The transport of fluids through pipes and channels is a foundational topic in fluid mechanics, with direct applications spanning many branches of science and engineering [...]
Thermodynamics, Descriptive and experimental mechanics
Yoshiya Igarashi, Norio Tanaka, Muhammad W. A. Junjua
et al.
To mitigate flood damage caused by overflow from a levee, it is essential to prevent the levee failure or extend the time to breaching. Although turfgrass on a levee slope is effective in suppressing erosion, insufficient maintenance can reduce its coverage. When overtopping occurs under such non-uniform turfgrass conditions, the flow tends to entrain air. In spillways, air entrainment is known to reduce friction loss; therefore, it may also contribute to lowering shear stress and erosion depth. This study conducted flume experiments with artificial turf arranged in various patterns on levee slopes to investigate flow patterns, air entrainment, and erosion. The flow pattern changed depending on the turf arrangement and overflow depth, and air entrainment occurred due to water surface fluctuations around the turfgrass. The inception point of air entrainment was found to be similar to or shorter than that observed in stepped spillways. Furthermore, the experiments showed a tendency for erosion depth to decrease once air entrainment is fully developed. This finding is significant because it suggests that erosion can potentially be minimized not only by reinforcing the levee structure itself but also by modifying flow characteristics through designs that promote air entrainment.
Thermodynamics, Descriptive and experimental mechanics
Modern fighter aircraft increasingly need to conjugate aerodynamic performance and low observability. In this paper, we showcase a methodology for a gradient-based bidisciplinary aero-stealth optimization. The shape of the aircraft is parameterized with the help of a CAD modeler, and we optimize it with the SLSQP algorithm. The drag, computed with the help of a RANS method, is used as the aerodynamic criterion. For the stealth criterion, a function is derived from the radar cross-section in a given cone of directions and weighed with a function whose goal is to cancel the electromagnetic intensity in a given direction. Stealth is achieved passively by scattering back the electromagnetic energy away from the radar antenna, and no energy is absorbed by the aircraft, which is considered as a perfect conductor. A Pareto front is identified by varying the weights of the aerodynamic and stealth criteria. The Pareto front allows for an easy identification of the CAD model corresponding to a chosen aero-stealth trade-off.
Thermodynamics, Descriptive and experimental mechanics
Abstract During the solidification of a sessile drop, the effect of heat exchange from the gaseous environmental medium is generally ignored. However, by combining experimental observations, direct numerical simulations, and a theoretical model, we have demonstrated that the environmental medium, particularly one with high thermal conductivity such as a liquid, has nonnegligible heat exchange with both the drop and the substrate, leading to accelerated cooling of the outer surface of the sessile drop. Consequently, it causes alterations in the geometry of the freezing front and ultimately results in the formation of a solidified shell that encloses the drop. Furthermore, the encapsulated liquid continues to solidify, which induces volume change and consequently changes the final outcome of the freezing process. This study highlights the importance of considering the properties of the environmental medium and provides novel strategies to manipulate the freezing rate and reshape the morphology of the solidified drop.
High-aspect-ratio (HAR) rectangular jets have attracted attention in automobile air conditioning (A/C) systems and turbulent jet applications owing to their excellent air delivery and mixing and attractive interior design. Active flow control (AFC) of rectangular jets using plasma actuators (PAs) has proven to be a promising technique because the actuator is simple, has low energy consumption, and can create flow features without interference. This research aims to understand the interaction between PAs and flow from a HAR rectangular nozzle using hot-wire anemometry, particle image velocimetry, and theoretical studies. Understanding how PAs affect the flow is beneficial for designing air vents to fit automobile A/C systems and various engineering applications by recreating the flow features with other AFC techniques and actuators. The combination of periodic excitation and vectoring effects transfers the flow’s mean energy to organized structures—known as spanwise vortexes—as large as 6 mm. The interaction between these coherent structures and the dissipative environment compresses the vortexes, resulting in the flow converging on the spanwise–streamwise (X–Z) plane and diverging on the transverse–streamwise (X–Y) plane. HAR rectangular jet flow features controlled by PAs can be predicted for specific cases by calculating the Strouhal number based on PA operating parameters.
Thermodynamics, Descriptive and experimental mechanics
This paper is devoted to the description of computational algorithms for modeling quasi-one-dimensional non-steady flows of a multicomponent reacting gas. The particularity of the developed modeling technique is that the paths of strong and weak discontinuities are mobile computational nodes, and the parameters for them are calculated using special algorithms. A set of programs has been developed, which can be used to solve the problems of the reacting gas dynamics that are of applied importance, as well as serve as an illustrator for physical gas dynamics training courses. The paper provides the results of the numerical modeling of the supersonic flow in a flat channel simulating the operation of experimental facilities of the Institute for Problems in Mechanics and the Institute of Physics and Technology. A satisfactory correlation between the calculated and experimental data has been obtained.
Thermodynamics, Descriptive and experimental mechanics
We focus on the fully developed turbulent flow in circular pipes and channels. We provide a comparison of the mean velocity profiles, and we compute the values of the global indicators, such as the skin friction, the mean velocity, the centerline velocity, the displacement thickness, and the momentum thickness. The comparison is done at low-to-moderate Reynolds numbers. For channel flow, we deduced the mean velocity profiles using an indirect turbulent model; for pipe flow, we extracted the needed information from a direct numerical simulation database available in the open literature. A one-to-one comparison of these values at identical Reynolds numbers provides a deep insight into the difference between pipe and channel flows. This line of reasoning allows us to highlight some deviations among the mean velocity profiles extracted from different pipe databases.
Thermodynamics, Descriptive and experimental mechanics
Phuc L. H. Ho, Canh V. Le, Phuong H. Nguyen
et al.
