Aspects of vibrations and fatigue of materials related to coherent structures of macroturbulent flows R.A.LOPARDO, C.A.FATTOR, J.M.CASADO & M.C.LOPARDO National Institute of Water Casilla Correo 21, B1804WAA – Ezeiza, Buenos Aires, Argentina Tel./Fax: (54-11) 4480-4500, e-mail: rlopardo@ina.gov.ar 1 INTRODUCTION Energy dissipation in stilling basins produces dynamic loads on the bottom slabs. Some characteristics of the coherent structures of the macroturbulent flow can be analysed from the calculation of the fluctuating loads on slabs of a free hydraulic jump stilling basin. The analysis of the autocorrelation function of the random loads at different locations along the bottom of the hydraulic jump allows estimating an approximate magnitude of the scale of times of the turbulence, as well as obtaining information regarding the main period of the fundamental vortexes. This last information can also be obtained from power spectra analysis of individual records of pressure fluctuations along the bottom of the hydraulic jump. The first purpose of this article is to analyse the characteristics of the coefficient of autocorrelation of fluctuating loads for different sizes of slabs, given through a/h 1 , where “a” is the length of the slab and “h 1 ” is the initial depth of the jump (Fig. 1). In this way, the relationship a/h 1 not only constitute a parameter of the dimensions of the slab but also an important reference of the size of the macroturbulent structures of the hydraulic jump compared to the slab length. Due to the fact that turbulent flow represents an alternating load on hydraulic structures, the second aim of this work is to present a methodology to estimate the reduction of the resistance of the metallic elements due to fatigue effects. ABSTRACT: Macroturbulent flows in stilling basins can be analysed by their coherent structures, being necessary to determine length and time scales to get a comprehensive understanding of the phenomena involved. The peak frequencies of the main vortexes transported for the mean flow can be evaluated if the main parameters of the coherent structures are determined, making possible the determination of the Strouhal number as well as its comparison with the natural frequency of the structures submerged in the macroturbulent flow and, in this way, estimating potential problems of resonance. On the other hand, the effect of the fatigue in an eventual loss of resistance of the steel bars embedded in the concrete can be experimentally determined by means of the zero-crossing frequency. In this way, the number of times of the steel bars passing from traction to compression and vice versa can be estimated. The experimental data involving vibrations aspects are limited, in this article, to a free hydraulic jump downstream of a vertical gate with incident Froude number F 1 = 3, and a rectangular slab of width b and non-dimensional length a/h 1 = 3, where a is the length of the slab and h 1 is the water depth at the inlet to the jump. As regards the analysis of fatigue, results for 3 < F 1 < 6 are presented, also considering a free hydraulic jump downstream of a vertical gate.