Hasil untuk "Descriptive and experimental mechanics"

Laura Lenters, Mathias Ulbricht, Heyko Jürgen Schultz

In stirred tank reactors, especially without using baffles, the liquid surface can deform, which in stirring technology is referred to a vortex. These vortices can be advantageous for some mixing tasks, such as obtaining emulsions, they can also impair a consistent product quality. Therefore, it is important for the production and process industry, to know whether a vortex occurs or not. Prediction is only possible with an outdated dimensionless baffle index and research on vortex formation with baffles is limited. In this study, two industrially important axial stirring systems—Propeller and Pitched-blade turbine—with different baffle geometries (rectangular, cylindrical, triangular) and numbers are assessed in regard to power input, vortex characteristics (depth, width, volume) and baffle state prediction. Power is recorded using strain gauges, while vortices are evaluated using an optical image evaluation method. The final vortex result is made dimensionless, accessible to the industry to enable improved predictions about the size of the vortices on an industrial scale in order to make the stirred tanks more economical and sustainable. Furthermore, an initial improvement of the baffle index for the investigated stirrers is given, because the original index incorrectly predicts the baffle state in 12.5% of cases.

Oleg Penyazkov, Pavel Grinchuk

Boiling crises are complex stochastic processes that are influenced by the physical phenomena of heat transfer and evaporation, as well as the shape and roughness of the boiling surface. When calculating the critical heat fluxes corresponding to the point of the first boiling crisis, it is important to know the numerical density of the formed bubbles per unit surface and volume. Most models consider only non-interacting bubbles. This greatly reduces their predictive accuracy. An analysis of the video footage of bubble boiling near the point of the first boiling crisis allows us to conclude that this is a typical picture for a continuum off-lattice problem of percolation theory. The main purpose of the work is to consider the point of the first boiling crisis as the percolation threshold for a three-dimensional problem. This threshold describes the transition from finite size inclusions (single bubbles and small groups of weakly interacting bubbles) to a percolation structure in which there is a macroscopic irregular bubble, the size of which is comparable to the size of the entire system. This hypothesis allows us to make estimates for the concentration of bubbles at the boiling point and to obtain estimates for critical heat fluxes at this point. The fundamental difference between the proposed approach and previous attempts to apply percolation theory to the description of boiling crises is the consideration of a three-dimensional problem in liquid volume, rather than a two-dimensional problem onto a hot boiling surface. It is shown, for the first time, that the proportionality constant in the Kutateladze–Zuber equation coincides with the percolation threshold for a three-dimensional continuum percolation problem on overlapping ellipsoids.

Laurel ONeill, Tim Kane

This work presents a method of directly imaging the growth and collapse of laser induced-bubbles in glycerol and water mixtures. The direct optical imaging is augmented with interferometric measurements of the vibration spectrum of the bubble-vessel system. Experimentation confirms the expectation that fluid viscosity affects the bubble formation and lifetime. During the experiment, deviation from the Rayleigh–Plesset equation is observed. Given this deviation of the observed bubble dynamics from the expected results, it is possible that the limited size of the sample volume and the walls of the container impact the bubble dynamics. The optical observations are supported by the observations of the system’s vibration spectrum.

Jaume Luis-Gómez, Francisco Martínez, Alejandro González-Barberá et al.

The control of ventilation systems is often performed by automatic algorithms which often do not consider the future evolution of the system in its control politics. Digital twins allow system forecasting for a more sophisticated control. This paper explores a novel methodology to create a Machine Learning (ML) model for the predictive control of a ventilation system combining Computational Fluid Dynamics (CFD) with Artificial Intelligence (AI). This predictive model was created to forecast the temperature and humidity evolution of a ventilated room to be implemented in a digital twin for better unsupervised control strategies. To replicate the full range of annual conditions, a series of CFD simulations were configured and executed based on seasonal data collected by sensors positioned inside and outside the room. These simulations generated a dataset used to develop the predictive model, which was based on a Deep Neural Network (DNN) with fully connected layers. The model’s performance was evaluated, yielding final average absolute errors of 0.34 degrees Kelvin for temperature and 2.2 percentage points for relative humidity. The presented results highlight the potential of this methodology to create AI-driven digital twins for the control of room ventilation.

Xiao Mei, Xiaoyu Fang, Liyang Zhang et al.

