Effect Of Boundary Layer Thickness Research Articles

Effect of boundary layer thickness and dimensionless wall distance y+ on numerical simulation of heat pipes

Effect of boundary layer thickness and dimensionless wall distance y+ on numerical simulation of heat pipes

Effect of boundary layer thickness on supersonic combustion in a scramjet combustor

In this study, the effect of boundary layer thickness on supersonic combustion in an ethylene-fueled scramjet combustor under Mach 5.5 flight conditions is studied by employing experiments, numerical simulations, and theoretical modeling. The plate flow quantitative results show that the absolute thickness of the boundary layer is higher for a larger combustor. But the relative thickness is thin. Although boundary layer thickness changes greatly as the combustion chamber scale enlarges, the influence on the transverse jet and cavity flow is limited. In contrast, the combustion chamber scale change has a great impact on the combustion. More violent flames exist in large-scale combustion chambers. A theoretical model demonstrated that a large subsonic region led to a long fuel residence time, resulting in larger Damköhler numbers. This indicated that a large combustor could maintain large combustion. Based on the above analysis, although the relative thickness of the boundary layer is thinner, the absolute subsonic region of the boundary layer full of combustion is increased, which is conducive to intense combustion. In addition, a verification calculation of the 50 mm shortening isolation section is carried out. It is found from the flame cloud diagram and the quantitative wall pressure data that the combustion is significantly weakened.

Effects of boundary layer thickness on the estimation of equivalent sandgrain roughness in zero-pressure-gradient boundary layers

Effects of boundary layer thickness on the estimation of equivalent sandgrain roughness in zero-pressure-gradient boundary layers

Droplet evaporation characteristics on heated superhydrophobic surface subjected to airflow

In a novel frost suppression method for air source heat pumps, the frost-free operation of the system is realized by vaporizing the condensed droplets on superhydrophobic heat exchanger fins before freezing. Due to the control requirements of this method, a multiphase flow model based on VOF method for droplet evaporation is proposed. The evaporation characteristics and survival time of a sessile water droplet on heated superhydrophobic surface subjected to airflow are investigated through the model. The convection and evaporative cooling effect are considered in the simulation, experiments showed that the prediction error for droplet survival time is within 8%. The effects of boundary layer thickness, substrate temperature, surface wettability, air temperature and humidity on the heat and mass transfer process of droplet evaporation are studied with this model. The survival time of water droplet with diameter below 100 μm evaporating in different evaporation modes is also calculated. The results showed that under the conditions of common parameters in winter, the survival time of a droplet evaporating in fully developed flow is in the range of 4∼20 s. The survival time of droplets evaporating in constant contact radius mode is 15.4% shorter than that evaporating in constant contact angle mode.

Effects of the boundary-layer thickness at the cavity entrance on rarefied hypersonic flows over a rectangular cavity

A comprehensive numerical study is performed to investigate rarefied hypersonic flows past a flat plate with a two- (2D) or three-dimensional (3D) rectangular cavity using the direct simulation Monte Carlo method. In this work, we obtain different boundary-layer thicknesses at the cavity entrance by varying the length of upstream flat plate. Then, the effects of boundary-layer thickness at the cavity entrance on flow characteristics and aerodynamic surface quantities are analyzed in depth. The concept of total-enthalpy boundary layer successfully identifies the boundary-layer edge while the traditional boundary layer based on velocity fails to captures the shock-wave front due to the presence of the weak shock wave in the rarefied hypersonic flow past a flat plate. The boundary-layer thickness at the cavity entrance has strong effects on the flow past the 2D cavity. As the boundary layer at the cavity entrance becomes thicker, the external stream can penetrate deeper into the cavity and transfer more momentum and energy to the cavity, so the cavity flow develops gradually from an open flow to a transitional one and ultimately to a closed one. In addition, the 3D effect plays an important role for the cases of extremely thick boundary layers at the cavity entrance, while it can almost be neglected when boundary layers at the cavity entrance are exceedingly thin.

Wind loading of a finite prism: aspect ratio, incidence and boundary layer thickness effects

A systematic set of low-speed wind tunnel experiments was performed at Re = 6.5x104 and 1.1x105 to study the mean wind loading experienced by surface-mounted finite-height square prisms for different aspect ratios, incidence angles, and boundary layer thicknesses. The aspect ratio of the prism was varied from AR = 1 to 11 in small increments and the incidence angle was changed from α = 0° to 45° in increments of 1°. Two different boundary layer thicknesses were used: a thin boundary layer with δ/D = 0.8 and a thick boundary layer with δ/D = 2.0–2.2. The mean drag and lift coefficients were strong functions of AR, α, and δ/D, while the Strouhal number was mostly influenced by α. The critical incidence angle, at which the prism experiences minimum drag, maximum lift, and highest vortex shedding frequency, increased with AR, converged to a value of αc = 18° ± 2° once AR was sufficiently high, and was relatively insensitive to changes in δ/D. A local maximum value of mean drag coefficient was identified for higher-AR prisms at low α. The overall behaviour of the force coefficients and Strouhal number with AR suggests the possibility of three flow regimes.

