Excitation using steprelaxation

In principle two main ways to excite a structure, viz. ambient excitation and externally forced excitation:

• Ambient excitation. Ambient excitation implies that the structure is tested in situ, using the operational vibrations as the excitation source. Examples are traffic in the case of bridges or wind in the case of wind turbines. Ambient excitation is frequently used to excite large structures where externally forced excitation becomes difficult, inconvenient and/or expensive. For example, wind excitation has been applied to extract the modal parameters of the 110-m-tall EOLE vertical-axis wind turbine [40] and has been used to validate an ADAMS/WT model of the Cannon Wind Eagle 300 downwind horizontal-axis wind turbine [134, 319];

• Externally forced excitation. In this case both the input force and the resulting responses are measured. The input forces may be generated in several ways (i.e. electromagnetic or hydraulic shaker, instrumented hammer, human excitation or step relaxation).

The preferred exciter is often the electromagnetic shaker which has the ability, when properly sized, to provide a flexible way of specifying sufficiently exciting inputs. Conversely, for parked wind turbine testing, human excitation, step relaxation, and wind excitation are the most useful [152]. Human excitation, however, is limited to moderately sized wind turbines, because small turbines have natural frequencies too high to be excited manually, while larger turbines may be too massive to excite. Step relaxation as well as wind excitation have been reported to work extremely well on vertical-axis wind turbines [40, 151, 152]. Of the two excitation techniques, step relaxation is the most time-consuming. However, the frequency content of the step function makes it ideal for testing large, flexible structures since a large amount of strain energy can be input to the structure. In addition, it has been observed that structural damping is difficult to estimate accurately from the measurements using wind excitation.

We have decided to use the step-relaxation method to excite the parked Lagerwey LW-50/750 wind turbine because its ability to estimate the damping and the fact that it is mechanically straightforward to implement. This method of structural excitation involves applying a static load to the structure by a cable anchored to the ground or to a deadweight, and then suddenly releasing this load via a quick release mechanism. The static load was transmitted by a 225 meter long TWARON® cable. TWARON is a lightweight, superstrong synthetic fibre made from the aramid polymer. It is preferred to steel because it is five times lighter at the same strength (the cable has a tensile strength of 170 kN and a total mass of 82.5 kg). The interested reader is referred to Appendix F.1 for a more detailed technical specification of the cable. Consequently, the mass loading is minimized. The only disadvantage is the lower modulus of elasticity, which is about half that of steel [47]. The cable has been secured to the tower at 40.5 m above the ground level using a cargo strap. A cargo strap was preferred to a fixture because mass loading is negligible and no special fixturing is required.

We have used a 20-ton truck as deadweight and used its hydraulic powered winch for loading the cable to the specified 40 kN. The tension in the cable is released by a firm pull on a nylon cord that activates the lever switch of the quick release mechanism. The cable is restrained at both ends by a chain preventing secondary hits or random banging after being released.

The left photograph in Figure 4.14 shows the initial position of the TWARON® cable, quick release mechanism, nylon cord, chain, force transducer, and the winch cable. The middle and right photograph show two successive frames of an actual release recorded with a Mini DV camera.

Figure 4.14: Step-relaxation hardware and recorded release. Left photograph: position of the TWARON cable, quick release mechanism, nylon cord, chain,, force transducer, and winch cable at T = 0.0 s, middle photograph: position at T = 0.08 s, and right photograph: position at T = 0.16 s.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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