Showing papers on "Drag coefficient published in 1986"

Force on a circular cylinder in viscous oscillatory flow at low Keulegan—Carpenter numbers

Abstract: This paper presents the in-line force coefficients for circular cylinders in planar oscillatory flows of small amplitude. The results are compared with the theoretical predictions of Stokes (1851) and Wang (1968). For two-dimensional, attached- and laminar-flow conditions the data are, as expected, in good agreement with the Stokes–Wang analysis. The oscillatory viscous flow becomes unstable to axially periodic vortices above a critical Keulegan–Carpenter number K (K = Um T/D, Um = the maximum velocity in a cycle, T = the period of flow oscillation, and D = the diameter of the circular cylinder) for a given β (β = Re/K = D2/vT, Re = Um D/v, and v = the kinematic viscosity of fluid) as shown experimentally by Honji (1981) and theoretically by Hall (1984). The present investigation has shown that the Keulegan—Carpenter number at which the drag coefficient Cd deviates rather abruptly from the Stokes—Wang prediction nearly corresponds to the critical K at which the vortical instability occurs.

On the large-scale structures in two-dimensional, small-deficit, turbulent wakes

Abstract: A systematic study of two-dimensional, turbulent, small-deficit wakes was carried out to determine their structure and the universality of their self-preserving states. Various wake generators, including circular cylinders, a symmetrical airfoil, a flat plate, and an assortment of screens of varying solidity, were studied for a wide range of downstream distances. Most of the generators were tailored so that their drag coefficients, and therefore their momentum thicknesses, were identical, permitting comparison at identical Reynolds numbers and aspect ratios. The flat plate and airfoil had a small, trailing-edge flap which could be externally driven to introduce forced sinuous oscillations into the wake. The results indicate that the normalized characteristic velocity and length scales depend on the initial conditions, while the shape of the normalized mean velocity profile is independent of these conditions or the nature of the generator. The normalized distributions of the longitudinal turbulence intensity, however, are dependent on the initial conditions.Linear inviscid stability theory, in which the divergence of the mean flow is taken into account, predicts quite well the amplification and the transverse distributions of amplitudes and phases of externally imposed sinuous waves on a fully developed turbulent wake generated by a flat plate. There is a strong indication that the large structures observed in the unforced wake are related to the two-dimensional instability modes and therefore can be modelled by linear stability theory. Furthermore, the interaction of the two possible modes of instability may be responsible for the vortex street-type pattern observed visually in the small-deficit, turbulent wake.

Variation of the drag coefficient and its dependence on sea state

Abstract: Using a Gill propeller vane anemometer and resistance wave wires over a water column depth of 15 m, simultaneous measurements of the momentum flux and sea surface wave spectra were acquired from the Pisa mast, 28 km offshore in the German Bight during autumn and winter 1979. These data were analyzed to identify the relationship between wind stress and surface waves. It was found that wind stresses for wind speeds above 15 m/s were regularly higher than open ocean wind stresses as reported by Smith (1980) and by Large and Pond (1981) for the same mean wind speed. These results, when described in terms of the drag coefficient, compared closely with the results of Sheppard et al. (1972), who collected surface layer statistics over Lough Neagh, Northern Ireland. After modeling the surface waves of the North Sea as a function of wave saturation (or wave age), it became evident that variations in the magnitude of the drag coefficient could be explained by coincident variations in the surface wave energy spectrum. By applying the wave dependent roughness length model described by Kitaigorodskii (1973), the North Sea drag coefficient was predicted to be larger than drag coefficients reported from the open sea.

A Parameter Describing Overall Conditions of Wave Breaking, Whitecapping, Sea-Spray Production and Wind Stress

Abstract: The dim ensionless parameter u * 2 /vσ can be used widely as a parameter to describe the overall conditions of air-sea boundary processes, where u* is the friction velocity of air, v is the kinematic viscosity of air and s is the spectral peak frequency of the wind waves A critical value of this parameter for the appreciable commencement of breaking of wind waves is 103 Beyond this value, the percentage of waves passing a fixed point that are breaking, α, and the percentage of whitecap coverage, P, are both approximately proportional to this parameter The number concentration of sea-salt particles containing salt in the vicinity of 10−10 g at the 6-m level is also proportional to this parameter The dimensionless roughness length associated with the air flow over water, u*Z0 /V, also correlates better with this parameter than with a parameter which does not contain the spectral peak frequency This gives an approximate relation of z0 σ/u* = 0025, and a corresponding formula for the drag coefficient is proposed The dimensional and physical interpretation of the parameteru */2/vσ is presented

