## Abstract

We report here the results of a series of careful experiments in turbulent channel flow, using various configurations of blade manipulators suggested as optimal in earlier boundary layer studies. The mass flow in the channel could be held constant to better than 0.1%, and the uncertainties in pressure loss measurements were less than 0.1 mm of water; it was therefore possible to make accurate estimates of the global effects of blade manipulation of a kind that are difficult in boundary layer flows. The flow was fully developed at the station where the blades were mounted, and always relaxed to the same state sufficiently far downstream. It is found that, for a given mass flow, the pressure drop to any station downstream is always higher in the manipulated than in the unmanipulated flow, demonstrating that none of the blade manipulators tried reduces net duct losses. However the net increase in duct losses is less than the drag of the blade even in laminar flow, showing that there is a net reduction in the total skin friction drag experienced by the duct, but this relief is only about 20% of the manipulator drag at most.

### List of symbols

*A, A′*log law constants

*c*chord length of manipulator

*D*drag of the manipulator

- d
*p*/d*x* pressure gradient in the channel

*h*half height of the channel

*H*height of the channel (2h)

*K*log law constant

*L*length of the channel

- L.E.
leading edge of the manipulator

*P*static pressure

*P*_{x}static pressure at a location

*x*on the channel*P*_{xm}static pressure at the location

*x*in the presence of manipulator*p*_{ref}static pressure at any reference location

*x*upstream of the manipulator*Re*Reynolds number

*t*thickness of the manipulator

- T.E.
trailing edge of the manipulator

*u*velocity in the channel

*U*friction velocity

*U*^{*}average velocity in the channel

*u*_{c}centre-line velocity in the channel

*U*_{+}*U/U*_{*}*u*_{m}velocities in the channel downstream of the manipulators

*u*_{ref}velocities in the channel at reference location upstream of the manipulators

*w*Coles's wake function

*W*width of channel

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