This study investigates the interaction between airflow and low-density bulk particles within vertical screw conveyors and examines its impact on conveying performance. A combined simulation approach integrating the Discrete Element Method and Computational Fluid Dynamics was employed to model both single-phase particle flow and gas–solid two-phase flow. A periodic model was developed based on the structural characteristics of the conveyor. Particle motion dynamics under both single-phase and coupled two-phase conditions were analyzed using EDEM and coupled Fluent-EDEM simulations. The effects of key operational parameters, including screw speed, filling rate, and helix angle, on mass flow rate were systematically evaluated. A comprehensive performance index was established to quantify conveying efficiency, and its validity was confirmed through analysis of variance on the regression model. Finally, the response surface methodology was applied to optimize parameters and determine the optimal combination of screw speed and filling rate to enhance mass flow efficiency. The results indicate that the gas–solid two-phase flow model provides a more accurate representation of real-world conveying dynamics. Future research may extend the model to accommodate more complex material conditions.

Sanichiro Yoshida

M.A.K. Chowdhuri, Z. Xia

Maria Vasilyeva, Nana Adjoah Mbroh, Mehrube Mehrubeoglu

In this work, we present a lower-dimensional model for flow and transport problems in thin domains with rough walls. The full-order model is given for a fully resolved geometry, wherein we consider Stokes flow and a time-dependent diffusion–convection equation with inlet and outlet boundary conditions and zero-flux boundary conditions for both the flow and transport problems on domain walls. Generally, discretizations of a full-order model by classical numerical schemes result in very large discrete problems, which are computationally expensive given that sufficiently fine grids are needed for the approximation. To construct a computationally efficient numerical method, we propose a model-order-reduction numerical technique to reduce the full-order model to a lower-dimensional model. The construction of the lower-dimensional model for the flow and the transport problem is based on the finite volume method and the concept of numerical averaging. Numerical results are presented for three test geometries with varying roughness of walls and thickness of the two-dimensional domain to show the accuracy and applicability of the proposed scheme. In our numerical simulations, we use solutions obtained from the finite element method on a fine grid that can resolve the complex geometry at the grid level as the reference solution to the problem.

Alessandro Mercuri, Pierluigi Fanelli, Giacomo Falcucci et al.

Water impacts provide a challenge for a wide range of applications, from aerospace, to marine, mechanical and civil engineering, due to the complexity conveyed by the coexistence of impulsive loads, large local deformations and high-amplitude vibrations. Thus, the need for reliable structural health monitoring (SHM) systems is emerging in the industrial field of fluid-structure interaction (FSI) applications. In this paper, we leverage the previous work on strain and displacement fields reconstruction to analyse a scale aluminium model subject to water vertical and oblique impacts. Fibre Bragg grating (FBG) sensors were installed on the hull ribs and used both as reconstruction sensors (to reconstruct the structure mechanical behaviour characteristics) and as control sensors, by using their signals to compare the real and reconstructed structural parameters, at the sensors locations. Finally, the effectiveness of different reconstruction layouts was investigated referring to the strain signal reconstruction quality in case of both vertical and oblique impacts. Results show the potential of the described method for the reconstruction of strain signal through a proper choice of the reconstruction sensors positions both in case of vertical and oblique impacts.

Maria Antonietta Boniforti, Maria Chiara Cesaroni, Roberto Magini et al.

Blood flow dynamics plays a crucial role in the growth and rupture of abdominal aortic aneurysms. The aim of this study was to analyze the possibility of predicting aneurysmal rupture by numerical investigations based on diagnostic images. The blood flow dynamics was analyzed in a patient-specific abdominal aortic aneurysm, reconstructed from CT images of an aneurysm while it was rupturing. The patient-specific geometry was virtually repaired in order to obtain a non-ruptured model representative of the geometry immediately preceding the rupture. To reproduce physiological conditions, numerical simulations were performed under pulsatile flow conditions, and blood was modelled as a non-Newtonian fluid, using the Carreau rheological model. Hemodynamic parameters that influence the rupture of the aneurysm were investigated, and their possible association with vascular disease was discussed. The results of the numerical simulations indicated regions of slow recirculation and low values of Time Averaged Wall Shear Stress (TAWSS) in the region of rupture. Unlike literature results, a high Oscillatory Shear Index (OSI) was not clearly found in this region. Nevertheless, just in the region where the rupture will occur, high values of Endothelial Cell Activation Potential index (ECAP) were found. This index is therefore extremely significant for assessing the vulnerability of the aortic wall and locating the critical rupture region.

Gang Li, Rui Ye, Mingjing Sang et al.

Abstract The BEPCII project is for upgrading the Beijing Electron Positron Collider to reach a higher luminosity. The cryogenic system has been firstly designed and deployed in BEPCII, which provides cooling capacity for SRF cavities, SCQ and SSM. The cryogenic control system consists of Siemens PLC for the refrigerator control and “PLC + IOC” for superconducting device control. The BEPCII machine commissioning with beam started in October 2006. This paper describes the design and the development of the cryogenic control system.

Byoung-Kwon Ahn, So-Won Jeong, Cheol-Soo Park et al.

