| An experimental study was performed to investigate the influence of freestream turbulence with coherent 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. Grids of parallel rods with diameters 2.86 cm, 1.59 cm and 0.95 cm were used to generate the freestream turbulence with well-defined primary vortex lines. The grids were placed at several locations upstream of the heat transfer model in orientations where the rods were perpendicular and parallel to the stagnation line. Hot-wire anemometry was used to measure the turbulence characteristics of the freestream turbulence. The freestream turbulence was characterized using the turbulence intensity, integral length scale, lateral velocity and vorticity fluctuating component. The turbulence intensity and the ratio of integral length scale to leading edge diameter were in the range of 3.93 to 11.78% and 0.07 to 0.7, respectively. Characteristics of coherent vortical structures downstream of the grids were examined by analyzing the isotropy of turbulence, lateral velocity and vorticity fluctuating components and the wavelet energy spectra of the lateral fluctuating velocity components downstream of the turbulence grids. Heat transfer coefficients were estimated by measuring the temperature distribution and the heat flux. The grids with rods perpendicular to the stagnation line, where the primary vortical structures are expected to be perpendicular to the stagnation line, result in higher heat transfer than those with rods parallel to the stagnation line. The difference between the two grid orientations was more pronounced for the bigger rod-grids. The measured heat transfer data and freestream turbulence characteristics were compared with existing correlation models. An attempt to predict the heat transfer augmentation at the stagnation line due to the turbulence with coherent vortical structures using a neural network was made. A new correlation for the stagnation line heat transfer, which includes the spanwise fluctuating vorticity components, has been developed. |
