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Coherent Structures in the far field of swirling jets

Fig 1: Jet rig at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC). Experimental setup for streamwise and crosswise stereoscopic PIV measurements in the far field of swirling and non-swirling jets.

The objective of this research project is the experimental and analytical investigation of coherent structure in the far-field of swirling and non-swirling jets. Swirling jets can be observed in nature as tornados or hurricanes and are used in various technical applications, e. g. in combustion systems (gas turbines) where the flow field provides beneficial conditions to aerodynamically stabilize the flame. Since swirling jets feature increased entrainment, these flows are often used to improve the cooling of components or to enhance mixing. Non-swirling and swirling jets are also well-known from the outlet flow of jet engines. Due to their versatile practical applications, the near field of both flow configurations has extensively been investigated for many decades. Beside their practical value, they also constitute popular flow configurations in fundamental research due to their basic and unique characteristics.

One of the most important characteristics of turbulent flows is the occurrence of coherent structures. They play a key role in fluid dynamics, as they usually are the most energetic fluctuating structures. Theyare responsible for turbulence production, noise emission, pressure and load fluctuations, mixing of fluids, and many other turbulence phenomena. An accurate state of knowledge about the structures is required in order to manipulate these phenomena by controlling the coherent structures. 

In the far-field of jet flows the accurate determination of coherent structures has proven challenging due their highly three-dimensional dynamics at a wide range of time- and length-scales.In contrast to the near field that often features only one or two dominant structures, the far field flow featuresa multitude of superimposed structures. Their determination requires either high-fidelity flow simulations or detailed measurements with a high spatial and temporal resolution and their extraction is more challenging than in the near field. For this reason, coherent structures in the far-field of swirling flows have never been studied comprehensively.

Fig 2: Extracted coherent structures of mode m=1 from experiments (left) and linear stability analysis (right) at various frequencies

The project is conducted in cooperation with the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University in Melbourne, Australia. Experiments were carried out in the jet rig shown in Fig. 1. Water is used as the working fluid to allow for the acquisition of time-resolved stereoscopic PIV measurements at high Reynolds number. Based on the acquired data, suitable methods for the extraction of coherent structures are assessed and developed. Furthermore, the applicability of linear stability analysis (LSA) on the far field is examined. Key goal of using this analytical tool is the reconstruction of the entire spectrum of large-scale coherent structures that dominate the flow. This analysis is simply based on the mean flow and turbulence profiles that can be obtained from low-cost experiments or numerical simulation. Some results of this approach are shown in Fig. 2 which compares the coherent structures of mode m=1 at various frequencies extracted from the experiment and determined by LSA [1]. 



[1] P. Kuhn, K. Oberleithner, J. Soria. Identification of coherent structures in the far field of a turbulent jet based on spectral proper orthogonal decomposition of time-resolved PIV snapshots. In 19th International Symposium on Applications of Laser and Imaging Techniques to Fluid Mechanics, Lisbon, Portugal, 16-19 July 2018.

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