This document summarizes research using 3D modeling to study mixing in a stratified urban pond with a thermal water inflow. The model simulated conditions over 7 days in winter. It reproduced the shape of the thermal plume, vertical temperature profiles measured at various depths, and low flow velocities measured near the shore. Different vertical deflector wall configurations were tested to improve water quality by enhancing circulation, but did not significantly influence conditions. A circulation system may be considered in future modeling to test its impact on mixing.

1. 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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2. 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

3. 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

4. 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

5. 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

6. 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

7. 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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8. Validation: Shape of the plume
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9. 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

10. Validation: Temperature profiles II.
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11. Validation: Temperature profiles III.
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12. 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

13. Validation: Velocities II.
Flow field at 0.25 m depth
: Modelled
: Measured
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14. Validation: Velocities II.
Flow field at 0.25 m depth
: Modelled
: Measured
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15. Validation: Velocities III.
Flow field at 0.25 m depth
: Modelled
: Measured
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16. 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

17. 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

18. Representation of stratification and water exchange
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19. 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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20. Wall configurations
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21. Wall configurations: Mixed layer depth
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22. Wall configurations: Water age
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23. 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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24. Thank you for your attention!
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