Rotor Aerodynamic Performance Research Articles

Investigation of Transonic Flow Features in an Axial Flow Compressor Rotor

Numerical investigation of an axial flow rotor is carried out for its performance characterization and aerodynamic behavior during the design and off-design operating conditions. The study focuses on capturing the transonic flow features from the choke point (CP) to the near stall (NS) point in the rotor. This includes the analysis of passage shock structure, its movement in the blade passage with varying back-pressure, shock boundary layer interaction, tip leakage flow structure, and resulting losses. The study is carried out from 60 % to 100 % of the design speed using steady and unsteady RANS simulations. Three turbulence models, namely; SST, k-ε, and Reynolds stress models, are employed. The SST model predicted the closest approximation to the experimental data. The rotor aerodynamic performance is predicted in terms of total pressure ratio, efficiency, and flow contours. Unsteady analysis revealed that the primary and secondary tip leakage vortices, combined with the suction side tip corner separation, are the major instabilities near the stall region.

Experimental investigation on aerodynamic and noise characteristics of Fenestron

It is difficult to simulate the strong interference and serious flow separation of Fenestron by the CFD method based on the widely used RANS equation, and the detailed experimental data, which could be used to validate the aerodynamic and noise numerical methods, is unavailable. The experimental investigation on the aerodynamic and noise characteristics of Fenestron is carried out. In view of the complex internal flow field in duct and the relative motion between the stationary duct and the rotating rotor, a comprehensive aerodynamics and pressure measurement scheme is designed based on the bottom support rig. In this measurement scheme, the thrust generated by the rotating rotor can be measured by the rotating shaft balance, the thrusts from Fenestron are measured by the external balance and the pressures on duct inner wall are monitored by a pressure measuring system. To fully capture the noise directionality of Fenestron, a series of noise observers located at an arc array are arranged. In terms of the Fenestron test models, the baseline model, the performance improvement model based on the high-performance tail rotor and the noise reduction model based on the non-uniform blade distribution are designed respectively. By the designed measurement scheme, aerodynamic forces and pressure distributions and noise were measured for the three different Fenestron models. The results show that the aerodynamic thrusts of the tail rotor and duct increase greatly and the noise increases slightly for the performance improvement model because of the larger aerodynamics. The rotor aerodynamic performance of the noise reduction model is reduced, but the modulation effect of the tail rotor improves the forces of the duct. The noise radiated by the noise reduction model is reduced and a good noise reduction result is obtained in frequency domain.

Experimental assessment of the ice protection effectiveness of icephobic coatings for a hovering drone rotor

A new installation at the Anti-Icing Materials International Laboratory was developed to study the effects of icing on the degradation of a hovering drone rotor aerodynamic performance and to test different potential ice protection solutions. Following a study on icing parameters effects on aerodynamic performance, this paper studies a methodology developed to assess the performances of coatings as a potential ice protection system under severe icing conditions and rotor speeds up to 4950 RPM. Four different coatings, commercial or under development, were applied to the rotor blades and their effect on the resulting ice accumulation and aerodynamic degradation were measured and compared to those previously obtained on similar, uncoated blades. Results show that passive icephobic coatings in general are a promising solution to limit the negative effect of icing conditions for drone applications, without any additional energy consumption from the system. Moreover, the repeatability assessment of tests under similar conditions and for the same coating showed agreement within ±5% between repetitions, showing no signs of ice protection efficiency degradation for any of the tested coatings.

Effects of the Shock Wave Structure on the Tip Clearance Leakage Flow in Transonic Compressor Rotors

