585 research outputs found
Spatio-temporal growth of disturbances in a boundary layer and energy based receptivity analysis
No full textIn fluid dynamical systems, it is not known a priori whether disturbances grow either in space or in time or as spatio-temporal structures. However, for boundary layers, it is customary to treat it as a spatial problem and some limited comparison between prediction and laboratory experiments exist. In the present work, the receptivity problem of a zero pressure gradient boundary layer excited by a localized harmonic source is investigated under the general spatio-temporal framework, using the Bromwich contour integral method. While this approach has been shown to be equivalent to the spatial study, for unstable systems excited by a single frequency source [T. K. Sengupta, M. Ballav, and S. Nijhawan, Phys. Fluids6, 1213 (1994)], here we additionally show, how the boundary layer behaves when it is excited (i) at a single frequency that corresponds to a stable condition (given by spatial normal-mode analysis) and (ii) by wideband frequencies, that shows the possibility of flow transition due to a spatio-temporally growing forerunner or wave front. An energy based receptivity analysis tool is also developed as an alternative to traditional instabilitytheory. Using this, we reinterpret the concept of critical layer that was originally postulated to explain the mathematical singularity of inviscid disturbance field in traditional instabilitytheory of normal modes
A new compact scheme for parallel computing using domain decomposition
No full textDirect numerical simulation (DNS) of complex flows require solving the problem on parallel machines using high accuracy schemes. Compact schemes provide very high spectral resolution, while satisfying the physical dispersion relation numerically. However, as shown here, compact schemes also display bias in the direction of convection – often producing numerical instability near the inflow and severely damping the solution, always near the outflow. This does not allow its use for parallel computing using domain decomposition and solving the problem in parallel in different sub-domains. To avoid this, in all reported parallel computations with compact schemes the full domain is treated integrally, while using parallel Thomas algorithm (PTA) or parallel diagonal dominant (PDD) algorithm in different processors with resultant latencies and inefficiencies. For domain decomposition methods using compact scheme in each sub-domain independently, a new class of compact schemes is proposed and specific strategies are developed to remove remaining problems of parallel computing. This is calibrated here for parallel computing by solving one-dimensional wave equation by domain decomposition method. We also provide the error norm with respect to the wavelength of the propagated wave-packet. Next, the advantage of the new compact scheme, on a parallel framework, has been shown by solving three-dimensional unsteady Navier–Stokes equations for flow past a cone-cylinder configuration at a Mach number of 4.Additionally, a test case is conducted on the advection of a vortex for a subsonic case to provide an estimate for the error and parallel efficiency of the method using the proposed compact scheme in multiple processors
Global spectral analysis of multi-level time integration schemes: Numerical properties for error analysis
No full textAn analysis is reported here for three-time level integration methods following the global spectral analysis (GSA) described in High Accuracy Computing Methods, T.K. Sengupta, Cambridge Univ. Press, USA. The focus is on the second order Adams–Bashforth (AB2) and the extrapolation in time (EXT2) methods. Careful distinction is made for the first time step at t=0 by either Euler forward or four-stage, fourth order Runge–Kutta (RK4) time schemes. The latter is used to solve a benchmark aeroacoustic problem. Several one-dimensional wave propagation models are analyzed: pure advection and advection-diffusion equations. Various spatial discretizations are discussed, including Fourier spectral method. Attention is paid to the presence of physical and numerical modes as noted in the quadratic equation obtained from the difference equation for the model 1D convection equation. It is shown that AB2 method is less stable and accurate than EXT2 method, with respect to numerical dissipation and dispersion. This is true for the methods, in which the physical mode dominates over the numerical mode. Presented analysis provides useful guide to analyze any three-time level methods
Effects of error on the onset and evolution of Rayleigh–Taylor instability.
