Showing papers on "Drag coefficient published in 2013"

On the exchange of momentum over the open ocean

Abstract: This study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 ...

Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds number boundary layer flow

Abstract: In this paper, we report the measurement of skin friction drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin friction drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ∼30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the drag in the turbulent flow. The results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.

Filtered two‐fluid models of fluidized gas‐particle flows: New constitutive relations

Abstract: New constitutive relations for filtered two-fluid models (TFM) of gas-particle flows are obtained by systematically filtering results generated through highly resolved simulations of a kinetic theory-based TFM. It was found in our earlier studies that the residual correlations appearing in the filtered TFM equations depended principally on the filter size and filtered particle volume fraction. Closer inspection of a large amount of computational data gathered in this study reveals an additional, systematic dependence of the correction to the drag coefficient on the filtered slip velocity, which serves as a marker for the extent of subfilter-scale inhomogeneity. Furthermore, the residual correlations for the momentum fluxes in the gas and particle phases arising from the subfilter-scale fluctuations are found to be modeled nicely using constitutive relations of the form used in large-eddy simulations of single-phase turbulent flows. V C 2013 American

A numerical study of the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow

Abstract: Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-friction drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-friction drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-friction drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing drag in turbulent flows than the direct effect of the slip, which led to a modest drag reduction in laminar flows. It was found that the drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units.

Automatic Differentiation Adjoint of the Reynolds-Averaged Navier-Stokes Equations with a Turbulence Model

Abstract: This paper presents an approach for the rapid implementation of an adjoint solver for the ReynoldsAveraged Navier–Stokes equations with a Spalart–Allmaras turbulence model. Automatic differentiation is used to construct the partial derivatives required in the adjoint formulation. The resulting adjoint implementation is computationally efficient and highly accurate. The assembly of each partial derivative in the adjoint formulation is discussed. In addition, a coloring acceleration technique is presented to improve the adjoint efficiency. The RANS adjoint is verified with complex-step method using a flow over a bump case. The RANS-based aerodynamic shape optimization of an ONERA M6 wing is also presented to demonstrate the aerodynamic shape optimization capability. The drag coefficient is reduced by 19% when subject to a lift coefficient constraint. The results are compared with Euler-based aerodynamic shape optimization and previous work. Finally, the effects of the frozenturbulence assumption on the accuracy and computational cost are assessed.

Flow adjustment at the leading edge of a submerged aquatic canopy

Abstract: [1] This paper describes the transition from open channel flow to flow over submerged vegetation using velocity measurements collected with acoustic Doppler velocimetry (ADV) and particle-image velocimetry (PIV). Submerged canopies were constructed from arrays of rigid circular cylinders of height h in water of depth H. Both the canopy density, described by the frontal area per volume (a), and degree of submergence (H/h) were varied. Flow adjustment occurs in three stages. First, velocity begins to decelerate upstream of the canopy, due to a high-pressure region generated at the canopy leading edge, and continues to decelerate within the canopy, due to canopy drag. Rapid flow deceleration within the canopy creates strong vertical flux out through the top of the canopy that extends over a length proportional to the canopy drag length scale, (CDa)−1, with CD being the canopy drag coefficient. Second, a mixing layer develops at the canopy interface, with the stress at the top of the canopy initially increasing, but eventually reaching a constant value. At this point, the flow within the canopy is fully developed. The length scale for mixing-layer development is related to canopy drag (CDa) and the depth ratio (H/h). In the third stage, the boundary layer above the mixing layer adjusts to the channel boundary conditions. A model is developed to predict the adjustment of vertically averaged velocity within the canopy. Measurements confirm that the flow adjustment is not dependent on canopy length.

