It’s better than a flat plate perpendicular to the flow. Most people seem to not expect that the back of the object affects the drag, but there’s a large low pressure zone due to the wake. With high pressure in the front and low pressure in the back (along with a somewhat negligible skin friction contribution), the drag is considerable. So you need to target both the front and back to have a low drag shape. Most aerodynamic shapes trade pressure drag for skin friction drag, as the latter is small (if the Reynolds number is high).
For “an aerodynamic shape” my intuition first gives me a stylized drop: hemispheric in the front and a long tail thinning to a point in the back. But after a couple of seconds it decides that a spindle shape would probably be better :-)
The “teardrop” shape is pretty good, though the name is a fair bit misleading as droplets almost never look like that. Their shape varies in time depending on the flow conditions.
Not quite sure what you mean by spindle shape, but I’m sure a variety of shapes like that could be pretty good. For the front, it’s important to not have a flat tip. For the back, you’d want a gradual decay of the radius to prevent the fluid from separating off the back, creating a large wake. These are the heuristics.
Which shape objects have minimum drag is a fairly interesting subject. The shape with minimum wave drag (i.e., supersonic flow) is known, but I’m not sure there are any general proofs for other flow regimes. Perhaps it doesn’t matter much, as we already know a bunch of shapes with low drag. The real problem seems to be getting these shapes adopted, as (for example) cars don’t seem to be bought on rational bases like engineering. This should not be surprising.
cars don’t seem to be bought on rational bases like engineering.
Of course, but I don’t see it as a bad thing. Typically when people buy cars they have a collection of must-haves and then from the short list of cars matching the must-haves, they pick what they like. I think it’s a perfectly fine method of picking cars. Compare to picking clothes, for example...
It’s better than a flat plate perpendicular to the flow. Most people seem to not expect that the back of the object affects the drag, but there’s a large low pressure zone due to the wake. With high pressure in the front and low pressure in the back (along with a somewhat negligible skin friction contribution), the drag is considerable. So you need to target both the front and back to have a low drag shape. Most aerodynamic shapes trade pressure drag for skin friction drag, as the latter is small (if the Reynolds number is high).
For “an aerodynamic shape” my intuition first gives me a stylized drop: hemispheric in the front and a long tail thinning to a point in the back. But after a couple of seconds it decides that a spindle shape would probably be better :-)
The “teardrop” shape is pretty good, though the name is a fair bit misleading as droplets almost never look like that. Their shape varies in time depending on the flow conditions.
Not quite sure what you mean by spindle shape, but I’m sure a variety of shapes like that could be pretty good. For the front, it’s important to not have a flat tip. For the back, you’d want a gradual decay of the radius to prevent the fluid from separating off the back, creating a large wake. These are the heuristics.
Which shape objects have minimum drag is a fairly interesting subject. The shape with minimum wave drag (i.e., supersonic flow) is known, but I’m not sure there are any general proofs for other flow regimes. Perhaps it doesn’t matter much, as we already know a bunch of shapes with low drag. The real problem seems to be getting these shapes adopted, as (for example) cars don’t seem to be bought on rational bases like engineering. This should not be surprising.
Of course, but I don’t see it as a bad thing. Typically when people buy cars they have a collection of must-haves and then from the short list of cars matching the must-haves, they pick what they like. I think it’s a perfectly fine method of picking cars. Compare to picking clothes, for example...