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AIRFOIL GENERATOR HOTWIRE WINDOWS
The size of the interrogation windows thus has to be as small as possible to resolve high gradients in the flow field. It is understood that the cross-correlation algorithms used for the PIV evaluation are of statistical nature and represent the mean velocity of the particles within each interrogation window. The particle displacements are determined by splitting the measurement image into several small interrogation windows and computing the cross-correlation between corresponding windows in both image frames. Where M is the magnification factor of the camera setup. The first applications of PIV on helicopter wind tunnel testing were made in the early 1990s and PIV has since undergone a rapid development, as indicated by the increasing number of published PIV-based rotor studies ( Fig. By using instantaneous measurements over an entire plane, the acquisition of unsteady flow features became experimentally feasible. 1998c) PIV has become the state-of-the-art measurement technique for rotor flow field measurements. However, the necessity to make measurements over an entire plane at a given instant of time led to the development of particle image velocimetry (PIV). Since the early 1970s, point-wise, non-intrusive techniques such as the laser Doppler velocimetry (LDV) have been developed and applied for rotor measurements ( Scully & Sullivan 1972, Boutier et al. a) transonic flow on the advancing side, b) tip vortex formation, and c) dynamic stall on the retreating side of the main rotorĮarly rotor flow field investigations were performed using flow visualization techniques and intrusive, point–wise techniques such as hot–wire anemometry and pressure probes (see e.g. Some specific problems in experimental helicopter aerodynamics illustrated on DLR’s EC135 test helicopter, after Conlisk (2001).
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In spite of the progress that has recently been made in the development of numerical prediction capabilities for isolated rotor components, detailed experiments are required to substantiate and confirm the results of complex CFD studies and to expand the understanding of flow phenomena related to helicopter flows ( Allan et al. The complexity of rotor flow requires experimental investigations to go hand in hand with numerical simulations. All these phenomena must be addressed to optimize the performance of helicopters with respect to hovering efficiency, cruise speed, range, loading capacity, and aeroacoustic emissions. 1c) affect the helicopter performance and limit the flight envelope ( Conlisk 2001). 1a), and dynamic stall on the retreating blades ( Fig. 1b), compressible flow on the advancing blades ( Fig. The concentrated blade tip vortices in the main rotor wake ( Fig. The unsteadiness and complexity of the flow field around helicopter rotor blades pose significant challenges for present aerodynamic investigations, as illustrated in Fig.