Experimental Study of Dry Granular Flow and Impact Behavior Against a Rigid Retaining Wall

Abstract: Shallow slope failure in mountainous regions is a common and emergent hazard in terms of its damage to important traffic routes and local communities. The impact of dry granular flows consisting of rock fragments and other particles resulting from shallow slope failures on retaining structures has yet to be systematically researched and is not covered by current design codes. As a preliminary study of the impact caused by dry granular flows, a series of dry granular impact experiments were carried out for one model of a retaining wall. It was indirectly verified that the total normal force exerted on a retaining wall consists of a drag force (Fd), a gravitational and frictional force (Fgf), and a passive earth force (Fp), and that the calculation of Fd can be based on the empirical formula defined in NF EN Eurocode 1990 (Eurocode structuraux. Base de calcul des structures, AFNOR La plaine Saint Denis, 2003). It was also indirectly verified that, for flow with Froude number from 6 to 11, the drag coefficient (Cd) can be estimated using the previously proposed empirical parameters.

The settling velocity of mineral, biomineral, and biological particles and aggregates in water

Abstract: [1] A new equation was developed to relate the size and settling velocity of particulate matter commonly recurring in aqueous ecosystems. This equation explicitly balanced the gravitational, buoyancy, viscous, and inertial forces as in Rubey (1933) but was amended to describe in one instance both individual particles and granular aggregates with an internal fractal architecture. This approach allowed for an algebraic solution of the settling velocity, thus overcoming earlier approaches that required iterative numerical solutions. The equation was tested with mineral, biomineral, and biological suspended particles and granular aggregates from 52 existing experimental data sets, and resulted in average correlation coefficients R between 71% and 93.9%, and normilized residuals between 14.3% and 24.8% over Reynolds numbers ranging within 10−7 and 102. Accuracy of these results was generally better than for the Stokes' law, the Stokes' law modified with the Schiller-Naumann drag coefficient, and Rubey's equation. Estimated parameters ranged within observed ones, thus suggesting that the equation was robust. An analysis of the drag showed that inertial force was negligible only for biological cells (isolated cysts), whereas it contributed by not less than 5% to the drag on large mineral particles and up to 20% for biomineral and biological aggregates. Finally, a correlation was found between the organic matter content and fractal properties of granular aggregates, which were described by empirical equations proposed here for the first time. The hypothesis that the settling velocity is a function of linear and nonlinear drag, and is ultimately determined by physical characteristics as much as biological composition and internal aggregate geometry, is supported here by quantitative analyses.

Experiments on Drag of Revolving Disks, Cylinders, and Streamline Rods at High Speeds

Abstract: An experimental investigation concerned primarily with the extension of test data on the drag of revolving disks, cylinders, and streamline rods to high Mach numbers and Reynolds numbers is presented.

Quantifying wave attenuation to inform coastal habitat conservation

Abstract: Coastal vegetation can protect people and property from erosion and flooding, potentially providing a win-win solution for conservation and development. However, the conditions under which natural habitats provide protection have been controversial, partly because the geomorphic, ecological, and hydrodynamic factors that determine wave attenuation vary greatly among locations, times, and studies. We re-analyzed existing wave attenuation studies in kelp, mangrove, marsh and seagrass habitats and found that much of the variation in wave attenuation can be explained by differences in vegetation characteristics and by the change in bulk drag with flow conditions. We found that vegetation can exert substantial drag on passing waves, but that the bulk drag coefficient declines in flow conditions characterized by high Reynolds numbers. This decline is important because storm conditions are highly turbulent (typical Reynolds numbers are greater than 104), and we lack empirical measurements of bulk drag coefficients from such conditions. Failing to account for the decline can over-estimate wave attenuation in storms by 19% to 1600%. These results suggest that larger habitat areas will need to be set aside for coastal protection than previously thought. Our approach provides predictions for designing practical habitat conservation and restoration plans that also protect humans and property from flooding and erosion.

Calculating the ecological impacts of animal-borne instruments on aquatic organisms

