MSc project: Grid-generated turbulence near the air/water surface of a partially filled water tank

Introduction
During the summer season cyanobacteria ('blauwalgen') may form scum layers ('drijflagen') in fresh water lakes such as the Volkerak-Zoommeer in The Netherlands (see picture taken from website of Rijkswaterstaat). These layers secrete chemical compounds which are toxic for swimmers and affect the water quality. When scum layers are formed quick and drastic measures are required such as a bathing prohibition and possible closure of a recreational area. The conditions under which scum layers are formed and disappear again are not well understood [3]. An accurate prediction of the formation of scum layers is required for a timely closing of recreational areas and for developing effective measures to prevent their formation such as the deployment of bubble plumes  for enhanced turbulent mixing.

Problem formulation
The underlying aim of this project is to gain understanding of the behavior of cyanobacteria near the air/water surface of a lake. To simplify the problem we will approximate cyanobacteria as small buoyant spheres and study the behavior of these spheres near the air/water surface of a partially filled water tank in our laboratory. Turbulence with well-defined properties will be generated by means of an oscillating grid located at some distance below the surface [2]. The main research questions which we want to answer are:
(1) What are the properties of grid-generated turbulence near the air/water surface? For example the profiles of the turbulent velocity fluctuations and length scales of turbulence as function of the distance to the surface?
(2) Under what flow conditions does a small buoyant sphere submersed in the water approach the water surface?
(3) If a sphere has reached the surface, under what flow conditions does it leave the surface?

Approach
In the current MSc project we will focus only on the first two research questions. To this purpose an experimental facility has to be designed and built, more specifically an oscillating grid in a water tank. The student will use optical diagnostics such as Particle Image Velocimetry [1] to study the turbulent flow near the surface. The results will be compared against data reported in literature [2]. Next, the behavior of small buoyant spheres in this flow will be studied in order to address to second research question.
The research project will be conducted within the laboratory of the Fluid Mechanics section at TU Delft. Dr. ir. Uittenbogaard from Deltares will be involved in the project as external adviser.

Literature
1. R.J. Adrian and J. Westerweel. Particle Image Velocimetry. Cambridge University Press, 2010.
2. B.H. Brumley and G.H. Jirka. Near-surface turbulence in a grid-stirred tank. J. Fluid Mech., 183:235-263, 1987.
3. A.K. Immers, R.E. Uittenbogaard and B.W. Ibelings. Gone with the wind - Cyanobacterial scum formation and dissipation under artificial mixing. In preparation.

Requirement
The student should have passed the exam for the MSc course on turbulence (WB1424ATU).

Supervision and contact information
1. dr. ir. C. Poelma (main supervisor), TU Delft, 3mE, section Fluid Mechanics
c.poelma@tudelft.nl | 015-27882620
2. dr. ir. W.-P. Breugem (second supervisor), TU Delft, 3mE, section Fluid Mechanics
w.p.breugem@tudelft.nl | 015-2788663
3. dr. ir. R.E. Uittenbogaard (external supervisor), Deltares, Rotterdamseweg 185, 2629 HD Delft, Rob.Uittenbogaard@deltares.nl