Blade Wake Interaction Research Articles

Flow Nonuniformities and Turbulent “Hot Spots” Due to Wake-Blade and Wake-Wake Interactions in a Multi-Stage Turbomachine

This experimental study provides striking examples of the complex flow and turbulence structure resulting from blade-wake and wake-wake interactions in a multi-stage turbomachine. Particle image velocimetry (PIV) measurements are performed within the entire 2nd stage of a two-stage turbomachine. The experiments are performed in a facility that allows unobstructed view of the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. This paper contains data on the phase-averaged flow structure including velocity, vorticity and strain-rate, as well as the turbulent kinetic energy and shear stress, at mid span, for several orientation of the rotor relative to the stator. Two different test setups with different blade geometries are used in order to highlight and elucidate complex phenomena involved, as well as to demonstrate that some of the interactions are characteristic to turbomachines and can be found in a variety of geometries. The first part of the paper deals with the interaction of a 2nd-stage rotor with the wakes of both the rotor and the stator of the 1st stage. Even before interacting with the blade, localized regions with concentrated mean vorticity and elevated turbulence levels form at the intersection of the rotor and stator wakes of the 1st stage. These phenomena persist even after being ingested by the rotor blade of the 2nd stage. As the wake segment of the 1st-stage rotor blade arrives to the 2nd stage, the rotor blades become submerged in its elevated turbulence levels, and separate the region with negative vorticity that travels along the pressure side of the blade, from the region with positive vorticity that remains on the suction side. The 1st-stage stator wake is chopped-off by the blades. Due to difference in mean lateral velocity, the stator wake segment on the pressure side is advected faster than the segment on the suction side (in the absolute frame of reference), creating discontinuities in the stator wake trajectory. The nonuniformities in phase-averaged velocity distributions generated by the wakes of the 1st stage persist while passing through the 2nd-stage rotor. The combined effects of the 1st-stage blade rows cause 10–12 deg variations of flow angle along the pressure side of the blade. Thus, in spite of the large gap between the 1st and 2nd rotors (compared to typical rotor-stator spacings in axial compressors), 6.5 rotor axial chords, the wake-blade interactions are substantial. The second part focuses on the flow structure at the intersection of the wakes generated by a rotor and a stator located upstream of it. In both test setups the rotor wake is sheared by the nonuniformities in the axial velocity distributions, which are a direct result of the “discontinuities” in the trajectories of the stator wake. This shearing creates a kink in the trajectory of the rotor wake, a quadruple structure in the distribution of strain, regions with concentrated vorticity, high turbulence levels and high shear stresses, the latter with a complex structure that resembles the mean strain. Although the “hot spots” diffuse as they are advected downstream, they still have elevated turbulence levels compared to the local levels around them. In fact, every region of wake intersection has an elevated turbulence level.

