Showing papers on "Drag coefficient published in 1978"

Drag and heat transfer on surfaces with small longitudinal fins

Abstract: An investigation is conducted regarding the drag characteristics of longitudinally ribbed surfaces using various configurations which attempt to confine the turbulent wall bursts to regions of small transverse extent in the initial birth region. The ratio of heat transfer increase to drag increase for the high area ratio rib configurations is determined and the heat transfer efficiency of the present surfaces is compared with data found in the literature. The drag data obtained indicate local 'wetted area skin friction' reductions but due to the increased surface area the planform drag is increased in most instances. There is no indication of drag reduction with the rectangular ribbed models that had sharp leading and fin tip edges. Heat transfer data indicate that several of the triangular grooved models have efficiencies approximately 10 percent higher than that of a smooth flat plate.

Bubble motion and mass transfer in non-Newtonian fluids: Part I. Single bubble in power law and Bingham fluids

Abstract: The Sherwood number and drag coefficient for a single gas bubble moving in a power law fluid and a Bingham plastic fluid are obtained using perturbation methods The perturbation parameters for power law and Bingham plastic fluids are m (= n – 1/2) and E (= R/U), respectively It is found that in the case of power law fluid, mass transfer and drag increase with increasing pseudoplasticity These theoretical results are found to be in good agreement with the available experimental data and the data obtained in the present study In the case of Bingham plastic fluid, mass transfer and drag are found to increase with increase in the Bingham number NB (= 2e) Contours of plug flow regions, where local stresses are less than the yield stress, are obtained as a function of the Bingham number NB These results qualitatively predict the zero terminal velocity observed for bubble motion in liquids with very high yield stress They are also in good agreement with the trends of the results obtained previously for solid sphere motion in Bingham plastic fluids

The Effect of Base Slant on the Flow Pattern and Drag of Three-Dimensional Bodies with Blunt Ends

Abstract: The paper describes an experimental investigation concerning the effects of slanting the blunt base of three-dimensional bodies having either an axisymmetric or a rectangular cross section. It was found that base slant can have a very dramatic effect on body drag, particularly in a relatively narrow range of slant angles where the drag coefficient exhibits a large local maximum (overshoot).

Drag on a sphere in unsteady motion in a liquid at rest

Abstract: A sphere was subjected to a simple harmonic motion in an otherwise undisturbed liquid. Records of the resistance of the liquid to the motion for various amplitudes and frequencies were obtained. The resistance was first represented by an equation consisting of three terms with empirical coefficients: the steady-motion drag, a term due to the ‘added mass’ and a term due to the history of the motion. It was found that the data could be correlated only with a large degree of scatter by this type of equation. Subsequently an attempt was made to represent the resistance by means of a single term, with an empirical coefficient C. It was found that C correlated well with the acceleration number Vd/V2 and the Reynolds number Vd/v, where V, V and d are the acceleration, velocity and diameter of the sphere respectively and v is the kinematic viscosity of the liquid. C increased with Vd/V2 and decreased in the limit to the steady-motion drag coefficient Cd when Vd/V2 became very small. The range of the Reynolds number in the experiments was 102 < Vd/v < 104 and the range of the acceleration number was 0 ≤ Vd/V2 ≤ 10·5.

Response of the Hurricane Boundary Layer to Changes of Sea Surface Temperature in a Numerical Model

Abstract: An axisymmetric, multilayer hurricane model is used to investigate the hurricane's response to sudden changes of sea surface temperature (SST). The model contains a parameterization of the planetary boundary layer (PBL) which includes matched formulations for the surface layer and the mixed layer. The heat, moisture and momentum fluxes are mutually dependent through Monin-Obukhov similarity theory. The height of the model hurricane PEL is 400–500 m, below which the potential temperature and specific humidity are nearly invariant with height. The flow in the hurricane PBL is characterized by subgradient tangential velocities and nearly uniform cross-isobaric flow angles. The sensible heating from the ocean is insignificant, but the evaporation is large. The magnitudes of the equivalent drag coefficients are approximately one-third those of the exchange coefficients for heat and moisture. As the SST is suddenly decreased (increased), the steady-state model hurricane experiences two stages of modifi...

