The Aerodynamic Correlation of a Scale Model with a Full Size Car

Doktorsavhandling, 2009

Reducing the Worlds consumption of energy has become an important part of the current political and environmental debate. The reasons are obvious: World population is increasing, continued growth in living standards, finite availability of resources and finally green house gas emissions from energy production have to be brought down to stabilize temperatures. Today road transportation is responsible for more than 20% of world CO2 emissions and this number is expected to double over the next few decades. Therefore vehicles with better efficiency must be developed. To do so requires, among other things; new and novel power units, lighter structures, better electronics and finally improved aerodynamics. Over a defined driving cycle over 40% of the fuel is consumed in overcoming the aerodynamic resistance, depending on vehicle type. At highway cruising speeds it can be considerably more. Therefore optimizing the aerodynamic properties constitutes one of the corner stones in the development of energy efficient vehicles. Due to the non-linear behaviour of fluid flows, but also to reach a balanced compromise with other vehicle attributes, the aerodynamic optimization is a long and iterative process. A process that requires both advanced wind tunnels and large computational resources. Small scale wind tunnel models offer great potential for increasing the efficiency of the aerodynamic development process and reducing the costs thereof. To reap the benefits it is believed that small scale models need to be highly detailed to consistently develop the aerodynamic properties of the real production vehicle. The objective of this thesis has been to investigate how detailed a small scale model of a car is required to be for its aerodynamic properties to correlate with those of its full size counterpart. Furthermore since an important issue in scale model development is the fluid mechanic scaling effects, these were studied to gain an understanding of how the details of the model affects the aerodynamic coefficients over changing Reynolds number. It was found that increments in the coefficient of drag and lift over various configuration changes could be accurately predicted with the detailed small scale wind tunnel model. Furthermore it was shown that the Reynolds number effects of the model are dependent on the level of detail. Thus it is shown that to carry out accurate development or research with small scale models, they need to be of considerable detail.