Abstract: The detailed nature of the three-dimensional flow over the blading, and in the wake, of an isolated compressor rotor has been studied experimentally in a large scale rotating rig. A variety of flow properties were measured including (1) blade profile and endwall surface flow visualization. (2) radial-circumferential arrays of pneumatic wake data acquired in the rotating frame and (3) full-span blade pressure distribution data. 23 refs.

Abstract: An experimental investigation of the near wake of a thin airfoil at various incidence angles is reported in this paper. The airfoil (NACA 0012 basic thickness form) was located in a wind tunnel, and the wake structure was measured using hot-wire sensors. The measurements of mean-velocity, turbulence intensity and Reynolds-stress components across the wake at several distances downstream show the complex nature of the near wake and its asymmetrical behavior. The asymmetry in the wake property, which is maintained up to a length of 1.5 chords downstream of the trailing edge of the blade, is dependent on the incidence angle of the inlet flow. The streamwise velocity defect in an asymmetric wake decays more slowly compared to that of a symmetric wake. The streamline curvature due to the blade loading has a substantial effect on the mean velocity profile as well as the turbulence structure. The numerical study of the same wake indicates that the existing turbulence closure models need some modification to account for the asymmetric characteristics of the 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. Volume I (published in October 1979, 32 pages) described the MAVSS principles of operation, the hardware developed, and the data reduction methods employed. Volume II (published in September 1981, 162 pages) described the analysis of MAVSS data to examine whether landing B-707 and DC-8 aircraft need to remain divided into Heavy and Large categories on the basis of the wake vortex hazard. Volume III (published in January 1982, 25 pages) summarized the results of Volume II in terms of the saftey implications of categorizing all landing B-707s and DC-8s as Large aircraft. In this volume, the statistical methods used to understand wake vortex decay are described and the data on all common jet transport aircraft are presented. (Author)

Abstract: A method is presented for modifying finite difference solutions of the potential equation to include the calculation of non-planar vortex wake features. The approach is an adaptation of Baker's 'cloud in cell' algorithm developed for the stream function-vorticity equations. The vortex wake is tracked in a Lagrangian frame of reference as a group of discrete vortex filaments. These are distributed to the Eulerian mesh system on which the velocity is calculated by a finite difference solution of the potential equation. An artificial viscosity introduced by the finite difference equations removes the singular nature of the vortex filaments. Computed examples are given for the two-dimensional time dependent roll-up of vortex wakes generated by wings with different spanwise loading distributions.

Abstract: The parameters characterizing a plane turbulent wake in its equilibrium state of development are determined on the basis of experiment and analysis. Measurements are made in an open-circuit suction-type wind tunnel with a contraction ratio of about 10 and a test-section of about 30 cm sq and 4.27 m long. Less than 1.5% variation in wind speed along the test section is attained by applying suitable divergence for the boundary layer growth. A two-dimensional wake is created behind a twin plate configuration at a freestream velocity of 21.3 m/s. The freestream turbulence level at this speed is about 0.15%.

Abstract: A brief discussion of the scalar electric potential generated by a swift charged particle in condensed matter is given. Collective modes give rise to an oscillatory wake stretching behind the particle, while electron-hole excitations are responsible for crispations that have the De Broglie wavelength of an electron travelling with the charged particle's velocity. Experimental manifestations of the wake are reviewed briefly. Completely quantal derivations of expressions for the wake potential are presented. The results are compared with the classically derived one.

