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Other Studies of Kelvin Helmholtz instabilities

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This part contains further developments of the study of the Kelvin Helmholtz instability to illustrate possibilities of Fluent: 
  1. Instability in a 3 phases layered fluid 
  2. 3D Instability

A) 3 phases
A-1 Mesh
The mesh is an evolution of the mesh used in the time wave study. This time the height is 0.3m the additional space being occupied by the hid phase.

Click here for a picture of this mesh.

A-2 Model
As with the mesh, the used model is the same as in the time wave study. The third phase is liquid gasoil which is slightly lighter than fuel. The fluids are layered with heavier on the bottom an lighter on top. As we use 3 phases the vortexes are moving in most settings. 

The interface 1 between water and fuel is perturbed whereas the interface 2 between fuel and gasoil is undisturbed 

A-3 Result
The results showed that the oscillation at the interface 1 affected the interface 2. I seems like the interaction between both interfaces has a stabilizing effect.

For example with water 2 m/s, fuel 0 m/s, gasoil 2 m/s. .
at time 0.14s


                                            at time 0.34s




With a water 2 m/s, fuel 0 m/s, gasoil -2 m/s setting, the oscillation are moving in opposite direction but there the same evolution occurs. 
B) 3D Instability
B-1 Mesh
As a 3D mesh uses a lot more computing power the domain is smaller: 0.1m in length (x-axis), 0.2m in height (y-axis),  0.1m in depth (new z-axis). We use a 4 mm space step size to keep the elements number down: 25*50*25=31250 elements.

The upper and lower faces are walls. The other faces are periodic ones.

Click here to see a picture of this mesh

B-2 Model
The model used and operating conditions are the same as for the time wave study. The initialization UDF init3d.c is an evolution of init.c. This time the is a perturbation on the x-axis and the z-axis. The wavelength in both direction is 0.1m. The fluid is flowing along the x-axis.
B-3 Result
The image below show the result of the computation after 0.1s. The surface is the 0.5 iso-value of fraction of fuel (i.e. the interface). The color is the velocity magnitude. 

Click here to see an animation of the instabilities formation with a 0.01 time step.

This image shows the particular shape of the instability. 

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