In many practical submerged objects, various types of cavitation such as bubble, sheet, and cloud cavitation occur according to flow conditions. In spite of numerous theoretical, numerical, and experimental studies, there are still many problems to be solved such as induced noise and damage potential due to cavitation. In this paper, an experimental investigation on coherent structures and induced noise characteristics of partial cavitation on a two-dimensional hydrofoil is presented. Experiments that focused on the dynamics of cavitation clouds were conducted in a cavitation tunnel. Using high-speed visualization, the series process consisting of inception, growth, and desinence of the partial cavity was investigated. The noise generated during the process was also measured, and the correlation with the cavity pattern was examined. The results show that the periodic behavior of cavitation clouds is directly reflected in the noise characteristics. In addition, the visualization of coherent structures within the sheet and cloud cavity provides a qualitative understanding of hairpin vortices and their packets, which play a dominant role in turbulent cavitating flows.

Furkan Kodakoglu, Sinan Demir, Damir Valiev et al.

A recent predictive scenario of premixed flame propagation in unobstructed passages is extended to account for obstructions that can be encountered in facilities dealing with explosive materials such as in coalmines. Specifically, the theory of globally-spherical, self-accelerating premixed expanding flames and that of flame acceleration in obstructed conduits are combined to form a new analytical formulation. The coalmining configuration is imitated by two-dimensional and cylindrical passages of high aspect ratio, with a comb-shaped array of tightly placed obstacles attached to the walls. It is assumed that the spacing between the obstacles is much less or, at least, does not exceed the obstacle height. The passage has one extreme open end such that a flame is ignited at a closed end and propagates to an exit. The key stages of the flame evolution such as the velocity of the flame front and the run-up distance are scrutinized for variety of the flame and mining parameters. Starting with gaseous methane-air and propane-air flames, the analysis is subsequently extended to gaseous-dusty environments. Specifically, the coal (combustible, i.e., facilitating the fire) and inert (such as sand, moderating the process) dust and their combinations are considered, and the impact of the size and concentration of the dust particles on flame acceleration is quantified. Overall, the influence of both the obstacles and the combustion instability on the fire scenario is substantial, and it gets stronger with the blockage ratio.

G. Vendroux, W. G. Knauss

Yorgos G. Stergiou, Athanasios G. Kanaris, Aikaterini A. Mouza et al.

The Abdominal Aortic Aneurysm (AAA) is a local dilation of the abdominal aorta and it is a cause for serious concern because of the high mortality associated with its rupture. Consequently, the understanding of the phenomena related to the creation and the progression of an AAA is of crucial importance. In this work, the complicated interaction between the blood flow and the AAA wall is numerically examined using a fully coupled Fluid-Structure Interaction (FSI) method. The study investigates the possible link between the dynamic behavior of an AAA and the blood viscosity variations attributed to the haematocrit value, while it also incorporates the pulsatile blood flow, the non-Newtonian behavior of blood and the hyperelasticity of the arterial wall. It was found that blood viscosity has no significant effect on von Mises stress magnitude and distribution, whereas there is a close relation between the haematocrit value and the Wall Shear Stress (WSS) magnitude in AAAs. This WSS variation can possibly alter the mechanical properties of the arterial wall and increase its growth rate or even its rupture possibility. The relationship between haematocrit and dynamic behavior of an AAA can be helpful in designing a patient specific treatment.

Hilde Breesch, Bart Merema, Alexis Versele

The test lecture rooms on Katholieke Universiteit Leuven (KU Leuven) Ghent Technology Campus (Belgium) are a demonstration case of Annex 62: Ventilative Cooling of the International Energy Agency’s Energy in Buildings and Communities programme (IEA EBC). The building is cooled by natural night ventilation and indirect evaporative cooling (IEC). Thermal comfort and the performances of ventilative cooling are evaluated. Long-term measurements of internal temperatures, occupancy, opening of windows and IEC were carried out in the cooling season of 2017. The airflow rates through the windows in cross- and single-sided ventilation mode were measured by both tracer gas concentration decay and air velocity measurements. In addition, the air flow pattern is visualized by measuring air temperatures in the room. The results show that good thermal summer comfort was measured except during heat waves and/or periods with high occupancy. Both nighttime ventilation and IEC operate very well. IEC can lower the supply temperature by day significantly compared to the outdoor temperature. The Air Changes Rates (ACR) of the night ventilation greatly depends on wind direction and velocity. The air temperature profile showed that the air is cooled down in the whole lecture but more in the upper zone. The extensive data monitoring system was important to detect malfunctions and to optimize the whole building performance.

Anwar Alwardi, Akram Alqesmah, R. Rangarajan

J. W. Dally, D. B. Barker

C. Bacon, J. Färm, J. L. Lataillade

K. M. Vernaza-Peña, J. J. Mason, M. Li