Mode decomposition methods for the analysis of cavitating flows in turbomachinery

Abstract The present work is aimed at the characterization of the cavitating flow regimes by applying the coupled POD/DMD technique to the vapor volume fraction field. The proposed approach provided an improved spatio-temporal-frequency description of the flow, based on the detection of the most energetic flow structures with information about their shape and size, and their decomposition into wave patterns oscillating with specific frequency and decay rate. The novel technique was applied to numerical results concerning the bubble cavitation and the supercavitation regimes of 2D water flows around a NACA hydrofoil at ambient temperature. Numerical simulations were performed by using a homogenous mixture model equipped with an extended Schnerr-Sauer cavitation model, in combination with a Volume of Fluid (VOF) interface tracking method. The proposed approached provided a better characterization of the unsteady cavitating flow, and allowed for a deeper insight about the dynamics of the vapor cavity, especially in cases involving the more chaotic regime of supercavitation. In particular, POD results figured out the most energetic coherent vapor structures associated to each cavitation regime: the first mode highlighted the main sheet cavity which grew on the hydrofoil up to detached, the second mode pointed out the cavitating/condensating doublet structures and the third mode figured out the smaller structures owning less energy but a higher frequency content. DMD modes performed a decomposition of the coherent structures detected by means of the POD analysis, into a subset of vapor pattern periodically evolving with a single frequency and a characteristic decay rate. Furthermore, results showed that the supercavitating flow structures owned characteristic frequencies which ranged from 5 to 26 Hz, while the less intensive bubble cavitation regime was characterized by frequencies ranging from 15 to 42 Hz

Implementation and validation of an extended Schnerr-Sauer cavitation model for non-isothermal flows in OpenFOAM

Abstract In the present work cavitation in liquid hydrogen and nitrogen was investigated by using the open source toolbox OpenFOAM. Simulations were performed by means of a mass transfer model, based on the homogeneous mixture approach in combination with the Volume of Fluid (VOF) method for the reconstruction the liquid-vapor interface. Two additional transport equations were considered, i.e. the liquid volume fraction advection and the temperature equation. The implementation of an extended Schnerr- Sauer model allowed for the introduction of the thermal effects in terms of latent heat release/absorption and convective heat transfer inside the liquid-vapor interface. A set of Antoine-like equations relate the saturation conditions to the local conditions

Modeling viscous effects on boundary layer of rarefied gas flows inside micronozzles in the slip regime condition

Abstract The present work provided a numerical investigation of the supersonic flow of rarefied gas into a planar micronozzle characterized by small depth and long divergent section. 2D and 3D computational fluid dynamics (CFD) computations were performed using the continuum Navier-Stokes equations in combination with partial slip conditions at walls, based on a the establishment of the slip regime related to a Knudsen number ranging between 1 x 10-3 and 1 x 10-1. Different partial slip conditions were considered, i.e. the ideal case of pure slip conditions and the full viscous case with Maxwellian slip conditions on sidewalls and planar walls, as well as the case of Maxwellian slip just on sidewalls. The Maxwell slip model was set with a tangential accommodation coefficient equal (TMAC) to 0.8. Comparisons were based on the estimation of the global performance of the micronozzle in terms of thrust force, specific impulse, discharge coefficient and Isp-efficiency. It resulted that when the nozzle depth was neglected, 3D simulations led to the same solution obtained by means of 2D computations inside the micronozzle. The boundary layer thicknesses experienced a linear growth on the sidewalls, and the viscous losses produced a reduction of the performance of about the 95%. Significant differences were found in the prediction of the jet plume, which took the typical bell-shape form in cases involving 2D computations, yet 3D simulations estimated a plume characterized by the succession of oblique shock waves and expansion fan waves. Instead, when the nozzle depth was considered, 3D simulations underlined a completely different behavior of the flow because of the establishment of the nozzle blockage and a viscous heating. The performance suffered an intense degradation of about the 47%, and the analysis of the jet plume highlighted the formation of the Mach disk followed by the typical diamond-shaped subsonic recirculation region

Mems vaporazing liquid microthruster: A comprehensive review

none4The interest in developing efficient nano and pico-satellites has grown in the last 20 years. Secondary propulsion systems capable of serving specific maneuvers are an essential part of these small satellites. In particular, Micro-Electro-Mechanical Systems (MEMS) Vaporizing Liquid Micro-thrusters (VLM), using water as a propellant, represent today a smart choice in terms of simplicity and cost. In this paper, we first propose a review of the international literature focused on MEMS VLM development, reviewing the different geometries and heating solutions proposed in the liter-ature. Then, we focus on a critical aspect of these micro thrusters: the presence of unstable phenom-ena. In particular, the boiling instabilities and reverse channel flow substantially impact the MEMS VLMs’ performance and limit their applicability. Finally, we review the research focused on the passive and active control of the boiling instabilities, based on VLM geometry optimization and active heating strategies, respectively. Today, these ones represent the two principal research axes followed by the scientific community to mitigate the drawbacks linked to the use of MEMS VLMs.openFontanarosa D.; Francioso L.; De Giorgi M.G.; Vetrano M.R.Fontanarosa, D.; Francioso, L.; De Giorgi, M. G.; Vetrano, M. R

