Single Rotor Blade Research Articles

Preliminary Design, Modeling, Production, and First Evaluation Tests of a Ti–Al Gas Turbine Blade

The aim of this work is to design a lightweight, creep-resistant blade for an axial single-stage micro-gas turbine. The selected process was casting of an intermetallic titanium/aluminum alloy. All the project phases are described, from the preliminary thermodynamic and geometric stage design, to its three-dimensional (3D) modeling and the subsequent finite element method–computational fluid dynamics (FEM-CFD) analysis, to the manufacturing process of the single rotor blade. The blade making (height 7 cm and chord 5 cm, approximately) consisted in a prototyping phase in which a fully 3D version was realized by means of fused deposition modeling and then in the actual production of a full-scale set of blades by investment casting in an induction furnace. The produced items showed acceptable characteristics in terms of shape and soundness. Metallographic investigations and preliminary mechanical tests were performed on the blade specimens. The geometry was then refined by a CFD study, and a slightly modified shape was obtained whose final testing under operative conditions is though left for a later study. This paper describes the spec-to-final product procedure and discusses some critical aspects of this manufacturing process, such as the considerable reactivity between the molten metal and the mold material, the resistance of the ceramic shell to the molten metal impact at high temperatures, and the maximal acceptable mold porosity for the specified surface finish. The CFD results that led to the modification of the original commercial shape are also discussed, and a preliminary performance assessment of the turbine stage is presented and discussed.

Investigation on Unsteady Flow Characteristics of a SCO2 Centrifugal Compressor

Supercritical carbon dioxide (SCO2) is a vital working fluid in the application of power units and its high density helps to achieve a compact mechanical structure. Centrifugal compressors are of vital use in various kinds of equipment. In this paper, a SCO2 centrifugal compressor of large input power and mass flow rate is designed and numerically investigated. A thorough numerical analysis of the unsteady flow field in the centrifugal compressor is performed in ANSYS-CFX. The computation adopts hexahedral mesh, finite volume method, and the RNG k-ε two-equation turbulence model. Streamlines, temperature, pressure, and Mach number distributions at different time steps in one revolution period are covered to present the unsteady effect of turbomachinery. Meanwhile, the force on a single rotor blade is monitored to investigate the frequency components of exciting force, thus providing the foundation for vibration analysis. Moreover, the torque, output power, pressure ratio, and isentropic efficiency in the steady and the unsteady time-averaged condition are calculated and compared with the design condition to measure the validity of the design. In summary, the unsteady computation result reveals that the unsteady flow characteristics are prominent in the designed compressor and the design of impeller and diffuser meet the requirement.

Long-wave instability of a helical vortex

We investigate the instability of a single helical vortex filament of small pitch with respect to displacement perturbations whose wavelength is large compared to the vortex core size. We first revisit previous theoretical analyses concerning infinite Rankine vortices, and consider in addition the more realistic case of vortices with Gausssian vorticity distributions and axial core flow. We show that the various instability modes are related to the local pairing of successive helix turns through mutual induction, and that the growth rate curve can be qualitatively and quantitatively predicted from the classical pairing of an array of point vortices. We then present results from an experimental study of a helical vortex filament generated in a water channel by a single-bladed rotor under carefully controlled conditions. Various modes of displacement perturbations could be triggered by suitable modulation of the blade rotation. Dye visualisations and particle image velocimetry allowed a detailed characterisation of the vortex geometry and the determination of the growth rate of the long-wave instability modes, showing good agreement with theoretical predictions for the experimental base flow. The long-term (downstream) development of the pairing instability leads to a grouping and swapping of helix loops. Despite the resulting complicated three-dimensional structure, the vortex filaments surprisingly remain mostly intact in our observation interval. The characteristic distance of evolution of the helical wake behind the rotor decreases with increasing initial amplitude of the perturbations; this can be predicted from the linear stability theory.

Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry

Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.

