Effects of acoustic excitation at resonance Strouhal numbers on characteristics of an elevated transverse jet

The effects of acoustic excitation on the flow behavior, penetration, and spread of the stack-issued and wall-issued transverse jets were studied experimentally. The jet flow was periodically excited by a loudspeaker that was driven with a square wave at resonance Strouhal numbers. The pulsed transverse jet was characterized by jet Reynolds number 2000. Streak pictures of the smoke flow patterns illuminated by the laser-light sheet in the median plane were recorded with a high-speed digital camera to illustrate the evolution process of the characteristic flow behavior within one excitation cycle. The binary edge-detection technique was used to determine penetration height and spread width. The tracer-gas concentration measurement provided jet dispersion information. The evolution processes of both the stack-issued and wall-issued transverse jets were characterized by a leading vortex ring and swing motion of the jet column near the jet exit as the jets were forced at resonance Strouhal numbers. A leading vortex ring appeared near the jet exit during the leading phase of excitation cycle and evolved subsequently to puffs of jet fluids in the upwind shear layer of the deflected jet. The swinging motion of the near-tube tip jet column induced up/down oscillation of the deflected jet. The excited stack-issued transverse jet exhibited significantly larger penetration height and spread width than the excited wall-issued transverse jet. The tracer-gas detection experiment results showed that the excited transverse jet disperses significantly faster and wider than the non-excited transverse jet. Pulsating the transverse jet at low resonance Strouhal numbers produced higher mixing and dispersion effects than pulsating the transverse jet at high resonance Strouhal numbers.