Wake Transition in Coaxial Jets

Abstract

A new flow pattern transition is observed in the near field of high Reynolds number coaxial jets. The configuration consists of a central jet of velocity u1 surrounded by an annular jet of velocity u2, and the study focuses on the limit of large velocity ratios r u = u2/u1. For moderate velocity ratios (i.e. for r u < r uc = 5 typically), the fast annular jet which dominates the near flow field development pinches the central slow jet at the end of the inner potential core at a frequency corresponding to the outer Strouhal jet mode (St = fD2/u2 = 0.35). The structure of the near flow field is strongly dependent on r u . It is observed experimentally that above the critical velocity ratio r uc , the inner core breaks down into an unsteady recirculation bubble. The size of the recirculation region corresponds to the inner jet diameter D1 and the velocity of the reverse flow is proportional to u2 (figure 1). The transition mechanism is explained by a simple model wdiose ingredients are the turbulent entrainment rate, governed by the outer jet, and mass conservation. This model satisfactorily predicts r uc . The transition to this recirculating regime results a wake type instability. The recirculation bubble oscillates periodically at a low frequency (St = fD1/u2 = 0.035) and a large amplitude relatively to Strouhal fundamental mocde (figure 2). This low frequency mode persists until x/D1 ≈ 3 downstream.