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Öğe Control of flow around a square prism by slot jet injection from the rear surface(ELSEVIER SCIENCE INC, 2010) Akansu, Yahya Erkan; Firat, ErhanThe flow around a square prism with slot jet injection from its base into the vortex formation region has been investigated experimentally for Reynolds number of 8000. For various injection rates, the mean and fluctuating pressure distributions on the square prism were measured and the velocity measurements were carried out by using a hot-wire anemometer. Flow visualization was performed by using the smoke-wire method. It was found that the slot jet injection has a substantial influence on the flow characteristics of the square prism. Increase of the injection ratio up to a certain value causes an important pressure recovery in the wake. For this lower range of blowing rates, blowing fluid tends to fill the wake region around the prism and extend it but the jet does not have enough momentum to penetrate the main vortices. By the further increasing of the blowing rate, the slot jet injection from the base side of the prism splits the near wake like a splitter plate. This causes a thinner wake width and weak vortices resulting in a higher vortex shedding frequency. (C) 2010 Elsevier Inc. All rights reserved.Öğe CONTROL OF FLOW AROUND A SQUARE PRISM BY USING SYNTHETIC JET(TECH UNIV LIBEREC, FAC MECHANICAL ENG, 2010) Akansu, Yahya Erkan; Firat, Erhan; Vit, T; Dancova, PControl of the flow around a square prism by means of synthetic jet through a single slot located on middle of upper surface of the model has been investigated experimentally for Reynolds number of 8000. Synthetic jet was implemented by using a pneumatic piston-cylinder which was driven by means of an electrical motor. The frequency and stroke of the piston were changed to obtain various values of the flow control parameters. For various excitation frequency and momentum coefficients of the synthetic jet, the mean and fluctuating pressure distributions on the square prism were measured. The velocity measurements at the slot exit and at a specific coordinate were conducted by means of constant temperature anemometer (CTA). Vortex shedding frequencies obtained by the spectral analysis of both pressure-time and velocity-time history. At a value of synthetic jet excitation frequency, vortex pairs generated at slot were perturbed the shear layer that separated from the upper leading edge of square model and disturbs it. When the synthetic jet excitation frequency closes enough to the vortex shedding frequency, the lock-in of vortex shedding frequency onto the exciting frequency was occurred. On the other hand, increasing excitation frequency of the synthetic jet causes a pressure recovery on the surface of the prism within the wake region. The efficiency of the synthetic jet is classified depending on change of the drag coefficient and vortex shedding frequency.Öğe Flow past a square prism with an upstream control rod at incidence to uniform stream(PERGAMON-ELSEVIER SCIENCE LTD, 2015) Firat, Erhan; Akansu, Yahya Erkan; Akilli, HuseyinIn the numerical study, it was mainly intended to test the capability of a control rod to reduce the drag and to suppress the fluctuating forces acting on the rod-square (total) system for various angles of incidence (alpha) and center-to-center spacing ratios (L/D). The Reynolds numbers (Re) based upon the diameter of control rod and the side length of the square prism are 50 and 200, respectively, for the control rod and the square prism. Seven distinct flow patterns were observed and it was demonstrated that the cavity flow pattern is the most effective in terms of simultaneous reduction of the time-averaged and RMS values of fluctuating force coefficients for both control rod and square prism. As the control rod located 2D or 3D upstream of the square prism at zero angle of incidence, the time-averaged drag coefficient of the total system i.e. the sum of the time-averaged drag coefficients of the control rod and the square prism is about 74% that of the square prism alone. Furthermore, the maximum reductions in RMS values of the fluctuating lift acting on the total system are 53% and 60%, respectively, for 2D and 3D. On the other hand, the effectiveness of control rod in reducing the time-averaged drag coefficient of the total system was generally diminished with increasing a. Instantaneous and time-averaged flow fields were also presented in order to identify the flow patterns around the rod-square system. (C) 2015 Elsevier Ltd. All rights reserved.Öğe Reduction of fluid forces acting on a square prism using a planar jet(Elsevier Science Inc, 2017) Akansu, Yahya Erkan; Firat, Erhan; Hacialiogullari, MustafaThis experimental study focused on the concurrent minimization of the mean and fluctuating forces acting on a square prism in crossflow by creating a continuous jet through a narrow spanwise slot in it. For this purpose, three different predetermined injection surfaces (i.e., front, top, and rear) were individually studied for the injection ratios (IR) of 0, 1, 1.5, 2, 2.5, and 3 at a Reynolds number (Re) of 8000. The results showed that the rear jet is the optimum injection surface. For the rear jet configuration, the optimum IR is 1.5 in terms of mean total drag coefficient reduction, < C-DT >. In this case, the reductions in the < C-DT > and the level of root mean square (RMS) of fluctuating pressure coefficient (C-p (RMs)) on the side surfaces are about 29.7% and 68%, respectively. However, the maximum reduction in the Cp (RMS) level on the side surfaces occurred at IR = 2. In this case, the reductions in the < C-DT > and the level of C-p (RMS) on the side surfaces are about 27.5% and 88%, respectively. The underlying mechanism of force reduction was also demonstrated. Various flow patterns were identified with respect to IR at incidence angle (alpha) of 0 degrees. It was also proved that, at relatively small angles of incidence, from alpha = 0 degrees to 20 degrees, the active flow control method is still effective in reducing < C-DT > and in suppressing the fluctuating side forces, which is indicative of periodic vortex shedding from either side of the square prism. (C) 2017 Elsevier Inc. All rights reserved.Öğe THE EFFECT OF ATTACK ANGLE TO VORTEX SHEDDING PHENOMENON OF FLOW AROUND A SQUARE PRISM WITH A FLOW CONTROL ROD(TURKISH SOC THERMAL SCIENCES TECHNOLOGY, 2011) Akansu, Yahya Erkan; Ozmert, Mehmet; Firat, ErhanIn this study, the effect of attack angle on vortex shedding phenomena in the case of flow around the square prism as a bluff body and the circular rod which used as flow control component was experimentally investigated. While the value of attack angle equal to 0 degrees, the upstream control rod and square prism are in tandem arrangement to free stream. Experiments were conducted at Reynolds number of 1.0 x 10(4), for the diameter ratio of d/D=0.25, the attack angle between 0 degrees <= alpha <= 90 degrees, and the value of normalized distance between two prism LID is 1, 2, 3, and 4. The velocity measurements were conducted by means of constant temperature anemometer (CTA) as well as vortex shedding frequencies obtained by the spectral analysis of velocity-time history. Characteristics of the vortex formation region and location of flow attachments, reattachments, and separations were observed by means of the smoke-wire flow visualizations. At the angle of attack 0 degrees of the location L/D=2 and 3, the flow structure that possess high vorex shedding frequency which causes remarkable reductions on the drag force and significant increase in the heat transfer was obtained. This flow structure continous to decrease with increasing attack angle up to around 10 degrees. By further increasing of attack angle, the effect of flow control rod on vortex shedding frequency disappears completely.
