Fixed Aspect Ratio Research Articles

Buoyant magnetic milliswimmers reveal design rules for optimizing microswimmer performance.

Magnetically-actuated swimming microrobots are an emerging tool for navigating and manipulating materials in confined spaces. Recent work has demonstrated that it is possible to build such systems at the micro and nanoscales using polymer microspheres, magnetic particles and DNA nanotechnology. However, while these materials enable an unprecedented ability to build at small scales, such systems often demonstrate significant polydispersity resulting from both the material variations and the assembly process itself. This variability makes it difficult to predict, let alone optimize, the direction or magnitude of microswimmer velocity from design parameters such as link shape or aspect ratio. To isolate questions of a swimmer's design from variations in its physical dimensions, we present a novel experimental platform using two-photon polymerization to build a two-link, buoyant milliswimmer with a fully customizable shape and integrated flexible linker (the swimmer is underactuated, enabling asymmetric cyclic motion and net translation). Our approach enables us to control both swimming direction and repeatability of swimmer performance. These studies provide ground truth data revealing that neither the first order nor second order models currently capture the key features of milliswimmer performance. We therefore use our experimental platform to develop design guidelines for tuning the swimming speeds, and we identify the following three approaches for increasing speed: (1) tuning the actuation frequency for a fixed aspect ratio, (2) adjusting the aspect ratio given a desired range of operating frequencies, and (3) using the weaker value of linker stiffness from among the values that we tested, while still maintaining a robust connection between the links. We also find experimentally that spherical two-link swimmers with dissimilar link diameters achieve net velocities comparable to swimmers with cylindrical links, but that two-link spherical swimmers of equal diameter do not.

The effect of slip on the development of flow separation due to a bump in a channel

A numerical study on the effect of surface slip on the flow in a constricted channel is presented, with the aim of exploring the use of surface slip to control flow separation. Our focus is on two-dimensional flow in a channel over a bump, with a fixed aspect ratio, upon which a Robin-type slip boundary condition is imposed. When the channel walls are fully no-slip, such a flow is known to develop a region of separation behind the bump, at sufficiently large Reynolds numbers. The effect of slip on the separation bubble dynamics occurring behind the bump is investigated, for Reynolds numbers $2000$ and $4000$ . It is shown that surface slip (i) attenuates the intensity of separation as it diminishes the minimum of the streamwise velocity within the recirculation region; (ii) delays the onset of flow separation, shifting it downstream, along the bump, and (iii) reduces the dimensions of the separation bubble behind the bump, allowing the flow to reattach sooner. Ultimately, slip inhibits separation, with both the points of separation and reattachment coalescing, for a slip length $\lambda$ of approximately $0.2$ .

Straintronic Nanomagnetic Logic Using Self-Biased Dipole Coupled Elliptical Nanomagnets

This work delineates the thermally stable, self-biased straintronic nanomagnetic logic (NML) by leveraging the dipole coupling between an array of nanomagnets using only two electrodes that discard the intricate voltage mechanism of multi-electrode systems. For efficient NML operation, the self-biased elliptical nanomagnets with fixed aspect ratio can be designed over a particular thickness range, limited by the thermal stability and shape anisotropy energy. Micromagnetic simulations show that the 180° magnetization switching of the nanomagnet is assisted by an external applied voltage and the dipole coupling, allowing controlled NML operations. The relationship between the critical switching frequency and critical strain is used to select the appropriate nanomagnets for different logic operations.

Turbulence Modulation by Slender Fibers

In this paper, we numerically investigate the turbulence modulation produced by long flexible fibres in channel flow. The simulations are based on an Euler–Lagrangian approach, where fibres are modelled as chains of constrained, sub-Kolmogorov rods. A novel algorithm is deployed to make the resolution of dispersed systems of constraint equations, which represent the fibres, compatible with a state-of-the-art, Graphics Processing Units-accelerated flow-solver for direct numerical simulations in the two-way coupling regime on High Performance Computing architectures. Two-way coupling is accounted for using the Exact Regularized Point Particle method, which allows to calculate the disturbance generated by the fibers on the flow considering progressively refined grids, down to a quasi-viscous length-scale. The bending stiffness of the fibers is also modelled, while collisions are neglected. Results of fluid velocity statistics for friction Reynolds number of the flow Reτ=150 and fibers with Stokes number St = 0.01 (nearly tracers) and 10 (inertial) are presented, with special regard to turbulence modulation and its dependence on fiber inertia and volume fraction (equal to ϕ=2.12·10−5 and 2.12·10−4). The non-Newtonian stresses determined by the carried phase are also displayed, determined by long and slender fibers with fixed aspect ratio λtot=200, which extend up to the inertial range of the turbulent flow.

