A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Total drag on a body consists of friction drag (viscous shear stress integrated over the surface) and form (pressure) drag (net pressure difference between windward and leeward sides). Streamlined bodies minimize form drag; bluff bodies experience large form drag due to flow separation. The ratio of form to friction drag depends on Reynolds number and geometry; understanding this decomposition helps optimize designs for different regimes.
When fluid flows over any solid object, it exerts two types of force. The first type arises from the viscous shear stress that the fluid applies directly along the surface — this is friction drag (also called skin friction drag). The second type comes from pressure: the fluid pushes harder on the windward face of the body than it pulls on the leeward face. The net rearward pressure force is form drag (or pressure drag). Total drag is the sum of these two, and understanding which dominates is essential for design.
Friction drag depends on how much surface area is exposed to the flow and how fast the velocity gradient is at the wall. From your boundary layer prerequisite, you know that the wall shear stress τ_w is proportional to the velocity gradient du/dy at y = 0. Summing τ_w over the entire wetted surface gives the friction drag. A thin flat plate aligned with the flow is the canonical friction-drag body: nearly all its drag comes from skin friction because the plate creates almost no wake and very little pressure imbalance front-to-back.
Form drag arises when flow separates. A blunt body — a flat plate perpendicular to the flow, a cylinder, or a truck cab — forces the flow to navigate a sharp pressure recovery on the leeward side that the boundary layer cannot accomplish before separating. The result is a large, low-pressure wake. The pressure difference between the high-pressure stagnation zone at the front and the near-ambient-pressure separated region at the back pushes backward: that is form drag. For a bluff cylinder at moderate Reynolds numbers, form drag can be 5–10 times larger than friction drag. The exact split depends on both geometry and Re — at very low Re (Stokes flow), viscous effects dominate everywhere and the distinction blurs; at high Re, separated wakes dominate and form drag is the primary concern.
Streamlining is the engineering practice of shaping a body to delay separation and minimize the separated wake. An airfoil or teardrop shape maintains an attached boundary layer over most of its surface, postponing the pressure recovery to a gradual rearward slope. The result is dramatically reduced form drag at the cost of somewhat more wetted surface (and therefore slightly more friction drag). The trade-off favors streamlining whenever Re is large enough that form drag would otherwise dominate — which is the case for vehicles, aircraft, and most engineering applications above pedestrian speeds. The ratio of maximum body thickness to chord length is a key design parameter: too blunt and form drag explodes; too thin and structural constraints become limiting before the aerodynamic benefit is fully realized.