1. What is Reynolds Number?

The Reynolds number (Re) is a dimensionless quantity used to predict flow patterns in different fluid flow situations. It compares inertial forces to viscous forces and helps determine whether a flow will be laminar or turbulent.

2. How Does the Calculator Work?

The calculator uses the Reynolds number formula:

Where:

• \( \rho \) — Fluid density (kg/m³)
• \( v \) — Flow velocity (m/s)
• \( L \) — Characteristic length (m)
• \( \mu \) — Dynamic viscosity (Pa·s)

Explanation: The drag coefficient is then estimated based on the Reynolds number range, with different approximations for laminar, transitional, and turbulent flows.

3. Importance of Reynolds Number

Details: The Reynolds number is crucial for predicting flow regimes, designing aerodynamic systems, calculating drag forces, and scaling fluid dynamics experiments.

4. Using the Calculator

Tips: Enter all values in SI units. The characteristic length depends on the object shape (diameter for spheres, chord length for airfoils, etc.). All values must be positive.

5. Frequently Asked Questions (FAQ)

Q1: What is the characteristic length?
A: It's a representative dimension of the object in the flow direction. For a sphere, it's the diameter; for an airfoil, it's the chord length.

Q2: How does Reynolds number affect drag coefficient?
A: At low Re (laminar flow), Cd is high. As Re increases, Cd typically decreases until turbulence begins, when it may increase again.

Q3: What are typical Reynolds number ranges?
A: Re < 2000: laminar; 2000 < Re < 4000: transitional; Re > 4000: turbulent (varies by geometry).

Q4: Why is my drag coefficient estimate approximate?
A: Exact Cd values depend on object shape, surface roughness, and flow conditions. This calculator provides a general estimate.

Q5: Can I use this for compressible flows?
A: This calculator assumes incompressible flow (Mach number < 0.3). For compressible flows, additional factors must be considered.