Diffusers are vital components of vertical wind tunnels. In this experimental study, the velocity distribution, pressure recovery coefficient, turbulence intensity, and flow separation of a 1:13 model of a diffuser with an octagonal inlet and square outlet for use in the skydiving vertical wind tunnel have been investigated. The experiments were carried out both in suction and blowing flow modes. Results showed no significant pressure loss and separation in the diffuser under study. Also, the pressure loss results were compared with the results obtained using semi-empirical correlations for a diffuser with a circular inlet and square outlet, which is geometrically the closest to the diffuser under consideration in the present work. It was observed that to correctly predict the pressure loss in the model diffuser using the semi-empirical correlations, the diffuser big-angle plane of 10.66° should be used in these correlations instead of the equivalent cone angle. Besides, if the effect of the non-uniformity of the velocity in the diffuser is not considered in the determination of the pressure loss, a significant error will incur in determining the pressure loss coefficient.

扩散器是垂直风洞的重要组成部分。在这项实验研究中，研究了用于跳伞垂直风洞的具有八角形入口和正方形出口的扩压器的1:13模型的速度分布，压力恢复系数，湍流强度和流分离。实验是在吸气和吹气两种模式下进行的。结果表明，在研究的扩散器中没有明显的压力损失和分离。同样，将压力损失结果与使用具有圆形入口和正方形出口的扩散器的半经验相关性进行了比较，该扩散器在几何上最接近本研究中的扩散器。已观察到，要使用半经验相关性正确预测模型扩散器中的压力损失，应在这些相关性中使用10.66°的扩散器大角度平面代替等效锥角。此外，如果在确定压力损失时未考虑扩散器中速度的不均匀性的影响，则在确定压力损失系数时将产生很大的误差。