Lattice Boltzmann simulations for micro-macro interactions during isothermal drying of bundle of capillaries

The fundamental understanding of the multiphase phenomena involved in drying of porous media is still a challenging task. Pore network models (PNMs) reveal the micro–macro interactions. However, PNMs have shortfalls in using true geometry, and in revealing capillary instabilities and film-effects. In this work, Shan Chen representation of Lattice Boltzmann Method (LBM) is applied to elucidate such complicated multiphase phenomena, i.e. micro-scale-dynamics evolution during drying. In a bundle of capillaries, liquid transport is explained with two pressure gradients. First, the pressure gradient between capillaries leading to capillary pumping, second, the pressure gradient within each capillary along the liquid–gas interface resulting in formation of hydraulic films. Transformation of hydraulic films to adsorbed films is revealed in irregular pore structures. Péclet number is employed to elucidate the effect of boundary layer thickness and temperature on drying kinetics. Characteristics of drying are re-established using the LBM for pore structures with bimodal size distribution.

Effect of boundary layer thickness on ammonia gas sensing of Cr2O3-decorated ZnO multipods

New insights into controlling boundary layer thickness are investigated in growth of Cr2O3-decorated ZnO multipods on a tilted silicon substrate. In the first step, ZnO multipods were grown on a tilted silicon surface using a vapor transport method in a quartz tube. Next step, low concentration of Cr2O3 nanoparticles was decorated on the ZnO multipods using a PVD machine. Both arm diameter and density of the Cr2O3-decorated ZnO multipods were found to be strongly dependent on the boundary layer thickness. The sensing properties of the samples measured by half bridge method indicated that decoration of the Cr2O3 nanoparticles on the ZnO multipods significantly enhanced the response to 100 ppm NH3 up to 85, CO2 up to 29 and NO2 up to 27. The response/recovery times were 25 s/48 s at 300 °C. Blue defect emission appeared in room temperature photoluminescence spectra of the sample grown in a thin boundary layer thickness.

Effect of boundary layer thickness on transverse sonic jet mixing in a supersonic turbulent crossflow

The surface boundary through which a sonic jet in supersonic cross flow is injected is shown to have a significant effect on the size, penetration, and mixing characteristics of the jet plume. A circular, high-pressure, sonic jet is injected into a M = 3.4 supersonic crossflow through a well-characterized turbulent boundary layer of two different thicknesses (δ/d = 0.6 and 6.1), with variable momentum ratios (J = 1.2, 2.6, and 5). Planar laser Mie scattering of condensed ethanol droplets is used to quantitatively image the injected fluid concentration in both side and end-views at multiple downstream locations. Jet penetration, plume area, and characteristic size and location of regions of intense mixing are compared. The jets injected through the thicker boundary layer are shown to have significantly enhanced jet penetration (∼50%), spread (∼100%), and mixing intensity (∼100%, especially in the near-field) over a wider area of the jet plume. Additionally, characterization of mixing is examined using the variance in the concentration field as well as probability density functions of concentration determined along contours of constant jet fluid concentration. From these results, the jet injections associated with the thicker boundary layer transition from shear dominated mixing zones on the windward side to more distributed mixing zones throughout the plume at earlier downstream locations and show influence of interactions between boundary layer vorticity and vortical structures within the jet leading to larger lateral expansion.

Numerical Analysis of Effect of Boundary Layer Characteristics on the Flow Field in S-shaped Inlet

In order to explore the effect of boundary layer thickness and pressure gradient on the perfor- mance of the flow field in the inlet, we design a high offset rate S-shaped inlet based on a certain unmanned aeri- al vehicle (UAV), and its author has analyzed the effect of boundary layer characteristics on the inlet with nu- merical simulation method. The suction of boundary layer which leads to separation zone not only becomes longer in the inlet, but also moves to the center plane of symmetry, the separation point of boundary layer ap- pears in advance as pressure gradient increases. Considering the influence of the boundary layer, various perfor- mance parameters all exceeds that of the uniform entrance inlet conditions, especially the circumferential total pressure distortion of outlet increased by 58.2% at most, obviously can't meet the engine to work properly, so we must consider and pay attention to the effect of the boundary layer characteristics on the flow field in the S-shaped inlet.

