Modification Of Boundary Layer Research Articles

Multivariable analysis of vortex generator-aided heat transfer enhancement for 18650 battery thermal management system

Currently, electric vehicles and hybrid cars have emerged as global solutions to replace internal combustion engine vehicles, with lithium-ion batteries being the most widely chosen option due to their low discharge rates and high energy density. Air-cooling systems, recognized for their cost-effectiveness and stability, are extensively employed in the thermal management systems of automotive batteries. However, conventional air-cooling models exhibit drawbacks in uniformly cooling battery packs. In this study, a numerical investigation is conducted to explore the enhanced cooling heat transfer facilitated by a vortex generator in the air-cooled battery thermal management system. The heat generation and transfer of a battery pack are analyzed using the experimentally validated 2C discharge heat dissipation model for batteries. The assessment focuses on the impact of a vortex generator on the heat transfer performance of a series-connected battery pack. The results indicate that fluid mixing, boundary layer modification, and flow instability are the primary factors contributing to enhanced heat transfer. Through the utilization of orthogonal experimental design and fuzzy grey relational analysis, the influence of three parameters (vortex generator position, angle of attack, and fin height) on thermal performance was systematically analyzed. The analysis reveals that the vortex generator position exerts the most significant impact on the heat transfer index, followed by height, with the angle having the least effect. Nusselt number and entropy generation were chosen as optimization indicators for a comparative analysis of enhancing heat transfer with and without optimized vortex generator assistance. The research results demonstrate that the optimal scenario occurs when entropy generation is minimized, resulting in a 56.4 % reduction in temperature difference (2.66 K) and a 12.1 % decrease in the maximum temperature (2.75 K). This study provides valuable guidance for further optimizing battery thermal management.

Simulation of flow and shear stress distribution on the Oceano Dunes, implications for saltation and dust emissions

Wind-driven shear stresses drive saltation and dust emissions over sand dunes. However, the complex topography of coastal dunes and practical difficulties in measuring shear stresses in the presence of blowing sand make a detailed view of this important information difficult to obtain. We combine computational fluid dynamics with field instrumentation and high resolution topographic data to investigate the shear stresses resulting from on-shore winds on the ground surface for a flat sloping beach area, nebkha foredune, and transverse dune at Oceano Dunes, CA. This approach allows for paired simulations over measured and modified topographies using the same boundary condition. Shear values from scenarios that account for the effects of vegetation and aerodynamic form separately are presented. Using available dust emissions data, an accounting of the emissions from the surface was performed. Sparse vegetation on the nebkha in the foredune was found to play a significant role in modulating the shear on the surface and initial boundary layer modification of the onshore wind profile.

Mechanistic study on modification of convective internal boundary layer by spanwise surface motion

Modification of a convective internal boundary layer (IBL) by spanwise motion of a warm surface is investigated by imposing different surface moving speeds in the present study. Our analysis shows that the spanwise surface motion reduces the Reynolds shear stress right after the increase in the surface temperature in the convective IBL. The maximum decreasing rate of the Reynolds shear stress is found to be approximately 75% at the largest moving speed of the warm surface considered in the manuscript. Due to the reduction of the Reynolds shear stress, the vertical momentum transport is fundamentally altered, and the mean flow accelerates immediately after the increase in the surface temperature. By scrutinizing the instantaneous and conditional averaged flow fields as well as the pre-multiplied energy spectra, we have attributed the reduction of the Reynolds shear stress to the suppression of the near-surface velocity streaks and quasi-streamwise vortices, and the delayed growth of the convective structures, such as thermal plumes. Our investigation suggests that the developments of the convective IBL can be influenced by a strong spanwise motion of the warm surface, which should be taken into consideration in the prediction model for practical applications.

