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U. Harlander

Publications and source records attributed to U. Harlander.

3 recordsLinked to original sources

Building transport models from baroclinic wave experimental data

In this paper we study baroclinic waves both from the experimental and the theoretical perspective. We obtain data from a rotating annulus experiment capable of producing a series of baroclinic eddies similar to those found in the mid-latitude atmosphere. We analyze the experimental outputs using two methods. First, we apply a technique that involves filtering data using Empirical Orthogonal Function (EOF) analysis, which is applied to both velocity and surface temperature fields. The second method relies on the construction of a simple kinematic model based on key parameters derived from the experimental data. To analyze eddy-driven fluid transport, we apply the method of Lagrangian descriptors to the underlying velocity field, revealing the attracting material curves that act as transport barriers in the system. These structures effectively capture the essential characteristics of the baroclinic flow and the associated transport phenomena. Our results show that these barriers are in good agreement with the transport patterns observed in the rotating annulus experiment. In particular, we observe that the structures obtained from the kinematic model, or the one derived in terms of filtered velocities, perform well in this regard.

nlin.CD

Experiments and long-term High Performance Computations on amplitude modulations of strato-rotational flows

The present paper describes a combined experimental and high performance computing study of new specific behaviors of the Strato-Rotational Instability (SRI). The confrontation of low frequency Particle Image Velocimetry (PIV) experimental data with Direct Numerical Simulations reveal new non-linear phenomena and patterns not yet observed in the SRI, that can contribute for our understanding of astrophysical flows, specially regarding phenomena related to accretion disk theories. The experiment designed to investigate these SRI related phenomena consists of a classical Taylor-Couette (TC) system with two concentric cylinders that can rotate independently under stable density stratification due to temperature gradients imposed in the axial direction. In the present investigations, Froude numbers are between 1.5<F<4., and Reynolds numbers vary between 300<Re<1300, while rotation ratio between outer and inner cylinders fixed at 0.35, a value slightly smaller than the Keplerian velocity profile, but beyond the Rayleigh limit. The same experimental configuration is used in the Direct Numerical Simulations investigation, that uses a parallel high-order compact scheme incompressible code that solves the Boussinesq equations. Both simulations and experiments reveal, in agreement with recent linear stability analyses, the occurrence of a return to stable flows with respect to the SRI when the Reynolds numbers increase. Low frequency velocity amplitude modulations related to two competing spiral wave modes, not yet reported, are observed both numerically and experimentally.

physics.flu-dyn

The stratorotational instability of Taylor-Couette flows of moderate Reynolds numbers

In view of new experimental data the instability against adiabatic nonaxisymmetric perturbations of a Taylor-Couette flow with an axial density stratification is considered in dependence of the Reynolds number Re of rotation and the Brunt-V\"ais\"al\"a number Rn of the stratification. The flows at and beyond the Rayleigh limit become unstable between a lower and an upper Reynolds number (for fixed Rn). The rotation can thus be too slow or too fast for the stratorotational instability. The upper Reynolds number above which the instability decays, has its maximum value for the potential flow (driven by cylinders rotating according to the Rayleigh limit) and decreases strongly for flatter rotation profiles finally leaving only isolated islands of instability in the (Rn/Re) map. The maximal possible rotation ratio $\mu_{\rm max}$ only slightly exceeds the shear value of the quasi-uniform flow with $U_\phi\simeq$const. Along and between the lines of neutral stability the wave numbers of the instability patterns for all rotation laws beyond the Rayleigh limit are mainly determined by the Froude number Fr which is defined by the ratio between Re and Rn. The cells are highly prolate for Fr>1 so that measurements for too high Reynolds numbers become difficult for axially bounded containers. The instability patterns migrate azimuthally slightly faster than the outer cylinder rotates.

physics.flu-dyn