PROPER ORTHOGONAL DECOMPOSITION: A TOOL TO STUDY THE UNDERLYING PHYSICS OF TURBULENCE

Wall bounded turbulence have been investigated by many authors to understand the underlying physics, experimentally as well as numerically with different techniques and methods. Enormous studies are reported in the literature in the field of turbulence to get the insight of near wall structure in a broad spectrum of Reynolds number. To recognize the contribution of different turbulent scales in a flow a well-known technique, Proper Orthogonal decomposition (POD) is used. Dynamical behaviour of coherent structure in a turbulent channel flow submitted to high temperature gradient is investigated via proper orthogonal decomposition (POD). The turbulent data is generated using thermal large eddy simulation (TLES) of channel flow at Re = 180 for two values of temperature ratio between hot and cold wall. The POD technique is applied to fluctuating part of the velocity to study the temporal evolution of the most energetic modes. It is observed that dominant flow structures are elongated in streamwise direction which further distorted due to the interaction with bean shaped propagating modes. The plotted average energy E(t) as a function of non-dimensional time shows a constant level of fluctuating energy with one little peak at t+ = 1000, all other smaller spikes are showing constant fluctuations about mean line. This behavior at quantitatively describe the role of temperature stratification which stabilizes the roll mode energy and prevent them from breakup into smaller scales thus affects the interaction process during turbulent burst event resulting in a chugging or relaminarizing phenomenon. It is observed that thermal stratification decreases the bursting rate by slowing the breaking mechanism of streamwise elongated modes to larger number of bean shaped modes which results in relaminarization of the flow near hot wall.