Symmetric Instability, Inertial Oscillations, and Turbulence at the Gulf Stream Front

AbstractThe passage of a winter storm over the Gulf Stream observed with a Lagrangian float and hydrographic and velocity surveys provided a unique opportunity to study how the interaction of inertial oscillations, the front, and symmetric instability (SI) shapes the stratification, shear, and turbulence in the upper ocean under unsteady forcing. During the storm, the rapid rise and rotation of the winds excited inertial motions. Acting on the front, these sheared motions modulate the stratification in the surface boundary layer. At the same time, cooling and downfront winds generated a symmetrically unstable flow. The observed turbulent kinetic energy dissipation exceeded what could be attributed to atmospheric forcing, implying SI drew energy from the front. The peak excess dissipation, which occurred just prior to a minimum in stratification, surpassed that predicted for steady SI turbulence, suggesting the importance of unsteady dynamics. The measurements are interpreted using a large-eddy simulation (LES) and a stability analysis configured with parameters taken from the observations. The stability analysis illustrates how SI more efficiently extracts energy from a front via shear production during periods when inertial motions reduce stratification. Diagnostics of the energetics of SI from the LES highlight the temporal variability in shear production but also demonstrate that the time-averaged energy balance is consistent with a theoretical scaling that has previously been tested only for steady forcing. As the storm passed and the winds and cooling subsided, the boundary layer restratified and the thermal wind balance was reestablished in a manner reminiscent of geostrophic adjustment.

Download Full-text

Insights into the mixing efficiency of submesoscale Centrifugal-Symmetric instabilities

10.31223/x5pg8g ◽

2021 ◽

Author(s):

Tomas Chor ◽

Jacob Wenegrat ◽

John Taylor

Keyword(s):

Boundary Layer ◽

Vertical Shear ◽

Mixing Efficiency ◽

Surface Boundary ◽

Stratified Turbulence ◽

Horizontal Shear ◽

Surface Boundary Layer ◽

Turbulent Dissipation ◽

Balanced Flow ◽

Shear Production

Submesoscale processes provide a pathway for energy to transfer from the balanced circulation to turbulent dissipation. One class of submesoscale phenomena that has been shown to be particularly effective at removing energy from the balanced flow are centrifugal-symmetric instabilities (CSIs), which grow via geostrophic shear production. CSIs have been observed to generate significant mixing in both the surface boundary layer and bottom boundary layer flows along bathymetry, where they have been implicated in the mixing and watermass transformation of Antarctic Bottom Water. However, the mixing efficiency (i.e. the fraction of the energy extracted from the flow used to irreversibly mix the fluid) of these instabilities remains uncertain, making estimates of mixing and energy dissipation due to CSI difficult.In this work we use large-eddy simulations to investigate the mixing efficiency of CSIs in the submesoscale range. We find that centrifugally-dominated CSIs (i.e. CSI mostly driven by horizontal shear production) tend to have a higher mixing efficiency than symmetrically-dominated ones (i.e. driven by vertical shear production). The mixing efficiency associated with CSIs can therefore alternately be significantly higher or significantly lower than the canonical value used by most studies. These results can be understood in light of recent work on stratified turbulence, whereby CSIs control the background state of the flow in which smaller-scale secondary overturning instabilities develop, thus actively modifying the characteristics of mixing by Kelvin-Helmholtz instabilities. Our results also suggest that it may be possible to predict the mixing efficiency with more readily measurable parameters (namely the Richardson and Rossby numbers), which would allow for parameterization of this effect.

Download Full-text

Large eddy simulation of turbulence in ocean surface boundary layer at Zhangzi Island offshore

Acta Oceanologica Sinica ◽

10.1007/s13131-013-0326-0 ◽

2013 ◽

Vol 32 (7) ◽

pp. 8-13 ◽

Cited By ~ 1

Author(s):

Shuang Li ◽

Jinbao Song ◽

Hailun He ◽

Yansong Huang

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Ocean Surface ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Eddy Simulation ◽

Large Eddy ◽

Zhangzi Island

Download Full-text

The Role of Whitecapping in Thickening the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0234.1 ◽

2015 ◽

Vol 45 (8) ◽

pp. 2006-2024 ◽

Cited By ~ 5

Author(s):

Gregory P. Gerbi ◽

Samuel E. Kastner ◽

Genevieve Brett

Keyword(s):