This paper presents a novel formulation for the computational homogenization analysis of materials at the limit state. The polynomial interpolations are employed to impose the periodic boundary conditions for the fluctuating term of the displacement field when using arbitrary finite element meshes. Second-order cone programming provides an efficient solution to solve the resulting optimization problems, and accurate load multipliers can be obtained with the minimum computational cost. Several asymmetrical material models are investigated to perform the efficiency of the proposed method. The collapse mechanisms of the representative volume elements are also presented.
Mechanical engineering and machinery, Descriptive and experimental mechanics
High-speed working units are very widely used in modern construction, in the destruction of durable materials and soils and more. When calculating the power and energy parameters of dynamic working units take into account changes in the nature of the interaction of the cutting element with the environment and the emergence and propagation of soil stresses from the action on the boundary of the array of the cutting element. This leads to the emergence of a stress-strain state in the soil mass, which has an oscillatory-wave character.
The nature of the stress-strain state is influenced by the state of the working environment and the speed of cutting (destruction) of the soil mass.
An unsolved problem in the dynamic destruction of soils is to take into account the kinematic features and technology of work with high-speed peripheral and front-end working units. The design parameters of these working units must take into account not only the dynamic parameters of the fracture process, but also the phenomenon of accumulation of fatigue deformations in the working environment.
This study evaluates the displacement sensitivity of a new compliant microgripper. The microgripper is designed based on a four-bar mechanism and the concept of a compliant mechanism. The effects of the width of the right circular hinge, the thickness of microgripper, and the material properties on the dis-placement sensitivity are considered via using the finite element method. In the beginning, the stress and deformation of the compliant microgripper are evaluated. Subsequently, the displacement of the microgripper is then analyzed. The results showed that the design parameter and the displacement sensitivity have a close relationship. To increase the grasping reliability and measure the displacement or force, a micro-displacement sensor is integrated with the proposed microgripper. Finally, the modeling and analysis of the proposed sensor are conducted.
Mechanical engineering and machinery, Descriptive and experimental mechanics
Alexander V. Babanin, Miguel Onorato, Luigi Cavaleri
We suggest that there exists a natural bandwidth of wave trains, including trains of wind-generated waves with a continuous spectrum, determined by their steepness. Based on laboratory experiments with monochromatic waves, we show that, if no side-band perturbations are imposed, the ratio between the wave steepness and bandwidth is restricted to certain limits. These limits are consistent with field observations of narrow-banded wind-wave spectra if a characteristic width of the spectral peak and average steepness are used. The role of the wind in such modulation is also discussed.
Thermodynamics, Descriptive and experimental mechanics
We put forth a robust reduced-order modeling approach for near real-time prediction of mesoscale flows. In our hybrid-modeling framework, we combine physics-based projection methods with neural network closures to account for truncated modes. We introduce a weighting parameter between the Galerkin projection and extreme learning machine models and explore its effectiveness, accuracy and generalizability. To illustrate the success of the proposed modeling paradigm, we predict both the mean flow pattern and the time series response of a single-layer quasi-geostrophic ocean model, which is a simplified prototype for wind-driven general circulation models. We demonstrate that our approach yields significant improvements over both the standard Galerkin projection and fully non-intrusive neural network methods with a negligible computational overhead.
Thermodynamics, Descriptive and experimental mechanics
The effect of wall velocity slip on the stability of a pressure-driven two-dimensional asymmetric channel flow is examined by considering Navier slip condition on the channel walls. The two-parameter families of mean velocity profiles are considered to approximate the underlying asymmetric basic flow. Competing effects of skewness and maximum velocity on the stability of the flow are explored for a range of model parameters. The Orr–Sommerfeld system of the asymmetric flow is solved using a Chebyshev spectral collocation method for both symmetric and non-symmetric type slip boundary conditions. Numerical results indicate that moderate asymmetry in the basic flow has a significant role on the stability of the Poiseuille-kind parallel/nearly parallel flows. Wall slip shows a passive control on the instability of the asymmetric flow by increasing or decreasing the critical Reynolds number and the set of unstable wave numbers. The stabilizing/destabilizing effect of slip velocity on the flow instability is weak or strong depending on the presence of velocity slip at the upper or lower wall. Velocity slip has a profound grip on the flow behaviour by changing the shear rate inside the perturbed flow.
Thermodynamics, Descriptive and experimental mechanics
In this paper responses of beams subjected to random loading are analyzed by the dual approach of the equivalent linearization method. The external random loading is assumed to be a space-wise and time-wise white noise in which the exact solutions of the modal equations can be found. A system of nonlinear algebraic equations for linearization coefficients of the modal linearized system is obtained in a closed form and is solved by the fixed-point iteration method. Results obtained from the proposed dual criterion are compared with the exact solution and those obtained from other approaches including energy method, and conventional linearization method. It is observed that the solution obtained by the dual criterion is in good agreement with the exact solution, especially, in the case of strong nonlinearity of beam.
Mechanical engineering and machinery, Descriptive and experimental mechanics
This research presents the effectiveness of different shear strengthening methods using epoxy bonded steel plates, steel strips, carbon fiber reinforced plastics (CFRP) sheets and externally anchored stirrups for enhancing the shear strength of RC beams. In this study, an experimental program including two series of twelve specimens was carried out to investigate the behavior of RC beams strengthened with the above methods. Further, a numerical analysis using finite element method (FEM) was performed to simulate the behavior of strengthened beams. The effectiveness of using epoxy bonded steel plates, CFRP sheets and externally anchored stirrups for shear strengthening of RC beams is confirmed through both numerical and experimental results.
Mechanical engineering and machinery, Descriptive and experimental mechanics