Hasil untuk "Descriptive and experimental mechanics"

M. R. Ajith Kumar, Prashanth R. Hanmaiahgari, Jaan H. Pu

A potential flow model of the semi-inverse type is proposed to simulate flow over round crested weirs. This technique involves the construction of only streamlines over the weir instead of constructing the entire flow net. A Serre–Green–Naghdi (SGN) equation is employed to determine the initial free-surface profile, which is solved using a combined finite volume-finite difference scheme. The potential flow equations were numerically solved using a five-point central finite difference scheme. The model was applied to define the pressure and velocity fields in channel controls involving transcritical flow, such as the Gaussian weir, parabolic weir, and semicircular weir. The impact of streamline curvature on pressure and velocity distributions was investigated in the study. The curvature of the streamline strongly influenced the rise and drop of the bed pressures along the test section. A semicircular weir experiment was also conducted to validate the pressure and velocity profiles obtained using the proposed 2-D fluid flow model. The computed pressure and flow profiles from the solution of the potential flow equation agree perfectly with the present experiment and similar experiments available in the literature. In conclusion, the SGN equation provides an excellent initial profile to solve a 2-D ideal fluid flow numerically.

Luca Pagliarini, Fabio Bozzoli

Despite a continuous effort devoted by the scientific community, a large-scale employment of Pulsating Heat Pipes for thermal management applications is still nowadays undermined by the low reliability of such heat transfer systems. The main reason underlying this critical issue is linked to the strongly chaotic thermofluidic behavior of these devices, which prevents a robust prediction of their working behavior for different geometries and operating conditions, consequently hampering proper industrial design. The present work proposes to thoroughly compare data referring to previous infrared investigations on different Pulsating Heat Pipe layouts, which have focused on the estimation of heat fluxes locally exchanged at the wall–fluid interfaces. The aim is to understand the beneficial contribution of local heat transfer quantities in the prediction of the complex physics underlying such heat transfer systems. The results have highlighted that, regardless of the considered geometry and working conditions, wall-to-fluid heat fluxes are able to provide useful quantities to be employed, to some extent, to generalize Pulsating Heat Pipe operation and to improve their existing numerical models.

Seyed M. Hajimirzaie

The mean wake structures of a cube (square cylinder) and circular cylinder of height-to-width aspect ratio 1.0, at a Reynolds number of 1.78 × 10⁴ based on the obstacle width, were investigated experimentally. The boundary-layer thickness was 0.14 of the obstacle height. The study was performed using thermal anemometry and two-dimensional digital particle image velocimetry (DPIV). Streamwise structures observed in the mean wake for both cylinders included well-known tip- and horseshoe (HS)-,vortex pairs as well as additional structures akin to the base vortices. In addition to tip-, base-, and HS-vortices, in the near wake of the cube, two more counter-rotating pairs of streamwise structures, including upper and inboard vortices, were observed. The existence of base vortices formed in the near wake for both obstacles is a unique observation and has not been previously reported for such low-aspect-ratio obstacles in thin boundary-layers. A model of arch-vortex evolution was proposed, in which arch structures were deformed by the external shear-flow to explain the observed base-vortices in the cylinder wake. A weak dominant-frequency of <i>St</i> = <i>f</i>₀<i>D</i>/<i>U∞</i> = 0.114 was observed across the height for the cube, while no discernible spectral peaks were apparent in the wake of the cylinder. Cross-spectral analysis revealed the shedding to be symmetric (in-phase) arch-type for the cylinder and predominantly anti-symmetric (out-of-phase) Karman-type for the cube. The study makes fundamental contributions to the understanding of the flow-field surrounding low-aspect-ratio cylinders.

Vimal Tiwari, Hari Mohan Dubey, Manjaree Pandit et al.

In this paper, the economically self-sufficient microgrid is planned to provide electric power and heat demand. The combined heat and power-based microgrid needs strategic placement of distributed generators concerning optimal size, location, and type. As fossil fuel cost and emission depend mainly on the types of distributed generator units used in the microgrid, economic emission dispatch is performed for an hour with a static load demand and multiple load demands over 24 h of a day. The TOPSIS ranking approach is used as a tool to obtain the best compromise solution. Harris Hawks Optimization (HHO) is used to solve the problem. For validation, the obtained results in terms of cost, emission, and heat are compared with the reported results by DE and PSO, which shows the superiority of HHO over them. The impact of renewable integration in terms of cost and emission is also investigated. With renewable energy integration, fuel cost is reduced by 18% and emission is reduced by 3.4% for analysis under static load demand, whereas for the multiple load demands over 24 h, fuel cost is reduced by 14.95% and emission is reduced by 5.58%.

