Abstract
Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.
Highlights
Since Walsh [1] and his team took the lead in the study of sharkskin and applied its surface structure to plates in the 1970s, many scholars have studied riblets, which are bionic surfaces, over the last several decades
In order to introduce riblets into the blade surface of a low-pressure fan, riblets with multiple lengths and heights are applied to a NACA4412 airfoil at the Reynolds number of 1 × 105
The amount of drag reduction considerably varies with the length and height of the riblet and the AOA of the airfoil flow
Summary
Since Walsh [1] and his team took the lead in the study of sharkskin and applied its surface structure to plates in the 1970s, many scholars have studied riblets, which are bionic surfaces, over the last several decades. Via research on flat plates, scholars found that the drag reduction of riblets is achieved by reducing the intensity of the turbulent boundary layer and the amplitude of Reynolds shear stress and vorticity fluctuations [15,16]. Wang et al [23], simulated the drag reduction capacity of riblets on the airfoil of a centrifugal fan blade and obtained a maximum value of 9.65%. A discussion of the influence of the sizes and placements of riblets on airfoil flow and resistance under different AOA conditions is important. The Reynolds number of the airfoil flow is 1 × 105 , under which the blades of low-pressure fans usually operate. The airfoil applying riblets with different lengths and groove widths are meshed by self-coded algebraic grids, and the corresponding flow fields are calculated by steady numerical simulation. The effect of riblets and their mechanisms on aerodynamic performance at various angles of attack (AOAs) are analyzed in detail
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