Numerical study of the stability of premixed flames propagating in half-open tubes

This paper studies premixed flame dynamics in half-open tubes by solving the two-dimensional, fully compressible, reactive Navier-Stokes equations on a dynamically adapting mesh using a high-order algorithm. A simplified chemical-diffusive model was used to describe the chemical reaction and diffusive transports in a stoichiometric hydrogen-air mixture. The influence of the length scale was examined by considering four tube heights at a fixed aspect ratio α = 7. The numerical simulations show that the flame evolves into a tulip flame (TF) in all the tubes shortly after being ignited at the open end. Variation in tube size leads to differences in the evolution of TF and generation of expansion waves. In a sufficiently large tube (d > 0.5 cm), the TF further develops into a series of more unstable distorted tulip flames (DTFs). By contrast, in a small tube (d < 0.5 cm), the TF shape remains until the end of the combustion. In addition, both the flame and pressure oscillate significantly almost in the entire process of flame propagation in the large tubes, while the oscillating behaviour in flame or pressure is negligible in the small tube after TF forms. It was found that the TF formation mechanism is length-scale dependent even for the same type of geometry and condition. A detailed examination of the interactions between flame, boundary layer, and pressure waves showed two mechanisms of TF formation: (1) boundary layer effect for the larger tubes (d ≥ 0.5 cm), and (2) Rayleigh–Taylor instability driven by compression waves for the smallest tube (d = 0.25 cm). The DTF formation in the half-open tubes is closely related to the expansion waves generated by the collapse of the TF cusp. The expansion waves cause a reverse flow in the boundary layer ahead of the flame front and consequently initiate the DTF.

Air entrained into viscous liquid by a disk: Confinement induced suppression of breakup

When a solid object falls into a bath of viscous liquid due to gravity with zero initial velocity, air can be entrained into the viscous liquid with the solid. We study such an air entrainment by a metal disk falling in the direction perpendicular to its axis in a confined space. We previously reported that the entrained air is extended and forms a thin sheet, which finally detaches from the disk with a topological change: The extended air breaks up into two parts, after which the lower part becomes a small bubble and the upper part starts retracting upwards [Phys. Rev. Fluids 3, 054004 (2018)]. In the present paper, we further confined the air sheet using thinner disks, to surprisingly find a complete suppression of a topological change: The entrained air detaches from the disk without breakup (i.e., without creating a small bubble) but with forming, instead of a sheet, an elliptic corn at the tip. The physical origin of the confinement induced suppression of breakup is discussed from the viewpoints of the inversion of curvature of an air-liquid interface and the movement of the contact line. Near the corn-forming detachment, three length scales characterizing the dynamics are all proportional to a single characteristic length as in the previous sheet-forming case, which leads to a fixed aspect ratio for the elliptic corn in the present case. The characteristic scale satisfies a simple scaling law, which is different from the previous sheet-forming case, and governs the self-similar dynamics near the detachment point. The present corn-forming case provides a remarkable scenario for the self-similar dynamics discussed in various fields such as statistical physics, applied mathematics, and hydrodynamic topology transitions.

Chaotic phenomena of natural convection for water in a V-shaped enclosure

The changes from steady-state to chaotic natural convection in a V-shaped differentially heated enclosure have been studied numerically because of their importance in nature, such as in a river or lake. The finite volume method (FVM) is applied to simulate the Navier-Stokes and Energy equations with proper boundary conditions. The working fluid is considered water with the Prandtl number, Pr = 7.0 for various Rayleigh numbers ranging from Ra = 100 to 106 with a fixed aspect ratio, A = 0.5. A group of bifurcations in the transitional flow is investigated, including a Pitchfork and a Hopf bifurcations. The occurrence of Pitchfork bifurcation happens between Ra = 1.3 × 104 and 1.4 × 104 and of Hopf bifurcation happens between Ra = 1.6 × 105 and 1.7 × 105. Moreover, another bifurcation occurs between Ra = 2.6 × 105 and 2.7 × 105 for the changeover from intervallic state to chaotic state. Power spectral density of temperature time series, phase space trajectories, and the largest Lyapunov exponents of the unsteady chaotic flows are also investigated. Additionally, heat and mass transfer are measured and explained elaborately with physical significance.

