Raman Lidar Chimney

Time period

September 2000

Project description

The Raman Lidar Trailer is a very versatile trailer-based system that measures various atmospheric elements. The receiver is a 27-inch (68.6-centimeter) diameter telescope, pointed in the horizontal direction. A large, flat mirror is oriented 45 degrees relative to the telescope axis. This allows the view of the telescope to be scanned from one edge of the horizon to the other, over 180 degrees. However, the most frequent position in which the system is used is pointing straight up. During daytime measurements, the instrument trailer is flooded with stray sunlight coming in through the window in the roof top. A baffling system was desired to reduce the amount of sunlight entering the trailer and therefore improve the performance of the system.

The trailer is routinely used at sites in Oklahoma and the Bahamas. At these sites, tornado and hurricane winds have been known to hit the trailer; therefore, the chimney design needs to survive at least 100 mph winds. The required footprint of the chimney is approximately 27 inches x 27 inches. The tallest possible chimney is desired since a longer baffle improves stray light blocking. Also, modifications to the existing hardware on the trailer had to be kept to a minimum. It was desired that all of the chimney modificaitons would be direct bolt on additions.

The trailer housing the instrument is a modified version of a regular trucking trailer. Therefore, the metal skin is approximately 1/16-inch thick aluminum sheet metal. This created a challenging enviroment for mounting a 27-inch square chimney protruding up to 6 feet (1.8 meters) from the roof of the trailer. Using basic fluid dynamics calculations, we determined the relationship between wind loading on the side of the chimney and the chimney's height. We quickly learned that 100 mph winds would easily rip all but the shortest designs from the roof of the trailer.

Using Pro/MEHCANICA structural analysis software, we modeled the design and the wind loadings. We quickly discovered that allowing the chimney to cantilever from the roof of the trailer created stress concentrations at the corners where the chimney base is bolted to the trailer roof. The solution was to add guy wires at the top and the mid-section of the chimney. This addition effectively made the load at the chimney mount to trailer interface independent of chimney height. A six-foot tall chimney was settled on for the final height so that it could be easily broken down into thin 3-foot (91.4-centimeter) x 6-foot sections.

Challenges and lessons learned

The overwhelming challenge on this project was schedule. The chimney needed to be designed, built, and ready to go for a deployment in Oklahoma by a strict deadline. The design, analysis, and creation of detail drawings were completed in approximately one week. Fabrication was completed in approximately two and a half weeks.

This was the first project where we made extensive use of structural analysis software. Without the software, we would not have been able to create a safe design in such a short period of time.