Blue: Top surface, Red: Bottom surface. Now this plot uses a real airfoil with a changing pressure coefficient over length, but with a flat plate the plot would look similar.
The much higher local speed gradient at the surface of the boundary layer in the rear part will add much more drag per length and the total drag will increase disproportionately. However, a drag decrease with increasing Reynolds numbers is nowhere to be seen. The thing is in the velocity profiles for laminar and turbulent boundary layers. Lets look at the picture below. The profiles are little different.
The turbulent profile is "fatter", or fuller, than the laminar profile. For the turbulent profile, from the outer edge to a point near the surface, the velocity remains reasonably close to the freestream velocity,then it rapidly decreases to zero at the surface. In contrast, the laminar velocity profile gradually decreases to zero from the outer edge to the surface. The key now is in the wall shear stress.
More shear stress results in higher skin friction drag. It is clear that velocity gradient near the surface for laminar flow is smaller than for the turbulent one, thus wall shear stress for the laminar flow is smaller than for the turbulent one. This means that laminar flow has smaller skin friction drag than the turbulent flow due to faster velocities near the surface.
The basic reason is that the energy to create the turbulence comes in the first place from the forward motion of the aircraft or air vehicle. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Why does a turbulent boundary layer cause more friction drag than a laminar boundary layer? Ask Question.
Asked 12 months ago. Active 11 months ago. The farther one moves away from the surface, the fewer the collisions affected by the object surface.
Laminar boundary layers - Oxford Scholarship
This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. Engineers call this layer the boundary layer because it occurs on the boundary of the fluid. The details of the flow within the boundary layer are very important for many problems in aerodynamics, including wing stall , the skin friction drag on an object, and the heat transfer that occurs in high speed flight. Unfortunately, the physical and mathematical details of boundary layer theory are beyond the scope of this beginner's guide and are usually studied in late undergraduate or graduate school in college.
We will only present some of the effects of the boundary layer at this time. On the slide we show the streamwise velocity variation from free stream to the surface. In reality, the effects are three dimensional.
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From the conservation of mass in three dimensions, a change in velocity in the streamwise direction causes a change in velocity in the other directions as well. There is a small component of velocity perpendicular to the surface which displaces or moves the flow above it. One can define the thickness of the boundary layer to be the amount of this displacement. The assumption of velocity-profile similarity is shown to reduce the partial differential boundary-layer equations to ordinary differential equations.
The chapter concludes with a qualitative account of the way in which aerodynamic lift is generated.
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