Before we can answer that question, we have to look at fluid dynamics! courtesy Cesareo de La Rosa Siqueira. [http://www.mcef.ep.usp.br/staff/jmeneg/cesareo/Cesareo_HomePage. html].
Laminar vs. Turbulent Flow Move the straw through the water very slowly. Describe the fluid movement around the straw Repeat, moving the straw more quickly. How does fluid movement differ with the speed?
We can model a copepod at large scale if we can match the Re What can we reasonably change in our model to accomplish a smaller Re?
Can we change the size of our model? Probably not. Even if possible, small size would be difficult to manage.
What about the speed of our swimmer? Again, possible but not practical. Winding slightly enough to effectively change the speed would be difficult at best.
Can we easily change density? Yes! But even the least dense liquid at room temperature is only 2/3 as dense as water. And that would only lower Re by the same amount.
That leaves us with viscosity! By replacing water with a shampoo mixture we can raise the viscosity by a factor or 3000! = dynamic viscosity of fluid (for water =.01)
Reynolds number in higher viscosity Wind the fish CAREFULLY! Time its swim across the tank. Measure the distance (cm) it travelled. Calculate its speed. 3 times and average Calculate its Reynolds number (Re)
Cnidarians fire small harpoons called nematocysts (or cnidoblasts) at their prey Courtesy of James Cook Uni
But a nematocyst is only 10mm, so Re should be low How do they overcome viscosity to fire these small projectiles so rapidly? Photo courtesy of Dr. Zoltan Takacs
What could compensate for the small size and make the nematocysts turbulent? Average velocity = 1860 cm/s Nearly 70 km/h or 42 mph Not impressed? –Speed is attained within 13 micrometers –Acceleration required is about 5,000,000 g’s! Figure from Nüchter et al. 2006.
Things behave differently at different size scales! As things get smaller, viscosity becomes more important. Re can be used to model movement at small scales.
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