Week 4.1: Project - Steady state simulation of flow over a throttle body

AIM

To simulate the flow over a throttle body in converge

OBJECTIVES

• Set up and run the steady state simulation for flow over throttle body.
• Post process the results and show pressure and velocity contours.
• Show the mesh (i.e surface with edges)
• Show the plots for pressure, velocity, mass flow rate and total cell count.

THEORY

Butterfly valve

A butterfly valve is a valve that isolates or regulates the flow of a fluid. The closing mechanism is a disk that rotates. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they cost less than other valve designs, and are lighter weight so they need less support. The disc is positioned in the center of the pipe. A rod passes through the disc to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow, so it induces a pressure drop, even when open. A butterfly valve is from a family of valves called quarter-turn valves. In operation, the valve is fully open or closed when the disc is rotated a quarter turn. The "butterfly" is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway. When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.

Effect of flow near constriction
When flowing through a constricted area of a pipe, a fluid's velocity increases and its static pressure decreases. This principle is known as the Venturi effect. The resistance to flow causes a drop in pressure as you proceed along the tube. When a positive pressure is applied to a tube with a constriction, the pressure in the constriction is lowered compared to that in the larger tube.

Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. In inviscid fluid dynamics, an incompressible fluid's velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy (Bernoulli's principle). Thus, any gain in kinetic energy a fluid may attain by its increased velocity through a constriction is balanced by a drop in pressure.

SIMULATION PROCEDURE

1. Setting up the case in converge
2. Running the simulation in cygwin
3. Post convert using cygwin/converge
4. Post processing in paraview

The first step is case setup in converge. Open converge and then import the geometry.

Go to boundary and create boundary fences as shown below

In this geometry there is a thickness to the elbow, which is not needed. So delete the outer pipe and its thickness.

Patch the open edges.

The next step is boundary flagging. Name the boundaries.

The orientation of normals should be checked such that it should be pointing inwards.

Then run a diagnosis check to find if there are any errors.

Geometry preparation is completed. The next step is case setup.

Inorder to run a steady state simulation firstly under application type in the setup tree, a time based simulation must be selected.

In materials turn on species and in predefined mixtures select air.

Choose the default properties in the gas simulation. In global transport parameters the default parameters are retained. In species tab, add O2 and N2.

The next one in the case setup tree is the simulation parameters. Next in the run parameters tab, select steady state solver.

The steady state monitor is a mandatory option for steady state simulations in order to assess convergence of the solution. Turn on steady state monitor.

Select the variable to be monitored in the steady state monitor tab. Then select the boundary at which the variable needs to be monitored. Choose oulet because an inlet boundary condition was given.

Set the simulation time parameters

Solver parameters:

Solver scheme - PISO

Solver type - density based

Pressure preconditioner - none

The maximum convection CFL limit should be less than one.

Give the boundary conditions as given below.

In grid control turn on fixed embedding. In base grid tab, the mesh sizes is to be entered.

The next step is to provide fixed embedding. Fixed embedding is used to refine the mesh to a value that is lower than the prevalent base size. This is especially useful if we are looking to refine near wall to capture the boundary layer or the wake of a body more accurately. Here fixed embedding is done in throttle boundary with an embedding scale of three.

Set the frequency of generation of output files. Once the case setup is completed, export the input files into a required directory. Then launch cygwin and navigate to the directory through CYGWIN in which all input files has been exported and then enter the command to run the simulation ' mpiexec.exe -n 1 converge.exe restricted'. After that post convert using converge and post process the results in paraview.

Results

Mesh

Mass flow rate

Average velocity

Total pressure

Total cell count

Velocity contour

The velocity contour is shown in the above figure. Near the walls of the pipe the velocity is zero, this is due to the no-slip condition provided in the boundary conditions.

Velocity vector plot

Pressure contour

The pressure contour is given in the above figure. As the fluid reaches near the throttle body the pressure is very high, due to the obstruction in the flow passage. Due to the presence of throttle body, the area of flow decreases which results in increase in velocity and decrease in static pressure.

Conclusion

• Steady state simulation of flow over a throttle body was done in converge.
• When flowing through a constricted area of a pipe, a fluid's velocity increases and its static pressure decreases. This is called as the venturi effect.