Mixing of a thermal plume in a highly stratified artificial

urban pond

A. Csibrán, T. Krámer and P. Torma

Budapest University of Technology and Economics, Department of Hydraulic and Water Resources Engineering, Budapest, Hungary

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Preliminary

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• Located in the heart of Budapest

• Inflow from a thermal bath • High stratification

• Bad water quality

Hydraulically dead zones?

3D thermodynamic modelling

The lake

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• Average water depth ≈ 1 meter

• Average lake temperature in winter ≈ 20.5 °C

• Steady inflow • 3500 m3/day, 32 °C

• Through a pipe at lower layers

• Surrounded by trees • Low wind stress on surface

Grid

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• Constraints of a curvilinear grid • Boundary-fitted and near-orthogonal

• 20 layers (sharp vertical gradients)

• 7600x20 cells

• Cell area: from 0.1 m2 to 5 m2

Grid

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• Constraints of a curvilinear grid • Boundary-fitted and near-orthogonal

• 20 layers (sharp vertical gradients)

• 7600x20 cells

• Cell area: from 0.1 m2 to 5 m2

Grid

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• Constraints of a curvilinear grid • Boundary-fitted and near-orthogonal

• 20 layers (sharp vertical gradients)

• 7600x20 cells

• Cell area: from 0.1 m2 to 5 m2

Validation

Validation in winter period Average air temperature during the simulation: 6.6 °C

Relatively long time simulation (7 days) Uncertain initial conditions

Objective to reproduce Shape of the thermal plume

Vertical temperature profiles (gradients)

Flow field

Primarily attempted to validate the turbulence model

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Validation: Shape of the plume

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Validation: Temperature profiles

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• 16 vertical profiles

• High Precision Thermometer

• At 4 depths • 0.05 m

• 0.25 m

• 0.45 m

• 0.65 m

Validation: Temperature profiles II.

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Validation: Temperature profiles III.

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Validation: Velocities

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• Nortek Vectrino Doppler • Suitable for low velocity

measurements

• 25 Hz

• 19 points

• At 0.25 m depth

• 1.5-2 m from the shore

Validation: Velocities II.

Flow field at 0.25 m depth

: Modelled

: Measured

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Validation: Velocities II.

Flow field at 0.25 m depth

: Modelled

: Measured

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Validation: Velocities III.

Flow field at 0.25 m depth

: Modelled

: Measured

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Mixed layer depth

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Mixed layer depth

0.2 °C

• Depth, where the difference

between maximum and

minimum temperatures is less

than 0.2 °C

• Describes stratification in every

point

Water age

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• Modelled as a conservative

tracer

• Advection-diffusion process

• The concentration of the tracer

within the inflow is decreased

by one unit each hour

Representation of stratification and water exchange

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How to improve water quality by hydrodynamics?

Different vertical deflector wall configurations

Passive and cheap solution

It has been proposed for similarly functioning cooling ponds

Circulation system?

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Wall configurations

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Wall configurations: Mixed layer depth

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Wall configurations: Water age

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Summary

The model was suitable to resolve low Reynolds number thermodynamics in a shallow setting Sharp gradients resolved

Through the use of water age we have revealed the zones where the water exchange is slower

We attempted to improve the water exchange by deflector walls however those did not influence the conditions significantly

In the near future we may consider to apply a circulation system in the model and test its impact

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Thank you for your attention!

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