Cavitation and Bubble Dynamics: Preface

Wind energy is one of the most promising renewable energy resources for power generation, and rapid growth has been seen in its acceptance since 2000. The most acceptable classification for wind turbines is by its axis of orientation: Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT). HAWTs are used in many countries for medium-to-large scale power projects, and most commercial installations around the globe are solely based on these turbines. On the other hand, HAWTs are not recognized as a viable option to harness the energy of the wind in urban areas, where the wind is less intense, much more chaotic and turbulent. VAWTs are suggested as a better choice for cities and isolated semi-urban areas. Several attributes have been suggested for the large-scale deployment of VAWTs, e.g., good performance under the weak and unstable wind, no noise and safety concerns, and aesthetically sound for integration in urban areas. Significant research has been published on wind turbine technology and resources assessment methodologies, and this review paper is a modest attempt to highlight some of the major developments of VAWTs, with a focus on the integration with urban infrastructure. Several recommendations have been drawn based on the state-of-the-art information on the subject for future studies and acceptance of wind turbines in the urban areas. It was concluded that further research is critical in making VAWTs a viable, dependable, and affordable power generation technology for many low and decentralized power applications.

A critical review of vertical axis wind turbines for urban applications

The refrigerator is an essential domestic appliance product and is available worldwide. Additionally, the technologies on which the majority of refrigerators are based involve high energy consumption, and environmental deterioration due to the type of energy input and the use of certain work fluids. Therefore, how refrigerators work is of great interest, and in recent years the development of diverse research in this field has intensified. As a consequence, in this article we present an exhaustive review of the research with the greatest impact on domestic refrigeration, based on vapor compression such as modeled, thermal stratification, control, environmentally innocuous refrigerants, thermal isolation, and hybrid systems, among others. Based on the above, we present the principal trends in this area, of interest to both industry and researchers.

Enhancements in domestic refrigeration, approaching a sustainable refrigerator – A review

A review on wind turbine noise mechanism and de-noising techniques

The building block of all economies across the world is subject to the medium in which energy is harnessed. Renewable energy is currently one of the recommended substitutes for fossil fuels due to its environmentally friendly nature. Wind energy, which is considered as one of the promising renewable energy forms, has gained lots of attention in the last few decades due to its sustainability as well as viability. This review presents a detailed investigation into this technology as well as factors impeding its commercialization. General selection guidelines for the available wind turbine technologies are presented. Prospects of various components associated with wind energy conversion systems are thoroughly discussed with their limitations equally captured in this report. The need for further optimization techniques in terms of design and materials used for the development of each component is highlighted.

/pdf/selection-guidelines-for-wind-energy-technologies-2flrrbn7cv.pdf

Selection guidelines for wind energy technologies

In this study, the aerodynamic noise characteristics of Savonius wind turbines were investigated using hybrid computational aero-acoustics techniques, and low-noise designs were proposed based on the understanding of the noise generation mechanism. First, the flow field around the turbine was analyzed in detail by solving three-dimensional unsteady incompressible Reynolds-averaged Navier–Stokes equations using computational fluid dynamics techniques. Then, the aerodynamic noise radiating from the wind turbine was predicted using the Ffowcs Williams and Hawkings equation with the obtained flow field information. Two distinct harmonic noise components—the blade passing frequency (BPF) and harmonics with a fundamental frequency that is much higher than the BPF—were identified in the predicted noise spectrum. The origin of the higher harmonic components was found to be related to vortex shedding from the rotating turbine. Based on this finding, the proposed low-noise design for Savonius wind turbines uses S-shaped blades. S-shaped blades were found to reduce the noise levels of Savonius wind turbines by up to 2.7 dB.

Development of low noise drag-type vertical wind turbines

Development of low-noise drag-type vertical wind turbines

In this study, cavitation flow of hydrofoils is numerically investigated to characterize the effects of turbulence models on cavitation-flow patterns and the corresponding radiated sound waves. The two distinct flow conditions are considered by varying the mean flow velocity and angle of attack, which are categorized under the experimentally observed unstable or stable cavitation flows. To consider the phase interchanges between the vapor and the liquid, the flow fields around the hydrofoil are analyzed by solving the unsteady compressible Reynolds-averaged Navier–Stokes equations coupled with a mass-transfer model, also referred to as the cavitation model. In the numerical solver, a preconditioning algorithm with dual-time stepping techniques is employed in generalized curvilinear coordinates. The following three types of turbulence models are employed: the laminar-flow model, standard k − e turbulent model, and filter-based model. Hydro-acoustic field formed by the cavitation flow of the hydrofoil is pr...

Numerical investigation on cavitation flow of hydrofoil and its flow noise with emphasis on turbulence models

In this study, cavitation flow around a hydrofoil and its radiated hydro-acoustic fields were numerically investigated, with an emphasis on the effects of viscous flux vectors. The full three-dimensional unsteady compressible Reynolds-averaged Navier–Stokes equations were numerically solved. The mass transfer model was adopted to model phase changes between liquid water and vapor. To resolve the numerical instability problem arising from the rapid changes in local density and speed of sound of the mixture, the preconditioning and dual-time stepping methods were employed. The filter-based turbulent model was applied to resolve the unstable cavitation flow field more accurately. In splitting the viscous terms, three approaches for dealing with viscous flux vectors were considered: the so-called viscous lagging, full viscous, and thin-layer models. Radiated hydro-acoustic waves were predicted by applying the Ffowcs Williams and Hawkings equations. The effects of the viscous flux vectors on the predicted flow fields and its radiated noise were then examined by comparing the hydro-dynamic forces, velocity distribution, volume fraction, far-field sound directivities, and sound spectrum of the three approaches. The results revealed that the thin-layer model can provide predictions as accurate as the full viscous model, but required less computational time.

Numerical Investigation into Effects of Viscous Flux Vectors on Hydrofoil Cavitation Flow and Its Radiated Flow Noise

A suction muffler is frequently used to control noise generated by impulsive pressure fluctuation induced by the piston/valve in a reciprocating compressor However, the suction muffler adversely affects the compressor performance because it causes an additional pressure drop and heat transfer In this study, with the aim of considering the complex interaction between acoustic waves and flow, both the flow performance and the acoustic performance of a suction muffler are simultaneously investigated by solving three-dimensional compressible Reynolds-averaged Navier–Stokes equations with high-order computational aeroacoustics schemes To confirm the validity of the proposed numerical scheme for the simulation of acoustic waves, the transmission loss of a simple expansion muffler is predicted and compared with theoretical and experimental results Then, performances of the target suction muffler are predicted and analyzed On the basis of the analysis results, a new muffler is designed and its performances are predicted and compared with those of the original one Finally, validation experiments are performed, whose results confirm improvements in the flow and acoustic performances of the new muffler design

Sanghyeon Kim

Papers

Development of low noise drag-type vertical wind turbines

Development of low-noise drag-type vertical wind turbines

Numerical investigation on cavitation flow of hydrofoil and its flow noise with emphasis on turbulence models

Numerical Investigation into Effects of Viscous Flux Vectors on Hydrofoil Cavitation Flow and Its Radiated Flow Noise

Investigation of flow and acoustic performances of suction mufflers in hermetic reciprocating compressor