Imperfect O(2) symmetry in counter-rotating split-cylinder flow

The effect of a small imperfection in the counter-rotating split-cylinder flow is studied numerically. The defect is characterized by a small parameter ϵ, corresponding to the difference in the magnitude of rotations in each half of the cylinder. With the two half cylinders not rotating exactly in counter rotation, the O(2) symmetry of the exact counter-rotating case (invariance to azimuthal rotations as well as to an involution consisting of reflections about the mid-plane composed of reflections about any meridional plane) is weakly broken. This small defect results in relevant variations in the flow. For slow rotations (characterized by a small Reynolds number), the system remains axisymmetric with the imperfection only breaking the reflection symmetry about the cylinder half-height. At larger Reynolds numbers, in the absence of the imperfection, axisymmetry is broken resulting in steady states with azimuthal wavenumber m. When axisymmetry is broken in the presence of the imperfection, a background rotation is introduced. Depending on the case and the level of imperfection, either rotating waves or slow-fast dynamics with mean background rotations are found instead. The interaction between azimuthal wavenumbers m = 2 and 3 plays a crucial role in the flow. The flow is analyzed in detail, varying ϵ from a very small value of 0.01%, typical of a natural imperfection in an experimental setup, to higher values corresponding to forced symmetry breaking. The ramifications of the imperfection on various solution states found in the exact counter-rotating case for a fixed aspect ratio are investigated.

On the steady laminar boundary-layer flow over the family of rotating tori

We present the laminar boundary layer flow over the standard tori rotating in an otherwise quiescent incompressible fluid. The mean flow equations are derived by using a specific form of the toroidal–poloidal coordinate system. A pseudo-spectral collocation method mixed with the first-order Euler scheme is employed to solve the related steady mean flow equations. A fixed aspect ratio R is defined as the ratio of radii of the torus and its tube that distinguishes each fixed torus within the general family of standard tori. Tori of spindle type turn into a sphere when the aspect ratio R=0 and thus the mean flow results for the rotating sphere are consistently reproduced. The influence of aspect ratio R on the steady mean velocity components and the wall shear stresses are presented in detail. In a precise manner, the mean flow results for each rotating torus are explainable in a consistent way to the already established case of steady laminar boundary layer flow over the rotating sphere. The significance of the current work is discussed in terms of the hydrodynamic instabilities of boundary layer flows over the family of rotating tori.

Optimization of porous fin location and investigation of porosity and permeability effects on hydro-thermal behavior of rectangular microchannel heat sink

Rectangular microchannel heat sink incorporated with porous fin is studied to improve the thermal performance of conventional solid fin microchannel. Three-dimensional microchannel with fixed aspect ratio and hydraulic diameter is considered and porous fin with three different thickness have been analyzed by mounting at various locations. Copper is used as porous medium with a porosity of 0.4 to 0.8 and permeability of 10−8 - 10−12 m2 are considered. The working fluid is water and the simulations were carried out for the Re range, 500 ≤ Re ≤ 1000. The performance improvement of the porous fin over solid fin is presented with the help of slip length. Pressure reduction efficiency is observed by comparing pressure drop values with and without slip effect. The parametric optimization of fin location has been done with the help of combined hydro-thermal performance. Further, the effect of porous parameters like porosity, permeability and quadratic drag factor on pressure drop and heat transfer coefficient is studied for various fin thicknesses. It is observed that the porous fin with porosity 0.4 and permeability 10−8 m2 has shown better heat transfer characteristics.

