S. C. Luo

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...

Characteristics of square cylinder wake transition flows

The wake transition regime in square cylinder flow is investigated experimentally. Mode A and B transitions similar to those captured in circular cylinder flows are found to exist in this regime and by using dye- and laser-induced fluorescence visualization, the spanwise and streamwise vortex structures are captured. Upon comparison with their corresponding modes in circular cylinder flows, some differences are noted. Most notably, the critical Reynolds numbers at which Mode A and B occur for square cylinder flows were estimated to be approximately 160 and 200, respectively, and are lower than those found in circular cylinder flows, which are generally agreed to be approximately 188–190 and 230–260, respectively. Also, the spanwise wavelengths for the two modes in square cylinder flows (5.2 Ds and 1.2 Ds for Modes A and B, respectively) are longer than their counterparts in circular cylinder flows (3–4Do and 0.8–1Do, respectively). Furthermore, by using a hot wire to measure the shedding frequency, it was...

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.

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.

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

Side Force on an Ogive Cylinder: Effects of Control Devices

This study is an extension of our earlier work, which examined the effectiveness of using an elliptic tip to control the side force acting on an ogive cylinder. In that study, only one tip was considered, and thus the effect of tip eccentricity on the side force was not known. In the present study, we examine another elliptic tip of a smaller eccentricity to get an insight into how tip eccentricity affects the local and overall side force distribution. Our measurements show that, although the smaller eccentricity tip has a side force distribution similar to that of the larger eccentricity tip, there are some major differences in their flow characteristics. For example, the larger eccentricity tip is found to reduce the onset angle of attack and delay the disappearance of the side force to a higher angle of attack. Furthermore, when α 60 deg, only the lower eccentricity tip displays a hysteresis effect in its side force distribution. To the best of our knowledge, this phenomenon has not been observed on an elliptic tip before, even though a similar phenomenon has been observed on a conical body with a rounded tip when the cone was subjected to unsteady bleeding

On the relation between centrifugal force and radial pressure gradient in flow inside curved and S-shaped ducts

Swirl flow in a curved duct and S-duct is governed by a centrifugal force and radial pressure gradient force between the sidewalls. In the present work, we introduce a dimensionless parameter that relates the ratio of these two forces and the duct’s center-line distance and show how the parameter is related to other more familiar dimensionless terms such as the pressure coefficient, Reynolds number, and Dean number. By using published data as well as our own measurements, it is shown that the data collapse reasonably well on a curve when the proposed parameter is plotted against dimensionless distance along the duct. The existence of collapsed curves for ducts of different curvature ratios indicates that the proposed parameter can be used to characterize the flow, although some scatter in the data exists, due to the presence of flow separation and streamwise vortices along the wall of the curved ducts. An attempt to suppress flow separation by using vortex generators in the S-duct leads to some improvement in the collapsed curve at the first bend of the duct. The results reported here are mainly focused on square and circular constant cross-sectioned 90° curved ducts and S-shaped ducts of different curvature ratios.

Three Techniques to Control Flow Separation in an S-Shaped Duct

The optimum aerodynamic performance of an S-shaped duct (or, more specifically, aircraft air-intake duct) demands relatively uniform flow in the duct withminimal swirl at the exit. Here, three different flow controlmethods are examined for a large curvature S-shaped duct of uniform square cross section; they are vortex generators, variable directional tangential blowing, and vortex generator jets. The flow control devices were implemented on a common test rig with the same test parameters and hence provide amore accurate comparison of their performance. While these devices are effective at suppressing flow separation at the inside wall of the first bend and reducing total pressure loss, it is shown that this happens at the expense of higher swirl at the duct exit. The increase in the swirl can be attributed partly to the increase in the radial pressure difference (or imbalance) between the side walls and partly to changes in the configuration and position of streamwise vortices near the outside wall of the second bend. The investigation thus illustrates competing aerodynamic performance when flow control devices are implemented in an S-shaped duct.

Aerodynamic forces and moments on an ogive cylinder at incidence

In flows over an ogive cylinder placed at incidence, it is well documented that a side force acts on the cylinder. However, little is known about the relation between the side force (C Fy ) and other force and moment components. In this note, the variations of three force and three moment components, acting on the ogive cylinder, with roll angle are measured simultaneously. The model was placed at four different angles-of-attack, namely a = 30°, 45°, 50° and 60°

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.