A critical assessment is made of tunnel wall interference corrections, indicated testing limits, and the data correlations for yawed truck models inside a solid-wall test section, using a modified pressure-signature method.
Two models, a full-size tractor-trailer and a similar half-scale model, were tested in the 9 x 9 meter wind tunnel of the National Research Council of Canada (NRCC). Both models were yawed through ±14° with full length and truncated trailer bodies for the purpose of determining wind-averaged drag and exploring the real aerodynamic testing limits due to tunnel interference.
The correction method employed an algorithm to perform an iterative solution to determine the strengths of potential sources, sinks, and horseshoe vortices for given inputs (measured ceiling pressures, side force, yaw and roll moments, as well as dimensional data). The output comprised corrections to tunnel dynamic pressure and yaw angle, and a horizontal buoyancy effect.
The calculated tunnel wall interference corrections were surprisingly large, even for the small model: the yaw-ranged-averaged drag and side-force coefficients of the half-scale model (2.2% blockage) were reduced by about 6% and 4%, respectively, and up to 22% and 15% in the case of the full-scale model (8.7% blockage). Appreciable increases in yaw angle were also indicated: up to 3% and 14% for the half- and full-scale model experiments, respectively. The study demonstrated that the modified-pressure-signature correction method resulted in correlating the full- and half-scale wind-averaged drag coefficient data within a 1% and 2% margin of repeatability for the 7 mph wind speed and 55 mph road speed (7 & 55 mph) and 7 & 30 mph condition, respectively.