Characterization of the Vibration and Strain Energy Density of a Nanobeam under Two-Temperature Generalized Thermoelasticity with Fractional-Order Strain Theory

In this work, fractional-order strain theory was applied to construct a novel model that introduces a thermal analysis of a thermoelastic, isotropic, and homogeneous nanobeam. Under supported conditions of fixed aspect ratios, a two-temperature generalized thermoelasticity theory based on one relaxation time was used. The governing differential equations were solved using the Laplace transform, and their inversions were found by applying the Tzou technique. The numerical solutions and results for a thermoelastic rectangular silicon nitride nanobeam were validated and supported in the case of ramp-type heating. Graphs were used to present the numerical results. The two-temperature model parameter, beam size, ramp-type heat, and beam thickness all have a substantial influence on all of the investigated functions. Moreover, the parameter of the ramp-type heat might be beneficial for controlling the damping of nanobeam energy.

A numerical study on free-fall of a torus with initial inclination angle at low Reynolds numbers

In this study, the sedimentation of a torus is studied numerically over a range of Reynolds number spanning 10 ≤ ReT≤ 90. The three-dimensional governing equations are solved using the fluid–structure interaction version of the SVD-GFD method on a hybrid Cartesian-cum-meshless grid. The torus has a fixed aspect ratio AR = 2, and is set to fall along the negative x direction at an initial inclination angle θ0, in the range of 0 ≤θ0≤ 80°. The flow behavior induced by the falling torus is characterized by the far field vortex sheet and the near field recirculation zone. At a given ReT, the flow reaches the same terminal state and the torus maintains the same final state, orientation and terminal velocity, regardless of θ0. The torus initially experiences a zigzag motion (along the x and z directions) in the global frame and the pitch motion in its body frame, which are more obvious at larger ReT and θ0. The offset of the torus centroid in the z direction between the initial and terminal states increases with increasing ReT and θ0 because of the θ0 effect. The falling time (Tf) for the torus traveling along the same distance in the x direction decreases with increasing θ0 when ReT≤ 50. However, when ReT > 50, Tf initially increases and then decreases with increasing θ0, with the maximum Tf occurring at θ0≈ 60°. A lift-like force is generated in the positive x direction due to the translational torus motion in the z direction, which originates from the pressure. This force is most pronounced at θ0≈ 60° for ReT > 50, which thus significantly decelerates the falling speed of the torus and increases Tf.

Property Assessment of High-Performance Concrete Containing Three Types of Fibers

This study investigated the mechanical properties and the effects of the volume fraction for carbon fibers (CF-VF), polypropylene fibers (PPF-VF) and aramid fibers (AF-VF) with a fixed aspect ratio of 650 for the carbon fibers, 400 for the polypropylene fibers and 900 for the aramid fibers in hybrid fiber-reinforced concrete (HFRC). Furthermore, compressive, splitting tensile and flexural tensile tests were carried out to obtain the optimal total volume fraction for the three types of fibers, as well as the optimal ratio between the CF-VF, PPF-VF and AF-VF. In addition, stress–strain curves of normal concrete and HFRC were examined to explore the whole mechanical process. The results indicated the CF-VF, PPF-VF and AF-VF have a significant effect on the tensile and flexural strengths of HFRC. The HFRC with a fiber additional ratio of 25:50:25 had the best hybrid effect. Moreover, a calculation method based on the compressive strength of normal concrete and HFRC and the volume fraction is proposed to calculate the strength of HFRC in engineering as a reference. Besides, a uniaxial compression constitutive mathematical model of normal concrete and HFRC is established.

Three-dimensional propulsion characteristics of counter-phase oscillating dual-foil propulsor

