Reynolds Number Scalability for Separation Control on a Laminar Airfoil

Wind tunnel tests are performed on a NACA 643-618 airfoil over a Reynolds number range of 0.06-4.0x10 in order to study several aspects of a laminar airfoil. Studies of blowing flow control and the effect of Reynolds number are the major topics of this effort. The tools used for investigation are surface pressure measurements for lift and wake surveys for drag. Preliminary testing at Re = 64,000 determined that four distinct flow regimes exist with respect to angle of attack: weak laminar separation, moderate laminar separation, laminar separation bubble, and strong leading edge laminar separation. A portion of the study investigates the cause of such dynamic flow physics. Attempts are then made to employ blowing to induce or imitate the laminar separation bubble. By creating the laminar separation bubble, significant lift increase and drag reduction are realized over a broader range of angles of attack. Normal, steady blowing is used because it is a well-characterized device that limits the number of parameters to be varied. Lift is increased significantly and separation is delayed for most cases, where attempts are made to describe the physical mechanisms that induce change. It is observed that the optimal blowing ratio changes between angle of attack regimes because different flow physics are required to induce a change. Studies of Reynolds number scaling found that the lift increased and drag decreased as Reynolds number increased. Importantly noted is that the laminar separation bubble becomes naturally effective at most angles of attack by Re = 180,000. Therefore, the value of flow control diminishes except in regions where strong leading edge separation is the limiting element of the airfoil. This research suggests that the laminar airfoil can be controlled in an energy efficient manner such that high performance is gained across all flight regimes with straightforward actuation.