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Ok this is a silly question but here it goes

Although it is good to have a laminar flow of the air around the object for low drag but the laminar flow is prone the phenomena called separation (sounds like breakup) which dramatically increases the drag on the object. On the other hand turbulent flow has a greater drag around the object in the beginning but is less prone to separation as compared to laminar flow , and this is the reason why golf balls have been introduces to dimples to create a controlled turbulent flow to get rid of separation.

So my question why don't surface of airplanes have dimples on them, as it would reduce the drag on the airplane and thus fuel consumtion, or does the effect which reduces the drag in the case of golf ball fails at higher speed and bigger size or is it something else

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  • $\begingroup$ My guess is that the surfaces used for lift and control of the plane would need to remain smooth. Also, I think drag problems can (mostly) be solved by flying higher so there is some economic tradeoff between the high cost of dimpling and just flying higher. $\endgroup$ – Brandon Enright Apr 21 '14 at 18:09
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    $\begingroup$ as much i know if you go too high the air density decreases and so the lift, eventually you have to cruse at a higher velocity to get same amount of lift which increases the fuel consumption, so dimples are not that bad idea $\endgroup$ – Dimensionless Apr 21 '14 at 18:11
  • $\begingroup$ Have the dimples got something to do with enabling rotation of the ball ? $\endgroup$ – Borat Sagdiyev Apr 21 '14 at 18:35
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This is a very good question! Drag due to viscous effects can be broken down into 2 components:

$$D = D_f + D_p$$

where $D$ is the total drag due to viscous effects, $D_f$ is the drag due to skin friction, and $D_p$ is the drag due to separation (pressure drag).

The equation above demonstrates one of the classic compromises of aerodynamics. As you mention, laminar boundary layers reduce the skin friction drag but are more prone to flow separation. Turbulent boundary layers have higher skin friction but resist flow separation.

$$D \quad\quad\quad=\quad\quad\quad D_f \quad \quad\quad+ \quad\quad \quad D_p\quad\quad\quad\quad\quad$$ $$\quad\quad\text{less for laminar}\quad\quad\text{more for laminar}$$ $$\quad\quad\text{more for turbulent}\quad\quad\text{less for turbulent}$$

Generally speaking the more "blunt" the body is (such as a golf ball) the more likely adding dimples to trip the boundary layer will reduce drag. Airplane wings are less prone to separation since they aren't as "blunt" and as a result skin-friction drag is more important.

For more information see Section 4.21 of Introduction to Flight by John D. Anderson

EDIT:

Laminar and turbulent boundary layers are fundamentally different in many ways but the important aspect for flow separation is how "full" the profile is. The figure below is a schematic comparing the mean velocity profile of a turbulent boundary layer to that of a laminar one. $V$ is the velocity tangent to the surface and $\eta$ is the distance away from the surface. As you can see, for turbulent boundary layers, the fluid close to the wall is moving faster than for the laminar profile.

Schematic of laminar and turbulent boundary layers.

What causes the flow to separate is an adverse pressure gradient, or $dp/dx < 0$ where $x$ is the coordinate along the surface. Generally fluid moves from high to low pressure. In the case of a boundary layer that is on the verge of separating, the flow is locally going from low to high pressure. The figure below illustrates the effect this has on the boundary layer. When the flow near the wall begins to reverse, the flow is beginning to separate. Because the fluid in a turbulent boundary layer near the surface is moving faster, a turbulent boundary layer is better able to resist an adverse pressure gradient than a laminar boundary layer.

Effect of adverse pressure gradient on a boundary layer.

Most objects that are designed with aerodynamics in mind are slender. This is done specifically to reduce the adverse pressure gradient ($dp/dx$) over the surface of the object and reduce the possibility of flow separation.

Drag on slender vs. blunt objects.

Figures are from Fundamentals of Aerodynamics by John D. Anderson.

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  • $\begingroup$ Ok but what about the fuselage isn't it a little bit more blunt than the wing $\endgroup$ – Dimensionless Apr 22 '14 at 2:12
  • $\begingroup$ The fuselage is, arguably, more blunt than the wing but it is still designed in such a way to minimize flow separation. If you looked at a cross-section of an airplane's fuselage it would look more like the slender body than the blunt body in Figure 15.7 above. $\endgroup$ – OSE Apr 22 '14 at 14:36
  • $\begingroup$ yup right about the fuselage $\endgroup$ – Dimensionless Apr 22 '14 at 17:21
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    $\begingroup$ Here is a demonstration on the differences of slender and blunt ends on a body: youtube.com/watch?v=jAYP6pWrdkc $\endgroup$ – Sampo Smolander Jul 5 '15 at 7:36
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This might be a simpler version of the answers above. Total drag is a sum of the surface friction drag and the form drag (pressure drag). About 90% of the drag of a smooth sphere shape is pressure drag and the rest is friction drag. Putting dimples on surface will increase the friction drag but will reduce the pressure drag by having the turbulent boundary layer attached farther before separation. Thus the loss by the frication increase is minimal compared to the gain by the pressure drag decrease. In airfoil shape case, the friction drag is about 90% of the total drag due to already optimized form drag. Adding dimples on such shape will only increase the total drag.

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Aircraft already utilize a different means to induce turbulent flow within the boundary layer. Some aircraft have installed along the leading edge, vortex generators. These tiny vertical tabs diverge and converge angles in a zig zag pattern. The energized boundary layer is then less prone to separation at higher angles of attack. This allows the aircraft to fly slower and generate more lift at a lower airspeed than without the generators.

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Dimpling aircraft surfaces will not reduce drag because the airflow does not separate (under normal conditions) from the surface like it does on the back of a flying golf ball. The dimples minimize the airflow separation. I think a dimpled wing top surface might be beneficial on an STOL aircraft where lift is a priority over drag--the dimpled surface might decrease the stall speed and allow the wing to produce lift at a higher angle of attack.

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