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Due to the high thermal load of the gas turbine, effective cooling of the blade is important, in which film cooling is a common used method. However, there is no universal consensus on the cooling hole arrangement, especially on the blade tip. In this paper, three typical cooling hole arrangements on blade tip with squealer structure have been investigated, namely Near Pressure Side (NPS), Camber Line (CL), and Near Suction Side (NSS). Numerical and experimental methods are applied to obtain the influence of cooling hole arrangement on cooling and aerodynamic performance at blade tip region. The distribution of film cooling effectiveness on blade tip surface and total pressure loss of blade cascade are analyzed. Results show that, with high blowing ratio (M), the coolant decreases the tip leakage flow with CL and NSS. For cooling effect, the CL and NPS arrangements result in better film cooling effectiveness than NSS arrangement. A topology model describing the cooling jet migration in the tip region is proposed, which shows that coolant coverage area increases where the cooling hole is arranged at the pressure side cavity vortex (PSCV) separation line. The proposed topology model provides direct guide on the optimized arrangement of cooling holes. Results show that much improved cooling effectiveness can be obtained, which is 68.6% higher than that of the CL arrangement. Albeit very simple, the proposed model can be used as a guideline for film hole arrangement on blade tip. [ABSTRACT FROM AUTHOR]

Vortex-induced vibrations (VIV) of an elastically supported circular cylinder (of diameter D) undergoing a wall interference are investigated for a mass-damping ratio 0.31, Reynolds numbers 1.77 × 103–1.24 × 104, and wall-cylinder gap height ratios e* = e / D = 0.1–1.6. Measurements of vibration responses and vortex shedding frequencies are done using a laser vibrometer and a hotwire anemometer, respectively, while particle image velocimetry techniques are employed to capture flow fields. This investigation aims to assimilate the effect of e* on vibration response, frequency response, VIV range, quality factor of VIV, added mass, added damping, lift force, and flow structure. A smaller e* is observed to increase the added mass, reduce the maximum vibration amplitude, and shorten the VIV range while enhancing the quality factor of VIV. The increase in added mass and the decrease in the maximum vibration amplitude in the VIV range are dramatic when e* is decreased from 0.6. Decreasing e* significantly impacts the flow structures and vortex shedding modes, resulting in intensified asymmetry in pressure distributions. The interference between the wall and the cylinder is further reflected by increasing time-mean lift forces and decreasing fluctuating lift forces with decreasing e* < 0.6. • Vortex-induced vibrations of a circular cylinder undergoing wall interference are investigated. • A smaller gap spacing ratio increases the flow-induced added mass and quality factor. • Interference between the cylinder and the wall is strong when the gap spacing ratio is less than 0.6. • A smaller gap spacing ratio reduces the time-mean stagnation pressure. [ABSTRACT FROM AUTHOR]

This study characterizes thin fuel opposed flow flame spread in simulated microgravity for a range of gap heights and airflow velocities in a Narrow Channel Apparatus (NCA). One objective was to estimate gap heights that suppress buoyancy without promoting excessive heat losses to the channel walls. A corollary of this objective was to assess the dependence of heat losses on the channel height. A second objective was to determine the influence of global combustion stoichiometry on simulated microgravity flame spread in the NCA. Whatman 44 filter paper was used for NCA gap heights ranging from 6–20 mm (half-gap below and above sample) and average opposed flow velocities 1–40 cm/s. Flames at low flows were fuel rich when the forced flows were of the same magnitude as the diffusive flow. For thin fuels, a full gap of 10 mm (5 mm half-gap) provided a compromise between buoyancy suppression and heat loss. Calculations were made of flame stoichiometry and of the influence of the velocity profile on flame spread rates (comparing it with previous theory). This part of the analysis provided support for the velocity gradient theory of flame spread. The information provided in this work on the theoretical nature of opposed flow flame spread over thin fuels is based on experimental measurements in simulated microgravity conditions. [ABSTRACT FROM AUTHOR]

