Planar dense wakes can be found in many industrial processes, such as combustion and paper-making. Confinement is known to make such wakes more locally absolutely unstable, but this destabilizing effect has not been comprehensively examined in real wakes bounded by a finite streamwise domain. For example, it is not known (i) how the confinement length and confinement ratio influence the dense wake stability and (ii) what the critical values of other operating parameters such as shear ratio should be for global instability to occur. In this experimental study, we try to answer these questions by examining a planar dense wake consisting of a central stream of CO₂ (dense gas) sandwiched by two identical outer streams of air (light gas). The wake is confined by solid walls of variabl...[ Read more ]

Planar dense wakes can be found in many industrial processes, such as combustion and paper-making. Confinement is known to make such wakes more locally absolutely unstable, but this destabilizing effect has not been comprehensively examined in real wakes bounded by a finite streamwise domain. For example, it is not known (i) how the confinement length and confinement ratio influence the dense wake stability and (ii) what the critical values of other operating parameters such as shear ratio should be for global instability to occur. In this experimental study, we try to answer these questions by examining a planar dense wake consisting of a central stream of CO₂ (dense gas) sandwiched by two identical outer streams of air (light gas). The wake is confined by solid walls of variable length, which act as an adjustable confinement. We find that the confinement has a strong influence on the hydrodynamic stability of the wake: (a) self-excited global oscillations appear only when the confinement length exceeds a critical value and (b) the planar dense wake is most globally unstable when moderately confined, at a confinement ratio of h = 0.7 and h = 1. The shear ratio also has significant influence on the wake stability. Knowledge of determining the confinement ratio and confinement length for global instability to occur under various conditions could be useful for optimizing industrial processes.