T. H. New

On the development of large-scale structures of a jet normal to a cross flow

It is well known that vortex rings are the dominant flow structures in the near field of a free jet, and this has led many researchers to believe that they also occur in a jet in cross flow (JICF). Previous studies have postulated that these vortex rings deform and fold as they convect downstream, which culminates in the formation of vortex loops at both the upstream and the lee-side of the jet column. In this paper, we take a fresh look at the vortical structures of JICF in water by releasing dye at strategic locations around the jet exit. The results show that there is no evidence of ring vortices in JICF, and the postulation that vortex loops are formed from the folding of the vortex rings does not reflect the actual flow behavior. The presence of a counter-rotating vortex pair (CVP) at the jet exit is found to inhibit the formation of the vortex rings. Instead, vortex loops are formed directly from the deformation of the cylindrical vortex sheet or jet column, without going through the vortex rings, i...

Elliptic jets in cross-flow

Flow structures of an elliptic jet in cross-flow were studied experimentally in a water tunnel using the laser-induced fluorescence technique (LIF), for a range of jet aspect ratio ( AR ) from 0.3 to 3.0, jet-to-cross-flow velocity ratio ( VR ) from 1 to 5, and jet Reynolds number from 900 to 5100. The results show that the effects of aspect ratio (or jet exit orientation) are significant only in the near field, and diminish in the far field which depends only on gross jet geometry. For low-aspect-ratio jets, two adjacent counter-rotating vortex pairs (CVP) are initially formed at the sides of the jet column, with the weaker pair subsequently entrained by the stronger pair further downstream. For high-aspect-ratio jets, only one CVP is formed throughout the jet column, but the shear layer develops additional folds along the windward side of the jet. These folds subsequently evolve into smaller scale counter-rotating vortex pairs, which we refer to as windward vortex pairs (WVP). Depending on its sense of rotation, the WVP can evolve into what Haven & Kurosaka (1997) referred to as unsteady kidney vortices or anti-kidney vortices, or, under some circumstances, interconnecting kidney vortices, which have not been reported previously. While Haven & Kurosaka (1997)s interpretation of the formation of kidney and anti-kidney vortices is topologically feasible, our observation reveals a slightly different formation process. Despite the differences in the near-field flow structures for different jet aspect ratios, the process leading to the formation of the large-scale jet structures (i.e. leading-edge vortices and lee-side vortices) for all cases is similar to that reported by Lim, New & Luo (2001) for a circular jet in cross-flow.

Influence of nozzle sharpness on the flow fields of V-notched nozzle jets

This study reports on an experimental investigation on laminar jets discharging from V-notched nozzles of different relative sharpness. Each nozzle consists of two smooth peaks and two sharp troughs and its sharpness is defined by the aspect ratio (AR) of the half ellipses which make up each half of the V-notch. Two ARs of AR=2 and 4 are used here to look at how changes in relative sharpness affect the vortex dynamics and quantifiable flow characteristics. Flow visualization results reveal the formation of coherent streamwise vortices at both peaks and troughs, as well as the bending of the large-scale vortex roll-ups. Digital particle-image velocimetry measurements reveal that peaks produce vortex roll-ups which are stronger than those associated with the troughs and that their strength difference increases with nozzle sharpness. Correspondingly, flow stresses show that the peaks and troughs tend to confer more significant effects upon the Reynolds normal stresses when nozzle sharpness increases, with Re...

Vortical structures in a laminar V-notched indeterminate-origin jet

A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a V-notched indeterminate-origin jet with two peaks and two troughs. The laminar jet (Re=2000) was studied under forcing and nonforcing conditions to investigate the resultant dynamics of coherent large- and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex pairs which spread outwards into the ambient fluid and the troughs generating similar vortex pairs but entraining ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex pairs. Laser cross sections further showed that the subsequent formation of azimuthal ring vortices causes these streamwise vortex pairs t...

Multigrid CCDPIV measurements of accelerated flow past an airfoil at an angle of attack of 30

Abstract Multigrid cross-correlation digital PIV (MCCDPIV) technique is used to measure the flow field around a NACA 0015 airfoil at a 30° angle of attack when the flow is accelerated to a constant velocity from a quiescent state. The experiments were conducted in an acceleration water tunnel using uniform accelerations of 50 and 100 mm/s 2 . The final uniform velocity was 100 mm/s for both cases, and the Reynolds number based on this velocity and the chord length of the airfoil was about 8000. The MCCDPIV measurements were carried out using a digital 2048 px×2048 px CCD camera to record single exposed images of seed particles illuminated with a Nd:YAG laser. These measurements have revealed a rich and complex unsteady flow structure, during both the acceleration phase and the constant velocity post-acceleration phase.