A long heated cylinder was placed near a cold wall under the incident of a Couette flow. The conventional fluid was chosen as water (H2O). The nanoparticle materials were selected as Al2O3 and CuO. The governing Navier-Stokes and energy equations were solved numerically through a finite volume method on a staggered grid system using QUICK scheme for convective terms and SIMPLE algorithm. The dependencies of hydrodynamic and heat transfer characteristics of the cylinder on non-dimensional parameters governing the nanofluids (Particle concentrations (φ), diameter (dnp), and particle materials) and the fluid flow (Peclet number Pe and gap height ratio L) were explored here. The shifting of the front stagnation point due to the addition of nanoparticles in the base fluid was investigated. A comparison between the heat transfer enhancement (NuM) of the cylinder and its drag coefficient's (CD) increment/reduction was made by presenting their ratio NuM/CD. The least square method was applied to the numerical results to propose NuM = NuM(Pe) and NuM = NuM(L). [ABSTRACT FROM AUTHOR]

Cathodic protection (CP) inside a long crevice, such as a disbonded coating area beneath a long insulating sheet covering buried steel pipe, was studied in the laboratory and in field tests for up to 7.2 y. Exposed steel inside a long crevice under an insulating sheet was simulated using a crevice model, a multi-electrode block that consists of 64 steel specimens arranged as a plane and a covering acrylic plate to form a crevice with a small or large gap to characterize the potential and current distribution on the steel inside the crevice. In laboratory tests, CP was effective for specimens inside the crevice with a depth more than 600 times larger than the crevice gap, even when both crevice ends were opened in a stagnant water. In a flowing water, sufficient CP was attained inside the crevice with a small gap; however, the CP was insufficient for that with a larger gap. In the field tests, CP was achieved on specimens with and without crevices. Without CP, the corrosion rate of the specimens inside the crevice was approximately 1/3 of that without the crevice because dissolved oxygen (DO) inside the crevice was reduced by natural corrosion. These results indicate that the CP effectiveness inside a crevice is controlled not only by the applied potential and IR drop but also by the reduction rate of DO in the crevice, which is consumed by natural corrosion and CP itself and is supplied by water flow and diffusion. [ABSTRACT FROM AUTHOR]

An intelligent EWOD top plate control system is proposed in this study. The dynamic top plate is controlled by a piezoelectric (PZT) cantilever structure. A High resolution laser displacement sensor is used to monitor the deflection of the top plate. The gap height optimization and the top plate vibration significantly improve the droplet velocity and decrease the droplet minimum threshold actuation voltage. The top plate vibration induced actuation velocity improvement is magnitude and frequency dependent. 100 μm and 200 μm vibrations are tested at 25 Hz. Vibration frequencies at 5 Hz, 10 Hz, and 20 Hz are tested while the magnitude is 200 μm. Results show greater improvements are achieved at larger vibration magnitudes and higher vibration frequencies. With a vibrated top plate, the largest reduction of the actuation voltage is 76 V RMS for a 2.0 μl DI water droplet. The maximum droplet instantaneous velocity is around 9.3 mm/s, which is almost 3 times faster than the droplet velocity without top plate vibration under insufficient driving voltages. Liquid that has different hysteresis such as acetonitrile with various concentrations are used as a control to show its compatibility with the proposed DMF chip. Contact line depinning under top plate vibration is observed, which indicates the underlying mechanism for the improvements in actuation velocity and threshold voltage. The top plate control technique reported in this study makes EWOD DMF chips more reliable when used for the clinical point-of-care diagnostic applications. [ABSTRACT FROM AUTHOR]

Differential Mobility Spectrometry (DMS) devices have been integrated with mass spectrometers to provide an orthogonal pre-separation for targeted quantitation. The distance between the DMS electrodes, referred to as the gap height, is a critical parameter for optimization of DMS performance. The literature includes descriptions of various atmospheric pressure DMS devices with gap heights spanning a range of approximately 0.035 to 2 mm. However, despite the presence of numerous publications describing different gap height devices, there is no systematic investigation that describes the effect of DMS gap height on separation peak capacity. This paper summarizes the results from experimental investigations of the effect of gap height on analytical performance (peak capacity and speed) for DMS devices with gap heights that range from 100 μm to 1.5 mm. In addition, this manuscript provides computer simulations of ion motion that shed some light on the relationship between DMS gap height, waveform frequency, and ion transmission. In order to provide accurate and precise experimental data for use in the simulations and development of models, a custom waveform generator has been designed to maintain separation waveform shape regardless of the capacitive load of the DMS cell and the separation waveform amplitude. The differential mobility function (alpha) has been calculated from experimentally measured compensation fields with each of the cells. When the experimental conditions were constant for a given compound, all tested cells provided similar alpha functions, confirming that similar separation mechanisms are observed regardless of gap height. On the basis of these comprehensive experimental data, it is suggested that a phenomenological model can be used for prediction of peak position for particular ion species in planar DMS devices with different gap height. A number of examples will be presented illustrating peak capacity and speed for separations at fixed E/N ratios, as well as at the maximum E/N achievable for each differential mobility filter. [ABSTRACT FROM AUTHOR]

