Boundary Layer Growth Research Articles

An experimental study to evaluate the performance of variable-width channel cold plates for cooling rectangular Li-ion batteries

The limited cooling capacity of cold plates employing conventional channel designs represents a key obstacle in thermal management systems. The persistent growth of the thermal boundary layer and the uneven distribution of flow across the channels are the principal challenges encountered in this context. To overcome these challenges, this study aims to analyse a modified cold plate of obstructed variable-width channels that can improve the hydrothermal performance and temperature uniformity. Different shapes of pin fins and grooves with variable sizes that match the width of the variable-width channels are proposed. Specifically, four types of pin fins–grooves were used: ellipse, drop, rhombus and trapezoid. The traditional cold plate served as a basis for evaluating the performance of new designs of cold plates. In addition to the traditional cold plate, the four proposed designs of cold plates were manufactured from copper. Experiments were conducted with water as a working fluid under a laminar flow for a flow rate range of 0.004–0.04 kg/s and a discharge C-rate range of 1C–2C. Results indicated that the use of variable-width channels with all shapes of fins–grooves effectively contributed to improving the heat dissipation of the cold plate, accompanied with an increase in hydraulic pumping power. At the highest flow rate, the best improvement percentage in the Nusselt number of 82.5% was achieved when trapezoidal pin fins–grooves were used; by contrast, the lowest pumping power increasing percentage of 33.2% was obtained when elliptical pin fins–grooves were adopted. The best hydraulic/thermal performance was achieved when trapezoidal pin fins were utilised, with the highest figure of merit of 1.685.

Understanding the Flow Field in a Highly Loaded Tandem Compressor Cascade

Abstract Large flow turning in compressor cascades with single airfoils requires an effective control of the boundary layer growth under the diffusing flow. An alternative approach consists of distributing the loading over two subsequent airfoils, using a so-called tandem blade, to, in some measure, restart the boundary layer before flow separation occurs. It is, however, not always clear whether the benefits of the two-blade setup justify the additional manufacturing complexity. The present work explores the tandem blade concept using a gradient-based optimization method to produce an efficient, highly loaded compressor cascade blade directly. A comparison between two-dimensional single and tandem configurations is first presented to clarify the benefits of one over the other. The geometry is optimized for each concept using a gradient-based optimization technique to improve the pressure loss coefficient at multiple operating points for a given flow-turning constraint. While the optimized single and tandem blade designs have similar performance, the lower solidity of the latter provides a lighter compressor stage for the considered operating range. A three-dimensional tandem compressor cascade based on the two-dimensional study is then optimized to account for secondary flows. The aerodynamic performance and the operating range are assessed and compared, along with a study of the physical phenomena surrounding the tandem configuration. The resulting geometry presents similar non-conventional features observed during the two-dimensional study that exploits the flow mechanism of two-airfoil configurations.

Quasi-one-dimensional non-equilibrium method for shock tube and stagnation line flows

Shock tube experiments are used to investigate non-equilibrium thermochemistry and radiative processes in reacting gas hypersonic flows. Boundary layer and shock structure are known to influence the spatial variation of the test slug state properties. This work derives and validates a novel method for viscous, quasi-one-dimensional, non-equilibrium flow in a shock tube assuming constant shock speed. The proposed method fully resolves the shock structure. Mirels' estimate for boundary layer growth around the test slug determines the dilation rate at the centerline. This, along with relevant boundary conditions, appropriately models core flow in a shock tube. The flow equations are discretized by finite differences on a staggered grid. The resulting highly non-linear set of algebraic equations is solved by Newton iterations. The Jacobian matrix is block tridiagonal with a Schur complement, allowing efficient inversion. This culminates in a unique and computationally efficient quasi-one-dimensional method offering improved modeling of the physical characteristics of shock tube experiments. Results of a 3 km/s, 66.6 Pa argon test case solved by a viscous, axisymmetric Navier–Stokes solution had agreement with the proposed method in temperature and pressure profiles to within 2% and post-shock velocity to within 15%. Reacting gas shock tube experiments in synthetic air and synthetic Titan atmospheres were analyzed. Radiance values in the non-equilibrium and equilibrium regions were compared under various assumptions for the shock structure and radial velocity distribution. These results highlight the necessity of a dedicated shock tube solver when analyzing shock tube thermochemistry, particularly when determining reaction rates and relaxation parameters.

