Hasil untuk "Descriptive and experimental mechanics"

Pengfei Zhao, Sai Raja Gopal Vadlamudi, Mi Zhou et al.

Abstract Droplet impact on solid surfaces plays a critical role in a wide range of applications, including inkjet printing, spray cooling, surface coatings, and microdroplet chemistry. Precise control of droplet–surface interactions is essential, but the fundamental mechanisms governing this process are still not fully understood. In this study, we demonstrate that large contact angle hysteresis (CAH) on hydrophobic nanoporous surfaces significantly amplifies post‐impact droplet oscillations. This reveals the critical influence of CAH on the redistribution of impact energy and the modulation of droplet–surface interactions. Using shape mode decomposition via Legendre polynomials and fast Fourier transform spectral analysis, we show that surfaces with larger CAH excite and sustain higher‐order droplet shape mode oscillations, leading to persistent capillary waves even after contact line pinning. The observed amplitude modulation and multiple frequency components within individual shape modes reveal nonlinear energy transfer between different modes. These amplified and coupled oscillations are shown to promote daughter droplet coalescence. This study presents a framework for understanding the role of CAH in storing and redistributing impact energy through nonlinear mode excitation and establishes CAH as a critical design parameter for controlling fluid dynamics on solid surfaces.

Arijit A. Ganguli, Sagar S. Deshpande, Mehul S. Raval

Natural convection in enclosures heated from both sides is a topic of interest in various space and safety applications in nuclear power reactors. The transient dynamics during natural convection in enclosures is critically dependent on micro-scaled boundary layers and also the timescales of micromixing. In the present work, a square enclosure operating at two high Rayleigh numbers (Ra = 3.27 × 10¹⁰ and Ra = 6.55 × 10¹⁰, with water as the working fluid) have been chosen for study. First, the velocity and timescales were found using Computational Fluid Dynamic (CFD) simulations for the square enclosure with Ra 3.27 × 10¹⁰ and compared with scaling laws that presently define them. An empirical correlation for heat transfer is then developed for the Ra range (1.3 × 10¹⁰ < Ra < 6.55 × 10¹⁰). Then, an existing DL framework (Proper Orthogonal Decomposition and Long Short-Term Memory (POD-LSTM)) network) is compared qualitatively and quantitatively with the CFD data. The transient data Ra = 6.55 × 10¹⁰ was chosen for this purpose. The scaling laws show a 30% deviation for the predictions of the transient length and time scales as compared to CFD and DL model predictions. Further, accurate results up to 99.6% have been obtained by the DL model when compared with the CFD model. The DL model is also found to require an order of magnitude less time than the one required for a CFD simulation.

Chuchen Yue, Qingwen Dai, Xiaolong Yang et al.

Abstract Preventing the accretion of droplets on surfaces is vital and slippery liquid‐infused porous surfaces (SLIPS) have promising application prospects, such as surface self‐cleaning and droplet transportation. In this work, controllable self‐transport of bouncing droplets on ultraslippery surfaces with wedge‐shaped grooves is reported. The impact behaviors of droplets on SLIPS under various impact velocities and diameters are explored, which can be classified as hover, total bounce, partial bounce, Worthington jet, and crush. SLIPS with wedge‐shaped grooves were designed to transport accreted droplets. An energy and transport model is established to explain the impact and self‐transport mechanism, where the Laplace pressure and moving resistance between droplets play a key role. Finally, SLIPS with branched wedge‐shaped grooves were designed for droplet self‐transport and demonstrated advantages. This work provides a general reference for spontaneous motion control of sessile droplets, droplets with initial impacting velocity, or even liquid films.

Metin Aydogdu

First of all Sentilkumar has to be congratulated for his paper [1]. In the paper, the author  presented the vibration of double walled carbon nanotubes. He claimed that the equations of motion given in [2]  are incorrect. The aim of this communication is to clarify this issue. 

Alexander Spethmann, Thomas Trottenberg, Holger Kersten et al.

Abstract Diagnostics with force probes in plumes of electric propulsion systems for spacecraft (thrusters) are presented. This contribution focuses on showing example measurements with force probes for the most common gridless ion thruster types. The gridless thrusters are investigated at typical operation modes with a force probe at fixed or variable positions in the plume. The probe measures the force on a tiny plate that is attached to a sensitive cantilever in the plume. The elastic deflection of the cantilever is measured interferometrically and translated into a proportional force. Several variants of the instrument have been developed. A HEMP thruster is investigated with a variant of the force probe that simultaneously measures two components of the force vector. With this vectorial probe, it is possible to determine the force vector acting on the probe target while moving the probe through the thruster plume. A Hall thruster is investigated with a simplified variant of the probe, which measures the force along one axis and is equipped with an additional shutter in front of the target. This allows monitoring over long periods of operation. Finally, a force probe is used as an indirect thrust balance for a FEEP thruster. In this case, the force probe can collect the entire plume.

Maxim Zhelnin, Anastasiia Kostina, Oleg Plekhov et al.

