About the atmospheric boundary layer
The UC Davis Atmospheric Boundary Layer Wind
Tunnel (UCD-ABLWT) was specifically designed to model the
turbulent characteristics of the atmospheric boundary layer. The
“atmospheric boundary layer” is a layer of air
covering the earth in which the airflow is influenced by
viscosity (fluid friction). Since the thickness of atmospheric
boundary layer is determined by the height at which surface
friction no longer affects the general flow of the wind, the
boundary layer thickness depends on the shape and condition on
the surface.
Over vast flat regions such as oceans or deserts
the boundary layer height may be as low as 500 feet, while above
large cities with lots of tall buildings, it may be as high as
1500 feet. Within this layer of air, motion is generally gusty
or “turbulent,” with the wind changing speed and direction
rapidly.
Wind tunnel facility
The tunnel is an open return
type with an overall length of 70 ft (21m). The entrance section
has a contraction area that minimizes the freestream turbulence
level of the air as it passes through the section. A
commercially available air filter follows the contraction area.
The filter reduces the large scale pressure fluctuations of the
flow and limits the size of airborne particles entering the wind
tunnel. Large scale turbulence is reduced by honeycomb flow
straighteners. Three triangularly shaped spires are placed
directly downstream of the flow straighteners to provide
favorable turbulent characteristics in the boundary layer flow.
The flow development section
is 40 ft (12.2 m) long and has walls that diverge to reduce the
pressure gradient in the flow direction. Roughness elements are
placed on the floor of this section to artificially thicken the
boundary layer. Thick boundary layer flow is desired, since
larger models can be tested and thus measurements of higher
resolution can be made. The wind tunnel was specifically
designed to develop a boundary layer thickness of about 3 ft (1
m) with a maximum full-scale wind speed of 13 ft/s (4.0 m/s) in
the test section.
The test section is 8 ft
(2.44 m) long, 5.5 ft (1.71 m) high, and 4 ft (1.2 m) wide in
the cross section. The ceilings of the flow development section
are adjustable for longitudinal pressure gradient control.
Access to the test section is through a framed acrylic door,
which serves as one of the two vertical acrylic walls. Six
clamps each on the top and bottom of the door, as well as two
large clamps at each end, are used to seal the door. Additional
sealing is achieved by the use of O-ring surgical tubing that
was pressurized and placed around the edges of the door and
between it and the metal frame.
In the test section, a three
dimensional probe positioning mechanism provides fast and
accurate sensor placement. The scissor arms of the mechanism,
which monitors vertical probe motion, are made of
aerodynamically shaped struts to minimize flow disturbances.
Gases can be injected into the tunnel either
for flow visualization or to simulate smoke stack emissions.
The diffuser section is 7.8
ft (2.37 m) long and has an expansion area that provided a
continuous transition from the rectangular cross sectional area
of the test section to the circular cross sectional area of the
fan. To eliminate upstream fan swirl effects and avoid flow
separation in the diffuser section, fiberboard and honeycomb
flow straighteners are placed between the fan and diffuser
sections.
The
fan, 6 ft (1.83 m) in diameter, has eight constant pitch blades
and is driven by a 75 horsepower variable speed DC motor. A dual
belt and pulley drive system couples the motor and the fan.
