A decomposition of the Navier-Stokes equations is used to identify the equivalent source term for broadband shock-associated noise (BBSAN). An analytical closed-form model to predict BBSAN is developed using an acoustic analogy based on the Navier-Stokes equations. The field-variables are decomposed into the base flow, aerodynamic fluctuations, and acoustic fluctuations. The spectral densities of fluctuating acoustic quantities are obtained by convolving the vector Green's function with the source terms involving the two-point cross-correlation of the aerodynamic quantities. The scaling of the source term with the off-design parameter β=(|Mj2−Md2|)1/2 is compared with experimental results. The base flow is obtained using a Reynolds-averaged Navier-Stokes solution, while the fluctuating statistical quantities are obtained using theoretical and experimental results. This paper identifies the equivalent source of BBSAN based on the scaling analysis and the physical mechanism of shock-associated noise. The identified source term resides within the Navier-Stokes equations without further rearrangement and correlates very highly with BBSAN. Predictions for BBSAN are made at multiple observer angles and nozzle pressure ratios using the identified source term, and these predictions compare favorably with the experimental results. Finally, identification of the source locations in the jet exhaust responsible for BBSAN at different Strouhal numbers is performed.
Blade vortex interaction noise is a problematic and dominant component of rotor noise. Plasma actuators strategically placed at the tip of the rotor blades can reduce the strength of the tip vortices. This reduction has the potential to significantly reduce blade vortex interaction noise. A combined experimental, numerical, and theoretical program shows supporting evidence that low power plasma actuators can effectively lower coherence of the blade tip vortex and reduce blade vortex interaction noise over-pressure by up to 80%. For a nominal small five-bladed unmanned aerial vehicle, we predict an approximate 8.88 maximum ΔdB reduction for a 150 m/s tip speed. Experimental, computational, and acoustic modeling support these predictions. This study represents a fundamental investigation in the fixed-frame, which provides evidence for higher level research and testing in a rotating framework.
Jet noise remains a major source of concern for military personnel working aboard aircraft carriers. Broadband shock-associated noise (BBSAN) and fine-scale mixing noise (FSMN) are the main components of noise in the sideline and upstream directions. A decomposition approach based on the Navier–Stokes equations is used to identify the source of BBSAN and FSMN. Statistical models are developed to predict BBSAN and FSMN using the identified source terms. The prediction models use Reynolds-averaged Navier-Stokes solution as an argument. Comparisons of the noise radiated due to BBSAN and FSMN by three different nozzle types, i.e., a method of characteristics nozzle, a bi-conic nozzle, and a faceted nozzle are performed at different operating conditions. Different noise reduction mechanisms such as fluidic injection and corrugations on the nozzle wall are implemented on the faceted nozzle. We examine the change in source statistics and its correlation on the intensity and directivity of the radiated noise for BBSAN and FSMN. Noise reduction predictions relative to the baseline cases are compared for each nozzle modification. Finally, we analyze the noise source locations at different Strouhal number for both components of jet noise in the sideline direction.
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