The interaction between shock waves and the tip clearance flow in a transonic compressor rotor has important effects on the tip clearance flow and the rotor aerodynamic performance. In this paper, two transonic rotors with a high pressure ratio are selected to study the tip flow, one of which has one normal shock wave at the blade tip and the other has two shock waves (an oblique shock wave and a normal shock wave) at the tip. The two rotors have the same meridional flow channel, design point flow rate, pressure ratio, and rotation speed to focus on the influence caused by the effect of the shock wave structure. The numerical results for the flow fields show the following conclusions. The strength of the two shock waves at the blade tip is weaker than that of one normal shock, and the former two shock waves are less stable than the latter. Therefore, with increasing tip clearance, the efficiency, pressure ratio, and stall margin of the rotor with the two shock waves decrease more rapidly. The static pressure difference between the pressure and suction sides of the tip clearance is the only driving factor of the tip clearance leakage flow, and the leakage flow depends on the local pressure difference and the secondary leakage caused by adjacent blades. The movement speed of the annular wall is less than that of the leakage flow, which has a minor blocking effect on the tip clearance leakage flow. The change in tip clearance has little effect on the chordal distribution of the static pressure difference and leakage flow rate per unit area, so the total leakage flow varies linearly with the tip clearance size.

Experimental Investigation of Icing Effects on a Hovering Drone Rotor Performance

A scaled version of the APT70 drone rotor, typical of small to medium UAV rotors, was tested in a 9-meter-high cold chamber for a wide range of icing parameters. The drone rotor used has four blades with varying chord and twist settings. The objective of this study was to investigate icing effects on the rotor aerodynamic performance, based on experimental data, for varying rotor speeds, precipitation rates, droplet sizes and air temperatures. Aerodynamic loads were measured using the built-in load cell, and data were compared to photographs taken during testing as well as ice thickness measurements at the end of tests. The impact of each test parameter and their variations on the degradation of the rotor’s performances was evaluated. The results show that larger droplets and lower RPMs and pitch angles generate a more rapid degradation of the performances due to the airflow around the blades and tip-vortex affecting the collection efficiency of the blades. With the smaller droplets, the air temperature did not affect the performance degradation, only the type of ice accumulation. However, with the larger droplets, degradation of the performances was less severe at warmer temperatures since almost no ice accumulated at the tip and droplets were expelled before freezing.

Experimental and numerical analysis of hovering multicopter performance in low-Reynolds number conditions

Unmanned Aircraft Systems (UAS) are state of the art in the aerospace industry and are involved in many operations. Although initially developed for military purposes, commercial applications of small-scale UAS, such as multicopters, are abundant today. Accurate engineering tools are required to assess the performance of these vehicles and optimize power consumption. The thrust and power curves of the rotors used by small-scale UAS are essential elements in designing efficient aircraft. The scarcity of experimental data and sufficiently accurate prediction models to evaluate rotor aerodynamic performance in the flight envelope are primary limitations in UAS science. In addition, for small-scale rotors at usual rotation rates, chord-based Reynolds numbers are typically smaller than 100,000, a flow regime in which performance tends to degrade. In this paper, experimental data on small-scale multicopter propulsion systems are presented and combined with a Computational Fluid Dynamics (CFD) model to describe the aerodynamics of these vehicles in low Reynolds numbers conditions. We use the STAR-CCM+ software to perform CFD simulations adopting both a dynamic-grid, time-accurate analysis and a static-grid, steady-state technique that solves the Navier-Stokes equations in a suitable framework. Comparing numerical simulation results on a conventional UAS propeller with related experimental data suggests that the proposed approach can correctly describe the thrust and torque coefficients in the range of Reynolds numbers characterizing the UAS flight envelope.

Robust design of a fan rotor blade by sweep and lean optimization with surface roughness

In order to reduce the fan rotor aerodynamic performance degradation caused by the increasing of blade surface roughness, optimal design of high bypass ratio fan rotor blade with surface roughness was carried out to improve the aerodynamic performance of the fan rotor in long-term flight operation. With the condition of surface roughness uniformly distributed on the blade, the fan blade was optimized by using sweep and lean technology, and the aerodynamic characteristics of the fan blade with different surface roughness were numerically investigated. Numerical results show that the overall aerodynamic performance of the fan rotor decreases as the equivalent sand roughness k s of the blade increasing from 3 μm to 150 μm. The isentropic efficiency of the original blade decreases by 1.56% when the surface roughness reaches 150 μm. Composite sweep and lean optimization was carried out when k s = 80 μm, the isentropic efficiency of the optimized blade at design point is increased by 0.28%, the second optimization based on the optimized blade above by using the reverse lean technology further improved the fan isentropic efficiency by 0.12%, the degradation caused by surface roughness was reduced by optimization, and the robust design of fan rotor blade was achieved.

