surface. I n today’s cost-conscious air transportation industry, fuel costs are a substantial economic concern. Drag reduction is an important way to increase fuel efficiency and reduce these costs. Even a 5% reduction in drag translates into estimated savings of millions of dollars in annual fuel costs. Organized structures that play an important role in turbulence transport may cause large sk...

Many studies in the last decade have revealed that patterns at the microscale can reduce skin drag. Yet, the mechanisms and parameters that control drag reduction, e.g. Reynolds number and pattern geometry, are still unclear. We propose an effective medium representation of the micro-features, that treats the latter as a porous medium, and provides a framework to model turbulent flow over patte...

Turbulence physics responsible for high skin-friction drag in turbulent boundary layers is first reviewed. A self-sustaining process of near-wall turbulence structures is then discussed from the perspective of controlling this process for the purpose of skin-friction drag reduction. After recognizing that key parts of this self-sustaining process are linear, a linear systems approach to boundar...

ABSTRACT The addition of wedge-like fairings onto the side missiles and space launch vehicles, to shield devices such as cameras reaction jet nozzles, creates additional drag, particularly when in supersonic hypersonic freestream flow. An experimental computational study was performed obtain aerodynamic data on simple representative configurations test accuracy theories for drag increment due t...

in this research, the effect of surface roughness and bubble injection on skin friction and hydrodynamic drag force has been numerically considered. this research is based on taylor-couette flow. considered flow includes two concentric cylinders which internal cylinder is movable and external is immovable. the effective equations on the flow includes: equations of momentum, continuous, turbulen...

A detailed investigation of the turbulent boundary layer on a flat plate of finite aspect ratio at zero incidence has been made, incorporating measurements of mean velocity profiles and skin friction over one surface of the plate. Skin friction was measured using Prof. Preston's surface-tube technique with the N.P.L. calibration, and this is justified by comparison with the measured momentum de...

A simple expression is derived of the componential contributions that different dynamical effects make to the frictional drag in turbulent channel, pipe and plane boundary layer flows. The local skin friction can be decomposed into four parts, i.e., laminar, turbulent, inhomogeneous and transient components, the second of which is a weighted integral of the Reynolds stress distribution. It is r...

In recent years a number of embankments were designedwith concrete overtopping protection shaped in a stepped fashion. During large overflows on stepped chutes, there is no skin friction between mainstream and steps, and flow resistance is basically form drag. Alterations of flow recirculation and of fluid exchanges between free-stream and cavity flow are discussed in this study. A comprehensiv...

Two simple feedback control laws for drag reduction are derived by applying a suboptimal control theory to a turbulent channel flow. These new feedback control laws require pressure or shear-stress information only at the wall, and when applied to a turbulent channel flow at Reτ = 110, they result in 16–22% reduction in the skin-friction drag. More practical control laws requiring only the loca...

A new adaptive controller based on a neural network was constructed and applied to turbulent channel ow for drag reduction. A simple control network, which employs blowing and suction at the wall based only on quantities measured at the wall, was shown to reduce the skin friction by as much as 20% in direct numerical simulations of low-Reynolds number turbulent channel ow. Also, a pattern was o...