Week 7: Shock tube simulation project

AIM:

• To simulate the flow across a shock tube. We intend to study the pressure and temperature distribution over time. Also, we intend to study the variation of cell count over time.

THEORY:

• The shock tube is a device that simulates and directs blast waves at a sensor or model to simulate actual explosions and their effects 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 at temperatures and pressures that other types of testing facilities find difficult to achieve. Shock tubes are also employed in the study of compressible flow phenomena and gas-phase combustion reactions. Shock tubes have recently been used.

Basic Concept:

• High-pressure driver gas expands upon the diaphragm opening, creating a shock wave
• Test gas is instantaneously compressed and heated to combustion temperatures by the incident and reflected shocks.
• High-temperature experiments were monitored near the end wall.

Shock Tube Characteristics:

• Instantaneous heating/compression from shock waves
• Accurately known incident- and reflected-shock conditions (from incident shock MS)
• Wide range of post-reflected shock Temperature and Pressure (600-4000 K, 0.1-1000 atm).

CASE SETUP:

1. Geometry:- STL file is imported into Converge. The geometry for consideration is given below,

• It consists of a tube which separated into two regions of different pressure. Nitrogen occupies a High-pressure region whereas oxygen occupies a low-pressure region.

a) Materials:- Pre-defined mixture was set to Air, Gas and species selected.

b) Global Transport Properties:- Default values retained,

Simulation Parameters:-

i) Run Parameters:- Transient enabled,

ii) Simulation Time Parameters:- Values set as shown below.

iii) Solver Parameters:- Solver scheme set to PISO,

Regions and Initialization:

- High-Pressure Region:- Pressure set to 6 bar. The mass fraction of N2 is set to 1.

-Low-Pressure Region- Pressure set to atm, Mass fraction of O2 is 1.

- Events: Events are used when two boundaries are in contact with each other and there is no boundary between them. the event flag creates triangles in between two regions called disconnect triangles.

- Events can be cyclic, sequential or permanent. Triangles are created when there is supposed to be no interaction between two regions and disconnected when there is supposed to be an interaction.

Boundary:

For all regions. Wall motion type set to Stationary. The surface movement is set to fix.

- Physical Models:- Turbulence modelling set to RNG K epsilon,

-Adaptive Mesh Refinement:- Adaptive mesh refinement feature in Converge was enabled. This was done to increase mesh resolution in the vicinity of the boundary such that cell size is sufficient to predict the boundary layer effects.

- Post Variable selection,

-Output files writing time interval is set as shown below,

RESULTS:

1) Base Mesh:- Mesh with dx=dy=dz=0.002m is shown below.

• Mesh with AMR:- Mesh with Adaptive Mesh refinement is shown below. Converge, monitors, the curvature properties and whenever it is more than the defined SGS criterion, it will refine the mesh,

2) Pressure Contour:- A shock wave is generated once the diaphragm is ruptured. The creation of a shock wave is characterised by an abrupt decrease in pressure on the high-pressure side initially. The reflected waves generate a back pressure resulting in an increase in pressure on the low-pressure side, which then moves on to cover the high-pressure side. There is an alternate fluctuation in pressure between regions as can be observed from the below animation as well as the plot

3) Temperature Contour:- The temperature contour also follows the same pattern as pressure. Temperature initially decreases in the N2 region. Reflected waves generate backpressure causing increases in temperature on the O2 side which then covers the high-pressure side. Alternate fluctuation in Temperature can be observed in the animation as well as the line plot,

4) Velocity:- A similar velocity profile characterised by fluctuations similar to pressure and temperature can be seen from the animation below.

5) Cell Count:- The plot below shows the variation of mesh size as a function of time. At 0.001sec when the diaphragm gets destroyed, the event is characterized by the destruction of disconnected cells. Therefore a sudden spike with rapid fluctuations is observed. Further variation post t=0.001sec can be explained due to the interaction of high and low-pressure regions during which AMR gets activated.

CONCLUSION:

• In the above analysis, flow through shock wave is simulated,

• The bursting of the diaphragm results in the creation of shock waves. Compression waves propagate in the low-pressure section whereas expansion waves in the high-pressure section. Shock waves get refracted back resulting in the mixing of the fluid.

• The mixing is characterised by abrupt fluctuation in pressure, density and velocity as can be observed from the plots.

• Cell count varies once the shock wave is created. The variation is primarily due to adaptive mesh refinement and rupturing of the diaphragm.