Low Rotation Research Articles

Performance Improvement of A Low Aspect Ratio Axial Flow Compressor Rotor by Means of Turbulence Generator

The present paper reports results of an experimental investigation aimed at improving the performance and operating range of an axial flow compressor using a simple passive means. A total of four configurations are tested, namely, (i) basic rotor configuration without any passive means, (ii) rotor with modified partial shroud (MPS), (iii) rotor with MPS and turbulence generator, TG, placed on the casing at the leading edge of the rotor blade and (iv) rotor with MPS and turbulence generator, TG1, placed on the casing at 44 mm upstream of the leading edge of the rotor blade. The turbulence generators are made of velcro tape. The rotor performance is determined by measuring static pressure on the casing, upstream and downstream of the rotor blade. In addition, five hole probe traverses are made at the rotor exit at five flow coefficients (at, above and below design flow coefficients). From the casing static pressure measurements, stall flow coefficient of rotor (with MPS + TG1) is found to be decreased. From the five hole probe measurements, rotor (with MPS + TG1) is found to have improved total and static pressure coefficients compared to rotor (with MPS). Slightly reduced performance of this rotor compared to the basic configuration is attributed to the adverse effects of MPS. The improved performance of the rotor (with MPS + TG1) is attributed to the energization of the casing wall boundary layer at the rotor inlet, due to vortexes generated by TG1.

Low-speed, low induction multi-blade rotor for energy efficient small wind turbines

To promote the deployment of small wind turbines (SWTs), thorough understanding of design parameter implications is essential. In-depth research is required to comprehend the influence of design TSR on off-design wind speed performance of multi-blade SWTs. In-house built blade element momentum algorithm was employed, which considered Reynolds number dependence of aerodynamic coefficients and correlated well with experimental results. Peak power coefficients were produced for 0.9, 1.5, 2 and 3 m diameter S826 rotors with blade numbers from 2 to 12 and design TSR of 2–10. Remarkably, regardless of diameter, greatest CP,max values were achieved around design TSR of 4. For peak efficiency, smaller the diameter, narrower the blade number and design TSR range. High-speed rotors have wider TSR range for high power coefficient. Yet, it was shown that operating TSR of low-speed rotors deviates less from design TSR as wind speed varies. It was revealed that low-speed (with a threshold design TSR of 3), low-induction multi-blade rotors provide high CP,max, better efficiency at off-design, shorter starting time and lower wind speed than three-bladed high-speed rotor. A small boost in operational TSR was found to effectively mitigate loss in off-design performance. These are key features to maximize energy harvesting.

Inter-shaft Bearing Fault Diagnosis Based on Aero-engine System: A Benchmarking Dataset Study

In this paper, the aero-engine test with inter-shaft bearing fault is carried out, and a dataset is proposed for the first time based on the vibration signal of rotors and casings. First, a test rig based on a real aero-engine is established, driven by motors and equipped with a lubricating system. Then, the aero-engine is disassembled and assembled following the specification process, and the inter-shaft bearing with artificial fault is replaced. Next, the aero-engine test is conducted at 28 groups of high and low pressure speeds. Six measuring points are arranged, including two displacement sensors to test the displacement vibration signals of the low pressure rotor and four acceleration sensors to test the acceleration vibration signals of the casing. The test results are integrated into an inter-shaft bearing fault dataset. Finally, based on the dataset in this paper, frequency spectrum, envelope spectrum, CNN, LSTM and TST are used for fault diagnosis, and the results are compared with those of CWRU and XJTU datasets. The results show that the characteristic fault frequency cannot be found directly in the spectrum and envelope spectrum corresponding to this paper's dataset but in CWRU and XJTU datasets. Using CNN, LSTM and TST for fault diagnosis of the dataset in this paper, the accuracy is 83.13%, 85.41% and 71.07%, respectively, much lower than the diagnosis results of CWRU and XJTU datasets. It can be seen that the dataset in this paper is closer to the actual fault diagnosis situation and is a more challenging dataset. This dataset provides a new benchmark for the validation of fault diagnosis methods. Mendeley data: https://github.com/HouLeiHIT/HIT-dataset.

