Aerodynamic study of a tricycle wheel subsystem for drag reduction

This paper deals with a computational fluid dynamics (CFD) and experimental drag analysis on an isolated rotating wheel subsystem (including its accessories: tire, suspension, A-arms, and fender) of a motor tricycle vehicle with two wheels in front. The main goal of the present work is to study the effect of the fender on the wheel subsystem drag and its optimization. The Star CCM+ commercial code was used for the numerical simulations. Different flow conditions were simulated and some results were validated by comparison to wind tunnel experimental results. To perform drag optimization, several aerodynamic fender shapes were designed and simulated as part of the subsystem. A drastic drag reduction up to 30.6% compared to the original wheel subsystem was achieved through numerical simulations

Experimental Evaluation of the Critical Flutter Speed on Wings of Different Aspect Ratio

In this work, wind tunnel experiments were conducted to evaluate the critical flutter speed of wings for three pertinent flight parameters (i) the aspect ratio (AR), (ii) the angle of attack (AoA), and (iii) the aircraft propeller excitation. Six symmetrical wings (NACA0012 design), of fixed chord length of 80 mm and varied AR from 8.75 to 15, were used for this purpose. These wings were mounted horizontally in the wind tunnel as fixed-free condition. The airflow speed is increased slowly until the wing flutters. The results show that the critical flutter speed decreases when the AR increases. For higher AR, the effect of the AoA on the flutter speed is minimal. However, for low AR, the AoA is vital in delaying the flutter instability of the wing. This critical speed spans low to moderate Reynolds numbers based on the wing chord length (Rec =7×104-2×105) which corresponds to the speed range of High Altitude and Long Endurance (HALE) aircraft. In contrast, for a propeller excitation outside the resonance region of the wing, its effect of the on flutter characteristics is not noticeable

The flow field in turbulent round free jets

A critical review of both experimental and computational studies of round turbulent jets is provided, beginning with the work of Tollmien (1926). This review traces the history, the major advances, and the various stages that the research community went through over the past 85-odd years - from statistical analyses through to the use of conditional sampling, proper orthogonal decomposition and structural eduction methods. It includes the introduction of novel experimental techniques as well as insights gained from recent large eddy and direct numerical simulations. Some direction where future research may prove beneficial is also provided. The review does not include the effects of passive or active control, scalar contaminant transport whether by heat or mass. It includes effects of Reynolds number, inlet conditions (excluding swirl) and considers both near- and far-field investigations. We have minimised reference to papers that utilise models of turbulence unless such works provide something of particular importance

A criterion for detection of the onset of Dean instability in Newtonian fluids

Dean instability for Newtonian fluids in laminar secondary flow in 180 degrees curved channels was studied experimentally and numerically. The numerical study used Fluent CFD code to solve the Navier-Stokes equations, focusing on flow development conditions and the parameters influencing Dean instability. An accurate criterion based on the radial gradient of the axial velocity was defined that allows detection of the instability threshold, and this criterion is used to optimize the grid geometry. The effects on Dean instability of the curvature ratio (from 5.5 to 20) and aspect ratio (from 0.5 to 12) are studied. In particular, we show that the critical value of the Dean number decreases with the increasing duct curvature ratio. The variation of the critical Dean number with duct aspect ratio is less regular. In the experimental study, flows were visualized in several tangential positions of a 180 degrees curved channel with aspect ratio 8 and curvature ratio 10. The flow is hydrodynamically developed at the entrance to the curved channel. The critical Dean number is detected and the development of secondary flow vortices by additional counter-rotating vortex pairs is observed. A diagram of different critical Dean numbers is established. (c) 2005 Elsevier SAS. All rights reserved

The Dean instability in power-law and Bingham fluids in a curved rectangular duct

The laminar flow of power-law and yield-stress fluids in 180, curved channels of rectangular cross section was studied experimentally and numerically in order to understand the effect of theological fluid behavior on the Dean instability that appears beyond a critical condition in the flow. This leads to the apparition of Dean vortices that differ from the two corner vortices created by the channel wall curvature. Flow visualizations showed that the Dean vortices develop first in the near-wall zone on the concave (outer) wall, where the shear rate is higher and the viscosity weaker; then they penetrate into the centre of the channel cross section where power-law fluids have high viscosity and Bingham fluids are unyielded in laminar flow. Based on the complete formation on the concave wall of the new pairs of counter-rotating vortices (Dean vortices), the critical value of the Dean number decreases as the power-law index increases for the power-law fluids, and the Bingham number decreases for the Bingham fluids. For power-law fluids, a diagram of critical Dean numbers, based on the number of Dean vortices formed, was established for different axial positions. For the same flow conditions, the critical Dean number obtained using the axial velocity gradient criterion was smaller then that obtained with the visualization technique. (C) 2009 Elsevier B.V. All rights reserved

A numerical study of dean instability in non-Newtonian fluids

We present a numerical study of Dean instability for non-Newtonian fluids in a laminar 180 deg curved-channel flow of rectangular cross section. A methodology based on the Papanastasiou model (Papanastasiou, T C., 1987, J. Rheol., 31(5), pp. 385-404) was developed to take into account the Bingham-type theological behavior. After validation of the numerical methodology, simulations were carried out (using FLUENT CFD code) for Newtonian and non-Newtonian fluids in curved channels of square or rectangular cross section and for a large aspect and curvature ratios. A criterion based on the axial velocity gradient was defined to detect the instability threshold. This criterion was used to optimize the grid geometry. The effects of curvature and aspect ratio on the Dean instability arc, studied for all fluids, Newtonian and non-Newtonian. In particular, we show that the critical value of the Dean number decreases with increasing curvature ratio. The variation of the critical Dean number with aspect ratio is less regular The results art compared to those for Newtonian fluids to emphasize the effect of the power-law index and the Bingham number The onset of Dean instability is delayed with increasing power-law index. The same delay is observed in Bingham fluids when the Bingham number is increased

Experiments and Large-Eddy Simulations of Lobed and Swirling Turbulent Thermal Jets for HVAC's Applications

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Detection of the onset of Dean instability and effects of the rheological behavior in non-Newotonian fluids

Start The onset of Dean instability in laminar secondary flow in 180 curved rectangular cross section channel was studied experimentally and numerically in Newtonian and non-Newtonian fluids. The development of the instability was observed; we showed that the Dean vortices develop first in the near wall zone on the concave wall, where the viscosity is weak and the shear rate is high, and than they penetrate the cross-section center characterized by a high viscosity for the pseudoplastic fluids and a solid (unsheared) zone for the yield fluids. Based on the complete formation of the Dean vortices, the critical value of the Dean number decreases with the increase of the power law index and with the decrease of the Bingham number. Contrarily to what is reported in the literature where the instability threshold was usually obtained visually, in this study a new criterion based on the radial gradient of the axial velocity to detect the instability threshold was defined. Comparison was made between numerical and experimental results concerning the instability threshold, obtained with the new criterion. Also, a comparison between the instability threshold using this new criterion and using the visualization technique is presented. We show that the value of the Dean number using the new criterion is comparable for the two studies, numerical and experimental. These values are weaker than those obtained with the visualization technique for the same conditions.your abstract here..

Revisión general sobre sistemas de acondicionamiento de aire en edificios ecológicos e inteligentes.

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