Bubble-Turbulence Dynamics and Dissipation Beneath Laboratory Breaking Waves

Turbulence within the upper ocean mixed layer plays a key role in various physical, biological, and chemical processes. Between September and November 2011, a dataset of 570 vertical profiles of the turbulent kinetic energy (TKE) dissipation rate, as well as conventional hydrological and meteorological data, were collected in the upper layer of the tropical Indian Ocean. These data were used to statistically analyze the vertical distribution of the TKE dissipation rate in the mixed layer. The arithmetic-mean method made the statistical TKE dissipation rate profile more scattered than the median and geometric-mean methods. The statistical TKE dissipation rate were respectively scaled by the surface buoyancy flux and the TKE dissipation rate at the mixed-layer base. It was found that the TKE dissipation rate scaled by that at the mixed-layer base exhibited better similarity characteristics than that scaled by the surface buoyancy flux, whether the stability parameter D/|LMO| was greater or less than 10, indicating that the TKE dissipation rate at the mixed-layer base is a better characteristic scaling parameter for reflecting the intrinsic structure of the TKE dissipation rate in the mixed layer, where D and LMO are respectively the mixed-layer thickness and the Monin-Obukhov length scale. The parameterization of the TKE dissipation rate at the mixed-layer base on the shear-driven dissipation rate and the surface buoyancy flux was further explored. It was found that the TKE dissipation rate at the mixed-layer base could be well fitted by a linear combination of three terms: the wind-shear-driven dissipation rate, the surface buoyancy flux, and a simple nonlinear coupling term of these two .

Estimation of the turbulent kinetic energy dissipation rate from 2D-PIV measurements in a vessel stirred by an axial Mixel TTP impeller

Near‐surface turbulent kinetic energy dissipation rates are altered by the presence of sea ice in the marginal ice zone, with significant implications for exchanges at the air‐ice‐ocean interface. Observations spanning a range of conditions are used to parameterize dissipation rates in marginal ice zones with relatively thin, newly formed ice, and two regimes are identified. In both regimes, the turbulent dissipation rates are matched to the turbulent input rate, which is formulated as the surface stress acting on roughness elements moving at an effective transfer velocity. In marginal ice zones with waves, the short waves are the roughness elements, and the phase speed of these waves is the effective transfer velocity. The wave amplitudes are attenuated by the ice, and thus, the size of the roughness elements is reduced; this is parameterized as a reduction in the effective transfer velocity. When waves are sufficiently small, the ice floes are the roughness elements, and the relative velocity between the sea ice and the ocean is the effective transfer velocity. A scaling is introduced to determine the appropriate transfer velocity in a marginal ice zone based on wave height, ice thickness and concentration, and ice‐ocean shear. The results suggest that turbulence underneath new sea ice is mostly related to the wind forcing and that marginal ice zones generally have less turbulence than the open ocean under similar wind forcing.

To calculate a turbulent kinetic energy dissipation rate from the microstructure vertical shear of the horizontal velocity via a spectral analysis, shear spectra need first to be cleaned from vibrations of the moving vehicle. Unambiguously, this study shows that the spectral cleaning must be applied all over the frequency range and not only at frequencies larger than 10 Hz, as a recent study suggested. For a Vertical Microstructure Profiler (VMP-6000), not correcting for vehicle vibrations below 10 Hz leads to overestimated dissipation rates from 50% to 700% for usual downcast velocities and for weak dissipation rates (ε < 1 × 10−9 W kg−1). Vibrations concern all vehicles, but the exact vibrational frequency signature depends on the vehicle shape and its downcast velocity. In any case, a spectral cleaning over the whole frequency range is strongly advised. This study also reports on a systematic low bias of inferred dissipation rates induced by the spectral cleaning when too few degrees of freedom are available for the cleaning, which is usually the default of the standard processing. Whatever the dissipation rate level, not correcting for the bias leads to underestimated dissipation rates by a factor 1.4–2.7 (with usual parameters), the exact amplitude of the bias depending on the number of degrees of freedom and on the number of independent accelerometer axes used for the cleaning. It is strongly advised that such a bias be taken into account to recompute dissipation rates of past datasets and for future observations.