Effect of Boundary Layer Modification and Enhanced Thermal Characteristics on Tribological Performance of Alumina Nanofluids Dispersed in Lubricant Oil

Effect of Boundary Layer Modification and Enhanced Thermal Characteristics on Tribological Performance of Alumina Nanofluids Dispersed in Lubricant Oil

Construction of an innovative adsorbent bed configuration in the adsorption chiller part 2. experimental research of coated bed samples

Enhancement of heat transfer conditions in the adsorption bed volume is one of the crucial tasks of improving adsorption chiller energy efficiency. The promising method is a modification of the heat exchanger boundary layer with thermally conductive glue. It ensures removing gaseous spaces, which strongly limit heat transfer from water to sorbent bed and in the opposite direction. Previously defined guidelines to build novel coated construction of adsorption beds published in Part 1 of this paper were applied to prepare coated bed samples and conduct experimental research. The thermal properties of the sorbent samples were examined with the laser flash method. Silica gel was used as a sorbent and two different kinds of epoxy resins were applied. Moreover, the scanning electron microscope (SEM) was employed to study the specific surface of the coated samples. The carried out analyses allowed to validate the selection criterion of coated bed components and indicate the sample with the best thermal properties. The maximum thermal conductivity of the coated layer was 0.533 W/(m·K) and it was obtained for a sample of silica gel with granulation of 710–800 μm with ER2 epoxy.

Numerical study of heat transfer enhancement using vortex generator for thermal management of lithium ion battery

A numerical study of two types of vortex generator (VG) have been conducted to enhance heat transfer of cooling in the battery thermal management system. The thermal behaviour and cooling effects have been investigated to evaluate the heat transfer performance of two types of VG. Air flow model coupled with a pouch cell model is carried out in this study. The results show that both types of vortex generators can enhance heat transfer before VGs, but only delta winglet VG can still enhance local heat transfer after it due to more vortices generated that can mix cold and hot air flow between the top and bottom thermal layers completely. It is found that the heat transfer enhancement is due to bulk fluid mixing, boundary-layer modification and flow destabilization by VGs. The maximum temperature of pouch cell is also numerically investigated by a validated battery model. The result shows that the maximum temperature of a pouch cell can be decreased by both delta winglet and rectangular rib. For the discharging rate at 5C, it can be decreased by 10% and the local Nusselt number can be increased by 38% compared to the baseline scenario without any VGs.

Bluff body boundary-layer modification and its effect on the near-wake topology

Boundary-layer modification over a bullet-shaped body has been accomplished using wall suction and its effect on the near-wake topology is reported. Two-dimensional velocity fields in the Reynolds number range 6.7×103−5.5×104 are acquired by means of particle image velocimetry and the mean, periodic, and random velocity components are analyzed. Wall suction reduces the boundary-layer thickness and increases the wall-normal velocity gradient. When the boundary layer is laminar up to the point of separation, the modification of the boundary layer leads to a vortex formation region that is up to 19% shorter compared with the case without wall suction. The modification of the boundary layer is accompanied by an increase of the Reynolds normal and shear stresses, which corresponds to larger forces acting on a fictive control area enclosing the mean recirculation region. The increase in Reynolds normal and shear stresses is due to an increase of the random fluctuation component, while the contribution of the periodic fluctuation component remains constant. The altered topology of the vortex formation region is accompanied by a significant change of the vortex shedding frequency and the base pressure. Empirical relations between the wake inlet conditions and the wake characteristics are reported, which hold for both laminar and turbulent boundary layers. The curvature of the streamlines enclosing the rear part of the body was also quantified and found to be proportional to the base pressure.

Heat transfer enhancement on the liquid side of an industrially designed flat-tube heat exchanger with passive inserts – Numerical investigation

Flow and heat transfer in a single tube of a flat-tube heat exchanger with passive inserts as means of heat transfer enhancement is numerically investigated. The tube geometry is taken from an industrial design with an elaborate pattern of passive inserts (bumps) distributed on the two walls of the channel. The channel is not symmetric in the spanwise direction, leaving a insert-free area on one side of the channel.In order to investigate the heat transfer in such a channel, the incompressible Navier-Stokes and energy equations are solved directly on a very fine computational mesh. These quasi-DNS simulations are carried out at three Reynolds numbers, spanning the threshold where laminar-turbulent regime transition is believed to occur based on previous experiments. Results suggest that turbulence does contribute to heat transfer enhancement at the highest Reynolds number. However, the boundary layer modification, which is the main heat transfer enhancement mechanism, is identically in action in both, laminar and turbulent, regimes. Results also show that the non-symmetric distribution of passive inserts is disadvantageous, particularly in the event of laminar-turbulence regime transition.