Flows around Two Tandem Circular Cylinders at Very High Reynolds Numbers

Abstract: Flows around two tandem circular cylinders in a stream up to the range of much higher Reynolds numbers than the critical one were investigated in a low speed wind tunnel. By making the surface of the upstream circular cylinder rough with two types of surface roughness, the critical Reynolds number of the cylinders was reduced. Drag coefficients and Strouhal numbers of the two tandem circular cylinders were measured and the flow patterns on the cylinders were visualized by the surface oil-flow technique, in the subcritical flow regime, in the supercritical one where there were laminar bubbles followed by turbulent separation and in the transcritical one where purely turbulent separation occurred without laminar bubbles but with an extremely regular vortex shedding behind the cylinders. The flow characteristics of the two tandem circular cylinders were discussed and the effects of Reynolds numbers and the gap spacing between the two cylinders were made clear.

Hypersonic Three-Dimensional Viscous Shock-Layer Flows over Blunt Bodies

Abstract: The viscous shock-layer equations have been extended to treat blunt three-dimensional bodies at angle of attack. Numerical solutions have been obtained on sphere-cones at angles of attack up to 38 deg. Comparisons are made with available experimental data, inviscid solutions, and solutions of the parabolized Navier-Stokes equations. The experimental data consisted of heat-transfer distributions, pressure distributions, and drag coefficients in a Mach number range from 10-18, Reynolds numbers of the order 1.3 x 10 4/ft, and a from 0-40 deg. Two cases were compared with the parabolized Navier-Stokes solutions at Mach numbers of 22.77 and 25.81 and altitudes of 180 and 240 kft at angles of attack of 23 and 38 deg, respectively. In general, the shocklayer predictions were in good agreement with the available experimental and numerical data, but the parabolic treatment of the crossflow viscous shock-layer equations prevented solutions on the leeward side of long bodies at large angles of attack. cp C'

The dependence of bulk evaporation coefficients on air-water interfacial conditions as determined by the isotopic method

Abstract: The analysis of the natural distribution of deuterium and oxygen 18 in moisture inside the turbulent boundary layer developed above a water surface makes possible the investigation of the mechanism of evaporation. The distribution of isotopes in water vapor allows the calculation of the relative contributions of molecular and turbulent transfer to the total mass transport (Merlivat and Coantic, 1975). We have applied this method to assess the influence of the conditions at the liquid surface on the evaporation flux. In particular, the influence of wind waves and superposed artificially generated waves was studied. Experiments have been carried out at the Institut de Mecanique Statistique de la Turbulence air-water tunnel, specially designed for the simulation of ocean-atmosphere energy exchanges. Here the wind velocities could be varied from 0.7 to 6 m/s. Waves were generated by a motor-driven paddle. The wave frequency was 1 Hz, and the maximum height 12 cm, while the surface roughness Reynolds number Res varied from 0.02 to 2. Mean wind velocity profiles and isotopic profiles were measured. Experimental drag coefficients are described quite well by Charnock's relation Z0 = u*2/ag., where a = 81.1. No specific difference is observed when waves are artificially generated. Agreement between experiment and theoretical isotopic distribution is checked for six models proposed to describe evaporation processes. Quite good agreement is found with Brutsaert's model (Brutsaert, 1975a,b) for a smooth surface if Res 1. Again, no specific effect due to the presence of artificially generated waves is observed. The above observations allow, then, the calculation of the drag and bulk evaporation coefficients, CD and Cq, as a function of the surface roughness Reynolds number: these coefficients, as well as their ratio, when calculated for a height of z = 10 m, vary from 0.99 × 10−3 to 1.24 × 10−3 and from 1.31 to 0.84, respectively, when Res increases from 0.02 to 10, corresponding to a mean wind speed range extending from 2 to 13 m/s. These evaporation bulk transfer coefficients are given for near-neutral stability conditions of the atmospheric layer. In the case of moderate instability, frequently observed over the sea, we would expect, and we observe, that the transfer coefficients are generally higher.

A Measurement of Mountain Drag

Abstract: The pressure drag on the Blue Ridge Mountain in the central Appalachians has been determined by measuring the pressure on each side of the ridge. During the first two weeks of January 1974 several periods with significant drag were observed with pressure differences typically 0.5 mb (50 Pa) across the ridge. This corresponds to a mountain drag which is at least equal to the skin friction drag in the Appalachian region if the other ridges in the area experience a similar drag. As expected, the drag seems to occur when the environmental wind has a component perpendicular to the ridge. Such a cross flow appears necessary but not sufficient for the production of drag. The measured mountain drag seems also to be associated with blocking of the surface flow upstream, a condition which occurs most frequently in stable nighttime conditions. While none of the possible drag mechanisms can be absolutely excluded or verified, drag due to flow separation appears unlikely and the mechanism of wave drag (of som...

