2,029 research outputs found
Acoustic and hydrodynamic analysis of the flow around an aerofoil with trailing-edge serrations
No full textDirect numerical simulations of the flow around a NACA-0012 aerofoil are conducted, employing an immersed boundary method to represent flat-plate trailing-edge extensions both with and without serrations. Properties of the turbulent boundary layer convecting over the trailing edge are similar for both cases. For cases with serrations, the trailing-edge noise produced by the flow over the aerofoil is observed to decrease in amplitude, and the frequency interval over which the noise reduction occurs differs depending on the serration length. The directivity and spanwise coherence of the trailing-edge noise appears largely unaffected by the serrations. The hydrodynamic behaviour in the vicinity of the trailing-edge extensions is investigated. The streamwise discontinuity imparted upon the turbulent flow by the straight trailing edge can clearly be observed in statistical quantities, whereas for the serrated case no spanwise homogeneous discontinuities are observed. The trailing-edge serrations appear to break up the larger turbulent structures convecting into the wake, and to promote the development of horseshoe vortices originating at the serrations themselves
Efficient parallel computing with a compact finite difference scheme
No full textThis paper proposes an efficient parallel computing approach based on a high-order accurate compact finite difference scheme in conjunction with a conventional domain decomposition method and MPI libraries. The proposed parallel computing approach consists of two major features: (a) a newly developed compact finite difference scheme with extended stencils containing halo points around subdomain boundaries, and (b) a predictor–corrector type implementation of a compact filter that effectively suppresses spurious errors from the subdomain boundaries. The current work employs three halo cells for the inter-node communication, based on which the coefficients of the new compact scheme at the subdomain boundaries are optimized to achieve as high level of resolution and accuracy as the interior compact scheme provides. Also, an optimal set of cut-off wavenumbers of the compact filter that minimizes spurious errors is suggested. It is shown that the level of errors from the proposed parallel calculations lies within the same order of magnitude of that from the single-domain serial calculations. The overall accuracy and linear stability of the new parallel compact differencing-filtering system are confirmed by grid convergence tests and eigenvalue analyses. The proposed approach shows a substantial improvement with respect to existing methods available
Direct numerical simulations of trailing-edge noise generated by turbulent boundary-layers
No full textDirect numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge (TE).The objective is to investigate the turbulent flow field in the vicinity of theTE and the associated broadband noise generation.To generate a turbulent boundary layer a short distance from the inflow boundary, a technique is employed which exploits the dynamical features of the outer and inner part of the boundary layer.The DNS data are Fourier transformed in time and point spectra at various locations are used to identify representative frequencies. Several flow quantities are visualized for these frequencies and compared to predictions made with an acoustic analogy and a modified version of Amiet's classical trailing edge noise theory. A rapid increase in skin friction and a decrease in boundary layer thickness is observed at the trailing edge, consistent with analytical predictions using triple deck theory, leading to suppression of very low frequencies as found from point spectra
Direct numerical simulations of low Reynolds number flow over airfoils with trailing-edge serrations
No full textDirect numerical simulations (DNS) have been conducted of NACA-0012 with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. For the low Reynolds number airfoil flows accessible by DNS, laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment occurs. Comparing results from simulations with serrated and un-serrated trailing-edge extensions, noise reduction for higher frequencies is shown using power spectra and one-third octave averaged pressure contours. The effect of the trailing-edge serrations on an acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied via cross-correlations, probability density functions of skin friction and spanwise wavenumber spectra. The results show that the presence of serrations leads to some spanwise variation of transitional structures in the separated shear layer, but does not significantly affect the overall hydrodynamic field on the airfoil upstream of the serrations. Two reasons for why the hydrodynamic field is not considerably affected by the presence of serrations are suggested.<br/
Edge waves and resonance on elastic structures: An overview
No full textCopyright @ 2011 SAGE PublicationsOver 50 years have elapsed since the first experimental observations of dynamic edge phenomena on elastic structures, yet the topic remains a diverse and vibrant source of research activity. This article provides a focused history and overview of such phenomena with a particular emphasis on structures such as strips, rods, plates and shells. Within this context, some of the recent research highlights are discussed and the contents of this special issue of Mathematics and Mechanics of Solids on dynamical edge phenomena are introduced
Numerical investigation of airfoil self-noise reduction by addition of trailing edge serrations
