Lec: FE2

16 important questions on Lec: FE2

How does FE work again?

External input is distributed/assigned to nodes based on inverse distance weighing

With the base function, an eq has been derived to determine which fraction is assigned to each node to achieve a more continuous gradient. Compared to FV assigning more strict boundaries.

Why do we integrate the internal flux between left and right node?

Internal flux varies between left and right node because the state varies

Give the formula for the mean internal flux between two nodes is calculated

E[XL,XR]*[Q] = 1/(Xr-Xl) integral Q(x)dx
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Why don't we take the flux of the average state?

The relation between state and flux can be highly non-linear, which actually often occurs

What is the order applied in FE 1D?

  • Calculate flux everywhere
  • Take average (by using the formula including an integral)

Name the two steps of the internal flux in 2D calculation

  1. Calculate the mean flux in the element (dark red arrow)
  2. Distribute this mean internal flux over the nodes of the element
    1. Determine amount of flow over each of the boundaries of our element/decompose of flux size for each side of the triangle, so it is perpendicular to boundaries
    2. Multiply with length over which it flows into/out of our element
    3. Assign this flux to next node

What is the difference with internal and external fluxes?

The internal flux has a direction in which it wants to flow, meaning it has an x and y component.

External flux just falls on your domain

What is the ironic part of FE and what is the term used to fix it?

Computers do not like integrals, as they include infinite concepts. Thus we need to use a technique to simplify our integrals, which is called Gaussian Quadrature

How does Gaussian Quadrature work?

Gaussian Quadratures only work for finite integrals
Can be used for set of eqs

gamma = weight
w = weighing factor

What are Gaussian Quadratures?

Equations to approach integrals

When does Gaussian Quadratures lead to errors?

Linearize our equation - we basically take average state and calculate the flux with that.
But we see the eq is not linear, so obv a l inear approach will lead to errors

If dealing with highly non-linear systems, linear GQ is not the best solution. More complex versions of GQ might be better.

Which versions of GQ do you have?

Linear
Quadratic
Cubic
Quartic

What is quite ironic about the GQ calculations?

This lecture is about a numerical technique to solve integrals while this whole course is about numerical techniques to solve differential equations.

How to predict which GQ technique to use for your equation?

The more  non-linear, the higher order methosd you would like to use. 
Even for a linear equation it might be better to use higher order GQ methods.

How is FE executed in a 2D case?


FE in 2D: we multiply with the element area rather than b-a from 1D case. And we take double integral.

Where do you solve your equation in GQ?

You solve your equation in your x-es

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