On the Influence of Swell Propagation Angle on Surface Drag

2019 ◽  
Vol 58 (5) ◽  
pp. 1039-1059 ◽  
Author(s):  
Edward G. Patton ◽  
Peter P. Sullivan ◽  
Branko Kosović ◽  
Jimy Dudhia ◽  
Larry Mahrt ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Three Dimensional ◽  
Surface Drag ◽  
Wave State ◽  
Propagation Angle ◽  
Predictive Skill ◽  
Large Eddy ◽  
Wave Effects ◽  
The Mean ◽  
Wave Models

AbstractA combination of turbulence-resolving large-eddy simulations and observations are used to examine the influence of swell amplitude and swell propagation angle on surface drag. Based on the analysis a new surface roughness parameterization with nonequilibrium wave effects is proposed. The surface roughness accounts for swell amplitude and wavelength and its relative motion with respect to the mean wind direction. The proposed parameterization is tested in uncoupled three-dimensional Weather and Research Forecasting (WRF) simulations at grid sizes near 1 km where we explore potential implications of our modifications for two-way coupled atmosphere–wave models. Wind–wave misalignment likely explains the large scatter in observed nondimensional surface roughness under swell-dominated conditions. Andreas et al.’s relationship between friction velocity and the 10-m wind speed under predicts the increased drag produced by misaligned winds and waves. Incorporating wave-state (speed and direction) influences in parameterizations improves predictive skill. In a broad sense, these results suggest that one needs information on winds and wave state to upscale buoy measurements.

scholarly journals Turbulent Flow Fields Over a 3D Hill Covered by Vegetation Canopy Through Large Eddy Simulations

Energies ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3624 ◽  
Author(s):  
Zhenqing Liu ◽  
Yiran Hu ◽  
Yichen Fan ◽  
Wei Wang ◽  
Qingsong Zhou
Keyword(s):  
Three Dimensional ◽  
Flow Fields ◽  
Large Eddy Simulations ◽  
Wind Tunnel Experiments ◽  
Turbulence Structures ◽  
Large Eddy ◽  
Turbulence Kinetic Energy Budget ◽  
The Mean ◽  
Spanwise Rotation ◽  
The Hill

The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed to examine the coherent turbulence structures of the flow fields over a vegetation-covered 3D hill. The numerical simulations were validated by the comparison with the wind-tunnel experiments. Besides, the flow fields were systematically investigated, including the examinations of the mean velocities and root means square of the fluctuating velocities. The distributions of the parameters are shown in a three-dimensional way, i.e., plotting the parameters on a series of spanwise slices. Some noteworthy three-dimensional features were found, and the mechanisms were further revealed by assessing the turbulence kinetic energy budget and the spectrum energy. Subsequently, the instantaneous flow fields were illustrated, from which the coherent turbulence structures were clearly identified. Ejection-sweep motion was intensified just behind the hill crest, leading to a spanwise rotation. A group of vertical rotations were generated by the shedding of the vortex from the lateral sides of the hill.

scholarly journals Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

2013 ◽  
Vol 721 ◽  
pp. 454-483 ◽  
Author(s):  
Mohammad Omidyeganeh ◽  
Ugo Piomelli
Keyword(s):  
Secondary Flow ◽  
Three Dimensional ◽  
Wall Stress ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Form Drag ◽  
Separated Shear Layer ◽  
Large Eddy ◽  
The Mean

AbstractWe performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18 900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune shape in the streamwise direction according to a sine wave. The statistics of the flow are discussed in 10 cases with in-phase and staggered crestlines, different deformation amplitudes and wavelengths. The results are validated qualitatively against experiments. The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which in turn may cause secondary flow across the stream, which directs low-momentum fluid, near the bed, toward the lobe (the most downstream point on the crestline) and high-momentum fluid, near the top surface, toward the saddle (the most upstream point on the crestline). The mean flow is characterized by a pair of counter-rotating streamwise vortices, with core radius of the order of the flow depth. However, for wavelengths smaller than the flow depth, the secondary flow exists only near the bed and the mean flow away from the bed resembles the two-dimensional case. Staggering the crestlines alters the secondary motion; the fastest flow occurs between the lobe and the saddle planes, and two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The distribution of the wall stress and the focal points of separation and attachment on the bed are discussed. The sensitivity of the average reattachment length, depends on the induced secondary flow, the streamwise and spanwise components of the channel resistance (the skin friction and the form drag), and the contribution of the form drag to the total resistance are also studied. Three-dimensionality of the bed increases the drag in the channel; the form drag contributes more than in the two-dimensional case to the resistance, except for the staggered-crest case. Turbulent-kinetic energy is increased in the separated shear layer by the introduction of three-dimensionality, but its value normalized by the plane-averaged wall stress is lower than in the corresponding two-dimensional dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer, respectively, affecting the turbulent kinetic energy.

scholarly journals Computational fluid dynamics approaches to drag and wake of a long-line mussel dropper under tidal current

