Week 5

This week was a bit of a bust. I didn’t really end up making much progress at all. I am still not able to get PyLith to work properly. I am having a lot of trouble getting gravity to work properly. At Brad’s advice, I rebuilt the simulation starting from a similar example provided in the code, changing just one element at a time. Unfortunately, I am still having big problems with gravity.

PyLith does not currently allow the user to specify any type of body force, only pressure, displacement, or point force conditions on “boundaries” which can be any collection of points in the mesh. But, in order to “turn on” gravity, the user must use the provided gravity field that is capable of applying body forces.

Unfortunately, the gravity is causing strange happenings in my simulations. It does not appear to be spatially uniform and is causing impossible curves. But, the σ-yy component of stress (the compressive stress in the y diretion – so basically how much any little section of ice is pressing against the ice above and below it) seems correct in distribution and magnitude.

A graph showing the displacement of the beam due to just gravity acting upon it. The ice beam is fixed in place on the left hand side and does not have the pressure from the water acting up. I would expect something mostly parabolic in shape or at the very least a figure that is concave down. The white bar shows the original position of the beam before imposing gravity. Figure made in Paraview.

The other problem that I have run in to is that the shape of my models that don’t include gravity but only a pressure condition don’t have the correct shape either.

Now I am going to go into the actual equations that we are dealing with so if you don’t want to deal with the math you can skip down a bit, but I promise it’s pretty simple.

So, because the code already builds in the way a material responds to forces, all the user needs to do is input the forces and material properties. One of the first things I did was figure out what kind of boundary conditions I need. This included the more simple conditions of fixing one end of the beam in place to simulate the part of the ice that is stuck on the ground, and having no normal or shear forces on the top free surface. This means that the air is not exerting a pressure directly into the ice or along the surface (friction is a good example of a shear force). I am neglecting air pressure because it is so small and also ends up cancelling out for the most part.

So the trickiest condition is the bottom boundary pressure. Since I only care about what is happening at the very bottom of the ice, the pressure condition does not depend on the height of the ice above the boundary since this is constant, the depth of the water is the only thing that matters.

Acting down on the ice is the weight of the overlying water column is

p = ρgH

where ρ is the density of ice (917 kg/m^3), g is the gravitational constant of  acceleration (9.81 m/s^2), and H is the thickness of the ice (for now constant at 700 m).

And the force acting up is the hydrostatic pressure which is equal to the weight of the overlying water column which can be understood with a little bit of fluid dynamics stuff. The way it works is we say that the liquid is always in hydrostatic equilibrium which means that there is no net acceleration happening in the fluid, which is a good assumption for water because it has low viscosity and therefore can flow much faster than the forces are being changed. The important condition that comes from hydrostatic equilibrium is that the pressure at any given depth is the same so the pressure is the same at the very bottom of the ice, where it is being acted upon by only ice and a little to the side at the same depth where it only is sensitive to the water above it (see points A and B in diagram below).


The height of the water is broken up into three components: (1) the average neutral height, (2) the tidal height change, and (3) the deflection from the neutral position. The average height is the position at which ice is in hydrostatic equilibrium at the average tidal height , determined with the assumption that the grounded ice is at the average height such that at average tidal height there is no deflection. this is calculated by setting the other two components to zero and solving the equation  0 = p_ice + p_water = -ρgH + ρ’gh for h.

This results in h = (ρ/ρ’)H meaning that h is the height of the ice times the ratio of the density of ice to the density of sea water. Having this constant makes it easy to represent the force on the bottom of the ice as a function of the tidal height and the deflection of the ice from equilibrium. This last part is a restoring force that increases the pressure acting upwards on the ice if it is below the equilibrium height and decreases that pressure if the ice is floating too high.

The pressure can then be expressed as

p = -ρgH + ρ’g(h + Δh – (H+y))

and substituting for h:

p = -ρgH + ρ’g((ρ/ρ’)H + Δh – (H+y)) =  ρ’g( Δh – (H+y)) which is a buoyancy force that interestingly only depends upon the density of the sea water.

So, that was a lot of math, but what ultimately is important to take away from this is that the pressure is dependent upon the instantaneous deflection of the beam and varies along the surface, which I think is where I am having issues with PyLith.

While, at the moment,  I still have some problems to work out but, as described above I think I have found their sources, the real issue is trying to fix them. I am getting a bit frustrated with the modelling, but I now understand it well enough that hopefully once the problems are resolved i will be able to progress quite quickly in increasing the complexity and hopefully accuracy of the model.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s