Non-modal perturbation growth in a laminar jet: an experimental study

Abstract

It is known that, apart from the growth of instability modes, the non-modal (algebraic) mechanism of linear growth plays an important role in near-wall flows. In unbounded flows, including submerged jets, the theoretical analysis of the non-modal growth mechanism started only in the last decade; this mechanism has not yet been identified in experiments. In the present work, experiments were conducted on the excitation of a non-modal ‘lift-up’ growth mechanism. Special wavy structures (deflectors) were introduced into a laminar round submerged jet, which excited a roller-like transverse motion. Based on experimental results, we definitely identify the non-modal ‘lift-up’ growth mechanism of introduced disturbances. The development of perturbations in the experiment qualitatively corresponds to the theoretically calculated optimal perturbations. The features of the transition to turbulence caused by non-modal growth are considered.

Experimental study of perturbation growth in a round laminar jet

“…PIV measurements in the transverse cut plane of the jet, which are described in Ivanov et al [20] in detail and are not included in this short paper, showed that the evolution of introduced perturbation matches qualitative features of theoretical optimal perturbations quite well. Namely, the following characteristic features of the "lift-up" growth mechanism have been confirmed.…”

“…The lengthening of the petals occurs linearly, therefore, the radial velocity perturbation is approximately constant, which is consistent with the theoretical calculations of stationary optimal perturbations of the jet (see fig. 6a at Ivanov et al [20]).…”

“…Close-to-linear growth of the axial velocity is in accordance with the development of theoretical optimal perturbations (see fig. 8 at Ivanov et al [20]).…”

“…Based on obtained experimental results, we definitely identify the non-modal 'lift-up' growth mechanism of introduced disturbances. The development of perturbations was tracked and compared with the theoretically calculated optimal perturbations [20].…”

“…To initiate the non-modal mechanism of perturbations growth, the cross-section distribution of axial velocity perturbation component could qualitatively replicate the same distribution for calculated optimal perturbations (see fig. 7 at Ivanov et al [20]) containing the zones of particles speeding-up and slowing-down so the longitudinal vortex pairs to create. It is also known that stationary perturbations have larger non-modal growth than non-stationary ones [17].…”