Abstract: Summary 1. Animal-borne instruments provide researchers with valuable data to address important questions on wildlife ecology and conservation. However, these devices have known impacts on animal behaviour and energetics. Tags deployed on migrating animals may reduce reproductive output through increased energy demands or cause phenological mismatches of foraging and nesting events. For marine organisms, the only tagging guidelines that exist are based on lift and thrust impacts on birds – concepts that do not translate well to aquatic animals. Herein, we provide guidelines on assessing drag from animal-borne instruments and discuss the ecological impacts on marine organisms. Of particular concern is the effect of drag from instruments to the welfare of the animals and for the applicability of collected data to wild populations. 2. To help understand how drag from electronic tags affects marine animals in the wild, we used marine turtles as model aquatic organisms and conducted wind tunnel experiments to measure the fluid drag of various marine turtle body types with and without commercially available electronic tags (e.g. satellite, TDR, video cameras). We quantified the drag associated with carrying biotelemetry devices of varying frontal area and design (squared or tear drop shaped) and generated contour plots depicting percentage drag increase as a framework for evaluating tag drag by scientists and wildlife managers. Then, using concepts of fluid dynamics, we derived a universal equation estimating drag impacts from instruments across marine taxa. 3. The drag of the marine turtle casts was measured in wind speeds from 2 to 30 m s 1 (Re 30 9 10 4 – 19 9 10 6 ), equivalent to 01–1 9ms 1 in seawater. The drag coefficient (CD) of the marine turtles ranged from 011 to 022, which is typical of other large, air-breathing, marine vertebrates (008–026). The CD of tags in reference to the turtle casts was 091 018 and most tags caused minimal additional drag ( 100%). The sensitivity of aquatic animals to instrument drag is a dynamic relationship between the fluid flow patterns, or CD, and the frontal area ratio of the animal and tag. 4. In this paper, we have outlined methods for quantifying the drag costs from animal-borne instrumentation considering the instrument retention time (time to release from the animal) and the activity of the instrumented animal. With this valuable tool, researchers can quantify the drag costs from animal-borne instrumentation and choose appropriate tags for their intended study organism and question. Reducing drag will ultimately reduce the impact on the instrumented animals and lead to greater biological realism in the collected data.

Intra-City Variation in Urban Morphology and Turbulence Structure in Helsinki, Finland

Abstract: Most atmospheric boundary-layer theories are developed over vegetative surfaces and their applicability at urban sites is questionable. Here, we study the intra-city variation of turbulence characteristics and the applicability of boundary-layer theory using building-morphology data across Helsinki, and eddy-covariance data from three sites: two in central Helsinki (400 m apart) and one 4 km away from the city centre. The multi-site measurements enable the analysis of the horizontal scales at which quantities that characterize turbulent transport vary: (i) Roughness characteristics vary at a 10-m scale, and morphometric estimation of surface-roughness characteristics is shown to perform better than the often used rule-of-thumb estimates (average departures from the logarithmic wind profile are 14 and 44 %, respectively). (ii) The drag coefficient varies at a 100-m scale, and we provide an updated parametrization of the drag coefficient as a function of z/zH (the ratio of the measurement height to the mean building height). (iii) The transport efficiency of heat, water vapour and CO2 is shown to be weaker the more heterogeneous the site is, in terms of sources and sinks, and strong scalar dissimilarity is observed at all sites. (iv) Atmospheric stability varies markedly even within 4 km across the city: the median difference in nocturnal sensible heat fluxes between the three sites was over 50W m−2. Furthermore, (v) normalized power spectra and cospectra do not vary between sites, and they follow roughly the canonical theory as developed over vegetated terrain.

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

Abstract: Analytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

Calculation of Drag Coefficient for Arrays of Emergent Circular Cylinders with Pseudofluid Model

Abstract: The emergent vegetation in open-channel flows is usually simulated using arrays of circular cylinders in laboratory experiments. Analysis of recent experimental data reveals that for a given Reynolds number, the drag coefficient of a cylinder in a dense array is larger than that of an isolated cylinder. A new approach is applied to parameterize the drag coefficient and Reynolds number for flows through arrays of emergent cylinders. The approach is developed based on the concept of pseudofluid, for which an analogy is made between the cylinder-induced drag in an open-channel flow and that induced by the cylinder settling in a stationary fluid. With the proposed parameterization, the experimental database is successfully reorganized in such a way that a generalized drag coefficient is related to a generalized Reynolds number by one single curve, which is valid for a wide range of solid fractions and Reynolds numbers. However, it should be mentioned that only rigid circular stems are considered in th...

