Statistics of fully turbulent impinging jets

Direct numerical simulations of sub- and supersonic impinging jets with Reynolds numbers of 3300 and 8000 are carried out to analyse their statistical properties. The influence of the parameters Mach number, Reynolds number and ambient temperature on the mean velocity and temperature fields are studied. For the compressible subsonic cold impinging jets into a heated environment, different Reynolds analogies are assesses. It is shown, that the (original) Reynolds analogy as well as the Chilton Colburn analogy are in good agreement with the DNS data outside the impinging area. The generalised Reynolds analogy (GRA) and the Crocco-Busemann relation are not suited for the estimation of the mean temperature field based on the mean velocity field of impinging jets. Furthermore, the prediction of fluctuating temperatures according to the GRA fails. On the contrary, the linear relation between thermodynamic fluctuations of entropy, density and temperature as suggested by Lechner et al. (2001) can be confirmed for the entire wall jet. The turbulent heat flux and Reynolds stress tensor are analysed and brought into coherence with the primary and secondary ring vortices of the wall jet. Budget terms of the Reynolds stress tensor are given as data base for the improvement of turbulence models

FluSI: A novel parallel simulation tool for flapping insect flight using a Fourier method with volume penalization

FluSI, a fully parallel open source software for pseudo-spectral simulations of three-dimensional flapping flight in viscous flows, is presented. It is freely available for non-commercial use under [https://github.com/pseudospectators/FLUSI]. The computational framework runs on high performance computers with distributed memory architectures. The discretization of the three-dimensional incompressible Navier--Stokes equations is based on a Fourier pseudo-spectral method with adaptive time stepping. The complex time varying geometry of insects with rigid flapping wings is handled with the volume penalization method. The modules characterizing the insect geometry, the flight mechanics and the wing kinematics are described. Validation tests for different benchmarks illustrate the efficiency and precision of the approach. Finally, computations of a model insect in the turbulent regime demonstrate the versatility of the software

Compressible starting jet: pinch-off and vortex ring–trailing jet interaction

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The dominant feature of the compressible starting jet is the interaction between the emerging vortex ring and the trailing jet. There are two types of interaction: the shock–shear layer–vortex interaction and the shear layer–vortex interaction. The former is clearly not present in the incompressible case, since there are no shocks. The shear layer–vortex interaction has been reported in the literature in the incompressible case and it was found that compressibility reduces the critical Reynolds number for the interaction. Four governing parameters describe the compressible starting jet: the non-dimensional mass supply, the Reynolds number, the reservoir to unbounded chamber temperature ratio and the reservoir to unbounded chamber pressure ratio. The latter parameter does not exist in the incompressible case. For large Reynolds numbers, the vortex pinch-off takes place in a multiple way. We studied the compressible starting jet numerically and found that the interaction strongly links the vortex ring and the trailing jet. The shear layer–vortex interaction leads to a rapid breakdown of the head vortex ring when the flow impacted by the Kelvin–Helmholtz instabilities is ingested into the head vortex ring. The shock–shear layer–vortex interaction is similar to the noise generation mechanism of broadband shock noise in a continuously blowing jet and results in similar sound pressure amplitudes in the far field.DFG, SFB 1029, TurbIn - Signifikante Wirkungsgradsteigerung durch gezielte, interagierende Verbrennungs- und Strömungsinstationaritäten in Gasturbine

The shifted proper orthogonal decomposition: A mode decomposition for multiple transport phenomena

Transport-dominated phenomena provide a challenge for common mode-based model reduction approaches. We present a model reduction method, which extends the proper orthogonal decomposition (POD) by introducing time-dependent shifts of the snapshot matrix. The approach, called shifted proper orthogonal decomposition (sPOD), features a determination of the multiple transport velocities and a separation of these. One- and two-dimensional test examples reveal the good performance of the sPOD for transport-dominated phenomena and its superiority in comparison to the POD.DFG, SFB 1029, Substantial efficiency increase in gas turbines through direct use of coupled unsteady combustion and flow dynamic

Global Stability Analysis of Compressible Leading-Edge Flow on a Swept Wing

International audienceThe global hydrodynamic stability of compressible leading-edge flow on a swept wing is addressed using Krylov-based iterative methods in conjunction with direct numerical simulations (DNS). Such a global hydrodynamic stability solver enables the analysis of complex fluid behavior by extracting global stability information directly from numerical simulations. Applying the DNS-based stability approach, unstable boundary-layer modes of the crossflow type and amplified as well as weakly-damped acoustic modes have been computed for a supersonic flow configuration. A parameter study reveals that, depending on the spanwise disturbance wavenumber β, boundary-layer modes or acoustic modes represent the dominant instability mechanism for the investigated parameter choices. Furthermore, the results of the present work clearly demonstrate the necessity of a global stability analysis to comprehensively understand the stability of swept leading-edge flow

Coherent structure detection and the inverse cascade mechanism in two-dimensional Navier-Stokes turbulence

Coherent structures in two-dimensional Navier-Stokes turbulence are ubiquitously observed in nature, experiments and numerical simulations. In spite of the steadily growing amount of literature with regard to flow coherence, the general influence of spatiotemporal coherence on the structure formation process that is associated with the inverse cascade of kinetic energy, is still subject of current research. The present study conducts a comparison between several structure detection schemes based on the Okubo-Weiss criterion, the vorticity magnitude, and Lagrangian coherent structures (LCSs), focusing on the inverse cascade in two-dimensional hydrodynamic turbulence. A recently introduced vortex scaling phenomenology [B. H. Burgess, R. K. Scott, J. Fluid Mech., 811:742-756, 2017] allows to determine the respective thresholds required by these methods based on physical properties of the flow. The resulting comparability allows to identify characteristic differences between the employed structure detection techniques. We observe that all schemes exhibit qualitatively similar results with respect to the cascade behavior. Furthermore, coherent structures contribute less to the inverse energy cascade than the residual incoherent parts of the flow. This can be understood by an increased misalignment of strain-rate and subgrid stress tensors within coherent structures. At the same time, these structures exhibit strong and localised nonlinear cross-scale interactions that appear to stabilize them. We quantify and interpret the resulting shape preservation of coherent structures by means of vortex thinning, which is analysed in terms of a recently introduced multi-scale gradient approach [G. L. Eyink, J. Fluid Mech., 549:191-214, 2006].Comment: 30 pages, 12 Figure