Effects of dry powder mixing on electrochemical performance of lithium-ion battery electrode using solvent-free dry forming process

This research examines how the morphology of the electrode powder mixture affects the structure and battery performance of lithium-ion battery electrodes fabricated using a dry electrode forming process. We demonstrate that the distribution of conductive and binding additives (CBA) on the active materials (AMs) can be controlled by adjusting the rotation speed of a dry mixer. Furthermore, we show that controlling the distribution of CBA can improve the rate performance of the batteries. The tortuosity of the CBA domain in the electrodes is analyzed from SEM images of electrode cross-sections as electronic and ionic conductive pathways. Low rotation speed dry mixing results in poor dispersion of CBA, leading to the formation of long-range conductive pathways with low tortuosity. Consequently, the capacity at high rates decreases due to fewer short-range conductive pathways between the AM particles. In contrast, high rotation speed dry mixing leads to excessive dispersion of CBA, resulting in high tortuosity in both long-range and short-range conductive pathways. As a result, the capacity decreases due to fewer efficient conductive pathways throughout the electrode. We demonstrate that medium rotation speed dry mixing achieves a favorable balance between short-range and long-range conductive pathways, providing high capacity at high rates.

Aeroacoustic radiation of low Reynolds number rotors in interaction with beams.

The radiation characteristics of rotor-beam interaction noise are studied experimentally for low Reynolds number small-scale rotors in interaction with beams of different shapes, sizes, and downstream positions. The number of blades ranges from two to four. For the two-bladed rotor, the presence of the beam has no effect on the mean aerodynamic performance. Moreover, the blade passing frequency (BPF) and the high frequency broadband noise (BBN) appear not to be affected by the presence of the beam. On the contrary, the magnitude of the 2×BPF-25×BPF harmonics increases up to 30 dB compared to the case without beam, with an envelope consisting of two humps: one centered around 5×BPF and another around 20×BPF-25×BPF. For the first hump, a dipole-like pattern with minimal amplitude aligned with the beam can be observed, whereas another dipole-like pattern is observed for the higher frequency hump, but with a minimal amplitude over all the rotor disk plane. Compared to the two-bladed rotor, the presence of the beam has an effect on the mean aerodynamic performance of the three- and four-bladed rotors, increasing both the torque and the thrust at iso-rotational speed. This change leads to a change in the directivity of the BPF tone that decreases at a latitude angle of θ=0° and increases at a latitude angle of θ=40°. Moreover, the same two competing humps are observed on the BPF harmonics envelope. Interestingly, the frequency range over which an amplification of the harmonic magnitude is observed seems not to be influenced by the number of blades. Finally, the magnitude of the low frequency hump increases with the beam diameter, the rotational speed, and the number of blades but decreases with the rotor-beam distance. That of the high frequency hump increases also with the rotational speed and the number of blades, but not anymore with the beam diameter, and reaches a maximum value when the rotor-beam distance is at an intermediate distance of L = 25 mm. This hump is also influenced, to a lesser extent, by the shape of the beam. The two different evolutions permit us to conclude that the noise generation mechanisms leading to the two humps must be different. Scaling laws of the acoustical energy are derived for all those parameters. As already done for previous experiments without beam, all of the results are made available as an open database, at https://dataverse.isae-supaero.fr/.

Sensorless Control With Switching Frequency Square Wave Voltage Injection for SPMSM With Low Rotor Magnetic Anisotropy

High-frequency signal injection sensorless algorithms are widely studied and used for rotor angle estimation in PMSM at low speed or standstill. One of the main drawbacks of such methods is the acoustic noise connected to the voltage injection. In order to minimize this problem, it is advisable to increase the frequency of the injected signal. Thus, many studies focus on square-wave injection at the switching frequency, which is the maximum theoretical frequency. Since these methods exploit the rotor magnetic anisotropy, it is relatively easy to use them in interior PMSMs, where the rotor anisotropy is high. On the contrary, it is hard to exploit them in surface PMSMs, which have an almost symmetric rotor, although a low rotor magnetic anisotropy is still present. In this paper, a sensorless algorithm with switching frequency square-wave injection is developed for surface PMSMs. To increase the signal-to-noise ratio, current oversampling is exploited. The benefits of such a technique are demonstrated with experimental results on a 2 Nm SPMSM.

Research on the wear performance of reactive plasma deposited TiB2-TiC0.3N0.7 matrix coating

Wear performance and behavior of as-sprayed TiB2-TiC0.3N0.7 matrix coating were investigated with different rotation speeds of counterpart and applied load. Analytical balance was used to weigh wear quantity. Scanning electron microscopy was used to observe surface morphology, abrasion scratch, and wear debris was researched through an energy dispersive spectrum. It is found the mass loss of matrix coating increases with the loading load increment at a low rotation speed of the counterpart. And wear loss decreases first and then increases with the increment of loading load at the high rotary speed of the counterpart. The wear mechanism of abrasive wear, adhesion wear, oxidation wear, and fatigue wear appear during the test of composite coatings at different rotation speeds of the counterpart and applied load.

