magistrsko delo
Jan Jamnik Srpčič (Author), Iztok Tiselj (Mentor)

Abstract

Najnatančnejša metoda za simulacije turbulentnih tokov je direktna numerična simulacija (DNS), ki zahteva zelo goste diskretne mreže in je računsko zelo zahtevna. Metoda velikih vrtincev (LES) deluje na mrežah, ki so v vsaki smeri tudi do desetkrat redkejše kot mreže v DNS, kar bistveno pospeši izračune. Metoda LES temelji na modelih, ki približno modelirajo turbulentno difuzijo v vrtincih, ki so manjši od resolucije mreže, medtem ko eksplicitno opiše večje vrtince. Metoda DNS prepozna vrtince vseh velikosti brez približkov. V magistrski nalogi obravnavamo točnost LES metode s primerjavo z obstoječimi rezultati, izračunanimi z metodo DNS v geometriji turbulentnega toka v kanalu s stopnico. Naša LES simulacija je potekala na približno 15-krat redkejši mreži in je porabila za dva velikostna reda manj računskega časa kot DNS. Simulaciji sta primerljivi zaradi enakega Reynoldsovega števila, ki znaša 7100. Primerjamo časovno povprečena hitrostna polja v različnih ravninah, iz katerih je razvidna velika podobnost med rezultati obeh metod v toku čez stopnico. Drobne, a opazne razlike v dimenzijah in oblikah vrtincev pripisujemo netočnosti metode velikih vrtincev. Točnost metode, uporabljene v magistrski nalogi, smo natančneje obravnavali še s primerjavo profilov časovno povprečenih komponent hitrosti ter hitrostnih fluktuacij na izbranih daljicah v računski domeni. Iz profilov je mogoče sklepati, da LES v sredini domene napove za nekaj odstotkov hitrejši tok in nekoliko počasnejši tok ob stenah kot metoda DNS. Kljub temu, da smo v LES metodi s približki opisali le najmanjše vrtince, pa ta aproksimacija vpliva tudi na obliko največjih vrtincev v opazovani geometriji.

Keywords

računska dinamika tekočin;metoda velikih vrtincev;turbulentni tok;enofazni tok;geometrija stopnice;

Data

Language: Slovenian
Year of publishing:
Typology: 2.09 - Master's Thesis
Organization: UL FMF - Faculty of Mathematics and Physics
Publisher: [J. Jamnik Srpčič]
UDC: 532.5
COBISS: 53018883 Link will open in a new window
Views: 516
Downloads: 122
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Other data

Secondary language: English
Secondary title: Accuracy of Large Eddy Simulation method for turbulent flow over backward facing step
Secondary abstract: Direct numerical Simulation is the most accurate method for simulating turbulent flows which require very dense discrete meshes and is therefore computationally very expensive. Large Eddy Simulation method (LES) is performed on meshes that are up to ten times coarser in each direction than the ones used in the DNS, which significantly decreases computation time. LES is based on semi-empirical models, which approximately describe turbulent diffusion in the smallest eddies that are not captured by the discrete grid. In the master’s thesis, we are analyzing the accuracy of the LES method through a comparison with existing results that are computed with the DNS method in turbulent flow geometry in a backward facing step domain. Our LES was running on approximately 15 times coarser mesh and consumed two orders of magnitude less computation time than the DNS. Simulations are comparable since they have the same Reynolds number, which is 7100. We are comparing the time-averaged velocity fields in different planes, which show considerable similarities between the methods in the flow throughout the step. Small but noticeable differences in dimensions and shapes of the vortices are attributed to the inaccuracy of the large eddy method. Additionally, we evaluated the accuracy of the LES through the comparison of time-averaged velocities and their fluctuations on selected line segments in the domain after the step. From the latter, we can conclude that the flow in the LES is faster in the center of the domain and slower near the walls in comparison to the flow in the DNS. Although the LES method uses empirical models only for the smallest eddies, this approximation affects the geometry of the largest vortices in the system.
Secondary keywords: computational fluid dynamics;Large Eddy Simulation method;turbulent flow;single-phase flow;backward facing step geometry;
Type (COBISS): Master's thesis/paper
Study programme: 0
Embargo end date (OpenAIRE): 1970-01-01
Thesis comment: Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages: 72 str.
ID: 12580103