Experimental study of the surface oscillations induced by a shallow flow past a lateral cavity

Abstract

The present work presents an experimental study in which resonant surface oscillations inside a lateral cavity are reconstructed, often denoted as se- iching, which are excited by a shallow main stream flowing past the horizontal basin. Firstly, the flow configurations that trigger transverse and/or longitu- dinal seiching are studied using pressure measurements in the corners of the cavity, which shows that a transitional Froude number exists, approximately 0.63, at which the dominant seiching mode changes from transverse to longi- tudinal seiching. For both types of resonant conditions, the surface shape is analyzed in detail using a three-dimensional particle tracking velocimetry (3D- PTV) setup. Based upon floating seeding particles, the 3D surface is recon- structed with a superior spatial resolution compared to traditional measurement techniques, which confirms the multimodal aspect of the surface oscillations.

Experimental study of bidirectional seiching in an open-channel, lateral cavity in the time and frequency domain

“…The first objective of this work is to refine the results presented in Ref. [22], in which the influence of the Froude number on the seiching waves was investigated, albeit with a nonconstant water depth. Therefore, a Fourier transform is applied to the surface registration (90 min in total) to compute the power spectral density (Psd FT ), i.e., the energy of the local surface deformation as a function of the frequency.…”

“…(2), the seiching mode with n l nodes in the longitudinal direction (along the x axis) and n t nodes in the transverse direction (along the y axis) will be denoted by m n l n t . Previous experiments on flow past a lateral cavity [18,22] showed that bidirectional seiching may occur in the specific case where the streamwise length scale L l = L equals half of the crosswise length scale L t = b + W , in which b is the width (along the y axis) of the main channel and L and W are the length (along the x axis) and width (along the y axis) of the cavity, respectively [see Fig. 1 3implies that the natural frequencies of unidirectional transverse seiching (n l = 0, n t = 0) equal half of the unidirectional longitudinal frequencies (n l = 0, n t = 0).…”

“…The first objective of this work is to refine previous results reported in Ref. [22] and investigate the relative magnitude of the different seiching modes (longitudinal and transverse) for a range of flow conditions with varying Froude number. Based on the registrations of the free surface in a series of laboratory experiments, the characteristic frequencies and corresponding amplitudes of the standing waves are first determined using a Fourier analysis of the measured recordings.…”

“…In contrast, for Fr > 0.60, Refs. [12,18,20,22] suggested a locked-on regime in which the frequency of the coupled oscillations remained fixed at, or locked-on, the natural frequency of the cavity over a range of Froude numbers. In this high-Froude-number regime, Refs.…”

“…[18] observed that waves in both the longitudinal and transverse direction can actually take place simultaneously. Reference [22] investigated the relative magnitude of transverse and longitudinal seiching in function of the Froude number of the main stream, in which they found a transitional Froude number for which the dominant seiching mode (i.e., having the largest amplitude) changed from transverse to longitudinal seiching.…”