Structure Of Boundary Layer Research Articles

Influence of surface layer schemes on tropical cyclone Hato (2017) intensity

Three surface layer schemes (MYNN, ETA, and MM5) in the Weather Research and Forecasting (WRF 4.1) model were selected to conduct simulation experiments for tropical cyclone (TC) Hato (2017). The influence of different surface layer schemes on TC intensity was compared. Although the planetary boundary layer and land surface schemes were the same, surface layer schemes minimally affected TC tracks but significantly affected TC intensity. The highest correlation coefficient and smallest bias and root mean square error between the simulated TC intensity in the ETA and observations. The TC intensity simulated using ETA was the strongest and closest to the observations. The surface latent heat fluxes simulated using ETA at the time of the TC landing were 1200–1500 W/m2, and is 800–1200 W/m2 by MM5 and MYNN, respectively. The surface sensible heat flux simulated using ETA was 400–800 W/m2 and that simulated using MM5 and MYNN was 200–400 W/m2. The equivalent potential temperature and radial wind simulated using ETA were significantly higher than those simulated using MM5 and MYNN. The larger the energy transmitted from the surface to the tropical cyclone, the stronger the TC. The dynamic structural characteristics were consistent with the tropical cyclone intensity. The ETA indicated the strongest unstable planetary boundary layer structure, stronger convergence at the lower layer, divergence at the upper layer, vertical transport, and absolute angular momentum, and induced the formation of the strongest TC. The comparison results revealed that the surface fluxes from different surface layer schemes can affect the dynamic and thermal structures of the planetary boundary layer owing to the different stability functions. The surface fluxes were also affected by the temperature and wind in the planetary boundary layer. This interaction between the surface layer and planetary boundary layer affects the eye structure and secondary circulation of the TC and TC intensity.

Impact of Seasonal Snow‐Cover Change on the Observed and Simulated State of the Atmospheric Boundary Layer in a High‐Altitude Mountain Valley

AbstractThe structure and evolution of the atmospheric boundary layer (ABL) under clear‐sky fair weather conditions over mountainous terrain is dominated by the diurnal cycle of the surface energy balance and thus strongly depends on surface snow cover. We use data from three passive ground‐based infrared spectrometers deployed in the East River Valley in Colorado's Rocky Mountains to investigate the response of the thermal ABL structure to changes in surface energy balance during the seasonal transition from low to high snow cover. Temperature profiles were retrieved from the infrared radiances using the optimal estimation physical retrieval Tropospheric Remotely Observed Profiling via Optimal Estimation. A nocturnal surface inversion formed in the valley during clear‐sky days, which was subsequently mixed out during daytime with the development of a convective boundary layer when snow cover was low. Over high snow cover, a very shallow convective boundary layer formed, above which the inversion persisted through the daytime hours. We compare these observations to NOAA's operational High‐Resolution‐Rapid‐Refresh model and find large warm biases on clear‐sky days resulting from the model's inability to form strong nocturnal inversions and to maintain the stable stratification in the valley during daytime when there was snow on the ground. We suggest several factors contributing to the large model errors. These are (a) the inability of the model to represent well‐developed thermally driven flows likely due to the too coarse horizontal grid spacing (3 km), (b) too much convective mixing during daytime, and (c) too strong vertical coupling between the valley atmosphere and the free troposphere.

Effect of d-type rib roughness on the turbulent structure of side wall boundary layer for wave-current combined flow

