You, Rui; Fırat, Erhan

This three-dimensional (3D) numerical study investigates the feasibility of controlling the intermittently reattaching flow (IRF) by using the moving-surface boundary-layer control (MSBC) method. To this end, a fixed square prism was preferred due to its geometric simplicity and was equipped with moving belts that placed side surfaces close to the trailing edges. The effects of three dimensionless key parameters including velocity ratio (i.e., VR, the ratio of the velocity of the moving belt to the freestream velocity), length ratio (i.e., LR, the ratio of the belt length to the edge length of the square prism), and gap ratio (i.e., GR, the ratio of the spanwise distance between the neighboring belts to the edge length of the square prism) mainly on the aerodynamic characteristics were investigated using large-eddy simulation (LES). The findings suggest that the rise in VR and LR parameters leads to a proportional decrease in the time-averaged drag, 〈CD〉, attributable to their contribution to the linear momentum transfer mechanism by the moving surfaces. This mechanism, firstly, provides an additional thrust to the prism in the opposite direction of the main flow. Secondly, this mechanism increases the base pressure by stretching the shear layers from the leading edges of the prism in the streamwise direction. For LR = 0.4 and VR = 4, a 17 % reduction in 〈CD〉 was observed. Additionally, the root-mean-square (RMS) values of fluctuating drag and lift coefficients, CD-RMS and CL-RMS, have decreased by 71 % and 55 %, respectively. Conversely, the increase in the GR parameter led to an increase in the spanwise distance between each individual moving belt and a reduction in the positive effect of the linear momentum transfer mechanism on the 〈CD〉 reduction. The segmentation of moving belts along the spanwise direction also triggered the vorticity redistribution mechanism concurrently. The study also elucidated the underlying mechanisms that led to the failure of the vorticity redistribution mechanism to adequately reduce the 〈CD〉.

İncele

Fırat, Erhan; Oruç, Vedat; You, Rui; Biçer, Uğur; Devecioğlu, Atilla G.

Flow around the cylinder, with/without a pair of airfoils symmetrical to the cylinder's horizontal axis, was simulated for a diameter-based Reynolds number (Re) of 100. The objective of this numerical study is to simultaneously reduce the values of the time-averaged and fluctuating forces acting on a system consisting of a pair of airfoils and a cylinder below those of a single cylinder by using different combinations of the parameters chord (c), angle of rotation (β), angle of attack (α), and the shortest distance between the aerodynamic center of the airfoil and the surface of the cylinder (g). The values of these key parameters are determined primarily through two approaches. The first approach involves emulating the flow pattern surrounding the system to match the potential flow pattern. The second approach involves disrupting the interaction between the shear layers from the cylinder by stretching them in the mean flow direction. The second approach has been demonstrated to exhibit superior performance in terms of reducing both time-averaged and fluctuating forces concurrently. The second approach has been demonstrated to exhibit superior performance in terms of reducing both time-averaged and fluctuating forces concurrently. The optimal geometrical parameters that yield the desired end results are determined to be c = 0.5D, α = 0°, β = 60°, and g = 0.2D. Utilizing these parameters has been demonstrated to result in a substantial reduction in the time-averaged drag, fluctuating drag, and fluctuating lift coefficients of the system, with the respective percentage reductions reaching 14.4 %, 92.1 %, and 73 %, respectively, when compared to the values obtained for a single cylinder.

AtıfScopus4·OpenAlex3

İncele

Akansu, Yahya Erkan; Fırat, Erhan; Hacıalioğulları, Mustafa

This 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, 〈CDT〉. In this case, the reductions in the 〈CDT〉 and the level of root mean square (RMS) of fluctuating pressure coefficient (CP 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 〈CDT〉 and the level of CP 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 (α) of 0°. It was also proved that, at relatively small angles of incidence, from α = 0° to 20°, the active flow control method is still effective in reducing 〈CDT〉 and in suppressing the fluctuating side forces, which is indicative of periodic vortex shedding from either side of the square prism.

AtıfScopus18·WoS18·OpenAlex18

İncele