On the enhancement of boundary layer skin friction by turbulence: an angular momentum approach

TL;DR

This paper introduces an angular momentum integral (AMI) equation to quantify and interpret the effects of turbulence, pressure gradients, and boundary layer growth on boundary layer skin friction, providing a new diagnostic framework.

Contribution

The paper presents a novel AMI equation that isolates skin friction effects and offers a unified approach to analyze boundary layer dynamics across different flow regimes.

Findings

Negative wall-normal velocity limits peak skin friction during transition.

The relative importance of AMI terms becomes independent of transition mechanisms in turbulence.

The AMI framework aids in understanding turbulence impact and flow control on skin friction.

Abstract

Turbulence enhances the wall shear stress in boundary layers, significantly increasing the drag on streamlined bodies. Other flow features such as freestream pressure gradients and streamwise boundary layer growth also strongly influence the skin friction. In this paper, an angular momentum integral (AMI) equation is introduced to quantify these effects by representing them as torques that alter the shape of the mean velocity profile. This approach uniquely isolates the skin friction of a Blasius boundary layer in a single term that depends only on the Reynolds number, so that other torques are interpreted as augmentations relative to the laminar case having the same Reynolds number. The AMI equation for external flows shares this key property with the so-called FIK relation for internal flows [Fukagata et al. 2002, Phys. Fluids, \textbf{14}, L73-L76]. Without a geometrically imposed boundary layer thickness, the length scale in the Reynolds number for the AMI equation may be chosen freely. After a brief demonstration using Falkner-Skan boundary layers, the AMI equation is applied as a diagnostic tool on four transitional and turbulent boundary layer DNS datasets. Regions of negative wall-normal velocity are shown to play a key role in limiting the peak skin friction during the late stages of transition, and the relative strengths of terms in the AMI equation become independent of the transition mechanism a very short distance into the fully-turbulent regime. The AMI equation establishes an intuitive, extensible framework for interpreting the impact of turbulence and flow control strategies on boundary layer skin friction.