RNDr. MgA. Viktor Hruška, Ph.D.

The amount and practical source of losses required by rectangular sonic black holes in air to effectively absorb low-frequency sound are analyzed numerically and experimentally. In the sole presence of viscothermal losses, only high-order (high frequency) Fabry-Perrot resonances are likely to be critically coupled in realistic rectangular sonic black holes. This results in sharp absorption peaks that do not reach unity at low frequencies, because the quality factors of the associated resonances are high. To avoid these drawbacks, slits of rectangular sonic black holes are partially filled with porous materials so that a profile of porous filled slits is superimposed on the sonic black hole profile itself. The improvement in the absorption coefficient is significant, particularly at frequencies below the viscous/inertial transition frequency of the porous material. This transition frequency is assumed to be the limit of possible perfect absorption of the bulk porous material layer. The numerical results are supported by experimental results, which also show that the vibrations of the plates forming the acoustic black hole must be considered with caution.

The phenomenon of bound states in the continuum and leaky resonances is employed to design a reactive silencer. It is targeted to prevent a significant spectral line from propagating in a flow-duct while maintaining low pressure drop and installation space (without any lateral extensions). The quasi bound states are achieved by tuning the geometrical parameters of thin plates embedded in a waveguide with dimensions and flow velocities typical for ventilation systems. In general, the procedure works for waveguides with plane wave propagation in the low Mach number regime. An optimization by evolution strategies used for this purpose is described and illustrated on two specific examples.

This paper presents a new Gradient Refractive Index (GRIN) structure, which utilizes a series of thin plates made of functionally graded material separated by narrow gaps, to control elastic P-wave fields. The proposed technique is demonstrated through two basic examples of wave manipulation: focusing and deflecting, but has the potential for a wide range of applications. To verify the theoretical calculations, COMSOL Multiphysics simulation software was utilized to solve the full Navier–Lamé equations. Overall, the results of this study demonstrate the effectiveness of the proposed GRIN structure in manipulating elastic P-wave fields.

In this article we address the issue of finding acoustic and elastic resonator shapes corresponding to given eigenfrequencies. This problem has a wide range of applications spanning from ultrasonics through the music acoustics to metamaterials. We employ the previously published solution to the Webster horn equation and use it to train an artificial neural network to solve the inverse eigenvalue problem. Subsequently, it is shown how the isospectrality and scalability properties might provide a whole class of fast designed resonator shapes.

The concept of bound states in the continuum and leaky resonances is utilized in the design of a reactive silencer that can effectively suppress significant spectral lines while maintaining a low-pressure drop within the flow duct and does not require additional installation space. By adjusting the geometrical parameters of thin plates that are embedded in a waveguide, quasi-bound states (or leaky resonances) can be achieved. An optimization algorithm is employed to fine-tune these parameters, and this process is illustrated through two specific examples. The resulting design is validated through numerical simulations that account for the effects of low Mach number flow. The investigations showed that it is possible to design a spectral silencer with low-pressure drop based on the chosen approach. By combining several leaky resonances, stopbands were created with a transmission loss of up to 17 dB in a frequency range of 10 Hz.

The goal of this research is to enhance sound absorption in rectangular acoustic black holes (ABHs), also known as metamaterial graded absorbers, by optimizing their design using evolutionary algorithms, specifically the covariance matrix adaptation evolution strategies (CMA-ES). The designs must have good practical feasibility for 3D printing. A model based on the Riccati equation is employed to optimize the reflection coefficient of the rectangular ABH. The shape of the ABH profile is controlled by a high order Bezier curve, providing greater flexibility than simpler profiles such as polynomials. Additionally, the study explores the use of porous material and the optimization of its macroscopic geometry. Results demonstrate that the use of ABHs can significantly improve sound absorption in a frequency range below the Biot frequency of the porous material.

There is only a limited amount of known analytical solutions to the Pridmore-Brown equation, mostly employing asymptotic behavior for a certain frequency limit and specifically chosen flow profiles. In this paper, we show the possibility of transformation of the Pridmore-Brown equation into the Schrödinger-like equation for the case of two-dimensional homentropic mean flow without critical layers. The corresponding potential that depends on the mean flow profile can then be approximated by a quartic polynomial, leading to a triconfluent Heun equation whose solution based on the triconfluent Heun functions is generally known. The quality of this approximation procedure is presented for the case of symmetric polynomial flow profiles for various values of polynomial order and the Mach number. A more detailed example is then shown for a quadratic mean flow profile, where the solution is accurate up to the third order of the Mach number.