Numerical Study on the Flow over a Simplified Vehicle Door Gap – an Old Benchmark Problem Is Revisited

A simplified automobile door gap model, defined at the Third Computational Aeroacoustics (CAA) Workshop on Benchmark Problems (Category 6) was investigated. After a thorough mesh study compressible and incompressible simulations were carried out and various turbulence models were tried. The influence of three dimensional effects and boundary layer thickness effects were examined too. In case of compressible simulations stability problems were encountered with the non-reflective boundary condition of CFX (beta version at the time of the simulations). It was found that for deep cavities incompressible simulations are not applicable. In spite of the diculties a good agreement between measurements and simulations was found when the flow speed was 50m/s. In case of 26.8m/s flow speed it was found that the presence of the upper channel wall, not taken into account by the previous authors simulating this problem plays an important role – a hitherto unexplained peak in the measured spectrum appeared this way in the simulation.

Effect of boundary layer thickness on secondary structures in a short inlet curved duct

The flow pattern in short inlet ducts with aggressive curvature has been shown to lead, in some cases, to an asymmetric flow field at the aerodynamic interface plane. In the present work, a two-dimensional honeycomb mesh was added upstream of the curved duct to create a pressure drop across it, and therefore to an increased velocity deficit in the boundary layer. This velocity deficit led to a stronger streamwise separation, overcoming the instability that can result in an asymmetric flow field at the aerodynamic interface plane. Experiments were conducted at Mach numbers of M=0.2, 0.44 and 0.58 in an expanding aggressive duct with rectangle to a square cross section with area ratio of 1.27. Steady and unsteady pressure measurements, together with Particle Image Velocimetry (PIV), were used to explore the effect of the honeycomb on the symmetry of the flow field. The effect of inserting a honeycomb was tested by increasing its height from 0 to 2.2 times the boundary layer thickness of the baseline flow upstream of the curve. Using the honeycomb, flow symmetry was achieved for the specific geometrical configuration tested with a negligible decrease of the pressure recovery.

Aeroelastic analysis of versatile thermal insulation (VTI) panels with pinched boundary conditions

Launch vehicle design and analysis is a crucial problem in space engineering. The large range of external conditions and the complexity of space vehicles make the solution of the problem really challenging. The problem considered in the present work deals with the versatile thermal insulation (VTI) panel. This thermal protection system is designed to reduce heat fluxes on the LH2 tank during the long coasting phases. Because of the unconventional boundary conditions and the large-scale geometry of the panel, the aeroelastic behaviour of VTI is investigated in the present work. Known available results from literature related to similar problem, are reviewed by considering the effect of various Mach regimes, including boundary layer thickness effects, in-plane mechanical and thermal loads, non-linear effects and amplitude of limit cycle oscillations. A dedicated finite element model is developed for the supersonic regime. The models used for coupling the orthotropic layered structural model with Piston Theory aerodynamic models allow the calculations of flutter conditions in case of curved panels supported in a discrete number of points. An advanced computational aeroelasticity tool is developed using various dedicated commercial softwares (CFX, ZAERO, EDGE). A wind tunnel test campaign is carried out to assess the computational tool in the analysis of this type of problem.

Identification of the effects of the nozzle-exit boundary-layer thickness and its corresponding Reynolds number in initially highly disturbed subsonic jets

The influence of the nozzle-exit boundary-layer thickness in isothermal round jets at a Mach number of 0.9 and at diameter Reynolds numbers ReD ≃ 5 × 104 is investigated using large-eddy simulations. The originality of this work is that, contrary to previous studies on the topic, the jets are initially highly disturbed, and that the effects of the boundary-layer thickness are explored jointly on the exit turbulence, the shear-layer and jet flow characteristics, and the acoustic field. The jets originate from a pipe of radius r0, and exhibit, at the exit, peak disturbance levels of 9% of the jet velocity, and mean velocity profiles similar to laminar boundary-layer profiles of thickness δ0 = 0.09r0, 0.15r0, 0.25r0, or 0.42r0, yielding 99% velocity thicknesses between 0.07r0 and 0.34r0 and momentum thicknesses δθ(0) between 0.012r0 and 0.05r0. Two sets of computations are reported to distinguish, for the first time to the best of our knowledge, between the effects of the ratio δ0/r0 and of the Reynolds number Reθ based on δθ(0). First, four jets with a fixed diameter, hence at a constant Reynolds number ReD = 5 × 104 giving Reθ = 304, 486, 782, and 1288 depending on δ0, are considered. In this case, due to the increase in Reθ, thickening the initial shear layers mainly results in a weaker mixing-layer development with lower spreading rates and turbulence intensities, and reduced sound levels at all emission angles. Second, four jets at Reynolds numbers ReD between 1.8 × 104 and 8.3 × 104, varying so as to obtain Reθ ≃ 480 in all simulations, are examined. Here, increasing δ0/r0 has a limited impact on the mixing-layer key features, but clearly leads to a shorter potential core, a more rapid velocity decay, and higher fluctuations on the jet axis, and stronger noise in the downstream direction. Similar trends can be expected for high-Reynolds-number jets in which viscosity plays a negligible role.

