POD and wavelet analyses on the flow structures of a polymer drag-reducing flow based on DNS data

Direct numerical simulation is carried out to study the inhomogeneous phenomena of polymer molecules in a drag reducing channel flow by polymer additives by using a spring-dumbbell model. Numerical results show that polymer concentration and elongation in buffer layer are highest, and most polymer molecules are almost parallel to the streamwise direction and vertical to the spanwise direction. Due to the elastic effect of polymer the turbulent kinetic energy balance is changed dramatically. Proper orthogonal decomposition method is employed to analyze the velocity and vorticity fluctuation features, particularly to extract the large scale structures from the original data. It is found that in the studied case the diameter of the largest streamwise velocity fluctuation structures of the drag reducing flow becomes double of that of Newtonian fluid flow. Wavelet transformation using Morelet wavelet and 3rd order Daubechies wavelet is applied to analyze the multi-resolution characteristics of velocity components. The wavelet analyses clearly show that energy cascade of the drag reducing flow is greatly suppressed with fewer energy branching.