Modified Savonius Wind Turbine for Wind Energy Harvesting in Urban Environments

Abstract The flow-induced rotation of the modified Savonius rotor, for which the blade consists of a semicircular profile and an elliptical shape, is studied using a series of unsteady computational fluid dynamics (CFD) simulations. This study first concentrates on the validation of the numerical scheme against Blackwell's experimental data of the conventional rotor. The computed flow physics around the modified rotor with the same diameter is then analyzed and compared with that of the conventional rotor during one rotation cycle. As the result, the modified rotor is outperforming the conventional one but keeping its unique features. The modified rotor offers exceeding performance at a tip speed ratio (TSR) greater than 0.8. The new peak of the power coefficient Cp is reached at TSR = 1.4 which is a typical operating condition of the wind turbine in urban areas. The remarkable finding is that the suppression of the flow separation on the blade is an effective way to improve the rotor's aerodynamic performance. As expected, the additional elliptical profile plays a key role in increasing the positive torque and in preventing the flow separation on the blade, especially at high TSR > 0.8. Finally, this study points to not only advances the fundamental understanding of flow mechanism around the rotor but also proposes a good practical energy harvesting application in urban environments.

Active vibration reductions of a lift-offset compound helicopter using individual blade pitch control with multiple harmonic inputs

PurposeThe purpose of this study is to use the individual blade pitch control (IBC), reduce actively both the rotor hub vibratory loads and airframe vibration responses for the lift-offset compound helicopter at a high-speed flight condition.Design/methodology/approachThe Sikorsky X2 technology demonstrator (X2TD) is used as the lift-offset compound helicopter. The X2TD lift-offset rotor is modelled and its rotor hub vibratory loads at a flight speed of 250 knots are predicted using a rotorcraft comprehensive analysis code, CAMRAD II, and the airframe structural dynamics is represented with a finite element analysis code, MSC.NASTRAN. When the propulsive trim methodology is applied for rotor trim, the best input condition for IBC using multiple harmonic inputs is searched to reduce the rotor vibration, while the rotor aerodynamic performance (the rotor effective lift-to-drag ratio) is improved or maintained at least. Finally, the reduction in airframe vibration responses is investigated when the best input condition for IBC with multiple harmonics is applied to the lift-offset rotor.FindingsWhen the IBC with the single harmonic input using the 2/rev actuation frequency, amplitude of 2° and control phase angle of 120° (2P/2°/120°) is considered for X2TD rotor, the rotor vibration is reduced by about 26.37% only and the rotor effective lift-to-drag ratio increases slightly by 0.98%. When X2TD rotor uses the IBC with multiple harmonic inputs (2P/2°/45° + 5P/1°/90°), the rotor hub vibratory loads and airframe vibration responses are reduced by 44.69% and from 0.48 to 79.10%, respectively, while rotor effective lift-to-drag ratio is improved by 0.77%, as compared to the baseline without IBC.Originality/valueThis study is the first study to use the 2/rev actuation for IBC to the four-bladed lift-offset coaxial rotor and to investigate to obtain simultaneously the rotor vibration reduction, rotor performance improvement and airframe vibration reduction, using IBC with multiple harmonic inputs.

Numerical investigation of effect of crossflow transition on rotor blade performance in hover

In the present study, the effect of crossflow transition on the rotor blade performance in hover was investigated numerically. The simulations were conducted for the Pressure Sensitive Paint (PSP) rotor in hover by using a Reynolds-averaged Navier-Stokes (RANS) solver based on unstructured meshes. The k−ωSST model was used for simulating the turbulent flow fields and for calculating the turbulent eddy viscosity. For predicting the laminar-turbulent onset phenomena involving crossflow-induced transition, the γ−Reθt−CF+ transition model was adopted. For the comparison of the transition locations without considering the crossflow transition effect, simulations were also carried out using the γ−Reθt transition model. The calculations were made for the PSP rotor at collective pitch angles from four to 12 degrees with an interval of two degrees. To investigate the effect of blade tip Mach number, two blade tip Mach numbers of 0.585 and 0.65 were tested. The predicted results such as the transition onset location and the rotor aerodynamic performance in terms of thrust coefficient, torque coefficient and figure of merit were compared with experimental data. It was found that proper consideration of the effect of crossflow transition is critical for the accurate prediction of the laminar-turbulent transition onset location on rotor blades. The figure of merit also compares better with the experimental data when the effect of laminar-turbulent transition is included. With the addition of the crossflow transition, the figure of merit is slightly decreased, particularly at low thrust levels.

