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Topic review
Updated time: 25 Jun 2021
Submitted by: Giorgio Palma
Definition: Metamaterials, man-made composites that are scaled smaller than the wavelength, have demonstrated a huge potential for application in acoustics, allowing the production of sub-wavelength acoustic absorbers, acoustic invisibility, perfect acoustic mirrors and acoustic lenses for hyper focusing, and acoustic illusions and enabling new degrees of freedom in the control of the acoustic field. The zero, or even negative, refractive sound index of metamaterials offers possibilities for the control of acoustic patterns and sound at sub-wavelength scales. The potential of metamaterial-based technologies has recently caught the interest of the aeronautics community. Their effect in the presence of realistic flows in the surrounding domains, with boundary layer, turbulence, is currently a hot research topic. The interaction with flow requires a careful design of the metamaterial to avoid detrimental effects and enabling the device maximum capabilities in aeronautics.
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Topic review
Updated time: 24 Nov 2021
Submitted by: Kai Zhang
Definition: With the rapid growth of space optics and aerospace technology, the applications of reflective optical systems are increasing year by year. A case in point is that off-axis three-mirror-anastigmat (TMA) systems have been widely used due to their unique advantages of large aperture and no central occlusion. Meanwhile, the growing requirements for optical systems’ performance, such as resolution and imaging quality, drive the development of focal length and aperture to larger sizes. In turn, the weight, surface accuracy, manufacturing time, and mirror cost are progressively demanding. In recent years, the lightweight space camera used for remote sensing with high resolution has become a global research hotspot in advanced optics.
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Topic review
Updated time: 09 Oct 2021
Submitted by: Loris Molent
Definition: Pitting corrosion is a major maintenance problem for aircraft aluminium alloys, and is also a problem for other materials. This paper has considered some service examples of corrosion-induced pitting or cracks exposed to service environments. These examples support the contention that fatigue crack growth, which is mainly due to in-flight dynamic loads, is not significantly influenced by the environment, unlike the largely static exposure on the ground.
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Topic review
Updated time: 16 Jun 2021
Submitted by: Anna Grincova
Definition: The nature of a civil aviation occurrence may be defined in three different categories while considering its severity. General categories include civil aviation accidents, serious incidents and incidents.
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Topic review
Updated time: 12 Nov 2021
Submitted by: Raman Singh
Definition: Stress corrosion cracking (SCC) is a vexing problem for load-bearing equipment operating in a corrosive environment in various industries, such as aerospace, chemical and mineral processing, civil structures, bioimplants, energy generation etc. For safe operation, effective maintenance and life prediction of such equipment, reliable design data on SCC (such as threshold stress intensity for SCC, i.e., KISCC) are invaluable. Generating reliable KISCC data invariably requires a large number of tests. Traditional techniques can be prohibitively expensive. Circumfrential notch tensile testing is a relatively recent, much simpler and cost-effective approach to generating accurate and reliable KISCC data.
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Topic review
Updated time: 25 Nov 2021
Submitted by: YuXiang Peng
Definition: The Global Navigation Satellite System (GNSS) plays a pivotal role in our modern positioning, navigation and timing (PNT) technologies. GNSS satellites fly at altitudes of approximately 20,000 km or higher. This altitude is above an ionized layer of the Earth’s upper atmosphere, the so called “ionosphere”. Before reaching a typical GNSS receiver on the ground, GNSS satellite signals penetrate through the Earth’s ionosphere. The ionosphere is a plasma medium consisting of free charged particles that can slow down, attenuate, refract, or scatter the GNSS signals. Ionospheric density structures (also known as irregularities) can cause GNSS signal scintillations (phase and intensity fluctuations). These ionospheric impacts on GNSS signals can be utilized to observe and study physical processes in the ionosphere and is referred to ionospheric remote sensing. This entry introduces some fundamentals of ionospheric remote sensing using GNSS.
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Biography
Updated time: 26 Sep 2021
Submitted by: William Wang
Abstract: Acoustics (ISSN 2624-599X) is a peer-reviewed, open access journal of acoustics science and engineering published quarterly online by MDPI. It is indexed by the Emerging Sources Citation Index (ESCI) in Web of Science. Manuscripts are peer-reviewed and a first decision provided to authors approximately 11.7 days after submission; acceptance to publication is undertaken in 2.9 days (median values for papers published in this journal in the second half of 2020).
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Topic review
Updated time: 08 Oct 2021
Submitted by: Ramesh Kumpati
Definition: Material failure may occur in a variety of situations dependent on stress conditions, temperature, and internal or external load conditions. Many of the latest engineered materials combine several material types i.e., metals, carbon, glass, resins, adhesives, heterogeneous and nanomaterials (organic/inorganic) to produce multilayered, multifaceted structures that may fail in ductile, brittle, or both cases.
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Topic review
Updated time: 12 May 2021
Submitted by: Riley Carriere
Definition: Sandwich panels are widely used in the design of unmanned satellites and, in addition to having a structural function, can often serve as shielding, protecting the satellites’ equipment from hypervelocity impacts (HVI) of orbital debris and micrometeoroids.
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Topic review
Updated time: 20 Oct 2021
Submitted by: Tomasz BLACHOWICZ
Definition: Cosmic radiation belongs to the challenges engineers have to deal with when further developing space travel. Besides the severe risks for humans due to high-energy particles or waves, the impact of cosmic radiation on electronics and diverse materials cannot be neglected, even in microsatellites or other unmanned spacecraft. Here, we explain the different particles or waves found in cosmic radiation and their potential impact on biological and inanimate matter.
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