Sidewall Effects Research Articles

A numerical study of the side-wall effects on turbulent bands in channel flow at transitional Reynolds numbers

We investigated the side-wall effects on turbulent bands in channel flow at transitional Reynolds numbers by direct numerical simulations using the open source spectral-element code Nektar++. The width-to-height aspect ratio of 50:1 is considered for this study. Our study shows that turbulent bands can survive the collision with the side wall above bulk Reynolds number of Re≃1000 but decay below Re≃975, i.e. the critical Reynolds number should be approximately between the two Reynolds numbers. We also discussed about the underlying mechanism for the decay of the band at low Reynolds numbers and potential effects of larger spanwise channel widths than that considered in our study. The results are informative for experimental studies of channel flow turbulence at transitional Reynolds numbers.

The effect of side walls on particles mixing in rotating drums

In this paper, we study the effects of the presence and shape of side walls and of the overall length of rotating cylindrical drums on the mixing of particles with differing sizes by application of the discrete element method (DEM). By varying the semi-axis of the spheroidally shaped side walls and the length of the overall drum, we observe the formation of circulation patterns near the side walls. Although there is a vast amount of literature studying mixing regimes in rotating drums, little is known about the effect of the side walls of the drum on particle mixing. The results of our study demonstrate that introducing curved side walls induces a strong circulation pattern near these side walls, but has, paradoxically, a negative impact on mixing and actually promotes segregation. The cause for this segregation is the difference in velocity of differently sized particles near the curved side walls. Large particles accumulate at the curved side walls, whereas small particles move away from the curved side walls. When the length of the drum is increased, the overall effect of the side walls is decreased, although it does remain observable, even in very large drums.

Wave-current interaction with three-dimensional bodies in a channel

A frequency-domain numerical model is developed to investigate the hydrodynamic properties of three-dimensional bodies of arbitrary geometry subjected to the action of waves and weak current in a channel based on higher order boundary element method. The problem is divided into a steady problem and an unsteady problem to be solved separately, where the unsteady potentials are given by two sets of integral equations based on linearizing the channel Green function and the velocity potential with the normalized current speed. The first-order wave force is obtained by the pressure integration over the body surface and the second-order drift force is evaluated by the far-field analysis. After examining the validation of the numerical model with some generalized relations derived in the present study, numerical studies for a Lewis-form ship and a four-column structure are performed to investigate the influence of channel walls on the hydrodynamic properties of bodies. Numerical results show that compared with the open-sea results, the side-wall effects become obvious when the oscillating frequencies are close to the transverse resonant frequencies of the channel, and compared with the cases without current, the current effects may enhance the interaction between the body and the reflected waves from the channel walls.

Clarification of the Mechanism of Pulse Laser Grinding of Nanosecond Lasers Using High-Speed Camera Imaging

Pulse laser grinding (PLG), as a cutting tool processing method, can not only achieve edge sharpening with high precision, but it can also produce surface modification. For example, polycrystalline cubic boron nitride (PCBN) tools processed by PLG can show increased hardness due to the reduction in defects. However, the mechanism of edge formation under PLG processing remains unclear. In this study, by observing the plasma generated during processing using a high-speed camera, the elementary process for each laser pulse of the PLG process was visualized. The plasma luminescence moved successively through four stages: multipoint luminescence, uniform luminescence, the downward movement of the luminous center, and faint luminescence. By comparing the results of three different laser pulse pitches (0.2, 2, and 20 μm), it was found that the pulse pitch had a significant influence on the PLG processing mode. When the pulse pitch was too small, the sidewall effect was likely to lead to local excess machining. The large pulse pitch resulted in processed surfaces that could not be fully covered by laser irradiation, and it was preferred to remove the decrease threshold subsequently. Thus, the moderate pulse pitch condition showed a superior processed surface compared to the others.