A study of the turbulence within a spiralling vortex filament using proper orthogonal decomposition

The stability and turbulence characteristics of a vortex filament emanating from a single-bladed rotor in hover are investigated using proper orthogonal decomposition (POD). The rotor is operated at a tip chord Reynolds number and tip Mach number of 218 000 and 0.23, respectively, and with a blade loading of $C_{T}/{\it\sigma}=0.066$. In-plane components of the velocity field (normal to the axis of the vortex filament) are captured by way of two-dimensional particle image velocimetry with corrections for vortex wander being performed using the ${\it\Gamma}_{1}$ method. The first POD mode alone is found to encompass nearly 75 % of the energy for all vortex ages studied and is determined using a grid of sufficient resolution to avoid numerical integration errors in the decomposition. The findings reveal an equal balance between the axisymmetric and helical modes during vortex roll-up, which immediately transitions to helical mode dominance at all other vortex ages. This helical mode is one of the modes of the elliptic instability. The spatial eigenfunctions of the first few Fourier-azimuthal modes associated with the most energetic POD mode is shown to be sensitive to the choice of the wander correction technique used. Higher Fourier-azimuthal modes are observed in the outer portions of the vortex and appeared not to be affected by the choice of the wander correction technique used.

무인비행체 블레이드 형상 변화에 따른 단일로터의 제자리 비행 추력성능 분석

무인비행체 블레이드 형상 변화에 따른 단일로터의 제자리 비행 추력성능 분석

Self-similar behaviour of a rotor wake vortex core

AbstractWe report a self-similar behaviour of solutions (obtained numerically) of the Navier–Stokes equations behind a single-blade rotor. That is, the helical vortex core in the wake of a rotating blade is self-similar as a function of its age. Profiles of vorticity and azimuthal velocity in the vortex core are characterized, their similarity variables are identified and scaling laws of these variables are given. Solutions of incompressible three-dimensional Navier–Stokes equations for Reynolds numbers up to $Re= 2000$ are considered.

An Experimental and Theoretical Study of Blade–Vortex Interaction Noise

An experimental method to study helicopter blade–vortex interaction (BVI) noise has been developed. Called the blade-controlled disturbance interaction, the method nearly replicates a rotor BVI by having a single-bladed rotor pass through a stationary gust field specially designed to simulate the highly impulsive induced velocity field of a vortex. This approach can be used to evaluate the effectiveness of blade design changes to noise radiation during a BVI event, as well as to study the effect of different interaction geometries on the resulting noise. The first set of experiments in this facility shows that the directionality of BVI noise radiation is very sensitive to the interaction angle. Oblique interactions spread the acoustics energy over wider azimuth angles compared to parallel interactions, as well as move the peak noise location closer to the rotor plane. Linear two-dimensional unsteady aerodynamic theory predicts the overall directionality trends reasonably well. However, the actual pulse shapes do not match and noise levels are underpredicted, particularly for oblique interactions.

Wind tunnel model tests of Magnus type wind rotors with a horizontal rotation axis

The paper presents results of wind tunnel tests of horizontal axis wind rotors of Magnus type. Firstly, measurements of aerodynamic side (lift) force and aerodynamic drag on rotating cylinder attached to horizontal aerodynamic balance were performed. Secondly, the model of single-blade rotor with counterbalance was tested.

Comparison of Experimental results with CFD for NREL Phase VI Rotor with Tip Plate

This article presents the results of 3D CFD rotor computations of a NREL Phase VI rotor with tip plate, a stall-regulated turbine with full-span pitch control, has a power rating of 20 kW. In this paper, A full three dimensional CFD–RANS approach by modeling the whole rotor of a wind turbine by means of periodicity was used. All the simulations were performed using the commercial multi-purpose CFD solver ANSYS CFX 12.1 with the transition model of Langtry and Menter is applied to a rotor at stationary wind conditions without wind shear. The comparisons were done for the blade with 0° yaw angle and 3° tip pitch angle on a single rotor blade without tower and nacelle. Reasonably good agreement is obtained when comparing modeled mechanical effects like power and thrust with findings from measurements. Similarly the spanwise force distributions are compared with experimental results for two wind speeds along with pressure distributions at five different spanwise locations. It is shown capability of 3D CFD computations that can be used to extract information about three-dimensional aerodynamic effects.

