Rotor In Hover Research Articles

Comparative Study of Coaxial Main Rotor Aerodynamics in the Hover with the Usage of Two Methods of Computational Fluid Dynamics

The paper is focused on numerical modeling of the aerodynamic characteristics of a full-scale coaxial main rotor in hover. The simulation was performed using two approaches of computational fluid dynamics (CFD): the original free wake model (FWM) developed by the authors and the unsteady Reynolds-averaged Navier–Stokes (URANS) equations method based on the Ansys Fluent software. The structure of the rotor vortex wake, flow images, vorticity and induced velocity fields, total and distributed aerodynamic characteristics of the rotor, including the rotor performance and figure of merit diagrams, have been analyzed. A comparison between FWM/URANS based calculations and calculated/experimental data by other authors has been performed. A satisfactory match of these data confirms reliability of used methods when modeling the aerodynamic characteristics of coaxial rotors. The FMW demonstrates a significant advantage in speed and resources intensity when calculating the total aerodynamic characteristics of a coaxial rotor. The URANS method makes it possible to model with significant accuracy the effects associated with the blade vortex interactions and pressure distribution over the blade surface. Finally, conclusions about most effectiveness of joint application of considered methods in solving complex problems of coaxial rotor aerodynamics has been made.

Application of Actuator Line Model for Large Eddy Simulation of Rotor Noise Control

A hybrid Large Eddy Simulation (LES)/Actuator Line Model (ALM) computational approach is employed to simulate the turbulent wake of a two-bladed rotor in hover. Validations are performed for the Knight and Hefner two-bladed rotor case with comparisons to previous LES predictions using the immersed boundary method, as well as hybrid RANS-LES using the Helios code. The Ffowcs-Williams Hawkings (FWH) acoustic analogy is then used for predicting rotor noise. Predictions from the combined LES/ALM/FWH model are next validated against recent PIV wake flow and acoustic measurements for a two-bladed rotor. Finally, the LES/ALM/FWH approach is exploited to demonstrate a novel way to explore active flow control strategies to reduce rotor noise by delaying flow separation and enabling a lower rate of rotation, resulting in lower noise while maintaining thrust.

Experimental study of the impact of the number of blades on the profile drag of UAV helicopter rotors in hover

Due to the motion of tip and root vortices of helicopter rotor blades and the inevitable blade-vortex interactions in multi-blade rotors, the profile drag of all individual blades is increased, when compared to the absence of any blade-vortex interaction. Different factors like the speed of rotation of the rotor and the number of blades greatly impact the magnitude of the profile drag increase. This effect is additionally worsened with the low- Reynolds operational conditions of small-sized UAV helicopter rotors. In addition, there is an initial upward motion of the tip vortex and thus there are three scenarios for the blade-vortex encounter, depending on whether the vortex emitted by the previous blade is still above, on or below the plane of rotation of the blades. All those uncertainties motivated the authors to carry- out this present experimental study. The results are shown in a convenient form. In hover, the profile drag increase is shown to be greater with increased number of blades and higher rotational speeds as there is less time for the preceding vortex to clear the path of the following blade.

Aerodynamic Characteristics of Helicopter with Ducted Fan Tail Rotor in Hover under Low-Speed Crosswind

The tail rotor of a helicopter operating under low-speed crosswind undergoes highly complex flow due to the interaction between the main rotor, fuselage, and tail rotor system. In this study, numerical simulations have been conducted on the complete configuration of a helicopter with a ducted fan tail rotor system (comprising a main rotor, ducted fan tail rotor, fuselage, and empennage) to analyze the wake interaction in hovering flight under various crosswind directions. The flow characteristics around the tail rotor, the tail rotor thrust, and the yawing moment of the helicopter are investigated and evaluated. The aerodynamic forces are compared with those of a helicopter with an open-type tail rotor. The results indicate that the aerodynamic performance of the ducted fan tail rotor is highly affected by the wakes of both the main rotor and port wing. Nevertheless, the helicopter with a ducted fan tail rotor is observed to be much more directionally stable under various crosswind directions, than that with an open-type tail rotor. This is because the rotor is protected by the fixed part of the tail rotor system in the former case.

Iterative Blade Element Momentum Theory for Predicting Coaxial Rotor Performance in Hover

This paper presents a new blade element momentum theory (BEMT) for a coaxial rotor in hover. The new BEMT iteratively solves the upper and lower rotor induced velocities to account for the mutual rotor-to-rotor interaction. The upper rotor induced velocity is affected by the lower rotor thrust and induced velocity, whereas the lower rotor induced velocity is affected by the upper rotor thrust and induced velocity. Two empirical constants are included in each rotor calculation. This new BEMT provides the performance of each rotor as a function of the rotor separation distance. The new BEMT is validated with measurement data for two coaxial rotor experiments. The first experiment validates the thrust to power coefficients at a given separation distance. The second experiment validates each rotor's figure of merit, thrust, power, interference loss factors, etc. as a function of the rotor separation distance. It is shown that the BEMT captures the trends and magnitudes of the performance as a function of the rotor separation distance compared to the measurement data. Detailed radial distributions of aerodynamic properties are also presented at several separation distances.

Experimental and computational investigation on comparison of micro-scale open rotor and shrouded rotor hovering in ground effect

Various investigations on open rotor (OR) hovering in-ground effect (IGE) are carried out, but few papers report shrouded rotor (SR) hovering IGE. This paper compares aerodynamic performance and flowfield characteristics of OR and SR hovering IGE by both experimental measurements and computational fluid dynamic (CFD) simulations. Experimental results reveal that in IGE flight, the aerodynamic performance of SR is more sensitive than that of OR. And at constant power, SR offers more thrust than OR at the same ground distance. Ground has a great influence on thrust for OR below 2.2 rotor radius distance, while for SR it shows obvious effect below 1.5 rotor radius distance. It is also shown that normalized aerodynamic coefficients of OR and SR are independent on rotor speed. In addition, for OR the rotor thrust coefficient changes nearly linearly with the logarithmic distance from ground, while for SR it changes nonlinearly. Flowfield analysis by CFD shows that shroud changes the tip flow features and expands the slipstream area of SR. When ground distance gets small, back pressure below the rotor-disk plane increases, which is more obvious for SR than OR. Furthermore, shroud thrust of SR decreases because of tip leakage flow and flow separation.

Improving predictions of transitional and separated flows using RANS modeling

In this paper, new concepts are presented from the aspects of viscous modeling and numerical scheme in an attempt to improve the prediction of rotor hover performance and associated viscous flow physics under a RANS statistic modeling framework. An improved viscous modeling is proposed to address the overprediction of flow separations faced by RANS modeling, which modifies the linear eddy viscosity models based on a new separation correction method. This concept is seamlessly implemented into two popular eddy viscosity models: the Spalart-Allmaras one-equation model and Menter's shear stress transport two-equation model. A new high order unstructured grid WENO scheme is adopted to improve the prediction of mean flow resolutions as well as viscous flow structures around the hovering rotor. Numerical results show significant improvements achieved in the prediction of rotor hover performance as well as viscous flow phenomena including boundary layer transitions and flow separations.

Validation of momentum source method in the analysis and prediction of the rotor finite ground effect in hover

Aiming at the complex flow phenomena with multiple speed scales and the high computational cost constraints of the rotor’s finite ground effect, the momentum source methods to satisfy the computational accuracy and efficiency are developed. Combined with the preconditioning methods, which cope with multiple speed scales, a numerical simulation method for the finite ground effect of the rotor is presented. On this basis, the validity of the numerical method is verified by hovering case firstly. The thrust only slightly increases by 3.26% compared with the experiment. Then, the effects of rotor deviation and height on the aerodynamic performance of the rotor are studied, and a surrogate mode with finite ground effect is established, which is tested by the testing samples on its prediction accuracy. The conclusion shows that the momentum source method can be applied to the analysis and prediction of the finite ground effect of rotor hovering, and the finite ground effect has a significant impact on the overall aerodynamic performance of the rotor.

Aerodynamic Interactions of Quadrotor Configurations

An advanced potential flow method is used to study the aerodynamic interactions between small rotors of quadrotor configurations and the impact on the overall flight performance. The aerodynamic analysis method is validated by comparing the thrust and power predictions of isolated rotors in hover and forward flight with experimental results. The aerodynamic model was next extended to consider the interactions of several rotors of various quadrotor configurations. Compared with an independent rotor, the interaction of multiple, closely located rotors increases the power requirements of the vehicle and causes a loss of thrust. For diamond configurations, the rotation direction of the lead rotors impacts the thrusts of the midrotors and, subsequently, the overall vehicle rolling moment. For square configurations, slightly higher total thrusts and slightly greater tendencies to pitch up are observed when the retreating blades of the lead rotors move back along the vehicle centerline than for the reversed rotational orientations. The diamond configuration exhibits up to 3.5% higher rotor efficiency and a lesser tendency to pitch up with increasing forward speed than the square configuration.

