1,721,127 research outputs found
On the evaluation of universal wake numbers for roughened circular cylinders in cross-flow
No full textNotes on the role of viscosity, vorticity and dissipation in incompressible flows
No full textA critical analysis is carried out of the incompressible flow equations, and it is shown that the assumption of the fluid being non-viscous, or that regions exist in the flow where viscosity may be neglected, is practically never necessary. On the other hand, the role of the generation and dynamics of vorticity is highlighted, as well as its importance for the description of the flow development and for assessing why and when simplified models may be devised. The evolution of some simple flows is also briefly described, and the connection between viscosity and the origin of lift on an airfoil is illustrated. Finally, the possible utility of the energy balance equation, which is usually neglected in incompressible flow analyses, is discussed. In particular, it is pointed out that dissipation exists and may be significant also in irrotational regions, and that its evaluation may often be carried out in a straightforward manner by means of the Bobyleff-Forsyth formula
Appraisal of universal wake numbers from data for roughened circular cylinders
No full textAn analysis was carried out to check whether certain existing universal wake numbers can characterize the cross-flow around roughened circular cylinders in transitional regimes. The results confirmed the soundness of the idea of the existence of a link between the drag coefficient of a bluff body, its pressure distribution, and the frequency of the shedding of vortices in its wake. In particular, Bearman's number and Griffin's number were shown to be able to describe this link with sufficient accuracy and to be a function of the Reynolds number based on the typical dimension of the surface roughness. A physical interpretation of Griffin's number was also given which permits to link the drag force with the velocity of the potential flow at separation and the frequency of vortex shedding
A note on Stokes’ hypothesis
No full textThe so-called Stokes’ hypothesis for a Newtonian fluid is reconsidered and a possible explanation is given of the fact that, in spite of its apparently weak physical justification, it permits to obtain good results for the description of most compressible flows. The explanation stands upon a closer analysis of the effect of the terms of the complete stress tensor in which the viscosity coefficients appear. An alternative formulation of the hypothesis is proposed, which also permits to clearly identify the very particular flow conditions in which it cannot justify the experimental evidence
The effect of surface roughness on the flow regime around circular cylinders
No full textExperimental research has been carried out to study the influence of surface roughness on transitions between the subcritical, critical supercritical and postcritical regimes around isolated circular cylinders in cross-flow. The experiments were performed in an open-jet wind tunnel, which was modified to obtain satisfactorily bidimensional conditions, with Reynolds numbers ranging from 2.6 × 10^4 to 2.8 × 10^5. The cylinders were covered with different types of standard commercial emery cloth, and the estimated relative roughness varied from 1.0 × 10^-3 to 12 × 10^-3. The signal from a hot-wire probe placed near the cylinder was analysed in order to ascertain the presence of vortex shedding and to obtain the Strouhal number. The mean pressure distribution around most of the cylinders were also measured, and then the drag coefficients were obtained by integration. The boundaries between the various flow regimes as a function of the degree of roughness and of the Reynolds number were tentatively established, assuming the critical regime to be characterised by the absence of regular vortex shedding, and the postcritical one by Reynolds-number independence. The results confirmed the striking influence of roughness on the flow regime; in particular, the critical regime can be impressively reduced, and it may even disappear in the case of highly roughened cylinders. The boundaries between the various regimes, as well as the supercritical and postcritical drag coefficients and Strouhal numbers, seem to be a function of both the size and the type of surface roughness. This is confirmation, therefore, that it is not possible to characterise the flow regime only by means of a Reynolds number based on the size of the roughness. Finally, it was demonstrated that strong supercritical and postcritical vortex shedding can take place from highly, but uniformly, roughened circular cylinders
Notes on incompressible flows
No full textA critical analysis is carried out of the incompressible flow equations, and it
is emphasized that the assumption of non-viscosity of the fluid, or even of the
existence of regions in the flow where viscosity may be neglected, is never
necessary. In effect, the form that is taken by the momentum equation for an
incompressible viscous fluid in a motion that is either irrotational or with vorticity
having a potential coincides with the equation that would be obtained if
the fluid were assumed to be non-viscous. Consequently, the role of the generation
and dynamics of vorticity is stressed, as well as its importance for the
description of the flow development and for assessing why and when simplified
models may be devised for its analysis or for the evaluation of the loads
acting on moving bodies. The conditions for the validity of the Bernoulli theorem
are also discussed, together with the significance of the conservation law
for the total vorticity in flows starting from rest. The evolution of some simple
flows is then briefly described, and the connection between viscosity and the
origin of lift on an airfoil is pointed out. Subsequently, the possible utility of
the energy balance equation, which is usually neglected in incompressible
flow analyses, is examined. In particular, it is shown that dissipation may be
significant also in irrotational regions, and that its evaluation for incompressible
flows may often be carried out in a straightforward manner by
means of the Bobyleff-Forsyth formula. Finally, the integral form of the kinetic
energy balance is applied to analyse several flows in different frames of
reference, highlighting the fact that a deeper comprehension may be gained
on the roles of the various terms, and on their connection with the drag acting
on moving bodies
Wind Tunnel Tests on a Tension Leg Platform Model
No full textDocumenti del Dipartimento di Ingegneria Aerospaziale. DDIA 85-
The characteristics of the forces acting on smooth and roughened circular cylinders in cross-flow near a plane surface
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