Cavitation, Cryogenic Cavitation and Turbomachinery Ref.

To request a copy of a publication send an email to antonio.ficarella02@gmail.com.

2015

"Monitoring Cavitation Regime from Pressure and Optical Sensors: Comparing Methods Using Wavelet Decomposition for Signal Processing", De Giorgi, M.G.; Ficarella, A; Lay Ekuakille, A., IEEE SENSORS JOURNAL, 15, 4684-4691, 2015.

A cavitating two-phase flow of water in a pipe with area shrinkage was experimentally investigated, acquiring at high sampling rate pressure signals and images of the cavitating flow-field. The fluctuations of the pressure measurements in the internal orifice and of the intensity level of the images' pixels were decomposed using the wavelet transform to analyze the frequency distribution of the signals energy with respect to the flow behavior and to highlight the influence of temperature on the phenomena under investigation. It was shown that the energy content at each frequency band of the acquired signals is well related to cavitation flow-field behavior.

"Cavitation Regime Detection by LS-SVM and ANN with Wavelet Decomposition Based on Pressure Sensor Signals", DE GIORGI, Maria Grazia; FICARELLA, Antonio; LAY EKUAKILLE, Aime, IEEE SENSORS JOURNAL, Volume 15, Issue 10, 5701-5708, 2015.

A cavitating two-phase flow of water in a pipe with area shrinkage was experimentally investigated, acquiring at high sampling rate pressure signals and images of the cavitating flow field. The time series of the pressure fluctuations was analyzed in terms of power spectral density and related to the cavitation regimes. Furthermore, the fluctuations of the pressure measurements were also decomposed using the wavelet transform to analyze the frequency distribution of the signals energy with respect to the flow behavior. The energy content at each frequency band of the acquire signals is well related to cavitation flow-field behavior. Moreover, the artificial neural network and the least squares support vector machine (LS-SVM) were implemented to identify the cavitation regime, using, as inputs, the power spectral density distributions of the pressure fluctuations, and some features of the decomposed signals, as the wavelet energy for each decomposition level and wavelet entropy. Results indicate the most accurate model to be used in the cavitation regime identification, underlining the enhanced capability of LS-SVM trained with the input data set based on the wavelet decomposition features.

2014

"An artificial neural network approach to investigate cavitating flow regime at different temperatures", D. Bello, M.G. De Giorgi, A. Ficarella, Measurement, Volume 47, Pages 971-981, ISSN 0263-2241, http://dx.doi.org/10.1016/j.measurement.2013.09.011, January 2014.

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.

2013

"An Artificial Neural Network Approach to Investigate Cavitating Flow Regime at Different Temperatures", M.G. De Giorgi, D. Bello, A.Ficarella, 4th Imeko TC19 Symposium on Environmental Instrumentation and Measurements, Protecting Environment, Climate Changes and Pollution Control, June 3-4, Lecce, Italy, 2013.

The aim of the present study is to implement an useful approach to predict and on-line monitoring the cavitating flow and to investigate the influence of the different parameters on the phenomenon by the application of Artificial Neural Network (ANN). 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. The 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 cavitation regimes and the impact of fluid temperature.

"Evaluating cavitation regimes in an internal orifice at different temperatures using frequency analysis and visualization", M.G. De Giorgi, A. Ficarella, M. Tarantino, International Journal of Heat and Fluid Flow, Volume 39, Pages 160–172. http://dx.doi.org/10.1016/j.ijheatfluidflow.2012.11.002, February 2013.

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.

2012

"Influence of convective heat transfer modeling on the estimation of thermal effects in cryogenic cavitating flows", M. G. Rodio, M. G. De Giorgi, A. Ficarella, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER (ISSN 0017-9310), Volume 55, Issues 23–24, pp. 6538-6554. DOI 10.1016/j.ijheatmasstransfer.2012.06.060, 2012.

