Fluid Mechanics – Lift on a body in a flowing fluid Identify the correct description of the lift force acting on an immersed body.

Introduction / Context:
When a body is immersed in a moving fluid, it experiences hydrodynamic forces due to pressure and viscous effects. These forces are decomposed into drag (parallel to the approach flow) and lift (normal to the approach flow). Understanding the lift definition is fundamental for aerodynamics and hydraulic structures.

Given Data / Assumptions:

• Steady external flow around a body.
• Approach velocity defines the reference direction for drag/lift components.
• Buoyancy (hydrostatic) considered separately from dynamic forces.

Concept / Approach:

Lift is the resultant dynamic force component normal to the free-stream direction, generated by asymmetric pressure distribution and circulation around the body. Drag is the parallel component. Buoyancy acts vertically upward independent of flow and is not “lift” in this context.

Step-by-Step Solution:

Verification / Alternative check:

For symmetrical bodies at zero angle of attack, lift ≈ 0; tilting or cambering the body produces a non-zero pressure difference and hence lift, confirming the definition.

Why Other Options Are Wrong:

(a) Buoyancy is hydrostatic, not part of dynamic lift. (b) Lift direction need not oppose gravity; it is defined normal to flow, which may be upward, downward, or lateral. (c) Wake contributes to drag and unsteady forces; lift is not caused “only” by the wake.

Common Pitfalls:

Confusing aerodynamic lift with buoyancy, or defining lift relative to vertical instead of the approach velocity direction.

Final Answer:

The dynamic fluid-force component normal to the approach velocity