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Aim of the paper is to investigate the flow in the close vicinity of flat-plate insert-type trailing edge serrations. It is now firmly established that aerofoil subjected to a serrated trailing edge is quiet. Recent advances in the analytical and numerical solutions have established several mechanisms that can help to explain the causes of the lower level of selfnoise radiation by a serrated aerofoil. Some high fidelity experimental works, such as the timeresolved Particle Image Velocimetry (PIV), have been used recently to track the evolution of a fully-developed turbulent boundary layer flow structures over a serrated trailing edge. However, due to the high cost and technical challenging, the time-resolved PIV measurements still have some limitations in the construction of a complete spatial and temporal evolution of these small-scale turbulent structures. The current work seeks to develop a simple technique by artificially generating the turbulent spots – the so called “building block” of the turbulent boundary layer, and to track in the space and time domains a complete evolution of these spots over the serrated trailing edge of an aerofoil using the ensemble-averaging analysis method for the hot-wire data. The test was conducted on a symmetrical NACA0008 aerofoil under zero-lift conditions and at a Reynolds number 11.1111 × 111155. Aero-acoustic measurements at these conditions confirm that significant broadband self-noise reduction can be achieved by a serrated trailing edge. After gathering the relevant boundary layer data, a preliminary examination of the velocity perturbation and turbulence intensity distributions near the straight trailing edge would confirm the classical turbulent spot structure that can typically be found in the literature. However, in the case of a serrated trailing edge, it is observed that each passage of a turbulent spot, especially at its so called ‘calm region’, would draw high momentum flow (i.e. positive velocity perturbation) periodically from the sawtooth gap towards the sawtooth surface. It is believed that this periodic calming and stabilization effect of the boundary layer might represent the general mechanism for the reduction of the selfnoise generated at the trailing edge, although currently more data analysis is still underway to dissect the internal flow details of the turbulent spot at the serrated trailing edge.

On The Spatial–Temporal Developments Of Turbulent Spots On a Serrated Trailing Edge