Evaluation of unsteady aerodynamic forces and pressure in wings and turbines at low Reynolds number by combining particle image velocimetry and proper orthogonal decomposition.

摘要

Aerodynamic unsteady forces in stationary and rotating wings are analyzed in this dissertation by using a combination of time-resolved particle image velocimetry (TR-PIV) and proper orthogonal decomposition (POD) techniques. Recent progress in experimental measurements has demonstrated the use of TR-PIV to calculate forces by applying the integral conservation of momentum equation in its different forms. However, a more accurate and robust method is needed for unsteady forces calculations. With this in mind, a modified pressure Poisson method is developed and applied in this work, showing its superior behavior compared to other methodologies described in the past. The independence of the calculated forces shows the robustness and stability of the method.;Whereas force calculations have been recently considered, the role of flow structures in force fluctuations has not been revealed yet and it is the main focus of this study. To elucidate these relations, a hybrid PIV-POD analysis is applied to reconstruct the velocity field from the most energetic modes of the flow. A model describing the vortex-force relations is proposed in terms of lift and drag variations during the vortex shedding process. A spectral analysis of the calculated forces suggests symmetric periodic lift, drag and circulation variations at the shedding frequency. Moreover, lift, drag and circulation signals are in phase, which supports lift-circulation proportionality. However, non-symmetric drag fluctuations are found at double the shedding frequency within a shedding cycle. For instance, when a positive or negative circulation vortex detaches, different values in the maximum and minimum drag are obtained.;The data and physical relations obtained in this work such as main frequencies, vortex-force fluctuations and behavior of reduced-order models can aid in the development of CFD applications at low Re. The methodology described can be applied to any moving or stationary wing at different Reynolds numbers and angles of attack which would provide additional data for numerical codes. Furthermore, accurate measurements of unsteady forces allow determining narrower and more precise safety margins. Moreover, these measurements could be of application in flow control since forces are related to flow features simultaneously.