Karman vortex street

Japanese: カルマン渦列 - カルマンうずれつ（英語表記）Kármán vortex street

When a flow hits a cylindrical object at a right angle, alternating vortices in opposite directions separate from the back of the object and flow downstream as two regular rows of vortex streets. In each row, vortices of the same direction are arranged at equal intervals, the two rows of vortices are in opposite directions, and the vortices in one row are always halfway between the other rows. This is called the Kármán vortex street. This vortex street has been known for a long time, but it was Theodor von Kármán who first gave a theoretical explanation for it in 1911, hence the name. If the spacing between vortex streets is h and the spacing between vortices in one row is a , Kármán's theory, which considers the fluid to be a perfect fluid, states that the vortex street is stable when h / a = 0.281, and unstable in any other configuration. In an actual viscous fluid, the Karman vortex street downstream of a cylinder is observed when the Reynolds number R = Ud /ν ( U is the velocity of the flow, d is the diameter of the cylinder, and ν is the dynamic viscosity of the fluid) is in the range from about 50 to about 500, and the ratio h / a is also larger than the theoretical value, close to 0.3. The Aeolian sound emitted by an electric wire in the wind is caused by the generation of the Karman vortex street.
In meteorological terms, the two-vortex line of stratocumulus clouds that appears in the lee of an isolated mountain is called the Karman vortex street. In the vicinity of Japan, they are often seen in the lee of Jeju Island, Yakushima Island, Rishiri Island, and the Kuril Islands. The ones that form in the lee of Jeju Island, an island in the south of South Korea, are particularly well-known. Their run is almost along the flow of low-level winds. In winter, they occur over the sea where the winter pressure pattern loosens and the inflow of cold air weakens, and low-level winds with a constant wind direction continue.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

柱状物体に直角に流れが当たるとき，物体の背後から交互に逆向きの渦がはがれて，2列の規則正しい渦列となって下流に流れ去る。各列には同じ向きの渦が等間隔に並び，2列の渦は互いに逆向きで，一つの列の渦は常に他の列の中間にある。これをカルマン渦列という。この渦列は古くから知られていたが，テオドール・フォン・カルマンが 1911年に初めて理論的説明を与えたので，この名がある。渦列の間隔を h，1列のなかでの渦の間隔を a とするとき，流体を完全流体とするカルマンの理論では，h/a＝0.281 のとき渦列は安定で，それ以外の配置では不安定である。実際の粘性流体においては，円柱の下流のカルマン渦列はレイノルズ数 R＝Ud/ν（U は流れの速度，d は円柱の直径，νは流体の動粘性率）の値が約 50から約 500の範囲において観測され，比 h/a も理論値より大きく，0.3に近い。風の中で電線が出すエオルス音は，カルマン渦列の発生が原因である。
気象用語としては，孤立した山などの風下に現れる，層積雲の 2本の渦の列をカルマン渦列と呼ぶ。日本付近では，チェジュ島，屋久島，利尻島，千島列島の風下などに多く見られる。特に大韓民国の南の島であるチェジュ島の風下にできるものは有名。走向はほぼ下層風の流れに沿っている。冬季，冬型気圧配置がゆるんで寒気の流入が弱まり，風向が一定した下層風が持続する海上で発生する。

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