Methods and apparatus for suppressing engine test cell howl

Methods and apparatus for suppressing cell howl with negligible impact on engine test conditions are described. The apparatus, in an exemplary embodiment includes a flow distorter configured to be positioned close to a nozzle exit of an engine nozzle, and a flow distorter support for maintaining the flow distorter at a selected location. The flow distorter is adjustably secured to the support so that a distance at which a tip of the distorter is located relative to support is adjustable.

BACKGROUND OF THE INVENTION 
This invention relates generally to turbine engines, and more specifically, 
to reducing, if not eliminating, an intense acoustic tone emitted during 
certain test operating conditions of such engines. 
For testing, turbine engines typically are enclosed within a test cell. The 
test cell is sufficiently large so that the engine is completed enclosed 
within the cell, and test operators can move about the cell to set test 
parameters and check engine performance. Under certain operating 
conditions, and with the engine located within a test cell, an intense 
acoustic tone is emitted. This tone sometimes is referred to as "cell 
howl". 
The intensity of the tone can sometimes damage the test cell and engine. 
Known attempts to eliminate cell howl, as described in Jones et al., The 
Acoustic Response Of Altitude Test Facility Exhaust Systems To 
Axisymmetric And Two-Dimensional Turbine Engine Exhaust Plumes, DGLR/AIAA 
92-02-131, May, 1992, include attempting to optimize a location of the 
cell exhaust collector relative to an engine nozzle exit, injecting water 
in an engine exhaust plume, locating volume resonators in the cell exhaust 
collector, and inserting a secondary concentric duct into the exhaust 
collector. These known attempts have not proven one hundred percent 
successful in eliminating cell howl for all engine nozzles, and the howl 
generated by some nozzles is not even always reduced. 
BRIEF SUMMARY OF THE INVENTION 
The present invention, in one aspect, relates to a flow distorter apparatus 
which suppresses cell howl with negligible impact on engine test 
conditions. The apparatus, in an exemplary embodiment, includes a flow 
distorter configured to be positioned close to a nozzle exit of an engine 
nozzle, and a flow distorter support for maintaining the flow distorter at 
a selected location. 
The flow distorter is adjustably secured to the support so that a distance 
at which a tip of the distorter is located relative to support is 
adjustable. More specifically, and in an exemplary embodiment, the support 
includes a base having an adjustable track to enable axial adjustment of 
the flow distorter relative to a nozzle exit plane. The support further 
includes a first support arm extending vertically from the base, and a 
second support arm extending angularly from the base to the first support 
arm. The first and second support arms are movable relative to the 
adjustable track and are secured thereto, for example, by bolts. 
Similarly, the flow distorter is secured to the first arm by a bolt, and 
the distorter can be adjusted relative to the first support arm so that 
the extent to which the distorter tip extends into the engine exhaust flow 
can be adjusted. 
Prior to testing operation of an engine in a test cell, the distorter axial 
position PAP and the distorter penetration P into the engine exhaust flow 
is selected. The distorter should be positioned so that it has minimal, or 
no, impact on engine performance but suppresses, if not totally 
eliminates, cell howl. 
The above described flow distorter apparatus is effective in reducing cell 
howl and is quick, easy, and inexpensive to fabricate and install in 
existing as well as newly fabricated test facilities. In addition, the 
flow distorter apparatus is external to the engine nozzle, which minimizes 
its impact on test measurements. By eliminating cell howl and minimizing 
test measurement impact, engine tests can be performed reliably and under 
consistent conditions, and cell howl damage is avoided.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic illustration of broadband supersonic jet noise, and 
FIG. 2 is a schematic illustration of a supersonic jet screech mode. In 
the broadband noise condition, air flows in a direction 10 from an exit 12 
of an engine nozzle 14. Random turbulence 16 generates noise, as shown in 
FIG. 1. Referring to FIG. 2, shock screech occurs when turbulence convects 
through a quasi-periodic shock cell system in an exhaust plume 18. 
Acoustic waves travel upstream to nozzle exit 12 and then generate new 
turbulence of the same wavelength as the acoustic waves. When the acoustic 
wavelength is close to the same spacing as the shock cells, a resonant 
feedback loop 20 is established which can produce extremely discrete and 
intense acoustic tones 22. These acoustic tone 22 generate cell howl. 