Abstract: In highly loaded turbines with large hub/tip ratios there is a marked increase in secondary flow effects. The optimization of the turbine flow requires detailed knowledge both of three-dimensional cascade flow and of the wake impinging on the downstream rows of airfoils. Therefore, in the DFVLR, thorough investigations of a single-stage turbine with cold air flow were performed. The stator of this turbine was designed for transonic flow and has a hub/tip ratio of 0.756 and an aspect ratio of 0.56. First, measurements were taken without the rotor in several sections behind the turbine stator with special regard to the mixing of the wakes and secondary vortices. Distributions of total pressure loss coefficient and flow direction give the order of magnitude of the mixing losses. Also, position, intensity, structure, and development of secondary vortices are shown. Some complementary measurements were carried out using five-hole probes. They confirm the above described results from two-dimensional measurements.Copyright © 1982 by ASME

Abstract: Three-dimensional flows and their influence on the stagnation pressure losses in a centrifugal compressor impeller have been studied. All 3 mutally perpendicular components of relative velocity and stagnation pressure on 5 cross-sectional planes, between the inlet and outlet of a 1 m dia shrouded impeller running at 500 rpm were measured. Comparisons were made between results for a flow rate corresponding to nearly zero incidence angle and two other flows, with increased and reduced flow rates. These detailed measurements show how the position of separation of the shroud boundary layer moved downstream and the wake’s size decreased, as the flow rate was increased. The wake’s location, at the outlet of the impeller, was also observed to move from the suction surface at the lowest flow rate, to the shroud at higher flow rates.Copyright © 1982 by ASME

Abstract: A fast-response, total-pressure probe was used with a periodically sampling and averaging data acquisition system to study the unsteady total-pressure field in an axial-flow turbomachine. Periodically unsteady total-pressure data were used to demonstrate some of the ways in which turbomachine blade wake transport and interaction influences the energy transfer involved. Observed trends of periodic variations in local total-pressure values could be explained in terms of the details of energy transfer associated with the different kinds of fluid particles (freestream, wake segment, blade surface boundary layer, mixed) moving through a blade row.

Abstract: This report discusses a cascade wake experiment intended to improve wake modeling in an airfoil analysis. The experiment included extensive measurements of the near and far wakes, trailing edge boundary layers, and airfoil surface static pressures. The measured wake displacement surface and computed boundary layers were used in conjunction with a cascade potential flow solver to demonstrate the feasibility of modeling the viscous aspects of cascade flow.Copyright © 1982 by ASME

Abstract: A device for measuring the velocity of fluid includes a vortex generating means positioned in the fluid stream so that vortices are formed in the wake of the generating means. Plates elongated in the direction of fluid flow are positioned adjacent the vortex generating means to improve detection of the vortices by increasing the signal to noise ratio.

Abstract: An experimental investigation of the effects of the speed of rotation on a turbomachinery rotor blade wake was conducted using a fan rotor in incompressible flow. Measurements were made at two different rotational speeds (1753 and 1010 rpm) with the same blade incidence angle. The blade incidence angle was also varied to discern the effect of blade loading. A three-sensor hot-wire probe mounted in a stationary frame of reference was used for the measurements made at several radial and axial stations in the near- and far-wake regions. The three-dimensional mean velocity and turbulence profiles, the wake defect, wake decay rate, turbulence intensities, and turbulence stresses are appreciably altered when the speed of rotation and the blade loading are changed. The wake defect is reduced and the radial velocity increased when the rotation speed is increased.

Abstract: A 3 m diameter horizontal axis wind turbine rotor has been tested in a large wind tunnel and in the field and the results are compared with theoretical predictions. The size of the rotor was chosen to obtain the most accurate data set possible under controlled conditions, minimising both scale effects and blockage corrections. The outdoor results illustrate the influence of turbulence on rotor performance and the value of the wind tunnel data was enhanced by making detailed measurements of the flowfield both upstream and downstream of the rotor. Measurements were made with hot wire probes and additional data sets were derived, for comparative purposes, using a laser doppler anemometer. These measurements have facilitated detailed corroborations of the predictive techniques, increasing the confidence with which these can be applied to derive estimates of the performance of large machines. The measurements made downstream of the rotor have made possible detailed studies of the physics of wake development; the relationship between the power generated by the rotor and the power extracted from the airstream has also been studied and compared with theoretical predictions. This information will be used to refine the methods for estimating power losses due to interactive effects within an array of machines.

