24 th Annual International Conference on Mechanical Engineering-ISME2016 26-28 April, 2016, Yazd University, Yazd, Iran 1 ISME2016-97228 Experimental Investigation of Flow Past an Elliptic Cylinder near a Plane wall Mohammad Javad Ezadi Yazdi 1 , Abdulamir Bak Khoshnevis 2 , Amir Reza Neshat 3 , Sajad Nazari 4 1 MSc student, Department of Mechanical Engineering, Hakim Sabzevari University; javadezadi2014@gmail.com 2 Associate Professor, Department of Mechanical Engineering, Hakim Sabzevari University; khosh1966@yahoo.com 3 MSc student, Department of Mechanical Engineering, Hakim Sabzevari University; amirne66at@gmail.com 4 Msc student, Department of Mechanical Engineering, Hakim Sabzevari University; pixy64@gmail.com Abstract The flow characteristics around an elliptic cylinder with an axis ratio of AR=2 located near a flat plate were investigated experimentally. The elliptic cylinder was located on the inside and outside a turbulent boundary layer region whose thickness (δ) is 0.38B. The Reynolds numbers based on the height of the cylinder cross- section were 15000 and 30000. The wake velocity profiles and turbulence intensities behind the cylinder were measured using hot-wire anemometry. The ground effect of the cylinder at a small gap ratio constrains the flow lower side of the cylinder wake. This constraint effect is more severe for the Re=15000, compared to the Re=30000. The wake region behind the elliptic cylinder is relatively small and the velocity profiles tend to approach rapidly to those of a flat plate boundary layer. Keywords: Wake-boundary layer interactions, Elliptic cylinder, Hot-wire anemometry, vortex shedding. Introduction Flow around bluff bodies has been widely investigated for their own academic interest as well as wide practical applications. Especially, the effect of a plane wall proximity on the flow around a circular cylinder has been widely investigated [1-11], as it represents a fundamental case of study as well as a case of great interest to some technological applications such as pipelines, marine structures and heat exchangers. The most relevant results found in the literature are summarized below. Zdravkovich [9] stated that the flow dynamics around a circular cylinder ''under'' the ground effect can be classified into three regions; (i) far region where the flow is not affected by the presence of the wall, (ii) intermediate region in which the wall effect starts to be seen and the vortex shedding becomes intermittent and (iii) small region where the vortex shedding vanishes. Price et al. [2] studied the effect of the gap-to- diameter ratio on the flow around a circular cylinder near a plane wall using flow visualization, particle image velocimetry, PIV, and hot-film anemometry for Reynolds numbers, based on cylinder diameter, between 1200 and 4960. They showed that the flow may be characterized by four distinct regions: (a) for very small gaps, G/D ≤ 0.125, the gap flow is suppressed or extremely weak, and separation of the boundary layer occurs both upstream and downstream of the cylinder. Although there is no regular vortex shedding, there is a periodicity associated with the outer shear-layer; (b) In the ''small gap ratio'' region, 0.125 < G/D < 0.5, the flow is very similar to that for very small gaps, except that there is now a pronounced pairing between the inner shear-layer shed from the cylinder and the wall boundary layer; (c) Intermediate gap ratios, 0.5 < G/D < 0.75, are characterized by the onset of vortex shedding from the cylinder; (d) For the fourth region, characterized by the largest gap ratios considered, G/D>1.0, there is regular vortex shedding and there is no separation of the wall boundary layer, neither upstream nor downstream of the cylinder. Using a moving wall, Nishino et al. [3] eliminated the effect of the boundary layer that develops on the plane wall. They performed PIV, forces measurements and flow visualization for Reynolds numbers of 0.4 × 10 5 and 1.0 × 10 5 with and without end-plates. In the case of the cylinder with end-plates, they observed a cessation of the wake for G/D < 0.3, according to the plots of the drag force on the cylinder. They explained this vortex cessation by a change in the instability mechanism that produces the wake vortices for G/D < 0.3 and they suggested that a very thin boundary layer still locally existed on the moving ground in this study and hence it might still be possible to argue that this thin boundary layer generates vorticity of opposite sign to interfere with the separated shear layer from the bottom side of the cylinder (just like the cases with a thick boundary layer formed on a fixed ground). In a recent study, Lin et al. [4] used flow visualization, PIV and FLDV measurements to study the flow characteristics of a circular cylinder placed horizontally above a plane boundary for Reynolds number ranging from 7.8 × 10 2 to 1.15 × 10 4 . For G/D < 0.5 regular alternate vortex shedding was suppressed and only vortices shed from the upper shear layer of the cylinder have been detected. Randomly switching gap flow was found and first put forward to be the main reason of multipeak or broadband spectral characteristics of the shedding event at a certain small gap ratios. It is also found that the streamwise velocity profiles of the upper shear layer, where periodic shedding eddies originate, exhibit well-behaved similarity. The interaction between the vortices shed from the upper shear layer and the jet-like flow for G/D < 0.3 still needs more investigation. A number of studies were performed in order to separate the effect of flow parameters for the better understanding of the changes in the flow regimes with