LES analysis of fire source aspect ratio effects on fire-wind enhancement

Study on temperature decay characteristics of fire smoke backflow layer in tunnels with wide-shallow cross-section

Narrow tines are among the most important tillage tools, which are used in chisel plows, cultivators and subsoilers. The draft of these blades is highly dependent on their geometric shape and working depth. The most important geometric specifications of these blades are their aspect ratio (depth/width ratio) and rake angle. Researchers’ efforts have been aimed at minimizing energy consumption and maximizing soil-loosening efficiency in using these types of blades. The objective of conducting this research was to investigate the effect of narrow tine aspect ratio on draft force; soil disturbed area and loosening efficiency. The tests were conducted in the soil-bin facilities of the Agricultural Machinery Department at Shiraz University. The experiment was conducted using a complete randomized block design with factorial arrangement in three replications. The tools had the same rake angle (30 degree) with four aspect ratios (2, 3, 4 and 5) and three blade widths (2.5, 5 and 7.5 cm). Draft force was measured by a recording load cell, and the soil-disturbed area was measured by using a profilemeter and developed by using Curve Expert and Matlab Ver. 5.6 softwares. The soil loosening efficiency was calculated by dividing the soil disturbed area by the draft force. The results showed that the effect of aspect ratio (d/w) on draft force was highly significant (p<0.01). The highest draft force for all blades was obtained at the aspect ratio of 5. The interactive effect of blade width and aspect ratio on draft force variations was also significant. The mean draft force at similar aspect ratios increased as the tool width was increased. The results also showed that the effect of blade aspect ratio on soil-disturbed area was highly significant. The soil-disturbed area increased with increasing the aspect ratio. The interactive effect of blade width and its aspect ratio on soil-disturbed area was significant, indicating that the effect of aspect ratio on soil-disturbed area depended on blade width. At similar aspect ratio, the disturbed area increased with increasing the blade width. In addition, the effect of aspect ratio on soil loosening efficiency was highly significant (p<0.01). The 7.5 cm wide blade had the highest efficiency at the aspect ratio of three, while for the 5 cm and 2.5 cm wide blades the highest efficiency was achieved at aspect ratios of 4 and 5, respectively. The highest and the lowest loosening efficiencies were obtained at aspect ratios of 3 and 2, respectively. Therefore, it is suggested that the narrow tillage tools be used at aspect ratio of 3 or higher. Statistical analysis showed significant interactive effect of aspect ratio and blade width (p<0.01) on soil loosening efficiency. Soil loosening efficiency had the highest and the lowest values at blade widths of 7.5 cm and 2.5 cm, respectively, indicating that the soil loosening efficiency was higher in wider blades. Key words: Narrow tillage tools, Draft, soil disturbed area, Soil loosening, Critical depth, Aspect ratio

A re-examination of entrainment constant and an explicit model for flame heights of rectangular jet fires

Recent interest in developing micro air vehicles (MAVs) for a variety of both civil and military uses has driven significant research into the aerodynamics of natural flyers, such as insects, birds and bats. Insects, in particular, exhibit desirable flight characteristics that MAV designers wish to incorporate into their designs, however our understanding of how these animals fly is still limited. Research into insect flight has shown that they employ a number of unsteady mechanisms, the most prevalent of these being the formation of a leading-edge vortex (LEV) which provides the wing with enhanced lift. While this research has greatly improved our understanding of insect flight, the effect of the wing's shape on these unsteady mechanisms is not well understood. This thesis describes an investigation into the effect of two wing morphological parameters, aspect ratio and camber, on the flow structures around flapping and rotating wings in an insect-like flight regime. The effect of wing aspect ratio is first explored at different Reynolds numbers using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Increasing the Reynolds number for an aspect ratio of 2.91 resulted in the development of a dual LEV structure, however increasing aspect ratio at a fixed Reynolds number generated the same flow structures. This result shows that the effects of Reynolds number and aspect ratio are linked. An alternate flow scaling method, using the wing span as the characteristic length, is presented to decouple the effects of Reynolds number from those of aspect ratio. This resulted in a span-based Reynolds number, which can be used to independently describe the development of the LEV. Indeed, universal behaviour was found for various parameters using this scaling. The effect of aspect ratio on the vortex structures was then assessed at different span-based Reynolds numbers and it was found the wing aspect ratio had the effect of shortening the wing's chord length relative to a fixed LEV size. Scaling the flow using the wing span was found to apply for revolving wings at large angles of attack, such that the flow separates from the leading-edge and a strong spanwise velocity is generated on the leeward side of the wing. These conditions are typical of those seen in nature, and hence this scaling could be applied to similar investigations involving insects and birds as well as nature-mimicking MAVs. The effect of wing aspect ratio was then explored at different advance ratios using a numerical model that mimicked a flapping insect wing. It was demonstrated that increasing the advance ratio enhances vorticity production at the leading-edge during the downstroke, and this results in more rapid growth of the LEV for non-zero advance ratios. This effect, combined with that of aspect ratio, determines whether the LEV remains stably attached to the wing or if it is shed. For high advance ratios and large aspect ratios the LEV was observed to quickly grow to envelop the entire wing during the early stages of the downstroke. Continued rotation of the wing resulted in the LEV being eventually shed as part of a vortex loop that peels away from the wing's tip. It is shown that the shedding of the LEV for high aspect ratio wings at non-zero advance ratios leads to reduced aerodynamic performance of these wings. This helps to explain why a number of insect species have evolved to have low aspect ratio wings, as they outperform high aspect ratios across a wide range of flight speeds. Finally, wing deformation is observed during the flight of some insect species, which results in the wing becoming cambered throughout each half stroke. In this study, the effect of wing camber on the flow structures and aerodynamic forces for insect-like wings is investigated using the rotating-wing numerical model. Both positive and negative camber was investigated at Reynolds numbers of 120 and 1500, along with the chordwise location of maximum camber. It was found that negatively cambered wings produce similar LEV structures to non-cambered wings at both Reynolds numbers, but high positive camber resulted in the formation of multiple streamwise vortices at the higher Reynolds number, which disrupt the development of the main LEV. Despite this, positively cambered wings were found to produce higher lift on drag ratios than flat or negatively cambered wings. It was determined that a region of low pressure near the wing's leading edge, combined with the curvature of the wing's upper surface in this region, resulted in a vertical tilting of the net force vector for positively cambered wings, which explains how insects can benefit from wing camber.

