Change In Boundary-layer Thickness Research Articles

Transpiration-induced convective heat transfer in ternary hybrid nanofluid flow due to dual stretching on wedge: Reverse flow analysis

Boundary layer separation is a common challenge in fluid dynamics, where flow separates from a surface due to adverse pressure gradients. Transpiration, the injection or suction of fluid through a surface, is a technique used to mitigate this issue. This study investigates transpiration’s effects on boundary layer separation over a stretching wedge conveying ternary hybrid nanoparticles, which is crucial for optimizing various engineering systems. The study establishes a mathematical model incorporating transpiration, dual stretching, viscous dissipation, and nanoparticle volume fractions. The model is reduced to ordinary differential equations using similarity transformations and then solved numerically with MATLAB’s bvp4c package. Here, the convective heat transfer increases with suction and decreases with blowing due to changes in boundary layer thickness. This insight provides valuable information for engineers seeking to control temperature gradients in their systems. Moreover, the study highlights transpiration’s efficacy in mitigating boundary layer separation induced by adverse pressure gradients. Furthermore, flow enhancement with blowing depends on specific conditions, such as the value of the blowing parameter, highlighting its nuanced influence.

The effect of sulfuration reaction rates with sulphur concentration gradient dependence on the growth pattern and morphological evolution of MoS2 in laminar flow.

Sulfuration reactions dominate the synthesis of transition-metal dichalcogenides via chemical vapor deposition. A neglected critical issue is the evolution of crystal domain morphology and growth models caused by boundary layer development. In this study, we propose two growth models within a laminar flow field to investigate the kinetic mechanism of uniformly grown MoS2. We used supercritical fluid pre-deposition to obtain a well-distributed and low-crystallinity Mo precursor on the surface of a substrate to avoid non-stoichiometric supply in sulfuration. The development of the boundary layer was suppressed through mainstream force by adjusting the substrate slope angle. For growth within the underdeveloped laminar boundary layer, monolayer MoS2 with a size of 50 μm uniformly distributed on the full substrate with R = 85% (relative change in boundary layer thickness). Moreover, the growth constrained by surface chemical reactions tended to promote spatially uniform growth. However, within the fully developed laminar flow, the crystal domains preferentially grew vertically, which was attributed to the excessive crystal growth rate (g). Our results provide new insights into the controllable preparation of two-dimensional materials.

Numerical study of ceramic catalytic turbine technology for emission reduction during vehicle warm-up

Ceramic Catalytic Turbine (CCT) Technology is expected to become an important means to reduce vehicle emissions, especially during engine warm-up. In this paper, catalytic reaction was numerically simulated in 3D for a CCT on a gasoline engine during the warm-up period. The results showed that turbulence in the turbine promotes catalytic activity; CCT starts to significantly affect exhaust pollution since turbine inlet temperature of 550 K; the conversion efficiency of harmful gas in exhaust rises sharply when turbine inlet reaches 575–625 K; when the inlet temperature is about 720 K, the conversion efficiencies of C3H6, CO, and NO reach 23.7%, 21.1%, and 15.5%, respectively. Meanwhile the gas temperature is increased by about 30 K at turbine outlet. In addition, during the process of numerical modeling and calculation, it is found that minor change in boundary layer thickness has a negligible impact on the simulation. However, an extremely thin boundary layer will cause computational divergence. The intensity of catalytic reaction can influence the convergence of numerical calculation, while the moving average of the catalytic reaction with the turbine inlet temperature, in return, can reveal the catalytic light-off process.

