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

Abstract

Large-scale tomographic particle image velocimetry (tomographic PIV) was used to study large-scale flow structures of turbulent convective air flow in an elongated rectangular convection cell. Three flow cases have been investigated, that is, pure forced convection and mixed convection at two different Archimedes numbers. The Reynolds number was constant at Re = 1.04 9 10 4 for all cases, while the Archimedes numbers were Ar = 2.1 and 3.6 for the mixed convection cases, corresponding to Rayleigh numbers of Ra = 1.6 9 10 8 and 2.8 9 10 8 , respectively. In these investigations, the size of the measurement volume was as large as 840 mm 9 500 mm 9 240 mm. To allow for statistical analysis of the measured instantaneous flow fields, a large number of samples needed to be evaluated. Therefore, an efficient parallel implementation of the tomographic PIV algorithm was developed, which is based on a version of the simultaneous multiplicative reconstruction technique (SMART). Our algorithm distinguishes itself amongst other features by the fact that it does not store any weighting coefficients. The measurement of forced convection reveals an almost two-dimensional roll structure, which is orientated in the longitudinal cell direction. Its mean velocity field exhibits a core line with a wavy shape and a wavelength, which corresponds to the height and depth of the cell. In the instantaneous fields, the core line oscillates around its mean position. Under the influence of thermal buoyancy forces, the global structure of the flow field changes significantly. At lower Archimedes numbers, the resulting roll-like structure is shifted and deformed as compared to pure forced convection. Additionally, the core line oscillates much more strongly around its mean position due to the interaction of the roll structure with the rising hot air. If the Archimedes number is further increased, the roll-like structure breaks up into four counter-rotating convection rolls as a result of the increased influence of buoyancy forces. Moreover, large-scale tomographic PIV reveals that the orientation of these rolls reflects a 'W'-like shape in the horizontal X-Z-plane of the convection cell.

Reversals of coherent structures in turbulent mixed convection

“…This corresponds to the trend of investigating flows with closer connections to engineering problems (Xia 2013). As displayed in figure 1(c), Kühn et al (2012) report that this flow arranges itself in a zigzag-like structure. 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.…”

“…This observed instability is similar to the wavy instability, which Clever & Busse (1991) explain and Pabiou, Mergui & Bénard (2005) . For mixed convection (c), the flow structure aligns in the shape of a zigzag with multiple convection roll segments or LSCs according to Kühn et al (2012). The displayed formation comprises four LSCs spanning between the sidewalls.…”

“…It shows the arrangement of the LED illumination (C), the camera system (D) and the measurement domain highlighted in green. To achieve particle images with sufficient contrast for PIV in a domain of this size, we chose the approach of LED-illuminated helium-filled soap bubbles similar to Kühn et al (2012). Further information on the PIV system and its configuration for the measured cases can be found in table 1.…”

Particle Image Velocimetry for Complex and Turbulent Flows

“…This approach results in significant speedup of a reconstruction by approximately a factor of 8 (Atkinson & Soria 2009). Additionally, hardware-related speedups have been achieved employing computer clusters with parallelized codes (Kühn et al 2012) or using GPU computing. Furthermore, some tomographic reconstruction strategies have been proposed recently that have the potential to reduce the computational load even more.…”

Measurements of the dynamics of thermal plumes in turbulent mixed convection based on combined PIT and PIV

“…Different arrangements of the LSCs are known to occur in the convection sample, e.g. three or four rolls and an alignment in a "W shape" (Kühn et al 2012;Schmeling et al 2013). However, to identify the arrangement based on the mean flow fields yields large uncertainties since the field of view is small.…”