Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection

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 measurements. 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 Â 10 4 and mixed convection has been studied at Ar = 3.3, Re = 1.01 Â 10 4 and Ra = 2.4 Â 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.

Large-scale tomographic PIV in forced and mixed convection using a parallel SMART version

“…Outside of the roll, the RMS values increase and are highest in the region of rising hot air. This finding is in a good agreement with the previous studies by Westhoff et al (2010). The highest observed RMS values amount to 0.1 m/s.…”

“…The core line positions obtained, presented in Fig. 5 (top), reveal a wave-like shape with a wavelength of approximately 0.5 m. This observation is in a very good agreement with the results of Schmeling et al (2011) and Westhoff et al (2010), who analysed stereoscopic PIV data of forced convection in the same apparatus at a similar Re. We suspect that the wavy shape is a result of the interaction of the flow with the side walls in cell's X-direction, and thus reflects the longitudinal aspect ratio of the apparatus.…”

Reversals of coherent structures in turbulent mixed convection

“…This means the single longitudinal convection roll (figure 1b), which exists in pure forced convection (Schmeling et al 2011), realigns in the shape of multiple diagonal segments or LSCs caused by the additional buoyancy forces. Furthermore, these LSCs can be observed as counter-rotating circulations in a vertical longitudinal section (Westhoff et al 2010). They then have a similar appearance as the multiple LSCs of pure RBC in the same geometry, see figure 1(a) (Kaczorowski & Wagner 2009;Podvin & Sergent 2012).…”

Experimental investigation of large-scale flow structures in an aircraft cabin mock-up

“…Due to the heat dissipation of the passengers, the thermal boundary condition of the cabin's thermal environment was similar to Poiseuille-Rayleigh-Benard (PRB) mixed convection flows [ 23 ]. From a technical point of view, the dynamics of the large-scale circulation and the effect of spatial distribution on thermal convection are very important [ 24 ]. In addition, the large-scale circulation was constantly fluctuating during the exercise [ 25 ].…”