Summary of Performance

Performance summaries of 10 % and 5 % settling time ratios from cases in Figure 4.6 are displayed in Figures 4.7(a) and 4.7(b), respectively. A 10 % and 5 % settling time is the time for the peak displacement to decay within 10 % and 5 % of the initial displacement (of 2.4 degrees). Settling time ratio is the controlled (with liquid) case’s settling time normalised by the uncontrolled (no liquid) case’s settling time at each structural frequency. The 10 % and 5 % values are chosen only as some indication of performance, and are not absolute by any measure. Of course, an effective case dissipates its initial energy quickly, resulting in the shortest settling time ratio. All cases are similar between 10 % and 5 % settling time ratios. Therefore, only the 5 % settling time ratios are discussed next. All mentions of settling time ratio in the discussion of Figure 4.7, refer to 5 % settling time ratio.

113 The case with a structural frequency of 0.5 Hz produces its smallest settling time ratio at an inclination of 4 degrees of approximately 0.04. This is a substantial improvement of about 96 % compared to the uncontrolled case. At an inclination of 4 degrees the static free surface length is tuned so that the liquid sloshing frequency matches the structural frequency. At structural frequencies of 0.7 Hz and 0.9 Hz, the shortest settling time is produced one inclination below the tuned inclination, with the tuned inclination having a slightly higher settling time. This difference is likely due to determining the inclination angle using a static free surface length and flat container’s static liquid height from the liquid frequency formula (Equation 4.1). However, using the inclined container’s static liquid height in this formula produces a sloshing frequency closer to the frequency of the oscillating structure. While the free surface length varies as the structure oscillates, the inclination angle that produces the highest energy dissipation and shortest settling time is slightly lower than the tuned inclination.

For a structural frequency of 0.7 Hz, the shortest settling time ratio, of about 0.12, occurs at an inclination of 6 degrees, where the tuning inclination of 8 degrees produces a slightly higher settling time ratio of 0.145. Similarly, the shortest settling time ratio for a structural frequency of 0.9 Hz is about 0.21 and occurs at an inclination of 10 degrees, where the inclination is tuned at 12 degrees producing a settling time ratio of 0.25. These differences of 2.5 % and 4 % are quite small when comparing the difference of 29 % at a structural frequency of 0.5 Hz between inclinations 4 and 12 degrees.

114 The shortest settling time ratio for all structural frequencies is about 4 % that occurs at 0.5 Hz at an inclination of 4 degrees. This is an increase of about 56 % from the second shortest settling time ratio of about 9 % at the same structural frequency and an inclination of 2.5 degrees. At a structural frequency of 0.5 Hz, shorter settling times occur from inclination angles 0 to 4 degrees than any inclination case at structural frequencies 0.7 Hz and 0.9 Hz. An average settling time ratio of approximately 9 % occurs at a structural frequency of 0.5 Hz at inclination angles from 0 to 4 degrees. This significant improvement of 81 % over a range of inclinations, is attractive for design purposes. This is due to enhanced energy dissipation being achieved without having to be too precise on the installed inclination angle. As long as the inclination angle is 4 degrees or smaller at a structural frequency of 0.5 Hz, enhanced energy dissipation can be achieved.

Average 10 % and 5 % settling times ratios over the range of frequencies, analysed at each inclination are displayed in Figures 4.8(a) and 4.8(b), respectively. This is to determine which inclination case is the most effective over the range of structural frequencies analysed. Effective energy dissipation over a range of structural frequencies is essential for industrial applications where a structure’s frequency can vary significantly. The most effective inclination angle of structural frequencies of 0.5 Hz to 0.9 Hz, for both 10 % and 5 % settling time ratios, is 4 degrees. This case produces the same average 10 % and 5 % settling time ratio of approximately 0.19 (19 %) or an improvement of 81 %.

A summary of performance for 10 % and 5 % settling time ratios with cases from Figures 4.7(a) and 4.7(b) are presented in Figures 4.9(a) and 4.9(b) with respect to liquid frequency

115 over structural frequency. Here, tuning is achieved when liquid frequency equals structural frequency or the value along the horizontal axis (fL/fs) is 1. All structural frequency cases

show similar trends where settling time reduces as fL/fs increases from 0.2 to around 1. For

fL/fs above 1, settling time increases. All structural frequencies produce shortest settling times

of an fL/fs of approximately 0.8 to 1 or around the tuned inclination angle. Performance is

enhanced at lower structural frequencies. As structural frequency increases so does settling time ratio, evenly at all inclinations angles. This is possibly due to increased initial energy within the system at higher structural frequencies.

Overall, substantial improvements are observed at all inclination angles over structural frequencies 0.5 Hz to 0.9 Hz as compared to the uncontrolled cases. The liquid sloshing absorbers can be tuned to achieve optimal energy dissipation through inclination alone. This is achieved by inclining the containers to achieve static free surface lengths that produce a liquid sloshing frequency that matches the frequency of the structure to be controlled. The optimal inclination angle is 4 degrees producing the shortest average 10 % and 5 % settling time ratios over structural frequencies 0.5 Hz to 0.9 Hz. This is attractive for design purposes as effective energy dissipation over a range of structural frequencies is essential for industrial applications where a large structure’s frequency can vary significantly. The inclination angle of 4 degrees is most effective at a structural frequency of 0.5 Hz where energy dissipation is enhanced by 96 % compared to the uncontrolled case.

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