FIG. 3 is a schematic illustration of a flow distorter apparatus 100 
located in an operative position relative an engine exhaust nozzle N and a 
test cell exhaust collector C. Apparatus 100, as shown in FIG. 3, includes 
a flow distorter 102 positioned close to a nozzle exit E of nozzle N, and 
a flow distorter support 104 for maintaining flow distorter 102 at a 
selected location. Flow distorter 102 has a square cross sectional shape. 
Distorter 102 can, however, have many other geometric shapes, and 
distorter 102 does not necessarily even have to be symmetric about an 
axis. In addition, and in an alternative embodiment, distorter 102 
includes a water passage for enabling cooling water to flow through 
distorter 102. 
Distorter 102 is adjustably secured to support 104 so that a distance at 
which a tip 106 of distorter 102 is located is adjustable. More 
specifically, support 104 includes a base 108 which is secured to an 
adjustable track 110 to enable axial adjustment of flow distorter 102 
relative to a nozzle exit plane. Base 108 is secured to track 110 by, for 
example, bolts. Support 104 further includes a first support arm 112 
extending vertically from base 108, and a second support arm 114 extending 
angularly from base 108 to first support arm 112. Similarly, flow 
distorter 102 is secured to first arm 112 by a bolt, and distorter 102 can 
be adjusted relative to arm 112 so that the extent to which tip 106 
extends into engine exhaust flow moving in a direction indicated by arrow 
116 can be adjusted. 
Flow distorter 102 is fabricated from stainless steel. Support 104 is 
fabricated from carbon steel. Of course, many different types of material 
can be used and the present invention is not limited to use in connection 
with any one particular material. 
Prior to testing operation of an engine in a test cell, and referring now 
to FIG. 4 which is a schematic illustration of the location of apparatus 
100 relative to nozzle exit E, distorter axial position PAP and distorter 
penetration P into an exhaust flow 118 are selected. Specifically, 
distorter axial position PAP refers to the distance from nozzle exit E to 
distorter 102. Distorter penetration P refers to the length of the section 
of distorter 102 located within, or penetrating into, engine exhaust flow 
118. 
The particular engine being tested, and the particular shape of distorter 
102 impact the desired axial position PAP and penetration P. Distorter 102 
should be positioned so that distorter 102 has no impact on engine 
performance but suppresses, if not totally eliminates, cell howl. In one 
particular test, it was empirically determined that a penetration P of 4.0 
inches and position PAP of 6.9 inches provided acceptable results. Again, 
the engine characteristics and the distorter characteristics all may have 
an impact on the specific position of the distorter relative to the engine 
nozzle exit. 
FIG. 5 is a perspective view of distorter apparatus 100. As clearly shown 
in FIG. 5, flow distorter 102 has a square cross sectional shape and is 
adjustably secured to support 104. Specifically, distorter 102 extends 
into an opening 120 in arm 112, and an adjustment screw 122 extends 
through arm 112 and is tightened against distorter 102 to maintain 
distorter 102 in position. 
Many variations of the above described flow distorter apparatus are 
possible. For example, the support as well as the flow distorter may have 
many different shapes and configurations. Also, the flow distorter may, 
for example, be supported from the ceiling rather than the floor of the 
test cell as shown in the drawings. Generally, the support performs the 
function of controlling the extent to which the flow distorter extends 
into the exhaust flow, and the axial position of the flow distorter 
relative to the nozzle exit. The distorter performs the function of 
disrupting the exhaust flow sufficiently so that cell howl is suppressed, 
but not to the extent of impacting engine operation. Specifically, the 
distorter eliminates the resonant feedback loops in the exhaust plumes, 
which eliminates the acoustic tones generated by such loops. Many 
different supports and distorters can be configured to perform these 
functions, and therefore, the present invention is not limited to the 
specific embodiments described and illustrated herein. 
While the invention has been described in terms of various specific 
embodiments, those skilled in the art will recognize that the invention 
can be practiced with modification within the spirit and scope of the 
claims.