Nektar++ is an open-source software framework designed to support the development of high-performance scalable solvers for partial differential equations using the spectral/hp element method. High-order methods are gaining prominence in several engineering and biomedical applications due to their improved accuracy over low-order techniques at reduced computational cost for a given number of degrees of freedom. However, their proliferation is often limited by their complexity, which makes these methods challenging to implement and use. Nektar++ is an initiative to overcome this limitation by encapsulating the mathematical complexities of the underlying method within an efficient C++ framework, making the techniques more accessible to the broader scientific and industrial communities. The software supports a variety of discretisation techniques and implementation strategies, supporting methods research as well as application-focused computation, and the multi-layered structure of the framework allows the user to embrace as much or as little of the complexity as they need. The libraries capture the mathematical constructs of spectral/hp element methods, while the associated collection of pre-written PDE solvers provides out-of-the-box application-level functionality and a template for users who wish to develop solutions for addressing questions in their own scientific domains.Program summaryProgram title: Nektar++Catalogue identifier: AEVV_v1_0Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEVV_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: MITNo. of lines in distributed program, including test data, etc.: 1052456No. of bytes in distributed program, including test data, etc.: 42851367Distribution format: tar.gzProgramming language: C++.Computer: Any PC workstation or cluster.Operating system: Linux/UNIX, OS X, Microsoft Windows.RAM: 512 MBClassification: 12.External routines: Boost, PFTW, MPI, BLAS, LAPACK and METIS (www.cs.umn.edu)Nature of problem: The Nektar++ framework is designed to enable the discretisation and solution of time-independent or time-dependent partial differential equations.Solution method: Spectral/hp element methodRunning time: The tests provided take a few minutes to run. Runtime in general depends on mesh size and total integration time. (C) 2015 The Authors. Published by Elsevier B.V.

Vortex shedding in the wake of bluff bodies is often an undesired phenomenon which generates unsteady loads, vibrations and fluctuations of the aerodynamic forces. Consequently, the attenuation or suppression of the self-sustained oscillations associated to the vortex shedding is a fundamental problem in a wide range of engineering applications. Three-dimensional control techniques to control the vortex shedding are characterised by a variation of the control input along the spanwise direction and offer a promising methodology due to their versatility and high potential efficiency 1. In the present paper, the control of vortex dynamics of the wake of a flow past a cylinder at Reynolds number Re = 180 is performed by means of spanwise distributed forcing 1 2; starting from a fully developed shedding, a sufficiently high spanwise forcing is introduced on the surface of the cylinder, close to the separation regions, to stabilise the near-wake in a time-independent state, similarly to the effect of a sinusoidal stagnation surface 3,4. The effects of the forcing on the drag reduction and the dynamics of the vorticity have been investigated using a spanwise gaussian forcing, which generates a significant redistribution of the spanwise vorticity into streamwise and vertical components was ob- served, leading to a minor susceptibility of the three-dimensional shear layers to roll-up into the vortex street 4? . An insight into the main physical mechanisms underlying the suppression is provided by the hydrodynamic stability theory. Three different regimes were found for different forcing amplitude and the computation of the leading modes helps to shed light on some mechanisms responsible for the suppression of the Von-Karman shedding.

In this paper we investigate the instabilities arising in a flow through a compressor passage using BiGlobal stability analysis. The adopted geometry comes from the results of previous experimental and numerical investigations on a linear low-pressure (LP) compressor cascade [6], [19], [20]. Specifically, we address the role of laminar separation of the boundary layers at Re=138,500, where such separation effects are enhanced by the strong adverse pressure gradients that the flow experiences, in contrast to the more commonly studied low-pressure (LP) turbines. The vortical structures downstream the separation bubble on the suction surface were recognised to show a well-defined time periodicity, which could be precisely detected. Floquet stability analysis was then used to investigate the response of the flow to infinitesimal perturbations. To overcome the difficulty of performing a Floquet stability analysis when the periodicity is restricted just to a small region of the domain, a phase-averaged base flow was computed, such that only the organised motions are extracted, neglecting all the background unsteadiness. The same technique allowed us to confirm the presence of strong energy transient growth phenomena, which are directly associated with convective instabilities occurring in the region downstream from the separation bubble.

Controlling the wake vortex dynamics of bluff bodies efficiently is a fundamental problem in many applications. Earlier direct numerical simulations (Darekar and Sherwin) of three-dimensional bluff bodies demonstrated that the introduction of a spanwise waviness at both the leading and trailing surfaces suppresses the vortex shedding and reduces the amplitude of the fluctuating aerodynamic forces. Under this motivation, starting from a fully developed shedding, a sufficiently high spanwise forcing is introduced on the surface of the cylinder, in the regions where separation effects occur, resulting in the stabilisation of the near wake in a time-independent state, similar to the effect of a sinusoidal stagnation surface. Stability analysis of the linearised Navier-Stokes equations was then performed on the three-dimensional flows to investigate the role of the spanwise modulation on the absolute instability associated with the von Kármán street.