@phdthesis{
author = "Janković, Novica",
year = "2020",
abstract = "U okviru disertacije se istražuje struktura turbulentnog vihornog strujanja u mlazu aksijalnog ventilatora. Generisano turbulentno strujanje je trodimenziono, nehomogeno i anizotropno. Primenjeni su složeni eksperimentalni, numerički i teorijski pristupi. Eksperimentalna instalacija je u radu detaljno opisana, kao i kvantifikacija merne nesigurnosti trokomponentnog sistema za Laser Dopler anemometriju (LDA). LDA sistem je, pri simultanom merenju tri komponente brzine, zahtevao precizna podešavanja merne zapremine u granicama prečnika do 100 μm. Ostvareno je precizno pomeranje ove zapremine duž mlaza. Poseban izazov je predstavljala numerička obrada dobijenih, u vremenu nejednako raspoređenih, mernih rezultata. Teorijskim razmatranjima, u okviru disertacije, bačeno je novo svetlo na klasične zapise Navije-Stoksovih i Rejnoldsovih jednačina u polarno-cilindričnim koordinatama. Detaljno su razmatrane empirijske radijalnoaksijalne raspodele ukupne, aksijalne, radijalne i obimske brzine, izmerene za dve brzine obrtanja ventilatora i tri ugla lopatica. Na ovaj način je bilo moguće istražiti uticaj Rejnoldsovog broja, kao i geometrijskih karakteristika ventilatora, tj. ugla lopatica, na karakter dejstva vihora na turbulenciju i evoluciju nivoa turbulencije, kako u samom mernom preseku, tako i u nizstrujnom razvoju mlaza. Analiza generisanja turbulencije i produkcije pojedinih Rejnoldsovih napona ukazala je na bitne karakteristike turbulentnog prenosa u slobodnom vihornom mlazu. Uočeno je da promena znaka gradijenta brzine ne izaziva uvek i promenu znaka pripadajuće komponente tenzora turbulentnog napona. U domenima strujnog polja u kojima se to dešava, mehanizam turbulentnog prenosa je nelokalnog karaktera, prisutna je negradijentna turbulentna difuzija i javlja se negativna produkcija kinetičke energije turbulencije. Izložena je diskusija o uticaju vihora na strukturu turbulencije i mehanizam turbulentnog prenosa. Analiza je ukazala na složena međudejstva osrednjenih i fluktuacionih polja u vihornom mlazu. Zaključeno je da generisanje energije fluktuacionog kretanja ne nastaje na osnovu kinetičke energije osrednjenog kretanja, već naprotiv. Smer prenošenja energije se menja, tako da osrednjeno kretanje „crpi” kinetičku energiju fluktuacionog kretanja, štoodgovara složenim strukturalnim svojstvima turbulencije. Zato su eksperimentalna istraživanja bila usmerena na merenje i analizu statističkih momenata višeg reda, i to ovde do šestog reda..., The dissertation deals with the structure of the turbulent swirl flow in an axial fan jet. The generated turbulent flow is three-dimensional, inhomogeneous and anisotropic. Complex experimental, numerical and theoretical approaches were applied. The experimental setup is described in detail, as well as the quantification of the measurement uncertainty of a three-component Laser Doppler Anemometry (LDA) system. The LDA system, while measuring the three velocity components simultaneously, requires precise adjustments of the measuring volume up to 100 μm in diameter. A precise displacement of this volume along the jet was achieved. A particular challenge was the numerical processing of the obtained unequally spaced data. Theoretical considerations, in the framework of the dissertation, have brought new light on classical notations of Navier-Stokes and Reynolds equations in polar-cylindrical coordinates. The empirical radial-axial distributions of total, axial, radial, and circumferential velocities measured for two fan speeds and three vanes angles are thoroughly analyzed. In this way, it was possible to investigate the influence of the Reynolds number as well as the geometric characteristics of the fan, i.e. angles of the blades, on the character of the swirl influence on turbulence and the evolution of the turbulence level, both in the measuring cross section and in the downstream development of the jet. An analysis of the generation of turbulence and the production of individual Reynolds stresses indicated the essential characteristics of turbulent transfer in a free swirl jet. It has been observed that a change in the sign of the velocity gradient does not always cause a change in the sign of the corresponding component of the turbulent stress tensor. In the fluid flow domains where this occurs, the turbulent transfer mechanism has nonlocal character, non-gradient turbulent diffusion and negative production of turbulence kinetic energy occur. Discussion of the effect of the swirl on the turbulence structure and the mechanism of turbulent transfer is presented. The analysis indicated the complex interactions of averaged and fluctuating fields in a swirl flow. It is concluded that the generation of fluctuating motion energy does not occur on the basis of kinetic energy of averaged motion, but on the contrary. The direction of energy transfer changes so that theaveraged motion "draws" the kinetic energy of the fluctuating motion, which corresponds to the complex structural properties of the turbulence. That is why experimental research has focused on measuring and analyzing higher order statistical moments, up to the sixth order here...",
publisher = "Univerzitet u Beogradu, Mašinski fakultet",
title = "Eksperimentalna i teorijska istraživanja strukture turbulentnog vihornog strujanja u mlazu aksijalnog ventilatora, Experimental and theoretical research of the structure of turbulent swirl flow in axial fan jet",
url = "https://hdl.handle.net/21.15107/rcub_nardus_18158"
}
