William L. Peirson

Tangential stress beneath wind-driven air–water interfaces

The detailed structure of the aqueous surface sublayer flow immediately adjacent to the wind-driven air-water interface is investigated in a laboratory wind-wave flume using particle image velocimetry (PIV) techniques. The goal is to investigate quantitatively the character of the flow in this crucial, very thin region which is often disrupted by microscale breaking events. In this study, we also examine critically the conclusions of Okuda, Kawai & Toba (1977), who argued that for very short, strongly forced wind-wave conditions, shear stress is the dominant mechanism for transmitting the atmospheric wind stress into the water motion - waves and surface drift currents. In strong contrast, other authors have more recently observed very substantial normal stress contributions on the air side. The availability of PIV and associated image technology now permits a timely re-examination of the results of Okuda et al., which have been influential in shaping present perceptions of the physics of this dynamically important region. The PIV technique used in the present study overcomes many of the inherent shortcomings of the hydrogen bubble measurements, and allows reliable determination of the fluid velocity and shear within 200 μm of the instantaneous wind-driven air-water interface. The results obtained in this study are not in accord with the conclusions of Okuda et al. that the tangential stress component dominates the wind stress. It is found that prior to the formation of wind waves, the tangential stress contributes the entire wind stress, as expected. With increasing distance downwind, the mean tangential stress level decreases marginally, but as the wave field develops, the total wind stress increases significantly. Thus, the wave form drag, represented by the difference between the total wind stress and the mean tangential stress, also increases systematically with wave development and provides the major proportion of the wind stress once the waves have developed beyond their early growth stage. This scenario reconciles the question of relative importance of normal and tangential stresses at an air-water interface. Finally, consideration is given to the extrapolation of these detailed laboratory results to the field, where the present findings suggest that the sea surface is unlikely to become fully aerodynamically rough, at least for moderate to strong winds.

Wave breaking onset and strength for two-dimensional deep-water wave groups

The numerical study of J. Song & M. L. Banner (J. Phys. Oceanogr. vol. 32, 2002, p. 254) proposed a generic threshold parameter for predicting the onset of breaking within two-dimensional groups of deep-water gravity waves. Their parameter provides a non-dimensional measure of the wave energy convergence rate and geometrical steepening at the maximum of an evolving nonlinear wave group. They also suggested that this parameter might control the strength of breaking events. The present paper presents the results of a detailed laboratory observational study aimed at validating their proposals. For the breaking onset phase of this study, wave potential energy was measured at successive local envelope maxima of nonlinear deep-water wave groups propagating along a laboratory wave tank. These local maxima correspond alternately to wave group geometries with the group maximum occurring at an extreme carrier wave crest elevation, followed by an extreme carrier wave trough depression. As the nonlinearity increases, these crest and trough maxima can have markedly different local energy densities owing to the strong crest–trough asymmetry. The local total energy density was reconstituted from the potential energy measurements, and made dimensionless using the square of the local (carrier wave) wavenumber. A mean non-dimensional growth rate reflecting the rate of focusing of wave energy at the envelope maximum was obtained by smoothing the local fluctuations. For the cases of idealized nonlinear wave groups investigated, the observations confirmed the evolutionary trends of the modelling results of Song & Banner (2002) with regard to predicting breaking onset. The measurements confirmed the proposed common breaking threshold growth rate of 0.0014 ± 0.0001, as well as the predicted key evolution times: the time taken to reach the energy maximum for recurrence cases; and the time to reach the breaking threshold and then breaking onset, for breaking cases. After the initiation and subsequent cessation of breaking, the measured wave packet mean energy losses and loss rates associated with breaking produced an unexpected finding: the post-breaking mean wave energy did not decrease to the mean energy level corresponding to maximum recurrence, but remained significantly higher. Therefore, pre-breaking absolute wave energy or mean steepness do not appear to be the most fundamental determinants of post-breaking wave packet energy density. However, the dependence of the fractional breaking energy loss of wave packets on the parametric growth rate just before breaking onset proposed by Song & Banner (2002) was found to provide a plausible collapse to our laboratory data sets, within the experimental uncertainties. Further, when the results for the energy loss rate per unit width of breaking front were expressed in terms of a breaker strength parameter b multiplying the fifth power of the wave speed, it is found that b was also strongly

