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Accuracy of Large Eddy Simulation method for turbulent flow over backward facing step
Full text linkNajnatančnejša 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.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
Large eddy simulation of turbulent flow in a pipe with a bend.
No full textV magistrskem delu so predstavljeni rezultati simulacij turbulentnega toka v cevi s kolenom. Simulacije smo izvedli s programom za računalniško dinamiko tekočin OpenFOAM. Uporabili smo metodo velikih vrtincev, predstavili pa smo tudi osnovno idejo direktne numerične simulacije. Simulacije smo izvedli na superračunalniku Vega za tok s tremi različnimi Reynoldsovimi števili in sicer z 1400, 5600 in 10000. V simulacijah smo predpostavili, da je tok nestisljiv in da je tekočina Newtonska. Za tokove z različnimi Reynoldsovimi števili smo uporabili različno goste računske mreže. Kvaliteto mrež in točnost simulacij smo preverili s primerjavo enodimenzionalnih profilov povprečne hitrosti in fluktuacij iz naših simulacij z rezultati direktne numerične simulacije. Posebej nas je zanimal vpliv kolena na simetrijo toka za kolenom. V ta namen smo vpeljali parameter asimetrije hitrosti, ki smo ga izračunali za vse obravnavane tokove. Primerjali smo odvisnosti parametra asimetrije vzdolž cevi za vse tokove. Analogno smo naredili še s fluktuacijami hitrosti. Primerjali smo odvisnosti parametra asimetrije fluktuacij vzdolž cevi za različne tokove. Propagacijo asimetrije v toku smo nato ocenili še z vpeljavo parametrov asimetrije strižne napetosti na steni cevi in velikosti vrtinčnosti.
Pokazali smo, da turbulentni tok v cevi pozabi na vpliv kolena, ko preteče približno 40 premerov cevi. V laminarnem toku, kjer koleno ustvari turbulenco, pa je ta razdalja precej daljša.In this master thesis the results of turbulent flow simulations in pipe with a bend are presented. Simulations were conducted using OpenFOAM, a code for computational fluid dynamics. We conducted large eddy simulation, however basic idea of direct numerical simulation is also presented. We simulated flows with different Reynolds numbers, namely with 1400, 5600 and 10000. We assumed incompressible flow of Newtonian fluid. We used different computational meshes for flows with different Reynolds number. Quality of meshes and accuracy of simulations was checked by comparing one dimensional profile results from our simulations with results from direct numerical simulations. We were especially interested in effect of the pipe bend on simmetry of the flow after the bend. For that reason we introduced a velocity asymmetry parameter, which we calculated for all considered flows. We compared asymmetry parameter dependency along the pipe for all flows. The same comparison was performed for the velocity fluctuations. We also compared fluctuations asymmetry parameter dependency along pipe for all flows. We then evaluated asymmetry of the flow with introduction of asymmetry parameters of wall shear stress and vorticity magnitude. Lastly we compared results of different parameters
Simulations of two-phase flows with surface tension using Smoothed Particle Hydrodynamics method
Full text linkMetoda SPH (Smoothed Particle Hydrodynamics), oziroma v slovenskem jeziku metoda hidrodinamike zglajenih delcev, je brezmrežna metoda, ki se uporablja za simuliranje problemov v računalniški dinamiki tekočin. Velja za alternativo tradicionalnim metodam, ki temeljijo na reševanju Navier-Stokesovih enačb na mrežah diskretnih točk. Tekočina je v metodi SPH predstavljena kot množica delcev, ki med seboj interagirajo. Približki vrednosti funkcij (hitrost, gostota, tlak) in njenih odvodov so določeni z uporabo funkcije glajenja, ki predstavlja utežno funkcijo delcev za računanje vrednosti teh funkcij v izbrani točki. Enačbe metode zglajenih delcev so Navier-Stokesove enačbe, zapisane za posamezni delec tekočine. Z metodo lahko simuliramo tudi večfazne tokove, pri katerih ima lahko velik vpliv površinska