With almost 60 years dedicated to the field of mechanics and the Department of Applied Mechanics, Hanoi University of Science and Technology, Prof. Dr. Sc., People's Teacher Nguyen Van Khang made a vital contribution to educating generations of outstanding students in mechanics and mechatronics, as well as to developing multi-body mechanics in Vietnam. His enthusiasm and passion for mechanical research lasted until the final days of his life. With a burning passion and endless creativity, he always strived hard and became the leading professor in Dynamics and Mechatronics in Vietnam. He also built and maintained close scientific cooperation with many famous professors in other countries. In the bottom of the hearts of colleagues, students and his family members, Prof. Khang remains a symbol of an excellent scientist with practical expertise, extensive knowledge and dedication, as well as a steadfast Party member with unwavering determination. The professor's spirit of dedication to science and teaching will always be a valuable lesson and a motivation for students and successors to follow.
Mechanical engineering and machinery, Descriptive and experimental mechanics
Three general imperfect interface models for thermal conduction are presented. The discontinuity of the microscopic field is described through the interface parameters: one characterizes the temperature jump, and another controls the heat flux jump. These models include the highly and lowly conducting ones as particular cases. The effective conductivity of composite materials with imperfect interfaces is derived from several effective medium approximations. Unlike the perfect interface, the effective conductivity of a composite containing imperfectly bonded inclusions depends on the size of the particles. The results for these models are illustrated by numerical calculations. Some discussions are also presented.
Mechanical engineering and machinery, Descriptive and experimental mechanics
Georgios C. Florides, Georgios C. Georgiou, Michael Modigell
et al.
We propose a methodology for the rheological characterization of a semisolid metal slurry using experimental squeeze-flow data. The slurry is modeled as a structural thixotropic viscoplastic material, obeying the regularized Herschel–Bulkley constitutive equation. All rheological parameters are assumed to vary with the structure parameter that is governed by first-order kinetics accounting for the material structure breakdown and build-up. The squeeze flow is simulated using finite elements in a Lagrangian framework. The evolution of the sample height has been studied for wide ranges of the Bingham and Reynolds numbers, the power-law exponent as well as the kinetics parameters of the structure parameter. Systematic comparisons have been carried out with available experimental data on a semisolid aluminum alloy (A356), where the sample is compressed from its top side under a specified strain of 80% at a temperature of 582 °C, while the bottom side remains fixed. Excellent agreement with the experimental data could be achieved provided that at the initial instances (up to 0.01 s) of the experiment, the applied load is much higher than the nominal experimental load and that the yield stress and the power-law exponent vary linearly with the structure parameter. The first assumption implies that a different model, such as an elastoviscoplastic one, needs to be employed during the initial stages of the experiment. As for the second one, the evolution of the sample height can be reproduced allowing the yield stress to vary from 0 (no structure) to a maximum nominal value (full structure) and the power-law exponent from 0.2 to 1.4, i.e., from the shear-thinning to the shear-thickening regime. These variations are consistent with the internal microstructure variation pattern known to be exhibited by semisolid slurries.
Thermodynamics, Descriptive and experimental mechanics
Andrew Craig-Jones, Daniel R. Greene, Jonathan J. Ruiz-Ramie
et al.
