Directivity of acoustic emissions from wave packets to the far field

We investigate the acoustic emission from wave packets to the far field. To this end, we develop a theory for one- and two-dimensional source fields in the shape of wave packets with Gaussian envelopes. This theory is based on an approximation to Lighthill's acoustic analogy for distant observers. It is formulated in the spectral domain in which a Gaussian wave packet is represented again by a Gaussian. This allows us to determine the directivity of the acoustic emission (e.g. superdirectivity and Mach waves) by simple geometric constructions in the spectral domain. It is shown that the character of the acoustic emission is mainly governed by the aspect ratio and the Mach number of the wave packet source. To illustrate the relevance of this theory we use it to study two prominent problems in subsonic jet aeroacoustic

Fluidmechanics of semicircular canals - revisited

Abstract.: In this work we find the exact solution for the flow field in a semicircular canal which is the main sensor for angular motion in the human body. When the head is rotated the inertia of the fluid in the semicircular canal leads to a deflection of sensory hair cells which are part of a gelatinous structure called cupula. A modal expansion of the governing equation shows that the semicircular organ can be understood as a dynamic system governed by duct modes and a single cupular mode. We use this result to derive an explicit expression for the displacement of the cupula as a function of the angular motion of the head. This result shows in a mathematically and physically clean way that the semicircular canal is a transducer for angular velocit

On the role of aortic valve architecture for physiological haemodynamics and valve replacement. Part I: flow topology and vortex dynamics

Aortic valve replacement has become a growing concern due to the increasing prevalence of aortic stenosis in an ageing population. Existing replacement options have limitations, necessitating the development of improved prosthetic aortic valves. In this study, flow characteristics during systole in a stenotic aortic valve case are compared with those downstream of two newly designed surgical bioprosthetic aortic valves (BioAVs) using advanced simulations. Our findings reveal that the stenotic case maintains a high jet flow eccentricity due to a fixed orifice geometry, resulting in increased vortex stretching in the commissural low-flow regions. One BioAV design introduces non-axisymmetric leaflet motion, which reduces the maximum jet velocity and forms more vortical structures. The other BioAV design produces a fixed symmetric triangular jet shape due to non-moving leaflets and exhibits favourable vorticity attenuation and significantly reduced drag. Therefore, this study highlights the benefits of custom-designed aortic valves in the context of their replacement through comprehensive flow analyses. The results emphasise the importance of analysing jet flow, vortical structures, momentum balance and vorticity transport for evaluating aortic valve performance

On the role of aortic valve architecture for physiological haemodynamics and valve replacement. Part II: spectral analysis and anisotropy

Severe aortic valve stenosis can lead to heart failure and aortic valve replacement (AVR) is the primary treatment. However, increasing prevalence of aortic stenosis cases reveal limitations in current replacement options, necessitating improved prosthetic aortic valves. In this study, we investigate flow disturbances downstream of severe aortic stenosis and two bioprosthetic aortic valve (BioAV) designs using advanced energy-based analyses. Spectral analysis shows kinetic energy (KE) decay variations, with the stenotic case aligning with Kolmogorov's theory, while BioAVs differ. We explore the impact of flow helicity on KE transfer and decay in BioAVs. Probability distributions of modal KE anisotropy unveil flow asymmetries in the stenotic and one BioAV case. Moreover, an inverse correlation between modal KE anisotropy and normalised helicity intensity is noted, with the coefficient of determination varying among the valve configurations. Leaflet dynamics analysis highlights a stronger correlation between flow and biomechanical KE anisotropy in one BioAV due to higher leaflet displacement magnitude. These findings emphasise the role of valve architecture in aortic turbulence and its significance for BioAV performance and energy-based design optimisation

Algebraically decaying modes and wave packet pseudo-modes in swept Hiemenz flow

The modal structure of the swept Hiemenz flow, a model for the flow near the attachment line of a swept wing, consists of eigenfunctions which exhibit (super-)exponential or algebraic decay as the wall-normal coordinate tends to infinity. The subset of algebraically decaying modes corresponds to parts of the spectrum which are characterized by a significant sensitivity to numerical discretization. Numerical evidence further suggests that a continuous spectrum covering a two-dimensional range of the complex plane exists. We investigate the family of uniform swept Hiemenz modes using eigenvalue computations, numerical simulations and the concept of wave packet pseudo-modes. Three distinct branches of the family of algebraically decaying eigenmodes are identified. They can be superimposed to produce wavefronts propagating towards or away from the boundary layer and standing or travelling wave packets in the free stream. Their role in the exchange of information between the free stream and the attachment-line boundary layer for the swept Hiemenz flow is discussed. The concept of wave packet pseudo-modes has been critical in the analysis of this problem and is expected to lead to further insights into other shear flows in semi- or bi-infinite domain

