Abstract: At low Reynolds numbers, three-dimensional features are frequently observed in the vortices shed behind a basically two-dimensional circular cylinder. This paper deals with the dependence of the configuration of the vortices on various end constructions. The cylinder is towed at a uniform speed in a water tank and simple flow visualization is used. It is found that the three-dimensional structure of the wake depends strongly on the flow configuration at each end of the cylinder. The boundary condition imposed on the nascent vortex lines determines the subsequent behaviour of the shed vortices. Consequently, the vortex street can be rendered more nearly two-dimensional by allowing the vortices to link outside the boundary as they approach that boundary normally. This is the case for the water–air interface when the water surface is clean. In the case of a contaminated water surface or of a solid surface acting as a boundary to the vortex street, the vortices link between themselves underneath the water surface and a strong interaction takes place behind the end of the cylinder. The subsequent effect is a bowing of the vortices towards the end of the cylinder. The free-end effect at the bottom end of the cylinder induces a strong bowing of the vortices towards that end and causes the wake to contract. It follows from the effect of surface contamination that the study of vortex wakes by the spreading of some surface contaminants might not necessarily show the true behaviour of the wake below the surface. It is postulated that slantwise shedding arises from a difference in the two end effects.

Abstract: A laser anomometer system employing an efficient data acquisition technique has been used to make measurements upstream, within, and downstream of the compressor rotor. A fluorescent dye technique allowed measurements within endwall boundary layers. Adjustable laser beam orientation minimized shadowed regions and enabled radial velocity measurements outside of the blade row. The flow phenomena investigated include flow variations from passage to passage, the rotor shock system, three-dimensional flows in the blade wake, and the development of the outer endwall boundary layer. Laser anemometer measurements are compared to a numerical solution of the streamfunction equations and to measurements made with conventional instrumentation.

Abstract: An experimental and theoretical investigation of the wake behind a porous disk which simulates the effect of a wind turbine is presented. Modeling of the wake in an environmental wind tunnel is described and experimental results for the three-dimensional model wake flowfield are presented. A three-dimensio nal turbulent flow analysis is developed to deal with prediction of the wake characteristics. The theoretical results are shown to display the experimentally observed features of the wake. I. Introduction T HERE is increasing need for accurate prediction of the interaction of wind turbine generators (WTG's) in arrays, as evidenced by literature published both here1'2 and abroad.3 The problem is the determination of the nature of the wake of a WTG and its development downstream prior to possible interaction with another WTG in the array. This is a case of relaxation of a wake in a turbulent shear flow, which is an important basic problem in fluid mechanics and, as such, deserves attention and study. Field tests are being carried out both here and abroad, but such approaches are expensive, difficult to carry out, and by their nature are incapable of being completely controlled. The lack of control contributes to the difficulty of interpreting such experiments and to the hazards of extrapolating from such results. To alleviate this problem it seems important to carry out a coupled experimental and theoretical program on laboratoryscale model wakes for the following reasons: First, such a laboratory investigation would provide information concerning model turbine wake characteristics over a wide range of controlled upstream conditions. Second, wake information thus obtained would be of value for checking the observations of actual wake characteristics and the predictions of various models to be developed in and for projected large-scale programs. Third, a relatively inexpensive tool for continuing assessment of wind turbine flowfields would be in hand. An exploratory investigation of laboratory-sc ale simulated WTG wakes has been carried out by Sforza et al.4 Results of that study are included here for completeness. Other studies have concentrated primarily on the overall effects of clusters of WTG's on global performance rather than on the detailed fluid dynamics of the wake re-energization process, i.e., Refs. 5-25. The objective of the present investigation is to establish the fluid dynamic processes occurring in the development of the flow in the wake, to determine the importance of some of the various simulated atmospheric boundary-layer characteristics on the flow, and to develop a reasonably accurate prediction method for the calculation of the simulated wake flowfield.

Abstract: An analysis is made of the wake structure and the forces on a delta wing as it undergoes nonsteady motion, wherein the flow separates at the leading edge. Comparisons of these predictions with existing experimental and theoretical data for the nonsteady linear and nonlinear motions indicate good agreement. It was found that the time-dependent, wake-shedding numerical procedure applied here for the wake rollup and the lift force calculation resulted in considerable saving of computer time over methods using the iterative wake rollup procedure. Calculated results for various motions of the delta wing, including the plunging motion, are presented for both the separated and the attached flow cases.

