Cavitation refers to the formation of vapor cavities in a liquid when the local pressure becomes lower than the saturation pressure. In many hydraulic applications, cavitation is considered as a ...non-desirable phenomenon, as far as it may cause performance degradation, vibration problems, enhance broad-band noise-emission, and eventually trigger erosion. In this Special Issue, recent findings about cavitation instabilities are reported. More precisely, the dynamics of cavitation sheets are explored at very low Reynolds numbers in laminar flows, and in microscale applications. Both experimental and numerical approach are used. For the latter, original methods are assessed, such as smooth particles hydrodynamics or detached eddy simulations coupled to a compressible approach.
•Cavitation dynamic behaviors of the hydrofoil with whalelike leading-edge protuberances are simulated.•The k-ω SST model with local compressibility correction for turbulent viscosity is implemented ...in OpenFOAM.•Interactions between cavitation and streamwise vortex pairs are discussed.•Influence factors of monopole and dipole noise in cavitating flow are obtained from Lighthill's acoustic analogy theory.•Time domain characteristics of monopole and dipole noise in cavitating flow are discussed.
In this paper, the open source software OpenFOAM is used to perform a numerical investigation of the cavitating flow around a modified NACA634-021 hydrofoil with bioinspired, wavy leading edge, with particular emphasis on study of the interactions between the cavitation and the streamwise vortices and the far-field radiation noise. A modified k-ω shear-stress transport (SST) model coupled with the Schnerr-Sauer cavitation model and the Ffowcs Williams-Hawkings (FW-H) acoustic analogy approach are introduced to the simulation. The transient cavitation structure and the streamwise vortices are captured well and the results show significant interactions between the cavitation and the streamwise vortices. Cavitation can promote fragmentation of the streamwise vortices, while the streamwise vortices cause cavitation inception to occur earlier and bind the cavitation within the trough region by changing the pressure distribution on the hydrofoil. The transformation of the FW-H equation's solution indicates that the monopole noise is directly related to the cavitation volume acceleration and the dipole noise is related to the mechanical force of the hydrofoil on fluids and the rate at which this force changes. The collapse of cavitation cloud and the collision of the re-entrant jet and main flow will cause violent fluctuation of the mechanical force thus produce instantaneous extreme dipole noise values, while the monopole noise is relatively strong in the cavitation collapse stage due to significant cavitation volume acceleration. The time domain hydroacoustic characteristics are similar for the modified hydrofoil and the baseline hydrofoil.
The modelling of internal and external flow phenomena of high pressure injection systems has developed significantly in the last few decades. The challenge currently however, is to model these areas ...of flow together accounting for different multi-phase phenomena that require a wide range of resolutions. More specifically, the highly turbulent nature of internal nozzle cavitation requires a high grid and temporal resolution, whereas the external nozzle atomisation processes exists over a comparatively much larger space with lower time and space resolution needed. The research described in this thesis is focused on coupling the internal nozzle and external processes using a single Eulerian Volume Of Fluid (VOF) multiphase model. First, investigations into the dynamics of internal nozzle cavitation is presented, through simulation of two phase nozzle flows without spray formation demonstrating sensitivities to discretization techniques and boundary conditions. Then, the simulation of internal nozzle cavitation with spray formation using a single model was achieved by the construction of a three phase VOF model with cavitation which is described. A non-condensable gaseous phase is considered alongside the liquid and vapours phases, the liquid interface is sharp with a diffusive interface between gaseous phases. Comparisons were made with both experimental data and previous numerical investigations. Finally, a new solver with the introduction of the Eulerian-Lagrangian Spray Atomization (ELSA) framework with the Interface Capturing Method (ICM) for surface density to the system to describe the liquid structures below the Sub-Grid Scale (SGS) is presented. Thus quantities such as droplet Sauter Mean Diameter (SMD) and droplet spray angle at these scales can be extracted, with comparisons made with experimental data. The coupling of the ELSA-ICM model with the three phase cavitating model allows for processes of the entire spray formation to be resolved. More specifically, for the first time, the evolving surfaces of the entire injection process, from internal nozzle cavitation to spray atomisation, can thus be tracked throughout even at the SGS. This allows for a direct insight into the interaction between the cavitation and atomisation processes.
•Big inconsistencies in studies of utilization of cavitation exist.•Studies are many times not repeatable.•Cavitation conditions cannot be described by the σ value.•Results show obvious issues of ...using σ value.•Effort should be put into description of methods.
