Week 7: Shock tube simulation project

Aim: Shock Tube Simulation

Objectives:
• To set up a transient shock tube simulation
• To plot the pressure and temperature history in the entire domain
• To plot the cell count as a function of time
• To show the mesh generation using ParaView.

Introduction:
The shock tube is an instrument used to replicate and direct blast waves at a sensor or a model to simulate actual explosions and their effects, usually on a smaller scale. Shock tubes 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. A shock wave inside a shock tube may be generated by a small explosion (blast-driven) or by the buildup of high pressures that cause the diaphragm to burst and a shock wave propagates down the shock tube.

Theory:
A simple shock tube is a tube, rectangular or circular in cross-section, usually constructed of metal, in which gas at low pressure and gas at high pressure is 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, for safety reasons, with a high speed of sound, but maybe 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 test begins with the bursting of the diaphragm. Several methods are commonly used to burst the diaphragm.
• A mechanically-driven plunger is sometimes used to pierce it or an explosive charge may be used to burst it.
• Another method is to use diaphragms of plastic or metals to define specific bursting pressures. Plastics are
used for the lowest burst pressures, aluminum and copper for somewhat higher levels, and mild steel and
stainless steel for the highest burst pressures. These diaphragms are frequently scored in a cross-shaped
pattern to a calibrated depth to ensure that they rupture evenly, contouring the petals so that the full section of the tube remains open during the test time.
• Yet another method of rupturing the diaphragm utilizes a mixture of combustible gases, with an initiator
designed to produce a detonation within it, producing a sudden and sharp increase in what may or may not be
a pressurized driver. This blast wave increases the temperature and pressure of the driven gas and induces a
flow in the direction of the shock wave but at a lower velocity than the lead wave.
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 a lower velocity than the lead wave. Simultaneously, a rarefaction wave, often referred to as the Prandtl-Meyer wave, travels back into the driver's gas.

5. Assigning Boundaries:
In converging every part of geometry is assumed to be a triangle. Selecting triangles and grouping to assign to specific boundaries.
boundaries.
Boundary> Flag> +> Create Boundaries> OK
a) High Pressure:
A as an entity selection-By Angle 20°, Using cursor pick option> Selecting the triangles as shown in the below image>
Assigning them to High-Pressure Boundary> Apply

a) Low Pressure:
' as an entity selection-By Angle 20 degrees> Using cursor pick option> Selecting the triangles as shown in below image>
Assigning them to Low-Pressure Boundary> Apply

Case Setup:

1. Application Type:
Selecting an application type as lime Based'.

2. Materials:
Gases are inside the shock tube.
Checking the solid simulation & Species option and unchecking the reaction mechanism option.

a) Regions and Initialization:
Selecting Add to create a new region,
Add 2 Regions, namely the high-pressure region and low-pressure region. i) High-Pressure Region:
i) High-Pressure Region:
for the High-Pressure Region setting the Stream ID as 0
Velocity - [0, 0, 0] m/s
Temperature as 300 K
Pressure as 600000 Pa.
Species N2 with Mass fraction value 1

ii) Low-Pressure Region:
for the Low-Pressure Region setting the Stream ID as 0
Velocity - [0, 0, 0] m/s
Temperature as 300 K
Pressure as 101325 Pa.
Species 02 with Mass fraction value I

b) Events:
When 2 regions are in contact with each other with no boundary in-between them, it is necessary to use events. The
events flag creates triangles in the place between the 2 regions called as disconnect triangles.

Events can be cyclic, permanent or sequential. Based on how the event should occur.
In this case, the event can repeat itself in a sequence. Based on this setting, Converge creates disconnected
triangles. These triangles are temporarily created to simulate the event. Triangles are created for when there should
be no interaction between the 2 regions and these triangles are deactivated when there is supposed to be an
interaction between the 2 regions.
At the start, 2 regions are not in contact and for that event is set as given below.
• Start second: 0 sec
• Region A: High pressure
• Region B: Low pressure
• Event: Close
After 0.001 seconds, to deactivate disconnected triangles and provide interaction between 2 regions the event is set
as:
• Start second: 0.001 sec
• Region A: High pressure
• Region B: Low pressure
• Event: Open

5. Boundary:
i) High Pressure:
Boundary Type- WALL - Slip
Velocity Boundary Condition:
Wall motion Type - Stationary Surface Movement: FIXED
Surface Movement: FIXED
Region: High-Pressure Region

ii) Low Pressure:
Boundary Type- WALL - Slip
Velocity Boundary Condition:
Wall motion Type - Stationary Surface Movement: FIXED
Surface Movement: FIXED
Region: High-Pressure Region

6. Physical Models:
• Turbulence Modeling:
Keep the Turbulence model as RNG k-epsilon
Keep all the settings default > OK

7.Output/ post processing

a) Post variable section

b) Output files: