Dubravka Pokrajac

Fluorescence spectroscopy of Rhodamine 6G: Concentration and solvent effects

Rhodamine 6G (R6G), also known as Rhodamine 590, is one of the most frequently used dyes for application in dye lasers and as a fluorescence tracer, e.g., in the area of environmental hydraulics. Knowing the spectroscopic characteristics of the optical emission is key to obtaining high conversion efficiency and measurement accuracy, respectively. In this work, solvent and concentration effects are studied. A series of eight different organic solvents (methanol, ethanol, n-propanol, iso-propanol, n-butanol, n-pentanol, acetone, and dimethyl sulfoxide (DMSO)) are investigated at constant dye concentration. Relatively small changes of the fluorescence spectrum are observed for the different solvents; the highest fluorescence intensity is observed for methanol and lowest for DMSO. The shortest peak wavelength is found in methanol (568 nm) and the longest in DMSO (579 nm). Concentration effects in aqueous R6G solutions are studied over the full concentration range from the solubility limit to highly dilute states. Changing the dye concentration provides tunability between ∼550 nm in the dilute case and ∼620 nm at high concentration, at which point the fluorescence spectrum indicates the formation of R6G aggregates.

Turbulence structure of open channel flows over permeable and impermeable beds: a comparative study

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels

Spatially Averaged Flows over Mobile Rough Beds: Definitions, Averaging Theorems, and Conservation Equations

AbstractThis paper reports the double-averaged (in space and in time) hydrodynamic equations for mobile-boundary conditions that are derived based on the refined double-averaging theorems, modified Reynolds decomposition, and improved definitions of the spatial and time bed porosities. The obtained double-averaged conservation equations provide a mathematical framework for studying mobile-boundary flows such as gravel bed rivers during flood events or flows over vegetated beds. These equations will help in designing measurement campaigns for obtaining mobile bed data and their interpretation and parameterization, eventually leading to improved and more robust predictive models.

Peculiar mean velocity profiles within a porous bed of an open channel

We present the velocity profile measured within a porous bed below rough turbulent open channel flow. Mean streamwise velocity has a minimum in the top pore, just below the bed surface, and then increases towards a constant value in the deeper pores. We argue that the velocity minimum in the top pore is due to enhanced turbulence generated within the external flow. This produces a flow regime similar to rough turbulent pipe flow, with very efficient momentum extraction. In the deeper pores turbulence intensities are damped so that momentum extraction is less efficient, allowing higher mean velocities to develop.

Subsurface processes generated by bore‐driven swash on coarse‐grained beaches

[1] Large‐scale laboratory experiments presented in this paper involved bore‐driven swash on permeable immobile coarse‐grained beaches. Two different sediments were used (d50 = 1.5 and 8.5 mm) resulting in different beach permeability and surface roughness. The experiments yielded detailed measurements of swash depth and velocities, wetting front, pressure, and groundwater levels across the swash zone. This paper is focused on the processes occurring within the beach. The measurements provide the shape of the wetting front and the groundwater table and reveal the behavior of air in the unsaturated region of the beach. Air is initially at atmospheric pressure, but the pressure builds up when air becomes encapsulated between the saturated region formed below the swash and the groundwater table. For the 1.5 mm beach, entrapped air significantly affected the water exchange between the swash and the subsurface. The considerable buildup of interstitial air pressure reduced vertical hydraulic gradients and thus infiltration rates. At the lower end of the beach the hydraulic gradients even became negative, indicating flow reversal and exfiltration. In contrast, for the 8.5 mm beach the rate of infiltration was only slightly affected by the buildup of pore‐air pressure. The vertical hydraulic gradients were more than twice the magnitude of those within the 1.5 mm beach. The results presented in the paper clarify the mechanisms that drive and impede the water exchange between the surface and subsurface flow. In particular, infiltration into the initially unsaturated part of the beach and the resulting air entrapment play a significant role in swash and similar flows.

Onset of turbulence in a regular porous medium: An experimental study

This paper presents the results from an experimental study on turbulent flows through a regular porous matrix of spheres packed in a cubic arrangement. The special pore geometry made it possible to measure the velocity within individual pores, using the ultrasonic velocity profiler (UVP). The UVP recorded the time series of the average cross-sectional velocity at a series of points along the pore. The measurements were performed for various hydraulic gradients resulting in a pore Reynolds number between 70 and 430. Three different regimes were detected: unsteady laminar, transition to turbulence, and turbulent. An additional measurement was performed using particle image velocimetry (PIV), which recorded two velocity components within a window which covered only the widest pore section. The PIV test had a Reynolds number of 370, which corresponds to transitional flow just before the onset of turbulence. The PIV result revealed the spatial distribution of mean velocities and turbulence intensities, as well...