Yi Zhu, Jian-Hua Pang, Fang-Bao Tian

This work presents a numerical study of the collective motion of two freely-swimming swimmers by a hybrid method of the deep reinforcement learning method (DRL) and the immersed boundary-lattice Boltzmann method (IB-LBM). An active control policy is developed by training a fish-like swimmer to swim at an average speed of 0.4 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>L</mi><mo>/</mo><mi>T</mi></mrow></semantics></math></inline-formula> and an average orientation angle of 0<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>∘</mo></msup></semantics></math></inline-formula>. After training, the swimmer is able to restore the desired swimming speed and orientation from moderate external perturbation. Then the control policy is adopted by two identical swimmers in the collective swimming. Stable side-by-side, in-line and staggered formations are achieved according to the initial positions. The stable side-by-side swimming area of the follower is concentrated to a small area left or right to the leader with an average distance of 1.35 <i>L</i>. The stable in-line area is concentrated to a small area about 0.25 <i>L</i> behind the leader. A detailed analysis shows that both the active control and passive self-organization play an important role in the emergence of the stable schooling formations, while the active control works for maintaining the speed and orientation in case the swimmers collide or depart from each other and the passive self-organization works for emerging a stable schooling configuration. The result supports the Lighthill conjecture and also highlights the importance of the active control.

Daniel J. Portillo, Eugene Hoffman, Matt Garcia et al.

Investigations of fluidic oscillators, or sweeping jet actuators, have primarily been conducted within the incompressible flow regime, which limits the accuracy of estimating fluidic oscillator performance for compressible flows. The objective of this study was to evaluate the effects of gas compressibility on the performance of a fluidic oscillator. A commonly used fluidic oscillator geometry (the Bray geometry) was scaled to five different sizes, 3D printed, and tested over a range of air flow rates. High-speed Schlieren images captured the sweeping jet exiting the fluidic oscillators, and custom MATLAB algorithms were used to calculate the oscillation frequencies and angles. A spectral proper orthogonal decomposition (SPOD) method was used to identify and compare the mode structures within the flow fields. All the results were compared using dimensionless parameters to observe performance trends. The results showed that the oscillation frequencies were directly proportional to the flow rate, while the oscillation angles were inversely proportional to the flow rate, regardless of scale size. The angular velocities were not proportional to the flow rate or scale size and exhibited maxima within the evaluated ranges. For all scale sizes, the mode structures were symmetric across the centerlines of the fluidic oscillators and extended further beyond the fluidic oscillators at higher flow rates. These results enable the prediction of fluidic oscillator performance, which can significantly improve the design process for an application where a fluidic oscillator may be used, such as aerospace applications, power generation, heat exchangers, or medical devices.

Robert A. Handler, Michael J. Buckingham

In a high viscosity, polymeric fluid initially at rest, the release of elastic energy produces vorticity in the form of coherent motions (vortex rings). Such behavior may enhance mixing in the low Reynolds number flows encountered in microfluidic applications. In this work, we develop a theory for such flows by linearizing the governing equations of motion. The linear theory predicts that when elastic energy is released in a symmetric manner, a wave of vorticity is produced with two distinct periods of wave motion: (1) a period of wave expansion and growth extending over a transition time scale, followed by (2) a period of wave translation and viscous decay. The vortex wave speeds are predicted to be proportional to the square root of the initial fluid tension, and the fluid tension itself scales as the viscosity. Besides verifying the predictions of the linearized theory, numerical solutions of the equations of motion for the velocity field, obtained using a pseudo-spectral method, show that the flow is composed of right- and left-traveling columnar vortex pairs, called vortex waves for short. Wave speeds obtained from the numerical simulations are within 1.5% of those from the linear theory when the assumption of linearity holds. Vortex waves are found to decay on a time scale of the order of the vortex size divided by the solution viscosity, in reasonable agreement with the analytical solution of the linearized model for damped vortex waves. When the viscoelastic fluid is governed by a nonlinear spring model, as represented by the Peterlin function, wave speeds are found to be larger than the predictions of the linear theory for small polymer extension lengths.

