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The present work discusses an experimental investigation of the flow into a silicon-based vaporizing liquid microthruster equipped with sensing capabilities and low-power on-channel secondary heaters. The sensing capabilities (resistance temperature detectors and capacitive void fraction sensors) are used to investigate the flow instability and to evaluate its performances. The device has a sandwich structure composed of a silicon substrate and a glass substrate. This last one allows optical access into the device. The main heating of the propellant is provided using a platinum resistive heater placed on the bottom of the silicon layer. By varying the electrical power supplied to the main heater at fixed mass flow rate, three flow regimes have been observed and investigated: fully liquid flow, two-phase flow, and fully vaporized flow. Their dynamics have been captured through high-speed micro-flow visualizations under rough vacuum conditions (about 29 kPa). Furthermore, the expansion of the exhaust vapor plume exiting from the micronozzle has been analyzed via Schlieren visualizations. Results highlighted the occurrence of a cyclic flow behavior during the two-phase flow regime. In contrast, the flow exhibits the presence of both liquid and vapor phases at the micronozzle exit. Once the fully vaporized flow regime is established, the two-phase flow dynamics into the inlet chamber become more stable with complete filling and more uniform distribution of the flow at the microchannels entrance. Schlieren imaging captured the increase of the exhaust plume spreading half-angle when moving from ambient condition towards rough vacuum.

In a typical rocket motor the propellant is burned in the combustion chamber, after the combustion products pass through a convergent-divergent nozzle where with a rapid expansion the exhaust jet is formed. The exhaust gases of rockets leave the nozzle outlet at very high temperatures, knowledge of thermal produced emissions is a crucial aspect during the design and the tests of the rocket motors. The thermal radiation signature produced by rocket engine exhaust plumes is a challenging phenomenon to analyze. The thermal emissions is affected by several characteristics, such as: motor type; propellant type, liquid or solid; additives in each propellant; burn time; rocket geometry and furthermore results to be influenced by other causes such as the trajectory of the rocket, the atmosphere conditions, the models used for the calculation of gas and particles radiative properties. In this paper, a signature analysis procedure was developed to understand the spectral characteristics of exhaust plumes in detail. Hence a detailed description of radiative particles properties as well as the measurement of the exhaust plume temperature and its distribution are important, because radiative heat transfer leads to a rise of the rocket surface temperature by several hundred degrees, resulting in a deterioration of the rocket base. Moreover, the rocket plume plays an important role in the operation of target detection, tracking, classification and identification. The rocket thermal radiation is in visible, infrared and ultraviolet regions. The infrared radiation mostly is given by the gas radiation of the some combustion products as CO2 and H2O, and the continuous spectral radiation of smoke solid micro particles. As a matter of fact in the solid rocket motor propellant generally there are additives such as boron and aluminum. The Al2O3 particles in the exhaust plume produce emission spectra in ultraviolet band due to the high temperature that reaches values in the range 2000-3000K. Moreover the high temperature iron oxide particles also emit in the ultraviolet spectrum. Finally the unburned fuels in the exhaust plume generate high concentrations of C, N, and H, among which the chemical reactions produce a large number of free photons, and the chemiluminescence participated by these free photons will also produce ultraviolet radiation [1]. To study the exhaust plume leaving the nozzle outlet, it is necessary to simulate compressible flow through and beyond rocket engine nozzle. For this purpose, commercial computational fluid dynamics software, ANSYS Fluent 2020 CFD, is used. The turbulent flow of the high speed combustion jet is described with the axisymmetric form of the Navier Stokes equation coupled with the turbulence model k - ?, solved with a pressure-based solver formulation with second-order weighted upwind central spatial discretization. Substantial efforts have been directed to studying the effects of afterburning reactions on IR signatures and predicting how different parameters of the propellant components affect the spectral radiation signature of the exhaust plume.