Heat exchangers can be found in a large number of technical systems and installations. They are usually operated in combination with other machines, such as axial fans, in order to remove or supply heat to a system. The heat exchanger can influence the existing flow field and thus lead to increased noise emission from fans located downstream of the heat exchanger. This can be observed, for example, in air conditioning units in which axial fans operate in combination with heat exchangers. Even though this mechanism is known, it is not yet understood how the heat exchanger affects the sound propagation of the sound produced by the downstream machine. For example, the heat exchanger may lead to a change in directional characteristics or specific frequencies may be attenuated. In order to better understand the interaction of the heat exchanger with the sound field, sound power measurements were carried out on various heat exchangers and the sound propagation was simulated numerically. It was shown that the sound attenuation due to the interaction with the periodic tube array is detectable in heat exchangers and that this leads to a sound reduction at the Bragg frequency. Based on its filling factor, the heat exchanger can reduce the sound propagation in certain frequency bands by up to 10 dB if the geometrirical properties are selected suitably. The simulations of a single unit cell confirm in very good agreement with the experimental results. This allows the conclusion that the approach presented in this paper is a cost-effective way to model acoustic effects of heat exchangers. Furthermore, sound attenuation effects by the heat exchanger were caused by thermoviscous effects on the cooling fins and dimensions of the heat exchanger housing.

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The study deals with a possibility to formulate the complex fluid–structure interaction problem of noise generation due to the unsteady flow in corrugated pipes by phenomenological formulation. Although the details of fluid dynamics are not accessible this way, the computational framework is much less demanding and the key features are still present. The van der Pol-like equations serve as a model of self-sustained sources coupled to the acoustic resonances of the tube. The effects of the sound field convection are discussed. A closer case-study investigation using linear stability analysis and a semi-analytical solution is presented. Numerical analysis investigating the flow velocity sweeps and fitting the model to the experimental data follows. The proposed system exhibits the key features known from experiments such as mode-locking and hysteresis and is capable of capturing the experimental data correctly.

Although the importance of sonic crystals serving as sound barriers is growing, only a little interest was given to the sound emission from a sonic crystal due to windy weather conditions. The Curle’s aeroacoustic analogy is reviewed in brief to obtain a suitable formulation for the low Mach number flow and rigid crystal structure. The radiation from the crystal has the dipolar characteristic and the dominating frequency corresponds to the Strouhal number 0.3. The values slightly exceeding 50 dB[A] were found for the radiation maxima at 20 m distance. It follows from the method design that it provides maximum estimate of the radiated power.

Self-sustained sources coupled to some sort of resonator have drawn attention recently as a subject of nonlinear dynamics with many practical applications as well as interesting mathematical problems from the chaos theory and the theory of synchronizations. In order to mimic the self-sustainability arising from physical background the van der Pol equation is commonly used as a model (e.g. vortex induced noise, flowstructure interactions, vocal folds motion etc.). In many cases the sound field inside the resonator is strong enough for weakly nonlinear formulation based on the Kuznetsov model equation to be employed. An array of sources governed by the inhomogeneous van der Pol equation coupled to the nonlinear acoustic wave equation is studied. The one dimensional constant cross-section open resonator with zero radiation impedance is assumed. The focus is on the main features such as mode-locking, harmonics generation and build-up from infinitesimal fluctuations.

Sonic crystals are promising installations in the field of noise reduction. This paper investigates the possibility that the low Mach number airflow through an array of rods forming the crystal may act as an unwanted sound source. The problem is approached by numerical simulations of the turbulent airflow and the numerical data are further treated by Curle’s aeroacoustic analogy. The results show that the crystal is capable of emitting the low-frequency sound with the fundamental frequency corresponding to the characteristic time of hydrodynamic instabilities given by the Strouhal law. Sensation of the noise is briefly discussed.

Na základě motivace ze základních aeroakustických vlnových rovnic je navržen fenomenologický model generování zvuku ve vroubkovaných trubicích. Numerické simulace velmi dobře korespondují s kvalitativním chováním známým z experimentů (zejména se jedná o efekty nelineární synchronizace a hystereze).

The sound generation in a corrugated pipe is studied based on a phenomenological approach relying on the feedback mechanism provided by the van der Pol type of governing equations. Necessary basics of vortex sound theory are presented. Effects of low Mach number flows and missing corrugations are simulated as well as different mean flow velocity pulses. The discussion aims for connecting the model features and parameters to the published experimental behavior of such pipes.