Search Results (29)

This study systematically investigates the evolution of the flow field and cavitation behavior during the underwater launch of a rotating sphere. By comparing surface pressure distribution, cavitation evolution, flow separation locations, and re-entrant jet formation under various rotational conditions, this study reveals the significant influence of rotation on both the cavitation processes and sphere’s motion trajectory. It is found that under rotational conditions, cavity detachment tends to occur earlier on the front side, and the re-entrant jet develops more fully, reaching maximum length and intensity at a moderate angular velocity. In additionally, rotation alters the cavity interface and overall flow structure, resulting in noticeable differences in surface wetting, pressure distribution, and separation behavior between the front and rear sides. As the rotational speed increases, flow separation points become less distinct, and pressure fluctuations on the rear side intensify, indicating that rotation plays a critical role in modulating underwater cavitation dynamics. The findings provide theoretical insights into flow control and cavitation risk assessment for underwater launches of rotating bodies. Full article

Hydrodynamic cavitation usually occurs in marine and ocean engineering and hydraulic systems and may lead to destructive effects such as an enhanced drag force, noise, vibration, surface damage, and reduced efficiency. Previous studies employed several passive and active control strategies to manage unstable cavitation and its adverse effects. This study reviews various passive and active control strategies for managing diverse cavitation stages, such as partial, cloud, and tip vortex. Regarding the passive methods, different control factors, including the sweep angle of the foil, roughness, bio-inspired riblets, V-shaped grooves, J grooves, obstacles, surface roughness, blunt trailing edge, slits, various vortex generators, and triangular slots, are discussed. Regarding the active methods, various injection methods including air, water, polymer, and synthetic jet and piezoelectric actuators are reviewed. It can be concluded that unstable cavitation can be controlled by both the active and passive approaches independently. However, in the severe conditions of cavitation and higher angles of attack, the passive control methods can only alleviate some re-entrant jets propagating in the downward direction, and proper control of the cavity structure cannot be achieved. In addition, active control methods mostly require supplementary energy and, consequently, lead to higher expenses. Combined passive active control technologies are suggested by the author, using the strengths of both methods to suppress cavitation and control the cavitation instability for a broad range of cavitating flows efficiently in future works. Full article

As an efficient drag reduction technique, ventilated cavity technology demonstrates significant application in underwater launch systems. This study employs numerical simulations to systematically examine the ventilated cavity flow characteristics and cavity–rudder interaction mechanisms for a rudder-equipped axisymmetric body. Numerical simulation predicts the gas leakage behavior, cavity geometry, and internal flow structure. The results indicate that the development of the ventilated cavity proceeds through three distinct stages: rapid growth, slow development, and quasi-periodic shedding. During this process, local high pressure at the leading edge of the rudder suppresses cavity growth, while cavity shedding is associated with re-entrant jet effects. Under the influence of the ventilated cavity, the overall load on the entire body and the local load on the rudder exhibit consistent patterns: F_x > F_y > F_z ≈ 0 and T_z > T_x ≈ T_y ≈ 0, with F_y and T_z fluctuating the most violently. The shedding cavity clusters are primarily concentrated at the rudder root during the quasi-periodic shedding stage. Full article

Accurate prediction of cavitating flows is essential for improving the performance and durability of marine and hydrodynamic systems. This study investigates the influence of different cavitation models—Kunz, Merkle, and Schnerr–Sauer—on the numerical prediction of cavitation around a hemispherical head-form body using computational fluid dynamics (CFD). Additionally, the effects of turbulence modeling approaches, including Reynolds-averaged Navier–Stokes (RANS) and partially averaged Navier–Stokes (PANS), are examined to assess their capability in capturing transient cavitation structures and turbulence interactions. The results indicate that the Schnerr–Sauer model, which incorporates bubble dynamics based on the Rayleigh–Plesset equation, provides the most accurate prediction of cavitation structures, closely aligning with experimental data. The Merkle model shows intermediate accuracy, while the Kunz model tends to overpredict cavity closure, limiting its ability to capture unsteady cavitation dynamics. Furthermore, the PANS turbulence model demonstrates superior performance over RANS by resolving more transient cavitation phenomena, such as cavity shedding and re-entrant jets, leading to improved accuracy in pressure distribution and vapor volume fraction predictions. The combination of the PANS turbulence model with the Schnerr–Sauer cavitation model yields the most consistent results with experimental observations, highlighting its effectiveness in modeling highly dynamic cavitating flows. Full article

