Abstract
Comparison of the experimental results of turbulent flow structures between a smooth sphere and a sphere with a vent hole, roughened, and o-ring is presented in the presence of a free-surface. Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from point of flow physics for the different sphere locations in the range of 0≤h/D ≤2.0 where h was the space between the top point of the sphere and the free surface. The ratio of ventilation hole to sphere diameter was 0.15, o-ring was located at 55 o with a 2 mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. The flow characteristics of instantaneous velocity vectors, vorticity contours, time-averaged streamline patterns, Reynolds stress correlations and streamwise and cross-stream velocity fluctuations for both the smooth and passively controlled sphere were interpreted.
Highlights
Unsteady flow structure occurs when flow pass a bluff body such as a sphere and the level of unsteady characteristics increases when the sphere is located under the free surface of the flow
Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel
The rate of entrainment due to the high rate of circulatory flow motion of vortical flow structure close to free-stream region for h/D = 0.1 and 0.25 is extremely high which results in causing unstable wake flow structures
Summary
Unsteady flow structure occurs when flow pass a bluff body such as a sphere and the level of unsteady characteristics increases when the sphere is located under the free surface of the flow. Hasanzadeh et al (2012) investigated the flow structure around a sphere beneath free surface for four different submerging elevation such as h/D = 0.25,0.50,1.0 and 2.00 for the Reynolds number of 5000 by using Large Eddy simulation and a dynamic Smagorinsky sub-grid stress. They stated that examination of the obtained results reveals that the rate of interaction between the free-surface and the wake structure downstream of the spherical body is maximum for the case of h/D = 0.25 and by increasing the submerging elevation, the rate of this interaction decreases. Comparison of three different passive control methods is made to emphasize the effect of flow control
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