The accuracy of numerical simulations for the prediction of cavitation in cryogenic fluids is of critical importance for the efficient design and performance of turbopumps in rocket propulsion systems. One of the main remaining challenges is efficiency in modeling of the physics, handling the multi-scale properties involved and developing robust numerical methodologies. Such flows involve thermodynamic phase transitions and cavitation bubbles that are on a smaller scale than the global flow structure. Cryogenic fluids are thermo-sensitive, and therefore, thermal effects and strong variations in fluid properties can alter the cavitation properties. The aim of this work is to address the challenge posed by thermal effects. The Rayleigh–Plesset equation is modified by the addition of a term for convective heat transfer at the interface between the liquid and the bubble coupled with a bubbly flow model to assess the prediction of thermal effects. We perform a parametric study by considering several values of and models for the convective heat transfer coefficient, hb, and we compare the resulting temperature and pressure profiles with the experimental data. Finally, the results of a 2D simulation with a commercial CFD code are presented and compared with the previous results. We note the importance of the choice of hb for the correct prediction of the temperature drop in the cavitating region, and we assess the most efficient models, underlining that the choice of hb estimation model in a cryogenic cavitating flow is more important in the bubble growth phase than in the bubble collapse phase.

"COMPUTATIONAL MODELING OF THERMO AND FLUID DYNAMIC EFFECT IN CAVITATING NOZZLES AND EXPERIMENTAL CHARACTERIZATION", M.G. De Giorgi, D. Bello, A. Ficarella, 67° Congresso Annuale ATI, Trieste (Italy), Sept. 11-14, 2012.

The aim of the present work was to investigate the influence of pressures and temperature on cavitation in an orifice flow. In particular image analysis and frequency analysis of the experimental pressure signals were used to identify different flow behaviors at different operating conditions. . In order to show the effect of the temperature at different cavitation numbers, the waterfall diagrams of the frequency components of the downstream and upstream pressure at 293-348 K were investigated. Bubbles growth and collapse generate pressure fluctuations so frequency spectra can be related to cavitation behavior. The amplitude of the upstream FFT is higher than the downstream one. In particular the frequency range 0-10 kHz is investigated. The amplitude of FFT were used to training an ANN. ANN permitted to identify the different cavitation regimes and to lightly the influence of each input parameters about the learning of the network and then about the cavitation phenomenon. Following, a theoretical analysis was performed to justify the results of the experimental observations. In this approach the nonlinear dynamics of the bubbles growth were described by the Rayleigh-Plesset equation in a one-dimensional code, so to couple the effects of the internal dynamic bubble with the other flow parameters (pressure, velocity, void fraction, temperature, etc..).

"A NEURAL NETWORK APPROACH TO ANALYSE CAVITATING FLOW REGIME IN AN INTERNAL ORIFICE", M.G. De Giorgi, D. Bello, A.Ficarella, ESDA2012-82205, Proceedings of The ASME 2012, Biennial Conference On Engineering Systems Design And Analysis, ESDA 2012, July 2-4, Nantes, France, 2012.

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.

2011

“A DATA ACQUISITION SYSTEM TO DETECT BUBBLE COLLAPSE TIME AND PRESSURE LOSSES IN WATER CAVITATION”, M. G. De Giorgi, A. Ficarella, M. Tarantino, International Journal on Measurement Technologies and Instrumentation Engineering (IJMTIE), Vol. 1 n. 1, Jan.-Mar. 2011.

This paper presents a data acquisition system oriented to detect bubble collapse time and pressure losses in water cavitation in an internal orifice. An experimental campaign on a cavitating flow of water through an

orifice has been performed to analyze the flow behavior at different pressures and temperatures. The experiments were based on visual observations and pressure fluctuations frequency analysis. Comparing the visual

observations and the spectral analysis of the pressure signals, it is evident that the behavior of the different cavitating flows can be correlated to the frequency spectrum of the upstream, downstream and differential

pressure fluctuations. The further reduction of the cavitation number and the consequent increase in the width of the cavitating area are related to a corresponding significant increase of the amplitude of typical frequency components. The spectrogram analysis of the pressure signals leads to the evaluation of the bubble collapse time, also compared with the numerical results calculated by the Rayleigh–Plesset equation.

2010 and before

“Thermodynamic effect on cavitation in water and cryogenic fluids”, De Giorgi M.G., M. G. Rodio M. G., Ficarella, A., ESDA2010-24694, 10th Biennal Conference on Engineering Systems Design and Analysis ESDA 2010, Istanbul, 12-14 Luglio, 2010.

The present study focuses on the formation of cavitation in cold and hot water and in cryogenic fluid, characterized by variations in fluid properties caused by a change in temperature. Cavitation phenomenon is investigated in water and nitrogen flows in a convergent-divergent nozzle through pressure measurements and the optical visualization method.