Abstract: A detailed experiment on the separated flow field at a sharp trailing edge is described and documented. The separated flow is a result of sustained adverse pressure gradients. The experiment was conducted using an elongated airfoil-like model at a transonic Mach number and at a high Reynolds number of practical interest. Measurements made include surface pressures and detailed mean and turbulence flow quantities in the region just upstream of separation to downstream into the near-wake, following wake closure. The data obtained are presented mostly in tabular form. These data are of sufficient quality and detail to be useful as a test case for evaluating turbulence models and calculation methods.

Abstract: An existing three-dimensional compressible integral boundary-layer method was modified to account for mean dilatation effects, to model transition properly, and to provide better numerical stability near computational boundaries Results of this method were compared against those from a three-dimensio nal finitedifference boundary-layer method on a difficult test case An interaction procedure was developed to couple this integral method with a number of wing-alone and wing-body transonic potential codes to account for viscous effects A strip wake model, including thickness and curvature effects, was developed and incorporated into this interaction procedure Results from this procedure were compared against experimental data and results from previous procedures, on test cases where viscous effects were large

Abstract: An analytical model of the vortex wake instability subject to forcing by unsteady lift fluctuations has been developed. These lift fluctuations can be caused either by variations in angle of attack or by flying through atmospheric turbulence. The effect of turbulence in the fluid itself, already studied by Crow and Bate, has been reviewed and incorporated into the analysis. A simple procedure is provided to estimate the time at which contact occurs between the two wake vortices as a function of certain characteristics of the forcing inputs. The behavior of the instability is somewhat altered by these forcing terms. In particular, it was found that contact between vortices can occur at much shorter wavelengths than predicted for the unforced case. Furthermore, in the presence of forcing, the minimum completion time for the instability does not correspond to the wavenumber for which the amplification rate is a maximum because the length of the vortex trajectories is also an important factor. Finally, the stability boundary is extended to larger wavenumbers by the presence of forcing, and the boundary location depends on the forcing level. factors are unsteady wing loading and the presence of disturbances in the flowfield. Each of these effects has already received some attention, as described below. The effect of an unsteady load distribution was investigated by Bilanin and Widnall7 for the case of constant net lift. The effect of differentially operated flaps was studied theoretically and experimentally and was shown to be a possible means of hastening the completion of the sinusoidal instability. Chevalier8 showed in flight tests that periodic angle-of-attack variations forced the instability and reduced the time required for the vortices to come into contact. Ap- parently the important case of varying the net lift has not yet been studied theoretically. In practice, load variations could arise either by the lifting surface encountering disturbances or by continual corrective motions applied by the pilot. The effect of turbulence in the fluid in which the sinusoidally deforming wake is embedded was investigated by Crow and Bate.9 The cases of very strong and very weak turbulence levels were considered and the results were used to form a composite curve for the wake lifespan (defined statistically) as a function of the turbulent energy dissipation rate. The turbulent eddies were assumed to lie in the Kolmogorov inertial subrange. The results show that the presence of turbulence will reduce the mean lifespan of the vortex wake, however the effect is relatively weak for low turbulence levels. Portions of the present work reflect the influence of Ref. 9. The present study has a somewhat different emphasis than the earlier basic work on the sinusoidal instability,1"3'6 which had as a major concern the identification of the underlying mechanism and the prediction of the amplification rate and stability boundaries. The present work builds on the previous results and seeks to describe the factors which force or provide initial conditions for the instability. Characterization of these factors allows the prediction of a definite time for the instability to go to completion. The result is therefore more precise than simply saying that the instability goes to com- pletion in a time on the order of the time scale associated with the amplification rate. Furthermore, the relative importance of the various factors which force the instability can be compared. This approach has also been taken in Refs. 7 and 9.