Focused on the unsteady property of a cavitating water jet issuing from an orifice nozzle in a submerged condition, this paper presents a fundamental investigation of the periodicity of cloud shedding and the mechanism of cavitation cloud formation and release by combining the use of high-speed camera observation and flow simulation methods. The pattern of cavitation cloud shedding is evaluated by analyzing sequence images from a high-speed camera, and the mechanism of cloud formation and release is further examined by comparing the results of flow visualization and numerical simulation. It is revealed that one pair of ring-like clouds consisting of a leading cloud and a subsequent cloud is successively shed downstream, and this process is periodically repeated. The leading cloud is principally split by a shear vortex flow along the nozzle exit wall, and the subsequent cloud is detached by a re-entrant jet generated while a fully extended cavity breaks off. The subsequent cavitation cloud catches the leading one, and they coalesce over the range of $x/d\approx 1.8~2.5$. Cavitation clouds shed downstream from the nozzle at two dominant frequencies. The Strouhal number of the leading cavitation cloud shedding varies from 0.21 to 0.29, corresponding to the injection pressure. The mass flow rate coefficient fluctuates within the range of $0.59~0.66$ at the same frequency as the leading cloud shedding under the effect of cavitation. Full article

Recognizing the urgent need for mitigating global warming, natural ventilation presents a potential strategy to reduce cooling energy demands, enhance thermal comfort, and contribute to indoor air quality. H-shaped buildings are prevalent worldwide, and they constitute the majority of the social housing construction in Brazil. Research suggests that the inadequate design of these buildings can result in poor ventilation; however, investigations about their natural ventilation performance are limited. Thus, the present contribution aims to determine the impact of the geometric characteristics of H-shaped buildings on the pressure distribution through wind tunnel experiments. Three models were tested in the wind tunnel experiments, representing different proportions. Their scales were configured to comply with the 5% obstruction limit allowed for wind tunnel testing, which was performed for 20 wind attack angles. Moreover, a scour test was carried out to allow a better understanding of the wind flow. Python scripting was developed to automate data processing, which is openly available in this paper. The results indicate that the proportion of the model influences the pressure distribution on roofs and leeward walls. Additionally, the depth of the recessed cavity affects its side surfaces and can result in a mirrored behavior on the frontal face of deep cavities (i.e., the wind direction is 45°). The model height influences the windward surfaces in its lower portion, since taller models present a recirculation vortex that modifies the pressure near the ground. Full article

This research investigates cavitation around a marine propeller, employing computational fluid dynamic (CFD) solvers, including an incompressible, isothermal compressible, and fully compressible flow. The investigation commenced with simulations utilizing an incompressible flow solver, subsequently extending to the two compressible flow solvers. In the compressible flow, there is a close interrelation between density, pressure, and temperature, which significantly influences cavitation dynamics. To verify computational methods, verification tests were conducted for leading-edge cavitating flows over a two-dimensional (2D)-modified NACA66 hydrofoil section at various cavitation numbers. The computational results were validated against the experimental data, with the solvers’ capability to predict cavitation forming the basis for comparison. The results demonstrate consistent predictions among the solvers; however, the fully compressible flow solver demonstrated a superior performance in capturing re-entrant jets and accurately modeling cavity closure regions. Furthermore, the fully compressible flow solver precisely estimated propeller hydrodynamic performance, yielding results closely aligned with experimental observations. Full article

We perform large-eddy simulations to study a cavitating flow over a two-dimensional hydrofoil section—a scaled-down profile (1:13.26) of guide vanes of a Francis turbine—using the Schnerr–Sauer cavitation model with an adaptive mesh refinement in intensive phase transition flow areas. In the test case, the guide vane is tilted at an angle of attack of $9^{°}$ to the direction of the flow, in which the Reynolds number, based on the hydrofoil chord length, equals $1.32\times {10}^6$, thus providing a strong adverse pressure gradient along the surface. The calculated time-averaged turbulence characteristics are compared with those measured by particle image velocimetry to verify that the flow is correctly reproduced in numerical simulations using the procedure of conditional averaging proposed and tested in our previous investigation. A re-entrant jet is identified as the primary source of vapor cloud shedding, and a spectral analysis of the cavitating flow over the profile midsection is conducted. Two characteristic frequencies corresponding to the cases, when an attached cavity detaches completely (as a whole) and two partially from the hydrofoil, are found in the flow. The study reveals that the natural frequency of partial cavity shedding is three times higher than that of full detachments. The examined regime exhibits an oscillatory system with two oscillation zones related to cavitation surge instability and unsteady cloud cavitation resulting from the re-entrant jet. Conditional averaging correlates cavitation structures with pressure distributions, forces, and torque on the guide vane. This modeling approach captures the fine details of quasi-periodic cavitation dynamics, providing insights into unsteady sheet/cloud cavitation and offering a method for developing control strategies. Full article

