A numerical model of turbulent transient flow is used to study the dynamics of turbulence during different periods of water hammer in a polymeric pipe. The governing equations of the transient flow are solved by using the finite difference (FD) method, and the effects of viscoelasticity are modeled by means of a two-dimensional (2D) Kelvin–Voigt model. The experimental data with the Ghidaoui parameter P in the order of one are chosen in which the generated shear wave propagates toward the center of the pipe, while the pressure wave passes the length of the pipe. By studying the turbulence shear force during different times, it is shown that the turbulence structure changes considerably in the first cycle of water hammer. In the accelerated phases, the dominant feature is the creation of a shear wave near the wall, and in the decelerated phases the dominant feature is the propagation of the shear wave created in the accelerated phase.
The ultrasound‐assisted pregelatinization starch (UAPS) is considered as a physical method for starch modification. In this study, the effect of ultrasonic control parameters, including power, probe size, temperature, and time was investigated in order to predict microbubble formation and cavitation process as a two dimensional axisymmetric computational fluid dynamics (CFD) model to optimize the UAPS process. The cavitation phenomenon and the bubble dynamics were further investigated with regards to the mentioned parameters. Results showed that with the increase in probe diameter from 20 mm to 100 mm, a new pattern of streamlines were generated at the bottom of the container, increasing the turbulence among bubbles. This is mainly due to collapsing bubbles, resulting in a huge amount of energy released which accelerates heat and mass transfer to the fluid. According to the results obtained from the dynamics of bubbles, the maximum radius of the bubbles had a rising pattern with the increase in the probe size (from 20 to 100 mm), amplitude (from 25 to 45%) and temperature (from 35 to 65°C); moreover, the energy of the bubble slightly increased with the increase in liquid temperature and probe size.
Practical applications
The physical modifications of starch structure are more widely employed since it is safer and without impurities, which have strong effects on different component functionalities. Pregelatinization is one of the most popular industrial methods of physically modifying starch. Pregelatinized. The ultrasound‐assisted pregelatinized starch as physical modification offers many advantages, mainly the declined utilize of chemicals and processing time, serving as an environment‐friendly processing technology, high selectivity method. Use of computational fluid dynamics is a suitable approach to assessing the data pertaining to the physical features of the ultrasonication process. We underscored the importance of filling the gaps of knowledge concerning modeling sonication process to produce the efficient ultrasound‐assisted pregelatinized starch under various ultrasound power, probe size and fluid temperature to predict microbubble formation, heat and energy transfer.
The functionality of most of the metasurfaces that have been investigated so far, especially in illuminations with arbitrarily linearly polarized incident light, are restricted to x- or y-polarized incoming light. In particular, filtering out one of the two orthogonal polarizations of the incoming electromagnetic wave loses the incident light energy and limits the potential performance of the metasurface. In this study, by utilizing the cross-shaped silicon metaatoms that support the simultaneous excitation of electric and magnetic dipoles under the illumination of both x- and y- orthogonal polarizations, we overcome the polarization-restricted functionality of the metalenses. By selecting the metaatoms arrangement in the metalens structure, which follows the hyperbolic phase profiles for both x- and y-polarized incoming light waves at the same time, we obtain the light intensity distribution with the extended depth of focus (EDOF) or enhanced intensities at the focal spot with the focusing efficiency 65% for the numerical aperture of 0.7. Utilizing metaatoms with the ability to control the two orthogonal incoming polarizations develops a new methodology for using the full potential and intensity of the arbitrary polarized incoming light. The present design concept of metaatoms has several advantages that are not limited to metalenses alone but can be applied in all metasurfaces realized to have good efficiency. Finally, the proposed metalenses are suitable for imaging, optical tweezers and lithography applications, where subwavelength light intensity distributions with EDOF are the most desirable property.
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