Boundary-layer resistances of artificial leaves in turbulent air: I. Leaves parallel to the mean flow

Boundary-layer resistance to heat transfer from plates was studied in a wind tunnel which produced turbulence with streamwise intensity in the range 3.5 to 25% and a longitudinal integral scale of the streamwise turbulence component (L _u,x) in the range from 8 to 100 mm. It was found that heat transfer enhancement occurred due to the turbulence, and that at any given intensity, this enhancement was determined by the ratio of L _u,x to the characteristic dimension of the plate.     

Boundary-layer flow over topography: Impacts of the Askervein study

One of the objectives of the Askervein Hill Project was to obtain a comprehensive and accurate dataset for verification of models of flow and turbulence over low hills. In the present paper, a retrospective of the 1982 and 1983 Askervein experiments is presented. The field study is described in brief and is related to similar studies conducted in the early 1980s. Data limitations are discussed and applications of numerical and wind-tunnel models to Askervein are outlined. Problems associated with model simulations are noted and model results are compared with the field measurements.     

The response of a propeller anemometer to turbulent flow with the mean wind vector perpendicular to the axis of rotation

The system transfer function ¦H(v)¦² at frequencyv (units of Hz) for a vertical velocity propeller anemometer in a statistically stationary and horizontally homogeneous turbulent flow is determined from: (1) experimental estimates of propeller velocity spectra; and (2) estimates of Eulerian vertical velocity spectra based on the hypothesis that degradation of the input vertical velocity Fourier components occurs in the inertial subrange. The experimental estimates of ¦H(v)¦² were adequately summarized with the mathematical expression for the system transfer function of a first-order system with parameterT which has units of time and is analogous to the time constant of a horizontal velocity propeller anemometer. Dimensional analysis techniques and the Monin-Obukhov similarity hypothesis were used to construct a model for the system parameterT which yielded the result that _w /D ₁ ( _w /)^1/3, where _w , andD ₁ denote the standard deviation of the input vertical velocity fluctuations, the horizontal mean wind speed, and the diameter of the propeller, respectively. The system parameterT is interpreted in terms of the time required for the propeller velocity statistics to become asymptotically independent of time upon being released from rest in a statistically stationary turbulent flow.Currently on leave of absence from the Indian Institute of Technology, New Delhi, India.     

A random-walk model for dispersion of heavy particles in turbulent air flow

A random-walk model is presented for calculating the dispersion of heavy particles in a turbulent air flow when only air turbulence statistics and the drag characteristics of the particle are known. Algebraic expressions for the modification of air velocity variance ² and Lagrangian autocorrelation tune-scale T _L,due to particle inertia effects, are derived. These expressions introduce only a very small computational overhead on the random-walk models for inertia-less particles of Wilson et al. (1983). Measurements of T _Land by Snyder and Lumley (1971) for four different particles are used to determine constants in the heavy-particle model. It is shown that the agreement between the model, for a single set of constants, and the dispersion measurements is good for the 47 m hollow glass, 87 m glass, and 47 m copper particles. The predictions for the 87 m corn pollen particles show less satisfactory agreement by underestimating dispersion measurements by 15% after 0.4s. Finally, some aspects of the model's application to spray dispersion in and above a crop canopy are considered.     

Boundary-layer turbulent transport and production/destruction of ozone during summertime smog episodes over the Swiss Plateau

Turbulent fluxes of sensible and latent heat and ozone were measured aboard a German Aerospace Research Establishment motorglider over the Swiss Plateau during the first field experiment of the Swiss air pollution experiment POLLUMET. In the lower part of the boundary layer, the ozone fluxes are negative (downward) throughout the day as a result of deposition and photochemical destruction at the surface. In the upper part of the boundary layer, the ozone fluxes tend to be negative until mid-afternoon and then become positive. The change in sign occurs after the ozone concentration in the boundary layer exceeds that in the reservoir above the inversion. Downdrafts bringing air parcels with ozone deficits across the inversion then become major contributors to the flux. The positive fluxes at upper levels result in an increase in flux divergence in mid-afternoon that is balanced by a relatively large source term in the ozone concentration budget.     

Estimation of the monthly and annual mean maximum and mean minimum air temperature values in Greece

In this study, regression equations to estimate the monthly and annual values of the mean maximum and mean minimum air temperatures in Greece are derived. For this purpose, data from 87 meteorological stations distributed all over Greece are used. Geographical parameters, i.e., altitude, latitude, longitude, minimum distance from the sea and an index of terrain morphology, are used as independent variables. These equations explain 79?C97% of the variance of the temperature values and have standard error of estimate between 0.59 and 1.20°C. Data from 37 other meteorological stations are used to validate the accuracy of the equations. Topographic or climatic factors, which could not be introduced into the equations, are responsible for most temperature residuals >0.5°C or <?0.5°C. Moreover, some particular emphasis has been given to the values of the regression coefficient for the altitude, since it is the estimator for the mean lapse rate of air temperature.     

