Week 4.1: Project Steady state simulation of flow over a throttle body
Objective
To perform the steady state simulation of flow over a throttle body and analyse the results.
Solution
The elbow stl file is imported into converge, and the geometry is scaled in order to convert units from mm to m.
The geometry consists of a thickness, because of an outer and an inner surface. The outer surfaces of the geometry are removed using the boundary fence option
The final geometry ready for boundary flagging is shown below
The two ends of the elbow are sealed using the Patch option, and the boundaries are flagged.
After using the diagnosis tool to ensure correctness of geometry, the case can be setup for simulation.
Case Setup
A time based simulation is selected with the material as Air.
The values for the global transport parameters and reaction mechanism are kept default.
Solver setup
A steady state, full hydrodynamic compressible solver is selected as shown below. The steady state monitor is turned on, and the use of shared memory is unchecked.
The steady state monitor can be defined as shown below
Simulation time parameters
An end time of 15000 cycles is selected with minimum time step of 1e-9 s and max time step of 1.0 s. The convection CFL limit it set as 1.0. The values of the other parameters are maintained default.
A Navier Stokes pressure based solver is selected, and under steady state solver control, the maximum CFL limit is given as 1.0 to reflect the value given under the simulation time parameters.
Boundary conditions and initialization
The sides of the geometry are flagged with the appropriate boundary, and assigned relevant boundary conditions.
Boundary flagging
The inlet, outlet and the elbow pipe surface are assigned separate boundaries.
The throttle body is assigned to a boundary using the selection by surface option.
Wall
The walls are defined with law of wall and zero normal gradient for the TKE.
Inlet
The pressure at the inlet is defined as 1.5 e^5 bar, with INFLOW boundary type and air as medium.
Outlet
The pressure at the outlet is defined as 1e^5 bar, with OUTFLOW boundary type and air as medium.
Throttle
The throttle is also defined as a wall boundary condition with law of wall.
Physical models
The RANS K-Epsilon model is selected as the turbulence model, with the values for the constants as shown below.
Grid size
The base grid is defined with the required size along all three axes. The size here is 2e-3.
Fixed embedding at the throttle body are added for further mesh refinement.
The output and post processing parameters are kept default, and the setup is completed for simulation.
The simulation is carried out using Cygwin, and the results are illustrated below.
Results
Grid size: dx - 2.0 e-3,dy - 2.0 e-3, dz - 2.0 e-3
Mass flow rate
The mass flow rate at the inlet and the outlet are conserved, at around 0.065 kg/s, when the flow attains steady state.
Velocity
The velocity at the inlet is around -200 m/s. The peak velocity at the outlet is roughly 250m/s. The velocity increases because the flow is accelerated due to the throttle effect.
Pressure
The pressure of 1.5 bar at the inlet remains constant. The pressure at the outlet increases, until it reaches a steady state value of around 1.35 bar.
Number of cells
With a mesh size of 2e-3 in all axes, and the refinement due to fixed embedding, the number of cells are roughly 35000.
Results
Paraview
Mesh
Velocity contour
The velocity contour shows that the flow is accelerated in the small gap between the throttle body and the walls of the tube, since the area is reduced. The velocity is maximum in this region (red). There are stagnation regions ahead of and behind the throttle body, where the flow separation occurs. The velocity here is minimum (blue). This is also highlighted in the glyph plot below.
Glyph plot - Velocity
Pressure contour
The pressure is lowest, at the gap between the throttle body and the walls (blue). This corresponds to the region where the velocity is maximum. The pressure is highest on the inlet face of the throttle body, where the flow separates and there is a stagnation point (red).
Throttle body
As seen above, the pressure is highest on the inlet face of the throttle body, because it is a region of stagnation.
The velocity contour over the throttle body shows maximum velocity at the gap between the throttle valve and the elbow, due to accelerated flow, when the air is compressed into a region with very small cross sectional area, its velocity increases.
Animation pressure
Animation velocity
Conclusion
Thus the steady state flow over a throttle body has been simulated and the results have been illustrated.