Turbulence and turbulent flux events in a small estuary

Relatively little systematic research has been conducted on the turbulence characteristics of small estuaries. In the present study, detailed measurements were conducted in a small subtropical estuary with a focus on turbulent flux events. Acoustic Doppler velocimeters were installed in the mid-estuary at fixed locations and sampled simultaneously and continuously for 50 h. A turbulent flux event analysis was performed for the entire data sets extending the technique of Narasimha et al. (Phil Trans R Soc Ser A 365:841-858, 2007) to the unsteady open channel flow motion and to turbulent sub-events. Turbulent bursting events were defined in terms of the instantaneous turbulent flux. The data showed close results for all ADV units. The very-large majority of turbulent events lasted between 0.04 and 0.3 s with an average of 1 to 4 turbulent events observed per second. A number of turbulent bursting events consisted of consecutive turbulent sub-events, with between 1 and 3 sub-events per main event on average. For all ADV systems, the number of events, event duration and event amplitude showed some tidal trends, with basic differences between high- and low-water periods. A comparison between the present estuary data and the atmospheric boundary layer results of Narasimha et al. (Phil Trans R Soc Ser A 365:841-858, 2007) showed a number of similarities and demonstrated the significance of turbulent events in environmental flows. A burstiness index of 0.85 was found for the present data

Turbulence in Small Sub-tropical Estuary with Semi-Diurnal Tides

In natural estuaries, contaminant transport is driven by the turbulent momentum mixing. The predictions of scalar dispersion can rarely be predicted accurately because of a lack of fundamental understanding of the turbulence structure in estuaries. Herein detailed turbulence field measurements were conducted at high frequency and continuously for up to 50 hours per investigation in a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was deemed the most appropriate measurement technique for such small estuarine systems with shallow water depths (less than 0.5 m at low tides), and a thorough post-processing technique was applied. The estuarine flow is always a fluctuating process. The bulk flow parameters fluctuated with periods comparable to tidal cycles and other large-scale processes. But turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of mechanisms. This resulted in behaviour which deviated from that for equilibrium turbulent boundary layer induced by velocity shear only. A striking feature of the data sets is the large fluctuations in all turbulence characteristics during the tidal cycle. This feature was rarely documented, but an important difference between the data sets used in this study from earlier reported measurements is that the present data were collected continuously at high frequency during relatively long periods. The findings bring new lights in the fluctuating nature of momentum exchange coefficients and integral time and length scales. These turbulent properties should not be assumed constant

Turbulence, turbulent mixing and diffusion in shallow-water estuaries

In natural waterways and estuaries, an understanding of turbulent mixing is critical to the knowledge of sediment transport, storm-water runoff during flood events, and release of nutrient-rich wastewater into ecosystems. The predictions of contaminant dispersion in estuaries can rarely be predicted analytically without exhaustive field data for calibration and validation. Why ? In natural estuaries, the flow Reynolds number is typically within the range of 105 to 108 and more. The flow is turbulent, and there is an absence of fundamental understanding of the turbulence structure. Any turbulent flow is characterised by an unpredictable behaviour, a broad spectrum of length and time scales, and its strong mixing properties. In small estuaries, the predictions of scalar dispersion can rarely be estimated accurately because of a lack of fundamental understanding of the turbulence structure. Detailed turbulent velocity and suspended sediment concentration measurements were performed simultaneously and continuously at high frequency for between 25 and 50 hours per investigation in shallow-water estuaries with semi-diurnal tides in Australia and Japan (Fig. 18). The detailed analyses provided an unique characterisation of the turbulent mixing processes and suspended sediment fluxes. Continuous turbulent velocity sampling at high frequency allowed a detailed characterisation of the turbulence field in estuarine systems and its variations during the tidal cycle. The turbulence was neither homogeneous nor isotropic. It was not a purely Gaussian process, and the small departures from Gaussian probability distribution were an important feature of the turbulent processes. A striking feature of the present data sets was the large and rapid fluctuations in all turbulence characteristics and of the suspended sediment fluxes during the tidal cycles. This was rarely documented, but an important characteristic of the newer data sets is the continuous high frequency sampling over relatively long periods. The findings showed that the turbulent properties, and integral time and length scales should not be assumed constant in a shallow estuary. The integral time scales for turbulence and suspended sediment concentration were similar during flood tides, but differed significantly during ebb tides. It is believed that the present results provided a picture general enough to be used, as a first approximation, to characterise the flow field in similar shallow-water estuaries with semi-diurnal tides. It showed in particular a different response from that observed in larger, deep-water estuaries. A turbulent flux event analysis was performed for a 50 hour long field study. The results showed that the large majority of turbulent events had a duration between 0.04 s and 0.3 s, and there were on average 1 to 4 turbulent events per second. A number of turbulent bursting events consisted of consecutive sub-events, with between 1 and 3 sub-events per event on average for all turbulent fluxes. A comparison with atmospheric boundary layer results illustrated a number of similarities between the two types of turbulent flows. Both studies implied that the amplitude of an event and its duration were nearly independent. Overall the present research highlighted some turbulent processes that were rarely documented in previous studies. However an important feature of the present analysis was the continuous high frequency sampling data sets collected during relatively long periods, as well as the simultaneous sampling of both turbulent velocities and suspended sediment concentrations

