Transient simulation of flow over a throttle body

Transient simulation of flow over a throttle body

The present project consists of a transient simulation of a throttle body using converge CFD software. The throttle body consists of a butterfly valve to let the fluid inside the throttle. The geometry of the throttle body and the valve are shown in figure 1.

Fig 1: throttle body and valve

The model imported from the design software is cleaned for simulation. The simulation is run from 0 to 1 ms with the initial time-step of 1e-09s. The main motive of this flow is the rotation of the valve in the throttle body at a different time.

Boundary conditions:

Inlet: Inflow pressure boundary condition: 0.15 Mpa

Outlet: Outflow pressure boundary condition: 0.1 Mpa

Elbow body: Wall

Throttle: Rotated at a certain angle at a different time. The rotation rate of the throttle value is shown below.

Time(milliseconds) Angle

0.0 0

2.0 25

4.0 25

8.0 0

The above table shows the that the valve rotates to 25 deg at 2 ms and stays there till 4ms. Then the valve turns back to the original position(0 deg) when it reaches 8 ms. From this transient flow, the velocity and the mass flow rate variation at both the inlet and outlet are observed.

The rotation of the valve is assigned through the centre of rotation of the valve. The values are as follows:

Rotation axis x y z

Rotation centre -0.0761534 0.03907741 0.0201742

Rotation about 0.7943662 0.6074391 0.0003278213

Meshing:

Meshing technique used by Converge studio is called as \"Cut cell cartesian method\" method. This makes sure that the geometry details are not loosened even when the mesh is coarsened, which represents that even the base grid is coarse, the mesh can be refined automatically in the regions where its needed to capture the flow. In the software, this can be done using a technique called Fixed embedding. In the present problem, we need to make sure that there are more cells present near the throttle valve. This gives accurate results.

Fixed embedding:

Embedding refers to local mesh refinement. In our problem, the embedding is done to the throttle valve with the following conditions.

Embedded layers: 2

Mode: Permanent

Scale: 3 is given for fine mesh

Scaling refers to the multiplication factor the mesh. In our case, the base grid for the entire domain is 2e-3 in all x, y and z directions. For the throttle valve, after scaling, the mesh on valve boundary scales are 6e-3 for the fine mesh.

The mesh changes for the throttle valve due to the change in a rotation for different time levels are shown in below pictures:

Fig 2: Mesh at time t = 0s.

Fig 3: Mesh at time t = 1ms ( Rotation = 12.5deg)

Fig 4: Mesh at time t = 5ms ( Rotation = 25 deg)

The total cells for the body keeps changing unlike the steady case. In the steady case, since the valve was steady, the mesh did not change and stayed stationery. Here, since the valve is rotating, the mesh changes as the time changes since mesh adapts dynamically to the position of the valve.

Result:

The simulation is run parallelly. The converge uses MPI programming to parallelize the solver, which means that the solver is equally distributed to given no of processors in the working computer. In the present problem, the solver is run with 4 processors which equally distributing the cell structure to each of the processors since the flow is steady.

The flow velocity variation with the variation of pressure on the valve for different time levels is shown from figures 5 to 8.

Fig 5: Flow velocity with valve pressure variation at t = 1ms.

Fig 6: Flow velocity and valve pressure variation at t = 2ms.

Fig 7: Flow velocity and valve pressure variation at t = 4ms.

Fig 8: Flow velocity and valve pressure variation at t = 8ms.

It is observed that as the valve rotates, the pressure on the valve surface decreases since it is closing and creates a barrier for the flow. For 2ms and 4ms, the pressure variation is very less since it stays at the same location.

The variation of mass flow rate for the inlet and the outlet is shown in figure 9.

Fig 9: Variation of mass flow rate with respect to time

It can be observed that the mass flow rate satisfies the mass conservation. The streamline flows for different time levels are shown below:

Fig 10: Streamline flow of velocity for t = 2ms.

Fig 11: Streamline velocity flow for t = 6ms.

Fig 12: Streamline velocity flow for t = 8ms.