Wind Velocity Profiles Research Articles

Simultaneous PIV and PTV measurements of wind and sand particle velocities

Wind-blown sand is a typical example of two-phase particle-laden flows. Owing to lack of simultaneous measured data of the wind and wind-blown sand, interactions between them have not yet been fully understood. In this study, natural sand of 100–125 μm taken from Taklimakan Desert was tested at the freestream wind speed of 8.3 m/s in an atmospheric boundary layer wind tunnel. The captured flow images containing both saltating sand and small wind tracer particles, were separated by using a digital phase mask technique. The 2-D PIV (particle imaging velocimetry) and PTV (particle tracking velocimetry) techniques were employed to extract simultaneously the wind velocity field and the velocity field of dispersed sand particles, respectively. Comparison of the mean streamwise wind velocity profile and the turbulence statistics with and without sand transportation reveal a significant influence of sand movement on the wind field, especially in the dense saltating sand layer (y/δ < 0.1). The ensemble-averaged streamwise velocity profile of sand particles was also evaluated to investigate the velocity lag between the sand and the wind. This study would be helpful in improving the understanding of interactions between the wind and the wind-blown sand.

Modelling wire-to-wire corona discharge action on aerodynamics and comparison with experiment

The purpose of this paper is to propose a simple model for plasma generation and effect on fluids at atmospheric pressure. Experiments are conducted using a wire-to-wire corona discharge actuator in a subsonic boundary layer flow. Velocity gains of several metres per second are observed. A quasi-2D numerical model of the discharge is proposed and explains the creation of two corona discharges around the electrodes. A one-way approach of the plasma aerodynamics coupling gives access to the ionic wind. It is confirmed that the actuator accelerates the flow from the anode to the cathode. Order of magnitudes of the ionic wind and flow velocity profiles are close to experiments. A first attempt to perform a 2D simulation of the wire-to-wire discharge is presented as the starting point of future works.

PREDICTION OF AIRFLOW RATE OF SINGLE-SIDED ROOM IN CROSS WIND

Openable windows of multiple dwelling houses or office buildings are often put on the single-sided wall because of restriction due to architectural planning. Conventional prediction method of airflow rate based on the differential pressure coefficient works on the assumption that openable windows exist on the both, windward and leeward sides of the room. This method is not useful for the room with single-sided opening. The prediction method of air flow rate of single-sided room beneficial to design process is not sufficiently prepared.In this study, the characteristics of wind induced ventilation with single-sided opening are analyzed with wind tunnel experiment, in which wind velocity profile of the approach flow is created as well as wind velocity profile around a building in build-up area. The normalized airflow rate 'k' is obtained from the wind tunnel experiment for simple prediction of airflow rate of single-sided room. The simple prediction method is mainly based on the normalized airflow rate 'k' and assumed wind velocity around a building, which is decided by CFD simulation or measurement.

Simulation of Snow Drift and the Effects of Snow Particles on Wind

Coupled equations between wind and saltating particles are presented for a stable wind blowing over an infinite plane bed and the equations are solved for a simplified particle‐bed impact process. The calculated results show that the saltating snow particles strongly affect the velocity distribution of the wind, causing a deviation from a logarithmically distributed wind velocity profile. The average height and length of saltating snow particle trajectories exponentially increase as the friction velocity increases; the ejected snow number flux and the streamwise snow transport rate also increase as the friction velocity increases.

A comparative study of structure of vertical motions in the lower troposphere over Pune, a tropical Indian station in March 2004 and 2005 using Wind Profiler data

Abstract. An extended heat wave condition affected Pune (18.31° N, 73.58° E), India and surrounding region during the pre-monsoon month of March 2004, when surface temperatures were observed to be above normal. In contrast, March 2005 showed a long spell of below normal temperatures. The vertical velocity measurements from UHF (404 MHz) Wind Profiler at Pune have been used to understand the role of vertical motions in the lower troposphere in maintenance of long spells of above (in March 2004) and below (in March 2005) normal surface temperatures over the station. The altitude profiles of vertical wind velocities showed different behavior in the two contrasting years 2004 and 2005. It is observed that for a major part of the month and at all heights, downward motions (subsidence) dominate during March 2004. Strong downward motions persisted for spells of 2 to 3 days when high surface temperatures (above 38°C) were recorded over this station. A positive relation between surface temperature anomalies and depth of subsidence points to the role of subsidence. March 2005 showed the persistence of upward motion for longer time duration during morning hours which prevented the surface temperatures from reaching high values with weak subsidence during afternoon hours. This is supported by observation of more frequent and organized thermal plumes extending right into the free troposphere. There are distinct episodes of advective warmings during March 2004 while March 2005 showed weak or absence of temperature advection from north. Hence a combination of meridional transport of heat and persistent subsidence in the lower troposphere seemed to have led to recording of above normal surface temperatures during March 2004 over the station.

