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【范围】
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Generally it is desirable during static engine noise testing
to simulate the noise-generating processes that occur during takeoff or
landing. One factor that may have significant Influence on fan noise from
turbofan engines during static testing is the airflow distortion encountered
by the low-pressure fan stage. .This report presents technical details on
devices that "condition the airflow entering the engine to more nearly
simulate flight operations without introducing a detrimental impact on sound
propagating from the engine.
Noise generated by the fan craaponent of a non-inlet-guide-vane turbofan
engine operating statically can be significantly different than when
operating with forward velocity, i.e., during flight operation or in a wind
tunnel. When operated statically, the airflow into the engine has higher
levels of both steady and unsteady distortions. As a result the fan may
generate noise levels higher than would be observed at equivalent engine
power settings in flight. The higher noise levels are more likely to occur
in the case of turbofan engines with a single stage fan operating at
conditions where the blade passing frequency has been designed to be
acoustically cut-off. This normally occurs at approach power. Reference 1
cites some of the early observations of static and flight noise differences
and offers possible explanations. A summary of observations on static noise
testing as deduced from industry and government sponsored research programs
as of the mid 1970 is provided in Reference 2.
A schematic representation of the stream of airflow into an inlet for the
Static and flight situations is provided in Figure 1. The flight situation
is visualized as one in which the inflow is captured in a slightly contracted
stream tube (see Figure 1(b)).
Static engine operation is visualized as a sink type flow, see Figure 1(a).
For the flight situation, the sound pressure level of the fan tone that
results from ingestion of normal levels of atmospheric turbulence has been
shown to be small. This conclusion is based on the analytical studies
presented in References 3 and 4 . There are several sources of distortion
during static operation: they include atmospheric turbulence, vorticity
caused by flow over the engine test stand structural and vorticity generated
by the presence of the ground plane. Atmospheric turbulence can be generated
by wind gusts, thermal gradients, flow over local terrain, (e.g., hills,-
trees, buildings), and reingestion of exhaust flow. The effect of the large
contraction of the static flow field on atmospheric turbulence and inducement
of flow over and around, the test stand structure can cause significant
distortion in the flow entering the fan. Noise can be produced by
interactions between the rotating fan blades and the distorted airflow. The
level of noise that can be generated depends upon the fan stage design,
engine power setting and the local environmental conditions. Some fan stage
designs are such that there may be negligible extraneous noise developed by
inflow distortion.
3. SCOPE: This Aerospace Information Report provides guidelines for hardware
design. Guidelines on such items as engine types# test arrangements, and
test purposes that would benefit from the use of an inflow control device
during static noise testing are outside the scope of this report.strRefField
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