The present work aims to propose a novel quasi-one-dimensional model for the performance estimation of a Vaporizing Liquid Microthruster (VLM). The analytical model was applied to the analysis of a MEMS-based VLM composed of a rectangular inlet chamber, a set of parallel microchannels as heating chamber, and a planar convergent–divergent micronozzle. It combines a steady-state boiling model for the analysis of the heater with a real nozzle flow model for the evaluation of actual thrust force and specific impulse, based on iterative procedure aiming at the convergence of the actual mass flow rate and the heat flux. For the purpose, a set of semi-empirical formulas found among both theoretical and experimental scientific works have been introduced for the estimation of the critical heat flux condition and the local heat transfer coefficient. In addition, the real nozzle flow model predicts the performance and the viscous losses due to the boundary layer growth inside the micronozzle. The last ones are estimated by introducing analytical expressions for the discharge coefficient and the I s p -efficiency into the isentropic nozzle flow theory. The resulting performance predictions of the 1D model referred to the on-design operating conditions. They well agreed with the experimental data, with a maximum estimated error of 7.3% on the thrust and the specific impulse. Furthermore, the analytical model of the micronozzle predicted a reduction of the mass flow rate up to about 8%, as well as thrust losses up to 15% due to the contraction of the cross sectional area. In addition, 2D and 3D computational fluid dynamics (CFD) simulations were performed in order to enforce the analysis of the viscous effects. Predictions of 2D computations overestimated the performances of the microthruster with respect to experiments, up to about 19% of the thrust and 20% of the specific impulse. On the other hand, the 3D predicted thrust approached to the experimental one with an error of about 9.2% below. In addition, a severe reduction of jet thrust in favor of the pressure thrust was observed at the nozzle exit. Furthermore, 3D computations pointed out the influence of the micronozzle depth on the boundary layer growth and the viscous losses. In particular, they revealed the establishment of the nozzle blockage and the thermal chocking of the supersonic flow owing to the subsequent viscous heating.

Small engines will be finding increasing applications in unmanned aerial vehicles (UAVs), drones and helicopters. However, their turbomachines exhibit lower efficiencies than those of large scale engines. In this context, the aerodynamic losses in the low-pressure turbines (LPTs) are largely accountable to flow separation at low Reynolds numbers operation, i.e. in cruise conditions. Active flow control is a promising technology to suppress separation, thus reducing losses, fuel consumption rates and therefore emissions. The present paper is focused on the experimental investigation of the potentialities of a Single Dielectric Barrier Discharge Plasma Actuator (SDBDPA) to reattach the separated flow at a Reynolds number of 2 ⋅ 10 4 . The influence of the high voltage (HV) waveform supplying the SDBDPA on both flow separation control and device power dissipation was studied. The investigated SDBDPA was manufactured by microfabrication techniques. Photolithography ensured thin metal deposition with high manufacturing reliability control. Due to the possible device degradation during operation, emphasis was put in selecting thin film materials that could withstand the plasma environment. Schott alkali-free borosilicate glass substrate was chosen as dielectric, while a multilayer tungsten (W)/titanium nitride (TiN) as electrode material. The experimental approach comprised the actuator testing over a curved wall plate, designed with a shape to reproduce the suction surface of a LPT rotor blade and installed in closed loop wind tunnel test section. The SDBDPA was located at the front side of the adverse pressure gradient area, in order to control flow separation. Different HV excitation waveforms (sinus, triangle and square) and amplitudes were tested and compared, aiming to identify the input signal that gave the best flow control authority and device energy conversion efficiency. The applied voltage and the discharge current were acquired in order to determine the actuator dissipated power. Two-dimensional (2-D) flow velocity measurements were carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Velocity results showed that the extension of the separation area was reduced by actuation. Moreover, when the actuator was on, the boundary layer thickness and the negative velocity magnitude decreased. Their reduction increased with the applied voltage (i.e. higher power dissipations). At comparable peak-to-peak applied voltages, the sinus waveforms slightly outperformed the other waveforms; however, while the sinus and triangle ones had comparable power dissipation, the square wave always dissipated the most.

