Purpose of streamlines in unsteady flow
I have seen diagrams of airfoil with streamlines drawn. It helps to show the velocity vectors of the flow around the airfoil.
However, does streamlines help in the analysis of unsteady flows, such as an airfoil moving up and down? Is there any useful information which can be extracted?
Streak lines can be useful for visualization.
Create an imaginary particle at points of interest (x, y).
Follow the particle in a Lagrangian frame.
Example: Dx/Dt = u, Dy/Dt = v, Dz/Dt = w so
x(t+dt) = x(t) + u*Dt, etc. following the particle through the flow.
The three equations can be integrated explicitly [use u(x, t)], implicitly [use some combination of u(x,t) and u(x+dx, t+dt) estimates]. The u's, v's and w's are interpolated from your CFD solutions at the various (x,y,z,t). Implicit schemes and various Runge-Kutta schemes are available.
I'm sure other folks will correct, extend, and clarify this outline!
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