Abstract: A variable gap size actuation (VGSA) mechanism is integrated into the digital microfluidic (DMF) system. The VGSA mechanism serves to optimize the aspect ratio during performing different microfluidic operations by changing the gap height between the top and bottom plates. This in effect will have a direct impact on the four main DMF operations including droplet transport, splitting, dispensing and merging as they are greatly affected by changing the aspect ratio. Experimental results demonstrate that the VGSA mechanism significantly enhances the principal DMF operations by retaining the appropriate gap height for each operation which is also dependent on droplets volumes when using fixed electrode size. Specifically, varying the gap height precisely between the two plates will enable us to transport the droplets more reliably, control the volume of the dispensed droplet, carry out splitting and merging more effectively, facilitate motion of residual droplets resulting from splitting or partial evaporation, enhance mixing at faster rates, achieve accurate positioning of droplets regardless of their volume, and minimize evaporation without complicating the DMF system with the use of a filler medium. The proposed mechanism is realized by accurate positioning of the top plate over the fixed bottom plate, instead of maintaining a fixed gap height during the operation. In this work, an experimental setup is constructed for the proof of concept to meet precise alignment requirements of the two parallel plates using a feedback-controlled positioning system. Three different methods viz., visual, capacitance-based and encoder values, are used to measure the gap height between the two plates precisely. For practical lab-on-chip devices, micro-actuators in conjunction with capacitance measurement feedback can be used to position the top plate during the operation. In this way, the proposed VGSA mechanism will introduce a mean for optimizing the parameters controlling the DMF operations. [Copyright &y& Elsevier]

Background: Japanese black bears, a large-bodied omnivore, frequently create small gaps in the tree crown during fruit foraging. However, there are no previous reports of black bear-created canopy gaps. To characterize physical canopy disturbance by black bears, we examined a number of parameters, including the species of trees in which canopy gaps were created, gap size, the horizontal and vertical distribution of gaps, and the size of branches broken to create gaps. The size of black bear-created canopy gaps was estimated using data from branches that had been broken and dropped on the ground. Results: The disturbance regime was characterized by a highly biased distribution of small canopy gaps on ridges, a large total overall gap area, a wide range in gap height relative to canopy height, and diversity in gap size. Surprisingly, the annual rate of bear-created canopy gap formation reached 141.3 m2 ha-1 yr-1 on ridges, which were hot spots in terms of black bear activity. This rate was approximately 6.6 times that of tree-fall gap formation on ridges at this study site. Furthermore, this rate was approximately two to three times that of common tree-fall gap formation in Japanese forests, as reported in other studies. Conclusions: Our findings suggest that the ecological interaction between black bears and fruit-bearing trees may create a unique light regime, distinct from that created by tree falls, which increases the availability of light resources to plants below the canopy. [ABSTRACT FROM AUTHOR]

Abstract: Numerically simulated results are presented for a family of rectangular cylinders (placed parallel to a wall and subjected to a uniform shear flow) with aspect ratios r ( with height a and width b) ranging from 0.1 to 1.0 (square cylinder) to gain a better insight into the dependency of the aerodynamic characteristics on the operational dimensionless parameters, namely aspect ratio r, gap height L and Reynolds number Re. The governing unsteady Navier–Stokes equations are solved numerically through SIMPLE algorithm with the QUICK scheme for convective terms. The critical Reynolds numbers at which an alternating vortex shedding commences from each of the rectangular cylinder are specified for different gap heights. A special attention is paid to search a rectangular cylinder of suitable thickness to produce the stronger vortices in the wake. The present numerical results are certified with some previous numerical and experimental findings. [Copyright &y& Elsevier]

