The flows

A flow is the movement of a fluid. A streamline is the materialization of the movement of the fluid in a flow.

There are 2 types of flows:

• Laminar: the current lines are almost parallel to each other.
• Vortex: the current lines are totally disordered.

The sails of a sports dinghy are most effective when the flow is laminar. It is highlighted by the penons and favors.

The flow remains laminar up to about 40 degrees of incidence, i.e. a broad reach / wide reach gait (depending on the speed of the dinghy).

Downwind, the flow is turbulent. The state of the flow depends on several factors: viscosity of the fluid ( \(V\) ), speed of the flow, density ( \(p\) ) of the fluid. In order to understand how the forces that drive our drifter are formed, let’s look at the different pressures along the flows.

The total pressure of a moving fluid is the sum of:

• Its static pressure ( \(Ps\) ): for the case of air, this is the meteorological pressure.
• Its dynamic pressure ( \(Pd\) ): which depends on its velocity squared \(Pd = 1/2 ρV2\)

Bernoulli states that within an incompressible flow, the total pressure remains constant along a streamline : \(V^2/2 + Ps/P = cste \)

Laminar flow generating displacement #

What we are interested in is understanding how a sail, immersed in a flow (the speed of the boat), can move the boat forward.

The sail surface (curve) deflects the flow. From a certain distance, the flow is no longer disturbed.

On the upper surface there is a tightening while on the lower surface there is a widening. This is a Venturi effect: the air is accelerated on the top surface while it is slowed down on the bottom surface (case of a “simple” sail).

There is therefore a decrease in static pressure on the top surface and vice versa on the bottom surface.

The static pressure differential creates a force: the aerodynamic force or buoyancy. The suction (top surface) is more important than the thrust (bottom surface).

Profile stall #

In a normal situation, the airflow is “attached” to both sides of the wing, top and bottom. The airflow sticks to the airfoil, which favors the lift.

At a certain angle of incidence, about 15 to 20°, depending on the characteristics of the wing - profile, aspect ratio, etc. - and the number of

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