2020 ◽  
Vol 103 (1) ◽  
pp. 003685041990123
Author(s):  
Zhijing Xu ◽  
Hongde Qin ◽  
Peng Li ◽  
Rujun Liu
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Research Work ◽  
Three Dimensional ◽  
Long Line ◽  
Large Eddy ◽  
Mussel Farms ◽  
Hydrodynamic Effects

Hydrodynamic effects of mussel farms have attracted increased research attentions in recent years. The understanding of the hydrodynamic impacts is essential for predicting the sustainability of mussel farms. A large mussel farm includes thousands of mussel droppers, and the combined drag on the mussel droppers is sufficient to possibly affect the longevity of the entire long-lines. This article intends to study the drag and wake of an individual long-line mussel dropper using computational fluid dynamics approaches. Two equivalent rough cylinders, namely, Curved-Model and Sharp-Model, have been utilized to simulate the mussel dropper, and each rough cylinder is assigned with surface roughness. The porosity is not considered in this article due to its complexity from inhalant and exhalant of mussels. Two-dimensional laminar simulations are conducted at Reynolds number from 10 to 200, and three-dimensional large eddy simulations are conducted at subcritical Reynolds number ranging from 3900 to [Formula: see text]. The results show that larger drag coefficients and Strouhal numbers are attributed to surface roughness and sharp crowns on the rough cylinder. The obtained drag coefficient ranges from 1.1 to 1.2 with respect to the diameter of the mussel dropper and the peak value of the tidal velocities. Wakes behind rough cylinders fluctuate more actively compared to those of smooth cylinders. This research work provides new insight for further investigations on hydrodynamic interactions between fluid and mussel droppers.

Dynamic Response of a Turbulent Cylinder Wake to Forced Excitation

2009 ◽  
Author(s):  
Efstathios Konstantinidis ◽  
Chunlei Liang
Keyword(s):  
Dynamic Response ◽  
Dimensionless Parameter ◽  
Excitation Frequency ◽  
Three Dimensional ◽  
Transverse Force ◽  
Cylinder Wake ◽  
Mean Velocity ◽  
Large Eddy ◽  
The Mean ◽  
Forced Excitation

Three-dimensional large-eddy simulations were carried out to determine the dynamic response of a turbulent cylinder wake to forced excitation at a subcritical Reynolds number of 2580. The excitation frequency was varied across the primary lock-on range while the amplitude of sinusoidal velocity perturbation and the mean velocity imposed at the inflow boundary were kept constant. The velocity fluctuations in the wake and the fluctuating forces on the cylinder are analyzed employing spectral, time-domain and phase-portrait methods. The results show that the dynamic response of the inline force is different than that of the transverse one on the border of the lock-on range; while the inline force exhibits a phase-locked response, the transverse force indicates an intermittent response. This behavior is linked to the wake dynamics which is similar to that of the transverse force. This result is explained on the basis of the Morison equation which shows that the inline force is biased by the inertial components associated with the added mass and pressure waves in unsteady flows. It is further shown that the existence of a mean velocity component alters radically the dynamics of the inline force and appropriate ranges of a dimensionless parameter are proposed to describe the response.

scholarly journals Coherent structure of the convective boundary layer derived from large-eddy simulations

1989 ◽  
Vol 200 ◽  
pp. 511-562 ◽  
Author(s):  
Helmut Schmidt ◽  
Ulrich Schumann
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Large Scale ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Stable Layer ◽  
Convective Boundary ◽  
Velocity Fluctuations ◽  
Large Eddy ◽  
The Mean

Turbulence in the convective boundary layer (CBL) uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for zero mean horizontal flow using large-eddy simulations (LES). The Rayleigh number is effectively infinite, the Froude number of the stable layer is 0.09 and the surface roughness height relative to the height of the convective layer is varied between 10−6 and 10−2. The LES uses a finite-difference method to integrate the three-dimensional grid-volume-averaged Navier–Stokes equations for a Boussinesq fluid. Subgrid-scale (SGS) fluxes are determined from algebraically approximated second-order closure (SOC) transport equations for which all essential coefficients are determined from the inertial-range theory. The surface boundary condition uses the Monin–Obukhov relationships. A radiation boundary condition at the top of the computational domain prevents spurious reflections of gravity waves. The simulation uses 160 × 160 × 48 grid cells. In the asymptotic state, the results in terms of vertical mean profiles of turbulence statistics generally agree very well with results available from laboratory and atmospheric field experiments. We found less agreement with respect to horizontal velocity fluctuations, pressure fluctuations and dissipation rates, which previous investigations tend to overestimate. Horizontal spectra exhibit an inertial subrange. The entrainment heat flux at the top of the CBL is carried by cold updraughts and warm downdraughts in the form of wisps at scales comparable with the height of the boundary layer. Plots of instantaneous flow fields show a spoke pattern in the lower quarter of the CBL which feeds large-scale updraughts penetrating into the stable layer aloft. The spoke pattern has also been found in a few previous investigations. Small-scale plumes near the surface and remote from strong updraughts do not merge together but decay while rising through large-scale downdraughts. The structure of updraughts and downdraughts is identified by three-dimensional correlation functions and conditionally averaged fields. The mean circulation extends vertically over the whole boundary layer. We find that updraughts are composed of quasi-steady large-scale plumes together with transient rising thermals which grow in size by lateral entrainment. The skewness of the vertical velocity fluctuations is generally positive but becomes negative in the lowest mesh cells when the dissipation rate exceeds the production rate due to buoyancy near the surface, as is the case for very rough surfaces. The LES results are used to determine the root-mean-square value of the surface friction velocity and the mean temperature difference between the surface and the mixed layer as a function of the roughness height. The results corroborate a simple model of the heat transfer in the surface layer.

