Increase of Unsteady Pressure at a Trailing Edge due to Inflow Turbulence

This paper presents new insights into the penetration of the freestream turbulence in a turbulent boundary layer. Experiments evidence an increase in the velocity fluctuations and integral length scale across the boundary layer in addition to an increase of the surface pressure fluctuations close to the airfoil trailing edge. This paper proposes a new empirical model to predict the integral length scale inside the boundary layer at the trailing edge of an airfoil subjected to inflow turbulence. The potential increase of the airfoil trailing noise is discussed. The flow mechanism studied in this paper is relevant to the noise prediction of many applications, including vertical-axis wind turbines subjected to high-inflow turbulence winds, generally observed in urban zones. Experiments performed on a NACA 0012 airfoil subjected to inflow turbulence of 10 and 20% intensity measured the boundary-layer turbulence and the surface pressure fluctuations close to the airfoil trailing edge. Measurements showed that the high-inflow turbulence penetrates the turbulent boundary layer reaching the inner sublayer. A freestream turbulence of 10% increases the velocity fluctuations inside the boundary layer caused by the larger turbulent structures, whereas an inflow of 20% turbulence intensity causes the increase of the velocity fluctuations caused by all turbulent scales. Furthermore, the freestream turbulence significantly increases the surface pressure fluctuations at the trailing edge. For an inflow with 10% turbulence intensity, the surface pressure spectrum increases by more than 6 dB in the entire frequency range. The increase is above 10 dB if the inflow turbulence intensity is 20%.