Abstract
We propose a new geometric disturbance called a variable pitch helically twisted elliptical (VPHTE) cylinder that was obtained by combining helically twisted elliptic (HTE) and asymmetric geometries for passive flow control. We confirmed that the new geometric disturbance reduces the drag and lift fluctuations compared with the smooth and HTE cylinders at the Reynolds number (Re) of 3000. Large eddy simulation was used for the evaluation of the flow control performance of the VPHTE cylinder at Re = 3000, which corresponds to the subcritical regime. A parametric study was conducted on the variable pitch ratio (VP). The smallest value of the time-averaged drag coefficient of the VPHTE cylinder is lower by about 16.7% and 4.1% than those of the smooth and HTE cylinders, respectively. Furthermore, a VPHTE cylinder reduces the root-mean-square value of the lift coefficient by 96.2% compared with a smooth cylinder. The vortex formation length related to the stabilization of the shear layer of the VPHTE cylinders is longer than those of the smooth and HTE cylinders, and it is consistently elongated as the VP increases. In addition, time-averaged pressure coefficients for different cylinders were compared to analyze the reason for the drag reduction. We systematically investigated the additional flow control performance of the VPHTE cylinders and VP effects by analyzing physical values such as the swirl strength, turbulent kinetic energy, and vorticity components around the VPHTE cylinders.
Highlights
Flow around a bluff body causes flow separation and vortex shedding when the Reynolds number (Re) is larger than a critical value, which induces periodic lift fluctuation and drag force on the structure
Because the helically twisted elliptic (HTE) cylinder already highly suppresses the fluctuation of CL, the discrepancy of values of CL between the HTE cylinder and variable pitch helically twisted elliptical (VPHTE) cylinders cannot be clearly clarified in Fig. 7(b), even if the proposed VPHTE
We proposed applying variable pitch to an HTE cylinder to improve the performance of flow control
Summary
Flow around a bluff body causes flow separation and vortex shedding when the Reynolds number (Re) is larger than a critical value, which induces periodic lift fluctuation and drag force on the structure. Kim et al., Wei et al., and Yoon et al. studied the performances of flow control of the helically twisted elliptical (HTE) cylinder with the structure of a daffodil stem at Re = 100 in the laminar flow regime. Hanke et al. observed the geometrical characteristics of harbor seal vibrissa (HSV) that identify objects in dark water by using experimental and numerical methods for a cylinder with HSV geometry at Re = 500 Their results showed the HSV cylinder reduces the drag coefficient by up to 40% compared with a smooth cylinder with an identical hydraulic diameter. The present study considers the VPHTE geometry as the passive control and investigates its effect on the force coefficients of the drag and lift fluctuation, which is induced by the wake vortices. For the purpose of comparison, we considered the smooth and HTE cylinders at the same Reynolds number of 3000 by employing a large eddy simulation
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