scholarly journals The effect of oscillating turbulent inflow on the shape of downstream turbulence

2021 ◽  
Author(s):  
Young-Woo Yi ◽  
◽  
Bhupendra Singh Chauhan ◽  
Hee-Chang Lim ◽  
◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Inlet Section ◽  
Mean Flow ◽  
Turbulent Inflow ◽  
Turbulent Statistics ◽  
Large Eddy ◽  
The Mean

Large Eddy Simulations (LES) has been widely applied and used in several decades to simulate a turbulent boundary layer in the numerical domain. In this study, we aimed to make a synthetic inflow generator (SIG) yielding an appropriate property of turbulent boundary layer in the inlet section and making quick development in the downstream of a three-dimensional domain. In order to achieve turbulent boundary layer quickly in a limited domain, the oscillating term was implemented in the well-defined boundary layer, which was expected to make faster convergence in the calculation. Cholesky decomposition was also applied to possess turbulent statistics such as the randomness and correlation of turbulent flow. In a result, the oscillating inflow did not show the faster convergence, but it indicated a possibility to improve statistical quantities in the downstream. In addition, regarding the mean flow characteristics were very close to the calculation without the oscillating flow. On the other hand, the turbulent statistics were improved depending on the oscillating magnitude.

Journal of Ship Research First and Second-Order Wave Effects on a Submerged Spheroid

1991 ◽  
Vol 35 (03) ◽  
pp. 183-190
Author(s):  
C. H. Lee ◽  
J. N. Newman
Keyword(s):  
Three Dimensional ◽  
Second Order ◽  
Slender Body ◽  
Regular Waves ◽  
Slender Body Theory ◽  
Pitch Moment ◽  
Wave Effects ◽  
The Mean ◽  
Approximate Result ◽  
Body Theory

Computations are presented for the linearized force and moment acting on a submerged slender spheroid in regular waves, the resulting pitch and heave motions, and the second-order mean force and moment. These numerical results, which are based on the use of a three-dimensional panel code, are compared with the approximations based on slender-body theory. The accuracy of the slender-body approximation is relatively good for the first-order forces and body motions, but substantial errors are revealed for the second-order mean drift force and pitch moment. Unlike the approximate result, the more correct numerical prediction of the mean pitch moment is non-zero, and generally acts in the bow down direction in head seas. To explain this result it is shown that the wave elevation directly above the spheroid increases in amplitude from bow to stern, thus causing a greater upward force on the afterbody relative to the forebody.

Electron Holographic Nano-Characterization of Gold Catalyst at Interface

2002 ◽  
Vol 727 ◽  
Author(s):  
S. Ichikawa ◽  
T. Akita ◽  
M. Okumura ◽  
M. Haruta ◽  
K. Tanaka
Keyword(s):  
Contact Angle ◽  
Titanium Oxide ◽  
Gold Catalyst ◽  
Three Dimensional ◽  
High Resolution Electron Microscopy ◽  
Electron Holography ◽  
Gold Particles ◽  
Oxide Support ◽  
The Mean ◽  
A Charge

AbstractThe catalytic properties of nanostructured gold catalyst are known to depend on the size of the gold particles and to be activated when the size decreases to a few nanometers. We investigated the size dependence of the three-dimensional nanostructure on the mean inner potential of gold catalysts supported on titanium oxide using electron holography and high-resolution electron microscopy (HREM). The contact angle of the gold particles on the titanium oxide tended to be over 90° for gold particles with a size of over 5 nm, and below 90° for a size of below 2 nm. This decreasing change in the contact angle (morphology) acts to increase the perimeter and hence the area of the interface between the gold and titanium oxide support, which is considered to be an active site for CO oxidation. The mean inner potential of the gold particles also changed as their size decreased. The value of the inner potential of gold, which is approximately 25 V in bulk state, rose to over 40 V when the size of the gold particles was less than 2 nm. This phenomenon indicates the existence of a charge transfer at the interface between gold and titanium oxide. The 3-D structure change and the inner potential change should be attributed to the specific electronic structure at the interface, owing to both the “nano size effect” and the “hetero-interface effect.”

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