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.

Recent Advances in Gas-Particle Nozzle Flows

Abstract: T USE of metallic fuel constituents in modern rocket engines has brought attention to nonequilibrium aspects of two-phase nozzle expansion processes. Since condensed metal oxide combustion products (which comprise 30 to 40% by weight of the total products of contemporary solid rockets) can do no expansion work, their presence in the rocket nozzle can only be deleterious to the effectiveness of the nozzle expansion process in converting thermal to kinetic energy. The condensed particles are accelerated in a nozzle almost exclusively by drag forces associated with lag or slippage of the particles relative to the expanding gas. Some performance loss relative to the calculated ideal no-slip expansion process must always be associated with macroscopic size particles, and experience has shown that the magnitude of the loss increases with the weight fraction of particles. Significant velocity and thermal lags thus have been suspected as a prime cause of rocket performance losses, and a number of studies have been directed toward delineation of the mechanism and magnitude of these lag effects. Early studies were summarized and extended in the review of Altman and Carter (l). Primarily, these early studies served to place bounds on the performance losses by examining the limiting cases of no-lag and complete lag. They demonstrated that thermal lag ordinarily has a lesser effect on specific impulse than does velocit}^ lag. Gilbert, Davis, and Altman (2) were the first to relate the losses to particle size. They solved the linear equation that results from assuming the drag force to be proportional to the velocity difference (Stokes' law) for the case of linearly accelerated nozzle gas. They demonstrated that, typically, a 1-ju diam particle follows the gas velocity closely, whereas a IQ-fj, diam particle has a significant lag. All of these early studies treated the nozzle expansion processes as though they are uncoupled, i.e., the thermal lag

A parameterization of the ice‐ocean drag coefficient

Abstract: [1] A parameterization of the ice-ocean drag coefficient (Cw) was developed through partitioning the oceanic drag force into three components: (1) form drag on the floe edge, (2) form drag on the ridge keel, and (3) skin friction on the ice bottom. Through these quantities, Cw was expressed as a function of observable sea ice geometric parameters. Sensitivity studies were carried out to investigate the influence of varying sea ice conditions on Cw. The results revealed that Cw increases first and then decreases with increasing ice concentration (A), similar to the observations of the air-ice drag coefficient, and which is mainly attributed to the nonmonotonic variation of the form drag on the floe edge with ice concentration. Moreover, the form drag on the floe edge is always the dominant component, having a proportion of more than 60% in sea ice with a large aspect ratio (draft/length ≥ 1/100), indicating the necessity of including this term in sea ice dynamic models, particularly for the marginal ice zone (MIZ). The form drag on the ridge keel becomes dominant only when the ridging intensity is extremely high (depth/spacing ≥ 1/20). Additionally, a large value of Cw cannot be caused only by the inclusion of form drag terms but also by large skin friction over rough ice bottoms. Finally, for typical situations in the MIZ with moderate ridging intensity, the parameterization will underestimate Cw by approximately 30% for a rough ice bottom and by over 80% for a smooth ice bottom if no form drags are considered.

Flow around spheres by dissipative particle dynamics

Abstract: The dissipative particle dynamics (DPD) method is used to study the flow behavior past a sphere. The sphere is represented by frozen DPD particles while the surrounding fluids are modeled by simple DPD particles (representing a Newtonian fluid). For the surface of the sphere, the conventional model without special treatment and the model with specular reflection boundary condition proposed by Revenga et al. [Comput. Phys. Commun. 121–122, 309 (1999)] are compared. Various computational domains, in which the sphere is held stationary at the center, are investigated to gage the effects of periodic conditions and walls for Reynolds number (Re)=0.5 and 50. Two types of flow conditions, uniform flow and shear flow are considered, respectively, to study the drag force and torque acting on the stationary sphere. It is found that the calculated drag force imposed on the sphere based on the model with specular reflection is slightly lower than the conventional model without special treatment. With the conventional...

Direct field measurement of wind drag on vegetation for application to windbreak design and modelling

Abstract: A field instrument was designed and field tested for measuring the applied wind load on trees and surface-mounted obstacles in a natural boundary layer. Using this instrument, the effect of vegetation porosity on the drag coefficient of small conifer trees (h=1·4 m) was determined directly in the field. Drag coefficients for two simple solid geometric forms (cone and cylinder) having approximately the same size (height and diameter) as the conifer trees were also measured over a relatively wide range of Reynolds numbers and the results compared to published drag curves for these shapes. The field study found that the porous element had a higher drag coefficient than a solid element, both for the solid element tested and for the drag coefficient suggested for surface-mounted solid obstacles. The drag coefficient changed on a continuum with porosity, rising initially from the value measured for the element as a solid, reaching a peak at an intermediate value and eventually falling to zero when the element was removed. This peak in the drag coefficient versus porosity curve corresponds to reports that shelterbelt efficiency peaks at medium-porosities, and is an important relationship in terms of modelling momentum extraction of vegetation, one which has not been shown previously in the literature. Findings of this study have direct application to the modelling of shelterbelts and windbreaks and the assessment of the amount of vegetation cover required to suppress wind erosion in rangeland vegetation communities. © 1998 John Wiley & Sons, Ltd.

Permeability calculations in three-dimensional isotropic and oriented fiber networks

Abstract: Hydraulic permeabilities of fiber networks are of interest for many applications and have been studied extensively. There is little work, however, on permeability calculations in three-dimensional random networks. Computational power is now sufficient to calculate permeabilities directly by constructing artificial fiber networks and simulating flow through them. Even with today’s high-performance computers, however, such an approach would be infeasible for large simulations. It is therefore necessary to develop a correlation based on fiber volume fraction, radius, and orientation, preferably by incorporating previous studies on isotropic or structured networks. In this work, the direct calculations were performed, using the finite element method, on networks with varying degrees of orientation, and combinations of results for flows parallel and perpendicular to a single fiber or an array thereof, using a volume-averaging theory, were compared to the detailed analysis. The detailed model agreed well with existing analytical solutions for square arrays of fibers up to fiber volume fractions of 46% for parallel flow and 33% for transverse flow. Permeability calculations were then performed for isotropic and oriented fiber networks within the fiber volume fraction range of 0.3%–15%. When drag coefficients for spatially periodic arrays were used, the results of the volume-averaging method agreed well with the direct finite element calculations. On the contrary, the use of drag coefficients for isolated fibers overpredicted the permeability for the volume fraction range that was employed. We concluded that a weighted combination of drag coefficients for spatially periodic arrays of fibers could be used as a good approximation for fiber networks, which further implies that the effect of the fiber volume fraction and orientation on the permeability of fiber networks are more important than the effect of local network structure.