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Drag01:23

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Drag is a resistive force opposing an object’s motion through a fluid, resulting from surface pressure and shear forces. It comprises two components: a perpendicular one from pressure and a tangential one from shear stress. Accurate drag calculations use pressure and wall shear stress distributions, often determined through Computational Fluid Dynamics (CFD) or wind tunnel testing. The drag coefficient, a dimensionless measure, depends on factors like shape, Reynolds number, Mach number,...
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Turbulent flow is characterized by unpredictable fluctuations in velocity and pressure, which result in a chaotic fluid movement distinct from the orderly patterns of laminar flow. While laminar flow is governed by smooth, parallel layers with minimal mixing, turbulent flow exhibits highly irregular, three-dimensional patterns. This behavior arises due to instabilities in the fluid's velocity profile, and amplifies as the flow velocity increases. Minor disturbances, known as turbulent...
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Updated: Feb 7, 2026

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions
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Turbulent Drag Reduction Using Anisotropic Permeable Substrates.

G Gómez-de-Segura1, A Sharma1, R García-Mayoral1

  • 1Department of Engineering, University of Cambridge, Trumpigton St, Cambridge, CB2 1PZ UK.

Flow, Turbulence and Combustion
|August 3, 2018
PubMed
Summary

Turbulent flow over permeable substrates can reduce drag, but wall-normal permeability may cause drag increase. Wavelength-dependent transpiration in realistic substrates mitigates this drag increase by inhibiting large spanwise structures.

Keywords:
DNSDrag reductionKelvin-HelmholtzPermeable substrates

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Area of Science:

  • Fluid dynamics
  • Turbulence research
  • Material science

Background:

  • Turbulent flow over surfaces is crucial in many engineering applications.
  • Permeable substrates offer potential for drag reduction through streamwise slip.
  • Anisotropic properties of substrates can influence flow behavior.

Purpose of the Study:

  • To investigate turbulent flow behavior over anisotropic permeable substrates.
  • To analyze the interplay between drag reduction and drag increase mechanisms.
  • To understand the role of wall-normal permeability and transpiration.

Main Methods:

  • Utilizing linear stability analysis and direct numerical simulations (DNS).
  • Modeling flow within the substrate using the Brinkman equation.
  • Deriving analytical boundary conditions at the substrate-channel interface.

Main Results:

  • Preferential streamwise slip on permeable substrates can reduce drag.
  • Wall-normal permeability can offset drag reduction by promoting spanwise structures (Kelvin-Helmholtz-like instability).
  • Wavelength-dependent transpiration in realistic substrates inhibits these drag-increasing structures.

Conclusions:

  • Wall-normal permeability is a key factor in the onset of drag-increasing Kelvin-Helmholtz rollers.
  • Realistic permeable substrates with wavelength-dependent transpiration can mitigate negative impacts of wall-normal permeability.
  • Optimizing substrate properties is crucial for effective drag management in turbulent flows.