Active Flow Control Methods for Aerodynamics and Aeroacoustics

: Aerofoil Trailing-edge Noise Applications

Abstract

The improvement of aerodynamic and aeroacoustic characteristics of aircrafts and wind
turbines are of fundamental importance for both their performance and operation. The
economic and operational viability of aviation and wind energy industry are closely related
to the aerodynamic efficiency of their components (which translates directly into costs and profit)
and also, to their compliance with the international noise regulations, which are becoming
increasingly stringent as it affects detrimentally the population’s health near airports and wind
farms. In the aviation industry, aerodynamically generated noise has gained attention from
the early 1970’s when the loud turbojet powered civilian aircraft started to operate. From this
time, jet noise annoyance has been extensively addressed and studied in the following decades,
which resulted in significant progress in our understating of the noise generation mechanisms
and development of technologies to reduce the noise. Consequently, other noise generation
mechanisms contributing to the total noise emissions, such as the airframe noise, started to gain
more attention.
This study investigates the use of active flow control techniques to assess the effectiveness
of such flow control methods on both the aerodynamic and the aeroacoustic performance of an
aerofoil. We are interested in the use of such active flow control methods for reducing trailing-edge
noise. While the previous studies have addressed the problem of trailing-edge noise using mostly
passive flow control methods, the active flow control technique has received less attention even
though it could provide better combined performance from both the aerodynamic and aeroacoustic
perspectives. Earlier studies have shown that the application of flow suction and/or blowing
could effectively change the turbulent flow structures which are responsible for lift augmentation,
drag reduction, stall management and noise scattering. Even though the technique has shown
promising outcomes, the literature lacks an in depth understanding of the underlying mechanisms,
especially those corresponding to the turbulence statistics for a non-zero pressure-gradient
structure. Therefore, the proposed study aims address this gap in the field of aerodynamics and
aeroacoustics and provide some reliable and high-quality data and analysis to investigate the
effectiveness of flow control techniques for reducing aerofoil self-noise at source.

The effects of the different flow control techniques are evaluated through Large Eddy Simulations
carried out at the University of Bristol High Performance Computing facility, Blue
Crystal. Simulations are carried out for a NACA 0012 aerofoil in a subsonic regime (Mach number
of M = 0.087) and chord-based Reynolds number of Rec = 4x105, immersed in a turbulent
boundary layer, triggered by a tripping-device. The LES simulations for the clean aerofoil configuration,
i.e. in the absence of flow control, was validated thoroughly against experimental
data available in the literature for the NACA 0012 aerofoil at the angle of attack of α= 0o. A
large number of high-quality LES simulations were then performed to study the effect of flow
control upstream of the trailing-edge on the hydrodynamics of the boundary layer and the noise generation mechanisms.

The numerical model and setup configurations were determined based on the available computational
resources, the data available in the literature and the author mesh and domain size
independency studies. Reynolds Averaged Navier-Stokes (RANS) simulations were carried out to
initiate the Large Eddy Simulations (LES). The LES turbulence modelling was implemented
using the dynamic sub-grid model formulation of Lilly (1992) [1] as closure. The Baseline results
are presented and validated against the experiments of Garcia-Sagrado (2007) [2], for the NACA
0012 at the angle of attack of α= 0o. The results of the current LES have shown good agreement
with the experimental data. The details of the simulations and the numerical model used for
evaluating the changes on the flow structure imparted by the flow control treatments for the
Baseline aerofoil, i.e. no flow control, are delineated and discussed in detail in Chapter 3.

The effects of the uniform flow suction applied upstream of the trailing-edge on the boundary
layer structures are discussed in Chapter 4. The pressure and velocity statistics show that the
technique can considerably change the time and length scales of turbulence and the dynamics
of both its large- and small-scale turbulent structures. The pressure frequency spectra results computed from the time-dependent pressure fluctuations show reduced power spectrum density
magnitudes of the flow control treated cases at low to mid-high frequencies for locations
downstream of the flow control device and near the trailing-edge. The increase of the boundary
layer structures length- and time-scales are also observed through the spatial-temporal cross correlations
analysis of the pressure field. Results are also presented for the convection velocity
of the flow structures over the trailing-edge area, showing a reduction relative to the Baseline
case.
The application of flow blowing for controlling the flow field near the aerofoil trailing-edge
is presented in Chapter 5. Results are presented for different flow blowing rate and blowing
angles. The evaluation of the pressure and velocity statistics show increased magnitudes of their
frequency-energy content at low frequencies while reduced PSD are observed at high frequencies
for the highest severity flow blowing case injected perpendicularly to the boundary layer. The use
of flow treatments is shown to lead to an increase in both the time and length-scales of turbulence
computed through the spatial-temporal cross correlations. The turbulent length-scales show
significant augmentation at high frequencies in the vicinity of the trailing-edge. An increase in
length-scale was also observed at low frequency ranges, where the inclined uniform flow blowing
case with the highest Cμ intensity (Cμ is the flow control severity), shows the more prominent
augmentation. The convection velocities decrease considerably for all the cases, where the inclined
flow blowing cases show the highest reductions. Furthermore, the streamwise lifespan of the flow
structures are seen to rise prominently for the inclined flow blowing.
Finally, the concepts of co-flow (zero-net mass flux) and boundary layer periodic excitations
(non-uniform inflow and outflow) are evaluated and compared to both the Baseline and the
uniform flow control manipulations, as reference. These results are presented in Chapter 6. While
the changes on the mean aerodynamic quantities are mild, the alterations on the flow turbulence
statistics are found to be substantial. The oscillating (periodic) boundary layer treatments show
significant augmentation on the time- and length-scales of the turbulent structures, further
than those accomplished by the uniform flow blowing or suction. Overall, the computed pressure
autocorrelations exhibit higher width, the spanwise correlations magnitudes are found to be
stronger and the decays, smoother. The boundary layer energy-frequency content computed from
the measured velocity fluctuations show a frequency shift of energy due to the changes to the
boundary layer flow structures. The details are provided in Chapter 6.
The presented flow control treatments have shown potential benefits to reduce trailing-edge noise as they have all shown to be able to cause changes to the turbulent flow structures responsible
for the trailing-edge noise generation. The effects of the studied flow control methods on the
turbulence energy content and its scales can directly influence the noise generation mechanisms.
The application of the proposed method will, however, depend on the aerodynamic and aeroacoustic
performance and the design requirements of the aero-component. The investigation here
provides a good starting point for a better understanding of the mechanisms involved in noise
generation and the effect of flow control on boundary layer structures and also demonstrates how
such techniques can be later used as an engineering solution for reducing noise from different
aero-components.

Date of Award	26 Nov 2020
Original language	English
Awarding Institution	University of Bristol
Sponsors	National Council of Scientific and Technological Development
Supervisor	Alberto M Gambaruto (Supervisor) & Mahdi Azarpeyvand (Supervisor)