Modelling waving crops in a wind tunnel

Analysis of movie films of a field of barley, combined with observations of the motions of individual plants, show that single stalks oscillate at a well-defined natural frequency even when stimulated by turbulent winds. Treating single stalks as resonant cantilevers allows the use of standard engineering methods to determine their elastic properties. Armed with these values, the application of similarity analysis to the equation of motion of a single stalk leads to criteria for aeroelastic modelling of wheat plants in the wind tunnel. A representative value for the spacing of stalks in a small section of model wheat field was calculated by referring to published data on momentum absorption in a variety of real and model canopies. Preliminary measurements of first and second moments of velocity in the model appear to confirm the importance of including elastic properties in wind-tunnel simulations of airflow in flexible crops.     

A study of internal waves in a wind tunnel

The combined use of a stratified flow wind tunnel and of periodic sampling methods in low Reynolds number flows allows the recovery of the instantaneous dynamics of internal waves. Several detailed examples are given of the thermal structure of large propagating and breaking internal waves and Kelvin-Hemholtz waves. Preliminary measurements of the stability of finite waves as a function of Richardson number are also reported.     

Characteristics of natural wind simulations in the TUM boundary layer wind tunnel

The boundary layer wind tunnel at the Technische Universit?t München was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universit?t München can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics.     

Disturbance of wind profile measurements by a slim mast

Wind-profile measurements for micrometeorological purposes are almost inevitably influenced by the supporting structures. Many investigators have studied the flow around obstacles in the wind tunnel but few comparisons with natural turbulent flows are available. The wind-speed error at reasonable distances from the obstacle is of the order of 2% only. Nevertheless, the effects on profiles may be large if the characteristics of flow around the obstacle vary with height, e.g., due to a variation of the geometry of supports, etc.Observations of flow around a cylindrical mast with horizontal supports are reported for different angles of attack. Modelling of the disturbed flow by a potential flow around a solid half-infinite body yields good agreement with observations. However, the amplitude of the disturbance exhibits no obvious relation with the geometry of the structure. This points to a Reynolds-number dependency of the flow distortion.Generally a nearly symmetrical mounting of anemometers on both sides of the mast yields only small errors in the fitted profiles. In contrast, a large error can result with an asymmetrical configuration.This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich Meeresforschung Hamburg.     

A wind tunnel study of turbulent flow over a two-dimensional ridge

We present a wind-tunnel simulation of adiabatic atmospheric flow normal to a rough, two-dimensional ridge. The data are analyzed in physical streamline coordinates, which are described in some detail. The mean velocity speed-up on the hill top is adequately predicted by existing formulae while the behaviour of the wake flow fits into a pattern that emerges from other wind-tunnel experiments. The turbulent stresses evolve in response to the extra strain rates induced by the hill, streamline curvature and acceleration: % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG1bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3456!\[u^{\bar 2}\]is coupled strongly to acceleration while % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaadaqdaaqaaiaadw% hacaWG3baaaaaa!3462!\[\overline {uw}\]and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG3bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3458!\[w^{\bar 2}\]follow curvature. These differing responses lead to significant phase differences between the changes in the component stresses as the hill is traversed. An analogous response is seen in the components of turbulent stress divergence, which are computed as part of streamwise momentum budgets. Only very close to the surface is turbulent stress divergence comparable to the inertial and pressure terms in the momentum budget; over most of the flow regime, the mean flow response is approximately inviscid. Finally, we compare our results with data from other wind tunnel models and from real hills.     

Estimation of liquid water content effects on the air temperature measurements in the clouds based on the wind tunnel experiments

Presented are the results of experiments carried out in the wind tunnel and aimed at the investigation of characteristics of high-frequency aircraft thermometer constructed for studying the atmosphere from the board of the aircraft-laboratory. It is demonstrated that the values of recovery coefficient of the thermometer obtained both in the wind tunnel and after numerical experiments practically coincided with the values that have been measured before, during the flight testing. The flow with the specified values of liquid droplet content that was created in the wind tunnel, enabled to determine both the degree of effects made by the cloud on the measured temperature and its fluctuations and the value of the coefficient for introducing the correction to the temperature and its fluctuations in the cloud. It is shown that the taking account of effects made by liquid water content on the measured temperature is absolutely necessary for the correct interpretation of aircraft-based results of studying the thermodynamic structure of clouds.     