Plasma actuation for lifted flame stabilization in coaxial methane-air flow

The flame stabilization represents a relevant issue in aero-engine design. In fact, the growing demand of pollutant emissions reduction without significant losses of the combustion efficiency has driven the efforts of the scientific community towards lean flames. Lean fuel mixtures, characterized by low temperature flames, could manifest an unstable behaviour which can easily lead to the flame extinction due to the establishment of the blowout condition. This requires the implementation of control systems to avoid flame instability occurrence. The present work shows an investigation of the impact of dielectric barrier discharge (DBD) plasma actuation on lifted flame stabilization in a methane CH4-air Bunsen burner at ambient conditions. Two different plasma actuator configurations, powered with a high voltage (HV)/high frequency sinusoidal signal, have been investigated. Once the best actuator configuration was selected, the efficiency of the plasma actuation has been evaluated in terms of the flame lift-off distance, length and shape. In particular, in order to change the actuator power dissipation, different peak-to-peak voltages Vpp were tested, while the actuation frequency was kept equal to 20 kHz. The application of plasma discharges to flame stabilization leads to plasma-attached flames or plasma-enhanced lifted flames, depending on the air and fuel flow rates. At air flow rate of 1.54 g/s, plasma actuation allowed to decrease the lift-off height until the fuel jet velocity was below about 0.05 m/s thanks to the extension of the flame region upstream, toward the burner exit section. Beyond this value, it had no significant impact on the flame lift-off height, even though the amplitude of the lift-off height oscillations reduced coupled with a more stable behaviour of the lifting flame. The benefit of the plasma actuation increased by reducing the air flow rate to 1.35 g/s. In this condition, plasma-assisted flame reattachment was evident at each fuel velocity, in combination with an increasing flame height proportionally to the fuel jet velocity

Numerical investigation of the performance of Contra-Rotating Propellers for a Remotely Piloted Aerial Vehicle

Abstract The present work aims at the numerical prediction of the performance of a Contra-Rotating Propellers (CRP) system for a Remotely Piloted Aerial Vehicles (RPAV). The CRP system was compared with an equivalent counter-rotating propellers configuration which was set by considering two eccentric propellers which were rotating at the same speed. Each contra-rotating test case was built by varying the pitch angle of blades of the rear propeller, while the front propeller preserved the original reconstructed geometry. Several pitch configurations and angular velocities of the rear propeller was simulated. Comparisons showed an improvement of the propulsive efficiency of the contra-rotating configuration in case of larger pitch angles combined with slower angular velocities of the rear propeller

Polychlorinated Biphenyls and Semen Quality in Healthy Young Men Living in a Contaminated Area

Polychlorinated biphenyls (PCBs) are persistent organic pollutants and endocrine disruptors that have been implicated in potential damage to human semen. However, the studies conducted so far provide contrasting results. Our study aimed to investigate the associations between PCB serum and semen levels and semen quality in high school and university students living in a highly PCB-polluted area of Italy. Subjects with a normal body mass index who did not make daily use of tobacco, alcohol, drugs, or medication were selected. All participants provided a fasting blood and a semen sample. Gas chromatography-mass spectrometry was used to determine the concentrations of 26 PCB congeners. The concentrations of PCB functional groups and total PCBs were also computed. A total of 143 subjects (median age 20, range 18–22 years) were enrolled. The median total PCB concentrations were 3.85 ng/mL (range 3.43–4.56 ng/mL) and 0.29 ng/mL (range 0.26–0.32 ng/mL) in serum and semen, respectively. The analysis of the associations between sperm PCB concentration and semen parameters showed (a) negative associations between some PCB congeners, functional groups and total PCBs and sperm total motility; (b) negative associations of total PCBs with sperm normal morphology; and (c) no association of PCBs with sperm concentration. Subjects at the highest quartile of semen total PCB concentration had 19% and 23% mean reductions in total motility and normal morphology, respectively, compared to those at the lowest quartile. The analysis of the associations of serum PCB levels with sperm parameters yielded null or mixed (some positive, other negative) results. In conclusion, the present study provides evidence of a negative effect of some PCB congeners and total PCBs in semen on sperm motility and normal morphology. However, the associations between the concentration of serum and semen PCB congeners and functional groups and sperm quality parameters were inconsistent

Numerical data concerning the performance estimation of a Vaporizing Liquid Microthruster

The data presented in this data article were on the basis of the study reported in the research articles entitled "A novel quasi-one-dimensional model for performance estimation of a Vaporizing Liquid Microthruster" (De Giorgi and Fontanarosa, 2018). The reference study presented a numerical analysis of the performance of the Vaporizing Liquid Microthruster (VLM) experimentally investigated in the data article entitled "Performance evaluation and flow visualization of a MEMS based Vaporizing Liquid Micro thruster" (Cen and Xu, 2010). For the purpose, a novel quasi onedimensional model was proposed, and results were compared with the numerical predictions provided by 2D and 3D CFD computations. Due to the scarcity of experimental data concerning the flow characterization inside a Vaporizing Liquid Microthruster, the present Data in Brief aims to provide the entire dataset coming from the numerical predictions for benchmark purposes and comparisons with different numerical approach

Comparison of numerical predictions of the supersonic expansion inside micronozzles of micro–resistojets

The present work provides a numerical investigation of the supersonic flow inside a planar micronozzle configuration under different gas rarefaction conditions. Two different propellants have been considered, namely water vapor and nitrogen, which relate to their use in VLMs (the former) and cold gas microthrusters (the latter), respectively. Furthermore, two different numerical approaches have been used due to the different gas rarefaction regime, i.e. the typical continuum Navier–Stokes with partial slip assumption at walls and the particle–based Direct Simulation Monte Carlo (DSMC) technique. As a result, under high–pressure operating conditions, both water and nitrogen flows supersonically expanded into the micronozzle without chocking in combination with a linear growth of the boundary layer on walls. However, when low–pressure operating condition are imposed and a molecular regime is established inside the micronozzle, a very rapid expansion occurred close to the nozzle exit in combination with a strong chocking of the flow and a micronozzle quality reduction of about 40%. Furthermore, water exhibited specific higher specific impulse than nitrogen above 60%

Characterization of cavitating flow regimes in an internal sharp-edged orifice by means of Proper Orthogonal Decomposition

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