Boundary Layer Control
Summary
- Boundary Layer Suction
- Tangential Slot Injection
- Boundary Layer Blowing
- Wall Cooling
- Diffusers
- Selected References
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 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).
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).
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.
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.
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 (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.
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.
Boundary Layer Suction
Figure 1: Boundary Layer Suction
Tangential Slot Injection
Boundary Layer Blowing
Wall Cooling
Diffusers
Figure 4: Diffuser Drag Reduction
Related Material
Selected References