Computational Aeroelastic method for rotor based on MBDYN

A framework of numerical formulations for the aeroelastic analysis of helicopter rotor is presented in this paper. The blade structural dynamics are modeled by an open source multibody dynamic software MBDYN, which solves finite element equation of elastic bodies in general motions. Then the structural deformation is transformed to blade surface grid by radial base function (RBF) interpolation, and volume grids are regenerated by RBF and TFI methods. Lastly, the fluid governing equations are solved. By integrating the above methods, S76 hovering rotors are simulated and compared to the test data. Results show that elastic torsion decreases local angle of attack. For status at [Formula: see text] and [Formula: see text], the shock and shock-induced separation are reduced on the outboard blade, which has remarkable effects on the prediction of rotor hovering performance.

An Unsteady Momentum Source Method and Its Application in Simulation of Hovering Rotor

The efficiency and accuracy of numerical simulation on power unit is the key to study the relevant aerodynamic layout with multiple rotating power units. However, the numerical simulation of the power unit using real geometry all faces the problem of low solution efficiency. Taking the rotor hovering state as an example, the real blade was firstly simplified and replaced by a thin mesh disk to establish the effective momentum source method. Then, using fan-shaped mesh region that changes with time to replace real blade and simulate the rotation, the unsteady momentum source method which could get the revolution of tip vortex was proposed. The results show that the momentum source method with the input of accurate blade force distribution can simulate rotor wake better, and the influence that blade geometry acts on wake mainly reflects in the blade force distribution. In addition, the unsteady momentum source method can simulate the revolution of tip vortex, and its consumptions of computing resources and calculation time are only about 1/8 of the unsteady numerical simulation based on the real geometry.

Small-Scale Rotor Design Variables and Their Effects on Aerodynamic and Aeroacoustic Performance of a Hovering Rotor

Abstract With the increased prominence of multicopter micro-aerial vehicles, more importance has been placed on the aerodynamic and acoustic performance of these systems, as their small-scale and lower Reynolds number regime provide results that are different from full-scale rotors. A computational methodology was employed in order to study the aerodynamic and aeroacoustic performance from different small-scale rotors used in a multicopter configuration. Three rotor design variables (twist, taper, and pitch) were investigated in order to understand their influence on aerodynamic and acoustic performance of a hovering rotor. Variables such as rotor rotation rate and rotor radius were kept constant. Common aerodynamic performance metrics such as the ratio of coefficient of thrust to coefficient of power and figure of merit (FM) were used to assess aerodynamic hover performance of the designed rotors. Acoustic performance was assessed by recording acoustic pressure in the far-field at two separate receivers. Acoustic results are presented in the frequency domain as one-third octave band data and as overall sound pressure level (SPL). Flow fields and pressure contours were calculated and displayed in order to help explain aerodynamic and acoustic results. From the results, insights are provided for rotor designs that are more aerodynamically and acoustically efficient in hover. Specifically, rotors that provided lower values of disk loading and higher values of power loading were typically more acoustically efficient. Using greater rotor twist and taper increased both aerodynamic and acoustic performance.

Breakdown of aerodynamic interactions for the lateral rotors on a compound helicopter

Auxiliary lift and/or thrust on a compound helicopter can introduce complex aerodynamic interactions between the auxiliary lift and thrust components and the main rotor. In this study high-fidelity computational fluid dynamics analyses were performed to capture the various aerodynamic interactions which are occurring for the Airbus RACER compound helicopter, featuring a box-wing design for auxiliary lift in cruise and wingtip-mounted lateral rotors in pusher configuration for auxiliary thrust in cruise and counter-torque in hover. Although the study was limited to a specific geometry, the effects and phenomena are expected to be to some extent applicable in general for compound helicopters and wingtip-mounted rotors in pusher configuration. A quantitative indication of the aerodynamic interaction effects could be established by leaving away different airframe components in the simulations. The downwash of the main rotor was found to cause a small negative angle of attack in cruise for the wings and lateral rotors and impinged directly on the lateral rotors in hover, resulting in moderate to very significant sinusoidally varying blade loading. The wing increased lateral rotor propulsive efficiency in cruise through its wingtip rotational flowfield and to a lesser extent through its wake. An upstream effect of the lateral rotors on the wing loading was also found. In hover the wing caused a net increase in left lateral rotor thrust as the deflection of the main rotor flow towards the rotor resulted in a local thrust decrease and the low momentum inflow to the rotor from the wake of the wing resulted in a local thrust increase. A small thrust decrease for the right lateral rotor was found due to the wing disturbing its slipstream as this rotor produced reversed thrust. In general, very significant aerodynamic interaction effects can be expected when a main rotor, lateral rotors and wing are in proximity to each other.

