The Effects of Large-Eddy Manipulator Devices on the Turbulent Spot and the Turbulent Boundary Layer

Recent experimental studies indicate that net drag reductions can be achieved in a turbulent boundary layer by placing a tandem configuration of large-eddy manipulator blades in the outer region of the boundary layer. However, the mechanisms responsible for the observed wall-shear reductions are not well understood. Furthermore, discrepancies exist among independent experimental studies regarding the magnitude of the attainable net drag reduction. A fundamental argument is made regarding the source of the observed wall-shear reductions. It is shown that the tandem manipulator is not a low-drag device. The implication is that the momentum deficit in the wake of the manipulator is a prominent contributor to the observed wall-shear reductions, not necessarily that the ability of the large eddies to transport momentum is hampered. The behavior of the wall shear downstream of the device, obtained using hot-film wall-shear sensors, is consistent with the entrainment, mixing, and consequent deceleration of low-momentum fluid from the wake of the manipulator. With the aid of direct measurements of wall shear, an upper bound is placed on the attainable net drag reduction by establishing a lower bound for the device drag. It is concluded that small net reductions (~ 5 percent) may be attained at large downstream distances (≳ 100 boundary-layer thicknesses). This conclusion is consistent with most net drag assessments made independently by others in which the momentum-balance technique was employed. However, the result is not consistent with reports of large net reductions (~ 20 percent) over shorter distances (~ 50 boundary-layer thicknesses). Efforts are also made to explain the observed effects in terms of turbulent structure. The turbulent spot is employed as a prototype structure for the large-scale, organized motions in the turbulent boundary layer. Dramatic wall-shear reductions occur in the region of the spot occupied by the large vortex structure. Such reductions are also evident when the spot is propagating in transitional and fully turbulent ambient boundary layers. Although the transport properties of the vortex structure may be affected by the manipulator, it is proposed that an important source of the wall-shear reductions is the transport by the large vortex of low-momentum fluid in the wake of the manipulator. Some effects of a three-dimensional manipulator are also explored. The design of the device is based on a crude model of the three-dimensional structure of the turbulent spot. Although it appears that the device inhibits the spanwise growth of the spot, its overall effect on the wall shear of the spot and the turbulent boundary layer does not represent an improvement over the effectiveness of a comparable tandem configuration.