hydrodynamics

Tiago M. Barbosa

Hydrodynamics play a significant role in the swimmer's performance. This chapter provides insight into how the water interacts with the swimmer and the forces acting upon the body. It examines the way water flows around the swimmer and how the forces produced can have an impact on the energy expended during swimming, and hence on the swimming efficiency. Swimmers, coaches, and researchers spend a good deal of time trying to understand how to optimize these key aspects. Cutting- edge devices are used to assess the swimmers, making them more efficient and improving their performance. These assessments can encompass the way the body is aligned, swimwear, or small details such as the position of the fingers during swimming.

What is the free body diagram of a swimmer?

-> What are the forces acting on me when I swim?

To have a deeper understanding of the swimmer's hydrodynamics we need to learn what the main forces acting upon the body are and how they interplay. Ultimately, swimming acceleration and speed depend on propulsive forces, resistive forces, and inertial parameters. Propulsive forces are those related to the thrust and forward movement. Resistive forces act opposite to the swimmer's direction of displacement. The inertial parameters are related to body features (anthropometric characteristics).

Thrust is due to steady and unsteady flow patterns (see here), and is the sum of propulsive drag, lift force, and the jet vortex (see here). The resistive force is also known as total drag force and results from three components — friction drag, pressure drag, and wave drag. Regarding the inertial parameters, these include the swimmer's body mass and the added mass of water.

Based on these external forces it is possible to model the swimming stroke and make a rough estimation of the acceleration-time and speed-time curves within the stroke cycle (see here). The swimmer does not move at a constant speed — that is, with uniform motion. Instead, over a stroke cycle, there are positive and negative accelerations. The typical profile of these curves depends on the swimming stroke, but in general positive accelerations occur when the thrust is greater than the drag force — for instance, during the pull phase. On the other hand, acceleration is negative when the thrust is smaller than the drag force — for example, during the arm's recovery. Therefore, one can swim faster by increasing the thrust while keeping the drag constant, by keeping the thrust constant and decreasing the drag, or by both increasing the thrust and simultaneously decreasing the drag force.

What is the influence of water flow on a swimmer's displacement?

-> How does water surround me as I swim?

Water, like any fluid, is a substance that flows and deforms when forces are applied to it. The way water flows around a swimmer affects several forces that act on the body, including the thrust (see here) and the resistance (see here). Fluids, such as water, are characterized by a set of properties. The most important are the density (mass or quantity of matter per unit of volume), and the viscosity (resistance to the movement of particles in the flowing substance).

The flow can be steady or unsteady. A flow is characterized as steady when there are no changes in the fluid velocity and pressure at a given point of the body over time. Conversely, if these properties change, the flow is unsteady. Both steady and unsteady flows play determinant roles in the production of thrust.

If the swimmer's limbs are moving at a constant or almost constant speed, with no significant changes in direction, then the water properties do not change over time and conditions are steady. Under these conditions, there are two propulsive forces, which are the drag and the lift produced by the hands and feet, whose speed and orientation are constant (see here).

On the other hand, when the swimmer's hands or feet accelerate or change direction suddenly, unsteady flows are created and this produces thrust. The speeds of the hands and the feet increase over their underwater trajectories. The hands accelerate from their entry into the water to their exit, and the feet also undergo accelerations as they kick up and down. In analyzing these unsteady conditions, we can assess the water circulation around the body or the limbs and observe, for instance, the generation of vortices.

Flow can also be described as laminar or turbulent. The flow is laminar when the layers of fluid are well organized and parallel to the swimmer's body. If the water layers show a random organization then this is turbulent flow.

How does drag force affect swimmers?

-> How does water resistance affect how I swim?

When an object moves through a real fluid such as water, which has viscosity and compressibility, there is always some resistance to overcome. This resistance is known as drag, because the object drags fluid particles along as it moves. Drag forces acting upon the body are a major concern because they slow us down, and can significantly affect performance.

The magnitude of the drag depends on a set of variables. The higher the fluid's density, the higher the drag, and this partly explains differences in salt water and fresh water performances. Water is about 800 times more dense than air, which means drag impacts a swimmer far more. Body surface also affects drag on a swimmer — the larger the area presented, the greater the drag. But the top determinant is the relative velocity between the body and the water — the faster you go, the more resistance there is to overcome. Drag forces can be broken down into three different components — skin friction drag (or viscous drag), pressure drag (or form drag), and wave-making drag.

Skin friction drag results from interaction between the water's viscosity and the body's surface. The water layer in contact with the skin sticks to it, and travels at the same speed as the body, so the relative speed is zero. This is the boundary layer. The next layer of water is decelerated by this layer, and so on, progressively further from the body. The higher the skin friction drag, the more water is dragged (or trailed) behind the body.

Pressure drag is related to the pressure difference between the leading and trailing edges of the body. At the front, there is high pressure where fluid particles are compressed. Particles then flow around the body, and eventually separate from the body at the boundary layer separation point. Beyond this, the flow reverses, producing vortices and a low-pressure region. The pressure differential means particles tend to move from the front to the rear of the body, "pushing" it backward — that is, producing pressure drag opposing the...