Abstract: The objectives are to provide a parametric description of the electrostatic interaction of a mesosonic, collisionless plasma with conducting bodies on the order of 1 to 10 Debye lengths in size, and to extend this description to the satellite-ionospheric interaction, where possible. Experimental findings include: the wake of the geometrically complex body appears to be a linear superposition of the wakes of its simple geometric components; and vector ion flux measurements show converging ion streams at the wake axis and direct evidence of ion streams deflected from the wake axis by the positive space charge potential associated with the axial ion peak. The extension to the satellite-ionospheric interaction utilizes qualitative scaling and indicates that similar, but smaller amplitude, wake structures may be expected for small or highly charged bodies. However, for large bodies at small potentials, the structure may be diffused by the thermal ion motion and the dispersion resulting for space charge potentials.

Abstract: Methods are developed for modeling the dynamic response of horizontal axis wind turbines to atmospheric turbulence. The dynamic system is made as simple as possible, while still retaining essential physical characteristics. The report describes how to model a turbine system using the 5 degrees-of-freedom to represent the lower frequency motions of the turbine system. The rotor is assumed to be rigid and is three-bladed for simplicity. Both the structural dynamic properties for this system and the aerodynamic influence coefficients which determine how the system will respond to atmospheric turbulence are developed. Two different wake models are used to compute the induced velocities at the rotor disk. The reader is introduced to the basic concepts involved in modeling the turbulent fluctuations of the wind. Using an approximation concept, the spatial variations of the turbulence are modeled with the first few terms of a series expansion. In addition, this report covers the use of filtered noise as a modeling approximation for the turbulence input and provides the necessary data for modeling the turbulence inputs for a wide range of turbine sizes. Finally, the governing equations for the turbine system and the turbulence inputs are combined to yield a state space description ofmore » the entire system. It is shown how decoupling of these equations allows power spectral densities of the various turbine responses to be computed. The computer program used to perform these computations is also discussed and listed.« less

Abstract: Downstream mixing of coaxial jets discharging in an expanded duct was studied to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the propulsion community for combustor flow modeling. Flow visualization studies showed four major shear regions occurring; a wake region immediately downstream of the inlet jet inlet duct; a shear region further downstream between the inner and annular jets; a recirculation zone; and a reattachment zone. A combination of turbulent momentum transport rate and two velocity component data were obtained from simultaneous measurements with a two color laser velocimeter (LV) system. Axial, radial and azimuthal velocities and turbulent momentum transport rate measurements in the r-z and r-theta planes were used to determine the mean value, second central moment (or rms fluctuation from mean), skewness and kurtosis for each data set probability density function (p.d.f.). A combination of turbulent mass transport rate, concentration and velocity data were obtained system. Velocity and mass transport in all three directions as well as concentration distributions were used to obtain the mean, second central moments, skewness and kurtosis for each p.d.f. These LV/LIF measurements also exposed the existence of a large region of countergradient turbulent axial mass transport in the region where the annular jet fluid was accelerating the inner jet fluid.

Abstract: Calculated and measured values of helicopter rotor flapping angles in forward flight are compared for a model rotor in a wind tunnel and an autogiro in gliding flight. The lateral flapping angles can be accurately predicted when a calculation of the nonuniform wake-induced velocity is used. At low advance ratios, it is also necessary to use a free wake geometry calculation. For the cases considered, the tip vortices in the rotor wake remain very close to the tip-path plane, so the calculated values of the flapping motion are sensitive to the fine details of the wake structure, specifically the viscous core radius of the tip vortices.

Abstract: Measurements are presented of the evolution of the probability density function of the temperature fluctuation along the centerline of the wake of a heated circular cylinder in the range 4⩽x/d⩽300.

