Rotor Blade Wake Research Articles

Transient analysis of the fluid flow on a pumpjet propulsor

This work discussed the transient characteristic of PJP and gave a comparative analysis between steady and transient simulations. The simulations employed Reynolds-averaged Navier-Stokes equations (RANSE) method with a focus on the transient flow field and tip clearance effect. The mutual interference between rotor and stator does not lead to a massive difference in PJP whole open water performance. It causes remarkable changes in stator and duct, and the increasing of tip clearance enhances the changes. The exciting force on blades can not be ignored and represents a different variation between rotor and stator with the effects of advance ratio or tip clearance and possesses a characteristics frequency corresponded to blade passing frequency (BPF) and their harmonics. The exciting force intensity mainly embodies in BPF. The stator mainly has better recycling on rotor blade wake, and the recycling mainly acts on circumference velocity. Comparing with axial velocity, which shares almost the whole energy in PJP wake, both the circumference velocity and radial velocity include a little. Tip sweeping vortex and tip leakage vortex are the typical vortices in PJP. Tip leakage vortex plays a vital role in the stator flow field and PJP wake.

Observed Thermal Impacts of Wind Farms Over Northern Illinois.

This paper assesses impacts of three wind farms in northern Illinois using land surface temperature (LST) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments onboard the Terra and Aqua satellites for the period 2003–2013. Changes in LST between two periods (before and after construction of the wind turbines) and between wind farm pixels and nearby non-wind-farm pixels are quantified. An areal mean increase in LST by 0.18–0.39 °C is observed at nighttime over the wind farms, with the geographic distribution of this warming effect generally spatially coupled with the layout of the wind turbines (referred to as the spatial coupling), while there is no apparent impact on daytime LST. The nighttime LST warming effect varies with seasons, with the strongest warming in winter months of December-February, and the tightest spatial coupling in summer months of June-August. Analysis of seasonal variations in wind speed and direction from weather balloon sounding data and Automated Surface Observing System hourly observations from nearby stations suggest stronger winds correspond to seasons with greater warming and larger downwind impacts. The early morning soundings in Illinois are representative of the nighttime boundary layer and exhibit strong temperature inversions across all seasons. The strong and relatively shallow inversion in summer leaves warm air readily available to be mixed down and spatially well coupled with the turbine. Although the warming effect is strongest in winter, the spatial coupling is more erratic and spread out than in summer. These results suggest that the observed warming signal at nighttime is likely due to the net downward transport of heat from warmer air aloft to the surface, caused by the turbulent mixing in the wakes of the spinning turbine rotor blades.

Identification of Spinning Mode in the Unsteady Flow Field of a Low Pressure Turbine

This paper presents an experimental investigation of the vane-blade unsteady interaction in an unshrouded low pressure (LP) turbine research rig with uneven blade/vane count (72 blades and 96 vanes). The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put on the adjustment of all relevant model parameters. In particular blade count ratio, airfoil aspect ratio, reduced mass flow, reduced speed, and Mach and Reynolds numbers were chosen to reproduce the full scale LP turbine at take off condition. Measurements by means of a fast-response pressure probe were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The probe has been fully traversed both in circumferential and radial traverses. The rotor exit is characterized by strong perturbations due to the tip leakage vortex and the rotor blade wake. Circumferential nonuniformities due to the upstream vane wake and to the downstream exit guide vane potential effects are also identified. Furthermore, in the present configuration with an uneven blade/vane count the nonuniformities due to the stator and rotor row are misaligned along the whole turbine circumference and create a spinning mode that rotates in direction opposite to the rotor at a high frequency. The aeroacoustic theory is employed to explain such further unsteady pattern. The variations of the exit flow angle within a cycle of such pattern are not negligible and almost comparable to the ones within the blade passing period.

Analysis of the Helishape Descent Database with Regards to Blade-Wake Interaction Noise

Helicopter rotor Blade Wake Interaction (BWI) noise is known to be significant during take-off and level flight. Less attention has been given to descent flight conditions, where Blade Vortex Interaction (BVI) noise is dominant. The present study investigates BWI noise in descent flight conditions by analyzing the HELISHAPE descent case database. Through signal analysis of both acoustic waveforms and spectra, the rotor azimuthal region responsible for BWI noise is localized and the dominance of BVI noise in the BWI frequency region is shown. Coherence analysis of the blade pressure data indicate significant chordwise coherence in the 3 to 4 Strouhal number range and absence of acoustic dipoles in the BWI frequency range. Comparison with analysis of a swept-back blade-tip database showed that blade-tip shape does not affect BWI noise. The findings of this study support BWI prediction models based on Amiet's theory and suggest that BWI noise can be ignored for predictions of rotor noise in descent flight conditions.

