Week 4.1: Project Steady state simulation of flow over a throttle body
Aim:
To simulate the flow over the throttle inside the elbow and to study pressure, velocity contours and plots for pressure, velocity, mass flow rate and total cell count.
Throttle:
A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases by the use of a throttle. The throttle refers to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal. In the current simulation the throttle is kept constant at the initial position and the steady state simulation is carried out to stydu the variation of fluid flow quantities due to throttle.
Simulation Process:
Import the file of geometry of the elbow in STL extension into Converge.
Delete the thickness of the geometry and patch the thus formed open edges in the geometry. Convert the scale to millimeters.
Run diagnosis and if there are any normals orientating outside the geometry, then go to transform, noraml and select any triangle and hit apply. All normals should be pointing inwards.
Then go to boundary > Flag and select create multiple boundaries.
Geometry clean up and Boundary Flagging:
Above figure shows the geometry of the elbow with the throttle inside it. Geometry is flagged into inlet, outlet, elbow wall and the throttle. Throttle is used to maintain the rate of the flow inside the elbow and can be seen below
Dimensions of Geometry:
Lx = 0.106m
Ly = 0.119m
Lz = 0.017m
Visualisation of Throttle:
The centre part is the throttle along with inlet and outlet above and below it respectively.
Case Setup:
1. Applications type: Time - Based
2. Materials:
Select default values for Gas Simulation and Global Transport Properties.
Species : Add N2 and O2 in Gas
3.Initial Condition and Events:
Add a region as Volumetric_Region in regions and Initialisation and intialise species value with below mass fraction
O2 - 0.23
N2 - 0.77 whose combination makes the Air as the working fluid for simulation.
4. Boundary Conditions:
Inlet
Boundary type - INFLOW.
Pressure - 150000 Pa
Temperature - 300 K
Species - N2 = 0.77 and O2 = 0.23
Outlet
Boundary type - OUTFLOW.
Pressure - 100000 Pa
Temperature - 300 K
Species - N2 = 0.77 and O2 = 0.23
This pressure difference between inlet and Outlet drives the flow from inlet and outlet .Also mention O2 and N2 species dor Backflow ( if it occurs due to adverse pressure)
Throttle
Since throttle is not moved/rotated in this case
Boundary type - WALL
Wall motion : Stationary
Apply Law of Wall for Velocity and Temperature Boundary Conditions.
Elbow_Wall
Boundary type - WALL
Wall motion : Stationary
Apply Law of Wall for Velocity and Temperature Boundary Conditions.
5. Simulation Parameters:
Run parameters:
Solver : Steady- State solver
Simulation mode: Full hydrodynamic
Turn off use shared memory and turn on steady state monitor Misc tab.
Steady State Monitor:
Add required parametres that need to be monitored so that if they converge within the given tolerance value the simulation would stop before reaching the given number of cycles to complete(End time)
Simulation Time Parameters:
Start time - 0
End time - 15000 cycles
Intial time step : 1e-9 s
Minimum time step:1e-9 s
Maximum Time step: 1 s
Maximum convection CFL limit: 1
Leave the rest as Default values.
Solver Parameters:
NS solver Scheme - PISO
NS Solver Type - Pressure Based Steady
Set the pre conditioner to none in Pressure in the Equation tab.
Set the maximum convection CFL limit to 0.5 in Steady state Solver Control tab and rest are left Default.
6. Physical Models:
Turbulence model : RNG k -`epsilon` in RANS
7. Grid Control :
Base grid : dx=dy=dx = 0.002 m
Add fixed Embedding in Grid control
Fixed Embedding:
At the throttle major changes in the flow variables occur.. In Order to capture the results accurately at the throttle, fine mesh will be need with more cells. So mesh size is reduces at the throttle with tthe help of fixed embedding. Scale of 3 makes the reduces the base mesh size by 8(2^3) times and embedded lalayers 2 means the mesh size is further reduced by 8 times at the throttle boundary.
8. Output/Post-processing
In post variable selection, under SPecies tab, add N2 and O2 in mass fraction box.