Abstract: : An improved theoretical method is presented for computing the effective wake of propulsors operating in thick stern boundary layers on axisymmetric bodies. The hydrodynamic interaction between the nominal velocities upstream of the propulsor and at the propulsor location is assumed to be inviscid in nature and the total energy is assumed to be conserved along a given streamline with and without the propulsor in operation. Theoretical predictions using the method are compared with experimental data obtained in the United States and Japan for five different propulsor/axisymmetric body configurations. For all five cases examined, the computed total velocity profiles immediately upstream of the propulsor (with the propulsor in operation) are in good agreement with the measured values. In addition, the volume-mean values of effective velocity profiles computed from the measured nominal velocity profiles are in good agreement with the measured values of the Taylor wake fraction (1-wT) for all five nominal wake distributions over a wide range of propulsor thrust loading coefficients. (Author)

Abstract: The wake of a turbomachinery rotor-blade is turbulent, highly three-dimensional, and nonisotropic with appreciable curvature in the trailing-edge and near-wake regions. The characteristics of the turbulence vary considerably with radius, blade loading, free-stream turbulence, Reynolds number, and the rotor-blade geometry. This paper is concerned with the turbulence properties of a moderately loaded compressor blade, particularly near the blade trailing-edge. The tangential variation of the axial, tangential and radial intensities and stresses across the wake, as well as their decay characteristics were measured with a tri-axial hot-wire probe in the rotor frame of reference. The decay of intensities and stresses were found to be very rapid in the trailing-edge and near-wake regions and slow in the far-wake region. The effects of inlet-guide-vane and the hub-wall boundary layers on the rotor wake turbulence spectra are also discussed. Similarity rules for the three components of intensity are also derived and presented in this paper.

Abstract: Downstream mixing of coaxial jets discharging in an expanded duct was studied to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the propulsion community for combustor flow modeling. Flow visualization studies showed four major shear regions occurring; a wake region immediately downstream of the inlet jet inlet duct; a shear region further downstream between the inner and annular jets; a recirculation zone; and a reattachment zone. A combination of turbulent momentum transport rate and two velocity component data were obtained from simultaneous measurements with a two color laser velocimeter (LV) system. Axial, radial and azimuthal velocities and turbulent momentum transport rate measurements in the r-z and r-theta planes were used to determine the mean value, second central moment (or rms fluctuation from mean), skewness and kurtosis for each data set probability density function (p.d.f.). A combination of turbulent mass transport rate, concentration and velocity data were obtained system. Velocity and mass transport in all three directions as well as concentration distributions were used to obtain the mean, second central moments, skewness and kurtosis for each p.d.f. These LV/LIF measurements also exposed the existence of a large region of countergradient turbulent axial mass transport in the region where the annular jet fluid was accelerating the inner jet fluid.

Abstract: An experimental investigation of the disturbed flow field created by conducting bodies in a mesosonic, collisionless plasma stream is reported. The mid-wake region is investigated, where, for bodies of the order of a Debye length in size, the focused ion streams converge to form a significant current density peak on the wake axis. A parametric description is obtained of the behavior of the amplitude, width, and position of this peak. The results also indicate that portions of the axial ion peak are created by additional mechanisms and that body geometry affects the mid-wake structure only when the sheath is sufficiently thin to conform to the shape of the body.

Abstract: This report is intended as an interim guide for those who routinely face air quality problems associated with near-building exhaust stack placement and height, and the resulting concentration patterns. Available data and methods for estimating wake flow and effluent dispersion near isolated block-like structures are consolidated. The near-building and wake flows are described, and quantitative estimates for frontal eddy size, height and extent of roof and wake cavities, and far wake behavior are provided. Concentration calculation methods for upwind, near-building, and downwind pollutant sources are given. For an upwind source, it is possible to estimate the required stack height, and to place upper limits on the likely near-building concentration. The influences of near-building source location and characteristics relative to the building geometry and orientation are considered. Methods to estimate effective stack height, upper limits for concentration due to flush roof vents, and the effect of changes in rooftop stack height are summarized. Current wake and wake cavity models are presented. Numerous graphs of important expressions have been prepared to facilitate computations and quick estimates of flow patterns and concentration levels for specific simple buildings. Detailed recommendations for additional work are given.

