 50000 pa initially. As we can see, before and after the throttle, there is a sudden decrease in pressure as the throttle body is acting as an obstacle and allowing a very narrow cross section of the elbow for the air to pass through it.
The pressur variation over the throttle is shown in the above image. The maximum pressuure is found at that side of the throttle body which is nearest to the inlet of the elbow which is equal to 120000 pa. The minimum pressure found at the side of the throttle body which is along the cylindrical axis of rotation. The value is 71000 pa.
Plots for Velocity at inlet and outlet boundaries for all the three mesh sizes vs the simulation cycles are shown below:
Average velocity at inlet and outlet boundaries vs the simulation cycles for mesh size 1 dx=dy=dz=2e-3 m:
Average velocity at inlet and outlet boundaries vs the simulation cycles for mesh size 2 dx=dy=dz=1.5e-3 m:
0
Average velocity at inlet and outlet boundaries vs the simulation cycles for mesh size 3 dx=dy=dz=1e-3 m:
The above plots for average velocity at inlet and outlet boundaries vs the number of simulation cycles can be observed and the following conclusions can be drawn. At the start of simulation, we can observe that there is a sudden change in the value of the average velocity both at inlet and after some cycles at the outlet boundary too. This is because the initial time step is taken as 1e^-9 s and afterwards the time-step increases. As this is a steady state simulation, the value of average velocity has reached a steady state at both inlet and outlet boundaries. When we compare the plots for the three mesh sizes we can say that the accuracy of the average velocity has increased. But one can observe during solving the simulation that it takes more time to solve as the grid size is made smaller.
Plots for Mass flow rate at inlet and outlet boundaries for all the three mesh sizes vs the simulation cycles are shown below:
Mass flow rate at inlet and outlet boundaries vs the simulation cycles for mesh size 1 dx=dy=dz=2e-3 m:
Mass flow rate at inlet and outlet boundaries vs the simulation cycles for mesh size 2 dx=dy=dz=1.5e-3 m:
Mass flow rate at inlet and outlet boundaries vs the simulation cycles for mesh size 3 dx=dy=dz=1e-3 m:
For the above mass flow rate at the inlet and outlet boundaries vs no. of simulation cycles the inflow mass flow rate is equal to the outflow mass flow rate which indicates that the steady state is reached.When we compare the plots for the three mesh sizes we can say that the accuracy of the mass flow rate has increased. But one can observe during solving the simulation that it takes more time to solve as the grid size is made smaller.
Plots for Static Pressure at inlet and outlet boundaries for all the three mesh sizes vs the simulation cycles are shown below:
Static Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 1 which is dx=dy=dz=2e-3:
Static Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 2 which is dx=dy=dz=1.5e-3:
Static Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 3 which is dx=dy=dz=1e-3:
The above plots show the variation of static pressure at inlet and outlet boundaries vs the number of simulation cycles. When we compare the plots for the three mesh sizes we can say that the accuracy of the static pressure has increased. But one can observe during solving the simulation that it takes more time to solve as the grid size is made smaller.
Plots for Total Pressure at inlet and outlet boundaries for all the three mesh sizes vs the simulation cycles are shown below:
Total Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 1 which is dx=dy=dz=2e-3:
Total Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 2 which is dx=dy=dz=1.5e-3:
Total Pressure at inlet and outlet boundaries vs simulation cycles for mesh size 3 which is dx=dy=dz=1e-3:
The above plots show the variation of total pressure at inlet and outlet boundaries vs the number of simulation cycles. One can observe that the static pressure is constant at the outlet boundary where as the total pressure is constant at the inlet boundary. When we compare the plots for the three mesh sizes we can say that the accuracy of the total pressure has increased. But one can observe during solving the simulation that it takes more time to solve as the grid size is made smaller.
Plots for Total cell count for all the three mesh sizes vs the simulation cycles are shown below:
Total cell count vs simulation cycles for mesh size 1 which is dx=dy=dz=2e-3:
Total cell count vs simulation cycles for mesh size 2 which is dx=dy=dz=1.5e-3:
Total cell count vs simulation cycles for mesh size 3 which is dx=dy=dz=1e-3:
The above three plots shows the variation of total cell count and also the number of cells processed by each processor vs the number of simulation cycles.
Velocity Vector plots for all the three mesh sizes are shown below:
Velocity vector plot for the mesh size 1 which is dx=dy=dz=2e-3:
Velocity vector plot for the mesh size 2 which is dx=dy=dz=1.5e-3:
Velocity vector plot for the mesh size 3 which is dx=dy=dz=1e-3:
The velocity vector plots shown above helps us to understand the direction of flow of the air mixture at various locations from inlet to outlet of the fluid volumetric region.
Here is the link to the velocity animations for all the three mesh sizes, which shows the development of velocity contours over simulation cycles:
Link for the animation for the velocity at the mesh size 1 dx=dy=dz=2e-3 m
Link for the animation for the velocity at the mesh size 2 dx=dy=dz=1.5e-3 m
Link for the animation for the velocity at the mesh size 3 dx=dy=dz=1e-3 m
Here is the link to the Pressure animations for all the three mesh sizes, which shows the development of pressure contours over simulation cycles:
Link for the animation for the pressure at the mesh size 1 dx=dy=dz=2e-3 m
Link for the animation for the pressure at the mesh size 2 dx=dy=dz=1.5e-3 m
Link for the animation for the pressure at the mesh size 3 dx=dy=dz=1e-3 m
From the above animations the following conclusions can be drawn:
In a steady state simulation, both velocity and pressure animations does not show the true nature of the flow of the fluid every time step.They rather show the values of velocity and pressure which the solver has obtained during each time step for each iteration. The true nature of the fluid flow can be observed at the end time step when steady state is reached.