Open-Loop Hover Testing of a Smart Rotor Model

Thedevelopmentand open-loop hovertesting ofa smartrotormodelwithtrailing-edgee apsforindividualblade control of helicopter vibration are presented. First, the University of Maryland Advanced Rotorcraft Code was used to sizethetrailing-edgee apandto determinethee ap-dee ectionrequirementsforvibration suppression in the wind tunnel. Next an analytic model for the coupled actuator-e ap-rotor system was used to develop a multilayer piezoelectric bender cone guration that was capable of meeting the e ap-dee ection requirements. Based on this study, a matched set of six smart rotor blades were fabricated in-house. The four-bladed smart rotor model was tested in the open-loop mode in hover using a Bell-412 Mach-scaled hub. Flap dee ections of §4 to §6 deg were recorded in the 1 ‐5/rev frequency range at the model operating speed of 1800 rpm. The e ap dee ection increased to §23 deg at 8/rev because of actuator resonant amplie cation. Rotor collective pitch was found to have negligible impact on actuator performance. The maximum control effectiveness was observed close to the blade e atwise bending and torsion natural frequencies. For 3/rev actuator excitation oscillatory thrust levels of up to §10 lb (45 N) (60% steady rotor blade thrust at 6-deg collective ) were recorded, thereby demonstrating the open-loop control authority of the actuator-e ap system. I. Introduction T HEhelicopterspendsa largeportionofitsoperation in forward e ight, resulting in an aerodynamic asymmetry between the advancingandretreatingsidesoftherotordisk.Asaresultofthis,the rotor e owe eld is extremely complex and can include transonic e ow ontheadvancingbladetip,dynamicstallontheretreatingsideofthe disk, highly yawed and reversed e ows, and blade-wake interactions with tip vortices from preceding blades. This highly complex and nonsteady aerodynamic environment and the dynamic response of the long and e exible rotor blades result in large vibratory forces, which are e ltered through the hub to the fuselage. Typically for an N-bladed rotor, the dominant N i1, N and N C1/rev blade loads are transmitted to the fuselage as an N/rev forcing. Reduction in vibration and noise levels of the helicopter yield benee ts in terms of improved passenger comfort, reduced crew fatigue, improved community acceptance, and increased fatigue life of structuralcomponents. Currently, passive isolators and absorbers are routinely used to reduce vibration. However these devices cause signie cant weight penalties (up to 3% of gross weight ) and rapidly degrade in performance away from the tuned e ight condition. In contrast, active vibration reduction schemes offer the promise of achieving reliable vibration reduction over a wide range of operating conditions with lower weight penalty than conventional passive methods. There are two types of rotating frame active control systems: higher harmonic control (HHC) and individual blade control (IBC). HHC involves excitation of the swashplate at N/rev. The major disadvantage of HHC is the high actuation power required to pitch the rotor blades particularly in extreme e ight conditions and large weight penalties associated with the hydraulic actuators. Another important limitation is that HHC is limited to N/rev excitation of the swashplate. However, for performance improvement, dynamic stall alleviation, acoustic control, and alleviation of gusts and ma

314 後流干渉効果による航空エンジン低圧タービンの性能向上に関する数値シミュレーション

314 後流干渉効果による航空エンジン低圧タービンの性能向上に関する数値シミュレーション

Experimental investigation of turbulent wake–blade interaction in axial compressors

Abstract The fundamentally unsteady character of flow in multi-stage turbomachines is traditionally neglected in most of the flow calculation methods. The rapid development of hard- and software for numerical flow simulation has opened possibilities for investigation of the unsteady flow in turbomachines. The still existing methodical problems in the simulation especially of turbulent flow makes the experimental validation of numerical results indispensable. In a low-speed research axial compressor with incompressible flow conditions and relatively large geometric dimensions, a flow measuring system based on split-film probes has been installed and extensively operated. Measurements of the unsteady flow field and the turbulence properties concerning the blade wakes have been made and reported. In the subsequent investigation, the passage of the IGV wakes and the rotor blade wakes of the first rotor through the adjacent stator blade row have been explored in one measuring plane in front of the stator, four measuring planes within the stator and a measuring plane behind the stator at the mean radius of the blading. Measurements of the unsteady absolute velocity field in the S1-plane and the turbulent fluctuations have been made. The transportation of the IGV wake through the stator is one result of the measurements. The decay of the fluctuation components inside the stator blade row has been investigated in detail.

Unsteady Transport Mechanisms in an Axial Turbine

A numerical analysis using a commercial unsteady Navier–Stokes solver has been performed on a pin/blade configuration, in order to assess the efficacy of a commercial code in calculating time-periodic interactions and to gain a better understanding of the unsteady flow physics in axial turbines. Two cases have been investigated, with the pin positioned at 25 and 50 percent of true chord ahead of the leading edge. Both configurations have been computed both two and three dimensionally. The two-dimensional case was used to examine the influence of numerical parameters, such as mesh, time, and space discretization. The three-dimensional case allowed insight into the complete flow field including the wake influence on the secondary flow and mixing processes of the blade row. The basic mechanisms of the wake–blade interaction proved, as expected, to be the same for both pin positions. Yet, as the closest pin wake interaction with the blade field was much stronger, its features have helped to identify the respective roles of wake fluid transport and blade potential field for both cases. The latter effect, noticeably strong with the thick leading edge blade form presented in this study, has often been neglected, and this study helps shed new light on this phenomenon. The code used had been validated in previous work for pin-free steady flow within the same blade row and the new time-dependent case has served to confirm the code range and limitations. [S0889-504X(00)02104-8]

