Roughness effects on the second-order turbulence statistics in oscillatory flows

Ghodke, Chaitanya D.; Apte, Sourabh V.

doi:10.1016/j.compfluid.2017.09.021

Cited by 13 publications

(16 citation statements)

References 34 publications

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“…As suggested by Julien (2010), with decreasing the drag coefficient should increase. Therefore, case C350B with a exhibits a larger drag coefficient when compared with the sand-grain type roughness elements with a (Raupach & Thom 1981; Ghodke & Apte 2017). The smaller value of observed for case WC350B suggests that despite the laminar nature of the wave, it greatly affects the mean flow for bumpy wall cases.…”

Section: Resultsmentioning

confidence: 96%

Drag enhancement by the addition of weak waves to a wave-current boundary layer over bumpy walls

Patil

Fringer

2022

J. Fluid Mech.

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We present direct numerical simulation results of a wave-current boundary layer in a current-dominated flow regime (wave driven to steady current ratio of 0.34) over bumpy walls for hydraulically smooth flow conditions (wave orbital excursion to roughness ratio of 10). The turbulent, wave-current channel flow has a friction Reynolds number of $350$ and a wave Reynolds number of $351$ . At the lower boundary, a bumpy wall is introduced with a direct forcing immersed boundary method, while the top wall has a free-slip boundary condition. Despite the hydraulically smooth nature of the wave-driven flow, the phase variations of the turbulent statistics for the bumpy wall case were found to vary substantially when compared with the flat wall case. Results show that the addition of weak waves to a steady current over flat walls has a negligible effect on the turbulence or bottom drag. However, the addition of weak waves to a steady current over bumpy walls has a significant effect through enhancement of the Reynolds stress (RS) accompanied by a drag coefficient increase of $11\,\%$ relative to the steady current case. This enhancement occurs just below the top of the roughness elements during the acceleration portion of the wave cycle: Turbulent kinetic energy (TKE) is subsequently transported above the roughness elements to a maximum height of roughly twice the turbulent Stokes length. We analyse the TKE and RS budgets to understand the mechanisms behind the alterations in the turbulence properties due to the bumpy wall. The results provide a mechanistic picture of the differences between bumpy and flat walls in wave-current turbulent boundary layers and illustrate the importance of bumpy features even in weakly energetic wave conditions.

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Section: Resultsmentioning

confidence: 96%

Drag enhancement by the addition of weak waves to a wave-current boundary layer over bumpy walls

Patil

Fringer

2022

J. Fluid Mech.

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“…The approach could be applied to shallow water waves to assess the effect of wind on increasing the bottom shear stress and linking it to disruption of the log layer. Turbulence greatly affects material transport; recently, Ghodke et al [49,50] in their groundbreaking and fully resolved numerical study reported the effects of turbulence on incipient material transport mechanisms. Such high-fidelity simulations can further provide time-accurate eddy viscosity data that otherwise are extremely challenging to obtain using experiments.…”

Section: Discussionmentioning

confidence: 99%

Material Transport under a Wave Train in Interaction with Constant Wind: A Eulerian RANS Approach Combined with a Lagrangian Particle Dispersion Model

Sinha

Golshan

2018

Fluids

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The interaction of a developed train of gravity deep water waves with suddenly applied winds is investigated in this manuscript. The direction of the wind is the same as that of the wave train (i.e., following) and its imposed surface shear stress is constant and steady. The focus of this study is on a micro-scale water wave field where the time scale is on the order of ten wave periods and the length scale is on the order of ten wave lengths. Accurate 2D Reynolds-averaged Navier-Stokes (RANS) multi-phase simulations of Navier-Stokes equations are performed in a Eulerian framework to capture the flow features, induced by the wave field and the surface wind. The sufficiently large spatial domain in the horizontal direction, combined with the sufficiently long simulation time, permits the development of surface currents and the consequent formation of a near-surface shear layer. The interaction of surface currents with the wave orbital velocity field results in the generation of spilling breaking waves. Downstream of the domain, vertical turbulent structures are observed as the result of such breaking waves. Lagrangian particle tracking is performed, using the RANS simulation velocity and eddy diffusivity data. A second order random acceleration particle tracking method is applied with the vitally important spatial gradients of the eddy diffusivity (Journal of Marine Science and Engineering, 2018, 6, 10.3390/jmse6010007.) also included in calculations. The spatial gradients of the eddy diffusivity were proved to be a key factor in material transport simulations. Our particle tracking results exhibit strong vertical mixing downstream of the domain and by means of visualizing the spiral trajectory of neutrally buoyant particles. Such enhanced vertical mixing (caused by horizontal winds) is the result of the strong near-surface advection (induced by currents) and the turbulence (induced by breaking waves). The objectives of this paper are twofold. Firstly, a numerical approach to simulate wind breaking waves is proposed based on: using K − RANS model to capture turbulence features, employing the Volume of Fluid Method (VOF) to model the free surface flow, and applying a constant shear stress body force at the interfacial cells to simulate the wind force. Such treatment of the winds eliminates the need for fully resolving the air phase. The computed eddy viscosity profiles are in good agreement with the experimental profiles reported in the literature (ν t = −κu * z,