Continuous high-frequency turbulence and suspended sediment concentration measurements in an upper estuary

The present study details new turbulence field measurements conducted continuously at high frequency for 50 hours in the upper zone of a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was used, and the signal was post-processed thoroughly. The suspended sediment concentration wad further deduced from the acoustic backscatter intensity. The field data set demonstrated some unique flow features of the upstream estuarine zone, including some low-frequency longitudinal oscillations induced by internal and external resonance. A striking feature of the data set is the large fluctuations in all turbulence properties and suspended sediment concentration during the tidal cycle. This feature has been rarely documented

Temperature dependence of the energy dissipation in dynamic force microscopy

The dissipation of energy in dynamic force microscopy is usually described in terms of an adhesion hysteresis mechanism. This mechanism should become less efficient with increasing temperature. To verify this prediction we have measured topography and dissipation data with dynamic force microscopy in the temperature range from 100 K up to 300 K. We used 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both materials exhibiting a strong dissipation signal at large frequency shifts. At room temperature, the energy dissipated into the sample (or tip) is 1.9 eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good agreement with an adhesion hysteresis mechanism. The energy dissipation over the PTCDA surface decreases with increasing temperature yielding a negative temperature coefficient. For the KBr substrate, we find the opposite behaviour: an increase of dissipated energy with increasing temperature. While the negative temperature coefficient in case of PTCDA agrees rather well with the adhesion hysteresis model, the positive slope found for KBr points to a hitherto unknown dissipation mechanism

Constructing the problem of initial teacher education in Aotearoa New Zealand: policy formation and risk, 2010-2018

This paper reports findings from an interpretive policy and discourse analysis of documents informing contemporary initial teacher education (ITE) policy development in Aotearoa New Zealand. The study first asks: what is the problem of teacher education as constituted in policy and associated documents in the period 2010-2018? We then compare the problems, suggested solutions, and recent evidence about the work of teacher education in New Zealand, to discuss the policy discourse, and theorise about the potential utility of solutions to address the problems raised. Our comparative analysis of the problems of ITE and proposed policy solutions with research evidence of teacher education work underscores the imperative of engagement with local and relevant evidence-based knowledge as a basis for informed policy decision making. &nbsp

Constructing the problem of initial teacher education in Aotearoa New Zealand: policy formation and risk, 2010-2018

This paper reports findings from an interpretive policy and discourse analysis of documents informing contemporary initial teacher education (ITE) policy development in Aotearoa New Zealand. The study first asks: what is the problem of teacher education as constituted in policy and associated documents in the period 2010-2018? We then compare the problems, suggested solutions, and recent evidence about the work of teacher education in New Zealand, to discuss the policy discourse, and theorise about the potential utility of solutions to address the problems raised. Our comparative analysis of the problems of ITE and proposed policy solutions with research evidence of teacher education work underscores the imperative of engagement with local and relevant evidence-based knowledge as a basis for informed policy decision making. &nbsp

Development of large-internal-surface-area nickel-metal plaques third quarterly progress report, dec. 19, 1964 - mar. 18, 1965

Electrodes impregnated with cadmium for porosity and electrochemical evaluatio