How horizontal transport and turbulent mixing impact aerosol particle and precursor concentrations at a background site in the UAE

Abstract. The optical, physical, and chemical properties of aerosol particles have been previously studied in the United Arab Emirates (UAE), but there is still a gap in the knowledge of particle sources and in the horizontal and vertical transport of aerosol particles and their precursors in the area. To investigate how aerosol particle and SO2 concentrations at the surface responded to changes in horizontal and vertical transport, we used data from a 1-year measurement campaign at a background site where local sources of SO2 were expected to be minimal. The measurement campaign provided a combination of in situ measurements at the surface and the boundary layer evolution from vertical and horizontal wind profiles measured by a Doppler lidar. The diurnal structure of the boundary layer in the UAE was very similar from day to day, with a deep, well-mixed boundary layer during the day transitioning to a shallow nocturnal layer, with the maximum boundary layer height usually being reached around 14:00 local time. Both SO2 and nucleation-mode aerosol particle concentrations were elevated for surface winds coming from the east or western sectors. We attribute this to oil refineries located on the eastern and western coasts of the UAE. The concentrations of larger cloud condensation nuclei (CCN)-sized particles and their activation fraction did not show any clear dependence on wind direction, but the CCN number concentration showed some dependence on wind speed, with higher concentrations coinciding with the weakest surface winds. Peaks in SO2 concentrations were also observed despite low surface wind speeds and wind directions unfavourable for transport. However, winds aloft were much stronger, with wind speeds of 10 m s−1 at 1 km common at night and wind directions favourable for transport; surface-measured concentrations increased rapidly once these particular layers started to be entrained into the growing boundary layer, even if the surface wind direction was from a clean sector. These conditions also displayed higher nucleation-mode aerosol particle concentrations, i.e. new particle formation events occurring due to the increase in the gaseous precursor.

Transient growth in diabatic boundary layers with fluids at supercritical pressure

Transient growth in diabatic boundary layers with fluids at supercritical pressure

Improving the photovoltaic thermal system efficiency with nature-inspired dolphin turbulators from energy and exergy viewpoints

The main purpose of this study is to enhance the efficiency of the photovoltaic thermal (PVT) system using nature-inspired dolphin turbulators designed based on thermal images of the dolphin dorsal fin. The turbulators are placed in the cooling channel of the PVT. The effects of the number and length scale of the turbulators are investigated on electrical, thermal, and overall efficiencies from energy and exergy perspectives, as well as the photovoltaic cell temperature at different times of the day. The dolphin turbulator enhances heat transfer to the coolant by preventing the boundary layer growth, accelerating the fluid flow, directing and impacting the coolant to the absorber plate, and improving the mixing of coolant. Consequently, turbulators reduce the photovoltaic cell temperature, increase the coolant temperature, and improve efficiencies. The enhanced PVT improves the electrical, thermal, and overall efficiencies by about 2.59 %, 16.9 %, and 14.78 % from the energy viewpoint and 2.59 %, 12.87 %, and 3.77 % from the exergy viewpoint compared to the conventional PVT. In addition, this study investigates how operational conditions, such as the coolant Reynolds number, the coolant inlet temperature, and ambient wind speed, affect the efficiencies from energy and exergy viewpoints.

Laminar Boundary Layer over a Serrated Backward-Facing Step

Laminar flow over a modified backward-facing step (BFS) was studied experimentally and computationally, with the results compared to a flight test on a Piper Cherokee wing. The BFS was modified with a serrated spanwise variation while maintaining a constant step height, and this modification is termed a serrated BFS (sBFS). A scaling law was proposed and then used to develop the experimental operation conditions. The experiments showed evidence that the transition to turbulence was delayed over the forward part of the serration (termed the valley). The boundary layer growth and characterization were used to validate the computational model, which was then used to examine details not available from the experiment, including the wall shear stress distribution and streamlines as they go over the sBFS. The wall shear stress showed the formation of low-shear diamonds downstream of the sBFS valley that were associated with laminar flow, which confirmed previous assumptions about the low-shear diamonds observed in the flight tests. The length of the low-shear diamonds was scaled with the sBFS geometry. Finally, the streamlines showed that the near-wall flow forward of the sBFS is pumped towards the sBFS peak, where it rapidly transitions to turbulence at that location.

