Questions: Form Drag and Pressure Drag: Decomposition of Total Drag

Question 1

Two objects move through air at the same speed: a smooth sphere and a teardrop-shaped body with the same frontal area. The teardrop has significantly more wetted surface area. Which has lower total drag, and what is the primary reason?

• A. The sphere — less surface area means less friction drag, which always dominates
• B. They are equal — frontal area determines drag and both are the same
• C. The teardrop — its streamlined shape prevents flow separation, eliminating the large pressure wake that dominates sphere drag at typical speeds
• D. The sphere — a symmetric shape has zero net pressure difference front-to-back

Answer: (C)

At practical Reynolds numbers, the sphere's bluff shape causes flow to separate before the rear, creating a large low-pressure wake. This pressure difference between the high-pressure stagnation zone at the front and the near-ambient separated region at the back produces form drag that far exceeds friction drag. The teardrop's gradual rearward taper delays separation, keeping the wake small. Its slightly higher friction drag (more wetted area) is far outweighed by its drastically reduced form drag. Streamlining specifically targets form drag — the dominant term for most engineering shapes at high Re.

Question 2

At high Reynolds numbers, a flat plate oriented perpendicular to the flow (bluff body) has dramatically higher drag than the same plate oriented parallel to the flow. What explains the difference?

• A. The perpendicular plate has more wetted surface area exposed to friction
• B. The perpendicular plate creates a large separated wake behind it, producing high form drag from the front-to-back pressure difference; the parallel plate has almost no separation
• C. The perpendicular orientation increases the velocity gradient at the wall, increasing viscous shear
• D. The parallel plate benefits from laminar flow while the perpendicular plate has turbulent flow

Answer: (B)

A plate perpendicular to the flow has a large high-pressure stagnation zone on the windward face and forces immediate flow separation at its edges, producing a massive low-pressure wake — nearly all of its drag is form drag. A plate parallel to the flow presents a thin profile with no adverse pressure gradient; the boundary layer stays attached and drag is almost purely friction. The difference in drag coefficient (C_D ≈ 1.2 vs. C_D ≈ 0.001 for the perpendicular vs. parallel flat plate) illustrates how separation geometry dominates drag.

Question 3

Streamlining a body reduces form drag but increases friction drag due to greater wetted surface area.

• T. True
• F. False

Answer: True

True — and this is the design trade-off engineers must balance. A streamlined shape extends the body rearward with a gradual taper to delay separation, but this elongation creates more surface area exposed to viscous shear, increasing friction drag slightly. However, at moderate to high Reynolds numbers where form drag would otherwise dominate, the friction drag increase is far smaller than the form drag reduction, making streamlining advantageous. At very low Re (Stokes flow), viscous effects dominate everywhere and the trade-off shifts.

Question 4

The most effective way to reduce drag on a bluff body (such as a truck cab or a cylinder) is to smooth the surface to reduce skin friction.

• T. True
• F. False

Answer: False

For bluff bodies at typical Reynolds numbers, form drag vastly exceeds friction drag — the separated wake causes a pressure imbalance that dwarf viscous shear forces. Smoothing the surface has minimal effect on this pressure-drag mechanism. The most effective interventions are geometric: streamlining the shape to delay or prevent separation (teardrop tails, boat-tailing on trucks), adding vortex generators to energize the boundary layer and delay separation, or using trip wires to force transition to turbulent flow (which paradoxically reduces drag on spheres by enabling the boundary layer to stay attached further around the body).

Question 5

Explain why a separated wake produces form (pressure) drag. What physical pressure distribution drives the rearward force on a bluff body?

Model answer: Flow approaching a bluff body stagnates on the windward face, creating high pressure there (the stagnation pressure). As flow accelerates around the body, it must decelerate and recover pressure on the leeward side — but for a bluff body the adverse pressure gradient is too steep, the boundary layer separates before reaching the rear, and the wake is filled with recirculating fluid near ambient pressure rather than the high recovered pressure that ideal attached flow would produce. The net result is high pressure at the front and near-ambient pressure at the back: this front-to-back pressure imbalance exerts a net rearward force on the body, which is form drag.

Form drag is directly tied to the size and pressure deficit of the separated wake. A large, low-pressure wake means high form drag; a small or nonexistent wake means low form drag. Streamlining works by giving the boundary layer a gentle enough pressure recovery that it stays attached to the body all the way to the rear, allowing the leeward pressure to recover toward the stagnation value and reducing the front-to-back imbalance.