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Showing papers on "Drag coefficient published in 1994"


Journal ArticleDOI
TL;DR: In this article, the authors explore concepts for active control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation technique and show that significant drag reduction is achieved when the surface boundary condition is modified to suppress the dynamically significant coherent structures present in the wall region.
Abstract: The objective of this study is to explore concepts for active control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation technique. Significant drag reduction is achieved when the surface boundary condition is modified to suppress the dynamically significant coherent structures present in the wall region. The drag reduction is accompanied by significant reduction in the intensity of the wall-layer structures and reductions in the magnitude of Reynolds shear stress throughout the flow. The apparent outward shift of turbulence statistics in the controlled flows indicates a displaced virtual origin of the boundary layer and a thickened sublayer. Time sequences of the flow fields show that there are essentially two drag-reduction mechanisms. Firstly, within a short time after the control is applied, drag is reduced mainly by deterring the sweep motion without modifying the primary streamwise vortices above the wall. Consequently, the high-shear-rate regions on the wall are moved to the interior of the channel by the control schemes. Secondly, the active control changes the evolution of the wall vorticity layer by stabilizing and preventing lifting of the spanwise vorticity near the wall, which may suppress a source of new streamwise vortices above the wall.

756 citations


Journal ArticleDOI
TL;DR: In this paper, the Navier-Stokes equations for constant viscosity were solved using the SPH method and the expected parabolic and paraboloid velocity profiles were obtained.
Abstract: present a new SPH method that can be used to solve the Navier-Stokes equations for constant viscosity. The method is applied to two-dimensional Poiseuille flow, three-dimensional Hagen­ Poiseuille flow and two-dimensional isothermal flows around a cylinder. In the former two cases, the temperature of fluid is assumed to be linearly dependent on a coordinate variable x along the flow direction. The numerical results agree well with analytic solutions, and we obtain nearly uniform density distributions and the expected parabolic and paraboloid velocity profiles. The density and ·velocity field in the latter case are compared with the results obtained using a finite difference method. Both methods give similar results for Reynolds number Re=6, 10, 20, 30 and 55, and the differences in the total drag coefficients are about 2~4%. Our numerical simulations indicate that SPH is also an effective numerical method for calculation of viscous flows.

369 citations


Journal ArticleDOI
TL;DR: In this paper, single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47, were analyzed in detail.
Abstract: We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.

313 citations


Journal ArticleDOI
TL;DR: Preliminary calculations show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these algae, suggesting that mechanical factors may be important in limiting thesize of intertidal macroalgae and that attention solely to biological constraints may be inappropriate.
Abstract: The intertidal zone of wave-swept rocky shores is characterized by high velocities and exceedingly rapid accelerations The resulting hydrodynamic forces (drag, lift, and the accelerational force) have been hypothesized both to set an upper limit to the size to which wave-swept organisms can grow and to establish an optimal size at which reproductive output is maximized This proposition has been applied previously to inter- tidal animals that grow isometrically, in which case the accelerational force is the primary scaling factor that constrains size In contrast, it has been thought that the size of wave- swept algae is limited by the interaction of drag alone with these plants' allometric pattern of growth Here we report on empirical measurements of drag and accelerational force in three common species of intertidal algae (Gigartina leptorhynchos, Pelvetiopsis limitata, and Iridaea flaccida) The drag coefficients for these species decrease with increased water velocity, as is typical for flexible organisms For two of these species, this decline in drag coefficient is dramatic, leading to small drag forces with concomitant low drag-induced mortality at plant sizes near those observed in the field However, all three species have surprisingly large inertia coefficients, suggesting that these plants experience large acceler- ational forces in surf-zone flows Preliminary calculations show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these algae This study further models the interplay between size-dependent survivorship and re- productive ability to predict the size at which reproductive output peaks This "optimal size" depends on the strength distribution and morphology of the algal species and on the flow regime characteristic of a particular site This study shows that the optimal size predicted for G leptorhynchos, calculated using velocities and accelerations typical of the moderately protected location where this species was collected, closely matches its observed mean size Similarly, the predicted optimal sizes of P limitata and I flaccida at the exposed site where these plants were sampled also match their mean observed sizes These data, although preliminary, suggest that mechanical factors (in particular the accelerational force) may be important in limiting the size of intertidal macroalgae and that attention solely to biological constraints may be inappropriate

