One evening, around high tide, near the old seamill in Ploumanach (The monk's place in Breton : St Guirec)...
The old seamill in the Ploumanach port is a granite building from the XVIIIth century which used the tidal energy to rotate its waterwheel.
It juts out from around 14 feet in the Ploumanach port and leans on a granite dike (14 feet high, 12 feet wide and 75 feet long). It separates the small harbour of Ploumanach from the Traouïeros valley towards which the sea flows. The place is wonderful, seagulls as well as river birds (herons, egrets, swans) may be watched there.
This dike has 2 parallelepipedic holes on each side of the mill. Each hole is about 7 feet high and 3 feet wide and let the tide flow in each direction.
In front of one of the holes, on the ground and upright to the dike, some middle-size granite rocks) make a slight obstacle to the flow perpendicular to the dike.
The vortex-type whirling phenomenon can be observed when the water level is sufficient, about one hour before the high tide time and for middle-amplitude tides.
Such a vortex is spawned from the local ebb and flow which goes through the dike hole, with an instability reinforced by the presence of obstacles on the ground.
At first sight, the typical funnel-like shape of a vortex appears and takes sometimes a brown or orange color (because of the particles aborbed by the vortex) and seems to dance underwater.
One first thinks to a boot fastened to a ring on the ground but there is no boot there at low tide. Your imagination is captured and runs far away...
Could it be a water viper ? This shape seems so lively !
No, it is a very thin helix-shaped tube where the light is caught. Bits of wood, bits of seaweed and other particles are captured there ceaselessly and, heavier than water, swept along the tube towards the vortex centre, quiet area of minimum energy before the current perpendicular to the dike draw them up further...
Keywords - Semantic relations sorted by different approach angles:
- Basic Ideas:
Instability in fluids - hydrodynamic Instability - aerodynamic Instability - Search for a minimum energy state - Transition channel between an instable state, with high energy levels and a quiet, stable state.
- Applications:
Cooling or Heat Exchange Systems (climate) - Particle-filtration Systems
- Patterns:
spiral, helix, solenoïd, helical motion, funnel, siphon
- Relation with Earth:
Coriolis force, axial rotation of Earth
- Dynamics:
Rotation, circulation, curl vector,streamlines, neutral point
- Visual associations: atmospheric phenomena:
cyclone, hurricane, eye of hurricane, tornado, typhoon
- Visual associations: instability phenomena in fluids:
maelström (whirlpool of major magnitude)
- Visual association: Space :
Spiral galaxy, accretion of rotating matter, matter attraction by a black hole.
Some explanations about the VORTEX phenomenon...
Definition
(Extract from Wikipedia - The free Encyclopedia)
A vortex is a spinning turbulent flow (or any spiral whirling motion) with closed streamlines.
The shape of media or mass rotating rapidly around a center forms a vortex.
It is a flow involving rotation about an axis.
Etymology
Latin : vortex which means "swirl"
The word "turbulence" comes from the Latin word: turbo,inis meaning "wheel".
Always underlying , the concept of rotation
Dynamics
A vortex can be any circular or rotary flow that possesses vorticity.
Vorticity is a mathematical concept used in fluid dynamics. It can be related to the amount of "circulation" or "rotation" in a fluid.
In fluid dynamics, vorticity is the circulation per unit area at a point in the flow field. It is a vector quantity (curl vector), whose direction is (roughly speaking) along the axis of the swirl.
Also in fluid dynamics, the movement of a fluid can be said to be vortical if the fluid moves around in a circle, or in a helix, or if it tends to spin around some axis. Such motion can also be called solenoidal. In the atmospheric sciences, vorticity is a property that characterizes large-scale rotation of air masses. Since the atmospheric circulation is nearly horizontal, the (3 dimensional) vorticity is nearly vertical, and it is common to use the vertical component as a scalar vorticity.
Observations
A vortex can be seen in the spiraling motion of air or liquid around a center of rotation.
Circular current of water of conflicting tides form vortex shapes.
Turbulent flow makes many vortices.
A good example of a vortex is the atmospheric phenomenon of a whirlwind or a tornado. This whirling air mass mostly takes the form of a helix, column, or spiral. Tornadoes develop from severe thunderstorms, usually spawned from squall lines and supercell thunderstorms, though they sometimes happen as a result of a hurricane. See Polar vortex.
A mesovortex is on the scale of a few miles (smaller than a hurricane but larger than a tornado).
On a much smaller scale, a vortex is usually formed as water goes down a drain, as in a sink or a toilet. This occurs in water as the revolving mass forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. This swirling flow structure within a region of fluid flow opens downward from the water surface.
Reynold's Number- Shape Characterization
The Reynold's number R is a quantity without dimension which characterizes the shape of a vortex or a turbulent flow.
As the magnitude of a turbulent flow (hence its shape) is :
- proportionnal to the size of the generating obstacle L
- proportionnal to the speed S of the fluid meeting the obstacle
- proportionnal to the amount (volumic density) r
of fluid hitting the obstacle
- inversely proportionnal to the viscosity h (because slowered) of the fluid meeting the obstacle
| Reynolds Number R |
(L .S.r / h) |
Instances
- Lift-induced drag of the wing of an aircraft,
- Primary cause of drag in the sail of a sloop,
- Sunspots,
- Tornados, hurricanes, whirlwinds, dust devils,
- Maelströms (whirlpools)
To see some vortex pictures , click on button "vortex pictures" !
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