scholarly journals The effect of a parallel free surface upon a submerged shallow synthetic jet

2020 ◽  
Author(s):  
Abhay Kumar ◽  
Ashish Karn
Keyword(s):  
Reynolds Number ◽  
Free Surface ◽  
Vortex Ring ◽  
Water Surface ◽  
Primary Objective ◽  
Synthetic Jet ◽  
Vortex Rings ◽  
Beam Laser ◽  
Jet Trajectory ◽  
Water Depths

The interaction of a submerged shallow synthetic jet with a parallel free surface has gathered substantial interest, owing to its relevance to the operation of marine vehicles viz. ships that move close to the water surface. However, despite exhaustive research on the perturbation on a free surface, very few studies have experimentally investigated the effect of unconfined water surface height on the evolution and propagation of a submerged synthetic jet. This study experimentally investigates a synthetic jet submerged in a quiescent flow at shallow depths ejecting parallel to the free surface, through qualitative analysis and quantitative measurements. The qualitative study includes the visualization of the flow using Plane Laser Induced Fluorescence (PLIF) technique, whereas the velocity measurements are carried out by a five-beam Laser Doppler Velocimetry (LDV) probe. The primary objective of these analysis and measurements is to gain a physical insight into the characteristics of vortex ring in a synthetic jet ejected from a fixed orifice at different water depths and at varying Reynolds number. Our studies indicate that the behavior of the vortex rings drastically changes as the depth of the jet crosses a certain threshold. Although no significant change in the path of synthetic jet is observed beyond a threshold depth in our experiments, the jet trajectory shows an interesting dependence on the Reynolds number based on circulation for shallow water depths. It has been found that in the shallow depths, the vortex ring drifts upwards and interacts with the free surface at lower Reynolds number, whereas for larger Reynolds number, the vortex ring rebounds near the free surface and moves downward. Based on our observations, it can be concluded that the phenomenon of upward/downward flection of vortex rings depends both upon its circulation and water depth.

Laminar to Turbulent Buoyant Vortex Ring Regime in Terms of Reynolds Number, Bond Number, and Weber Number

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Xueying Yan ◽  
Rupp Carriveau ◽  
David S. K. Ting
Keyword(s):  
Reynolds Number ◽  
Vortex Ring ◽  
Weber Number ◽  
High Speed ◽  
Reynolds Numbers ◽  
Bond Number ◽  
Vortex Rings ◽  
Transition Regime ◽  
Turbulent Vortex ◽  
Buoyant Vortex Rings

When buoyant vortex rings form, azimuthal disturbances occur on their surface. When the magnitude of the disturbance is sufficiently high, the ring will become turbulent. This paper establishes conditions for categorization of a buoyant vortex ring as laminar, transitional, or turbulent. The transition regime of enclosed-air buoyant vortex rings rising in still water was examined experimentally via two high-speed cameras. Sequences of the recorded pictures were analyzed using matlab. Key observations were summarized as follows: for Reynolds number lower than 14,000, Bond number below 30, and Weber number below 50, the vortex ring could not be produced. A transition regime was observed for Reynolds numbers between 40,000 and 70,000, Bond numbers between 120 and 280, and Weber number between 400 and 800. Below this range, only laminar vortex rings were observed, and above, only turbulent vortex rings.

Formation of vortex rings from falling drops

1967 ◽  
Vol 29 (1) ◽  
pp. 177-185 ◽  
Author(s):  
David S. Chapman ◽  
P. R. Critchlow
Keyword(s):  
Reynolds Number ◽  
Vortex Ring ◽  
Liquid Drop ◽  
Prolate Spheroid ◽  
Ring Formation ◽  
Vortex Rings ◽  
Surface Tensions ◽  
Wide Range ◽  
The Moment

