The project commenced with knowledge transfer from the lead scientist and his research team to the researcher. The researcher acquired knowledge in advanced techniques in system identification and control theory. The researcher summarized this knowledge transfer in the book chapter, “Closed-loop Active Flow Control,” published by Wiley in “Advance UAV Aerodynamics, Flight Stability and Control: Novel Concepts, Theory and Applications”. In the next stage, the researcher worked on development of sensors, actuators, standalone data acquisition and control system to be embedded into rotating blades. To incorporate complex control systems, the internal structure of the model was manufactured with advanced rapid prototyping methods. Active flow control in the form of wall normal blowing was incorporated inside a blade. One main challenge was presented by the limited size of an airfoil model that could fit into the unique gust wind tunnel facility at TU Berlin. The facility consists of a main wind tunnel that generates the baseline flow and a second wind tunnel capable of generating unsteady gusts. In this respect, a comprehensive CAD design study was performed to find an airfoil shape, actuator geometry, and sensor locations together with an appropriate choice of sensors. The aerodynamic experiment was complimented by measurements of the small rotor acoustic signature. The objective of these experiments was to understand the source of aerodynamic noise in a rotating frame. Aeroacoustic measurements were recorded simultaneously with free-field microphones in an anechoic room. The extraneous rotor noise comprised the tonal peak at blade passing frequency and broadband noise components. The effect of spanwise tripping (passive technique) was investigated. The results showed that it is possible to reduce the level of low frequency tones by 2-6 dB while shifting the acoustic energy to high frequencies. The ability to reduce low frequency tones is of great importance given that under atmospheric conditions, low frequencies travel longer distances since atmospheric absorption is relatively low. This work was presented at the 57th IACAS (Israel Annual Conference on Aerospace Sciences) and the researcher was awarded the Neev-Ya Hadas Durban Prize. The experimental work was supported by mathematical models development to describe blade aerodynamic and aero-acoustic behaviour in unsteady inflow conditions. The aerodynamic model is based on classical unsteady thin airfoil theory and the aero-acoustic model on the Amiet theory. Although fluid flow systems, as described by the Navier-Stokes equation, are inherently non-linear, much can be achieved in closed-loop-exploiting linear concepts. Thus, the mathematical model was oriented toward development of a closed-loop reliable physics-based low order model that is based on physical principals.