To the purpose of this study was to compare muscle oscillation, muscle activation time, and oxygen consumption while wearing compression pants vs. a control garment during running. Methods. Eleven injury-free and recreationally active participants (26.73 ± 12.74 years) were recruited for this study. Participants ran in full-leg compression pants (COMP) and a loose-fitting control garment (CON). Participants ran for 6 min at three submaximal speeds: preferred speed (PS), preferred speed minus 10% (PS − 10%), and preferred speed plus 10% (PS + 10%). The muscle activity of the leg was measured through electromyography (EMG). Muscle oscillation (MO) was measured with accelerometers attached to the thigh and shank. The rate of oxygen consumption (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo>.</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub>) and heart rate (HR) were recorded during each condition. MO was assessed over the 0–60 Hz range by averaging power across 10 Hz bins per leg segment. EMG data was processed to identify the activation time. Following each condition, a belief score was recorded. Dependent variables were each compared between conditions using 2 (garment) × 3 (speed) repeated measure ANOVAs (α = 0.05). The relationship between the belief score and dependent variables (compression-control) was analyzed using Pearson’s product-moment correlation (α = 0.05). Results. MO was lower with the full-leg compression pants vs. the control garment (<i>p</i> < 0.05). The muscle activation time for each muscle was shorter while wearing the full-leg compression pants (<i>p</i> < 0.05). Neither the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo>.</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub>, RPE, SF, nor the HR were influenced by the garments (<i>p</i> > 0.05). There was no significant correlation between changes in the dependent variables and belief. Conclusion. Wearing compression pants resulted in reduced MO and activation time; however, these changes did not translate into a reduction in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo>.</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub>.
Mechanics of engineering. Applied mechanics, Descriptive and experimental mechanics
Abstract Programmable droplet manipulation based on external stimulation is in high demand in various modern technologies. Despite notable progress, current manipulation strategies still suffer from a common drawback such as single control means of modulating the external stimulation input, which leads to huge challenges in sophisticated and large scale‐up droplet handling. Herein, a unique pattern‐reconfiguration‐driven droplet manipulation method is developed on conductive/nonconductive pattern surfaces under charge deposition. Contactless charge deposition induces the “edge barrier” phenomenon at the boundaries of conductive/nonconductive patterns, analogous to an invisible and tunable wall guiding droplet behaviors. The edge barrier effect can be flexibly tuned by the nonconductive surface pattern. Thus, with charge deposition, surfaces are endowed with protean control functionality. The design of conductive/nonconductive patterns can effectively enable multifunction droplet manipulations, including track‐guided sliding, sorting, merging, and mixing. Moreover, dynamical pattern reconfiguration drives programmable fluidics with sophisticated and large scale‐up droplet handling capabilities in a low‐cost and simple approach.
Particle Image Velocimetry (PIV) is a widely used experimental technique for measuring flow. In recent years, open-source PIV software has become more popular as it offers researchers and practitioners enhanced computational capabilities. Software development for graphical processing unit (GPU) architectures requires careful algorithm design and data structure selection for optimal performance. PIV software, optimized for central processing units (CPUs), offer an alternative to specialized GPU software. In the present work, an improved algorithm for the OpenPIV–Python software (Version 0.25.1, OpenPIV, Tel Aviv-Yafo, Israel) is presented and implemented under a traditional CPU framework. The Python language was selected due to its versatility and widespread adoption. The algorithm was also tested on a supercomputing cluster, a workstation, and Google Colaboratory during the development phase. Using a known velocity field, the algorithm precisely captured the time-average flow, momentary velocity fields, and vortices.
Thermodynamics, Descriptive and experimental mechanics
This study predicts how the Free Surface Level (FSL) variations around finite length vegetation affect flow structure by using a numerical simulation. The volume of fluid (VOF) technique with the Reynolds stress model (RSM) was used for the simulation. Multizone Hexahedral meshing was adopted to accurately track the free surface level with minimum numerical diffusion at the water–air interface. After the validation, finite length emergent vegetation patches were selected based on the aspect ratio (<i>AR</i> = vegetation width-length ratio) under constant subcritical flow conditions for an inland tsunami flow. The results showed that the generation of large vortices was predominated in wider vegetation patches (<i>AR</i> > 1) due to the increase and decrease in the FSL at the front and back of the vegetation compared to longer vegetation patches (<i>AR</i> ≤ 1), as this offered more resistance against the approaching flow. The wider vegetation patches (<i>AR</i> > 1) are favorable in terms of generating a large area of low velocity compared to the longer vegetation patch (<i>AR</i> < 1) directly downstream of the vegetation patch. On the other hand, it has a negative impact on the adjacent downstream gap region, where a 14.3–34.9% increase in velocity was observed. The longer vegetation patches (<i>AR</i> < 1) generate optimal conditions within the vegetation region due to great velocity reduction. Moreover, in all the <i>AR</i> vegetation cases, the water turbulent intensity was maximum in the vegetation region compared to the adjacent gap region and air turbulent intensity above the FSL, suggesting strong air entrainment over this region. The results of this study are important in constructing vegetation layouts based on the <i>AR</i> of the vegetation for tsunami mitigation.
