Modelling the hydrodynamics of swimming fish. From individuals to infinite schools

PhD ceremony: Mr. D.A.P. Reid, 13.15 uur, Aula Academiegebouw, Broerstraat 5, Groningen

Dissertation: Modelling the hydrodynamics of swimming fish. From individuals to infinite schools

Promotor(s): prof. C.K. Hemelrijk

Faculty: Mathematics and Natural Sciences

Swimming in groups is in almost all cases more efficient than alone. Thus, schooling is beneficial not only due to the often-cited predator defence and mate-finding benefits but also due to energetic benefits, cocludes Daniel Reid in his thesis. His thesis concerns the hydrodynamics of fish that swim by steadily undulating their body.

He has studied this using a computer model (the so-called Mulitparticle Collision Dynamics method) and a meta-analysis of published data on swimming fish. In the computer model the water and its dynamics are simulated by means of millions of particles that move and collide. From this behaviour at the micro-scale, correct hydrodynamics emerge at the macro scale. Reid has discovered that the addition of long tail-like plates to the downstream side of a cylinder increases its drag coefficient at low Reynolds numbers but increases it at higher ones, possibly explaining why microscopic swimmers use round flagellae for propulsion instead. By studying a swimming fish he show ed that constraining their acceleration in the lateral direction (as many simulations of fish swimming do) may influence the results: the patterns of force and flow are exagerrated so that they resemble those of an unconstrained fish with a higher tailbeat frequency.

Reid discovered in simulations of infinitely large schools that in most cases it is beneficial as regards speed and efficiency to swim in groups, even in a long line behind each other. Through a meta-analysis of the data from scientific literature on steadily swimming fish he found several general principles that apply across species, e.g. that the rearwards speed V of the propulsive body wave is the most important factor in determining the forwards swimming speed U, and that the so-called 'slip ratio' U/V depends on the Reynolds number.