A two-dimensional temperature correction method for pressure-sensitive paint measurement on helicopter rotor blades

Pressure-sensitive paint (PSP) has become an important technique for global pressure measurement on rotating surfaces such as helicopter rotor blades. Due to the temperature sensitivity of PSP, it is essential to apply accurate temperature correction method in PSP measurements. In previous studies, one-dimensional (1D) correction methods were generally adopted which only considered the radial temperature gradient on the blade. However, in the current study, it was found that the systematic error of this 1D assumption was non-negligible in large-scale experiments due to the growing chordwise temperature gradient. Accordingly, a two-dimensional (2D) temperature correction method considering both radial and chordwise temperature gradients was developed to improve the accuracy of PSP. In this method, a 2D temperature map was firstly generated based on the TSP results. Calibrations were then conducted under a series of temperature and pressure values to generate a calibration surface. According to the calibration results, the pressure and temperature fields on the rotor blade were solved directly. The proposed 2D method was validated through PSP measurements on a 2 m diameter BO-105 helicopter rotor in hover. Compared with 1D temperature correction, the results from the current method showed clearly improved agreement with the CFD results and reduced overall measurement error.

Dynamic mode decomposition of rotorcraft blade tip vortex in hovering state

The flow field structure of a rotorcraft is complex; specifically, the rotor tip vortex structure has a great influence on the rotor performance. Therefore, in this paper, the evolution characteristics of rotor tip vortices and the dynamic mode decomposition (DMD) of rotor tip vortices in a rotor hovering state are studied. Through a time-resolved particle image velocimetry experiment, a comparative study of the blade tip vortex flow field at a fixed rotation speed (1500 rpm) and a collective pitch of 6° and 9° was performed. The method of DMD is used for the reduced-order analysis of the vorticity field of the blade tip vortex in the hovering state. By this method, these important vortex structures are extracted and discussed; meanwhile, the future flow field is also reconstructed. The results of flow visualization indicate that the trajectory of the blade tip vortex is moving down the axis, while moving toward the hub in the radial direction in the hovering state. The results of DMD analysis show that during the evolution of the blade tip vortex, different modes have different contributions to the rotor as a whole. In addition, the larger the collective pitch, the larger the modal coefficient amplitude and the slower the stabilization speed.

Experimental analyses of synthetic jet control effects on aerodynamic characteristics of helicopter rotor

ABSTRACTExperimental analyses of synthetic jet control (SJC) effects on aerodynamic characteristics of rotor in steady state and in hover were conducted. To ensure the structural strength of rotor and enough interior space for holding the synthetic jet actuators (SJAs), a particular blade with a frame-covering structure was designed and processed, and the experiment was conducted with low free stream velocities and rotor rotation speeds. There were three test conditions. In steady state, there were three free stream velocities (10m/s, 15m/s and 20m/s). In hover state, the rotor was worked with two rotation speeds of 180RPM and 240RPM. In forward flight, the rotor was worked with a rotation speed of 180RPM and a free stream velocity of 7.5m/s. To measure the synthetic jet control effect on rotor in stall, the range of collective pitch was set from 10° to 28° in steady state. The aerodynamic forces and sectional velocity field were measured by using the six-component balance and the Particle Image Velocimetry (PIV) system in the wind tunnel. Flow control effects on the blade based on the synthetic jets (SJ) were experimentally investigated with different jet parameters, such as jet locations, jet angles, and jet velocities. In steady state, the jet closer to the leading edge, and the jet angle of 90° had more advantages in improving the aerodynamic characteristics. Furthermore, the aerodynamic forces and sectional velocity field measurement of rotor in hover were conducted, it showed that SJAs could increase flow velocity at the upper surface, which led to lower upper surface pressure. As a result, the normal forces of rotor with two rotation speeds were increased significantly. These results indicated that the synthetic jet has a capability of increasing the normal force and delaying or preventing the stall of rotor.