Hubble Space Telescope survey of Magellanic Cloud star clusters: UV-dim stars in young clusters

ABSTRACT Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex colour–magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in star clusters younger than ∼2 Gyr, the clusters younger than ∼800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ∼1.7-Gyr-old cluster NGC 1783 detected a group of eMSTO stars extremely dim in ultraviolet (UV) bands. Here, we use multiband Hubble Space Telescope photometry to investigate five star clusters younger than ∼200 Myr, including NGC 1805, NGC 1818, NGC 1850, and NGC 2164 in the Large Magellanic Cloud, and the Small Magellanic Cloud cluster NGC 330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC 1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a by-product, we show that the star clusters NGC 1850 and BHRT 5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.

Lightweight Human Ear Recognition Based on Attention Mechanism and Feature Fusion

With the development of deep learning technology, more and more researchers are interested in ear recognition. Human ear recognition is a biometric identification technology based on human ear feature information and it is often used for authentication and intelligent monitoring field, etc. In order to make ear recognition better applied to practical application, real time and accuracy have always been very important and challenging topics. Therefore, focusing on the problem that the mAP@0.5 value of the YOLOv5s-MG method is lower than that of the YOLOv5s method on the EarVN1.0 human ear dataset with low resolution, small target, rotation, brightness change, earrings, glasses and other occlusion, a lightweight ear recognition method is proposed based on an attention mechanism and feature fusion. This method mainly includes the following several steps: First, the CBAM attention mechanism is added to the connection between the backbone network and the neck network of the lightweight human ear recognition method YOLOv5s-MG, and the YOLOv5s-MG-CBAM human ear recognition network is constructed, which can improve the accuracy of the method. Second, the SPPF layer and cross-regional feature fusion are added to construct the YOLOv5s-MG-CBAM-F human ear recognition method, which further improves the accuracy. Three distinctive human ear datasets, namely, CCU-DE, USTB and EarVN1.0, are used to evaluate the proposed method. Through the experimental comparison of seven methods including YOLOv5s-MG-CBAM-F, YOLOv5s-MG-SE-F, YOLOv5s-MG-CA-F, YOLOv5s-MG-ECA-F, YOLOv5s, YOLOv7 and YOLOv5s-MG on the EarVN1.0 human ear dataset, it is found that the human ear recognition rate of YOLOv5s-MG-CBAM-F method is the highest. The mAP@0.5 value of the proposed YOLOv5s-MG-CBAM-F method on the EarVN1.0 ear dataset is 91.9%, which is 6.4% higher than that of the YOLOv5s-MG method and 3.7% higher than that of the YOLOv5s method. The params, GFLOPS, model size and the inference time per image of YOLOv5s-MG-CBAM-F method on the EarVN1.0 human ear dataset are 5.2 M, 8.3 G, 10.9 MB and 16.4 ms, respectively, which are higher than the same parameters of the YOLOv5s-MG method, but less than the same parameters of YOLOv5s method. The quantitative results show that the proposed method can improve the ear recognition rate while satisfying the real-time performance and it is especially suitable for applications where high ear recognition rates are required.

Low Barrier Methyl Internal Rotations and 14N Quadrupole Coupling in the Microwave Spectrum of 2,4-Dimethylthiazole.

The microwave spectrum of 2,4-dimethylthiazole was recorded using a pulsed molecular jet Fourier-transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Torsional splittings into quintets were observed for all rotational transitions due to internal rotations of two inequivalent methyl groups. Hyperfine structures arising from the nuclear quadrupole coupling of the 14N nucleus were fully resolved. The microwave spectra were analyzed using the modified version of the XIAM code and the BELGI-Cs-2Tops-hyperfine code. The barriers to methyl internal rotation of the 4- and 2-methyl groups were determined to be 396.707(25) cm-1 and 19.070(58) cm-1, respectively. The very low barrier hindering the 2-methyl torsion was a challenge for the spectral analysis and modeling, and separately fitting the five torsional species together with combination difference loops was the key for a successful assignment. The methyl torsional barriers were compared with those of other thiazole derivatives, showing the influence of the methyl group position on the barrier height. The experimental results were supported by quantum chemical calculations.

