Question

In terms of the concepts examined in this experiment, why can a stingray swim much faster horizontally than vertically in the ocean, if its orientation is horizontal, like the coffee filters in the experiment?

Note: Experiment was about examining an object moving at a relatively high speed in air (with a higher Reynolds number) and an object moving at a relatively low speed in oil (with a lower Reynolds number). The goal was to examine the relationship between the drag force and the speed of the object.

Answer 1

From the knowledge of Ryalonds Number we know that

Where

• ρ is the density of the fluid (SI units: kg/m³)
• u is the velocity of the fluid with respect to the object (m/s)
• L is a characteristic linear dimension (m)
• μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/m·s)
• ν is the kinematic viscosity of the fluid (m²/s).

It is evident that for small Reynolds number the speed is slow, as it is directly proportional.

And the Drag Equation is

Where

• speed u,
• fluid density ρ
• Area A

So from the above two equations it is clear that when the stingray travels vertically it has larger area . And the drag force is directly proportional to it, so it will experience more drag due to which the speed will be less. So it can move faster when it is in horizontal direction.