Showing papers on "Drag coefficient published in 1991"

Polymer Electrolyte Fuel Cell Model

Abstract: We present here an isothermal, one-dimensional, steady-state model for a complete polymer electrolyte fuel cell (PEFC) with a 117 Nation | membrane. In this model we employ water diffusion coefficients electro-osmotic drag coefficients, water sorption isotherms, and membrane conductivities, all measured in our laboratory as functions of membrane water content. The model pre.dicts a net-water-per-proton flux ratio of 0.2 H20/H § under typical operating conditions, which is much less than the measured electro-osmotic drag coefficient for a fully hydrated membrane. It also predicts an increase in membrane resistance with increased current density and demonstrates the great advantage of a thinner membrane in alleviating this resistance problem. Both of these predictions were verified experimentally under certain conditions.

Transient eddy formation around headlands

Abstract: Eddies with length scales of 1–10 km are commonly observed in coastal waters and play an important role in the dispersion of water-borne materials. The generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models. Using shallow water depth-averaged vorticity dynamics, eddies are shown to form when flow separation occurs near the tip of the headland, causing intense vorticity generated along the headland to be injected into the interior. An analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring (accelerating) to adverse (decelerating), and its occurrence depends principally on three parameters: the aspect ratio [b/a], where b and a are characteristic width and length scales of the headland; [H/CDa], where H is the water depth, CD is the depth-averaged drag coefficient; and [Uo/σa], where Uo and σ are the magnitude and frequency of the far-field tidal flow. Simulations with a depth-averaged numerical model show a wide range of responses to changes in these parameters, including cases where no separation occurs, cases where only one eddy exists at a given time, and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist, interacting strongly with each other. These simulations also demonstrate that in unsteady flow, a strong start-up vortex forms after the flow separates, leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow.

Unsteady drag on a sphere at finite reynolds number with small fluctuations in the free-stream velocity

Abstract: Unsteady flow over a stationary sphere with small fluctuations in the free-stream velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady Stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the force has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & Mckee (1984) wherein for a particle introduced to a turbulent flow the initial velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effects of Reynolds number on the unsteady drag due to the fluctuating free-stream velocity are presented. The implications for particle motion in turbulence are discussed.

Drag Coefficient and Fall Velocity of nonspherical particles

Abstract: The most commonly encountered particles in the areas of sediment transport, fluidization, sedimentation, etc. are nonspherical. The role of these particles can be ascertained only through the knowledge of drag coefficient and fall velocity. Graphical relationships for the determination of drag coefficient and fall velocity are available in the literature. The graphical relationships are less accurate because they are subject to errors of judgment. Furthermore, these relationships cannot be used for any analytical purposes. In 1954, Schulz et al. collected extensive data for the drag coefficient and fall velocity of nonspherical particles. Based on these data, empirical equations for the drag coefficient and fall velocity of nonspherical particles of natural and mechanical origin are derived from curve fittings and presented. It is hoped that these equations will find use in the analytical modeling of various engineering problems involving fluid-nonspherical particles interaction.

The roughness of wind waves

Abstract: In this paper, a new dynamic roughness equation for airflow over wind waves is proposed, based on relationships of dimensionless parameters and the 1986 HEXMAX field data. The equation can be considered as a modification of the Charnock formula. The data are also compared with the parameterizations of Toba and Koga, and of Hsu, and the consequences of the new equation for aerodynamic drag are discussed. In the new equation, the drag coefficient is expressed as a function of both wind speed and wave age c p/u*. This testifies to the strong wave-age dependence of the drag, found in several experimental studies. It is also in good agreement with results of a theoretical model of the airflow-seawave interaction, proposed by Janssen (1989).

Drag balance for hypervelocity impulse facilities

Abstract: A new technique is described for measuring drag with 100-/*s rise time on a nonlifting model in a free piston shock tunnel. The technique involves interpretation of the stress waves propagating within the model and its support. A finite element representation and spectral methods are used to obtain a mean square optimal estimate of the time history of the aerodynamic loading. Thus, drag is measured instantaneously and the previous restriction caused by the mechanical time constant of balances is overcome. The effectiveness of the balance is demonstrated by measuring the drag on a cone with 15-deg semivertex angle in nominally Mach 5.6 flow with stagnation enthalpies from 2.6 to 33 MJ/kg. Measurement repeatability of about 10% is achieved.