An experimental study has been carried out in a laboratory flume to characterize the turbulence structure and turbulence anisotropy in the boundary layer over smooth and rough side walls for both current alone and wave-current combined flow situations. The rough side wall of the flume comprises a train of circular ribs (diameter, k) attached vertically maintaining uniform spacing p along the streamwise direction. The experiments are performed for smooth surface and rough (ribbed) surfaces with p/k = 2, 3, and 4 to reproduce different cases of d-type rib roughness. The effect of wave-current interaction has been investigated by superposing waves of two different frequencies. Time series data of three velocity components are obtained using Acoustic Doppler Velocimeter. At the near wall region, roughness with higher p/k value enhances the level of turbulent intensity and Reynolds stress significantly. In a channel with smooth side wall, the wave-current combined flow produces lesser turbulence intensity than the current alone flow near the wall. However, for a ribbed wall case, the effect is completely opposite that is, wave-current interacting flow induces higher intensities compared to the reference current alone flow. Substantial decline in the turbulent length scales at the near wall region are observed for ribbed walls, which reveals the strong effect of roughness elements on the turbulent structure. Superposition of wave reduces the length scales even more for both smooth and rough wall cases. As the spacing between two ribs ( p/ k ratio) increases, the energy dissipation rate increases. The analysis of anisotropy invariant map demonstrates a reduction of anisotropy in the vicinity of ribbed wall compared to that for a smooth wall. For wave-current combined flow, the anisotropy invariant data of Reynolds stress tensor varies dramatically within the boundary of map, reflecting significant changes in the state of turbulence.

Evaluation of Vertical Profiles and Atmospheric Boundary Layer Structure Using the Regional Climate Model CCLM during MOSAiC

Regional climate models are a valuable tool for the study of the climate processes and climate change in polar regions, but the performance of the models has to be evaluated using experimental data. The regional climate model CCLM was used for simulations for the MOSAiC period with a horizontal resolution of 14 km (whole Arctic). CCLM was used in a forecast mode (nested in ERA5) and used a thermodynamic sea ice model. Sea ice concentration was taken from AMSR2 data (C15 run) and from a high-resolution data set (1 km) derived from MODIS data (C15MOD0 run). The model was evaluated using radiosonde data and data of different profiling systems with a focus on the winter period (November–April). The comparison with radiosonde data showed very good agreement for temperature, humidity, and wind. A cold bias was present in the ABL for November and December, which was smaller for the C15MOD0 run. In contrast, there was a warm bias for lower levels in March and April, which was smaller for the C15 run. The effects of different sea ice parameterizations were limited to heights below 300 m. High-resolution lidar and radar wind profiles as well as temperature and integrated water vapor (IWV) data from microwave radiometers were used for the comparison with CCLM for case studies, which included low-level jets. LIDAR wind profiles have many gaps, but represent a valuable data set for model evaluation. Comparisons with IWV and temperature data of microwave radiometers show very good agreement.

Characterization of the vertical evolution of urban nocturnal boundary layer by UAV measurements: Insights into relations to cloud radiative effect

The complex structure of the nocturnal boundary layer (NBL) and its impact on air pollution remain poorly understood. In this study, we present in-situ nocturnal flight measurements onboard an unmanned aerial vehicle (UAV) during the wintertime of 2022 at an urban site in Hefei, China. Besides, co-located measurements of radiation intensity and total amount of cloud were conducted. The vertical distribution of temperature, particle number concentration, and relative humidity were obtained to study the structure of NBL and the key factors driving the evolution of the NBL. A multi-layer inversion boundary layer was observed during haze and fog episodes, which affects the vertical diffusion of particles near the surface and leads to a vertical gradient of particle number concentrations. The particle size distribution demonstrates a drastic vertical variation over different sections of the nocturnal boundary layer: homogeneously mixed in the SBL and the RL layer, sharply reduced in the IL. It is found that the temperature and particle number concentration differences between near-surface and at 500 m are highly related to variations of the radiation intensity and the amount of cloud. The decreased cloud cover enhances the surface cooling, creating a shallow NBL with multiple inversion layers, which reinforces the suppression of vertical diffusions and consequently promotes the accumulation of aerosols within the NBL. This reveals an important mechanism for the impact of NBL evolution modulated by cloud radiative effect on the formation of urban haze.

A New Conceptual Picture of the Trade Wind Transition Layer

Abstract The transition layer in the trades has long been observed and simulated, but the physical processes producing its structure remain little investigated. Using extensive observations from the Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) field campaign, we propose a new conceptual picture of the trade wind transition layer, occurring between the mixed-layer top (around 550 m) and subcloud-layer top (around 700 m). The theory of cloud-free convective boundary layers suggests a transition-layer structure with strong jumps at the mixed-layer top, yet such strong jumps are only observed rarely. Despite cloud-base cloud fraction measured as only 5.3% ± 3.2%, the canonical cloud-free convective boundary layer structure is infrequent and confined to large [O(200) km] cloud-free areas. We show that the majority of cloud bases form within the transition layer, instead of above it, and that the cloud-top height distribution is bimodal, with a first population of very shallow clouds (tops below 1.3 km) and a second population of deeper clouds (extending to 2–3 km depth). We then show that the life cycle of this first cloud population maintains the transition-layer structure. That is, very shallow clouds smooth vertical thermodynamic gradients in the transition layer by a condensation–evaporation mechanism, which is fully coupled to the mixed layer. Inferences from mixed-layer theory and mixing diagrams, moreover, suggest that the observed transition-layer structure does not affect the rate of entrainment mixing, but rather the properties of the air incorporated into the mixed layer, primarily to enhance its rate of moistening. Significance Statement The physical processes producing the structure of the trade wind transition layer, a thin atmospheric layer thought to be important for regulating convection, are not yet well understood. Using extensive observations from a recent field campaign, we find that the cloud-free convective boundary layer structure, with an abrupt discontinuity in thermodynamic variables, is infrequent, despite cloud-base cloud fraction being small. We show that very shallow clouds both forming and dissipating within the transition layer smooth vertical gradients compared to a jump, except in large [O(200) km] cloud-free areas. This condensation–evaporation mechanism, which is fully coupled to the mixed layer, does not appear to affect the rate of entrainment mixing, but rather the properties of air incorporated into the mixed layer.

A two-phase density-based solver for simulating wet steam flows with non-equilibrium condensation. II. Effects of 3D surface topography through nozzles

In Paper I, we developed a new two-phase flow solver and assessed its reliability and effectiveness through a series of numerical simulations. In Paper II, we utilized this solver to study the steam flow through nozzles characterized by three-dimensional surface topography. To generate random rough surfaces, we employed a multivariate Weierstrass–Mandelbrot function. Our numerical simulation results exhibited good agreement with the reported experimental data for pressure distribution. Moreover, we examined the changes in pressure, temperature, humidity, Mach number, average Nusselt number, and thrust as the wall surface morphology varied. We observed that the pressure field was prone to fluctuations, the boundary layer structure thickened and changed, the degree of condensation decreased with reduced humidity, three-dimensional Mach waves occurred, and the averaged Nusselt number decreased while thrust deviation increased. These findings contribute to a better understanding of the effects of fractal dimensions and condensation on random roughness in steam flow through nozzles and highlight the significance of incorporating surface topography into numerical simulations to improve accuracy and predictability in engineering applications.

A significant study on radioactive cross fluid model with melting boundary near stagnation region

The current research endows the mathematical modeling for the stagnation point flow along with melting nonlinear radiative heat transfer for cross fluid. It has many applications such as synthesis of polymer solutions, plastic manufacturing, food processing, preparation of synthesis, adhesives, etc. A radially stretching sheet has started the flow keeping the assumption of rotational symmetry. Transformation approach is adopted to attain the nonlinear ordinary differential equations based on standard conservation laws of mass, linear momentum and also energy. Numerical solutions of the governing problem are computed via the shooting technique. Characteristics of the parameters appearing in modeling like the melting parameter, power–law index, local Weissenberg number, nonlinear radiation parameter, temperature ratio parameter and Prandtl number are comprehensively analyzed through graphical behavior. In addition, comparative analysis is presented for the cases when the value of free stream velocity overtops the stretching velocity and vice versa. Results are authenticated by taking special cases and excellent compatibility is achieved with the existing literature. The detailed analysis anticipates that the boundary layer structures could be strengthened by growing the melting parameter. Elevation in Weissenberg number caused velocity along with temperature profiles to demonstrate for [Formula: see text]. Further, the temperature behaves oppositely analogous to development in nonlinear radiation parameter besides the temperature ratio parameter.

