A hybrid method for particle image velocimetry (PIV) is developed to overcome the limitations of the optical flow method applied to PIV images with large displacements. The main elements of the hybrid method include a cross-correlation scheme for initial estimation, a shifting scheme for generating a shifted image, and an optical flow scheme for obtaining a refined high-resolution velocity field. In addition, a preprocessing scheme is used for correcting the illumination intensity change. The accuracy of the hybrid method is evaluated through simulations in a parametric space in comparison with the typical correlation methods and optical flow method. Further quantitative comparisons are made in PIV measurements in a circular air jet.

This paper describesOpenOpticalFlow_PIV, an open source Matlab program integrating the optical flow method with the cross-correlation method for extraction of high-resolution velocity fields from particle images with large displacements. This hybrid method provides an additional tool to process PIV images, which combines the advantages of the optical flow method and cross-correlation method and overcomes the intrinsic issues of the two methods. The principles of the hybrid method are concisely described, including the cross-correlation method for initial coarse-grained estimation and the optical flow method for refined high-resolution estimation. This paper gives more detailed descriptions of the main program, relevant subroutines and selection of the relevant parameters for computation. Examples are given to demonstrate applications of the hybrid method.

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The relationship between skin friction and the surface optical flow (SOF) in viscous flows is discussed based on the evolution equations of surface temperature, scalar and enstrophy where the SOF is defined as the convection velocity of these quantities. It is found that the SOF is proportional to skin friction, which can be determined by solving the optical flow equation re-cast from these evolution equations. This optical flow method can be applied to surface temperature and mass transfer visualizations to extract skin friction fields in experiments. To examine this method, it is first applied to complex surface enstrophy structures obtained in direct numerical simulation (DNS) data of a turbulent channel flow. Further, it is applied to surface temperature structures obtained in time-resolved temperature sensitive paint (TSP) measurements in a flow over a National Advisory Committee for Aeronautics (NACA) 0015 airfoil model and an impinging jet.

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This paper describes an exploratory study of a nonconventional wind power converter with a pair of oscillating wings, which is called an oscillating-wing unit. The working principles of the oscillating-wing unit are described, including the aerodynamic models, kinematical, and dynamical models. The performance of the oscillating-wing unit is evaluated through computational simulations and the power scaling in comparison with conventional horizontal-axis wind turbines. Then, a model oscillating-wing unit is designed, built, and tested in a wind tunnel to examine the feasibility of the oscillating-wing unit in extraction of the wind energy in comparison with the theoretical analysis. The theoretical analysis and experimental data indicate that the oscillating-wing unit has the power efficiency comparable to the conventional horizontal axis wind turbine and it can operate at low wind speeds.