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A.I.VE.LA. – the Italian Association of Laser Velocimetry and non-invasive diagnostics is a non-profit cultural association whose objective is to promote and support research in the field of non-contact or minimally invasive measurement techniques, particularly of electromagnetic-based and optical techniques.

Through its Annual Meeting, AIVELA aims at creating an active and stimulating forum where current research results and technical advances can be exchanged and the development of new systems for laboratory use, field testing and industrial application can be promoted. The techniques addressed include Laser Doppler Anemometry – LDA, Phase Doppler Anemometry – PDA, Image Velocimetry – PIV, Flow visualization techniques, Spectroscopic measurement techniques (LIF, Raman, etc.), Laser Doppler Vibrometry – LDV, Speckle Pattern Interferometry – ESPI, Holographic techniques, Shearography, Digital Image Correlation – DIC, Moiré techniques, Structured light techniques, Infrared imaging, Photoelasticity, Image based measurement techniques, Ultrasonic sensing, Acoustic and Aeroacoustic measurements, etc.

The first Annual Meeting was held back in October 1992 and, since then, the event has met large consensus among the research and scientific communities worldwide for the high scientific interest of the papers presented.

The XXVII AIVELA Annual National Meeting was organised in collaboration with Istituto Motori – CNR and was held at the premises of Istituto Motori – CNR, in Naples, on 30-31 October 2019.

This volume contains a selection of the papers presented at the event.

The full Programme of the Meeting can be found at: http://www.aivela.org/XXVII_Meeting/final-programme.html.

Trusting our Association and its initiatives will meet your interest, I wish to thank you in advance for your kind attention and hope to meet you soon at one of our events.

EDITOR

Enrico Primo Tomasini

President of A.I.VE.LA.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The aim of the article is to propose a robust and reliable engineering method for identifying and characterizing vortical structures within a flow field measured with a classic two-component PIV measurement system. Some of the most popular vortex-detection criteria are briefly presented for comparison purposes. Many of these fail if spurious vectors are present within the flow field due to poor PIV image quality. The proposed method was tested both on synthetic images of ideal vortices, having different spatial resolutions and different noise levels in order to perform a parametric assessment, and on real PIV images of a four-bladed rotor wake.

The paper deals with the introduction of an alternative method for the evaluation of the fluid flow rate through a square pipe, by directly measuring the flow mean velocity at a fixed location in the section area. The authors of this paper have previously introduced a way for the evaluation of the fluid flow rate, by measuring a single point flow velocity in square pipes. In this study, the authors perform a series of considerations aimed at the validation of the proposed methodology, by comparing it with prescribed procedures described by ISO 3966:2008 standard. In this standard a method for the evaluation of fluid flow rate (under specific fluid dynamic conditions) is stated from velocity measurements performed in a-priori fixed points (in a given test section and to be defined according to the cross-section shape), using a static Pitot-tube (or equivalent equipment). In this work, the authors validate the proposed simplified method against the Log-Thebycheff procedure (according to the same standard). A Laser Doppler Anemometer (LDA) has been used.

Contra-rotating propellers represent a non-conventional approach for marine propulsion whose main advantage lies in the increase of propulsive efficiency. This is achieved by recovering part of the energy loss due to the rotational flow generated by a single propeller by means of a contra-rotating downstream propeller. The hydrodynamics of such configuration is quite complex due to the interaction between upstream and downstream propellers and a deep understanding of their features is critical to driving the design phase. In this work a methodology based on planar (2D-2C) Particle Velocimetry is presented to investigate on the flow in the wake of two contra-rotating propellers. An ad-hoc mixed hardware and software phase-locking technique is developed in order to analyze the contribution of each propeller to the overall hydrodynamics of the system.

Measurement of velocity fields is a fundamental topic in fluid dynamics. Image-based analysis methods such as Particle Image Velocimetry or Laser Doppler Velocimetry are usually used. However, these techniques need complex instrumentation and particular test conditions. In this work, a computer vision-based approach is developed in order to obtain vapour velocity field map in effective, robust and economic way. Moreover, iterative filtering algorithm is applied to improve the results. The implemented method is tested on a suction system for domestic use, and the obtained velocity maps are validated by hot-wire anemometry, leading to totally comparable results, both in terms of profile and mean velocity. Uncertainty analysis shows acceptable results, considering the random nature of the phenomenon.

This paper is aimed to the study of spontaneous radiant energy emitted by trapped vortex flame, focusing on the intermittent occurrence of flame blowout during thermoacoustic oscillation. We tackle this issue by a wavelet–based auto–conditioning technique, able to detect intermittent structures in the radiant energy signal, which we identify as a trace of extinction–reignition events. Intermittent events are found to be in phase with the tonal component of the radiant energy signal, although they occur randomly in time. Despite the wavelet auto–conditioning technique is a well established method in fluid dynamics and aeroacoustics, our work represents the first application in combustion field.

