We report on the absorption properties of polarization-insensitive transmissive and reflective metamaterial absorbers based on two planar aluminium periodic structures and SU-8 epoxy resist. These absorbers were investigated using numerical simulation and experimental methods in the terahertz range (below 2 THz). SU-8 is a very promising organic material for dielectric layers in planar metamaterials, because its application simplifies the process of fabricating these structures and significantly reduces the fabrication time. The experimental absorption of the metamaterial absorbers has narrowband characteristics that were consistent with the numerical simulations. Power flow analysis in the transmissive metamaterial unit cell shows that the absorption in the terahertz range occurs primarily in the SU-8 layer of the absorber.
Pressure pulsations occurring in volumetric compressors manifold are still one of the most important problems in design and operation of compressor plants. The resulting vibrations may cause fatigue cracks and noise. Accuracy of the contemporary method is not sufficient in many cases. The methods for calculating pressure pulsation propagation in volumetric compressors manifolds are based on one-dimensional models. In one-dimensional models, the assumption is made that any installation element may be simplified and modeled as a straight pipe with given diameter and length or as a lumped volume. This simplification is usually sufficient in the case of small elements and long waves. In general, the geometry of the element shall be also considered. This may be done using two ways: experimental measurements of pressure pulsations, which lead to transmittance approximation for the investigated element, or CFD analysis and simulation for the acoustic manifold element. In this paper, a new method based on Computational Fluid Dynamics (CFD) simulation is presented. The main idea is to use CFD simulation instead of experimental measurements. The impulse flow excitation is introduced as a source. The results of simulation are averaged in the inlet and outlet cross sections, so time only dependent functions at the inlet and outlet of the simulated element are determined. The transmittances of special form are introduced. On the basis of introduced transmittances, the generalized four pole matrix elements and impedance matrix elements may be calculated. The method has been verified on the basis of experimental measurements.
Ice formed on radome surfaces causes communication disruption due to radio-frequency interference (RFI), which reveals the importance of de-icing systems for radomes. As a radome de-icing application, in this work, carbon nanotube (CNT) thin films were fabricated using a spray-coating method, and influence of process parameters on RF transmittance and electrothermal properties was investigated. With the increase of spraying time, sheet resistance of the fabricated film decreases, which results in a decrease of the RF transmittance and improvement of the heating performance. Also, the de-icing capability of the fabricated CNT film was evaluated at –20oC, and efficient removal of ice under cold conditions was demonstrated.
In this study, agar-based nanocomposite films containing ultra-porous silica aerogel particles were fabricated by gel casting using an aqueous agar/silica aerogel slurry. The silica aerogel particles did not show significant agglomeration and were homogeneously distributed in the agar matrix. Transmission electron microscopy observations demonstrated that the silica aerogel particles had a mesoporous microstructure and their pores were not incorporated into the agar polymer molecules. The thermal conductivities of the agar and agar/5 wt.% silica aerogel nanocomposite films were 0.36 and 0.20 W·m–1·K–1, respectively. The transmittance of the agar films did not decrease upon the addition of silica aerogel particles into them. This can be attributed to the anti-reflection effect of silica aerogel particles.
Small-signal transmittances: input-to-output and control-to-output of BUCK converter power stage working in CCM or DCM mode are discussed. Ideal converter case and converter with parasitic resistances are considered separately. Derivations of small-signal transmittances, based on different approaches to finding the converter averaged models, are presented and the results are compared. Apart from theoretical considerations, some results of numerical calculations are presented.