Orr-Sommerfeld mode
= Orr-Sommerfeld mode
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An Orr-Sommerfeld mode has nonzero wall-normal velocity satisfying the <Orr-Sommerfeld equation>. Its wall-normal vorticity satisfies the accompanying forced <Squire equation>. The triangular coupling separates the velocity eigenproblem from the vorticity forcing; the vorticity need not vanish in three dimensions.