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# On the Impact of the Aspect Ratio on the Formation of Large-Scale Structures in Turbulent Mixed Convection in a Cuboidal Sample

Wessels, Malte and Schmeling, Daniel and Bosbach, Johannes and Wagner, Claus (2019) On the Impact of the Aspect Ratio on the Formation of Large-Scale Structures in Turbulent Mixed Convection in a Cuboidal Sample. International Journal of Heat and Fluid Flow, 76, pp. 231-241. Elsevier. doi: 10.1016/j.ijheatfluidflow.2019.01.004. ISBN 0142-727X. ISSN 0142-727X.

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## Abstract

Temperature measurements combined with Particle Image Velocimetry (PIV) were performed in a cuboidal convection sample with a square cross section (width / height). The objective of the study is to analyze the influence of the aspect ratio $\Gamma_{xz}=L/H$ and the Archimedes number $Ar$ on the large-scale coherent structures in mixed convection. Air at ambient pressure was used as working fluid ($Pr=0.7$). The aspect ratio was varied between $1\leq\Gamma_{xz}\leq4$ (length / height) and the variation of the Archimedes number was achieved by prescribing the inflow velocity of an air jet, which was superimposed on the thermally induced flow via an inlet and outlet channel. The Rayleigh number was kept constant at $Ra=2.0\times10^8$. A model, adopted from pure thermal convection, was used to extract the number of convection rolls from the circumferential temperature distribution measured along the sidewalls of the convection sample. For a fixed Archimedes number ($Ar\approx4$), the number of thermally induced large-scale circulations (LSC$_\mathrm{TC}$) increased with the aspect ratio of the sample. Specifically, starting with $\Gamma = 2$, a linear dependence with unity slope was found. A decrease of the Archimedes number led to a transition from the single LSC$_\mathrm{TC}$ structures to FC-dominated LSCs (LSC$_\mathrm{FC}$) for $\Gamma=1$ and $\Gamma=2$, whereas the multi-roll structures found for $\Gamma=3$ and $\Gamma=4$ were more stable even for higher inflow velocities.

Item URL in elib:https://elib.dlr.de/119227/
Document Type:Article
Title:On the Impact of the Aspect Ratio on the Formation of Large-Scale Structures in Turbulent Mixed Convection in a Cuboidal Sample
Authors:
AuthorsInstitution or Email of AuthorsAuthor's ORCID iD
Wessels, MalteMalte.Wessels (at) dlr.deUNSPECIFIED
Schmeling, DanielDaniel.Schmeling (at) dlr.dehttps://orcid.org/0000-0003-2712-9974
Bosbach, JohannesJohannes.Bosbach (at) dlr.dehttps://orcid.org/0000-0002-1531-127X
Wagner, Clausclaus.wagner (at) dlr.deUNSPECIFIED
Date:2 March 2019
Journal or Publication Title:International Journal of Heat and Fluid Flow
Refereed publication:Yes
Open Access:No
Gold Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:76
DOI :10.1016/j.ijheatfluidflow.2019.01.004
Page Range:pp. 231-241
Publisher:Elsevier
ISSN:0142-727X
ISBN:0142-727X
Status:Published
Keywords:Mixed Convection, Large-Scale Circulations, Aspect Ratio Dependence, PIV, Temperature Probes
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Transport
HGF - Program Themes:Terrestrial Vehicles (old)
DLR - Research area:Transport
DLR - Program:V BF - Bodengebundene Fahrzeuge
DLR - Research theme (Project):V - Next Generation Train III (old)
Location: Göttingen
Institutes and Institutions:Institute for Aerodynamics and Flow Technology > Ground Vehicles
Institute for Aerodynamics and Flow Technology > Experimental Methods, GO
Deposited By: Schmeling, Dr Daniel
Deposited On:04 Mar 2019 13:02