Bubbles and drops in the vicinity of turbulent/non-turbulent interface in turbulent boundary layers
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RESEARCH ARTICLE
Bubbles and drops in the vicinity of turbulent/non‑turbulent interface in turbulent boundary layers Di Wu1 · Jinjun Wang1 · Chong Pan1 Received: 22 April 2020 / Revised: 6 October 2020 / Accepted: 10 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The properties of low-vorticity pockets within the turbulent region (named as bubbles) and high-vorticity pockets within the potential flow region (named as drops) in the vicinity of turbulent/non-turbulent (T/NT) interface in turbulent boundary layers are first experimentally investigated using two-dimensional time-resolved particle image velocimetry. The T/NT interface is detected with a turbulent kinetic energy (TKE) criterion. The bubbles and drops which enlarge the T/NT interface area will promote the entrainment process. A detailed study revealed that the T/NT interface above bubbles is generally higher than the mean interface position, whereas the T/NT interface below drops is lower. Due to different sizes of bubbles and drops, though the temporal probability of bubbles is much larger than drops, the spatial probability is similar. Moreover, the conditional mean velocity distribution and the convection speed of the bubbles and drops are first obtained, especially in experiments. The streamwise velocity of fluids in bubbles at different Reynolds number is larger than surrounding turbulent flow, while in drops, the streamwise velocity is smaller than surrounding free-stream. The conditional mean streamwise velocity difference can be well scaled with respective friction velocity. Besides, bubbles and drops will convect downstream with the velocity of surrounding fluids. Graphic abstract
1 Introduction * Jinjun Wang [email protected] 1
Key Laboratory of Fluid Mechanics (Ministry of Education), Beijing University of Aeronautics and Astronautics, Beijing 100191, China
The turbulent/non-turbulent (T/NT) interface is an irregular boundary between turbulent flow and irrotational flow, which exists in many kinds of flow fields such as combustion flame front, turbulent patches in atmosphere and ocean, impinging jets, turbulent boundary layers, etc. Due to the
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importance in mixing and entrainment process, the properties of T/NT interface have received vast attention (Corrsin and Kistler 1955; Townsend 1976; Jimenez et al. 2010; da Silva et al. 2011; Gampert et al. 2014; Zhou and Vassilicos 2017). In the investigation of geometrical properties, it has been found that the T/NT interface consists of two layers, the viscous superlayer and the turbulent sublayer (da Silva et al. 2014a, b; Watanabe et al. 2016). The distribution of the interface height agrees with the Gaussian function, while the intermittency profile fits with an error function (Chauhan et al. 2014a). T/NT interface shows fractal-like properties with the fractal dimension of Df ≈ 2.3 (de Silva et al. 2013). As for the dynamical properties, many previous studies in literatures focus on the velocity jump and
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