A study of the formation of vortex rings when a liquid drop falls into a stationary bath of the same liquid has been made. The investigation covered liquids with a wide range in surface tensions, densities and viscosities. The results confirm the reported existence of optimum dropping height from which the drop develops into a superior vortex ring. The optimum heights are analysed, by a photographic study, in terms of the liquid drop oscillation. It is found that vortex rings are formed best if the drop is spherical and changing from an oblate to a prolate spheroid at the moment of contact with the bath. A Reynolds number has been determined for vortex rings produced at optimum dropping heights; these numbers are approximately 1000. A possible mechanism for the ring formation is suggested.

scholarly journals Investigation of Vortex Rings for Free Jet and Synthetic Jet at Various Reynolds Numbers and Strouhal Numbers

2020 ◽  
Vol 2020 ◽  
pp. 1-15
Author(s):  
Jianlong Chang ◽  
Shizhen Zheng ◽  
Yang Du ◽  
Junjie Xu ◽  
Yanchen Liu ◽  
...  
Keyword(s):  
Vortex Ring ◽  
Strouhal Number ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Vortex Rings ◽  
Close Link ◽  
Free Jet ◽  
Good State ◽  
Break Up

The behaviors of vortex rings for free jet and synthetic jet with various Reynolds numbers and Strouhal numbers are numerically studied by the k-ε model. The positions of the leading vortex of free jet are investigated under the three different conditions, and the effect of Strouhal number on the vortex ring is analyzed in detail. The results show that different Reynolds numbers lead to different positions for the shedding of the vortex rings. During the movement of the vortex rings, the symmetry of the vortex ring in synthetic jet retains in good state, and the vortex rings do not break up with well-arranged distributions. The distance between two adjacent vortex rings near the synthetic jet exit has the close link between the Strouhal number yet is independent of the Reynolds numbers.

Evolution of elliptic synthetic jets at low Reynolds number

2019 ◽  
Vol 868 ◽  
pp. 66-96 ◽  
Author(s):  
Xu-Dong Shi ◽  
Li-Hao Feng ◽  
Jin-Jun Wang
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Ring ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Stereoscopic Particle Image Velocimetry ◽  
Entrainment Rate ◽  
Streamwise Vortices ◽  
Primary Vortex ◽  
Stroke Length

The influence of the nozzle aspect ratio ($AR=1$, 2 and 4), stroke length ($L_{0}=1.85$, 3.7 and 5.55) and Reynolds number ($Re=79$, 158, 316 and 632) on the behaviour of elliptic synthetic jets is studied experimentally. Laser-induced fluorescence and two-dimensional and stereoscopic particle image velocimetry are used to analyse the vortex dynamics and evolution mechanism. It is found that the fluid elements around the major axis of an elliptic vortex ring move downstream faster and tend to approach the centreline, while the fluid elements around the minor axis move downstream at a slower speed and away from the centreline, thereby resulting in the occurrence of the well-known axis-switching phenomenon for elliptic synthetic jets. During this process, a pair of arc-like vortices forms ahead of the primary vortex ring, and they are constituted by streamwise vortices in the leg part and spanwise vortices in the head part; two pairs of streamwise vortices form from the inside of the primary vortex ring and develop in the tails. The streamwise vortices are pushed away progressively from the centreline by the synthetic jet vortex rings that are formed during the subsequent periods. These additional vortical structures for non-circular synthetic jets show regular and periodic characteristics, which are quite different from the previous findings for non-circular jets. Their mutual interaction with the vortex ring causes significant changes in the topology of elliptic synthetic jets, which further results in the variation of the statistical characteristics. Increasing the aspect ratio, stroke length and Reynolds number will make the evolution of the synthetic jet become more unstable and complex. In addition, the entrainment rate of an elliptical synthetic jet is larger than that of a circular synthetic jet and it increases with the nozzle aspect ratio ($AR\leqslant 4$) and Reynolds number. It is indicated that the formation of streamwise vortices could enhance the entrainment rate. This finding provides substantial evidence for the potential application of elliptic synthetic jets for effective flow control.