High-speed photographic recordings have been made, the cavitation phenomena evolution and the related frequency content are investigated by means of pixel intensity time series data. The results obtained concur with those obtained with the spectral analysis of the pressure signals. In the case of cryogenic fluid frequency peaks are shifted towards lower frequencies, with respect to cold water and the magnitude of the signal rises, in particular at low frequencies, for nitrogen and hot water. This can be due to thermal effects that contribute also to the low frequencies in the case of cryogenic fluid. To verify the validity of this assumption, a simple model based on the resolution of Rayleigh equation is used.

"Analysis of Thermal Effects in a Cavitating orifice Using Rayleigh Equation and experiments", M.G. De Giorgi, D. Bello, A. Ficarella, ASME Internationa Journal Of Gas Turbine And Power, VOL. 132 ISSUE 4, APRIL 2010.

The cavitation phenomenon interests a wide range of machines, from internal combustion engines to turbines and pumps of all sizes. It affects negatively the hydraulic machines’ performance and may cause materials’ erosion. The cavitation, in most cases, is a phenomenon that develops at a constant temperature, and only a relatively small amount of heat is required for the formation of a significant volume of vapor, and the flow is assumed isothermal. However, in some cases, such as thermosensible fluids and cryogenic liquid, the heat transfer needed for the vaporization is such that phase change occurs at a temperature lower than the ambient liquid temperature. The focus of this research is the experimental and analytical studies of the cavitation phenomena in internal flows in the presence of thermal effects. Experiments have been done on water and nitrogen cavitating flows in orifices at different operating conditions. Transient growth process of the cloud cavitation induced by flow through the throat is observed using high-speed video images and analyzed by pressure signals. The experiments show different cavitating behaviors at different temperatures and different fluids; this is related to the bubble dynamics inside the flow. So to investigate possible explanations for the

influence of fluid temperature and of heat transfer during the phase change, initially, a steady, quasi-one-dimensional model has been implemented to study an internal cavitating flow. The nonlinear dynamics of the bubbles has been modeled by Rayleigh–Plesset equation. In the case of nitrogen, thermal effects in the Rayleigh equation are taken into account by considering the vapor pressure at the actual bubble temperature, which is different from the liquid temperature far from the bubble. A convective approach has been used to estimate the bubble temperature. The quasisteady one-dimensional model can be extensively used to conduct parametric studies useful for fast estimation of the overall performance of any geometric design. For complex geometry, three-dimensional computational fluid dynamic (CFD) codes are necessary. In the present work good agreements have been found between numerical predictions by the CFD FLUENT code, in which a simplified form of the Rayleigh equation taking into account thermal effects has been implemented by external user routines and some experimental observations.

“An Experimental Investigations Of The Influence Of Thermal Effects On Inception Of Cavitation In Sharp-Edged Orifices”, M.G. De Giorgi, M. Tarantino, A. Ficarella, ExHFT, 7th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Krakow, Poland, June 28 - July 03, 2009.

Cavitation remains a persistent problem for internal flow devices due to both performance reduction and the ability of cavitation bubbles to cause significant component surface damage during bubble collapse. The focus of this study is the experimental characterization of cavitation in a sharp-edged orifice in presence of thermal effects, considering water at different temperatures and cryogenic fluid, in particular nitrogen. The flow is investigated by means of visual observations and pressure signals. By means of high-frequency response pressure transducers strategically placed in the orifice cavitation could be sensed by the correlation of the power spectrum of the pressure signal measured with a cavitation index.

“Simulation Of Cryogenic Cavitation By Using Both Inertial And Heat Transfer Control Bubble Growth”, M. G. De Giorgi, A. Ficarella, AIAA 39th Fluid Dynamics Conference and Exhibit, San Francisco, USA, June 2009.

The study is concerning the modeling of cavitation in cryogenic flows. The energy equation for the mixture is solved in conjunction with the mass and momentum conservation, and the evaporative cooling effects of cavitation are accounted for. Usually the cavitating flows are modeling with the basic hypothesis of isothermal flow, however it’s known that there is a temperature decrease in the vapor cavity in the case of cryogenic fluids. These fluids, in fact, are characterized by larger compressibility if compared with fluids, such as water, at room temperature, by a small difference in density between vapor and liquid phases and by a small latent heat of vaporization. The aim of this paper is a numerical investigation of this phenomenon, using a multiphase formulation that accounts for the energy balance, variable thermodynamic properties of the fluid and nucleation transport equation.