Hydraulic Control of Sill Flow with Bottom Friction

Abstract: The hydraulics of strait and sill flow with friction is examined using a reduced gravity model. It is shown that friction moves the critical (or control) point from the sill to a location downstream. If the strait has constant width, the control point lies where the bottom slope is the negative of the drag coefficient Cd. If -Cd exceeds the bottom slope everywhere, the flow cannot be controlled (in the classical sense that energy and flow force are minimized). Friction also decreases the minimum obstacle height required to establish hydraulically controlled flow in the classical laboratory towing experiment. Also, friction greatly encourages the establishment of stationary hydraulic jumps in the lee of the sill and, under certain conditions, gives rise to stationary jumps on the upstream face of the obstacle. Some consequences of these results for deep-ocean overflows are given using the Iceland-Faroe overflow as an example.

Bulk transfer coefficient over a snow surface

Abstract: The drag coefficient C D and the bulk transfer coefficient for sensible heat C H over a flat snow surface were determined experimentally. Theoretical considerations reveal that C D depends on the friction velocity u * as well as on the geometrical roughness h of the snow surface. It is found that C D increases with increasing u * and/or h. The dependency of C H on u * and h is so small that it is possible to consider C H as a constant for practical purposes: C H, 1 = 2.0 × 10−3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at C E, 1 = 2.1 × 10−3 for a reference height of 1 m.

Effect of Tripping Wires on the Flow around a Circular Cylinder Normal to an Airstream

Abstract: Experimental studies have been made to make clear the effect of tripping wires on the transition in a boundary layer on a circular cylinder in a cross flow. The flow patterns are variable according to the angle φR, namely, a contact point of the tangent to the cylinder from the top of the roughness, and to the roughness Reynolds number Rek, defined by the roughness height and the velocity at the outer edge of the boundary layer. In the range of φR < 70°, the roughness has no influence on the transition up to Rek =620, while, at Rek≥1220, it is found to has full effect on the transition. In this case, the Strouhal number S increases to 0.30 and the drag coefficient CD decreases to 0.60. In the range of 70°≤φR≤75°, the transition occurs at Rek = 620. Beyond φR =76°, the flow is separated by the roughness, and the values of S and CD are about 0.175 and 1.55, respectively.

A Mechanism for the Increase of Wind Stress (Drag) Coefficient with Wind Speed over Water Surfaces: A Parametric Model

Abstract: A mechanism is proposed for a physical explanation of the increase in wind stress (drag) coefficient with wind speed over water surfaces. The formula explicitly incorporates the contribution of both winds and waves through the parameterizations of an aerodynamic roughness equation. The formula is consistent with measurements from the field and with results obtained by numerical models for storm surges and water level fluctuations.

Drag Coefficients of Latticed Towers

Abstract: Recent building codes require that wind loads be determined by multiplying a calculated velocity pressure by the drag coefficient of the structure. It is suggested that the drag coefficients given ...

The drag on spheres in viscoelastic fluids with significant wall effects

Abstract: Using boundary element anf finite-element methods, numerical results for the creeping motion of viscoelastic fluids past a sphere in the presence of a wall are presented. It is found that the fluid elasticity and the fluid shear-thinning can reduce the drag coefficient of the sphere remarkably. Drag reduction by up to 25% is observed at a Weissenberg number Wi = 0.7 in the absence of fluid shear-thinning and in the presence of fluid shear-thinning the drag is reduced as much as 40% for the same value of the Weissenberg number. A shift of the streamlines in the downstream direction for moderate Weissenberg number is clearly demonstrated.

A fundamental study of drag and an assessment of conventional drag-due-to-lift reduction devices

Abstract: The integral conservation laws of fluid mechanics are used to assess the drag efficiency of lifting wings, both CTOL and various out-of-plane configurations. The drag-due-to-lift is separated into two major components: (1) the induced drag-due-to-lift that depends on aspect ratio but is relatively independent of Reynolds number; (2) the form drag-due-to-lift that is independent of aspect ratio but dependent on the details of the wing section design, planform and Reynolds number. For each lifting configuration there is an optimal load distribution that yields the minimum value of drag-due-to-lift. For well designed high aspect ratio CTOL wings the two drag components are independent. With modern design technology CTOL wings can be (and usually are) designed with a drag-due-to-lift efficiency close to unity. Wing tip-devices (winglets, feathers, sails, etc.) can improve drag-due-to-lift efficiency by 10 to 15% if they are designed as an integral part of the wing. As add-on devices they can be detrimental. It is estimated that 25% improvements of wing drag-due-to-lift efficiency can be obtained with joined tip configurations and vertically separated lifting elements without considering additional benefits that might be realized by improved structural efficiency. It is strongly recommended that an integrated aerodynamic/structural approach be taken in the design of (or research on) future out-of-plane configurations.