Many industrial and transportation applications use combustion in dedicated chambers. Combustion implies, depending upon the nature and the amount of precursors, production of carbon dioxide, pollutants and dusts in terms of particulate matters. With the aim of reducing emission, lean combustion is of great interest. However flame stability within the combustor chamber is a key issue under lean conditions. In fact under lean conditions burners exhibit flame instability, flashback or lean blow out, until the flame extinction. Hence the online monitoring of these phenomena related to combustion instability is essential. One the most used techniques is to check temperature and flame stability by means of sensing probes resisting to high temperatures. Increasing the number of probes, it is possible to perform a 2D and 3D monitoring. However since these probes are costly and require heavy maintenance procedures, it could be wise to exploit imaging processing through cameras directed to portholes across which we can see inner parts, and atmosphere of the furnace/chamber. This paper illustrates findings related to monitoring the flame behaviour different operating conditions chamber by an advanced image processing. A specific algorithm has been developed to characterize the flame, hence, to perform measurements. Myriad filters have been utilized to enhance flame features.

Different input waveforms applied to a Single Dielectric Barrier Discharge Plasma Actuator (SDBDPA) were compared for flow separation control on low-pressure turbines (LPTs). The investigated Reynolds number (Re) was 2·104. The work aim was the device optimization in terms of materials and excitation conditions for enhancing its durability and performances. The SDBDPA was manufactured by microfabrication techniques. Device materials that could withstand the plasma environment were selected. Sine, square and triangle waveforms were compared in terms of actuator dissipated power and induced velocity. At comparable peak-to-peak applied voltage, the sinus outperformed the other waveforms, while the square dissipated the most.

This paper investigates the performance of a 960 kWP photovoltaic (PV) system, located in southern Italy. Monitoring data over a 43 months period are used to evaluate the monthly average of energy yields, losses and efficiency. The performance ratio and the capacity factor were 84.4% and 15.6%, respectively. The yearly average module efficiency was 15.3% and the system efficiency was 14.9%, whereas the capture and system losses were 0.6 h/day and 0.1 h/day respectively. The degradation rate of the PV system, after continuous outdoor exposure, has been estimated at about 1.48%/year by applying the Classical Seasonal Decomposition (CSD) method. A comparison in terms of degradation rate is made with other PV plants installed in different climates to address the need of standard methods to determine this crucial parameter. The actual performance of the studied PV system is compared with the expected outcomes by using two widely used PV simulation tools, SAM and PVsyst. A detailed difference analysis is carried out to evaluate the accuracy of the SAM and PVsyst models. Results show that the yearly average energy injected into the grid was under-estimated by 3.0% by SAM and by 3.3% by PVsyst, but overall PVsyst outperforms the SAM tool. Results provide reliable data on the plant and demonstrate its good performance compared to other plants located in the same climate. The results can be used to improve the prediction of the performance of future PV of systems in the Mediterranean climate, and also provide a useful dataset for comparative studies.

Sloshing refers to the motion of the free liquid/gas interface inside a partially filled container subjected to an external excitation with a consequent motion of the system center of gravity. Several parameters related to the fluid container, the fluid properties, the excitation levels, and the gravitational environment govern this phenomenon. Moreover, it is a significant cause of disturbance in embarked reservoirs. In this paper, it is shown that the absolute liquid/gas interface position η A during moderate sloshing (i.e., in the absence of splashing and wave breaking) can be measured over the full surface domain by the Reference Image Topography technique. It is applied to sloshing in steady and in transient regimes, underlining its potential. The simultaneous measurement of the liquid/gas interface position, in several points, has allowed us to identify swirling regimes and to evaluate the spatial distribution of fluid damping.

The aim of the present work is the experimental investigation of the effects of the addition of water and urea into jet fuels, on the reduction of nitrogen oxides (NOx) emissions and eventually improvement of the lean flame stability in aeroengine combustors. Experiments have been carried out using a 300-kW liquid-fueled swirling combustor. Various urea and/or water concentrations have been tested at the same fuel/air ratio. In order to study the flame behavior, non-invasive optical diagnostic techniques, as charge-coupled device (CCD) cameras in different spectral ranges (Visible and UV ranges, with different optical filters), have been adopted to analyze the shape and the brightness of the flame structure. Measurements of exhaust emissions (NOx, SO2, CO, CO2 and O2) have also been performed in order to evaluate the impact of emulsification on the entire combustion process. Finally, the thermal efficiency losses with respect to the neat jet test case were also analyzed for each emulsified fuel condition.