Variational Estimation of the Wind Stress Drag Coefficient and the Oceanic Eddy Viscosity Profile

Abstract: A variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic Ekman layer model. An identical twin experiment is discussed first in which the “observations” are created by the dynamic model. The field measurements from the LOTUS-3 (Long-Term Upper Ocean Study-3) buoy are then analysed. By fitting the model results to the data, the unknown boundary condition (the wind stress drag coefficient) and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data, and an optimal estimate of the current field is obtained. Though the model is simple, the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution.

Viscous flow in a channel with periodic cross-bridging fibres : exact solutions and brinkman approximation

Abstract: A general solution of the three-dimensional Stokes equations is developed for the viscous flow past a square array of circular cylindrical fibres confined between two parallel walls. This doubly periodic solution, which is an extension of the theory developed by Lee & Fung (1969) for flow around a single fibre, successfully describes the transition in behaviour from the Hele-Shaw potential flow limit (aspect ratio B [Lt ] 1) to the viscous two-dimensional limiting case (B [Gt ] 1, Sangani & Acrivos 1982) for the hydrodynamic interaction between the fibres. These results are also compared with the solution of the Brinkman equation for the flow through a porous medium in a channel. This comparison shows that the Brinkman approximation is very good when B > 5, but breaks down when B [les ] O(1). A new interpolation formula is proposed for this last regime. Numerical results for the detailed velocity profiles, the drag coefficient f, and the Darcy permeability Kp are presented. It is shown that the velocity component perpendicular to the parallel walls is only significant within the viscous layers surrounding the fibres, whose thickness is of the order of half the channel height B′. One finds that when the aspect ratio B > 5, the neglect of the vertical velocity component vz can lead to large errors in the satisfaction of the no-slip boundary conditions on the surfaces of the fibres and large deviations from the approximate solution in Lee (1969), in which vz and the normal pressure field are neglected. The numerical results show that the drag coefficient of the fibrous bed increases dramatically when the open gap between adjacent fibres Δ′ becomes smaller than B′. The predictions of the new theory are used to examine the possibility that a cross-bridging slender fibre matrix can exist in the intercellular cleft of capillary endothelium as proposed by Curry & Michel (1980).

The added mass, Basset, and viscous drag coefficients in nondilute bubbly liquids undergoing small-amplitude oscillatory motion

Abstract: The motion of bubbles dispersed in a liquid when a small‐amplitude oscillatory motion is imposed on the mixture is examined in the limit of small frequency and viscosity. Under these conditions, for bubbles with a stress‐free surface, the motion can be described in terms of added mass and viscous force coefficients. For bubbles contaminated with surface‐active impurities, the introduction of a further coefficient to parametrize the Basset force is necessary. These coefficients are calculated numerically for random configurations of bubbles by solving the appropriate multibubble interaction problem exactly using a method of multipole expansion. Results obtained by averaging over several configurations are presented. Comparison of the results with those for periodic arrays of bubbles shows that these coefficients are, in general, relatively insensitive to the detailed spatial arrangement of the bubbles. On the basis of this observation, it is possible to estimate them via simple formulas derived analytically for dilute periodic arrays. The effect of surface tension and density of bubbles (or rigid particles in the case where the no‐slip boundary condition is applicable) is also examined and found to be rather small.

Drag reduction and turbulent structure in two- dimensional channel flows

Abstract: A two-component laser velocimeter has been used to determine the effect of wall strain rate, polymer concentration and channel height upon the drag reduction and turbulent structure in fully developed, low concentration, two-dimensional channel flows. Water flows at equal wall shear stress and with Reynolds numbers from 14430 to 34640 were measured for comparison. Drag reduction levels clearly depended upon wall strain rate, polymer concentration and channel height independently.However, most of the turbulent structure depended only upon the level of drag reduction. The slope of the logarithmic law of the wall increased as drag reduction increased. Similarly, the root-mean-square of the fluctuations in the streamwise velocity increased while the r.m.s. of the fluctuations in the wall-normal velocity decreased with drag reduction. The production of the streamwise normal Reynolds stress and the Reynolds shear stress decreased in the drag-reduced flows. Therefore it appears that the polymer solutions inhibit the transfer of energy from the streamwise to the wall-normal velocity fluctuations. This could occur through inhibiting the newtonian transfer mechanism provided by the pressure-strain correlation. In six drag-reducing flows, the sum of the Reynolds stress and the mean viscous stress was equal to the total shear stress. However, for the combination of highest concentration (5 p.p.m.), smallest channel height (25 mm) and highest wall strain rate (4000 s - 1 ), the sum of the Reynolds and viscous stresses was substantially lower than the total stress indicating the presence of a strong non-newtonian effect. In all drag-reducing flows the correlation coefficient for uv decreased as the axes of principal stress for the Reynolds stress rotated toward the streamwise and wall-normal directions.