napetost. To magistrsko delo metodo predstavi ter vanjo vključi dva načina vključitve površinske napetosti. Predstavljena sta dva večfazna primera, ki prikazujeta vplive različnih parametrov metode, vpliv vključitve sil površinske napetosti ter zmogljivost metode SPH. Prvi primer je deformacija kvadratne kapljice, drugi pa je Rayleigh-Taylorjeva nestabilnost.The Smoothed Particle Hydrodynamics or SPH method is a meshless numerical method applicable to various problems in computational fluid dynamics (CFD). It is considered as an alternative to the traditional methods based on Navier-Stokes equations solved on meshes of discrete points. In the SPH method, the fluid is discretized with interacting particles. The SPH model consists of the aproximations of function values (velocity, density, pressure) and their derivatives using the smoothing function. Equations of SPH model are Navier-Stokes equations written for a specific particle. The method can also be used in simulations of multiphase flows where surface tension force can have a major impact. In this master thesis, a SPH method with surface tension force model is presented. Capabilities of the SPH method were demonstrated on two multiphase flow problems, where effects of surface tension force and other model parameters are presented. They also prove that SPH method is truly a powerful tool. First problem is deformation of a square droplet and the second one is famous Rayleigh-Taylor instability
Numerical Simulation of Vortex Flow Meter
No full textTa zaključna naloga razišče natančnost numeričnih simulacij vrtinčnih merilnikov pretoka z odprtokodnimi programi. Mreža vrtinčnega merilnika pretoka s posebej zasnovanim zastojnim telesom je bila narejena s programom Salome-Meca, numerične simulacije so bile izvedene s programom OpenFOAM. Simulacije so bile izvedene pri različnih Reynoldsovih številih od 5000 do 50000. Rezultate simulacij smo primerjali s podatki, pridobljenimi s poskusi pri enakih Reynoldsovih številih. Glavni cilj naloge je natančno predvideti frekvenco nastajanja vrtincev in njeno linearno odvisnost z masnim pretokom. Simulacije so bile izvedene z različnimi simulacijskimi metodami, kot so metoda direktne numerične simulacije (ang. Direct Numerical simulation - DNS), metoda velikih vrtincev (ang. Large Eddy Simulation - LES) in metoda Reynoldsovega povprečenja po časovnih intervalih (ang. Unsteady Reynolds Averaged Navier Stokes - URANS). Pri višjih Reynoldsovih številih sta bili metodi DNS in LES računsko preveč zahtevni, zato smo simulacije za Reynoldsova števila nad 35000 izvedli le z metodo URANS.This thesis explores the accuracy of numerical simulations of vortex flow meters using an open-source software. Mesh of a vortex flowmeter with a specially designed bluff body was created with Salome-Meca and numerical simulations were performed with the OpenFOAM code. Simulations were carried out for different Reynolds numbers ranging from 5000 to 50000. Results were then compared to the experimental data at the same Reynolds numbers. The main goal of the study is to accurately predict the vortex shedding frequency and to demonstrate its linear dependency on the mass flow rate. Flow simulations were performed using different simulation techniques such as Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Navier Stokes (URANS). However, at higher Reynolds numbers both DNS and LES simulations are computationally too demanding so only URANS was used at Re above 35000
Large Eddy Simulations of turbulent heat transfer near heated foil
Full text linkNa Odseku za reaktorsko tehniko Inštituta Jožef Stefan se postavlja nov eksperiment v Laboratoriju za termohidravliko večfaznih tokov (THELMA). Namen eksperimenta je študija temperaturnih fluktuacij na greti kovinski foliji, ki je hlajena s turbulentnim tokom. Načrtovanje eksperimenta temelji na numeričnih simulacijah, v katerih smo analizirali vpliv različnih aproksimacij na temperaturne fluktuacije na greti foliji. To delo opisuje rezultate simulacij.
