Geoffrey B. Smith

Direct numerical simulations of free convection beneath an air–water interface at low Rayleigh numbers

Direct numerical simulations of a cooling air–water interface were employed to determine the structure of the temperature, velocity, and vorticity fields in the thin thermal boundary layer formed at the free surface. The simulations were performed at low to moderate Rayleigh numbers. In this flow, the turbulence is initiated by the Rayleigh instability at the interface and is maintained by buoyant production. Visualizations of the flow reveal that the temperature field at the interface is composed of large warm patches surrounded by cooler dense fluid which accumulates in thin bands. The cool fluid associated with the bands initially falls in sheets, but rapidly forms descending tubes and plumes. The turbulence statistics were scaled both with outer and inner variables. The latter scaling is based on the so-called surface strain model which is essentially consistent with Townsend’s inner scaling. It is found that the temperature statistics collapse well using inner variables. On the other hand, the vertical velocity scales well with inner variables within the thermal boundary layer, but at greater depths it becomes more appropriate to use outer scaling. The anisotropic nature of the velocity statistics in the core of the flow is ascribed to the relatively low Rayleigh numbers used in the simulations. An explanation for this anisotropy is offered based on a detailed examination of the turbulence kinetic energy balances.

The thermal structure of an air-water interface at low wind speeds

High-resolution infrared imagery of an air–water interface at wind speeds of 1 to 4 ms-1 wasobtained. Spectral analysis of the data reveals several important features of the thermal structureof the so-called cool skin. At wind speeds for which wind waves are not generated, the interfacialboundary layer appears to be composed of buoyant plumes that are stretched by the surfaceshear as they reach the interface. The plumes appear to form overlapping laminae with ahead–tail structure which we have termed fish-scales. At higher wind speeds, gravity wavesappearing on the surface give rise to distinct signatures in the infrared imagery. The wavesystem appears to modulate the surface temperature with sufficient strength so that the lengthand time scales of the waves are readily revealed in a k–ɷ spectrum. A surface drift speed canalso be easily inferred from the spectrum. A direct numerical simulation of the cool-skin of asheared water interface has also been performed. For Richardson numbers less than about 10-3, the simulations reveal a surface temperature pattern dominated by a streaky structure with acharacteristic spanwise length scale on the order of 100l+ where l+=v/u*. The simulationsconfirm that this streaky structure is formed as slow moving fluid originating from belowencounters a surface shear. The thermal structure of the surface appears virtually unchangedwhen buoyancy is turned off in the simulations and shear remains. This indicates that the fishscalepattern has universal features in the sense that it forms independently of the mechanismby which the turbulence is generated. The simulations are found to be in remarkable agreementwith the experimental results for which the same streaky, fish-scale structure was observed andthe same streak spacing was obtained.

The effect of a surfactant monolayer on the temperature field of a water surface undergoing evaporation

The surface temperature field of a body of water undergoing evaporation was measured using infrared imaging techniques, demonstrating for the first time the effect of surfactant monolayers on the spatial structure of this field. Measurements were obtained from a water surface which was covered with a monolayer of the surfactant oleyl alcohol, and also from a surface which was free of surfactants. The oleyl alcohol and surfactant-free experiments were compared at equivalent heat fluxes. The presence of surfactants increased the characteristic length scale of the surface temperature field. This conclusion is supported by both visual observation of the infrared imagery and spatial Fourier transforms of the temperature fields. The presence of the surfactant monolayer had a small effect on the root mean square of the temperature field but significantly affected the skewness, creating a more positively skewed probability density function for the surfactant covered field. These observations were found to hold when comparison between the clean and surfactant case was made at heat fluxes varying by a factor of ∼11.

Surfactant effects on passive scalar transport in a fully developed turbulent flow

Direct numerical simulations of fully developed turbulence in an open channel were performed. Effects of surfactants on heat transfer and the underlying turbulent structures were investigated. As surface elasticity is increased turbulent fluctuations are damped and the mean surface temperature is decreased. A surface strain model is introduced to explain this behavior in a heuristic manner. A nondimensional parameter representing the ratio of surface elastic forces to local inertial forces is introduced. It is concluded that for values of the parameter of order one, surfactants have strong effects on surface turbulence, whereas an effectively clean surface can be obtained for parameter values less than O(10−3).

Near-surface turbulence for evaporative convection at an air/water interface

Turbulence measurements are reported for the flow beneath an air/water interface undergoing evaporative convection. Measurements were obtained using a two component laser Doppler velocimeter system. Two hydrodynamic boundary conditions were considered for the free surface: a shear free surface, which is the case when surfactants are absent, and a constant elasticity surface, created by depositing a monolayer of oleyl alcohol. The shear free boundary condition case results in significantly higher levels of near surface turbulence than the constant elasticity case. This difference between the two cases decreases with distance from the free surface. Profiles of the turbulent fluctuations were obtained for the horizontal and vertical velocity components and are compared with the somewhat analogous case of a heated solid wall.

