Propagation speed of inertial waves in cylindrical swirling flows

Thermo-acoustic combustion instabilities arise from feedback between flow perturbations and the unsteady heat release rate of a flame in a combustion chamber. In the case of a premixed, swirl stabilized flame, an unsteady heat release rate results from acoustic velocity perturbations at the burner inlet on the one hand, and from azimuthal velocity perturbations, which are generated by acoustic waves propagating across the swirler, on the other. The respective time lags associated with these flow–flame interaction mechanisms determine the overall flame response to acoustic perturbations and therefore thermo-acoustic stability. The propagation of azimuthal velocity perturbations in a cylindrical duct is commonly assumed to be convective, which implies that the corresponding time lag is governed by the speed of convection. We scrutinize this assumption in the framework of small perturbation analysis and modal decomposition of the Euler equations by considering an initial value problem. The analysis reveals that azimuthal velocity perturbations in swirling flows should be regarded as dispersive inertial waves. As a result of the restoring Coriolis force, wave propagation speeds lie above and below the mean flow bulk velocity. The differences between wave propagation speed and convection speed increase with increasing swirl. A linear, time invariant step response solution for the dynamics of inertial waves is developed, which can be approximated by a concise analytical expression. This study enhances the understanding of the flame dynamics of swirl burners in particular, and contributes physical insight into the inertial wave dynamics in general.</jats:p

Transient vortex events in the initial value problem for turbulence

A vorticity surge event that could be a paradigm for a wide class of bursting events in turbulence is studied to examine how the energy cascade is established and how this event could serve as a new test of LES turbulence models. This vorticity surge event is tied to the formation of the energy cascade in a direct numerical simulation by the traditional signatures of a turbulent energy cascade such as spectra approaching -5/3 and strongly Beltramized vortex tubes. A coherent mechanism is suggested by the nearly simultaneous development of a maximum of the peak vorticity $\|\omega\|_\infty$, growth of the dissipation, the appearance of a helically aligned local vortex configuration and strong, transient oscillations in the helicity wavenumber spectrum. This coherence is also examined for two LES models, a traditional purely dissipative eddy viscosity model and a modern method (LANS$-\alpha$) that respects the nonlinear transport properties of fluids. Both LES models properly represent the spectral energy and energy dissipation associated with this vorticity surge event. However, only the model that preserves nonlinear fluid transport properties reproduces the helical properties, including Beltrami-like vortex tubes.Comment: 4 pages, 6 figure

Kinetic oxygen measurements by CVC96 in L-929 cell cultures

Generally animal and human cells use oxygen during their whole life. Consequently the oxygen use is a simple indicator to test the vitality of cells. When the vitality decreases by the delivery of toxic substances the decrease can be observed directly by the oxygen-use of the cells. To get fast information of the vitality of cells we have measured the O(2)-tension by testing a new model of a bioreactor, the Cell Vitality Checker 96 (CVC96), in practical application. With this CVC96, soon a simple test will exist for the measurement of the oxygen use. In this respect the question had to be answered whether the use in the laboratory is easy and whether oxygen as a parameter in the vitality test can also be applied in future for problems in the field of material testing

Modeling heat transfer and skin friction frequency responses of a cylinder in cross-flow : a unifying perspective

The dynamic behavior of skin friction and heat release of a cylinder in pulsating cross-flow are investigated. Existing analytical solutions are presented as transfer functions versus frequency, known from control theory. Newly found expressions are given for Reynolds number ranges, where no appropriate model exist until now. These expressions are obtained by the combination of CFD simulation and system identification (CFD/SI). In the CFD/SI approach time series are generated by exciting inlet velocity fluctuations over a wide range of frequencies in one single CFD simulation. Time series are acquired for heat release, skin friction and velocity forcing, and then post-processed with system identification tools. Direct numerical simulations are conducted for mean flow Reynolds numbers between 0.1 and 40, solving the incompressible Navier-Stokes equations in a 2D domain using a finite volume approach. The system identification framework provides methods to identify a mathematical model for the response in heat release and skin friction to velocity fluctuations from data series. It can be confirmed that Bayly’s model for heat release fluctuations performs well at low Reynolds numbers. Lighthill’s model, often used in the assessment of Rijke tubes, is more accurate for high Reynolds numbers, but the time constant was underpredicted for Reynolds numbers of order 10. For the range above a Reynolds number of 0.4 a unifying model could be developed. This model especially excels at Reynolds numbers of order 10. Available models for skin friction usually match the simulated data up to a point, but do not give any dependence on Reynolds number which is corrected here. The expressions presented allow insight in the physics of the dynamic behavior of a cylinder in pulsating cross flow and also facilitate the use of these models in further investigations.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Enhanced heat transfer in laminar pulsating flow past a flat plate

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Heat transfer in laminar pulsating flow past a heated flat plate is investigated. In contrast to previous investigations, the response of the wall heat flux to both harmonic velocity and temperature fluctuations is considered over a wide range of pulsation amplitudes and Strouhal numbers. The reason for choosing the flat plate configuration is that for one given oscillation frequency, both low- and high-frequency regimes can be observed in a single simulation. Due to the spatial development of the hydrodynamic and thermal boundary layers along the length of the plate, the Stokes length is first larger (upstream section), then (downstream section) smaller than the local boundary layer thickness. Firstly, the case with constant wall temperature is studied, where fluid temperature fluctuations are locally generated by velocity perturbations in the presence of mean flow gradients. Depending on the local Strouhal number, which controls boundary layer phase lags, the local wall heat flux can be enhanced as well as decreased. This effect is due to a nonlinear interaction between velocity and temperature fluctuations. Secondly, periodic temperature fluctuations of the wall temperature are imposed too, and the deviations from the stationary case are stronger. At high velocity amplitudes, secondary flows induced by viscous forces near stagnation points come into play and can either support or impede the enhancement of heat transfer.dc201

Simulation of Pure Sedimentation of Raindrops using Quadrature Method of Moments

The quadrature method of moments (QMOM) is used for simulation of pure sedimentation of raindrops in a one-dimensional rainshaft. The moments have been calculated in three ways, based either on droplet diameter using two and three nodes or on droplet volume using two nodes. The method gives useful information on the range of sizes and also on spatial segregation of the droplets. The results show that all three methods give useful information about the transport processes involved and compare satisfactorily with the spectral (bin) method although use of diameter as internal coordinate is preferable from the viewpoint of quantitative agreement

Response of a swirl flame to inertial waves

Acoustic waves passing through a swirler generate inertial waves in rotating flow. In the present study, the response of a premixed flame to an inertial wave is scrutinized, with emphasis on the fundamental fluid-dynamic and flame-kinematic interaction mechanism. The analysis relies on linearized reactive flow equations, with a two-part solution strategy implemented in a finite element framework: Firstly, the steady state, low-Mach number, Navier–Stokes equations with Arrhenius type one-step reaction mechanism are solved by Newton’s method. The flame impulse response is then computed by transient solution of the analytically linearized reactive flow equations in the time domain, with mean flow quantities provided by the steady-state solution. The corresponding flame transfer function is retrieved by fitting a finite impulse response model. This approach is validated against experiments for a perfectly premixed, lean, methane-air Bunsen flame, and then applied to a laminar swirling flame. This academic case serves to investigate in a generic manner the impact of an inertial wave on the flame response. The structure of the inertial wave is characterized by modal decomposition. It is shown that axial and radial velocity fluctuations related to the eigenmodes of the inertial wave dominate the flame front modulations. The dispersive nature of the eigenmodes plays an important role in the flame response