Using Satellite and ARM Observations to Evaluate Cold Air Outbreak Cloud Transitions in E3SM Global Storm‐Resolving Simulations

AbstractThis study examines marine boundary layer cloud regime transition during a cold air outbreak (CAO) over the Norwegian Sea, simulated by a global storm‐resolving model (GSRM) known as the Simple Cloud‐Resolving Energy Exascale Earth System Model Atmosphere Model (SCREAM). By selecting observational references based on a combination of large‐scale conditions rather than strict time‐matched comparisons, this study finds that SCREAM qualitatively captures the CAO cloud transition, including boundary layer growth, cloud mesoscale structure, and phase partitioning. SCREAM also accurately locates the greatest ice and liquid in the mesoscale updrafts, however, underestimates supercooled liquid water in cumulus clouds. The model evaluation approach adopted by this study takes advantages of the existing computational‐expensive global simulations of GSRM and the available observations to understand model performance and can be applied to assessments of other cloud regimes in different regions. Such practice provides valuable guidance on the future effort to correct and improve biased model behaviors.

Optimization of contraction cone length in an open-circuit wind tunnel

The studies in the literature mostly focus on the curvature of the contraction cone, while research regarding the impact of contraction length on flow characteristics is limited. This study aims to determine the optimal length of the contraction cone, which ensures uniform flow at the outlet section, considering the wall shear factor. Four contraction cones of varying lengths were designed based on the hydraulic radius of the inlet. Numerical analysis was conducted to obtain static pressure and velocity distributions for the designed geometries. It was observed that wind tunnels designed with contraction cone lengths 2, 3, and 3.5 times the inlet hydraulic radius exhibited similar flow patterns. However, a longer contraction curve minimizes flow disturbances and turbulence, thereby enhancing flow uniformity and steadiness, while thicker boundary layers result from increased wall shears due to boundary layer growth. Consequently, it was concluded that a wind tunnel configured with a contraction cone length of 2 times the inlet hydraulic radius (square contraction cone) is the optimal choice, considering the combined effects of wall shear and flow uniformity.

Accurate assessment of land–atmosphere coupling in climate models requires high-frequency data output

Abstract. Land–atmosphere (L–A) interactions are important for understanding convective processes, climate feedbacks, the development and perpetuation of droughts, heatwaves, pluvials, and other land-centered climate anomalies. Local L–A coupling (LoCo) metrics capture relevant L–A processes, highlighting the impact of soil and vegetation states on surface flux partitioning and the impact of surface fluxes on boundary layer (BL) growth and development and the entrainment of air above the BL. A primary goal of the Climate Process Team in the Coupling Land and Atmospheric Subgrid Parameterizations (CLASP) project is parameterizing and characterizing the impact of subgrid heterogeneity in global and regional Earth system models (ESMs) to improve the connection between land and atmospheric states and processes. A critical step in achieving that aim is the incorporation of L–A metrics, especially LoCo metrics, into climate model diagnostic process streams. However, because land–atmosphere interactions span timescales of minutes (e.g., turbulent fluxes), hours (e.g., BL growth and decay), days (e.g., soil moisture memory), and seasons (e.g., variability in behavioral regimes between soil moisture and latent heat flux), with multiple processes of interest happening in different geographic regions at different times of year, there is not a single metric that captures all the modes, means, and methods of interaction between the land and the atmosphere. And while monthly means of most of the LoCo-relevant variables are routinely saved from ESM simulations, data storage constraints typically preclude routine archival of the hourly data that would enable the calculation of all LoCo metrics. Here, we outline a reasonable data request that would allow for adequate characterization of sub-daily coupling processes between the land and the atmosphere, preserving enough sub-daily output to describe, analyze, and better understand L–A coupling in modern climate models. A secondary request involves embedding calculations within the models to determine mean properties in and above the BL to further improve characterization of model behavior. Higher-frequency model output will (i) allow for more direct comparison with observational field campaigns on process-relevant timescales, (ii) enable demonstration of inter-model spread in L–A coupling processes, and (iii) aid in targeted identification of sources of deficiencies and opportunities for improvement of the models.

