Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A24: Acoustics I |
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Chair: Daniel Bodony, University of Illinois at Urbana-Champaign Room: 319 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A24.00001: The effect of sweep on forward-step noise Jin Hao, Meng Wang The flow and acoustic fields of swept forward-facing steps in low-Mach-number turbulent boundary layers are investigated using large-eddy simulation in combination with Lighthill's theory. The step height is $13\%$ of the thickness of the unperturbed boundary layer at $Re_\theta = 4755$, and the step sweep angle is varied from $0^\circ$ to $45^\circ$ with an increment of $15^\circ$. For the same incoming flow and step height, larger sweep angles produce smaller perturbations to the boundary layer, leading to weaker surface pressure fluctuations and acoustic radiation and speedier recovery towards an equilibrium boundary layer in the downstream. With proper normalization using the free-stream velocity component normal to the step face, the sweep-independence principle is found to be approximately valid for the reattachment length, mean step-normal velocity profiles and pressure and skin friction coefficients at all sweep angles examined. In terms of surface-pressure fluctuations and radiated acoustics, sweep independence is observed for sweep angles up to $30^\circ$ at relatively low frequencies. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A24.00002: Computational Study of Shock-Associated Noise Characteristics Using LES J. Liu, A. Corrigan, K. Kailasanath, N. Heeb, D. Munday, E. Gutmark Shock-associated noise generation has been investigated by using large-eddy simulations to compute jet flows at an underexpanded jet condition with three jet temperatures. To better understand shock-associated noise generation, shock-free jets with the same fully expanded jet conditions have also been simulated. The predictions agree well with the available experimental data in both the near and far field. It is found that shock cells at this underexpanded jet condition have little impact on the jet core length and the turbulence kinetic energy distribution, whereas the heating effect has a much larger impact by increasing the initial shear-layer spreading and shortening the jet core length. Shock-associated noise dominates in the upstream direction, and the broadband peak frequencies move to higher values in downstream direction. This frequency increase is initially small in the upstream direction, but becomes much larger in the downstream direction. In addition, it is found that the heating effect increases the broadband peak frequency. Overall the heating effect increases the mixing noise and slightly reduces the shock-associated noise. This reduces the difference between the shock-containing jets and the shock-free jets as the temperature increases. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A24.00003: Noise prediction from external flows using Ffowcs-Williams and Hawkings techniques Zane Nitzkorski, Krishnan Mahesh We investigate noise production from turbulent flow using the Ffowcs-Williams and Hawkings (FWH) acoustic analogy for general hydrodynamic flow configurations. We describe our methodology of using porous implementations of the FWH equations to calculate far-field sound from sources that are computed by either incompressible or compressible LES/DNS. We discuss a novel endcap methodology for the quadrupole source terms. The methodology allows for estimation of volumetric noise computed over a small volume as opposed to the common approach of ignoring the entire volume term while providing fewer limitations on the propagation function. We compute the noise from cylinders at various Reynolds' numbers (Re $=$ 150, 10k, 89k) and trailing edge configurations and compare our results, base flow as well as acoustic data, to available computations and experiments. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A24.00004: Vortex Noise Reductions from a Flexible Fiber Model of Owl Down Justin Jaworski, Nigel Peake Many species of owl rely on specialized plumage to reduce their self-noise levels and enable hunting in acoustic stealth. In contrast to the leading-edge comb and compliant trailing-edge fringe attributes of owls, the aeroacoustic impact of the fluffy down material on the upper wing surface remains largely speculative as a means to eliminate aerodynamic noise across a broad range of frequencies. The down is presently idealized as a collection of independent and rigid fibers, which emerge perpendicularly from a rigid plane and are allowed to rotate under elastic restraint. Noise generation from an isolated fiber is effected by its interaction with a point vortex, whose motion is induced by the presence of the rigid half-plane and the elastically-restrained fiber. Numerical evaluations of the vortex path and acoustic signature furnish a comparison with known analytical results for stationary fibers, and results from this primitive model seek to address how aerodynamic noise could be mitigated by flexible fibers. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A24.00005: Interaction of a turbulent boundary layer with a cavity-backed circular orifice and tonal acoustic excitation Qi Zhang, Daniel Bodony Acoustic liners are effective reducers of jet exhaust and core noise and work by converting acoustic-bound energy into non-radiating, vorticity-bound energy through scattering, viscous, and non-linear processes. Modern liners are designed using highly-calibrated semi-empirical models that will not be effective for expected parameter spaces on future aircraft. The primary model limitation occurs when a turbulent boundary layer (TBL) grazes the liner; there are no physics-based methods for predicting the sound-liner interaction. We thus utilize direct numerical simulations to study the interaction of a Mach 0.5 zero pressure gradient TBL with a cavity-backed circular orifice under acoustic excitation. Acoustic field frequencies span the energy-containing range within the TBL and amplitudes range from 6 to 40 dB above the turbulent fluctuations. Impedance predictions are in agreement with NASA Langley-measured data and the simulation databases are analyzed in detail. A physics-based reduced-order model is proposed that connects the turbulence-vorticity-acoustic interaction and its accuracy and limitations are discussed. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A24.00006: Experimental validation of the directional sensitivity of the acoustic radiation force to particle diameter Weiyu Ran, J.R. Saylor A review of existing theories for the acoustic radiation force on a particle reveals a contradiction. Some theories predict that this force exhibits a change in sign at a critical particle diameter (all other parameters held constant), while other theories predict no such sign change. To ascertain which result is correct, experiments were conducted using an ultrasonic standing wave field in air. Particles were injected into this field whereupon, as expected, they migrated toward the pressure nodes of the standing wave field. The average diameter of these particles was gradually decreased. Under such conditions, the particles should either (i) continue to migrate to the pressure nodes, or (ii) migrate to the pressure anti-nodes at a critical diameter, if a change in sign of the acoustic radiation force exists. The results of these experiments will be presented, along with their implications on extant theories. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A24.00007: Acoustic Radiation Force on a Finite-Sized Particle due to an Acoustic Field in a Viscous Compressible Fluid Subramanian Annamalai, Manoj Parmar, Balachandar S. Particles when subjected to acoustic waves experience a time-averaged second-order force known as the acoustic radiation force, which is of prime importance in the fields of microfluidics and acoustic levitation. Here, the acoustic radiation force on a rigid spherical particle in a viscous compressible medium due to progressive and standing waves is considered. The relevant length scales include: particle radius ($a$), acoustic wavelength ($\lambda$) and viscous penetration depth ($\delta$). While $a/\lambda$ and $a/\delta$ are arbitrary, $\delta \ll \lambda$. A farfield derivation approach has been used in determining the radiated force. Expressing the flow-field as a sum of the incident and scattered fields, an analytical expression for the force is obtained as a summation over infinite series (monopole, dipole and higher sources). These results indicate that the contributions from monopole, dipole and their cross-interaction are sufficient to describe the acoustic radiation force. Subsequently, the monopole and dipole strengths are represented in terms of the particle surface and volume averages of the incoming velocity. This generalization allows one to evaluate the radiation force for an incoming wave of any functional form. However acoustic streaming effects are neglected. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A24.00008: Sequencing of acoustic events in the near field of subsonic jets Jacques Lewalle, Pinqing Kan Our group has developed several pattern recognition algorithms to identify short events common to near-and far-field signals. Here we are treating far-field and near-field pressure data as well as TR-PIV (10kHz) sections through the near jet. Our algorithms are based on wavelet transforms (band-pass filtering) and cross-correlations, identifying short excerpts in the time-frequency-lag domain that contribute most to the correlations. Matching such events between multiple signal pairs exposes the sequencing of near-field activity. We consider only near-field events (NFEs) matched with a loud far-field event (FFE). The NFEs are based on the correlation of velocity, vorticity, 2-D divergence, Q-index and Kulite signals with far-field pressure. The timing of the NFEs maps out possible sequences of events related to loud coherent noise emission. Results at several subsonic Mach numbers are compared. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A24.00009: Full-scale simulation and reduced-order modeling of a thermoacoustic engine Carlo Scalo, Jeff Lin, Sanjiva Lele, Lambertus Hesselink We have carried out the first three-dimensional numerical simulation of a thermoacoustic Stirling heat-engine. The goal is to lay the groundwork for full-scale Navier-Stokes simulations to advance the state-of-the-art low-order modeling and design of such devices. The model adopted is a long resonator with a heat-exchanger/regenerator (HX/REG) unit on one end - the only component not directly resolved. A temperature difference across the HX/REG unit of 200 K is sufficient to initiate the thermoacoustic instability. The latter is a Lagrangian process that only intensifies acoustic waves traveling in the direction of the imposed temperature gradient. An acoustic network of traveling waves is thus obtained and compared against low-order prediction tools such as DeltaEC. Non-linear effects such as system-wide streaming flow patterns are rapidly established. These are responsible for the mean advection of hot fluid away from the HX/REG (i.e. thermal leakage). This unwanted effect is contained by the introduction of a second ambient heat-exchanger allowing for the establishment of a dynamical thermal equilibrium in the system. A limit cycle is obtained at +178 dB. [Preview Abstract] |
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