Laser Doppler Anemometry Measurements in an Axial Compressor Stage

The aim is to provide unsteady experimental data of the e owe eld in a benchmark one-stage axial compressor, to improvetheunderstanding ofthee owfeatures.Theinvestigationisconductedwitha two-componentlaserDoppler anemometer, and 32 axial locations are surveyed, from the rotor inlet to the stator outlet. Because laser Doppler anemometry is a nonintrusive device, data are obtained from the rotor row, providing information in a region dife cult to explore with other techniques. Limitation of blade shadowing is addressed with an adjustment of the laser head orientation, which permits reaching any point inside the rotor. Furthermore, a dual probe orientation gives two sets of measurements yielding three velocity components. The dynamics of the rotor tip clearance vortex is discussed in the interrow gap. There is a strong interaction between this vortex and the rotor bladewake. Phaseaveraged results show the unsteady e owe eld and the rotor ‐stator interaction. The rotor wakes are cut up by the stator row and are visible until the exit of the axial zone of measurements. Their interaction with the stator blades boundary layers creates strong time-dependent e ow structures resulting in phase lags between the rotor wakes at the stator outlet.

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.

Measurement of unsteady flow and turbulence in a low speed axial compressor

Abstract A two-stage experimental axial compressor with incompressible flow conditions and relatively large geometric dimensions is utilized for a detailed investigation of the unsteady flow field and the structure of turbulence. Hot-film probes in split-film configuration operated in constant temperature (CT) mode in different forms and orientations are applied. Evaluation of the measurements in a mainflow/crossflow/radial flow reference system renders turbulence data and Reynolds-stress terms compatible with the physics of the wake flow. Turbulence between the rotor blade wakes is relatively low with 3% and nearly isotropic. Turbulence in the wakes increases about threefold to 10%, has a characteristic structure and is definitely anisotropic. It depends distinctly on the operating point of the compressor. A comparison of the different probe types shows good results between the rotor blade wakes. In the wake region the circumferential extension of one probe type enables a correct measurement of the unsteady flow components.

Measured Rotor Wake and Potential Forcing Functions, Including Blade Row Interactions

A fundamental experiment is directed at the acquisition and analysis of data dee ning compressible forcing functions generated by a rotor, including interactions with the upstream-generated inlet guide vane (IGV) wakes, for application to wake forcing function models in turbomachine forced-response design systems. The research fan facility consists of an IGV row and a downstream rotor. IGV‐ rotor axial spacing is variable, with the IGV row able to be indexed circumferentially, thereby enabling the rotor wakes to be measured both in the IGV freestream and wakes. At a rotor-relative Mach number of 0.6 unsteady measurements are made of the rotor wake pressure and velocity e elds for two IGV‐ rotor axial spacings and with the rotor wake measured in the IGV freestream and wake regions. The decay characteristics of the rotor blade wakes are compared to empirical correlations. After Fourier decomposition, a vortical‐ potential gust splitting analysis is implemented and applied to the e rst-harmonic data to determine the vortical and potential harmonic wake gust forcing functions both upstream and downstream of the rotor. The potential gust component of the rotor wakes upstream of the rotor is found to be dominated by the e rst-harmonic component with small contributions from the second and third harmonics. Higher harmonics of the vortical gust component of the rotor wakes measured both in and out of the IGV wakes are found to be signie cantly reduced in the IGV wake regions.

Active aerodynamic control of wake-airfoil interaction noise - Experiment

A proof of concept experiment has been performed that shows the potential for active aerodynamic control of rotor wake/stator interaction noise in a simplified manner. A single airfoil model representing the stator was fitted with a movable trailing-edge flap controlled by a servomotor. This flap allowed control of the unsteady lift of the airfoil and acted as the active aerodynamic element in the system. Upstream of the airfoil, a disturbance generator created a periodic wake, similar to an actual rotor blade wake, which impinged on the airfoil. A control system moved the motor-driven flap in the correct angular displacement rate and phase to reduce the unsteady load on the airfoil during the wake interaction. The motion could be arbitrary, permitting the airfoil-flap combination to respond to generic inflow disturbances. Noise reduction results showed that this concept works very well in the bandwidth at which the servomotor drive actuated the flap (currently up to 100 Hz). The peak-to-peak acoustic interaction pulse was reduced by a factor of 2, whereas the corresponding noise was reduced by a significant 10 dB at some frequencies. A companion paper by £. J. Kerschen, entitled Active Aerodynamic Control of Wake-Airfoil Interaction Noise—Theory, demonstrates the ability to predict the required flap motion to achieve these dramatic noise reductions.