Additional information about blow–out are inferred by analysing the average signature computed from the radiant power time series: re–ignition velocity appears higher than extinction velocity. This result confirms the effectiveness of the radiant power for combustion diagnostics.

A vertical plunging jet has been investigated experimentally by means of an innovative volumetric shadowgraphy technique. A space carving algorithm has been used for the measurement of the size and concentration of the air bubbles, to follow the air bubbles path inside the investigated volume, providing both spatial and temporal evolution of the same. Furthermore, the air bubble tracking has been performed by means of an algorithm based on the Lucas-Kanade optical flow algorithm. Results highlighted a distribution of the air bubbles that follows the free jet spread inside the investigated volume with a dependence of speed and size from the action exerted by the vertical jet. The volumetric shadowgraph technique has proven effective in characterizing air bubbles, also in presence of relevant void fraction.

Drones are a growing business in Europe, delivering services in all environments, including urban areas: delivery of goods and e–commerce, precision agriculture, mapping, infrastructure inspections. In order to allow the integration of this platform in urban areas and to obtain public acceptance, it is necessary to significantly reduce the noise produced by the propellers. Currently, the level of noise generated by propellers is too relevant in terms of annoying effects induced on living beings. In particular, low altitude flights have been found to disturb citizens and frighten animals. With the aim to tackle such an issue, this paper describes an experimental parametric analysis of a passive noise control technique to reduce the signature generated by small–scale propeller employed for mini unmanned aerial propulsion.

The noise control strategy here proposed is based on a serration pattern realized at the trailing edge (TE) of the blade. Despite the same approach has already been successfully employed to mitigate noise generated by wind turbines and fixed wing aircraft, few studies have been devoted to investigate the effects of a serration pattern on small propellers. A large number of different serration were realized by varying height, width and teeth distribution. The idea is to find the optimal serration geometry in terms of acoustic requirements and to highlight the role of teeth number.

Results shows a sensible reduction in the noise propeller signature. Serration effects seems to depend strongly from the polar angle identifying different region of operation. Spectral Analysis points out that the serration affect the low frequency region and even the tonal component of aerodynamic noise. Statistical analysis shows a departure from a gaussian distribution and an effect of damping on distribution tails.

End of Line tests have a crucial role in industrial processes to validate the products' quality. In automotive industry, engines at the end of assembling process are fully tested within hot test cells at different working conditions by mean specific test cycles. Vibration monitoring is widely used in order to identify potential faults in the assembly process. This contact technique leads to several disadvantages, mainly due to the accessibility and the characteristics of surfaces for accelerometers mounting (e.g. rotating parts, geometries complexes, high temperatures) and to the time-consuming setup procedures. These limitations can be overcome by using microphone sensors. This paper describes the development phases of an acoustic based fault diagnosis system, its integration on hot test cells and the results obtained over large number of engines tested. By mean some specific technical solutions the acoustics approach in the EOL fault diagnosis of combustion engines has showed itself to be compatible with the characteristics of the production industry environment, providing reliable and repeatable results. The latter has been demonstrated also by comparison with the traditional accelerometer technique.

Acoustic simulations provide today a valid tool to simulate complex environments and complex interaction between acoustic and structure. Multiple methods are nowadays available with different degrees of accuracy and different applications. Simulation methods cover a wide frequency range with FE methods dominating the low frequency range. SEA mostly covers high frequency range with BEM covering an intermediate frequency range. Ray- tracing can work on the entire frequency range and is used when a large domain must be simulated. These methods require acoustic properties of materials to be implemented such as acoustic impedance or absorption and STL. The aim of this paper is to show different methods to provide these properties and discuss about the equivalence/difference of the numerical and experimental approaches under specific assumptions.

The gear wheels are the most common mechanism for the transmission of the torque between two organs. The tribological conditions can compromise both their performance and lifetime. Vibration analysis is a valid tool for the assessment of the health of a mechanical system. The following work illustrates a non-linear methodology for evaluating the vibrational dynamics of a single pair of gears, under different tribological conditions. The proposed methodology highlights the presence of damage on the surface of the teeth of the gear pair and evaluates effect on micro-vibratory dynamics due to different lubrication and rotation regimes. Once the vibratory dynamics features of the structure are known, a statistical model has been developed capable of classifying the different tribological regimes.

