Hypoxia, defined as dissolved oxygen (DO) < 2 mg/L, in the central basin of Lake Erie has been studied since the mid‐1900s. Even so, spatial patterns of hypoxia, and episodic hypoxia in nearshore areas where drinking water plant intakes are located, are not well characterized owing to limited observations and short‐term dynamics. We evaluated a physically based, DO model with respect to patterns of hypoxia observed in Lake Erie. The DO model used assigned rates of sediment and water column oxygen demand that were temperature dependent but otherwise spatially and temporally uniform. The DO model was linked to National Oceanic and Atmospheric Administration's (NOAA) Lake Erie Operational Forecasting System hydrodynamic model, an application of the Finite Volume Community Ocean Model (FVCOM). Model temperature and DO were compared with observations from ship‐based studies, real‐time sensor networks and an array of moored sensors that we deployed in 2017. In years with dominant southwesterly winds, persistent downwelling occurred along the south shore, which resulted in a thinner thermocline and earlier initiation of hypoxia along the south shore than the north. Occasional northeast winds temporarily reversed this pattern, causing upwelling along the south shore that brought hypoxic water to nearshore locations and water intakes. The DO model reproduced observed spatial and temporal patterns of hypoxia and revealed locations subject to episodes of hypoxia, including nearshore Ohio, north of Pelee Island, and near the Bass Islands. Model skill was limited in some respects, highlighting the importance of accurate simulation of the thermal structure and spatial patterns of oxygen demand rates.
Hypoxic conditions continue to be an environmental concern in lakes, including those with shallow and well‐mixed basins, such as the western basin of Lake Erie, in which hypoxia is not anticipated. We investigated the dynamics and causes of hypoxia using field measurements at two locations in the western basin during the late summer of 2017. Two hypoxic events (dissolved oxygen [DO] concentrations < 2 mg L−1) were recorded that were caused by upwelling of hypolimnetic water from the central basin of the lake following winds from the south and southwest. In this case, instantaneous stratification occurred when cool central basin water (i.e., 15.7°C) intruded as a 2.5‐m‐thick layer above the bottom under the warm western basin waters (i.e., 23.9°C). A third hypoxic event, which was associated with more typical thermal stratification from atmospheric warming, occurred during a calm and warm period near the end of the deployment. In this case, we observed a continuous decline in hypolimnetic DO from ≈ 8 to < 5 mg L−1, which likely declined to < 2 mg L−1 in 14 d using a one‐dimensional model. Interbasin exchange flows generated instantaneous hypoxia multiple times within a year and were the dominate cause (63% of 11 cases) of hypoxia identified during August fishing trawls in the study area over the past 30 yr. Results of this work should help with the prediction and understanding of hypoxia in lakes with multiple basins, which will be informative for water quality and fisheries management.
Well-resolved numerical simulations of turbulent open channel flows are analyzed to evaluate the accuracy of the 2nd order structure function method (SFM) in estimating the rate of dissipation of turbulent kinetic energy within boundary layers. The objective is to assess the variation in the 2/3 Kolmogorov constants due to flow anisotropy with distance from the wall. Comparison of the dissipation calculated directly from the numerical data, with that from the SFM shows that usage of the canonical constants, based on the assumption of local isotropy, can result in considerable error (>50%) in the prediction of dissipation when using the vertical or spanwise velocity components. From the numerically calculated dissipation, optimal Kolmogorov 2/3 constants were obtained and empirical relations, which account for near-wall effects, were proposed. Usage of the optimal constants will improve estimation of the dissipation rate when the SFM is applied to compute dissipation in geophysical boundary-layer flows.
This paper investigates the boundary-layer characteristics on a helical blade of large chord length, enclosed in an annulus and rotating in a fluid otherwise at rest. The three-dimensional form of momentum integral equations is derived, and used to predict the boundary-layer growth and limiting streamline angles on the blade surface. The measurements are in general agreement with the predictions. The wall shear stress correlation, which includes both Reynolds number and rotation parameters, valid for a rotating blade operating at zero pressure gradient, is derived. Radial and tangential velocity profiles, the tangential component of turbulence intensity and blade static pressures are measured at several locations on the blade surface. The nature of flow near the blade tip is discussed. An expression for the radial velocity profile, valid in the outer region of the boundary layer, is derived theoretically.
We evaluated the accuracy of the inertial dissipation method to estimate the rate of dissipation of turbulent kinetic energy within boundary layers by performing well‐resolved numerical simulations of turbulent channel flows and comparing the dissipation calculated directly from the data, with that deduced from the frequency spectra. The convection velocity, commonly used to convert frequency spectra into wave number spectra is found to be larger than the local mean velocity by approximately a factor of 2 near the bed and about 10% in the logarithmic layer and beyond. Usage of the standard Kolmogorov constants (particularly in the vertical and spanwise directions) also leads to significant errors (50% or more) in computation of dissipation. Usage of the optimal Kolmogorov constants and the convection velocity, obtained from the simulation results could result in improved accuracy in dissipation calculation with the inertial method.
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