Control of Laminar Separation Bubbles

In the present paper active separation control of a transitional laminar separation bubble is investigated by means of direct numerical simulations. The specific influence of different forcing amplitudes and disturbance modes on the size and shape of the laminar separation bubble are illustrated. These consist of steady and unsteady twoand three-dimensional Tollmien– Schlichting-(TS)-like boundary layer disturbances which are amplified by instability mechanisms within the laminar boundary layer and in the separation bubble itself leading to an upstream shift of laminar-turbulent transition such that the streamwise extent of the laminar separation bubble is controlled. It is shown that an appropriate selection of the forcing frequency together with a suitable location and length of the disturbance strip for exciting sinusoidal disturbance waves can be found with the help of linear stability theory. The present results demonstrate that unsteady forcing is much more efficient than steady forcing of the flow in order to provoke laminar-turbulent transition to shorten a laminar separation bubble. For an active control method based on these results a criterion to detect the extension of the bubble is investigated as well and several control schemes are suggested and compared to each other. The paper ends with an outlook on their realization possibilities for future applications. 1.0 INTRODUCTION A transitional laminar separation bubble (LSB) is characterized by laminar separation (S) because of an adverse pressure gradient, laminar-turbulent transition (T), and turbulent reattachment (R). Despite the occurrence of negative skin friction within the bubble LSBs may cause an undesired drag rise because of a considerable influence on the global pressure distribution of the airfoil. Unfortunately, their spatial extend is difficult to predict by theory or in wind-tunnel experiments, because of their Reynolds number dependence and their sensitivity to background disturbances which are difficult to take into account. Small environmental changes may have a sudden influence and operating an airfoil at “off-design conditions” can make it prone to LSBs. However, such considerations will become completely obsolete once a suitable separation bubble control becomes available for operational use. The key of LSB control is to control laminar-turbulent transition since an earlier transition will move the re-attachment upstream. Gad-el-Hak [4] has termed this “the easy task of flow control” (compared to turbulent separation control). Mainly because it suffices to provoke laminar-turbulent transition by some appropriate means without an urgent need for some highly sophisticated controller. In fact, a simple switch that turns LSB control on or off when appropriate could be sufficient, once the