Autoregressive Conditional Heteroscedasticity in daily wind speed measurements

Summary It is argued that the predictability of meteorological variables is not constant but shows regular variations. This is shown for the daily mean wind speeds and its meridional and zonal components at Shearwater, Canada, for the period 1963–1988. To capture this feature, a Generalised Auto Regressive Conditional Heteroscedastic model is proposed. In this model, the conditional variance of an observation depends linearly on the conditional variances of the previous observations and on the previous prediction errors. Here, conditional heteroscedasticity models are used which let the variance depend on previous prediction errors, in conjunction with an autoregressive model for the mean, using the Gamma distribution for the wind speed and the Normal distribution for its components. It is shown that these heteroscedastic models outperform their homoscedastic versions, and that heteroscedastic features are more clear in the wind speed component records.With 6 Figures     

Wind tunnel wake measurements of heavier-than-air gas dispersion near a two-dimensional obstacle

The dispersion of a uniform two-dimensional flow of carbon dioxide gas in air over a square two-dimensional obstacle was studied experimentally in an atmospheric boundary-layer wind tunnel. The obstacle Reynolds number was about 6000 based upon the undisturbed velocity at the height of the obstacle, and the Froude number was about 1.75. Carbon dioxide was injected vertically upward from the surface at a rate of 10% of the freestream velocity. The injection surface area began one obstacle length upstream and extended upstream one and one-half obstacle lengths. A gas concentration measurement system for carbon dioxide and air mixtures was developed for use in an atmospheric wind tunnel. Centerline steady-state concentration profiles were measured for various locations downstream of the leading edge of the obstacle. The maximum concentrations of carbon dioxide occurred atop the obstacle in the thin recirculation zone above the obstacle. Immediately behind the obstacle within the larger downstream recirculation zone, concentration levels significantly decreased. Further downstream, the concentration levels continued to diminish.Experimental results were compared with the numerical solutions of Sutton et al. (1986). The experimental measurements of concentration were lower than the predicted results of the numerical solutions, which may be attributed to the higher value of the Froude number present in the experimental measurements.Associate Professor.     

A wind tunnel study of mean and fluctuating concentrations in a plume dispersing over a two-dimensional hill

This paper describes an experimental investigation of the behaviour of the statistics of concentration fluctuations in a passive plume dispersing over a two-dimensional hill of moderate steepness. Recently developed high frequency response Flame Ionization Detector (FID) technology with a frequency response in excess of 200 Hz was utilized to obtain an extensive set of measurements of the mean and fluctuating plume concentrations. Plumes dispersing over flat terrain and over a hill with a maximum slope of 0.3 were studied. For both cases, extensive turbulent flow measurements were also carried out.The measured mean plume concentration profiles were of a generally Gaussian form and showed the expected effects of surface reflection for the flat terrain and hill. Plume intermittency and concentration fluctuation intensity were calculated at all measurement locations. Conditional and unconditional plume concentration statistics were calculated. The conditional (in-plume) concentrations and intensities were more uniform with height than for the unconditional ones.     

Accuracy of soil heat flux plate measurements in coarse substrates – Field measurements versus a laboratory test

Summary The in-situ performance of heat flux plates within coarse porous substrates might be limited due to poor contact between plate and substrate. We tested this behaviour with a simple laboratory set-up. Two test substrates were placed above a reference material of known thermal conductivity between a warm and a cold plate to establish a vertical heat flux. The temperature gradients and the response of a soil heat flux plate were measured. By means of the Fourier law of heat conduction the thermal conductivity of each test substrate was calculated, thus incorporating all heat transfer within the volume and representing the “effective” conductivity. The laboratory method had an accuracy of up to ±7% (±13% for a smaller set-up). In comparison, heat flux plate-derived heat fluxes showed errors of up to 26%. Use of heat flux plates in coarse substrates is not recommended without additional measurements.     

Analysis of wind tunnel dispersion data over two-dimensional obstacles

An analysis of wind tunnel data of dispersion from elevated point sources over a flat floor and gently sloping, 2-D hills is performed. (The data were obtained by Khurshudyan et al., 1981.) Formulas for the mixing lengths and the Lagrangian time scales are tested, suitable for use in various dispersion models. Some expressions for the vertical first moments of the plume concentration distribution suitable for shear flow (Hunt, 1985) are also tested.Then, a normalization is suggested, based on the source mean flow and turbulence parameters, for the ground-level concentration maximum value and position along the plume centerline. Using this normalization, the maximum position is almost constant, regardless of source height variation and of whether the hill is present or not, at least for source positions upstream or over the top.The maximum values allow the determination of normalized terrain amplification factors TAF_N, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization.     

