• Vorticity
  • Measure of the rotational spin about an axis at some point within a fluid (or wind field). 
Abstract from DBPedia
    In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along with the flow. It is an important quantity in the dynamical theory of fluids and provides a convenient framework for understanding a variety of complex flow phenomena, such as the formation and motion of vortex rings. Mathematically, the vorticity is the curl of the flow velocity : where is the nabla operator. Conceptually, could be determined by marking parts of a continuum in a small neighborhood of the point in question, and watching their relative displacements as they move along the flow. The vorticity would be twice the mean angular velocity vector of those particles relative to their center of mass, oriented according to the right-hand rule. In a two-dimensional flow, is always perpendicular to the plane of the flow, and can therefore be considered a scalar field.

    渦度(うずど、かど、英: Vorticity)は、流れの回転するありさまを表現する量である。渦度はベクトル量(さらに言えば擬ベクトル)であり、流れの速度ベクトルのなすベクトル場の回転である。 渦度ベクトル Ω は流速ベクトル v = (vx, vy, vz) により、以下のように表される。 渦度ベクトルを流線のようにつなげた曲線を渦線という。流れの中のある閉曲線上の各点を通る渦線によってできる曲面を渦管という。断面積を無限小にした渦管を渦糸という。渦糸が閉曲線になっている場合、これを渦輪という。

    (Source: http://dbpedia.org/resource/Vorticity)

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