Vortex control of bifurcating jets: A numerical study

Abstract

Direct and large-eddy simulations (DNS/LES) are performed to analyze the vortex dynamics and the statistics of bifurcating jets. The Reynolds number ranges from ReD=1.5×103 to ReD=5.0×104. An active control of the inlet conditions of a spatially evolving round jet is performed with the aim of favoring the jet spreading in one particular spatial direction, thus creating a bifurcating jet. Three different types of forcing, based on the information provided by a LES of a natural (unforced) jet, are superimposed to the jet inlet in order to cause its bifurcation. The different forcing types mimic the forcing methods used in experimental bifurcating jets (Lee and Reynolds, Parekh et al., Suzuki et al.), but using excitations with relatively low amplitudes, which could be used in real industrial applications. The three-dimensional coherent structures resulting from each specific forcing are analyzed in detail and their impact on the statistical behavior of bifurcating jets is explained. In particular we focus on the influence of the forcing frequency and of the Reynolds number on the jet control efficiency. Analysis of the coherent vortex dynamics shows that an inlet excitation which combines an axisymmetric excitation at the preferred frequency and a so-called flapping excitation at the subharmonic frequency is the most efficient strategy for jet control, even at high Reynolds numbers.

Parametric Analysis of Excited Round Jets - Numerical Study

“…Numerical simulations performed with Direct Numerical Simulation (DNS) [10][11][12][13] and Large Eddy Simulation (LES) [14][15][16] reproduced and confirmed these experimental results. Moreover, new findings were revealed, among others, for low Reynolds number jets (Re = 1500) the flapping excitation is sufficient to obtain jet bifurcation and there is no need to also include the axial excitation [12] as was later confirmed in [15]. Such behaviour was attributed to the existence of strong primary vortex rings which at low Reynolds number are not distorted by natural turbulence or flapping forcing.…”

“…In the present study we add the excitation as a component of the velocity prescribed at the inlet. Similarly as in [12,15,16] the inlet velocity profile used in the present work is defined as:…”

“…Such behaviour was attributed to the existence of strong primary vortex rings which at low Reynolds number are not distorted by natural turbulence or flapping forcing. In contrast, at higher Reynolds numbers the flapping excitation rapidly distorts these vortices and therefore axial forcing is included to amplify the vortices and retain the two-fold split jet [12,15]. The effect of the forcing frequency was studied in [15] with respect to alteration of the flapping frequency f h only, while f a was kept equal to f p .…”

“…In contrast, at higher Reynolds numbers the flapping excitation rapidly distorts these vortices and therefore axial forcing is included to amplify the vortices and retain the two-fold split jet [12,15]. The effect of the forcing frequency was studied in [15] with respect to alteration of the flapping frequency f h only, while f a was kept equal to f p . A different setting of the forcing frequencies was studied in [13] where a combination of the axial and flapping forcing was used as an optimisation parameter.…”

Turbulent jet manipulation using two unsteady azimuthally separated radial minijets

“…process is also observed in the numerical study of a bifurcating jet [40]. In that study, it was reported that the jet splits into two separate jets.…”