Hydrodynamics of the interceptor on a 2-D flat plate by CFD and experiments

AbstractNowadays, the use of interceptor by both partial and total dynamic lift crafts is quite common. In this article, a lot of evidence is given regarding the effectiveness of interceptor. The interceptor, when placed at the stern region, changes the pressure distribution around the craft. Its presence affects drag force, lifting force and the position of pressure's center leading to a new trim. This study focuses on hydrodynamic effects of interceptors on a 2-D flat plate based on both computational fluid dynamic (CFD) and experimental approaches. The Reynolds average Navier-Stokes (RANS) equations are used to model the flow around a fixed flat plate with an interceptor at different heights and attack angles. Based on finite volume method and SIMPLE algorithm which uses static structures, this model can be analyzed and the RANS results can be compared with the experimental data obtained in the current channel of the laboratory of waves and current of COPPE/UFRJ (LOC in Portuguese acronym). According to the results, the increase of pressure at the end of the flat plate was proportional to the interceptor height. In addition, the existence of interceptors can significantly increase the lift force coefficient at high angles of attack also proportional to the interceptor height. The presence of interceptor at the end of the flat plate increased both the lift coefficient and the drag coefficient but hydrodynamic drag did not grow as fast as the lift coefficient did. The lift coefficient increased much more. Furthermore, the results showed that the interceptor effectiveness is proportional to the boundary layer thickness at the end of the flat plate. As the interceptor was inside the boundary layer alterations of flow speed led to changes in boundary layer thickness, directly affecting interceptor's efficiency. Optimum choice of interceptor height had a great effect on its efficiency, and in choosing it the flow speed and length of the boat must be taken into consideration.

Energy-saving continuous fractionation and deep purification of organic materials by zonal melting

Energy-saving continuous fractionation and deep purification of organic material by zonal melting is considered. This approach permits the combination of various purification methods: fractionation by directional and/or counterflow solidification; and deep purification by single-zone or multizone melting. Simplified (for engineering use) and more detailed mathematical descriptions of the solidification of a binary melt with a stable phase boundary are considered. The rates of crystal nucleation and growth in binary melts are taken into account, as well as the influence of mass transfer on the heat transfer, the thermal diffusion, and the need for considerable temperature gradients. The mathematical descriptions employ effective distribution coefficients of the components that take account of the change in boundary-layer thickness on solidification (melting). The discrepancy between practical experience and the calculation results is no more than 10%, with 95% probability. An industrial system for continuous deep purification by zonal melting is proposed. It is based on a roller crystallization unit. A general mathematical description of the solidification of a binary melt is formulated and provides the basis for a simplified mathematical description and an algorithm for its solution in engineering calculations.

CFD Modeling of the Atmospheric Boundary Layer in Short Test Section Wind Tunnel

The aim of this paper is to provide a contribution to algorithms for the numerical simulation of the atmospheric boundary layer (ABL) in short test section wind tunnel, with the lowest pressure loss possible, for large Re, similar to the high values observed in nature. Different turbulent models have been examined for their relative suitability for the atmospheric boundary layer airflow with and without the implementation of buoyancy effects with modified turbulence model constants for the atmosphere. Validation of turbulent models through comparison with wind tunnel experiments is essential for practical applications. It has been observed that the k-e model is most suitable tool for generation of an ABL in short-chamber wind tunnel. A comparison has been made with the available experimental data, from literature, and the predicted CFD values are very close to the corresponding experimental measurements. The simulation results show the importance of turbulence model constant (Cµ), the non-uniform velocity and turbulence intensity profiles. Also, the significance of y+ for consistent assessment is confirmed. However, it has been found that the buoyancy force makes significant change in boundary layer thickness without a major impact on computation time.

Evaluation of the carbon content of aerosols from the burning of biomass in the Brazilian Amazon using thermal, optical and thermal-optical analysis methods

Abstract. Aerosol samples were collected at a pasture site in the Amazon Basin as part of the project LBA-SMOCC-2002 (Large-Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate). Sampling was conducted during the late dry season, when the aerosol composition was dominated by biomass burning emissions, especially in the submicron fraction. A 13-stage Dekati low-pressure impactor (DLPI) was used to collect particles with nominal aerodynamic diameters (Dp) ranging from 0.03 to 0.10 μm. Gravimetric analyses of the DLPI substrates and filters were performed to obtain aerosol mass concentrations. The concentrations of total, apparent elemental, and organic carbon (TC, ECa, and OC) were determined using thermal and thermal-optical analysis (TOA) methods. A light transmission method (LTM) was used to determine the concentration of equivalent black carbon (BCe) or the absorbing fraction at 880 nm for the size-resolved samples. During the dry period, due to the pervasive presence of fires in the region upwind of the sampling site, concentrations of fine aerosols (Dp<2.5 μm: average 59.8 μg m−3) were higher than coarse aerosols (Dp> 2.5 μm: 4.1 μg m−3). Carbonaceous matter, estimated as the sum of the particulate organic matter (i.e., OC × 1.8) plus BCe, comprised more than 90% to the total aerosol mass. Concentrations of ECa (estimated by thermal analysis with a correction for charring) and BCe (estimated by LTM) averaged 5.2 ± 1.3 and 3.1 ± 0.8 μg m−3, respectively. The determination of EC was improved by extracting water-soluble organic material from the samples, which reduced the average light absorption Ångström exponent of particles in the size range of 0.1 to 1.0 μm from >2.0 to approximately 1.2. The size-resolved BCe measured by the LTM showed a clear maximum between 0.4 and 0.6 μm in diameter. The concentrations of OC and BCe varied diurnally during the dry period, and this variation is related to diurnal changes in boundary layer thickness and in fire frequency.

