Increase In Friction Velocity Research Articles

Study on whitecapping dissipation process for wave modelling during tropical cyclones

The atmosphere-wave interaction is an important physical process during tropical cyclones. Understanding and modelling of this process are of great significance for the technical and functional design of coastal and harbor structures. At the high wind velocities of tropical cyclones, foams and sprays that are blown away from the sea form a slip layer between the atmosphere and the sea surface. This slip layer makes the atmosphere-wave interaction exhibit different characteristics compared with that at low wind velocities. The significant effect of this layer on the atmosphere is the reduction of aero-dynamical surface roughness, which has been used to improve the expression of the drag coefficient. On this basis, the effect of the slip layer on the sea surface is further explored in this study. The whitecap coverage may reach a low limit at high wind velocities, and a modified numerical method of whitcapping dissipation for the wave spectrum model is proposed based on the classic field observations of whitecaps. According to these observations, when developing waves appear, the variation characteristics of whitecap coverage are different from those of developed waves with low wind velocities. Thus, the critical friction velocity of wave states should be defined, which can be expressed by the threshold steepness of developed waves due to the negative correlation between wave age and wave steepness. The dissipation mode is then modified to gradually reach the limit with the increase of friction velocities, which is validated during 24 tropical cyclones measured with 26 buoys. The negative Bias of the default mode generally decreases with the increase of friction velocity, even reaching −0.8 m, while the Bias of the modified mode is mostly maintained between 0.2 m and −0.2 m.

The Influence of Refined Urban Morphological Parameters on Dynamical and Thermal Fields in a Single-Layer Urban Canopy Model

In this study, localised and non-uniform urban morphology (UM) and urban fraction (UF) parameters are implemented in a single-layer urban canopy scheme in the Weather Research and Forecasting (WRF) mesoscale meteorological model. The purpose of this research is to evaluate the effect of the refined parameterisation scheme on the simulation of dynamic and thermal fields in the urban canopy of the Guangzhou metropolitan area. The results showed that, compared with the default urban canopy parameters of the WRF model, using the localised UM parameters resulted in the most significant improvement in the 10 m wind speed simulation. In urban districts, the mean bias between the observed and simulated 10 m wind speed was reduced significantly by 59% from 2.63 m/s to 1.09 m/s during the daytime. For the thermal environment simulation during the daytime, higher UF and UM values resulted in lower surface albedos and generated narrower street canyons compared with the default modelling setting, which caused more heat to be trapped in the urban canopy and ultimately led to an increase in the surface skin temperature (TSK) and a largely increased ground heat flux (GRD). As a result, at night, more heat was transferred from the ground to the surface, producing a higher TSK. The effect of the localised UF on the sensible heat flux (HFX) was closely related to the near-surface temperature gradient. The UM caused the HFX to increase during the daytime, which was related to the near-surface heat exchange coefficient in the lower model layers. As the high-resolution UM significantly altered the urban geometry, the dynamic environment simulation resulted in a large increase in friction velocity and a decrease in wind speed.

Baseline Subsoil CO2 Gas Measurements and Micrometeorological Monitoring: Above Canopy Turbulence Effects on the Subsoil CO2 Dynamics in Temperate Deciduous Forest

Accurate and continuous measurement of the subsoil CO2 is critical to better understand the terrestrial and atmosphere gas transfer process. This work aims to develop and field test a specific flow system to continuously measure the soil gas concentration (χc) and understand its main physical drivers. Hourly data measured in situ were collected through two dedicated wells at 1 m and 6 m depth coupled with micrometeorological measurement. Our study shows that χc at -1 m was at the lowest in winter and highest in summer. Meanwhile, the seasonal variation of χc at -6m is somewhat unclear. While it is inevitable that temperature plays a significant role, this factor related to biological activity cannot fully explain the variation. The decrease in χc at both depths in summer coincides with an increase of friction velocity, especially during dry periods with R2 of 0.68, which shows strong empirical evidence that wind turbulence plays a significant role in driving the deep soil CO2. A monitoring strategy for gas measurement combining borehole and micrometeorological measurement offers excellent long-term monitoring possibilities to derive the vertical distribution of CO2 and better understand the main physical drivers of gas exchange.