The Effects of Free-Stream Turbulence and Flow Pulsation on Heat Transfer From a Cylinder in Crossflow

The heat transfer from a circular cylinder is a fundamental problem with practical applications ranging from tubular heat exchangers to the leading edge of gas turbine blades. The flow field across the surface is often unsteady due to natural instabilities or forced oscillations of the fluid. Examples are the periodic natural shedding of vortices form cylinders and the wakes of rotor blades in gas turbine engines. In addition, active vibration of the fluid is a possible means of augmenting the heat transfer. When the frequencies are near the natural shedding frequency of the cylinder, active fluid vibration also gives the possibility of frequency control through lock-on. The present work is an experimental study of the combined effects of flow pulsation and turbulence on the local heat transfer from a circular cylinder. In particular, the possibility of a coupling between the two effects is investigated.

Measurement and analysis of unsteady flow structures in rotor blade wakes

Time-variant data are obtained to investigate the exit flow field from a rotor in a research compressor. In the free-stream region, the instantaneous data are analogous to one another and to the ensemble averaged free-stream results. However, in the wake region, some of the instantaneous signals are similar to one another and to the ensemble averaged wake, but others differ significantly. These variations in the instantaneous data are interpreted and shown to be due to a vortex street structure in the wake. This is accomplished by: (1) developing a mathematical model of the rotor blade exit flow field based on a wake vortex street structure analogous to the unsteady flow field behind bluff bodies due to classical von Karman vortex shedding; and (2) correlating predictions of both the ensemble averaged and instantaneous rotor blade exit flow fields as well as the velocity probability density distributions from this vortex wake flow field model with the corresponding data. The correlation of the ensemble averaged rotor blade exit flow fields is very good and the flow angle distribution correlation excellent. The predicted instantaneous rotor blade exit flow field exhibits many of the flow features found in the data. Also, the probability density distributions for the data and the vortex wake flow field model are analogous to one another.

Aircraft Turbofan Noise

Recent advances in the understanding of turbofan noise generation and suppression in aircraft engines are reviewed with particular emphasis on NASA research. The review addresses each link in the chain of physical processes which connect unsteady flow interactions with fan blades to far field noise. Mechanism identification and description, duct propagation, radiation, and acoustic suppression are discussed. Recent advances in the experimental technique of fan inflow control assure that inflight generation mechanisms are not masked by extraneous sources in static tests. Rotor blade surface pressure and wake velocity measurements aid the determination of the types and strengths of the generation mechanisms. Approaches to predicting or measuring acoustic mode content, optimizing treatment impedance to maximize attenuation, translating impedance into porous wall structure, and interpreting far field directivity patterns are illustrated by comparisons of analytical and experimental results. A persistent theme of the review is the interdependence of source and acoustic treatment design to minimize far field noise. Areas requiring further research are discussed and the relevance of aircraft turbofan results to quieting other turbomachinery installations is addressed.

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.

Interaction of Compressor Rotor Blade Wake With Wall Boundary Layer/Vortex in the End-Wall Region

This paper reports the experimental study of the three-dimensional characteristics of the mean velocity of the rotor wake inside the annulus- and hub-wall boundary layers. The measurements were taken with a rotating three-sensor hot wire behind the rotor. This set of measurements probably represents the first set of comprehensive measurements taken inside the annulus- and hub-wall boundary layers. The wake was surveyed at several radial locations inside the boundary layer region and at several axial locations. Interaction of the wake with the annulus-wall boundary layer, secondary flow, tip-leakage flow, and the trailing vortex system results in slower decay and larger width of the wake. The presence of a strong vortex and its merger with the wake is also observed. The end-wall boundary layers and the secondary flow were found to have a substantial effect on both the decay characteristics and the profile of the wake. These and other measurements are reported and interpreted in this paper.

Effects of Rotation and Blade Incidence on Properties of Turbomachinery Rotor Wake

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.

