Numerical Analysis of Flatback Trailing Edge Airfoil to Reduce Noise in Power Generation Cycle

Abstract

Turbo machinery is an essential part in the power generation cycle. However, it is the main source of noise that annoys workers and users, and contributes to environmental problems. Thus, it is important to reduce this noise when operating the power generation cycle. This noise is created by a flow instability on the trailing edge of the rotor blade-an airfoil that becomes a section of the rotor blade of the rotating machine-manufactured as a blunt trailing edge (T.E.), with a round or flatback shape, rather than the ideal sharp T.E. shape, for the purposes of production and durability. This increases the tonal noise and flow-induced vibrations at a low frequency, owing to vortex shedding behind T.E. when compared with a sharp T.E. In order to overcome this problem, the present study investigates the oblique T.E. shape using numerical simulations. In order to do so, flow was simulated using large eddy simulation (LES) and the noise was analyzed by acoustic analogy coupled with the LES result. Once the simulation results were verified using the flatback airfoil measurements of the Sandia National Laboratories, numerical prediction was performed to analyze the flow and the noise characteristics for the airfoils, which were modified to have oblique trailing edge angles of 60 • , 45 • , and 30 •. From the simulation results of the oblique T.E. airfoil, it could be seen that the vortex shedding frequency moves in accordance with the oblique angle and that the vortex shedding noise characteristics change according to the angle, when compared to the flatback T.E. airfoil. Therefore, it is considered that modifying the flatback T.E. airfoil with an appropriate oblique angle can reduce noise and change the tonal frequency to a bandwidth that is suitable for mechanical systems.

Aerodynamic and Acoustic Simulations of Thick Flatback Airfoils Employing High Order DES Methods

“…Despite that, there is only limited knowledge in this area due to the complexity of the case and its difficulty to be computed numerically, even when high fidelity computational fluid dynamics (CFD) codes were employed. [ 16,18–20,23–27 ] Stone et al. [ 23 ] demonstrated that the prediction of drag coefficient varies significantly depending on the employed codes and computational meshes.…”

“…Furthermore, most studies were focused on the force assessments but little effort has been spent for wake analysis. [ 16,18–20,23–27 ] On the other hand, further evaluating the wake flow characteristics is needed to understand the flow physics because the forces acting on the airfoil and the noise emission are dominated by the flow fluctuations in the wake.…”

Effect of Tailing-Edge Thickness on Aerodynamic Noise for Wind Turbine Airfoil

“…Shin et al [32] adopted the LES method to numerically study the noise of turbine machinery with BTE blades. The trailing edge flow field and noise characteristics were analyzed.…”