Project Transient simulation of flow over a throttle body

Objective: To Simulate (Transient state) flow of fluid over a throttle body in converge studio and post-process the results.

1. To Show the mesh (i.e surface with edges)
2. To post-process the results and show pressure and velocity contour
3. To Show the plots for pressure, velocity, mass flow rate and total cell count.
4. To show the calculations on how you calculated an end time for the simulation.
5. To create an animation in which its throttle movement should be visible.

Given: We have an elbow geometry created and saved as .STL file by Solidworks. Drag and drop the geometry in Converge studio for the further analysis part.
First, we should transform or scale the geometry into the meter (m), this is the unit required by Converge studio because the geometry we have is in millimeter (mm).

Throttle profile info:

• 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.

Theory:

Here, we are dealing with the flow over moving throttle of the elbow. The throttle body is moving with time. Hence, it becomes a transient problem. Since we are stimulating moving parts we need to do a transient simulation to capture all dynamic effects which include angular velocity, Velocity, vorticity, Angular Momentum, etc.

As shown in the below image, throttle has moved slightly from the original position i.e. 25 degrees.

Case Setup:

First, we do geometry clean up i.e. looking for all types of surface errors in the diagnosis section. Specifically, we will be looking for Intersections, nonmanifold problems, Open edges, Normal orientation with one additional checkpoint is that all the normals are pointing to the interior. Later we proceed for Boundary flagging step.

We are going to simulate transient flow over the throttle body so that solver in Run Parameter must be selected as TRANSIENT. As we select solver to transient soon simulation time parameters will be changed to SECONDS instead of CYCLE.

We need to calculate the end time of simulation based on the following parameters
1. Velocity at Inlet
2. Characteristics Length of Domain through which fluid flow going to happen
3. Number of passes of fluid through the domain in order to reach convergence.

To know the dimensions of a domain, mark tick on the geometry bounding box from the Options menu of Geometry Dock.

`**`Ideally, we should know the complete length of the elbow.

Characteristics length= 0.2m (Approx.)
Avg. Velocity @ Inlet = 191.43 m/s

Time to pass a fluid through the elbow= (Ch. Lenght/Avg. velocity)
T= 0.2/191.43

T= 0.00104 sec. (for one time pass)

To make a pass of fluid 10 times through elbow.

T= 0.00104*10
T= 0.01 ms (Approx.)

RUN TIME PARAMETER:

As we are interested in rotating throttle since it is a transient problem. This will be done in the following ways.

Select Boundary: Throttle
Wall Motion Type: Rotating
Surface Movement: Moving

Rotation center and Rotation about(axis) will be evaluated by:

Go to Measure `->`Direction`->`Arc Normal `->`Select any 3 vertices along arc `->`Apply

OR

Go to Measure `->`Direction`->`Triangle Normal `->`Select any Triangle `->`Apply

In message log results we will get as,
Arc center as Rotating center
Arc Normal(Cell Normal) as Rotating About (axis)
Arc Diameter as Cylinder Diameter

Put all the values in the respective fields. Till now we have given rotation axis and rotation center about which throttle will rotate.
Now we have to provide Rotation rate to the throttle.

Throttle profile info:

• 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.

Here, we have given throttle rotation information.

Output/ Post Processing:

Time interval to write 3D output files= (running simulation time)/(Required nos. of output files)

t= 0.01/10
t= 0.001 sec

Import all the files to the required folder for post-processing.

3D post-processing results:

Mesh generation:

Here we can clearly see that mesh around the throttle keeps on changing as throttle moves along the rotation axis because that it helps to give accurate results around the throttle valve.

Pressure and Velocity Contour:

Here we can see as throttle rotates to 25 degrees, velocity and pressure both reduced in later part of the Elbow channel. At the front face of the throttle, it experiences high pressure and high velocity when the throttle is at 0 degrees. There is a drastic reduction in static pressure when the valve opens to 25 degrees that\'s causes flow separation.

Streamline of velocity:

Streamline of velocity distorted due to rotational movement of the throttle valve causing variation in velocity value.

The following different characteristics can be observed from the velocity contours obtained during throttle openings –

1. The spread of the wake region decreases as the throttle opening increases.
2. The stagnation region can be observed at the upstream face of the throttle valve for all the valve positions.
3. The velocity increases at the throttle valve edges and the velocity of airflow through the clearance between the valve edge and the throttle body increases as the throttle valve opening angle increases. 

Plots for pressure, velocity, mass flow rate and total cell count:

Boundary id_3: Inlet
Boundary id_4: Outlet

All the below plots represents some physical realistic fluid flow variables changes with respect to time.

The total airflow rate through the throttle body increases with an increase in the throttle valve opening angle.

In the vorticity plot, we can say that as the throttle valve opens vortex in the fluid-flow keeps increasing till it is fully opened and remain at the same position for a particular period of time thereafter there is a drop-in swirling of fluid during throttle valve closer.

Total cell count:

Here, we can conclude that the major reason behind getting this erratic structure in the plot is the Rotation of throttle. Since we rotate the throttle valve to 25 degrees for 2ms to 4ms total cell count has a specific value when the throttle valve starts rotating fixed embedding helps us to provide required no. of grid elements to capture data throughout rotation process hence, total cell count keeps on changing.

Dynamic data:

Angular momentum and Angular velocity plots are shown below throughout the process.

Conclusion:

The following different characteristics of the velocity and pressure contours can be used to compare and analyze the flow across different throttle valve positions.
1. 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.
2. The throat region has the minimum area available for the air to flow. In order to reduce energy loss, the flow should be accelerated in the throat. Hence, the pressure drop at the throat should be minimum.
3. The region of re-circulating flow, formed downstream of the throttle shaft, is known as the wake region. The recirculation of air is caused by the viscous airflow around the throttle plate surface. The medium is displaced when it leaves the surface of throttle shaft, resulting in turbulent eddies being formed. The presence of eddies make the flow through the wake region turbulent and hence, it should be minimum.
4. There is a region of adverse pressure gradient formed due to excessive loss of momentum near the flow contacting surface. Due to the adverse pressure gradient, the flow near the surface of a body is decelerated. Thus, this pressure gradient precludes the flow from progressing downstream past a certain point, called the point of separation. At this point, the flow becomes separated from the contacting surface of the throttle plate. The position of the point of separation is also an important factor to be considered while analyzing the pressure contours
5. The stagnation point is a region formed in the airflow field where the local velocity of air approaches zero. In the throttle flow, the stagnation point is formed at the tip of the leading edge of the throttle plate where the air is brought to rest due to sudden obstruction inflow. In this region, the static pressure increases as the velocity drops.