Environmental Fluid Mechanics

Nonstationary convection forced by distributed buoyancy sources is a fundamental environmental fluid mechanics process, particularly in ice-covered freshwater waterbodies. In this paper, we present novel field-based results that characterise the diurnal evolution of the main energetics of radiatively-driven convection in ice-covered lakes that is the radiatively-induced buoyancy flux, B, and the kinetic energy dissipation rate, $$\varepsilon$$ . To estimate the spatiotemporal distribution of $$\varepsilon$$ , we applied scale similarity of the velocity structure functions to identify the fine turbulence scales from high-frequency velocity measurements. The field study was carried out at Lake Vendyurskoe, Russia, in April 2016. Small-scale velocity fluctuations were measured using acoustic Doppler current profiler in a 2 m layer beneath the ice cover. The method was proven to be valid for low-energy convection without mean shear. The inertial subrange, covering order of magnitude in the spatial domain, was identified by fitting the $$^2/_3$$ scaling power law to the structure function method, thus confirming the regime of fully developed turbulence. The calculated rate of dissipation of turbulent kinetic energy $$\varepsilon$$ reaches values up to $$3 \times 10^{-9} \hbox { m}^{2}\hbox {s}^{-3}$$ . Although a strong correlation between $$\varepsilon$$ and B was observed, $$\varepsilon$$ picks up about 1 h later after the onset of the heating-phase. This delay roughly corresponds to the turnover time of the energy containing eddies. We finally observed a decay of $$\varepsilon$$ at night, during the relaxation-phase, but, interestingly, the level remained above the statistical error.

A sensitivity study is performed to examine the impact of lateral boundary conditions (LBCs) on the NOAA-EPA operational Air Quality Forecast Guidance over continental USA. We examined six LBCS: the fixed profile LBC, three global LBCs, and two ozonesonde LBCs for summer 2006. The simulated results from these six runs are compared to IONS ozonesonde and surface ozone measurements from August 1 to 5, 2006. The choice of LBCs can affect the ozone prediction throughout the domain, and mainly influence the predictions in upper altitude or near inflow boundaries, such as the US west coast and the northern border. Statistical results shows that the use of global model predictions for LBCs could improve the correlation coefficients of surface ozone prediction over the US west coast, but could also increase the ozone mean bias in most regions of the domain depending on global models. In this study, the use of the MOZART (Model for Ozone And Related chemical Tracers) prediction for CMAQ (Community Multiscale Air Quality) LBC shows a better surface ozone prediction than that with fixed LBC, especially over the US west coast. The LBCs derived from ozonesonde measurements yielded better O3 correlations in the upper troposphere.

Surface discharges of negatively buoyant jets into moving ambient water create a range of complex flow patterns. These complexities arise through the interplay between the discharge’s initial fluxes and the motion of the ambient current. In this study a series of laboratory experiments were conducted for negatively buoyant surface discharges into crossflow to investigate flow patterns under different discharge and ambient conditions. The results compared with simulations of the CORMIX model, an expert system for ocean outfall design. In CORMIX, the simulation module DHYDRO for dense discharges has been used. Finally the flow different patterns were arranged in a dimensionless diagram to propose a modified flow classification system with new criteria.

Experimental tests were conducted on a rectangular flume with different grid-type dam grid sizes, channel slopes and debris flow bulk density. We use the Bernoulli equation of energy conservation to deduce a formula to calculate debris-flow discharge. By combining the flume experiments with dimensional analysis methods, we mainly consider the bulk density of the debris flow, grid size of the grid-type dam, and flume slope to establish the discharge-coefficient correction formula and propose a method of calculating debris-flow discharge through a grid-type dam. The practical value of the formulas was illustrated using the Niwan gully, a tributary of Bailong River in Gansu province, China, as an example, which can be used as a reference in further practical engineering applications.

A large panel of instruments was deployed in the Eastern English Channel to measure the evolution of bedload fluxes during a tidal cycle for two different sites. The first one was characterized by a sandy bed with a low dispersion in size while the other study site implied graded sediments with grain sizes ranging from fine sands to granules. The in situ results obtained were compared with predictions of total bedload fluxes by classical models. A good agreement was found for homogeneous sediments with these formulas. In the case of size heterogeneous sediments, a fractionwise approach, involving a hiding-exposure coefficient and a hindrance factor, provided better predictions of bedload fluxes, but still some discrepancies were noticed. Present results revealed that the consideration of particle shape in formulas through the circularity index enhanced the estimations of bedload transport rates. A new adjustment of Wu et al.’s (J Hydraul Res 38:427–434, 2000) formula was proposed and a very good agreement was obtained between the measured and predicted values.