Flow-induced vibration and impact of a cylinder between two close sidewalls

The dynamics of a cylinder arranged between two sidewalls in a uniform flow is experimentally investigated. To investigate the effect of sidewalls on flow-induced vibration, both circular and square cylinders are considered. The gap between cylinder and sidewalls is sufficiently small to induce the oscillating cylinder to periodically impact the walls under certain conditions. The dynamic responses of a circular cylinder change dramatically depending on whether the cylinder impacts the walls. An impacting circular cylinder can oscillate with a large amplitude beyond a critical reduced velocity, the magnitude of which is restricted by the gap distance, whereas a non-impacting circular cylinder only oscillates in a lock-in region of the reduced velocity. The periodic impact with the sidewalls, rather than lock-in with vortex shedding, allows the large-amplitude oscillations of the impacting cylinder to persist. The impacting circular cylinder exhibits strong hysteresis, which is not observed for the non-impacting cylinder. Furthermore, the oscillation frequency of the impacting cylinder is proportional to the reduced velocity. The periodic impact acts to improve the power extracted by a damping mechanism in a broader range of reduced velocity. Meanwhile, for a square cylinder between sidewalls, oscillation by galloping is suppressed, and no impacts occur over the entire range of the reduced velocity. The suppression is caused by shear-layer reattachment on the side surface of the square cylinder, which is generally observed at a large angle of incidence for an isolated square cylinder.

Performance enhancement for a dual-chamber OWC conceived from side wall effects in narrow flumes

Performance enhancement is always the focus for the exploration in oscillating water column (OWC) wave energy converter devices. In the present study, the hydrodynamic characteristics of a dual-chamber OWC system composed of two 3D circular sub-units is examined under the framework of OpenFOAM. The model-scale simulations are firstly carried out in a wave flume with different widths under the excitation of regular waves. Then, conceived from the side wall effects in narrow flumes, a pair of harbor walls parallel to the wave propagation direction is introduced; by tuning its deployed location and geometric dimension, the enhancement of wave power extraction can be expected. The results indicate that significant estimation errors in the capture width ratio may occur when the incident wave length is nearly equal to the flume width or the resonant frequency of water column is close to the wave frequency in a narrow flume. In addition, the integrated system consisting of OWC and harbor walls shows that the distance between above two structures can be tuned to a threshold value to maximize the capture width ratio by strengthening the fluids and structures interaction. The medium submerged depth of harbor walls may be an alternative when the trade-off between performance enhancement and economic costs has to be considered during practical construction. Moreover, a well agreement between the model-scale and prototype-scale results proves that the performance enhancement can be scalable to large scales by integrating OWC device with harbor walls.

Numerical Investigation of Effusion Cooling in Gas Turbine Combustor Liner

In this study, a numerical analysis was performed to find the aerothermal characteristics of the effusion cooled gas turbine combustor liner. The analyzed geometric model is a scale model of an actual combustor liner. The study aims to investigate the effect of different blowing ratios by validating an experimental test setup. In experimental studies on effusion cooling, the sidewall effect is a serious problem that can distort the results. Numerical analyses provide advantages in visualizing temperature and velocity contours in different sections of physical model. The counter rotating vortex pairs, the horseshoe vortex and the recirculation zone are the main flow features of the jet mixture. At a blowing ratio of 3.35, numerical analyzes gave the highest value of cooling effectiveness. Although the blowing ratio slightly changes the cooling effectiveness in experimental data, it has been found that the effect of blowing ratio is more pronounced on the numerical results, especially at high blowing ratios.

Axial compression behaviours of tubular sectioned C-shape continuous-supported steel walls in MSB

The ultimate strength behaviour of C-shaped continuous-supported walls employing hot-rolled structural hollow section (SHS) columns in modular steel buildings (MSB) was evaluated using two full-scale axial compression tests. The columns' width-to-length (D/B), width-to-thickness (D/t), and material strength were considered experimental parameters. A three-dimensional (3D) finite element (FE) model was validated against test results, accounting for geometry and material nonlinearities while projecting ultimate strength and failure behaviour. The size, spacing, number of columns, stiffener thickness and sidewall effects were evaluated in a parametric analysis. The columns compressive resistance was predicted using prediction equations from EC3:1-1, AISC360, and GB50017. The results revealed that the column nominal strength was reduced to achieve compression yielding due to columns stiffness differences, mutual constraints, and elastic buckling. If the buckling length is considered or omitted, the EC3:1-1 slenderness limit for Class 3 steel members becomes less conservative. Reducing column size and spacing or increasing stiffener size and number improves column and wall compressive behaviour; however, the sidewalls improved the corner columns constraint while weakening the front wall. GB50017 predictions were the safest; whereas, EC3:1-1 made the most unsafe predictions when buckling was not taken into account.