Andriy Boychenko, Dmitry Mishchuk

Исследование кинематических структур многозвенных манипуляторов является нетривиальной задачей. Сложности проявляются при исследовании кинематических схем у которых количество подвижных независимых частей превышает количество обобщенных независимых степеней подвижности, так как в таких случаях имеет место переизбыток подвижности системы и для одной позиции может существовать больше двух конфигураций манипулятора. Подобная неоднозначность сильно усложняет поиск зависимостей между обобщенными координатами системы и конечной точкой перемещения рабочего органа. Также необходимо учитывать, что в дальнейшем при составлениях динамических уравнений движений для подобных систем, такая неоднозначность усложняет исследование задач динамики. Решения подобных проблем осуществляется разными методами и в данной статье рассматривается возможность применения нейронных сетей для исследований подобных неоднозначных задач на примере кинематического исследования шарнирно-сочлененного манипулятора. В представленной работе рассмотрены возможности использования библиотеки Tensor-Flow от компании Google при помощи которой создаются модели нейронных сетей и «нейронов», а также функции нахождения весовых коэффициентов настройки созданной модели сети. В ходе исследования была создана нейронная сеть для расчета функции регрессии зависимостей между геометрическими координатами точки перемещения захватного устройства манипулятора и обобщенными координатами его стреловой системы. Целью данного исследования является демонстрация возможностей нейронных сетей при решении инженерных задач, которые требуют сложных математических преобразований. Использование архитектур нейронных сетей в системах управления манипуляторами позволяет создавать универсальные механизмы для выполнения различных технологических процедур, при этом снижая затраты на разработку подобных решений. В целом нейронные сети не являются универсальными средствами для решения всех инженерных задач, так как требуют большего набора тестовых выборок для настройки (обучения) параметров своих моделей, однако существует ряд проблем при решении которых нейронные сети имеют большие преимущества.

Viona Ojiambo, Mark Kimathi, Mathew Kinyanjui

Carlo Gualtieri, Athanasios Angeloudis, Fabian Bombardelli et al.

Computational Fluid Dynamics (CFD) has consolidated as a tool to provide understanding and quantitative information regarding many complex environmental flows. The accuracy and reliability of CFD modelling results oftentimes come under scrutiny because of issues in the implementation of and input data for those simulations. Regarding the input data, if an approach based on the Reynolds-Averaged Navier-Stokes (RANS) equations is applied, the turbulent scalar fluxes are generally estimated by assuming the standard gradient diffusion hypothesis (SGDH), which requires the definition of the turbulent Schmidt number, Sct (the ratio of momentum diffusivity to mass diffusivity in the turbulent flow). However, no universally-accepted values of this parameter have been established or, more importantly, methodologies for its computation have been provided. This paper firstly presents a review of previous studies about Sct in environmental flows, involving both water and air systems. Secondly, three case studies are presented where the key role of a correct parameterization of the turbulent Schmidt number is pointed out. These include: (1) transverse mixing in a shallow water flow; (2) tracer transport in a contact tank; and (3) sediment transport in suspension. An overall picture on the use of the Schmidt number in CFD emerges from the paper.

Eugene S. Benilov

This paper presents a model which yields examples of stable vortices in a continuously stratified rotating fluid, thus providing a possible explanation of the observed longevity of oceanic eddies. The model is based on two assumptions. Firstly, the ocean comprises a thin upper (active) layer and a thick lower (passive) one, with large and small vertical gradients of density, respectively. Secondly, the Rossby number is small, justifying the use of the geostrophic and quasi-geostrophic approximations for the active and passive layers (the two are treated differently because the vortex-induced displacement of the isopycnal surfaces is comparable to the depth of the active layer, but is much smaller than that of the passive one). Using the asymptotic equations derived on the basis of the above assumptions, we prove a stability criterion and thus identify a class of stable vortex profiles. This class is much wider than the one following from the standard requirement that the potential vorticity be monotonic in the whole bulk of the fluid.

Tran Thu Ha, Nguyen Anh Son, Duong Ngoc Hai

When a slender body moves very fast through water at sufficient speed the cavity phenomena is happened. In a cavity model the body’s lengths and cavitator diameter are determined by the differential variationoptimal method so that the velocity of body is maximum.

Abbas Kareem Wanas

Nguyen Minh Triet

This paper extends the dual equivalent linearization  technique (ELT) to obtain flutter speeds of a two-dimensional airfoil  with nonlinear stiffness and damping in pitch degree of freedom.  Although the use of dual ELT has been investigated in some previous papers,  this paper presents an extension of dual ELT using the global-local  approach, in which the local equivalent linearization coefficients are averaged in the  global sense. The numerical calculation shows that the extended dual ELT  gives more accurate flutter speeds in comparison with the ones of classical  ELT.

Harold E. Gascoigne

Y. Rabin, D. Rittel

V. Bindiganavile, N. Banthia

C. Y. Poon, M. Kujawinska, C. Ruiz

Peter G. Ifju

L. E. Asp, L. A. Berglund