No full textHere we investigate effects of error in simulating Rayleigh Taylor instability (RTI). The error metrics are evaluated based on the correct spectral analysis of a model equation by Sengupta et al. (J Comput Phys 226:1211–1218, 2007) [13]. The geometry for RTI consists of a rectangular box with a partition at mid-height separating two volumes of air kept at a temperature difference of 70 K. This helps avoiding Boussinesq approximation and the present time-accurate computations for compressible Navier–Stokes equation (NSE) in 2D are reported. Computations for CFL numbers of 0.09 and 0.009 shows completely different onset of RTI, while the terminal mixed stage appears similar. The difference is traced to very insignificant difference in the value of numerical amplification factor for the two CFL numbers
Non-equilibrium Thermodynamics of Rayleigh-Taylor instability
No full textRayleigh-Taylor instability (RTI) has been studied here as a non-equilibrium thermodynamics problem. Air masses with temperature difference of 70K, initially with heavier air resting on lighter air isolated by a partition, are allowed to mix by impulsively removing the partition. This results in interface instabilities, which are traced here by solving two dimensional (2D) compressible Navier-Stokes equation (NSE), without using Boussinesq approximation (BA henceforth). The non-periodic isolated system is studied by solving NSE by high accuracy, dispersion relation preserving (DRP) numerical methods described in Sengupta T.K.: High Accuracy Computing Method (Camb. Univ. Press, USA, 2013). The instability onset is due to misaligned pressure and density gradients and is evident via creation and evolution of spikes and bubbles (when lighter fluid penetrates heavier fluid and vice versa, associated with pressure waves). Assumptions inherent in compressible formulation are: (i) Stokes' hypothesis that uses zero bulk viscosity assumption and (ii) the equation of state for perfect gas which is a consequence of equilibrium thermodynamics. Present computations for a non-equilibrium thermodynamic process do not show monotonic rise of entropy with time, as one expects from equilibrium thermodynamics. This is investigated with respect to the thought-experiment. First, we replace Stokes' hypothesis, with another approach where non-zero bulk viscosity of air is taken from an experiment. Entropy of the isolated system is traced, with and without the use of Stokes' hypothesis. Without Stokes' hypothesis, one notes the rate of increase in entropy to be higher as compared to results with Stokes' hypothesis. We show this using the total entropy production for the thermodynamically isolated system. The entropy increase from the zero datum is due to mixing in general; punctuated by fluctuating entropy due to creation of compression and rarefaction fronts originating at the interface and reflecting from the walls
First Maltese record of Stephanopachys quadricollis (Marseul, 1879) (Coleoptera, Bostrichidae)
No full textThree specimens of Stephanopachys quadricollis (Marseul, 1878) were recently found in Malta in UV light
traps and represent the first record of this species for this country. Although S. quadricollis is native to
the Mediterranean basin, it is not yet clear if these Maltese records are due to a natural population or to
an interception. Distributional, nomenclatural and biological data on this species are summarized, and a
new synonymy is established: Stephanopachys quadricollis (Marseul, 1879) = Stephanopachys quadraticollis
Kocher, 1956, syn. n.peer-reviewe
Correction: Corrigendum: Intravenous administration of lidocaine directly acts on spinal dorsal horn and produces analgesic effect: An in vivo patch-clamp analysis
No full textScientific Reports 6: Article number: 26253; published online: 18 May 2016; updated: 01 June 2017. The Author Contributions statement in this Article is incomplete, where: “M.K. and T.K. designed the experiments; M.K. conducted experiments and analysed the data; H.F. and T.K. wrote the manuscript. All authors reviewed the manuscript”.</jats:p
Esperienza e apprendimento, commento a K. Sengupta, T.K. Abdel-Hamid, L.N. Van Wassenhove, “ La trappola dell’esperienza”
No full textT.K. Cureton's Tip: How the Kinesiology Department Founder and Professor at the University of Illinois Led Roger Bannister to Break the Four Minute Mile
No full textThis project investigates the events leading up to Roger Bannister???s record-breaking four minute mile in 1954. It focuses on the role of T.K. Cureton, founder of the kinesiology department and professor of physical education at the University of Illinois, who conducted research on training habits of athletic champions. On the basis of literature, internet, and archival research, the study finds that Cureton advised Bannister to train the whole body rather than sport specific areas during their meeting in London, which was the tip leading to Bannister???s record. The author further discusses the careers of these two men, the context of their meeting, and the myth of the impossible.unpublishedU of I OnlyRequested by Student
Productiviteit van overheidsbeleid: Deel 1: Het Nederlandse onderwijs, 1980-2012
No full textValues Technology and InnovationTechnology, Policy and Managemen
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