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

Steady-state simulation of flow over a throttle body using CONVERGE CFD

Aim:

Simulating the Steady-state flow over a throttle body using CONVERGE CFD.

Objectives:

• Able to simulate the over a throttle body using CONVERGE CFD.
• 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.
• Create an animation in which its throttle movement should be visible.

Flow simulation in CONVERGE done in 3 steps:

1. Geometry preparation and Case setup :
   •  We use Converge GUI(Graphical User Interface) to set up the case and export input files.
2. Solving the problem using input files:
   • These input files are solved by Converge in the Terminal (Command-line interface). We USed Cygwin for the command line terminal.
3. Post-processing results:
   • We convert our solution into Paraview binary format to post-process the results in Paraview

Throttle:

A throttle is a mechanism by which fluid flow is managed by constriction or obstruction.

Engine power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal. What has often been termed a throttle (in an aviation context) is also called a thrust lever particularly for jet engine-powered aircraft. For a steam locomotive, the valve which controls the steam is known as the regulator.

Steady-state:

Steady-state means we don't account for time, so in simple terms, it's the result when time reaches infinity.

• Steady-state simulation computes the fully developed solution that does not change in time.
• When steady-state is used the mean values are computed.
• The steady-state simulation is way faster than transient.

Transient:

The transient solver solves each time step, so your result will have reached a finite time:

• Transient simulation computes the instantaneous values at each time for each quantity.
• Transient simulation has much higher requirements compared to steady-state. The CPU time, disk storage, evaluating results, ...
• The results of transient simulation may be more comprehensive.

Geometry preparation in CONVERGE CFD:

• Geometry preparation:
  • Import the ".STL" for the throttle body. 
    
    • 
    • 
  • For this kind of flow analysis, we need a closed surface that can act as a fluid domain.
  • CAD Cleanup:
    • We don't need a thick pipe wall because it is nowhere useful to participate in the simulation. So, we can delete outer surfaces which are acting as thick surfaces.
      •  
    • Deleting Outer surfaces:
      • 
      • 
      • 
      • 
    • By deleting the outer faces we end up with Open edges. we have to close the open edges to make it a closed body.
      • use Patch commend to close the Open edges.
        • 
      • Inlet Patch:
        • 
      • Outlet Patch:
        • 
• Boundary setting:
  • We have to set Inlet, Outlet, Throttle & Walls boundaries for the above-created geometry.
  • By default In CONVERGE, all the surfaces are made up of triangles we have to select respective triangle faces for the boundary.
    • 
    • 
• Normal Transformation:
  • All the face normal directions should be facing towards the flow region.
  • In this fluid is flowing through the channel but normals are facing outwards. We have to transform the normal's direction.
  • 
  • 
• Diagnosis:
  •  Checking for any interactions, open edges, etc.  
  • 

Case Setup in CONVERGE CFD:

1. Application Type :
   • 
2. Materials:
   • Set default values in Gas Simulation & Global transport parameters.
   • Species: Add Gas species O2  & N2. 
     • 
3. Initial Conditions and Events:
   • Add Volumetric region in regions
   • Add species by modifying their concentrations so that we can clearly observe the difference in post-processing.
   • 
4. Boundary conditions:
   • Defining boundary type and conditions:
     • 

   • Boundary Name	Type	Boundary Condition	Value	Species
     Inlet	INFLOW	Total Pressure	1.5e5 Pa	O2 : 0.23 & N2 : 0.77
     Outlet	OUTFLOW	Pressure	1.0e5 Pa	O2 : 0.23 & N2 : 0.77
     Throttle	WALL	No-Slip
     Wall	WALL	No-Slip

     • INLET:
       • 
     • OUTLET:
       • 
     • Wall:
       
       • 
     • Throttle
       • 
5. Simulation Parameters:
   • Run Parameters:
     • Selecting the Solver & solver type. informing additional things to solve (Energy & Momentum equations) and monitoring steady-state.
     • 
   • Steady-state monitor:
     
     • In this, we have to Add monitor variables to be monitored during the simulation at different boundaries.
     • 
   • Simulation Parameter:
     • defining initial, Minimum & Maximum Time step sizes.
     • 
   • Solver Parameters:
     • Solver Scheme to PISO and Density-based solver.
     • under Equations window set precondition to None for all the variables.
     • 
     • 
6. Physical Models:
   • usingRenoulds Average Navier stroke equation
     • 
7. Grid Control:
   •  This is the section where we define meshing size(Grid size).
   • Enable the Fixed embedding to refine the mesh at walls. 
     • 
8.  Output/Post-Processing
   • Post Variable selection: select the variables to be monitored for post-processing.
     • 
     • 
9. Exporting Input files:
   • Export the input files
     • 

Solving Channel Flow in Cygwin:

1. Go to the directory where input files are exported
2. Make mpiexec.exe & converge.exe files globally available.
3. "mpiexec.exe -n 4 converge.exe restricted logfile &"
   
4. Use above mentioned line to start calculation in the background
5. mpiexec.exe & converge.exe are executable applications to solve the problem
6. -n 4:- number of processors 
7. restricted:- my license is a restricted one.
8.   logfile:- creates temporary memory to save data with name as logfile at a particular iteration
9. & :- To run Calculation in Background 
10. "tail -10 logfile" command is used to view the last 10 lines of the logfile.
11. "mpiexec.exe -n 1 post_convert_30_serial_64" command used to convert output files to paraview binary format, so that they can be processed in the paraview.

Post Processing and Results from Paraview:-

• open "..vtm" group file in paraview. This group contains all the solution files.
• use the slice tool to create a contour section plane and plot the contours
• Stream tracer is used to trace the particles.

Plots:

• Mesh
  • 
  • 
• Velocity Contour
  • 
• Velocity Vector
  • 
• Pressure Contour
  • 
  • 
• Temperature Coutour
  • 
• Stream tracer :
  • 
  • 
  • 
• Graphs:
  • bound_id_2 = inlet
  • bound_id_3 = outlet
  • 
  • 
  • 
  • 
  • 

Result Table:

mesh size dx=dy=dz=	Total cells	Mass flow rate(Kg/s)		Velocity(m/s)		Pressure (Pa)
(m)		Inlet	Outlet	Inlet	Outlet	Inlet(Total)	Outlet(Total)
0.002	157237	0.0455913	0.0455871	120.814	170.942	150000	120864

Conclusion:

• Successfully Simulated the Steady-state flow over a throttle body using CONVERGE CFD & extracted various Plots and contours.  
• we can observe a red zone near the throttle wall it is due to the sticking of high-pressure air coming from the inlet.