Air speeds of migrating birds observed by ornithodolite and compared with predictions from flight theory

Abstract: We measured the air speeds of 31 bird species, for which we had body mass and wing measurements, migrating along the east coast of Sweden in autumn, usinga VectronixVector 21 ornithodolite and a GillWindSonicanemometer. We expected each species’ average air speed to exceed its calculated minimum-power speed (Vmp), and to fall below its maximum-range speed (Vmr), but found some exceptionsto both limits. To resolve these discrepancies, we first reduced the assumed induced power factor for all species from 1.2 to 0.9, attributing this to splayed and up-turned primary feathers, and then assigned body drag coefficients for different species down to 0.060 for small waders, and up to 0.12 for the mute swan, in the Reynolds number range 25000–250000. These results will be used to amend the default values in existing software that estimates fuel consumption in migration, energy heights on arrival and other aspects of flight performance, using classical aeronautical theory. The body drag coefficients are central to range calculations. Although they cannot be measured on dead bird bodies, they could be checked against wind tunnel measurements on living birds, using existing methods.

Induced-Drag Calculations in the Unsteady Vortex Lattice Method

Abstract: α angle of attack, rad Γ circulation, m2s−1 λ wake wavelength, m ρ∞ free-stream air density, kgm −3 τ panel tangential vector ω angular velocity, rad s−1 a non-dimensional distance between aerofoil mid-chord and centre of rotation A panel area, m b semi-chord, m ∆b panel span, m B wingspan, m c aerofoil chord, m ∆c panel chord, m C Theodorsen’s function, C(k) = F (k) + iG(k) Cd sectional drag coefficient CD wing drag coefficient Cl sectional lift coefficient Cs sectional leading-edge suction coefficient ∗Graduate Student, Department of Aeronautics. AIAA Student Member. †Senior Lecturer, Department of Aeronautics. E-mail: rpalacio@imperial.ac.uk. AIAA Member. ‡Lecturer, Department of Mechanical Engineering Sciences. AIAA Member.

Drag force scaling for penetration into granular media.

Abstract: Impact dynamics is measured for spherical and cylindrical projectiles of many different densities dropped onto a variety non-cohesive granular media. The results are analyzed in terms of the material-dependent scaling of the inertial and frictional drag contributions to the total stopping force. The inertial drag force scales similar to that in fluids, except that it depends on the internal friction coefficient. The frictional drag force scales as the square-root of the density of granular medium and projectile, and hence cannot be explained by the combination of granular hydrostatic pressure and Coulomb friction law. The combined results provide an explanation for the previously observed penetration depth scaling.

Tsunami damping by mangrove forest: A laboratory study using parameterized trees

Abstract: . Tsunami attenuation by coastal vegetation was examined under laboratory conditions for mature mangroves Rhizophora sp. The developed novel tree parameterization concept, accounting for both bio-mechanical and structural tree properties, allowed to substitute the complex tree structure by a simplified tree model of identical hydraulic resistance. The most representative parameterized mangrove model was selected among the tested models with different frontal area and root density, based on hydraulic test results. The selected parameterized tree models were arranged in a forest model of different width and further tested systematically under varying incident tsunami conditions (solitary waves and tsunami bores). The damping performance of the forest models under these two flow regimes was compared in terms of wave height and force envelopes, wave transmission coefficient as well as drag and inertia coefficients. Unlike the previous studies, the results indicate a significant contribution of the foreshore topography to solitary wave energy reduction through wave breaking in comparison to that attributed to the forest itself. A similar rate of tsunami transmission (ca. 20%) was achieved for both flow conditions (solitary waves and tsunami bores) and the widest forest (75 m in prototype) investigated. Drag coefficient CD attributed to the solitary waves tends to be constant (CD = 1.5) over the investigated range of the Reynolds number.

Depth-independent drag force induced by stirring in granular media.

Abstract: The drag force experienced by a horizontal cylinder rotating around the vertical axis in a granular medium under gravity is experimentally investigated. We show that, for deeply buried objects, the drag force dramatically drops after half a turn, as soon as the cylinder crosses its own wake. Whereas the drag during the first half turn increases linearly with the depth, the drag after several rotations appears to be independent of depth, in contradiction with the classical frictional picture stipulating that the drag is proportional to the hydrostatic pressure. We systematically study how the saturated drag force scales with the control parameters and show that this effect may be used to drill deeply in a granular medium without developing high torques.