An optimum suppression of fluid forces by controlling a shear layer separated from a square prism

Abstract: This paper deals with the suppression of the fluid forces by controlling a shear layer on one side separated from a square prism. The control of the separated shear layer was established by setting up a small circular cylinder (the control cylinder) in it on one side. Experimental data were collected to examine the effects on the fluid forces and vortex shedding frequency due to variation of the position and diameter of the control cylinder. The results show that (i) the maximum reduction of the time-mean drag and fluctuating lift and drag occurred when the control cylinder was located near what would ordinarily be considered the outer boundary of the shear layer; (ii) the control of the separated shear layer by means of a small cylinder appeared to be effective in suppressing the fluctuating lift and drag rather than the time-mean drag; (iii) in the case of the control cylinder of 6 mm in diameter, the time-mean drag was reduced to about 30 percent, and the fluctuating lift and drag were reduced to approximately 95 and 75 percent, respectively; (iv) the fluid forces and the frequency of vortex shedding of the square prism were mainly dependent on the characteristics of a very thin region near the outer boundary of the shear layer.

On the effect of riblets in fully developed laminar channel flows

Abstract: The effect of longitudinal riblet surfaces on viscous drag in fully developed laminar channel flows was investigated. Unlike turbulent flows, drag reduction was not obtained in the laminar flows. Results were independent of Reynolds number. Wall‐shear rates on most regions of the cross‐sectional perimeter of riblets were smaller than that of corresponding plane channel flow even though the net drag was increased.

Simulation of wind-distorted sprinkler patterns

Abstract: Single‐sprinkler wind‐distorted distribution patterns are simulated utilizing drop trajectory computations, and compared with measured patterns. It is found that the exact formulation of the drag coefficient of single drops is not critical for applications focusing on water distribution. A drag correction factor, k is introduced to account for the effect of the incidence angle between drag force and orientation of a jet segment. The agreement between measured and computed patterns improves considerably when the proper value of k has been used. Three measures of similarity of computed to measured single patterns are put forth. The most comprehensive measure, denoted by σ, is a normalized sum of squares of differences between the measured and computed local application rates. The values of k that produce the best agreement in terms of σ seem to be an increasing function of the range of a sprinkler. Single patterns computed with the best values of k were used to calculate uniformity coefficients for a realis...

Dragonfly Flight: Power Requirements at High Speed and Acceleration

Abstract: Summary Most studies of insect flight deal primarily with hovering or with forward flight at constant, moderate speed. This paper reports investigations of flight characteristics that are especially relevant to the performance of dragonflies at high and/or changing velocity. Dragonflies were filmed in free flight in the field to determine velocity and acceleration. The power required for repeated acceleration is shown to be large, in some circumstances, relative to the estimated maximum available power and probable top power requirements for steady flight. Distributions of velocity and acceleration, and concomitant power requirements, differ markedly among species, however. In addition, parasite drag was measured in winds of 2-7ms" 1 and drag coefficients determined to be about 0.40 at Reynolds number greater than 10 4 . This result implies substantially lower power requirements at high speeds, compared to previous estimates. Other aspects of power output, including the probable magnitude of inertial power, are considered in relation to published data.

On the wave age dependent drag coefficient and roughness length at sea

Abstract: The wave age dependent drag coefficient and roughness length at sea have been studied theoretically by using the simple Phillips spectrum with a wave age dependent Phillips coefficient. It is shown that the turbulent surface stress, which is computed in a reference frame moving with the surface waves according to Kitaigorodskii's ideas, leads to a wave age dependent Charnock constant in the range 0.010 to 0.022. The wave-induced stress is computed by using well-established empirical functions of wave growth and assuming that there is a cutoff frequency above which the atmospheric momentum goes to the turbulent stress only. This leads to a total surface stress depending on wave age such that the drag coefficient has a maximum for intermediate wave age numbers (∼10). The total surface stress (turbulent and wave induced) is used to compute an effective Charnock constant, which is found to vary from 0.012 to 0.085. The results are compared with published observations.

Base drag reduction by control of the three-dimensional unsteady vortical structures

Abstract: The present paper deals with the wake of a 2D body equipped with a drag reduction device. The device is a 3D trailing edge consisting of alternate segments of blunt base and spanwise cavity. The aerodynamic mechanisms acting on the near wake are studied in a water tunnel from schlieren observations by thermally marking large scale structures. The results show that the efficiency of the device is directly related to the presence of longitudinal vortices. An optimization of the shapes in subsonic compressible flow had led to a decrease of more than 40% of the total drag of the profile.

