Lec: FD2

12 important questions on Lec: FD2

Define the advec/diff eq in a different way, showing the divergence of two fluxes (q). Why is this useful?

X

Change at certain point is in a way the derivative of the flux, which means it is the balance of what comes in/out.   Thus flux q is displacing the state variable, which is visible in the displaced peak.

What happens to the peak of the advection/diffusion eq if only diffusion is increased?

Peak wil flatten and moves to side, but stays in the same x-location

What happens to the peak of the advection/diffusion eq if both diffusion and advection are increased?

Peak moves to the right and flattens to same shape as diffusion-only case

Advection thus only displaces
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What is the minus sign in the advection/diffusion equation?

It indicates diffusion wants to move the state variable into the lowest concentration direction

What are i and n used for?

i = space
n = time

Give the definition and properties of the Crank-Nicolson method. Also write down the stencil and the formula

Crank-Nicolson is also midpoint + centered method combined, where the slope of the function at the beginning and end of the timestep are taken and averaged.

Combines properties of im- and explicit methods: smoothness of implicit method and CN gives more accuracy while remaining stable (in contrast to explicit)

Explain the definitions and difference between unstability, and oscillation

Unstability: error blows up over time, meaning the error grows exponentially with each timestep

Oscillatnig: fluctuating around the correct solution, but never becomes the correct solution; may have varying amplitudes.

DIfference: Error is not blowing up over time, it is just consistently wrong.

Why use matrix-vector algebra and matrix inversion?

By building a matrix we can solve all values of s at once. The matrix takes care of all the cross-dependencies of all the different variables.

Give the formula for a matrix inversion using the terms M, v and s

M*s = v
M^-1 *M*s = M^-1 * v
I * s = M^-1 * v
s = M^-1 * v

Why BCs only at corners of matrix?

Solving this example with 2 dirichlet BCs: 0 and 1 (and opposite)

When solving system, the end solution will look like the graphs (see notes).

BCs just apply at one single point, but it propagates into the system when you start solving it.

What is needed in order to solve a model (6)?

  1. BCs (Dirichlet, Neumann, Robin)
  2. Initial conditions
  3. Parameter values
  4. Numerical method
  5. Duration
  6. Domain description

Explain and give the formulas of the Dirichlet, Neumann and Robin BCs?

Dirichlet: fixed head/state at boundary has fixed value

Neumann: no flow/gradient of the state at the boundary has a fixed value. Neumann is used when we want to control flux over the boundary

Robin: linear combi of Dirichlet and Neumann

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