Surface Layer Turbulence Research Articles

強風時における接地気層の乱流の空間構造に関する性質について

強風時における接地気層の乱流の空間構造に関する性質について

Experimental Study of Turbulent Stratified Shearing Flow

Two-dimensional turbulent stratified shearing flow was simulated in the laboratory by letting fresh water flow over the surface of an otherwise stationary body of saline water. The behavior of the turbulent surface layer was observed to depend on upstream condition as well as downstream condition in a manner similar to free surface flow in open channel. Velocity and concentration were measured by hot-film anemometer and conductivity probe in a region upstream of a density jump where flow is jet-like and is supercritical. The velocity scale for turbulent intensity, relative to the mean velocity scale, was found to diminish to zero at an overall Richardson number approximately equal to 0.15. Turbulent entrainment velocity, obtained from numerical integration of the mean velocity profiles, was found to be approximately proportional to the velocity scale of longitudinal turbulent intensity.

The Vertical Transports of Kinetic Energy by Turbulence and Pressure in the Boundary Layer

The transport of energy by turbulence and pressure is examined for a turbulent surface layer over a dry prairie grassland. Both terms are found to be significant in the budget of turbulent kinetic energy. The values of the divergences are deduced by considering the variations of the non-dimensionalized terms with respect to z/L. It was found that the divergence of the turbulent kinetic energy flux was about equal to minus the buoyancy production. The main contribution was due to the divergence of the vertical transfer of the vertical component of kinetic energy. The divergence of pressure transfer was found to be of opposite sign and of about equal magnitude to the divergence of turbulent energy flux. The dissipation of turbulent kinetic energy about equalled the local production. The spectral components of the energy budget are briefly examined. It was found that the non-dimensional frequency range with no appreciable production nor dissipation was only about a decade wide whereas the spectral flux was constant to within 10% over about a two-decade band.

Statistics of Surface Layer Turbulence over the Tropical Ocean

Atmospheric surface layer turbulent statistics measured during the Barbados Oceanographic and Meteorological Experiment 8 and 30 m above mean sea level are presented. The budget equations of turbulent kinetic energy, humility variance and temperature variance are examined. Within discussed limitations it is concluded that production equals dissipation in the case of turbulent kinetic energy and humidity variance. The analysis of the temperature variance budget revealed large differences between productions and dissipations computed assuming standard similarity functions derived from other data sets. Initial computation of fluxes revealed large systematic decreases with height in the shear stress and heat flux. Comparison with other results suggested corrections which would eliminate these differences. Comparison between profile fluxes and direct measurements suggests strong similarity of momentum and water vapor transfer.

Spectral Characteristics of Surface Layer Turbulence Over the Tropical Ocean

Spectra of wind, temperature and humidity fluctuations measured during BOMEX in the atmospheric surface layer are discussed. Strong similarities exist between dimensionless humidity and horizontal wind velocity spectra and between dimensionless cospectra of the water vapor flux and the momentum flux. In contrast, the peaks of the temperature spectra and the sensible heat flux cospectra occur at frequencies an order of magnitude greater than those of the other two variables. These differences are related to the mean vertical planetary boundary layer profiles of the three variables as well as to the effects of heating-cooling by divergence of radiation in the PBL.

On the analysis of wind wave-induced disturbances in the atmospheric turbulent surface layer

Simultaneous measurements of wind velocity, air humidity and sea surface wave-elevation fluctuations obtained on a platform in the open Caspian Sea are analyzed. It is shown that wave noises exist in the spectra and cospectra of the lower part of the atmospheric boundary layer, not only for the frequency of the main energy transporting component of the sea waves, but also in other parts of the spectra, at both lower and higher frequencies. The high frequencies are noncoherent with the sea waves and could be considered as measurement ‘errors’ due to the existence of the waves. A method of elimination of the coherent wave noises from the spectra and cospectra is suggested and the effectiveness of its application is demonstrated. The essential difference between cases of developing and decaying wind waves is demonstrated.

Cospectral similarity in the atmospheric surface layer

AbstractThere are two asymptotic cases where the four parameters which determine surface layer turbulence structure reduce to three. In the unstable limit, which we call local free convection, the surface stress is no longer important. In the stable limit z no longer enters; we propose the name ‘local z‐less stratification’ for this state. In both cases explicit predictions result for various properties of the cospectra of stress and heat flux. Although the predictions are based on asymptotic arguments and therefore might be expected to hold only at large ∣z/L∣ values, data from the 1968 Kansas experiments show that most predictions are valid fairly close to neutral.

Spectral characteristics of surface-layer turbulence

Spectral characteristics of surface-layer turbulence

Efficiency of heat shielding of a plane wall during blowing of air through a slot at an angle to the shielded surface

Results are presented of an experimental study of the effect of the angle of secondary air injection on the efficiency of heat shielding of a plane surface at the turbulent surface layer.

Concerning the similarity theory of A. S. Monin and A. M. Obukhov for the turbulent structure of the thermally stratified surface layer of the atmosphere

AbstractThis paper considers the theoretical basis of the Monin‐Obukhov similarity theory for the turbulent, thermally stratified, surface layer of the atmosphere. It is shown that the classical form of the similarity theory cannot be applied legitimately to the variances of the horizontal components of the wind velocity fluctuation. This limitation of the theory has recently been suspected on the basis of observational data, although its theoretical demonstration emphasizes the need for caution when attempting to apply similarity theory to other characteristics of the surface layer turbulence that are currently of interest.

Diffusion in the diabatic surface layer

Diffusion from point and line sources in an adiabatic surface layer were briefly discussed using a similarity method by Ellison, following a suggestion made at the 1958 Oxford Symposium by Batchelor. The chief result is that the ground concentration at a downwind distance x, from a point source at ground level, is found to be proportional to xm, where m lies in the range −1.85 to −1.9. Assuming that Lagrangian similarity of the surface layer turbulence obtains also in the diabatic case, the power index, m, of the downwind ground level concentration is shown, in the present work, to vary from about −1.35 in very stable conditions to −3.0 in superadiabatic conditions. This result is in good agreement with recent detailed concentration measurements.

The diurnal variation of wind over tropical Africa

AbstractIn the Central Sudan wind speed decreases from morning to midday. Upper wind observations made twice daily at Khartoum during 1935 and 1936 provided data for the investigation of this unusual type of variation. It appears from mean monthly vector winds for morning and midday that the relationship of surface winds to geostrophic winds at a height above the surface layer of turbulence is similar to that in other latitudes and that a decrease in geostrophic wind occurs at midday. A corresponding decrease in pressure gradient can be shown only on the synoptic charts based on mean monthly sea level pressures for March, gradients for other months being indefinite and data scanty. Horizontal temperature gradients are worked out from the observed upper wind distribution, which agree with the temperature gradients obtained from surface temperatures and assumed lapse rates. These indicate a decrease from morning to midday, confirming a similar variation in pressure gradient. It is shown that this diurnal variation is typical of a wide belt of tropical Africa and, based on upper air temperature observations at Khartoum, the suggestion is made that it is due to the midday rise in temperature within the region of the thermal equator being less than that in regions north and south of it, at the height of the geostrophic wind level.