Heat transfer surface enhancement through the use of longitudinal vortices: a review of recent progress

The mean streamwise development of pairs of longitudinal vortices embedded in an otherwise two-dimensional turbulent boundary layer was studied. Planes of closely spaced measurements of the three components of mean velocity were obtained at several streamwise locations, and the vorticity and circulation were calculated. Skin-friction measurements were also made. It was found that the rate of vorticity spreading in a vortex was greatly increased by close proximity of other vortices. The rate of streamwise circulation decrease was significantly greater for corotating vortices than for counter rotating vortices. Boundary-layer thinning and increased skin friction occured in regions where the secondary flow induced by the pairs was directed toward the wall; the boundary layer was thickened and skin friction reduced where the secondary flow was directed away from the wall.

Experimental study of the development of longitudinal vortex pairs embedded in a turbulent boundary layer

On management and control of turbulent shear flows

The results are presented of turbulence measurements on an ‘infinite’ swept wing, simulated by a duct attached to a blower tunnel. The configuration is close to that used at the Netherlands NLR except that the boundary layer does not quite separate. The measurements include triple products, and a balance of the transport equation for turbulent energy is presented. The results confirm the NLR finding of a significant decrease in the magnitude of shear stress compared with an equivalent two-dimensional boundary layer: this is evidently the effect of crossflow on large eddies that have initially developed in a two-dimensional boundary layer. This unexpected effect of three-dimensionality is at least as important in prediction of real-life flows as the better-known lag between the direction of the shear stress and that of the mean-velocity gradient. Tentative suggestions for modelling the reduction in shear-stress magnitude are advanced.

Measurements in the turbulent boundary layer on an ‘infinite’ swept wing

The central intent of this paper is to describe a number of analyses and experi- ments that could eventually serve to elucidate the physics of fluid mechanics phe- nomena such as the generation of whirls, the quasi-two-dimensionalization of tur- bulence near the free surface, and the reverse energy cascade at pure and contam- inated interfaces. Discussed in some detail are the vorticity flux; surface-tension effects; basic equations and boundary conditions; the role of surfactants; topol- ogy of interfacial interactions; characteristics of connections, disconnections, and reconnections; a number of canonical flows (single and paired vortices, jets, and vortex rings); and the need for the development of prediction methods for turbulent flows in a region bounded by a free-surface, solid-fluid juncture boundary layer and the nonlinear interactions of shear flow wake turbulence with a free surface.

Vorticity, Free Surface, and Surfactants

Detailed mean-flow and turbulence measurements have been made in a low-speed turbulent boundary layer in zero pressure gradient with an isolated, artificially generated vortex imbedded in it. The vortex was generated by a half-delta wing on the floor of the wind-tunnel settling chamber, so that the vortex entering the working section had the same circulation as that originally generated, while axial-component velocity variations were very much reduced, relative to the local mean velocity, from values just behind the generator. The measurements show that the circulation around the vortex imbedded in the boundary layer is almost conserved, being reduced only by the spanwise-component surface shear stress. Therefore the region of flow affected by the vortex continues to grow downstream, its cross-sectional dimensions being roughly proportional to the local boundary-layer thickness. The behaviour of the various components of eddy viscosity, deduced from measured Reynolds stresses, and of the various triple products, suggests that the simple empirical correlations for these quantities used in present-day turbulence models are not likely to yield flow predictions which are accurate in detail.

Longitudinal vortices imbedded in turbulent boundary layers. Part 1. Single vortex

Longitudinal vortices imbedded in turbulent boundary layers

Sufficiently strong lateral deflection (“skewing”) of the mean streamlines in a shear layer can lead to the generation of discrete longitudinal vortices. The mechanism is basically in viscid, given the presence of an initial shear: this “skew-induced” vorticity should be distinguished from the generation of streamwise vorticity by Reynolds stresses, which is important only in very long, straight streamwise corners and is confined in practice to noncircular ducts. In most cases of practical interest the boundary layer and the imbedded vortex will be turbulent but the decay of the vortex under the action of Reynolds stresses is slow, because its circulation is reduced only by the effect of the spanwise component of skin friction. That is, the skew-induced vortices can influence the flow for a very long distance downstream. The present paper is a description of measurements of the mean and turbulent properties of decaying vortices and vortex pairs; the process of formation (being “ in viscid “) does not require such detailed study as the slow decay. The configurations used are idealizations of those found in practice, but the data should be useful for developing and testing calculation methods intended for reallife cases.

I. M. M. A. Shabaka

Papers

Longitudinal vortices imbedded in turbulent boundary layers. Part 1. Single vortex

Longitudinal vortices imbedded in turbulent boundary layers

Imbedded longitudinal vortices in turbulent boundary layers

Longitudinal vortices imbedded in turbulent boundary layers