=1.5× 10
m²/s.Average axial velocity at inlet (x=-25 mm): U
=11.6 m/s. Reynolds number: U
D/=202,000. Atmospheric pressure at outlet.
Swirling boundary layer developing in a conical diffuser.
The conical diffuser is placed 100 mm downstream of a rotating swirl generator of
diameter D=260 mm and discharges into the atmosphere at X=510 mm. It has a 20°
included angle and an area ratio of 2.84.
The swirling flow is created by a rotating cylinder including a honeycomb screen at its inlet. At its outlet, the inlet swirl is close to solid-body rotation. Along the diffuser, the swirl is of sufficient magnitude to prevent boundary layer separation but just insufficient to cause recirculation in the core flow. The axial pressure gradient and the curvature of the streamlines have been found to be the dominant perturbations imposed to the swirling boundary layer as it exits the cylindrical part and enters the conical diffuser. The swirl is responsible for severe radial gradients near the wall for most of the turbulence quantities.
Air with a kinematic viscosity:
=1.5× 10 m²/s.
Average axial velocity at inlet (x=-25 mm): U=11.6 m/s.
Reynolds number: UD/=202,000.
Atmospheric pressure at outlet.
The following measurements are provided at station -25, located at x=-25
mm, 75 mm downstream of the swirl generator and 25 mm upstream of the
diffuser entrance. The swirl is close to solid-body rotation with a nearly uniform axial
velocity in the core region outside the boundary layers. The swirl number is Wmax/U=0.59
where Wmax is the maximal circumferential velocity. The wall
shear stress is 
wx/U²=0.00282 in x direction
and wz/U²=0.00190 in z direction. The
wall streamline angle is 
= tan
(W/U)
=0=34°.
/U
, 
/U 
/U² , 
/U²
, 
/U² 
/U² , 
/U²
, 
/U² ² (deduced) Hot-wire velocity measurements have been carried out using a single wire probe for the
mean quantities and an X-wire probe for the turbulence quantities. It has been possible to
measure all Reynolds stresses using the technique of rotating the probes ±45°. It
is worth mentioning that the velocity measurements are made in traverses normal to the
diffuser wall along y axis (y is perpendicular to x but not to X).
Wall stress estimated using the logarithmic law of the wall. The
two components wx
and wz are
determined using the value of .
Static pressure measurements using wall taps. The pressure coefficient is defined as 
=2
/
U².
(
The following measurements are available at 7 stations along the diffuser: x=025, 060, 100, 175, 250, 330, 405 mm ($$$ in the file names)
/U , /U /U² , /U²
, /U² /U² , /U²
, /U² ² (deduced) (files Mm$$$.dat) wx/U², wz/U² (files Mm$$$.dat) The following measurements are available along the diffuser.
The calculations should be performed for the whole diffuser (not only for the boundary layer).
The calculation of the duct flow should be started at station x=-25 mm using the experimental values provided as inlet conditions.
The diffuser discharges to the atmosphere at X=510 mm. Zero gradients may be assumed for the flow variables.
The following results should be plotted and compared with the data.
normalized mean velocity, Reynolds
stress and k profiles against y (perpendicular to diffuser wall) distribution wx/U² and wz/U²
distributions Armfield et al. (ref. 2.) have used a k-
and an algebraic Reynolds stress turbulence model
with a two-layer wall function to calculate this case. The use of a two-layer, rather than
a single-layer, wall function has been found to be necessary to accurately predict the
level, location and the axial variation of the near-wall peak in turbulence quantities.
