Firat, Erhan; Seyhan, Mehmet; Ozkan, Gokturk M.

Scale-adaptive simulations (SAS) of three-dimensional flow around a circular cylinder fitted with a passive version of a novel flow control method (passive Venturi actuator, PVA) are performed at a diameter-based Reynolds number of Re = 28 000. The PVA consists of one or more narrow slits located at the top and/or bottom sides of the cylinder that connected to the throat of the axial Venturi slit in this cylinder. The main purpose of the study is to investigate the influence of divergence angle and narrow slit location relative to the axial Venturi slit on the aerodynamic performance of the cylinder. To this end, four models were designed with various PVAs. Additional models, a plain cylinder (unmodified model) and a cylinder fitted with an axial Venturi slit (model without a narrow slit), were also used for quantitative comparison. SAS predicts that an additional 5% reduction in the time-averaged drag coefficient, ⟨ C D ⟩ , was observed when two narrow slits located on the surface at an angle of ±80° from the front stagnation line were fitted to the cylinder with an axial Venturi slit. Reducing the divergence angle of the PVA leads to improvements in ⟨ C D ⟩ and root mean square of fluctuating force coefficients, C D − rms and C L − rms . It is found that a cylinder with a PVA that has two narrow slits and a divergence angle of 6° can produce a 28.6% reduction in ⟨ C D ⟩ , a 58.5% reduction in C D − rms , and an 81.2% reduction in C L − rms , when compared to the plain cylinder.

AtıfScopus4·OpenAlex4

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

The main objective of this study is to assess the effectiveness of attached permeable plates in suppressing nominally two-dimensional vortex shedding from a fixed cylinder. The permeable plate along the entire span of the cylinder was made of a chrome-nickel screen and was attached normal to the cylinder surface. The main parameters of the study are porosity of the permeable plate, β, and the plate angle of the cylinder-plate arrangement, θ, with respect to the freestream. Experiments were performed using Particle Image Velocimetry (PIV) and Flow Visualization Techniques. Additionally, drag force measurements were carried out in a wind tunnel to analyze the effects of permeable plates on drag coefficient of the circular cylinder. The results revealed that the use of a permeable plate successfully suppresses the vortex shedding downstream of the circular cylinder by reducing the velocity fluctuations in the wake, elongating the vortex formation region further downstream and attenuating the vortex shedding frequency. The porosity values of β=0.4 and 0.5 with plate angles in the range of 35°≤θ≤90° were found to be effective on suppressing the vortex shedding. Furthermore, both the drag coefficients of the cylinder-plate arrangement and the <TKE>maxwere reduced almost for all porosity ratios for θ<30°.

AtıfScopus63·WoS55·OpenAlex59

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