INFLUENCE OF DIHYDROQUERCETIN ON FRICTION OF SUPER-HIGH-MOLECULAR POLYETHYLENE BY STEEL AND ALLOY Ti6Al

The main reasons reducing the lifetime and failure of implants are tribo-oxidative processes. Stabilizing additives are used to inhibit tribo-oxidative activity and increase the lifetime of the implant. The most common stabilizer is α-tocopherol. The biocompatible stabilizer of a number of flavonoids, dihydroquercetin, was selected as the thermo- and tribo-oxidative stabilizer of UHMWPE. Dihydroquercetin (DHQ) stabilizes peroxide radicals formed as a result of the tribological process (friction of UHMWPE) by detaching a hydrogen atom from OH groups with the formation of a non-reactive phenoxyl radical. A comparative analysis of the samples, stabilized with DHQ and α-tocopherol, was carried out. The samples studied were prepared by direct compression molding at a temperature of 190ºС. The comparative study of thermo- and tribo-oxidative stability of UHMWPE samples modified with DHQ and α-tocopherol evidences that the thermo-oxidative stability induced with DHQ is higher that of α-tocopherol. In the first case the thermal-oxidative stability of the composition is maintained up to a temperature of 230 °C. Evolution of friction compositions at temperatures higher than that of the human body was investigated. Such tests were conducted using a gradual increase in friction velocity. During the tests, an increase in weight of initial UHMWPE and the composition with α-tocopherol induced by tribo-oxidative processes was observed. The introduction of dihydroquercetin, a highly effective biostabilizer of thermal oxidation, leads to the conservation of the original weight of the samples. When selecting a counterbody for the developed composition with DHQ, a study was conducted of friction on steel 3X13 and Ti6Al alloy used in medicine and in endoprostheses of artificial joints. It was found that the best results are obtained in the case of using a steel counterbody.

Numerical simulation of the effects of land use and cover change on the near-surface wind speed over Eastern China

In this study, the Weather Research and Forecasting (WRF) model is used to quantify the effects of land use and cover change (LUCC) on the near-surface wind speed (SWS). The simulated results show that the SWS based on land use cover data for the 2010s (LUC10) is lower than that based on land use cover data for the 1980s (LUC80) by a difference of 0.17 m s−1; the LUCC effects also result in a decrease of 9.0% of the probability of strong wind. The LUCC effects induce significant alteration of the roughness length, causing changes in the drag coefficient and friction velocity, and thereby decrease SWS. A 0.1 m increase in roughness length could cause a 0.003 increase in drag coefficient and a 0.015 m s−1 increase in friction velocity. The contributions of LUCC to the SWS changes vary among different regions. The increase of SWS in Northeastern China is caused by the changes from deciduous broadleaf to deciduous needleleaf forests, mixed forests and croplands, and these changes decrease the surface roughness length, drag coefficient and friction velocity. The significant decrease of SWS over the middle reaches of the Yangtze River is induced by the changes from closed shrubland and cropland/natural vegetation mosaic to evergreen broadleaf and deciduous broadleaf forest. The slowdown of SWS over the Shandong Peninsula, the Beijing–Tianjin–Hebei region, the Yangtze River Delta, and the Pearl River Delta can be attributed to the extension of urban and built-up areas and the decrease of croplands and the cropland/natural vegetation mosaic. The slowdown in SWS caused by LUCC is also revealed by the friction wind model (FWM); however, the FWM presented more significant effects of LUCC on decrease in SWS.

Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

Abstract. Flux towers provide essential terrestrial climate, water, and radiation budget information needed for environmental monitoring and evaluation of climate change impacts on ecosystems and society in general. They are also intended for calibration and validation of satellite-based Earth observation and monitoring efforts, such as assessment of evapotranspiration from land and vegetation surfaces using surface energy balance approaches. In this paper, 15 years of Skukuza eddy covariance data, i.e. from 2000 to 2014, were analysed for surface energy balance closure (EBC) and partitioning. The surface energy balance closure was evaluated using the ordinary least squares regression (OLS) of turbulent energy fluxes (sensible (H) and latent heat (LE)) against available energy (net radiation (Rn) less soil heat (G)), and the energy balance ratio (EBR). Partitioning of the surface energy during the wet and dry seasons was also investigated, as well as how it is affected by atmospheric vapour pressure deficit (VPD), and net radiation. After filtering years with low-quality data (2004–2008), our results show an overall mean EBR of 0.93. Seasonal variations of EBR also showed the wet season with 1.17 and spring (1.02) being closest to unity, with the dry season (0.70) having the highest imbalance. Nocturnal surface energy closure was very low at 0.26, and this was linked to low friction velocity during night-time, with results showing an increase in closure with increase in friction velocity. The energy partition analysis showed that sensible heat flux is the dominant portion of net radiation, especially between March and October, followed by latent heat flux, and lastly the soil heat flux, and during the wet season where latent heat flux dominated sensible heat flux. An increase in net radiation was characterized by an increase in both LE and H, with LE showing a higher rate of increase than H in the wet season, and the reverse happening during the dry season. An increase in VPD is correlated with a decrease in LE and increase in H during the wet season, and an increase in both fluxes during the dry season.

A summertime near-ground velocity profile of the Bora wind

While effects of the atmospheric boundary layer flow on engineering infrastructure are more or less known, some local transient winds create difficulties for structures, traffic and human activities. Hence, further research is required to fully elucidate flow characteristics of some of those very unique local winds. In this study, important characteristics of observed vertical velocity profiles along the main wind direction for the Bora wind blowing along the eastern Adriatic coast are presented. Commonly used empirical power-law and the logarithmic-law profiles are compared against unique 3-level high- frequency Bora measurements. The experimental data agree well with the power-law and logarithmic-law approximations. An interesting feature observed is a decrease in the power-law exponent and aerodynamic surface roughness length, and an increase in friction velocity with increasing Bora wind velocity. This indicates an urban-like velocity profile for smaller wind velocities and rural-like velocity profile for larger wind velocities, which is due to a stronger increase in absolute velocity at each of the heights observed as compared to the respective velocity gradient (difference in average velocity among two different heights). The trends observed are similar both during the day and night. The thermal stratification is near neutral due to a strong mechanical mixing. The differences in aerodynamic surface roughness length are negligible for different time averaging periods when using the median.For friction velocity the arithmetic mean proved to be independent of the time record length, while for the power-law exponent both the arithmetic mean and the median are not influenced by the time averaging period. Another issue is a large difference in aerodynamic surface roughness length when calculating using the arithmetic mean and the median. This indicates that the more robust median is a more suitable parameter to determine the aerodynamic surface roughness length than the arithmetic mean value. Variations in velocity profiles at the same site during different wind periods are interesting because, in the engineering community, it has been commonly accepted that the aerodynamic characteristics at a particular site remain the same during various wind regimes.

Effects of micro-Al(OH)3 and nano-SiO2 on wear behavior of GF/epoxy composites in high-friction velocity

Effects of micro-Al(OH) 3 powders on wear behavior of GF/EP composites are studied in friction velocity from 15 to 45 m/s. The results show that friction coefficients change insignificantly with increase of loads and addition of powders, but change significantly with the variation of friction velocity. Abrasion rates increase dramatically with the increase of friction velocity and load. The addition of nano-SiO2 increases the abrasion rate under the same load, the addition of Al(OH)3 powders decreases the abrasion rate. The wear mechanisms are adhesive and fatigue, which do not change with addition of either micro-Al(OH)3 powders or nano-SiO2. In the previous study, abrasive friction is the major wear mechanism in lower velocity friction condition. Different from low-velocity friction, the friction between the GF/EP composites and the transfer film is predominating in high-friction velocity, which results in dramatical increase in wear rate and decrease in friction coefficient in higher friction velocity. The effects of Al(OH) 3 powders on wear rate are attributed to the heat absorption when Al(OH) 3 powder decomposes, which is testified by the endothermic effect between 200°C and 300°C in DSC curves.