Refraction corrections to source localization using an acoustic array suspended below an aerostat

Acoustic sensor arrays suspended below tethered aerostats have several advantages in detection and localization of sources on the ground in comparison with ground-based sensors. Aerostats can be elevated up to 1-2 Km above the ground where the temperature and wind velocity can significantly differ from their values close to the ground. Therefore, due to refraction, a ray trajectory that connects a source on the ground and an aerostat can bend significantly resulting in different directions of sound propagation along this trajectory. However, so far, refraction has been ignored in source localization using aerostats. In this paper, a theory is developed that allows one to find the source coordinates more accurately by taking into account refraction of sound signals emitted by the source. The source coordinates are expressed in terms of direction of sound signal propagation near the aerostat, the coordinates, and orientation of the aerostat, and the vertical profiles of temperature and wind velocity. Other meteorological parameters may also be incorporated, if available, such as time-of-day, geographical features, and historical weather trends.

Simulation of the effect of windbreaks on odour dispersion

Computational fluid dynamic (CFD) models are capable of defining best management practices associated with the wide use of natural windbreaks to help disperse livestock odours. The Fluent shear stress transport (SST) k – ω model was used to simulate odour dispersion downwind from natural windbreaks and to test the effect of tree characteristics (tree porosity, type and height, and windbreak distance from the odour source). The airflow inertial resistance of windbreaks was defined as proportional to the square of the tree diameter. The SST k – ω model was initially calibrated for air velocity recovery rate on the downwind side of the windbreak, using a two-dimensional simulation. Following calibration with field odour measurements, the model was used to compare the size and length of odour plumes developing downwind from windbreaks with different tree characteristics and odour source position. With the air velocity recovery rate calibration, the SST k – ω model predicted the wind velocity profile with an acceptable error of 4.5%. The calibration of the SST k – ω model for odour dispersion required the use of a function transforming the simulated odour mass concentration into simulated absolute hedonic tone. Once calibrated for odour dispersion, the SST k – ω model was able to accurately predict odour concentration downwind from the windbreak. By comparison, the SST k – ω simulations indicated that a dense (aerodynamic porosity of 0.2 compared to 0.66) and tall (9.2 m compared to 4.6 m) natural windbreak produced a shorter odour plume, but the odour concentration immediately downwind from the windbreak was much higher. For the same aerodynamic porosity, tree type or structure had limited effect on odour plume size. As compared to a source located further away (60 m), an odour source located close (15 m) to the windbreak produced a shorter plume because a closer source resulted in less dilution upwind from the windbreak and a greater mass of odourous air trapped downwind from the windbreak. Thus, a tall and dense natural windbreak, located no more than 15 m downwind from the odour source, minimises the length of the odour plume, but increases the intensity of the odour level immediately downwind from its position.

Wind velocity profiles with a blowing sand boundary layer: Theoretical simulation and experimental validation

Describing wind velocity profiles modified by the movement of blown sand is of continuing significance because they are thought to bear rich information about wind‐sand interaction in a blowing sand cloud. This paper treats the saltating cloud as a fluidized particle flow and develops a mathematical model of the wind‐sand interactions. The force exerted on the air by saltating particles is characterized by introducing a drag coefficient of the fluidized aeolian particle flow that is modified after the drag coefficients proposed for liquid fluidized particle flow. The resulting model was then used to simulate wind velocity profiles in the presence of a saltation cloud on the basis of the mean particle velocity and particle concentration profiles of the fluidized particle flow obtained from wind tunnel tests. The simulated velocity profiles agree well with those directly measured in the wind tunnel. The blowing sand boundary layer is divided into a saltation subboundary layer, in which saltating particles participate in momentum exchange with the wind, and an outer subboundary layer above the saltation subboundary layer. The wind velocity profiles within the saltation subboundary layer are characterized by convex‐upward curves in a log (height)‐linear (wind velocity) plot rather than the straight lines in the velocity profiles of clean wind. The upward convexity of the velocity profiles increases as wind velocity increases, suggesting that more wind momentum is absorbed by the saltating particles as the sand transport rate increases. The wind velocity profiles in the outer subboundary layer approach the Prandtl logarithmic law with the saltation subboundary layer providing increased aerodynamic roughness. Our results suggest that the wind shear velocity in the presence of a saltation cloud is a physical dimension that can reflect the effect of saltation.