Effects of Perpendicular Blade-Vortex Interaction, Part 2: Parameter Study

Experiments have been performed to document the turbulent flow produced downstream of an airfoil encountering an intense streamwise vortex. In the second part of two-part paper, measurements revealing the effects of blade-vortex separation, blade angle of attack, and vortex strength are presented. The purpose is to provide the data needed to improve blade-vortex and blade-wake interaction noise prediction schemes to account for these effects. A total of 48 flows were studied by measuring three-component velocity and turbulence stress profiles and spectra through the vortex core downstream of the interaction. Most measurements were made 15 chordlengths downstream of the blade trailing edge. These measurements show, consistent with an inviscid model, that the effects of the interaction are only weakly dependent on blade angle of attack and vortex strength and on whether the vortex passes to the suction or pressure side of the blade. The interaction has little influence on the properties of the vortex core for separations greater than about 0.3 chords

Broadband Helicopter Noise Generated by Blade Wake Interactions

Broadband Helicopter Noise Generated by Blade Wake Interactions

Blade-wake interactions and rotary boundary layers

The flows studied here are provoked mainly by a rotating configuration having one or more thin blades, with or without an external stream normal to the axis of rotation. The applications range from helicopters and airborne seeds to food mixers and hover mowers. Computations and analysis are described for the three-dimensional rotary boundary layer, which generally is steady only in the absence of the stream. The computational method appears robust and flexible, yielding results for a variety of rotary blade arrangements. The analysis then deals with cases of large gaps, small gaps, many blades and the far outboard response controlling the global spread influence. Interesting new phenomena are found concerning multiple blade-wake interactions, inner-outer-flow interactions at tiny angles of incidence, viscous-inviscid interactions especially near corners or trailing edges and blade tips, and a doubly viscous structure induced by a dense blade-gap arrangement. Further developments and discussion are presented at the end. © 1996 The Royal Society.

Flow Structure in a Radial Flow Pumping System Using High-Image-Density Particle Image Velocimetry

Use of high-image-density particle image velocimetry (PIV) allows characterization of the instantaneous structure of wake and wake-blade interactions in a simulated rotating machine. The distribution of vorticity over an entire plane within the pumping system is related to the instantaneous pressure source terms in the wake of the impeller. Comparison of instantaneous and ensemble-averaged vorticity contours shows that limited ensemble-averaging can produce a substantial reduction in vorticity levels associated with the instantaneous pressure source terms. When the wake from the impeller interacts with a stationary diffuser blade, the instantaneous processes of flow separation and reattachment can be effectively characterized using combinations of instantaneous streamline patterns and contours of constant vorticity. Moreover, active control of the inflow into the pumping system allows substantial modification of these vorticity distributions.

Actively Controlled Radial Flow Pumping System: Manipulation of Spectral Content of Wakes and Wake-Blade Interactions

A unique, actively controlled pumping system allows independent control of inflow and impeller perturbations, as well as the phase shift between them. The basic configurations of an impeller and an impeller-diffuser blade system have been investigated, with the objective of manipulating the spectral content of the unsteadiness of the near-wake at the impeller discharge. Substantial alteration of the discrete spectral components can be attained. A central feature is the generation of a number of nonlinear interaction components, corresponding to sum and difference frequencies, of the forcing- and blade passing-frequencies and their harmonics. With proper choice of perturbation conditions, it is possible to attenuate the inflow perturbation, as well as to alter the magnitudes of the blade passing component and its harmonics.