Tok in prenos toplote v novem eksperimentu smo napovedali s simulacijami z metodo velikih vrtincev (ang. Large Eddy Simulation) s pomočjo računalniškega programa OpenFOAM. Prevajanja toplote v foliji nismo modelirali. Folijo smo opisali z idealiziranim robnim pogojem konstantnega toplotnega toka. Simulacije so predpostavljale nestisljiv, polno razvit turbulentni tok Newtonske tekočine z Reynoldsovim številom 10000. V prvih simulacijah je bila dodana temperatura kot pasivni skalar, torej brez povratne zanke na enačbo za ohranitev gibalne količine. Drugi del simulacij pa je upošteval spreminjanje viskoznosti vode s temperaturo ter vzgonske sile. Določili smo ustrezno toplotno moč folije v eksperimentu ter moč, pri kateri je mogoče zanemariti vpliv temperature na materialne lastnosti vode.
Posebej nas je zanimala porazdelitev temperature na vroči foliji v turbulentnem režimu. Z analizo koherentnih termičnih struktur v simulacijah smo uspeli določiti potrebno resolucijo in frekvenco visokohitrostne termične kamere pri danemu Reynoldsovemu in Prandtlovemu številu. V zaključku je predstavljena kratka primerjava naših simulacij s preliminarnimi meritvami temperaturnih fluktuacij na greti foliji.At Reactor Engineering Division of Jožef Stefan Institute a new experiment is being designed in the THELMA laboratory to study temperature fluctuations at the heated foil cooled by turbulent flow. Detailed design of the experiment is based on Computational Fluid Dynamics (CFD) studies which are described in the present work.
The flow and heat transfer in the new experiment predicted with a wall-resolved Large Eddy Simulation (LES) using a wall-adapting local eddy-viscosity (WALE) model in OpenFOAM computer code. Foil was modeled with approximation of the constant heat flux. Simulations assumed incompressible, fully developed turbulent flow of Newtonian fluids with Reynolds number of 10000. In the first part of the study, heat transfer has been included to the study using temperature as a passive scalar, so there is no feedback loop on mass and momentum conservation equation. The second part of the simulations took into account the influence of the temperature on the water viscosity and density. We have identified the power of the foil in the experiment and the maximum foil temperatures at which the water temperature can be described with the passive scalar approximation.
The most relevant result of our simulations were temperature fluctuations on the foil. With observation and analysis of coherent thermal structures in simulations we have predicted the required resolution and frequency needed by thermographic high-speed camera at given Reynolds and Prandtl numbers.
In the Conclusions we present the comparison of our simulations with the preliminary measurements
Direktna numerična simulacija turbulentnega prenosa toplote z metodo spektralnih elementov
Full text linkMeasurement of turbulent flow over backward facing step with Particle Image Velocimetry
Full text linkIzvajali smo meritve hitrostnih polj v toku vode skozi kanal s stopnico. Pri temsmo uporabili metodo slikanja delcev (angleško: Particle Image Velocimetry - PIV).Meritve smo opravljali v prozorni testni sekciji z geometrijskim presekom pravoko-tnika, ki se po stopnici razširi v kvadratni presek. Testna sekcija je bila dolžine1.2 metra, omenjeni eksperiment pa smo izvajali pri toku z Reynoldsovim številomokrog 7100. Za snemanje eksperimenta smo uporabili hitro kamero, sinhroniziranoz laserjem, nato pa smo s komercialnim programom LaVision pognali obdelavo po-snetkov, kjer smo na podlagi dveh zaporednih slik pripravili hitrostne profile toka vrazličnih ravninah z dveh zornih kotov, in jih nato povprečili. Rezultati opravljeniv tem delu so uporabni za primerjavo z direktno numerično simulacijo opravljeno venaki geometrijiLiquid velocity fields in the flow over the backward facing step were measured withthe Particle Image Velocimetry (PIV) technique. A transparent experiment test sec-tion with the geometry of backward facing step flow and the length of 1.2 meter wasdesigned. The experimental campaign described in the present work was performedin turbulent flow with Reynolds numbers 7100. LaVision system with a single highspeed camera and a pulse laser was used for a series of two-dimensional measure-ments of the velocity field at several cross-sections from two different perspectives.The experimental setup and the results presented in this work are expected to beuseful for comparison with accurate numerical simulations
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