Effect Of Separation Distance Research Articles

The effect of separation distance on hydrogen spillover in Os promoted Co@HCS catalysts

Decreasing the carbon shell thickness increased the reducibility of Co3O4. The reducibility of Co3O4 was further achieved by adding an Os promoter outside of the sphere showing that H spillover was favoured over H2 diffusion/Co reduction by carbon.

Effects of separation distance, lateral offset, and yaw on the drag of a truck and SUV platoon

The recent popularity of electric vehicles and upcoming governmental regulations on emissions are forcing automotive manufacturers to build more efficient vehicles. An approach to improve the efficiency of a road vehicle is by reducing its aerodynamic resistance. One way of achieving this is through platooning, vehicles drive in close-proximity. Despite of the extensive literature available on platooning of multiple vehicle types, most studies have focused on the drag and flow effects around the truck only. This work aims to increase the understanding and recognize the usefulness of a truck-SUV platoon system as a whole using wind tunnel measurements on two scale models and CFD simulations. From the results of this study, it was found that the effects on the SUV are large and contribute to a significant portion of the drag reduction of the system, especially at short inter-vehicle distances where the drag of the car can become negative. The drag reduction for the truck is generally limited as the truck is a much larger vehicle. For the system, the results show that for very short distances it is beneficial for the truck to lead the platoon, while at longer separations there is no preferable order.

Acoustic scattering of a pair of rigid spheroids based on the T-matrix method

In this study, the T-matrix method combined with the addition theorems of spherical basis functions is applied to semi-analytically compute the underwater far-field acoustic scattering of a pair of rigid spheroids with arbitrary incident angles. The involvement of the addition theorems renders the multiple scattering fields of each spheroid to be translated into an identical origin. The accuracy and convergence property of the proposed method are verified and validated. The interference of specular reflection wave and Franz wave can be spotted from the oscillations of the form function. Furthermore, the propagation paths of specular reflection and Franz waves are quantitatively analyzed in the time domain with conclusions that the Franz waves reach the observation point subsequent to specular reflection waves and the time interval between these two wave series is equal to the time cost of the Franz waves traveling along the sphere surfaces. Finally, the effects of separation distances, aspect ratios (the ratio of the polar radius to equatorial radius), non-dimensional frequencies, and incidence angles of the plane wave on the far-field acoustic scattering of a pair of rigid spheroids are studied by the T-matrix method.

Capillary forces and capillary bridges between a three-finger microgripper and a plate

ABSTRACT The presence of surface tension in liquids, which induces a strong capillary force between wetting particles in micro/nano scale, has aroused extensive attention. In present study, a fundamental investigation on capillary forces and rupture behaviors of capillary bridges between a three-finger microgripper and a plate is conducted in quasi-static state. Theoretical analysis is performed for solutions of the capillary force. The capillary bridges between a three-finger microgripper and a plate are established based on the principle of energy minimization. An analytical approach for computing the capillary force for the three-finger/plate geometry is proposed by means of variables obtained from the simulation models. The comparison of the single-finger capillary bridge and three-finger capillary bridge is investigated based on the developed models. The effects of separation distance, capillary bridge volume, radial distance and contact angle on the capillary force of three-finger capillary bridges are analyzed in detail. The results demonstrate that the variation of capillary force with separation distance and volume changing is not monotonic, which is caused by the edge effect of the three-finger microgripper. Capillary force measurements were experimentally characterized to demonstrate the reliability of the simulation models and the capillary force solution method based on an established experimental platform.

Upward Flame Spread over Two Parallel Paper Sheets under Reduced Ambient Pressure and Elevated Oxygen Concentration