Device for reducing the frictional drag of moving bodies

Abstract: A device for reducing the frictional drag inherent in flow mechanics in airborne, waterborne and space vehicles in which the surface of a body in a flowing medium is provided with an asymmetrical microstructure in the form of grooved profiles whose dimensions do not essentially exceed the average free travel length of the molecules of the medium.

On the effective roughness length for use in numerical three-dimensional models

Abstract: We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.

Explicit guidance of drag-modulated aeroassisted transfer between elliptical orbits

Abstract: This paper presents the complete analysis of the problem of minimum-fuel aeroassisted transfer between coplanar elliptical orbits in the case where the orientation of the final orbit is free for selec- tion in the optimization process. The comparison between the optimal pure propulsive transfer and the idealized aeroassisted transfer, by several passages through the atmosphere, is made. In the case where aeroassisted transfer provides fuel saving, a practical scheme for its realization by one passage is proposed. The maneuver consists of three phases: A deorbit phase for non zero entry angle, followed by an atmospheric fly-through withdvariable drag control and completed by a post atmospheric phase. An explicit guidance formula for drag control is derived and it is shown that the required exit speed for ascent to the final orbit can be obtained with a very high degree of accuracy.

Airfoil large-eddy breakup devices for turbulent drag reduction

Abstract: It was determined from the present LaRC experiments that tandem, airfoil-shaped large eddy breakup (LEBU) devices can reduce local skin friction as much as 30 percent with a recovery region extending more than 100 boundary layer thicknesses downstream. These airfoils experience near laminar skin friction device drag and produce net drag reductions of up to 7 percent. In contrast to the thin plates used in previous experiments, these airfoils are more than 1000 time stiffer and hence have the potential to withstand the real flight environment (dynamic pressure 36 times larger than in low-speed wing tunnels). In addition, the higher Reynolds numbers of the present tests indicate drag reduction performance is at least as good (or better) as at lower Reynolds numbers.

Penalty finite-element analysis of coupled fluid flow and heat transfer for in-line bundle of cylinders in cross flow

Abstract: The two-dimensional steady state Navier-Stokes equations and the energy equation governing laminar incompressible flow are solved using a penalty finite-element model for the case of flow across an in-line bundle of cylinders. Two cases of in-line cylinder bundles, one five rows deep and the other an infinite bundle, are considered with pitch-diameter ratios of 1.25, 1.5 and 1.8 Reynolds numbers studied range from 100 to 600 and Prandtl number is taken as 0.7. Velocity field vectors, stream lines, vorticity, pressure and temperature contours, local and average Nusselt numbers, pressure and shear stress distribution around the cylinder walls and drag coefficients are presented. The results obtained agree well with available experimental and numerical data.

Incorporation of Stratification Effects on the Oceanic Roughness Length in the Derivation of the Neutral Drag Coefficient

Abstract: Based on the assumption that, over the sea, the roughness length of the wind profile scales with the wind stress, a new formulation that describes the drag coefficient as a function of the given neutral drag coefficient and stability is derived. The new formulation is compared to an earlier formulation where roughness changes with stability were ignored. The two are then illustrated with data collected from both the Marine Remote Sensing Project (1979) and the Tower Ocean Wave and Radar Dependence Experiment (1984). It was found that when the surface roughness was allowed to depend on wind stress (and therefore stability), the stratification correction to the neutral drag coefficient was larger than for the case when the roughness length was not allowed to vary.

Circulation control airfoils as applied to rotary-wing aircraft

Abstract: Nomenclature c — airfoil chord cd = corrected section drag coefficient cdg = effective drag coefficient rake = d drag coefficient from wake survey ct = section lift coefficient cm = pitching moment about the half-chord C^ = blowing momentum coefficient, =m-Vj/(qco'S) h = slot height m =jet mass flow A^ = freestream Mach number qx = freestream dynamic pressure s = chordwise slot location S =wing reference area V = velocity a. = angle of attack

Direct drag measurements for a flat plate with passive boundary layer manipulators

