Research on data assimilation approach of wind turbine airfoils in stall conditions

Abstract

This study aims to analyze the applicability of the turbulence model constants obtained through data assimilation on various airfoils in stall conditions, thereby offering the potential for computational resource savings. Through the wind tunnel experiments and published test data at Reynolds numbers ranging from the order of 10⁵ to 10⁶, the ensemble Kalman filter method was proposed to optimize the constants of the SA (Spalart-Allmaras) model, and its efficacy was validated. The assimilated constants obtained from YA-21 and S809 airfoils were applied separately to other airfoils with similar separation degrees for comparative analysis of their assimilation effects. Based on this, the influence of each model constant on numerical simulation was explored, and the pressure distributions were compared before and after assimilation as well as on other airfoils. Additionally, the impact of variations in airfoil thickness and Reynolds number on assimilated results was investigated. The results suggest that the constants that impact assimilation outcomes appreciably under stall conditions pertain to production and diffusion terms. When the Reynolds numbers are on the order of 10⁵, assimilated constants derived from the 21 % thickness airfoil exhibited optimization effects on the NACA4415 and S809 airfoil, providing a more accurate depiction of separated flow over an airfoil than the original constants conditions. The optimization effect persisted when the Reynolds number reached the order of 10⁶. As the primary factor in the production term, C_b1 became sensitive to changes in Reynolds number exceeding other constants. However, the applicability of thick airfoils is slightly degenerated. Thicker airfoils are more susceptible to changes in the constants of the production and diffusion terms, which makes the assimilated constants need to be applied with caution. These findings demonstrate the feasibility of the mentioned approach, suggesting that assimilated constants from a medium-thickness airfoil can be partially used to replace the self-assimilated constants of other airfoils.