The present study investigates different design strategies to produce non-wettable micropatterned surfaces. In addition to the classical method of measuring the contact angle, the non-wettability is also discussed by means of the immersion test. Inspired by non-wettable structures found in nature, the effects of features such as reentrant cavities, micropillars, and overhanging layers are studied. We show that a densely populated array of small diameter cavities exhibits superior non-wettability, with 65% of the cavities remaining intact after 24 h of full immersion in water. In addition, it is suggested that the wetting transition time is influenced by the length of the overhanging layer as well as by the number of columns within the cavity. Our findings indicate a non-wetting performance that is three times longer than previously reported in the literature for a small, densely populated design with cavities as small as 10 μm in diameter. Such properties are particularly beneficial for neural implants as they may reduce the interface between the body fluid and the solid state, thereby minimiing the inflammatory response following implantation injury. In order to assess the effectiveness of this approach in reducing the immune response induced by neural implants, further in vitro and in vivo studies will be essential. Full article

The V-band frequencies are becoming popular due to their application potential towards secure high data rate communications. This article reports bandwidth enhancement of an 11-cavity V-band Klystron amplifier employing staggered tuning. A systematic approach is presented to stagger-tune the periodically allocated multiple cavities of the Klystron operating at 60.1 GHz. Using the three-dimensional particle-in-cell (PIC) simulation, it is shown that, employing the proposed approach, the −3 dB bandwidth of the device (with peak tuned configuration) has been increased from 165 MHz to 540 MHz, demonstrating a 260% increment. The −1 dB bandwidth of the device is estimated to be 270 MHz. The proposed approach of stagger tuning may be employed for similar devices employing multiple RF cavities to meet the requirement of wide bandwidth. Full article

Fabric permittivity is critical for the manufacturing of wearable sensors and antennas as well as predicting how fabrics interact with electromagnetic fields. Engineers should also understand how permittivity changes under different temperatures, densities, and moisture content values, or when several fabrics are mixed in aggregates, when designing future applications such as microwave dryers. The permittivity of cotton, polyester, and polyamide fabric aggregates is investigated in this paper for a wide range of compositions, moisture content levels, density values, and temperature conditions around the 2.45 GHz ISM band using a bi-reentrant resonant cavity. The obtained results show extremely comparable responses for all characteristics investigated for single and binary fabric aggregates. Permittivity always increases as temperature, density, or moisture content levels rise. Moisture content is the most influential characteristic, causing enormous variations in the permittivity of aggregates. Fitting equations are supplied for all data, with exponential functions used to accurately model variation in temperature and polynomial functions employed to precisely model density and moisture content variations with low error levels. The temperature permittivity dependence of single fabrics without the influence of air gaps is also extracted from fabric and air aggregates by using complex refractive index equations for two-phase mixtures. Full article

Herein, we describe a 54-year-old patient with a congenital ventricular diverticulum (CVD), referred to our emergency department for presyncope episodes and multiple re-entrant ventricular tachycardias (VT). Significantly, echocardiographic findings were not clear, and the diagnosis was made by cardiac magnetic resonance imaging (CMRI), which showed the presence of an apical accessory cavity connected to the ventricle and contracting synchronously. CMRI allowed the differential diagnosis with other outpouching cardiac defects. The patient underwent a subcutaneous implantable cardioverter defibrillator (S-ICD) implant and was referred for heart transplantation (HT). The diagnosis, treatment, and main findings of the CVD are discussed in this case report. Full article

Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect. Full article

V-band Klystrons find applications in satellite communications and 5G technology. Here, we present a V-band Klystron using radial re-entrant square cavities (RRSCs). The RRSCs are easy to fabricate, assemble, align, and tune to the operating frequency, which are significant concerns in V-band. We have optimized the number of cavities and the gap for best possible gain and bandwidth. The eigenmode and particle-in-cell (PIC) simulation results of the CST microwave studio are presented. The optimum design uses eleven equidistantly placed RRSCs, producing the maximum gain of 27.17 dB at 60.1 GHz with a bandwidth of ∼150 MHz. Full article

The current study is focused on improving the thermal performance of the microchannel heat sink (MCHS) using the passive reentrant cavity approach. The MCHS physical model’s single channel was used in a three-dimensional numerical simulation. The basic geometrical layout of the MCHS’s computational domain was drawn from previously published research and verified using numerical and analytical correlations that were already in existence. The innovative conical-shaped microchannel heat sink’s (CMCHS) properties for heat transmission and fluid flow were examined numerically under steady-state conditions with laminar flow and a constant heat flux. At various flow velocities and configurations, the impacts of the geometrical parameters on pressure drops and heat transfer were examined. The outcome demonstrates a tremendously positive thermal performance with a significantly greater pressure drop than the traditional straight channel. In the microchannels with the conical-shaped reentrant cavities and minimal pressure loss, convection heat transfer is significantly improved. The findings of the present investigation demonstrate that the conical-shaped MCHS is practical and has a good chance of being used in real-world settings. Full article