engleski

Physical modeling of natural winds, transient aerodynamic loads on vehicles, sheltering efficiency of wind barriers

Studies on atmospheric boundary layer (ABL) flow characteristics, in general, lead to an increased understanding of air pollutant dispersion and wind loading of structures. Commonly, these studies are carried out in boundary layer wind tunnels where it is necessary to compromise between two contradictory demands: on the one hand, the ABL wind-tunnel model should be as large as possible to provide better insight into details of the simulated flow, while on the other hand, a structural wind tunnel model that is too large can cause undesirable blockage effects in the wind-tunnel test section. These scaling issues were comprehensively studied at the Munich Institute of Technology, Germany where widely accepted ‘Counihan’ vortex generators were redesigned for purposes of part-depth ABL simulations, i.e. modeling only the lower portion of the ABL. In this seminar, a basic concept of the Counihan method for ABL wind-tunnel simulations will be briefly reported and results of several part-depth ABL simulations using newly developed vortex generators will be presented. A major threat to traffic in coastal Croatia is a strong, cold and gusty north-easterly bora wind blowing over the Dinaric Alps along the eastern Adriatic. Its most prominent feature is strong gustiness. In severe bora cases mean hourly wind speeds averaged 17 m/s, while gust velocities as large as 69 m/s were observed. Therefore, the bora-wind-induced instability of road vehicles represent a major problem, as in the past roads and bridges along the Croatian coast have had to be closed to traffic often due to safety requirements. Transient aerodynamic loads on a vehicle exposed to cross-wind bora-like gusts were experimentally simulated in the NatHaz Laboratory at the University of Notre Dame, IN. Wind gusting of a vehicle on a bridge was simulated by exposing the vehicle model to an air flow consisting of a velocity time history that alternates within a predefined velocity threshold. Larger steady aerodynamic loads were observed during the simulated wind gusts, characterizing bora winds, compared to the normal wind periods. Furthermore, two different unsteady aerodynamic loading phenomena were reported originating from periodicity of the simulated bora-wind gusting as well as from shed vortices from the bridge, and their interactions with the vehicle model. Recently, wind-tunnel tests were carried out at the University of Florence, Italy to determine an optimal aerodynamic design of wind barriers for Croatian freeway bridges. Particle Image Velocimetry (PIV) measurements were taken on bridge models in the wake of the wind barrier for different angles of attack, barrier porosity and height, design of barrier elements, incoming turbulence intensity etc. In this seminar, some representative results on sheltering efficiency of wind barriers will be reported.

Wind tunnel experiments; atmospheric boundary layer; transient winds; cross-wind loads on vehicles; aerodynamic design of wind barriers

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