Abstract: The oceanic dispersion of industrial waste barged to Deep Water Dumpsite 106 is effected in the first instance by barge-bound eddies (“wake dispersion”), then by naturally occurring mean shear and turbulence. Wake dispersion is very efficient, producing initial dilution up to a factor of 104, and is controlled by the dimensions and the forward speed of the barge. Subsequent dispersion by natural oceanic processes is slow under stratified summer conditions, describable by an effective diffusivity of order 300 cm2 sec−1, or a diffusion velocity in the neighborhood of 0.2 cm sec−1, for the first 12 hours or so after release. A comparison of the observed data with shear-dispersion theory shows that: (a) horizontal dispersion in the upper 10–20 m of a stratified ocean is mainly due to mean shear-vertical mixing interaction; (b) the low observed rates of dispersion may be attributed to the fact that an early phase of shear diffusion was in evidence. Extrapolation to diffusion times of the order of several days should be possible using a constant diffusion velocity, or an effective diffusivity increasing in direct proportion to time.

Abstract: It has been pointed out by a number of investigators that aerofoils with blunt trailing edges have many advantages, both structural and aerodynamic, at transonic and supersonic speeds. However, a profile with a blunt trailing edge unfortunately creates a high base drag at subsonic speeds and this could result in the total drag being considerably higher than that for a profile with a sharp trailing edge. This base drag arises due to the periodic vortex shedding from the blunt base which causes low pressures in the wake thus creating the base drag. In order to reduce the base drag of a blunt trailing edge at subsonic speeds, this periodic vortex shedding must be suppressed. This has been achieved by various methods — a splitter plate behind the base, the base bleed, base cavities — and these have shown varying degrees of drag reduction.

Abstract: Development of the wind turbine sound prediction code began as part of an effort understand and reduce the noise generated by Mod-1. Tone sound levels predicted with this code are in good agreement with measured data taken in the vicinity Mod-1 wind turbine (less than 2 rotor diameters). Comparison in the far field indicates that propagation effects due to terrain and atmospheric conditions may amplify the actual sound levels by 6 dB. Parametric analysis using the code shows that the predominant contributors to Mod-1 rotor noise are (1) the velocity deficit in the wake of the support tower, (2) the high rotor speed, and (3) off-optimum operation.

Abstract: A device for measuring the velocity of fluid includes a vortex generating means, a pressure sensor means and a processing means. The vortex generating means is positioned in the fluid stream so that vortices are formed in the wake of the generating means. The pressure sensor means is positioned in the wake of the generating means for sensing pressure variation caused by the passage of the vortices. The processing means is coupled to the pressure sensor for determining the fluid flow rate as a function of pressure changes sensed by the pressure sensor means.

Abstract: Mean and turbulent wake properties at three axial locations behind the rotor of an aerodynamically loaded 1.2 pressure ratio fan were measured using a stationary cross film anemometer in an anechoic wind tunnel. Wake characteristics at four radial immersions across the duct at four different fan speeds were determined utilizing a signal enhancement technique. The shapes of the waveforms of the mean rotor relative and mean upwash velocities were shown to change significantly across the span of the blades. In addition, an increase in fan rotational speed caused an increase in the maximum wake turbulence intensity levels near the hub and tip. Spectral analysis was used to described the complex nature of the rotor wake.

Abstract: : A Monostatic Acoustic Vortex Sensing System (MAVSS) was installed at Chicago's O'Hare International Airport to measure the strength and decay of aircraft wake vortices from landing aircraft The MAVSS consists of an array of acoustic antennas which measure the vertical profile up to 60-m altitude of the vertical component of the wind The decay in wake vortex strength is measured as the vortex passes over successive antennas in the array In this volume, the data are analyzed to examine whether landing B-707 and DC-8 aircraft need to be divided into Heavy and non-Heavy categories on the basis of wake vortex hazard Volume I (published in October 1979) described the MAVSS principles of operation, the hardware developed, and the data reduction methods employed

Abstract: The near field wakes downstream of circular cylinders and of 12 sided cylinders were surveyed in a wind tunnel. Local velocity and velocity deficit diagrams are presented. The variation of turbulence in the wake was surveyed and the frequency of the periodic component of wake motion was determined. Differences between wakes of circular cylinders and of 12 sided cylinders are discussed. Also effects of strakes, orientation of the 12 sided cylinders, and rounding of the corners are noted.