Week 4.2: Project - Transient simulation of flow over a throttle body

ransient simulation of flow over a throttle body using CONVERGE CFD

Aim:

Simulating the transient 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.
    
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  • 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.
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    • Deleting Outer surfaces:
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    • 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.
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      • Inlet Patch:
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      • Outlet Patch:
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• 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.
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• 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.
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• Diagnosis:
  • Checking for any interactions, open edges, etc.
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Case Setup in CONVERGE CFD:

1. Application Type :
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2. Materials:
   • Set default values in Gas Simulation & Global transport parameters.
   • Species: Add Gas species O2 & N2.
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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.
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4. Boundary conditions:
   • Defining boundary type and conditions:
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   • 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:
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     • OUTLET:
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     • Wall:
       
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     • Throttle:
       • In this problem throttle is a rotating body
         • First of all, we have to find the center of rotation and axis of rotation.
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         • Co-ordinates
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5. Simulation Parameters:
   • Run Parameters:
     • Selecting the Solver & solver type. informing additional things to solve (Energy & Momentum equations).
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   • Simulation Parameter:
     • defining star, end time, Minimum & Maximum Timestep sizes.
     • Simulation Time calculation.
       • From steady state analysis we got inlet velocity as 120 m/s
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       • length of the domain L is < L_x + L_y
         • L < 0.106314 + 0.119998 = 0.226312
         • Time = Distance / Velocity
           • T = 0.226312 / 120
           • T = 0.0018 sec
           • So simulation time should be greater than 0.0018 sec
           • For this analysis we taken simulation time as 0.01 seconds.
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   • Solver Parameters:
     • Solver Scheme to PISO and Density-based solver.
     • under Equations window set precondition to None for all the variables.
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6. Physical Models:
   • usingRenoulds Average Navier stroke equation
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7. Grid Control:
   • This is the section where we define meshing size(Grid size).
   • Enable the Fixed embedding to refine the mesh at walls.
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8. Output/Post-Processing
   • Post Variable selection: select the variables to be monitored for post-processing.
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9. Exporting Input files:
   • Export the input files
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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 </dev/null> 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. </dev/null> 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
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• Velocity Contour
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• Pressure Contour
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• Temperature Coutour
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• Stream tracer :
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• Graphs:
  • bound_id_2 = inlet
  • bound_id_3 = outlet
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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(Static)	Outlet(Total)
0.0015	173493	0.03635	0.03633	94.024	137.57	150000	112988

Conclusion:

• Successfully Simulated the transient flow over a throttle body using CONVERGE CFD & extracted various Plots and contours.
• By this transient analysis, we can observe the valve motion during the simulation.
• As the vale is fully opened at the zero timesteps and closing slowly and stoped at 25 degrees at 0.002 seconds and maintained same position through the simulation.
• s the valve is getting closed it is acting as an obstacle i.e, we can observe more pressure red zone at inlet side.