A novel social distance model reveals the sidewall effect at bottlenecks

Intermittent and periodic outbreaks of infectious diseases have had profound and lasting effects on societies throughout human history. During the global spread of SARS-CoV-2 and the resulting coronavirus disease (COVID-19), social distance has been imposed worldwide to limit the spread of the virus. An additional deliberate intention of keeping a minimum safety distance from neighbors can fundamentally alter the “social force” between individuals. Here, we introduce a new “social distance” term inspired by gas molecular dynamics and integrate it into an existing agent-based social force model to describe the dynamics of crowds under social-distanced conditions. The advantage of this “social distance” term over the simple increasing of the repulsive range of other alternatives is that the fundamental crowd properties are precisely described by our model parameters. We compare the new model with the Helbing and Molnar’s classical model and experimental data, and show that this new model is superior in reproducing experimental data. We demonstrate the usability of this model with a bottleneck motion base case. The new model shows that the bottleneck effect can be significantly alleviated through small wall modifications. Lastly, we explain the mechanism of this improvement and conclude that this improvement is due to spatial asymmetry.

(Invited) The Application of ALD and ALE Passivated MicroLED Display

Micro LED has been engaged to the next generation display technology and evolved for various applications from several electronic manufacturers and institutions. The less than 10×10 μm2 area of micro-LEDs is desired for the high pixel per inch (PPI) of micro display but the sidewall effect from etching process will drop the quantum efficiency. Therefore, a good passivation process is essential to be introduced into micro-LED process to overcome the efficiency droop issue. The passivation technique of atomic layer deposition (ALD) enables to improve the sidewall damages from dry etching and hence the quantum efficiency of micro-LEDs can be enhanced [1, 2]. Furthermore, the improvement can be enhanced within smaller size of micro LEDs. In particular, due to the excellent uniformity and step coverage passivation by ALD Al2O3 thin films, the interest in efficient ALD equipment has gradually increased. The atomic layer etching (ALE) is an advance etching process that promises an excellent depth control in shallow device structures. This paper provides an overview of ALD and ALE process for improving the micro-LED performance.The epitaxial structure of the micro-LED structures was grown on a c-plane sapphire substrate using metal-organic chemical vapor deposition (MOCVD). Following the development of MOCVD, a transparent and ohmic p-contact of indium tin oxide (ITO) with a thickness of 100 nm was deposited by electron-beam evaporation as a transparent and ohmic p-contact. The ITO layer can result in efficient electrical current spreading and provide a transparent emission window for topside emitting devices. N-type GaN, p-type GaN, and the multiple quantum wells (MQWs) active region, which consists of twelve pairs of InGaN wells and a GaN barrier with an emission wavelength of around 445 nm, are the major epitaxial layers. The growth phases of micro-LED processing included mesa etching, ohmic contact metallization, sidewall passivation deposition using Al2O3 of thickness 21.3 nm and SiO2 of thickness 300 nm, and interconnects. The inductively coupled plasma (ICP) dry etching process is used for the mesa etching method, and SiO2 and Al2O3 passivation layers are deposited using PECVD and ALD techniques. Since Al2O3 and SiO2 are well-known dielectric materials for photonic applications, they were selected. Because of the field effect passivation produced by negative fixed charges, Al2O3 has emerged as a unique passivation material for commercial solar cells. The passivation layers tend to prevent surface recombination and reabsorption of emitted light near the etched surface during forward-biased activity. Following the deposition of the dielectric passivation layers, a hole was created using the ICP etching technique for the deposition of metal contacts. The electrode metal consisted of 50/300 nm of Ti/Au for p- and n- metal contacts deposited using electron-beam evaporation followed by rapid thermal annealing. The process flow of ALD passivated micro-LED is shown in Fig. 1(a).Fig. 1(b) showed the effect of the ALD-Al2O3 passivation technique on the EQE enhancement of µ-LEDs. According to the literature, the peak EQE of a small LED is considerably small due to the surface and nonradiative recombination of the mesa layer caused by plasma-assisted dry etching. ALD sidewall passivation is an important method for reducing plasma damage in LEDs. We can observe the 21.3 nm Al2O3 passivation on micro-LED in Fig.1(a). Therefore, Fig. 1 (b) presents the EQEs of 10 × 10-µm2 µ-LEDs with and without an Al2O3 passivation layer deposited using the ALD technology for reducing plasma damage. Compared with the device not subjected to ALD-Al2O3 passivation, the 10 × 10-µm2 array subjected to ALD-Al2O3 passivation exhibited an EQE enhancement of 37.5%. This result proves that ALD sidewall passivation can effectively reduce the sidewall damage caused by SRH nonradiative recombination and thus increase the EQE of small LEDs.

