Mean velocity and turbulence measurements in a pulsed jet

An extensive investigation into a fully pulsed jet has been performed with a single wire hotwire anemometer operated in a constant temperature mode to study the influences of varying mean exit velocity of the jet and pulsing frequency on the velocity field and turbulence quantities. The results of radial profile test across the jet show that the pulsed jet is axisymmetric. The test of jet centerline location demonstrates that the jet follows a straight line trajectory to propagate downstream. From the axial measurements, it is shown that the pulsed jet behaves like a steady jet after the pulsation effects have diminished towards furthest downstream locations, hence the existence of two distinct regions namely the pulsed dominated region and the high turbulence steady jet region is proved. However, the existence of the two distinct regions is strongly dependent on the mean exit velocity and pulsing frequency. At a high mean exit velocity and a low pulsing frequency, the high turbulence steady jet region can not be identified displaying no transition region. Moreover, at high mean exit velocities with a constant pulsing frequency, the aggregate turbulence intensity decreases more slowly along the downstream positions. Amplifications of the root-mean-square level of centerline-aggregate turbulence associated with an increasing mean exit velocity contribute to this case. At higher pulsing frequencies with a constant mean exit velocity, the aggregate turbulence intensity decreases more rapidly along the downstream positions as a result of the attenuations of the root-mean-square level of centerline-aggregate turbulence. Skewness and flatness along the downstream positions depart from the symmetry value of 0 and Gaussian value of 3, respectively as the mean exit velocity increases with a constant pulsing frequency. At higher pulsing frequencies with a constant mean exit velocity, skewness and flatness become lower in which approach the symmetry value of 0 and Gaussian value of 3, respectively. From the radial measurements, at an increasing pulsing frequency with a constant mean exit velocity the radial profile of mean axial velocity becomes narrower. As the mean exit velocity is increased at a constant pulsing frequency, narrower radial profiles result. Also, the radial distribution of aggregate turbulence intensity has smaller levels as the pulsing frequency is increased at a constant mean exit velocity. As the jet propagates downstream the levels of radial distribution of the aggregate turbulence intensity across the jet reduce gradually. At a constant pulsing frequency, amplification of the root-mean-square level of aggregate turbulence occurs as the mean exit velocity is increased which leads to the higher levels of radial distribution of aggregate turbulence intensity. Moreover, the radial profile of relative turbulence energy becomes broader as the pulsing frequency is increased at a constant mean exit velocity. However, there is some evidence demonstrating the narrower radial profiles as the pulsing frequency increases. As the mean exit velocity is increased at a constant mean exit velocity, a broader radial profile is produced. The levels of triple-products across the jet reduce as the pulsing frequency is increased at a constant mean exit velocity but the levels increase as a result of an increasing mean exit velocity at a constant pulsing frequency. Skewness and flatness in the radial direction are relatively lower at an increasing pulsing frequency with a constant mean exit velocity whereas at an increasing mean exit velocity with a constant pulsing frequency their values become higher. Furthermore, the jet volume flow rate relative to that at the jet exit significantly reduces as the Reynolds number increases. The jet growth and normalized volume flow rates are found to be higher than that of steady jets even up to further downstream position associated with the broadening radial profiles of mean axial velocity. At the low mean exit velocity of 13.7 m/s, the jet growth and jet volume flow rate at the low pulsing frequency of 10 Hz are higher than those at the high pulsing frequency of 25 Hz. However, as compared with the jet growth and volume flow rate phenomena at the low mean exit velocity of 13.7 m/s, a striking issue appears at the high mean exit velocity of 34.4 m/s demonstrating that the jet growth and jet volume flow rate at the low pulsing frequency are lower than those at the high pulsing frequency.