 Hi everybody, welcome to this interview with Dr. Sef Alunemans, responsible for acoustics at Siemens-Kamesa. Today we are going to talk a bit about, with you about airfoil self-noise. If I fold this piece of paper, I can say that it looks like an airfoil, a wind turbine blade. Could you please explain us which are the most important sources of noise for a wind turbine? Yes, so this airfoil, you can see this as a blade section, part of a blade, and this blade is moving through the air. So you have an airflow going from the leading edge over the airfoil to the trailing edge. And the most important source of wind turbine noise is trailing edge noise. It's when the flow passes the trailing edge, when it generates sound at the trailing edge. Another potential source of noise could be inflow turbulence noise. When you have turbulence in the oncoming flow, it hits the leading edge of the airfoil, and that could also generate sound of a wind turbine, but that depends on the environment around the wind turbine, if there's a lot of atmospheric turbulence. The third potential source could be a three-dimensional effect, which is the tip of the blade. When you have a pressure difference, you could get a so-called tip vortex, the flow going from the pressure side to the suction side, and that can create also noise from the tip of the blade. Okay, interesting. So you say that the most important one is trailing edge noise. Is there any way to reduce noise of the source? Yes, there are different ways to reduce this trailing edge noise. One of them, the most obvious one, is to reduce the flow speed, because the sound depends very strongly on the flow speed, and that can be done by reducing the RPM of the wind turbine. The second way you could reduce trailing edge noise is by changing the shape of the airfoil in such a way that you maintain the aerodynamic performance, but reduce the thickness of the turbine boundary layer. That also reduces the sound. But the most important way to reduce trailing edge noise is by using blade add-ons, which can be retrofitted on a blade, and the most used example of that is trailing edge serrations. And I brought one here, so this is what they look like. So it's a triangular teeth at the trailing edge, which are mounted like this. So we look basically like this. Of course, in reality, the blades are much bigger, but this is just to give an idea, and this reduces the sound. And can you explain us a bit briefly how do they work? Yes, so normal trailing edge noise, if you have a straight trailing edge, then the turbulence passes the trailing edge at a straight angle. So there's a large discontinuity. If you add the serrations, then the turbulence passes the trailing edge at an angle, at an oblique angle, and this reduces the efficiency of the acoustic radiation. Okay, nice. And is there a way to improve somehow the actual trailing edge serration that you showed us? Yes, it's interesting you ask. Because I brought our latest noise reduction technology, which looks like this. So we still have the teeth here, but we also have finer combs in between the teeth. And what this concept is actually inspired by the silent flight of the owl, the bird. It can fly much quieter than other birds, and people think it's because of special features on the wings of an owl. And we try to mimic this natural feature by applying these combs in between the teeth, and that gives us an even larger noise reduction than with the standard serrations. Okay, very nice. Interesting. So thank you. Thank you for being with us and for your time. It was really interesting to learn about this. You're welcome. Thanks again for watching us.