Compute streamfunction from numerical velocity field

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Discussion Overview

The discussion revolves around the computation of the streamfunction ψ from a discrete two-dimensional velocity field (u, v) in order to plot streamlines. Participants explore methods for integrating the governing equations and the implications of the grid structure on the calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how to integrate the equations governing the streamfunction, particularly when starting from a specific grid point.
  • Another participant inquires about the grid structure and whether the velocity field satisfies the incompressible continuity equation, suggesting these factors are crucial for the analysis.
  • A later reply confirms that the velocity field is on a rectangular grid, is equally spaced, and adheres to continuity, but notes the absence of known streamlines.
  • One participant proposes that the outer boundary of the rectangular region must be a streamline, suggesting that the streamfunction value on this boundary can be set to zero. They also suggest a numerical integration approach to estimate streamfunction values at interior grid points, including a method for correcting values if integration does not yield zero at the boundary.

Areas of Agreement / Disagreement

Participants generally agree on the grid structure and continuity of the velocity field. However, there is no consensus on the best method to compute the streamfunction values at interior grid points, with differing approaches suggested.

Contextual Notes

There are limitations regarding the assumptions made about the integration methods and the dependence on the grid structure. The discussion does not resolve how to handle potential discrepancies in streamfunction values at the boundaries.

Niles
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Homework Statement


I have a discrete two-dimensional velocity field (u,v). I want to plot the streamlines by finding the streamfunction ψ and from that plot the streamlines by finding the curves where ψ=constant.

Homework Equations


In order to find ψ I then have to solve the equations (see link)

$$
udy = d\Psi \\
vdx = -d\Psi
$$

The Attempt at a Solution


My main issue is that I'm not sure how to integrate these two equations. Say I start with the component u at point (i=1, j=1). If I have to integrate (=sum) this along y, then I basically get a number for the row i=1. But is this the way to do it?
 
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Is it on a rectangular grid? Are the velocities known to satisfy the incompressible continuity equation? Do you know any of the bounding streamlines?
 
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Chestermiller said:
Is it on a rectangular grid? Are the velocities known to satisfy the incompressible continuity equation? Do you know any of the bounding streamlines?

It is on a rectangular 2D grid, grid points equally spaced. It is a simulated velocity field, so it obeys continuity. I don't know any of the streamlines by default
 
Niles said:
It is on a rectangular 2D grid, grid points equally spaced. It is a simulated velocity field, so it obeys continuity. I don't know any of the streamlines by default
Well, the outside surface of the rectangular region has to be a streamline, since there is no flow across this boundary. Call the value of the stream function on this boundary zero. I don't know the best way to get the stream function values at the interior grid points, but a crude method would be to integrate each of the two equations numerically, one of them vertically and the other horizontally, and then take the average at each grid point. If integration along a vertical line does not give a value of zero of the stream function at the far boundary, I would distribute the excess uniformly among all the grid points in that column. The same goes for the horizontal direction.
 

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