In this study, the flow dynamics of intrusive gravity currents past a bottom-mounted obstacle were investigated using highly resolved numerical simulations. The propagation dynamics of a classic intrusive gravity current was first simulated in order to validate the numerical model with previous laboratory experiments. A bottom-mounted obstacle with a varying non-dimensional height of $$\tilde{D}=D/H$$ , where D is the obstacle height and H is the total flow depth, was then added to the problem in order to study the downstream flow pattern of the intrusive gravity current. For short obstacles, the intrusion re-established itself downstream without much distortion. However, for tall obstacles, the downstream flow was found to be a joint effect of horizontal advection, overshoot-springback phenomenon, and associated Kelvin-Helmholtz instabilities. Analysis of the numerical results show that the relationship between the downstream propagation speed and the obstacle height can be subdivided into three regimes: (1) a retarding regime ( $$\tilde{D}$$ $$\approx $$ 0–0.3) where a 30 % increase in obstacle height leads to a 20 % reduction in propagation speed, simply due to the obstacle’s retarding effect; (2) an impounding regime ( $$\tilde{D}$$ $$\approx $$ 0.3–0.6) where the additional 30 % increase in obstacle height only leads to a further (negligible) 5 % reduction in propagation speed, due to the accelerating effect of upstream impoundment and downstream enhanced mixing; and (3) a choking regime ( $$\tilde{D}$$ $$\approx $$ 0.6–1.0) where the propagation speed is dramatically reduced due to the dominance of the obstacle’s blocking effect. The obstacle thickness was found to be irrelevant in determining the downstream propagation speed at least for the parameter range explored in this study. The present work highlights the significance of topographic effects in stratified flows with horizontal pressure forcing.

Variation of temporal scour depth around circular piers under clear-water scour condition was studied experimentally for a variability of configuration studies, including different size of circular piers. A total of 96 experiments were carried out with more than 800 data sets of temporal scour depth in non-uniform gravel bed. The experimental data were used to calculate the temporal scour depth at different planes i.e. 0°, 90°, 180° and 270°, 0° plane indicates the maximum and 180° plane indicates minimum scour zones. Some of the previous methods of estimating temporal scour depth at 0° plane are studied in this study. Three temporal scour depths equations for 0°, 90°, 270° and 180° planes have been proposed in this study. The predictions of the temporal scour depth improved after the revised methods of Kothyari et al. (J Hydraul Eng 133(11):1229–1240, 2007). Results of the present proposed equation for 0° plane were compared with latest available relationships in literature and found in good agreements with observed values.

The leakage of fluid leaving a puncture in a pressurized vessel immersed in a quiescent, miscible medium is studied under steady flow conditions. This problem has engineering applications in submerged pipelines and hazardous gas lines. The leakage rate for the puncture is characterized as functions of various hydrodynamic and geometric conditions. Dimensional analysis shows that the leakage percent, Q*, is a function of the Reynolds number, the pressure ratio between the center of the tube and the external hydrostatic pressure, P*, and the hole-to-main tube diameter ratio, D*. The effect of puncture shape is also examined, rectangular and circular. A 3-D finite volume computational model is constructed for laminar flow of a Newtonian fluid under steady conditions and validated with supporting experiments. The results show that the fractional leakage rate Q* increases with P* and approaches a constant value at high P* for a fixed Reynolds number. In addition, it is found that the leakage rate increases with decreasing Reynolds number at a fixed pressure ratio. The geometric effect of the diameter ratio is shown to have a more pronounced effect near a pressure ratio of two with more fluid exiting the puncture for larger diameter ratios. The results of the shape analysis show that the circular puncture has the largest fractional leakage when compared to a rectangle with an equivalent cross-sectional area.

Shallow turbulent flows are omnipresent in hydrosystems. Examples include flows past islands, in river confluences and longshore currents. A characteristic feature of a shallow flow is the existence (under certain conditions) of two-dimensional coherent structures (2DCS). This paper discusses and illustrates the usefulness and limitations of reduced dimensionality models in conjunction with hydrodynamic stability theories in illuminating the onset and subsequent dynamics of 2DCS in shallow flows. The paper follows closely the keynote lecture given by the second author on the 4th International Symposium on Shallow Flows which took place in Eindhoven in 2017. The paper gives the reader a comprehensive review of the reduced dimensionality models and hydrodynamic stability theories used in analyzing the dynamics of shallow flows.