Effects of crossflow on puff and oscillation modes of a pulsed elevated transverse jet

Abstract

The effects of crossflow on the flow behaviour and mixing characteristics of an acoustically excited elevated transverse jet were investigated experimentally in a wind tunnel. The jet was excited at a frequency around the resonance condition by using a loudspeaker. The temporally varying flow structures of the excited elevated transverse jet in the median plane were captured by a high-speed digital camera. Visual penetration height and spread width were obtained by using an image processing method. The measured tracer-gas concentration revealed the dispersion characteristics. Forcing the transverse jet around the resonance frequency induced two distinct characteristic flow modes, puff and up–down oscillation of the deflected jet. A series of puffs travelling in the shear layer of the deflected jet were formed as the crossflow Reynolds number was lower than about 2300. For crossflow Reynolds numbers larger than 2300, large up–down oscillations of the deflected jet were observed. Acoustic excitation also eliminated the downwash effect induced by the shear of the transverse stream. The deflected transverse jet excited in the puff flow mode generated a significantly larger penetration height and spread width when compared with those in the oscillation flow mode and with the un-excited transverse jet. The measured tracer-gas concentration distributions demonstrated that the excited transverse jet has much better mixing and dispersion characteristics than the un-excited transverse jet. Besides, the puff flow mode has better mixing and dispersion characteristics than those in the oscillation flow mode.