253 citations


Journal ArticleDOI
Sze-Foo Chien1
TL;DR: In this article, a new correlation was developed to predict the settling velocity of irregularly shaped particles in Newtonian and non-Newtonian fluids for all types of slip regimes.
Abstract: A new correlation has been developed to predict the settling velocity of irregularly shaped particles in Newtonian and non-Newtonian fluids for all types of slip regimes The correlation was derived from extensive data on the drag coefficients and particle Reynolds numbers of irregularly shaped particles The effective fluid viscosity at the settling shear rate is used in the correlation A trial-and-error or numerical iteration method is required to predict the settling velocity for non-Newtonian fluids The correlation predicted and verified the effects of fluid properties, particle properties, and operation parameters on the settling velocity

212 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the potential flow of incompressible fluids admits a pressure (Bernoulli) equation when the divergence of the stress is a gradient as in inviscid fluids, viscous fluids, linear viscoelastic fluids and second-order fluids.
Abstract: Potential flows of incompressible fluids admit a pressure (Bernoulli) equation when the divergence of the stress is a gradient as in inviscid fluids, viscous fluids, linear viscoelastic fluids and second-order fluids. We show that in potential flow without boundary layers the equation balancing drag and acceleration is the same for all these fluids, independent of the viscosity or any viscoelastic parameter, and that the drag is zero when the flow is steady. But, if the potential flow is viewed as an approximation to the actual flow field, the unsteady drag on bubbles in a viscous (and possibly in a viscoelastic) fluid may be approximated by evaluating the dissipation integral of the approximating potential flow because the neglected dissipation in the vorticity layer at the traction-free boundary of the bubble gets smaller as the Reynolds number is increased. Using the potential flow approximation, the actual drag D on a spherical gas bubble of radius fi rising with velocity U(t) in a linear viscoelastic liquid of density ‰ and shear modules G(s) is estimated to be D = 2 …a 3 ‰ ˙

160 citations


Journal ArticleDOI
TL;DR: In this article, the effects of the interparticle distance and particle Reynolds numbers on the drag forces of both the trailing and leading particles are examined, and an empirical equation is obtained to describe the drag force variation of a single particle trailing in the wake of a leading particle.

124 citations


Journal ArticleDOI
TL;DR: In this paper, the authors measured the mean velocity and the corresponding Reynolds shear stresses of Newtonian and non-Newtonian fluids in a fully developed concentric flow with a diameter ratio of 0.5 and at a inner cylinder rotational speed of 300 rpm.
Abstract: Mean velocity and the corresponding Reynolds shear stresses of Newtonian and non-Newtonian fluids have been measured in a fully developed concentric flow with a diameter ratio of 0.5 and at a inner cylinder rotational speed of 300 rpm. With the Newtonian fluid in laminar flow the effects of the inner shaft rotation were a uniform increase in the drag coefficient by about 28 percent, a flatter and less skewed axial mean velocity and a swirl profile with a narrow boundary close to the inner wall with a thickness of about 22 percent of the gap between the pipes. These effects reduced gradually with bulk flow Reynolds number so that, in the turbulent flow region with a Rossby number of 10, the drag coefficient and profiles of axial mean velocity with and without rotation were similar. The intensity of the turbulence quantities was enhanced by rotation particularly close to the inner wall at a Reynolds number of 9,000 and was similar to that of the nonrotating flow at the higher Reynolds number. The effects of the rotation with the 0.2 percent CMC solution were similar to those of the Newtonian fluids but smaller in magnitude since the Rossby number with the CMC solution is considerably higher for a similar Reynolds number. Comparison between the results of the Newtonian and non-Newtonian fluids with rotation at a Reynolds number of 9000 showed similar features to those of nonrotating flows with an extension of non-turbulent flow, a drag reduction of up to 67 percent, and suppression of all fluctuation velocities compared with Newtonian values particularly the cross-flow components. The results also showed that the swirl velocity profiles of both fluids were the same at a similar Rossby number.