AFM Special Issue Summary - Integrating Surface Flux with Boundary Layer Measurements

To help bridge science topics related to land-atmosphere interactions, we organized a virtual special issue in this journal (Agricultural and Forest Meteorology [AFM]) entitled, “Land-Atmosphere Interactions: Integrating Surface Flux with Boundary Layer Measurements.” The motivation for the special issue was driven by existing disciplinary barriers between research areas that all address land-atmosphere interactions. In particular, it addressed research silos between those who study features of the land surface, surface fluxes (including water, energy, and trace gases), atmospheric boundary layer growth and thermodynamics, and atmospheric composition and aerosols. The special issue sought to bring these communities together to integrate multiple observations across the soil-vegetation-atmosphere continuum with the aim of 1) improving broader understanding of land-atmosphere interactions, feedbacks, and coupling, 2) fostering new collaborations between atmospheric and surface flux scientists, and 3) identifying new paths for integrative research. Here, we provide an overview and synthesis of the special issue.

An extension of Thwaites’ method for turbulent boundary layers

Thwaites (Aeronaut. Q., vol. 1, 1949, pp. 245–280) developed an approximate method for determining the evolution of laminar boundary layers. The approximation follows from an assumption that the growth of a laminar boundary layer in the presence of pressure gradients could be parameterized solely as a function of the Holstein–Bohlen flow parameter, thus reducing the von Kármán momentum integral to a first-order ordinary differential equation. This method is useful for the analysis of laminar flows, and in computational potential flow solvers to account for the viscous effects. In this work, an approximate method for determining the momentum thickness of a two-dimensional, turbulent boundary layer is proposed following Thwaites’ work. It is shown that the method provides good estimates of the momentum thickness for multiple boundary layers, including both favourable and adverse pressure gradient effects, up to the point of separation. In the limit of high Reynolds numbers, it is possible to derive a criterion for the onset of separation from the proposed model, which is shown to be in agreement with prior empirical observations (Alber, 9th Aerospace Sciences Meeting, 1971). The sensitivity of the separation location with respect to upstream perturbations is also analysed through this model for the NASA/Boeing speed bump and the transonic Bachalo–Johnson bump.

Effects of thermal radiation on Hiemenz slip flow and heat transfer of MHD non-Newtonian fluid in porous media due to permeable plate

This article is to presents a numerical study on effects of thermal radiation on stagnation point slip flow and heat transfer of MHD fluid through porous media due to permeable plate. The governing equations are P DE’s which are transformed into ODE’s by using similarity method. These ODE’s are numerically solved. It is observed that, the heat transfer rate increases with enhancing parametric values of Prandtl number. The results of the present work revels a good correlation with earlier published work. It is noted that temperature enhances as Casson parametric values enhanced. The velocity and temperature gradients are analyzed through graphs and tables. Based on the obtained scientific computations, it is claimed that the reported results can play a useful role in manufacturing processes and improvement in energy and heat resources. It is found that the effect of nonlinear thermal radiation stabilizes the thermal boundary layer growth.

Understanding the Effect of Three-Dimensional Design in Tandem Blade

Abstract In order to maximize the pressure ratio and efficiency, compressor designers have tried several unconventional design approaches. Tandem blading is one such unconventional design that promises a higher pressure ratio per stage through a higher diffusion factor. The nozzle shape created between the forward and aft blades of a tandem configuration acts as a passive boundary layer control mechanism. The boundary layer growth over the aft rotor is therefore effectively controlled with the help of this gap-nozzle flow. The flow complexity is likely to increase in the case of a tandem rotor due to the twin leakage vortices, twin wake regions, and their interaction with the hub and casing boundary layers. Modern compressor blades are often designed with three-dimensional blade techniques such as sweep, lean, dihedral, end bent, etc., to reduce the various losses and achieve optimum performance. However, to the best of the author’s knowledge, the effect of 3D blade designs on the performance of tandem rotors has not been fully explored so far. A comprehensive numerical investigation is undertaken to understand the effect of 3D designs on the performance of tandem blades. Axial sweep and dihedral failed to improve the performance of the tandem rotor. Significant improvement in the stall margin is observed for the forward chordwise-swept and negative lean tandem rotors and is largely attributed to lower tip incidence. The performance penalty of the forward-swept and negatively leaned cases can be reduced by integrating compound or variable lean and sweep into the design.