Research on Similarity Scaling of Structure for Compressor Disc

To study the similarity scaling of structure for compressor disc, based on the basic rules of similarity scaling, physical speed of five linear scale factors (LSF) is determined, and the ratio of between centrifugal force and LSF is established, then the centrifugal force of single rotor blade and single rim flange with different LSF is obtained. Using the method of finite element numerical simulation, strength of four assessment projects including circumferential stress of disc heart, circumferential stress of cylindrical surface, radial stress of cylindrical surface and circumferential stress of meridian plane is analyzed. The results show that the maximum relative error among four assessment projects is only 2.60%, and stress level of disc corresponding to the different LSF is very close to each other. Then a conclusion can be got that the stress level of disc is little difference with itself geometry size, and the structure of disc can be designed pro rata scaling.

Experimental hover performance evaluation on a small-scale rotor using a rotor test stand

This paper presents the work being carried out in order to deduce hover performance of a small-scale single rotor blade as a preliminary study of a small coaxial rotor helicopter development. As an initial research, a test stand capable of measuring thrust and torque of a small-scale rotor blade in hover state was constructed and fabricated. The test stand consists of three parts; a rotating device, a load measuring sensor and a data acquisition system. Thrust and torque were measured with varying collective pitch angle at fixed RPM. Through this research, hover performance tests were conducted for a small-scale single rotor blade operating in low Reynolds numbers (Re ≈ 3 × 105). The rotor blades investigated in this paper have maximum FM values varying from 0.59 to 0.65, which are low relative to modern full-scale helicopters. From these differences in FM between a small and a full-scale helicopter, the induced power factor is determined as varying from 1.35 to 1.42. Through this study, tests of hover performance were conducted for a single small-scale rotor blade, as well as verifying the test stand itself for the acquisition of hover performance.

3D Numerical Investigation of Tandem Airfoils for a Core Compressor Rotor

The tandem airfoil has potential to do more work as a compressor blade than a single airfoil without incurring higher losses. The goal of this work is to evaluate the fluid mechanics of a tandem rotor in the rear stages of a core compressor. As such, the results are constrained to shock-free fully turbulent flow with thick endwall boundary layers at the inlet. A high hub-to-tip ratio 3D blade geometry was developed based on the best-case tandem airfoil configuration from a previous 2D study. The 3D tandem rotor was simulated in isolation, in order to scrutinize the fluid mechanisms of the rotor, which had not been previously well documented. A geometrically similar single blade rotor was also simulated under the same conditions for a baseline comparison. The tandem rotor was found to outperform its single blade counterpart by attaining a higher work coefficient, polytropic efficiency, and numerical stall margin. An examination of the tandem rotor fluid mechanics revealed that the forward blade acts in a similar manner to a conventional rotor. The aft blade is strongly dependent on the flow it receives from the forward blade, and tends to be more three-dimensional and nonuniform than the forward blade.

Eulerian laser Doppler vibrometry: Online blade damage identification on a multi-blade test rotor

Laser Doppler vibrometry enables the telemetry-free measurement of online turbomachinery blade vibration. Specifically, the Eulerian or fixed reference frame implementation of laser vibrometry provides a practical solution to the condition monitoring of rotating blades. The short data samples that are characteristic of this measurement approach do however negate the use of traditional frequency domain signal processing techniques. It is therefore necessary to employ techniques such as time domain analysis and non-harmonic Fourier analysis to obtain useful information from the blade vibration signatures. The latter analysis technique allows the calculation of phase angle trends which can be used as indicators of blade health deterioration, as has been shown in previous work for a single-blade rotor. This article presents the results from tests conducted on a five-blade axial-flow test rotor at different rotor speeds and measurement positions. With the aid of artificial neural networks, it is demonstrated that the parameters obtained from non-harmonic Fourier analysis and time domain signal processing on Eulerian laser Doppler vibrometry signals can successfully be used to identify and quantify blade damage from among healthy blades. It is also shown that the natural frequencies of individual blades can be approximated from the Eulerian signatures recorded during rotor run-up and run-down.

