Tracking water level in the test of a tuned liquid damper

Posted March 26th, 2008 by chris

The problem

A tuned liquid damper is a simple way of reducing the response of a building to dynamic loads such as earthquake shocks. It consists of a tank of water with the depth to length ratio set so that the natural frequency of the water sloshing in the tank matches that of the structure.

To test the concept a tank of water was carried on a base plate suspended from a framework on an earthquake table, with the forces between the plate and the tank monitored as the earthquake table was activated. Forces between the tank and support could be readily measured, as could accelerations of the plate. However, there was also a need to monitor the depth of the water with time as well as the pendulum displacements of the plate and the displacements of the support frame.

The Solution

Lines were drawn on the glass sides of the tank and the intersections of the water's surface and those lines were captured as templates. At the same time more conventional target types were attached to other elements of the support frame and base plate to allow the monitoring of water levels as the tuned liquid damper was excited by the earthquake table. The water level indications functioned very well providing a real-time tracking of water levels throughout the tank.

Advantages of the Video Gauge

The use of the video gauge allowed a completely non-contact measurement of water level to be made without any disturbance to the flow in the tank as might have been caused by using floats and LVDTs to provide water depth measurements. Minimal preparation was required to set up the measurements and the measuring system could very readily be reconfigured to provide the data needed for different tests.

This is an excellent example of the way that innovative uses are found for the Video Gauge once users become familiar with the ease with which it can capture data. Whilst the use of a liquid level as a target is unlikely to give optimum resolution this application did not require the maximum level of resolution.