Transient Flow Simulation over the Throttle Body

Transient Flow Simulation over the Throttle Body

Objective

The primary objective of this simulation is to calculate the end time of the flow and the throttle valve shall rotate about its axis as per the following given information-

From 0 to 2 milli secs, the throttle valve should rotate by 25 degrees.

From 2 to 4 milli secs, the throttle valve should stay at 25 degrees.

After 8 milli secs, the throttle valve should come back to 0.

Geometry

The following elbow geometry was imported in to Converge Studio and after checking and eliminating the error, boundary flagging was performed. We can see the Throttle Valve in the middle.

Case Set-up

The most important part in the case set up is to make the throttle rotate about its axis as per the given information. The idea is to observe how the instantaneous change affects the flow, as well as, how the flow re-adapts to the new steady state.

The important settings are as detailed below-

Solver

• Transient

Fluid

• Air (O2 + N2)

Simulation Parameters

• Start Time: 0 Seconds
• End Time: 0.01 Seconds

Boundary Conditions

• Pressure at Inlet= 150000 Pa
• Pressure at Outlet= 100000 Pa
• Throttle-
  • Wall Motion Type- Rotating
  • Surface Movement- Moving
  • Rotation rate file-

Type-Sequential

second	Angle
0.0	0.0
0.002	25.0
0.004	25.0
0.008	0.0

Initial Conditions in Region 0

• Pressure= 101325 Pa
• Temperature= 300 K

Physical Models

• Turbulence modelling- RNG k-ε

End time calculation

To calculate the end time, lets first look at the total length of the elbow. We can see from the geometry bounding box above, the approximate length of the elbow is 0.4m.

Then the average Inlet Velocity in the previous Steady State Simulation, as shown below, is calculated as 191 m/s.

Therefore, the time taken for the flow to pass through elbow once = 0.4/191= 0.00209 second or roughly 2 ms.

Therefore, we must simulate the flow 5 times of this calculated value so we can satisfy the given objective perfectly.

The end time is therefore considered as 10 ms.

Throttle Movement

In the Below Animation, as per the applied settings in the Case Set-up, we can see the throttle is moving to 25 degrees at 2 ms, stays at 25 degrees until 4 ms and comes back to it’s original position at 8 ms.

Mesh

The mesh is generated at Base grid of 0.002m and Fixed Embedding is introduced around the throttle valve with Scale = 3, and Layers =2.

Since this is a transient simulation, the mesh shall also recreate at each time step with the throttle movement. We can see that mesh is being recreated in the below animation.

Results

Velocity Profile

We can see in the below animation that as the throttle moves to 25 degrees, the velocity is highest at 330 m/s. This can be observed by the red streamlines that are created.

But when the valve moves to 25 degrees, it is also obstructing the flow. Thereby creating a chaotic, turbulent flow downstream. There is an area behind the valve, where the velocity is zero.

When the valve moves back to its original position, the flow recovers and become steady again.

Pressure Profile

When the valve is at 0 degrees, the valve does not experience a lot of pressure. But as soon as it moves to 25 degrees, it starts obstructing the incoming flow and experiences the maximum pressure of 1.5e5 Pa (equal to Inlet pressure settings in boundary conditions). This observed on the side of the valve that is facing the incoming flow.

This is because the total kinetic energy of the flowing fluid is converted into pressure energy when the flow is obstructed by the valve.

Mass Flow Rate and Average Velocity Plots

The Average Velocity and Mass Flow rates almost look identical over the span of 10ms. In the initial time steps, when the throttle valve is moving from 0 to 25 degrees, both mass flow rate and velocity steeply rises at the outlet. This peak is different from the inlet values. This is because the velocity increases in some regions due to the throttle move and it is at the highest at that point of time. That causes the flow to go out rapidly than compared to the inlet velocity.

The flow is still experiencing obstruction until 4 ms but as soon as the valve starts moving to its original position, the obstruction is removed gradually, and therefore, with no obstruction, the flow starts moving freely and we can see a gradual rise in both mass flow rate and velocity.

After 8 ms, there is little or no obstruction, therefore we see a constant value for both the plots until the end time.

Total Cell Counts

Since this is a transient simulation, we have the moving mesh as explained earlier. And as the mesh is constantly changing and adapting to the moving throttle, we are obviously observing the change in cell counts over the span of time.

We see that as the throttle moves from 0 to 2ms, there is a rise in cell counts. And as the throttle valve moves to its original position, from 4 to 8 ms, we see the decrease in the cell counts. The cell count remains constant when the throttle valve is at stationary, corresponding to the throttle movement.