81 citations


Journal ArticleDOI
TL;DR: In this article, a detailed account of the origin of induced air motion within spray jets is given, and this lays the basis for a new one-dimensional model for predicting the induced axial air velocity.
Abstract: A study of the fundamental mechanics of the droplet and gas motion in liquid sprays is presented in this paper. Only vertical sprays without any externally applied gas flow are considered. First a detailed account of the origin of induced air motion within spray jets is given, and this lays the basis for a new one-dimensional model for predicting the induced axial air velocity. Two main flow zones (zone I and zone II) are identified, where the droplet velocity is much greater and of the same order as the induced air velocity respectively. Within zone I there is a near-sub-zone I , close to the nozzle where the droplet velocities deviate little from their initial values, and it is found that the air velocity decreases or increases to a maximum value, depending on whether its initial value is greater or less than a critical value, which itself is a function of the drag coefficient, the initial spray radius and the droplet velocity. In this zone the average induced air velocity decays more slowly, as z -½ ( z being the downstream distance) than the rate of decay, as z -1 , in regular unforced jets. Further downstream in the adjacent forced jet sub-zone , the drag of the faster moving droplets forces an air jet to develop with a rate of growth that is determined by the turbulence if the angle of the spray droplets is small or by the angle of the spray if the angle is large. In this second sub-zone, which typically extends to the stopping distance of the droplets, the flow is largely independent of the flow in the near sub-zone. The 1D model was applied to a rose-head axisymmetric spray and a flat-fan agricultural spray. The calculations agree closely with experimental observations. To calculate the radial variation of the air velocity a 2D axisymmetric model was developed where the air velocity was obtained in the form of a similarity solution. The predictions were in good agreement with the measurements of Binark & Ranz (1958). Finally it is shown that the 1D and the 2D models are consistent with each other.

75 citations


Journal ArticleDOI
01 Aug 1994-Icarus
TL;DR: In this article, the process of resonance trapping due to radiation pressure and Poynting-Robertson drag is discussed in the frame of the planar circular restricted problem of three bodies.

73 citations


Journal ArticleDOI
TL;DR: In this paper, a simulation of the flow of a power-law fluid past a single sphere located on the axis of a cylindrical pipe is simulated numerically and two different regimes are considered: in the settling regime, the spherical particle moves through a quiescent fluid.
Abstract: This paper is concerned with the prediction of critical conditions for the behaviour of particles in simulated oil-drilling fluids. In practice, the rheology of many oil-drilling fluids can be represented adequately by a power-law model and the behaviour of spherical particles can give valuable information on the behaviour of less regularly shaped particles. In this paper, the flow of a power-law fluid past a single sphere located on the axis of a cylindrical pipe is simulated numerically. Two different regimes are considered. In the settling regime, the spherical particle moves through a quiescent fluid. For this regime, drag coefficients for a wide range of power-law indices and particle Reynolds numbers are computed. Power-law indices range from n = 0.4 to n = 1 and Reynolds numbers (based upon the radius of the sphere and the settling velocity) are in the range 0.2 ⩽ Rep ⩽ 100 for sphere/cylinder diameter ratios of 1 30 and 1 50 . An expression is proposed which determines the drag coefficient in power-law fluids as a function of power-law index n and particle Reynolds number Rep. Numerical results are compared with both experimental results and other published numerical results. In the transport regime, the fluid is in motion relative to the pipe and the sphere, so that the fluid in the cylinder is sheared. Drag coefficients in the mid-Reynolds-number range for the transport regime are compared with those obtained in the settling regime. Finally, the numerical results have been presented in the form of two graphs which can be used to predict velocities of particles settling in power-law fluids and also to predict the maximum-sized particle which can be transported upwards by the flow of a power-law fluid.

