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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: The pattern of air flow over bird wings, as indicated by pressure-distribution data, is consistent with aerodynamic theory for aeroplane wings at low Reynolds numbers, and with the observed lift and drag coefficients.
Abstract: The aerodynamic properties of bird wings were examined at Reynolds numbers of 1-5 × 10 4 and were correlated with morphological parameters such as apsect ratio, camber, nose radius and position of maximum thickness. The many qualitative differences between the aerodynamic properties of bird, insect and aeroplane wings are attributable mainly to their differing Reynolds numbers. Bird wings, which operate at lower Reynolds numbers than aerofoils, have high minimum drag coefficients (0·03-0·13), low maximum lift coefficients (0·8-1·2) and low maximum lift/drag ratios (3–17). Bird and insect wings have low aerofoil efficiency factors (0·2-0·8) compared to conventional aerofoils (0·9-0·95) because of their low Reynolds numbers and high profile drag, rather than because of a reduced mechanical efficiency of animal wings. For bird wings there is clearly a trade-off between lift and drag performance. Bird wings with low drag generally had low maximum lift coefficients whereas wings with high maximum lift coefficients had high drag coefficients. The pattern of air flow over bird wings, as indicated by pressure-distribution data, is consistent with aerodynamic theory for aeroplane wings at low Reynolds numbers, and with the observed lift and drag coefficients.

133 citations

Journal ArticleDOI
TL;DR: In this article, the momentum equations describing the steady cross-flow of power law fluids past an unconfined circular cylinder have been solved numerically using a semi-implicit finite volume method.
Abstract: The momentum equations describing the steady cross-flow of power law fluids past an unconfined circular cylinder have been solved numerically using a semi-implicit finite volume method The numerical results highlighting the roles of Reynolds number and power law index on the global and detailed flow characteristics have been presented over wide ranges of conditions as 5 ≤ Re ≤ 40 and 06 ≤ n ≤ 2 The shear-thinning behaviour (n 1) show the opposite behaviour Furthermore, while the wake size shows non-monotonous variation with the power law index, but it does not seem to influence the values of drag coefficient The stagnation pressure coefficient and drag coefficient also show a complex dependence on the power law index and Reynolds number In addition, the pressure coefficient, vorticity and viscosity distributions on the surface of the cylinder have also been presented to gain further physical insights into the detailed flow kinematics

133 citations

Journal ArticleDOI
TL;DR: In this article, a conformal transformation of the fluid flow onto the exterior of a polygon, and thence onto the interior of a unit circle is presented, where the initial irrotational flow is represented by a logarithmic vortex at the centre of the circle.
Abstract: Although the form and dimensions of steep vortex ripples are well studied in relation to the oscillating flow which generates them, nevertheless the accompanying fluid motion is not yet understood quantitatively. In this paper we present a method of calculation based on the assumption that the sand-water interface is fixed and that the effect of sand in suspension is, to a first approximation, negligible. The method employs a simple conformal transformation of the fluid flow onto the exterior of a polygon, and thence onto the interior of a unit circle. The initial, irrotational flow is represented by a logarithmic vortex at the centre of the circle. Other vortices within the fluid are each represented by a symmetric system of P vortices and their images in the unit circle, P being the number of sides of the original polygon. Typically P is equal to 5. However, P is not limited to integer values but may be any rational number greater than 2 (see § 15). To proceed with the calculation it is assumed that separation of the boundary layer takes place at the sharp crests of the ripples, and that the shed vorticity can be represented by discrete vortices, with strengths given by Prandtl's rule. (For a typical time sequence see figures 7 and 8.) After a complete cycle, a vortex pair is formed, which can escape upwards from the neighbourhood of the boundary. The total momentum per ripple wavelength and the horizontal force on the bottom are expressible very simply in terms of the shed vortices at any instant. The force consists of two parts: an added-mass term which dissipates no energy, and a ‘vortex drag’, which extracts energy from the oscillating flow. The calculation is at first carried out with point vortices, in a virtually inviscid theory. However, it is found appropriate to assume that each vortex has a solid core whose radius expands with time like [e( t − t n )] ½ , where t n denotes the time of birth, and e is a small parameter analogous to a viscosity. The expansion of the vortex tends to reduce the total energy (which otherwise would increase without limit) at a rate independent of e. If the cores of two neighbouring vortices overlap they are assumed to merge, by certain simple rules. Calculation of the effective vortex drag in an oscillating flow yields drag coefficients $\overline{C}_D$ of the order of 10 −1 , in good agreement with the measurements of Bagnold (1946) and of Carstens, Nielson & Altinbilek (1969). The tendency for the highest drag coefficients to occur when the ratio 2 a / L of the total horizontal excursion of the particles to the ripple length is about 1·5 is confirmed. When 2 a / L = 4, the drag falls to about half its value at ‘resonance’.

132 citations

Journal ArticleDOI

132 citations

Journal ArticleDOI
TL;DR: In this article, the authors measured the spanwise spacing and bursting rate of the wall-layer structure of a turbulent channel flow of water and showed that when the additives are confined entirely to the linear sublayer of the water flow and there is no evidence of drag reduction, the span-wise streak spacing increases and the average bursting rate decreases.
Abstract: When drag-reducing additives are confined entirely to the linear sublayer of a turbulent channel flow of water, both the spanwise spacing and bursting rate of the wall-layer structure are the same as those for a water flow and there is no evidence of drag reduction. Drag reduction is measured downstream of the location where the additives injected into the sublayer begin to mix in significant quantities with the buffer region (10 y + The superscript + denotes a dimensionless quantity scaled with the kinematic viscosity ν and the wall shear velocity v * = (τ w /ρ) ½ . of the channel flow. At streamwise locations where drag reduction does occur and where the injected fluid is not yet uniformly mixed with the channel flow, the dimensionless spanwise streak spacing increases and the average bursting rate decreases. The decrease in bursting rate is larger than the corresponding increase in streak spacing. The wall-layer structure is like the structure in the flow of a homogeneous, uniformly mixed, drag-reducing solution. Thus, the additives have a direct effect on the flow processes in the buffer region and the linear sublayer appears to have a passive role in the interaction of the inner and outer portions of a turbulent wall layer.

132 citations


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