In output files, under Writing time intervals tab, specify 100 cycles as time interval for writing 3D output data files and restarting files.
Export All these input files to a directory. Open CYGWIN, navigate to the directory where all these folders are saved and run the following command ;
mpiexec.exe -n 4 converge.exe restricted </dev/null> logfile_006 &
which runs the simulation with 4 processors using converge executable and outputs are dumped into a file called logfile_006 with job being ran in backend.
Outputs/Post -Processing:
Post processing is performed in the paraview software using the group files with ..vtm extension created in the post processor of Converge
Geometry:
The pink colored element is the throttle places inside geometry. Elbow, inlet, outlet can also be seen
Mesh:
Finer Mesh at the Throttle
The finer mesh near throttle using the fixed embedding in order to obtain better results. The mesh on tge complete geometry can be seen.
Pressure Contour:
Since the flow will be stopped in front of the throttle, the kinetic energy converts into static pressure hence the increse in the static pressure can be seen. In the wake region where the flow rate will be less the static pressure decreases. above the throttle, where there is reduction in the area, the velocity increases reducing the pressure. The clear picture of the pressure distribution near throtlle can be seen in below pictures.
Pressure distribution near Throttle:
Pressure on the Throttle:
Above the throttle, since the velocity will be increased due to reduction in area, pressure can be seen reducing. Also pressure at outlet is lesser than the same at the inlet.
Velocity Contour:
The velocity distribution at the outlet is higher than inlet. This is due to the reduction in the area near the throttle. The constriction of the area at the throttle makes the fluid drive at the faster velocity above the below the throttle. So the maximum velocity is produced near throttle. The velocity increase will also results in the increase of dynamic pressure and thus makes the compensation for reduction in static pressure.
Velocity distribution near Throttle:
Velocity distribution near the throttle can be seen clearly in the above figure. Since there ist termination of flow due to the throttle, velocity is much lesser in that region. Also in wake region, where fluid flow is less, velocity is reduced. Velocity at the outlet is higher than inlet due to the reduction in the area for flow at throttle which increases the flow velocity.
Orientation of flow / Visualisation of Velocity vector:
The fashion in which the fluid flows inside the elbow can be with the help of vectors which are oriented in the direction of fluid flow. There are more vectors near the throttle indicating the increased flow rate and velocity.
Velocity vectors near Throttle:
Some of the vectors can be seen directing above the throttle.
Outputs from the Plotting module of Converge:
Variation of flow quantities at Inlet and Outlet of geometry:
Mass Flow Rate :
Total Pressure:
Increase in the total pressure at the outlet is due to the increased velocity of flow after the throttle which increases the dynamic pressure thereby total pressure. Velocity at inlet is the given initial condition of 150000 Pa.
Average Velocity:
Average velocity at inlet is around 190m/s and 220 m/s at outlet
Total Number of Cells:
The simulation converges at around 6450 cycles and the number of cells is around 35500 inclusive of the given fixed embedding which resulted in more number of cells.
Streamline Flow inside the Elbow geometry:
Streamlines near the Throttle:
The steamlines indicates the velocity flow in the elbow. At throttle, streamlines clearly indicates the flow velocity on its 4 sides. Velocity is maximum in the consticted region due to reduced area. Velocity increases past the throttle.
Animation of Pressure:
Animation of Velocity:
Animations are created in the Paraview software with 4 frames per second. These animations shows the distribution of pressure and velocity inside elbow over the cycles and steady state results after solution converges.
Conclusion:
Studied the effect on flow variables due to the presence of the throttle inside the body.
Increase of static pressure at throttle due to flow termination along with reduction in velocity is analysed.
In the wake region, the flow rate reduces and it results in reduction of both pressure and velocity of flow.
In the constricted region near throttle, velocity reaches its maximum due to reduced area and it results in dynamic pressure too. Velocity increases past the throttle and maximum velocity is around 300m/s.
Better results are captured with more of number of cells near throttle which are created using fixed embedding. More number of cells, finer the mesh, higher the simulation time.