Shock tube simulation project

Shock Tube Simulation:

Objectives:

• Setup a transient shock tube simulation
• Plot the pressure and temperature history in the entire domain - Explain the result
• Plot the cell count as a function of time - Explain the result

It is expected to show the mesh generation using ParaView.

Also, create animation and upload it on Youtube and provide the Youtube link for an animation in your report.

Theory:

The shock tube is an instrument used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale. Shock tubes (and related impulse facilities such as shock tunnels, expansion tubes, and expansion tunnels) can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities. Shock tubes are also used to investigate compressible flow phenomena and gas phase combustion reactions. More recently, shock tubes have been used in biomedical research to study how biological specimens are affected by blast waves.

A shock wave inside a shock tube may be generated by a small explosion (blast-driven) or by the buildup of high pressures which cause diaphragm(s) to burst and a shock wave to propagate down the shock tube (compressed-gas driven).

Generally, there are two types of Shock tubes:

Both compression-driven and blast-driven shock tubes are currently used for scientific as well as military applications. Compressed-gas driven shock tubes are more easily obtained and maintained in laboratory conditions; however, the shape of the pressure wave is different from a blast wave in some important respects and may not be suitable for some applications. Blast-driven shock tubes generate pressure waves that are more realistic to free-field blast waves. However, they require facilities and expert personnel for handling high explosives.

Working:

A simple shock tube is a tube, rectangular or circular in cross-section, usually constructed of metal, in which a gas at low pressure and a gas at high pressure are separated using some form of a diaphragm. The diaphragm suddenly bursts open under predetermined conditions to produce a wave propagating through the low pressure section. The shock that eventually forms increases the temperature and pressure of the test gas and induces a flow in the direction of the shock wave. Observations can be made in the flow behind the incident front or take advantage of the longer testing times and vastly enhanced pressures and temperatures behind the reflected wave.

The low-pressure gas, referred to as the driven gas, is subjected to the shock wave. The high-pressure gas is known as the driver gas. The corresponding sections of the tube are likewise called the driver and driven sections. The driver gas is usually chosen to have a low molecular weight, (e.g., helium or hydrogen) for safety reasons, with high speed of sound, but may be slightly diluted to 'tailor' interface conditions across the shock. To obtain the strongest shocks the pressure of the driven gas is well below atmospheric pressure (a partial vacuum is induced in the driven section before detonation).

The bursting diaphragm produces a series of pressure waves, each increasing the speed of sound behind them, so that they compress into a shock propagating through the driven gas. This shock wave increases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave but at lower velocity than the lead wave. Simultaneously, a refraction wave, often referred to as the Prandtl-Meyer wave, travels back in to the driver gas.

Applications:

Biomedical research like inserting of medicine · Military applications like missile launching · Mining purposes · Blockage removal in a Sewage plant.

Simulation setup:

The following Simulation tools will be incorporated:

1.Converge studio:- To pre-process the data that needs to simulate the case. It plays an important role in generating important 3d input files only using GUI. It does not run simulation directly, however it generates inputs files which use to run the simulation.

2. Cygwin:- Cygwin is a collection of GNU and open-source tools that provide functionality similar to Linux distribution on windows.
3. Converge msmpi 3.0:- To run the simulation using these input files in Cygwin in series or in parallel.
4. Post convert:- To convert all the output files which are processed by converge msmpi into the files which will be readable for post-processing in Paraview.
5. Paraview :- To post-processing to obtain the contour, plots, etc. It is an open-source, multi-platform data analysis and visualization software.

Geometry Setup:

Case Setup/Simulation Setup:

MATERIAL:

Gas Simulation:

• As only air is our working medium hence uncheck Reaction Mechanism.
• Predefined mixture: air
• Enable species(not in mech.dat) as we want to assign the participating species of Air like O2 and N2

Global Transport Properties

• Turbulent Prandtl number: 0.9
• Turbulent Schmidt number: 0.78

Species:

• Click on Gas species and add O2 and N2

SIMULATION PARAMETER:

Run Parameter:

• Solver: Density-based Transient
• Simulation Mode: Full Hydrodynamic
• Gas flow Solver: Compressible (as air is working medium)

Simulation Time Parameter:

• End time: 0.003s
• Start time: 0s
• Initial time step: 1e-09 s
• Minimum time step: 1e-09 s
• Maximum time step: 1 s

REGION & INITIALIZATION:

• Click on Region and Initialization and add 2 Regions.
• Region 0 as High Pressure region-Stream ID 0: temperature 300k; Pressure 6bar
• Species Air
• Region 1 as Low pressure- Stream ID 1:
• Species Air.

Events (to connect or disconnect the flow between two regions):

The flow is restricted till 0.001s and the flow will be taking place after 0.001s to the end time.

BOUNDARY:

• Naming:For High-pressure region: HP1, HP2, HP3.
• For low-pressure region: LP1, LP2, LP3
• Boundary condition:
• For all the Boundary HP1, HP2, HP3, LP1, LP2, LP3 we are providing Boundary Type as Wall and Velocity BC as Slip(as the shock, expansion waves are getting reflected from the Boundaries)
• Note that we are not providing the Inlet and Outlet BC conditions as in other cases because it is a closed vessel in which air is not entering nor leaving the system.

Turbulence model:

• RNG k-epsilon

Base grid:

• dx=0.0015m; dy=0.0015m; dz=0.0015m.

Enable Adaptive Mesh Refinement- (to capture the small change in flow by the automatic refinement of the grid at only where there is a small change in flow):

• Select High and Low-pressure Regions in Active Regions under AMR group 1.
• Maximum cells: 200000 (the maximum number of cells possible in the mesh).
• Enable species parameter.
• Embed type: Subgrid-scale(SGS)- SGS is used to show the small change in flow parameter (using subgrid criteria) by scale it down, depends on Max embedding level.
• Max embedding level: 2
• Formula for subgrid : Grid size after subgrid(smallest cell size after subgrid)=Base grid size/(2^Max embedding level)=0.004/(2^2)=0.8/4=0.001metre.
• Timing control type: PERMANENT.

Post-variable selection:

Output files:

The time interval for writing 3d output data files=3e-05 s

The time interval for writing restarting output=3e-05 s

Export input files and Running the Simulation:

Now we have successfully set up the Case and need to export the input files to run the Simulation.

Run the Simulation in Cygwin using a parallel processor.

Post Converting:

Now we need to convert all the output files which are processed by converge msmpi into the files which will be readable for post-processing in Paraview

Paraview:

We need to Post-process the output files to obtain the contour.

Results:

Pressure Plots for High and Low-pressure regions:

There is no pressure transfer takes place till 0.001s, after that the pressure is transferring from the high-pressure region to low pressure region and vice versa due to the Slip Boundary condition.

Temperature Plots for High and Low-pressure regions:

There is no temperature transfer that takes place till 0.001s, after that the heat transfer takes place from the high-pressure region to low pressure region and vice versa due to the Slip Boundary condition.

Contour:

Mesh:

Mesh size is 2mm.

Pressure contour:

Temperature contour:

Animation:

Pressure Temperature:

Conclusion:

• As the pressure in the high-pressure region exceeds maximum diaphragm pressure the diaphragm gets ruptured, and due to this, there is a pressure difference created. The high-pressure regions tend to more towards the low-pressure region which leads to formation of shock waves in that region which increases sudden pressure and Temperature on that low pressure region. On the other side, in the high-pressure region, the expansion waves will be created due to pressure relaxation, where Pressure and Temperature decrease.
• Both the Shock and Expansion waves are interacting with each other after reflecting from the wall due to the Slip Boundary condition of the Wall.