Experimental investigation of tip anhedral effects on the aerodynamics of a model helicopter rotor in hover

This study experimentally investigates the effects of tip anhedral on the rotor aerodynamic performance and the tip vortex characteristics in hovering flight. A five-bladed scaled helicopter rotor with blades that have either rectangular (baseline) or anhedral tip geometries was used as the experimental model. Thrust and torque measurements were performed at the tip Mach numbers (Mtip) of 0.3 and 0.4 at five different pitch angles. In addition, flow field measurements via phase-locked particle image velocimetry technique were conducted at Mtip = 0.4 for a selected pitch angle. Thrust and torque measurements reveal that the anhedral tip configuration reaches a maximum Figure of Merit (FOM) value of 0.67 yielding approximately 5% FOM improvement when compared to the rectangular blade at Mtip = 0.3. It is observed that the tip anhedral changes the tip vortex trajectory by increasing the radial contraction of the wake and decreasing the average axial convection speed in the range of the wake age considered in this study. The meandering amplitude of the tip vortex increases abruptly after the passage of the proceeding blade in both cases yet a greater increase is observed in the anhedral tip configuration. Moreover, the axial convection of the previously shed tip vortices in the downstream direction is significantly slowed down during the course of the blade passage such that they reside close to the rotor disc in the anhedral case. The resultant proximity of the tip vortices to the blade with anhedral tip configuration promotes the induced flow in the radially inwards direction, which probably attenuates the strength of the newly forming tip vortex. Accordingly, the maximum tangential velocity and circulation of the tip vortex are reduced by 20% and 13%, respectively, when compared to the rectangular blade case. The reduced tip-vortex circulation is consistent with the increased FOM value. A good agreement was found with the Vatistas vortex model using n=3 for the rectangular blade and n=2.2 for the blade with tip anhedral.

Effect of Growth in Turbine Size on Rotor Aerodynamic Performance of Modern Commercial Large-Scale Wind Turbines

In this study, the impact of growth in turbine size on aerodynamic rotor performance of modern commercial large-scale wind turbines was investigated. For this examination, three different commercial large-scale wind turbines of models VESTAS V80-2MW, V90-3MW, and V126-3.3MW operating on three existing onshore wind farms were selected. In order to determine the effects of turbine size growth on aerodynamic rotor performance, turbine power output P, turbine power coefficient, Cp, turbine rotational speed, Ω, thrust force, T, tip speed ratio, λ, and flow angle, ϕ, values obtained from these turbines were compared at the same magnitude of free-stream wind speeds, U∞. The results reveal that decreasing specific power decreases the rated output wind speed of the wind turbines; hence, the reduction in the rated output of wind speed contributes to the tendency of higher power capacity. Moreover, it was determined that the enlargement in the turbine size has a profound influence on the parameters that affect the performance of the turbine such as rotational speed, Ω, thrust force, T, tip-speed ratio, λ, and flow angle, ϕ. For instance, higher thrust loads and tip-speed ratios were acquired for the turbines having a larger rotor diameter which results in better aerodynamic efficiency of the turbine.