Modelling the mean and turbulent structure of the summertime Arctic cloudy boundary layer

This paper addresses the problem of modelling the summertime Arctic cloudy boundary layer. Specifically we consider the problem of multi-layered clouds in the boundary layer that includes the decoupling of the turbulence between upper and lower clouds. A high-resolution one-dimensional model with second-order turbulence closure and spectral radiative transfer is used to simulate a case study that was obtained during the 1980 Arctic Stratus Experiment. The effects of radiation, large-scale vertical motion and drizzle are investigated in sensitivity studies. Results of this study show that radiative transfer is important to the maintenance of the multiple cloud layers, and suggest that weak rising vertical motion is the most favorable situation to maintain two separate cloud layers.     

Evidence of pressure-forced turbulent flow in a forest

Lagged cross-correlation analyses between streamwise velocity at several heights within and above a forest, and between streamwise velocity and surface pressure, provide evidence that turbulence in the sub-crown region of the forest is to a large extent driven by pressure perturbations. The analyses support earlier results based on examination of coherent structures observed in the same forest. The phase of the streamwise velocity signal exhibits an increasing delay with decreasing height, indicative of a downwind tilted structure, until the upper region of the forest is reached, at which point the effect is reversed. It is suggested that positive pressure perturbations ahead of advancing microfronts induce streamwise accelerations in the trunk space. This link between the pressure pattern and the wind field explains why velocity spectra in the trunk space are depleted in the higher frequencies, relative to levels above.     

Boundary-layer flow over analytical two-dimensional hills: A systematic comparison of different models with wind tunnel data

Two mass consistent models (MATHEW and MINERVE) and two dynamic linearized models (MS3DJH/3R and FLOWSTAR) are used to simulate the mean flow over two-dimensional hills of analytical shape and of varying slope. The results are compared with detailed wind tunnel data (RUSHIL experiment at US EPA). Different numerical experiments have been performed, varying input data and control parameters, to test the data-processing methodology and to evaluate the minimum input data (for mass consistent models only) necessary to obtain a reliable flow field. The models behave differently according to the physical assumptions made and numerical procedure used: an assessment is then made in order to identify the proper solution for the different conditions of topography and wind data.     

RETRACTED ARTICLE: Validation of mean and turbulent parameters measured from the aircraft in the marine atmospheric boundary layer

The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés Océaniques/ Recherche Expérimentale) experiment, which took place between 04 Oct. and 17 Nov. 1993, was conducted over the oceanic Azores current located in the Azores basin. The SST (Sea Surface Temperature) field was characterized in the SEMAPHORE area (31°–38°N; 21°–28°W) by a large meander with a SST gradient of about 1°C per 100 km. In order to study the evolution of the MABL (Marine Atmospheric Boundary Layer) over the ocean, the mean and the turbulent data were evaluated by the measurement with two aircraft and a ship in different meteorological conditions. Three cases of low pressure and three cases of high pressure are mainly presented here. For the six cases, the satellite images (NOAA) did not show any relation between the SST field and the cloud cover. At each flight level, the decrease of the SST with the altitude due to the divergence of the infrared radiation flux from the ocean is 0.25°C per 100 m. For the comparison between the two aircraft, the mean thermodynamic and dynamic parameters show a good agreement except for the temperature. The dispersion of the sensible heat flux is larger than that of the latent heat flux due to the weak sensible heat flux over the ocean both in the intercomparison between two aircraft and in the comparison between the aircraft and the ship.     

The sensitivity of large-eddy simulation of turbulent shear flow to subgrid models

Previous large-eddy simulations of turbulent shear flows have shown significant sensitivity to some aspects of the subgrid-scale turbulence models. In particular, Mason and Thomson (1992) demonstrated a marked improvement in simulations through the inclusion of stochastic stress variations in the subgrid model. Here a series of simulations is presented with a view to exploring the general sensitivity to such a subgrid model and in particular to the size and nature of the stochastic stress variations. The model proposed by Mason and Thomson is found to be fairly satisfactory but some quantitative uncertainty remains.     

A wind-tunnel study of turbulent flow close to regularly arrayed rough surfaces

Recent observations of flux-gradient anomalies in atmospheric flow close to forests, and similar rough surfaces, prompted a wind-tunnel investigation in which cross-wire anemometry was used to study the vertical development and horizontal variability of adiabatic flow over five regularly arrayed rough surfaces, encompassing a 32-fold range of roughness concentration . The roughness elements were cylinders, 6 mm in both height and diameter.Below a layer in which the velocity profile is semi-logarithmic, two surface influences upon the mean velocity field can be distinguished: wake diffusion and horizontal inhomogeneity. The wake diffusion effect causes non-dimensional vertical velocity gradients to be smaller than in the semi-logarithmic region; at least for elements with aspect ratios l/h 1, it is governed by the transverse dimension l of the roughness elements, and is observed when z > h + 1.5l (where z is height above the underlying surface, and h is the height of the roughness elements). A simple diffusivity model successfully describes the horizontally averaged velocity profiles in the region of wake influence, despite conceptual disadvantages. The horizontal inhomogeneity of the flow is negligible when z > h + D (D being the inter-element spacing), and does not entirely mask the wake diffusion effect except over very sparsely roughened surfaces ( 0.02). A criterion for negligibility of both effects, and hence for applicability of conventional turbulent diffusivity theory for momentum, is z > h + 1.5D. These results are compared with atmospheric data, and indicate that wake diffusion may well cause some underestimation of the zero-plane displacement d over typical vegetated surfaces.     