Abstract. Microstructure measurements of shear and temperature can be used to calculate ocean turbulent dissipation rates and diffusivities. Here microstructure observations are taken along an transect in the North Atlantic, that includes observations of very weak deep ocean turbulence. In this paper we show the necessity of using the thermistor probes, instead of the more common shear probes, to calculate dissipation rates when they are smaller than 1×10-10 W kg−1. Profiles of combined dissipation rates from the shear and thermistor probes are then compared to the finescale strain parameterization and Thorpe sorting method. Based on this comparison, recommendations and restrictions are suggested for applying both parameterizations in a weakly turbulent environment. The results indicate that temperature-based strain provides improved estimates of dissipation rates in the deep ocean where density gradients are small, while density-based strain provides better results otherwise. We find that Thorpe based estimates are very accurate when pre-existing knowledge of the turbulent kinetic energy dissipation rate ε is used. When this knowledge is not available, using climatological mean estimates of ε can allow for more detailed estimates of dissipation by applying the Thorpe resorting method. Finally, we employ the triple decomposition framework to get more insights in the relative roles of dianeutral and isoneutral mixing processes, and use this to calculate the dianeutral and isoneutral diffusivities. It turns out that the triple decomposition is generally not a good predictor of the isoneutral diffusivity. Overall, this paper has assessed the potential of direct observations and parameterizations of dissipation and showed that dissipation rates can be estimated quite well within a factor 5 between different methods, but it becomes difficult to achieve higher accuracy.

Direct numerical simulations (DNS) of a shear layer with salt-fingering-favorable stratification have been performed for different Richardson numbers Ri and density ratios Rρ. In the absence of shear (Ri = ∞), the primary instability is square planform salt fingering, alternating cells of rising and sinking fluid. In the presence of shear, salt fingering takes the form of salt sheets, planar regions of rising and sinking fluid, aligned parallel to the sheared flow. After the onset of secondary instability, the flow becomes turbulent. The continued influence of the primary instability distorts the late-stage structure and hence biases isotropic estimates of the turbulent kinetic energy dissipation rate ε. In contrast, thermal and saline gradients evolve to become more isotropic than velocity gradients at their dissipation scales. Thus, the standard observational methodology of estimating the turbulent kinetic energy dissipation rate ε from vertical profiles of microscale gradients and assuming isotropy can underestimate its true value by a factor of 2–3, whereas estimates of thermal and saline dissipation rates using this approach are relatively accurate. Likewise, estimates of Γ from vertical profiles overestimate the true Γ by roughly a factor of 2. Salt sheets are ineffective at transporting momentum. Thermal and saline effective diffusivities decrease with decreasing Ri, despite the added energy source provided by background shear. After the transition to turbulence, the thermal to saline flux ratio and the effective Schmidt number remain close to the values predicted by linear theory.

The present paper reports PIV (Particle Image Velocimetry) measurements of turbulent velocity fluctuations statistics in development region of an axisymmetric free jet (Re = 28 000). To minimize measurement uncertainty, adaptive calibration, image processing and data post-processing algorithms were utilized. On the basis of theoretical analysis and direct measurements, the paper discusses effect of PIV spatial resolution on measured statistical characteristics of turbulent fluctuations. Underestimation of the second-order moments of velocity derivatives and of the turbulent kinetic energy dissipation rate due to a finite size of PIV interrogation area and finite thickness of laser sheet was analyzed from model spectra of turbulent velocity fluctuations. The results are in a good agreement with the measured experimental data. The paper also describes performance of possible ways to account for unresolved small-scale velocity fluctuations in PIV measurements of the dissipation rate. In particular, a turbulent viscosity model can be efficiently used to account for the unresolved pulsations in a free turbulent flow

It has been recognized that modulated wave groups trigger wave breaking and generate energy dissipation events on the ocean surface. Quantitative examination of wave-breaking events and associated turbulent kinetic energy (TKE) dissipation rates within a modulated wave group in the open ocean is not a trivial task. To address this challenging topic, a set of laboratory experiments was carried out in an outdoor facility, the Oil and Hazardous Material Simulated Environment Test Tank (203 m long, 20 m wide, 3.5 m deep). TKE dissipation rates at multiple depths were estimated directly while moving the sensor platform at a speed of about 0.53 m s−1 toward incoming wave groups generated by the wave maker. The largest TKE dissipation rates and significant whitecaps were found at or near the center of wave groups where steepening waves approached the geometric limit of waves. The TKE dissipation rate was O(10−2) W kg−1 during wave breaking, which is two to three orders of magnitude larger than before and after wave breaking. The enhanced TKE dissipation rate was limited to a layer of half the wave height in depth. Observations indicate that the impact of wave breaking was not significant at depths deeper than one wave height from the surface. The TKE dissipation rate of breaking waves within wave groups can be parameterized by local wave phase speed with a proportionality breaking strength coefficient dependent on local steepness. The characterization of energy dissipation in wave groups from local wave properties will enable a better determination of near-surface TKE dissipation of breaking waves.

