On the effects of initial conditions on statistical properties of single and twin turbulent jets

Abstract

An experimental study was designed to investigate the effects of Reynolds number and nozzle geometry on the development and the structures of free turbulent single and twin jets. The velocity measurements were performed using a particle image velocimetry (PIV). Measurements were conducted in the symmetry planes of single round, elliptic, rectangular, triangular, cross, daisy, star and square jets, and the Reynolds number studied ranged from 2500 to 20000. The effects of the initial conditions on the jets’ properties are studied using the mean velocities, Reynolds stresses and triple velocity correlations. Also, several methods are employed to extract and examine the turbulent structures of the jets which are responsible for their different mixing performance. The results show that noncircular jets have shorter potential core length, faster growth of centerline turbulence intensity and higher velocity decay and spread rates. Due to the specific topology of the daisy and triangular jets, the normalized profiles of mean velocity and higher order turbulent statistics in their minor and major planes are close to each other, therefore, the turbulent kinetic energy can be approximated by k=0.5((u^2 )+2(v^2 )). In addition, proper orthogonal decomposition (POD) analysis shows that the fractional and cumulative energy of turbulent structures in both minor and major planes of the triangular jet are almost identical. In the elliptic and rectangular jets, the statistical quantities in the minor and major planes differ significantly. The swirling strength analysis shows that the vortical structures in the minor plane of the elliptic and rectangular jets are more intense compared to those in their major plane as well as in the round and square jets leading to a higher jet spreading in the minor planes. It is observed that an increase in Reynolds number decreases the decay and spread rates, however, at Re > 10000 they become asymptotic. The joint probability density function (JPDF) and weighted joint probability density function (WJPDF) analysis demonstrate that the dominant events that contribute towards the Reynolds shear stress are different upstream of the axis-switching point in the minor and major planes of the elliptic jet. Consequently, a sign change occurs in the distribution of the Reynolds shear stress in the major plane but not in the minor plane. In twin jets, upstream of the merging point, the levels of turbulence intensities and Reynolds shear stress are higher in the inner shear layers, but the opposite trend is observed downstream of the merging point. The swirling strength analysis reveals that the vortical structures are more intense in the inner shear layers upstream of the merging point, their strengths are equal at the merging point and eventually more intense in the outer shear layers after the merging point. The analysis performed at the turbulent/non-turbulent interface of the jets reveals no Reynolds number effect on conditional streamwise velocity and spanwise vorticity distribution. However, the values of conditional transverse velocity are significantly larger at the lowest Reynolds number. The two-point correlation functions point out that the turbulent structures in the outer shear layers are larger compared to the inner shear layers.