Abstract: Drag measurements for a flat plate equipped with passive boundary layer turbulence manipulators, the so‐called Large Eddy Break Up (LEBU) devices, have been carried out in a towing tank at plate length Reynolds numbers of 5–20 million. The towing tank facility allowed direct drag measurements as opposed to indirect methods based on, e.g., measurement of momentum loss thickness of the boundary layer. The LEBU devices consisted of two hydrofoils arranged in tandem, with a spacing of five to eight boundary layer thicknesses at the manipulator position (δm) and a height in the boundary layer of 0.5δm–0.9δm. Four different manipulator configurations were tested, with and without a trip wire on the test plate. The LEBU foil section was 10% thick and symmetric. The ratio between the LEBU chord and δm ranged from 0.8 to 1.1, and the chord Reynolds number ranged from 25 000 to 100 000. Only for the lowest Reynolds number was a 2% drag reduction obtained. However, the main body of the experiments showed no drag reduction in contrast to previously reported experiments.

Two‐dimensional turbulent boundary layers over rigid and moving swept wavy surfaces

Abstract: The two‐dimensional, incompressible, turbulent boundary layer over a flexible wall is analyzed. The wall consists of sinusoidal waves that are swept with respect to the flow direction. The mean flow is a boundary layer with wave‐induced stresses. These stresses are evaluated from the solution of the linear problem. It is found that the small mean skin friction reduction, observed for the case of zero sweep, persists. The pressure drag reduces for swept wavy wall from its value for no sweep. There is a small drag reduction for the moving wall cases even for the lowest phase speed considered. Within the assumptions of the turbulence model used, a possible drag reduction mechanism is discussed.

Wall temperature control of low-speed body drag

Abstract: The use of thermal means to control drag under turbulent boundary layer conditions is examined Numerical calculations are presented for both skin friction and (unseparated) pressure drag for turbulent boundary-layer flows over a fuselage-like body with wall heat transfer In addition, thermal control of separation on a bluff body is investigated It is shown that a total drag reduction of up to 20 percent can be achieved for wall heating with a wall-to-total-freestream temperature ratio of 2 For streamlined slender bodies, partial wall heating of the forebody can produce almost the same order of total drag reduction as the full body heating case For bluff bodies, the separation delay from partial wall cooling of the afterbody is approximately the same as for the fully cooled body

The Effects of Surfactant on Certain Air—Sea Interaction Phenomena

Abstract: A comprehensive laboratory study has been made on the following typical phenomena at the air-sea interface both for ordinary tap water and for water containing a soluble surfactant (NaC12H25SO4): (1) generation of wind waves; (2) wind shear stress and wind setup; and (3) growth of regular waves by the wind. The addition of the surfactant to the water shows a large suppression of the wind-generated waves, and its effect increases with increasing concentration of the surfactant within the range of our experiment. When the wind waves attenuate partially, the spectral density near the dominant frequency region shows similarity. For the maximum concentration used (2.6 x 10-2%) wind waves are almost completely suppressed up to a wind speed U10 ≅ 15 m/s, where U10 is the wind speed at height z = 10m. For U10>19m/s, wind waves are generated which are similar to those on tap water. A large decrease of the wind shear stress is observed when the wind waves are suppressed almost completely by the surfactant. An empirical relation for the drag coefficient has been obtained for this case, which is slightly different from that by Van Dorn (1953). An empirical relation for the drag coefficient covering a very wide range of wind speed (U10:8–35 m/s) has also been obtained for ordinary tap water. The surface slope has been measured and related to the friction velocity of the wind. It is shown that the same relation holds for tap water and for water containing surfactant, if the friction velocities measured for the respective waters are used in the relation. The measured growth rate of the fundamental frequency component of regular waves on tap water is greater, by a factor of 2, than Miles’ (1959) growth rate, and a little greater than Snyder and colleagues’ (1981) growth rate. The growth rate of the regular waves is greatly reduced by the addition of surfactant. However, the relation between the growth rate and the friction velocity of the wind is little affected by the surfactant, because the friction velocity is diminished by the presence of surfactant.

Experiment on fluid drag and viscosity with an oscillating sphere

Abstract: The fluid drag on a spherical ball performing oscillatory motion is studied by measuring the decay of amplitude of its free oscillations. For small amplitude of oscillations, the drag force is found to be a linear function of the instantaneous velocity of the ball. This is in agreement with theory. The linear relationship is used to determine the coefficient of viscosity of air. For oscillations involving large amplitude, the relationship between the drag force and the instantaneous velocity is nonlinear. An empirical relationship between the two, consistent with the experimental data, is established.