Investigation Of Aeroelastic Instability Mechanism Of Tensioned Membrane Structures

Membrane structures tend to deform and vibrate significantly under wind load due to their lightweight and flexibility, the deformation and vibration affect the flow field around the membrane at the same time. Under special conditions, the wind-structure interaction effect makes the amplitude of the membrane increase sharply with the wind speed, and cause the aero-elastic instability like bridge and airfoil structures. As aero-elastic instability is a big threat to the safety of the structure, thus the study on the mechanism of the aero-elastic instability has been an important issue in wind engineering field. However, most of the researches in this area were mainly concentrated on the bridge structure, rather than the membrane structure. One of the most important reason is that the dynamic characteristics of membrane structure is very complicated, and it is very difficult to simplify the membrane structure as a bridge which can be can be simplified to be a section model that only have translational and rotational degrees of freedom. The wind-induced collapses of membrane structures at home and abroad in the recent years show that the wind-resistant design theory at this stage still have some shortcomings. As a result, it is necessary to study the wind-induced disaster mechanism of membrane structure and discuss the possibility of aero-elastic instability and to find the measures to control it.Based on the research background mentioned above, a series of aero-elastic model wind tunnel tests were carried out in this study. The change laws of a variety of response characteristic parameters with coming flow speed were studied to make clear the mutual action mechanism of wind and structure, to reveal the aero-elastic instability mechanism of membrane structures, to put forward the wind resistant design method that considering the aero-elastic instability. The main contents in this paper are shown as following:1. Proposed a research method for the aero-elastic instability of membrane structures based on the full load domain and multiple response characteristics.For the geometric nonlinearity and multi-modal vibration characteristics are obviously in wind-induced response of membrane structure, so that the conventional aero-elastic instability research method is not suitable for membrane structures and a new research method is need for the multi-freedom soft system. As a result, a research method based on the full load domain and multiple response characteristics was proposed. This method is based on the aero-elastic model wind tunnel experiment method and the joint use of numerical simulation and analytical methods. When using this method, two aspects, the response characteristics and change laws of flow field are mainly studied. The aero-elastic instability mechanism of membrane structure is revealed through the studies of the correlation between displacement and wind velocities above the roof at different wind speeds, the change laws of amplitude, major vibration modes and system damping. After establishing the overall research ideas, the research methods used in this study, such as modal identification method, damping identification method and CFD numerical simulation method based on the moving boundary technique, were discussed and verified.2. Designed and finished a series of aero-elastic model wind tunnel experiments for typical tensioned membrane structures.Aero-elastic model wind tunnel experiment, especially aero-elastic model wind tunnel experiment for membrane structure, has always been a difficult problem in the field of structure wind engineering. The problems, such as how to choose the model material and how to avoid the influence of the measurement devices on the wind field and structure vibration are needed to be solved. Based on the discussions of the aero-elastic model similar theoretical, non-contact measurement technology and method to apply the pre-tension for membrane structures, designed and finished the open-type one-way tensioned membrane, closed-type one-way tensioned membrane and closed-type saddle-shaped tensioned membrane, obtained the wind-induced response and flow field data. Based on the analysis of the change laws of the amplitude and dominant vibration modes at different wind speed, found that the amplitude increases sharply and the dominant mods jumped from one mode to another, and preliminary judged that the phenomenon was related to the aero-elastic instability.3. Revealed the aero-elastic instability mechanism of membrane structures by joint using of several research methods.Based on the correlation analysis of displacement response and flow field wind speed, determined the wind speed range for the aero-elastic instability. Analyzed the dominant frequencies of displacement and spectrum by Power Spectrum Density analysis method and found that lock-in phenomenon existed in the dominant frequencies of wind speeds in the aero-elastic instability wind range. To verify the phenomena, a series of CFD simulations that considering the influence of average deformation and vibration on flow field were carried out. The research found that, the frequency of the vortices changed with the deformation of the membrane; when it was clear to the frequency of the vibration mode, a large amplitude oscillation was induced. At the same time, the large amplitude oscillation controlled the frequency of the vortices and the lock-in phenomenon occurred. The change laws of the damping ratios of the system were also analyzed. It’s found that damping ratios of the system increased with the wind speeds firstly and then dropped significantly. Its peak occurred at the wind speed that just before the aero-elastic instability. This phenomenon can be interpreted as that the damping ratio of the dominant mode increase with the wind speed due to the change of the aerodynamic damping of the vibration mode; this makes the structure vibration mode jump from the lower order to the higher mode and reveals the reason for the mode jump. Based on the research results mentioned above, it can be considered that aero-elastic instability of membrane structure is a kind of vortex-induced resonance caused by vortex shedding; it was characterized by a sharply increase of the amplitude, a suddenly change of the dominant vibration mode or a dramatically decrease of the system damming.4. Derived the analytical expressions of added mass and aerodynamic damping for tensioned membrane structures.Analytical expressions of added mass and aerodynamic damping for open-type one-way tensioned membrane vibrating in uniform flow were derived based on the aerodynamic acoustic and quasi-static theories and verified by a series of aero-elastic wind tunnel experiments. This method simplified the wind load on the membrane to be the sum of the aerodynamic acoustic pressure and quasi-static pressure, The former is caused by the compressive action of the vibrating membrane on the air and can be evaluated by aerodynamic acoustic theory. The latter is equal to the mean wind pressure on the corresponding quasi-static model at any instant, which changes because of the shape-change of the membrane. This can be evaluated by wind tunnel tests with rigid models or Computational Fluid Dynamics(CFD) simulations of several instantaneous shapes in a vibration period.The results showed that the differences of the added mass and aerodynamic damping can be controlled to be within 9.0% and 20%; both the added mass and aerodynamic damping increases with the on-coming flow velocity; the added mass can be reaches 5 times structural mass and the aerodynamic damping can be reaches 10 times the structural damping. As a result, the added mass and aerodynamic damping should not be neglected in the wind-induced vibration response analysis.5. Proposed the wind resistant design method that considering the aero-elastic instability.Two processes, critical wind speed determination and additional aerodynamic force evaluation processes, were added in the current wind resistant design process of membrane structures to considering the influences of the aero-elastic and wind-structure interaction effect. Then the methods to determine the critical wind speed and the additional aerodynamic force were given; the influences of the pre-tensions on the critical wind speed were discussed and the method to improve the critical wind speed was suggested.