Impact of jet intermittency on surface-structured heat sinks for electronics liquid cooling

The boosting heat generation rate of high-performance processors is challenging the conventional cooling techniques. Passive surface structure designs no longer satisfy the heat dissipation requirement. The present work proposes the combined design of active jet intermittency and passive surface modification in heat sinks, and evaluates the heat transfer enhancement through numerical and experimental approaches. The trapezoidal-waveform flow rate at the inlet has constant time-averaged Reynolds number of 763 and the Strouhal number ranges from 0.01 to 0.04. The surface geometry includes cylindrical groove and hemispherical dimple with different arrangements. The underlying physics of thermal performance improvement is explained by complex flow circulation that disrupts the thermal boundary layer growth and dynamic vortex motion that promotes mixing between near-wall fluid and mainstream coolant. Higher pulsation frequency and smaller duty cycle are beneficial for cooling enhancement while the percentage improvement of dimple structure is higher than groove structure, which agrees with the experimental results. Compared to the steady-flow flat heat sink, a maximum enhancement of 45% is obtained by adopting intermittent jet at grooved surface.

On the Physics of High CAPE

Abstract Large values of convective available potential energy (CAPE) are an important ingredient for many severe convective storms, yet there has been comparatively little research on how, physically, such large values arise or why they take on the observed values and climatology. Here we build on recently published observational and theoretical work to construct a simple, one-dimensional coupled soil–atmosphere model of preconvective boundary layer growth, driven by a single diurnal cycle of prescribed net surface radiation. Based on this model and previously published research, we suggest that high CAPE (>∼1000 J kg−1) results when air masses that have been significantly modified by passage over dry, lightly vegetated soils are advected over moist and/or moderately vegetated soils and then exposed to surface solar heating. Several diurnal cycles may be needed to raise the moist static energy of the boundary layer to levels consistent with high CAPE. The production of CAPE and erosion of convective inhibition (CIN) are strongly affected by the potential temperature of the desert-modified air mass, the level of near-surface soil moisture (and root-zone soil moisture if significant vegetation is present), the type of soil, and the characteristics of the vegetation. Consequently, CAPE production and severe convective weather may be significantly affected by regional-scale land-use changes and by climate change. Significance Statement The energy available for severe convective storms depends strongly on the properties of the underlying soil and vegetation and the temperature of air masses formed over dry terrain upstream. This implies that the severity of convective storms can be strongly affected by changes in land use and by climate change.

Model for core catcher ablation and cavity shape in the film regime

During core meltdown in a nuclear reactor the corium may propagates toward the lower head and hit the vessel or structural elements as a coherent liquid jet. This could occur for instance in sodium fast reactors (SFR) if discharge tubes and an in-vessel core-catcher are used as mitigation devices. This can result in the ablation of the core-catcher and potential loss of its integrity. During ablation, a cavity is formed within the solid. Therefore the ablation of a solid structure by a liquid jet is studied here, with a focus on the cavity shape to improve future core-catcher designs. During the cavity formation, liquid flows over the cavity being formed as a liquid film in the film ablation regime, and ultimately results in the formation of a liquid pool. In the film ablation regime, two types of cavity shapes have been identified previously. To describe the process of the cavity formation, a mathematical model valid in this film ablation regime is here proposed which links the local melting rate to the local curvature of the cavity. It is applied with two sets of assumptions applicable to the two types of cavity previously identified. For the first assumption set, the heat transfer coefficient varies along the liquid/solid interface while the liquid temperature is held constant, it is representative of laminar boundary layer growth in laminar film. For the second assumption set, the heat transfer coefficient is held constant while the temperature varies along the liquid/solid interface, this is consistent with a turbulent liquid film. The two modeling approaches are compared with experimental data acquired from the HAnSoLO experimental setup. Numerical predictions show promising agreements with experimental observations, particularly for the laminar boundary layer growth zone of the cavity.

