Flow, Turbulence and Combustion vol:91 issue:1 pages:63-77
Large scale periodic structures can exist in selected flow fields. Examples are the Precessing Vortex Core in swirling flows, vortex shedding behind a cylinder or the wake of an annular jet. A number of techniques are available to extract these large scales from the turbulent fluctuations in the flow field. In this paper, an analysis is made of three such methods: Eulerian Time Filtering (ETF), Proper Orthogonal Decomposition (POD) and non-linear least-squares regression POD (NLSR-POD). The accuracy of the three different extraction methods is compared quantitatively with phase averaged data of an annular wake flow. This flow was chosen as a test case, since it is widely used in industrial applications, such as for example bluff-body burners. It was shown that all three methods were able to reconstruct the flow field with reasonable accuracy. These techniques are therefore applicable to a number of periodic flows. The big advantage of these extraction methods is that they require 20 times less experimental data compared to phase averaging. All three methods require more or less the same computational time and since the computational time is a few orders of magnitude lower than the measurement time, application of these techniques results in a very large reduction in the total time to obtain the flow field characteristics. This results in a significant reduction of time in the design process of such flows.