Simulacija velikih turbulentnih vrtloga bure kraj Maslenice

master's thesis

• Title (english): Large-eddy simulation of a bora case near Maslenica
• Author: Daniel Bihar
• Mentor: Branko Grisogono (mentor)
• Committee member: Branko Grisogono (predsjednik povjerenstva)
• Committee member: Željko Večenaj (član povjerenstva)
• Committee member: Ivica Sović (član povjerenstva)
• Committee member: Zvjezdana Bencetić-Klaić (član povjerenstva)
• Committee member: Josip Stipčević (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Geophysics); Zagreb
• Defense date and country: 2020-10-15, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Geophysics; Meteorology and Climatology

Abstract

Nastanak, razvoj i kraj bure dugo su bili nepoznati sa znanstvenog stajališta. Različite ideje o termalnom tjeranju bile su vodeće sve do osamdesetih godina 20. stoljeća kada su nova avionska mjerenja donijela svježa saznanja o generiranju bure. Simulacijom velikih vrtloga (eng. ”Large-eddy simulation”, LES [4]) u ovoj radnji potvrđujemo sadašnja saznanja o buri. Bura je, vertikalno gledano, nelokalni vjetar niz planinski obronak, a njenu karakterističnu turbulenciju generiraju mehanički procesi naginjanja, savijanja i, u olujnom slučaju bure, loma planinskih valova. U ovom radu usporedili smo mjerenja događaja bure 30. prosinca 2015. s direktnom simulacijom velikih vrtloga danog slučaja na terenu kraj Maslenice, blizu južnog Velebita veličine 66 km × 33 km te 10 km vertikalne domene. To konkretno donosi horizontalnu rezoluciju od 32 m te najfiniju vertikalnu rezoluciju pri dnu modela od 1 m do 100m pri vrhu domene modela. Prema trenutnom saznanju ovo su prvi dokumentirani uspješni meteorološki LES rezultati bure u Hrvatskoj.
Simulacija kojom smo pokazali ispravnost teorije nastanka bure prilagođena je verzija nizozemskog LES-a (naziva ”microhh 1.0”) otvorenog koda. Primijetili smo da su glavni strukturni elementi turbulencije bure vrtlozi skale od 100 m naviše. Vrtlozi manji od oko 100 m više ne utječu, u prosjeku, na mezoskalno gibanje zraka nego se spontano raspadaju dalje prema znanom kraju turbulentne kaskade u viskoznoj disipaciji.
Model također uspoređujemo s in situ mjerenim podacima u trajanju od 20 sati. Za ovaj rad korišteni su podaci bure dana 30. prosinca 2015., gdje vidimo odlično slaganje mjerenja i metode LES-a. Pokazano je dobro slaganje sa statističkim i mehaničkim svojstvima bure kao što su brzina, spektar brzine te smjer vjetra i njegove varijacije, što je važan dokaz valjanosti ovih simulacija.

Abstract (english)

Start, development and ultimate end of bora wind were largely unknown until recent times. Most of the 20th century thermal forcing on a mountain side was the leading explanation. In 80s, with the introduction of airborne measurements in relevant areas, a breakthrough in the understanding of bora basic dynamics was achieved. In this thesis Large-eddy simulation (LES) [4] of the particular bora case validates current understanding of the matter. Bora is, vertically, nonlocal wind above and down the mountain slope. Its characteristic turbulent nature is due to asymmetric sloping, deforming, overturning, and in extreme cases, breaking of the mountain waves. In this thesis, we have compared measurements of bora wind dated 12/30/2015 with Large-eddy simulation (LES) of the same case located near Maslenica (South of the mountain of Velebit range, northern Dalmatia, Croatia). The simulated terrain had width and height of 66 km and 33 km with the vertical dimension of 10 km. Horizontal resolution in the simulation was 32 m with varying vertical resolution of 1 m at the bottom to 100 m at the top of the domain. At the moment, our results are the first successful meteorological LES results of Croatian bora wind published publicly.
A simulation which we used and modified is made on the basis of Duch open source LES model (”microhh 1.0”). We have noticed that the main structural element of bora are big eddies of 100 m and more. Vortices of smaller scale (less than 100 m) no longer contribute significantly to mesoscale air motion. These turbulent eddies, on average, inevitably cascade all the way to viscous scales.
LES model is also validated against 20 hours of in situ measurements (dated 12/30/2015). Agreement of measured and simulated data is tested on wind velocities component-wise, wind speed spectra, wind direction, etc... In all aspects, the LES appears to have high correspondence to measured data.

• Keywords: LES; DNS; bura; simulacija; vrtlozi; Maslenica; Velebit; valovi
• Keywords (english): LES; DNS; Large-eddy simulation; bora; Maslenica; Velebit; mountain waves
• Language: croatian
• URN:NBN: urn:nbn:hr:217:614509
• Study programme: Title: Graduate university study pf Physics - Geophysics; specializations in: Seismology and Physics of the Earth's Interior, Meteorology and Physical Oceanography; Course: Meteorology and Physical Oceanography; Study programme type: university; Study level: graduate; Academic / professional title: magistar/magistra fizike – geofizike (magistar/magistra fizike – geofizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Public note: Uz diplomski rad postoje i video-materijali sa isječcima vizualizacije rezultata simulacija koje možete dobiti na uvid ako se javite u Geofizičku knjižnicu ([e-mail]). In addition to the thesis, there are also video materials with clips of visualization of simulation results, to which you can gain access if you contact the Geophysics Library ([e-mail]).
• Created on: 2020-10-20 09:37:31