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The effect of surface wave on the double-averaged (DA) current-only flow properties is examined over two different types of rough beds comprising hemispherical elements with regular and staggered arrangements. For each case, the relative roughness spacing of p/r = 4 is maintained, where p = pitch distance and r = hemisphere height. The changes in turbulence characteristics against the water depth are analysed for superimposed surface waves propagating along the current with 1 and 2 Hz frequencies. The effect of regular and staggered roughness arrangements on different turbulence characteristics is explored. Results demonstrate that the DA stream-wise velocity decreases and increases at the outer region and within the roughness region, respectively, due to wave propagation. Owing to the surface wave, the DA intensity and form-induced intensity increase above the roughness layer and decrease within the bottom boundary layer. The values of form-induced intensities are smaller (greater) than the DA turbulence intensities at each considered flow depth within the roughness (outer) layer. For the entire flow depth, the values of DA Reynolds stress decrease due to wave interactions along a steady current. Again, the turbulence is observed to be close to isotropic for the current-only flow at the outer zone, while the flow transits to an anisotropic state with wave imposition on current. Further, the computed values of the turbulence indicator and dispersion functions are used for the identification of the level of turbulence and dispersion at each flow depth for the locations upstream and downstream of a hemisphere in the hemispherical bed. [ABSTRACT FROM AUTHOR]

An experimental study has been carried out in a laboratory flume to characterize the turbulence structure and turbulence anisotropy in the boundary layer over smooth and rough side walls for both current alone and wave-current combined flow situations. The rough side wall of the flume comprises a train of circular ribs (diameter, k) attached vertically maintaining uniform spacing p along the streamwise direction. The experiments are performed for smooth surface and rough (ribbed) surfaces with p/k = 2, 3, and 4 to reproduce different cases of d -type rib roughness. The effect of wave-current interaction has been investigated by superposing waves of two different frequencies. Time series data of three velocity components are obtained using Acoustic Doppler Velocimeter. At the near wall region, roughness with higher p/k value enhances the level of turbulent intensity and Reynolds stress significantly. In a channel with smooth side wall, the wave-current combined flow produces lesser turbulence intensity than the current alone flow near the wall. However, for a ribbed wall case, the effect is completely opposite that is, wave-current interacting flow induces higher intensities compared to the reference current alone flow. Substantial decline in the turbulent length scales at the near wall region are observed for ribbed walls, which reveals the strong effect of roughness elements on the turbulent structure. Superposition of wave reduces the length scales even more for both smooth and rough wall cases. As the spacing between two ribs (p / k ratio) increases, the energy dissipation rate increases. The analysis of anisotropy invariant map demonstrates a reduction of anisotropy in the vicinity of ribbed wall compared to that for a smooth wall. For wave-current combined flow, the anisotropy invariant data of Reynolds stress tensor varies dramatically within the boundary of map, reflecting significant changes in the state of turbulence. [ABSTRACT FROM AUTHOR]

The present study demonstrates that a round turbulent jet issued from a cylinder surface towards the upstream against the current can be a controlling parameter for vortex synchronization (VSC) in the wake region (WR) of the vertical cylinder. The results depict that the VSC state can be reached when the jet excitation frequency and the natural shedding frequency (NSF) are almost equal, or the jet excitation frequency is double that of the NSF. Precisely for VSC case, the turbulent production and dissipation terms of the turbulent kinetic energy budget equation contribute majorly to the total energy at the reattachment region with shifting of energy from the pressure and the turbulent diffusion terms. The enhancement of intermittency in turbulence for the VSC cases is also pertinent. Key words: probability density function (pdf), joint pdf, Shannon entropy, vortex synchronization, vorticity. La presente etude demontre qu'un jet tourbillonnaire rond emis a partir d'une surface de cylindre en amont par rapport au courant peut etre un parametre de controle pour la synchronisation des vortex (SCV) dans la region de sillage du cylindre vertical. Les resultats montrent que l'etat de SCV peut etre atteint lorsque la frequence d'excitation du jet et la frequence de decollement naturelle sont presque egales, ou que la frequence d'excitation du jet est le double de celle de la frequence de decollement naturelle. Precisement pour le cas de SCV, les termes de production turbulente et de dissipation turbulente de l'equation budgetaire de l'energie cinetique turbulente contribuent majoritairement a l'energie totale dans la zone d'amorcage de l'attachement avec le deplacement de l'energie des termes de la pression et de la diffusion turbulente. L'amelioration de l'intermittence de la turbulence pour les cas de SCV est egalement pertinente. [Traduit par la Redaction] Mots-cles : fonction de densite, fonction de densite a probabilite commune, entropie Shannon, synchronisation de vortex, vorticite., Introduction Severe structural vibrations and acoustic noise are generated due to the vortex shedding (VS) around a bluff (also referred to as a blunt) body. Choi et al. (2008) expressed [...]