Interacting Effects in a Multistage Axial Compressor Using Shrouded and Cantilevered Stators

This paper describes the analysis of the flowfield in a four-stage low-speed axial compressor at the design point investigated both numerically and experimentally, with particular emphasis on the impact of stator hub flow leakage in a section of the machine featuring a change of stator configuration. The goal of the work is to give insight into the effect of the stator hub configuration on the aerodynamic performance in terms of loss and efficiency of downstream blade rows as well as the resulting stage matching effects. The investigation is focused on aerodynamic behavior and stage interaction, which the stator hub configuration of a given stage induces in a downstream stage with respect to its performance and flowfield. Two shrouded and one cantilevered stator hub configurations, such as those commonly employed in industry, are investigated and discussed with respect to local flow phenomena and stagewise aerodynamic effects. The results show that the stator hub configuration of a given stage has a drastically high impact on the downstream rotor aerodynamic performance, whereas the downstream stator as well as the upstream rotor are only slightly influenced by the stator hub configuration.

Aerodynamic Performance of an Octorotor SUAV with Different Rotor Spacing in Hover

To study the aerodynamic performance of hovering octorotor small unmanned aerial vehicles (SUAV) with different rotor spacing, the computational fluid dynamics (CFD) method is applied to analyze the flow field of an octorotor SUAV in detail. In addition, an experimental platform is built to measure the thrust and power of the rotors with rotor spacing ratios L/D of 1.0, 1.2, 1.4, 1.6, and 1.8, sequentially. According to the theory of momentum, rotor aerodynamic performance is obtained with qualitative analysis. Further analysis with numerical simulation is presented with the flow field of the octorotor SUAV, the vorticity distribution, velocity distribution, pressure distribution, and streamline. The results show that the aerodynamic performance varies with the rotor spacing. Specifically, the aerodynamic performance is poor at L/D = 1.0, which is accompanied with strong interaction of wake and tip vortexes and interaction with each other. However, the aerodynamic efficiency is much improved with a larger rotor spacing, especially achieving the highest at L/D = 1.8, which is considered to be the best rotor spacing ratio for this kind of octorotor SUAV.

Numerical-experimental correlation of hovering rotor aerodynamics in ground effect

This work presents the correlation between experimental measurements and numerical simulations concerning the aerodynamics of a hovering rotor operating in ground effect above parallel and inclined surfaces. The capability of a potential-flow, free-wake aerodynamic solver based on a boundary-element-method formulation to simulate in-ground-effect problems is examined in detail. In particular, two approaches for including ground effect are outlined and compared, namely the bounded domain method and the mirror image method. Analyses of rotor aerodynamic performance and flow-field visualization reveal that the experimental data and the numerical predictions given by the solver based on the mirror-image method are in good agreement, thus proving the suitability of this developed computational tool to simulate rotor aerodynamics in the presence of close ground. Concerning the bounded domain method, it has been proven that it is suitable for the evaluation of rotor thrust and wake shape close to the rotor, but does not provide accurate simulation of the flow-field in proximity to the ground and corresponding inflow over the rotor disk.

A general numerical model for rotor aerodynamics based on added mass force

When wind turbines operate in complex atmospheric environments, the flow fields around their blades are complex and unsteady. To capture the lag characteristics of induced velocity when the dynamic environment of a rotor changes, this study further modified the equilibrium wake model using the blade element momentum and acceleration potential theories. In addition, a novel expression for the added mass force mA=128/75ρR3 was derived. Consequently, a numerical aerodynamics model of a rotor with an added mass force was developed. The model was used to calculate the aerodynamic load of a 5 MW wind turbine under steady wind, turbulence, gust, wind direction change, and rapid pitching. The results demonstrated that the numerical model with added mass force could reflect the rotor aerodynamic performance more accurately than its counterpart. The fluctuation amplitudes of the flapwise and edgewise bending moments of the blade root were smaller than those from the equilibrium wake model. The overshoot in loading, which was caused by the added mass force during pitching, could not be ignored. The research results were compared with experimental data from the National Renewable Energy Laboratory. It was found that the proposed model provided a new strategy for calculating the added mass force and predicted rotor aerodynamic loads more accurately than the model without added mass force.