소형 로터 블레이드의 제자리 비행 성능 시험장치 개발 및 검증

본 논문은 동축반전 로터 블레이드의 성능 특성 파악을 위한 선행연구로서, 단일 로터 블레이드에 대한 공력 데이터 획득 및 성능 특성을 확인하기 위한 연구에 중점을 두었다. 이를 위해 제자리 비행 상태에서 추력과 토크의 측정이 가능한 소형 로터 블레이드용 회전 시험장치를 구성하고 제작하였다. 로터 회전 시험장치는 회전장치부, 센서부, 그리고 데이터 획득 시스템으로 구성되었으며, 고정된 회전속도에서 콜렉티브 피치각을 변화시키면서 추력과 토크를 측정할 수 있도록 하였다. 이를 통하여 저 레이놀즈 수 (<TEX>$Re{\approx}3{\times}10^5$</TEX>) 영역에서 운용되는 단일 로터의 제자리 비행 성능 시험을 수행하였고 소형 로터 블레이드의 제자리 비행 성능을 획득하여 로터 회전 시험장치의 성능을 검증하였다. This paper presents the work being carried out in order to deduce hover performance of a small-scale single rotor blade as a preliminary study of a small coaxial rotor helicopter development. As an initial research, a test stand capable of measuring thrust and torque of a small-scale rotor blade in hover state was constructed and fabricated. The test stand consists of three parts; a rotating device, a load measuring sensor and a data acquisition system. Thrust and torque were measured with varying collective pitch angle at fixed RPM. Through this research, hover performance tests were conducted for a small-scale single rotor blade operating in low Reynolds number (<TEX>$Re\;{\approx}3{\times}10^5$</TEX>), as well as for verifying the test stand itself for acquiring hover performance.

High Resolution Wake Capturing Methodology for Hovering Rotors

A high-resolution Reynolds-averaged Navier‐Stokes (RANS) solver is applied to study the evolution of tip vortices from rotary blades. The numerical error is reduced by using high-order accurate schemes on appropriately refined meshes. To better resolve the vortex evolution, the equations were solved on multiple overset grids that ensured adequate resolution in an efficient manner. For the RANS closure, a one equation wall-based turbulence model was used with a correction to the production term to account for the stabilizing effects of rotation in the core of the tip vortex. While experimental comparison of the computed vortex structure beyond a few chord lengths downstream of the trailing edge is lacking in the literature, reasonable validation of the vortex velocity profiles is demonstrated up to a distance of 50 chord lengths of evolution for a single-bladed rotor. For the two-bladed rotor case, the tip vortex could be tracked up to two rotor revolutions with minimal diffusion. The accuracy of the computed blade pressures and vortex trajectories confirm that the inflow distribution and blade-vortex interaction are represented correctly. The accuracy achieved in the validation studies establishes the viability of the methodology as a reliable tool that can be used to predict vortex evolution and the aerodynamic performance of hovering rotors.

A wind-tunnel based study of helicopter tail rotor blade vortex interaction

AbstractThe interaction of a helicopter tail rotor blade with the tip vortex system from the main rotor is a significant source of noise and, in some flight states, can produce marked reductions in control effectiveness. This paper describes a series of wind-tunnel tests to simulate tail rotor blade vortex interaction with a view to providing data for the development and validation of numerical simulations of the phenomenon. In the experiments, which were carried out in the Argyll wind-tunnel of Glasgow University, a single-bladed rotor located in the tunnel’s contraction was used to generate the tip vortex which travelled downstream into the working section where it interacted with a model tail rotor. The tail rotor was instrumented with miniature pressure transducers that measured the aerodynamic response during the interaction. The results suggest that the rotor blade vortex interaction is similar in form to that measured at much higher spatial resolution on a fixed, non-rotating blade. The combination of the two datasets, therefore, provides a valuable resource for the development and validation of predictive schemes.