For heat rejection, small air-cooling towers are widely used in mine ventilation systems. However, the thermal efficiency of the cooling towers can be significantly affected by their geometrical arrangement and crosswind conditions. In certain ambient conditions, heated air coming from an exit of one tower can flow to intakes of other towers, which leads to a reduction in the thermal efficiency of the entire ventilation system. The aim of this study was to investigate the influence of crosswind speed and tower spacing on the temperature and moisture content of intakes of cooling towers. For this purpose, a three-dimensional CFD model of the non-isothermal turbulent flow of moist air around cooling towers is proposed. The model is based on the Reynolds-averaged Navier–Stokes equations with a standard turbulence model which are supplemented by heat transfer and moisture transport equations. The investigation of the effects of the crosswind speed and the tower spacing was carried out for two cooling towers by multiparametric numerical simulation using the CFD model. It was shown that the upstream tower protects the downstream one from the effect of the crosswind. The increase in the crosswind speed causes a rise in temperature and moisture content at the intakes of the downstream tower. The increase in the tower spacing, in general, contributes to a decrease in air temperature at the intakes of the downstream tower. However, at low crosswind speed, the heat transfer at the intakes can rise with the tower spacing due to a reduction in the protection possibilities of the upstream tower. Results of the numerical simulation of airflow around three cooling towers indicated that the increase in the number of cooling towers contributes to a rise in temperature and moisture content at the intakes.

Nissrine Akkari, Fabien Casenave, Elie Hachem et al.

This paper presents a new nonlinear projection based model reduction using convolutional Variational AutoEncoders (VAEs). This framework is applied on transient incompressible flows. The accuracy is obtained thanks to the expression of the velocity and pressure fields in a nonlinear manifold maximising the likelihood on pre-computed data in the offline stage. A confidence interval is obtained for each time instant thanks to the definition of the reduced dynamic coefficients as independent random variables for which the posterior probability given the offline data is known. The parameters of the nonlinear manifold are optimized as the ones of the decoder layers of an autoencoder. The parameters of the conditional posterior probability of the reduced coefficients are the ones of the encoder layers of the same autoencoder. The optimization of both sets of the encoder and the decoder parameters is obtained thanks to the application of a variational Bayesian method, leading to variational autoencoders. This Reduced Order Model (ROM) is not a regression model over the offline pre-computed data. The numerical resolution of the ROM is based on the Chorin projection method. We apply this new nonlinear projection-based Reduced Order Modeling (ROM) for a 2D Karman Vortex street flow and a 3D incompressible and unsteady flow in an aeronautical injection system.

Efrizon Umar, Nathanael Panagung Tandian, Ahmad Ciptadi Syuryavin et al.

The development of new practices in nuclear research reactor safety aspects and optimization of recent nuclear reactors needs knowledge on forced convective heat transfer within sub-channels formed between several nuclear fuel rods or heat exchanger tubes, not only in the fully developed regime but also in the developing regime or laminar flow regime. The main objective of this research was to find a new correlation equation for calculating the convective heat transfer coefficient in the vertical square sub-channels. Recently, a simulation study was conducted to find a new heat transfer correlation equation for calculating the convective heat transfer coefficient within a vertical square sub-channel in the developing regime or laminar flow regime for Reynolds number range 400 ≤ <i>Re</i> ≤ 1700. Simulations were carried out using a computational fluid dynamics (CFD) code and modeling already defined in the software. The novelty of the research lies in the analysis of the entrance effect for the sub-channel by proposing a new empirical correlation that can then be inserted into the STAT computer code. The surface temperature distribution around the tangential direction of the active cylinders shows that the implementation of active and dummy cylinders in the current study can simulate sub-channels that exist in a real nuclear reactor core. The current study shows that the flow simulated in this study is in its developing condition (entrance region). A new forced convective heat transfer correlation for the developing region in the form of <i>Nu</i> = 2.094(<i>Gz</i>)^0.329 for the Graetz number range 161 ≤ <i>Gz</i> ≤ 2429 was obtained from the current study.

Ove Tobias Gudmestad

In marine engineering, the dynamics of fixed offshore structures (for oil and gas production or for wind turbines) are normally found by modelling of the motion by a classical mass-spring damped system. On slender offshore structures, the loading due to waves is normally calculated by applying a force which consists of two parts: a linear “inertia/mass force” and a non-linear “drag force” that is proportional to the square of the velocity of the particles in the wave, multiplied by the direction of the wave particle motion. This is the so-called Morison load model. The loading function can be expanded in a Fourier series, and the drag force contribution exhibits higher order harmonic loading terms, potentially in resonance with the natural frequencies of the system. Currents are implemented as constant velocity terms in the loading function. The paper highlights the motion of structures due to non-linear resonant motion in an offshore environment with high wave intensity. It is shown that “burst”/“ringing” type motions could be triggered by the drag force during resonance situations.