An efficient algorithm for high accuracy particle tracking in finite elements

Abstract We propose an algorithm for particle tracking based on Chengs method [Int. J. Numer. Meth. 39 (1996) 1111–1136]. Velocities in a flow field are known at a series of points and interpolated between them by finite element local functions. Tracking is performed in local coordinates, element by element, using any standard ODE solution method. The exit from an element is found using the polynomials to interpolate between the tracking points. The algorithm was tested and compared to Pollocks and Chengs method in a series of numerical experiments, in which the Euler, Runge–Kutta 2, Runge–Kutta 5(4) and Runge–Kutta 6(4) ODE solution methods were combined with first-, second-, third- and fifth-order exit polynomials. The known velocities had a random error with standard deviation of 0%, 0.1% and 1% of the velocity. Meaningful results were obtained only when the spatial interpolation error and the error of the tracking method were calculated separately, otherwise some results were misleading. The numerical experiments confirmed that the accuracy of the exit polynomial has to be consistent with the ODE solution method. Quadratic interpolation of velocities on a coarser mesh often gives more accurate path lines and requires less computational time than linear interpolation. Pollocks method for particle tracking is viable only if input data are rather inaccurate and path lines nearly straight. Chengs method is appropriate for moderately accurate input data, while the proposed algorithm with Runge–Kutta 5(4) or Runge–Kutta 6(4) method and fifth-order exit polynomial has excellent accuracy. Computational time is about 10 times longer than for Chengs method while the accuracy is increased by several orders of magnitude.

Interrelation of wastewater and groundwater management in the city of Bijeljina in Bosnia

Abstract The city of Bijeljina lies on alluvial deposits of the river Drina, which provide groundwater storage of great capacity with good hydraulic properties. Groundwater is abstracted by wells which are located in a field close to the city, and used for public water supply. In the summer of 1993, large numbers of coliform bacteria were found repeatedly in two of the wells. A study was initiated with the main objective of providing a short-term solution to the problem, as well as some guidelines for the long-term groundwater management strategy. A regional groundwater flow model was developed, calibrated and verified. In situ tracer experiments were used to improve the reliability of particle tracking computations. The results were used to prove that the sources of pollution were domestic septic tanks upstream of the well field. The problem was temporarily solved by closure of five wells capturing polluted water and opening some new wells in the less affected zone. The particle tracking results show why this strategy works, but also show that it is only a temporary solution. As a long-term solution, a sewerage system and network of monitoring wells were proposed. The sewerage system has been constructed recently. The role of monitoring is to check if bacterial pollution has ceased following the construction of the sewerage system, as well as to detect other types of pollutants. Groundwater levels have been monitored for one year already, so the data for model re-calibration has been collected. The re-calibrated model can be used for more accurate delineation of protective zones, prediction of the risks related to other pollutants, analysis of the possibility to reactivate the closed wells, etc. As the major objective in long-term water resources management, it is proposed that an integrated approach should be adopted, with simultaneous management of water supply, wastewater and solid waste.

On the role of infiltration and exfiltration in swash zone boundary layer dynamics

Boundary layer dynamics are investigated using a 2-D numerical model that solves the Volume-Averaged Reynolds-Averaged Navier-Stokes equations, with a VOF-tracking scheme and a k - ϵ turbulence closure. The model is validated with highly resolved data of dam break driven swash flows over gravel impermeable and permeable beds. The spatial gradients of the velocity, bed shear stress, and turbulence intensity terms are investigated with reference to bottom boundary layer (BL) dynamics. Numerical results show that the mean vorticity responds to flow divergence/convergence at the surface that result from accelerating/decelerating portions of the flow, bed shear stress, and sinking/injection of turbulence due to infiltration/exfiltration. Hence, the zero up-crossing of the vorticity is employed as a proxy of the BL thickness inside the shallow swash zone flows. During the uprush phase, the BL develops almost instantaneously with bore arrival and fluctuates below the surface due to flow instabilities and related horizontal straining. In contrast, during the backwash phase, the BL grows quasi-linearly with less influence of surface-induced forces. However, the infiltration produces a reduction of the maximum excursion and duration of the swash event. These effects have important implications for the BL development. The numerical results suggest that the BL growth rate deviates rapidly from a quasi-linear trend if the infiltration is dominant during the initial backwash phase and the flat plate boundary layer theory may no longer be applicable under these conditions.

Higher Order Analytical Solutions of Water Table Fluctuations in Coastal Aquifers

The groundwater response of coastal aquifers to tidal forcing is described by Laplaces equation coupled with the nonlinear phreatic-free surface boundary condition. Here we describe fluctuations in the water table using two small parameters, extending previous work by proposing an ansatz to compute higher order, semi-analytical solutions. The new solutions are compared with known lower order solutions. The relative difference between the linear solution and higher order solutions can reach up to 30% of the linear solution for shallow beaches. The new solutions exhibit a reduction in the over height of the groundwater fluctuations compared with the lower order solutions. In addition, the super elevation of the water table, both near shore and as the aquifer tends landward, is examined with the inclusion of higher order terms.