Observation of the vertical structure of atmospheric boundary layer in subtropical UHI by radiosonde

Existing studies on the urban heat island (UHI) effect tend to focus on the planar scale. However, the UHI effect has obvious three-dimensional properties. This research focuses on the vertical structure of UHI. The study combined big data from sounding balloon and surface measurements in the Database for Atmospheric and Hydrologic Research, Taiwan. Data organization and classification were conducted using the Python programming language, the NumPy mathematical function library, and the Pandas data manipulation library. Results confirmed the existence of the atmospheric boundary layer and the canopy. Furthermore, the temperature inversion of the atmospheric boundary layer fell between heights of 130 m and 667 m. The height of the inversion is affected by the wind speed and cloudiness. Thereby demonstrating that the inversion fluctuates with weather scenario changes. The research results can be applied as the direction of urban planning thinking for urban heat island countermeasures.

Submeso Wave-Like Structures in the Atmospheric Boundary Layer and Their Parameters Measured with the Help of Sodar in Moscow Region

The paper presents study of the parameters of wave-like structures based on the data of long-term continuous sodar monitoring of the atmospheric boundary layer (ABL). Submesoscale internal gravity waves (IGWs) of non-orographic origin trapped in a stably stratified ABL (SBL) are considered. Statistical data on the parameters of two classes of IGWs are presented: internal gravity-shear waves (IGSWs) of the Kelvin-Helmholtz billow (KHB) type and buoyancy waves (BW). Identification and classification of IGWs was carried out by the visual analysis of sodar echograms. The measurements carried out in the Moscow region were used. The seasonal and diurnal variability of the frequency of registration of waves of both classes were studied, the values of the parameters of the observed waves were analyzed, and the ranges and average values of these quantities were compared.

Wind turbine wake: bridging the gap between large eddy simulations and wind tunnel experiments

The wind industry has experienced rapid growth over the past two decades and wind energy is expected to continue leading the way in the global energy transition. Additional wind farms are expected to be installed in the coming years, both offshore and onshore. In view of exploiting any suitable terrain, an increased number of turbines are installed in close proximity to existing structures, thus exposing these to the wake turbulence. For instance, electrical cables are regularly exposed to those higher levels of turbulence compared to design loads, potentially leading to premature failure of the power line. It is hence essential to have a good understanding of these phenomena to be able to mitigate their effects. Scale-resolving investigations using Large Eddy Simulation (LES) can provide insight into site-specific loading conditions when combined with an Actuator Line Model (ALM), which reproduces the reaction force of each blade on the flow while still using a relatively coarse grid. The ALM used in this framework was newly-implemented in the spectral element code Nek5000, a highly efficient code with reduced cost per degree of freedom and exponentially fast convergence. The present work aims to bridge the gap between LES and a well controlled wind tunnel test of a scaled wind turbine, for validation purposes.First, a small scale wind turbine of diameter 80 cm is designed and tested in a well controlled wind tunnel environment. Experimental data are gathered ahead of it and in its wake, and the mean velocity and turbulence intensity profiles are presented.Although blockage effects are kept to a minimum, they will be accounted for in the simulations to ensure a better comparison with the measurements. In this work, we focus on a single inflow velocity field and on a fixed rotation speed.Simulating the produced wake using LES combined with ALM remains challenging as the results are very sensitive to the turbulent inflow conditions and also to the airfoil aerodynamics used in the ALM. In the experiment, the incoming profile is also that of a developing flow and accurately reproducing its main characteristics is required for proper inflow to the LES. A “Recycle and Rescale Method” (R2M) will be hence be used as it is well suited for such case. The method will allow for the relevant turbulent boundary layer structures to be properly reproduced numerically, hence ensuring that the wind turbine is exposed to similar inflow conditions than in the experiment.With the developed framework, the comparison between the wake produced in the experiment and that obtained using LES with ALM will provide the necessary data to further adjust the parameter and grid size used for the LES and the polar model used in the ALM.

Stability of drawing of microstructured optical fibres

We consider the stability of the drawing of a long and thin viscous thread with an arbitrary number of internal holes of arbitrary shape that evolves due to axial drawing, inertia and surface tension effects. Despite the complicated geometry of the boundaries, we use asymptotic techniques to determine a particularly convenient formulation of the equations of motion that is well-suited to stability calculations. We will determine an explicit asymptotic solution for steady states with (a) large surface tension and negligible inertia, and (b) large inertia. In both cases, we will show that complicated boundary layer structures can occur. We will use linear stability analysis to show that the presence of an axisymmetric hole destabilises the flow for finite capillary number and which answers a question raised in the literature. However, our formulation allows us to go much further and consider arbitrary hole structures or non-axisymmetric shapes, and show that any structure with holes will be less stable than the case of a solid axisymmetric thread. For a solid axisymmetric thread, we will also determine a closed-form expression that delineates the unconditional instability boundary in which case the thread is unstable for all draw ratios. We will determine how the detailed effects of the microstructure affect the stability, and show that they manifest themselves only via a single function that occurs in the stability problem and hence have a surprisingly limited effect on the stability.

Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations

Abstract. Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface. Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top. In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.

Condensation characteristics of air–water vapor mixture on the surface of vertical flat plate

The condensation heat transfer of air–water mixed vapor is an important heat transfer phenomenon in production life. An experimental system for condensation heat transfer of the air–water vapor mixed vapor is built in this paper. The condensation characteristics of the mixed vapor and the double boundary layer structure characteristics were investigated. The study mainly discussed the influence of flow velocity on the condensation performance of mixed vapor and summarized the experimental correlation equations considering flow velocity. Results indicate that the condensation performance decreases significantly as the air concentration increases. The mixed vapor flow velocity was increased from 1 to 3 m/s, and the condensation heat transfer coefficient was increased by 34.9–121.0%. The enhanced effect of flow velocity was also observed at high air concentration and low subcooling. The ratio of convective and condensation heat transfer coefficients increases with the rise in air concentration, reaching a maximum of 0.12. The proportion of convective heat transfer can be reduced by increasing the mixed vapor flow velocity. The gas film thickness is substantially higher than the liquid film thickness in mixed vapor condensation, and the gas/liquid film thermal resistance ratio can be more than 30 times. And increasing the flow velocity can significantly reduce the gas film/liquid film thermal resistance ratio. A new experimental correlation equation is fitted based on the experimental results, and this equation can be used as a reference for the design of related heat exchangers.

Turbulent characteristics in complex coastal areas assessed using BSWO observations and WRF-LES simulation results

Studies exploring the turbulent structure of the planetary boundary layer (PBL) over complex topographies are very limited, especially at night, because of the lack of available observations and difficulty of highly resolved simulation. In this study, micro-meteorological and turbulent characteristics over complex coastal areas were investigated based on high spatial-temporal resolution by using the Weather Research and Forecasting–Large Eddy Simulation (WRF-LES) model. Then, the simulations were compared to comprehensive observations obtained at the Boseong Standard Weather Observatory (BSWO), which is equipped with various observation facilities, such as a meteorological observation tower with a height of 307 m, and aiding ground-based remote sensing measurements used for PBL research. The comparison between the WRF-LES model results and the measurements by the wind lidar system at the BSWO showed that the LES approach in this study reproduced the spatial/vertical wind field structure and land-sea wind circulation distinctly better than the conventional PBL schemes. High turbulent kinetic energy (TKE) was observed near the surface level at night due to the increased nocturnal shear production following the evolution of land and mountain breezes. The calculated TKE from WRF-LES model was applied to identify the mixing intensity based on the mixing length and eddy diffusion coefficient within the boundary layer, and then those were evaluated with the observed extinction coefficient from the aerosol Micro-Pulse Lidar system (MPL). In addition, the planetary boundary layer height (PBLH) at night was explored by using the TKE threshold method combined with the bulk Richardson number threshold method for the daytime PBLH. Compared with the observed value from a ceilometer at the BSWO, we found that the TKE threshold method proposed in this study could yield highly accurate PBLH values, especially at night. It is also expected that the enhanced application of WRF-LES like to this study can lead to understanding and prediction with the high confidence for much detail and accurate spatiotemporal turbulent structure over complex coastal areas, which will contribute to more comprehensive air quality modeling studies.

