Shock tube simulation
Project objective:
To set up a transient shock tube simulation, to study the changes in various properties and parameters of fluid involved in the simulation.
Software tools used :
- Simulation - Converge CFD studio (v2.3)
- Command editor - Cygwin
- Post-processing - Paraview
Geometry:
Case setup :
- General flow gas simulation
- Solver type: Transient
- Turbulence model: RNG k-ε
- Wall temperature: 300k
- Grid size: 1 mm
- Simulation run time: 0.003 seconds
Low-pressure region:
- Pressure - 101325 Pa
- Species - `O_2`
High-pressure region:
- Pressure - 600000 Pa
- Species - `N_2`
Adaptive mesh refinement:
Adaptive mesh refinement has been enabled based on species. Converge refines the mesh whenever a change is detected with respect to the given data range.
- Species selected: `N_2`
- Type: Permanent
- Maximum embedding level: 3
- Minimum cells: 1
- Maximum cells: 200000
As a physical diaphragm is not suitable for this type of simulation, using the 'Events' feature, a virtual diaphragm has been enabled which opens at 0.001 seconds after the start of simulation thereby shock waves are generated.
Simulation outputs:
Base mesh - 1mm
Mesh with Adaptive mesh refinement
Nitrogen mass fraction
From the above animation, the nitrogen mass fraction variation can be seen along the length of the shock tube. we know that flow will always drive from the high-pressure region to the low-pressure region. In this case, the nitrogen gas in the high-pressure region tries to flow towards the low-pressure region, while doing so it breaks through the diaphragm between the two regions, ultimately generating a shock wave. As there is no outlet, the wave reflects back and forth until the pressure in the tube stabilizes.
Velocity magnitude contour
Pressure contour
The pressure variation along the shock tube can be visualized from the above animation. As said above, the reflection takes place until the pressure between the low-pressure region and high-pressure region stabilizes.
Temperature vs time:
Temperature contour
Pressure vs time:
From the above plot of pressure vs time, it can be seen that the simulation has not yet converged to a steady value between the low-pressure region and high-pressure region. But it can be seen that the plots intersect at around 3.5 bar pressure which would be the pressure at which the shock tube gets stabilized and the reflection dies out.
NOTE: The simulation has been carried out only till 0.003 seconds as it is time and resource intensive.
Total cells vs time:
The total cell count vs time has been plotted above. Until 0.001 seconds(before the opening of diaphragm) the cell count is maintained at 2000 cells, after which it increases and decreases based on the adaptive mesh refinement and various other parameters involved in the simulation.