The present study intends to investigate the boundary layer characteristics to assess the potentiality of the single dielectric barrier discharge plasma actuators (SDBDPAs) to reattach the separated flow at low Reynolds numbers. The effect of the actuator geometrical parameters and of the Reynolds number on the device control authority was experimentally investigated. For this aim, a curved wall plate, which profile shape was designed to reproduce the suction surface of a low-pressure turbine (LPT) blade, was installed in the test section of a closed loop wind tunnel and a groove was made over it, at the front of the adverse pressure gradient region, for allocating a SDBDPA. Three actuators, characterized by different streamwise width, were manufactured by photolithography technique and they were tested. The velocity flow field, in both presence and absence of external flow, was investigated by particle image velocimetry (PIV) measurements. When the actuator was turned on, a sinusoidal voltage excitation with amplitude of 8 kV and frequency of 2 kHz was applied and the dissipated power ( P ‾ el ) was retrieved by electrical characterization. The effect of the active flow control was firstly estimated by analyzing the plasma induced velocity fields in absence of external flow. Subsequently the wind tunnel inlet free stream velocity ( v x , ∞ in ) was set to 1.54 m/s. The velocity, turbulence intensity (Tu) and vorticity (ωz) fields together with the boundary layer shape factor ( H 12 ) and momentum coefficient ( c μ ) were evaluated in both presence and absence of actuation. All the aforementioned analyses together with the estimation of the device electrical-to-fluidic energy conversion efficiency (ηfm) allowed identifying the best actuator geometry. Then, that configuration was chosen to investigate the effects of the wind tunnel velocity on the device control authority. The tested v x , ∞ in values ranged from 1.54 m/s up to 3.16 m/s. In absence of actuation, a large reverse flow and high turbulence intensity was observed in the separation region. Considering the actuated cases, it was found that at P ‾ el ≈ 7 W, the SDBDPA operation always implied a reduction of the separated region, of the flow angle and of the turbulence intensity. Moreover, the plasma induced jet had a larger impact on the flow at lower velocities and a low flow control effect was noticed at the highest v x , ∞ in values. The H 12 factor evaluation confirmed the flow regimes at the different tested velocities (i.e. c μ values). The whole data set allowed to evaluate the actuator success for separation control and to identify a threshold value of the c μ coefficient delimiting the still detached flow from the reattached one.

The power forecasting plays a significant role in the electrical systems. Furthermore the high-dimensional data reduction without losing essential information represents an important advantage in the forecasting models. Low computational costs and short execution time together with high predicted performance are the main goals to be reached in the development of a prediction method. In this paper a hybrid method based on an active selection of the support vectors, using the quadratic Renyi entropy criteria in combination with the principal component analysis (PCA), is shown to dimensionally reduce the training data in the forecasting models. The reduced data have been used to implement the Least Squares Support Vector Machines (LS-SVM) in order to predict the photovoltaic (PV) power in the day-ahead time horizon. The model has been validated using historical data of a PV system in the Mediterranean climate. Additionally the weather variations have been taken into account to evaluate the outcome of the sunny and cloudy condition in the PV forecasting models. The proposed technique gives fulfill results. A training data size same as 30% original dimension allows to improve the forecasting accuracy and reduces the computational time of 70% respect to an implementation without dimensionality reduction data. A dimensionally reduction technique, based on the Renyi entropy and PCA is shown.LS-SVM models are applied to predict the PV output power in a one-day head frame.Photovoltaic forecasting model is performed using the historical PV power data.The predicted method considers the weather conditions as sunny and cloudy days.The accuracy and executive time of the hybrid method has been investigated.

