Delft University of Technology Experimental and numerical study of cavitating flow around a surface mounted semi- circular cylinder Ghahramani, Ebrahim; Jahangir, Saad; Neuhauser, Magdalena; Bourgeois, Sébastien; Poelma, Christian; Bensow, Rickard E. DOI 10.1016/j.ijmultiphaseflow.2019.103191 Publication date 2020 Document Version Final published version Published in International Journal of Multiphase Flow Citation (APA) Ghahramani, E., Jahangir, S., Neuhauser, M., Bourgeois, S., Poelma, C., & Bensow, R. E. (2020). Experimental and numerical study of cavitating flow around a surface mounted semi-circular cylinder. International Journal of Multiphase Flow, 124, [103191]. https://doi.org/10.1016/j.ijmultiphaseflow.2019.103191 Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. 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International Journal of Multiphase Flow 124 (2020) 103191 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow Experimental and numerical study of cavitating flow around a surface mounted semi-circular cylinder ∗ Ebrahim Ghahramani a, , Saad Jahangir b, Magdalena Neuhauser c, Sébastien Bourgeois c, Christian Poelma b, Rickard E. Bensow a a Mechanics and Maritime Sciences, Chalmers University of Technology, Gothenburg, 412 96, Sweden b Department of Process & Energy (Faculty 3mE), Delft University of Technology, 2628 CA Delft, The Netherlands c R&D Department, ANDRITZ Hydro, Vevey 1800, Switzerland a r t i c l e i n f o a b s t r a c t Article history: In this paper, the cavitating flow around a bluff body is studied both experimentally and numerically. The Received 19 June 2019 bluff body has a finite length with semi-circular cross section and is mounted on a surface in the throat Revised 1 November 2019 of a converging-diverging channel. This set-up creates various 3D flow structures around the body, from Accepted 21 December 2019 cavitation inception to super cavities, at high Reynolds numbers ( Re = 5 . 6 × 10 4 − 2 . 2 × 10 5) and low cav- Available online 23 December 2019 itation numbers ( σ = 0 . 56 − 1 . 69 ). Earlier studies have shown this flow to be erosive and the erosion Keywords: pattern varies by changing the flow rate and w/o the cylinder; hence, this study is an attempt to under- Cavitation stand different features of the cavitating flow due to the cylinder effect. In the experiments, high-speed Bluff body imaging is used. Two of the test cases are investigated in more detail through numerical simulations us- High speed imaging ing a homogeneous mixture model. Non-cavitating simulations have also been performed to study the Multiphase flow effect of cavitation on the flow field. Based on the observed results, vortex shedding can have different patterns in cavitating flows. While at higher cavitation numbers the vortices are shed in a cyclic pattern, at very low cavitation numbers large fixed cavities are formed in the wake area. For mid-range cavita- tion numbers a transitional regime is seen in the shedding process. In addition, the vapour structures have a small effect on the flow behaviour for high cavitation numbers, while at lower cavitation numbers they have significant influence on the exerted forces on the bluff body as well as vortical structures and shedding mechanisms. Besides, at very low cavitation numbers, a reverse flow is observed that moves upstream and causes the detachment of the whole cavity from the cylinder. Such a disturbance is not seen in non-cavitating flows. ©2019 Published by Elsevier Ltd. 1. Introduction itation physics is of considerable importance for reliable prediction and control of its occurrence in the design of hydraulic and ma- Cavitation is a phenomenon in which liquid is converted into rine systems as well as its application in biomedical treatment and vapour due to the local drop of fluid pressure below the vapour chemical systems. pressure. It is a common and mostly undesirable event in hydraulic For this purpose, various experimental and numerical studies systems such as pumps, turbines and ship propellers. Cavitation have been performed on simpler geometries and benchmark test can cause noise, load variations, blockage in the machinery and cases to recognize different flow characteristics and parameters system vibrations which may lead to fatigue failure ( Balas et al., that lead to abrupt pressure drop, cavity generation and its vio- 20 06; Van Terwisga, 20 09 ). Also, cavitation erosion may lead to lent