Coupled physics analysis of blended-wing-body underwater glider equipped with electromagnetic active flow control

Lift-drag ratio is an important index to determine the hydrodynamic performance, glide performance, and economy of the underwater glider(UG). Limited by many factors, the shape optimization method has been unable to meet the needs of UG hydrodynamic performance optimization. Aiming at this crucial problem, the electromagnetic active flow control method for the UG is proposed in this paper. The coupled physical analysis of the blended-wing-body underwater glider(BWB-UG) equipped with electromagnetic active flow control is carried out by numerical method. The numerical calculation model of electromagnetic active flow control is established. The coupled physical field structure of the BWB-UG equipped with electromagnetic active flow control is studied by Detached-Eddy Simulation. The mechanism of electromagnetic active flow control on the hydrodynamic performance of the BWB-UG is further revealed by coupling physics analysis. The results show that a layer of the periodically distributed electromagnetic force is formed on the surface of BWB-UG, and the boundary layer structure can be effectively controlled. By equipping with electromagnetic active flow control, the lift-drag ratio of BWB-UG can be increased by 11.659%.

Investigations of correlation and coherence in turbulence from a large-eddy simulation

Abstract. Microscale flow descriptions are often given in terms of mean quantities, turbulent kinetic energy, and/or stresses. Those metrics, while valuable, give limited information about turbulent eddies and coherent turbulent structures. This work investigates the structure of an atmospheric boundary layer using coherence and correlation in space and time with a range of separation distances. We calculate spatial correlations over entire planes of velocity fluctuations, from which we can evaluate the correlation along different directions at different spacings. Similarly, coherence of the three velocity components over separations in the three directions is also investigated. We apply these analyses to a mesoscale–microscale coupled scenario with time-varying conditions and examine nuances in spatial correlations that are often overlooked. Through these analyses and results, this work highlights important differences observed in terms of coherence when comparing large-eddy simulation data to simpler models and suggests ways to improve these simpler models. We note that such differences are important for disciplines like wind energy structural dynamic analysis, in which blade loading and fatigue depend strongly on the structure of the turbulence. We emphasize the additional wealth of data that can be provided by typical atmospheric boundary layer large-eddy simulation when correlation and coherence analysis is included, and we also state the limitations of large-eddy simulation data, which inherently truncate the smaller scales of turbulence.

Drag Reduction Performance of Triangular (V-groove) Riblets with Different Adjacent Height Ratios

Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.

The Near‐Bed Flow Structure and Bed Shear Stresses Within Emergent Vegetation

AbstractThe structure of the bottom boundary layer (BBL) in aquatic flows influences a range of biophysical processes, including sediment transport, hyporheic exchange, and biofilm formation. While the structure of BBL above bare sediment beds has been well studied, little is known about the complex near‐bed flow structure within aquatic vegetation. In this study, we used high‐resolution laboratory measurements and numerical Large Eddy Simulations to investigate the near‐bed mean and turbulent flow properties within staggered‐ordered emergent vegetation under a wide range of flow conditions and densities. There is strong spatial variability of key near‐bed flow characteristics on the scale of the vegetation elements. Measurement locations that provide single‐point flow characteristics closest to the spatially averaged values were identified. The spatially averaged BBL thickness is influenced strongly by vegetation density. This impact of vegetation density is engendered through its direct control of near‐bed turbulent kinetic energy (TKE), which in turn is negatively correlated with BBL thickness, both locally in a given flow and across the range of flow conditions studied here. A model based on the near‐bed TKE is developed to predict the BBL thickness and, ultimately, the bed shear stress. Model predictions were in close agreement with the experimental and numerical results. These findings provide new insights into the physical links between near‐bed flow variables and therefore contribute to the understanding of some of the complex biophysical processes present in vegetated flows.

Airborne Quantum Key Distribution Performance Analysis under Supersonic Boundary Layer.

Airborne quantum key distribution (QKD) that can synergize with terrestrial networks and quantum satellite nodes is expected to provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the random distributed boundary layer that surrounding to the surface of the aircraft when the flight speed larger than Mach 0.3. Here, we investigate the airborne QKD performance with the BL effects. Furthermore, we take experimental data of supersonic BL into the model and compare the airborne QKD performance under different conditions. Simulation results show that, owing to the complex small-scale turbulence structures in the supersonic boundary layer, the deflection angle and correspondingly drifted offset of the beam varied obviously and randomly, and the distribution probability of photons are redistributed. And the subsonic and supersonic boundary layer would decrease ~35.8% and ~62.5% of the secure key rate respectively. Our work provides a theoretical guidance towards a possible realization of high-speed airborne QKD.