In this study the use of a nm thick plasma polymerised primer layer as a means of enhancing the adhesion of silicone elastomer to stainless steel was investigated. This primer layer was formed from tetraethoxysilane (TEOS) and polyhydrogenmethyl siloxane (PHMS). This primer mixture was nebulized into helium atmospheric pressure plasma formed using the PlasmaStream™ system. The resulting primer coatings were characterised using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, optical profilometry, scanning electron microscopy, spectroscopic ellipsometry and water contact angle measurements. The adhesion of the silicone elastomer was accessed using 45° peel strength measurements. This study investigated the effect of plasma jet to substrate distance on primer performance. Increased coating oxidation is observed as this gap between the jet orifice and substrate increases. The plasma power, measured using current and voltage probes was observed to increase with gap height. The corresponding coating thickness decreased from 450 to 100 nm as the gap height increased from 2 to 12 mm. Associated with this change there was a 38% reduction in silicone elastomer adhesion to the stainless steel surface, demonstrating that silicone elastomer bonded to the thinner and less oxidised primer layer exhibited superior adhesion. [ABSTRACT FROM AUTHOR]

There is a strong gap flow exists between the pump rotor tip and the inside of the duct of the pump jet propulsor. To predict the hydrodynamic performance of a pump jet propulsor more accurately, considering the influence of fluid viscosity in the gap area, a gap flow model suitable for a pump jet propulsor is proposed. Through an analysis of the hydrodynamic performance of the pump jet propulsor, it is determined that it is optimal for the gap height to be 0.98–1.0 times of the complete gap height, Additionally, it is verified that the flux coefficient (C Q) range of 0.8–0.84, which can accurately simulate the energy loss of the fluid at the gap flow, is reasonable. Moreover, through a comparative analysis of the hydrodynamic performance, pressure, and circulation distribution of the pump jet propulsor, it is found that, when the influence of the gap flow model is considered, the wall pressure and circulation distribution of the rotor blade and duct are more consistent with the calculation results of viscous flow. In addition, the introduction of the gap flow model only improves the prediction accuracy of the hydrodynamic performance of the pump jet propulsor. • The gap flow model can improve the prediction accuracy of the hydrodynamic performance of pump jet propulsion, especially the greatest impact on the improvement of duct performance. • This article showed that when the gap flow model height was within the range of 0.98–1.0 times the full height, the calculation result converged. • The calculation result of the selection of the flux coefficient indicated that the flux coefficient selection range of 0.8–0.84 was reasonable and could truly reflect the energy attenuation at the gap flow of the pump jet propulsion. • The gap flow model truly reflects the circulation and pressure distribution of the rotor blade as well as the pressure differential force distribution between the blade surface and back of the blade. [ABSTRACT FROM AUTHOR]

• A novel vertical T-junction based droplet generator was fabricated using a 3D printing technology. • The device was simply assembled by a screw-and-nut combination. • The vertical T-junction gap height could be tuned by the rotational degree of the screw. • Precise control of the droplet size was easily achieved by the rotation of the screw. • Accordingly, different size of the PEGDA beads could be synthesized. Herein, we present the fabrication of a novel vertical T-junction based droplet generator using a desktop 3D printer and the production of a variety of size of polymer particles by tuning the vertical T-junction gap height (h g) with ease. The droplet generator device consists of two main components: an internal thread nut with a vertical T-junction droplet generator unit, and a screw with an external thread for adjusting h g. The two components were fabricated by a 3D printer and assembled simply by a bolt-and-nut combination. The clockwise or counter-clockwise rotation of the screw is converted into the downward and upward linear motion of the screw, adjusting the h g value. The h g value can be tuned precisely from 0 to 750 μm by designing 75 control teeth on the top of the screw. The size of the produced droplets is proportional to the h g value. The size range of the droplets was 39–1404 μm, when h g changed from 20 to 200 μm. We synthesized poly(ethylene glycol) diacrylate (PEGDA) particles using the droplet as a reactor, and accordingly the PEGDA particle size could be controlled. Our proposed 3D printed droplet generator is rapid, simple and cost-effective to be fabricated without need of lithographic processes. [ABSTRACT FROM AUTHOR]