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ISSN オンライン: 2642-0554

LOCALIZED SECONDARY INSTABILITIES OF VELOCITY STREAKS IN PLANE CHANNEL FLOW

Fredrik Lundell
Dept. of Mech., KTH, S-100 44 Stockholm, Sweden

要約

Secondary, wavy, instabilities of velocity streaks are studied in a plane channel flow. As opposed to previous studies, a random signal was used to force the secondary instability (at a fixed streamwise position). Momentary, the random signal exhibits specific frequencies. Eventually the frequency will be amplified by the streak, forming a (localized) growing secondary instability with a length of a few wavelengths. Earlier work, theoretical, numerical and experimental alike, have studied the streamwise growth of infinite long harmonic oscillations. If amplified by the streak, the short localized secondary disturbances might ultimately develop into a turbulent spot. Phase-averaged hot-wire measurements, together with proper evaluation of the data, gives information of the disturbances. The amplitude of the localized disturbances grow exponentially. It is indicated that the growth rate of the disturbance decreases as the length of the disturbance decreases. It is also seen that the propagation speed of the disturbances are close to the local velocity at the position where the maximum disturbance energy appear.