Within the last years there has been a substantial increase in reports of utilization of hydrodynamic cavitation in various applications. It has came to our attention that many times the results are poorly repeatable with the main reason being that the researchers put significant emphasis on the value of the cavitation number when describing the conditions at which their device operates.
In the present paper we firstly point to the fact that the cavitation number cannot be used as a single parameter that gives the cavitation condition and that large inconsistencies in the reports exist. Then we show experiments where the influences of the geometry, the flow velocity, the medium temperature and quality on the size, dynamics and aggressiveness of cavitation were assessed. Finally we show that there are significant inconsistencies in the definition of the cavitation number itself.
In conclusions we propose a number of parameters, which should accompany any report on the utilization of hydrodynamic cavitation, to make it repeatable and to enable faster progress of science and technology development.
Hydrodynamic cavitation is considered to be a promising technology for process intensification, due to its high energy efficiency, cost-effective operation, ability to induce chemical reactions, and ...scale-up possibilities. In the past decade, advancements have been made in the fundamental understanding of hydrodynamic cavitation and its main variables, which provide a basis for applications of hydrodynamic cavitation in radical-induced chemical reaction processes. Here, we provide an extensive review of these research efforts, including the fundamentals of hydrodynamic cavitation, the design of cavitation reactors, cavitation-induced reaction enhancement, and relevant industrial applications. Two types of hydrodynamic cavitation reactors—namely, stationary and rotational—are compared. The design parameters of a hydrodynamic cavitation reactor and reactor performance at the laboratory and pilot scales are discussed, and recommendations are made regarding optimal operation and geometric conditions. The commercial cavitation reactors that are currently on the market are reviewed here for the first time. The unique features of hydrodynamic cavitation have been widely applied to various chemical reactions, such as oxidization reactions and wastewater treatment, and to physical processes, such as emulsion generation and component extraction. The roles of radicals and gas bubble implosion are also thoroughly discussed.
The combined application of electric fields and ultrasonic waves has shown promise in controlling cell membrane permeability, potentially resulting in synergistic effects that can be explored in the ...biotechnology industry. However, further clarification on how these processes interact is still needed. The objective of the present study was to investigate the atomic-scale effects of these processes on a DPPC lipid bilayer using molecular dynamics simulations. For higher electric fields, capable of independently forming pores, the application of an ultrasonic wave in the absence of cavitation yielded no additional effects on pore formation. However, for lower electric fields, the reduction in bilayer thickness induced by the shock wave catalyzed the electroporation process, effectively shortening the mean path that water molecules must traverse to form pores. When cavitation was considered, synergistic effects were evident only if the wave alone was able to generate pores through the formation of a water nanojet. In these cases, sonoporation acted as a mean to focus the electroporation effects on the initial pore formed by the nanojet. This study contributes to a better understanding of the synergy between electric fields and ultrasonic waves and to an optimal selection of processing parameters in practical applications of these processes.
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•Computational analysis of electric fields and ultrasound sinergy in lipid membranes.•Ultrasonic waves had no effect on pore formation under high electric fields.•The waves thinned the bilayer, catalyzing electroporation in lower electric fields.•Cavitation enhances electroporation by directing its effects with a water nanojet.
Cavitation-enhanced delivery of therapeutic agents is under development for the treatment of cancer and neurodegenerative and cardiovascular diseases, including sonothrombolysis for deep vein ...thrombosis. The objective of this study was to quantify the spatial and temporal distribution of cavitation activity nucleated by Definity infused through the EKOS catheter over a range of acoustic parameters controlled by the EKOS endovascular system.
Three insonation protocols were compared in an in vitro phantom mimicking venous flow to measure the effect of peak rarefactional pressure, pulse duration and pulse repetition frequency on cavitation activity energy, location and duration. Inertial and stable cavitation activity was quantified using passive cavitation imaging, and a metric of cavitation dose based on energy density was defined.
For all three insonation protocols, cavitation was sustained for the entire 30 min Definity infusion. The evolution of cavitation energy during each pulse duration was similar for all three protocols. For insonation protocols with higher peak rarefactional acoustic pressures, inertial and stable cavitation doses also increased. A complex relationship between the temporal behavior of cavitation energy within each pulse and the pulse repetition frequency affected the cavitation dose for the three insonation protocols. The relative predominance of stable or inertial cavitation dose varied according to insonation schemes. Passive cavitation images revealed the spatial distribution of cavitation activity.
Our cavitation dose metric based on energy density enabled the impact of different acoustic parameters on cavitation activity to be measured. Depending on the type of cavitation to be promoted or suppressed, particular pulsing schemes could be employed in future studies, for example, to correlate cavitation dose with sonothrombolytic efficacy.
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