Estimation of Stator Vibration in DFIM Under Various Types of Rotor Excitation

Variable speed power generation using Doubly Fed Induction Machine requires an adjustable frequency supply at the rotor terminals which can be achieved using power converters. Ideally, a variable frequency sinusoidal voltage should be applied, but it is not possible without the help of power converters. For a typical 250 MW DFIM with a speed variation of -10.73% to +8.33%, excitation power of 25 MW at 3.3kV and 11600 Amps is required. With this much high rotor current at low fundamental rotor frequency (3.75 Hz) electromagnetic forces in the air gap contain harmonic force components. Also, any time harmonic of radial force density in the rotor will be induced in the stator as a distinct time harmonic, modified by the influence of space harmonics in DFIM with regular three phase integral slot winding on both stator and rotor and vice versa. So, if the frequency and order of electromagnetic force components conflicts with mode shape and natural frequencies of the machine stator strong magnification in vibration will occur or more severely develops mechanical resonance. The stator vibration of electrical machine is generated by the electromagnetic forces acting on the surface of the stator tooths. This paper analyzes stator vibration for 250 MW DFIM in multiphysics simulation with its rotor winding excited through pure sinusoidal excitation at fundamental rotor frequency of 3.75 Hz and with two-Level voltage source converter at 3.75 Hz fundamental rotor frequency. Hardware results with supported multiphysics simulation on a laboratory prototype of 2.2 kW rating DFIM.

Preparation of PP/PC light‐diffusing materials with UV‐shielding property

AbstractIn this study, we presented a one‐step melt‐blending method to fabricate polypropylene (PP)/polycarbonate (PC) light‐diffusing materials. A promising balance between haze and transmittance was obtained by the addition of small amount of PC to the matrix. On the basis of Mie scattering theory and morphology analysis, the correlation between processing conditions and light‐diffusing properties was built. Low rotor speed and high viscosity ratio were beneficial to optical properties of PP/PC composites. To extend the advanced application, two general anti‐ultraviolet additives nanoscale titanium dioxide (Nano‐TiO2) and 2‐(2‐hydroxy‐5‐methylphenyl) benzotriazole (UV‐P) were added to impart UV‐shielding property to the composites. Their specific influences on the morphology and comprehensive properties of PP/PC composites were studied. It was found that the introduction of Rayleigh scattering raised by Nano‐TiO2 could strengthen the scattering effect and cause a significant reduction in light transmittance, while UV‐P had little effect on the optical properties of the light‐diffusing composites. In summary, this study prepared the PP/PC composites with high‐performance light scattering and UV‐shielding properties applying a facile blending method, which showed a potential application in flexible lightings and LCD displays.

Evaluating the wear of cutting tools using a tunnel boring machine laboratory simulator

Purpose. One of the most common problems in mechanized excavation is the cutting tool wear, which has a great impact on the final cost of the project and its duration. Also, one of the most important factors affecting the wear of cutting tools is the operating parameters of the tunnel boring machine (TBM). Within the framework of this research, a tunnel boring machine laboratory simulator was designed and constructed to investigate the tunnel excavation process in the laboratory. Methods. A few of the features of this device are that it operates horizontally, has a low rotation speed, keeps the pins in contact with fresh soil throughout the test, and has the possibility of measuring the torque of the device during the test. A study of the cutting tool wear was conducted using granulation prepared from Tabriz metro line 2, as well as using operating parameters of mechanized excavation machines, such as penetration rate and cutter head rotation speed. Findings. The research results showed that by reducing the rotation speed of the cutter head from 35 to 10 rpm, the average wear of cutting tools is reduced by 63%. Also, by reducing the excavation time from 80 to 10 minutes, the cutting tool wear is reduced by 58%. The wear of cutting tool increases with increasing moisture content from 0 to 10%, and then decreases with increasing moisture content from 10 to 25%. Originality. During this research, a new device was designed and built to simulate tunnel excavation mechanisms. This laboratory simulator measures wear percentage, penetration rate and torque. Practical implications. There has been significant progress in predicting soil abrasion rates, but there are few accepted models for predicting cutting tool wear and soil abrasion rates. During the design and construction of the tunnel boring machine laboratory simulator, the effect of operating parameters on wear of cutting tool was examined.

