Study on the influence of geometric dimensions on the profile design of flexible nozzle in wind tunnel

With the numerical simulation method, the nozzle design results on a basic model and a scaled model are studied through analyzing symmetric plane Mach number distribution, density gradient distribution and central axis Mach number distribution curve in the wind tunnel test section. The main purposes of the study are: to compare the flow field uniformity of the two models, including the root mean square deviation of Mach number in the first diamond region, axial Mach number gradient and average Mach number, to obtain the preliminary influence of geometric dimensions on nozzle design results and to provide guidance for nozzle design in wind tunnels of different scales. The results show that: with the increase of the geometric dimensions of the wind tunnel, the Reynolds number of the flow field increases, the wall viscosity weakens, the boundary layer is thinner and the flow is easier. Under the same flow condition, the thickness of the boundary layer of the scaled model accounts for about 8% of the cross-section dimension, while that of the basic model accounts for only about 5%. Smaller boundary layer thickness has less influence on laminar flow profile. The adverse effect caused by boundary layer correction is weakened, thus improving the quality of the flow field. Compared with the scaled model, the average Mach number in the first diamond region of the basic model increases by 2-3 times, the root mean square deviation of Mach number basically improves by more than 30%, and the axial Mach number gradient increases by at least one order of magnitude. The average Mach number of the basic model is closer to the design value; the root mean square deviation of Mach number and the axial Mach number gradient are smaller than those of the scaled model; the flow field improves comprehensively.