Determination of equivalent sound speed profiles for ray tracing in near-ground sound propagation

The determination of appropriate sound speed profiles in the modeling of near-ground propagation using a ray tracing method is investigated using a ray tracing model which is capable of performing axisymmetric calculations of the sound field around an isolated source. Eigenrays are traced using an iterative procedure which integrates the trajectory equations for each ray launched from the source at a specific direction. The calculation of sound energy losses is made by introducing appropriate coefficients to the equations representing the effect of ground and atmospheric absorption and the interaction with the atmospheric turbulence. The model is validated against analytical and numerical predictions of other methodologies for simple cases, as well as against measurements for nonrefractive atmospheric environments. A systematic investigation for near-ground propagation in downward and upward refractive atmosphere is made using experimental data. Guidelines for the suitable simulation of the wind velocity profile are derived by correlating predictions with measurements.

Wind velocity profile reconstruction from intensity fluctuations of a plane wave propagating in a turbulent atmosphere

Reconstruction of a wind profile based on the statistics of plane-wave intensity fluctuations in a turbulent atmosphere is considered. The algorithm for wind profile retrieval from the spatiotemporal spectrum of plane-wave weak intensity fluctuations is described, and the results of end-to-end computer experiments on wind profiling based on the developed algorithm are presented. It is shown that the reconstructing algorithm allows retrieval of a wind profile from turbulent plane-wave intensity fluctuations with acceptable accuracy.

Similarity theory and calculation of turbulent fluxes at the surface for the stably stratified atmospheric boundary layer

In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved surface flux calculation technique for use in operational models.

Strong wind observations in the atmospheric surface layer

The content of this article is a contribution to the limited amount of available strong-wind multi-level tower observations in the atmospheric surface layer, and is primarily intended for those engineers and scientists engaged in the field of wind engineering. The observations were used to evaluate the correctness of the predictions obtained from theoretical and empirical models, the latter used frequently by the wind engineering community. The comparisons included profiles of mean wind, turbulence intensity, and gust velocities. To test the mean-velocity models for the prediction of wind speeds at locations where no recording stations were present, observations at a reference location were used to predict and to compare with the simultaneous observations at a number of locations where wind speed observations were available. The analysis of the data revealed that under strong wind conditions thermal stability effects should not be ignored. For obstacle-free open terrain significant variations of the aerodynamic roughness length are observed. The height of the surface layer that increases with roughness and wind speed is at least 150 m. Davenport's “gradient” height, not a function of wind speed, is approximately twice the height of the surface layer that applies to the strong wind data analyzed. Estimation of wind speed at locations where normally no observations are available may exceed the actual speed by as much as 50%.

Atmospheric Phase Error in Coherent Laser Radar

Coherent laser radar (ladar) deduces the shape, velocity and vibration of a remote target by measuring the round-trip optical phase or the Doppler shift of target-scattered light. Phase fluctuations from wind-driven atmospheric turbulence generate micro-Doppler or velocity error at mum/sec levels. The turbulence-induced noise spectra of all geometric phase components of the ladar, the average phase power spectral densities of arbitrary ladar receiver modes, and the resultant Doppler noise spectra, are derived in the Rytov approximation for any isotropic stationary turbulence, any wind velocity profile, any Fresnel number and any source illumination pattern. In Kolmogorov turbulence the piston phase noise spectrum, and the average phase shift over a time interval, are well-approximated by simple universal asymptotic forms dependent only on atmospheric coherence length, turbulence-weighted average wind speed Vmacr, and aperture size D. The spectrum slope changes at a critical frequency of order Vmacr/D. The approximate results do not depend on the usually-unknown outer scale of turbulence

Experience in measuring the wind-velocity profile in an urban environment with a Doppler sodar

The experience of long-term acoustic remote measurements of vertical wind-velocity profiles at two sites in Moscow is reported. Equipment performances and measurement conditions are described. Acoustic measurement features characteristic of a large city with high traffic noise and spurious reflections from buildings are discussed. Criteria and techniques of rejecting noisy and false signals are described as well as the methods of statistical data processing suitable in the case of a signal-to-noise ratio rapidly varying in time and a significant number of rejected signals. Preliminary results of measurements are given.

Wind-induced fatigue of structures under neutral and non-neutral atmospheric conditions

The authors of this paper have carried out a wide research project aimed at determining the wind-induced fatigue of structures, hypothesising the wind field as neutrally stratified, as it is typical of evaluations addressed to ultimate loading conditions. As wind-induced fatigue is sensitive to moderate wind velocities, for which stable or unstable atmospheric conditions can occur, this paper extends the procedures previously established to non-neutral thermal stratifications. The mean wind velocity profile and the cross-power spectral density functions of the atmospheric turbulence are expressed in terms of the Monin–Obukhov length. The occurrence of stable, unstable and neutral conditions is taken into account by means of the joint probability distribution of the mean wind velocity and direction and of the Monin–Obukhov length. The effects of non-neutral stratifications on the structural response and on the mean fatigue life are preliminary discussed with reference to an example concerning a steel chimney.