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“…Ikeda & Durbin 2007; Cardillo et al. 2013; Yuan & Piomelli 2015; Ghodke & Apte 2018 a ) in terms of Kolmogorov length scales and wall units.…”

Section: Numerical Methodologymentioning

confidence: 99%

“…Using DANS equations, Giménez-Curto & Corniero Lera (1996) confirmed that Sleath's (1987) findings could be explained by the presence of dispersive stresses resulting from flow separation from roughness elements. Ghodke & Apte (2018 a ) utilised the DANS equations to investigate the effect of roughness on momentum transfer mechanisms in oscillatory flow in the transitional and very rough turbulent regimes. Using DNS, they showed that dispersive stresses were significant mainly below roughness crests and up to twice the roughness diameter above roughness crests for larger and smaller hexagonally packed spherical roughness elements, respectively.…”

Section: Introductionmentioning

confidence: 99%

An experimental and numerical study of turbulent oscillatory flow over an irregular rough wall

Dunbar

A²,

Scandura³

et al. 2023

J. Fluid Mech.

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The hydrodynamics of turbulent oscillatory flow over a gravel-based irregular rough wall is investigated using laser-Doppler anemometry measurements of velocities in a large oscillatory flow tunnel and direct numerical simulation (DNS) of the Navier–Stokes equations. The same periodic irregular roughness was used for both experiments and DNS. Four flow shapes are investigated: sinusoidal, skewed, asymmetric and combined skewed–asymmetric. The experiments were conducted for target Reynolds numbers (based on the Stokes length and standard deviation of free-stream velocity) of $R_{\delta,\sigma }=800$ and $R_{\delta,\sigma }=1549$ ; DNS was conducted for flows with target $R_{\delta,\sigma }=800$ . Boundary layer thickness, bottom phase lead and friction factor are in good agreement with previous studies. For the first time, evidence of Prandtl's secondary flows of the second kind in oscillatory flow is presented. Turbulence structure is visualised using isosurfaces of $\lambda _{2}$ (Jeong & Hussain J. Fluid Mech., vol. 285, 1995, pp. 69–94), revealing densely packed structures that grow stronger and weaker in correspondence with the free-stream velocity. Reynolds and dispersive stresses peak just below the highest roughness crest, with dispersive stress vanishing a short distance above the roughness. Bursts of turbulence kinetic energy and wake kinetic energy are generated each flow half-cycle, with variable behaviour depending on flow shape. Non-Gaussian turbulence statistics are observed that originate near the wall, becoming increasingly non-Gaussian far from the wall. Probability density functions of turbulence statistics can be closely approximated by a fourth-order Gram–Charlier distribution at most phases and elevations, though when statistics deviate more strongly from Gaussian, streamwise and wall-normal (spanwise) statistics are better described by a Pearson type IV (VII) distribution.

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Roughness effects on the second-order turbulence statistics in oscillatory flows

Cited by 13 publications

References 34 publications

Drag enhancement by the addition of weak waves to a wave-current boundary layer over bumpy walls

Drag enhancement by the addition of weak waves to a wave-current boundary layer over bumpy walls

Material Transport under a Wave Train in Interaction with Constant Wind: A Eulerian RANS Approach Combined with a Lagrangian Particle Dispersion Model

An experimental and numerical study of turbulent oscillatory flow over an irregular rough wall

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