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 $O_2$

and the $N_2$

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 $O_2$

and $N_2$

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$=\frac{L}{U}\ast 10$

which is 0.0165s

Calculating CFL number 

$CFL=U\ast \frac{△t}{△x}$

In order to attain numerical stability, the CFL number is set as 1, the value of the time step

is $1.947847421381563e-6s$

. This value of time step corresponds to the laminar boundary layer thickness which is, in this case, is $△x$

 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.