32
2 Wind Tunnels and Other Aerodynamic Test Facilities
Fig. 2.3 Effect of temperature on the performance of a wind tunnel (© ETW)
2.3 Reynolds Number Effect and Laminar to Turbulent
Transition
Due to the insufficient Reynolds number in most wind tunnels, the laminar to turbulent
transition of the boundary layer developing on the model wall can occur further
downstream compared to flight. Yet, the state of the boundary layer has a decisive
influence on transfer phenomena (skin friction, wall heat transfer) as well as on
boundary-layer separation and shock-wave/boundary-layer interactions. On an actual
wing profile, the laminar to turbulent transition, if any, occurs very close to the
leading edge (a few percent of the chord), whereas it is further downstream on the
profiles tested in a wind tunnel (often at mid chord or further downstream), when
tested a lower Reynolds number. One method to overcome this difficulty is to trigger
transition by artificial roughness on the model, a technique often referred as boundary
layer tripping. The height of the roughness or boundary layer trip devices (sometimes
called turbulators) is adapted to the thickness of the boundary layer at the trip location
and to the Reynolds and Mach numbers of the test.
The principle of the trip device is to fix the location of transition; its height is
of similar order to the boundary layer and can be calculated using boundary-layer
theory. For 2D wings, the transition can be tripped by a simple wire glued to the
wall near the leading edge. A stripe of tape works in some cases and a rough band
consisting of carborundum grains glued to the wall is also used. The more recent trip
device consists of a thin saw tooth tape. The jagged pattern is effective at generating
2 Wind Tunnels and Other Aerodynamic Test Facilities
Fig. 2.3 Effect of temperature on the performance of a wind tunnel (© ETW)
2.3 Reynolds Number Effect and Laminar to Turbulent
Transition
Due to the insufficient Reynolds number in most wind tunnels, the laminar to turbulent
transition of the boundary layer developing on the model wall can occur further
downstream compared to flight. Yet, the state of the boundary layer has a decisive
influence on transfer phenomena (skin friction, wall heat transfer) as well as on
boundary-layer separation and shock-wave/boundary-layer interactions. On an actual
wing profile, the laminar to turbulent transition, if any, occurs very close to the
leading edge (a few percent of the chord), whereas it is further downstream on the
profiles tested in a wind tunnel (often at mid chord or further downstream), when
tested a lower Reynolds number. One method to overcome this difficulty is to trigger
transition by artificial roughness on the model, a technique often referred as boundary
layer tripping. The height of the roughness or boundary layer trip devices (sometimes
called turbulators) is adapted to the thickness of the boundary layer at the trip location
and to the Reynolds and Mach numbers of the test.
The principle of the trip device is to fix the location of transition; its height is
of similar order to the boundary layer and can be calculated using boundary-layer
theory. For 2D wings, the transition can be tripped by a simple wire glued to the
wall near the leading edge. A stripe of tape works in some cases and a rough band
consisting of carborundum grains glued to the wall is also used. The more recent trip
device consists of a thin saw tooth tape. The jagged pattern is effective at generating