Turbomachinery unsteady load predictions with nonuniform inflow

A two-dimensional frequency domain source-doublet based potential paneling formulation is developed to predict steady and unsteady turbomachinery blade loadings. The method employed blade-to-blade periodic boundary condition and interblade phase angles to simulate the complex blade-wake interactions in turbomachinery stages. The method is used in the present paper to simulate the unsteady response on Space Shuttle main engine high-pressure oxidizer turbopump diffuser blades. The amplitude of unsteady pressure and the wave propagation characteristics are discussed by comparing similar studies.

Prediction of blade wake interaction noise based on a turbulent vortex model

Blade wake interaction is defined as the broadband noise generated by the interaction of helicopter rotor blades with their own wake. Experimental observations have shown that this is a strong function of advance ratio and tip path plane angle. This paper describes how this noise source can be associated with the blade vortex interactions in the forward sector of the rotor. Measured levels of turbulence in the vortex core are used to predict the broadband noise levels with some success. However, more detailed information on the turbulence spectrum and the trajectory of the shed vortices is required before more accurate noise predictions can be made.

Panel method for counter-rotating propfans

A time domain source-doublet surface paneling method is developed for analyzing the unsteady loads on counter rotating propellers in incompressible irrotational flow. A scheme for treating the blade-wake interaction problem is described. Sample results for single rotation propellers are given, with comparisons to alternate theories and experiment. Finally, some preliminary results for quasi-steady and fully unsteady loads on counter rotating systems are presented.

A Note on Blade Wake Interaction Influence on Compressor Stator Row Aerodynamic Performance

Attention is given to the effect of blade wake interactions on the performance of an axial flow compressor's stator row, for the case of compressor and fan stator flows without shocks. The measured midspan loss of total pressure can be related to stator surface boundary layers, the chopped rotor wakes passing through the stator row, and the interaction between these flows. The interaction between rotor blade wake segments and stator blade surface boundary layers generates much higher losses than expected in both the stator wake and in the region of flow between stator boundary layers/wakes, if interaction effects are ignored.

Measurement and Analysis of Total-Pressure Unsteadiness Data From an Axial-Flow Compressor Stage

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.

Mean Velocity and Decay Characteristics of the Guidevane and Stator Blade Wake of an Axial Flow Compressor

Pure tone noise, blade row vibrations, and aerodynamic losses are phenomena which are influenced by stator and IGV blade wake production, decay, and interaction in an axial-flow compressor. The objective of this investigation is to develop a better understanding of the nature of stator and IGV blade wakes that are influenced by the presence of centrifugal forces due to flow curvature. A single sensor hot wire probe was employed to determine the three mean velocity components of stator and IGV wakes of a single stage compressor. These wake profiles indicated a varying decay rate of the tangential and axial wake velocity components and a wake profile similarity. An analysis, which predicts this trend, has been developed. The radial velocities are found to be appreciable in both IGV and the stator wakes. This wake data as well as the data from other sources are correlated in this paper. Appreciable static pressure gradient across the wake is found to exist near the trailing edge of both stator and IGV.

Periodically Unsteady Flow in an Imbedded Stage of a Multistage, Axial-Flow Turbomachine

A periodic-average flow measurement technique involving a hot-wire sensor was used to measure the periodically unsteady velocity field in the first stage of a low-speed, multistage, axial-flow research compressor. In portions of the compressor annulus, the periodic-average velocity patterns for imbedded rotor and stator exit flows showed appreciable sequential variation with the systematically changed data sampling position of the rotor blades. Representative examples of periodic-average flow field variation with rotor blade sampling position in stop-action sequence are shown for various locations in the compressor. A simple, first-order approximation physical description of blade wake flow transport and interaction based on experimental data interpretation is proposed to organize and thus help understand the data obtained.