The effects of the aerodynamic interaction on the performance of two Flettner rotors

Flettner rotors are nowadays becoming a widespread solution for wind-assisted propulsion. To increase the fuel savings of the ship on which they are installed, multiple devices are typically used. However, in the performance estimate of these hybrid ships, it is currently assumed that Flettner rotors operate independently, regardless of the number of devices employed and their relative position on the ship’s deck. The present investigation deals with a wind-tunnel experimental campaign aimed at understanding the aerodynamic interaction effects on the performance of two similar Flettner rotors. The study indicates that the aerodynamic performance of the two Flettner rotors is affected by their interaction, and, generally, this is most noticeable when the devices are set closer to each other and when they are aligned with the wind direction. It is demonstrated that, depending on the apparent wind direction, the layout of the Flettner rotors on the ship’s deck has a remarked influence on the driving and heeling force coefficients of the entire rig. Lastly, the velocity ratio is found to play a key role in the determination of how the interaction affects the Flettner rotor aerodynamic performance.

Strongly-coupled aeroelastic free-vortex wake framework for floating offshore wind turbine rotors. Part 2: Application

This two-part paper presents the integration of the free-vortex wake method (FVM) with an aeroelastic framework suitable to model the rotor-wake interactions engendered by floating offshore wind turbine (FOWT) rotors in operation. Part 1 of this paper introduces the numerical development and validation of an aeroelastic framework. Due to a lack of experimental aeroelastic benchmarks for FOWTs, a series of validation studies are conducted against the rotor aerodynamic and structural performance of the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine. Part 2 of this paper focuses on the modeling and simulating different aeroelastic operational conditions of FOWTs. Numerical results of the current framework capture consistently the aerodynamic rotor performance, such as power, thrust, and torque of wave-induced pitching FOWTs. In addition, the presented aeroelastic framework yields additional information about the power, thrust, and torque fluctuations due to the out-of-phase blade passing frequency and corresponding blade deflections. The fidelity of the presented framework demonstrates, for the first time, an FVM-based aeroelastic method capable of carrying out investigations on rotor-wake interactions and relevant aeroelastic phenomena of FOWTs.

Aerodynamic Analysis of a Three-Bladed Pivoted Savonius Wind Turbine: Wind Tunnel Testing and Numerical Simulation

In this study, a three-bladed pivoted vertical axis Savonius wind turbine is subjected to numerical and experimental studies. The experiments are carried out in a subsonic open-jet type wind tunnel, where the instantaneous position of the opening/closing blades are also determined via high speed imaging. The effects of adding end plates and the rotor aspect ratio on the turbine torque and power coefficients are investigated experimentally. Results show that adding end plates greatly enhances the rotor aerodynamic performance, in terms of both the maximum power coefficient and also the working range of the turbine. Similar effects are also observed for the effects of increasing the aspect ratio. Comparing numerical results with the experimental data demonstrated that the numerical results are in a convincing agreement with the experimental data of a wind rotor with an aspect ratio of 2.0 equipped with end plates. Although there are several two-dimensional numerical simulations for the drag-based vertical axis wind turbines in the literature, the results of the current study suggests that two-dimensional numerical results are not comparable with the experimental data of the rotors with small aspect ratios, especially without end plates.

Strongly-coupled aeroelastic free-vortex wake framework for floating offshore wind turbine rotors. Part 1: Numerical framework

This two-part paper presents the integration of the free-vortex wake method (FVM) into an aeroelastic framework suitable to model the rotor-wake interactions engendered by floating offshore wind turbine (FOWT) rotors in operation. Part 1 of this paper introduces the numerical development and validation of an aeroelastic framework. Because no experimental aeroelastic benchmarks exist for FOWTs, a series of validation studies are conducted against the rotor aerodynamic and structural performance of the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine. Part 2 of this paper focuses on the modeling of different aeroelastic operational conditions of FOWTs. Numerical results of the current framework captures consistently the aerodynamic rotor performance, such as power, thrust, and torque of wave-induced pitching FOWTs. In addition, the presented aeroelastic framework captures additional information about the power, thrust, and torque fluctuations due to the out-of-phase blade passing frequency and corresponding blade deflections. The fidelity of the presented framework showcases, for the first time, an FVM-based aeroelastic method capable of carrying out investigations on rotor-wake interactions and relevant aeroelastic phenomena of FOWTs.