Wave propagation and attenuation in waveguides with flexible components play a critical role in noise control applications such as HVAC systems, aero-engines, and gas turbines. Conventional analyses typically address either rigid-walled ducts, single-region impedance linings, or flexible shells in isolation, leaving the combined effects of shell flexibility, bifurcated chamber geometry, membrane-mediated coupling, and dissipative linings underexplored. The non-orthogonal eigenfunctions arising from shell flexibility further complicate standard mode-matching approaches. A semi-analytical framework is developed for a thin, flexible cylindrical shell enclosing a bifurcated expansion chamber with concentric sub-regions coupled by membrane discs and an internal acoustic liner. The shell dynamics follow Donnell–Mushtari thin-shell theory, while the fluid is treated as compressible and inviscid. A Galerkin-augmented mode-matching formulation handles the non-orthogonal eigenfunctions and higher-order derivatives at flexible boundaries. Convergence and consistency are confirmed through energy-balance checks and reconstruction of the interface matching conditions. Resistive linings dissipate a significant portion of the incident acoustic energy and yield broadband transmission loss, whereas purely reactive linings produce sharp but narrow resonance peaks with negligible absorption. Parametric studies show that the inner radius, intermediate radius, outer radius, and cavity half-length act as independent handles for trading off reflection, absorption, and transmission. The proposed framework reliably models coupled fluid-structure wave propagation in flexible, membrane-coupled, bifurcated waveguides with acoustic linings, providing practical design guidance for thin-walled waveguide structures in noise-control applications.
The facile fabrication of core–shell Co/N-CNFs by electrospinning and their catalytic applications in aerobic oxidative coupling of thiols are presented.
Civil aviation continues to face evolving threats of unlawful interference, including terrorism, hijacking, sabotage, and cyber-enabled attacks. Although regulatory frameworks, technological capabilities, and international cooperation have strengthened considerably, evaluating the effectiveness of aviation security remains largely centered on procedural compliance and incident-based indicators. This study adopts a practice-oriented perspective by examining how aviation security effectiveness is interpreted and assessed by professionals directly involved in implementing security measures. Drawing on in-depth interviews with experienced aviation security and safety practitioners, professional narratives were analyzed thematically to explore how deterrence, vulnerability, and security effectiveness are understood in operational settings. Guided by deterrence theory, the analysis demonstrates that practitioners associate effective security not only with technological capability and regulatory compliance, but also with consistent implementation, professional judgment, organizational coordination, and alignment with international standards, particularly those established by the International Civil Aviation Organization (ICAO). Participants also identified persistent challenges associated with human factors, resource constraints, and evolving threat environments, while emphasizing insider threats as a context-dependent source of vulnerability requiring sustained organizational vigilance. Rather than offering broadly generalizable conclusions, the study provides context-specific insights into how deterrence is interpreted and enacted in contemporary aviation practice. In doing so, it complements predominantly technical and regulatory perspectives by highlighting the value of practitioner narratives for informing future research, operational decision-making, and policy discussions concerning aviation security.
This study investigates the metaphysics of fasting according to the hugely influential mystic Muhyi al-Din ibn ‘Arabi (d. 638/1240). Ibn ‘Arabi argues that fasting holds an unparalleled position in ritual worship. While his predecessor Abu Hamid al-Ghazali (d. 505/1111) in his magnum opus—The revival of the religious sciences (Ihya’ ‘ulum al-din)—addresses the ethical dimension of fasting, Ibn ‘Arabi’s concern is the metaphysical reality of it. There are six principal reasons Ibn ‘Arabi gives for fasting being superior to other forms of worship, all of which revolve around fasting’s uniqueness that adverts to the uniqueness of God: (1) Fasting is elevated because God has connected it to Himself in prophetic traditions. All other acts of worship are connected to humans. Fasting is thus elevated from the servant to God. (2) Fasting is not an action like other forms of worship; it is an inaction since it entails refraining from eating, drinking, and sexual intercourse. This makes its essence incomprehensible as it is not an entity but the lack of one. The incomprehensibility of the essential inaction of fasting connects it to the incomprehensibility of God. (3) Fasting, insofar as it displays independence from food, drink, and sexual intercourse, mirrors the divine attribute of true independence from all things (samadaniyya). (4) Fasting is described as a shield (junna) in prophetic traditions because although it is not an action in itself, the state of fasting becomes a protection against evil actions. Awareness of God (taqwa), likewise, protects against evil actions. Thus, fasting is related to God as it begets awareness of God. (5) The breath of the person who fasts, though malodourous to humans, is said to be fragrant for God by Prophet Muhammad. This ‘fragrance’ is produced by the breath of the person who fasts, which adverts to the Breath of the Compassionate (nafas al-Rahman) that brings all things into existence according to Ibn ‘Arabi. (6) The people who fast shall enter heaven through the Gate of Rayyan (quenched thirst) as reported in prophetic traditions. Ibn ‘Arabi argues that the quenching of thirst represents an endpoint or ‘perfection’ (kamal) after which one does not require more drink. This ‘perfection’ (of satiation) mirrors God’s perfection. Fasting is the only form of worship that has a gate that alludes to its perfection, which demonstrates that it is unique. In all these ways, then, there is nothing like fasting, which connects it to God because there is nothing like God.