Investigations of boundary layer structure, cloud characteristics and vertical mixing of aerosols at Barbados with large eddy simulations

Abstract. Large eddy simulations (LESs) are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, we use the cell perturbation method based on finite amplitude potential temperature perturbations. In this work, this method is for the first time tested and validated for moist boundary layer simulations with open lateral boundary conditions. Observational data obtained from the SALTRACE field campaign is used for both model initialization and a comparison with Doppler wind and Raman lidar data. Several numerical sensitivity tests are carried out to demonstrate the problems related to “gray zone modeling” when using coarser spatial grid spacings beyond the inertial subrange of three-dimensional turbulence or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties in the downwind area west of Barbados are markedly affected in these kinds of simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and location. Saharan dust layers that reach Barbados via long-range transport over the North Atlantic are included as passive tracers in the model. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom at z ≈ 1800 m become apparent. The exact position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability (especially inversion strength) and wind shear. Comparisons of LES model output with wind lidar data show similarities in the downwind vertical wind structure. Additionally, the model results accurately reproduce the development of the daytime convective boundary layer measured by the Raman lidar.

On the interaction between turbulence grids and boundary layers

Turbulence grids are widely used in wind tunnels to produce representative turbulence levels when testing aerodynamic phenomena around models. Although the purpose of the grid is to introduce a desired turbulence level in the freestream flow, the wall boundary layers of the tunnel are subjected to modification due to the presence of such grids. This could have major implications to the flow around the models to be tested and hence there is a need to further understand this interaction. The study described in this paper examines wind tunnel wall boundary layer modification by turbulence grids of different mesh sizes and porosities to understand the effect of these parameters on such interaction. Experimental results are presented in the form of pressure loss coefficients, boundary layer velocity profiles and the statistics of turbulence modification.

Aircraft‐based observations of atmospheric boundary‐layer modification over Arctic leads

Leads are elongated channels in sea ice which play an important role for the heat and moisture exchange between the polar ocean and atmosphere. The aircraft campaign STABLE aimed to improve our current understanding of the formation of convective plumes over leads and their impact on the polar atmospheric boundary layer. It was carried out over the pack ice in the northern Fram Strait in March 2013. We present case studies of the boundary layer modification and turbulent fluxes over four wide leads, which differed strongly with respect to lead characteristics and environmental conditions. The observed near‐surface sensible heat fluxes ranged from 15 to 180 W m−2. The leads also induced an increase of the near‐surface temperature of up to 3.2 °C and a humidity increase of up to 0.2 g kg−1. In one of the cases, large entrainment fluxes exceeding 30% of the surface fluxes were observed. Vertical profiles of turbulent sensible heat and momentum fluxes were nonlinear downstream of the leads with a distinct flux maximum in the core of the convective plumes. In two cases, the plumes also strongly affected the wind field within the atmospheric boundary layer. Low‐level jets that existed in those cases in the region upstream of the leads disappeared in the plume region. Finally, it is shown that large errors can occur when flux measurements are derived from lead orthogonal flight legs only. Therefore, complex flight patterns, as presented in this study, are necessary to accurately determine the energy fluxes in the environment of leads.

Boundary layer modification by means of wall suction and the effect on the wake behind a rectangular forebody

The wake characteristics of a two-dimensional rectangular forebody with a smooth leading edge and a blunt trailing edge are investigated. Wall suction is applied along the forebody in order to modify the developing boundary layer. An initially laminar boundary layer subject to suction yields an asymptotic suction boundary layer at the trailing edge of the body, whereas a high enough suction coefficient relaminarizes an initially turbulent boundary layer. The critical suction velocity required to achieve this significant modification of the boundary layer properties is typically in the order of 1% of the free-stream velocity, where the critical suction coefficient depends on the Reynolds number. We show that a thinner boundary layer induces a higher vortex shedding frequency and a lower base pressure. Furthermore, the boundary layer state, laminar or turbulent, has a significant influence on the wake. For example, the Strouhal number based on the effective body thickness is being reduced by 25% from laminar to turbulent inlet conditions.