ABSTRACT The effects of separation distance and environmental parameters (such as ambient pressure and oxygen concentration) on the upward flame spread over two parallel paper sheets were investigated in this work, which would be conducive to enriching the fire dynamic theories and providing a reference for the risk evaluation in building and space fires. It was found that, with the increase of separation distance, the flame spread regimes can be roughly divided into four classes: the flame quenching regime (without inner flame), where the interaction of two parallel papers is mainly dominated by the solid surface radiation; the complete fusion regime, limited by the oxygen diffusion; the partial fusion regime, dominated by the convection; the separated flames regime, dominated by the gas-phase radiation. The flames inside the narrow space are fused under the flame-induced horizontal flow, and extinguished for the oxygen starvation, which reflects the interaction of two parallel samples. Interestingly, although the flames inside and outside the parallel sheets often have different flame lengths and burning intensities, they spread at almost the same rate, which is ascribed to the overlapping of the pyrolysis region on both sides of the thermal thin paper. A critical separation distance was found, where the flame spread rate is largest due to the stoichiometric mixture fraction. The orange-blue layered flames were analyzed, which is related to the boundary layer of the flow field and the mixture of fuel and fresh air. The oxygen concentration has nearly equivalent impacts on the flame-induced buoyancy and horizontal flows, and thus a constant flame fusion angle is maintained. Nevertheless, the enhanced ambient pressure mainly promotes the buoyancy and reduces the flame fusion angle. The flames inside the narrow space are apt to be extinguished under lower ambient pressure with weak buoyancy similar to the reduced gravity and microgravity environments. The transformation of the dominating regimes were observed at a moderate separation distance: the flame quenching regime at pressures below 18 kPa, the complete fusion regime at pressures between 18 ~ 29 kPa, and the partial fusion regime at pressures over 29 kPa.

Effects of a vertical wall on wave power absorption with wave energy converters arrays

The application of in-house transient wave-body interaction ITU-WAVE computational tool is extended to predict the wave power absorption with Wave Energy Converters (WECs) arrays in front of a vertical wall using time dependent boundary integral equation method. The vertical wall effect is taken into account with method of images which considers the perfect reflection of incident waves from a vertical wall. The effects of separation distance between WECs as well as a vertical wall and WECs, and heading angles are studied to predict wave power absorption, mean and individual interaction factors which determine the performances of WECs arrays. The numerical results of WECs arrays in front of vertical wall show that both radiation and exciting force parameters are quite different from those of without a vertical wall. The numerical investigations also demonstrate that wave power absorption with an array system in front of a vertical wall are significantly greater than those of without a vertical wall. This is due to nearly trapped and standing waves between a vertical wall and WECs. The prediction of hydrodynamic parameters in front of a vertical wall with present ITU-WAVE are validated against other published numerical, analytical, and experimental results which show satisfactory agreements.

Packing characteristics of sub-micron particles under van der Waals force and electrostatic force via discrete element method

Benefiting from the large specific surface area and controllable physical behaviors, sub-micron sized polymer particles (100–1000 nm) have been fabricated as sensors, color displays and energy management devices. The formation mechanisms of sub-micron particles' structure are still not clear. In this work, a DEM model for sub-micron particles is built by applying van der Waals force and electrostatic force in the model. The packing patterns and porosity are validated with experimental results. The packing characteristics including packing patterns, porosity, coordination number (CN) and forces are examined. Clusters and chains structure is found for sub-micron particles, with large porosity but also large coordination number, different with the relationship of porosity and CN of micron particles. Meanwhile, the effect of separation distances and Ha constant and charge amount are investigated. Suitable separation distances for van der Waals force calculation are obtained for different sized sub-micron particles.

On the study of the psychological effects of blocked views on dwellers in high dense urban environments

This paper studies the effects of neighbourhood views of densely packed high-rise buildings on human perceptions of oppressiveness and noise annoyance. Earlier studies have found that views containing close-by tall buildings would not only negatively affect annoyance induced by road traffic noises but also develop an oppressive feeling. However, there is very limited understanding about the extent to which views blocked by buildings with tight spacing and separation distance affects the responses of oppressiveness and noise annoyance. Moreover, few studies have explored the potential of vertical greening and façade condition on attenuating those responses. This study aimed at formulating multivariate models to predict the probability of evoking perceived oppressiveness and noise annoyance when exposed to blocked views from indoors in a canyon-like setting conditioned by those spatial and façade attributes at road traffic noise levels between 55 and 65 dBA. Sixteen audio-visual composite scenarios were presented to 53 participants through immersive virtual reality for annoyance and oppressiveness ratings. The results showed that the probability of high noise annoyance responses could be significantly lowered with a larger separation distance, the presence of vertical greening, and maintenance on building façades. In addition, decreasing noise level, increasing separation distance, presence of building spacing, vertical greening and well-maintained building façades were likely to reduce oppressive feeling. Further, the model revealed that participants had higher sensitivity to vertical greening than to the openness of view when perceiving noise annoyance. The effect of separation distance was significantly stronger on perceived oppressiveness than on noise annoyance.

