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Drag coefficient

About: Drag coefficient is a research topic. Over the lifetime, 14471 publications have been published within this topic receiving 303196 citations. The topic is also known as: drag factor.


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Journal ArticleDOI
TL;DR: In this article, it was shown that reentrant sea spray can explain the enhanced transfer of enthalpy and momentum in tropical cyclones at high wind speeds. But, this was only applied to a hurricane, and not to hurricanes of greater than marginal hurricane intensity.
Abstract: The intensity of tropical cyclones is sensitive to the rates at which enthalpy and momentum are transferred between sea and air in the high-wind core of the storm. Present models of the wind dependence of these transfer rates suggest that the effective drag coefficient is more than twice the effective enthalpy transfer coefficient at wind speeds above 25 m s21. Using this ratio in numerical models, however, makes it impossible to sustain storms of greater than marginal hurricane intensity. Some other physical process must, therefore, enhance enthalpy transfer at very high wind speeds. This paper suggests that re-entrant sea spray explains this enhanced transfer. When a spray droplet is ejected from the ocean, it remains airborne long enough to cool to a temperature below the local air temperature but not long enough to evaporate an appreciable fraction of its mass. The spray droplet thus gives up sensible heat and returns to the sea before it has time to extract back from the atmosphere the heat necessary to continue its evaporation. Microphysical modeling, combined with data from the Humidity Exchange over the Sea Experiment (HEXOS), makes it possible to derive an expression for the net enthalpy transfer of re-entrant spray. This spray enthalpy flux is roughly cubic in wind speed. When this relation is used in a numerical simulation of a hurricane, the spray more than compensates for the observed increase in the ratio of drag and enthalpy transfer coefficients with wind speed. The momentum flux associated with sea spray is an important energy sink that moderates the effects of this spray enthalpy flux. Including a parameterization for this momentum sink along with wave drag and spray enthalpy transfer in the hurricane simulation produces results that are similar to ones based on equal transfer coefficients.

239 citations

Journal ArticleDOI
TL;DR: In this paper, the Strouhal number of a dimpled circular cylinder over the Reynolds number range from 2 x 104 to 3 x 10s is reported. And the ratio of the depth of the dimples to the diameter of the cylinder is 9xlO~ 3.
Abstract: Measurements are reported of the drag coefficient and Strouhal number of a dimpled circular cylinder over the Reynolds number range from 2 x 104 to 3 x 10s. The ratio of the depth of the dimples to the diameter of the cylinder is 9xlO~ 3. In common with sand-roughened cylinders, the dimpled cylinder has a lower critical Reynolds number than a smooth cylinder. After the drag coefficient minimum, the CD does not rise to the high values that are typical of cylinders with sand roughness but is found to be closer to that for a smooth cylinder. Over a Reynolds number range from about 4xl04 to 3xl05, a dimpled circular cylinder has a lower drag coefficient than a smooth cylinder.

239 citations

Journal ArticleDOI
28 Feb 2007-Langmuir
TL;DR: It is shown that the displacement sensitivity of the tweezers is modulated by a standing light wave between the microsphere and the surface, and the data suggest that microspheres can experience attractive and/or repulsive forces close to surfaces.
Abstract: Optical tweezers are widely used to measure molecular forces in biology. Such measurements are often influenced by a nearby surface that can perturb both the calibration of the tweezers as well as ...

238 citations

Journal ArticleDOI
TL;DR: It was found that flexibility can enhance aerodynamic performance and that the best performance is realized when the wing is excited by a non-linear resonance at 1/3 of the natural frequency, which points to the importance of considering non- linear resonances for enhancing aerodynamics performance.
Abstract: In the present study, a computational investigation was carried out to understand the influence of flexibility on the aerodynamic performance of a hovering wing. A flexible, two-dimensional, two-link model moving within a viscous fluid was considered. The Navier-Stokes equations governing the fluid dynamics were solved together with the equations governing the structural dynamics by using a strongly coupled fluid-structure interaction scheme. Harmonic kinematics was used to prescribe the motions of one of the links, thus effectively reducing the wing to a single degree-of-freedom oscillator. The wing's flexibility was characterized by the ratio of the flapping frequency to the natural frequency of the structure. Apart from the rigid case, different values of this frequency ratio (only in the range of 1/2 to 1/6) were considered at the Reynolds numbers of 75, 250 and 1000. It was found that flexibility can enhance aerodynamic performance and that the best performance is realized when the wing is excited by a non-linear resonance at 1/3 of the natural frequency. Specifically, at Reynolds numbers of 75, 250 and 1000, the aerodynamic performance that is characterized by the ratio of lift coefficient to drag coefficient is respectively increased by 28%, 23% and 21% when compared with the corresponding ratios of a rigid wing driven with the same kinematics. For all Reynolds numbers, the lift generated per unit driving power is also enhanced in a similar manner. The wake capture mechanism is enhanced, due to a stronger flow around the wing at stroke reversal, resulting from a stronger end of stroke vortex at the trailing edge. The present study provides some clues about how flexibility affects the aerodynamic performance in low Reynolds number flapping flight. In addition, it points to the importance of considering non-linear resonances for enhancing aerodynamic performance.

238 citations

Journal ArticleDOI
TL;DR: In this paper, a NavierStokes solver, the e N method transition model, and a Reynolds-averaged two-equation closure were coupled to study the low Reynolds number flow characterized with laminar separation and transition.
Abstract: 4-10 5 . In order to gain better understanding of the fluid physics and associated aerodynamics characteristics, we have coupled (i) a NavierStokes solver, (ii) the e N method transition model, and (iii) a Reynolds-averaged two-equation closure to study the low Reynolds number flow characterized with laminar separation and transition. A new intermittency distribution function suitable for low Reynolds number transitional flow is proposed and tested. To support the MAV applications, we investigate both rigid and flexible airfoils, which has a portion of the upper surface mounted with a flexible membrane, using SD7003 as the configuration. Good agreement is obtained between the prediction and experimental measurements regarding the transition location as well as overall flow structures. In the current transitional flow regime, though the Reynolds number affects the size of the laminar separation bubble, it does not place consistent impact on lift or drag. The gust exerts a major influence on the transition position, resulting in the lift and drag coefficients hysterisis. It is also observed that thrust instead of drag can be generated under certain gust condition. At α=4 o , for a flexible wing, self-excited vibration affects the separation and transition positions; however, the time-averaged lift and drag coefficients are close to those of the rigid airfoil.

236 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023307
2022688
2021489
2020504
2019504
2018456