| description abstract | Considerable effort has been devoted to the control of natural laminar-turbulent transition using various techniques, e.g., Refs. 1234. Out of all the techniques tested, suction has been found useful not only for delaying transition, but also for postponing separation 56. For example, Gad-el-Hak 6 mentioned the possibility of maintaining laminar flow downstream of a spanwise slot through the combination of massive suction with other various control strategies. Also, Lingwood 7, in his review mentioned the work of Pfenninger and Bacon 8, who studied the control of transition in a laminar boundary layer. They found that suction stabilizes the attachment-line flow at the leading edge of swept wings. The possibility of reducing the skin friction through suction was also addressed 59, and in particular, the application of suction on a turbulent boundary layer through a single narrow porous wall strip or open slit has been widely studied for quite a number of reasons, e.g., see Refs. 9, 1011121314. For example, Merigaud et al., 11, and Pailhais et al. 13 have suggested that the application of suction to the turbulent boundary layer on the fuselage of an airplane may alleviate the likely contamination of the leading edge of a swept wing by the fuselage. Under certain conditions, relaminarization can be achieved immediately downstream of the strip 912. Antonia, Zhu, and Sokolov 9 studied the effect of concentrated wall suction, applied through a short porous wall strip, on a low Reynolds number turbulent boundary layer. They showed that, when the suction rate is sufficiently high, pseudo-relaminarization occurs almost immediately downstream of the suction strip. Further downstream, there is transition followed by a slow return to a fully turbulent state. Further, Oyewola, Djenidi, and Antonia 1215 showed that both the suction rate, σ (≡Vwb/θ0U1, where Vw is the suction velocity, b is the effective width of the strip, θ0 is the momentum thickness of the boundary layer at the leading edge of the porous strip with no suction, and U1 is the free stream velocity), and the momentum thickness Reynolds number, Rθ, played an important role in the relaminarization process. They argued that the ratio Rθ0/σ should not exceed an (as yet undetermined) critical value if relaminarization is to occur. They also suggested that the retransition which follows the relaminarization could be controlled. They proposed that a series of suction strips could produce an effective means for controlling both the relaminarization and re-transition. | |
