Direct measurements of entrainment by acoustically pulsed axisymmetric air jets flowing into surrounding air have been made for a range of orifice sizes, Strouhal numbers, and excitation powers. The entrainment was considerably increased, by up to 5.8 times at distances greater than 15 diameters axially downstream of the orifice exit plane. The entrainment of the excited jet varied linearly with downstream distance. The jet response varied nonlinearly with excitation strength, indicating that there may be a practical upper limit to the acoustic augmentation of entrainment. The response depends on Strouhal number and appears to be optimum at about 0.25.

Mean velocity and turbulence profiles, downstream of the jet orifice, in a 10 m/s crossflow have been measured over a range of Strouhal numbers and excitation powers for a jet/crossflow velocity ratio range of 1.3 to 4.6. This showed that acoustically exciting a jet flow causes significant increases in jet spread, penetration (up to 92% increase), and mixing. The jet mixing length was strongly reduced. Toroidal vortices were shown to be shedding from the jet orifice and produced profound changes in the jet structure. Increase of jet penetration and turbulence (hence mixing) began to saturate by about 80-W driving power, thus only small further gains were possible up to the maximum power used of 160 W. The jet turbulence and penetration data showed that the response appeared to be optimum at about a Strouhal number of 0.22. Overall, the jet mixing processes were significantly improved, in a controllable manner, by pulsating the jet flow. Nomenclature a = jet axis, point of maximum velocity c = location on centerline in orifice exit plane D = jet orifice diameter F = function / = driving frequency H = tunnel height j = approximate boundary of the jet zone /?«, = crossflow static pressure at X/D --3.66 R ej = Reynolds number of jet at the orifice R eoo = Reynolds number of crossflow based on hydraulic diameter St = jet Strouhal number at the orifice =fD/Uj si = approximate position of maximum shear defining jet lower boundary su -approximate position of maximum shear defining jet upper boundary 7}= temperature of the jet flow at orifice exit plane TO, = crossflow temperature at X/D = -3.66 U = local mean velocity in X-Y plane at a particular X location U e = jet velocity excitation pulsation amplitude, or pulsation strength, at the orifice exit plane center (unsteady flow) Uj = steady jet velocity at the orifice exit plane center rms = local rms value of fluctuating velocity (turbulence) in X-Y plane at a particular X location rms = overall (average over H) turbulence in X-Y plane at a particular X location = change in overall turbulence, caused by acoustic excitation, in X-Yplane at a particular X location £/<» = average crossflow velocity at X/D = -3.66 cross section v = approximate edge of core region of the toroidal vortices shedding from the orifice W = power at acoustic driver X, Y,Z = rectangular coordinates; see Fig. 1 for origin location

The mixing by an acoustically pulsed axisymmetrical air-jet, flowing into the atmosphere, has been studied by means of velocity and temperature profile measurements. The strength of the velocity pulsation imparted to the jet flow and of the associated toroidal vortices were also measured. The entrainment rate was increased by up to two times, with the majority of the extra entrainment occurring over the first five diameters downstream of the jet orifice, where toroidal vortices are formed and attain their greatest strength. The jet response depends on Strouhal number and appears to be optimum at about 0.25. The response starts to saturate at the limit of pulsation strength used.

The mixing by an acoustically pulsed axisymmetrical air-jet, flowing into the atmosphere, has been studied by means of velocity and temperature profile measurements. The strength of the velocity pulsation imparted to the jet flow and of the associated toroidal vortices were also measured. The entrainment rate was increased by up to two times, with the majority of the extra entrainment occurring over the first five diameters downstream of the jet orifice, where toroidal vortices are formed and attain their greatest strength. The jet response depends on Strouhal number and appears to be optimum at about 0.25.The response starts to saturate at the limit of pulsation strength used.

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