Thermodynamics, Descriptive and experimental mechanics
The major objective of this current investigation is to examine the unsteady flow of a thermomagnetic reactive Maxwell nanofluid flow over a stretching/shrinking sheet with Ohmic dissipation and Brownian motion. Suitable similarity transformations were used to reduce the governing non-linear partial differential equations of momentum, energy and species conservation into a set of coupled ordinary differential equations. The reduced similarity ordinary differential equations were solved numerically using the Spectral Quasi-Linearization Method. The influence of some pertinent physical parameters on the velocity, temperature and concentration distributions was studied and analysed graphically. Further investigations were made on the impact of the Eckert number, Prandtl number, Schmidt number, thermal radiation parameter, Brownian motion parameter, thermophoresis parameter and chemical reaction parameter on the skin friction coefficient, surface heat and mass transfer rates. The results were displayed in a tabular form. Obtained results reveal that the Maxwell parameter and the unsteadiness parameter reduce the Maxwell nanofluid velocity and the fluid temperature is increased with an increase in the Eckert number and thermal radiation parameter.
Thermodynamics, Descriptive and experimental mechanics
Max Koch, Juan Manuel Rosselló, Christiane Lechner
et al.
The dynamics of a laser-induced bubble on top of a solid cylinder is studied both experimentally and numerically. When the bubble is generated close to the flat top along the axis of the cylinder and its maximum radius exceeds the one of the flat top surface, it collapses in the form of a mushroom with a footing on the cylinder, a long stem and a hat-like cap typical for a mushroom head. The head may collapse forming a thin, fast liquid jet into the stem, depending on bubble size and bubble distance to the top of the cylinder. Several experimental and numerical examples are given. The results represent a contribution to understand the behavior of bubbles collapsing close to structured surfaces and in particular, how thin, fast jets are generated.
Thermodynamics, Descriptive and experimental mechanics
The problem of laminar to turbulent transition in a boundary layer flow subjected to an adverse pressure gradient is relevant to many engineering applications. Under such conditions, the initially laminar flow within the boundary layer can undergo separation and then become turbulent upon reattachment, as transition is triggered by instabilities within the separated shear layer. In turbomachinery blades with high loading, the transition mechanism is further complicated by the presence of periodic wake disturbances shed by blades that move relatively in the upstream flow. The paper reports an experimental study of the effect of wake disturbances generated upstream on the development of a laminar boundary layer over a flat plate imposed with an adverse pressure gradient that is typical of a highly loaded front-stage compressor blade. Detailed velocity measurements using a hotwire are performed along the plate and the results are analysed both in the time domain and the frequency domain. Description of the major features identified is provided and the leading mechanisms that trigger the transition process are identified to be a possible combination of amplified Tollmien–Schlichting waves and the roll-up of vortices due to the Kelvin–Helmholtz instability of the separated shear layer.
Thermodynamics, Descriptive and experimental mechanics
Singular spectrum analysis (SSA) has been employed effectively for analyzing in the time-frequency domain of time series. It can collaborate with data-driven models (DDMs) such as Artificial Neural Networks (ANN) to set up a powerful tool for mechanical fault diagnosis (MFD). However, to take advantage of SSA more effectively for MFD, quantifying the optimal component threshold in SSA should be addressed. Also, to exploit the managed mechanical system adaptively, the variation tendency of its physical parameters needs to be caught online. Here, we present a bearing fault diagnosis method (BFDM) based on ANN and SSA that targets these aspects. First, a multi-feature is built from pure mechanical properties distilled from the vibration signal of the system. Relied on SSA, the measured acceleration signal is analyzed to cancel the high-frequency noise. The remaining components take part in building a multi-feature to establish a database for training the ANN. Optimizing the number of the kept components is then carried out to obtain a dataset called Tr_Da. Based on Tr_Da, we receive the optimal ANN (OANN). In the next period, at each checking time, another database called Test_Da is set up online following the same way of building the Tr_Da. The compared result between the encoded output and the output of the OANN corresponding to the input to be Test_Da provides the bearing(s) health information. An experimental apparatus is built to evaluate the BFDM. The obtained results reflect the positive effects of the method.