The present work was performed to investigate the employment of micro dielectric barrier discharge plasma actuators for mitigating separation, thereby decreasing wake losses and increasing efficiency, on a highly loaded compressor cascade. To this aim, the experimental characterization of the control device was initially done. A dedicated activity was devoted to microelectronic technology adoption for micro plasma actuator fabrication, together with batch production of electrodes with photolithographic techniques. The actuation effect on quiescent flow was evaluated by measuring the induced wall-jet with particle image velocimetry. The actuator power consumption was estimated by recording the applied voltages and resulting currents. Experimental results were then used to calibrate a multi-physics numerical model, for the prediction of the body forces induced by plasma actuator. Different algebraic models were compared. Numerical modelling was applied to predict the capability of micro plasma actuation to suppress flow separation into a highly-loaded subsonic compressor stator cascade. At first, simulations of the compressor cascade without active flow control were carried out and the results were compared with the literature experimental data. A good agreement was found between the experimental and the numerical results. Active flow control by the micro plasma actuator was then tested under different sinusoidal voltage amplitudes. It was found that the compressor pressure losses were reduced by increasing the applied voltage; actuation brought to a reduction in the pressure loss coefficient up to 14% and to an increase in static pressure up to 3%. When the actuator was on, the isosurface of the Q-criteria showed the reduction of secondary flow structures and the shape factor at the trailing edge of the midspan section was always lower than 2.2, confirming a reattachment of the flow. Furthermore, a conventional macro actuator found in the literature was also modelled and its actuation effect was compared to the one of the micro plasma actuator. In conclusion, the analysis of the actuation cost underlined that the adoption of micro actuation allowed reaching a higher gain when operating at lower voltage and same frequency.

The present work is aimed at the numerical spatio-temporal characterization of the unsteady cavitating flow structures on a hydrofoil by means of Proper Orthogonal Decomposition (POD) and Fast Fourier Transform (FFT) techniques. Three different cavitation regimes have been investigated: bubble cavitation, cloud cavitation and supercavitation. The homogeneous mixture approach has been used, in combination with an extended Schnerr–Sauer cavitation model. The accuracy of the numerical predictions has been improved by means of the implementation of a Density Correction Model of the turbulent viscosity, and a simplified Population Balance Modeling (PBM) which solved the spatial distribution and the temporal evolution of the nuclei. In particular, the PBM has led to a reduction of the intensity of the evaporation inside the vapor cavities and a consequent condensation enhancement at the cavity closure and in the wake downstream. This phenomenon mainly impacted on the vapor cavity dynamics during supercavition by facilitating the formation of the re-entrant jet and the vapor cavity detachment. Also, during supercavitation the nuclei density nb exhibited maximum variations of about 35.6% with respect to the inlet nuclei density. As the cavitation number increased, both the intensity and the extension of oscillations significantly reduced, and in bubble cavitation regime nb fluctuated at amplitudes of about 10% of the inlet nuclei density. The characterization of the cavitation regimes revealed that the bubble cavitation regime had a more stable and periodic dynamics highlighted by a higher hydrodynamic efficiency and a reduced root mean square of the lift force. The cloud cavitation and the supercavitation exhibited a more violent bubble detachment which caused stronger oscillations of the vapor cavity as well as the pressure upstream. This was retrieved in an increase of the average drag coefficient of about the 38% due to the presence of vapor cloud transported downstream, which promoted the surge of the flow. The vorticity analysis underlined that the formation of the re-entrant jet and the bubble detachment were promoted by the baroclinic vorticity, while the dilatation vorticity drove the dynamics of the detached clouds, governed by the phase change phenomena. The FFT analysis of the dynamics of the vapor cavity and the pressure upstream led to the detection of the most representative frequencies and Strouhal numbers of each cavitation regimes, in particular ( f s = 16.6 Hz , S t = 0.355 ) for bubble cavitation, ( f s = 10.74 Hz , S t = 0.358 ) for cloud cavitation, and ( f s = 8.79Hz , S t = 0.300 ) during supercavitation. The POD analysis allowed for the detection of the most relevant cavitating structures, in relation to the vapor cavity fluctuations and their frequency content. Furthermore, the FFT analysis of the temporal eigenfunctions demonstrated that the first POD mode of the liquid volume fraction described the overall unsteady behavior previously detected. Instead, high order POD modes revealed frequency values well above the overall ones of the main flow previously detected.

Recently, the Micro Single Dielectric Barrier Discharge Plasma Actuator has become attractive for application in aeronautics and micropopulsion thrusters. The present work carried out a preliminary characterization of such device, acting on initially quiescent air by experimental and numerical approaches. Sinusoidal voltage excitation with amplitude up to 7 kV and frequency up to 2.5 kHz was applied. The induced flow was investigated by particle image velocimetry and the measured velocity fields were used to estimate experimentally the time-averaged induced body force distributions by a differential method. Plasma induced forces were modeled by following three different approaches, later implemented as a source term in the Navier–Stokes equations for the fluid flow simulations. Potentialities, advantages and disadvantages of the considered force modeling methods were investigated. Quantitative comparison of the experimental and numerical induced force, as well as of the velocity fields, allowed establishing which model best predicted the actuator effects. The algebraic Dual Potential Model provided a good agreement between experimental and simulated results, in terms of flow velocities and thickness of the induced wall-jet. The downstream decay of the wall-jet velocity, experimentally observed, was also successfully predicted. A maximum induced velocity of ≈2 m/s was obtained and a jet thickness of ≈3 mm.

