1. Problem and Purpose Hypotheses Design Plan Background Information Currently, school buses are very fuel inefficient, averaging 7 mpg (Laughlin, 2004). A major part of their fuel consumption is towards overcoming aerodynamic drag. Their box-like shape has a high coefficient of drag and because the power needed to overcome drag increases with the cube of speed, a heavy vehicle moving at highway speeds spends over half of its fuel overcoming drag(“The tyre: Rolling resistance and fuel savings”, 2003). There are estimated ~480,000 school buses in USA that total to about 4.4 billion miles per year (“School Bus Safety Data,” 2009). According to Laughlin’s estimate of 7mpg, that is about 630 million gallons of diesel burned by school buses yearly. If an economically viable drag reduction device could be made for school buses, millions of gallons of fuel could be saved and CO 2 prevented from going into the atmosphere. Purpose: To design and evaluate the practicality of 3 different drag reduction devices using CFD and then optimize their geometries to achieve the highest reduction in C d per monetary unit. An object that moves through a fluid experiences aerodynamic drag. A drag force is created because fluids are viscous and resist deformation. At typical automobile speeds, the drag force varies with the square of velocity and the power needed to overcome it varies with the cube of velocity. The drag coefficient, C d, is a unit less number that summarizes how aerodynamically efficient an object is. The C d for a streamlined airfoil can be around 0.05, while the C d for a box is over 1. It is calculated by the rearranging the drag equation to F D is the drag force in N ρ is the density of the fluid, in kg/m 3 C D is the coefficient of drag A is the cross sectional area C D =2F D /(ρAv 2 ) CFD, computational fluid dynamics, is using computers to approximate solutions to the Navier-Stokes equations, the ones that govern fluid flow. It is very versatile and with the growth of computing power, it can produce accurate results compared to a wind tunnel. CFD allows many designs to be tested and allows a user to visualize the results. Current CFD programs can perform a wide range of fluid simulations and measure many physical quantities such as pressure, drag forces, temperature, and more. The two main components of aerodynamic drag on a vehicle are skin friction and pressure drag, also known as form drag. Skin friction is caused by the air rubbing against the surface of the vehicle. It is not significant for heavy vehicles like a bus and contributes less than 10% to the total drag force(Patten, McAuliffe, Mayda, & Tanguay, 2012). Pressure drag is the significant component and occurs because a moving vehicle creates a region of high pressure air in the front that pushes the vehicle back and a region of low pressure air called the wake where the air stagnates and pulls the vehicle back((Edgar, 2008). CFD results(right) vs. experimental(left) Image from http://www.mathematik.uni-dortmund.de/~kuzmin/cfdintro/lecture1.pdfhttp://www.mathematik.uni-dortmund.de/~kuzmin/cfdintro/lecture1.pdf 1.All of the three designs, rear spoiler, back plates, and frontal fairing will reduce the total drag force and the C d of the school bus. 2.At least one of the designs will be able to reduce C d by 10% or more. 3.One of these devices, after optimization, will be economically practical to implement if manufacturing costs could be brought down low enough. Rationale: Previous experiments on semi-trailers have shown that simple devices can reduce C d by a large amount.