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Description |  |
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 20o 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.
= 1.5 × 10-5 m2/s. = 202,000.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/Uo = 0.59 where Wmax is the maximal circumferential velocity.
The wall shear stress is 
/Uo2 = 0.00282
in the x direction and 
/Uo2 = 0.00190
in the z direction. The wall streamline angle is 
= tan-1(W/U)y=0 = 34o.
Profiles of
/Uo2,
/Uo2,
/Uo2,
/Uo2,
/Uo2,
, 
/Uo2Experimental Details | |
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 ±45o. 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
and are
determined using the value of .
Static pressure measurements using wall taps. The pressure coefficient is defined as
Cp = 2p/(
Uo2).
(U),
(W): 2%;
(Reynolds stresses): 10%;
(positions): ±0.01 mm.
Available Measurements | |
The following measurements are available at 7 stations along the diffuser: x = 025, 060, 100, 175, 250, 330, 405 mm ($$$ in the file names)
/Uo2,
/Uo2,
/Uo2,
/Uo2,
/Uo2,
/Uo2 (files Mm$$$.dat)/Uo2,
/Uo2 (files Mm$$$.dat)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.
At x= 025, 060, 100, 175, 250, 330, 405 mm:
Along the diffuser wall:
/Uo2 and
/Uo2
distributions.Previous Numerical Studies | |
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.
Main references | |
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