In this work two calibration methodologies, able to characterize the digital sensitivity of MEMS accelerometers, are presented and compared, to identify the contributions for the evaluation of the reproducibility in the low frequency range. The methodologies are different from the point of view of test bench, test procedure and data processing method. In particular, different vibration actuators are used, a linear slide and an electro-dynamic shaker, different sensors as a reference for the calibration, piezoelectric accelerometers and a Laser Doppler Vibrometer (LDV). A group of 5 accelerometers is tested for the purpose of developing the calibration techniques and evaluate a first reproducibility estimate. The experimental results provided by the two calibration procedures show significant differences. Some elements that could explain these differences have been identified, and will be further investigated in future work.

In recent years aeronautical systems are becoming increasingly complex, as they are often required to perform various functions. New intelligent systems are required capable of self-monitoring their operation parameters, able to estimate their health status, and possibly perform diagnostic or prognostic functions. For these purposes, these systems frequently need to acquire several different signal types; although it is sometimes possible to implement virtual sensor techniques, it is usually necessary to implement dedicated sensing hardware. On the other hand, the installation of the required sensors can, however, significantly increase the complexity, the weight, the costs and the failure rate of the entire system. To overcome these drawbacks, new types of optical sensors, minimally invasive for measuring the system parameters and having a high spatial resolution and a minimum added complexity are now available. Fiber Bragg Gratings (FBGs) sensors are suitable for measuring various technical parameters in static and dynamic mode and meet all these requirements. In aerospace, they can replace several traditional sensors, both in structural monitoring and in other system applications, including mechatronic systems diagnostics and prognostics. This work reports the results of our experimental research aimed at evaluating and validating different FBG installation solutions such as deformation, bending, vibration, and temperature sensors. These were compared with numerical simulations results and measurements performed with traditional strain gauges and accelerometers.

Several prototypes of mechanical fingers composing anthropomorphic prosthetic hands have been developed using underactuated mechanisms. These mechanisms require specific experimental procedure to measure their properties. Due to their low weight, the addition of further sensors for measurement purposes could modify the trajectory of the single parts and the overall prosthetic device behaviour. Vision system methods represent a route to overcome these drawbacks. In this study, an experimental methodology to acquire the dynamics of an underactuated prosthetic finger is presented due to the employment of a vision-based technique, the Digital Image Correlation. The experimental characterization aids the development of a dynamical model of the finger behaviour.

The paper deals with discussion of research activities within ITEMB (InTEgrated Full Composite Main landing gear Bay Concept) framework, an EU Clean Sky 2 program coordinated by Airbus. The driving motivation for the investigation on such a technology was found in the opportunity to design a main landing gear bay in a full composite configuration: rational approaches have been implemented in an efficient testing stage providing the necessary database for the static qualification of the conceived design. Advanced and innovative solutions for a "more integrated" system were duly analysed and experimentally validated thus proving the overall device compliance with industrial standards and applicable airworthiness requirements.

The procedure of deriving laminar burning parameters from high-speed, high-resolution shadowgraph recordings of spherical expanding flames allows to obtain experimental data in lifelike conditions of high pressure and temperature. Straightforward in principle, relatively simple to implement, the approach has been applied in several labs worldwide, to the extent of achieving a "common practice" status, sometimes resembling a codified protocol. Simple as it may look on paper, yet the technique hides a number of pitfalls, which can impair the end result. After the introduction of solid-state high-speed cameras (CCD, then CMOS), gathering high-speed videos of single combustion events became a breeze: this, along with the possibility of easy stockpiling of data, lend to underrate data analysis: sort of trading the accuracy of measurement for the repeatability. Another critical phase is dealing with the stretch, which affects any real flame: measured data must be processed to obtain the unstretched flame speed and, ultimately, the laminar burning velocity. The relationship between flame speed and stretch will be discussed, being a key factor for the deconvolution of experimental, stretch-affected data. In the present work a critical discussion is proposed, from experimental data acquisition and processing to stretch analysis: the underlying hypotheses of each step will be used as the guidance to a "good" rather than "common" practice. Reference will be made to a specific test case: the combustion of CH₄ in air at P₀ = 6 bar, spanning the whole flammability range.

A rising global attention to the environmental impact of anthropogenic activities pushes towards a continuous reduction of such impact and a control, through monitoring techniques, of the main sources of pollution. The transport sector is involved, as all other field of anthropogenic activities in the efforts towards a drastic abatement of emissions. Shipping activities generate greenhouse effect gases (GHGs), affecting the environment on a global scale, and other pollutants harmful for human health and the ecosystem on a local scale. These latter aspects are particularly relevant in ports where berths are close to densely inhabited areas. Many efforts have been and will be spent to predict and quantify these emissions with the aim of controlling them but direct measurements aimed at the identification and quantification of particularly polluting substances are to be considered a key point to achieve an effective control of emissions. In the context of ports, as in any transportation infrastructure, an identification of polluting vehicles and a quantification in an objective way of their emissions is crucial to implement any control activity of the polluting emissions. The development of optical remote sensing techniques appears to be particularly promising. This work provides a state of the art of the main techniques based on optical sensors (LIDAR, LIDAR DIAL, DOAS, camera UV) possibly applied in the maritime sector, highlighting advantages and potentialities. Results of a preliminary measurement campaign are reported to show the first encouraging feedback for the feasibility of the application.

