Week 7: Shock tube simulation project

AIM: To run the transient flow simulation for a shock tube using Converge CFD.

OBJECTIVE:

• To set up the transient shock tube case setup for the simulation.
• And to plot the pressure, temperature contours and study the history of entire domain and to explain the result.
• Plot for the cell count as a function of time and explain the result.
• Show the mesh generated using Paraview.

DESCRIPTION:

The report contains a detailed description of the shock tube simulation using converge CFD and solving using Cygwin and post-processing the result using Paraview.

This Case study contains a rectangular duct that has two sections along with a diaphragm in between where a high pressure region is connected to the low pressure region.

A shock tube is a device used for studying shock waves. Prior to starting an experiment, a diaphragm inside the tube blocks any flow. We can increase the pressure on one side such as the compressor and then start the flow by rupturing the diaphragm. The gas expands down the other half of the tube.

STEPS USED FOR SIMULATION OF SHOCK TUBE:

Loading the .stl file in converge studio as follows

Using the given geometry we create 2 regions i.e a high pressure region and a low pressure region and create a diaphragm using events (time difference) we observe the results in the shock tube.

Creating boundaries for High pressure and low pressure by regions

For high pressure wall is

For High pressure 2d wall is

For Low pressure wall is

For Low pressure 2d front and rear is

Checking for normals

Making them point inwards as fluid flow is inside using Transform>normal

Now checking for diagnosis we see no errors

Now setting up the case with Time based

For materials, Gas simulation with Air and species

Species O2 and N2

Run parameters as transient and default

For simulation run parameters as 3milli seconds

Solver PISO

For regions we set two regions one for High pressure and one for low pressure

For high pressure with N2 mole fraction 1 and 6bar pressure

And for low pressure O2 as mole fraction and 1atm pressure

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.

For Boundaries,

For high pressure, with wall and slip condition

For High pressure front and rear face is

For low pressure, with wall and slip

For low pressure 2d front and back is

Turbulence model with RNG k-epsilon

Base grid of 1mm

Adaptive mesh refinement is used as this increases the number of cell in the vicinity of the region where shock takes place

For post variable

Output files

Then running the case set-up after exporting the input files to the folder using Cygwin.

By using the application mpiexec.exe & converge.exe by command line 'mpiexec.exe -n 2 converge.exe restricted' we run the simulation till results are converged.

Then using converge and paraview post processing results are observed and studied.

RESULTS:

The Mesh looks are follows

The velocity contour is

The Velocity Animation is

The pressure contour is

The Pressure Animation is

The Temperature contour is

The Temperature Animation is

The Pressure plots for High & low pressure regions is

The Temperature plots are

The cell count plots is

CONCLUSION:

High pressure N2 gas compresses the low pressure O2 and after some time of compression a recirculation takes place and the low pressure gas passes the N2.From the problem, we understand the compressibility of gas and study the flow.

Due to adaptive mesh refinement, the cell count is varying all the time when the expansion and contraction are taking place inside the shock tube.

From the pressure plot, it is observed that after some time both the species will have common pressure as one pressure is increasing and the other is decreasing with time.

The temperatures at the beginning fluctuate and then will attain equilibrium position after some time.