11,248 research outputs found
Stabilization of acoustic modes using Helmholtz and Quarter-Wave resonators tuned at exceptional points
Acoustic dampers are efficient and cost-effective means for suppressing
thermoacoustic instabilities in combustion chambers. However, their design and
the choice of their purging air mass flow is a challenging task, when one aims
at ensuring thermoacoustic stability after their implementation. In the present
experimental and theoretical study, Helmholtz (HH) and Quarter-Wave (QW)
dampers are considered. A model for their acoustic impedance is derived and
experimentally validated. In a second part, a thermoacoustic instability is
mimicked by an electro-acoustic feedback loop in a rectangular cavity, to which
the dampers are added. The length of the dampers can be adjusted, so that the
system can be studied for tuned and detuned conditions. The stability of the
coupled system is investigated experimentally and then analytically, which
shows that for tuned dampers, the best stabilization is achieved at the
exceptional point. The stabilization capabilities of HH and QW dampers are
compared for given damper volume and purge mass flow.Comment: 34 pages, 19 figures, acepted in the Journal of Sound and Vibratio
The giant acoustic atom --- a single quantum system with a deterministic time delay
We investigate the quantum dynamics of a single transmon qubit coupled to
surface acoustic waves (SAWs) via two distant connection points. Since the
acoustic speed is five orders of magnitude slower than the speed of light, the
travelling time between the two connection points needs to be taken into
account. Therefore, we treat the transmon qubit as a giant atom with a
deterministic time delay. We find that the spontaneous emission of the system,
formed by the giant atom and the SAWs between its connection points, initially
decays polynomially in the form of pulses instead of a continuous exponential
decay behaviour, as would be the case for a small atom. We obtain exact
analytical results for the scattering properties of the giant atom up to
two-phonon processes by using a diagrammatic approach. We find that two peaks
appear in the inelastic (incoherent) power spectrum of the giant atom, a
phenomenon which does not exist for a small atom. The time delay also gives
rise to novel features in the reflectance, transmittance, and second-order
correlation functions of the system. Furthermore, we find the short-time
dynamics of the giant atom for arbitrary drive strength by a numerically exact
method for open quantum systems with a finite-time-delay feedback loop.Comment: To be published on Physical Review
Viscous theory of surface noise interaction phenomena
A viscous linear surface noise interaction problem is formulated that includes noise production by an oscillating surface, turbulent or vortical interaction with a surface, and scattering of sound by a surface. The importance of viscosity in establishing uniqueness of solution and partitioning of energy into acoustic and vortical modes is discussed. The results of inviscid two dimensional airfoil theory are used to examine the interactive noise problem in the limit of high reduced frequency and small Helmholtz number. It is shown that in the case of vortex interaction with a surface, the noise produced with the full Kutta condition is 3 dB less than the no Kutta condition result. The results of a study of an airfoil oscillating in a medium at rest are discussed. It is concluded that viscosity can be a controlling factor in analyses and experiments of surface noise interaction phenomena and that the effect of edge bluntness as well as viscosity must be included in the problem formulation to correctly calculate the interactive noise
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