scholarly journals Turbulent Vortex Ring/Free Surface Interaction

1995 ◽  
Vol 117 (3) ◽  
pp. 374-381 ◽  
Author(s):  
A. Weigand ◽  
M. Gharib
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Surface Interaction ◽  
Vortex Rings ◽  
Small Scale ◽  
Turbulent Vortex ◽  
Turbulent Transition ◽  
Vortex Loops ◽  
Scale Structures ◽  
Small Scale Structures

The interaction of turbulent vortex rings that approach a clean water surface under various angles is experimentally investigated. The temporal evolution of the vortex rings with an initial Reynolds number of Re0 = 7500 is characterized by the laminar/turbulent transition and asymptotic relaminarization of the flow. Using the shadowgraph technique, two major flow cases were identified as a result of the vortex-ring/free-surface interaction: a trifurcation case that results from the interaction during the transition stage, and a bifurcation case that evolves during the fully-developed turbulent stage. In contrast to the laminar interaction, the turbulent bifurcation pattern is characterized by the reconnection and mutual interaction of many small-scale structures. Simultaneous digital particle image velocimetry (DPIV) and shadowgraph measurements reveal that the evolution of the small-scale structures at the free surface is strongly dominated by the bifurcation pattern, which in turn is a consequence of the persisting laminar sublayer in the core regions of the reconnected turbulent vortex loops.

The evolution of an elliptic vortex ring

1981 ◽  
Vol 109 ◽  
pp. 189-216 ◽  
Author(s):  
M. R. Dhanak ◽  
B. DE Bernardinis
Keyword(s):  
Reynolds Number ◽  
Vortex Ring ◽  
Ideal Fluid ◽  
Vortex Rings ◽  
Core Size ◽  
Vorticity Distribution ◽  
The Core ◽  
Moderate Reynolds Number ◽  
Break Up

The evolution of a vortex ring in an ideal fluid under self-induction from a flat and elliptic configuration is followed numerically using the cut-off approximation (Crow 1970) for the velocity at the vortex. Calculations are presented for four different axes ratios of the initial ellipse. A particular choice is made for the core size and vorticity distribution in the core of the vortex ring. When the initial axes ratio is close to 1, the vortex ring oscillates periodically. The periodicity is lost as more eccentric cases are considered. For initial axes ratio 0·2, the calculations suggest a break-up of the ring through the core at one portion of the ring touching that at another, initially distant, portion of the ring.Results from quantitative experiments, conducted at moderate Reynolds number with the vortex rings produced by puffing air through elliptic orifices, are compared with the calculations. The agreement is fairly good and it is found that a vortex ring produced from an orifice of axes ratio 0·2 breaks up into two smaller rings. The relevance of the results to the vortex trail of an aircraft is discussed.

Electronic Cooling Using Synthetic Jet Impingement

2006 ◽  
Vol 128 (9) ◽  
pp. 897-907 ◽  
Author(s):  
Anna Pavlova ◽  
Michael Amitay
Keyword(s):  
Reynolds Number ◽  
Low Frequency ◽  
Jet Impingement ◽  
Constant Heat Flux ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Electronic Cooling ◽  
Vortex Rings ◽  
Jet Formation ◽  
Image Velocimetry

The efficiency and mechanisms of cooling a constant heat flux surface by impinging synthetic jets were investigated experimentally and compared to cooling with continuous jets. Effects of jet formation frequency and Reynolds number at different nozzle-to-surface distances (H∕d) were investigated. High formation frequency (f=1200Hz) synthetic jets were found to remove heat better than low frequency (f=420Hz) jets for small H∕d, while low frequency jets are more effective at larger H∕d. Moreover, synthetic jets are about three times more effective in cooling than continuous jets at the same Reynolds number. Using particle image velocimetry, it was shown that the higher formation frequency jets are associated with breakdown and merging of vortices before they impinge on the surface. For the lower frequency jets, the wavelength between coherent structures is larger such that vortex rings impinge on the surface separately.

scholarly journals Scalings for rectangular synthetic jet trajectory in a turbulent boundary layer

2021 ◽  
Vol 915 ◽  
Author(s):  
Girish K. Jankee ◽  
Bharathram Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Synthetic Jet ◽  
Jet Trajectory

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