Uniform Wind Flow Research Articles

Effect of complex orography on numerical simulations of a downburst event in Spain

A downburst is a localized and intense downdraft of air that descends quickly from the middle troposphere and reaches the Earth's surface. It is frequently originated by a thunderstorm or a supercell. Downburst winds can cause significant damage to buildings, infrastructure, and pose a great threat to aviation traffic. On July 1, 2018, many supercells were spotted near the Zaragoza Airport (Spain), and at least one of them generated a downburst that affected the airport, causing significant damage in the surrounding area. This event is here simulated using the Weather Research and Forecasting (WRF-ARW) numerical weather prediction model. Three different WRF-ARW orography experiments are carried out to investigate if the region's complex orography has an important role in supercell and downburst development over the research area. One of the three experiments uses the default orography as control; another one uses a 90 % smoothed orography, and the third experiment is configured with a high-resolution dataset. Several atmospheric and convective variables are compared for each orography experiment. Results show that MUCAPE is clearly higher when the orography complexity is reduced. The smoothing process leads to a more uniform wind flow, contributing to the formation of numerous supercells. However, supercells channel through valleys and mountains in the control and high-resolution orography experiments, where the surface wind divergences are uniquely reproduced, and the highest reflectivity values are observed. Moisture advection from the Mediterranean Sea is essential in the process, reaching more deeply into the study region in the smoothed orography experiment due to the lack of orographic barriers. Orography affects dynamic and thermodynamic features, which have considerable effects on the formation and development of downbursts.

Nonlinear Aeroelastic in-Plane Behavior of Suspension Bridges under Steady Wind Flow

The nonlinear aeroelastic behavior of suspension bridges, undergoing dynamical in-plain instability (galloping), is analyzed. A nonlinear continuous model of bridge is formulated, made of a visco-elastic beam and a parabolic cable, connected each other by axially rigid suspenders, continuously distributed. The structure is loaded by a uniform wind flow which acts normally to the bridge plane. Both external and internal damping are accounted for the structure, according to the Kelvin-Voigt rheological model. The nonlinear aeroelastic effects are evaluated via the quasi-static theory, while structural nonlinearities are not taken into account. First, the free dynamics of the undamped bridge are addressed, and the natural modes determined. Then, the nonlinear equations ruling the dynamics of the aeroelastic system, close to the bifurcation point, are tackled by the Multiple Scale Method. This is directly applied to the partial differential equations, and provides the finite-dimensional bifurcation equations. From these latter, the limit-cycle amplitude and its stability are evaluated as function of the mean wind velocity. A case study of suspension bridge is analyzed.

Wind effect on the evolution of two obliquely interacting random wave trains in deep water

A pair of coupled nonlinear evolution equations for the spectral functions are derived in a situation of crossing sea states characterized by two narrowband Gaussian random surface wave systems in the presence of uniform wind flow. These two equations are employed to perform stability analysis of two initially homogeneous wave spectra subject to infinitesimal perturbations. It is found that the results of the stability analysis remain qualitatively similar with the corresponding deterministic situation. The notable difference is that the growth rate of instability reduces slightly due to the effect of randomness. But, it is much higher than the growth rate of a single wave system. As the spectral bandwidth increases the growth rate of instability decreases.

Effect of uniform wind flow on modulational instability of two crossing waves over finite depth water

The effect of uniform wind flow on modulational instability of two crossing waves is studied here. This is an extension of an earlier work to the case of a finite-depth water body. Evolution equations are obtained as a set of three coupled nonlinear equations correct up to third order in wave steepness. Figures presented in this paper display the variation in the growth rate of instability of a pair of obliquely interacting uniform wave trains with respect to the changes in the air-flow velocity, depth of water medium and the angle between the directions of propagation of the two wave packets. We observe that the growth rate of instability increases with the increase in the wind velocity and the depth of water medium. It also increases with the decrease in the angle of interaction of the two wave systems. doi:10.1017/S1446181118000172

EFFECT OF UNIFORM WIND FLOW ON MODULATIONAL INSTABILITY OF TWO CROSSING WAVES OVER FINITE DEPTH WATER

The effect of uniform wind flow on modulational instability of two crossing waves is studied here. This is an extension of an earlier work to the case of a finite-depth water body. Evolution equations are obtained as a set of three coupled nonlinear equations correct up to third order in wave steepness. Figures presented in this paper display the variation in the growth rate of instability of a pair of obliquely interacting uniform wave trains with respect to the changes in the air-flow velocity, depth of water medium and the angle between the directions of propagation of the two wave packets. We observe that the growth rate of instability increases with the increase in the wind velocity and the depth of water medium. It also increases with the decrease in the angle of interaction of the two wave systems.

Nonlinear evolution equations in crossing seas in the presence of uniform wind flow

The effect of uniform wind flow in a situation of crossing sea states is studied here on the stability analysis for a pair of obliquely interacting uniform wave trains. It is observed that the growth rate of instability in crossing seas in the presence of wind flow is much higher than that in the absence of wind flow. It is also found that the growth rate of instability increases as the wind flow velocity increases up to some critical velocity beyond which the wave becomes linearly unstable and consequently the present analysis does not remain valid. The growth rate of instability obtained here is also greater than that for a single wave packet propagating in the presence of uniform wind flow.