Effect of modeling sidewalls on tire vibration and noise

Low-noise tire design demands the potent model, which regards accessible parameters in the design process before manufacturing a tire. Unlike most previous analytical models that integrated tire treadband with sidewalls with an assumption of identical properties, this research segregates them. The separation clarifies the effects of each tire part on vibration and noise individually, which has not been presented in previous publications and is noteworthy in design. The model is developed considering three connected plates, describing treadband and sidewalls, on an elastic foundation derived from vibro-acoustic coupling inside the tire. Natural frequencies are determined by the Galerkin method using modes shapes satisfying all boundary conditions. The vibration response of a tire rolling on the road is then formulated utilizing Green’s function and convolution integral. Eventually, vibrational tire noise is calculated by the boundary element method. Comparing the proposed model with the repeatedly used integrated plate model has indicated the dissimilarity of treadband and sidewall responses with a difference of 1.4 dB(A) in total noise level. Moreover, implemented parametric study based on a small central composite design has revealed their parameters’ distinct influences on generated noise. For instance, increment in treadband thickness reduces sound level, while decreasing sidewall thickness effectively leads to noise reduction. So, the proposed model is worth employing instead of the previous overused integrated model to predict and reduce tire noise.

Numerical investigation of side-wall effects on the seakeeping performance of a ship advancing in waves

During seakeeping tests of a ship model in a towing tank, the hydrodynamics will show discrepancies when compared to the open-sea results because of side-wall effects. Here, a three-dimensional panel method based on a Rankine-type tank-wall Green function is proposed to solve this problem. This tank-wall Green function, which satisfies the non-penetration condition on the side walls, is in the form of an infinite series, which has slow convergence. Therefore, an approximate computation method is established, and this is found to have satisfying efficiency and accuracy. The present numerical results are compared to the corresponding experimental results from a floating spheroid of beam–length ratio 1/5 and a Lewis-form ship, both advancing with forward speed at Froude number 0.1. Good agreement between the test measurements and the numerical results is obtained in all cases, indicating that the present panel method is useful and applicable. According to the propagation characteristics of the three-dimensional translating–pulsating source Green function, an approximate discrimination method is proposed for identifying side-wall effects. The present numerical method can be further extended to investigate the interactions between side-wall effects and shallow-water effects.

Effects of boundary walls on the properties of settling spheres

Abstract Numerical Simulations are performed, using Eulerian two fluid model (TFM) to investigate the effects of solid volume fraction and no-slip side walls on the settling particles. It is found that average settling velocity decreases with increasing volume fraction for both gas-solid (GS) and liquid-solid (LS) systems, in good agreement with the Richardson-Zaki 1 − ϕ n ${\left(1-\phi \right)}^{n}$ law. It was also noted that average velocity is independent of the boundary condition for both gas-solid (GS) and liquid-solid (LS) systems. The root mean square value of the solid volume fraction shows the increasing trend with volume fraction, caused by the many particle interactions. Furthermore, no-slip sidewalls were found to damp the velocity fluctuations quantitively, while following the well-known ϕ 1 / 2 ${\phi }^{1/2}$ scaling with volume fraction. Side walls were found to act as kinetic trap for the particles, damping the fluctuation near the walls and plateauing in the mid plane. These simulations showed that the GS system shows the higher solid fraction fluctuations that the LS system at the same Reynolds number, mainly because of the higher collision frequency (higher Stokes number) among the particles.

