Search

Elastic metamaterials with double negativity can manipulate the propagation of elastic waves at sub-wavelength scales by inducing multiple resonances to achieve different negative effective parameters. Numerous efforts have been made to control the dynamic behavior by directly tuning the frequency range of double negativity in elastic metamaterials. This study examines the impact of tensile loads on double negativity in relation to ligament inclination angle. The findings will aid in the direct tunability of double negativity in single-phase chiral metamaterials. The study examines a simple single-phase four-ligament chiral unit with low-order double negativity. The presence of double negativity was verified through band structure analysis and calculation of the four effective dynamic parameters. Additionally, the effects of two geometrical factors on the frequency ranges of negative parameters were investigated through parametric scanning. The results indicate that the frequency range of double negativity reaches its maximum at a ligament inclination angle of around 45° and disappears as the angle approaches 65°. Given that the ligament inclination angle of the unit cells can be easily altered by external tension, this intriguing outcome is leveraged to achieve the emergence and vanishing of double negativity. This characteristic is confirmed through the examination of negative refraction phenomena via simulation examples. [ABSTRACT FROM AUTHOR]

Pressure fluctuations in the draft tube mainly arise from the rotational motion of the vortex rope. The formation and development of the vortex rope are significantly influenced by cavitation. However, the precise mechanism underlying the progression of cavitation from its initial weak state to a more severe level remains unclear. The objective of this study is to investigate cavitation in a pump-turbine operating under part load turbine conditions. Numerical simulations were conducted to analyze the impact of the cavitation coefficient on various parameters, including vapor volume, efficiency, and head. With increasing degree of cavitation, the vortex rope's pattern in the draft tube undergoes a transition from a helical type to a torch-like type. This transition consequently leads to a shift in the dominant frequency of pressure fluctuations within the draft tube. Furthermore, the helical vortex rope exhibits a higher rotational speed compared to the torch-like vortex rope, thereby causing a more pronounced effect on the pressure field. The factors contributing to the morphological transition of the vortex rope were explored, with particular emphasis on the accelerating reverse axial flow and the reduction in the draft tube's circumferential velocity. Additionally, this study examined the effect of the cavitation coefficient on the swirl number and flow pattern, while also assessing its impact on entropy production. These findings provide valuable insight into the control of cavitation flow in pump-turbines operating under part load conditions. Moreover, they carry significant implications for the design and optimization of pump-turbine systems. [ABSTRACT FROM AUTHOR]

Pumped storage hydropower is perhaps the most reliable solution to the energy crisis, but its development and application are limited by hydraulic instability, such as S-shaped characteristics. In this paper, based on the S-shaped region pressure fluctuation test of a model pump turbine, the connection between the guide vane opening and S-shaped pressure fluctuation is established, and a guide vane opening of 28 mm is selected for numerical analysis to expand the discussion of the S-shaped pressure fluctuation characteristics to large guide vane openings. The frequency components of the vaneless space are extracted by fast Fourier transform analysis, and the rotational stall frequency is identified. Among them, the blade passage frequency and its multiplier are induced by the rotor–stator interaction, and the low-frequency component of pressure fluctuations is induced by the rotational stall and vortex rope. Based on this, the internal flow and flow rate along the S-shaped characteristic curve and their correlation with the rotational stall are analyzed. In addition, the percentage of and variation in the entropy production rate in the S-shaped region are also analyzed. [ABSTRACT FROM AUTHOR]