Research of Wind Shear Dynamic Characteristics of Wind Wheels

In order to accurately analysis the aerodynamic characteristic variations of wind turbines under shear, the influence of axial and shear on the aerodynamic characteristic of a horizontal-axis wind turbine is simulated in this paper by using a sliding grid method based on FlowVision. By using the TJÆREBORG wind turbine as the object of study, a three-dimensional model of a uniform wind flow can be created. The CFD calculation results, the experimental results and the Bladed results can be used to confirm the reliability of the model. In order to investigate the effect of wind shear with regard to three-dimensional unsteady flow characteristics and a three-dimensional flow field in a wind turbine impeller, an analysis of wheel wind speed of 11m/s and an investigation of the influence of wind shear on turbine performance are carried out.

Structural design of a natural windbreak using computational and experimental modeling

Computational and experimental approaches are employed for the structural design of a natural windbreak. It is intended to find the optimum tree shelterbelt to obviate the uneven wind speed distribution across the width dimension of a high-level competition rowing channel. The experimental results, obtained in a wind tunnel, and consisting of erosion-technique images and local wind-speed measurements, are used to benchmark the computational model. A good agreement between the two sets of results is obtained. Several windbreak configurations, considering one or two rows of different cross-sectional shape and porosity, are computationally modeled. For the shortest row a rectangular shape, with porosity of 35%, is considered; for the tallest row, which aims the modeling of a poplar tree, a porosity of 87% is assumed at the trunk level, and 60% at the crown. The optimum shelterbelt consists of two rows, composed by bamboo and poplar trees, which allows the attainment of a low and nearly uniform wind flow across the width of the channel.

Measuring Ammonia Concentrations and Emissions from Agricultural Land and Liquid Surfaces: A Review

Aerial ammonia concentrations (C g) are measured using acid scrubbers, filter packs, denuders, or optical methods. Using C g and wind speed or airflow rate, ammonia emission rate or flux can be directly estimated using enclosures or micrometeorological methods. Using nitrogen (N) recovery is not recommended, mainly because the different gaseous N components cannot be separated. Although low cost and replicable, chambers modify environmental conditions and are suitable only for comparing treatments. Wind tunnels do not modify environmental conditions as much as chambers, but they may not be appropriate for determining ammonia fluxes; however, they can be used to compare emissions and test models. Larger wind tunnels that also simulate natural wind profiles may be more useful for comparing treatments than micrometeorological methods because the latter require larger plots and are, thus, difficult to replicate. For determining absolute ammonia flux, the micrometeorological methods are the most suitable because they are nonintrusive. For use with micrometeorological methods, both the passive denuders and optical methods give comparable accuracies, although the latter give real-time C g but at a higher cost. The passive denuder is wind weighted and also costs less than forced-air C g measurement methods, but it requires calibration. When ammonia contamination during sample preparation and handling is a concern and separating the gas-phase ammonia and aerosol ammonium is not required, the scrubber is preferred over the passive denuder. The photothermal interferometer, because of its low detection limit and robustness, may hold potential for use in agriculture, but it requires evaluation. With its simpler theoretical basis and fewer restrictions, the integrated horizontal flux (IHF) method is preferable over other micrometeorological methods, particularly for lagoons, where berms and land-lagoon boundaries modify wind flow and flux gradients. With uniform wind flow, the ZINST method requiring measurement at one predetermined height may perform comparably to the IHF method but at a lower cost.

Lindstedt series for periodic solutions of beam equations with quadratic and velocity dependent nonlinearities

We prove the existence of small amplitude periodic solutions, for a large Lebesgue measure set of frequencies, in the nonlinear beam equation with a weak quadratic and velocity dependent nonlinearity and with Dirichelet boundary conditions. Such nonlinear PDE can be regarded as a simple model describing oscillations of flexible structures like suspension bridges in presence of an uniform wind flow. The periodic solutions are explicitly constructed by a convergent perturbative expansion which can be considered the analogue of the Lindstedt series expansion for the invariant tori in classical mechanics. The periodic solutions are defined only in a Cantor set, and resummation techniques of divergent powers series are used in order to control the small divisors problem.

302 一様風中のドップラー効果(GS-3 流れ)

302 一様風中のドップラー効果(GS-3 流れ)

Numerical study of the performance of tornado-type wind energy systems

The tornado-type wind energy system was proposed to utilize the pressure drop created by an intens vortex in a tower. The tower serves as a low pressure exhaust for the turbine. The author carried out a numerical solution of the tower flow, using the two-equation (kappa-epsilon) turbulence model, for the small size system tested by Yen, with a 0.127-m- (5-in.-) diam tower standing in a uniform wind flow. A comparison of the results with the measured pressure values verified the model. In the present work the same numerical model is used for a system with a tower that is completely embedded in an atmospheric boundary layer. The results show, for a tower in a boundary layer of the one-seventh power-law profile, about a 28% reduction of the values of power coefficient from those for a tower in a uniform stream. Calculations for the performance of a system with different tower heights and the same tower and turbine diameters show a sharp decrease of power for H/D less than or equal to 0.5. Analysis of the tangential velocity shows that the vortex should be maintained up to heights /ZETA//D less than or equal to 0.9 to avoid the decrease in powermore » extraction. Numerical predictions are made for systems with tower diameters between 0.5 and 8 m (1.64 and 26.24 ft) using the same system geometry and approach flow conditions. The results show that the scale effect is negligible for systems of 4-m tower diameter or larger.« less