Steady-state simulation of flow over a throttle body

Aim:

This particular project aims at obtaining ta steady-state simulation of flow in an elbow-shaped channel through throttle body that is fitted in it.

Governing Equations:

The simulation is run on the basis of the Navier Stokes equations. The NS equation consists of three momenta, one continuity, and one energy equation. The energy equation is solved only during the time where there is a heat transfer taking place. This is done so to reduce unnecessary computational time.

Objective:

The objective is to obtain a comparison between the flow of a fluid over throttle body when it is steady (not moving) and when the throttle body is dynamic. This project deals only with the former half of the comparison. The latter half i.e the dynamic simulation will be carried out in the next project. The results obtained are later evaluated and conclusions will be drawn accordingly.

The workflow involves:

1. Creating a geometry of the backward flowing step model using the create function in the converge studio
2. Flagging the boundaries starting from the flow into the model and the fluid exiting from it.
3. It is important to make sure that the throttle body is selected as a whole and submitted as a particular boundary.
4. Creating the required case setup and setting up parameters according to the requirements.
5. Transporting the input files and running the simulation
6. Convert the obtained 3D files into a corresponding viewable form.
7. Obtaining the contours and animation and drawing results from them using a 3D viewing platform such as ParaView.

The case setup:

The simulation was run with a base mesh size of 2mm. The boundary was first flagged into 4 different regions. The five different regions being the inlet, outlet, throttle body, and elbow wall as seen in Figure1. The regions have been indicated based on the colors. Once the boundary flagging is finished, the case setup is started, starting with time-based as the application type. Air is selected as a gas mixture. Global transport parameters were selected to be the default with the Prandtl number being 0.9 and Schmidt number being 0.78.

The solver was selected to be a steady-state solver for now since we will be needing a steady-state output. The simulation was run for 15000 cycles with an initial time step 1e-9seconds. The solution was obtained using the NS equation. The solver scheme was selected to be PISO. PISO stands for Pressure Implicit with Splitting of operator. All the boundaries were dropped under a single region. Turbulence modeling was run according to the RNG k-ε equation. Coming to setting the grid control, the base grid was set to 2mm. Fixed embedding was enabled to capture the very minute area that is present between the channel and the throttle body. For this particular project, the setup is as seen in Figure2 below. The output tab was selected to be the same all default. Once the setup was completed, the files were exported to a particularly saved output folder.

Figure1. Screenshot involving boundary flagging window during a backward-facing step simulation

Figure2. Screenshot involving fixed embedding window during a backward-facing step simulation

Results and Figures:

The base grid size was selected to be 2mm with a fixed embedding enabled having a scale of 1 with 2 layers. This helped the simulation to capture the small area present around the throttle body and the channel. Since this resulted in a large number of cells as seen in Figure3, the simulation ran for a longer time before converging. The closely packed cell grid is clearly visible in the sliced section view of Figure4 with smaller cell blocks present around the throttle body.

Figure3. The plot of number of cells with a mesh size of 1mm in all directions during a backward-facing step simulation

Figure4. The plot showing surface with edges during an elbow flow simulation.

The velocity contour at end of the simulation was as obtained as Figure5. The flow shows that the velocity reaches a peak of 200 m/s near the outlet starting from around 60m/s at the inlet. The flow before the throttle body forms a pattern such that the fluid is concentrated at the periphery of the tube. This can be attributed to the throttle body present at the center which is blocking the flow of the fluid. As a result, the fluid at the periphery experiences a velocity of 120 m/s. The fluid coming in direct contact with the throttle body slows down considerably and flows around the throttle body. This is where the separation of fluid takes place. As a result, the velocity immediately after the throttle body is lower than the previous value. The fluid once after combining at the very end has the highest velocity of around 200m/s. The inlet average velocity wobbles and converges to around 100 m/s at 200 cycles and the outlet velocity increases to 150 m/s at around 200 cycles as seen in Figure6.

Figure5. velocity contour plot during an elbow flow simulation.

Figure6. The plot of average velocity (m/s) of inlet and outlet during an elbow flow simulation.

The pressure contour at end of the simulation was as obtained as Figure7. The inlet pressure was set to 1.5 bar and the outlet was set to 1bar. This difference in pressure was the driving force for the fluid. This 1.5 bar of high pressure is visible in the contour plot at the inlet which gradually reduces as the flow progresses and reaches 1.2 bar before coming in contact with the throttle body. The pressure after the throttle body is at 1.2bar. The pressure is still considerably high because of the low velocity in the region and also because of the separation of the fluid that takes place. The pressure value is at a minimum at the very end of the tube at around 1 bar and this is attributed to the high velocity experienced by the fluid at the exit. Figure8 shows that static inlet pressure reduces to 1.4bar and the outlet total pressure converges at 1.1bar at around 200 cycles from the start of the simulation.

Figure5. pressure contour plot during an elbow flow simulation.

Figure6. The plot of pressure (Pa) of inlet and outlet during an elbow flow simulation.

The mass flow rate plot as seen in Figure9 has an inlet rate of 0.004kg/s before it converges and the outlet flow increases by the same amount at around 200 cycles from the start of the simulation.

Figure11. The plot of mass flow rate (kg/s) of inlet and outlet during an elbow flow simulation

Conclusion:

The elbow flow with a throttle body shows the flow of the fluid is physically sound meaning that the behavior of the flow was as expected. The base mesh was set to be 2mm along with the use of fixed embedding. As a result, the simulation time ran for a longer time. This project serves as a comparison with the next project i.e flow with a dynamic throttle body. The link for the youtube video would be https://youtu.be/o56glRJ3ox8.