Vertical dense jet in flowing current


The discharge of brackish water, as a dense jet in a natural water body, by the osmotic power plants, undergoes complex mixing processes and has significant environmental impacts. This paper focuses on the mixing processes that develop when a dense round jet outfall perpendicularly enters a shallow flowing current. Extensive experimental measurements of both the salinity and the velocity flow fields were conducted to investigate the hydrodynamic jet behavior within the ambient current. Experiments were carried out in a closed circuit flume at the Coastal Engineering Laboratory (LIC) of the Technical University of Bari (Italy). The salinity concentration and velocity fields were analyzed, providing a more thorough knowledge about the main features of the jet behavior within the ambient flow, such as the jet penetration, spreading, dilution, terminal rise height and its impact point with the flume lower boundary. In this study, special attention is given to understand and confirm the conjecture, not yet experimentally demonstrated, of the development and orientation of the jet vortex structures. Results show that the dense jet is almost characterized by two distinct phases: a rapid ascent phase and a gradually descent phase. The measured flow velocity fields definitely confirm the formation of the counter-rotating vortices pair, within the jet cross-section, during both the ascent and descent phases. Nevertheless, the experimental results show that the counter-rotating vortices pair of both phases (ascent and descent) are of opposite rotational direction.

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A 0 :

Jet source area (m2)

B :

Mean channel width (m)

B 0 :

Initial buoyancy flux (m4 s−3)

c :

Local fluid conductivity (S m−1)

c a :

Ambient fluid conductivity (S m−1)

c 0 :

Initial jet fluid conductivity (S m−1)

C :

Dimensionless jet excess salinity (conductivity) (−)

D :

Jet source diameter (m)

F :

Jet densimetric Froude number (−)

g :

Gravity acceleration (m s−2)

g′ :

Initial reduced gravity (m s−2)

H :

Flow depth (m)

l M :

Jet-to-plume length scale (m)

l m :

Jet-to-crossflow length scale (m)

l Q :

Discharge length scale (m)

l B :

Plume-to-crossflow length scale (m)

M 0 :

Initial momentum flux (m4 s−2)

Q 0 :

Initial discharge volume flux (m3 s−1)

Re 0 :

Initial jet Reynolds number (−)

Ri d :

Richardson number (−)

S,s :

Dilution (−)

S i ,s i :

Minimum dilution at the impact point (−)

S t ,s t :

Minimum dilution at the position of the terminal rise height (−)

U,V,W :

Streamwise, spanwise and vertical time-averaged velocity (ms−1)

U a :

Mean ambient channel velocity (m s−1)

U c :

Velocity scale (m s−1)

U 0 :

Initial jet velocity (m s−1)

u r :

Ratio of ambient to jet velocity (−)

x, y, z :

Longitudinal, lateral and vertical coordinates, respectively (m)

x i :

x-Position of the jet impact point (m)

x t :

x-Position at which the jet attains its maximum rising height (m)

z L :

Thickness of the bottom layer of the spreading density current (m)

z mt :

Jet terminal rise height (m)

z t :

Jet centerline rising height (m)

z 0 :

Jet port height (m)

Φ, Φ′ :


ν 0 :

Initial jet kinematic viscosity (m2 s−1)

θ :

Angle relative to the horizontal (°)

ρ :

Local fluid density (kg m−3)

ρ a :

Ambient fluid density (kg m−3)

ρ 0 :

Initial jet fluid density (kg m−3)


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This research was supported by a grant from the Italian national project “Hydroelectric energy by osmosis in coastal areas”, PRIN 2010-2011. The experiments were carried out at the Coastal Engineering Laboratory of the Dpt. of Civil, Environmental, Building Engineering and Chemistry, Technical University of Bari, Italy.

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Correspondence to M. Mossa.

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Ben Meftah, M., Malcangio, D., De Serio, F. et al. Vertical dense jet in flowing current. Environ Fluid Mech 18, 75–96 (2018).

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  • Dense jet
  • Shallow flow
  • Salinity
  • Velocity
  • Penetration
  • Dilution
  • Vortices