Eddy-correlation measurements of sensible heat fluxes over a grass surface

Direct measurements of sensible heat fluxes were conducted over a grass surface at Ladner, British Columbia, using yaw-sphere-thermometer eddy-correlation systems. The results show that for half-hour averaging periods, there is no phase-lag between sensible heat and net radiation flux densities. Field comparison of two yaw-sphere-thermometer systems gave good and consistent agreement. At a height of 2 m above ground and a horizontal crosswind separation of 1.5 m, less than 5% variability was noted in the measured heat fluxes. For a 19-m horizontal separation, the variability was less than 20%. The aridity index (α) advanced by Priestley and Taylor (1972), is shown to be a potentially useful climatic indicator.     

华南暴雨试验期间香港风廓线雷达资料的评估

联合应用5部风廓线雷达、多普勒天气雷达组网观测资料,并结合海河流域自动气象站资料详细分析了2012年7月21日海河流域强降水发生、发展过程中温度场、风场演变特征。结果表明：这次强降水天气具有明显的中尺度天气特征,强降水主要发生在露点温度梯度区内。低空急流指数变化对短时强降水的峰值有一定的指示作用,它的快速增加意味着强降水将要开始。垂直风切变对短时强降水的形成具有参考价值：降水开始前,2 500-3 000 m的风速和风切变首先开始增大,之后大风向1 000 m以下扩散,风切变梯度增强预示着短时强降水开始;1 000-1 500 m风速迅速减小,高层也随之减小,强降水趋于减弱或结束。风廓线雷达中风向风速的变化能够指示系统的过境时间, 以上结论对预报强降水的起始及降水的持续时间具有使用价值。

     

Sensible heat flux measurements with a yaw sphere and thermometer

A yaw sphere has been designed that measures a quantity that is proportional to the vertical heat flux. The method is described, and some comparisons are given with measurements made with a three-dimensional pressure-sphere anemometer, in combination with a fast-response thermometer.     

A wind tunnel study of turbulent flow over model hills

Detailed wind tunnel measurements have been made of mean flow and turbulence over a two-dimensional ridge and a circular hill, both having cosine-squared cross-section and maximum slope about 15 °. The measurements were made in an artificially thickened neutrally stratified boundary layer, and have been compared with results from linear models and rapid distortion theory as appropriate.Our study shows that linear theory gives generally good predictions of the mean flow on the upwind side of the hills, and especially of the flow speedup at the hill top, but that the turbulence is less well predicted. In particular, the measurements show a major increase in the vertical component of turbulence and in the shear stress on the upwind slope of both the two- and three-dimensional hills which is not predicted by either equilibrium or isotropic rapid-distortion theories, although this may be partly due to the effect of streamline curvature. Rapid-distortion theory is successful only in describing the streamwise component of turbulence in the outer region of the flow, while in the upper part of the inner region of the flow, the turbulence measurements show disagreement with both the equilibrium and the rapid-distortion theories. Our experiments also confirm that the equilibrium region is a very thin layer close to the surface, while above this region and below the outer region, there is a transitional region where all terms in the stress equation are important.The measurements over the three-dimensional hill suggest that the mean flow and turbulence are broadly similar to those over the two-dimensional ridge, but with reduced perturbation amplitudes. The major differences between the two cases are found on the upwind slope and in the wake where, respectively, horizontal divergence and convergence of the three-dimensional flow are most pronounced.     

Scale effects in wind tunnel modeling of an urban atmospheric boundary layer

Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations.     

Detection of a katabatic wind event with GPS meteorology measurements at Scott Base Antarctica

A katabatic wind event which dramatically affects the polar climate has been detected using GPS meteorology measurements. GPS-derived precipitable water vapor (GPS PWV) variability and its relation to a katabatic event at Scott Base station, Antarctica was investigated. The investigations using the data gathered from 21 to 30 November of 2002. They showed that the water vapor profile exhibited an irregular pattern with a maximum PWV of 7.38?mm (~6?mm on average). This event was strongly influenced by relative humidity than by wind speed activity. The dominant wind flow during this period was from the North to Northeast (blowing from the Ross Sea) with a mean speed of 3.79?ms^?1. The PWV increased when the temperature was between ?15 and ?11°C. During the katabatic event identified between 21:30 UT of 28 November and 18:40 UT on 29 November, the wind blew from the Southeast to South direction (from the Ross Ice Shelf) with a maximum speed of 10.92?ms^?1. The PWV increased ~1.0?mm (23%) from the mean value accompanied by severe wind had pronounced effect on GPS observations.