In cavitating flow it has been assumed that there are plenty of nuclei for the inception of cavitation. Thus, the primary focus is on proper accounting of bubble growth and collapse. The expansion/contraction rate of the vapor bubble, is dominated by two mechanisms: a momentum that is needed to push away/pull up ambient liquid, and a heat transfer that is needed to drive phase change at the interface between the bubble and ambient liquid. They are called the inertia (momentum) control and the heat transfer control, respectively. In the expansion process the bubble radial movement is dominated by the inertia only at first and then changes to the heat transfer control. Over the whole contraction process it is controlled by the inertia. In the present work a cavitation model has been implemented by the user in the code Fluent 12.0, according this bubble behavior. Numerical results have also compared with the results obtained by the Schneer-Sauer cavitation model.

“Numerical Study And Experiments Of Cryogenic Cavitating Flows”, MG. De Giorgi, A.Ficarella, Festival dell’innovazione- Giornata Sulla Ricerca Nel Settore Aerospaziale In Puglia, Bari (Italy), Dec. 4th, 2008.

“CFD Modeling of Two Phase Cryogenic Flow in an Internal Orifice”, M.G. De Giorgi, A. Ficarella, M.G. Rodio, ANSYS Italy Conference 2008, 16-17 Ottobre 2008 - Mestre (VE).

The study is concerning the modeling of cavitating cryogenic flows, present in many engineering applications as superconductivity technology, liquefied natural gas plants, aerospace components, and many other fields. In this paper we re-examine previously developed cavitation models, used for water liquid, to adapt that to the simulation of cryogenic fluids. Usually the cavitating flows are modeling with the basic hypothesis of

isothermal flow, however it’s known, for cryogenic fluids, that there is a temperature decrease in the vapor cavity. Cryogenic fluids, in fact, are characterized by larger compressibility if compared with fluids, such as

water, at room temperature, by a small difference in density between vapour and liquid phases and by a small latent heat of vaporization. The aim of this paper is a numerical investigation of this phenomenon, using a

multiphase formulation that accounts for the energy balance, variable thermodynamic properties of the fluid and nucleation transport equation. The numerical analysis has been performed by the commercially available code Fluent release 6.3 and the Fluent beta version v.12.0. After the validation of two different cavitation models by comparison with experimental data present in literature, numerical simulations have been performed to analyze and better understand some experimental observations obtained by an experimental apparatus of University of Salento, where the two-phase cryogenic flow passing through an internal nozzle has been studied. Numerical cavitation patterns and pressure distribution, obtained by the mechanical equilibrium model, are found to agree very well with experimental data. In all cases the results show that as vapor fraction get larger, temperature becomes lower in liquid nitrogen. Vapor fraction increase corresponds to gas phase generation, therefore, latent heat should be absorbed from the fluid. As a result, temperature decreases as vapor fraction increases.

“Flow Visualization Study on Two-Phase Cryogenic Flow”, M.G. De Giorgi, A. Ficarella, M.G. Rodio, D. Laforgia, 22nd European Conference on Liquid Atomization and Spray Systems ILASS 2008, Sep. 8-10, 2008, Como Lake, Italy.

In cryogenic fluid cavitation phenomenon is similar to boiling. Generally, these two processes can be distinguished by the fact that cavitation is the process of nucleation in a liquid when the pressure falls below the saturated vapor pressure, while the boiling is the process of nucleation that occurs when the temperature is raised above the saturated vapor temperature. In cryogenic fluids is very difficult to distinguish between these two different causes of phase change, and a lack of experimental investigations is present in literatures. In aerospace field, cryogenic fluids are usually used as rocket propellant obtained as mixture of liquid oxygen (LOx) and liquid hydrogen (LH2). These fluids can be employed under particular conditions as low temperature, microgravity and the environmental space, but they are susceptible to cavitation phenomena, because to a few change of temperature is combined to a change of phase. The aim of this paper is to examine the results of a flow visualization study on two-phase cryogenic flow passing through a internal nozzle. The transient growth process of the cloud cavitation induced by flow through the throat is observed using high-speed video images and analyzed by pressure and accelerometer signal.

“Shape Optimization For Cryogenic Cavitating Flows Past An Isolated Hydrofoil” M.G. De Giorgi, M.G. Rodio, P. M. Congedo, A. Ficarella, FEDSM 2008 ASME Fluids Engineering Conference, paper FEDSM2008-55119 - August 10-14, 2008 Florida, USA.