Prediction of ice shapes and their effect on airfoil performance

Abstract: Calculations of ice shapes and the resulting drag increases are presented for experimental data on a NACA 0012 airfoil. They were made with a combination of LEWICE and interactive boundary-layer codes for a wide range of conditions which include air speed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil. In all cases, the calculated results account for the drag increase due to ice accretion and, in general, show good agreement.

Interference-free measurements of the subsonic aerodynamics of slanted-base ogive cylinders

Abstract: Drag, lift, pitching moment, and base-pressure measurements have been made, free of support interference, on a range of slanted-base ogive cylinders, using the NASA Langley Research Center 13-in magnetic suspension and balance system. Test Mach numbers were in the range 0.04-0.2. Two types of wake flow were observed, a quasi-symmetric turbulent closure or a longitudinal vortex flow. Aerodynamic characteristics differ dramatically between the two wake types. Drag measurements are shown to be in agreement with previous tests. A hysteretic behavior of the wake with varying Reynold's number has been discovered for the 45-deg base. An interaction between forebody boundary-layer state and wake flow and base pressures has been detected for higher slant angles.

Living and fossil scallop shells as airfoils: an experimental study

Abstract: Selected shell specimens of extant and fossil streamlined pectinids, which have or pre- sumably had level-swimming ability, were examined experimentally to elucidate their hydrody- namic properties, in particular, airfoil efficiency estimated by lift-drag ratio. Using a stationary water tank for nautical engineering, lift and drag forces were measured at various attack angles. Of the examined species, Amusium japonicum, which is characterized by an unusually shiny surface, upward- cambered commissure and sharpened trailing edge, is the most efficient level swimmer and has the lowest drag coefficient and the highest value of lift-drag ratio at any attack angle. Amussiopecten praesignis from the Plio-Pleistocene may have swum horizontally because its airfoil efficiency is superior to that of a living level swimmer, Placopecten magellanicus. Although its shell shape is analogous to that of Placopecten, Camptonectes (Maclearnia) cinctus from the Lower Cretaceous shows a much inferior efficiency and a significant flow separation from the surface. Bernoulli's effect in convex-upward species may contribute to increase lift, but a certain attack angle is always required for level flight. The strategy of level swimming probably evolved independently in several pecti- nacean lineages in which swimming rather than shell robustness became the preferred defense against predators. One problem that must be solved is that some feedback mechanism is required to check pitching, rolling, and yawing of the shell to attain stability in level flight.

On the Settling Velocity in a Nonstationary Atmosphere

Abstract: A general drag coefficient has been used in the equation of motion for solid spherical particles. The time constants, stopping times, and settling velocities in a still atmosphere are computed for a wide range of Reynolds numbers. The settling times are compared with the times calculated when a particle is falling in a fluctuating atmosphere. It is found that such particles will get significantly longer settling times owing to an enhancement in the drag coefficient caused by an increase of the relative velocity between the particle and the fluid. Surprisingly, this enhancement is present for a horizontal wind field due to a coupling between particle motion in different directions, but it is also present for a vertical field. The effect is most pronounced in the intermediate Reynolds number region, slightly above the Stokes range, where the increase in settling time can be more than 10% for certain fluctuation frequencies and amplitudes. This indicates that such particles must be carefully treated when the...

Virtual mass and drag in two-phase flow

Abstract: We study virtual mass and drag effects in a fluid suspension consisting of spherical particles immersed in an incompressible, nearly inviscid fluid. We derive average equations of motion for the fluid phase and the particle phase by the method of ensemble averaging. We show that the virtual mass and drag coefficients may be expressed exactly in terms of the dielectric constant of a corresponding dielectric suspension with the same distribution of particles. We make numerical predictions for the case of an equilibrium distribution of hard spheres.

Effect of rounding side-corners on aerodynamic forces and turbulent wake of a cube placed on a ground plane

Abstract: This paper describes the experimental study of a flow past a cube with rounded side-corners placed in a ground plane under the condition of δ/D < 1, where δ is the thickness of the upstream boundary layer. The experiment was carried out in an N.P.L. type wind-tunnel having a working section of 500 mm×500 mm × 2,000 mm at a Reynolds number 4.74×104. The suface-pressure distribution on the cube was measured, and the drag coefficient was determined from the surface-pressure distribution. Furthermore, two kinds of vortices generated around the cube were observed. The distribution of velocities and turbulent intensities in the turbulent wake behind the cube with rounded side corners were measured, and compared with those of a two-dimensional cylinder. As a result, it was found that the drag coefficient decreases rapidly in the range of 0 ≦ 2R/D ≦ 0.3, and the Strouhal number for the arch-vortex shedding increases as the radius of the corner increases.