Effect of hydraulic diameter and aspect ratio on single phase flow and heat transfer in a rectangular microchannel

To explore the effect of aspect ratio (AR) of carbon nanotubes (CNT) on the piezoresistive behavior of the composites, four kinds of multiwalled carbon nanotubes (MWNT) with different nominal aspect ratios (AR = 62, 133, 433, and 833) were well dispersed in a thermoplastic elastomer (TPE) via melt blending. The piezoresistivity of the MWNT/TPE nanocomposites was found to be dependent on the nominal MWNT aspect ratios. However, their relationship is non-linear and non-monotonic. By introducing the effective MWNT aspect ratios which are length-dependent and diameter-dependent, it has been demonstrated that the piezoresistivity will decrease with the increase of effective aspect ratios. The length-dependent increase of aspect ratio results in one hundred-fold or more decrease of piezoresistivity, but the diameter-dependent increase of aspect ratios only leads to a slight marginal change of the piezoresistivity. The proper selection of MWNT aspect ratios could enable their utilization to tailor as well as finely tune the piezoresistivity of the MWNT/TPE nanocomposites.

We examine the effect of varying roughness-element aspect ratio on the mean velocity distributions of turbulent flow over arrays of rectangular-prism-shaped elements. Large-eddy simulations (LES) in conjunction with a sharp-interface immersed boundary method are used to simulate spatially-growing turbulent boundary layers over these rough surfaces. Arrays of aligned and staggered rectangular roughness elements with aspect ratio >1 are considered. First the temporally- and spatially-averaged velocity profiles are used to illustrate the aspect-ratio effects. For aligned prisms, the roughness length ( $$z_\mathrm{o}$$ ) and the friction velocity ( $$u_*$$ ) increase initially with an increase in the roughness-element aspect ratio, until the values reach a plateau at a particular aspect ratio. The exact value of this aspect ratio depends on the coverage density. Further increase in the aspect ratio changes neither $$z_\mathrm{o}$$ , $$u_*$$ nor the bulk flow above the roughness elements. For the staggered cases, $$z_\mathrm{o}$$ and $$u_*$$ continue to increase for the surface coverage density and the aspect ratios investigated. To model the flow response to variations in roughness aspect ratio, we turn to a previously developed phenomenological volumetric sheltering model (Yang et al., in J Fluid Mech 789:127–165, 2016), which was intended for low to moderate aspect-ratio roughness elements. Here, we extend this model to account for high aspect-ratio roughness elements. We find that for aligned cases, the model predicts strong mutual sheltering among the roughness elements, while the effect is much weaker for staggered cases. The model-predicted $$z_\mathrm{o}$$ and $$u_*$$ agree well with the LES results. Results show that the model, which takes explicit account of the mutual sheltering effects, provides a rapid and reliable prediction method of roughness effects in turbulent boundary-layer flows over arrays of rectangular-prism roughness elements.

