Multifunctional Material Systems: A state-of-the-art review

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation. However, for seismic excitation, where energy is mostly carried by surface waves, energy reflection and redirection might lead to harming surrounding regions. Here, we propose a seismic metabarrier able to convert seismic Rayleigh waves into shear bulk waves that propagate away from the soil surface. The metabarrier is realized by burying sub-wavelength resonant structures under the soil surface. Each resonant structure consists of a cylindrical mass suspended by elastomeric springs within a concrete case and can be tuned to the resonance frequency of interest. The design allows controlling seismic waves with wavelengths from 10-to-100 m with meter-sized resonant structures. We develop an analytical model based on effective medium theory able to capture the mode conversion mechanism. The model is used to guide the design of metabarriers for varying soil conditions and validated using finite-element simulations. We investigate the shielding performance of a metabarrier in a scaled experimental model and demonstrate that surface ground motion can be reduced up to 50% in frequency regions below 10 Hz, relevant for the protection of buildings and civil infrastructures.

/pdf/engineered-metabarrier-as-shield-from-seismic-surface-waves-30mbz6rdfq.pdf

Engineered metabarrier as shield from seismic surface waves.

Acoustic metamaterial plates for elastic wave absorption and structural vibration suppression

Acoustic multi-stopband metamaterial plates design for broadband elastic wave absorption and vibration suppression

Recently, with advances in acoustic metamaterial science, the possibility of sound attenuation using subwavelength structures, while maintaining permeability to air, has been demonstrated. However, the ongoing challenge addressed herein is the fact that among such air-permeable structures to date, the open area represents only small fraction of the overall area of the material. In the presented paper in order to address this challenge, we first demonstrate that a transversely placed bilayer medium with large degrees of contrast in the layers' acoustic properties exhibits an asymmetric transmission, similar to the Fano-like interference phenomenon. Next, we utilize this design methodology and propose a deep-subwavelength acoustic metasurface unit cell comprising nearly 60% open area for air passage, while serving as a high-performance selective sound silencer. Finally, the proposed unit-cell performance is validated experimentally, demonstrating a reduction in the transmitted acoustic energy of up to 94%. This ultra-open metamaterial design, leveraging a Fano-like interference, enables high-performance sound silencing in a design featuring a large degree of open area, which may find utility in applications in which highly efficient, air-permeable sound silencers are required, such as smart sound barriers, fan or engine noise reduction, among others.

Ultra-open acoustic metamaterial silencer based on Fano-like interference

Physical properties of the ideal Bose gas with non-integer dimensions between D = 2 and D = 3 are theoretically investigated. Calculation shows that there exists a hierarchy of condensation transitions with changing fractional dimensionality. The results obtained on the specific heat and the condensed density in non-integer dimensions are similar to those for liquid helium-4 in porous media. This results suggest that the geometrical factor may be important for the physical properties of liquid helium-4 in porous media.

The ideal Bose gas in non-integer dimensions and liquid 4He in porous media

A control apparatus of an engine system, the engine system including an engine, a VGT configured to control a boost pressure applied to the engine by adjust an angle of a vane provided in a turbine, and an EGR valve configured to control an amount of a recirculated exhaust gas, includes a target value determiner determining a target boost pressure and a target intake air amount based on an engine speed and a fuel injection amount, an EGR valve controller calculating a target EGR mass flow rate based on the target boost pressure and the target intake air amount and determining an EGR valve opening rate, and a turbocharger controller performing a sliding mode control using the calculated target EGR mass flow rate as a parameter to calculate a target compressor power and a target turbocharger mass flow rate and determining a turbine vane opening rate.

Apparatus and method of controlling engine system

A principle of an acoustic Eaton lens array and its application as a removable tsunami wall is proposed theoretically. The lenses are made of expandable rubber pillars or balloons and create a stop-band by rotating the incoming tsunami wave and reduce the pressure by canceling each other. The diameter of each lens is larger than the wavelength of the tsunami near the coast, that is, order of a kilometer. The impedance matching on the border of the lenses results in a little reflection. Before a tsunami, the balloons are buried underground in shallow water near the coast in folded or rounded form. Upon sounding of the tsunami alarm, water and air are pumped into the pillars, which expand and erect the wall above the sea level within a few hours. After the tsunami, the water and air are released from the pillars, which are then buried underground for reuse. Electricity is used to power the entire process. A numerical simulation with a linear tsunami model was carried out.

Acoustic Eaton lens array and its fluid application

The superfluid fraction of a charged boson fluid at non-integer dimensions between 2 and 3 is studied. Interaction between particles is assumed to be Coulombic, and Bogoliubov approximation is applied for a weak coupling regime. The theoretical results are compared with the superfluid densities of superconducting films, showing an universal splitting behavior. The qualitative similarity between the charged boson fluid and the superconducting films gives a strong clue for the origin of the universality observed in the superconducting films.

Superfluid fraction of a charged boson fluid at non-integer dimensions and superconducting films

A many-body theoretical scheme based on the functional Schrodinger picture method is formulated for the study of a dilute Bose gas system. The standard ground state properties are obtained by solving the infinite dimensional Schr�dinger equation variationally with a Gaussian trial wavefunctional. It is shown that a shifted Gaussian trial functional enables us to calculate a higher order correction of order na3, which corresponds to the fluctuation constribution from the condensate.

Sang-Hoon Kim

Papers

The ideal Bose gas in non-integer dimensions and liquid 4He in porous media

Apparatus and method of controlling engine system

Acoustic Eaton lens array and its fluid application

Superfluid fraction of a charged boson fluid at non-integer dimensions and superconducting films

Functional Schrödinger picture approach to a dilute bose gas and fluctuation correction to the ground state energy density