The Drag Related Flow Field Characteristics of Trucks and Buses

Abstract: Non-aerodynamic factors are largely responsible for the size and shape of contemporary trucks and buses. The results of wind tunnel experiments with 1/7-scale tractor-trailer and bus models are used to identify major drag producing regions of the flow fields, and to document some of the detailed characteristics. Some modifications of both the forebody and base flow fields are made in order to explore the practical potential for drag reduction. The largest drag reductions are shown to be achievable by changing the forebody flow field. By controlling flow separation from leading edges, either by modifying body contours or by employing add-on devices, apparent minimum drag limits have been identified. The possibility of even lower drag levels within existing constraints is analyzed. At the end, non-zero yaw drag characteristics are briefly discussed.

Interaction Effects on the Drag of Bluff Bodies in Tandem

Abstract: The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations, and from various add-on devices designed to reduce their drag.

Studies of single wire parallel stream type HGMS

Abstract: The initial buildup process for three different paramagnetic particle slurries (Mn 2 P 2 O 7 , Mn 2 O 3 , and Cr 2 O 3 ) on a single wire is studied for parallel stream type HGMS. It is found experimentally that the accumulation radius R a as a function of time t is expressed by the equation R_{a}^n} = At + 1 . The exponent n and the coefficient A depend on the slurry concentration as well as the magnetic field. The value of n varies from about 3 to 4. These results are shown to be consistent with a model which takes into account the drag coefficient for the wide range of Reynolds numbers encountered here.

Experimental Study of Aerodynamic Noise vs Drag Relationships for Circular Cylinders

Abstract: An experimental program has been conducted to determine the quantitative relationship between far-field noise and drag coefficient for circular cylinders. The test program included smooth and roughened cylinders with Reynolds numbers between 45,000 and 450,000 and Mach numbers between 0.1 and 0.5. The resulting drag coefficients varied between 0.75 and 1.2. The results show a strong dependence of SPL on drag coefficient varying as 50 to 90 times log (CD) depending on directivity angle and bandwidth. This represents the first controlled laboratory data available to directly verify the quantitative relationship between noise and drag coefficient at constant Mach numbers. aF b

Drag due to a striated boundary in slow Couette flow

Abstract: A highly viscous fluid separates a smooth plate moving in its own plane and a wavy plate. Due to the wavy striations, the mean flow is decreased and the mean drag is increased. The waviness makes the fluid appear to be anisotropic.

Eddy flux measurements over the ocean and related transfer coefficients

Abstract: Eddy correlation measurements of vertical turbulent fluxes made during AMTEX 1975 are used to assess the reliability of flux prediction from established bulk transfer relations, using both surface-layer and planetary boundary-layer formulations. The surface-layer formulae predict momentum and latent heat fluxes to an accuracy comparable to the direct eddy correlation method, using transfer coefficients of CDN (at 10m and in neutral conditions) increasing with wind speed, and a constant CEN ∼- 1.5 × 10−3. The data suggest CCHN, for sensible heat, increases significantly with wind speed and is on average 30% lower than CCEN The boundary-layer drag coefficient, CGD, agrees within about 40% of recently published values using a vertically averaged geostrophic wind to the height of the lowest temperature inversion, corrected for trajectory curvature. Values of θ*/δθ from which CCGH is derived, are in excellent agreement if the published values are modified to account for inappropriate surface temperatures used in their derivation. Preliminary values of CGE are also presented.

