Sergey A. Karabasov

Jet Noise: Acoustic Analogy informed by Large Eddy Simulation

A novel approach to the development of a hybrid prediction methodology for jet noise is described. Modeling details and numerical techniques are optimized for each of the three components of the model. Far-field propagation is modeled by solution of a system of adjoint linear Euler equations, capturing convective and refraction effects using a spatially developing jet mean flow provided by a Reynolds-averaged Navier―Stokes computational fluid dynamics solution. Sound generation is modeled following Goldsteins acoustic analogy, including a Gaussian function model for the two-point cross correlation of the fourth-order velocity fluctuations in the acoustic source. Parameters in this model describing turbulent length and time scales are assumed to be proportional to turbulence information also taken from the Reynolds-averaged Navier―Stokes computational fluid dynamics prediction. The constants of proportionality are, however, not determined empirically, but extracted by comparison with turbulence length and time scales obtained from a large eddy simulation prediction. The large eddy simulation results are shown to be in good agreement with experimental data for the fourth-order two-point cross-correlation functions. The large eddy simulation solution is then used to determine the amplitude parameter and also to examine which components of the cross correlation are largest, enabling inclusion of all identified dominant terms in the Gaussian source model. The acoustic source description in the present approach is therefore determined with no direct input from experimental data. This model is applied to the prediction of sound to the experimental configuration of the European Union JEAN project, and gives encouraging agreement with experimental data across a wide spectral range and for both sideline and peak noise angles. This paper also examines the accuracy of various commonly made simplifications, for example: a locally parallel mean flow approximation rather than consideration of the spatially evolving mean jet flow and scattering from the nozzle; the assumption of small radial variation in Green function over the turbulence correlation length; the application of the far-field approximation in the Green function; and the impact of isotropic assumptions made in previous acoustic source models.

Compact Accurately Boundary-Adjusting high-REsolution Technique for fluid dynamics

A novel high-resolution numerical method is presented for one-dimensional hyperbolic problems based on the extension of the original Upwind Leapfrog scheme to quasi-linear conservation laws. The method is second-order accurate on non-uniform grids in space and time, has a very small dispersion error and computational stencil defined within one space-time cell. For shock-capturing, the scheme is equipped with a conservative non-linear correction procedure which is directly based on the maximum principle. Plentiful numerical examples are provided for linear advection, quasi-linear scalar hyperbolic conservation laws and gas dynamics and comparisons with other computational methods in the literature are discussed.

New efficient high-resolution method for nonlinear problems in aeroacoustics

This paper is devoted to a new efficient numerical method for aeroacoustic applications. The method is second-order accurate, and it has a very compact numerical stencil. It combines traditional merits of finite volume and finite difference approaches such as shock capturing and linear Fourier accuracy on coarse grids.

Transonic Helicopter Noise

Helicopter noise is an increasingly important issue, and at large forward-flight speeds transonic rotor noise is a major contributor. A method for predicting transonic rotor noise, which is more computationally efficient than previous methods and which furthermore offers physical insight into the noise generation, is developed. These benefits combine to make it of potential use to helicopter rotor designers. The permeable surface form of the Ffowcs Williams-Hawkings (FW-H) equation is used to express the sound field in terms of a distribution of monopole and dipole sources over a permeable control surface and a distribution of quadrupole sources over the volume outside of this surface. By choosing the control surface to enclose the transonic flow regions, the noise from the quadrupole distribution becomes negligible. Only the more straightforward surface sources then need be considered, making the acoustic approach computationally efficient. By locating the control surface close to the blade subject to enclosing the transonic flow regions, efficiency in the computational-fluid-dynamics (CFD) approach is also attained. To perform noise predictions, an Euler CFD method to calculate the flowfield was combined with an acoustic method incorporating the retarded time formulation of the FW-H equation. Several rotor blades in hover and steady forward flight were considered, all of which involved transonic flows but for which shock delocalization did not occur. The predictions showed very good agreement with experimental data and with predictions obtained using more computationally intensive methods.