Experimental Study of the Sidewall Effect on Three-Dimensional Turbulent Wall Jet

Abstract An experimental study on the effect of sidewalls on the flow characteristics of a three-dimensional turbulent square wall jet is carried out at a Reynolds number of 25,000. The sidewalls are defined as the two parallel plates along the vertical jet centerline. Four different sizes of sidewall enclosure (here after referred to as ‘SWE’) are placed at the lateral positions (z) of ±3.5h, ±4h,±4.5h, and ±5h from the vertical jet centerline plane, where h is the height of square jet. The mean characteristics of fluid flow in wall normal (y) and lateral (z) directions at different downstream locations (x/h=0.2−45) are measured using a hotwire anemometer. The velocity measurements are also performed in the z – y lateral plane at four downstream locations (x/h=30, 35, 40, and 45). Results indicate that the mean velocity profile in lateral and wall normal directions behaves differently depending on the size of SWEs. The decay rate of the maximum mean velocity increases with decrease in size of SWEs after the downstream location (x/h≥20). The decay rate of the maximum mean velocity increases about 5.6% in 140 mm SWE as compared to 200 mm SWE. It is noted that spread of the jet in wall normal and lateral directions increases with decrease in size of SWEs after the attachment of the flow stream on the sidewalls. In the far-field location of the present case, the smaller size of SWE (140 mm SWE) has 10.7% and 23.1% higher spread rate as compared to larger size of SWE (200 mm SWE) in wall-normal and lateral directions, respectively. It is also seen that the self-similar profile gets delayed in wall-normal direction as compared to lateral direction for all the cases. The wall-normal self-similar profile is obtained early with increase in the size of SWEs and it is obtained at x/h=30, 27, 24, and 20 for 140 mm, 160 mm, 180 mm, and 200 mm SWEs, respectively. The flow stream seems to climb the sidewall and this tendency increases with increase in size of SWEs.

Nature-Based Coastal Protection by Large Woody Debris as Compared to Seawalls: A Physical Model Study of Beach Morphology and Wave Reflection

Anchored Large Woody Debris (LWD) is increasingly being used as one of several nature-based coastal protection strategies along the north-western coasts of Canada and the US. As an alternative to conventional hard armoring (e.g., seawalls), its usage is widely considered to be less harmful to the coastal ecosystem while maintaining the ability to protect the beaches against wave attack and erosion. The effects of seawalls on beaches have been extensively studied; however, the performance and efficacy of LWD and its potential as a suitable alternative to seawalls (and other shoreline protection structures) are still understudied in current research. This paper presents and compares the effects of a conventional vertical seawall with two different LWD structures on beach morphology and wave reflection through large-scale physical modeling in a wave flume at a 1:5 scale. An assessment of techniques used to measure beach morphology and an assessment of model effects were included in the study. It was found that the wave reflection could be reduced by using a single log instead of a wall structure, while changes in the beach morphology response largely depended on the type of the LWD structure. A stacked log wall showed near-identical behavior as a conventional seawall. Visible model effects from the experiments, including the effect of the flume sidewalls on the beach morphology, were quantified and analyzed to inform future research.

