Comparison of turbulent jets issuing from rectangular nozzles with and without sidewalls

Abstract

This paper reports a systematic study of a turbulent jet issuing from a rectangular slot nozzle of high-aspect ratio, AR (≡ w/ h, where w and h are the long and short sides of the slot, respectively) tested with and without sidewalls. The solid sidewalls were flush with each of the slot’s short sides vertically and extend axially along the streamwise direction of the jet. Hot-wire measurements were conducted at a Reynolds number based on slot-width ( h) and exit centerline velocity of Re h ≈ 7000 (for AR = 30 and 60) and at 10,000 (for AR = 30) up to 160 h downstream. All jets have a potential core in which the local centerline velocity is approximately constant, followed by a transition region and then a statistically two-dimensional (2-D) region where the centerline mean velocity, U c ∼ x −1/2. The potential core of the jet without sidewalls is shorter than that with sidewalls. Near field power spectral analysis reveals that the primary vortex shedding rate is higher for the jet without sidewalls than the jet with sidewalls. The 2-D region of the jet with sidewalls is found to extend over a longer axial distance than that of the jet without sidewalls. It is also demonstrated that both the decay and spread rates of the jet within the 2-D region are lower for the case with sidewalls. Beyond the 2-D region, the jet without sidewalls enters into a far field transitional phase and then tends to behave statistically like an axisymmetric jet with U c ∼ x −1. The centerline turbulence intensity of the jet with sidewalls becomes asymptotic closer to the nozzle exit than the jet without sidewalls. The skewness and flatness factors confirm further statistical differences between the jet with and without sidewalls.