No full textDNS of the flow around a NACA-0012 airfoil are conducted, employing an immersedboundary method to represent flat-plate trailing-edge extensions both with and withoutserrations. Properties of the turbulent boundary layer convecting over the trailing-edgeare similar for all cases. For cases with serrations, the trailing-edge noise produced by theflow over the airfoil is observed to decrease in amplitude, and the frequency interval overwhich the noise reduction occurs differs depending on the serration length. The trailing-edge noise appears otherwise largely unaffected by the serrations in terms of its directivityand spanwise coherence. The hydrodynamic behaviour in the vicinity of the trailing-edgeextensions is investigated. The streamwise discontinuity imparted upon the turbulent flowby the straight trailing-edge can clearly be observed in statistical quantities, whereas forthe serrated case no spanwise-homogeneous discontinuities are observed. The turbulentflow through the serrations promotes the development of horshoe vortices originating atthe serrations themselves, which appear to promote a more rapid mixing within the airfoilwake
Explicit models for flexural edge and interfacial waves in thin elastic plates
Get PDFThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.In the thesis explicit dual parabolic-elliptic models are constructed for the Konenkov flexural edge wave and the Stoneley-type flexural interfacial wave in case of thin linearly elastic plates. These waves do not appear in an explicit form in the original equations of motion within the framework of the classical Kirchhoff plate theory. The thesis is aimed to highlight the contribution of the edge and interfacial waves into the overall displacement field by
deriving specialised equations oriented to aforementioned waves only. The proposed models consist of a parabolic equation governing the wave propagation along a plate edge or plate junction along with an elliptic equation over the interior describing decay in depth. In this
case the parabolicity of the one-dimensional edge and interfacial equations supports
flexural wave dispersion. The methodology presented in the thesis reveals a dual nature of edge and interfacial plate waves contrasting them to bulk-type wave propagating in thin elastic structures. The thesis tackles a number of important examples of the edge and interfacial wave propagation. First, it addresses the propagation of Konenkov flexural wave in an elastic isotropic plate under prescribed edge loading. For the latter, parabolic-elliptic explicit models were constructed and thoroughly investigated. A similar problem for a semi-infinite orthotropic plate resulted in a more general dual parabolic-elliptic model. Finally, an analogous model was derived and analysed for two isotropic semi-infinite Kirchhoff plates under perfect contact conditions
Solution of the problem of composite charge using R.D.38
No full textIn this paper the author has solved the problem of internal ballistics of composite charge using 'R.D.38' method which is based upon the usual isothermal approximation. A linear law of burning has been assumed
Direct numerical simulations of airfoil self-noise
No full textDirect numerical simulations (DNS) of airfoil self-noise were conducted. For the low Reynolds number airfoil flows accessible by DNS, the occurrence of laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment leads to additional noise sources other than the traditionally studied trailing-edge noise. Cross-correlations of acoustic and hydrodynamic quantities in conjunction with rayacoustic theory are used to identify the main source locations for a NACA-0006 airfoil. It is found that the contribution of trailing edge noise dominates at low frequencies while for the high frequencies the radiated noise is mainly due to flow events in the reattachment region on the suction side. DNS have also been conducted of NACA-0012 airfoils with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. Noise reduction for higher frequencies is shown and the effect of the trailing edge serrations on the acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied
Direct numerical simulations of trailing edge noise generated by boundary-layer instabilities
No full textDirect numerical simulations (DNS) are conducted of noise generated at an infinitely thin trailing edge (TE). The aim is to predict the far-field sound and the near-field hydrodynamics, thereby providing an insight into the physical mechanisms of sound generation at airfoil TEs and potentially helping to validate acoustic theories. One of the theories widely used is the classical inviscid theory of Amiet, where the far-field sound can be evaluated in closed form if the convecting surface pressure spectrum upstream of the TE is known. For the first time, data from DNS including viscous effects are compared to the classical inviscid TE noise theory. In the present investigation, Tollmien–Schlichting waves are introduced close to the inflow boundary. The disturbances propagate downstream producing pressure fluctuations at the TE. In conducting two-dimensional DNS the theoretical method requires modification to account for the radiation of the total pressure difference in two dimensions only, as opposed to the three-dimensional sound radiation originally considered by Amiet. The modified theoretical analysis and a comparison between DNS and theoretical results are presented, scrutinizing the assumptions made in the derivation. Amiet's surface pressure jump transfer function is found to predict the scattered pressure field accurately. Directivity plots of DNS data show that viscous effects appear to smear individual lobes and that a downstream pointing lobe is present at higher Mach number which is attributed to an additional wake source
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