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200<br />
On the time evolution of a pulsed jet<br />
G. Querzoli 1, M. Falchi 2 and G. P. Romano 2<br />
The formation of vortex rings from sudden release of fluid through an orifice or a<br />
nozzle has been extensively investigated in the past a. Anyway most of authors<br />
focussed on flow rate that were constant or increasing until the vortex ring complete<br />
formation or stopped suddenly at a given instant, forcing the end of the vortex<br />
formation. In the present work, the velocity field generated by a pulsed jet is<br />
investigated. The flow-rate is periodic. It is modulated gradually so that there is a well<br />
defined phase of acceleration, followed by a gradual deceleration. Different flow-rate<br />
laws have been tested varying the overall shape of the curve and the relative duration<br />
of the acceleration and deceleration phases, and stroke-volumes, whereas the period<br />
was kept constant (T = 1 s). For each phase and curve 50 velocity field have been<br />
acquired and the average is considered.<br />
The jet is generated by a sharp-edged orifice (3.0 cm in diameter) on a thin plate<br />
placed inside a parallelepipedal vessel, 110 x 40 x 40 cm. On the upstream side of the<br />
vessel a small chamber hosts the water inlet and the piston-cylinder device that drives<br />
the flow. The flow develops in a 60 cm long chamber. Both water inlet and outlet are<br />
connected to a constant-head tank through two one-way valves in order to avoid<br />
back-flow during the periodic motion of the piston. The velocity field is measured on<br />
a middle plane by means of the Robust Image Velocimetry (RIV) b. RIV is a<br />
velocimetry based on image analysis that uses robust statistics instead of crosscorrelation<br />
(as PIV does) to compare interrogation windows. The measuring plane is<br />
illuminated by a Nd-Yag laser and couples of particle images are acquired by a crosscorrelation<br />
camera, 1376 x 1040 x 12 bit in resolution.<br />
An example of the resulting velocity (vectors) and vorticity (colour) field is shown<br />
in figure 1. In figure 2 the time laws of the piston motion used during the experiment<br />
are shown. The analysis of the results suggests that the vortex ring is affected only by<br />
the accelerated ejection.<br />
Figure 1 Figure 2<br />
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1 Dipartimento di Ingegneria del Territorio, Università di Cagliari, Italia. e-mail: querzoli[@unica.it<br />
2 Dipartimento di Ingegneria Meccanica e Aeronautica, Università di Roma "La Sapienza", Italia<br />
a Shariff and Leonard, Ann. Rev. Fluid Mech., 24, 235, (1992); Gharib M; Rambod E; Shariff K, J. Fluid<br />
Mech., 360, 121(1998)<br />
b Falchi, Querzoli Romano, Experiment in Fluids, submitted (2005).
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