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

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

Depression in Heart Failure with Reduced Ejection Fraction, an Undervalued Comorbidity: An Up-To-Date Review

Introduction: Depression is a common and severe comorbidity among individuals with heart failure (HF). Up to a third of all HF patients are depressed, and an even higher proportion have symptoms of depression. Aim: In this review, we evaluate the relationship between HF and depression, explain the pathophysiology and epidemiology of both diseases and their relationship, and highlight novel diagnostic and therapeutic options for HF patients with depression. Materials and Methods: This narrative review involved keyword searches of PubMed and Web of Science. Review search terms included ["Depression" OR "Depres*" OR "major depr*"] AND ["Heart Failure" OR "HF" OR "HFrEF" OR "HFmrEF" OR "HFpEF" OR "HFimpEF"] in all fields. Studies included in the review met the following criteria: (A) published in a peer-reviewed journal; (B) described the impact of depression on HF and vice versa; and (C) were opinion papers, guidelines, case studies, descriptive studies, randomized control trials, prospective studies, retrospective studies, narrative reviews, and systematic reviews. Results: Depression is an emergent HF risk factor and strongly relates with worse clinical outcomes. HF and depression share multiple pathways, including platelet dis-reactivity, neuroendocrine malfunction, inappropriate inflammation, tachi-arrhythmias, and frailty in the social and community setting. Existing HF guidelines urge evaluation of depression in all HF patients, and numerous screening tools are available. Depression is ultimately diagnosed based on DSM-5 criteria. There are both non-pharmaceutical and pharmaceutical treatments for depression. Regarding depressed symptoms, non-pharmaceutical treatments, such as cognitive-behavioral therapy and physical exercise, have shown therapeutic results, under medical supervision and with an effort level adapted to the patient's physical resources, together with optimal HF treatment. In randomized clinical studies, selective serotonin reuptake inhibitors, the backbone of antidepressant treatment, did not demonstrate advantage over the placebo in patients with HF. New antidepressant medications are currently being studied and could provide a chance to enhance management, treatment, and control of depression in patients with HF. Conclusions: Despite the substantial link between depression and HF, their combination is underdiagnosed and undertreated. Considering the hopeful yet unclear findings of antidepressant trials, further research is required to identify people who may benefit from antidepressant medication. The goal of future research should be a complete approach to the care of these patients, who are anticipated to become a significant medical burden in the future

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

Active Control of Unsteady Cavitating Flows Over Hydrofoil

A preliminary two-dimensional (2D) numerical investigation of the active control of unsteady cavitation by means of one single synthetic jet actuator (SJA) is presented. The investigation involves the cloud-cavitating flow of water around a NACA 0015 hydrofoil with an angle of attack of 8-deg and ambient conditions. The SJA locates on the suction side at a distance of 16% of the chord from the leading edge; it has been modeled by means of a user-defined velocity boundary conditions based on a sinusoidal waveform. A Eulerian homogeneous mixture model has been used, coupled with an extended Schnerr–Sauer cavitation model and a volume of fluid interface tracking method. As first, a sensitivity analysis allowed to evaluate the influence of the main control parameters, namely, the momentum coefficient Cμ⁠, the dimensionless frequency F+, and the jet angle αjet. As a result, the best performing SJA configuration was retrieved at Cμ=0.0002, F+=0.309⁠, and αjet=90 deg⁠, which led to a reduction of both the average vapor content and the average torsional load in the measure of 34.6% and 17.8%. The analysis of the coupled dynamics between vapor cavity–vorticity and their proper orthogonal decomposition (POD)-based modal structures highlighted the benefit of the SJA lies in preventing the growth of a thick sheet cavity, which causes the development of the highly cavitating cloud dynamics after the cavity breakup. This is mainly due to an additional vorticity close to the hydrofoil surface just downstream the SJA, as well as a local pressure modification close the SJA during the blowing stroke

Myeloid sarcoma of the rib: An atypical isolated chest finding

Myeloid sarcoma is a rare extramedullary myeloid tumor typically occurring in association with acute myeloid leukemia. We report an atypical case of myeloid sarcoma arising from the rib of a healthy young man with no specific blood test abnormalities. Once the malignant nature of the tumor was confirmed, a complete surgical excision was performed and definitive diagnosis was achieved by way of an exhaustive histopathological examination of surgical specimens. Systemic treatment was administered and currently neither systemic nor local relapse has been identified. Our experience suggests surgical resection could be a valid treatment in isolated myeloid sarcoma patients