Article: Flow patterns and drag coefficients of cephalopod shells

Streamlining is important in the adaptive strategy of swimmers because it minimizes waste of propulsive energy (i.e. maximizes hydrodynamic efficiency). Streamlining of cephalopod shell form was evaluated by analysing the pattern of fluid flow past shells and by calculating shell drag coefficients. Flow visualization experiments show that shell flow patterns are characterized by boundary layer separation along the flank of the outer whorl, and by turbulence in the umbilicus and behind the shell. Tow-tank measurement of drag and velocity shows that variation in shell geometry causes significant variation in drag coefficient. Inflated, depressed, and widely umbilicate shells have high drag coefficients (generally greater than 0-6-0-7). Shells that delay separation (e.g. compressed, involute oxycones) have low drag coefficients (about 0-1), but this is more than an order of magnitude greater than drag coefficients of rapid-swimming fish and squids. For most shell types change in shell orientation during swimming results in slightly higher drag coefficients as velocity increases. Analogy with aircraft and ship appendages suggests that extension of the body behind the shell has virtually no effect on drag coefficient.