Increased tornado hazard in large metropolitan areas

The tornado climate was compared between large metropolitan areas and neighbouring non-metro cities using modern tornado reports in the Storm Prediction Center (SPC) archives. Twenty large metro areas in the higher-risk region of the U.S. were used to boost sample sizes hence robustness of results. Observational biases were minimised by using the most densely populated zips. The analysis found 50% greater tornado frequency and a thicker-tailed severity distribution in metro areas compared to the non-metro cities. These differences are significant at the 1% level.Regarding tornado frequency, the primary question is whether the raised occurrence rates in metro areas are due to observation biases or real differences in tornado climate. Past studies found no relative biases at the population densities used here, whereas there are two potential urban drivers of tornadogenesis. First, the urban heat island raises the storm severity above and downwind of main urban areas, as recorded in precipitation and lightning datasets. Second, the increased surface roughness over metro areas raises low-level shear which in turn has been found to be favourable for tornadogenesis. Modification of convective storms over large metro areas is the more plausible explanation of raised tornado frequency.The drivers of a thicker-tailed tornado severity distribution in metro areas are less certain. Potential causes include: increased debris-loading in metro tornadoes; modification of storms' lower boundary layer by increased surface roughness in metro areas; the reduced density of damage indicators in non-metro cities.

Experimental Investigation of Base-Drag Reduction via Boundary-Layer Modification

This investigation identifies and characterizes the changes in the near wake of a bluff body produced by boundary-layer thickening, which, in turn, result in an overall reduction in the base drag. For this study, wedge and flat-plate models with both smooth and rough forebody surfaces were used. The forebody surface roughness on both models was increased to modify/thicken the forebody boundary layer. Hot-wire surveys were carried out to characterize the boundary-layer state ahead of the separation point, and base-pressure measurements were used to quantify base-drag reduction associated with boundary-layer modification. In addition, particle image velocimetry measurements in the near wake were performed for both models, and proper orthogonal decomposition analysis was used to analyze the velocity data. Low-order near-wake velocity reconstructions obtained from proper orthogonal decomposition analysis accurately represented the near-wake behavior for the studied cases. It was observed from these low-order reconstructions that the shed vortices became more organized, convected faster, and decreased in strength when the boundary layer was thickened/modified. These changes in the wake behavior resulted in an increase in base pressure and an overall decrease in the base drag.

Wind-tunnel simulation of thick turbulent boundary layer

An experimental study aimed at revealing the possibility of simulation, in a subsonic wind tunnel, of enhanced Reynolds numbers Re** via modeling a thick flat-plate boundary layer possessing the properties of a Clauser-equilibrium shear flow is reported. We show that turbulators prepared in the form of variable-height cylinders of height h and diameter d = 3 mm and installed in two rows along the normal to the streamlined wall offer rather an efficient means for modification of turbulent boundary layer in solving the problem. In the majority of cases, mean and fluctuating characteristics of the boundary layer exhibit values typical of naturally developing turbulent boundary layers at a distance of 530 cylinder diameters. The profiles of mean velocity with artificially enhanced boundary-layer thickness can be well approximated, in the law-of-the-wall variables, with the well-known distribution of velocities for canonical boundary layer.

Turbulence Structure Modification and Drag Reduction by Microbubble Injections in a Boundary Layer Channel Flow

Turbulent boundary layer modification in a channel flow using injection of microbubbles as a means to achieve drag reduction was studied. The physical mechanism of this phenomenon is not yet fully understood. To obtain some information related to this phenomenon, single-phase (pure water) flow and two-phase (water and microbubbles) channel flow measurements are taken. The void fraction conditions were varied while maintaining a Reynolds number of 5128 based on the half channel height. The study indicates that the presence of microbubbles within the boundary layer modifies the turbulence structure such that variations in time and space turbulent scales are observed, as well as ejection and sweep phenomena.

Development of a MEMS-Based Control System for Compressible Flow Separation

A MEMS-based sensor and actuator system has been designed and fabricated for separation control in the compressible flow regime. The MEMS sensors in the system are surface-micromachined shear stress sensors and the actuators are bulk-micromachined balloon vortex generators (VGs). A three-dimensional (3-D) wing model embedded with the shear stress sensors and balloon VGs was tested in a transonic wind tunnel to evaluate the performance of the control system in a range of Mach number between 0.2 and 0.6. At each Mach number tested, the shear stress sensors quantify the boundary layer on the surface of the wing model while the balloon VGs interact with the boundary layer in an attempt to provide flow control. The shear stress measurements indicate the presence of a separated flow on the trailing ramp section of the wing model at all Mach numbers tested when the balloon VGs are not activated. This result is confirmed by total pressure measurements downstream from the wing model where a wake profile is observed. When the balloon VGs are activated, the shear stress level on the trailing ramp increases with the Mach number. At the highest Mach number tested, this increase elevates the shear stress on the ramp to almost the same level as the unseparated flow, suggesting the possibility of a boundary layer reattachment. This result is supported by the downstream pressure measurements which show a large pressure recovery when the balloon VGs are activated. The wind tunnel experiment successfully demonstrated two aspects of the MEMS flow control system: the effectiveness of the microshear stress sensors in measuring the separation characteristics of a high-speed compressible flow and the ability of the microballoons in positively enhancing the aerodynamic performance of a high-speed wing through boundary layer modification.