Jose Libreros, Maria Trujillo

Anode baking is critical in carbon anode production for aluminium extraction. Operational and geometrical parameters have a direct impact on the performance of anode baking furnaces (ABF), and hence on the resulting anode quality. Gas flow patterns, velocity field, pressure drop, shear stress and turbulent dissipation rate are the main operational parameters to be optimised, considering a specific geometry that is discretised as a mesh. Therefore, this paper aims to establish the need to generate an appropriate mesh to perform accurate numerical simulations of three-dimensional turbulent flow in a single section of an ABF. Two geometries are considered for generating three meshes, using COMSOL and cfMesh, with different refinement zones. The three meshes are used for creating nine incompressible isothermal turbulent flow models, with varying operational parameters. Velocity field, convergence and turbulent viscosity ratio in the outlet of fuel inlet pipes are the quantification criteria. Quantification criteria have shown that a better physical representation is obtained by refining in the whole combustion zone. COMSOL Multiphysics’ built-in mesh generator allows quadrilateral, tetrahedron and hexahedron shapes. Adaptive cell sizes and shapes have a place within modelling, since refining a mesh in appropriate zones brings the Peclet number down when the incompressible isothermal turbulent flow is simulated.

Jinyong Feng, Liangyu Xu, Emilio Baglietto

The accuracy of computational fluid dynamics (CFD) predictions plays a fundamental role in supporting the operation of the current nuclear reactor fleet, and even more importantly the licensing of advanced high-efficiency reactor concepts, where local temperature oscillations driven by thermal striping, cycling and stratification can limit the structural performance of vessels and components. The complexity of the geometrical configurations, coupled to the long operational transients, inhibits the adoption of large eddy simulation (LES) methods, mandating the acceptance of the more efficient Reynolds-averaged Navier-Stokes (RANS)-based models, even though they are unable to provide a complete physical description of the flow in regions dominated by complex unsteady coherent structures. A new strategy has been proposed and demonstrated at Massachusetts Institute of Technology (MIT) toward the enhancement of unsteady Reynolds-averaged Navier-Stokes (URANS) predictions, using local resolution of coherent turbulence, to provide higher fidelity modeling in support of safety-related issues. In this paper, a comprehensive assessment of the recently proposed Structure-based (STRUCT-<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ε</mi></semantics></math></inline-formula>) turbulence model is presented, starting from fundamental validation of the model capabilities and later focusing on a representative safety-relevant application, i.e., thermal mixing in a T-junction. Solutions of STRUCT-<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ε</mi></semantics></math></inline-formula>, the widely used Realizable <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>k</mi><mo>−</mo><mi>ε</mi></mrow></semantics></math></inline-formula> model (RKE) and Large Eddy Simulation with Wall-Adapting Local Eddy-viscosity subgrid scale closure (LES-WALE) are compared against the experimental data. Both the velocity and temperature fields predicted by the STRUCT-<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ε</mi></semantics></math></inline-formula> model are in close agreement with the high-fidelity data from the experiments and reference LES solutions, across all validation cases. The approach demonstrates the potential to address the accuracy requirements for application to nuclear safety-related issues, by resolving the turbulent flow structures, while the computational efficiency provides the ability to perform consistent uncertainty quantification.

Enriquez Bazán, Javier Arcenio

Jamer Guillén Cruzado

Rocio Manuela Rosado Sequeiros, R. Condori

Lina Mariana

Vu Lam Dong, Pham Duc Chinh

In our previous paper, we constructed bounds on the effective bulk modulus of isotropic multicomponent composites using minimum energy principles and modified Hashin-Shtrikman polarization trial fields. In this paper, following the variational approach, we construct more sophisticated bounds on the effective shear modulus. Applications to particular models are presented. 

Ghozali Mahmud

S. Chokhandre, C. Bennetts, Jason P. Halloran et al.

K. Matsuno

Practicing experiments on the origins of life within the framework of quantum mechanics comes to face a task of distinguishing the descriptive spaces of the object between a space of physical states and a space of probability distributions. One candidate for accommodating both the physical and the probabilistic description in a mutually tolerable manner is to apply first-second person descriptions to the space of physical states while letting the space of probability distributions addressable in third person descriptions be accessible via first-second person descriptions. The mediator or messenger for accommodating these two types of description is the process of probability flow equilibration. The relative state formulation of quantum mechanics opens a possibility for the likelihood that a simple atom such as a carbon atom may carry a message for holding the process of probability flow equilibration. An experimental example demonstrating a carbon atom serving as a messenger is found in the running of the citric acid cycle in the absence of biological enzymes.

Nguyen Xuan Toan

The study of fluctuations of structures in general and bridge structures in particular under the influence of moving loads considering the impact of vehicle braking forces draws the attention of many scientists. However, due to the complexity of this problem a static method has been so far applied for approximate calculation in bridge design standards. In this article the author introduces the equation of bending vibrations of beam elements according to the model of dynamic interaction between beam elements and moving vehicle loads considering vehicle braking forces.