In this experimental investigation the shock structure and shear layer growth in the near field of non-circular supersonic jets impinging orthogonally on a flat surface were studied under two different nozzle expansion ratios (NPRs). The supersonic jets were of triangular and hexagonal cross-sectional shapes with equal area at the nozzle exit. The expansion ratio of all the nozzles used in the experiments was the same at 1.44, with a throat area of 75 mm2. This corresponded to a design exit Mach number of 1.8 for optimum expansion (NPR = 5.87). Experiments were conducted for three different impinging distances (LP) from the nozzle exit plane. The distances in the experiments were 2Dh, 4Dh, and 6Dh, where Dh was the hydraulic diameter of the nozzle exit shape. Experiments were carried out at NPR = 8 and 11(both under expansion conditions) at each impinging distance. Flow visualization of the impinging jet's shock structure was recorded using a Z-type Schlieren system employing concave mirrors which were polished to λ/6. The experiments were repeated for circular jets at the same experimental conditions and the shock structures of both circular and non-circular jets were compared. The shear layer thickness and its variation in the near field were obtained from the image processing of the schlieren images. The impinging distance was observed to have a significant effect on the growth of shear layer via modified shear layer shock interactions in the near field due to the presence of the orthogonal wall and its distance from the nozzle exit. The impinging plate was square in cross-section with a side dimension of 175 mm. The effect of impinging distance on the shock structure was found to be different for circular and non-circular jets. The jet shocks in the impinging jet, the plate shock in the impinging zone and the tail shocks near the high-speed radial wall jet exhibited a varied behavior depending upon the shape of the jet and its impinging distance. Hexagonal and triangular jets did not produce any Mach disk in the near field at shorter impingement distances such as Lp = 2Dh. The introduction of the impinging effect and subsequent reduction in Lp decreased the Mach disk distance from the nozzle exit of both shapes, till the Mach disk disappeared at Lp = 2Dh. The length of the jet shocks near the nozzle exit also decreased with an increase in Lp for both circular and non-circular shapes. A decrease in Lp impeded the shear layer growth along the jet boundary in the near field for all jet shapes. The triangular jet exhibited a thicker layer along the jet boundary originating from the corner, when compared to the other two jet shapes.

中文翻译：

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膨胀三角形和六边形超音速射流下正交撞击的近场特征


在这项实验研究中，在两种不同的喷嘴膨胀比 （NPR） 下，研究了非圆超声速射流正交撞击平面的激波结构和近场中的剪切层增长。超音速射流具有三角形和六边形横截面形状，在喷嘴出口处的面积相等。实验中使用的所有喷嘴的膨胀率相同，均为 1.44，喉部面积为 75 mm2。这对应于最佳膨胀的设计出口马赫数为 1.8 （NPR = 5.87）。对距喷嘴出口平面的三种不同撞击距离 （LP） 进行了实验。实验中的距离为 2Dh、4Dh 和 6Dh，其中 Dh 是喷嘴出口形状的水力直径。在每个撞击距离的 NPR = 8 和 11（均在膨胀条件下）进行实验。使用采用凹面镜的 Z 型纹影系统记录撞击射流激波结构的流动可视化，该系统被抛光至 λ/6。在相同的实验条件下对圆形射流重复实验，并比较了圆形和非圆形射流的冲击结构。剪切层厚度及其在近场的变化是通过纹影图像的图像处理获得的。由于正交壁的存在及其与喷嘴出口的距离，观察到撞击距离通过改进的剪切层冲击相互作用在近场对剪切层的增长产生显着影响。撞击板的横截面为方形，边尺寸为 175 mm。发现撞击距离对激波结构的影响对于圆形和非圆形射流是不同的。 撞击射流中的射流激波、撞击区的板激波和高速径向壁射流附近的尾部激波根据射流的形状及其撞击距离表现出不同的行为。六边形和三角形射流在较短的撞击距离（例如 Lp = 2Dh）下在近场不产生任何马赫盘。撞击效应的引入和随后的 Lp 降低减小了两种形状的喷嘴出口的马赫盘距离，直到马赫盘在 Lp = 2Dh 时消失。对于圆形和非圆形，喷嘴出口附近的射流冲击长度也随着 Lp 的增加而减小。Lp 的降低阻碍了所有射流形状在近场中沿射流边界的剪切层增长。与其他两种射流形状相比，三角形射流沿源自拐角的射流边界表现出更厚的层。