M. S. Chong

A general classification of three‐dimensional flow fields

The geometry of solution trajectories for three first‐order coupled linear differential equations can be related and classified using three matrix invariants. This provides a generalized approach to the classification of elementary three‐dimensional flow patterns defined by instantaneous streamlines for flow at and away from no‐slip boundaries for both compressible and incompressible flow. Although the attention of this paper is on the velocity field and its associated deformation tensor, the results are valid for any smooth three‐dimensional vector field. For example, there may be situations where it is appropriate to work in terms of the vorticity field or pressure gradient field. In any case, it is expected that the results presented here will be of use in the interpretation of complex flow field data.

A theoretical and experimental study of wall turbulence

In this paper the dimensional-analysis approach to wall turbulence of Perry & Abell (1977) has been extended in a number of directions. Further recent developments of the attached-eddy hypothesis of Townsend (1976) and the model of Perry & Chong (1982) are given, for example, the incorporation of a Kolmogoroff (1941) spectral region. These previous analyses were applicable only to the ‘wall region’ and are extended here to include the whole turbulent region of the flow. The dimensionalanalysis approach and the detailed physical modelling are consistent with each other and with new experimental data presented here.

On the mechanism of wall turbulence

In this paper an attempt is made to formulate a model for the mechanism of wall turbulence that links recent flow-visualization observations with the various quantitative measurements and scaling laws established from anemometry studies. Various mechanisms are proposed, all of which use the concept of the horse-shoe, hairpin or ‘A’ vortex. It is shown that these models give a connection between the mean-velocity distribution, the broad-band turbulence-intensity distributions and the turbulence spectra. Temperature distribut’ions above a heated surface are also considered. Although this aspect of the work is not yet complete, the analysis for this shows promise.

The vortex-shedding process behind two-dimensional bluff bodies

Using a variety of flow-visualization techniques, the flow behind a circular cylinder has been studied. The results obtained have provided a new insight into the vortex-shedding process. Using time-exposure photography of the motion of aluminium particles, a sequence of instantaneous streamline patterns of the flow behind a cylinder has been obtained. These streamline patterns show that during the starting flow the cavity behind the cylinder is closed. However, once the vortex-shedding process begins, this so-called ‘closed’ cavity becomes open, and instantaneous ‘alleyways’ of fluid are formed which penetrate the cavity. In addition, dye experiments also show how layers of dye and hence vorticity are convected into the cavity behind the cylinder, and how they are eventually squeezed out.

A study of the evolution and characteristics of the invariants of the velocity-gradient tensor in isotropic turbulence

Since the availability of data from direct numerical simulation (DNS) of turbulence, researchers have utilized the joint PDFs of invariants of the velocity gradient tensor to study the geometry of small-scale motions of turbulence. However, the joint PDFs only give an instantaneous static representation of the properties of fluid particles and dynamical Lagrangian information cannot be extracted. The Lagrangian evolution of the invariants of the velocity gradient tensor is studied using conditional mean trajectories (CMT). These CMT are derived using the concept of the conditional mean time rate of change of invariants calculated from a numerical simulation of isotropic turbulence

Turbulence structures of wall-bounded shear flows found using DNS data

This work extends the study of the structure of wall-bounded flowsn using the topological properties of eddying motions as developed by Chong etn al . (1990), Soria et al . (1992, 1994), and as recently extended by Blackburnn et al . (1996) and Chacin et al . (1996). In these works,n regions of flow which are focal in nature are identified by being enclosed by an isosurface of a positive small value of the discriminantn of the velocity gradient tensor. These regions resemble the attached vortex loopsn suggested first by Theodorsen (1955). Such loops are incorporated in the attached-eddyn model versions of Perry & Chong (1982), Perry et al . (1986), and Perry & Marusic (1995), which are extensions of a model first formulated by Townsend (1976). Then direct numerical simulation (DNS) data of wall-bounded flows studied here aren from the zero-pressure-gradient flow of Spalart (1988) and the boundary layer withn separation and reattachment of Na & Moin (1996). The flow structures are examinedn from the viewpoint of the attached eddy hypothesis.

A series-expansion study of the Navier–Stokes equations with applications to three-dimensional separation patterns

An algorithm has been developed which enables local Taylor-series-expansion solutions of the Navier-Stokes and continuity equations to be generated to arbitrary order. Much of the necessary algebra for generating these solutions can be done on a computer. Various properties of the algorithm are investigated and checked by making comparisons with known solutions of the equations of motion. A method of synthesising nonlinear viscous-flow patterns with certain required properties is developed and applied to the construction of a number of two- and three-dimensional flow-separation patterns. These patterns are asymptotically exact solutions of the equations of motion close to the origin of the expansion. The region where the truncated series solution satisfies the full equations of motion to within a specified accuracy can be found.

Vortical flow. Part 1. Flow through a constant-diameter pipe

This paper describes an exploration of the behaviour and properties of swirling flow through a constant-diameter pipe. The experiments reveal a complicated transition process as the swirl intensity Ω is increased at fixed pipe Reynolds number Re ≈ 4900. For Ω [les ] 1.09, the vortex was steady, laminar, axisymmetric, and developed slowly with streamwise distance. The upstream velocity profiles were similar to those commonly appearing in the literature in similar apparatus. Spiral vortex breakdown appeared in the test section for 1.09 [les ] Ω [les ] 1.31 and was associated with a localized transition from jet-like to wake-like mean axial velocity profiles. Further increase in Ω caused the breakdown to move upstream of the test section. Downstream, the core of the post-breakdown flow was unsteady and recovered toward jet-like profiles with streamwise distance. At Ω = 2.68, a global transition occurred in which the mean axial velocity profiles suddenly developed an annular axial velocity deficit. At the same time, disturbances began to appear in the outer flow. Further increase in Ω eventually led to an annulus of reversed axial flow and a completely unsteady vortex.

Turbulent channel flow: comparison of streamwise velocity data from experiments and direct numerical simulation

© 2009 Cambridge University Press. Online edition of the journal is available at http://journals.cambridge.org/FLM

A note on the cause of rebound in the head-on collision of a vortex ring with a wall

It is well documented that a trailing vortex pair approaching the ground, and a vortex ring colliding head-on with a rigid plane, experience a reversal in axial velocity which is commonly referred to as “rebound”. One explanation of this phenomenon suggests that it is essentially an inviscid process due to the effect of the finite core-size, whereas another and more widely accepted explanation attributes it to the influence of a secondary vortex which is generated at the surface by viscous effects. The aim of this paper is to assess experimentally the validity of these competing explanations. To achieve this, flow visualization studies of the collision of a vortex ring with a wall are compared with those of the head-on collision of two identical rings. The head-on collision is designed to mimic the inviscid, free-slip case of a ring/wall interaction. This paper describes the experimental findings.