Copyright © A. Filippone (1996-2001). All Rights Reserved.

Boundary Layer Control


Boundary layer control (BLC) is a generic definition to classify all those methods that can be used to reduce the skin friction drag, by controlling the turbulent transition, the development of the turbulent flows, and the separation (laminar as well as turbulent), all phenomena occurring within the boundary layer.

Research in boundary layer flows at all speeds (including transition and separation) is a wide topic in aerodynamics. Understanding the stability of the boundary layer is in fact of great importance for the development of technology for skin friction drag reduction. Some technical methods for BLC, both powered and unpowered, are described below.

Boundary Layer Suction

Boundary layer suction is is used on aircraft wings to prevent laminar and turbulent separation, by removing flow of low momentum. This technique is also used in some wind tunnels to remove the boundary layer. The method consists in operating a powered system to suck boundary layer flow from closely spaced vertical slots (see Fig. 1).

Boundary Layer Suction
Figure 1: Boundary Layer Suction

The technique can be so effective that in laboratory experiments it was observed fully laminar flow for very high Reynolds numbers and supersonic speeds. The corresponding drag levels are very low (close to a flat plate drag, that is a theoretical minimum).

The development of a boundary layer suction system is quite complicated, since it involves considerations on optimum slot placement, structural modifications, power system, amount of suction, etc. (Pfenninger, 1977).

Tangential Slot Injection

Slot injection flow is the result of a sudden nozzle acceleration that occurs between two closely spaced wing elements (Fig. 2). Flow from the slot is at high speed and pushes a slower boundary layer on the suction side of the flap. The effect is that boundary layer separation is delayed even at large flap angles.

Slot Injection

Figure 2: slot injection scheme

The method of slot injection is also used to cool downstream high temperature elements (gas turbines and engine components), in some wind tunnels (ground boundary layers), in three dimensional flows, at both subsonic and supersonic/hypersonic speeds.

Skin friction drag reduction can be evaluated with momentum balance downstream the jet flow after measuring the shear flow, or indirect methods, such as force balance.


Boundary Layer Blowing

Blowing is somewhat similar to the slot injection. Blowing a boundary layer on high temperature components (blades of gas turbines) can enhance the heat exchange between gas and blade surface, or maintain a thin layer of colder flow that allows the systems to function with very high gas flow.

Wall Cooling

Wall cooling (in air) is used to damp the Tollmien- Schlichting waves, which increases the minimum critical Reynolds number for transition. The physical result is sketched in the figure below.

Wall Cooling

Figure 3: Wall cooling effect on critical Re

Skin friction drag reduction rates of up to 18 % have been measured in laboratory conditions on a flat plate. The cooling method is however unpractical in general cases (for ex. aviation), and might be confined to laboratory devices.


Diffusers can be used to reduce the viscous drag of internal flows. These devices are divergent channels that exploit the momentum deficiency in the boundary layer with an increased turbulence created by the in the channel. The diffuser has an optimum length and divergence angle. Skin friction drag is minimum at the optimum point, as shown in the figure below.

Diffuser Drag Reduction
Figure 4: Diffuser Drag Reduction

Related Material

Selected References

  • Lachmann GV (editor), Boundary Layer and Flow Control, Pergamon Press, New York, 1961.

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Copyright © A. Filippone (1996-2001). All Rights Reserved.