Investigation of the physical scaling of sea spray spume droplet production

[1] In this paper we report on a laboratory study, the Spray Production and Dynamics Experiment (SPANDEX), conducted at the University of New South Wales Water Research Laboratory in Australia. The goals of SPANDEX were to illuminate physical aspects of spume droplet production and dispersion; verify theoretical simplifications used to estimate the source function from ambient droplet concentration measurements; and examine the relationship between the implied source strength and forcing parameters such as wind speed, surface turbulent stress, and wave properties. Observations of droplet profiles give reasonable confirmation of the basic power law profile relationship that is commonly used to relate droplet concentrations to the surface source strength. This essentially confirms that, even in a wind tunnel, there is a near balance between droplet production and removal by gravitational settling. The observations also indicate considerable droplet mass may be present for sizes larger than 1.5 mm diameter. Phase Doppler Anemometry observations revealed significant mean horizontal and vertical slip velocities that were larger closer to the surface. The magnitude seems too large to be an acceleration time scale effect. Scaling of the droplet production surface source strength proved to be difficult. The wind speed forcing varied only 23% and the stress increased a factor of 2.2. Yet, the source strength increased by about a factor of 7. We related this to an estimate of surface wave energy flux through calculations of the standard deviation of small-scale water surface disturbance, a wave-stress parameterization, and numerical wave model simulations. This energy index only increased by a factor of 2.3 with the wind forcing. Nonetheless, a graph of spray mass surface flux versus surface disturbance energy is quasi-linear with a substantial threshold.

Aqueous surface layer flows induced by microscale breaking wind waves

Microscale breaking wind waves cover much of the surface of open waters exposed to moderate wind forcing. Recent studies indicate that understanding the nature and key features of the surface skin flows associated with these small waves is fundamental to explaining the dramatic enhancement of constituent exchange that occurs in their presence. We describe a laboratory study in which velocity measurements were made within a few hundred micrometres of the surface of microscale breaking wind waves without bubble entrainment, using flow visualization and particle image velocimetry (PIV) techniques for a range of wind speed and fetch conditions. Our measurements show that for each experiment, the mean surface drift directly induced by the wind on the upwind faces and crests of these waves is (0.23 ± 0.02)u a * in the trough increasing to (0.33±0.07)u a * at the crest, where u a * is the wind friction velocity. About these mean values, there is substantial variability in the instantaneous surface velocity up to approximately ±0.17u a * in the trough and ±0.37u a * at the crest. This variability can be attributed primarily to the modulation of the wave field, with additional contributions arising from fluctuations in wind forcing and near-surface turbulence generated by shear in the drift layer or by the influence of transient microscale breaking. We observed that in a frame of reference travelling with a microscale breaking wave, the transport in the aqueous surface layer is rearward along its entire surface, except within and immediately upwind of the spilling region. Moreover, we found that transport of surface fluid rarely occurs forward over the crest and into the spilling region. This is in marked contrast with previously envisaged wind drift layer flow structures. Visualizations and PIV measurements demonstrate the important role of microscale wind-wave breaking in the direct transport of fluid from the surface to the highly turbulent domain below. Many hundreds of surface flow visualization images were carefully examined. These showed that at the toe of each microscale breaker spilling region, there is an intense and highly localized convergence of surface fluid, with convergence rates generally exceeding 100 s -1 . By comparison, observations of surface convergence attributable to parasitic capillary activity are modest. The changes in mean surface drift along the upwind faces of the waves are equivalent to mean surface divergences of between 0.2 and 1.3 s -1 . Flow visualizations of the surface layer along the upwind (windward) faces of these waves revealed the occurrence of locally intense flow divergence. However, maximum values of the divergence rate were observed to be only of order 10 s -1 . Hence these divergence zones are much more diffuse than the convergence zones at the toes of spilling regions. Overall, our measurements strongly support the view that microscale breaking is likely to be the dominant process in the enhancement of sea surface exchange at moderate wind speeds, as has been suggested by a number of previous investigators. Using a simple model based on our observations, it is shown that microscale breaking is potentially a very effective process in the observed enhancement of constituent transfer for U 10 ≥ 4 m s -1 (where U 10 is wind speed measured 10 m above the surface) and this mechanism exceeds by a wide margin the strength of other previously proposed mechanisms.

Linking reduced breaking crest speeds to unsteady nonlinear water wave group behavior

Observed crest speeds of maximally steep, breaking water waves are much slower than expected. Our fully nonlinear computations of unsteadily propagating deep water wave groups show that each wave crest approaching its maximum height slows down significantly and either breaks at this reduced speed, or accelerates forward unbroken. This previously noted crest slowdown behavior was validated as generic in our extensive laboratory and field observations. It is likely to occur in unsteady dispersive nonlinear wave groups in other natural systems.

Hydrophobically-associating cationic polymers as micro-bubble surface modifiers in dissolved air flotation for cyanobacteria cell separation

