A device was developed to directly measure skin friction on an external test plate in transonic flight conditions. The tests would take place on the FTF-II flight test plate mounted underneath a NASA F-15 aircraft flying at altitudes ranging from 15,000 to 45,000 ft. at Mach numbers ranging from 0.70 to 0.99. These conditions lead to predicted shear levels ranging from 0.3 to 1.5 psf. The gage consisted of a floating element cantilevered beam configuration that was mounted into the surface of the test plate in a manner non-intrusive to the flow it was measuring. Strain gages mounted at the base of the beam measured the small strains that were generated from the shear forces of the flow. A non-nulling configuration was designed such that the deflection of the floating head due to the shear force from the flow was negligible. Due to the large vibration levels of up to 8 grms that the gage would experience during transonic flight, a vibration damping mechanism needed to be implemented. Viscous damping had been used in previous attempts to passively dampen the vibrations of skin friction gages in other applications, yet viscous damping proved to be an undesirable solution due to its leakage problems and maintenance issues. Three methods of damping the gage without a fluid filled damper were tested. Each gage was built of aluminum in order to maintain constant material properties with the test plate. The first prototype used a small internal gap and damping properties of air to reduce the vibration levels. This damping method proved to be too weak. The second prototype utilized eddy current damping from permanent magnets to dampen the motion of the gage. This mechanism provided better damping then the first prototype, yet greater damping was desired. The third method utilized eddy current damping from an electromagnet to dampen the motion of the gage. The eddy current damper achieved a much larger reduction in the vibration characteristics of the gage than the previous designs. In addition, the gage was capable of operating at various levels of damping. A maximum peak amplitude reduction of 33 % was calculated, which was less than theoretical predictions. The damping results from the electromagnetic gage provided an adequate level of damping for wind tunnel tests, yet increased levels of damping need to be pursued to improve the skin friction measurement capabilities of these gages in environments with extremely high levels of vibration. The damping provided by the electromagnet decreased the deflections of the head during 8 grms and 2 grms random noise vibrations bench tests. This allowed for a greater survivability of the gage. In addition, the reduction of the peak amplitude provided output with vibration induced noise levels ranging from 24 % to 5.9 % of the desired output of the gage. The gage was tested in a supersonic wind tunnel at shear levels of tw=3.9 to 5.3 psf. The shear levels encountered during wind tunnel verification tests were slightly larger than the shear levels encountered on the F-15 flight test plate during the flight tests, but the wind tunnel shear levels were considered adequate for verification purposes. The experimentally determined shear level results compared well with theoretical calculations / Master of Science

The optical power of satellites such as the Hubble telescope is directly related to the size of the primary mirror. However, due to the limited capacity of the shuttle bay, progress towards the development of more powerful satellites using traditional construction methods has come to a standstill. Therefore, to allow larger satellites to be launched into space significant interest has been shown in the development of ultra large inflatable structures that can be packaged inside the shuttle bay and then deployed once in space. To facilitate the packaging of the inflated device in its launch configuration, most structures utilize a thin film membrane as the optical or antenna surface. Once the inflated structure is deployed in space, it is subject to vibrations induced mechanically by guidance systems and space debris as well as thermally induced vibrations from variable amounts of direct sunlight. For the optimal performance of the satellite, it is crucial that the vibration of the membrane be quickly suppressed. However, due to the extremely flexible nature of the membrane structure, few actuation methods exist that avoid local deformation and surface aberrations. One potential method of applying damping to the membrane structure is to use magnetic damping. Magnetic dampers function through the eddy currents that are generated in a conductive material that experiences a time varying magnetic field. However, following the generation of these currents, the internal resistance of the conductor causes them to dissipate into heat. Because a portion of the moving conductor's kinetic energy is used to generate the eddy currents, which are then dissipated, a damping effect occurs. This damping force can be described as a viscous force due to the dependence on the velocity of the conductor. While eddy currents form an effective method of applying damping, they have normally been used for magnetic braking applications. Furthermore, the dampers that have been designed for vibration suppression have typically been ineffective at suppressing structural vibration, incompatible with practical systems, and cumbersome to the structure resulting in significant mass loading and changes to the dynamic response. To alleviate these issues, three previously unrealized damping mechanisms that function through eddy currents have been developed, modeled and tested. The dampers do not contact the structure, thus, allowing them to add damping to the system without inducing the mass loading and added stiffness that are typically common with other forms of damping. The first damping concept is completely passive and functions solely due to the conductor's motion in a static magnetic field. The second damping system is semi-active and improves the passive damper by allowing the magnet's position to be actively controlled, thus, maximizing the magnet's velocity relative to the beam and enhancing the damping force. The final system is completely active using an electromagnet, through which the current can be actively modified to induce a time changing magnetic flux on the structure and a damping effect. The three innovative damping mechanisms that have resulted from this research apply control forces to the structure without contacting it, which cannot be done by any other passive vibration control system. Furthermore, the non-contact nature of these dampers makes them compatible with the flexible membranes needed to advance the performance of optical satellites. / Ph. D.

This thesis investigates an alternative solution to deal with the civil structure vibration. Non-contact electromagnetic or Eddy current damping is selected as a score of vibration suppression. Electromagnetic damping relies on the interaction between a permanent magnet and conductor. An electromagnetic damper (EMD) is applied both to a laboratory footbridge structure and 6-storey model-scale aluminium moment resisting frame (AMRF). In this first study the EMD is connected in series with an electronic shunt circuit to construct an electromagnetic shunt damper (EMSD). A robust optimisation method is applied to develop the corresponding optimal design formula of the EMSD. The principle of an EMSD is to convert mechanical energy to electrical energy. Hence, the induced electromotive force (emf) is generated by electromagnetic induction. This emf induces an amount of shunt damping, which is fedback to the structure to achieve vibration suppression. It was found that when the impedance was applied, the shunt damping feature was of a similar nature to viscous dampers. In contrast, when an RLC (resistance-inductance-capacitance) circuit is connected, the shunt damping is analogous to a tuned mass damper. A second form of EMD is Eddy current damper (ECD), which relies on a geometrical arrangement of permanent magnets and conductors to produce damping forces. The vertical and horizontal orientation of the magnet, unidirectional and alternative pole projection and moving different direction of the conductor are investigated. A theoretical study involving the infinite boundary and finite boundary (the method of images current) is carried out to obtain an analytical calculation of the damping force. On the basis of this analysis, one type of ECD prototype was physically built. A performance test was carried out to determine the damping characteristics of the ECD, which agreed with the results of the numerical analysis. In addition, the ECD was applied to control the dynamics of the 6-storey AMRF. It was found that, the ECD can effectively increase system damping and have a satisfactory control effect.