Lec: FD3

9 important questions on Lec: FD3

What would be good arguments to pick a certain L? What happens if you have a very small vs very wide plume

If concentration plume would be only mm big, diffusion would kill shape immediately.
Gigantic wide plume, meaning gradients at sides would be really weak, advection is equally important, but diffusion is much smaller

Therefore, size/width of plume is important and L is taken as the width.

All information about our problem is captured in one dimensionless nr (D/uL)

True

Name the amazing quality of dimensionless nrs

Give you a nice way to compare two processes to eachother and in importance
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Why are dimensionless nrs handy:

  • Compare processes
  • Build scale models: as long as Peclet nr is the same, the size of u/L/D do not matter. Thus you don't have to build a lab with a 100 m wide river in it. If we then reduce L by factor 20, we need to increase u or decrease D accordingly in order to maintain Pe nr. 
Thus: you can analyse problem, determine dimensionless nr, as long as you keep your dimensionless nr the same, you are studying the same problem. You only need to adjust your other variables accordingly in order to maintain same magnitude.

When is the explicit (Euler forward) time integration scheme stable? Give the formulas and Peclet nrs

If advection dominates (high Pe nrs):
u * dt/dx <=1

If diffusion dominates: 2Ddt/(dx)2 <=1

What happens if we do not keep to the stability rules as defined by CFL? Also make a drawing

We start skipping cells, effectively meaning oscillations in flow occur. The flow is not nicely distributed over the grid cells anymore, but the first grid cell is overemptied.

What is the relation between the Peclet nr and stability analysis?

The Peclet nr indicates the relative importance of advection vs diffusion and thereby indicates how large the stepsizes and cellsizes should be in order to maintain stability (for eg Euler forward).

How do you make an equation dimensionless?

  1. Factor out all the typical magnitudes so everything has the same magnitudes/units/dimensions
  2. Make the formula dimensionless by multiplying with L/uS

At which Peclet nrs does diffusion/advection dominate and when are they equally influential?

Pe = 1
Pe > 1 Diffusion is dominant
Pe < 1 Advection is dominant

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