Future workInvestigation of further experiments with varying turbulence intensities (i.e. varying heat fluxes) to obtain detailed knowledge of the interaction between the bouyant-convective turbulence and the gas transfer pro-cess. Reliable qualitative and quantitative results are expected from the wide spectrum of turbulence intensities investigated.

Institut für HydromechanikAbteilung Misch- und Transportvorgänge

Postanschrift: Besuchsanschrift: www.ifh.uni-karlsruhe.de A project supported byKaiserstr.12 Otto-Ammann-Platz 176131 Karlsruhe Gebäude Nr. 10.81

A. Niepelt, H. Herlina, G. H. Jirka

Six images of the simultaneous oxygen concentration and velocity maps(3×3cm²) at three different time steps are presented on the right. Theinstantaneous concentration and flow field is shown on the top, thecorresponding fluctuation map is illustrated below. The superimpositionof the concentration map onto the velocity maps in one graph gives a better insight into the interaction between the turbulent flow field and gas transfer process.

Results – Qualitative ObservationsTwo sequences of eight instantaneous oxygen concentration and velocity maps are presented from experiments with a Rayleigh number Ra of about 3⋅109 (∆T =3.3°C) and a net heat flux of jnet ≈160W/m². The simultaneous LIF and PIV sequences show the correlation between the sinking plumes and the transport of oxygen-rich fluids from the surface i

Methods Experimental Set-Up: The Convection Tank• turbulence generation through sensible and latent

heat transfer across the water surface• closed and thermally well controlled system• headspace filled with cold and relatively dry air• cold air released through four diffusers (ua ≈0 m/s)

BackgroundGas transfer across the air-water interface plays an important role in geophysical processes and in environmental engineering. An essential example of gas transfer process in nature is the reaeration process by which oxygen is transferred across the surface of a water body. In lakes or reservoirs that are topo-graphically sheltered the dominant driving mechanisms for gas transfer are not only wind-shear, but also buoyant-convective instabilities.

The convective process commences at night when the water surface undergoes cooling and a thin cool surface layer is formed. The cool layer becomes heavier than the ambient fluid and consequently tends to sink down as negatively buoyant plumes. The main goal of this study is to gain fundamental understanding of the actual physical mechanisms of this process through detailed laboratory experiments.

into the deeper oxygen-poor region. Being heavier than the ambient fluid, the cold upper layer sinks down as plumes. As cold plumes start to sink, warm water from below replaces the cold water above. The continuity of sinking and rising plumes induces stirring in the convection tank and is likely the dominating transport mechanism for the gas transfer in the investigated system.

x (mm)

z (m

m)

t=4.18min

c’ (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30

−2.0

−1.0

0.0

1.0

2.0

t=4.18min

x (mm)

z (m

m)

C (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30 0

2

4

6

8

10

x (mm)

z (m

m)

t=2.91min

c’ (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30

−2.0

−1.0

0.0

1.0

2.0

t=2.91min

x (mm)

z (m

m)

C (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30 0

2

4

6

8

10

t=1.08min

x (mm)

z (m

m)

C (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30 0

2

4

6

8

10

x (mm)

z (m

m)

t=1.08min

c’ (

mg/

l)

25 30 35 40 45 50

0

5

10

15

20

25

30

−2.0

−1.0

0.0

1.0

2.0

dimensions: 50×50cm² bottom, 65cm deep, h = 42cm

Two neighbouring plumes are being advected horizontally to the left. Both plumes are falling with approxi-mately the same velocity of about 1.7 mm/s.

CV, t=4minwmax

=1.9mm/s

z (m

m)

0

10

20

30

40

50 +7swmax

=1.6mm/s +14swmax

=1.5mm/s +21swmax

=1.6mm/s

+28swmax

=1.8mm/s

x (mm)

z (m

m)

0 10 20 30 40 50 60 70

0

10

20

30

40

50 +35swmax

=1.8mm/s

x (mm)0 10 20 30 40 50 60 70

+42swmax

=1.7mm/s

x (mm)0 10 20 30 40 50 60 70

+49swmax

=1.8mm/s

x (mm)

sink

ing

velo

city

(m

m/s

)

= 2 mm/s

0 10 20 30 40 50 60 700.0

1.0

2.0

3.0

4.0

CV t=4min

z (m

m)

0

10

20

30

40

50 +7s +14s +21s

+28s

x (mm)

z (m

m)

0 10 20 30 40 50 60 70

0

10

20

30

40

50 +35s

x (mm)0 10 20 30 40 50 60 70

+42s

x (mm)0 10 20 30 40 50 60 70

+49s

x (mm)

(C−

Cb)/

(Cs−

Cb)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

Two sinking plumes are moving closer to each other. In the vector fields they are seen to merge forming a larger plume which continuous to fall with a velocity almost two times faster than before they merged.

CV, t=2.75minwmax

=1.5mm/s

z (m

m)

0

10

20

30

40

50 +5swmax

=2.0mm/s +10swmax

=2.9mm/s +15swmax

=3.7mm/s

+20swmax

=3.5mm/s

x (mm)

z (m

m)

0 10 20 30 40 50 60 70

0

10

20

30

40

50 +25swmax

=2.1mm/s

x (mm)0 10 20 30 40 50 60 70

+30swmax

=1.3mm/s

x (mm)0 10 20 30 40 50 60 70

+35swmax

=1.3mm/s

x (mm)

sink

ing

velo

city

(m

m/s

)

= 2 mm/s

0 10 20 30 40 50 60 700.0

1.0

2.0

3.0

4.0

CV t=2.75min

z (m

m)

0

10

20

30

40

50 +5s +10s +15s

+20s

x (mm)

z (m

m)

0 10 20 30 40 50 60 70

0

10

20

30

40

50 +25s

x (mm)0 10 20 30 40 50 60 70

+30s

x (mm)0 10 20 30 40 50 60 70

+35s

x (mm)

(C−

Cb)/

(Cs−

Cb)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

Oxygen contour and vector maps: instantaneous concentration and flow field (top) & corresponding fluctuation map.

The LIF technique is based the oxygen quenching method in which the fluorescence intensity is inversely proportional to theoxygen concentration.

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Synoptic measurements of velocity and concentration fields using a combined Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) system that allows direct quanti-fication of the turbulent gas flux (c′w′) across the interface.

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Gas transfer at the air-water interfaceinduced by buoyant-convective turbulence