Week 7: Shock tube simulation project

AIM -

To set up a transient shock tube simulation

Plot the pressure and temperature history in the entire domain

Plot the cell count as a function of time 

SHOCK TUBE-

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 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 or by the buildup of high pressures which cause diaphragm to burst and a shock wave to propagate down the shock tube. In the simulaiton event is created where until given time the diaphragm is present but then at a given time condition are set as such that the two seperate region become a single region.

PERFORMING THE SIMULATION IN CONVERGE-

Geometry:

The shock tube is create in 3D but then it will be used as a 2D, because the length is much greater than the other sides

Here the length of the tube is 0.2 m and the square cross section is of 0.01X0.01 m^2.

Case Setup: 

Application type = Time based.

Materials -

Keeping the gas simulation and the global transport parameters as the same.

Species = N2, O2

Simulation Parameters-

Run parmeters = 

Using the solver type as transient.

Simulation time parametes =

The start time is 0 seconds and the end time is 0.003 seconds.

using initial time step as 1e-7 s an the maximum time step as 0.001.

The maximum Mach CFL limit is set as 1.0

Solver Parameters -

Solver type = Density Based.

Boundary-

For both the regions we will consider that the boundary type is WALL 

wall is stationary and the surface movement is FIXED and using slip velocity condition.

The temperature and pressure is neumann boundary condition.

Initial conditions and events

Regions-

There are two regions as shown below

(1) High pressure region - it has N2 at 6 Bar and 300 K.

(2) Low Pressure Region - it has O2 at 1 atm pressure and 300K.

Events-

The sequential event is introduced here, at 0 s both the regions are CLOSE , this means that there is diaphragm present between the interface of both the regions.

At 0.001 s the regions become OPEN this means that there is not diaphragm present between the interface of the two regions.

Turbulance model is set as RNG K epsilon.

Grid Control

Base Grid-

The base grid is set at 0.001 m in all three directions.

Adaptive Mesh Refinement 

The maximum cells is set as 200000. Both regions are kept as active.

Using the Species AMR, considering N2 the SGS embed value is at 0.001 for the complete run time and the embed scale is at 3 with Default embedding level as 3. 

Results 

Mesh-

The total cell count is 20000 just before 0.001 s and after that it is almost 200000

just before the breaking of two zones. 

At the end of the time 

Pressure Plot and Contour:

The pressure of the high pressure region reduces while the pressure of the low pressure region increases. This flow has a damping nature.

Below is the pressure contour at time 0.003 s

Temperature Plot and Contour:

The Temp of high pressure region reduces while in the low pressure region increases.

Temperature contour at time 0.003 s  

Mass Fraction of N2-

Pressure Contur Animation

Temperature Contour Animation

Mass Fraction Contour for N2

CONCLUSION -

1.Bursting of the diaphragm creates a shock wave in the tube.

2. There are compression wave in the low pressure region and in the high pressure region there are expansion waves.

3. The reflaction of the wave from the wall results in mixing of the two fluids.

4. the sudden increase in the cell count is due to the adaptive mesh refinement.  

5. the mixing of the fluids can be observed from the pressure, density and velocity contours.