[show abstract][hide abstract] ABSTRACT: The present study report direct numerical simulation (DNS) of a circular jet and the effect of a large scale perturbation at the jet inlet. The perturbation is used to control the jet for increased spreading. Dual-mode perturbation is obtained by combining an axisymmetric excitation with the helical. In the fluid dynamics videos, an active control of the circular jet at a Reynolds number of 2000 for various frequency ratios (both integer and non-integer) has been demonstrated. When the frequency ratio is fixed to 2, bifurcation of the jet on a plane is evident. However, for a non-integer frequency ratio, the axisymmetric jet is seen to bloom in all directions. Comment: 2 pages
[show abstract][hide abstract] ABSTRACT: Abstract Direct numerical simulation of incompressible, spatially developing round and square jets at a Reynolds number of 1,000 is
performed. The effect of two types of inlet perturbation on the flow structures is analyzed. First, dual-mode excitation,
which is a combination of axisymmetric perturbation at preferred mode frequency and helical perturbation at sub-harmonic frequency
is used, having a disturbance frequency ratio equal to R
= 2. It is observed that the circular and square jets bifurcate and spread on one of the orthogonal planes forming a Y-shape
jet in the downstream while no spreading is visible on the other plane. The second type of perturbation is a flapping excitation
at a sub-harmonic frequency, St
F = 0.2. It leads to a Y-shape bifurcation for both square and circular jets. On the other hand, for flapping excitation at
the preferred mode frequency, namely, St
F = 0.4, a circular jet bifurcates into a Ψ-shape whereas the square jet reveals simple spreading.
Journal of Visualization 01/2010; 13:141-149. · 0.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: Direct numerical simulation (DNS) of incompressible, spatially developing circular jets in the Reynolds number range of 500–2000 is reported. The flow field has been explored by solving three-dimensional unsteady Navier–Stokes equations using higher order spatial and temporal discretization. The main objective of the present work is to predict the transition sequence without resorting to stability analysis and understand the role of instabilities that appear during the evolution of a circular jet at various Reynolds numbers. In most calculations, small-scale perturbations are superimposed over the incoming velocity profile. Flow patterns corresponding to individual instabilities are identified with the help of vortical structures. The DNS calculation reveals the progressive appearance of instabilities with increasing Reynolds number. The present study also shows that the point of appearance of jet instability depends crucially on Reynolds number. In addition, the critical points are dependent on the small scale perturbation superimposed on the inflow velocity. The critical Reynolds numbers for the onset on unsteadiness of unperturbed and perturbed jets fall in the range of 900–925 and 500–525 respectively. The most amplified mode during evolution switches from the helical to the axisymmetric as the Reynolds number increases from 1000 to 2000. The ratio of jet half-width and the local momentum thickness, namely δ1/2/θ, changes consistently with the instability modes. Small-scale perturbations at the inlet cause early saturation of the KH-instability and hence early transition to turbulence closer to the nozzle exit. These factors result in faster decay of the centerline velocity and higher jet spreading.