Week 7: Shock tube simulation project
AIM:
To observe the flow of shocks generated in a shock tube and also to plot the pressure velocity and temperature plots.
PROCEDURE:
what are shock tubes?
A shock tube is a long tubular hollow duct of various shapes, used to determine chemical kinetics. Shock wave can help us determine the pressure, temperature and various quantities using this experimental rig. These are also used for compressible flow phenomenon and gas phase combustion reactions.
The model used here consists of 2 sections where one section is filled with nitrogen of 1 molar and other section has oxygen of one molar. A diaphram separates both the regions and nitrogen is kept at 6bar at 300k and oxygen is kept at 1bar 300k. When the diaphragm open the pressure diffrence of 5bar drives the nitrogen into oxygen at very fast pase which is why we see shock waves and also contaction and expansion waves. After a while the shocks die down and settle but fluid takes really long for settling.
The stl file is downloaded and model is imported into converge.
The dimensions of the model are
along x = 0.2m
along y = 0.01m
along z = 0.01m
Transform all the normals inwards and delete the interface boundaries and we can start boundary flagging.
The boundaries are as follows
1.high pressure region
boundary walls, front and back.
Low pressure region
Boundary wall, front and back.
now it is necessary to understand that we require two regions one for nitrogen and other is for oxygen. The nitrogen is inside high pressure section while the oxygen is in loow pressure region.
Now as we have finished boundary flagging we can start our case setup.
Case setup:
1. Application type
Time_based
2. Materials
Gas Simulation - default
Gas transport parameters - default
Species - O2 and N2
3. Simulation Parameters
Run parameters-
Solver - Transient
Temporial type - Time based
Gas flow solver - Compressible
Simulation time parameters
Start time - 0s
End time - 0.005s
Initial time step - 1e-9s
Minimum time step - 1e-9s
maximum time step - 1s
Solver parameters
Navier-Stokes solver type: Pressure based
4. Boundary Conditions
Boundary
All boundaries are to be set as slip walls and conditions have to be default.
specify regions as per the high pressure and low pressure sections.
5. Initial Conditions and Events
Regions and Initialization
Region 0
Pressure - 6bar
Temperature - 300k
Species - N2 mass fraction 1
Region 1
pressure - 1bar
Temperature - 300k
Species - O2 mass fraction 1
Events - sequential
start event by Region A Region B Event
0 Regions Region 0 Region 1 close
0.001 Regions Region 0 Region 1 Open
6. Physical Models
Turbulence modeling - default
7. Grid Control
Base Grid - 0.002m
Adaptive-mesh Refinement
Maximum cells - 200000
bring the available regions to the right side in AMR group
Select species box
Name N2, Timing typr permanent, S embed 0.001
8. Output/Post-Processing
Post variable selection - default
Output files
Time interval for writing 3D outpur files - 1e-5s
Time interval for writing text output - 1e-6s
Timeinterval for writing restarting output - 0.001s
Here we end up in case setup and let us move on to exporting the files and execution in CYGwin and we can convert the files into ensight form and visualtize in paraview.
RESULTS:
Mesh
Animation link: https://youtu.be/LUoOAuEuEvo
Cell_count
Since we are dealing with shocks its highly necesssary to have adaptive mesh technique to refine the mesh when we have higher density of shocks flowing which eventually will increase the cell count which can be seen above.
Pressure
This is the average pressure in the tube which suddenly drops due to shock generation and slowly over time it tries to settle down to a common pressure all over the duct.
Max pressure
Here it is clearly understandable that the high pressure region which is in 6bar pressure slowly drops due to flow of the N2 gas into the O2 region which will reduce the pressure inside the tube which is dropping gradually over time and will reach a steady state.
Min pressure
The O2 region which was initially at 1bar starts to experience high pressure after the break of diaphragm due to the inflow of high pressure N2 and will increase the minimum pressure value over taime and will reach a steady state.
Pressure flow between regions
The flow of N2 from region 0 to region 1 is the one responsible for this graph. We can observe that the as soon as diaphragm is broken N2 starts to flow into O2 region at approximately 4bar and reverbs back and again flows front this happens various times intil shocks are faded and pressure has to reach a equilibrium condition.
Pressure_animation: https://youtu.be/AbUqhVV4W08
Temperature max
The max temperature value rises very high due to the sudden shock formation and this temperature will slowly reach an equlibrium value and settle down.
Min Temperature
The minimum temperature occurs in the low pressure regions that are formed due to shocks and the value is gradually increasing because of high temperature formed which can be seen in the max temp graph. These wave formations are due to the continues too and fro motion of waves along the duct.
Temperature between the regions
All the graphs clearly show a wavy nature which will settle over time to a constant pressure and temperature. This type of graph is formed due to the variation in properties which are happening suddenly due to shocks. The pressure and temperature in the graphs for the flow from region 0 to 1 show that the shocks are at very high pase moving to and fro and these make the properties to reach an equlibrium value very fast. We can see that the properties have 50% settled in 5milli seconds which is really fast.
CONCLUSION:
Hence shock tubes are highly essential and are very useful for combustion experiments where we do not have a spark plug and the fuel ignites due to the shocks. The properties over the domain change at a high pase but also reach equilibrium very soon in no time.