Development of a methodology for measuring the wheel ventilation drag in a 1:1 scale wind tunnel

The modern world’s challenges regarding the CO2 emission levels and the increasing cost of fuels established new development targets for the automotive industry. The birth of electric vehicles put a new option on the market for trying to reduce pollutants emissions; their weak points, however, highlight the need for research and development in order to offer in the future a product that can be comparable to the most successful cars equipped with internal combustion engines. This new generation of ground vehicles, indeed, mainly suffer from a reduced range. For this reason, nowadays, great focus is put on cars’ aerodynamic development for trying to reduce any form of aerodynamic drag (conventional aerodynamic drag and ventilation drag); this approach is as valid as a weight reduction of the entire vehicle in order to lower the power required by the engine for winning these energy losses. However, the accomplishment of this purpose through a mass decrease of the car would be much more complicated due to the fact that would result in conflicting goals compared to the classical project requirements such as safety and comfort but also with the new ones like the battery weight required for the desired ranges. The main and only driving factor in the automotive industry has always been the exterior design; this was true until a direct application of aeronautical principles was seen due to the different oil crisis that happened in the past. Different attempts were needed in order to find the required and optimal shapes for satisfying the aerodynamic goals. As a matter of fact, differently from airplanes (identified as streamlined bodies), a car belongs to the category of bluff bodies that move near the ground; therefore, the main drag contribution is represented by the pressure drag rather than the skin friction one. In any case, a car develops three forces and three moments among which the drag is the driving parameter for fulfilling the modern emission regulations and range requirements. Given the intense aerodynamic research of the last decades, great focus is now put also on the interaction between the airflow and the rotating wheels for assessing their influence on the fuel/energy consumption. Even in an increasingly digitalized world where computational methodologies are progressively establishing themselves, wind tunnel facilities assume a fundamental role for development and correlation purposes. Therefore, the aim of this thesis, developed in the Pininfarina wind tunnel, was that of identifying a methodology for the evaluation of the ventilation drag; this is an additional action generated by the uneven pressure distribution around a rotating wheel and that opposes its rotation. A series of experimental test was conducted for the purpose of defining the ventilation drag coefficient that quantifies this supplementary drag contribution which usually is not provided by typical wind tunnel measurement campaigns. All in all, the definition of an equivalent aerodynamic drag coefficient that takes into consideration both the classic aerodynamic drag coefficient and the ventilation drag coefficient better allows to evaluate the effective efficiency of the whole vehicle.