Algebraically diverging modes upstream of a swept bluff body

Classical stability theory for swept leading-edge boundary layers predicts eigenmodes in the free stream with algebraic decay far from the leading edge. In this article, we extend the classical base flow solution by Hiemenz to a uniformly valid solution for the flow upstream of a bluff body, which includes a three-dimensional boundary layer, an inviscid stagnation-point flow and an outer parallel flow. This extended, uniformly valid base flow additionally supports modes which diverge algebraically outside the boundary layer. The theory of wave packet pseudomodes is employed to derive analytical results for the growth rates and for the eigenvalue spectra of this type of mode. The complete spectral analysis of the flow, including the algebraically diverging modes, will give a more appropriate basis for receptivity studies and will more accurately describe the interaction of perturbations in the free stream with disturbances in the boundary laye

Vortical flow in the utricle and the ampulla: a computational study on the fluid dynamics of the vestibular system

We present a computational study of the fluid dynamics in healthy semicircular canals (SCCs) and the utricle. The SCCs are the primary sensors for angular velocity and are located in the vestibular part of the inner ear. The SCCs are connected to the utricle that hosts the utricular macula, a sensor for linear acceleration. The transduction of angular motion is triggered by the motion of a fluid called endolymph and by the interaction of this fluid with the sensory structures of the SCC. In our computations, we observe a vortical flow in the utricle and in the ampulla (the enlarged terminal part of the SCCs) which can lead to flow velocities in the utricle that are even higher than those in the SCCs. This is a fundamentally new result which is in contrast to the common belief that the fluid velocities in the utricle are negligible from a physiological point of view. Moreover, we show that the wall shear stresses in the utricle and the ampulla are maximized at the positions of the sensory epithelia. Possible physiological and clinical implications are discusse

Rethinking sociality and health through transfiguration

In this introductory article to the Special Section, we intend to literally bring sociality to (bodily) life and ask what medical anthropology might gain by using the lens of sociality for a better understanding of the phenomena it is concerned with. Conversely, we probe how the field of health and illness – including themes concerning embodiment, vulnerability, suffering, and death – might help to further spell out the notion of sociality both conceptually and methodologically. Drawing on the contributors’ ethnographic enquiries into contemporary health phenomena in East Africa, South America, and Western Europe, we do so by bringing sociality into conversation with transfiguration. By this we refer to: (1) the constantly unfolding processes of particular extended figurations encountering, affecting, and becoming enmeshed in each other; as well as (2) the (temporarily) stabilized figurational arrangements emerging from these enmeshments. It is our hope that this notion of transfiguration will help render visible the modalities through which human engagements with each other and the world form diverse arrangements. Moreover, we aim to better understand the processes by which these arrangements – which we term ‘extended figurations’ – interact with each other, change over time, and possibly vanish and make way for others. A detailed appreciation of the workings of these extended figurations, we believe, can significantly enhance our comprehension of the particular processes of change that stand at the center of our ethnographic interest. In this sense, the concept of transfiguration constitutes one possible way of structuring the messiness and complexity of sociality for analytical purposes

Stabilizing a Leading-edge Boundary Layer Subject to Wall Suction by Increasing the Reynolds Number

AbstractLaminar fiow in the boundary layer at the leading edge of swept airplane wings typically becomes transitional and turbulent shortly downstream of the attachment line. Flow control techniques to maintain the fiow laminar, such as suction into the wall, therefore must focus on this instability, which otherwise leads to turbulent fiow and thus contaminates the fiow over the entire wing chord.The present paper presents new results on how the linear leading-edge boundary layer (LEBL) instability of swept-cylinders fiow, which models swept-wing fiow, may be avoided. The classical Reynolds number definition is employed, which is based on the far-field velocity Q∞, the cylinder radius R*, and the sweep angle Λ. It is demonstrated that the fiow can be stabilized by increasing the Reynolds number at constant wall suction through an increase of R* or Λ, but not of Q∞.The stability analysis is carried out for the swept Hiemenz boundary layer (SHBL), a widely used fiat-plate approximation of the swept-cylinder LEBL. As demonstrated recently 1, the SHBL with suction becomes similar to the two-dimensional asymptotic suction boundary layer (ASBL) when increasing the classical Reynolds number ReSH to large values. In the limit of ReSH→∞, the SHBL with suction becomes identical to the highly stable ASBL, and hence inherits its linear stability properties. The transformation of these recent findings concerning the linear stability of the SHBL with suction to the swept-cylinder LEBL unveils that stabilization of fiow with constant suction can be observed by increasing ReSH