Airfoil gust load alleviation by circulation control

This paper presents a numerical investigation on the gust load alleviation effects based on circulation control (CC) via blowing over airfoil trailing-edge Coanda surface. The NACA0012 airfoil was chosen for the study from subsonic to transonic speeds. The field velocity method is introduced to the unsteady Reynolds averaged Navier-Stokes solutions to simulate responses of the airfoil to arbitrarily shaped gust penetrations. For validation, gust responses to a step change in angle of attack, sharp-edged gusts and one-minus-cosine gusts from the numerical solutions are compared against available theories and other numerical references. Experimental data are used to validate the numerical results of CC. Thereafter circulation control via steady blowing with different momentum coefficients are tested for the gust load alleviation effects in terms of lift coefficients. The results show that circulation control can effectively suppress the lift disturbances caused by the gust for subsonic flows. Most importantly, CC has a fast frequency response characteristic as more than 50% of the total change in lift coefficient can be obtained within the non-dimensional time s = U∞t/b = 1. Based on the rapid response characteristic, circulation control via unsteady blowing with dynamically adaptive momentum coefficients proportional to the vertical gust velocities is proposed and tested. The results demonstrate that a near constant lift coefficient can be achieved under gust condition for subsonic incoming flow indicating its potential for real-time adaptive load control. It has been demonstrated that CC is also able to reduce gust load at transonic speed, but it is less effective as compared with that at subsonic speed.