ERCOFTAC Bulletin - Centre Acoustique

and potential fatigue. However, the convection of the hot
spots (entropy waves) also generates noise [11]. Hence,
these two elements are intrinsically linked the hot zones
both influencing aerodynamics, aeroacoustics and life.
The flow in passage containing the blades, is connected
by a cavity like (rim seal) zone to the internal cooling system.
Rim seal flows are also, in themselves, intrinsically
unsteady (see O’Mahoney et al. [12]). They involve complex
three-dimensional shear layers. These impinge on
downstream surfaces, thus generating an acoustic feedback
loop. The blade passing may also potentially excite
shear layer cavity interaction resonances or underling unsteady
rotor-stator cavity flows.
Through the development bypass ratio engines the jet
noise is substantially reduced. Consequently, Tyler and
Sofrin [13] explored compressor/fan noise arising from
rotating pressure fields spinning with the compressor rotors.
Fan noise has both tonal and broadband components
[14]. Also, the unsteady rotor-stator interactions
will produce sound, notably the interaction of the
rotor wakes and tip vortices with downstream stators.
These mechanisms will also give rise to aerodynamic loss.
Hence, there is a key link between aeroacoustics and
aerodynamics. This is particularly so when it is noted
that acoustic feedback can substantially modify the mean
flow. When exploring flow control, the link can become
even clearer. For example, trailing edge ejection can be
used to fill in low momentum zones in wakes. This will
improve aerodynamic performance but also can reduce
noise. Perforated aerodynamic surfaces on intakes can
both be used to reduce noise and potentially improve
aerodynamic performance [15].
(a) (b)
(c) (d)
Figure 2: Open rotor intake and duct grid views along
with total pressure contours (mid-section plane of a nonbifurcated
intake).
As outlined in Eastwood [16], in a real engine upstream
of the nozzle, there is a compressor, combustor and turbine.
There are also numerous other associated geometrical
features. It has been supposed for some time that
these may influence the jet noise itself [17] [18]. As noted
by Moore [17] the engine internal noise can modify the jet
structure. Hence, as can be seen from the above although
noise is a byproduct of turbulent flow it can also influ-
ence the flow structure itself. Indeed, Epstein et al. [19]
explored to use noise to control compressor stability and
[20] separation control on airfoils. Ffowcs-Williams [21]
also outlines the potential of noise to both reduce/cancel
noise and also enhance aerodynamic performance. The
latter are not unchartered waters and pose great research
challenges. Nonetheless, the rich flow physics and acoustics
insights provided by LES, along with the potential
to precisely control excitations might enable us to make
advances in the area of using sound to reduce sound and
also aerodynamic losses. Hence, there seems a crucial
need to treat aerodynamics and aeroacoustics more holistically.
(a) (b)
Figure 3: Multiblock grid zones and instantaneous axial
velocity contours for a heated jet nozzle-pylon-wing-flap
simulation.
Turbochargers (see Baines [22], Winterbone, Nikpour
and Frost [23]) are subjected to pulsating flow from engine
exhausts. As shown by Baines this can alter the
efficiency by nearly 30%. This pulsation might also be
a factor in generating excessive turbocharger noise. For
radial compressors the impeller blade wakes, as well as
resulting in an aerodynamic performance penalty can result
in fatigue and also substantial acoustic related problems
[24]. Blanco et al. [25] observe that there is a substantial
acoustic coupling between the rotor, volute and
piping system for centrifugal pumps. Hence, they perform
CFD simulations that are coupled to a model for
the piping system acoustic modes
1.3 Role of LES
The use of acoustics to positively control flow is far from
a new idea. Unfortunately, acoustic fluctuation amplitudes
are low relative to for example that of the turbulence.
The acoustic energy levels needed to exert substantial
flow control can be high (sound pressure levels of
around 100 dB) [20] and there is frequency dependence.
However, an area of worthwhile exploration would perhaps
be to see if ambient noise could be used to produce
some modest positive gain. Certainly, with aero
engines fractions of a percent in efficiency are of tremendous
economic importance. LES solutions readily reveal
both flow structural and acoustic information (see,
for example, the Figure 1 chevron jet flow LES). Hence,
complex interactions can be more readily observed and
understood. Techniques are available to correlate acoustic
signatures with flow events. An adjoint process does
not seem unreasonable where the influence of the acoustic
events on the flow can be explored and adjustments
made to use the acoustic signatures to positively influence
the flow. At a simple level, an acoustic source term
at a fixed frequency could be added to numerical solutions.
The resulting flow field could be Fourier analyzed.
Variations in response at the frequency of excitation or
harmonics of it could be indicative of the impact of the
acoustic source on the acoustic or flow field. These two
aspects could be separated out by a spatial Fourier transform
and hence looking at phase velocities.
ERCOFTAC Bulletin 90 41