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8.2 Dimensionless Parameters in Fluid Mechanics

3 min readLast Updated on July 19, 2024

Dimensionless parameters in fluid mechanics are crucial for understanding flow behavior. These ratios, like Reynolds number, Froude number, and Mach number, help engineers predict flow regimes and characteristics without complex calculations.

By comparing forces acting on fluids, these parameters simplify analysis. They allow us to determine if a flow is laminar or turbulent, affected by gravity, or compressible. This knowledge is key for designing efficient systems and solving real-world fluid problems.

Dimensionless Parameters in Fluid Mechanics

Key dimensionless parameters in fluid mechanics

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  • Reynolds number (ReRe) represents the ratio of inertial forces to viscous forces in a fluid flow, defined as Re=ρVDμRe = \frac{\rho VD}{\mu} where ρ\rho is fluid density, VV is characteristic velocity, DD is characteristic length (pipe diameter), and μ\mu is dynamic viscosity
  • Froude number (FrFr) represents the ratio of inertial forces to gravitational forces in a fluid flow, defined as Fr=VgLFr = \frac{V}{\sqrt{gL}} where VV is characteristic velocity, gg is gravitational acceleration, and LL is characteristic length
  • Mach number (MaMa) represents the ratio of the flow velocity to the speed of sound in the fluid, defined as Ma=VcMa = \frac{V}{c} where VV is flow velocity and cc is speed of sound in the fluid

Physical significance of dimensionless parameters

  • Reynolds number (ReRe) characterizes the relative importance of inertial forces and viscous forces, determining the flow regime as laminar, transitional, or turbulent
    • Higher ReRe indicates a greater influence of inertial forces, leading to turbulent flow (chaotic motion)
    • Lower ReRe indicates a greater influence of viscous forces, resulting in laminar flow (parallel layers)
  • Froude number (FrFr) characterizes the relative importance of inertial forces and gravitational forces, relevant in open-channel flows and flows with a free surface (rivers, spillways)
    • Determines the flow regime in open-channel flows as subcritical (Fr<1Fr < 1, gravitational forces dominate), critical (Fr=1Fr = 1, inertial and gravitational forces are balanced), or supercritical (Fr>1Fr > 1, inertial forces dominate)
  • Mach number (MaMa) characterizes the compressibility of the fluid flow, determining the flow regime based on compressibility effects
    • Ma<0.3Ma < 0.3 indicates incompressible flow where density changes are negligible
    • 0.3<Ma<0.80.3 < Ma < 0.8 indicates subsonic flow where compressibility effects become significant
    • 0.8<Ma<1.20.8 < Ma < 1.2 indicates transonic flow where shock waves may form
    • Ma>1.2Ma > 1.2 indicates supersonic flow where compressibility effects dominate

Calculation of dimensionless parameters

  1. Identify the given variables and their units (velocity, density, viscosity, length)
  2. Ensure consistent units for all variables (convert if necessary)
  3. Substitute the values into the appropriate dimensionless parameter formula (ReRe, FrFr, or MaMa)
  4. Calculate the result and interpret its meaning based on the flow conditions (laminar/turbulent, subcritical/critical/supercritical, incompressible/compressible)

Dimensionless parameters vs flow regimes

  • Reynolds number (ReRe) and flow regimes
    • Laminar flow occurs when Re<2300Re < 2300 (for pipe flow) with fluid particles moving in parallel layers and no mixing between layers, dominated by viscous forces
    • Transitional flow occurs when 2300<Re<40002300 < Re < 4000 (for pipe flow) exhibiting characteristics of both laminar and turbulent flows with intermittent fluctuations and instabilities
    • Turbulent flow occurs when Re>4000Re > 4000 (for pipe flow) with chaotic and irregular motion of fluid particles, mixing between layers, and dominated by inertial forces
  • Froude number (FrFr) and open-channel flow regimes
    • Subcritical flow occurs when Fr<1Fr < 1 with gravitational forces dominating, flow velocity less than wave velocity, and downstream conditions affecting upstream flow (backwater effects)
    • Critical flow occurs when Fr=1Fr = 1 with inertial and gravitational forces balanced and flow velocity equaling wave velocity
    • Supercritical flow occurs when Fr>1Fr > 1 with inertial forces dominating, flow velocity greater than wave velocity, and upstream conditions controlling the flow (hydraulic jumps)
  • Mach number (MaMa) and compressibility effects
    • Incompressible flow occurs when Ma<0.3Ma < 0.3 with negligible density changes and velocity field independent of pressure field
    • Compressible flow occurs when Ma>0.3Ma > 0.3 with significant density changes, coupled velocity and pressure fields, and potential shock wave formation at higher Mach numbers (supersonic jets, rocket exhaust)
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© 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.
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