Define Reynolds number and discuss its impact on airfoil performance.

Define Reynolds number and discuss its impact on airfoil performance.

• A. Re = ρ V L / μ; higher Re reduces viscous effects, delays transition, and influences boundary-layer development, affecting lift and drag.
• B. Re = μ /(ρ V L) and higher Re increases viscous effects.
• C. Re is the ratio of density to viscosity and has no impact on lift.
• D. Re is defined by the Mach number.

Answer: (A)

Reynolds number tells you whether inertial forces or viscous forces dominate the flow around an airfoil. It is defined as Re = ρ V L / μ, where ρ is air density, V a representative flow speed, L a characteristic length (like chord length), and μ the dynamic viscosity. Because it compares inertia to viscosity, increasing Re means viscosity plays a smaller role relative to inertia. That tends to make the boundary layer thinner and more capable of staying attached to the surface under adverse pressure gradients, so separation occurs later. With the flow remaining attached longer, you can achieve higher lift before stall and see changes in drag, since pressure drag tends to decrease when the flow sticks better, even though skin-friction drag grows with Re. In short, higher Reynolds numbers reduce the relative importance of viscous effects, shift boundary-layer behavior and transition, and thereby influence lift and drag characteristics of the airfoil. The other statements misstate the definition or the effect: one uses the inverse expression, another suggests viscous effects rise with Re, and another claims Re has no impact on lift or is defined by Mach number.

Reynolds number tells you whether inertial forces or viscous forces dominate the flow around an airfoil. It is defined as Re = ρ V L / μ, where ρ is air density, V a representative flow speed, L a characteristic length (like chord length), and μ the dynamic viscosity. Because it compares inertia to viscosity, increasing Re means viscosity plays a smaller role relative to inertia. That tends to make the boundary layer thinner and more capable of staying attached to the surface under adverse pressure gradients, so separation occurs later. With the flow remaining attached longer, you can achieve higher lift before stall and see changes in drag, since pressure drag tends to decrease when the flow sticks better, even though skin-friction drag grows with Re. In short, higher Reynolds numbers reduce the relative importance of viscous effects, shift boundary-layer behavior and transition, and thereby influence lift and drag characteristics of the airfoil.

The other statements misstate the definition or the effect: one uses the inverse expression, another suggests viscous effects rise with Re, and another claims Re has no impact on lift or is defined by Mach number.