Computational fluid dynamics modelling of multiphase flows in double elbow geometries

This study investigates the effects of elbow on the transition and development of multiphase flow using computational fluid dynamics modelling techniques. The Eulerian - Multifluid VOF model coupled with an Interfacial Area Transport Equation has been employed to simulate air-water two-phase flow in a pipe with two standard 90 degree elbows mounted in series. Turbulence effects were accounted for by the RNG k-ε model. The effects of separation distance on two-phase flow development have been studied for initial slug and churn flow regimes. Computational fluid dynamics simulation results of phase distribution and time series of void fraction fluctuations were obtained and they showed good agreement with available experimental data. The results show that for initial slug flow regime, there is no flow regime transformation upstream and downstream of the two elbows. While at initial churn flow regime, flow regime transformation occurs at different sections of the flow domain before and after the two elbows. It was noticed that irrespective of the flow regime, the amplitudes and frequencies of void fraction fluctuation become smaller as the fluid flows along the pipe. Changes in the separation distance between the two elbows have larger effects on the flow at churn flow regime.

Computational Study on Surface Bonding Based on Nanocone Arrays.

Surface bonding is an essential step in device manufacturing and assembly, providing mechanical support, heat transfer, and electrical integration. Molecular dynamics simulations of surface bonding and debonding failure of copper nanocones are conducted to investigate the underlying adhesive mechanism of nanocones and the effects of separation distance, contact length, temperature, and size of the cones. It is found that van der Waals interactions and surface atom diffusion simultaneously contribute to bonding strength, and different adhesive mechanisms play a main role in different regimes. The results reveal that increasing contact length and decreasing separation distance can simultaneously contribute to increasing bonding strength. Furthermore, our simulations indicate that a higher temperature promotes diffusion across the interface so that subsequent cooling results in better adhesion when compared with cold bonding at the same lower temperature. It also reveals that maximum bonding strength was obtained when the cone angle was around 53°. These findings are useful in designing advanced metallic bonding processes at low temperatures and pressure with tenable performance.

Varying VAWT Cluster Configuration and the Effect on Individual Rotor and Overall Cluster Performance

The effect of separation distance between turbines on overall cluster performance were simulated using computational fluid dynamics software and we found that at a distance equivalent to two rotors, there was an improvement of +8.06% in the average performance of the cluster compared to a single, isolated turbine. A very small improvement in performance was noted at the equivalent distance of 12 rotor diameters. The performances of three individual turbines in pyramid- and inverted pyramid-shaped vertical axis wind turbine clustered farm configurations with varying oblique angles at a fixed spacing of two equivalent rotor diameters were also investigated. The design experiment involves the simulation of test cases with oblique angles from 15° to 165° at an interval of 15° and the turbines were allowed to rotate through 18 full rotations. The results show that the left and right turbines increase in performance as the angle with respect to the streamline axis increases, with the exception of the 165° angle. The center turbine, meanwhile, attained its maximum performance at a 45° oblique angle. The maximum cluster performance was found to be in the configuration where the turbines were oriented in a line (i.e., side by side) and perpendicular to the free-stream wind velocity, exhibiting an overall performance improvement of 9.78% compared to the isolated turbine. Other array configurations show improvements ranging from 6.58% to 9.57% compared to the isolated turbine, except in the extreme cases of 15° and 165°, where a decrease in the cluster performance was noted due to blockage induced by the left and right turbines, and the center turbines, respectively.

Critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires of various distances: Experiments and a revisited model

Fire smoke in a tunnel fire is a key factor causing casualties, thus the critical longitudinal ventilation velocity is a key parameter in tunnel fire safety ventilation design. The correlation of critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires has rarely been studied experimentally and quantified before, despite the fact that this scenario exists in practice. In this paper, the critical longitudinal ventilation velocities for smoke control in the tunnel, induced by two nearby fires were experimentally quantified, involving various separation distances between the two fires of both axi-symmetrical and linear sources of various heat release rates, with a total of 72 test conditions. Results show that the effect of separation distance between the two fires on the critical longitudinal ventilation velocity, decreases with the increase of fire heat release rates. The critical longitudinal ventilation velocities are found to firstly decrease with increasing separation distance between the two fires, and then remain unchanged when the separation distance between two fires beyond a critical value. A new modified model, involving two fires of various separation distances, is proposed. The model is shown to well represent the experimental results.

