Week 4.2: Project - Transient simulation of flow over a throttle body
Throttle
A throttle is a mechanism by which the fluid flow is managed by construction or obstruction. An engines power can be
increased or decreased by the restriction of inlet air due to the presence of throttle. The term throttle can be used to increase
the efficiency of the car, the car accelerator peddal can be used as throttle to increase the efficiency of the engine. It is used
as throttle in the field of aviation, known as thrust lever in jet engine powered aircraft. For steam locomotive the valve which
controls the regulation of gas is called as reguator.
Application of throttle
The throttle is a mechanism which is used to increase or decrease the power of engine by restricting the flow of inlet gases
coming through it. There are numerous applications of the throttle mechanism which are listed below
Internal Combustion Engines
Fuel Injected Engines
Reciprocating Engine Aircraft
Steam locomotives
Automobiles
Case-setup
The elbow. STL file is loaded into the Converge-CFD set-up.
The internal flow simulation does not involve the usage of the external boundary, the external boundary is of special interest
when there is a CHT simulation setup. The external boundary can be deleted by selecting the triangle option from the ribbon
and then hitting the "D" button of the keyboard by enabling the hotkeys. The other alternative is selecting the "Repair" option
from the "Geometry Dock" window and selecting the entity and hitting "Apply".
The diagnosis dock is selected from the "View" option and the "Find" option is selected. It is found that there are 211 open
edges in the geometry ("Intersections(0)","Nonmanifold Problems(0)","Open Edges(211)", "Overlapping Tris(0)","Normal
Orientation(0)","Isolated tris (0)".
To remove open edges from the geometry, the "Repair" option in the "Geometry Dock" is selected, under "Patch" the "Open
Edges" are selected viz (inlet and outlet) of the geometry and then "Boundary Flagging" is done to make "Open Edges" in the
resulting geometry to "0".
The "Normal Toggle" is selected from the ribbon, upon selecting the normal toggle it was observed that the normal is pointing
outside the volume, technically the normal should point inside the volume where the fluid is flowing. Therefore in the
geometry dock, the "Transform" option is clicked. The "Normal" tab is selected from the geometry dock and one of the
triangles containing the normal pointing outward is selected. The "Apply" option is selected for removing the normals.
The "Case Setup Dock" is selected from the "View" and "Begin Case Setup" is executed.
The "Time" based "Application" type is selected. The "Material" is selected and the predefined mixture is selected as air. The
"Reactant mechanism" is checked off because it is not a combustion problem. The species is selected and the "Apply" is
clicked. Under "Gas simulation" the Equation of state is selected as "Redlich Kwong", the critical temperature is 133K and the
critical pressure is 3770000Pa. The Turbulent Prandtl number is 0.9 and the Turbulent Schmidt number is 0.78 under Global
Transport parameters. The
and the
are selected as species.
The "Geometry Bounding Box" present under the options ribbon in the "Geometry Dock" helps us to analyze whether the
geometry is scaled properly or not. It also helps us to approximate the total length of the geometry.
Calculating Rotate center and Rotate about for the throttle body
The converge gives us the cool option to visualize how the throttle body rotates. The only input the converge wants from us
is that to give the arc center and the arc normal. The "Measure" option in the ribbon provides a "Direction" under which there
are three options namely, "Two Vertices", "Triangle Normal", "Arc Normal". The "Arc Normal" option is selected and randomly
three points are selected and the Converge gives the desired values as tabulated below
The Rotation rate file is named "throttle_position.in" where the profile information of the rotation of the throttle body is
stored as below
Boundary conditions
The boundary conditions for the different named selections are tabulated below
In the "Region and Initialization" the mass fraction of
and
were created and named as "Volumetric Region User Defined" and the same is updated in the boundary condition region
name. These are the initial condition and will be washed out after the solution has converged.
Determination of Transient Parameters for Mesh-2
The total length of the pipe approximated as 0.2m
The velocity at the inlet calculated from Mesh-2 of the steady-state simulation is 120.646 m/sec
Simulation End Time
which is 0.0165s
Calculating CFL number
In order to attain numerical stability, the CFL number is set as 1, the value of the time step
is
. This value of time step corresponds to the laminar boundary layer thickness which is, in this case, is
during the entire transient simulation which however is not possible as per the Cartesian cut cell method to use the uniform
grid size away from the boundary and to use smaller or irregular cells wherever the boundary intersects the cells. This allows
us to use the "Variable Time Step Algorithm". The algorithm involves the usage of the initial, minimum, maximum time
steps, maximum convection CFL limit, maximum diffusion CFL limit, and the maximum Mach CFL limit. Since the throttle
body is moving so the mesh size is changing at every time step, therefore each of the above variables corresponds to each of
the mesh sizes which would result in the maximum permissible timestep in the control volume to make the iteration stable.
Results
Velocity contour
Pressure contour
Mass flow rate contour
Line plots
Velocity
Pressure
Mass flow rate
Total cell count
Summary
Conclusion
The transient numerical analysis of flow over a throttle body was carried successfully using Converge software and Cygwin
command prompt terminal.
The movement of the throttle body was studied and visualized by providing the relevant boundary condition which is
"Moving" as Wall motion type and "Rotating" as surface type.
The change in the position of the throttle body helps us to visualize how pressure, mass flow rate, and velocity varied
quantitatively around the throttle body, and different flow structures were observed.
The transient simulation was carried for base grid size 2mm from the steady-state simulation. The time taken for the
simulation on a single mesh size was 1.5 days. Thus there was a need for high computing power which could have
significantly brought down the program run time and thus the same simulation could have been performed on different mesh
sizes.