Firat, Erhan; Ozkan, Gokturk M.; Akilli, Huseyin

Abstract: The flow over fixed, hollow, rigid circular cylinders with and without two series of equally spaced holes of equal diameter (0.2 cylinder diameter) along the surface of the cylinder has been studied by means of dye injection flow visualization, particle image velocimetry (PIV), and force measurements. Holes are connecting the front and rear stagnation lines or the upper and lower sides (i.e., they are positioned at ± 90° from the stagnation line) of the cylinder. The center-to-center distance between holes was 7-hole diameter. The experimental data were obtained from water channel and wind tunnel measurements performed at Reynolds numbers of 6900 and 20000, respectively. It was found that cylinder with holes located on the surface at an angle ± 90° from the front stagnation line behaves like a double-chamber synthetic jet actuator when subjected to cross-flow. The interaction of synthetic jet arrays with external flow over the side surfaces instigates the three-dimensional instabilities in the near wake and produces a 14.4% drag reduction compared to the drag of a plain cylinder. It was shown that cylinder with holes located on the surface at angles of 0° and 180° from the front stagnation line also instigates the three-dimensional instabilities in the near wake, but the primary mechanism responsible for drag reduction is self-bleed through downstream holes. This system producing drag reduction was found to have a drag coefficient equal to about 78.9% of that for a plain cylinder. Important flow structures inside and behind the cylinders with holes were also identified. Graphic abstract: [Figure not available: see fulltext.].

AtıfScopus17·WoS17·OpenAlex16

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Firat, Erhan; Ozkan, Gokturk M.; Akilli, Huseyin

The wake flows behind fixed, hollow, rigid circular cylinders with two rows of holes connecting the front and rear stagnation lines were investigated using particle image velocimetry (PIV) for various combinations of three hole diameters, d = 0.1D, 0.15D, and 0.20D, six hole-to-hole distances, l = 2d, 3d, 4d, 5d, 6d, and 7d, and ten angles of incidence (α), from 0° to 45° in steps of 5°, at a Reynolds number of Re = 6,900. Time-averaged velocity distributions, instantaneous and time-averaged vorticity patterns, time-averaged streamline topology, and hot spots of turbulent kinetic energy occurred through the interaction of shear layers from the models were presented to show how the wake flow was modified by the presence of the self-issuing jets with various momentums emanating from the downstream holes. In general, as hole diameter which is directly related to jet momentum increased, the values of time-averaged wake characteristics (length of time-averaged recirculation region, vortex formation length, length of shear layers, and gap between the shear layers) increased. Irrespective to d and l tested, the values of the vortex formation length of the models are greater than that of the cylinder without hole (reference model). That is, vortex formation process was shifted downstream by aid of jets. It was found that time-averaged wake characteristics were very sensitive to α. As α increased, the variation of these characteristics can be modeled by exponential decay functions. The effect of l on the three-dimensional vortex shedding patterns in the near wake of the models was also discussed.

AtıfScopus26·WoS25·OpenAlex28

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