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

Journal Article · Tue Apr 12 00:00:00 EDT 2022 · Journal of Fluid Mechanics

DOI:https://doi.org/10.1017/jfm.2022.264· OSTI ID:1866492

Elnahhas, Ahmed ; Johnson, Perry L.

Turbulence enhances the wall shear stress in boundary layers, significantly increasing the drag on streamlined bodies. Other flow features such as free stream pressure gradients and streamwise boundary layer growth also strongly influence the local 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 most relevant to the flow's engineering context, 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. , Phys. Fluids , vol. 14, 2002, pp. 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 direct numerical simulation 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.

• Research Organization: Stanford Univ., CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); National Aeronautics and Space Administration (NASA)
• Grant/Contract Number: NA0002373; NNX15AU93A
• OSTI ID: 1866492
• Alternate ID(s): OSTI ID: 1977731
• Journal Information: Journal of Fluid Mechanics, Journal Name: Journal of Fluid Mechanics Vol. 940; ISSN 0022-1120
• Publisher: Cambridge University Press (CUP)
• Country of Publication: United Kingdom
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
turbulent boundary layers
turbulent transition