Journal ArticleDOI
01 Dec 1994
TL;DR: In this article, a finite element method for the transport of a particle-containing liquid through a capillary pore has been studied using the center-line approach, where flow fields, drag correction factors, and pressure drops for single particles and short chains of particles are derived.
Abstract: The transport of a particle-containing liquid through a capillary pore has been studied using a finite element method. Direct calculation has been made of flow fields, drag correction factors, and pressure drops for single particles and short chains of particles using the center-line approach. Three cases have been considered a moving sphere in a stationary liquid , a stationary sphere in a moving liquid, and a moving sphere in a moving liquid. The correction factors for the inner sphere in short chains agree well with the results of complex stream function calculations for infinite chains of particles. Two topics have been particularly addressed. First covered is the use of the numerical calculations to identify the limiting particle spacings for which single-sphere calculations give close agreement with the calculations for such inner spheres. It is shown that single-sphere calculations have a wide range of applicability, considerably simplifying the effort involved in numerical calculation. Second, we carry out calculations up to large values of the particle radius/pore radius ratio. The case of a moving sphere in a moving liquid is directly relevant to the third topic of the paper—transport of particles through microfiltration membranes. Application of the numerical results for conditions corresponding to a commercial capillary pore microfiltration membrane show that hydrodynamic interactions can result in the maximum achievable flux for particle-containing fluids being significantly less than the pure water flux. Such hydrodynamic flux reduction has previously been neglected by membrane researchers.

Journal ArticleDOI
TL;DR: In this article, an experimental study was conducted to measure rise velocities of air bubbles along an inclined flat wall for small Weber numbers and good agreement was obtained with the theoretical drag coefficient values of Ryskin and Leal.

Journal ArticleDOI
TL;DR: In this paper, it is shown that the drag reduction process is linked with the presence of an air concentration boundary layer next to the channel bottom, which increases the effective viscosity of the mixture and the sublayer thickness.
Abstract: In supercritical open channel flows, air is entrained at the free surface. Such air-water flows, called selfaerated flows, exhibit smaller friction losses than non-aerated flows. New data on drag reduction in selfaerated flows are presented. It is shown that the drag reduction process is linked with the presence of an air concentration boundary layer next to the channel bottom. An analogy with dilute polymer solutions and micro bubble modified boundary layers is developed and it is suggested that the presence of air next to the bottom increases the effective viscosity of the mixture and the sublayer thickness. A parallel with sediment laden flows is also developed. Although the distribution of suspended sediments differs from the distribution of air bubbles, it is suggested that the mechanisms of drag reduction observed in suspended sediment flows are similar to those in self-aerated flows.

Journal ArticleDOI
TL;DR: In this paper, the authors present some of the experimental results that have been obtained during an extensive investigation of the behaviour of urban trees in high winds, including stiffness, natural frequency, and damping.

