Session A05: Acoustics: General

Viscosity, surface tension, and gravity effects on acoustic reflection and refraction

The idea of the present work is to study the effects of viscosity, surface tension, and gravity on Snell's law of reflection and refraction of an acoustic field from an interface between two fluids. The analysis leads to a number of non-trivial phenomena and corrections to Snell's law. While all these effects can be considered individually due to separation of the associated time scales, the contributions of surface tension to the gravity and viscosity cases are treated as well.   

Investigation of Propeller Noise Using LES and an Acoustic Analogy

The flow field of a five-bladed marine propeller operating at design condition is used to calculate far-field sound. The flow field is obtained using large eddy simulation (LES) and far-field sound is computed using the Ffowcs-Williams and Hawkings acoustic analogy. The physics of sound generation is examined by analyzing both the near-propeller flow field and far-field sound. While the blades generate strongly tonal noise, the overall acoustic signature of the propeller is broadband. The propeller is found to be acoustically compact at frequencies up to 100 times the rotation rate. The majority of sound is generated by localized unsteadiness at the blade tip, due to shedding of the tip vortex. Two other notable sound generation mechanisms are: convection of boundary layer turbulence past the blade trailing edge and interaction of the blade root wakes with the propeller-hub surface. Acoustic energy is found to be distributed among higher frequencies as local Reynolds number increases radially along the blades.   

Computation of wall pressure fluctuations in compressible turbulent channel flows

An accurate description of the spatiotemporal characteristics of the fluctuating surface pressure in wall-bounded flows is important for predictions of structural vibration and acoustic radiation.  In this study direct numerical simulations are performed using high-order non-dissipative schemes to investigate the unsteady wall pressure in compressible channel flows at Mach numbers of 0.4 and 1.5 and friction Reynolds numbers of up to 510.  The simulations are shown to capture both convective and acoustic contributions to the wavenumber-frequency spectra of wall-pressure fluctuations.  At fixed frequencies acoustic peaks are identified at streamwise acoustic wavenumbers.  These peaks are much lower than the convective peaks and decrease with decreasing Mach number.  At Mach 0.4, the acoustic peaks are several orders of magnitude weaker than the convection peaks, indicating very small acoustic contributions, which is in contrast to predictions from semi-empirical models.  The effect of a small surface hump on the wall pressure statistics will also be discussed.   

Statistics fo Dynamic Pressure Measurements in a Rectangular Cavity

Historically, record length and processing methods have limited the physics that can be extracted from a dynamic pressure measurement in a rectangular cavity.  Limited record length, window size, overlap, and averaging have produced resulting spectra with sharp tonal peaks and relatively smooth broadband noise.  This has led to a simplified understanding of the cavity, even though CFD and numerous non-intrusive experimental methods have shown different spectral features.  A recent experiment captured 8 seconds at Mach 0.7 and 15 seconds at Mach 1.5 of dynamic pressure measurement at 100 kHz sample rate in a rectangular cavity with a Length to Depth ratio of 5.6.  The Reynolds number for both Mach numbers were 2e^6/ft.  Using statistical analysis we will show how the tonal peaks vary in both the frequency and amplitude when the window size is varied.  Using histograms we will show that the historical average may not be the statistical average.