An experimental investigation of the surface temperature field during evaporative convection

Measurements of the surface temperature field are presented for a water surface undergoing evaporation. These temperature fields were measured using an infrared camera for a range of heat fluxes q″=30–500 W/m2. Experiments were conducted for water surfaces with and without a surfactant monolayer. A statistical analysis of the data is presented which shows the effect of heat flux and surfactants on the root mean square and skewness of the field. The data reveals a linear increase in the rms with increasing heat flux, which is similar for clean and surfactant conditions. In contrast, the skewness is markedly different for the clean and surfactant-covered cases. For clean surface conditions, the skewness attains large, negative values, becoming increasingly negative as q″ increases. When the surface is covered with a surfactant monolayer, however, the skewness exhibits small, negative values which approach zero as the heat flux increases. This behavior is reflected in the pdf which is clearly asymmetric in t...

Infrared imaging of the surface temperature field of water during film spreading

Deposition of a spontaneously-spreading film on a clean water surface creates a front which propagates radially outward from the point of deposition. This rapidly spreading film was used as a tool to quickly change the boundary condition of a water surface from one which is shear-free, to a boundary condition which supports shear. Infrared images of a water surface experiencing evaporative convection were recorded as this film spread. These images were converted to surface temperature fields. The amount of turbulent structure present in these fields changes dramatically across the front. Ahead of the front, significant variations at large and small spatial scales are evident, while behind the front the small scale structures are eliminated. The time scale at which this damping occurs is short and has not been reported on heretofore. In addition to being relevant to free surface turbulence, these results demonstrate the utility of infrared imaging in the study of spreading films.

Observations of the Structure of the Surface Temperature Field at an Air-Water Interface for Stable and Unstable Cases

The thermal structure of an air-water interface is investigated by examining thermal imagery obtained from a high resolution infrared (IR) sensor. The experiments were performed at the ASIST facility at the University of Miami for wind speeds ranging from approximately 2 ms−1 to 10 ms−1 and for flux based Richardson numbers ranging from about 10−2 to 10−5. Two cases were examined: (1) the so-called cool-skin case where the water surface was significantly cooler than the bulk water temperature and (2) the warm-skin case where the water surface was warmer than the bulk. In the cool-skin case, the low wind speed results reveal a cellular structure reminiscent of earlier results in which the lateral length scale of the cells (or fish-scales) varies as the inverse of the friction velocity. The imagery clearly reveals the progression from non-breaking gravity waves, to a system of omnidirectional breaking which seems to create a nearly isotropic surface temperature field. Though no wind waves were present at low wind speeds, the thermal imagery reveals the existence of persistent, highly coherent, Langmuir-like cell structures which were marked by surface convergent zones in which ambient surfactant may have accumulated. Imagery obtained for the case in which the water-side thermal boundary layer is stable constitutes a novel aspect of this work. In this warm-skin case, the cellular (fish-scale) structure appears as it does in the unstable case, strongly suggesting that these small scale features are due to shear instabilities in the surface layer. In addition, they are more clearly revealed as the natural convective instability of the thermal boundary layer is suppressed. This appears to reduce the appearance of the smallest scales of surface turbulence.

Drifter Observations of Submesoscale Flow Kinematics in the Coastal Ocean

Fronts and eddies identified with aerial guidance are seeded with drifters to quantify submesoscale flow kinematics. The Lagrangian observations show mean divergence and vorticity values that can exceed five times the Coriolis frequency. Values are the largest observed in the field to date and represent an extreme departure from geostrophic dynamics. The study also quantifies errors and biases associated with Lagrangian observations of the underlying velocity strain tensor. The greatest error comes from under-sampling, even with a large number of drifters. A significant bias comes from inhomogeneous sampling of convergent regions that accumulate drifters within a few hours of deployment. The study demonstrates a Lagrangian sampling paradigm for targeted submesoscale structures over a broad range of scales, and presents flow kinematic values associated with vertical velocities O(10) m h-1 that can have profound implications on ocean biogeochemistry.

Whitecap lifetime stages from infrared imagery with implications for microwave radiometric measurements of whitecap fraction

Quantifying active and residual whitecap fractions separately can improve parameterizations of air-sea fluxes associated with breaking waves. We use data from a multi-instrumental field campaign on FLoating Instrument Platform (FLIP) to simultaneously capture the signatures of active and residual whitecaps at visible, infrared (IR) and microwave wavelengths using, respectively, video camera, mid-IR camera, and a radiometer at 10 GHz. We present results from processing and analyzing IR images and correlating this information with radiometric time series of brightness temperature at horizontal and vertical polarizations TBH and TBV. The results provide evidence that breaking crests and decaying foam appear in mid-IR as bright and dark pixels clearly distinguishing active from residual whitecaps. We quantify the durations of whitecap lifetime stages from the IR images and identify their corresponding signatures in TB time series. Results show that TBH and TBV vary in phase during the active and in anti-phase during the residual whitecap stages. A methodology to distinguish active and residual whitecaps in radiometric time series without a priori IR information has been developed and verified with corresponding IR and video images. The method uses the degree of polarization P (the ratio between the sum and difference of TBV and TBH) to capture whitecaps as prominent spikes. The maximum and zero-crossing of the first derivative of P serve to identify the presence of active whitecaps, while the minimum of dP marks the transition from active to residual whitecap stage. The findings have implications for radiometric measurements of active and total whitecap fractions. This article is protected by copyright. All rights reserved.