An Analytical Model to Describe the Efficiency of an Immersion Rinsing Process

In this paper, a simple model is presented, describing an immersion rinsing process for flat solid substrates (e.g., semiconductor wafers). It is assumed that together with the solid a layer of processing liquid is transferred into a tank filled with rinsing liquid. The model calculates the replacement of the processing liquid with rinsing liquid as a function of the rinse time and the position in the tank. For static rinsing systems an analytical expression is readily obtained. For dynamic systems (where liquid convection plays a role) the concept of a convective boundary layer is introduced. It is assumed that within this boundary layer diffusion is the dominant transport mechanism while outside the boundary layer convection is dominant. Under these assumptions an analytical expression can also be obtained for dynamic rinsing with the thickness of the convective boundary layer as a process parameter. The resulting analytical expressions can be readily used to calculate the concentration of remaining processing liquid in the vicinity of the solid substrate as a function of rinse time. This provides a good way to determine the efficiency of the rinsing process. Moreover, these expressions allow us to assess the effect of changes in boundary layer thickness and thus can help with the optimization of existing rinsing processes.

Atmospheric turbulent boundary layer development due to a change in surface roughness

The turbulent wind flow passing from a surface of one roughness to another is investigated analytically. The wind flow is modelled by a deep atmospheric turbulent boundary layer that assumes the possibility of self-preserving development in the course of flow modification. Closed form solutions are obtained describing the entire flow development and the characteristics of the wind velocity, surface stress changes and frictional velocity changes. It is found that as the wind flows past the roughness discontinuity, the flow adapts to the new surface roughness through adjustment by a vertical displacement of streamlines. This leads to a change in boundary layer thickness and the length of the acceleration region, in which the adjustments depend obviously on the difference in roughness between the new and the original region. By including the effect of frictional velocity and higher-order terms, the theory also shows that a greatly different value of shear stress is possible in the new regime, as opposed to what Townsend [J. Fluid Mech. 26 (1966) 255] has predicted.

Control of the spanwise structure of a bluff-body wake by changes in boundary-layer thickness at separation

Flow separation from a blunt trailing-edge gives rise to inclined vortex formation in a manner similar to the well-known structure from a circular cylinder. By extracting a portion of the boundary layer on one side of the trailing edge, it is possible to generate large-scale vortex formation having a high degree of spanwise uniformity. This two-dimensional, large-scale vortex formation is accompanied by a substantial decrease in the near-wake fluctuation level.

Analysis of low Reynolds number separation bubbles using semiempirical methods

The formation and growth of transitional separation bubbles can significantly affect boundary-layer development on airfoils operating at low chord Reynolds numbers. Of primary concern is the change in boundary-layer thickness between laminar separation and turbulent reattachment. This can be estimated using semiempirical methods, such as the one devised by Horton (1968), which are based on solutions to the integral forms of the boundary-layer equations. The applicability of these methods at low Reynolds numbers was investigated using hot-wire measurements of bubbles formed on an NACA 66(3)-018 airfoil at chord Reynolds numbers of 50,000-200,000. The momentum thickness growth between separation and transition was found to be similar to that predicted for a laminar half-jet and appears to be influenced by the momentum thickness Reynolds number at separation. This parameter also was found to have a noticeable effect on the Reynolds number based on the length of a bubble's laminar portion.