A unified explanation of energy growth sources for unstable modes in flat-plate boundary layers

The detailed energy sources that sustain the eigenmodal exponential growth in boundary layers are currently unclear. In the present study, the phase of each term in the linear stability equation is examined to identify the significant physical sources for a wide range of Mach numbers and wall temperature ratios. The Tollmien–Schlichting mode for incompressible flows, the oblique first mode for supersonic flows and the Mack second mode and supersonic mode for hypersonic flows share some similar features. The unique appearance of obliqueness for the most unstable first mode is accompanied by the enhancement of Reynolds shear stress. By contrast, the weakened Reynolds thermal stress prevents the oblique second mode from being the most unstable state. Wall cooling stabilises the oblique first mode by rendering Reynolds thermal stress and dilatation fluctuations out of phase with the internal energy fluctuation. It destabilises the second mode by a newly generated pronounced region of wall-normal internal energy transport beneath the second generalised inflection point. In comparison, the porous coating destabilises the oblique first mode by significantly enhancing the mean-shear production while it stabilises the second mode similarly to wall heating. Finally, the relatively weak supersonic mode has the feature that the phase destruction of wall-normal transport near the critical layer results in a low contribution to the internal energy growth. Connections and consistencies are also highlighted with the previous inviscid thermoacoustic interpretation for the second mode (Kuehl, AIAA J., vol. 56, 2018, pp. 3585–3592) and for the supersonic mode. The pronounced sources along the critical layer and near-wall regions provide a unified understanding of the local energy amplification mechanisms of the inviscid modes in hypersonic boundary layers.

Machine Learning–Adjusted WRF Forecasts to Support Wind Energy Needs in Black Start Operations

Abstract The push for increased capacity of renewable sources of electricity has led to the growth of wind-power generation, with a need for accurate forecasts of winds at hub height. Forecasts for these levels were uncommon until recently, and that, combined with the nocturnal collapse of the well-mixed boundary layer and daytime growth of the boundary layer through the levels important for energy generation, has contributed to errors in numerical modeling of wind generation resources. The present study explores several machine learning algorithms to both forecast and correct standard WRF Model forecasts of winds and temperature at hub height within wind turbine plants over several different time periods that are critical for the anticipation of potential blackouts and aiding in black start operations on the power grid. It was found that mean square error for day-2 wind forecasts from the WRF Model can be improved by over 90% with the use of a multioutput neural network, and that 60-min forecasts of WRF error, which can then be used to adjust forecasts, can be made with an LSTM with great accuracy. Nowcasting of temperature and wind speed over a 10-min period using an LSTM produced very low error and especially skillful forecasts of maximum and minimum values over the turbine plant area.

Loss reduction in the compressor corner region via blade cooling

With Reynolds number (Re) decreasing to a critical value (approximately 2.0 × 105), the laminar-to-turbulent transition on the blade/endwall surface accelerates the loss generation in the corner region of compressors. Thus, how to effectively control the three-dimensional turbulent mixing loss at a low Re is of great importance. In this study, the mechanism of loss variation in a high-speed compressor cascade as Re decreases from 5.6 × 105 to 1.5 × 105 is investigated by large eddy simulations (LESs). Then, an isothermal cooled wall condition with a constant temperature of 0.8T01 (T01 is the inlet total temperature) is specified on the blade surface to explore the potential of loss reduction via wall cooling at Re=1.5 × 105. At Re=5.6 × 105, the boundary layers on the blade suction surface strongly interact with the endwall boundary layers and multiple passage vortex structures near the spanwise location of x/H = 0.3, inducing a large-scale concentrated shedding vortex (CSV) and determining the loss generation rate in the corner region. With Re decreasing to 1.5 × 105, the blade boundary layers grow much thicker due to the delayed transition. Detailed loss classification result indicates that the stronger interaction between the thickened blade boundary layers and secondary flow accelerates the local generation of turbulent fluctuations, which is mainly responsible for the performance deterioration in the corner region at a low Re. As the blade surface is cooled, the growth of blade boundary layers and the migration of secondary flow in the blade passage can be controlled simultaneously. Thus, the interaction between the blade boundary layers and secondary flow near the spanwise location of x/H = 0.3 is weakened, leading to a reduction in the overall total pressure loss by 13.3%. The results hope to guide in controlling the loss in the corner region of compressors operating at a low Re.