ИСТИНА

In the linear stability theory of shear flows, only a few classical results have been carefully tested experimentally (Blasius boundary layer [1, 2], the plane Poiseuille flow [3], Poiseuille flow in a round pipe[4]) Stability of free shear flows, e.g. jets and wakes, and perturbation growth in them are much less studied experimentally, because of low values of critical Reynolds numbers (4.02 for the Bickley jet and 37.6 for axisymmetric jet of "far-downstream" profile) and free jets breakdown not far from the origine. Recently, we invented a novel method for the formation of laminar jets [5] producing the jet of D=0.12m in diameter, which stays laminar at the distance 5.5D from the orifice for the Reynolds number ~ 10000. The jet with such characteristics provides a way to conduct detailed experimental study of perturbation growth in a free jet. The stability of the jet velocity profiles, obtained in experiments using above mentioned device, are theoretically studied by the modal stability theory. Two branches of growing perturbations are found, the frequency range for the fastest growing perturbations is 4 - 6 Hz for both of them, but the wavelengths for the first branch are shorter than for the second one (Figure (a)). In experiments, the perturbations of the jet are produced by the oscillating foil (Figure (b)). The jet is visualised, frequencies and wavelengths of perturbations are measured. Experimental wavelengths of perturbations are in agreement with theoretical predictions (Figure (a)). The shortest laminar region of the perturbed jet is observed for excitation frequencies 5 and 6Hz; whereas frequences higher than 10Hz, do not significantly affect the laminar portion length, which is in agreement with theoretical predictions (Figure(c)). [1] Schubauer and Skramstad, NACA-TR-909 (1948). [2] Boiko et al., J. Fluid Mech. 281 (1994). [3] Nishioka et al., J. Fluid Mech. 72, 4 (1975). [4] Eckhardt, Phil. Trans. R. Soc. A. 367 (2009). [5] Zayko et al., Physics of Fluids in press, (2018).