A three-dimensional computational study was conducted on the propulsive performance of auto-pitch wing-in-ground effect oscillating foil propulsors (APWIGs) using an unsteady Reynolds Averaged Navier-Stokes solver. This novel propulsor is characterized as the combination of dual-foil configuration and spring-based pitching motion. Both the counter-phase oscillating dual-foil arrangement with produced wing-in-ground (WIG) effect and the auto-pitch mechanism based on attached torsional springs are expected to be favorable for performance improvement. To clearly identify the role of two concerned parameters for APWIGs, the study was divided into two parts of simulations to examine aspect ratio and torsional spring stiffness separately. Firstly, the effect of aspect ratio on the hydrodynamic characteristics was investigated by a fully prescribed oscillating dual-foil configuration. The current computations covering a wide range of aspect ratio from 1 to 10 indicated that the three-dimensional effect tends to dominate the propulsion hydrodynamics with a value of lower than 2. The maximum drop of 14.85% in propulsive efficiency due to the finite-span effect was found at the aspect ratio of 1, while a moderate aspect ratio of 4 leads to an acceptable efficiency loss of 3.22%. Secondly, the three-dimensional hydro-elasticity characteristics of APWIGs as a function of spring stiffness were studied by employing a fixed aspect ratio. A relatively low aspect ratio in which the finite-span effect is still dominant was selected to compare the three-dimensional simulations with two-dimensional predictions. It was observed that the torsional spring stiffness has a significant influence on both hydrodynamic performance and vortex structures of finite-span APWIGs. There exists an optimum spring stiffness for finite-span APWIGs corresponding to the highest efficiency, which resembles the hydro-elasticity characteristics of two-dimensional cases. An averaged efficiency loss of around 10% was reported owing to the low-aspect-ratio effect for three-dimensional APWIGs.

A combined experimental-analytical modeling study of the artificial aging response of Al–Mg–Si alloys

The isothermal aging response at 170 °C of three extruded Al–Mg–Si alloys that had been solution-treated and quenched was experimentally tracked to understand the influence of Mg + Si content and the Mg/Si ratio on microstructure and mechanical behavior. Transmission electron microscopy (TEM) was employed to quantitatively characterize the needle-shaped β'' precipitates in the underaged, peak-aged and overaged conditions; while the alloy with a high Mg + Si content raised the number density of β'' precipitates, a higher Mg/Si ratio increased the number density but decreased the size of β'' precipitates in the peak-aged condition. In the overaged condition, coarsening of β'' precipitates occurred primarily along its length. Single crystal micropillar compression tests were conducted in the naturally-aged, under-aged, peak-aged and over-aged conditions to correlate the microstructure to the critical resolved shear stress (CRSS). In the peak-aged condition, the alloy with the high Mg + Si content and a Mg/Si ratio corresponding to the β'' stoichiometry (Mg5Si6) exhibited the highest CRSS. A precipitation kinetics model that assumed a cylindrical morphology for the β'' phase with a fixed aspect ratio was developed to describe β'' phase microstructure evolution as a function of alloy chemistry, solution treatment temperature and artificial aging parameters. The results from this model served as input to a dislocation-precipitate interaction model to calculate single crystal yield strength. The predictions from these analytical models were validated with experimental results from this study and from the literature and fundamental insights were obtained on the role of microstructural parameters in affecting yield strength for non-spherical precipitates.

Numerical and experimental investigation of modified V-shaped turbine blades for hydrokinetic energy generation

The Savonius rotor is one of the simple and cost-effective vertical axis drag type devices for hydropower generation. The main drawback of the Savonius hydrokinetic turbine is its low performance due to negative torque developed by returning blade profile. In this paper, the performance of modified V-shaped rotor blades with different V-angles ranging from 90° to 40°, by maintaining fixed edge length, arc radius and aspect ratio of 0.7 is investigated. The numerical analysis is carried out to estimate the optimum V-angle by maintaining 70 mm depth of water with an inlet velocity of 0.3090 m/s. The numerical study revealed that, for 80° V-angle rotor blade profile, the maximum coefficient of power was found to be 0.2279 at a tip speed ratio of 0.9. This optimum V-angle model was used for experimental analysis to study the effect of aspect ratio ranging from 0.7 to 1.75 using top, middle and bottom plates by maintaining 140 mm depth of water and inlet velocity of 0.513 m/s. The rotor blade with two endplates and one middle plate with an aspect ratio of 1.75 has shown a significant increase of performance by 86.13% at a tip speed ratio of 0.86 as compared to turbine blade with two endplates.

The vibration of a viscothermoelastic nanobeam of silicon nitride with variable thermal conductivity induced by ramp-type thermal loading

In the present paper, the analysis has been constructed for thermoelastic homogeneous isotropic nanobeams. A generalized model of the viscothermoelasticity theory of one relaxation time with variable thermal conductivity has been used under simply supported conditions for fixed aspect ratios. Laplace transform has been applied for the governing differential equations. The inverse of the Laplace transform has been calculated by using the Tzou approximation method. The numerical results have been validated for a thermoelastic rectangular nanobeam of silicon nitride as a case when it is subjected to ramp-type heating and simply supported. The numerical results have been shown in figures to stand on the viscothermoelastic parameter's effects and the ramp-type heat parameter. The effect of the variability of thermal conductivity is a very significant consideration. The nanobeam thickness has a significant impact on all the studied state functions.

