Velocity field of isolated turbulent puffs

The velocity field of isolated turbulent puffs was measured using the particle image velocimetry\udtechnique and was compared with the steady jet flow field. Puffs were generated by injecting air\udthrough a 5 mm diameter nozzle into a flow chamber with a weak coflow. Isolated puffs with a\udReynolds number of 5000 were examined in the range of 40???75 diameters downstream of the\udnozzle. The injection time was varied in order to assess the effects of injection volume and\udequivalent stroke ratio on the puff structure. The results from phase-locked measurements indicate\udthat as the injection volume increased, puffs elongated in the axial direction and became similar to\udstarting jets in the range considered. The largest scaled fluctuating velocities and turbulent shear\udstress within the puffs were twice the steady jet values. Inspection of the vorticity field revealed the\udpresence of vorticity throughout the puff volume. Entrainment takes place on the portion of the puff\udclosest to the nozzle and the entrainment rate is greater for the puffs with the smaller injection\udvolume. This is consistent with the observations of rapid mixing and combustion of puffs in\udprevious studies