Mechanical Engineering Department
BS in Mechanical Engineering
The Aerodynamic Test Platform (ATP) for the Cal Poly HPV Club is a system that was designed by Cal Poly mechanical engineering students to measure aerodynamic characteristics of a human-powered vehicle (HPV). The HPV team desired a system that could quantify the lift, drag, and other aerodynamic qualities of a full scale HPV at various orientations in oncoming airflow. Established methods for determining aerodynamic characteristics include computational fluid dynamics (CFD) and wind tunnel testing of scaled models. The ATP was devised to simulate the test results given by a full-scale wind tunnel without requiring a wind tunnel large enough to test a full-size HPV. The basic premise was that the HPV could be mounted to the ATP and located on the roof of a motor vehicle and driven through still air to approximate the oncoming airflow present in the wind tunnel. The project sponsor and design team agreed upon a list of full specifications including size, function, durability, and usability. The primary goal of the system was to safely attach an HPV to the roof of the van and measure lift and drag. A budget of $2000.00 was specified for the project. The vehicle was chosen to be the Cal Poly ME department van. Many different concepts and designs were developed in parallel with preliminary testing and calculation of the distortion of the airflow field around the van. A small scale model was made out of wood to better understand the geometry of the structure and its eventual ability to resist vibration. Ultimately an ATP design was determined which could provide the different orientations specified by the sponsor and reasonably obtain the desired results. The system would draw primarily on load cells fitted with strain gauges to measure the aerodynamic forces. With the design of ATP established, components were sized relative to a worst-case loading condition, in part associated with a 30 mph crosswind. Each part was meticulously considered in sizing calculations due to the dangers posed to other drivers in the event of a system failure. Hand calculations of simplified loading cases and large factors of safety led to initial sizes of parts. Next, finite element analysis was performed critical system components to further verify the hand analysis. The effects of noise due to road vibrations (pot holes, uneven road, etc.) vibrations (pot holes, uneven road, etc.) in the lift and drag signals was investigated. An accelerometer was attached to the roof of the van and the recorded accelerations were used as inputs into a transient finite element analysis. The analysis suggested that road noise would not noticeably distort the strains being measured in the load cells. With the parts satisfactorily sized, they were purchased from various suppliers including local steel suppliers, hardware stores, and some specialty online business. Manufacturing was split up according to familiarity with design and manufacturing capability of each team member. Most of the manufacturing processes consisted of cutting to raw materials to length, machining assorted holes and notches, and welding. The system was completely built in a few weeks of fabrication. Previously established safety tests, such as verifying that the system could handle the worst-case design loads without failure, were implemented. Strain gauges were attached at the necessary locations of the load cells. Due to a lack of access to a more sophisticated circuit board, a crude data acquisition system was employed with a spring scale to measure the output signal of individual strain gauge bridges over a drag force range of 0 to 30 N. The data acquisition system clearly measured a change in the signal under the test loads, proving strain could be accurately measured in the load cells. The system was delivered to the HPV club and once calibrated, will be ready for use.