Boundary-layer bleed is an invaluable tool for controlling the
airflow in supersonic aircraft engine inlets. Incoming air is
decelerated to subsonic speeds prior to entering the compressor
via a series of oblique shocks. The low momentum flow in the boundary
layer interacts with these shocks, growing in thickness and, under
some conditions, leading to flow separation. To remedy this, bleed
holes are strategically located to remove mass from the boundary
layer, reducing its thickness and helping to maintain uniform
flow to the compressor. The bleed requirements for any inlet design
are unique and must be validated by extensive wind tunnel testing
to optimize performance and efficiency.
To accelerate this process and reduce cost, researchers at the
NASA Lewis Research Center initiated an experimental program to
study the flow phenomena associated with bleed holes. Knowledge
of these flow properties will be incorporated into computational
fluid dynamics (CFD) models that will aid engine inlet designers
in optimizing bleed configurations before any hardware is fabricated.
This ongoing investigation is currently examining two hole geometries,
90° and 20° (both with 5-mm diameters),
and various flow features (see the following figure):
The tests are being conducted in NASA Lewis' 15- by 15-Centimeter
Supersonic Wind Tunnel, which is an open-loop, continuous-flow
facility. Mach number variation is provided by interchangeable,
fixed-geometry nozzle blocks. The tunnel can be run subsonically
from Mach 0.2 to 0.8, and supersonically at discreet Mach numbers
of 1.67, 2.0, 2.5, and 3.0.
To date, several of these areas have been investigated. For both
90° and 20° holes, pitot and five-hole probe surveys downstream
of single bleed holes revealed an oblique barrier shock originating
from the back lip of the hole. Pressure sensitive paint applied
to the wall indicated flow expansion and compression regions in
the vicinity of these orifices (see the figure below).
For the 20° hole, alignment with the local flow direction was found
to be critical for supersonic Mach numbers. The misalignment of
a slanted hole fabricated in a rotatable plug was varied between
0° and 30° at Mach numbers of 0.60, 1.67, and 2.50. For each angle,
a mass flow survey was taken for plenum pressures varying between
full bleed and no bleed. It was found that at Mach 2.50 as little
as one degree of flow misalignment resulted in a drop in mass
flow through the orifice. The lower Mach number test showed similar
behavior; however, mass flow hindrance proved less severe.
In inlet applications, holes are arranged in fields, each on the
order of two to three diameters away from neighboring holes, so
a two-hole interaction for this type of arrangement was studied.
Two 90° holes with the rotatable plug geometry were located two
diameters from each other: one hole was at the center of the plug
and the other was offset from the center. The plug was then rotated
between 0° (holes lie in a plane normal to the flow direction)
and 90° (holes lie in a plane parallel to the flow direction).
Mass flow surveys were again taken for each angle. As with the
20° misalignment experiment, sensitivity to location increased
with Mach number. However, although shallow orientations resulted
in a drop in the flow coefficient, at angles above 50°, the flow
coefficient improved. This shows the potential for multihole configurations
to exhibit more efficient bleed properties than their single-hole
counterparts.
Continuing research is being conducted to study various other
phenomena. As air enters a 90° hole, it separates from the hole
wall, creating a blockage throat that hinders the hole's performance.
Flow-visualization experiments using oil trace techniques verified
this feature; however, flow-field measurements in the hole are
yet to be recorded. Surveys of the trailing vortices created by
bleed holes are also planned, as well as studies of the problems
and benefits of bleed hole scaling.
Bodner, J.P.; and Greber, I.: Experimental Investigation of the Effect of a Single Bleed Hole on a Turbulent Supersonic Boundary-Layer. AIAA Paper 96-2797, 1996.
Davis, D.O.; Grimes, M.; and Schoenenberger, M.: Effect of Flow
Misalignment and Multi-Hole Interaction on Boundary-Layer Bleed
Hole Flow Coefficient Behavior. Prepared for the 1996 International
Mechanical Engineering Congress and Exhibit, ASME, Nov. 17-24,
1996.
Previous articleLast updated May 5, 1997
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