Transition of the thermal boundary layer and plume over an isothermal section-triangular roof: An experimental study

TL;DR

This experimental study investigates flow transitions and bifurcations in buoyancy-driven thermal boundary layers over an isothermal triangular roof, revealing complex bifurcation routes and the influence of environmental stratification.

Contribution

First experimental observation of flow transition and bifurcation routes over an isothermal triangular roof using phase-shifting interferometry and temperature measurements.

Findings

Flow transitions from laminar to chaotic at high Rayleigh numbers.

Presence of two fundamental frequencies indicating complex bifurcation routes.

Environmental thermal stratification significantly influences flow transition.

Abstract

The development of thermal boundary layers and plume near a section-triangular roof under different isothermal heating conditions have been the focus of numerous numerical studies. However, flow transition in this type of flow has never been observed experimentally. Here, phase-shifting interferometry and thermistor measurements are employed to experimentally observe and quantify the flow transitions in a buoyancy-driven flow over an isothermal section-triangular roof. Visualisation of temperature contours is conducted across a wide range of Rayleigh numbers from laminar at $10^3$ to chaotic state at $4 \times 10^6$. Power spectral density of the temperature measurements reveals the type of bifurcations developing as the Rayleigh number is increased. This flow transition is characterised as a complex bifurcation route with the presence of two fundamental frequencies, a low and a high frequency. We found that the thermal stratification in the environment plays a significant role in the flow transition. The spatial development of flow is also quantitatively and qualitatively described. In addition to clarifying flow transition in experiments, the work demonstrates the implementation of phase-shifting interferometry and punctual temperature measurements for characterisation of near-field flow over heated surface.