Simulation of Flow Over a Throttle Body Converge Studio - Transient

The objective of this part of the project, is to simulate the transient flow through an elbow pipe with a throttle valve in the middle. This valve will be rotating from zero to 25 degrees, and then stay steady. 

Case Setup.

In the steady state part of the project, the geometry was already imported from a CAD model and the boundaries were flagged. In this part, the first thing to do is to create the rotation motion desired. For this, we make the throttle to rotate about its axis and center, from 0 to 25 degrees at 2ms, and then make it stop. The idea for this is to see, not only how the change affects instantaneously to the flow, but also how the flow re-adapts to the new steady state. The main case parameters are summarized as follows:

  • Inlet Pressure = 1.5 bar
  • Inlet Temperature = 300 K
  • Outlet Pressure = 1 bar
  • Outlet Temperature = 300 K
  • End time = 0.01 s
  • Turbulence Model = RNG k-ε">ε
  • Min. Time Step = 1e-9
  • Max Time Step = 1.0
  • Base Grid Size = 0.002 m
  • Fixed Embedding Scale = 3
  • Fixed Embedding Embedded Layers = 2

Geometry

Mesh

Mesh Detail (Initial)

Mesh Detail (Moving Mesh)

The geometry and the mesh grid size are the same than in the previous part of the project. However, note that being a transient simulation, the mesh needs to be re-created at each time step. The most important part for this, is around the throttle. In the previous picture, note how the mesh gets recreated when the throttle rotates.

Postprocessing and Results.

Again, the graphs interface of Converge was used to follow the progress of the simulation. Note that in this case, the plot only shows the results once they converge at each time step. This is, now what we see is not how the numerical calculations are being made (the error, the convergence etc) but how the flow variables are changing with time.

Simulation Progress:

As expected, there is a change in the mass flow rate at both inlet and outlet during the part where the valve is rotating. At 2 ms, the valves stops moving and then the flow start to become steady. Note however that the process is not immediate, as the flow needs to readapt to the new state.

Density Contour

Pressure Contour

The previous two animations show the sudden rise of the pressure and density when the valve blocks the fluid. It is clear that this produces at the same time an important gradient between the flow upstream and downstream. At the beggining, the gradient is higher, because the flow gets accumulated upstream, but once the valve stops rotating, the distribution gets a little bit smoother, reaching a new steady state. This is going to have an important impact in the velocity field, as it is shown below.

Velocity Contour

The valves suddenly blocks the fluid when it starts rotating. The result is a sudden drop in the flow after the valve. Actually, it produces a deattachment of the flow right after the valve. We can see how these blue areas evolve when the valve stops moving, and that at the end a new steady state is reached (we can appreciate minor changes due to the time of the simulation, but a clear patron is shown).

Stream Lines Tracer

This previous animation might be the most clear one. It shows what happens to the flow and the velocity field (it is coloured by velocity) once the valve starts rotating. The main impact it has, is to create a turbulent, caotic flow downstream. While the angle of rotation is still small, the main consequence is just a deviation of the flow and a change in the velocity field. However, when it continues rotating to higher angles, then the flow is not able to re-attach at the throttle downstream, creating areas where there is no flow at all, and a turbulent flow afterwards. The end of the video shows how, after a while, the flow readapts to the new state, and althought the flow is still turbulent, it becomes steady.


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The End