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Attività di analisi CFD su diversi piani velici
No full textReport Consorzio Pisa Ricerche, Aug. 2010
Experimental characterization of the influence of boundary layer thickness on the base pressure and near-wake flow of a bluff body.
No full textThe influence of the thickness of the boundary layer developing over the surface of an axisymmetric bluff body upon its base pressure and near-wake flow is analysed experimentally. The model, whose diameter and length are d = 70 mm and l = 400 mm, has a forebody with an elliptical contour and a sharp-edged flat base; it is supported above a plate by means of a faired strut. The pressure distributions over the body lateral and base surfaces were obtained using numerous pressure taps, while the boundary layer profiles and wake velocity field were measured through hot-wire anemometry. The tests were carried out at Re = u∞ l/n = 5.50 x 10^5, at which the boundary layer over the lateral surface of the body becomes turbulent before reaching the base contour. Strips of emery cloth were wrapped in various positions around the body circumference in order to modify the thickness and characteristics of the boundary layer. The results show that the base suctions decrease with increasing boundary layer thickness. This variation is found to be connected with an increase of the length of the mean recirculation region behind the body and with a corresponding reduction of the curvature of the streamlines at separation
Modeling infrared imaging data for the assessment of functional impairment in thermoregulatory processes
No full textConnection between base drag, separating boundary layer characteristics and wake mean recirculation length of an axisymmetric blunt-based body
Full text linkThe variation of the base drag of an axisymmetric bluff body caused by modifications of the boundary-layer separating at the sharp-edged contour of its base is analysed through different numerical simulations, and the results are compared with those of a previous experimental investigation. Variational MultiScale Large-Eddy Simulations (VMS-LES) are first carried out on the same nominal geometry and at the same Reynolds number of the experiments. Subsequently, Direct Numerical Simulations (DNS) are performed at Reynolds numbers that are roughly two orders of magnitude lower, in order to investigate on the sensitivity of the main findings to the Reynolds number. The results of experiments, VMS-LES and DNS simulations show that an increase of the base pressure - and thus a decrease of the base drag - may be obtained by increasing the boundary layer thickness before separation, which causes a proportional increase of the length of the mean recirculation region behind the body. In spite of the different setups, Reynolds numbers and turbulence levels in the experiments and numerical simulations, in all cases the base pressure is found to be directly proportional to the length of the mean recirculation region, which is thus a key index of the base drag value. In turn, the recirculation length seems to be connected with the location of the incipient instability of the detaching shear layers, which can be moved downstream by an increase of the thickness of the separating boundary layer and upstream by an increase of the turbulence level
On the influence of boundary layer thickness on the base drag and on the near-wake flow of an axisymmetric blunt-based body
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Control of laminar and turbulent flows in diffusers by means of multiple contoured cavities
No full textUnsteady Flow Regimes in a T-Shaped Micromixer: Mixing and Characteristic Frequencies
Full text linkExperiments and direct numerical simulations are used jointly to study the asymmetric and symmetric time-periodic regimes occurring in a T-shaped micro mixer for larger Reynolds numbers than those of steady regimes. The first is characterized by a large mixing degree, whereas the flow in the second regime Always exhibits a nearly double mirror symmetry in the mixing channel, which strongly hampers mixing. The characteristic nondimensional frequency, calculated using the hydraulic diameter of the mixing channel and the bulk velocity, augments with the Reynolds number in both periodic regimes, but a large discontinuity is observed at the transition between the two regimes. A detailed description of the main flow features is given to provide a physical explanation on the Strouhal number variation. The present analysis can be exploited in practice to design active control strategies, e.g., by exciting the flow at the frequencies typical of the asymmetric unsteady regime
Separation control and efficiency improvement in a 2D diffuser by means of contoured cavities
Full text linkThe performance of a passive control method aimed at reducing and, possibly, eliminating boundary layer separation is evaluated by means of numerical simulation. The passive control, which consists
of introducing appropriately-shaped cavities in solid walls, is applied to a plane diffuser. The Reynolds number is such that the turbulence can be neglected (Re = 500, based on the diffuser half-width at the
inlet section and the inlet velocity on the axis). A configuration characterized by an area ratio of 2 and a divergence angle of 7 degrees is chosen, so that, without the introduction of the control, the flow is characterized by a large zone of steady asymmetrical boundary layer separation. In order to reduce the separated zone and to increase the efficiency of the diffuser, a couple of symmetric contoured cavities is introduced in the diverging walls. An optimization procedure is developed to obtain the cavity geometry
that maximizes the pressure recovery in the diffuser and minimizes the boundary layer separation extent. The introduction of the optimal cavities leads to an increase in pressure recovery of the order of 13% and to a strong reduction of the separation extent. This result is due to a favourable modification of the velocity and vorticity fields in the near-wall region. The most important geometrical parameters are also identified and the robustness of the control to small changes in their values is investigated. It is found that the contoured cavities are effective as long as the flow is able to reattach immediately downstream of the cavities
Base-drag reduction of an axisymmetric bluff body through boundary-layer and near-wake modifications.
No full textIn the present work, the main findings of an experimental and numerical research activity aimed at characterizing and reducing the base drag of bluff bodies are presented. We consider the flow around an axisymmetric body, which can be viewed as a simplified model of a road vehicle. The results of experiments, LES and DNS simulations show that the base suctions - and thus the base drag - decrease in direct proportion to the increase of the length of the mean recirculation region behind the body which, in turn, can be obtained by increasing the boundary layer thickness before separation. Although the different set-ups and Reynolds numbers in the experiments and numerical simulations imply significant differences in the near wake dynamics, in all cases the length of the mean recirculation region present behind the body seems to be connected with the location of the incipient instability of the detaching shear layers. It is shown that the location of this instability can be moved downstream, and thus base drag can be reduced, by increasing the thickness of the separating boundary layer. The results of the present analysis may be useful to devise further strategies for pressure drag reduction
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