Modeling And Simulation Of Piezoelectric Wind Energy Harvesters Enhanced By Interaction Between Vortex-induced Vibration And Galloping

The piezoelectric wind energy harvesters(PWEHs)based on wind-induced vibrations are able to convert ambient wind energy into electricity.As the ideal energy source for various low-power wireless nodes,the PWEH has the advantages of long lifespan,ease of miniaturization and free of pollution.From low to medium wind speed range,the electrical output of PWEHs can be significantly improved utilizing the interaction between vortex-induced vibration(VIV)and galloping.However,owing to the late start of related researches,the geometrically linear models(GLMs)are currently used for the analyses of the output performance of the VIV-galloping interactive PWEHs.The GLMs would produce large errors when applied to the large amplitude vibrations scenarios such as VIV-galloping interactions.Therefore,to provide reliable theoretical supports for the design of VIV-galloping interactive PWEHs,it is of great significance to develop a geometrically nonlinear model(GNM)of PWEHs subjected to VIV-galloping interactions.This thesis systematically analyzes some problems in the current theoretical models of PWEHs.The GNM of PWEHs subjected to VIV-galloping interactions is developed by consistently processing geometrically nonlinear terms and by considering the effect of bluff body rotation on the angle of attack.The efects of the consistent treatment of geometrical nonlinearity on the model accuracy are analyzed and an ease-to-implement identification method of the mechanical damping ratio for the low-coupling PWEHs is proposed in this thesis.The proposed GNM was verified by wind tunnel tests using the PWEHs with square-cross-sectioned bluff bodies.An aerodynamic parameter extraction method utilizing Computational Fluid Dynamics(CFD)is proposed in this thesis and was validated by wind tunnel tests using the PWEH with hexagonal-cross-sectioned bluff body.Finally,a wireless temperature and humidity sensor node is successfully driven using the fabricated VIV-galloping interactive PWEHs prototypes.The main content of this thesis includes:(1)Aiming at the problems of the current GNMs in dealing with the high-order geometrically nonlinear terms,two GNMs of PWEHs subjected to VIV-galloping interactions is developed by consistently handling the geometrical nonlinearity using the force balance method and the variational approach,respectively.In order to analyze the effect of consistent treatment of the geometrical nonlinearity,the base excitation is introduced into the model,and the analytical solution of the GNMs under the base excitation is derived using the multiple scales method,and the accuracy of the analytical solutions is analyzed via comparisons with the numerical solutions.The results indicate that the model accuracy is remarkably improved by the consistent treatment of the geometrical nonlinearity when subjected to large amplitude vibrations.In addition,due to considering the effects of the axial aerodynamic force,the accuracy of GNM derived by variational approach is slightly higher than that based on the force balance method.(2)The effects of the rotation of the bluff body on the relative direction of the incoming flow is systematically analyzed,and the aerodynamic force including the rotation effect of the bluff body is derived.Based on the experimental data reported in the literature,the aerodynamic coefficients of the square cross-sectioned bluff body are obtained using the piecewise curve fitting method,improving the fitting accuracy of the aerodynamic coefficients and expands the scope of application for the aerodynamic coefficients.In addition,in this thesis,not only the transverse aerodynamic force,but the axial aerodynamic force is derived to enhance the model accuracy.The aerodynamic force model of the galloping-based PWEHs or VIV-galloping interactive PWEHs is further improved through the studies mentioned above.(3)In order to verify the proposed GNM of PWEHs with or without VIV-galloping interactions,two prototypes with square cross-sectioned bluff bodies were fabricated and tested in a small wind tunnel.Aiming at the need of model verification,an ease-toimplement identification method of mechanical damping ratio for low coupling PWEHs is proposed.Wind tunnel tests show that both the current GLM and the proposed GNM successfully predict the interaction or separation of VIV and galloping.The differences of the simulated responses of both GLM and GNM are sufficiently small at relatively low wind speed,but the accuracy of the proposed GNM is much higher than the current GLM at medium to high wind speed range.For instance,in the wind speed range from 5.0m/s to 12m/s,for the PWEH prototype with VIV-galloping interaction,the maximum relative errors of GLM and GNM are 24.20% and 6.86%,respectively;for the PWEH prototype without VIV-galloping interaction,the maximum relative errors of GLM and GNM are28.54% and 8.56%,respectively.The results show that the accuracy of the proposed GNM meets the design requirements of the VIV-galloping interactive PWEHs.(4)For the bluff body with unknown aerodynamic parameters,taking the hexagonal cross-sectioned bluff body as an example,the extraction method of aerodynamic parameter based on CFD simulation is studied.Using the CFD model based on Shear Stress Transport(SST),the lift and drag coefficients of the hexagonal cross-sectioned bluff body at different angles of attack are obtained.The Den Hartog criterion validate the galloping potential for the hexagonal cross-sectioned bluff body.The extraction method of the aerodynamic parameters for the wake oscillator is studied based on CFD simulation.For the hexagonal cross-sectioned bluff body,the Strouhal number,the dimensionless half length and the dimensionless width of the wake oscillator,and the constant related to acceleration of bluff body and rotation of wake oscillator are determined using this extraction method.A PWEH prototype with hexagonal crosssectioned bluff body is fabricated and tested to validate the identified aerodynamic parameters,and good agreement is found between model simulations and experiments.(5)The performance of PWEHs with square and hexagonal cross-sectioned bluff bodies are theoretically compared,and wind tunnel tests are then conducted to verify the theoretical results.Finally,a wireless temperature and humidity sensor node is successfully driven using the fabricated VIV-galloping interactive PWEH prototypes.At wind speed of 9.4m/s,the wireless sensor node is capable of measuring the temperature and humidity twice during one discharge cycle of the energy storage capacitor.