Aerodynamic Characteristics of a Hex-Rotor MAV With Three Coaxial Rotors in Hover

In order to study the aerodynamic characteristics of a Hex-rotor micro aerial vehicle (MAV) with different rotor spacing, both experiments and numerical simulations are performed in this paper. First of all, a series of aerodynamic parameters to characterize the hover performance of the Hex-rotor MAV are analyzed theoretically. Secondly, tests on the Hex-rotor MAV with different rotor spacing ratios ( $i=0.5$ , 0.56, 0.63, 0.71, 0.83, 1.00) is presented in detail. The thrust and power of the Hex-rotor MAV are obtained from the self-designed test platform. In the meanwhile, pressure and velocity distribution of the Hex-rotor MAV are obtained by Computational Fluid Dynamics (CFD) simulation. The results show that the aerodynamic performance of the Hex-rotor MAV is varied with the rotor spacing. Specifically, the rotor interference is inclined to improve the aerodynamic performance of the Hex-rotor MAV with the proper rotor spacing. Owing to the increase of rotor spacing, the interactions between rotors are weakening gradually. In addition, the uniformity of velocity distribution, the stability of downwash distribution and the integrity of vortex are the necessary conditions for a larger thrust of the Hex-rotor MAV. Finally, combined with the test and simulation results, it is also found that there is larger thrust characterized with better aerodynamic performance at $i=0.63$ , which is assumed as the best rotor arrangement of the Hex-rotor MAV. Especially for ${{Re}}=1.16\times 10^{5}$ , the thrust increases by 6.09%, and the hover efficiency increases by about 9.17% in general.

Thrust and Acoustic Performance of Small-Scale, Coaxial, Corotating Rotors in Hover

The findings from a test campaign aimed at understanding the thrust and acoustic performance of a small-scale, coaxial, corotating rotor in hover are discussed. Measurements of axial thrust are acquired alongside surveys of the near-field acoustics at eight observer positions above and below the tip path plane of the upper rotor. The variables of interest are the index angle and stacking distance between the upper and lower rotors, as well as rotor speed. Standard metrics like thrust coefficient, sound pressure spectrum level, and the sound pressure level of the various blade harmonics and subharmonics are studied. Human ear effects are also considered using the A-weighting standard, which is shown to have a profound influence on the perceived noise levels. The analysis demonstrates how rotor thrust and sound pressure levels are more dependent on rotor index angle than stacking distance. However, the index angle where the peak thrust coefficient resides is more sensitive to stacking distance at low rotation speeds than it is at high rotation speeds. Various rotor speed and index angle combinations of the stacked rotor are also shown to produce the same rotor thrust, all the while resulting in uniquely different sound levels of the various rotor harmonics.

Entire Flow Field Modeling Including Wake Region through Improved Finite-State Method

In the recent decades, the accuracy and robust of finite state inflow model to provide the velocity on the disk have been greatly improved. It is adequate to compute the thrust, pitch and roll moment real time for normal flight conditions. For lifting rotor flying close to a non-penetrable surface, the wake will interact with the inflow of the lifting rotor. Additionally, for co-axial rotors the wake of the upper rotor influences the lower rotor. Therefore, it is necessary to develop a model to compute the flow field in the wake since the Peters-He model cannot give the induced velocity below the rotor disk. In this paper, the improved finite state model will be introduced to compute the axial induced velocity everywhere in the flow field including wake and downstreamregions. The adjoint theorem will be introduced, and the computational efficiency of the convolution method and numerical algorithm for computing the adjoint varibales with time delay will be compared. With the parameters of a Harrington rotor, the results in the time domain for such a rotor in hover and forward flight conditions will also be illustrated.

Towards silent micro-air vehicles: optimization of a low Reynolds number rotor in hover

The demand in micro-air vehicles is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of micro-air vehicles is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotorcrafts. At micro-air vehicle scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5000 to 100,000 and low Mach number of approximately 0.1. This knowledge, however, is crucial for aeroacoustic optimization and blade noise reduction in drones. This contribution briefly describes a low-cost, numerical methodology to achieve noise reduction by optimization of micro-air vehicle rotor blade geometry. Acoustic power measurements show a reduction of 8 dB(A). The innovative rotor blade geometry allowing this noise reduction is then analysed in detail, both experimentally and numerically with large eddy simulation using lattice Boltzmann method. Turbulence interaction noise is shown to be a major source of noise in this configuration of low Reynolds number rotor in hover, as a result of small scale turbulence and high frequency unsteady aeroadynamics impinging the blades at the leading edge.