The Effect of Separation Distance Between Informal Dwellings on Fire Spread Rates Based on Experimental Data and Analytical Equations

Globally, the number of informal settlement dwellings are increasing rapidly; these areas are often associated with numerous large fires. Unfortunately, until recently, very little research has been focused on informal settlement fire issues leaving any attempts to improve their fire safety lacking the evidence base to support effective-decision making. However, over the past 4 years, a limited number of researchers have looked at better understanding these fires through full-scale experimentation and numerical modelling; starting to provide the necessary evidence base and future research directions. It is with this background in mind that this paper seeks to provide a more fundamental understanding of the effect of dwelling separation distance on informal settlement fire spread based on full-scale experiments and analytical equations. In this paper two full-scale experiments were conducted. Both experiments consisted of multiple dwellings, with the main difference between the experiments being the separation distance. Fire spread times, heat release rates, door and window flow velocities, ceiling temperatures and incident heat fluxes were recorded and are reported for both experiments. Theoretical neutral planes are derived and compared to the experimental neutral planes, which show relatively good correlation. The paper continues by calculating the expected incident radiation and time-to-ignition, using the flux-time product method, of the two fire scenarios (i.e., the two experiments) through means of analytical equations, and these findings are compared to the experimental results. Through configuration factors, the paper shows the effect of separation distance, dwelling height and dwelling length on the times-to-ignition, where it is clear that the heat flux received by an adjacent dwelling decrease approximately exponentially as the distance between dwellings increases, and consequently, the time-to-ignition increases exponentially as the separation distance between dwellings increases.

Experimental study on hypersonic shock–body interaction between bodies in close proximity using translucent fast pressure- and temperature-sensitive paints

This work presents a successful implementation of fast-responding pressure- and temperature-sensitive paints for study of hypersonic shock–body interaction between stage separation bodies. Fast PSP and TSP were applied symmetrically on two adjacent surfaces with a minimum separation of 5 mm, and the time-resolved pressure and temperature fields were obtained in a Ma = 6 flow using the intensity-based approach. The technical barrier of limited optical access was overcome through the development of translucent paints that allowed back-illumination and imaging through a glass wall. The in situ calibration was generally sufficient to remove the temperature-induced error for cases with weak/mild shock impingement on the surface. For cases with strong shock impingement and large temperature gradient, temperature correction was applied on the PSP data based on the TSP results prior to the in situ calibration. The PSP results with temperature correction showed good agreement with the transducer data. The complex flow structures due to shock–body interaction were clearly visualized by PSP and TSP, which allowed detailed analysis on the effects of separation distance and inclined angle.

Interaction of two counterpropagating laser induced plasmas and shock waves in air

The interaction of two counterpropagating plasmas and shockwaves due to a 7 ns laser induced breakdown (LIB) of ambient atmospheric air was studied using the 2D emission and shadowgraphic imaging technique. The effect of separation distance (d) between the two plasma sources and their energy ratio on plasma and shockwave evolution is presented. These low density and low temperature plasma sources form a stagnation layer in the interaction zone that showed an interesting dependence on the input energy of the sources. The interaction dynamics of the two counterpropagating plasma plumes and the shock waves in ambient air are compared with that of the respective single plasma source to explicitly bring out the energy exchange during the interaction. The evolution of a plasma jetlet, studied by 2D emission images and shadowgraphic imaging, has shown very good correlation giving valuable insight into the role of a plasma induced shockwave on the expanding plasma. The propagation of the shock front of the high energy source through the plasma of a low energy source is observed to predominantly lead to the formation of a jetlet and its evolution.

Study of flow field and heat transfer characteristics for an interacting pair of counter-rotating dual-swirling impinging flames

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of counter rotating dual-swirling flames impinging on a flat surface. Effect of separation distance (H/Dh = 3, 4, 6 and 8), inter-jet spacings (S/Dh = 4, 7 and 10) and Reynolds number on flow interactions and impingement heat transfer have been studied under stoichiometric conditions. The inner non-swirling flames are seen to bend towards interacting side due to asymmetric interactions developed in a pair of dual-swirling flames. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting dual-swirling impinging flames. Numerical simulation predicted the deflection of inner flames to be primarily due to large distorted RCZ developed from asymmetric interactions. Comparisons of co-and-counter swirling configurations suggest that the presence of plate has a stronger influence on flow structure of impinging flames than sign of rotation. High resolution heat fluxes have been obtained using steady state 1-D heat balance from temperature readings obtained on rear side of quartz impingement plate through IR thermal camera. 1-D heat balance has been corrected for lateral conduction taking place inside the impingement plate using an iterative procedure. For counter swirl case, cross-flow leaves as a straight flow along the transverse direction due to the nature of interactions. Cross-flow causes heat flux contours in the interaction region to be asymmetric and preferential heating is observed along the transverse direction. The increase in Re(o) causes wider flame spread and severe interactions result in increased tendency for preferential heating of the impingement plate. High average heat fluxes are obtained at smallest H/Dh and S/Dh whereas minimum relative deviation is observed at S/Dh = 10 for H/Dh = 4.

