T. T. Lim

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...

Flow Visualization, Techniques and Examples

Keywords: ecoulement ; experimental Reference Record created on 2005-11-18, modified on 2016-08-08

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.

Wake-Structure Formation of a Heaving Two-Dimensional Elliptic Airfoil

This paper is prompted by a recent numerical study that shows that for a two-dimensional (2-D) elliptic airfoil undergoing prescribed heaving motion in a viscous fluid, both leading-edge vortices and trailing-edge vortices contributed to the formation of the wake structures. However, an earlier dye-visualization study on a heaving NACA 0012 airfoil appears to show that the wake structures were derived from trailing-edge vortices only. The dissimilarity in the two studies remains unclear because there is no corresponding experimental data on a 2-D heaving elliptic airfoil. In this study, digital particle image velocimetry technique was used to investigate the wake-structure formation of a 2-D elliptic airfoil undergoing simple harmonic heaving motion. For the range of flow conditions investigated here, our results show that the type of wake structures produced is controlled by when and how the leading-edge vortices interact with the trailing-edge vortices

Flowfield Around Ogive/Elliptic-Tip Cylinder at High Angle of Attack

We present the results of experimental investigations on the e owe eld around a conventional sharp-nose ogive cylinder and an elliptic-tip ogive cylinder. The studies include simultaneous side-force and surface pressure mea- surements in a wind tunnel as well as e ow visualization in a water tunnel. The results show that changes in the direction of the side force are related to changes in the asymmetry of the pressure distribution along the body. Of the two tip shapes investigated, it is found that the variation of the side forcewith the roll anglefor theelliptictip is more predictable than that for the sharp ogive tip. Although the e ow visualization study shows that the elliptic-tip cylinder with the major axis transverse to the freestream is more effective in delaying the onset of e ow asymmetry to a higher angle of attack, the maximum side forces for the two tip geometries are almost the same.

On the breakdown of vortex rings from inclined nozzles

In a recent experimental investigation, Webster and Longmire [Phys. Fluids 9, 655 (1997)] reported that the large-scale jet structures from inclined nozzles, which consisted of continuous inclined vortex rings, would undergo breakdown if the inclined angle of the nozzle was sufficiently large. They attributed the breakdown to the presence of longitudinal vorticity, but did not elaborate on the mechanism involved. In this paper, we examined the above issue by focusing primarily on the large-scale structures of the inclined jet (i.e., the inclined vortex rings). To the author’s knowledge, this area of research remains relatively unexplored. A study of it would certainly help to shed light on the mechanism involved in the breakdown of the inclined jets. Here we investigated the effects of the Reynolds number, the nozzle’s inclined angle, and L/D (see below) on the evolution of inclined vortex rings. Nozzles with the inclined angle of 5°, 10°, 20°, and 45° were considered, and the Reynolds number of the flow ...

A new flow regime in a Taylor–Couette flow

In this Brief Communication, we report a new finding on a Taylor–Couette flow in which the outer cylinder is stationary and the inner cylinder is accelerated linearly from rest to a desired speed. The results show that when the acceleration (dRe/dt) is higher than a critical value of about 2.2 s−1, there exists a new flow regime in which the flow pattern shows remarkable resemblance to regular Taylor vortex flow but is of shorter wavelength. However, when the acceleration is lower than 2.2 s−1, a wavy flow is found to occur for the same Reynolds number range. To our knowledge, this is probably the first time that such a phenomenon has been observed. For completeness, the case of a decelerating cylinder is also investigated, and the results are found to be almost the same.

A note on the leapfrogging between two coaxial vortex rings at low Reynolds numbers

In the early 1970s, Maxworthy [J. Fluid. Mech. 51, 15 (1972)] and Oshima et al. [J. Phys. Soc. Jpn. 38, 1159 (1975)] attempted to produce leapfrogging between two coaxial vortex rings at low Reynolds numbers in their laboratories. However, the rings failed to undergo the classical leapfrogging behavior and merged to form a single ring. In a recent numerical study, Shariff et al. (NASA Tech. Memo. 102257, 1989) attributed the failure partly to the effect of core distortion. They pointed out that vortex cores at low Reynolds numbers are thicker and therefore more susceptible to distortion during the leapfrogging. In this Brief Communication, it is shown through a systematic investigation that the initial generating condition also plays an important role in determining the success or failure of leapfrogging. The common belief that leapfrogging can be achieved simply by generating two vortex rings in quick succession may not be true for all flow conditions, especially when the Reynolds number is low. This finding may help to explain the results of Maxworthy and Oshima et al.

The interaction of the piston vortex with a piston-generated vortex ring

This paper presents the results of an experimental investigation of the effects of a piston vortex on the vorticity evolution of a vortex ring. The rings are produced by the roll-up of a shear layer at a circular orifice in a plane wall and have a Reynolds number of 2000 based on the ejection velocity and orifice diameter. The generation mechanism is a piston moving inside a cylinder with a stroke length of two piston diameters. The experimental apparatus is similar to that used by Glezer & Coles (1990) where the piston finishes flush with the orifice, with the result that a piston vortex produced by the apparatus interacts with the vortex ring. Instantaneous velocity field measurements using cross-correlation digital particle image velocimetry reveal that the piston vortex not only increases the circulation of the ring but also creates an asymmetric vorticity distribution of the vortex core. It is found that ‘imperfect’ merging of the piston vortex with the primary vortex ring promotes the growth of an instability which leads to early transition to turbulence of initially laminar vortex rings.

The impact of a vortex ring on a porous screen

In this paper, we re-examine the issue raised by Morton (2000) regarding the impact of a vortex ring on a porous screen. We show, through a systematic experiment, that Mortons observation can only occur at very low Reynolds number or above a certain critical Reynolds number, which is dependent on the porosity of the screen. In particular, we found that a primary vortex ring of moderate Reynolds number impacting a porous screen can also undergo vortex rebound similar to that observed in vortex ring–solid wall interaction. At a higher Reynolds number, the self-induced flow of the primary vortex ring around the axis of symmetry passes through the screen as a jet-like flow, which later regenerates into a new vortex ring of the same sign but weaker circulation. At an even higher Reynolds number, the primary vortex ring has sufficient energy to pass through the screen via a possible mechanism of vortex stretching and reconnection, and continues as a modified ring in its lee.