In this study, the effects of aspect ratio and loading of multiwalled carbon nanotubes (MWCNTs) on the dynamic mechanical, thermal, and flammability properties of low-density polyethylene (LDPE)/MWCNT nanocomposites prepared by the melt blending technique were investigated. At low CNT loading, CNT with low aspect ratio acted as a plasticizer in LDPE. The storage modulus of the nanocomposites increased with the increase in aspect ratio and CNT loading. The increase in scan rate for the composites results in the decrease in total crystallinity, crystallization peak temperature, and a late onset of crystallization. The flammability properties like heat release capacity, peak heat release rate, and total heat release decrease with the increase in both aspect ratio and loading of CNTs in the composites.

In this paper, the effect of wing aspect ratio and kinematics on wing-wake interaction at Re∼104, which matched the flight regime of flapping-wing micro air vehicle (FWMAV), was investigated. The dynamically scaled-up robotic model submerged in a water tank environment revealed that the wing-wake interaction augmented lift across a decrease in both aspect ratio and wing pitching duration. At such high Re, a time-course digital particle image velocimetry (DPIV) measurement showed the entire flow was strongly dominated by trailing-edge vortices (TEV). A pair of counter-rotating TEV was found to induce a jetlike flow towards the windward side of the wing at stroke reversal. The transfer of momentum from the accelerated flow to the wing caused the enhanced lift. The size of the pair vortex decreased for an increase in both aspect ratio and wing pitching duration. The size of the TEV pair was the key feature found to generate the observed aerodynamic force characteristics.

The role of jet inlet geometry in impinging jet heat transfer, modeling and experiments

In this study, the condensation heat transfer coefficient and pressure drop of steam are obtained in small rectangular tubes with different aspect ratios. The experiments were carried out on three rectangular tubes with aspect ratios of 1:2, 1:3 and 1:5, with mass flux between 25 and 45 kg/m2s, and vapor qualities between 0.1 and 0.8. The experimental data were analyzed to determine the effect of vapor quality, mass flux, and aspect ratio on the heat transfer coefficient and pressure drop. The results showed that the effect of aspect ratio on condensation heat transfer coefficient appears to be dependent on the flow pattern. For stratified flow, the condensation heat transfer coefficient increases as the mass flux increases. For annular flow, the condensation heat transfer coefficient hardly changed. The pressure drop always increases as the aspect ratio increases. Previous studies on round tube heat transfer and pressure drop correlations have not successfully predicted the small rectangular tube data; therefore, modified Shah correlation and Lockhart &amp; Martinelli correlation are proposed, which predict the data with 20% and 23% RMS error, respectively.

We present recent results on the effect of crack/pore aspect ratio on seismic wave velocities. Our comparative studies of experimental and theoretical investigations show that the crack/pore aspect ratio may be an important reservoir parameter in fractured and porous rocks. For low-porosity rock, the effect of aspect ratio on velocities at low effective pressure is greater than that of porosity. We found that the change of wave velocities with effective pressure may well be modeled by an extented Biot’s theory with the following relationship between aspect ratio and effective pressure: + where is effective pressure, is the initial aspect ratio at zero effective pressure, is the terminal aspect ratio, and is a constant for a given rock. We determine these model parameters for 4 rocks: granite, limestones (Bedford and Leuders), sandstone, having porosities from 0.65% to 21.7%. Variations in velocity with differential pressure correspond with aspect ratio changes from 0.0015 to 0.125. In low-porosity Casco granite, Vp increases with pressure from 3.3 km/s to 6.4 km/s and is associated with more than an 80 fold increase in aspect ratio from 0.0015 to 0.125. The Bedford limestone is populated with fractures and cracks of aspect ratio 0.0085; the Leuders limestone has an average aspect ratio of 0.03. Gulf Coast sandstone has an average aspect ratio of about 0.05. The model results can be used for high-resolution forward modeling and inversion of reservoir properties and to estimate in situ porosity, fluid content, and pore pressure from seismic data.

This paper reports a numerical investigation of the effect of aspect ratio (AR) on the flow structure around the 35° Ahmed body. The AR is defined as the ratio of length to the height of the model. A total of five ARs are considered at a Reynolds number based on the height of the Ahmed body of 7.8 105. The flow governing equations are solved in the FLUENT solver using the SST K-Omega turbulence model. It is found that the drag coefficient (Cd) slightly increases at the AR1 4.2%. As the aspect ratio increases, the drag coefficient decreases. The analysis reveals that early flow separation in the AR1 causes a reattachment at the rear slant end, which leads to an increase in drag. It also highlights that there exists a minimum aspect ratio beyond which the drag increases. and there is a limit in the increasing order beyond which the drag will begin to reduce. Further, it demonstrates that there is no change in the recirculation region and consequently drag coefficient is not a function of the recirculation region. Thence, a clear understanding of the effect of the length-based aspect ratio can help to optimize the length during design.

Phenomenological characteristics of hydrogen/air premixed flame propagation in closed rectangular channels

Effect of aspect ratio on the compaction characteristics and micromorphology of copper powders by magnetic pulse compaction