High-efficiency regenerative shock absorbers considering twin ball screws transmission for increasing the vehicles’ driving range using a hybrid approach

This article proposes a hybrid system for increasing the electric vehicles’ (EVs) driving range considering regenerative shock absorbers. The proposed hybrid approach is a combination of atomic orbital search (AOS) and wild horse optimizer (WHO); later, it was called AOSWHO method. The major objectives of the proposed work are to extend vehicle range, reduce the charging time of the vehicle, reduce the battery size, and increase the efficiency of vehicle. In this proposed work, linear Permanent magnet (PM) generators are used as the sole instrument and on-board source is utilized to absorb vibration energy of the moving vehicle. Here, the proposed method has four key modules: (a) suspension vibration input, (b) transmission mechanism, (c) generator module, and (d) power storage. A suspension vibration is mostly pretentious by road roughness and speed vibration, which is optimally handled by the proposed hybrid technique. The conversion of the alternating linear vibration into unidirectional rotation of generator shaft is processed by proposed hybrid technique. Based on the volume and efficiency of the dampers, a small volume, brushless DC motor with low rotor inertia is selected to produce electricity. By then, the proposed AOSWHO run on MATLAB site and its performance is related to several existing systems. From the simulation result, it is concluded that proposed system delivers an State of Charge (SOC) of 40.33, which is high compared to existing approaches.

Stability characteristics of the spiral Poiseuille flow induced by inner or outer wall rotation

In this paper we reveal, that the stability behavior of the spiral Poiseuille flow in an annular gap with rotating inner or outer cylinder exhibits a striking similarity at low and intermediate rotation rates, while both cases differ significantly at higher rotation rates. Based on the work of Vasanta Ram (2019) we formulate the flow problem by means of the inner swirl parameter (Si), the outer swirl parameter (So) and the curvature parameter (ϵ). Six different values of the curvature parameter {0.005; 0.025; 0.111; 0.25; 0.333; 0.785} and a swirl range of 10−5<Si;So<105 are investigated. We reveal that for the rotating outer cylinder case three characteristic jumps of the critical axial and azimuthal wavenumbers can be identified. Similar features can be identified for the inner rotation case. Based on this, we define three regimes delimited for both the rotating inner and rotating outer cylinder which enables us to perform a systematic comparison. The first regime transition occurs at low values of the swirl parameter (Si; So>10−4) and is accompanied by a rapid decrease of the critical Reynolds number. Our computations reveal that this regime transition does not occur for a small curvature parameter of ϵ= {0.005; 0.025} in the outer rotation case. The second wavenumber jump occurs at higher swirl parameters of around Si>2.5 and So>1.6. While the critical Reynoldsnumber decreases monotonously in the inner cylinder rotating case, we show that in the case of rotating outer cylinder a rapid increase of Rec is observed as So increases further. To gain an insight into the mechanisms involved that induce such wavenumber jumps, a detailed comparative study on the associated Reynolds shear stress distributions is performed. For larger ϵ it is shown that the distributions of the Reynolds shear stresses change significantly over the first jump exhibiting similar features for both flow cases, leading to the conclusion that in both cases the same centrifugal instability is triggered here. Instead, the distributions of the Reynolds shear stresses before and after the second jump are fundamentally different for inner and outer rotation. While the Reynolds shear stresses spread over the whole gap height for the inner rotation case, they are nonzero only close to a small regime near the inner cylinder for the outer rotation case at high rotation rates. Overall, analyzing the shear stresses, we reveal that a similar instability arises in the Spiral Poiseuille flow with inner and outer cylinder rotation at intermediate swirls, but at higher swirl outer rotation seems to give rise to an instability associated with a critical layer that is inherently different to the inner rotation case.

Mathematical modeling of a hybrid magnetic gear for an autonomous low-power wind turbine