Experimental investigation of shock\u2013shock interactions with variable inflow Mach number

Experiments on shock–shock interactions were conducted in a transonic–supersonic wind tunnel with variable free-stream Mach number functionality. Transition between the regular interaction (RI) and the Mach interaction (MI) was induced by variation of the free-steam Mach number for a fixed interaction geometry, as opposed to most previous studies where the shock generator angles are varied at constant Mach number. In this paper, we present a systematic flow-based post-processing methodology of schlieren data that enables an accurate tracking of the evolving shock system including the precise and reproducible detection of RIrightleftarrows MI transition. In line with previous experimental studies dealing with noisy free-stream environments, transition hysteresis was not observed. However, we show that establishing accurate values of the flow deflections besides the Mach number is crucial to achieve experimental agreement with the von Neumann criterion, since measured flow deflections deviated significantly, up to 1.2^{circ }, from nominal wedge angles. We also report a study conducted with a focusing schlieren system with variable focal plane that supported the image processing by providing insights into the three-dimensional side-wall effects integrated in the schlieren images.

Investigation on vertical temperature profile of thermal ejected plume from a compartment fire with an adjacent side wall

This paper presents an investigation of vertical temperature profile of the thermal ejected plume from a compartment fire with an adjacent side wall, which has not been quantified in the literatures. A full-scale compartment model with a vertical facade and an adjacent side wall positioned at the side of the window has been set up in the numerical simulations. The vertical temperature distribution upon the facade with various window dimensions, total heat release rates and side wall separation distances is measured. By comparing with former study, the validity of the numerical simulation results has been confirmed, which shows good agreement with the previous experimental data. Result reveals that only when the side wall separation distance (D) is less than a critical value for each window dimension, will the vertical temperature profile of thermal ejected plume affect by the constraining boundary of adjacent side wall, otherwise it will remain basically the same. Then, a new correlation is proposed to describe the constraining effect of side wall on the vertical temperature profile by introducing a coefficient α. Numerical simulation results for various window dimensions, total heat release rates and side wall separation distances can be well collapsed by the newly proposed correlation.

Investigation on temperature distribution under the coupling action of transverse position and fire sealing of linear fire in tunnel

By sealing the portal at one end of tunnel and changing the transverse location of linear fire source in tunnel, the hot smoke layer, longitudinal temperature distribution and maximum temperature of linear fire source were investigated. It is found that the flame will bend under the influence of curved ceiling and hot smoke layer will be thicker when the linear fire source approaches to the side wall. The variation law of heat release rate is analyzed by considering the limiting effect of side wall and thermal feedback. By modifying the previous formula of effective ceiling height, an empirical formula for predicting longitudinal temperature distribution of linear fire sources with different aspect ratios is proposed, and the ventilation coefficient is obtained to characterized the temperature under sealing and without sealing. Moreover, the accuracy of predicted temperature under sealing is verified by using the ventilation coefficient. Based on dimensional analysis, it's concluded that the maximum temperature mainly depends on the heat release rate, and an empirical formula is proposed to predict the maximum temperature rise of linear fire sources with different aspect ratios in several transverse positions. The error of predicted values and actual values is within 13%.

Investigation of Electrical and Optical Properties of AlGaInP Red Vertical Micro-Light-Emitting Diodes With Cu/Invar/Cu Metal Substrates

This research studies the performance of n-side up thin-film AlGaInP-based vertical micro-light-emitting diodes (V- μLEDs) with four different chip sizes, 100×100, 70×70, 50×50, and 25×25 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , on a 50 μm thick composite metal (copper/Invar/copper; CIC) substrate. The LEDs were fabricated to understand the electrical and optical properties of AlGaInP V- μLEDs as functions of chip sizes. For device performance, the small LEDs provide a larger current density under the same voltage and present smaller forward voltage, a low red-shift phenomenon, and low output power density. For the external quantum efficiency (EQE) of device, larger LEDs exhibit maximum EQE at lower current density as compared to smaller LEDs. The injection of a small current at the same current density obtains the emission image. The obtained data suggest that the smallest V- μLEDs exude a sidewall effect that could impact the device performance.

Increase in the Efficiency of III-Nitride Micro-LEDs: Atomic-Layer Deposition and Etching

MICRO-LEDs ( n-LEDs) HAVE been engaged in the next-generation display technology and evolved for various applications from several electronics manufacturers and institutions. The area of n-LEDs (less than 10 # 10 nm2) is desired for the high pixelsper-inch (PPI) value of the microdisplay, but the sidewall effect from the etching process will drop the quantum efficiency