Abstract: The present results demonstrate that the normalized wake growth behind a solid strip is much faster than behind a porous strip with /3 = 0.425. According to the findings of Castro, the wake of the solid strip is dominated by a vortex street which does not form behind the /3 = 0.425 porous strip. Taken together, these results indicate that the twodimensional far wake does depend on the shape of the wake producing object insofar as the shape affects the formation of large vortical structures.

Abstract: Experimental investigations have shown that the bound circulation distribution on a rotor blade is critically dependent on the location of the vortices in the wake. Consequently, in order to optimize rotor performance, compute blade loads, and determine acoustic signatures, it is necessary to use analytical methods which are capable of predicting wake geometry. The factors which contribute to the complexity of the problem both in hovering and forward flight are discussed. A simplified approach to a free wake analysis of rotors in hover is outlined and the analytical results compared with experimental data for two- and four-bladed rotors. ELICOPTER rotor aerodynamic analysis remains one of the more challenging of the unsolved problems of modern fluid mechanics. Although solutions are not required for successful design, as evidenced by the growing use of the helicopter, the availability of a consistent aerodynamic theory for the rotor could lead to formal rather than heuristic design optimization and would help to reduce the costly flight testing required to minimize vibration, noise and rotor fatigue loads. This paper will discuss some of the unsolved problems of rotor aerodynamics and suggest a simplified approach to defining the complex vortex structure of the rotor and its wake. Such simplification could facilitate the expansion of the mathematical models to include, for example, certain im- portant real fluids effects. Geometric Complexity In hovering flight the lifting rotor blade generates a spiralling, rapidly contracting wake which, unlike a lifting wing where the wake is transported rapidly downstream, remains in the vicinity of the rotor at least for the initial spiral. The bound circulation distribution along a following blade is strongly influenced by this wake because of its proximity to the blade and rapid initial contraction. Figure 1 sketches the geometry at the crucial first encounter of a blade with the rolled up tip vortex in the wake. Figure 2 shows a typical bound circulation distribution and wake geometry in hovering flight. The wake continues to distort under the influence of the self-induced velocities as it descends, which complicates the computation of induced velocities both at the blade and in the wake. In the forward flight case the wake geometry may be somewhat simplified since only the first spiral has any ap- preciable influence on the blade loads, further spirals being carried away from the rotor as it advances. On the other hand, the forward flight case is complicated by the existence of time varying airloads requiring the inclusion of a shed, as well as a trailing wake structure and the use of nonstationary flow theory. In the case of hovering flight, because of sym- metry, the flow is essentially stationary when viewed in the rotating system. Several alternative approaches to handling the geometric complexities outlined above have been proposed. The simplest

Abstract: : This report presents the results of a study of seven flows involving the interaction between a normal shock wave and a two-dimensional turbulent boundary layer. The measurements were made at free-stream Mach numbers of 1.3, 1.4 and 1.5 and at Reynolds numbers based on an effective streamwise run of 10 to 30-million. The results were obtained from comprehensive traverses with both pilot and static probes. Standard boundary-layer integral parameters based on wall and measured static pressures are presented, together with velocity profiles and the Mach number distribution over the interaction region. An investigation has been made of the 'law of the wall' and the 'law of the wake' under the influence of strong normal pressure gradients.

Abstract: Mean and turbulence 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 describe the complex nature of the rotor wake.

Abstract: The problem of the potential incompressible flow about a helicopter rotor blade is solved using an unsteady vortex-panel method where the mutual interaction between the blade and the distorting free wake is taken into account The present method alleviates the need to rely upon measured-wake geometries or p5escribed-wake models in order to calculate the airloads A computer program has been developed which is capable of predicting the geometry of the time-dependent three-dimensional (3-D) wake and the instantaneous loadings for a single blade in hover, climb, and forward flight The solution is obtained by using a time-accurate step-by-step procedure The complex-wake geometry at any time is presented graphically with a computer graphics system Calculated results are compared with published data for a rotor blade in both hover and forward flight The code has also been applied to the study of the effect of changing blade tip geometry