Observations of SST-Induced Perturbations of the Wind Stress Field over the Southern Ocean on Seasonal Timescales

Abstract The surface wind stress response to sea surface temperature (SST) over the latitude range 30°–60°S in the Southern Ocean is described from the National Aeronautics and Space Administration's QuikSCAT scatterometer observations of wind stress and Reynolds analyses of SST during the 2-yr period August 1999 to July 2001. While ocean–atmosphere coupling at midlatitudes has previously been documented from several case studies, this is the first study to quantify this relation over the entire Southern Ocean. The spatial structures of the surface wind perturbations with wavelengths shorter than 10° latitude by 30° longitude are closely related to persistent spatial variations of the SST field on the same scales. The wind stress curl and divergence are shown to be linearly related, respectively, to the crosswind and downwind components of the SST gradient. The curl response has a magnitude only about half that of the divergence response. This observed coupling is consistent with the hypothesis that SST modification of marine atmospheric boundary layer (MABL) stability affects vertical turbulent mixing of momentum, inducing perturbations in the surface winds. The nonequivalence between the responses of the curl and divergence to the crosswind and downwind SST gradients suggests that secondary circulations in the MABL may also play an important role by producing significant perturbations in the surface wind field near SST fronts that are distinct from the vertical turbulent transfer of momentum. The importance of the wind stress curl in driving Ekman vertical velocity in the open ocean implies that the coupling between winds and SST may have important feedback effects on upper ocean processes near SST fronts.

The influence of swash infiltration–exfiltration on beach face sediment transport: onshore or offshore?

Measurements were obtained from the swash-zone of a high-energy macrotidal dissipative beach of pore-pressure at four levels below the bed, and cross-shore velocity at a single height above the bed. Time-series from relatively high ( H s≈2.0 m) energy conditions were chosen for analysis from the mid-swash-zone at high tide. Calculation of upwards-directed pore-pressure gradients shows that, in this case, fluidisation of the upper layer of sediment, leading to enhanced backwash transport, is unlikely. An apparent conflict exists in the literature regarding the net effect of infiltration–exfiltration on the sediment transport, through the combined effects of stabilisation–destabilisation and boundary layer modification. This is examined for the data collected using a modified Shields parameter. Inferred instantaneous transport rates integrated over a single swash cycle show a decrease in uprush transport of 10.5% and an increase in backwash transport of 4.5%. Sensitivity tests suggest that there is a critical grain size at which the influence of infiltration–exfiltration changes from offshore to onshore. The exact value of this grain size is highly sensitive to the method used to estimate the friction factor.

Moisture uptake in the boundary layer above the Agulhas Current: A case study

Shipboard meteorological measurements and radiosonde ascents are used to describe the modification of the marine atmospheric boundary layer as air is advected across the warm Agulhas Current toward the coast. The data were collected on April 28, 1995, during the Agulhas Current Air‐Sea Exchange Experiment south of Port Alfred, South Africa, in a region characterized by a strong sea surface temperature gradient. The surface latent heat flux increased from approximately 160 W m−2 over the seaward border to 270 W m−2 over the current (approximately 80–90 km wide) and decreased dramatically to approximately 40 W m−2 over the cool shelf. This spatial heat flux gradient was reflected in the overlying atmosphere by a transition in atmospheric stability from the warm current to the cool continental shelf. A convective boundary layer over the current was replaced by a stable boundary layer over the cool shelf, where a deep residual moisture layer indicated the former vertical extent of convective eddies. A progressive accumulation of moisture occurred within the boundary layer as the air mass was advected over the current. Mean specific humidity and precipitable water vapor content in the boundary layer over the shelf were 20–25% higher than over the seaward border of the current. The results give credence to the assertion by many investigators that moisture uptake above the Agulhas Current may contribute significantly to moisture convergence and rainfall over the interior of South Africa.