Chris Rehmannís Research Group:Mixing in Natural Flows





An efficient method to measure the dispersion coefficient in rivers


Accurate models of contaminant transport are needed to predict the effect of both point and non-point sources of pollution on downstream water supplies.Models of river mixing and contaminant transport require a dispersion coefficient K, which determines the spreading rate of a contaminant cloud and the rate of decrease of the peak concentration. The dispersion coefficient is usually estimated either with costly field tracer experiments or empirical formulas that predict K within an order of magnitude. We are developing and testing a method for using acoustic Doppler current profiler (ADCP) measurements to estimate the dispersion coefficient more efficiently. The method uses a result from river mixing theory that relates the dispersion coefficient to the transverse profile of velocity.†† Use of the theoretical result in practice has been impeded by the difficulty of measuring detailed velocity profiles and bathymetry, but now that the USGS routinely uses ADCPs to verify discharge measurements at its gaging stations, the USGS can use our proposed method to estimate K as well as discharge with little or no change to its measurement protocol.More efficient, accurate, and widespread measurements of K will improve predictions of contaminant transport and help with issues of water supply. (South Florida Water Management District)


Differential transport of salt and heat in a diffusively stable flow


In ocean modeling, salt and temperature are usually assumed to mix at equal rates. However, differential transport of heat and salt has been observed in laboratory experiments, simulations, and field measurements. Differences in the transport of salt and heat can affect ocean modeling and the interpretation of oceanic vertical mixing.We are conducting laboratory experiments to study effects of molecular diffusivity on mixing in a turbulent flow stratified with both salt and temperature. These experiments allow us to quantify the differential transport and determine the conditions under which the mixing rates for salt and temperature differ. (National Science Foundation)


Mixing at a sheared, salt fingering interface


In many places in the ocean, the temperature and salinity distributions are conducive for double-diffusive phenomena, such as salt fingers.Understanding vertical mixing in the ocean often requires an understanding of the interaction of double diffusion and mechanical processes, such as shear-driven turbulence. To assess the relative contributions of mechanically generated turbulence and double-diffusive convection to mixing, a laboratory model of a nonrotating gravity current subject to salt fingers was developed. The gravity current can be arrested by releasing cold, fresh water into an opposing flow of warm, salty water. The resulting steady state allows the overall mixing rate, interface properties, and spatial evolution of the temperature and salinity differences to be measured.(National Science Foundation)


Turbulence and mixing due to a bubble plume


Bubble plumes are used to promote mixing and improve water quality in lakes and reservoirs. A particular example is McCook Reservoir, an element of a plan to reduce the harmful effects of combined-sewer overflows in the Chicago area.This reservoir will capture and store nearly 3 x 107 m3 of combined sewage until it can be treated, and bubble plumes are planned to mix and aerate the sewage to prevent anaerobic conditions. Because mixing and aeration in the McCook project will cost $80 million, an increase in the efficiency of mixing could save much effort and money.We have conducted in two unique facilities, a 14-m diameter, 7-m deep tank filled with both non-potable water and sewage and a 35-m deep navigation lock on the Snake River in Washington.We have produced the first ensemble average profiles of the dissipation of turbulent kinetic energy near a bubble plume. (U.S. Army Corps of Engineers)


Metapopulation dynamics of the zebra mussel in rivers and estuaries


This project seeks to reduce the destructive effects of zebra mussels by determining how they are transported in rivers and estuaries.Because a patch of mussels cannot sustain itself without a constant supply of larvae, the number of zebra mussels in an entire river can be drastically reduced if the larval supply can be blocked.The success of this and other control measures depends on the details of the river flow and the biology of the zebra mussel.In particular, if zebra mussels can establish local populations in areas with low flow, such as side embayments, the effectiveness of a dispersal barrier could be reduced. The goal of this project is to understand the effect of embayments on zebra mussel populations. (National Sea Grant College Program, Illinois-Indiana Sea Grant)


Evaluation of a scheme to control zebra mussels


The goal of project was to evaluate a scheme to control zebra mussels in the Illinois River system.The hypothesis that small-scale turbulence can increase the mortality of zebra mussel larvae was tested.Laboratory experiments showed that the mortality increases when the size of the larvae is comparable to the smallest scale of the turbulence. The possibility of using bubble screens as a dispersal barrier in the Chicago Sanitary and Ship Canal was evaluated. (Illinois Water Resources Center)




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