In this paper, a methodology is described aiming at assessing the variability causes, affecting the measurement instruments typically used in the automotive sector for the analysis of dimensional defects of coupled components, to check the conformity of the width of the gap between two adjacent parts and the profile, i.e. the alignment of the two surfaces. The inspection may be performed by means of different gap/flush tools, such as feeler gages, calipers and Laser scanner devices; depending on the choice, a different measurand has to be considered. The obtained measurements need to be consistent, because these devices are used indistinctly, depending on the severity of the control and on the typology of the control itself. The paper proposes a methodology to verify the metrological conformity of measurements provided by both traditional and laser-based instruments, so that it is possible to interpret them in an interchanging manner. A 3D geometric model is realized, used as a basis for the analysis of the most influencing uncertainty causes and measurement biases. The methodology will demonstrate to be able to identify and evaluate in a parametric way, the typical variability causes of the instruments considered. It will also highlight possible improvements in the integration of the obtained results.

As the technical level of modern engines increases to fulfill the emissions requirements, the techniques used to investigate in-cylinder phenomena need to update and to improve. Optical diagnostics provide precious information about the injection and combustion processes. To visualize the fuel vapor phase, a light source with specific wavelength and energy is needed; multiple optical accesses and additional optical components are required; the techniques are susceptible to the directionality of the light source and to the fuel composition. Recently, Infrared imaging has been used to overcome some of the drawbacks of well-known optical techniques. A peculiarity of infrared imaging is the ability to detect the energy emitted by a body as electromagnetic waves, from 0.76 to 1000 μm wavelength. This work illustrates the application of infrared imaging in a compression ignition engine for the analysis of the injection and combustion processes. The diesel fuel vapor penetration is experimentally measured and then compared to a 1d model of spray injection. Another application of IR can be the evaluation of the CO₂ in the cylinder, that is a key species in the combustion process, the wavelength of 4.2 μm, relative to the asymmetric stretch of this molecule, is investigated to follow its distribution within the cylinder for different, conventional and non-conventional combustion modes.

In the present paper the effect of a synthetic jet, coaxial with an annular type combustor, has been investigated in order to infer aerodynamic interaction between the two gaseous flows. The experimental investigation has the purpose to find the existence of synthetic jet excitation frequency able to extend the stability limits of a lean air/propane mixture flame. Lean flame is widely adopted in low thermal NOx emission combustors. As it is well known for gaseous premixed flame increasing the air/fuel ratio (lean flame) the flame propagation velocity is decreasing until it is getting comparable to the velocity of supplied fresh gaseous mixture. At this point, the flame is flushed away from the combustor mouth and the combustion process is quenched (blow off). In order to avoid those effects, for very low air/combustible ratio the modern premixed industrial combustors control the flame stability by means of the so called "pilot flame", a small secondary relatively rich flame. The pilot flame, fed by a few percent of the total amount of supplied combustible, represent a relatively high source of thermal NOx. The experimental investigations have been performed by means of a Particle Image Velocimetry (PIV), in order to obtain instantaneous two dimensional velocity distribution overall the entire gaseous reacting jet, in order to stretch the influences of coherent structures generated in the jet, by means of natural aerodynamic effect and by means of perturbation forced into the flow by means of the synthetic coaxial jet under combustion conditions.

Domestic Vertical Axis Wind Turbines (VAWT) are able to operate also when the wind speed is low and can be installed in the open spaces of housing areas. The Darrieus turbine is the most promising since it is characterized by higher efficiency, even if for applications at higher wind speeds with respect to Savonius VAWTs. Several researchers try to optimize the aerodynamics of the Darrieus rotors, allowing wind turbines to start rotation even in the presence of low wind speeds. In this paper the authors propose a numerical investigation of the flow field around a 1:4 scaled model of a pair of blades installed on an innovative Darrieus-type VAWT. A Reynolds Averaged Navier-Stokes-based CFD model was validated on the basis of the wind tunnel data, and the scaled turbine performance was analysed by numerically evaluating the effect of different geometric configurations and rotation angles on the turbine torque coefficients. The results of the simulations confirmed the capabilities of the proposed configuration to give valuable performance even for low wind speeds.