In traditional wind turbines, a mechanical gearbox is used to transmit the torque. This device converts the low rotation speed of the wind turbine blades into the high rotation speed of the generator shaft. Mechanical gearboxes are characterized by low reliability. They consist of rotating gears, all the torque between which is transmitted through the contact of the teeth at one point, which is accompanied by friction. Magnetic multipliers are more efficient than gear reducers. They contain no wearing parts and have a relatively high torque density. The modulator transforms the magnetic field between the inner and outer rotor, due to which the speed of rotation changes. This device has a number of advantages over a mechanical gearbox - the interaction between rotating elements (torque transmission) occurs over their entire area, while the gears of mechanical gearboxes perceive all the transmitted force at one point of contact between them. Research of autonomous wind turbines built on the basis of magnetic gearboxes is a relevant direction. This will reduce operating costs, increase the efficiency of converting wind energy into electricity, and increase the reliability of the wind turbine as a whole. The purpose of the work is the development of a two-dimensional field mathematical model of a hybrid magnetic gearbox for the evaluation of its parameters and characteristics and the optimization of its geometric dimensions. A generator with a built-in magnetic multiplier is the object of this study. The magnetic multiplier during operation creates a rotating magnetic field that can be used to induce emf in the generator winding. Such a generator is more compact than a gear drive, so this option was chosen as a prototype in this study. Geometrical models of a hybrid generator with a magnetic gearbox were developed and numerical field mathematical models were developed for the analysis of its parameters and characteristics. An analysis of the electromagnetic field and characteristics of the basic generator was carried out in the COMSOL Multiphysics software complex, on the basis of which optimization of its geometric dimensions was carried out to optimize mass and dimensional indicators

A finger-snapping inspired bistable mechanism for converting low-frequency vibrations to high-speed rotation

Low-frequency vibrations can be exploited to drive a series of rotation-based devices (e.g. miniaturized centrifuges and energy harvesters), but their practical applications are hindered by the low rotation speeds of vibration-to-rotation conversion mechanisms. To address this issue, we report herein a finger-snapping inspired bistable mechanism that can achieve high-speed rotation out of low-frequency vibrations (<5 Hz). The proposed bistable mechanism consists of two sprung-cranks, a proof mass attached with a curved beam, and a pawl, in which the bistability is owed to the coupling of the potential energy of the springs with that of the deformed beam. Both theoretical simulations and experimental tests have been done to show the feasibility of the bistable mechanism. When triggered by vibrations with frequencies varying from 3.2 Hz to 4.5 Hz, the bistable mechanism can drive a rotor to rotate uni-directionally with high speeds ranging from 900 rpm to 1300 rpm. At a low vibration frequency of 3.2 Hz, around 290% increase in the rotation speed can be achieved by the bistable mechanism as compared with the corresponding linear mechanism (rack-and-pinion mechanism). The finger-snapping inspired bistable mechanism is thus a promising candidate in the tapping of ambient low-frequency vibrations as a green energy source for some mechatronic devices.

Small sample measurements at the low energy muon facility of Paul Scherrer Institute

The low energy muon spin rotation spectroscopy (LE-μSR) is primarily used to investigate thin films, surfaces, and interfaces of materials, which has matured at the Paul Scherrer Institute (PSI) and is routinely employed by users of the low energy muon (LEM) facility. However, because of the large beam spot and low implanted muons rate, LE-μSR measurements on small samples are difficult, requiring an optimal sample size of 25×25mm2. Recently, we have boosted our ability to measure small samples, down to 5×5mm2 area, by beam collimation and tuning. This achievement is crucial for the measurements of many magnetic and superconducting materials. Furthermore, we have devised a method that allows us to measure five small area samples mounted together on the same sample plate. We expect this method to further improve the efficient use of beam time at LEM.

Noise reduction on low Reynolds number rotors by boundary layer transition

Following force balance and acoustic measurements in an anechoic chamber, we have shown that promoting earlier boundary layer transition on an idealised low Reynolds number rotor using 3-D cylindrical roughness elements could lead to an interesting decrease in the noise emitted by the rotor. For most tripped cases, the aerodynamic performance of the rotor was impaired by tripping, but a reduction of the broadband noise and the high amplitude blade passing frequency harmonics were observed. On the contrary, the blade passing frequency and its first harmonics did not seem to be significantly affected by the presence of the tripping. The tripping was more effective at low rotational speeds, where the broadband noise contribution dominates the overall sound pressure level (OASPL), whereas the effect on OASPL is almost negligible at high rotational speeds, where the blade passing frequency noise becomes dominant. Careful selection of the roughness height with respect to the boundary thickness and inter-roughness spacing showed that aerodynamic performance loss can be minimised while preserving the gain in noise reduction.

Suspension of large inertial particles in a turbulent swirling flow

We present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles are heavier than the working fluid so that their dynamics results from a competition between gravitation and turbulent agitation. Two suspension regimes are observed. At low rotation rate particles are confined near the bottom for any size or density. At high rotation rate, particles are loosely confined: small particles are nearly homogeneously distributed and large ones found near the top as if gravity was reversed. We discuss these observations using a minimal random walk model accounting for particle inertia.