The aim of this paper is shape optimization of a cryogenic flow past an isolated hydrofoil in order to reduce the cavitation. The numerical simulation of cavitating flows has been performed by way of the commercially available code Fluent (release 6.3), implementing a cavitation model by using external routines. The model is based on a simplified Rayleigh-Plesset equation, and takes into account both nucleation and thermal effects. This study has been divided in two parts. Firstly the cavitation model has been validated by comparison with experimental data, in particular, water cavitation on a NACA0015 airfoil and hydrogen cavitating flow over an external profile. Secondly, Fluent has been coupled with a multi-objective genetic algorithm (MOGA). Genetic algorithms have proved their interest with respect to gradient-based methods because of their high flexibility, and also because of their ability to find global optima of multi-modal problem. The representation of the design space has been previously investigated through a Design of Experiment (DOE) procedure. A shape optimization of an hydrofoil has been computed in order to minimize the vapor volume in different operating conditions.

“Cavitation Modeling in Cryogenic Fluids for Liquid Rocket Engine Applications”, M.G. De Giorgi, A. Ficarella, M.G. Rodio, AIAA-2008-3842, AIAA 38th Fluid Dynamics Conference and Exhibit, 23-26 giugno 2008, Seattle, USA, 2008.

The aim of this investigation is to compare the results of the numerical simulation of cryogenic flows using different cavitation models, taking into account other additional hypothesis as bubble number variation and

thermal effects. Different cavitation models have been analyzed. A particular focus is for the so called “full cavitation” model that presents, in the expression of the mass transfer due to cavitation, some empirical

coefficients tested in the past for water and not for cryogenic flows. Then thermal and nucleation effects have been taking into account, solving the transport equations for the bubble density and energy conservation. The performances of the different models have been compared with the experimental data of a flowfield of liquid hydrogen flowing in a Venturi device. The calculated pressure, temperature and vapor profiles have been compared for the different models with experimental data.

“Modeling Nucleation Phenomena in Cavitating Flow”, M.G. De Giorgi, A. Ficarella, D. Laforgia, AIAA 2007-4459, 18th AIAA Computational Fluid Dynamics Conference, 25 - 28 June 2007, Miami, FL.

The aim of this investigation is to implement a cavitating flow model taking into account the nucleation for both water and cryogenic fluids, in particular hydrogen. In this paper we re-examine previously developed

cavitation models, used for water liquid, including thermal effect for simulations of cryogenic fluids and transport equation for the bubble number. Thermal effects substantially impact the cavitation dynamics of

cryogenic fluids. Different numerical cavitation models, based on different physics of mass exchange between liquid and vapor phases, and taking into account the nucleation, were used to simulate the flowfield in

internal flow. The transport equation for bubble number is solved in conjunction with the mass and momentum conservation. Performances of the reported cavitation models are tested, comparing the predicted

pressure with the experimental data, with non-cryogenic and cryogenic fluids. For non-cryogenic the phase change during rapid depressurization of water have been studied and the role of vapor bubbles nucleation

and growth and the effect on the system fluid dynamics were modeled. For cryogenic fluid the cavitation phenomena of liquid hydrogen flowing in a Venturi has been investigated and the results have been compared

with experimental data.

“ExperimentalStudy Of Thermal Cavitation In An Orifice”, M.G. De Giorgi, F. Chiara, A. Ficarella, ASME Paper ESDA 2006-95406, Proceedings of ESDA2006 8th Biennial ASME Conference on Engineering Systems Design and Analysis, July 4-7, 2006, Torino, Italy.

Some experimental tests were performed studying the flow of water thought an orifice, in order to investigate the onset of the various cavitation structures and the effects of cavitation at different cavitation numbers and temperature.

Different flow rates have been tested for different values of temperature. The results show that the cavitation originates at the inlet of the flow constriction area. It grows intensively and transforms into a cavitating cloud. As flow rate was increased, it was observed that the cavitating cloud travels downstream of the hole oscillating around the exit position and it is connected to the hole inlet through a sheet having a complex turbulent

structures The decrease in the cavitation number causes a corresponding increase of the width of the cavitating area especially in proximity of the critical cavitation number. In particular, it was observed that the critical cavitation number increases as the temperature increase. The behavior of the cavitation phenomenon can be related to the pressure fluctuations measured downstream of the orifice; a Fourier Transform of the downstream pressure signal was performed.