The influence of three different channel contractions on the mean flow and turbulent characteristics has been measured experimentally in a recirculating flume. The velocity deficit, turbulence intensity, Reynolds stress, and turbulence scales data within the channel contractions were obtained from three-dimensional velocity fluctuations measurements using Acoustic Doppler Velocimeter. The mean flow profile shows undulation towards the free surface for all the contraction ratios, indicating the presence of the velocity-dip phenomenon due to the secondary currents. These currents changed the pattern of turbulence intensities and Reynolds shear stresses significantly within the channel contraction compared to the without contraction case. The results reveal that with the increase of channel contraction ratio, the strength of the secondary current increased considerably and resulted in a significant change in flow patterns close to the free surface compared to the bottom. The length scales of turbulent flow indicate eddy sizes in different ranges to be smaller at near-bed region within the contraction zone due to the breakdown of larger eddies. [ABSTRACT FROM AUTHOR]

Combined wave-current turbulent characteristics are investigated over a rough bed comprising of hemispheres positioned at different spacings (p/r = 4, 6, and 8; p is the patch distance and r is the height of the hemisphere) in terms of time-space averaged (double-averaged) flow components. An Acoustic Doppler Velocimetry was used to measure the instantaneous velocity components for all background flow cases that comprised current-only flow and wave superimposed on current (wave frequency 0 Hz, 1 Hz, and 2 Hz). This study attempts to characterize the double-averaged velocity, relative and formed induced turbulence intensity, and Reynolds shear stress for the specific flow layers due to the interaction of wave-current flow over the hemispherical rough bed. The study further explores how these parameters are modulated with the change in spacing between the roughness elements. The relative contribution of the different turbulent busting events is analyzed for the different positions with respect to the hemispherical elements. Results show that sweep and ejection are the major contributors to all flow conditions and roughness cases explored herein. Furthermore, the anisotropic structure of turbulence due to the superposition of waves for different rough bed conditions is explored in the double-averaged perspective. [ABSTRACT FROM AUTHOR]

This paper reports the findings of an experimental study carried out in a laboratory flume to investigate the interaction of a surface wave with a unidirectional current over submerged tandem (bed mounted two in-line) hemispheres. The pitch-to- height ratio of the hemispheres was varied to spawn d-type (L/d = 2), intermediate (L/d = 4), and k-type (L/d = 8) relative spacing. The observations are focused on the changes induced in the mean velocity, turbulence intensity, Reynolds shear stress, and the frictional contributions of burst-sweep cycle to shear stress due to superposition of surface waves on the ambient flow. The consequences from the present experiment showed that the stream-wise velocity fluctuation follows the normal Gaussian distribution at upstream of hemispheres. Further, the joint probability density function showed that the superimposed surface waves on the current are capable of modulating the turbulent eddies with a larger band of energy and frequency spectrum compared to current-only flow. Key words: turbulence, wave-current, quadrant analysis, probability density function. Dans cet article, on presente une etude effectuee dans un canal d'experimentation afin d'examiner l'interaction d'une onde de surface a courant unidirectionnel sur des hemispheres submerges en tandem (montes sur lit deux en ligne). On a varie le rapport d'espacement a hauteur des hemispheres afin d'engendrer un espacement relatif de type d (L/d = 2), de type intermediaire (L/d = 4) et de type k (L/d = 8). Les observations portent sur les changements induits au niveau de la vitesse moyenne, de l'intensite de turbulence, de la tension de Reynolds et des contributions frictionnelles du cycle rafale-balayage a la tension en raison de la superposition d'ondes de surface sur l'ecoulement ambiant. Les conclusions de cette experience ont revele que la fluctuation de vitesse dans le sens de l'ecoulement suit une distribution gaussienne normale en amont des hemispheres. De plus, la fonction de densite de probabilite commune a montre que les ondes de surface superposees sur le courant sont capables de moduler les remous turbulents avec une plus grande bande d'energie et de spectre de frequences comparativement au debit de courant seul. [Traduit par la Redaction] Mots-cles : turbulence, courant d'ondes, analyse par quadrant, fonction de densite., 1. Introduction The turbulent flow past a bluff body mounted on a rigid plane yields a three-dimensional turbulent velocity field at the wake region that comprises a recirculation zone, horseshoe [...]