Environmental Fluid Mechanics Laboratory
The Environmental Fluid Mechanics Laboratory is housed within the Department of Civil and Environmental Engineering at Colorado State University. Our research focus in on fundamental and complex fluid dynamics problems with broad applications in engineering, oceanography and atmospheric science. We use computational fluid dynamics (CFD) simulation tools in conjunction with theoretical analysis and experimental methods (as well as collaborative research with field observationalists) to study turbulent flows in both natural and engineered systems. Our emphasis is on fundamental understanding of fluid flow processes in order to develop improved models for turbulent flows.
Some of our current ongoing reseach projects include:
- CAREER: Internal Waves, Turbulence and Diapycnal Mixing in Oceanic Flows (funded by the National Science Foundation).
- Internal wave driven mixing and transport in the coastal ocean (YIP award funded by Office of Naval Research (ONR)).
- Flow Dynamics and turbulent mixing arounds obstacles in oceanic flows (funded by ONR).
- Baffle factors of small system disinfection contact basins (funded by the Colorado Department of Public Health and Environment (CDPHE)).
Postdoctoral Research : Mixing and Transport of Dissolved Wastes from Aquaculture Pens (2006 - 2007) Advisors - Professors Oliver Fringer and Jeff Koseff
The rapid expansion of marine aquaculture is a potential solution to the problem of overfishing and fisheries depletion worldwide, but also a major threat to ocean ecosystems. For example, wastes and nutrient levels from aquaculture pens alter the character of the surrounding ecosystem. The dispersal of wastes from fish pens released into the coastal ocean may not necessarily be ``Gaussian'' (monotonically decreasing from the source in all directions), and 'dilution may not be the solution'. The evidence from previous field and laboratory studies suggests that wastes may be transported in plumes which retain their coherence and possess relatively high concentrations over large distances. This pattern of dispersal could result in much higher concentrations of wastes at certain points on the coastline, even at considerable distances from the source. In this study, our objective is to understand the dispersal of dissolved wastes such as nitrogen from an aquaculture pen using high-resolution two- and three-dimensional numerical simulations under different time-varying flows. Previous work on scalar dispersion under such flow conditions has shown how strong oscillatory effects dominate the character of the plume over time intervals of several periods. Our emphasis in this study is to understand the behavior of the waste plume evolution under oscillatory flow conditions superimposed on a constant mean current compared to the classical spreading that would occur under unidirectional flow conditions in the "near-field'' downstream of a pen.
Ph.D. Research dissertation: Energetics and Dynamics of Internal Waves on Slopes Using Numerical Simulations (2003 - 2006) Advisor - Professor Oliver Fringer
The presence of internal waves in the ocean has long been recognized. However, only in the past decade, has there been a significant resurgence of interest in breaking internal waves. This has been accentuated largely due to a hypothesis that eddy diffusivity is very large in small, localized turbulent patches caused by the interaction of internal waves with topography in the ocean (Munk & Wunsch 1998). Indeed, evidence from many noteworthy open ocean experiments indicates that turbulent mixing is driven by breaking internal waves and shear instabilities at boundaries, supporting the conjecture that the internal wave field is the only serious candidate for supply of energy for vertical mixing in the open ocean.
There are numerous in-situ and remote-sensing observations that clearly show the presence of nonlinear internal waves (hereinafter referred to as NLIWs) in marginal seas and coastal waters. The generation mechanism of these waves is widely accepted to be from the interaction of long first-mode internal tides with bottom topography However, very little is known about the structure of these highly nonlinear internal waves and their ultimate fate, especially as they propagate onshore into shoaling regions. These NLIWs are likely to be one of the primary pathways through which the energy in the internal wave field is fed into the dissipative scales. Therefore, understanding the dynamics of these NLIWs has far reaching implications for numerous applications in the coastal environment.
A key component in gaining improved understanding of NLIWs deals with their energy flux budget. For a small-amplitude linear wave with a wave characteristic slope, s, encountering a shelf break with topographic slope g, most of the energy is transmitted (forward-reflected) and the slope is said to be subcritical. The converse is true for a supercritical slope (g > s) where most of the wave energy is reflected backwards from the topography. A critical slope is obtained when g = s, for which the reflected wave is parallel to the topography and focusing of wave energy takes place. This leads to enhanced dissipation and mixing in the bottom boundary layer. The dynamics are well understood for linear waves (Phillips 1977), however, the energy flux distribution across a shelf break for NLIWs is poorly understood and deserves more attention. Our study aims toward a better understanding of the energy flux distribution for waves spanning the gap between the conditions where linear theory holds and the conditions where nonlinear internal waves occur.
In this research, we present results from high-resolution 2D and 3D numerical simulations of the interaction of a first-mode internal wave field with an idealized shelf break. Our emphasis is to obtain an understanding of the partitioning of the internal wave energy over the course of the interaction process and gain insight into the dynamics of the onshore propagating internal boluses that form as a result of the interaction.
MScEng Research Thesis: Turbulent Mixing and Dispersion in Environmental Flows (2001-2002) Advisor - Professor Derek Stretch..
Direct numerical simulations are used to study mixing and dispersion in decaying stably stratified turbulence from a Lagrangian perspective. The change in density of fluid particles due to small scale mixing is extracted from the simulations to provide insight into the mixing process. These changes are driven by temporally and spatially intermittent events that are strongly suppressed as the stratification increases and overturning motions disappear. This occurs for times Nt > 2&pi i.e. after one buoyancy period, where N is the buoyancy frequency. The role of small scale mixing processes in the density (or buoyancy) flux is analyzed. After an initial transient, we find that diapycnal displacements due to mixing dominate the dispersion of fluid particles, even in weak stratification. The relationship between the diapycnal diffusivity and vertical dispersion coefficients is found to be strongly dependent on stratification. Models for the mixing following fluid particles are investigated. The timescale for the density changes due to small scale mixing is shown to be approximately independent of N and instead remains linked to the energy decay timescale which is relatively insensitive to stratification. There are large changes in the structure of these flows as they evolve under the influence of buoyancy forces. We investigate these changes and their relationship to mixing. We find that strong mixing events are closely linked to the presence of overturning regions in the flow, and that they occur close to (but not within) these regions. The results reported here have implications for the development of improved models of diffusion in stably stratified turbulence. Further Development of this work has led to several new insights into modelling mixing in such flows and this work has been published in the Journal of Fluid Mechanics .
BScEng final year dissertation: Mixing Efficiency in Environmental Flows (2000).
My final year (undergraduate) dissertation was on mixing efficiency in environmental flows using direct numerical simulations, supervised by Professor Derek Stretch. My interest in water engineering lead me to undertake my final year (4th year of BScEng degree) design project on the design of a gravity dam on the Mooi River in South Africa, covering hydrological, hydraulic and structural engineering aspects. This project was supervised by Professor G G S Pegram. The project received an award for excellence from the Concrete Society of South Africa.