Numerical Analysis of Turbulent Wakes of Turbomachinery Rotor Blades

The wakes of turbomachinery rotor blades are turbulent, three-dimensional, and are subjected to curvature and rotation effects. The objective of this study is to predict the development of such wakes and compare the predictions with the existing data. A finite difference procedure is employed in the numerical analysis of the wake utilizing the continuity, momentum, and turbulence closure equations in the rotating curvilinear and non-orthogonal coordinate system. The turbulence closure is affected by the modified Reynolds stress model. The effects of curvature and rotation on the turbulence structure are accounted for with this turbulence closure model. The pedictions from the present turbulence model agree well with the mean velocity and the turbulence wake data.

Prediction of Two- and Three-Dimensional Asymmetrical Turbulent Wakes, Including Curvature and Rotation Effects

Twoand three-dimensional turbulent wakes which develop under the influence of streamline curvature and rotation were calculated with three turbulence closure models. The first model was comprised of transport equations for the turbulent kinetic energy and the rate of energy dissipation. The second model was comprised of equations for the rate of turbulent kinetic energy dissipation and Reynolds stresses, but the effects of the convection and diffusion in the Reynolds stress transport equation were handled collectively. The third model utilizes equations of turbulent kinetic energy dissipation and Reynolds stresses in nearly exact form. All three of the models were modified for the effects of streamline curvature. The second and third models include the effects of rotation through the rotation-originated redistribution term in the transport equation of Reynolds stresses. The numerical results for the wakes of airfoils, cascades, and turbomachinery rotor blades demonstrate that the second and third models provide accurate predictions, but computer time and storage can be considerably saved with the second model.

The Time-Variant Aerodynamic Response of a Stator Row Including the Effects of Airfoil Camber

An experimental investigation was conducted to quantitatively determine the validity and applicability of state-of-the-art transverse gust cascade analyses. This was accomplished by obtaining fundamental time-variant forced response data at realistic values of key parameters in a large-scale, low-speed, single-stage research compressor. The forcing function, the velocity defect created by the rotor blade wakes, was measured with a crossed hot-wire probe. The resulting time-variant aerodynamic response was measured by means of flush mounted high response pressure transducers on both flat plate and cambered airfoil stator vane rows over a wide range of incidence angles. These dynamic data were then analyzed to determine the chordwise variation of the unsteady pressure difference in terms of a dimensionless dynamic pressure coefficient and an aerodynamic phase lag referenced to a transverse gust at the leading edge of the vanes. These dimensionless pressure difference data were all correlated with predictions obtained from a state-of-the-art compressible transverse gust, flat plate cascade analysis. Correlation of the classical flat plate unsteady data with the predictions permitted the range of validity of the analysis to be assessed in terms of incidence angle. Correlation of the cambered vane unsteady data with those for the flat plate and with the predictions allowed the effects of airfoil camber as well as the applicability of the flat plate prediction to realistic cambered airfoils to be quantitatively determined.

Multistage Axial-Flow Turbomachine Wake Production, Transport, and Interaction

To aid in the understanding of rotor and stator viscous wake interaction, slowand fast-response instrumentation has been employed to obtain time-average and instantaneous periodic-average flowfield measurements between the blade rows in a low-speed multistage axial-flow compressor. Quantitative measurements indicate that the rotor blade wake pattern can vary significantly when influenced by an upstream stationary blade row, and that relative circumferential positioning of the stationary blade rows can have considerable influence on downstream rotor and stator flowfields. Differences in aerodynamic and acoustic characteristics resulting from changes in unsteady wake interaction patterns were observed.

Rotor Blade Wake Flutter—A Comparison of Theory and Experiment

During early whirl testing of the hingeless main rotor of the AH‐56A Cheyenne helicopter, a high‐frequency (7P), highly coupled, flap‐torsion‐inplane flutter occurred at rotor overspeed at a condition of near zero lift at the rotor tips. The flutter disappeared at higher and lower values of rotor lift rather than being nearly lift independent as had been predicted by quasisteady aerodynamic theory. Wake flutter was suspected and coroborated by subsequent analysis. This discussion covers the theoretical flutter analyses and the effects on flutter of design changes made both to eliminate the flutter and to improve vehicle stability and control. A theoretical analysis employing quasi‐steady aerodynamics was the basis for predicting the complicated reactionless interblade mode shape and frequency, but not the lift level of instability. The flutter stability analysis was then amended to include the effects of previous blade passage wakes and interblade mode coupling, in the four‐bladed rotor, on unsteady aerodynamics and the new analysis results agreed very well with the experimental flutter. The flutter was shown to occur in the specific region of lift near zero, being stable at higher and lower levels, and at a critical overspeed rpm with stability reappearing with increase of rpm above the critical.