The development of the cavitation phenomenon for lower cavitation numbers affects the pressure frequency components, related to the impacts due to vapor bubbles implosions. Finally, soma numerical simulations have been performed; the simulation results were compared with the experimental ones.

“Experimental and Numerical Investigation on Cavitating Flows”, M. G. De Giorgi, A. Ficarella, F. Chiara, D. Laforgia, 35th AIAA Fluid Dynamics Conference and Exhibit, Toronto (Canada), June 6-9, 2005.

Cavitating flows have been investigated using both numerical and experimental methods. Three different cavitation models, based on mechanical or thermal equilibrium hypotesis, have been implemented in a

general-purpose CFD code. As an external flows example, the behavior of a NACA 0015 hydrofoil was investigated. The cavitating flow field over the hydrofoil was predicted, and the results compared with

experimental data, reported in literature. The general characteristics of the cavitating flow were well predicted. Especially, the cavity length was calculated with reasonable accuracy. Further, the cavitation

models were applied to flows through an orifice, and the computed results were compared with the experimental observations, obtained with a CCD camera. The cavitation originates at the inlet of the flow

constriction area. It grows intensively and transforms into a dense cloud. Shedding is observed in this stage.

As flow rate was increased, it was observed that the cloud travels downstream of the hole oscillating about the exit position and it is connected to the hole inlet through a sheet having a complex turbulent structures.

“Comparison of Different Physical Models for Simulation of Cavitating Flows around a Hydrofoil”, M. G. De Giorgi, A. Ficarella, D. Laforgia, ASME Congress, Houston (USA), June 19-23, 2005, Asme Paper FEDSM2005-77142.

The simulation of cavitating flow is a difficult task, due to the large density ratio between liquid and vapor phases. Investigation of currently known cavitation and turbulence models and their mutual interaction can be a significant asset to improving their performance. Several previous works present results on cavitating flows obtained by various physical models, that mainly differ by the treatment of the mass transfer calculation between vapor and liquid phases. In this study different cavitation models have been implemented in a commercial, general-purpose CFD code (Fluent), and numerical results were compared to experimental ones.

Different turbulence models have been also compared, in particularly, to improve numerical simulations by taking into account the influence of the compressibility of two-phase medium on turbulence, a modified k-e RNG model and compared with the standard model. The simulations were carried out on 2D hydrofoil with an Eppler E817 cross section. The inception cavitation number, the cavity length and the pressure coefficients have been compared with the experimental data.

"Cavitating Flow Simulations in Turbopumps", in coll. con M. G. De Giorgi, D. Laforgia, Atti del 58° Congresso Nazionale ATI, Padova-San martino di Castrozza, 9-12 settembre 2003.

“Cavitation Effects and Transient Behavior for the Control Valve of a High-Pressure Diesel Injection System”, M. G. De Giorgi, A. Ficarella, H. Breitbach, SAE Paper 2001-01-1979, International Spring Fuels & Lubricants, Orlando, Florida (USA), 7-9 maggio 2001; SAE 2001 Transactions - Journal of Fuels and Lubricants, vol. 110, pp. 1310-1319, 2001.

“Modeling of Cavitation and of the Related Behaviour of the Control Valve in a Fuel Injection System”, in collaborazione con M. G. De Giorgi, V. Landriscina, P. Barthelet, C. Genco, 55° Congresso Nazionale ATI, Matera (Italia), 15-20 settembre 2000.

“Cavitation Modeling to Understand the Behaviour of ControlSystems”, M. G. De Giorgi, A. Ficarella, D. Laforgia, C. Genco, 5th Biennial Conference On Engineering Systems Design & Analysis ESDA 2000, Montreux, Switzerland, July 10-13, 2000.

“Experimental and Numerical Investigation on Cavitating Flows in Diesel Injection Systems”, in collaborazione con D. Laforgia. Meccanica, vol. 33, pp. 407-425, Aug. 1998.

"Development of an ENO Scheme for Computing Cavitating-liquid Flows", in collaborazione con M. Napolitano, 4th International Symposium on Computational Fluid Dynamic, 1992.

"Cavitation Problems of Diesel Engine Injection Systems", in collaborazione con N. Intini e D. Laforgia, ATA, vol. 45, n. 3, pag. 115-122, marzo 1992. Presentato anche alla International Conference on Mechanics of Two-Phase Flows, Taipei (Taiwan), 12-15 giugno 1989.