Down-mixing of phytoplankton above filter-feeding mussels—interplay between water flow and biomixing

Filter-feeding bivalves may have a pronounced grazing impact on the phytoplankton biomass in many shallow marine areas. The blue mussel Mytilus edulis, which lives in dense beds, can filter more than 100 m 3 m -2 d -1 , but the grazing impact is highly influenced by hydrodynamic pro- cesses. Without externally generated currents or turbulent mixing, only a thin layer of near-bottom water would be subject to the down-mixing that causes an important supply of food to the mussels. Here, food-depleted jets of water expelled through the exhalant opening of a mussel may not only prevent the water, once filtered, from re-entering the animal, but the substantial speed of the jet may also help to mix the near-bottom water. The extent of such biological mixing—'biomixing'—caused by a dense population of M. edulis (200 cm long bed, filtering at 147 m 3 m -2 d -1 ) was studied experi- mentally at 2 flow speeds (4 and 8 cm s -1 ) in a laboratory flume channel at natural (low) algal concen- trations in order to determine its relative importance compared to current-generated turbulence. Dis- tributions of algal cells a distance of 162 cm from the start of the bed showed a near-bottom depletion of about 58 and 45%, respectively, for the 2 flow speeds, which indicate the degree of refiltration. In addition, flow structures were quantified in terms of distributions of velocity, turbulent shear stress and turbulent kinetic energy in the benthic boundary layer at 3 levels of mussel filtration-activity (maximal, reduced and zero). A description is given of this filtration activity of M. edulis with and without added suspensions of algal cells, which influence its valve-opening degree and filtration rate. It is concluded that biomixing enhances the flow-induced down-mixing of phytoplankton and can be identified as peaks in profiles of turbulent kinetic energy and turbulent shear stress. The associated increase in friction velocity over the length of the mussel bed at maximal filtration activity amounted to 56 and 49% for the 2 flow speeds studied. This shows the functionality of biomixing to be most helpful at low speed, where it is most needed due to the low levels of flow induced turbulence con- tributing to down-mixing of phytoplankton.

WIND OF AN AIR LAYER CHARACTERIZED BY VEGETATIONS

The purpose of the present paper is to clarify the behaviour of the roughness length zo and the zero-plane displacement d against friction velocity and to explain the vertical wind profile within a canopy using introduced theoretical relationships between physical quantities. Measurements were carried out at the heat and water balances observation field in the Environmental Research Center of the University of Tsukuba, Ibaraki, Japan. The field surface was covered by pasture grass (rye grass, Secale Cereale) with a height of 0.46m. The observations under the near neutral condition show that mean values of zo and d increase with the growth of plant height h. The relations are expressed by z0=0.065 h and d=0.401 h respectively. The value of z0/(h-d)=0.109 is smaller than those found by many other canopies, for instance, 0.44 over wheat field, 0.26 over needle-leaned tree, 0.24 over bean field and 0.12 over corn field (Hayashi and Kotoda, 1980). It is noteworthy that the pasture canopy contributes less effectively to aerodynamical roughness than the other canopies. Tall, flexible vegetations are more deformed with the increase of wind speed. Therefore, z0 and d change in accordance with wind speed at a fully rough surface. Then z0 increases slightly with the increase of the friction velocity u* as follows (Fig. 6): z0=0.044 u*+0.018 On the other hand d diminishes with the increase of friction velocity as follows (Fig. 7): d=-0.361 u*+0.288 This is attributed to the phenomena that the wind penetrates more into the canopy as the speed increases and that the aerodynamical surface, a level of no downward momentum flux, becomes lower. Under such conditions, tall vegetations are exposed more to the wind, and the canopy changes its aerodynamical roughness. For the flat surface, the friction velocity u* is in direct proportion to the wind speed uh. But for the pasture field, the friction velocity settles down at a value of u*=0.370m/s and deviates from the linear relation between u and u* in the range of uh_??_2.0m/s. The main cause for the deviation of the value of u* is considered to be the difference in the shape of the canopy under such windy conditions (Fig. 8). The relation between the momentum diffusivity KM and the friction velocity is expressed essentially by two straight lines as follows (Fig. 9): KM=0.093 u* (u* 0.3m/s) We can also recognize that the drag coefficient is independent of the friction velocity and the mean value of the drag coefficient CD is 3.07×10-2 at z=0.46 m, although the deviation is not small (Fig. 10). For the purpose of verification of the theoretical relationships between the physical quantities within the canopy, a numerical solution of the wind profile is done and compared with the results of the field observation. Under near neutral condition the wind profile over the pasture canopy is represented by a logarithmic law. But in the canopy layer, the following relations are given: τ=ρKMdu/dz KM=βζ0muh(1-ra) where ζ0 is the normalized roughness length, i.e. ζ0=z0/h, /h, τ the shearing stress, p the air density, Cd the drag coefficient of canopy element, a the leaf area density, and α, β and m are nondimensional constants, and r is the newly defined luxuriant length.

