[en] This paper assesses the suitability of the inflow Reynolds number defined by Re_o ≡ U_oD/ν (here U_o and D are respectively the initial jet velocity and diameter while ν is kinematic viscosity) for a round air/air jet. Specifically an experimental investigation is performed for the influences of U_o, D and Re_o on the mean-velocity decay and spread coefficients (K_u, K_r) in the far field of a circular air jet into air from a smoothly contracting nozzle. Present measurements agree well with those previously obtained under similar inflow conditions. The relations K_u ∝ U_o and K_r ∝ 1/U_o for U_o < 5 m/s appear to work, while each coefficient approaches asymptotically to a constant for U_o > 6 m/s, regardless of the magnitudes of Re_o and D. It is revealed that Re_o may not be an appropriate dimensionless parameter to characterize the entire flow of a free air/air jet. This paper is the first paper that has challenged the suitability of Re_o for turbulent free jets. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)

[en] This paper analyses the downstream developments of the mean and the turbulent velocity fields of a plane jet. Based on the conservation of mass and the conservation of momentum, the mean-velocity half width (reflecting the jet spread rate) and the relative mass flow rate (jet entrainment) are related to the decay rate of the centreline mean velocity. These relations are not subject to self-preservation. Both analytical and experimental results suggest that the jet spread rate (K₁) and the entrainment rate (K₃) (and thus the decay rate K₂) can be well estimated from the centreline velocity, i.e., K₁ ≈ 0.6 K₂ and K₃ ∝ K₂. The effect of initial mean velocity and RMS velocity profiles on the downstream mean velocity field appears to be embodied in the constants K₁ K₂ and K₃. The analytical relationship for the self-preserving Reynolds shear stress, obtained for the first time, works well. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)

[en] A free triangular jet (TJ1) and its counterpart initially passing a short circular chamber (TJ2) are numerically modeled using large eddy simulation (LES). This paper compares the near-field characteristics of the two jets in detail. To enable some necessary experimental validations, the LES conditions of TJ1 and TJ2 are taken to be identical to those measured by Xu et al. (Sci. China Phys. 56 1176 (2013)) and England et al. (Exp. Fluids. 48 69 (2010)), respectively. The LES predictions are found to agree well with those measurements. It is demonstrated that a strong swirl occurs near the chamber inlet plane for the TJ2 flow. At the center of the swirl, there is a cluster of three sink foci, where each focus is aligned midway between the original triangular apexes. In the vortex skeleton constructed from the time-averaged flow field, the vortices arising from the foci are helically twisted around the core of the jet. As the flow passes through the chamber, the foci merge to form a closed-loop “bifurcation line”, which separates the inward swirling flow and the outward oscillating jet. This global oscillation is regarded as a source node near the centerline of the chamber. If the chamber is removed for a “free” jet, i.e., TJ1, a cluster of three pairs of counter-rotating foci is produced and the net swirl circulation is zero, so the overall oscillation of the jet does not occur. (paper)

[en] The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is investigated using large eddy simulation (LES). The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5×10⁴. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8–30d_e (exit diameter). Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5–1.0d_e. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)

[en] The present paper reports the first investigation on a turbulent jet issuing from a diamond orifice (hereafter termed a “diamond jet⁾ with an aspect ratio of 1.7. Velocity measurements were conducted in the transitional region, and the exit Reynolds number of the jet was 50000. For comparison, a round jet with identical normalized boundary conditions was also measured. It is shown that the diamond jet decays and spreads faster than the round jet does over the measured flow region. The axis-switching phenomenon is observed in the diamond jet. Although both jets display primary coherent structures in the near field, these structures are found to break down more rapidly in the diamond jet, due to the higher three-dimensionality of the flow. Moreover, the streamwise components of the Reynolds normal stress and all the shear stresses reach their maxima around the location of the maximal mean shear while the maxima of the lateral components of the Reynolds normal stresses occur around the centreline of the jet

[en] We report an investigation of the active control of a round air jet by multiple radial blowing mini-jets. The Reynolds number based on the jet exit velocity and diameter is 8000. It is found that once the continuous minijets are replaced with pulsed ones, the centerline velocity decay rate K can be greatly increased as the pulsing frequency of mini-jets approaches the natural vortex frequency of the main jet. For example, the K value is amplified by more than 50% with two (or four) pulsed mini-jets blowing, compared with the continuous mini-jets at the same ratio of the mass flow rate of the mini-jets to that of the main jet. (fundamental areas of phenomenology(including applications))

[en] In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters (D_e) and their aspect ratios (AR) range from 1 to 6.5. Planar particle image velocimetry (PIV) is used to measure the velocity field at the same Reynolds number of Re = 5 × 10⁴, where Re = U_eD_e/v with U_e being the exit bulk velocity and v the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core, expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet (AR = 6.5) produces the greatest decay rate of the near-field mean velocity, postpones the position at which the 'axis-switching' phenomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet. (electromagnetism, optics, acoustics, heat transfer, classical mechanics, and fluid dynamics)