Promocijas darbs

Meteoroloģisko novērojumu staciju dati Kurzemes Baltijas jūras piekrastē rāda, ka gada siltajā periodā (marts – oktobris) diennakts siltajā laikā vēja ātruma sadalījumam ir maksimumi, kas atbilst krastam paralēliem vējiem gan no ziemeļu, gan dienvidu virziena. 2013. gada 3. un 6. maijs tika izvēlēti tālākai analīzei. Tika veikta matemātiskā modelēšana izmantojot WRF (Weather Research and Forecast) modeli, kā arī novērojumu analīze, iekļaujot ASCAT attālinātās izpētes vēja ātruma un virziena mērījumus no satelīta. Rezultāti rāda, ka ziemeļu virziena plūsmas var izskaidrot ar piekrastes zemo līmeņu plūsmu (coastal low-level jet), bet dienvidu virziena plūsmas var skaidrot ar krastam paralēlajām strūklām, kas veidojas, kad plūsma sastop virsmas raupjuma lēcienu temperatūras inversijas gadījumā. Atslēgas vārdi: Piekrastes meteoroloģija, WRF, ASCAT, jūras brīze, piekrastes plūsmasData from meteorological observation stations located on the Kurzeme coast of Baltic Proper show that during the warm season (March – October) between sunrise and sunset the distribution of wind direction has maxima that correspond to coast parallel winds both from northern and southern direction. Two dates (3 of May 2013 and 6 of May 2013) were chosen for case studies, where the numerical modelling using WRF (Weather Research and Forecast) model was carried out together with the analysis of both conventional observations and ASCAT satellite wind speed and direction measurements. Results show that northern flows can be explained using Coastal Low-level Jets. The southern flow can be explained by jets created when the shoreward flow crosses a stepwise change in surface roughness when a temperature inversion is present. Keywords: Coastal meteorology, WRF, ASCAT, sea breeze, coastal flow

Mathematical modelling of bubble dynamics induced by ultrasonic cavitation

Kavitācija ir burbuļu veidošanās process šķidrumā spiediena pazemināšanās rezultātā, kas nozīmīgs silīcija plākšņu tīrīšanā. Šajā darbā matemātiski modelētas sfērisku kavitācijas burbuļu rādiusa izmaiņas harmoniski mainīga akustiskā lauka iespaidā, izmantojot Releja-Plesē un Kellera – Mikša modeļus. Tiek pētītas arī burbuļu pārvietošanās stāvviļņu akustiskajā laukā pirmā Bjerknesa spēku ietekmē, kā arī citu burbuļu radītā otrā veida Bjerknesa spēku ietekmē. Vienādojumu skaitliskai atrisināšanai izmantota Runges – Kutta – Fēlberga metode, kā arī daļēji slēgtā Eilera metode.Cavitation is the process of bubble formation in liquids caused by decreasing pressure. It plays a significant role in the megasonic cleaning of silicon wafers. The changes in the radius of a spherical cavitation bubble in an oscillating acoustic field are investigated using numerical results obtained by solving the Rayleigh-Plesset and Keller-Miksis equations. The bubble movement in a standing-wave acoustic field caused by primary Bjerknes force and the interaction between two bubbles with secondary Bjerknes force are explored as well. Runge-Kutta-Fehlberg and Semi-implicit Euler methods are employed

Mathematical modeling of cavitation bubble size distribution in megasonic cleaning process