Scaling of Trajectories of Elliptic Jets in Crossflow

A jet in crossflow is a fundamental flow phenomenon that is important to a variety of engineering applications, such as aerodynamic flow control, film cooling of turbines and combustors, and jet-mixing enhancements, just to name a few. Over the past 60 years, numerous experimental and computational studies have been conducted on various aspects of the flowfield with much of the attention focused on the large-scale flow structure development, jet trajectories, scalar-mixing and transport properties, and other associated flow phenomena [1–27]. Although the scaling of the trajectories of a circular jet in crossflow has been studied for many years, there is still no generally accepted scaling parameter for the jet trajectory. The scaling parameters that have been proposed by researchers include d (by Kamotani and Greber [1], andChassaing et al. [2]), rd (byPratte andBaines [3]) and rd (byKeffer andBaines [4]), where r is the velocity ratio defined as jet velocity/crossflow velocity. Pratte and Baines [3], who derived the rd-scaling-based dimensional analysis, argued that the jet trajectory should not be normalized by d or rd, and went on to propose the power-law formulation

Dynamics of laminar circular jet impingement upon convex cylinders

Flow dynamics associated with a laminar circular jet impinging upon a convex cylinder has been investigated by laser-induced fluorescence and digital particle-image velocimetry techniques. Cylinder-to-jet diameter ratios of 1, 2, and 4 were investigated, while the jet-to-cylinder separation distance was kept at four jet diameters throughout. Flow visualization and λ2 criterion results show that once the jet ring-vortices impinge upon the cylindrical surface, they move away from the impingement point by wrapping themselves partially around the surface. As the cylinder diameter increases, wall boundary layer separation, vortex dipole formation, and separation locations are initiated earlier along the cylindrical surface, producing significantly larger wakes. Along the cylinder straight-edges, ring-vortex cores are significantly smaller after impingement. This is due to accentuated vortex-stretching caused by partial wrapping around the cylindrical surface by the ring-vortices, on top of their movement away ...

Effects of Corrugated Aerofoil Surface Features on Flow-Separation Control

W ITH increased awareness of the potential engineering benefits in emulating certain aspects of insect wings or insect flight mechanics, it is not surprising that there is recent surge of interest in their investigations for lift generation or stall mitigation. Other than exploring how the exact flapping/heaving mechanisms employed by insects contribute toward their agility during flight [1–8], understanding how unique surface geometries and features of insect wings enable these insects to maneuver the way they do is also one of the major research motivations for some recent studies. Of interest to the present study are investigations conducted by Hu and Tamai [9], Murphy and Hu [10], and Levy and Seifert [11] recently, where they looked at the flow dynamics of aerofoils based on dragonfly wing cross sections. Hu and Tamai [9] and Murphy and Hu [10] studied corrugated aerofoils with cross sections resembling typical dragonfly wing cross sections and observed favorable aerodynamic behavior. They noted that flow-separation vortices trapped within the corrugationvalleys draw fluid toward the aerofoil wall region and reduce the overall extent of the flow-separation region. These unique flow features mean that flow separations can be delayed until a higher angle of attack with accompanying increases in lift-to-drag ratios for these corrugated aerofoils up to a chord Reynolds number of Re 125; 000. On the other hand, the corrugated aerofoil studied by Levy and Seifert [11] had far fewer corrugations. Instead, their aerofoil had only two corrugations close to the leading edge, followed by a “saddle” and convex trailing-edge “hump”. Because of this geometric difference, the mechanisms with which this aerofoil is able to delay flow separation are different. In this case, flow separations arising from the upstream corrugations reattach back to the trailingedge hump regularly, which translates into fewer flow-separation events propagating beyond the trailing edge. In particular, a recirculating vortex is observed to form at the saddle, which is believed to play an important role in controlling flow separation. It should be mentioned that Hu and Tamai [9] performed their experiments at Re 34; 000, and Murphy and Hu [10] conducted theirs at Re 58; 000 to 125,000, whereas Levy and Seifert [11] performed their investigations atRe < 8000. In addition, the ranges of angle of attack investigated between these studies were also different. It is clear from the earlier studies that the vortex formation and behavior along the upper surfaces of corrugated aerofoils drive the favorable flow effects seen so far. Although some insights into their behavior have been provided by the earlier studies, direct comparisons between them were difficult due to the different test conditions used. To do that, they have to be studied under similar flow conditions, and this provided the primary motivation for the present study. To accomplish that, an experimental flow visualization and particle image velocimetry (PIV) investigation was performed in this study to compare the differences in the near-field vortical behavior and the extent to which flow separation is mitigated between these two corrugated aerofoils at a fixed chord Reynolds number of Re 14; 000. The use of a relatively lowReynolds number herewill not only provide additional insights into the basic aerodynamic characteristics of dragonfly wings but also shed light on the use of different corrugated aerofoils in micro aerial vehicles as well.

Effects of noncircular collars on an axisymmetric jet

An experimental investigation was carried out to study the effects of noncircular collars on an axisymmetric jet using surface flow visualization and hot-wire anemometry. Circular, square, and triangular collars with expansion ratios of 1.20, 1.35, and 1.54, respectively, with collar lengths of up to two jet diameters were used. Flow visualization shows that circular collars led to equidistant flow reattachments along the collar wall, while square and triangular collars resulted in the formation of a pair of counter-rotating vortex-pairs on each side of the collar wall. These vortex-pairs are caused by the presence of the three-dimensional velocity gradients between locations of minimum and maximum step-heights, which drove fluid from the collar wall centerlines towards the corners. Time-averaged velocity measurements show that the circular collar required the shortest collar length to achieve maximum centerline velocity decay, followed by square and triangular collars. Centerline turbulence intensity and...

A Visual Study on Elliptical Jets in Cross Flow

Laser-induced fluorescence technique is used to study the near-field of an elliptical jet exiting normally into a cross flow, for elliptic jet having aspect ratio of 2 and 3. Results show that the non-uniform curvature of the elliptic geometry causes the leading-edge vortices at the interface between the jet and the cross flow to behave differently from that of a circular jet. In particular, when the major-axis is aligned with the cross flow there is an intense interaction between the leading-edge vortices which culminates in the pairing of adjacent vortices. The pairing is suppressed when the major axis is perpendicular to the cross flow.