Mechanical engineering and machinery, Descriptive and experimental mechanics
The flow formed by the discharge of inclined turbulent negatively round buoyant jets is common in environmental flow phenomena, especially in the case of brine disposal. The prediction of the mean flow and mixing properties of such flows is based on integral models, experimental results and, recently, on numerical modeling. This paper presents the results of mean flow and mixing characteristics using the escaping mass approach (EMA), a Gaussian model that simulates the escaping masses from the main buoyant jet flow. The EMA model was applied for dense discharge at a quiescent ambient of uniform density for initial discharge inclinations from 15° to 75°, with respect to the horizontal plane. The variations of the dimensionless terminal centerline and the external edge’s height, the horizontal location of the centerline terminal height, the horizontal location of centerline and the external edge’s return point as a function of initial inclination angle are estimated via the EMA model, and compared to available experimental data and other integral or numerical models. Additionally, the same procedure was followed for axial dilutions at the centerline terminal height and return point. The performance of EMA is acceptable for research purposes, and the simplicity and speed of calculations makes it competitive for design and environmental assessment studies.
Thermodynamics, Descriptive and experimental mechanics
Anvar Gilmanov, Alexander Barker, Henryk Stolarski
et al.
When flow-induced forces are altered at the blood vessel, maladaptive remodeling can occur. One reason such remodeling may occur has to do with the abnormal functioning of the aortic heart valve due to disease, calcification, injury, or an improperly-designed prosthetic valve, which restricts the opening of the valve leaflets and drastically alters the hemodynamics in the ascending aorta. While the specifics underlying the fundamental mechanisms leading to changes in heart valve function may differ from one cause to another, one common and important change is in leaflet stiffness and/or mass. Here, we examine the link between valve stiffness and mass and the hemodynamic environment in aorta by coupling magnetic resonance imaging (MRI) with high-resolution fluid−structure interaction (FSI) computational fluid dynamics to simulate blood flow in a patient-specific model. The thoracic aorta and a native aortic valve were re-constructed in the FSI model from the MRI data and used for the simulations. The effect of valve stiffness and mass is parametrically investigated by varying the thickness (<i>h</i>) of the leaflets (<i>h</i> = 0.6, 2, 4 mm). The FSI simulations were designed to investigate systematically progressively higher levels of valve stiffness by increasing valve thickness and quantifying hemodynamic parameters known to be linked to aortopathy and valve disease. The computed results reveal dramatic differences in all hemodynamic parameters: (1) the geometric orifice area (GOA), (2) the maximum velocity <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>V</mi> <mrow> <mi>max</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula> of the jet passing through the aortic orifice area, (3) the rate of energy dissipation <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mover accent="true"> <mi>E</mi> <mo>˙</mo> </mover> <mrow> <mi>diss</mi> </mrow> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> </mrow> </semantics> </math> </inline-formula>, (4) the total loss of energy <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mrow> <mi>diss</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula>, (5) the kinetic energy of the blood flow <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mrow> <mi>kin</mi> </mrow> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> </mrow> </semantics> </math> </inline-formula>, and (6) the average magnitude of vorticity <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>Ω</mi> <mi mathvariant="normal">a</mi> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>)</mo> </mrow> <mo>,</mo> </mrow> </semantics> </math> </inline-formula> illustrating the change in hemodynamics that occur due to the presence of aortic valve stenosis.
Thermodynamics, Descriptive and experimental mechanics