Journal ArticleDOI
TL;DR: In this paper, the authors used FIDAP to analyze Laminar fluid flow and forced convection heat transfer over equally spaced linear arrays of spheres using the finite element package.
Abstract: Laminar fluid flow and forced convection heat transfer over equally spaced linear arrays of spheres are analyzed using the finite element package FIDAP. For the arrays, sphere spacings of 1.5, 2, and 3 diameters are examined at Reynolds numbers of 40, 80, and 120 and Prandtl numbers ranging from 0.73 to 7.3. Average Nusselt numbers and drag coefficient data for a linear array of eight spheres (as an approximation to the developing region) and a single sphere with periodic boundary conditions (as an approximation to fully developed flow) are presented and correlated.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the ability of Coanda jet blowing to modify the base pressure of a cylindrical body aligned axially in a flow, and thereby, produce overall drag reduction.
Abstract: This work investigates the ability of Coanda jet blowing to modify the base pressure of a cylindrical body aligned axially in a flow, and thereby, produce overall drag reduction. It is found that blowing through one or two slot jets concentric to the outer body circumference can significantly influence the entire base flow region. The recirculating wake is eliminated and is replaced by freestream fluid entrained by the Coanda blowing. Base pressure rises significantly and leads to drag reduction of up to 30% beyond the thrusting action of the Coanda jet. A comparison between the power savings through drag reduction and the power requirement of the Coanda jet demonstrates that net benefits are attainable at certain body geometries and flow conditions. By judiciously selecting the jet blowing velocity, it is possible to produce a nearly flat wake velocity profile requiring little net power. RAG reduction of immersed bodies is a subject with a long history, leading to the early concept of streamlining. The net drag force on a body may be considered the sum of viscous drag and pressure drag forces. For streamlined aero- dynamic and hydrodynamic bodies, the pressure drag is small, and current research is directed towards the application of laminar flow control (e.g., suction), turbulent viscous drag reduction (e.g., riblets and large eddy breakup devices (LEBUs)), and the use of nonlinear aerodynamics for induced drag reduction (e.g., winglets or crescent wing planforms). For bluff bodies, on the other hand, streamlining is usually not an option for reducing drag because the bluff shape is often dictated by other constraints. This situation is particu- larly true for trucks, buses, and most automobiles. For bluff bodies then, where pressure drag dominates, drag reduction is primarily through base flow modification, including flow separation control using airfoils, the use of plates, cavities, base bleed, and suction/blowing. Bluff bodies may be considered of two general classifica- tions—high and low aspect ratio. The former may be modeled as bodies of large span relative to a characteristic height, and are generally two-dimensional in nature. Examples would be the classic cylinder in crossflow, or a symmetric airfoil with a thick, blunt trailing edge. Drag reduction for such bodies has been performed1"3 with drag reductions of up to 64% reported in the literature. Low aspect ratio bodies have also been studied. Such bodies are characterized by three-dimensionality or axisymmetry, with a sphere being the classic example. Other examples would be cylinders of rectangular or circular cross section aligned axially in the flow direction. Work on drag reduction of such bodies has been reported in Refs. 4-12. A variation of these studies would include inclined base regions such as fast-back auto- mobiles and cargo transport aircraft. During the energy crisis of the seventies, renewed interest in vehicle drag led to many important new findings which are summarized in the books

Journal ArticleDOI
TL;DR: In this article, the authors measured wave-induced parallel and normal forces on an armor unit on a berm breakwater and found that the wave forces at the location of the measurements are drag dominated.
Abstract: Wave‐induced parallel and normal forces on an armor unit on a berm breakwater have been measured. The armor unit was located on the flattened part of the reshaped berm breakwater. Simultaneously with the force measurements water‐particle velocities in the vicinity of the armor unit have been measured with a laser Doppler velocimeter. The parallel forces have been analyzed as a Morison‐type force. Drag coefficient CD and inertia coefficients CM were obtained by a least‐square method. The average value of CD was approximately 0.35 and of CM approximately 0.2. The wave forces on an armor unit at the location of the measurements are drag dominated. Hence the data are best conditioned for obtaining CD values and not so well conditioned by obtaining CM values. We believe that this is a major reason for the low CM values, since normally the CM values should be larger than 1.0. An unsuccessful attempt has been made to model the normal forces.

Journal ArticleDOI
TL;DR: In this article, the magnetic moment of individual living magnetic bacteria was determined by motion analysis in a time-dependent magnetic field, and the experimental and numerical determination of the drag coefficients agree within their error bounds.
Abstract: The magnetic moment of individual living magnetic bacteria was determined by motion analysis in a time-dependent magnetic field. For this purpose we had to estimate the drag exerted on the moving bacterium by the surrounding liquid. First, the bacterium was approximated by an ellipsoid. In order to determine drag coefficients for more complicated (and realistic) forms, a model experiment was built. In this experiment enlarged models of bacteria were rotated in a viscous liquid and the torque acting upon them was measured. Computing algorithms were developed in order to calculate drag coefficients of magnetic bacteria and to simulate their motion in magnetic fields. The experimental and numerical determination of the drag coefficients agree within their error bounds. Besides the determination by motion analysis, the bacterial magnetic moment was also calculated from the number and size of magnetic particles contained in the bacterium as seen in an electron microscope. The results of both calculations agree well.

