Comparison of les and rans predictions with experimental results of the fan of a turbofan

This paper aims at validating LES capability if applied to an actual turbofan configuration at nominal regime, if compared to RANS and experimental measurements. For assessment, averaged radial profiles are compared in 3 axial planes – before the stage, between the rotor blade and stator vanes, and downstream of the stator. RANS and LES results are in very good agreement, but found to be shifted compared to the measurements and for some quantities. An analysis of the unsteady axial velocity is then proposed, investigating root-mean square of axial velocity. Tip-leakage, as well as two boundary layer transitions are evidenced in the rotor. An estimation of the integral turbulent timescale is finally proposed in the whole domain, using autocorrelation of the axial velocity. Suction- side horseshoe vortices are found to be very coherent, as well as the stator corner vortices. Regions of large timescale are moreover evidenced between rotor and stator wakes

Induction in a von Karman flow driven by ferromagnetic impellers

We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impeller's electric and magnetic characteristics differ from that of the fluid. Implications in regards to the VKS dynamo are discussed.Comment: 14 pages, 7 figure

Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence

International audienceWe analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von Kármán swirling flow between two counterrotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field and a case with self-sustained magnetic fields. Evidence of long-term memory and 1/f noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von Kármán flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large-scale magnetic field

Robust estimate of dynamo thresholds in the von Kármán sodium experiment using the extreme value theory

International audienceWe apply a new threshold detection method based on the extreme value theory (EVT) to the von Kármán sodium (VKS) experiment data. The VKS experiment is a successful attempt to get a dynamo magnetic field in a laboratory liquidmetal experiment. We first show that the dynamo threshold is associated with a change of the probability density function of the extreme values of the magnetic field. This method does not require the measurement of response functions from applied external perturbations and thus provides a simple threshold estimate. We apply our method to different configurations in the VKS experiment, showing that it yields a robust indication of the dynamo threshold as well as evidence of hysteretic behaviors. Moreover, for the experimental configurations in which a dynamo transition is not observed, the method provides a way to extrapolate an interval of possible threshold values

Experimental Observation of Spatially Localized Dynamo Magnetic Fields

International audienceWe report the first experimental observation of a spatially localized dynamo magnetic field, a common feature of astrophysical dynamos and convective dynamo simulations. When the two propellers of the von Ka ́rma ́n sodium experiment are driven at frequencies that differ by 15%, the mean magnetic field's energy measured close to the slower disk is nearly 10 times larger than the one close to the faster one. This strong localization of the magnetic field when a symmetry of the forcing is broken is in good agreement with a prediction based on the interaction between a dipolar and a quadrupolar magnetic mode

Chaotic dynamos generated by a turbulent flow of liquid sodium

International audienceWe report the observation of several dynamical regimes of the magnetic field generated by a turbulent flow of liquid sodium (VKS experiment). Stationary dynamos, transitions to relaxation cycles or to intermittent bursts, and random field reversals occur in a fairly small range of parameters. Large scale dynamics of the magnetic field result from the interactions of a few modes. The low dimensional nature of these dynamics is not smeared out by the very strong turbulent fluctuations of the flow

The von Karman Sodium experiment: Turbulent dynamical dynamos

International audienceThe von Karman Sodium (VKS) experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Karman geometry). We first report observations related to the self-generation of a stationary dynamo when the flow forcing is R-pi-symmetric, i.e., when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is not symmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow

A characteristic inlet boundary condition for compressible, turbulent, multispecies turbomachinery flows

A methodology to implement non-reflecting boundary conditions for turbomachinery applications, based on characteristic analysis is described in this paper. For these simulations, inlet conditions usually corre- spond to imposed total pressure, total temperature, flow angles and species composition. While directly imposing these quantities on the inlet boundary condition works correctly for steady RANS simulations, this approach is not adapted for compressible unsteady Large Eddy Simulations because it is fully re- flecting in terms of acoustics. Deriving non-reflecting conditions in this situation requires to construct characteristic relations for the incoming wave amplitudes. These relations must impose total pressure, total temperature, flow angle and species composition, and simultaneously identify acoustic waves reach- ing the inlet to let them propagate without reflection. This treatment must also be compatible with the injection of turbulence at the inlet. The proposed approach shows how characteristic equations can be derived to satisfy all these criteria. It is tested on several cases, ranging from a simple inviscid 2D duct to a rotor/stator stage with turbulence injection

The VKS experiment: turbulent dynamical dynamos

International audienceThe VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Karman geometry). We report observations related to the self-generation of a stationary dynamo when the flow forcing is symmetric, i.e. when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical, with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is asymmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow

Analysis of rotor/stator interactions in a high-speed low-pressure turbine cascade using Large-Eddy Simulations.

peer reviewedTurbomachinery flows are highly turbulent and prone to different types of instabilities. These flow unsteadiness affect the aerodynamic performance of the machines mainly when approaching off-design conditions. The understanding of these complex phenomena is thus crucial in the design phase of a new aeronautical architecture, especially the study of the interaction between consecutive static and rotating parts. These interactions involve potential blade effects, wake interactions and many other sources. This paper focuses on rotor/stator interactions in low-pressure turbine which has the particularity to have a locally transonic flow regime in its vein. The geometry considered consists of a linear blade cascade in front of which bars rotate, acting as wake generators that impact periodically the leading edges of the blades, influencing all the aerodynamics of the cascade. Large-Eddy simulations (LES) of this configuration are carried out and compared to experimental results acquired at the Von Karman Institute during the Cleansky project SPLEEN. The main objective is the thorough investigation of the effect of the bars in such conditions. To do so, an incremental investigation is done, comparing a case with bars only, an isolated vane and finally the complete experimental setup. In particular the influence of the configuration on the flow turbulence is studied followed by the impact on the blade loading and the nature of the fluctuations in the flow downstream of the cascade. Finally, the variation of the blade wakes and the influence on the general topology of the flow are presented. In particular, the recirculation bubble present on the pressure side and the position of the shock waves in the inter-blade channel are detailed