Formation of free round jets with long laminar regions at large Reynolds numbers

Abstract

The paper describes a new, simple method for the formation of free round jets with long laminar regions by a jet-forming device of ∼1.5 jet diameters in size. Submerged jets of 0.12 m diameter at Reynolds numbers of 2000–12 560 are experimentally studied. It is shown that for the optimal regime, the laminar region length reaches 5.5 diameters for Reynolds number ∼10 000 which is not achievable for other methods of laminar jet formation. To explain the existence of the optimal regime, a steady flow calculation in the forming unit and a stability analysis of outcoming jet velocity profiles are conducted. The shortening of the laminar regions, compared with the optimal regime, is explained by the higher incoming turbulence level for lower velocities and by the increase of perturbation growth rates for larger velocities. The initial laminar regions of free jets can be used for organising air curtains for the protection of objects in medicine and technologies by creating the air field with desired properties not mixed with ambient air. Free jets with long laminar regions can also be used for detailed studies of perturbation growth and transition to turbulence in round jets.

A feedback model of the edge tone, using the adjoint Orr–Sommerfeld equation

“…The strange mode found in our previous work (Nagy & Paál 2017) and experimentally (Zayko et al. 2018) was excluded because it exists only near to the orifice. In the linear approach it has no effect downstream of the point of its disappearance.…”

Comparison of Experimental and Theoretical Characteristics of Linear Waves in the Submerged Air Jet

“…In [11,12] the jet with a long laminar region and the facility forming it are described thoroughly. Hereinafter, the theoretical analysis and experiments are conducted for the flow with Re=5030, based on the diameter D and average velocity (the axis speed Uc=1.5 m/s).…”

“…Hereinafter, the theoretical analysis and experiments are conducted for the flow with Re=5030, based on the diameter D and average velocity (the axis speed Uc=1.5 m/s). The velocity profiles at different distances from the orifice were measured by a thermoanemometer, then compared with profiles obtained from stationary laminar calculation of the jet (see [12]), and finally approximated by analytical functions (Fig. 1).…”

“…At the Institute of Mechanics of Lomonosov Moscow State University the facility has been created that generates a submerged jet of 0.12 m in diameter (D) and with a long laminar region ~5D at the Reynolds numbers (Re) in the range 5000 -10000 [11,12]. Such jet provides the possibility to conduct detailed experimental observations of perturbations growth and to compare them with predictions of the linear stability theory applied to velocity profiles at different distances from the orifice.…”

Characteristics of the circular subsonic laminar jets flow with an initial Poiseuille profile

“…The laminar-turbulent transition in such flows occurs within the initial section (x/D < 4-6) [5][6]. The outflow from jet sources of different geometry (holes, pipes and channels, diffusers) has been studied much less [7][8][9][10]. At the same time, such configurations are encountered in practice.…”