Dissolved air flotation (DAF), an effective treatment method for clarifying algae/cyanobacteria-laden water, is highly dependent on coagulation-flocculation. Treatment of algae can be problematic due to unpredictable coagulant demand during blooms. To eliminate the need for coagulation-flocculation, the use of commercial polymers or surfactants to alter bubble charge in DAF has shown potential, termed the PosiDAF process. When using surfactants, poor removal was obtained but good bubble adherence was observed. Conversely, when using polymers, effective cell removal was obtained, attributed to polymer bridging, but polymers did not adhere well to the bubble surface, resulting in a cationic clarified effluent that was indicative of high polymer concentrations. In order to combine the attributes of both polymers (bridging ability) and surfactants (hydrophobicity), in this study, a commercially-available cationic polymer, poly(dimethylaminoethyl methacrylate) (polyDMAEMA), was functionalised with hydrophobic pendant groups of various carbon chain lengths to improve adherence of polymer to a bubble surface. Its performance in PosiDAF was contrasted against commercially-available poly(diallyl dimethyl ammonium chloride) (polyDADMAC). All synthesised polymers used for bubble surface modification were found to produce positively charged bubbles. When applying these cationic micro-bubbles in PosiDAF, in the absence of coagulation-flocculation, cell removals in excess of 90% were obtained, reaching a maximum of 99% cell removal and thus demonstrating process viability. Of the synthesised polymers, the polymer containing the largest hydrophobic functionality resulted in highly anionic treated effluent, suggesting stronger adherence of polymers to bubble surfaces and reduced residual polymer concentrations.

Municipal gravity sewers: an unrecognised source of nitrous oxide.

Nitrous oxide (N2O) is a primary ozone-depleting substance and powerful greenhouse gas. N2O emissions from secondary-level wastewater treatment processes are relatively well understood as a result of intensive international research effort in recent times, yet little information exists to date on the role of sewers in wastewater management chain N2O dynamics. Here we provide the first detailed assessment of N2O levels in the untreated influent (i.e. sewer network effluent) of three large Australian metropolitan wastewater treatment plants. Contrary to current international (IPCC) guidance, results show gravity sewers to be a likely source of N2O. Results from the monitoring program revealed hydraulic flow rate as a strong driver for N2O generation in gravity sewers, with microbial processes (nitrification and possibly denitrification) implicated as the main processes responsible for its production. Results were also used to develop a presumptive emission factor for N2O in the context of municipal gravity sewers. Considering the discrepancy with current IPCC Guidelines, further work is warranted to assess the scale and dynamics of N2O production in sewers elsewhere.

Managing Adaptation of Urban Water Systems in a Changing Climate

Current evidence is that climate change is occurring, it is largely manmade and it will have significant implications for human civilisation. Australia is particularly vulnerable to the anticipated effects of climate change, creating major challenges for water resource management and water supply security. Climate change adaptation offers a means by which we can reduce our exposure to future climate change risks, whilst at the same time exploiting any potential benefits that may arise from climatic changes. This review outlines the current major climate change adaptation challenges facing the water supply industry at large, with a particular focus on these challenges in an Australian context. It also aims to highlight the critical knowledge gaps and strategies required to assist in the formulation of adaptation responses to the range of potential impacts on water infrastructure and future water security. A diverse range of management and assessment techniques are used by relevant professions in industry. Here, an adaptive management approach is presented highlighting the important information required for robust assessment.

Stress above Wind-Plus-Paddle Waves: Modeling of a Laboratory Experiment

A model based on wind-over-waves coupling (WOWC) theory is used to simulate a laboratory experiment and to explain the observed peculiarities of the surface stress distribution above a combined wave field: wind-generated-plus-monochromatic-paddle waves. Observations show the systematic and significant decrease in the stress as the paddle wave is introduced into the pure wind-wave field. As the paddle-wave steepness is further increased, the stress level returns to the stress level characteristic of the pure wind waves. Further increase in the paddle-wave steepness augments the stress further. The WOWC model explains this peculiarity of the stress distribution by the fact that the paddle waves significantly damp the wind waves in the spectral peak. The stress supported by these dominant waves rapidly falls when the paddle wave is introduced, and this decrease is not compensated by the stress induced by the paddle wave. With further increase in the steepness of the paddle wave, the stress supported by dominant wind waves stays at a low level while the stress supported by the paddle waves continues to grow proportional to the square of the steepness, finally exceeding the stress level characteristic of the pure wind-wave field.

Water wave attenuation due to opposing wind

A laboratory investigation of the attenuation of mechanically generated waves by an opposing wind has been completed. Wave attenuation was quantified by measurements of the decline in surface variance. These measurements show higher effective levels of monochromatic wave attenuation than predicted by air-side measurements: approximately an order of magnitude higher than measurements by Young & Sobey (1985) and, a factor of 3 higher than those of Donelan (1999) for waves in a JONSWAP spectrum. Furthermore, they show that theoretical estimates currently underestimate the attenuation rates by a factor of at least 3. This study has shown that the magnitude of wave attenuation rates due to opposing winds is approximately 2.5 times greater than the magnitude of wave growth rates for comparable wind forcing. At high wave steepnesses, detailed analysis suggests that air-side processes alone are not sufficient to induce the observed levels of attenuation. Rather, it appears that energy fluxes from the wave field due to the interaction between the wave-induced currents and other subsurface motions play a significant role once the mean wave steepness exceeds a critical value. A systematic relationship between the energy flux from the wave field and mean wave steepness was observed. The combination of opposing wind and wind-induced water-side motions is far more effective in attenuating waves than has previously been envisaged.