An analysis of extreme non-synoptic winds

This paper presents an analysis of wind velocity data relating to 11 extreme non-synoptic events obtained from velocity time series pertaining to flow over rural and coastal terrain. For the purposes of the current analysis, an extreme event is deemed to have occurred when the local velocity increases rapidly by 50% or more and decreases within a relatively short time. The data were measured at a number of locations within Northern Europe and in all cases a sufficient array of anemometry was present to obtain an indication of the velocity profile. The velocity profile is shown to alter significantly during such events with three different trends identified. Despite the large variations in wind speed and velocity profile, wavelet analysis is used to illustrate that the underlying turbulent structures exhibit similar behaviour to those expected to occur during extreme wind speed events in boundary layer flow.

Experimental Study for Wind Velocity Profiles in and around a Rice Canopy at Different Growing Stages

Experimental Study for Wind Velocity Profiles in and around a Rice Canopy at Different Growing Stages

Evaluation of CFD Accuracy for the Ventilation Study of a Naturally Ventilated Broiler House

Using the CFD model, a new ventilation system design will be found later taking into consideration the ventilation efficiency such as uniformity, stability, and suitability of environmental factors in a naturally ventilated broiler house. Because conducting a field experiment for the ventilation study presented so many difficulties, a reliable 3-dimentional computational fluid dynamics (CFD) model had to be developed to investigate the natural ventilation. Before investigating its accuracy, a wind tunnel and particle image velocimetry (PIV) test was initially conducted to find their best experimental conditions and improve the PIV accuracy13,15. A 1/20 scale model of a naturally ventilated broiler house was used to get qualitative and quantitative airflow distribution in the broiler house using the PIV and CFD. To improve the CFD accuracy, the PIV and CFD computed airflows in the broiler house were compared, particularly on the distribution, local air velocity, and turbulent intensity in the house. The quality of the mesh density and the design of the boundary condition, especially the wind velocity and turbulence profiles, were found to be very important for getting accurate results. Assuming the PIV results were accurate, the most accurate CFD results were obtained when using a RNG k-ε turbulence numerical model. The average error of the CFD computed air velocity when using the RNG k-ε models was -6.2%.

Two-phase measurements of wind and saltating sand in an atmospheric boundary layer

Wind-blown sand movement is a particle-laden two-phase flow related to wind erosion in which the velocity distributions of both wind and sand are of particular interest. In the present study, two types of natural sand, one collected from the Pohang beach (diameter d = 200–300 μm) in South Korea and the other from the Taklimakan desert ( d = 100–125 μm) in China, were tested in a simulated atmospheric boundary layer. A high-speed digital camera system was used to capture images of the saltating sand particles at 2000 fps with an exposure time of 1/3000 s. Instantaneous velocity fields of the saltating sand particles were obtained using a particle tracking velocimetry (PTV) method. From these data, the particle resultant velocity, volume concentration, and streamwise mass flux were estimated as a function of height. The results reveal that the resultant particle velocity has an approximate log-linear profile with vertical height. Both the particle concentration and streamwise mass flux decay dramatically in the near surface region ( z < 20 mm for the beach sand, and z < 15 mm for the desert sand), then decline mildly beyond this region. To investigate the modification of the surrounding wind by the saltating sand particles, a hot-wire anemometry with a robust hot-film probe was employed to measure the wind velocity profiles with and without saltation. The present experimental data on both the saltating sand and wind provide useful information that enhances our understanding of saltation transport and further development of control techniques of wind erosion.

Application of a ray theory model to the prediction of noise emissions from isolated wind turbines and wind parks

AbstractVarious propagation models have been developed to estimate the level of noise near residential areas. Predictions and measurements have proven that proper modelling of the propagation medium is of particular importance. In the present work, calculations are performed using a ray theory methodology. The ray trajectory and transport equations are derived from the linear acoustics equations for a moving medium in three dimensions. Ground and atmospheric absorption, wave refraction and diffraction and atmospheric turbulence are taken into account by introducing appropriate coefficients in the equations. In the case of a wind turbine (W/T) it is assumed that noise is produced by a point source located at the rotor centre. Given the sound power spectrum, the noise spectrum at the receiver is obtained by solving the axisymmetric propagation problem. The procedure consists of (a) finding the eigenrays, (b) calculating the energy losses along the eigenrays and (c) synthesizing the sound pressure level (SPL) by superposing the contributions of the eigenrays. In the case of a wind park the total SPL is calculated by superposing the contributions of all W/Ts. Application is made to five cases of isolated W/Ts in terrains of varying complexity. In flat or even smooth terrain the predictions agree well with the measurements. In complex terrain the predictions can be considered satisfactory, taking into account the assumption of constant wind velocity profile. Application to a wind park shows clearly the influence of the terrain on the wind velocity and consequently on the SPL. Copyright © 2006 John Wiley &amp;Sons, Ltd.