Turbulence Observations in the Northern Bight of Monterey Bay from a Small AUV

Vertical profiles of the mean horizontal currents U(z), the vertical momentum fluxes τ(z), and the turbulence kinetic-energy dissipation rates (TKE dissipation rate) e(z) in the upper water layer (UWL) are considered and their joint analysis is carried out. For this purpose, data from laboratory measurements performed in the wind-wave channel of the Institute of Applied Physics, Russian Academy of Sciences (RAS) [1, 2], are used. They correspond to conditions of strong wind and breaking wind waves. The profiles of the currents and momentum fluxes are estimated for x and z components of the velocity at five horizons in the UWL at four various wind values. The empirical estimates of e(z) obtained from the same data in the previous work [3] are used in the joint analysis. It is established that (a) velocity of currents U(z) increases noticeably when compared to the values of U(z) in the absence of waves, (b) the momentum flux in the water τw(z) decreases noticeably when compared to that in the air τa(z), and (c) τw(z) significantly attenuates with depth according to ratio τw(z) ~ 1/z2. The mentioned anomalies of profiles U(z) and τ(z) in the UWL are analyzed together with the previously determined pattern of TKE dissipation-rate falloff with depth according to ratio e(z) ~ 1/z2 in order to search for an interpretation of the results.

Utilizing an optically index-matched facility and high-resolution particle image velocimetry measurements, this paper examines flow structure and turbulence in a rough-wall channel flow forReτin the 3520–5360 range. The scales of pyramidal roughness elements satisfy the ‘well-characterized’ flow conditions, withh/k≈ 50 andk+= 60 ~ 100, wherehis half height of the channel andkis the roughness height. The near-wall turbulence measurements are sensitive to spatial resolution, and vary with Reynolds number. Spatial variations in the mean flow, Reynolds stresses, as well as the turbulent kinetic energy (TKE) production and dissipation rates are confined toy< 2k. All the Reynolds stress components have local maxima at slightly higher elevations, but the streamwise-normal component increases rapidly aty<k, peaking at the top of the pyramids. The TKE production and dissipation rates along with turbulence transport also peak near the wall. The spatial energy and shear spectra show an increasing contribution of large-scale motions and a diminishing role of small motions with increasing distance from the wall. As the spectra steepen at low wavenumbers, they flatten and develop bumps in wavenumbers corresponding tok− 3k, which fall in the dissipation range. Instantaneous realizations show that roughness-scale eddies are generated near the wall, and lifted up rapidly by large-scale structures that populate the outer layer. A linear stochastic estimation-based analysis shows that the latter share common features with hairpin packets. This process floods the outer layer with roughness-scale eddies, in addition to those generated by the energy-cascading process. Consequently, although the imprints of roughness diminish in the outer-layer Reynolds stresses, consistent with the wall similarity hypothesis, the small-scale turbulence contains a clear roughness signature across the entire channel.

Historical CTD dataset and associated processed dissipation rate using an improved Thorpe method in the Indonesian seas

Biomimicry is introduced as an approach to develop an ecologically-informed design solution. The study proposes a regenerative design solution through hydrodynamic effects of the complex mangrove roots system that is applied to a biomimetic structural column design, with consideration on interactions between root porosity, water flows, and sediment transportation. Sangga Besar River, Perak is chosen as the key parameters of the flow. The roots are modelled with a circular array of cylinders (patch) with different porosities and varying spacing ratios. 15 patches are derived from 3 main sets where each set consisted of five patches with varying porosities. The simulation of each case to understand the impacts of different patch porosity on the flow structure is carried out using Ansys Fluent Student Version 2021. The study found that turbulence is impacted, but not directly proportional to the patch porosity where there could be an optimal porosity of the patch that generates maximum yield of turbulent kinetic energy dissipation rate with minimum sediment erosion for the onset of sediment deposition. The patch with φ=40% has the highest turbulence dissipation rate which relatively coherent with the range of porosity that would likely be encountered in natural mangrove ecosystem.

An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Karman constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate. An average value of the von Karman constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Karman constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results. The flux-prof...

In a system with the 3-way regulating valve, low energy consumption high efficiency based on fluid dynamics CFD software, for the 3-way regulating valve of the 3 d simulation for internal flow property research, obtained the pressure within the valve flow turbulence kinetic energy and turbulence dissipation rate etc. The results show that the flow field information with the increase of the opening, the seat area on the turbulence kinetic energy and turbulence dissipation rate increases sharply, change law instead of the seat position. The existence of transition cone can greatly reduce the turbulent kinetic energy and the turbulent dissipation rate of the upper seat in the wide opening range and the valve in the small opening range. Long-term avoidance of the valve working in the minimum-opening condition will be beneficial to the protection of the valve.