Influence of freestream vortical structures on stagnation region heat transfer

An experimental study was performed to investigate the influence of freestream vortical structures on stagnation region heat transfer. A heat transfer model with a cylindrical leading edge was tested in a low speed wind tunnel at Reynolds numbers ranging from 67,750 to 142,250 based on leading edge diameter of the model. Turbulence generating grids of parallel rods of diameter, 2.86 cm, 1.59 cm and 0.95 cm, were placed upstream of the heat transfer model in horizontal and vertical orientations to generate freestream turbulence with different orientations of vortical structures. The rods in horizontal orientation were perpendicular to the stagnation line and those in vertical orientation were parallel to the stagnation line of the heat transfer model. The distance between the grid and heat transfer model was varied from 25 to 125 rod diameters. The grids with rods in the horizontal orientation, where the primary vortical structures are expected to be perpendicular to the stagnation line, result in higher heat transfer than with the grids where the rods are in the vertical orientation. The difference in heat transfer with the two grid orientations decreases with increasing grid-to-model distance for a given rod-grid. The difference also decreases with decreasing rod size for a given normalized grid-to-model distance. For the 2.86 cm rod-grid, the difference in heat transfer augmentation between horizontal and vertical grid-orientations is highest at the stagnation line and decreases with streamwise distance. This difference, however, remains fairly constant over the whole stagnation region for the 1.59 cm and 0.95 cm rod-grids.