Journal ArticleDOI
TL;DR: In this paper, a wave-current interaction model of the bottom boundary layer is used to compute bed drag coefficients and evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification.
Abstract: This paper describes a hydrodynamic model with turbulent energy closure that uses a simplified wave-current interaction model of the bottom boundary layer to compute bed drag coefficients. The coupled model is used to investigate the interaction of the upper and lower boundary layers with the geostrophic core flow for simple shelf geometry and forcing, and to evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification. The thickness of the bottom boundary layer predicted by the model ranges from 10 to 35 m and is consistent with observations from the California shelf. The increased bottom friction calculated by the coupled model in intermediate water depths increases bottom Ekman veering (leftward in the Northern Hemisphere) by as much as 10° if stratification is strong, thus enhancing downwelling and upwelling. Currents along isobaths in shallow water are uniformly decreased by as much as 25% in the coupled model for both summer and winter init...

Journal ArticleDOI
TL;DR: In this paper, a combination of modified LEWICE and interactive boundary-layer codes for a wide range of values of parameters such as airspeed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil is presented.
Abstract: Calculation of ice shapes and the resulting drag increases are presented for a NACA 0012 airfoil. The calculations were made using a combination of modified LEWICE and interactive boundary-layer codes for a wide range of values of parameters such as airspeed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil. Based on experimental data, an improved correlation of equivalent sand-grain roughness was developed. Calculated ice shapes are in good agreement with experimental data for rime ice, but some differences are shown between predictions and experimental data for glaze ice. Calculated drag coefficients generally follow trends shown by the experimental data.

01 Jan 1994
TL;DR: In this paper, a combination of modified LEWICE and interactive boundary-layer codes for a wide range of values of parameters such as airspeed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil is presented.
Abstract: Calculation of ice shapes and the resulting drag increases are presented for a NACA 0012 airfoil. The calculations were made using a combination of modified LEWICE and interactive boundary-layer codes for a wide range of values of parameters such as airspeed and temperature, the droplet size and liquid water content of the cloud, and the angle of attack of the airfoil. Based on experimental data, an improved correlation of equivalent sand-grain roughness was developed. Calculated ice shapes are in good agreement with experimental data for rime ice, but some differences are shown between predictions and experimental data for glaze ice. Calculated drag coefficients generally follow trends shown by the experimental data.

Journal ArticleDOI
TL;DR: A physical model of the swimming appendage of a larval Artemia was oscillated and translated through a tank of glycerine to determine how such a shape may be used to generate thrust at the intermediate Reynolds numbers at which it operates, suggesting that passive extension of setae can be influenced by relative limb and body speed.
Abstract: A physical model of the swimming appendage (second antenna) of a larval Artemia was oscillated and translated through a tank of glycerine to determine how such a shape may be used to generate thrust at the intermediate Reynolds numbers at which it operates. Force on the model was measured by strain gauges and used to calculate coefficients of drag at a series of speeds and frequencies that represented flow regimes of different larval stages. Measured coefficients of drag (Cd) over this Reynolds number range ({approx}1-10) suggest that an expression for a cylinder perpendicular to flow at intermediate Reynolds number (Cd = 1 + 10 Re-2/3) best represents the changes in drag coefficients for this geometry. Unsteady forces were found to be a negligible portion of the force on the model in spite of a high ratio of frequency of oscillation to forward translational velocity (i.e., Strouhal number). Comparison of the thrust generated by the model with its fan of setae rigidly fixed versus passively flexing sugges...

Journal ArticleDOI
TL;DR: In this paper, the drag coefficient of a gas bubble in steady motion in an unbounded axisymmetric shear flow is obtained numerically for different values of the Reynolds and Weber numbers.
Abstract: The drag coefficient of a gas bubble in steady motion in an unbounded axisymmetric shear flow is obtained numerically for different values of the Reynolds and Weber numbers.

Book ChapterDOI
01 Jan 1994
TL;DR: In this paper, the authors used pressure sensors and moored current meters to measure the seabed drag coefficient for reversing tidal flows over the asymmetric sand waves, suggesting a small form drag component.
Abstract: Seabed drag coefficients have been measured at a site within the Norfolk Banks and at a site within the sand wave field in the southern North Sea, using pressure sensors and moored current meters. At the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used. The results are generally in agreement with values used in numerical models. At the sand banks site, the drag coefficient increases with wind conditions, but at the sand waves site there is a reduction during the highest wave conditions, attributed to the drag reduction caused by sand resuspension from the bed. This result suggests that sediment effects must be considered if wave/current interaction is included in numerical models of the region. The drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face, suggesting a small form drag component.

