Statistical properties of turbulent free jets issuing from nine differently-shaped nozzles

Abstract

This paper reports the centerline evolutions of turbulent statistical properties in nine air jets issuing from differently-shaped nozzles into still air surroundings. All nozzles of investigation have nominally identical opening areas or equal equivalent diameters (D e ) and their aspect ratio (AR) varies from AR = 1 (circle) to AR = 2.5 (isosceles triangle). Present measurements were made at the Reynolds number (based on D e ) of approximately 15,000. Results show that the loss of jet-axisymmetry at the exit generally causes the mean velocity decaying faster, and the fluctuating intensity growing, in the near field, thus indicating the increased overall entrainment rate. It is also shown that a change of shape of the nozzle exit does not affect the asymptotic decay rate of the centreline velocity in the far field. The near-field structure of the isosceles-triangular jet is deduced to be most three-dimensional, compared with the circular counterpart from smooth contraction being least. These discrepancies, however, weaken as the downstream distance x is increased. Beyond x/D e = 20–30, the normalized velocity spectra for all jets of small AR collapse well, indicating similar statistical behaviors over a wide range of scales in the central region. Indeed, sufficiently downstream from the exit, insignificant differences occur in jets’ velocity probability density function (PDF), the related skewness and flatness factors, and also in their Taylor and Kolmogorov microscales. It is demonstrated that all the length scales grow approximately linearly with x at x/D e ≥ 20.