Numerical simulation of forced circular jets: Effect of flapping perturbation

Abstract

The forcing in round jets has large scale effects on the development and characteristics of the flow field such as the entrainment, spreading rate, and mixing of the flow. In the present work, an active control technique called the large scale flapping perturbation is studied for influencing the evolution of the circular jet. Direct numerical simulation of incompressible, spatially developing circular jets at a Reynolds number of 1000 is reported. The flow field has been explored by solving three-dimensional unsteady Navier-Stokes equations using second order spatial and temporal discretization. Among the considered variables (apart from the excitation amplitude), the perturbation frequency is the most important controlling parameter. For a circular jet at an excitation frequency of 0.1, there is evidence of a Y-shaped bifurcation on the bifurcating plane, while no bifurcation is observed on the other orthogonal plane. With an increase in the excitation frequency to 0.3, the circular jet issued from the orifice divides itself into three parts and a ψ-shaped bifurcation is obtained on the bifurcating plane. The development of the ψ-shaped bifurcation may be due to the interaction of the consecutive vortex ring and the formation of elongated vortices. However, with further increase in the excitation frequency, the spreading rate becomes weaker with the evidence of formation of the ψ-shaped bifurcation.