The following work details the experimental and numerical characterization of jet flow control in a coaxial Bunsen burner obtained by an alternate current driven dielectric barrier discharge plasma actuator (AC-DBDPA). Fuel and oxidizer flows could be inverted between the inner circular tube and outer annular tube. Use of the AC-DBDPA was intended to improve lean combustion performance. The high voltage (HV) electrode of the AC-DBDPA was powered with sinusoidal voltage excitation at 20 kHz frequency and peak-to-peak voltage of about 12 kV. Two burner fueling configurations were tested, characterized by different flow rates in the inner and annular tubes. Values of actuator power dissipation, which were deduced through electrical characterization, ranged between 25 and 28 W. A preliminary analysis of the air-methane flame behavior in presence and absence of plasma actuation was performed. Images of the flame and chemiluminescence emissions showed that the flame luminosity changed and OH radical production increased in presence of the discharge. The thermal and fluid dynamic effects of the plasma actuation on the flow were investigated by means of a non-reacting flow characterization (absence of combustion), where air was used as both fuel and oxidizer. The flow pattern and turbulence in both presence and absence of actuation were studied by laser Doppler velocimetry (LDV) measurements. Flow temperature was acquired using thermocouples. The non-reacting flow experimental results showed that, for a fixed inlet volumetric flow rate, the plasma actuation increased jet velocity and air temperature. Interesting results were obtained from a spectral analysis using the time resolved velocity signals: plasma actuation affected jet mixing and impacted energy distribution between the different flow scales. Complementary numerical simulations identified increasing temperature as primarily responsible for increasing flow velocity. Moreover, numerical predictions permitted better identification of flow regimes under different test conditions.

Cavitating flow regimes of water at ambient temperature inside a sharp-edged orifice have been characterized by means of a snapshot POD combined with high-speed visualizations. The density related gray level of each experimental image was processed at each pixel point. An estimation of the energy contribution in image reconstruction of each POD eigenvalue was made. Coherent flow structures related to the most significant eigenfaces were detected. Finally an FFT analysis of the temporal eigenfunctions led to the identification of the main modal dynamic properties. It was found that the flow structure of the first POD mode was defined by a single symmetric cavity. This cavity is related to the extension of the cavitating cloud. Conversely polarities and asymmetries in flow structures of higher modes reveal the establishment of advecting vortices and swirling motions. The frequency analysis of the first four POD modes has been used for the characterization of the different cavitation regimes. As the cavitation number decreases, the jet cavitation is characterized by a rapid increase of the frequencies of modes 3 and 4, up to 1000 Hz. Instead, developed cavitation and supercavitation are characterized by a growth in frequency of the second mode up to 300 Hz, as well as they exhibit the highest FFT magnitudes of the first mode.

This erratum corrects errors that appeared in the paper “Analysis of the Performance of Plasma Actuators Under Low-Pressure Turbine Conditions Based on Experiments and URANS Simulations” which was published in Proceedings of ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition, Volume 2A: Turbomachinery, V02AT40A034, June 2017, GT2017-64867, doi: 10.1115/GT2017-64867.

The study is focused on the monitoring of the performances of a photovoltaic system, influenced of climatic characteristics of a particular geographical area, in which the PV system is installed. This paper presents the results obtained by monitoring, from March to October 2012, a 960 kW p photovoltaic system installed on the parking lots in the campus of the University of Salento, Italy and divided in two subfields with different tilt angle (3–15°) and different nominal powers (353.3 kW p and 606.6 kW p ). The values of energy and power generated, final yield, reference yield, photovoltaic system efficiency, performance ratio and cell temperature losses are analyzed and linked to the climatic characteristics of the site, such as insolation (for the two distinct subfield), ambient temperature and wind speed. This study is part of the European Project “Buildings Energy Advanced Management System, BEAMS”, and its aim is to develop an advanced, integrated management system for buildings and special infrastructures of public use. This work offers a tool to estimate the performances of plants to be installed in sites with climatic characteristics similar to South-eastern Italy, and it is also useful as comparison to investigate the productivity of plants placed in different areas.

