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1. Title – Shock Tube Simulation. 2. Objective – To create the tube in converge Software. To set up the case with proper grid size, material and boundary conditions. To solve the case using Cygwin terminal and convert the result for Paraview software. To import output files in Paraview and generate required…
Lokesh Seervi
updated on 22 Apr 2021
1. Title – Shock Tube Simulation.
2. Objective –
3. Theory –
A shock tube is an instrument used to study gas phase combustion reactions, and aerodynamic flow. At its simplest, a shock tube is a metal tube with a diaphragm. In experiments using a shock tube, the high-pressure gas is referred to as the driver gas, while the low-pressure gas is the driven gas. The gases need not be of the same chemical composition. The gasses are pumped into or out of the tube on each side of the diaphragm until the desired pressure is reached on each side. To begin an experiment with a shock tube, the diaphragm may be ruptured using a plunger with a blade attached, though the required mechanism is complex.
In this study we will simulate shock wave generation in close channel, which is known as shock tube. In converge we can not provide an obstruction or diaphragm physically, but it allows us to hold fluid apart without a boundary. The holding of fluid will be regulated by time, after a certain time (defined by user) fluids will mix and this will replicate like a diaphragm has burst. Collision gases with wall will create shock waves.
4. Procedure –
Geometry -
In this study we simulated shock wave generation in shock tube, for that the following procedure was followed –
1. Starting with application type, which is time based, since it is not related to IC engine applications.
2. In material gas simulation was enabled and we went with default gas parameters.
3. In run parameters transient state chosen as solver type, simulation mode was chosen as full hydrodynamic because we want to simulate with effects of fluid also.
4. In simulation time parameter start time 0s, end time 0.003s, initial time step 1e-09s, minimum time step 1e-09s, maximum time step 1s was chosen.
5. In solver parameters density-based solver was chosen, other parameters kept unchanged.
6. In boundary conditions six boundaries were defined, three at high pressure side and three at low pressure side. Front, top and bottom sides were treated as wall and other were two D boundaries.
Boundaries -
High pressure and Low pressure -
remaining -
7. We went with default turbulence model which was Reynolds Averaged Navier Stokes k-epsilon model.
8. In this simulation two regions were defined one was named as high pressure and second one was low pressure. In high pressure region we gave pressure value 6e5 and temperature were kept at 300k. in low pressure region parameters were kept default. At high pressure side nitrogen was there and at low pressure side oxygen was there.
9. Events option was enabled, because we wated to regulate the fluid flow. At the beginning of simulation nitrogen and oxygen are held apart by an imaginary boundary. After one 0.001s we allowed the mixing, its like rupture of diaphragm.
10. We went with only one mesh with size 0.004m. addition to that adaptive mesh refinement was enabled in grid control option. This option allows us to rescale the mesh with a particular criterion like in this simulation we wanted to refine mesh where mass fraction of nitrogen gets minimum value 0.001. embedded scale was selected as 3.
Embedded mesh = base grid/2^n
n = embedded scale
embedded mesh = 0.00423 = 0.0005m
so finest mesh of this simulation was 0.0005m.
11. Post variable selection allows us to select variables which we want to analyse in post processor. Converge has some variables selected as defaults. We went unchanged.
12. In output files 1e-5s was given as time interval to write 3D outputs, 1e-6 for restarting output and 0.001s for writing text output.
13. Input files exported in a folder and these solved using Cygwin terminal.
14. Files were converted into converge 3D postprocessing option and postprocessed in Paraview.
5. Results -
Pressure Plot -
Temperature Plot -
Cell Count -
Pressure contour(at 50th time step) -
Velocity contour (at 50th time step) -
Pressure animation -
https://drive.google.com/file/d/1dvhdk0Xx1DsXjh-oOEIRVhThDnFrwKlJ/view?usp=sharing
Velocity animation -
https://drive.google.com/file/d/10YIdVm1R7O3_FfSDobM5qFGWcgXuwYNz/view?usp=sharing
Velocity and pressure animations and plots has created to get good insights. Mixing of Nitrogen and Oxygen was regulated by event. From the contours t can be observed that at the beginning of simulation there is no flow across imaginary diaphragm after 0.001s, simulation replicated diaphragm rupture and shock wave generation can be seen there. Nitrogen flow towards oxygen because it has high pressure.
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