A truck is moving with the velocity of 7 m / s and the wind blows in opposite to the truck with

step 1 Relative air speed is the truck speed plus the opposing wind speed. So that would be 7 plus 3 or

A truck is moving with the velocity of 7 m / s and the wind...

A truck is moving with the velocity of $7 \mathrm{~m} / \mathrm{s}$ and the wind blows in opposite to the truck with $3 \mathrm{~m} / \mathrm{s}$. If the air has a temperature of $20^{\circ} \mathrm{C}$, determine the friction drag acting on the flat side $B C F E$ of the truck. Assume the boundary layer is completely turbulent.

Key Concepts

Reynolds Number

The Reynolds number is a dimensionless quantity that measures the relative significance of inertial forces to viscous forces in a fluid flow. It is instrumental in predicting the transition between laminar and turbulent flow regimes within the boundary layer. A high Reynolds number typically signifies turbulent flow, which in turn affects friction drag calculations. Understanding the Reynolds number helps in applying appropriate empirical formulas or correlations in drag analysis.

Skin Friction Drag

Skin friction drag is the component of aerodynamic drag that arises from the shear stress due to the viscosity of the fluid acting tangentially on the surface of an object. It is computed using the skin friction coefficient, which is influenced by factors such as surface roughness, boundary layer characteristics, and the state of the flow (laminar or turbulent). This drag is particularly relevant in determining the overall aerodynamic performance of vehicles and other bodies moving through a fluid.

Boundary Layer Theory

Boundary layer theory describes the thin region adjacent to a solid surface where viscous forces are significant, and the velocity of the fluid changes from zero (due to the no-slip condition) to the free stream velocity. Understanding this layer is vital in analyzing frictional forces and heat transfer between the fluid and the surface, as it directly affects the drag experienced by the object.

Relative Velocity

Relative velocity in fluid dynamics refers to the effective speed at which a fluid (such as air) moves past a solid object. It is crucial in determining how much aerodynamic force is exerted on a body, as the drag force depends on the square of the relative speed between the object and the flow. This concept is essential when calculating forces acting on moving bodies in the presence of wind or other fluid flows.

Turbulent Flow

Turbulent flow is characterized by chaotic and irregular fluid motion, which enhances mixing and momentum transfer within the boundary layer. In the context of friction drag calculations, the turbulent nature of the boundary layer implies different friction coefficients compared to laminar flow. This concept is important when estimating skin friction, as turbulent boundary layers generally increase momentum transfer and reduce the thickness of the viscous sublayer.

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