Boundary Layer ◽

Initial Conditions ◽

Ocean Surface ◽

Breaking Waves ◽

Length Scale ◽

Surface Boundary ◽

Transfer Velocity ◽

Surface Boundary Layer ◽

Turbulent Length Scale ◽

Shear Production

AbstractThe effects of wind-driven whitecapping on the evolution of the ocean surface boundary layer are examined using an idealized one-dimensional Reynolds-averaged Navier–Stokes numerical model. Whitecapping is parameterized as a flux of turbulent kinetic energy through the sea surface and through an adjustment of the turbulent length scale. Simulations begin with a two-layer configuration and use a wind that ramps to a steady stress. This study finds that the boundary layer begins to thicken sooner in simulations with whitecapping than without because whitecapping introduces energy to the base of the boundary layer sooner than shear production does. Even in the presence of whitecapping, shear production becomes important for several hours, but then inertial oscillations cause shear production and whitecapping to alternate as the dominant energy sources for mixing. Details of these results are sensitive to initial and forcing conditions, particularly to the turbulent length scale imposed by breaking waves and the transfer velocity of energy from waves to turbulence. After 1–2 days of steady wind, the boundary layer in whitecapping simulations has thickened more than the boundary layer in simulations without whitecapping by about 10%–50%, depending on the forcing and initial conditions.

Download Full-text

Lagrangian Investigation of Wave-Driven Turbulence in the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0081.1 ◽

2019 ◽

Vol 49 (2) ◽

pp. 409-429 ◽

Cited By ~ 4

Author(s):

Tobias Kukulka ◽

Fabrice Veron

Keyword(s):

Boundary Layer ◽

Turbulent Transport ◽

Ocean Surface ◽

Inertial Subrange ◽

Breaking Wave ◽

Surface Boundary ◽

Lagrangian Analysis ◽

Surface Boundary Layer ◽

Near Surface ◽

Eddy Simulation

AbstractTurbulent processes in the ocean surface boundary layer (OSBL) play a key role in weather and climate systems. This study explores a Lagrangian analysis of wave-driven OSBL turbulence, based on a large-eddy simulation (LES) model coupled to a Lagrangian stochastic model (LSM). Langmuir turbulence (LT) is captured by Craik–Leibovich wave forcing that generates LT through the Craik–Leibovich type 2 (CL2) mechanism. Breaking wave (BW) effects are modeled by a surface turbulent kinetic energy flux that is constrained by wind energy input to surface waves. Unresolved LES subgrid-scale (SGS) motions are simulated with the LSM to be energetically consistent with the SGS model of the LES. With LT, Lagrangian autocorrelations of velocities reveal three distinct turbulent time scales: an integral, a dispersive mixing, and a coherent structure time. Coherent structures due to LT result in relatively narrow peaks of Lagrangian frequency velocity spectra. With and without waves, the high-frequency spectral tail is consistent with expectations for the inertial subrange, but BWs substantially increase spectral levels at high frequencies. Consistently, over short times, particle-pair dispersion results agree with the Richardson–Obukhov law, and near-surface dispersion is significantly enhanced because of BWs. Over longer times, our dispersion results are consistent with Taylor dispersion. In this case, turbulent diffusivities are substantially larger with LT in the crosswind direction, but reduced in the along-wind direction because of enhanced turbulent transport by LT that reduces mean Eulerian shear. Our results indicate that the Lagrangian analysis framework is effective and physically intuitive to characterize OSBL turbulence.

Download Full-text

Large Eddy Simulation of Tandem Blade Stator Cascades

Volume 2A: Turbomachinery ◽

10.1115/gt2017-64286 ◽

2017 ◽

Author(s):

Pratik Mitra ◽

Jahnavi Kantharaju ◽

Rohan Rayan ◽

Joseph Mathew

Keyword(s):

Boundary Layer ◽

Leading Edge ◽

Immersed Boundary ◽

Surface Boundary ◽

Suction Surface ◽

Surface Boundary Layer ◽

Substantial Impact ◽

Eddy Simulation ◽

Large Eddy ◽

Rapid Transition

Large eddy simulations of tandem blade compressor cascades have been performed with an explicit filtering method. A low speed case was simulated using the public domain code Incompact3d which solves incompressible flow with an immersed boundary method for embedded solid bodies, obviating the effort expended on preparing good quality meshes around blading. The LES successfully captures transition on the front blade and yields a significantly different loading compared with RANS solutions obtained before. The less substantial impact on the rear blade is traced to rapid transition forced by the turbulent wake of the front blade. LES with a refined grid was found to shorten the transition width due to the crucial role of small scales during transition. A complementary study with an in-house compressible LES solver was conducted for a transonic tandem cascade at the inlet Mach number of 0.89. Flow expands around the leading edge of the front blade and is terminated by a shock which interacts with the suction surface boundary layer. The beneficial effect of tandem blading was found to be achieved by limiting this separation. The shock-induced separation also marks a rapid transition of the suction surface boundary layer that is readily captured in the LES, showing pre-transitional streaks, but could prove difficult even for current transition-sensitive RANS.

Download Full-text

Ocean Surface Boundary Layer Response to Abruptly Turning Winds

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0198.1 ◽

2021 ◽

Author(s):

Xingchi Wang ◽

Tobias Kukulka

Keyword(s):

Boundary Layer ◽

Wave Model ◽

Ocean Surface ◽

Stokes Drift ◽

Navier Stokes ◽

Surface Boundary ◽

Navier Stokes Equation ◽

Surface Boundary Layer ◽

Eddy Simulation ◽

Budget Analysis

AbstractTurbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Based on a Large Eddy Simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik-Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments, whose winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases due to reduced TKE production. Stage 2 is characterized by TKE increasing with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values due to inertial resonance and non-equilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts due to slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian TKE production exceeding Stokes TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

Download Full-text

Parameterization of Wave-Induced Mixing Using the Large Eddy Simulation (LES) (I)

Atmosphere ◽

10.3390/atmos11020207 ◽

2020 ◽

Vol 11 (2) ◽

pp. 207 ◽

Cited By ~ 1

Author(s):

Haili Wang ◽

Changming Dong ◽

Yongzeng Yang ◽

Xiaoqian Gao

Keyword(s):

Boundary Layer ◽

Large Eddy Simulation ◽

Ocean Surface ◽

Breaking Waves ◽

Global Climate Models ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Eddy Simulation ◽

Large Eddy ◽

Wave Induced

Turbulent motions in the thin ocean surface boundary layer control exchanges of momentum, heat and trace gases between the atmosphere and ocean. However, present parametric equations of turbulent motions that are applied to global climate models result in systematic or substantial errors in the ocean surface boundary layer. Significant mixing caused by surface wave processes is missed in most parametric equations. A Large Eddy Simulation model is applied to investigate the wave-induced mixed layer structure. In the wave-averaged equations, wave effects are calculated as Stokes forces and breaking waves. To examine the effects of wave parameters on mixing, a series of wave conditions with varying wavelengths and heights are used to drive the model, resulting in a variety of Langmuir turbulence and wave breaking outcomes. These experiments suggest that wave-induced mixing is more sensitive to wave heights than to the wavelength. A series of numerical experiments with different wind intensities-induced Stokes drifts are also conducted to investigate wave-induced mixing. As the wind speed increases, the influence depth of Langmuir circulation deepens. Additionally, it is observed that breaking waves could destroy Langmuir cells mainly at the sea surface, rather than at deeper layers.

Download Full-text

Prediction of Atmospheric Turbulence Characteristics for Surface Boundary Layer using Empirical Spectral Methods

Revista Brasileira de Meteorologia ◽

10.1590/0102-77863540083 ◽

2020 ◽

Author(s):

Yagya Dutta Dwivedi ◽

Vasishta Bhargava Nukala ◽

Satya Prasad Maddula ◽

Kiran Nair

Keyword(s):

Boundary Layer ◽

Time Series ◽

Surface Roughness ◽

Wind Speed ◽

Atmospheric Turbulence ◽

Surface Boundary ◽

Low Frequencies ◽

Surface Boundary Layer ◽

Power Spectral ◽

Mean Wind Speed

Abstract Atmospheric turbulence is an unsteady phenomenon found in nature and plays significance role in predicting natural events and life prediction of structures. In this work, turbulence in surface boundary layer has been studied through empirical methods. Computer simulation of Von Karman, Kaimal methods were evaluated for different surface roughness and for low (1%), medium (10%) and high (50%) turbulence intensities. Instantaneous values of one minute time series for longitudinal turbulent wind at mean wind speed of 12 m/s using both spectra showed strong correlation in validation trends. Influence of integral length scales on turbulence kinetic energy production at different heights is illustrated. Time series for mean wind speed of 12 m/s with surface roughness value of 0.05 m have shown that variance for longitudinal, lateral and vertical velocity components were different and found to be anisotropic. Wind speed power spectral density from Davenport and Simiu profiles have also been calculated at surface roughness of 0.05 m and compared with k−1 and k−3 slopes for Kolmogorov k−5/3 law in inertial sub-range and k−7 in viscous dissipation range. At high frequencies, logarithmic slope of Kolmogorov −5/3rd law agreed well with Davenport, Harris, Simiu and Solari spectra than at low frequencies.

Download Full-text