Control of Sublayer Streaks Using Microjet Actuators

A combination of numerical techniques is employed to model the interaction of a microjet actuator and the near-wall coherent structures found in turbulent boundary layers. These streak-like flow structures are modeled by using an analogy between them and Klebanoff modes. The Klebanoff mode can be modeled by seeking an optimum response within a Blasius boundary layer to a freestream streamwise vorticity source generated by an applied body force. In a similar way, except that a turbulent mean-velocity profile is used and the streamwise vorticity source is now located within the boundary layer, sublayer streaks can be generated. The results of these simulations agree well with previous experimental and numerical observations of streak dynamics and structure. The linear models of the streak and boundary layer are combined to simulate the basic realization of streak control using microjet actuation. It is concluded that to increase turbulence production, low-speed streaks should be targeted, and to decrease it high-speed streaks should be targeted. Spanwise and streamwise arrays of actuators are studied and found to be more effective than isolated actuators. In particular, a streamwise array of smaller actuators is found to be much more effective than a single larger actuator producing the same mass flow as the actuator array. A coupled simulation of the actuator, streak, and boundary layer is performed. The intensification of a low-speed streak is demonstrated. The pressure footprint of the convecting streak can have a significant influence on the output velocity of a microjet. This suggests that the weak microjets might respond violently and unpredictably to the nondeterministic pressure fluctuations of the turbulent boundary layer.