collapse. Three types of flow geometries are used frequently material loss and degradation of the system. However, it is a de- as they can be more representative of different fundamental cavi- sirable event in some other situations such as ultrasonic cleaning tation patterns. They include (1) flow over lifting bodies (e.g. Foeth and ultrasonic drug delivery ( Ibsen et al., 2013 ) and mixing two et al., 2008; Asnaghi et al., 2017 ) in which the pressure drop on the or more dissimilar fluids such as in marine diesel engines ( Avellan suction side is the major cause of cavitation; (2) liquid flow inside et al., 1991; Habchi et al., 2014 ). Therefore, understanding the cav- converging-diverging nozzles (e.g. Jahangir et al., 2018; Ganesh et al., 2016 ) where the variations in flow area leads to consider- able variations of velocity and pressure magnitudes, which may be ∗ Corresponding author. accompanied by various cavity patterns ( Jahangir et al., 2018 ); and E-mail address: [email protected] (E. Ghahramani). https://doi.org/10.1016/j.ijmultiphaseflow.2019.103191 0301-9322/© 2019 Published by Elsevier Ltd. 2 E. Ghahramani, S. Jahangir and M. Neuhauser et al. / International Journal of Multiphase Flow 124 (2020) 103191 (3) flow around bluff bodies in which the low pressure wake re- and prior to separation and cavitation inception the flow is at- gion is susceptible to cavity generation. As compared to the first tached to the body surface and in the particular case of the sphere, two groups, the flow around a bluff body is generally more peri- a re-entrant jet can develop that leads to shedding of cavitating odic which makes the measurements more reproducible. Besides vortices, as stated above. Yamagata et al. (2015) studied the Ae- that, in similar testing conditions, cavity structures around a bluff olian tone (sound generated by flow over objects) from a semi- body can be considerably more aggressive and more erosive. The circular cylinder in a uniform flow at different angle of attacks work of Escaler et al. (2003) , for example, shows that mounting and compared the results with a circular cylinder. According to a small bluff body on a hydrofoil induces severe localised erosion, this study, the wake width in the transverse direction for the flat where cavities collapse on a narrow region in the wake of the bluff front semi-circular body is 1.7 D which is considerably wider than body. circular and flat back semi-circular bodies ( width ≈ 1.1 D ), and it The characteristics of the cavitation around a bluff body can be can be explained due to the flow separation at the cylinder edges. highly dependent on the shape of the body. The flow around a cir- Also, the flat front semi-circular case had the highest sound pres- cular cylinder has been the subject of various studies in literature sure level as compared to other angle of attacks and its corre- for both cavitating and non-cavitating conditions as it has broad sponding Strouhal number ( St = 0 . 12 ) was lower than any other engineering applications. Matsudaira et al. (1992) studied the flow body orientation and the circular case ( St = 0 . 19 ), which can be pattern in a cavitating von Kármán vortex street behind a circular related to its wake width ( Yamagata et al., 2015 ). When facing cylinder for different Reynolds and cavitation numbers and found the flow with their flat surface, semi-circular cylinders can sig- that the bubbles frequently collapse when the von Kármán vortex nificantly increase the flow erosiveness. While it is usually tried separates from the cylinder or the attached cavity. By investigat- to avoid cavitation erosion in engineering applications, the bluff ing the cavitation collapse and impact behind a circular cylinder, bodies can be used in the resistance test of different materials Saito and Sato (2003) observed that the von Kármán vortex cavities and coatings as they accelerate the erosion process and it is of are divided into different parts and the structures located near the particular importance to understand possible sources of their ero- channel wall collapses rapidly with a colliding motion towards the siveness. In an experimental work, Escaler et al. (2001) mounted wall. Also, it was seen that the collapses have three different pat- semi-circular obstacles on the suction side of a hydrofoil to alter terns, namely: 3D-radial collapse, 2D-radial collapse and axial col- the flow field in order to accelerate the erosion process.