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis. The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data. The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature . The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions. As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident. The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature. The vapor fraction was estimated using an image processing algorithm. The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.

The forecasting techniques are affected by the renewable sources randomness. Improvements of the prediction models with more accurate results and lower error are necessary for future development of the microgrids projects and of the economic dispatch sector. The LS-SVM (Least Square Support Vector Machine), a relatively unexplored neural network known as GMDH (Group Method of Data Handling) and a novel hybrid algorithm GLSSVM (Group Least Square Support Vector Machine), based on the combination of the first two models, were implemented to forecast the PV (Photovoltaic) output power at several time horizons up to 24 h. In order to improve the forecasting accuracy, each model was combined with three strategies for multi-step ahead forecast (Direct, Recursive and DirRec). A detail analysis of the normalized mean error is carried out to compare the different forecasting methods, using the historical PV output power data of a 960 kWP grid connected PV system in the south of Italy. The outcomes demonstrate the GLSSVM method with the DirRec strategy can give a normalized error of 2.92% under different weather conditions with evident improvements respect to the traditional ANN (Artificial Neural Network).

The performance assessment of photovoltaic (PV) systems is a complex process. Several meteorological data sources are available to evaluate the PV system generation. Different computing models can be applied to determinate the solar irradiance on the plane of the array (POA). The cooling effect of the PV module due to the wind speed should not be neglected. The present study may support several users to perform more accurate PV energy predictions, providing important suggestions to develop future PV system projects with more high reliability. Perez and Hay-Davies models for the computing of the irradiance on tilted surfaces are combined with three meteorological datasets, characterized by different monitoring period and meteo station location, to estimate the POA irradiance, the module temperature and PV energy output for a PV system located in the Mediterranean climate area. Prediction results are performed by the PVsyst tool and compared with the actual data. Simulations are carried out taking into account the wind effects on the PV module performance. Results demonstrate that the geographic features of the location, in which the weather station is located, have higher impact on the estimations of the PV system performance than the distance between the PV system and the meteo station. Perez and Hay-Davies models provide predictions of the PV energy and the module temperature with a difference up to 3% and 1% respectively. Yearly average wind speed in the range 2–4 m/s fosters a cooling effect up to 3% higher than one due to the wind magnitude less of 2 m/s, increasing the PV energy up to 1%.

The effects of active separation control on a highly loaded subsonic compressor stator cascade were numerically studied by comparing the behavior of different devices: continuous jet actuator (CJA), zero-net mass-flux synthetic jet actuator (SJA), and plasma actuator (PA). For the jet actuator modeling, a suction/blowing type boundary condition was used, imposing a time-constant velocity for the CJA case and a prescribed sinusoidal time-varying velocity for the SJA case. For the PA case, the body force, which represents the effect of the plasma actuator on the flow, was added to the momentum equations in the computational fluid dynamics (CFD) code. The PA is slightly more efficient for the reduction of flow separation in the region just downstream of the blade actuators. However, at the same mechanical power delivered by the actuator to the fluid, the SJAs are more advantageous than the CJAs and slightly outperform plasma actuator application from the pressure loss reduction and pressure rise viewpoints. The fluidic jets have low power requirements, whereas the power consumption would be prohibitive for the PA configuration that shows low fluid mechanic efficiency

