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Bora wind, Wind speed vertical profile, Logarithmic law, Power law
Klemen Bergant, Samo Stanič, Marija Bervida, 2018, published scientific conference contribution

Abstract: Bora is cold and gusty downslope wind with variable gust frequency and duration, appearing on the lee side of Dinaric Alps. Its flow characteristics are unique and theoretically still not fully described, especially for modeling purposes. We present an analysis of the wind speed vertical profiles at Razdrto, which lies in a gap between the Nanos and Javorniki plateau in southwest Slovenia and is strongly exposed to Bora. An analysis of the vertical wind speed profiles during Bora episodes is based on experimental wind data, provided by Helikopter energija, for six Bora events of different duration, appearing between April 2010 and May 2011. Average wind speed in 10-minute intervals was collected at four different heights (20, 31, 40 and 41.7 m above the ground)at the wind turbine site in Razdrto using cup anemometers. Wind direction data with same temporal resolution was obtained from a single wind vane placed at 40.9 m above the ground. Based on the collected data, the applicability of the empirical power-law and the logarithmic law profiles, commonly used for the description of neutrally stratified atmosphere, was investigated for the case of Bora. The parameters for the power-law and logarithmic law were obtained by fitting the wind speed data using linear regression method and are compared to standard values for that particular type of terrain. The quality of fits was very good with r2 above 0.9, indicating that both power-law and logarithmic law adequately describe mean horizontal Bora wind. The median value of the power-law coefficient was found to be 0.16±0.03, which is consistent with standard value for neutral atmosphere (0.143). The aerodynamic roughness varied from 0.003 m to 0.22 m with the median value of 0.09±0.07, which describes open level country terrain with some trees. The event in November 2010 with large roughness is expected to be due to specific wind direction and surface conditions.
Found in: osebi
Keywords: Bora wind, Wind speed vertical profile, Logarithmic law, Power law
Published: 07.02.2019; Views: 914; Downloads: 7
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Near-Ground Profile of Bora Wind Speed at Razdrto, Slovenia
Klemen Bergant, Samo Stanič, Marija Bervida, Benedikt Strajnar, 2019, original scientific article

Abstract: Southwest Slovenia is a region well-known for frequent episodes of strong and gusty Bora wind, which may damage structures, affect traffic, and poses threats to human safety in general. With the increased availability of computational power, the interest in high resolution modeling of Bora on local scales is growing. To model it adequately, the flow characteristics of Bora should be experimentally investigated and parameterized. This study presents the analysis of wind speed vertical profiles at Razdrto, Slovenia, a location strongly exposed to Bora during six Bora episodes of different duration, appearing between April 2010 and May 2011. The empirical power law and the logarithmic law for Bora wind, commonly used for the description of neutrally stratified atmosphere, were evaluated for 10-min averaged wind speed data measured at four different heights. Power law and logarithmic law wind speed profiles, which are commonly used in high resolution computational models, were found to approximate well the measured data. The obtained power law coefficient and logarithmic law parameters, which are for modeling purposes commonly taken to be constant for a specific site, were found to vary significantly between different Bora episodes, most notably due to different wind direction over complex terrain. To increase modeling precision, the effects of local topography on wind profile parameters needs to be experimentally assessed and implemented.
Found in: osebi
Keywords: Bora wind, logarithmic law, power law, roughness length, wind profile
Published: 04.10.2019; Views: 493; Downloads: 23
.pdf Fulltext (5,90 MB)

Bora wind effects on common structures in the Vipava valley
Marija Bervida, doctoral dissertation

Abstract: Strong and gusty north-east wind called Bora is common in south-west regions of Slovenia, as well as along the Adriatic coast. Its intermittent behavior, related to variable strength, frequency and duration, has brought out scientific curiosity for decades. Bora affects human life and causes problems for structures built in Bora affected areas. In Slovenia, Bora is the strongest in the Vipava valley. The motivation for this research is the need to evaluate Bora wind effects on structures, commonly found in the Vipava valley region, using a high resolution computational fluid dynamics (CFD) modeling approach. To date, there are several experimental and computational constraints for accurate representation of Bora in a CFD model, therefore, the main aim of this dissertation is to build foundations for Bora wind simulations using CFD and its method of finite volumes. The dissertation incorporates the analysis of experimental measurements of Bora wind, as well as numerical modeling studies. Vertical mean wind speed profile characteristics of Bora were analyzed based on experimental measurements at Razdrto just above the Vipava valley. The obtained results contributed to the choice of Bora mean wind profiles applied at the inflow of computational models. Guidelines regarding the choice of the associated wind profile parameters were given and a new relationship between these parameters was found. As orographic barriers to the north of the Vipava valley are known to give rise to Bora and to define the specific properties of the Bora flow, numerical modeling studies were in the first place focused on the implementation of the real-scale complex terrain into a CFD model. Simulation of wind flow over orographic barrier in Vipava valley was performed using Raynolds averaged Navier-Stokes approach, providing a first estimation of the flow field over a small hill of Zemono. As resolving the turbulence characteristics of Bora is very important for the estimation of wind loads on structures, modeling studies converged towards a more appropriate approach - Large eddy simulations (LES). A crucial step in setting up an accurate LES is the generation of appropriate inflow, which was investigated for the case of atmospheric boundary layer (ABL) flow. The synthetic method PRFG^3 for the generation of unsteady inflow was tested and adapted as a source of an ABL flow with desired turbulence flow properties. Based on its performance, in particular on adequate reproduction of target turbulence intensities and length scales, it was found that PRFG^3 method may be used to generate velocity inflow with desired turbulence properties in LES. Finally, simulations of wind flow coming from Bora direction over the Vipava valley were performed with the aim to depict the effects of underlying orography on the flow within and above the valley. Modeling results were found to be comparable with the results of lidar based remote sensing of vertical atmospheric structures within and above the valley.
Found in: osebi
Keywords: Vipava valley, Bora wind, Wind profile, Orography, Atmospheric boundary layer, Computational fluid dynamics, Numerical simulations
Published: 17.06.2020; Views: 239; Downloads: 2
.pdf Fulltext (35,12 MB)

Lidar Observations of Mountain Waves During Bora Episodes
Longlong Wang, Marija Bervida, Samo Stanič, Klemen Bergant, William Eichinger, Benedikt Strajnar, 2020, published scientific conference contribution

Abstract: Airflows over mountain barriers in the Alpine region may give rise to strong, gusty downslope winds, called Bora. Oscillations, caused by the flow over an orographic barrier, lead to formation of mountain waves. These waves can only rarely be observed visually and can, in general, not be reliably reproduced by numerical models. Using aerosols as tracers for airmass motion, mountain waves were experimentally observed during Bora outbreak in the Vipava valley, Slovenia, on 24-25 January 2019 by two lidar systems: a vertical scanning lidar positioned just below the peak of the lee side of the mountain range and a fixed direction lidar at valley floor, which were set up to retrieve two-dimensional structure of the airflow over the orographic barrier into the valley. Based on the lidar data, we determined the thickness of airmass layer exhibiting downslope motion, observed hydraulic jump phenomena that gave rise to mountain waves and characterized their properties.
Found in: osebi
Keywords: Bora, mountain waves, lidar observations
Published: 08.07.2020; Views: 151; Downloads: 0
.pdf Fulltext (3,66 MB)

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