Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection Andreas Westhoff * , Johannes Bosbach, Daniel Schmeling, Claus Wagner German Aerospace Center (DLR) Göttingen, Institute of Aerodynamics and Flow Technology, Bunsenstr. 10, D-37073 Göttingen, Germany article info Article history: Received 7 December 2009 Received in revised form 21 April 2010 Accepted 28 April 2010 Available online xxxx Keywords: Large-scale circulations Particle image velocimetry Proper orthogonal decomposition Turbulent mixed convection Forced convection Low-frequency oscillations abstract The formation and dynamics of large-scale circulations in forced and mixed convection has been studied at ambient and elevated fluid pressure by means of particle image velocimetry and temperature mea- surements. The study has been conducted in two rectangular containers of the same shape and aspect ratios of C xz = 1 and C yz = 5. For the measurements at high fluid pressure the dimensions of the cell have been scaled down by a factor of 5. Air with Pr = 0.7 has been used as fluid in both configurations. Forced convection has been investigated at Re = 1.01  10 4 and mixed convection has been studied at Ar = 3.3, Re = 1.01  10 4 and Ra = 2.4  10 8 . In this configuration low-frequency oscillations in the heat transfer between the inlet and outlet have been found for mixed convection. Instantaneous velocity vector fields obtained from particle image velocimetry have been analysed using proper orthogonal decomposition and an algorithm to detect the core and the core centre position of large-scale circulations. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction In many large-scale convective flows the transport of heat strongly depends on the dynamics of large-scale flow structures. In mixed convection (MC) the heat transfer is further determined by the interaction of forced convection (FC) and thermal convec- tion (TC). Particularly turbulent MC has gained to a lot of interest during the last decades and is of utmost importance, e.g. in geo- physics, astrophysics (Kupka, 2003), indoor climatisation (Costa et al., 2000; Linden, 1999) or in industrial processes and applica- tions (Sillekens et al., 1998). The motivation of the current study is to improve the under- standing of the physical mechanisms which drive the flow struc- ture formation and heat transport in turbulent MC. Our main attention is given to the inquiry of:  How does the flow structure formations depend on the dimen- sionless parameters, particularly the ratio of buoyancy and iner- tia forces?  How do structure and dynamics of the large-scale circulation (LSC) influence the global heat transfer?  How far can the spatial dimensions of MC be scaled by using classical concepts, e.g. by adjustment of fluid pressure and inflow velocity (Westhoff et al., 2007, 2008)? The present study addresses mainly the second issue; the for- mation and dynamics of the LSC and their influence on the heat transfer is studied experimentally in a rectangular container, i.e. a fluid layer, which is heated from below, cooled from above, and further exposed to forced convection under well-defined conditions. This system is characterised by five dimensionless parameters, i.e. the Rayleigh number Ra  DTbgH 3 j/m, the Reynolds number Re  UH/m, the Prandtl number Pr  m/j and the aspect ratios of the rectangular container C xz  W/H and C yz  L/H. Here b denotes the isobaric thermal expansion coefficient, g the acceleration due to gravity, DT the applied temperature difference, j the thermal diffusivity, m the kinematic viscosity, U the characteristic velocity, W the width, L the length, and H the height of the cell. An addi- tional parameter to describe mixed convection is the Archimedes number Ar = Ra/(Re 2  Pr)= DTbgH/U 2 , which is the ratio of buoy- ancy and inertia forces. For Ar  1 the flow is primarily driven by inertia forces, while for Ar  1 the flow is dominated by buoyancy forces. Flows with Ar  1 are termed MC. The limiting case of pure TC is still under heavy investigation. Most of the studies consider the so-called Rayleigh–Bénard con- vection (RBC), where a fluid layer is confined between two hori- zontal parallel plates heated from below and cooled from above. In RBC the transport of heat often leads to the generation of ther- mal ‘‘plumes’’, which are emitted as hot plumes from the bottom thermal boundary layer and as cold plumes from the top thermal boundary layer. In a broad range of Ra these plumes drive LSC structures. The understanding of the motion of the plumes and 0142-727X/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2010.04.013 * Corresponding author. E-mail address: andreas.westhoff@dlr.de (A. Westhoff). International Journal of Heat and Fluid Flow xxx (2010) xxx–xxx Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff ARTICLE IN PRESS Please cite this article in press as: Westhoff, A., et al. Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed con- vection. Int. J. Heat Fluid Flow (2010), doi:10.1016/j.ijheatfluidflow.2010.04.013