A numerical investigation is performed to analyse the suppression of the boundary layer separation first on a NACA 0015 airfoil and subsequently in a highly loaded subsonic compressor stator cascade using two different active flow control techniques: synthetic jet actuators (SJA) and continuous jet actuators (CJA). In particular, the effort is concentrated towards understanding the physics of the phenomenon, which makes one configuration perform better than the other one as an active flow control (AFC) system. The analysis of the interaction of the jet with the boundary layer was performed, with a description of the vortical structures, which are beneficial for the mixing of the boundary layer and the entrainment of energy from the external flow towards the most inner layers. Regarding airfoils, the comparisons of the SJA with the CJA were performed considering two similarity conditions: identical momentum coefficients and identical amounts of energy fed into the cross flow. Finally, the flow behaviour and the topology structure in a highly loaded compressor cascade with and without AFC are examined. Active flow control using synthetic jet actuators proved to be attractive because it could exploit the unsteady phenomena, inhibit the separation, improve and control the aerodynamics of the flow in both external aerodynamics and turbomachinery components. Furthermore, comparing the CJA and the SJA for active flow control on the compressor at similar momentum coefficients shows that the relative reduction of the total pressure losses for the SJA is approximately twice as large as that for the CJA. It should also be remarked that even if the two AFC configurations present similar effects on the reduction of secondary flow structures, the SJA is more advantageous than the CJA from the regaining energy viewpoint.

Identification of cavitating regime is an important issue in a wide range of fluid dynamic systems. The cavitation behavior is affected by several parameters, as the operating pressure and the fluid temperature. In the present study the cavitating behavior of water inside an orifice was analyzed by images analysis and by pressure signals. Four cavitation regimes were characterized: no-cavitation, developing cavitation, super cavitation and jet cavitation. A three-layer Elman neural network was designed to predict the cavitation regime, from the frequency content of the pressure fluctuations, recorded upstream and downstream the internal orifice. Cavitation regimes were successfully predicted. The designed neural networks were useful also to underline the influence of each operating parameter on the phenomena under investigation; in particular it was possible to identify the frequency ranges that characterize the different cavitation regimes and the influence of the fluid temperature.

Preventing the flow separation could enhance the performance of propulsion systems and future civil aircraft. To this end, a fast detection of boundary layer separation is mandatory for a sustainable and successful application of active flow control devices, such as plasma actuators. The present work reports on the design, fabrication and functional tests of low-cost capacitive pressure sensors coupled with dielectric barrier discharge (DBD) plasma actuators to detect and then control flow separation. Finite element method (FEM) simulations were used to obtain information on the deflection and the stress distribution in different-shaped floating membranes. The sensor sensitivity as a function of the pressure load was also calculated by experimental tests. The results of the calibration of different capacitive pressure sensors are reported in this work, together with functional tests in a wind tunnel equipped with a curved wall plate on which a DBD plasma actuator was mounted to control the flow separation. The flow behavior was experimentally investigated by particle image velocimetry (PIV) measurements. Statistical and spectral analysis, applied to the output signals of the pressure sensor placed downstream of the profile leading edge, demonstrated that the sensor is able to discriminate different ionic wind velocity and turbulence conditions. The sensor sensitivity in the 0–100 Pa range was experimentally measured and it ranged between 0.0030 and 0.0046 pF Pa−1 for the best devices.

The identification of the water cavitation regime is an important issue in a wide range of machines, as hydraulic machines and internal combustion engine. In the present work several experiments on a water cavitating flow were conducted in order to investigate the influence of pressures and temperature on flow regime transition. In some cases, as the injection of hot fluid or the cryogenic cavitation, the thermal effects could be important. The cavitating flow pattern was analyzed by the images acquired by the high-speed camera and by the pressure signals. Four water cavitation regimes were individuated by the visualizations: no-cavitation, developing, super and jet cavitation. As by image analysis, also by the frequency analysis of the pressure signals, different flow behaviours were identified at the different operating conditions. A useful approach to predict and on-line monitoring the cavitating flow and to investigate the influence of the different parameters on the phenomenon is the application of Artificial Neural Network (ANN). In the present study a three-layer Elman neural network was designed, using as inputs the power spectral density distributions of dynamic differential pressure fluctuations, recorded downstream and upstream the restricted area of the orifice. Results show that the designed neural networks predict the cavitation patterns successfully comparing with the cavitation pattern by visual observation. The Artificial Neural Network underlines also the impact that each input has in the training process, so it is possible to identify the frequency ranges that more influence the different cavitation regimes and the impact of the temperature. A theoretical analysis has been also performed to justify the results of the experimental observations. In this approach the nonlinear dynamics of the bubbles growth have been used on an homogenous vapor-liquid mixture model, so to couple the effects of the internal dynamic bubble with the other flow parameters.