On the Mechanisms of Trailing Edge Noise Reduction by Upstream Treatments

Over the past years, different treatments including tandem configuration, porous materials and Kevlar have been applied extensively to control the leading edge noise, by installing them on the leading edge or downstream of the leading edge. One of the underlying mechanisms, commonly referred to as "edge-to-edge interference", has traditionally been linked to communication between the suction and pressure sides of the surface. Noise reduction peaks attributed to this mechanism are observed to collapse at non-dimensional frequencies f(l+l_0)/U_c = (n − 1)/2, where l is the length of the treatment section and l_0 is the length from the leading edge to the start of the treatment section, as established in [1]. This paper applied these treatments to trailing edge noise problems, by installing the treatments upstream of the trailing edge. In addition to the above-mentioned materials, non-porous (flexible) materials were also tested and the noise reduction peaks consistently collapsed at the same non-dimensional frequencies, which challenges the conventional understanding of the edge-to-edge interference mechanism. Numerical models using commercial software COMSOL MultiPhysics were conducted to explore the fundamental principles behind the edge-to-edge interference and other noise reduction mechanisms for tandem configuration. Simulation results showed that in addition to the waves from the upstream and downstream plates being out of phase with each other, their magnitude ratio was close to unity which is another essential condition to trigger the edge-to-edge interference. The second condition implies that the edge-to-edge mechanism only works at higher frequencies, while at lower frequencies, the noise reductions are due to the reductions of source strengths ("short chord effect"). Experiments showed that with upstream trailing edge treatments, up to 5 dB far-field noise reduction can be achieved.