Positive Tail Loads for Minimum Induced Drag of Subsonic Aircraft

Abstract: By applying Prandtl's relation for the induced drag of a biplane to typical wing-tail combinations, it can be shown that the minimum induced drag occurs with a positive tail upload. This fact has been overlooked because the reduction in the total induced drag by a tail download was overestimated by using the total downwash of the wing on the tail, while neglecting the downwash produced on the wing by the tail. It is proved that, regardless of the relative size of the tail, the downwash produced by a tail download increases the induced drag of the wing so as to cancel the additional "tail thrust," and keep the mutually induced drag of a wing-tail combination the same as that induced upon the tail alone when it is in the wing's Trefftz-plane. At any finite tail length the bound circulation vortex of the wing produces a downwash that increases the induced drag of a tail upload. However, the circulation vortex system of the tail upload produces an upwash on the wing that results in a "wing thrust" component that cancels the increased drag on the tail so that the total induced drag is a minimum with a positive tail load. In order to facilitate the calculation of the mutually induced drag of typical wing-tail combinations, an explicit relation is derived for the limiting case of a small-span tail at any distance above or below a large-span wing.

Study of Flow past a Circular Cylinder by an Inviscid Model

Abstract: Two-dimensional vortex shedding behind a circular cylinder is simulated by a point vortex array separating from the boundary layer. The flow round the body is impulsively started from rest. The boundary layer is divided into partitions and each of them is replaced by a point vortex with the same circulation as the corresponding partition. This point vortex array separates from the surface of the cylinder, forming Karman's type wake. The calculated drag coefficients have a peak initially and decrease gradually to the value of about 0.6 and begin to oscillate around the value between 0.6 and 0.8. The lift also begins to oscillate more clearly than the drag. The results are in good agreement with those obtained by much more elaborate finite-difference methods.

Evaluation of the aerodynamic method of determining fluxes over natural grassland

Abstract: Profiles of wind speed, temperature, humidity, and CO2 content up to a height of 6.5 m were measured over a period of several months above natural grassland in an area with an average fetch of 2km. Profile shapes have been analysed in relation to atmospheric stability. In unstable conditions, temperature and CO2 profiles exhibit similar shapes and depart more from logarithmic than does the wind profile. All profiles are relatively well described by existing empirical stability corrections. In stable conditions all profiles have approximately the same shape: they depart from the logarithmic profile at moderate stabilities and return to it at strong stabilities. However, the oft-quoted log-linear profile does not provide a very good fit. Empirical stability corrections have been used to develop a generalized aerodynamic method for flux computation. This method tends to underestimate (by up to 20%) fluxes during unstable conditions and to overestimate (by up to 40%) in moderately stable conditions. These results stem from a comparison of aerodynamic and Bowen ratio estimates of sensible heat flux. They are in essential agreement with the analysis of profile shapes. The drag coefficient of vegetation may be predicted from the leaf area index. This allows the use of a simplified aerodynamic method, based on measurements of wind speed at one level and of temperature (or H2O or CO2) at two levels.

Current-induced flux motion in type-I superconducting films studied at 100-ns time resolution

Abstract: The motion of magnetic flux tubes in the current-induced resistive state of superconducting Pb films several microns thick has been studied with magnetooptical flux detection in conjunction with a high-speed stroboscope, permitting the time-resolved observation of rapid flux-tube dynamics. A computer model of flux-tube motion has been developed which, when suitably modified, is in reasonable agreement with the experimental results. By measuring the flux-tube velocity directly, it has been determined that the phenomenological expression for the viscous drag coefficient is inadequate and can account for only about 20% of the observed drag force. A recalculation of the viscous drag including quasiparticle diffusion and relaxation effects is offered as an explanation of this disparity.

Miniature drag sphere velocity probe.

Abstract: The development and testing of a miniature drag sphere velocity probe is described. Impetus for its development arose from a need to investigate the flow structure and turbulence beneath breaking surface water waves, i.e., a requirement for a fast‐response small velocity probe which is useable in oscillatory flows. However, the presence of both drag (velocity) and inertial (acceleration) forces limits the measurable fluctuations to those having scales at least an order of magnitude larger than the sphere diameter, while the sphere diameter is itself limited by the requirement that the drag coefficient be independent of velocity. The resulting probe, with a sphere diameter of 0.43 cm, while not free of temperature drifts, pressure sensitivity, and cross‐channel talk, yields encouraging results.

The influence of sea waves on the wind profile

Abstract: From wind profile and wave measurements performed during the JONSWAP II experiment, relations between the dimensionless profile slope and the significant wave height are derived. It is shown that the wind profile is distorted by the waves especially in the vicinity of the water surface. The wave influence on the profile seems to be restricted to heights below about three wave heights. Above this level, the dimensionless profile slope is an approximately constant value corresponding to a drag coefficient of about 1.15 × 10−3.