Kavitācija ir gāzes burbuļu rašanās un oscilāciju process šķidrumā akustiskā lauka iedarbībā. Burbuļu oscilācijas ir nozīmīgas akustiskās tīrīšanas procesos. Tās ir atkarīgas no burbuļa izmēriem. Šī darba mērķis ir uzlabot matemātisko modeli burbuļu sadalījumam pa miera stāvokļa rādiusiem. Šī darba ietvaros tiek skaitliski realizēta vienādojumu sistēma, kas apraksta burbuļa svārstības un gāzes apmaiņas starp burbuli un šķidrumu. Tad literatūrā aprakstītais burbuļa sadalījuma modelis tiek papildināts ar gāzes apmaiņas procesu, burbuļu mijiedarbības spēku, kā arī burbuļu telpiskās pārvietošanās aprakstu, izmantojot darba pirmajā daļā iegūtos rezultātus.Cavitation is the process of bubble formation and subsequent bubble radius oscillations in the liquid that is under the influence of an acoustic field. The bubble oscillations have an important role in megasonic cleaning process and depend on the bubble size. The aim of this work is to improve the mathematical model of equilibrium bubble size distribution. In this paper, first, a system of equations is solved to calculate the gas exchange between bubbles and liquid (rectified diffusion), and, second, an existing bubble size distribution model is improved adding the gas exchange between bubbles and liquid, reciprocal force acting on bubbles and spatial bubble movement using the results previously obtained

The making of the New European Wind Atlas – Part 1: Model sensitivity

This is the first of two papers that document the creation of the New European Wind Atlas (NEWA). It describes the sensitivity analysis and evaluation procedures that formed the basis for choosing the final setup of the mesoscale model simulations of the wind atlas. The suitable combination of model setup and parameterizations, bound by practical constraints, was found for simulating the climatology of the wind field at turbine-relevant heights with the Weather Research and Forecasting (WRF) model. Initial WRF model sensitivity experiments compared the wind climate generated by using two commonly used planetary boundary layer schemes and were carried out over several regions in Europe. They confirmed that the most significant differences in annual mean wind speed at 100 m a.g.l. (above ground level) mostly coincide with areas of high surface roughness length and not with the location of the domains or maximum wind speed. Then an ensemble of more than 50 simulations with different setups for a single year was carried out for one domain covering northern Europe for which tall mast observations were available. We varied many different parameters across the simulations, e.g. model version, forcing data, various physical parameterizations, and the size of the model domain. These simulations showed that although virtually every parameter change affects the results in some way, significant changes in the wind climate in the boundary layer are mostly due to using different physical parameterizations, especially the planetary boundary layer scheme, the representation of the land surface, and the prescribed surface roughness length. Also, the setup of the simulations, such as the integration length and the domain size, can considerably influence the results. We assessed the degree of similarity between winds simulated by the WRF ensemble members and the observations using a suite of metrics, including the Earth Mover¿s Distance (EMD), a statistic that measures the distance between two probability distributions. The EMD was used to diagnose the performance of each ensemble member using the full wind speed and direction distribution, which is essential for wind resource assessment. We identified the most realistic ensemble members to determine the most suitable configuration to be used in the final production run, which is fully described and evaluated in the second part of this study (Dörenkämper et al., 2020).The European Commission (EC) partly funded the NEWA project (New European Wind Atlas) through FP7 (topic FP7-ENERGY.2013.10.1.2) The authors of this paper acknowledge the support the Danish Energy Authority (EUDP 14-II, 64014-0590, Denmark); the German Federal Ministry for the Economic Affairs and Energy, on the basis of the decision by the German Bundestag (ref. no. 0325832A/B); Latvijas Zinatnu Akademija (Latvia); Ministerio de Economía y Competitividad (Spain, ref. nos. PCIN-2014-017-C07-03, PCIN-2016-176, PCIN-2014-017-C07-04, and PCIN-2016-009); the Swedish Energy Agency (Sweden); and the Scientific and Technological Research Council of Turkey (grant no. 215M386). Andrea N. Hahmann additionally acknowledges the support of the Danish Ministry of Foreign Affairs, administered by the Danida Fellowship Centre under the project “Multiscale and Model-Chain Evaluation of Wind Atlases” (MEWA) and the ForskEL/EUDP (Denmark) project OffshoreWake (PSO-12521, EUDP 64017-0017)