Investigation of the factors contributing to skin friction coefficient in a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) flow using direct numerical simulation (DNS)*

DNS is performed to solve the incompressible Navier-Stokes equation for pressure and velocity in a self-similar APG-TBL.² We study the contributions to the skin friction coefficient (C_f) from turbulent fluctuations, viscous effects and pressure gradient. C_f is calculated based on the formulation given by Renard and Deck for boundary layer (BL) flows.³ The non-dimensional pressure gradient (β) is defined as β = δ₁ P_e,x / τ_w where δ₁ is the displacement thickness, P_e,x is the far-field pressure gradient and τ_w is the mean wall shear stress. The C_f decomposition is studied for three different BL flows with β = 0, 1 and 39. With increasing β, C_f approaches zero and the flow becomes more like a free shear layer. It is found that the contribution from viscous effects reduces as β changes from 0 to 39. The Reynolds stress remains the dominant contributor for all β and its contribution to C_f has a peak at y = δ₁ for β = 39 where y is the wall normal direction.