13th International Symposium on Particle Image Velocimetry – ISPIV 2019 Munich, Germany, July 22-24, 2019 Transitional flow in a 90° pipe bend Johannes Burkert 1* ,R¨ udiger Schwarze 1 , Katrin Bauer 1 1 TU Bergakademie Freiberg, Institute of Mechanics and Fluid Dynamics/Fluid Dynamics and Turbomachinery, Freiberg, Germany * johannes.burkert@imfd.tu-freiberg.de Abstract The transitional flow in pipe bend flows has not been well explored in contrast to transitional flow in straight pipes. During the last decade the interest in understanding the flow development in pipe bends for different Reynolds numbers Re has increased. In order to fill the gap especially for flow transition, a new test rig which covers a range of 500 < Re < 7000 has been designed and manufactured. Initial high speed particle image velocimetry (PIV) measurements were carried out in order to test the new set-up and to characterize the evolving flow structures in the relevant Reynolds number range. For this purpose the measurements were concentrated on the stream wise flow of the bend and upstream the bends outlet region. The results show that the test rig is operating and that it does not distort the flow development. 1 Introduction Transitional flow in a straight pipe has been well explored and understood up to date. Most importantly, Hof et al. (2004) observed nonlinear traveling waves denoting the onset of turbulent flow. Further work on the transition in pipe flow was done by Eckhardt et al. (2007) and Van Doorne and Westerweel (2007) who determine the transition process as a mostly chaotic process which is dominated by nonlinear traveling waves. During transition hairpin like structures grow up and become big vortex areas. The onset of this process is not periodically what is characteristic for the transition from laminar to turbulent flow. However, the transitional behavior of flow in pipe bends is still not fully understood. It is known that at above a critical value of the Dean number (De > 10) the so called Dean vortices develop as a pair of sec- ondary vortices Dean (1927); Fresconi and Prasad (2007). For higher Reynolds numbers (Re = 50000 - 200000) in the fully turbulent flow regime, Tunstall and Harvey (1968) first found additional swirl super- posed to the Dean vortices downstream the pipe bend. The swirl orientation changes periodically and is referred to as swirl switching, probably caused by flow separation (Tunstall and Harvey (1968)). Thirty years later, Br¨ ucker (1998) observed a characteristic frequency for the swirl switching which identifies this phenomenon as a combination of low and high frequency flow structures. Recent studies e.g. by Huf- nagel et al. (2017) in a similar Reynolds number-range confirmed these characteristic frequencies found by Br¨ ucker (1998). The origin of the swirl switching is supposed directly in the bend itself and may be caused by shear layer instabilities (R ¨ utten et al. (2005)). Moreover, the phenomenon is not a movement of the Dean cells but a change between two distinct states. In other words, one Dean cell is suppressing the other cell in a periodic manner (R ¨ utten et al. (2005)). Kalpakli et al. (2010); Kalpakli and ¨ Orl¨ u (2013); Kalpakli et al. (2015, 2016) together with Carlsson et al. (2015) have done several experimental as well numerical work on the swirl switching process for different Reynolds numbers. They found the same range of frequencies to be an identification for the swirl switching and that the curvature ratio has the most impact besides the Reynolds number on the switching intensity. Noorani and Schlatter (2016) found the swirl switching process in a closed toroidal pipe, too. However, up to date the onset or rather the responsible structures of swirl switching has not been fully understood. Moreover, the development of characteristic flow in 90° pipe bends during laminar-turbulent transition has to the best of our knowledge not been investigated experimentally, yet.