Spaceships: Hull Configuration and Streamlining
Why is a tear drop shape more aerodynamic with a round head, than with a sharp head (as if mirroring the back end)?
6 Answers
Suryanarayanan R
Suryanarayanan R, Amateur Aero-Engineer
Answered Mar 26 2015 · Upvoted by Achilleas Vortselas, Ph.D. Mechanical Engineer · Author has 154 answers and 130.6k answer views
The tear drop is somewhat the most aerodynamic (i.e. has least drag) under the assumption of a subsonic flow of moderately high Reynold's number (say Re ~ 10^4) on an axisymmetric body.
Overall, The following affect the drag on an axisymmetric body under these assumptions :
Pressure distribution over the surface of the body,
The skin friction, and
The location of flow separation.
The obvious answer to your question is that, the pressure distribution and the skin friction on the sharp head is such that it's drag coefficient is higher as compared to the one with a relatively blunt head.
The Drag Coefficient for a Half Cone (with the sharp edge pointing forward) is 0.50 while that of a Half Sphere with the same frontal area is 0.42.
Source : Drag coefficient
On thinking this through, I surmise that at the front end, we would like to minimize skin friction drag, hence keep a lower length for the flow to pass through - but not completely a vertical flat plate(in which case the Drag Coefficient = 1)!
At the rear end, we would want to avoid flow separation for as far as possible and avoid the turbulent wake, as this will monumentally increase drag.
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John Palmore
John Palmore, PhD student in Aerospace Engineering
Answered Mar 26 2015 · Author has 105 answers and 72.9k answer views
Separation.
Airfoils are designed in the way that they are designed so that the air going over them will not separate. This is important because the separation will cause a type of drag force to appear. This will make your airplane less efficient, because it will have to overcome more force to fly. The configuration that you give will certainly separate at the rear.
I found a video on YouTube of a triangular wedge by Andreas Muller. It shows the problem of separation it quite well. A vortex forms behind the triangle which indicates separation. In the frame I grabbed, the center of the vortex is marked by a horizontal red line.
That vortex means separation. Separation means more drag. While the case you're describing is a bit different from what's in the video, the same phenomenon will result. The flow will separate near the rear end of the airfoil, and drag will ensue.
Here's the full video.
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Dave Robinson
Dave Robinson, More than 1/4 of a century working in Aerospace Design for some of the world's top Aerospace companies
Answered Mar 27 2015 · Upvoted by Achilleas Vortselas, Ph.D. Mechanical Engineer · Author has 587 answers and 839.8k answer views
Thank you for the A2A opportunity.
It isn't exactly the case. Aerofoil shapes tuned to provide Lift tend to have a rounded (radius) nose in order to enhance the stall characteristics of the Aerofoil. It should be noted that aerofoils tuned for supersonic flow past them tend to have fairly sharp leading edges.
However, if the aerofoil's principle purpose is to reduce Drag and not generate Lift then a slender symmetrical shape (about its centre) may very well produce the least aerodynamic Drag; by maintaining Laminar flow past the surfaces of the body for subsonic velocities.
In most applications, some sort of structural strength is required at the leading edge of an aerodynamic shape and this necessitates some thickness in order to provide volume in which to fit the structure.
* A fineness ratio, between 4 and 12, typically applies to the cylindrical shaped fuselage of a commercial transport aircraft and indicates the lowest Drag configuration. It represents the ratio of fuselage length and its average diameter.
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Thomas Moura
Thomas Moura, M.S. Mechanical Engineering & Aerospace and Aeronautical Engineering, University of Campinas (2015)
Answered May 27 2017 · Author has 333 answers and 48.5k answer views
This shape is only the best for lowish speeds, for transonic and supersonic the best shape is called sears-haack body. At low speeds, the shape of a tear minimizes the detachment of the flow, that would generate drag due to turbulence. This flow detachment is caused by an condition called “adverse pressure gradient” where the local pressure of the flow is increasing in the direction of the of the flow in essence “braking” it up to the point the flow separates.
So this is the shape that minimizes this adverse pressure gradient.
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Sai Kiran
Sai Kiran, Aeronautical engineer, Lateral thinker & a lifelong learner
Answered Mar 31 2015 · Author has 289 answers and 120.7k answer views
For easy understanding: A tear drop is symmetrical in shape, imagine a fish in water, it can swim easily due to its nature of streamline as its shape can allow it to pass through water easily. Similarly imagine you are holding a cardboard and you have dipped in water in such a way that, when you move cardboard in water the flow acts on the surface, while moving you feel resistance which is the drag(in simple words force which prevents movement) created by water.
The drop can't be as streamline as a fish because of drag produced by air, the leading part of drop bulges but maintains its symmetric nature.
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Ramesh Agarwal
Ramesh Agarwal
Answered Mar 8 2017
I won’t be able to give you the scientific proof but I’ll present an example. The secret is in the name itself. Which shape does a tear or a rain drop assume when falling down?
That’s right the tear drop shape; hence its name.
Why that’s the most aerodynamic? Well suppose you’re going on a motorcycle, obviously you would bend to decrease your drag. Similarly the drop tries to reduce the drag and thus becomes the tear drop shape which we all know so well.
Of course there are other things related to the water droplet such as the surface tension and the viscous force but this is the simplest answer which I can write.
Teardrop - sphere or streamlined?
