Computational Fluid Dynamics in Swimming

Author: Mehulkumar K.

In the quest for excellence in competitive swimming, athletes and coaches are constantly searching for innovative methods to enhance performance and gain a competitive edge. One of the most significant advancements in sports engineering has been the application of Computational Fluid Dynamics (CFD), which has revolutionized the way swimmers approach training and competition strategies. At its core, swimming is a battle against the resistance of water, and success depends not only on a swimmer’s physical strength and technique but also on how effectively they navigate through the water. By simulating how water flows around a swimmer’s body, CFD provides a detailed analysis of drag, lift, and turbulence, crucial factors impacting performance and speed. For better swimming performance, the hydrodynamic drag force resisting motion should be minimized and the propelling force maximized at the same time. The hydrodynamic drag force on a swimmer can be divided into three components: the friction drag due to the viscosity of water, the form drag due to the complex form of the swimmer’s body and the wave drag due to the interaction of the swimmer with the free surface [1]. Understanding these components can be advantageous to the coaches and athletes to optimize body position, stroke technique, and even swimwear design for improved hydrodynamics. In swimming, drag is typically categorized into two types: passive and active [2]. Passive drag refers to the resistance a swimmer experiences when in a stable position, such as during gliding without kicking or pulling, while active drag occurs when the swimmer is in motion, facing resistance during one of the four strokes: freestyle, backstroke, breaststroke, or butterfly. By calculating and analysing both types of drag, CFD helps coaches and athletes better understand how to reduce resistance and improve overall performance.

In a sport where mere fractions of a second can determine success, as seen in the dramatic 2008 Beijing Olympics men’s 100m butterfly final, where Michael Phelps triumphed over Milorad Cavic by just 0.01 seconds, it is clear that even the smallest improvements in technique and body positioning can make a world of difference. This relentless pursuit of perfection has driven extensive research into how swimmer morphology and streamlined motion can reduce drag. Recent CFD simulations have revealed that male body shapes generally experience lower drag than female body shapes, with variations in chest and hip
dimensions causing substantial differences in drag [2-3]. These findings emphasize the importance of body shape in optimizing hydrodynamic efficiency. For example, males typically swim 10% faster than females on average, a trend still evident today. The Olympic record for the men’s 100m freestyle is 10.7% faster than the women’s, with Pan Zhanle’s 46.40 second record during the 2024 Paris Olympics surpassing Emma McKeon’s 51.96 seconds during the 2020 Tokyo Games. This difference is largely attributed to variations in body shape, particularly drag resistance, highlighting how body position and form play an integral role in performance.

Beyond swimmer morphology, CFD also plays a vital role in optimizing swimwear design, which is another key factor in reducing drag. CFD has become an essential tool for swimwear manufacturers, enabling them to test and optimize materials and designs in virtual environments before creating physical prototypes. This capability has led to the development of advanced swimwear that minimizes drag, enhances buoyancy, and improves overall performance. The famous Speedo LZR Racer suit, for example, was a product of CFD-driven design, contributing to numerous world records and groundbreaking achievements in swimming. The application of CFD in swimwear design shows how every aspect of swimming performance down to the very fabric and cut of the suit can be optimized for maximum efficiency in the water.

To analyse and calculate drag, coaches and scientists use several methods, including towing, flume testing, inverse dynamics, and CFD [1]. The first three methods are experimental and rely on the swimmer’s actual motion and dynamics for calculations, while CFD uses advanced modelling techniques to simulate a swimmer’s movement through water. Unlike traditional methods, CFD allows for detailed analysis of the water flow around a swimmer’s body, providing valuable insights into the impact of various factors like finger and thumb positions, hand and forearm alignment, head position, and different gliding depths on swimming propulsion and drag [4-6]. This ability to simulate and visualize how water interacts with the swimmer’s body enables coaches and athletes to fine-tune techniques with remarkable precision, making CFD an indispensable tool in training and performance optimization. By optimizing these variables, swimmers can achieve smoother, more efficient movement through the water, ultimately improving speed and reducing energy expenditure.

The field of CFD has greatly advanced our understanding of complex hydrodynamic phenomena in swimming. By combining CFD with Particle Image Velocimetry (PIV), researchers can visualize and better understand the flow fields around swimmers. Experimental data, such as pressure and force measurements from devices like pressure sensors, are also crucial in verifying CFD results and enhancing the interpretation of PIV images. Additionally, the Swimming Human Simulation Model (SWUM), which incorporates experimental data on fluid forces and swimmer velocity, helps uncover the mechanisms behind unsteady fluid forces during swimming. SWUM simulations offer valuable insights for coaches and swimmers, enabling them to optimize movements for improved performance and training. Furthermore, the integration of CFD, PIV, and SWUM presents a powerful tool to advance swimming research [6]. Studies are also being conducted to develop numerical models to study the drag forces acting on a swimmer near the water surface, considering wave drag. In these simulations, the motion of the free surface is captured using the Volume of Fluid (VOF) method, further enhancing our understanding of fluid dynamics in swimming. In conclusion, the advancement and application of CFD in swimming has revolutionized the way athletes, coaches, and manufacturers approach performance optimization, from understanding drag forces and swimmer morphology to refining swimwear design, providing unparalleled insights that continue to push the boundaries of efficiency and speed in the sport.

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