Aerodynamic Study of Bulk Commodity Tractor Trailers

Our project goal is to test aerodynamic features of two existing tractor trailers and measure the reduction in fuel consumption as well as emissions. This will be accomplished by studying and comparing the drag coefficients of each design. Our scaled-down models will be created with 3D-printing and laser cutting technology.

Problem Definition
Semi-trucks are used all over the world for transporting goods. They provide easy and affordable methods to keep economies running. The downside is that semi-trucks produce a lot of carbon-dioxide pollution from their exhaust. If the trailers they tow are improved aerodynamically, that carbon-dioxide production can be minimized. The Western Trailer Company, headquartered in Boise, ID, has a multitude of tractor trailers that have different aerodynamic characteristics. The Trailer Park Boys are tasked with testing and simulating these trailers in various configurations so improvements can be made to meet new tractor trailer requirements that will be implemented in 2018.

Design Goals & Deliverables
At the end of the project, the client will receive:

•	Scale models of selected Western Trailer products suitable for testing in the University of Idaho's Mechanical Engineering Wind Tunnel

•	Wind Tunnel data on aerodynamic drag associated with different scale models at different wind speeds

•	Methodology for translating changes in scale model drag coefficients to changes in fuel efficiency and emissions for full-size trailers (following appropriate federal guidelines)

•	Quantify changes in trailer performance associated with changes in trailer geometry for specific trailer models

•	Final design report that documents the team’s model development and testing process

Model Design
At the start of the project, our team was first tasked with created scaled-down models of various Western Trailer products so that we could use the wind tunnel. The wind tunnel test section is 18" x 18", and the blockage ratio cannot exceed 10%. We decided to base our entire scaling system off our model height of exactly 3". This resulted in a scaling factor of 1:38.33 and an overall blockage ratio of 2.47%. The resulting length and width of the 48-foot trailer model was 15.018" and 2.671", respectively. The A-Train models also followed this scaling factor, resulting in a height of 2.039" and a width of 2.560". The lead trailer is 10.106" long and the pup trailer is 7.915" long.

Five panels were cut out of 1/8" birch wood on the laser cutter at the University of Idaho. The five panels included the front, sides, top, and bottom panels of the trailer models. An intuitive tooth design was used to ensure correct alignment of all the panels on each model. Wood glue was placed on the edge of each tooth when the panels were assembled.

Internal Design
Testing procedures required we attached various features to the trailers and observe how the aerodynamic forces change. Through the use of internal magnets, quick and easy model modifications were created to simplify the wind tunnel testing process. The magnets were glued to both the inside surfaces of the wooden trailers and to recessed surfaces present in all 3D-printed parts.

Testing Matrix
The testing matrix for the 48 foot trailer configuration is located on the left, while the testing matrix for the A-Train configuration is located on the right. The spacing between the lead and pup trailer in the A-Train configuration will also vary. The table above gives the five different lengths that were tested.

Data Processing
The data will be processed and analyzed as follows:

1) Drag force values are acquired from electronic displays that are connected to a force balance within the wind tunnel.

2) The wind speeds are correlated to the frequency of the fan driver input on the wind tunnel.

3) Values such as density and projected cross-sectional area are determined and held constant for the entire analysis of each model.

4) The Coefficient of Drag for each model tested is calculated. Plots will be created that compares the drag coefficients of similar models, such as the 48-foot trailer with different front-end attachments.

5) Constants C1 through C5 are determined for each model. For our testing, the same constants are used for every model. According to official documentation, a trailer is considered long if it is over 50 feet. The longest model we have is a 48-foot dry box van trailer. The Tire Rolling Resistance Level (TRRL) and Weight Reduction (WR) was taken from an example from Cornell Law School that discussed the details of the new regulations.

6) The fuel consumption rate per 1000 ton-miles is calculated.

Testing Results
Early testing showed a relatively linear relationship between CD and wind velocity past ~12.5 m/s. We will run future tests from ~12.5 m/s to ~30 m/s in 2.5 m/s increments as the coefficient of drag levels out after that interval. Each model will be run twice and we will average the voltage data to reduce error.

Wind Tunnel Learning
When testing scale models, geometric similitude is very important. The ratio of model length to actual length must be identical with model width to actual width, and so on with all other dimensions. The Buckingham-pi theorem allows dimensional analysis to simplify testing and data processing.

3-D printing models accurately and effectively has taken many unexpected modification. This is mainly due to the fact that the 3-D printer can only print and 8" x 8". The upper left hand image shows a part that needed to be printed in sections. The upper right hand image shows that tabs and inserts were needed to be designed into the part.