Topic
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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TL;DR: In this article, the authors examined the hydraulics of the strait and sill flow with friction and showed that friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and under certain conditions, gives rise to stationary jumps on the upstream face of the obstacle.
Abstract: The hydraulics of strait and sill flow with friction is examined using a reduced gravity model. It is shown that friction moves the critical (or control) point from the sill to a location downstream. If the strait has constant width, the control point lies where the bottom slope is the negative of the drag coefficient Cd. If -Cd exceeds the bottom slope everywhere, the flow cannot be controlled (in the classical sense that energy and flow force are minimized). Friction also decreases the minimum obstacle height required to establish hydraulically controlled flow in the classical laboratory towing experiment. Also, friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and, under certain conditions, gives rise to stationary jumps on the upstream face of the obstacle. Some consequences of these results for deep-ocean overflows are given using the Iceland-Faroe overflow as an example.
101 citations
TL;DR: A mechanistic model of drag was developed with explicit functions to describe changes in shape and size of macroalgae in flow and allows for more reliable predictions of drag at high, ecologically relevant water velocities.
Abstract: Rocky intertidal organisms experience large hydrodynamic forces due to high water velocities created by breaking waves. Flexible organisms, like macroalgae, often experience lower drag than rigid organisms because their shape and size change as velocity increases. This phenomenon, known as reconfiguration, has been previously quantified as Vogel's E, a measure of the relationship between velocity and drag. While this method is very useful for comparing reconfiguration among organisms it does not address the mechanisms of reconfiguration, and its application to predicting drag is problematic. The purpose of this study was twofold: (1) to examine the mechanisms of reconfiguration by quantifying the change in shape and size of a macroalga in flow and (2) to build a mechanistic model of drag for reconfiguring organisms. Drag, frontal area and shape of the intertidal alga Chondrus crispus were measured simultaneously in a recirculating flume at water velocities from 0 to approximately 2 m s(-1). Reconfiguration was due to two separate mechanisms: whole-alga realignment (deflection of the stipe) at low velocities (<0.2 m s(-1)) and compaction of the crown (reduction in frontal area and change in shape) at higher velocities. Change in frontal area contributed more to drag reduction than change in drag coefficient. Drag coefficient and frontal area both decrease exponentially with increasing water velocity, and a mechanistic model of drag was developed with explicit functions to describe these changes. The model not only provides mechanistic parameters with which to compare reconfiguration among individuals and species, but also allows for more reliable predictions of drag at high, ecologically relevant water velocities.
101 citations
TL;DR: In this article, a group of photographic stations spaced at intervals along the trajectory were used to record an image of the sphere and of the pattern of the surrounding shock waves, and the successive times of operation of each station were recorded on a precision chronograph and were combined with the distances to determine the retardation and, hence, the drag.
Abstract: Smooth spheres, 9 / i6 in. in diameter, were fired through a group of photographic stations spaced at intervals along the trajectory. Each station recorded an image of the sphere and of the pattern of the surrounding shock waves. The successive times of operation of each station were recorded on a precision chronograph and were combined with the distances to determine the retardation and, hence, the drag. The firings were carried out at Mach Numbers from 0.29 to 3.96 and at the corresponding Reynolds Numbers from 9.3 X 10 to 1.3 X 10. The results show that the drag coefficient is essentially constant a t subsonic velocities, rises rapidly through a relatively broad transonic region, and decreases slowly with further increase in velocity in the supersonic region. At subsonic velocities, the drags agree well with the measurements made elsewhere below the critical Reynolds Number. However, a close examination of drags and separation points showed no evidence of a critical Reynolds Number for this size sphere. It is believed tha t separation phenomena on small spheres at supersonic velocities are controlled by compressibility effects rather than boundary-layer conditions. Additional firings were carried out with rough /i6-m. spheres, with smooth /32-in. spheres, and with smooth iy 2 in . spheres to study the effects of roughness and size. At supersonic velocities these effects change the drag but little. At low transsonic velocities the drag coefficients of the P/Vin. spheres fall well below the Vie-in. sphere curve, and this difference, combined with changes in the wake flow pattern, demonstrates the occurrence of a critical Reynolds Number for the P/Vin. sphere.
101 citations
TL;DR: In this article, a series of wavy cylinders with different combinations of dimensionless wavelength and wave amplitude were studied in detail at a Reynolds number of Re ∞Dm/ν = 100, where U∞ is the free-stream velocity and Dm is the mean diameter of a wavy cylinder.
Abstract: Three-dimensional numerical simulations of laminar flow around a circular cylinder with sinusoidal variation of cross-section along the spanwise direction, named ‘wavy cylinder’, are performed. A series of wavy cylinders with different combinations of dimensionless wavelength (λ/Dm) and wave amplitude (a/Dm) are studied in detail at a Reynolds number of Re = U∞Dm/ν = 100, where U∞ is the free-stream velocity and Dm is the mean diameter of a wavy cylinder. The results of variation of mean drag coefficient and root mean square (r.m.s.) lift coefficient with dimensionless wavelength show that significant reduction of mean and fluctuating force coefficients occurs at optimal dimensionless wavelengths λ/Dm of around 2.5 and 6 respectively for the different amplitudes studied. Based on the variation of flow structures and force characteristics, the dimensionless wavelength from λ/Dm = 1 to λ/Dm = 10 is classified into three wavelength regimes corresponding to three types of wake structures. The wake structures at the near wake of different wavy cylinders are captured. For all wavy cylinders, the flow separation line varies along the spanwise direction. This leads to the development of a three-dimensional free shear layer with periodic repetition along the spanwise direction. The three-dimensional free shear layer of the wavy cylinder is larger and more stable than that of the circular cylinder, and in some cases the free shear layer even does not roll up into a mature vortex street behind the cylinder. As a result, the mean drag coefficients of some of the typical wavy cylinders are less than that of a corresponding circular cylinder with a maximum drag coefficient reduction up to 18%. The r.m.s. lift coefficients are greatly reduced to practically zero at optimal wavelengths. In the laminar flow regime (60 ≤ Re ≤ 150), the values of optimal wavelength are Reynolds number dependent.
101 citations
TL;DR: In this paper, a new correlation law has been found, which is based on experimental measurements of the terminal velocities of irregular particles falling in fluids of different densities and viscosities.
101 citations