MODIFICATION OF LARGE EDDIES IN TURBULENT BOUNDARY-LAYERS

Turbulent boundary layers were altered with a tandem array of manipulators arranged to produce a maximum drag reduction. The Reynolds number based on the momentum thickness, Reθ, at the first manipulator position was between 1700 and 2400. Temperature was used as a passive contaminant to explore the dynamical relationship between the near-wall and outer regions of the manipulated layer. Heat was introduced by warming the wall uniformly to 15 °C above the ambient temperature or with a line heater in the wake of the manipulator. Temperature and velocity measurements showed a reduction in fluctuation amplitude and a strong decrease in larger scale mixing accompanied by a reduction of the Taylor microscale and integral lengthscale. Isocorrelations indicated that the eddy size was decreased in all three directions. The net result of the manipulators was a marked decrease in the entrainment of irrotational fluid into the boundary layer. The results suggest that the manipulators do not directly affect the wall region but rather decrease the entrainment, and hence the growth of the boundary layer, leading to possible drag reduction. © 1990, Cambridge University Press. All rights reserved.