Deep Learning-Based Subsurface Target Detection From GPR Scans

Ground penetrating radar (GPR) has been widely used as a non-destructive technique to detect subsurface objects. Manual interpretation of GPR data is tedious and time-consuming. To address this challenge, an automatic method based on a deep instance segmentation framework is developed to detect and segment object signatures from GPR scans. The proposed method develops the Mask Scoring R-CNN (MS R-CNN) architecture by introducing a novel anchoring scheme. By analyzing the characteristics of the hyperbolic signatures of subsurface objects in GPR scans, a set of anchor shape ratios are optimized and selected to substitute the predefined and fixed aspect ratios in the MS R-CNN framework to improve the signature detection performance. In addition, transfer learning technique is adopted to obtain a pre-trained model to address the challenge of insufficient GPR dataset for model training. The detected and segmented signatures can then be further processed for target localization and characterization. GPR data of tree roots were collected in the field to validate the proposed methods. Despite the noisy background and varying signatures in the GPR scans, the proposed method demonstrated promising results in object detection and segmentation. Computational results show that the improved MS R-CNN outperforms the other state-of-the-art methods.

An Experience-Based Direct Generation Approach to Automatic Image Cropping

Automatic Image Cropping is a challenging task with many practical downstream applications. The task is often divided into sub-problems - generating cropping candidates, finding the visually important regions, and determining aesthetics to select the most appealing candidate. Prior approaches model one or more of these sub-problems separately, and often combine them sequentially. We propose a novel convolutional neural network (CNN) based method to crop images directly, without explicitly modeling image aesthetics, evaluating multiple crop candidates, or detecting visually salient regions. Our model is trained on a large dataset of images cropped by experienced editors and can simultaneously predict bounding boxes for multiple fixed aspect ratios. We consider the aspect ratio of the cropped image to be a critical factor that influences aesthetics. Prior approaches for automatic image cropping, did not enforce the aspect ratio of the outputs, likely due to a lack of datasets for this task. We, therefore, benchmark our method on public datasets for two related tasks - first, aesthetic image cropping without regard to aspect ratio, and second, thumbnail generation that requires fixed aspect ratio outputs, but where aesthetics are not crucial. We show that our strategy is competitive with or performs better than existing methods in both these tasks. Furthermore, our one-stage model is easier to train and significantly faster than existing two-stage or end-to-end methods for inference. We present a qualitative evaluation study, and find that our model is able to generalize to diverse images from unseen datasets and often retains compositional properties of the original images after cropping. Our results demonstrate that explicitly modeling image aesthetics or visual attention regions is not necessarily required to build a competitive image cropping algorithm.

A Fast-Acting Method for Simulating Precipitation During Heat Treatment of Superalloy 718

A fast-acting, mean-field method for simulating precipitation of the γ′′ and γ′ phases during aging of superalloy 718 following super-delta-solvus solution treatment was formulated and validated using observations in the literature. The approach assumed classical (homogeneous) nucleation and diffusion-controlled growth (N&G) of disk/ellipsoidal-shaped-γ′′ and spherical-γ′ particles. For the γ′′ precipitates in particular, the evolution equations for both nucleation and growth incorporated corrections for the non-spherical shape, assuming a fixed aspect ratio. In addition, special attention was paid to the choice of input material properties for simulations. These parameters included the bulk free energies of transformation, particle-matrix (misfit) elastic strain energy (for γ′′), effective diffusivities, and the γ′′–γ and γ′–γ interface energies. The applicability of the diffusivities and interface energies chosen for the N&G simulations was established by their consistency in replicating previously measured rate constants for the diffusion-controlled coarsening of both γ′′ and γ′. The N&G formulation was discretized to obtain numerical (spreadsheet) solutions via the Kampmann–Wagner approach. Simulation results for the temporal evolution of volume fraction and average size of the precipitates showed good agreement with experimental measurements. The sensitivity of model predictions to various input parameters was also quantified.