Study of the mean monthly air temperatures during the successive sunspot cycles

In the first part of this paper (I–III) the mean monthly air temperatures during successive sunspot cycles are examined for six stations of Central and Northwestern Europe. Based upon the observed data the variations between two consecutive 11-year periods of solar activity are studied for the following elements:

1. a)
   The differenceT _h?T_c between the mean temperatures of the four warmer (May–August) and the four colder (November–February) months of the year.     
   
   

On the initiation of surface waves by turbulent shear flow

An analytical model is developed for the initial stage of surface wave generation at an air–water interface by a turbulent shear flow in either the air or in the water. The model treats the problem of wave growth departing from a flat interface and is relevant for small waves whose forcing is dominated by turbulent pressure fluctuations. The wave growth is predicted using the linearised and inviscid equations of motion, essentially following Phillips [Phillips, O.M., 1957. On the generation of waves by turbulent wind. J. Fluid Mech. 2, 417–445], but the pressure fluctuations that generate the waves are treated as unsteady and related to the turbulent velocity field using the rapid-distortion treatment of Durbin [Durbin, P.A., 1978. Rapid distortion theory of turbulent flows. PhD thesis, University of Cambridge]. This model, which assumes a constant mean shear rate Γ, can be viewed as the simplest representation of an oceanic or atmospheric boundary layer.For turbulent flows in the air and in the water producing pressure fluctuations of similar magnitude, the waves generated by turbulence in the water are found to be considerably steeper than those generated by turbulence in the air. For resonant waves, this is shown to be due to the shorter decorrelation time of turbulent pressure in the air (estimated as ∝ 1/Γ), because of the higher shear rate existing in the air flow, and due to the smaller length scale of the turbulence in the water. Non-resonant waves generated by turbulence in the water, although being somewhat gentler, are still steeper than resonant waves generated by turbulence in the air. Hence, it is suggested that turbulence in the water may have a more important role than previously thought in the initiation of the surface waves that are subsequently amplified by feedback instability mechanisms.     

Model of air flow and air pollution concentration in urban canyons

A model for the calculation of the turbulence flow field and air pollutant concentrations in urban canyons is developed. A two-dimensional set of hydrodynamical equations and a threedimensional diffusion equation are solved numerically with a personal computer. Different boundary conditions were investigated. Three flow regimes were found: without vortex, with one vortex, and with two vortexes, within an urban canyon. The influence of building density and wind speed components along the street was also investigated.     

Power spectrum analysis of the time-series of annual mean surface air temperatures

Maximum Entropy Spectral Analysis of the annual mean surface temperature series for land masses and sea in the northern and southern hemispheres indicated long-term linear warming trends of (0.12 to 0.56) °C/century with superposed significant periods in the ranges T = 5–6 yr, 10–11 yr, 15 yr, 20 yr, 28–32 yr, and 55–80 yr. Extrapolation in future indicated for 2000–2030 a departure of (+0.4 °C) above the 1950–70 level. However, for the 1980s, the observed values are above the expected level, probably indicating large greenhouse effects due to human intervention. In that case, our predictions would be underestimates.     

Investigation of small-scale droplet concentration inhomogeneities in a turbulent flow

Summary This paper is focused on the numerical investigation of spatial characteristics of droplet concentration fluctuations, originating in a turbulent flow. The turbulent flow is simulated using a model of a statistically stationary, homogeneous and isotropic turbulent flow. We found that preferential droplet concentration with the maximum of spatial spectrum at about 1.5 cm is formed regardless of the droplet size (up to the droplet radius of 20 μm). The amplitude of fluctuations depends on the droplet radius and constitutes a significant part of mean concentration. Analogous simulations with non-inertial droplets (passive scalar) do not reveal such concentration fluctuations. Our main conclusion is, therefore, the following: preferential droplet concentration at the cm-scale forms in a turbulent flow due to droplets inertia.     

Evolution of large scale disturbances and their interaction with mean flow in a rotating barotropic atmosphere—Part II

In part I of this paper, we have discussed two problems: the general properties of two-dimensional baro-tropic motion and the evolution and structure of both Rossby wave packet and inertio-gravity wave packet. In this part, we shall continue our discussion. Third, normal modes and continuous spectra of both quasi-geostrophic and non-geostrophic models, their different behaviour, and the comparison of normal mode approach to the wave packet approach. Fourth, weakly nonlinear theory of interaction based on the analysis of eddy transports. A nonzonal basic flow as well as non-geostroptaic model is also included in the consideration. The last, the fully nonlinear theory, making emphasis on the conditions for the maintenance of nonzonal disturbances and the conditions for their continuous and complete absorption by the zonal flow. A comparison of Rossby wave absorption to energy cascade in the two-dimensional turbulence is also given.