On the identification of flow instabilities in a differentially-heated closed cavity: Non-intrusive measurements

Abstract

Transition from laminar to turbulent flow in differentially heated closed cavities has been studied quite extensively using numerical simulations, however, experimental studies are rather limited. In one of the numerical works Henkes and Hoogendoorn (1990), it has been indicated that two different mechanisms simultaneously lead to instabilities in the flow, which, in turn, are identified in the form of two frequencies in flow oscillations. The aim of the present work is to identify and investigate these two mechanisms using non-intrusive experiments. As a definite advancement over the conventional ways of predicting flow instabilities using physical probes, which often lead to undesirable flow perturbation(s), the present measurements have been performed in a complete non-intrusive manner. To achieve this, the experimental test section has been mapped using a laser-based Mach Zehnder interferometer. Interferometry-based results are further corroborated using smoke visualization technique. The experiments are conducted for three Rayleigh numbers in the transition regime. The dynamics of the flow is analyzed near one of the corners of the cavity where instability is likely to appear first. Using spectral analysis of the real-time temperature field, frequencies of oscillations have been identified. It is noted that as one moves inside the boundary layer, the amplitude corresponding to one of the two frequencies becomes quite dominant, which indicates towards the presence of Tollmien-Schlichting (TS) instability. Outside the boundary layer, another frequency, along with the above-mentioned frequency, is observed. This additional frequency is due to the instability caused by the hydraulic jump. With further increment in Rayleigh number, more frequencies are found in the flow, which lead to sudden enhancement in heat transfer from the thermally active walls of the cavity.