Estimating Active Drag Based on Full and Semi-Tethered Swimming Tests.

During full tethered swimming no hydrodynamic resistance is generated (since v = 0) and all the swimmer's propulsive force (FP) is utilized to exert force on the tether (FT = FP). During semi-tethered swimming FP can be made useful to one of two ends: exerting force on the tether (FST) or overcoming drag in the water (active drag: Da). At constant stroke rate, the mean propulsive force (FP) is constant and the quantity FP - FST (the "residual thrust") corresponds to Da. In this study we explored the possibility to estimate Da based on this method ("residual thrust method") and we compared these values with passive drag values (Dp) and with values of active drag estimated by means of the "planimetric method". Based on data obtained from resisted swimming (full and semi-tethered tests at 100% and 35, 50, 60, 75, 85% of the individual FT), active drag was calculated as: DaST = kaST.vST2 = FP - FST ("residual thrust method"). Passive drag (Dp) was calculated based on data obtained from passive towing tests and active drag ("planimetric method") was estimated as: DaPL = Dp.1.5. Speed-specific drag (k = D/v2) in passive conditions (kp) was )25 kg.m-1 and in active conditions (ka) )38 kg.m-1 (with either method); thus, DaST > Dp and DaST > DaPL. In human swimming active drag is, thus, about 1.5 times larger than passive drag. These experiments can be conducted in an ecological setting (in the swimming pool) by using basic instrumentation and a simple set of calculations.

Coordination and stroking parameters in the four swimming techniques: a narrative review.

Swimming performances are multifactorial and primarily include anthropometric, hydrodynamic, bioenergetic and biomechanical factors whose contributions depend on age, gender, swimming distance and swimming stroke. An integrative and multivariate approach to swimming captures the complexity and various pathways of performance, but swimming technique is generally examined through such parameters as stroke index, propelling efficiency, stroke length and stroke rate. The first originality of our narrative review is to present the state of art of the methods to collect and measure inter-limb coordination in the four swimming techniques, with a particular focus on the effect of skill. The second part provides readers with an overview of the current findings on the main factors that influence inter-limb coordination (i.e., swimming speed, drag and the manipulation of stroke rate) following a physical approach.

Coalescence of bubbles and drops in an outer fluid.

When two liquid drops touch, a microscopic connecting liquid bridge forms and rapidly grows as the two drops merge into one. Whereas coalescence has been thoroughly studied when drops coalesce in vacuum or air, many important situations involve coalescence in a dense surrounding fluid, such as oil coalescence in brine. Here we study the merging of gas bubbles and liquid drops in an external fluid. Our data indicate that the flows occur over much larger length scales in the outer fluid than inside the drops themselves. Thus, we find that the asymptotic early regime is always dominated by the viscosity of the drops, independent of the external fluid. A phase diagram showing the crossovers into the different possible late-time dynamics identifies a dimensionless number that signifies when the external viscosity can be important.