Question

is parachute a streamline shape? Why?

Answer 1

Have you heard of the Von Karmen vortex? It's a periodically alternating spin of air downstream of anything moving through the air. It tends to push, or more precisely, twist the object alernating back and forth. The more streamlined the shape, the less force there is. And the larger the object, the lower the oscillation frequency.

You can see the Von Karmen vortex behind every flag pole, as it makes the flag wave. But in stronger wind, you can see it pushing the flagpole from side to side.

The Tacoma Narrows Bridge fell due to Von Karmen vortex, in harmony with the natural frequency of the structure. On rebuilding, the not only moved the natural frequency elsewhere, but in putting strategically placed holes in the structure, they allowed the wind to reduce the low pressure region behind the structure, damping the vortex strength.

For a round parachute, the most un-streamlined shape possible, the Von Karmen vortex is slow and very strong. It really twists the canopy all over the place. If you put a hole in the top, it relieves some of the extreme low pressure up there and damps the vortices, at the cost of ideal steady-state sink rate. But in practice, the tipping of the unvented canopy makes for a higher average sink rate, so you gain that from the hole, too.

Modern square steerable parachutes should be thought as more of a wing than a bucket