Aeroacoustics Flashcards
What is aerofoil self noise?
Noise generated by the interaction an aerofoil with its boundary layers and wake
- dominant at lower wind speeds/higher frequency
What is In-flow turbulence?
Noise caused by upstream atmospheric turbulence
- dominant at higher wind speeds/lower frequency
What is trailing edge noise?
Turbulent boundary layer interacts with trailing edge
- Minimised through design of aerofoil section at trailing edge
What is laminar boundary layer vortex shedding?
Instability in separated laminar flow from lower edge of blade
- Minimised by preventing separation through blade design
- Serrated leading edge effective
What is boundary layer separation and stall noise?
Separated boundary layer becomes turbulent and interacts with blade surface
- Increases with angle of attack
- Minimised by blade pitch regulation
What is trailing edge bluntness vortex shedding?
Instability in wake due to thickness of trailing edge
- Minimised by using a sharper blade profile
What is tip vortex formation?
Separated vortex flow interacts with blade surface
- Minimised through design of the tip shape
8 ways to reduce aerofoil noise
- serrated aerofoils
- porous treatment
- morphing structures
- surface treatment
- flow suction
- flow injection
- micro jets
- sweeping jets
Pros and Cons of Surface treatment to reduce noise?
+ very effective in controlling noise
+ minimum impact on aerodynamic performance
- Involves very complex flow physics
Where is future research focused for surface treatment?
- better understanding of flow physics and noise control mechanism
- more novel/effective treatment
- application to other aeroacoustic applications
When is FWH (Ffowcs Williams–Hawkings) used?
Aeroacoustic sources in relative motion with respect to a hard surface
Turbulence is moving
Two distinct regions of fluid flow
What are free space Green’s functions?
Adds a volume source to the continuity equation
Adds an external force to the momentum
Combined to form a ‘forced’ wave equation
What do monopoles, dipoles and quadrupoles look like in the context of the FWH integral
Monopiles have no spatial derivatives (only a temporal)
Dipoles have one spatial derivative (x_i)
Quadrupoles have two spatial derivatives (x_i and x_j)