A simulation model for wind turbine blade fatigue loads

The paper describes a horizontal axis wind turbine time domain simulation and fatigue estimation program written using the Delphi TM language. The program models the flapwise motion of a single rotor blade to determine the blade-root fatigue damage of a medium size wind turbine. The effects of turbulence intensity, mean wind speed, wind shear, vertical wind component, dynamic stall, stall hysteresis, and blade stiffness were examined. When all these effects were simulated it is found that a reduction in life of about 2 occurs between a low wind speed low turbulence intensity site, compared to a high wind speed high turbulence intensity site.

Vorticity Concentration at the Edge of the Inboard Vortex Sheet

Flow visualization and velocity measurements are used to show that the outer edge of the trailing vortex sheet from an untwisted rotor blade rolls up into a discrete vortical structure which is opposite in sense and comparable in strength to the tip vortex. This occurs both in hover and forward flight, and at both upstream and downstream edges of the wake in forward flight. The roll-up process and the trajectories of the tip vortex and the counterrotating vortex are examined using digitized laser sheet video images. Azimuth-resolved velocity data, obtained under an isolated, two-bladed teetering rotor in a wind tunnel at low advance ratio, are used to quantify vortex strengths. INTRODUCTION Figure 1 shows the classical model of the rotor wake in hover, developed from the early experimental work of Gray1 and Landgrebe2. The wake is dominated by the tip vortices, which concentrate vorticity in small core regions, and carry a large amount of kinetic energy with them. Early attempts to model the behavior of the rotor wake used simple models of the velocity field induced by these vortices. To get better accuracy in modeling the wake and the loads on the rotor blades, it was found necessary to include the vorticity trailed 1: Graduate Research Assistant. Member, AHS 2: Associate Professor. Member, AHS 3: Post-Doctoral Fellow. member, AHS behind the blade in a thin shear layer, which can be modeled as an vortex There is also some rollup of vorticity at the hub into a root vortex; however, this contains relatively little kinetic energy. Examination of Figure 1(a) shows that the outer edge of the vortex sheet moves down faster than the corresponding tip vortex. This can be seen by considering the signs of vorticity of the tip vortex and the vortex sheet, as shown in Figure 1(b). Unlike the circulation distribution of a fixed wing, the bound circulation of the rotating blade generally peaks well inboard of the tip, so that the sign of DΓ/Dr changes. Thus, the tip vortex leaves the blade with a sense of rotation opposite to that of the inboard vortex sheet. Mutual induction between the tip vortex and the vortex sheet must thus inhibit the downward motion of the tip vortex, and accelerate that of the edge of the sheet. Figure 1 was developed by detailed observation of patterns formed by clouds of microscopic particles, illuminated by strobed lights or continuous lights. The patterns were captured using cameras with short shutter exposure times. The figure leaves unspecified the continuity between the tip vortex and the vortex sheet. This aspect of the rotor wake is the subject of this paper. Early experimental investigations of the rotor wake were motivated by pursuit of precise rotor performance prediction. Gray1 modeled the rotor wake by a tip vortex filament and an inboard vortex sheet composed of several vortex filaments based on the visualization of a single-bladed rotor in hover. In his experiments, the visible portions of the inboard vortex sheet and the tip vortex were not close enough to permit observation of the phenomena at the junction. Distortion of the inboard vortex sheet was not observed in the visualization. Landgrebe et al.2 extended such models to multi-bladed rotors. The interaction between the vortex sheet and the tip vortex was neglected and consequently the inboard vortex sheet was prescribed as a rigid sheet of distributed vorticity fixed to the spatial locations of blade passage. Later vortex modeling of prescribed wake analysis for the rotor wake in hover3 or in forward flight4 also treated the inboard vortex sheet as an undistorted sheet of vortex filaments. These models contributed to a very large