Fiveable
Fiveable

10.2 Drag and Lift Forces

3 min readLast Updated on July 19, 2024

Drag and lift forces shape fluid dynamics around objects. Drag resists motion parallel to flow, while lift acts perpendicular. These forces arise from pressure differences and shear stress on surfaces, crucial for understanding aerodynamics and hydrodynamics.

Drag and lift coefficients quantify these forces, depending on factors like geometry and flow conditions. Reynolds number, relating inertial to viscous forces, significantly influences drag. Understanding these concepts is essential for designing efficient vehicles and structures in fluid environments.

Drag and Lift Forces

Drag vs lift forces

Top images from around the web for Drag vs lift forces
Top images from around the web for Drag vs lift forces
  • Drag force acts parallel to fluid flow direction
    • Resists motion of body through fluid (car driving on highway)
    • Caused by combined pressure and shear stress distributions on body surface
  • Lift force acts perpendicular to fluid flow direction
    • Generated by asymmetric pressure distribution or viscous forces (airplane wing)
    • Occurs in various geometries, not just airfoils and wings (spinning soccer ball)

Pressure and shear stress contributions

  • Pressure contribution to drag known as form drag or pressure drag
    • Results from pressure difference between upstream and downstream sides of body (blunt objects like spheres)
    • Dominates in high Reynolds number flows and blunt bodies
  • Shear stress contribution to drag called skin friction drag
    • Caused by viscous shear stress acting on body surface (streamlined shapes like airfoils)
    • Prevails in low Reynolds number flows and streamlined bodies
  • Pressure contribution to lift arises from asymmetric pressure distribution on upper and lower surfaces
    • Higher pressure on bottom surface and lower pressure on top surface (airplane wings)
  • Shear stress contribution to lift occurs in specific cases like Magnus effect
    • Spinning body generates lift due to asymmetric boundary layer development (spinning cylinders or spheres)

Drag and lift coefficient calculations

  • Drag coefficient (CDC_D) quantifies drag force on body in non-dimensional form
    • CD=FD12ρU2AC_D = \frac{F_D}{\frac{1}{2} \rho U^2 A}
      • FDF_D: drag force
      • ρ\rho: fluid density
      • UU: freestream velocity
      • AA: reference area (frontal area or planform area)
  • Lift coefficient (CLC_L) quantifies lift force on body in non-dimensional form
    • CL=FL12ρU2AC_L = \frac{F_L}{\frac{1}{2} \rho U^2 A}
      • FLF_L: lift force
      • ρ\rho, UU, and AA same as in drag coefficient equation
  • Drag and lift coefficients depend on multiple factors
    1. Body geometry (shape, size, orientation)
    2. Flow conditions (Reynolds number, Mach number, angle of attack)
    3. Surface properties (roughness)

Reynolds number and drag relationship

  • Reynolds number (ReRe) characterizes ratio of inertial forces to viscous forces in fluid flow
    • Re=ρULμRe = \frac{\rho U L}{\mu}
      • ρ\rho: fluid density
      • UU: characteristic velocity
      • LL: characteristic length
      • μ\mu: fluid dynamic viscosity
  • Drag coefficient depends on Reynolds number for a given geometry
    • In laminar flow (low ReRe), CDC_D inversely proportional to ReRe: CD1ReC_D \propto \frac{1}{Re}
    • In turbulent flow (high ReRe), CDC_D relatively independent of ReRe
    • Transition from laminar to turbulent flow occurs at critical ReRe, which depends on body geometry and surface roughness
  • Reynolds number influences boundary layer development on body surface
    • Laminar boundary layer at low ReRe results in lower skin friction drag (flat plate)
    • Turbulent boundary layer at high ReRe leads to higher skin friction drag but may delay flow separation and reduce form drag (golf ball dimples)
© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary