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

Steady state simulation of flow over a throttle body

Aim:-

• Set the steady-state simulation for flow over throttle body case setup in the converge.
• Export all the input files and run the simulation using Cygwin.
• Plot velocity and pressure contours plot for the flow inside the elbow pipe.
• Show mesh (i.e surface with edges) by creating a mid surface/slice along the z-direction.
• Converge plot for velocity, pressure, mass flow rate, and total cell count.
• Explain the plots and write a detailed report on the project.
• Create animation in Paraview representing flow over throttle at the steady initial stage.

Objective:-

• Import the throttle geometry model in the converge using the create option.
• Perform Geometry cleanup.
• Define the boundary to each surface of the geometry.
• Set normal direction on the inward side of the surfaces. means, define the direction towards the volume where the fluid is present.
• In case of setup define the material, simulation parameters, fluid region, boundary condition, no of cycles, grid definition.
• Extract the setup files in the simulation folder.
• Run exported input setup files in Cygwin.
• Post-process all the results into 3D outputs and open in Paraview.
• Show mesh generation on the 2D mid surface by using the slice command from the Paraview.
• Plot velocity and pressure contour over the sliced surface.
• present pictures of the fluid flow over the throttle valve using streamlines and vectors. 
• Plot converge plot for velocity, pressure, mass flow rate, and total cell count.
• find out the location of separation and the reason for the separation of flow.
• Create animation in the Paraview to show the flow over the throttle valve.
• Explain the plots and write a detailed report on the project.

 Throttle Valve :-

• A throttle valve is a valve designed to regulate the supply of a fluid (as steam or gas and air) to an engine and operated by a handwheel, a lever, or automatically by a governor.
• Especially it is the valve in an internal-combustion engine incorporated in or just outside the carburetor and controlling the volume of vaporized fuel charge delivered to the cylinders

Steady State Simulation :-

These are the simulation we run if are just interested in the end result and not in how those results are getting formed with time.

If we want to see how results are changing w.r.t time and how the solution is getting formed over time then we need to run the TRANSIENT STATE SIMULATION.

The time is in Cycles for STEADY STATE SIMULATION whereas time is in Seconds for TRANSIENT STATE SIMULATION.

1.Geometry Creation

First of all, the Elbow body along with throttle step geometry was setup using converge CFD software. The geometry was separately prepared using different CAD package ie. Solidworks, then .stl file was imported to Converge Studio to set up the case study.

                                              Image 1: Elbow body along with Throttle

*Each face of the step was split into multiple triangles.

The Geometry can be downloaded from this link: Elbow Geometry

To create the geometry, the geometry was uploaded from the different CAD software package. Then the geometry clean up was done along with inlet and outlet catch creation. All the normal vector were pointing out for this geometry, so we wanted it to point inside and normal to each surface. Normals point the direction of the fluid region generally. The command "Transform" was used to make the normal inside pointing the direction of the fluid region.

The Geometry was a check for if the geometry has any open surface.

Then the normal ere checked and from the diagnostic dock, everything was checked.

2.Boundary Creation

After creating the elbow Geometry, The Boundry tag was created along with different boundary id and coloring. A total of 4 different boundaries tag was created for so many different split triangles of the elbow with each face split into multiple triangles. To create the boundaries naming, the "Boundary" command in the "Geometry" dock was used, and then the faces were appointed to appropriate tags along with id.

                                                      Image 2: Boundary creation representing the different id and color.

The five different boundaries were Inlet, Outlet, Elbow Wall, and Throttle.

3.Case-Study Setup

The various parameter was considered while setup this case study. The case study setup dock was used to set up the problem which appears on the right-hand side of the converge studio.

Image 3: Case Setup Dock.

3.1 Application Type:

In order to run a steady-state simulation, firstly, under the Application type in the setup tree, a "Time-based" simulation must be selected. 

3.2 Materials:

Under the material tab "Gas simulation" was selected and "Air" as a predefined mixture.

Gas: Air

Global Transport Parameters: Fixed value

by default, the following value is considered.

     Prandtl Number  : 0.9

     Schmidt Number : 0.78

Species: Oxygen-O2 & Nitrogen-N2

Reaction Mechanism (mech.dat) was selected for this problem.

3.3 Simulation Parameters

The Steady-state solver was selected for running this case study along with a "Pressure based" solver.

Steady State Simulation :-

These are the simulation we run if are just interested in the end result and not in how those results are getting formed with time. The time is in CYCLES for STEADY STATE SIMULATION.

The Following Simulation parameters were selected for running each simulation.

3.4: Boundary Conditions

In this tab, The volumetric region and physics for each boundary were defined.

3.5: Initial Conditions and Events

 Volumetric Region was created in order to assign this to all the boundary tag.

Species:- O2 (0.23) & N2 (0.77)

Pressure :101325Pa

Temperature:300K

3.6: Physical Models

A turbulent model was selected out of so many models.

3.7: Grid Control

The case study was run using fine different mesh grid size. So different case study was set up in order to study the effect of value when the mesh is more refined.

3 different base mesh sizes are,

Fixed embedding was done in order to capture more cells near to throttle in order to get more precise data.

Scale : 3 & Embed Layers: 2

3.7 Output/Post-Processing

Post-Variable Selection-  Existing value

Output Files- Time interval for writing 3D output files-100.0

                    The time interval for writing restart files -100.0

The case study was solved using CYGWIN and the post-processing was done in ParaView.

For post converting we used the following command

Meshed Geometry

Case Study: dx=dy=dz=2e-3 m With Boundary Embeding

Image 3: Meshed Geometry with Fixed Embedding on Throttle.

Result

Line Plots & Contours

Velocity

The velocity contour of a section cut along the plane normal to he z-axis is as shown below. The maximum velocity of 240m/s is predicted near the gapping between the throttle and elbow wall which is understandable due to the reduced flow area at that section and also lowest velocity is noticed behind the valve where is flow is obstructed and due to low pressure zone. Duw to high pressure flow near to throttle and elbow the velocity is too high. The hitting surface of throttle has thin boundary layer.

Pressure Contour:

The Pressure contour of a section cut along the plane normal to the z-axis is as shown below. The maximum pressure of 1.4 Bar is predicted near the gapping between the throttle and elbow wall which is understandable due to the reduced flow area at that section and also the pressure of .5 bar is noticed behind the valve where is flow is obstructed.

Mass Flow Rate

Total Cells

Animation Video :-

Conclusion:-

• Geometry cleanup is important to set the required fluid domain for the simulation.
• Fixed embedding is a great tool to refine mesh near the required surfaces of the parts. 
• Pressure drops because of friction near the turning section of the elbow pipe and the lowest pressure is observed near the end side of the Valve surface where eddies form.
• As the pressure drops because of the throttle valve acting as an obstacle velocity near the outlet area starts to increase which obeys Bernoulli's principle which is the principle of conservation of energy.