Effects of aspect ratio and sidewall boundary-layer in airfoil testing

A correction to account for the sidewall boundary-layer effects in two-dimensional airfoil testing is presented by taking into consideration the nonlinear variation of the crossflow velocity across the width of the tunnel. The crossflow effects in the wind tunnel are represented in an empirical manner by considering the inviscid, compressible flow between a straight and a wavy wall. The analysis shows significant reduction in sidewall boundary-layer effects on airfoil midspan measurements with increasing aspect ratio of the model. Application of the correction to wind-tunnel data on airfoils demonstrated the method to be satisfactory in correlating shock location and also in giving good agreement between the measured pressure distribution and computed free air predictions. shed due to loss of lift in the boundary layer and a consequent change in the effective angle of incidence. The second approach, proposed independently by Barn well2 and Winter and Smith,3 assumes the changes in the boundary-layer thickness due to model-induced pressure field to have a significant effect on the flow over the airfoil. Using the small disturbance equation and accounting for the changes in the width of the flow passage, Barnwell presented a simplified correction for the measured forces in the form of a modified Prandtl-Glauret rule to account for the attached sidewall boundary-layer effects. Barn well's correction gave good agreement with the experiments of Bernard-Guell e4 at low Mach numbers. Sewall5 extended Barn well's approach to transonic speeds using the von Karman similarity parameter. Recently, an alternative simpler form of the correction encompassing both the Barnwell and Sewall methods has been proposed by Murthy.6 The Barnwell-Sewall correction appears to be effective in giving a satisfactory comparison between the measured and calculated pressure distributions on several airfoils tested in the NASA Langley 0.3-m Transonic Cryo- genic Tunnel.79 These studies suggest that the change in the sidewall boundary-layer thickness due to the airfoil pressure field can be a significant source of blockage correction, particularly at transonic speeds. The Barnwell-Sewall correction has been derived under certain assumptions of simplified boundary-layer treatment and linear variation of the crossflow velocity across the width of the tunnel. These assumptions imply that the airfoil chord is sufficiently large so that the effect of the sidewall boundary layers can be considered to be quasi-one-dimensional. Barn- well10 has shown recently that the linear crossflow assumption is justified provided (4d*/b)(b/c)2 is small. Hence, this assumption is likely to become less accurate when the width of the tunnel is much larger than the airfoil chord (i.e., for high- aspect-ratio models).

The interaction between waves and a turbulent current: waves propagating with the current

This paper describes an experimental programme carried out in a laboratory channel with rough and smooth beds, to investigate the interaction between gravity waves and a turbulent current. In particular, changes induced in the mean-velocity profiles, turbulent fluctuations, bed shear stresses and wave attenuation rates are considered for a range of wave heights, keeping the wave period constant. The smooth-boundary tests were carried out as a necessary preliminary to the more-realistic rough-boundary condition.A directionally sensitive laser anemometer was used to measure horizontal, vertical, and 45° velocity components in the oscillating fluid, and an on-line minicomputer was programmed to produce ensemble averages of velocities, Reynolds stresses and wave-elevation data. The cycle was sampled at 200 separate phase positions, with 180 observations at each position. Measurements were made at up to 30 points in the vertical.Preliminary tests were carried out on the unidirectional current and on the waves alone. These show that mean-velocity profiles and turbulence parameters of the current agree satisfactorily with previous experiments, and that the waves are approximated closely by Stokes’ second-order theory.For combined wave and current tests, mean-velocity profiles are generally found to differ from those suggested by a linear superposition of wave and current velocities, a change in boundary-layer thickness being indicated. However, shear stresses at the smooth boundary are found to be described by such a linear addition.

Local effects of horizontal and vertical sound fields on natural convection from a horizontal cylinder

Experiments were made with a heated horizontal circular cylinder subjected to transverse horizontal and vertical standing sound fields, and supported at a velocity antinode. Shadowgraphs of the air around the cylinder are presented for sound fields having frequencies of about 710 and 1470 Hz, with closely-spaced intervals of intensity. The observations confirm the local changes in boundary layer thickness and heat transfer indicated in previous work, and provide new evidence of the progressive splitting of the rising plume with a horizontal sound field and the development of a reverse-flow bubble at the bottom of a cylinder in a vertical sound field; the latter flow pattern has previously been predicted by analysis. The local changes are in general agreement with the trends predicted by boundary-layer theory.