Shark Skin Riblets 2D — Boundary-Layer Drag Model

Riblet Geometry

Riblet spacing s 30 µm

Height / spacing ratio h/s 0.50

Flow

Flow speed U 5.0 m/s

Reference length L 2.0 m

Live Readouts

Riblet spacing s⁺

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Friction velocity u_τ

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Skin-friction drag change ΔC_f

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Wall shear stress τ_w (smooth → riblet)

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Reynolds number Re_L / C_f0

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How it works

Shark denticles carry parallel microscopic ridges ("riblets") aligned with the flow. They don't reduce drag by being smooth — they reduce it by controlling the streamwise vortices that dominate turbulent skin friction. The grooves pin those vortices above the ridge tips, shrinking the wetted area they can drag momentum across.

Turbulent flat-plate friction (Schlichting fit):
  Re_L = U·L / ν            C_f0 = 0.074 · Re_L^-0.2
  u_τ  = U · sqrt(C_f0 / 2)  (friction velocity)

Riblet spacing in wall units:
  s⁺ = s · u_τ / ν

Drag response (fit to Bechert et al. 1997 towing-tank data,
peak reduction ≈ 8% near s⁺ ≈ 15, crossing back to drag-increasing past s⁺ ≈ 30):
  ΔC_f/C_f0 (%) = 8 · [ 1 − ((s⁺ − 15) / 15)² ]   (clamped to [-30, 9])

  τ_w = C_f · ½ρU²   (ρ = 1000 kg/m³ for water)

Top panel — flow-streak view: left half is the smooth reference plate, right half the riblet surface; streak colour and speed reflect the local drag factor.
Middle panel — the actual groove cross-section, drawn to scale from the spacing/ratio sliders. Drag it left/right to scroll along the ridge row.
Bottom panel — the full ΔC_f/C_f0 vs s⁺ curve with your current operating point marked live, so you can see exactly where you sit on the peak-reduction hump.
Below s⁺ ≈ 15 the grooves are too small to matter much; near s⁺ ≈ 15 they reach peak drag reduction; above s⁺ ≈ 30 they start protruding into the high-speed flow and increase drag instead.

Real-world relevance: riblet films modelled on shark skin have flown on Olympic swimsuits and Airbus wing panels, and are being trialled as low-friction hull coatings for cargo ships.

Fast / low-friction streak Slow / high-friction streak Riblet cross-section — drag to scroll