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.

An improved bulk air–sea surface flux algorithm, including spray-mediated transfer

Abstract: A bulk air–sea flux algorithm couples the ocean and the lower atmosphere through flux boundary conditions and can be used in various analyses and in numerical models. The algorithm described here has two features not present in any other existing bulk flux algorithm. First, it has a new air–sea drag relation. Here, for wind speeds above about 9 m s−1, the friction velocity u✻, which is related to the square root of the surface stress, is linearly related to UN10, the neutral-stability 10 m wind speed. When extrapolated to hurricane-strength winds, this drag relation has better properties than relations formulated in terms of a drag coefficient or a roughness length. The second unique feature of this flux algorithm is that it recognizes two routes by which heat and moisture cross the air–sea interface: one is the interfacial route, which is controlled by molecular processes right at the air–sea interface; the second is the spray-mediated route, which is governed by microphysical processes at the surface of sea spray droplets. Through microphysical theory and our analysis of 4000 sets of eddy-covariance measurements of the scalar fluxes, we separate the measured fluxes into the interfacial and spray contributions and thereby produce the only spray flux algorithm tested and validated against oceanic data. Because all components of our flux algorithm are physics-based and validated with data for winds up to 25 m s−1, one application is extrapolating this algorithm to hurricane-strength winds, where sea spray plays a dominant role in scalar transfer.

Drag and lift forces on a bubble rising near a vertical wall in a viscous liquid

Abstract: The two components of the force acting on a clean almost spherical bubble rising near a plane vertical wall in a quiescent liquid are determined experimentally. This is achieved by using an apparatus in which a CCD camera and a microscope follow the rising bubble. This apparatus allows us to measure accurately the bubble radius, rise speed and distance between the bubble and the wall. Thereby the drag and lift components of the hydrodynamic force are determined for Reynolds numbers Re (based on bubble diameter, rise velocity U, and kinematic viscosity ν) less than 40. The results show the existence of two different regimes, according to the value of the dimensionless separation L* defined as the ratio between the distance from the bubble centre to the wall and the viscous length scale ν/U. When L* is O(1) or more, experimental results corresponding to Reynolds numbers up to unity are found to be in good agreement with an analytical solution obtained in the Oseen approximation by adapting the calculation of Vasseur & Cox (1977) to the case of an inviscid bubble. When L* is o(1), higher-order effects not taken into account in previous analytical investigations become important and measurements show that the deformation of the bubble is significant when the viscosity of the surrounding liquid is large enough. In this regime, experimental results for the drag force and shape of the bubble are found to agree well with recent theoretical predictions obtained by Magnaudet, Takagi & Legendre (2002) but the measured lift force tends to exceed the prediction as the separation decreases.

A numerical study of three-dimensional stratified flow past a sphere

Abstract: A numerical study is described of the Boussinesq flow past a sphere of a viscous, incompressible and non-diffusive stratified fluid. The approaching flow has uniform velocity and linear stratification. The Reynolds number Re (= 2ρ0Ua/μ) based on the sphere diameter is 200 and the internal Froude number F(= U/Na) is varied from 0.25 to 200. Here U is the velocity, N the Brunt-Vaisala frequency, a the radius of the sphere, μ the viscosity and ρ0 the mean density. The numerical results show changes in the flow pattern with Froude number that are in good agreement with earlier theoretical and experimental results. For F < 1, the calculations show the flow passing round rather than going over the obstacle, and confirm Sheppard's simple formula for the dividing-streamline height. When the Froude number is further reduced (F < 0.4), the flow becomes approximately two-dimensional and qualitative agreement with Drazin's three-dimensional low-Froude-number theory is obtained. The relation between the wavelength of the internal gravity wave and the position of laminar separation on the sphere is also investigated to obtain the suppression and induction of separation by the wave. It is also found that the lee waves are confined in the spanwise direction to a rather narrow strip just behind the obstacle as linear theory predicts. The calculated drag coefficient CD of the sphere shows an interesting Froude-number dependence, which is quite similar to the results given by experiments. In this study not only CD but also the pressure distribution which contributes to the change of CD are obtained and the mechanism of the change is closely examined.

On the Linear Parameterization of Drag Coefficient over Sea Surface

Abstract: Combining the logarithmic law with the Charnock relation yields a drag coefficient that is a function of wind speed with the Charnock coefficient as a parameter. It is found that the function is nearly linear within the typically measured range of the drag coefficient. The slope of the linear function is dominated by the Charnock coefficient. When the Charnock relation is extended to a wave age‐dependent function, the drag coefficient remains a near-linear function of wind speed after invoking the 3/2 power law. The slope of the linear function is dominated by wave steepness.