Effects of Computational Parameters on Solidification Structures Simulated with Phase-Field Method for a Ni-Cu Binary Alloy

In this study W was found to have strong effects on solidification structure of the binary alloy. Based 0.5 W0、W0 and 1.5W0 base W0, three conditions with free energy base W equal to 0.5 W0, W0 and 1.5W0 respectively were computed. The results showed that the variation of energy base contributed a lot to the development of side branches of equiaxed dendrite structures, the lower W, the more developed side branches, the higher W, the more restrained side branches. Furthermore, the effects of boundary layer thickness δ on solidification structures were also studied. It is found that when the layer thickness δ is as small as Δx/0.94, computational errors occurred at the base of the dendrite and when the layer thickness δ is as big as Δx/0.54, some irreal phenomena, such as coarsened dendrite trunks and maladjusted side branches, could be caused. Further study on space step Δx showed that bigger space steps made higher computational errors. It is given that the relationships among phase-field perturbation, space step and solidification structure.

Study on the compressible viscous flows through a straight pipe

When sonic nozzles of significantly smaller diameter are used as standard flow meters, the critical backpressure ratio is affected by the boundary layer at the nozzle throat. It is known that, as the flow pathway is complicated in actual valves, the critical pressure ratio differs from the ideal value. However, the effect of the boundary layer thickness on choking criteria is still controversial. Choking phenomenon in a sonic nozzle fitted with a straight circular pipe of variable length is investigated numerically and the results are compared with experimental and theoretical ones. The results show that the actual critical pressure ratio is less than the ideal value and it decreases with increase in pipe length. The effect of boundary layer thickness on main flow Mach number is also made clear in this work. In addition, sonic conductance and pressure loss in actual viscous flows computed are compared with the ISO 6358 standard.

New visions in experimental investigations of a supersonic under-expanded jet into a high subsonic crossflow

A series of experiments was performed to investigate the interaction of an under-expanded axisymmetric supersonic jet exhausted from a flat plate with a high subsonic crossflow. The goal was to study the effect of boundary layer thickness (δ) and jet to freestream dynamic pressure ratio ( J) on flow field pressure distributions. The resulting measurements upstream of the jet showed that with increasing boundary layer thickness, the magnitude of the pressure coefficient decreases, whereas downstream of the jet, the recovery of the back-pressure moved closer to the nozzle exit. Flow field measurements indicated that with increasing boundary layer thickness, the jet plume dissipation rate increased, whereas the strength of the counter-rotating vortex pair (CRVP) did not vary significantly. In addition, it was clearly observed that with increasing J, the CRVP penetrated into the crossflow and the magnitude of the pressure coefficient on the surface upstream and downstream of the jet increased.

EFFECTS OF BOUNDARY-LAYER THICKNESS ON FLOW OSCILLATIONS INSIDE OPEN CAVITIES AT SUPERSONIC SPEEDS

An experiment was conducted in a wind-tunnel to study the internal acoustic field and flow oscillations inside rectangular-box cavities. The length-to-depth ratio of the cavities was 8. The data presented herein was obtained over a Mach number of 1.5 at a Reynolds number of 2.26×107 per meter with different boundary-layer thicknesses of approximately 24 mm and 5.5 mm. The experimental angle of attack, yawing and rolling angles were 0°. The rules were revealed as governing the effects of boundary-layer thickness on flow oscillations and unsteady characteristics by comparing the experimental results with flow oscillation modes predicated by Rossiter's and Heller's modified equation. The results indicate that a decrease in the ratio of boundary-layer thickness to cavity-depth (δ/D) induces flow oscillations amplification, peak oscillation frequency splitting and shifting phenomena of open cavity tones in the low-frequency region.

Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

AbstractThis study presents a three‐dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn‐shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn‐shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high‐turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20261

丘陵の後流中の渦構造と熱伝達に及ぼす境界層厚さの影響に関する数値解析(熱工学,内燃機関,動力など)

丘陵の後流中の渦構造と熱伝達に及ぼす境界層厚さの影響に関する数値解析(熱工学,内燃機関,動力など)