Journal ArticleDOI
TL;DR: In this paper, the energy transfer equation for wind-generated ocean waves is solved numerically for fetch-limited conditions in waters of limited depth, and the resulting growth curves for the total energy and the peak frequency are obtained for five different bottom friction dissipation formulations: an empirical Joint North Sea Wave Project (JONSWAP) expression, three expressions based on the drag law friction model (Hasselmann and Collins, 1968; Collins, 1972; Madsen et al., 1988a) and a form based on eddy viscosity friction model.
Abstract: The energy transfer equation for wind-generated ocean waves is solved numerically for fetch-limited conditions in waters of limited depth. The resulting growth curves for the total energy and the peak frequency are obtained for five different bottom friction dissipation formulations: an empirical Joint North Sea Wave Project (JONSWAP) expression (Hasselmann et al., 1973), three expressions based on the drag law friction model (Hasselmann and Collins, 1968; Collins, 1972; Madsen et al., 1988a) and a form based on the eddy viscosity friction model (Weber, 1991a). The effects of different bottom friction dissipation formulations on the energy balance are intercompared and evaluated. The results are also compared with the Coastal Engineering Research Center growth curves. Numerical simulation and mathematical analysis show that only the drag law formulations with a fixed drag coefficient scale in terms of the air friction velocity. The empirical JONSWAP, the drag law formulation with dynamically changing drag coefficient, and the eddy viscosity model do not scale. It is possible to tune the dissipation coefficients for one particular wind friction velocity but different water depths so that all five formulations give almost the same growth curves for the total energy and the peak frequency. This can be explained by the fact that the five tuned bottom friction dissipation models have the same effect on the energy balance when integrated over the frequency and angle space.

Journal ArticleDOI
TL;DR: In this paper, a new approach for dense-spray modeling is employed to develop a model for multi-droplet interaction effects, which enables the calculation of the drag coefficient, evaporation rate and heat transfer of a droplet in a cloud based on models for a single droplet.

18 Jul 1994
TL;DR: In this article, a phenomenological model for accommodation coefficients due to Hurlbut, Sherman, and Nocilla is used to obtain values for the accommodation coefficients for average satellite materials, thermosphere constituents and temperatures, and satellite velocities using a number of laboratory measurements.
Abstract: : Calculation of Cd for satellites using accommodation coefficients is reviewed. A phenomenological model for accommodation coefficients due to Hurlbut, Sherman, and Nocilla is used to obtain values for the accommodation coefficients for average satellite materials, thermosphere constituents and temperatures, and satellite velocities using a number of laboratory measurements. There is a significant difference between these results and the traditional method of calculating Cd. These differences contribute as much as 20% error in use of thermosphere models for calculation of satellite drag.

Journal ArticleDOI
TL;DR: In this article, a finite volume numerical model, using a nonorthogonal adaptive grid, has been developed to examine both steady deformed and transient deforming droplet behavior.
Abstract: A finite volume numerical model, using a nonorthogonal adaptive grid, has been developed to examine both steady deformed and transient deforming droplet behavior. The model has been tested by comparison with existing numerical solutions and experimental data. Computations of the steady state evaporation of n-heptane droplets in high-temperature air (T∗ ∞ = 1000 K, 10 ≤ Re∞ ≤ 100, We∞ ≤ 10) show deformed oblate shapes with major axes perpendicular to the mean flow direction. Using volume equivalent diameters, predictions based on existing Nusselt and Sherwood number correlations for spherical droplets are in good agreement with the numerical results. A new correlation is presented for the drag coefficient of deformed vaporizing droplets.

Journal ArticleDOI
TL;DR: The energy losses of protein ions passing through a collision cell filled with inert gas have been modeled as the aerodynamic drag on a projectile at high Knudsen number and derived protein cross sections are ∼0.8 of those found with the simple collision model used by Covey and Douglas.