This paper investigates the effect of external expansion ramps on supersonic flow emitted from high-aspect-ratio rectangular nozzles with a focus on additional sources of noise due to ramps as well as their effectiveness as noise reduction devices. Schlieren videos showed that the extended ramps stabilized the jet, reduced spreading, and elongated the shock cell systems. Far-field acoustics revealed that the most severe noise penalties arose from the impingement of the shear layer on the ramps and the expansion of the jet at the trailing edge of the ramp. For transonic jets, which contain only broadband sources of noise, noise due to ramps was louder than turbulence mixing noise. The penalty increased with the ramp length. On the contrary, the ramps were effective at reducing noise at higher jet Mach numbers, where the shock-associated components of noise were strong and turbulence mixing noise dominated jet scraping noise. The extent of reduction was dependent on the relative length between the ramp and the jet potential core. A ramp approximately as long as, but no longer than, the potential core minimizes peak mixing noise, shock-associated noise, and noise arising from the interaction between flow and the ramp.

本文研究了外部膨胀斜坡对高纵横比矩形喷嘴发出的超音速流的影响，重点是斜坡引起的额外噪声源及其作为降噪装置的有效性。纹影视频显示，延长的坡道稳定了射流，减少了扩散，并拉长了冲击单元系统。远场声学显示，最严重的噪声惩罚来自斜坡上剪切层的撞击和斜坡后缘处射流的膨胀。对于仅包含宽带噪声源的跨音速喷气机，斜坡产生的噪声比湍流混合噪声更大。惩罚随着斜坡长度的增加而增加。相反，斜坡能有效降低喷气马赫数较高时的噪音，其中与冲击相关的噪声成分很强，湍流混合噪声主导了喷气刮擦噪声。减少的程度取决于斜坡和喷射势核之间的相对长度。大约与潜在核心一样长但不长于潜在核心的斜坡最小化峰值混合噪声、冲击相关噪声以及由流动和斜坡之间的相互作用引起的噪声。