Understanding jet noise

Jets are one of the most fascinating topics in fluid mechanics. For aeronautics, turbulent jet-noise modelling is particularly challenging, not only because of the poor understanding of high Reynolds number turbulence, but also because of the extremely low acoustic efficiency of high-speed jets. Turbulent jet-noise models starting from the classical Lighthill acoustic analogy to state-of-the art models were considered. No attempt was made to present any complete overview of jet-noise theories. Instead, the aim was to emphasize the importance of sound generation and mean-flow propagation effects, as well as their interference, for the understanding and prediction of jet noise.

Adjoint linearised Euler solver in the frequency domain for jet noise modelling

The paper is devoted to extending the new efficient frequency -domain m ethod of adjoint Green’s function calculation to curvilinear multi -block RANS domains for middle and far field sound computations. Numerical details of the method such as grids, boundary conditions and convergence acceleration are discussed. Two acoustic s ource models are considered in conjunction with the method and acoustic modelling results are presented for a benchmark low -Reynolds -number jet case.

Concurrent multiscale modelling of atomistic and hydrodynamic processes in liquids

Fluctuations of liquids at the scales where the hydrodynamic and atomistic descriptions overlap are considered. The importance of these fluctuations for atomistic motions is discussed and examples of their accurate modelling with a multi-space–time-scale fluctuating hydrodynamics scheme are provided. To resolve microscopic details of liquid systems, including biomolecular solutions, together with macroscopic fluctuations in space–time, a novel hybrid atomistic–fluctuating hydrodynamics approach is introduced. For a smooth transition between the atomistic and continuum representations, an analogy with two-phase hydrodynamics is used that leads to a strict preservation of macroscopic mass and momentum conservation laws. Examples of numerical implementation of the new hybrid approach for the multiscale simulation of liquid argon in equilibrium conditions are provided.

Water−Peptide Dynamics during Conformational Transitions

Transitions between metastable conformations of a dipeptide are investigated using classical molecular dynamics simulation with explicit water molecules. The distribution of the surrounding water at different moments before the transitions and the dynamical correlations of water with the peptides configurational motions indicate that the water molecules represent an integral part of the molecular system during the conformational changes, in contrast with the metastable periods when water and peptide dynamics are essentially decoupled.

Multiscale modelling: approaches and challenges

Multiscale systems that are characterized by a great range of spatial–temporal scales arise widely in many scientific domains. These range from the study of protein conformational dynamics to multiphase processes in, for example, granular media or haemodynamics, and from nuclear reactor physics to astrophysics. Despite the diversity in subject areas and terminology, there are many common challenges in multiscale modelling, including validation and design of tools for programming and executing multiscale simulations. This Theme Issue seeks to establish common frameworks for theoretical modelling, computing and validation, and to help practical applications to benefit from the modelling results. This Theme Issue has been inspired by discussions held during two recent workshops in 2013: ‘Multiscale modelling and simulation’ at the Lorentz Center, Leiden (http://www.lorentzcenter.nl/lc/web/2013/569/info.php3?wsid=569&venue=Snellius), and ‘Multiscale systems: linking quantum chemistry, molecular dynamics and microfluidic hydrodynamics’ at the Royal Society Kavli Centre. The objective of both meetings was to identify common approaches for dealing with multiscale problems across different applications in fluid and soft matter systems. This was achieved by bringing together experts from several diverse communities.

Cabaret Scheme for Computational Aero Acoustics: Extension to Asynchronous Time Stepping and 3D Flow Modelling

Explicit time stepping renders many high-resolution schemes in Computational Aeroacoustics to become less efficient when dealing with non-uniform meshes in multiple dimensions. In the present paper, the Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) Euler scheme is extended to asynchronous time-stepping which allows it to significantly boost computational performance with non-uniform grids. Numerical examples for 1D and 2D CABARET with asynchronous time stepping are provided. For further demonstration of the CABARETs capabilities in 3D where the asynchronous time stepping is expected to bring a step change in the scheme efficiency, Large Eddy Simulation of subsonic flow over a NACA0012 airfoil at zero angle of attack is conducted. The aerodynamic and acoustic results obtained with a single time stepping CABARET method are compared with the experimental data available.