潤滑油含浸陽極酸化皮膜の耐摩耗性

Most of studies on abrasion resistance of anodic oxide films have been performed on dry or wet contact surface, and very few reports have been found for the films impregnated with lubricating oils.This paper describes the study on the abrasion resistance of anodic oxide films impregnated with a lubricating oil by using Okoshi's Rapid Wear Testing Machine.The results obtained were as follows:(1) Lubricating Oils of high viscosity and silicone oil remarkably increased the abrasion resistance at a low friction velocity, but the resistance decreased with the increase in friction velocity.(2) The abrasion resistance of the films treated in boiling solution of zinc sulfate was higher than those of films treated with other chemical treatments, and it was also found that the resistance decreased with the increase in number of crazings on the films.(3)The maximum value of resistance was obtained when the films were heat-treated at 200°C; but the value gradually decreased when the treating temperature was higher than 400°C.

On Zero-plane Displacement and Roughness Length in the Wind Velocity Profile Equation over a Corn Canopy

In order to check TAKEDA's theory (equation (2)), observations of wind velocity profiles were made over the sorgo canopy in August 1967. Furthermore, to test the theory another set of observations were made over the corn canopy in Tanashi, Tokyo, in July and August 1968. The author wants to know if the theory can be applied to all wind velocity profiles over different kinds of plant canopies at different stages of growth and development.The wind velocity profiles were measured with five small cup-anemometers. The plant height (h), the leaf area index (L.A.I.) the mean leaf area density (M.L.A.D.=L.A.I./h) and the rate of heading were measured, too.Five results were obtained from these observations as follows:I. An opposite relationship between the zero-plane displacement (d) and the roughness length (z0) was found, in which a decrease in d and an increase in z0 with wind velocity were shown (see Fig. 3).II. A good correspondence was found between d versus u* (friction velocity) and M.L.A.D. with growth and development of the corn canopy (see Fig. 3).III. A good correspondence was found between z0 vs. u* and the heading of corn plants with growth and development of the corn canopy (see Fig. 3).IV. It was found that the values d and z0 moved with the particular characteristic on the rough surface diagram introduced by TAKEDA (see Fig. 4).V. It was found that the variation of H (effective plant height), obtained from equation (2), in which α is taken to be 0.087, vs, u* decreased slightly with the increase of friction velocity on 24.25 July, that H was constant, even if u* increase, on 1-3 August and that the variation of H vs. u* increased slightly with the increase of u*, furthermore the plotting points scattered under the condition of u*>80cm/sec on 5-9 August (see Fig. 5).Finally it was concluded that the theory was applicable even to the corn canopy taking structures of canopy on 24.25 July and on 1-3 August into account, and that, on the contrary, observed results on 5-9 August disagreed with the theory probably due to the oscillatory movements of plants with long periods.

The flow in a turbulent boundary layer after a change in surface roughness

The changes of surface stress in a deep boundary layer passing from a surface of one roughness to another of different roughness are described fairly accurately by theories that assume self-preserving development of the flow modifications. It has been shown that the dynamical conditions for self-preserving flow can be satisfied if the change in friction velocity is small and if log l0/z0 is large (l0 is the depth of the modified flow and z0 is the roughness length of the surface). In this paper it is shown that, if the change of friction velocity is not small, the dynamical conditions can be satisfied to a good approximation over considerable fetches if log l0/z0 is large. The flow modification is then locally self-preserving, that is, the fields of mean velocity and turbulence are in a moving equilibrium but one which changes very slowly with fetch and depends on the ratio of the initial to the current friction velocity. In the limit of a very large increase in friction velocity, the moving equilibrium is essentially that of a boundary layer developing in a non-turbulent free stream. Equations describing the flow development are derived for all changes of friction velocity, and the form of the velocity changes is discussed. For large increases of friction velocity, the depth of the modified layer is substantially less than would be expected from the theories of Elliott and of Panofsky & Townsend.