Stokes stream function

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For two-dimensional flows, see Stream function.
Streamlines around a sphere in axisymmetric Stokes flow. At terminal velocity the drag force Fd balances the force Fg propelling the object.
Streamlines around a sphere in axisymmetric Stokes flow. At terminal velocity the drag force Fd balances the force Fg propelling the object.

In fluid dynamics, the Stokes stream function is used to describe the streamlines and flow velocity in a three-dimensional incompressible flow with axisymmetry. A surface with a constant value of the Stokes stream function encloses a streamtube, everywhere tangential to the flow velocity vectors. Further, the volume flux within this streamtube is constant, and all the streamlines of the flow are located on this surface. The velocity field associated with the Stokes stream function is solenoidal—it has zero divergence. This stream function is named in honor of George Gabriel Stokes.

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[edit] Cylindrical coordinates

A point plotted with cylindrical coordinates.
A point plotted with cylindrical coordinates.

Consider a cylindrical coordinate system ( ρ , φ , z ), with the z–axis the line around which the incompressible flow is axisymmetrical, φ the azimuthal angle and ρ the distance to the z–axis. Then the flow velocity components uρ and uz can be expressed in terms of the Stokes stream function ψ by:[1]

\begin{align} u_\rho &= - \frac{1}{\rho}\, \frac{\partial \psi}{\partial z}, \\ u_z &= + \frac{1}{\rho}\, \frac{\partial \psi}{\partial \rho}. \end{align}

The azimuthal velocity component uφ does not depend on the stream function. Due to the axisymmetry, all three velocity components ( uρ , uφ , uz ) only depend on ρ and z and not on the azimuth φ.

The volume flux, through the surface bounded by a constant value ψ of the Stokes stream function, is equal to 2π ψ.

[edit] Spherical coordinates

A point plotted using the spherical coordinate system
A point plotted using the spherical coordinate system

In spherical coordinates ( r , θ , φ ), r is the radial distance from the origin, θ is the zenith angle and φ is the azimuthal angle. In axisymmetric flow, with θ = 0 the rotational symmetry axis, the quantities describing the flow are again independent of the azimuth φ. The flow velocity components ur and uθ are related to the Stokes stream function ψ through:[2]

\begin{align} u_\theta &= - \frac{1}{r\, \sin(\theta)}\, \frac{\partial \psi}{\partial r}. \\ u_r &= + \frac{1}{r^2\, \sin(\theta)}\, \frac{\partial \psi}{\partial \theta}, \end{align}

Again, the azimuthal velocity component uφ is not a function of the Stokes stream function ψ. The volume flux through a stream tube, bounded by a surface of constant ψ, equals 2π ψ, as before.

[edit] Zero divergence

In cylindrical coordinates, the divergence of the velocity field u becomes:[3]

\begin{align} \nabla \cdot \mathbf{u} &= \frac{1}{\rho} \frac{\partial}{\partial \rho}\Bigl( \rho\, u_\rho \Bigr) + \frac{\partial u_z}{\partial z} \\ &= \frac{1}{\rho} \frac{\partial}{\partial \rho} \left( - \frac{\partial \psi}{\partial z} \right) + \frac{\partial}{\partial z} \left( \frac{1}{\rho} \frac{\partial \psi}{\partial \rho} \right) = 0, \end{align}

as expected for an incompressible flow.

And in spherical coordinates:[4]

\begin{align} \nabla \cdot \mathbf{u} &= \frac{1}{r\, \sin(\theta)} \frac{\partial}{\partial \theta}\Bigl( u_\theta\, \sin(\theta) \Bigr) + \frac{1}{r^2} \frac{\partial}{\partial r}\Bigl( r^2\, u_r \Bigr) \\ &= \frac{1}{r\, \sin(\theta)} \frac{\partial}{\partial \theta} \left( - \frac{1}{r} \frac{\partial \psi}{\partial r} \right) + \frac{1}{r^2} \frac{\partial}{\partial r} \left( \frac{1}{\sin(\theta)} \frac{\partial \psi}{\partial \theta} \right) = 0. \end{align}

[edit] References

  • Batchelor, G.K. (1967). An Introduction to Fluid Dynamics. Cambridge University Press. ISBN 0521663962. 
  • Lamb, H. (1994). Hydrodynamics, 6th edition, Cambridge University Press. ISBN 9780521458689.  Originally published in 1879, the 6th extended edition appeared first in 1932.

[edit] Notes

  1. ^ Batchelor (1967), p. 78.
  2. ^ Batchelor (1967), p. 79.
  3. ^ Batchelor (1967), p. 602.
  4. ^ Batchelor (1967), p. 601.