Heat transfer and fluid flow characteristics of a pair of interacting dual swirling flame jets impinging on a flat surface

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of dual interacting swirling flames impinging on a flat surface. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting isothermal swirling impinging jets. Inverse heat conduction procedure (IHCP) has been used to calculate the impingement heat fluxes from the surface temperatures captured by Infra-red camera. Effect of separation distance (H/Dh = 2.5, 4, 6 and 8) and inter-jet spacings (S/Dh = 4, 6, 8 and 10) have been studied at various Reynolds numbers (Re(o) = 7000, 9000, 11000, 13,000 and Re(i) = 700, 1000, 1300) under stoichiometric conditions. Strong interactions between adjacent dual swirling flames result in high heat transfer due to increased mixing and turbulence in the interaction region. The inner non-swirling flames are seen to deflect towards interacting side due to asymmetric interactions. Numerical simulation predicted this deflection to be primarily due to large recirculation bubble developed from asymmetric interactions. Tilted cross-flow, emerging from interaction region has been observed due to momentum exchange taking place between cross-flow and swirling flames (jets). Area weighted average of local heat flux and relative deviation from averaged value has been calculated at various H/Dh and S/Dh. High average heat fluxes are obtained at smallest H/Dh and S/Dh. It has been concluded that for a system of burners considered for the present study, H/Dh = 2.5 and S/Dh = 8 is the optimum configuration on the basis of minimum relative deviation.

(Invited) Energy Transfer and Catalysis in Metal Organic Framework Arrays

Metal Organic Frameworks (MOFs) provide a promising platform for the control of energy transfer within light-harvesting arrays. The efficiency of energy transfer is related to three major factors - the degree of spectral overlap between the donor (emission) and acceptor (absorption), the distance between donor and acceptor, and the orientation of the donor and acceptor in 3D space. Our group has pioneered work in the investigation in effects of separation distance and orientation on energy transfer in chromophore-modified MOFs. Given our results, design parameters for MOF energy transfer candidates will be provided. Additionally, we will explore the capability of MOF arrays to promote electron transfer catalysis. Our results indicate that MOFs promote various energy-related catalytic reactions including oxygen reduction, carbon dioxide reduction, and water oxidation. The mechanism of electron transport through these catalysts-modified MOFs is one of ambi-polar redox hopping. Through MOF incorporation, the current resultant under electrochemical conditions is increased by two orders of magnitude.

Low‐profile quad‐band perforated rectangular dielectric resonator antenna for wireless communications

A novel low-profile, high-efficiency quad-band perforated rectangular dielectric resonator antenna (RDRA) for wireless communication applications has been introduced. The quad-band perforated RDRA consists of a single dielectric sheet virtually divided into four sections. The perforation technique is used to alter the dielectric constant of each section. The effective dielectric constant is changed according to the perforation radii and their separation distances. The quad-band RDRA is made of four blocks of different materials. An investigation of the radiation characteristics of the quad-band perforated RDRA has been introduced. Two quad-band perforated RDRA array elements are introduced for multiple-input–multiple-output applications. The effect of separation distances between the array elements for two different array arrangements has been studied. Cutting slots in the ground plane is employed to minimise the mutual coupling between the perforated RDRA array elements. The antenna structure is designed and analysed using the finite-integration technique, and confirmed using the finite-element technique.

Performance analysis of solo Duck wave energy converter arrays under motion constraints

This paper studies the power capture performance of solo Duck wave energy converter (WEC) arrays. The barrier function method combined with a quasi-Newton BFGS optimization algorithm is applied to find the maximum captured power of the array when the Ducks are under motion constraints. Based on this optimized maximum captured power, the effects of separation distance, wave period, incident wave direction and Duck width on the array performance are investigated. For the two Ducks array, results show that the alternative constructive and destructive interaction stripes in the contour plot of the q-factor variation with non-dimensional separation distance are resulted from the diffracted wave pattern from each Duck, and the hydrodynamic interaction strength is reduced when constraints affect the performance. For the three Ducks array, the middle Duck shows larger variability of captured power than the side Ducks due to experiencing double in phase diffracted wave from the side ones. The captured power of the solo Duck WEC array is sensitive to incident wave direction, and arrays with Ducks of smaller width are found to have better performance in power capture efficiency.