We analyse the currently popular vortex identification criteria that are based on point-wise analysis of the velocity gradient tensor. A new measure of spiralling compactness of material orbits in vortices is introduced and using this measure a new local vortex identification criterion and requirements for a vortex core are proposed. The inter-relationships between the different criteria are explored analytically and in a few flow examples, using both zero and non-zero thresholds for the identification parameter. These inter-relationships provide a new interpretation of the various criteria in terms of the local flow kinematics. A canonical turbulent flow example is studied, and it is observed that all the criteria, given the proposed usage of threshold, result in remarkably similar looking vortical structures. A unified interpretation based on local flow kinematics is offered for when similarity or differences can be expected in the vortical structures educed using the different criteria.

On the relationships between local vortex identification schemes

The most widely used definitions of a vortex are not objective: they identify different structures as vortices in frames that rotate relative to each other. Yet a frame-independent vortex definition is essential for rotating flows and for flows with interacting vortices. Here we define a vortex as a set of fluid trajectories along which the strain acceleration tensor is indefinite over directions of zero strain. Physically, this objective criterion identifies vortices as material tubes in which material elements do not align with directions suggested by the strain eigenvectors. We show using examples how this vortex criterion outperforms earlier frame-dependent criteria. As a side result, we also obtain an objective criterion for hyperbolic Lagrangian structures.

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An objective definition of a vortex

The identification issue of coherent vortices is investigated on the basis of direct numerical simulation (DNS) and large-eddy simulations (LES) of turbulent flows. It is first shown that the pressure Laplacian is positive within a low-pressure tube of small cross section enclosed by convex isobaric surfaces in a uniform-density flow. Since this quantity is related to the second invariant Q of ∇ u , the Q criterion (region where Q is positive) is a necessary condition for the existence of such tubes. This eduction scheme is compared to other classical methods in incompressible simulations of isotropic turbulence: a mixing layer, a channel flow and a backward-facing step. Q-isosurfaces turn out to display very nice coherent vortices. This criterion is also used in combination with a conditional sampling method to discuss the characteristics of quasi-longitudinal vortices in a manipulated channel flow. The contribution of near-wall vortical structures to velocity and vorticity fluctuations is clearly isolat...

On coherent-vortex identification in turbulence

In the future, solar energy, along with other renewable resources, could play a key role in mass production of fine chemicals. It could also potentially solve environmental problems, as demonstrated by recent developments in the use of solar energy, such as solar photocatalysis. The solar photocatalytic technology has been demonstrated to be effective for: • Treating groundwater, drinking water, industrial wastewater, and air and soil pollution, • Water disinfection, and • Industrial production of fine chemicals. This report summarizes the current status of solar photocatalysis and identifies future opportunities for research and industry in this field, including recent relevant bibliography. The main commercial solar photocatalytic applications are described, included the technologies based on sunlight for antifogging and self-cleaning of coating materials, glass, and concrete. An overview of several different solar photoreactors and the main operating process parameters are also provided. For the estimation of capital costs, it is suggested the use of appropriate “figures of merit”. The present review would be of interest for researchers, technologists, engineers, and industrialists.

Solar photocatalysis: Materials, reactors, some commercial, and pre-industrialized applications. A comprehensive approach

Three proposed local criteria for the identification of vortices are analyzed and discussed; they are based on the analysis of invariants of the velocity gradient tensor ▿u or invariants of the tensor S 2 +Ω 2 , where S and Ω are the symmetric and antisymmetric parts of ▿u. Moreover, a tentative non-local procedure is proposed, which takes advantage of the observation that vortices tend to be made up of the same fluid particles; this leads to the definition of a Galilean invariant quantity, which can be computed and used to identify vortical structures. Three analytical flow fields are used for a comparative evaluation of both local and non-local criteria, which allows a deeper understanding of the physical meaning of the considered techniques.

On the effectiveness and limitations of local criteria for the identification of a vortex

Since all long-distance trade in the Roman world travelled by water, Roman harbour design and construction have special importance. Harbour excavation must be supplemented by analysis of the components of the hydraulic concrete, structural analysis of the cementing materials, and consideration of the design of the wooden formwork. The authors have begun collecting large cores from concrete blocks at Roman harbours and other maritime structures, analysing the materials used, the method of placement, and the structural characteristics of the resulting concrete. These data have provided new information on the engineering properties of Roman concrete, the process of funding and execution, and the trade in the volcanic ash which was the crucial component of hydraulic concrete.



© 2004 The Nautical Archaeology Society

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The ROMACONS Project: a contribution to the historical and engineering analysis of hydraulic concrete in Roman maritime structures

The authors have completed structural and compositional analysis of Roman hydraulic concrete using large cores taken from a variety of maritime structures. In 2005 an 8 m 3 block of hydraulic, pozzolanic concrete was built in the sea at Brindisi (Italy), applying the materials and procedures specified by the Roman architect Vitruvius. Cores were taken at 6 months and 12 months after construction and subjected to the same analyses as the first-century BC cores from pilae associated with the Villa of the Domitii Ahenobarbi at Santa Liberata. Results show that a slight variation on the Vitruvian formula yields results closest to the Roman material, and that substantial curing requires 12 months.

A comparison of the chemical and engineering characteristics of ancient roman hydraulic concrete with a modern reproduction of vitruvian hydraulic concrete

Reproducing a Roman maritime structure with Vitruvian pozzolanic concrete

The present invention concerns a composite cement paving with photocatalytic action for reducing urban polluting agents, composed of a bituminous or cement foundation, a resin having the function of interface, a superficial cement layer with photocatalytic properties able to reduce organic and inorganic polluting agents, said paving also comprising reinforcing materials and possible suitably positioned fibrous materials. The paving thus realised presents a strong photocatalytic action, prolonged overtime, even in the presence of high mechanical stress caused by heavy traffic.

R. Cucitore

Papers

On the effectiveness and limitations of local criteria for the identification of a vortex

The ROMACONS Project: a contribution to the historical and engineering analysis of hydraulic concrete in Roman maritime structures

A comparison of the chemical and engineering characteristics of ancient roman hydraulic concrete with a modern reproduction of vitruvian hydraulic concrete

Reproducing a Roman maritime structure with Vitruvian pozzolanic concrete

High durability photocatalytic paving for reducing urban polluting agents