What Is an Internal Wave (Tide) and How to Use the Energy of Internal Waves
Internal waves (tides) are waves that are formed in the water column of oceans and seas at different depths. The first to take notice of internal waves was Fridtjof Nansen, who during his expedition in the Arctic Ocean on the ship “Fram” (“Forward”) observed cyclic variations of salinity and temperature at the same depth.
It is rather difficult to observe internal tides directly. One of the existing methods of studying these waves is an acoustic depth monitoring of sonic scattering levels rising and falling on the internal tides. Huge internal waves can be discovered with the help of bathyscaphes and mesoscaphes.
What causes the existence of internal tides?
This kind of waves originates from the direct influence of outer forces or by resonance interaction between surface waves and their own oscillations in the density layer.
The most stable and widespread waves of this type are tidal waves. Their periods may vary from 6 hours to 14 days, but the most clearly defined waves have a diurnal or semidiurnal period.
Wind and air pressure, affecting the water surface with unalike force and different duration, create a broad range of internal waves of meteorological origin.
In the ocean almost always one can observe internal inertial waves related to free oscillations. They arise after outer forces (for example, meteorological) have stopped their influence on the heterogeneous in density body of water, which strives to return to the former state, which results in cyclical oscillations near the equilibrium position.
These waves can be formed by currents when the latter have great velocity gradients in the density level.
Which phenomenon is called dead water?
Internal waves can cause an interesting phenomenon known as dead water. It was first recorded by the above mentioned Norwegian oceanographer Fr. Nansen. When this event happens, a vessel may suffer from strong resistance preventing it from moving forward in seemingly calm conditions. This happens when the vessel is sailing in rather fresh water with the level depth comparable to the vessel’s draught.
What are the properties (velocity, height, length, etc.) of internal waves?
Internal tides move much slower than, for instance, surface wind-generated waves. Phase velocity of these waves doesn’t normally exceed several meters per second. Usually, the velocity of these waves is 7-10 times lower than the velocity of surface waves of the same length.
The height of internal tides is bigger than that of surface waves, as the energy needed to rise a layer of water in the air is considerably greater than the energy needed to rise a layer of water in the water, close in density. The height of these waves may reach 20 – 30 meters. Fr. Nansen described waves with 100 meters of height.
The frequencies are usually much lower and the amplitudes are higher than those of surface waves because the density differences within a fluid are a lot smaller than the fluid density. The lengths of the waves vary from several centimeters to many kilometers, the periods, as was already mentioned, from seconds to hours and even days.
How does the energy propagate and how can it be used?
The energy of internal tides propagates horizontally and at the same time vertically, unlike surface waves, where energy spreads only horizontally. This difference can be explained by the nature of the stratification which supports the waves. Surface waves exist due to the sharp change in density between air and water; it is restricted to the surface – and so is their energy propagation. Internal waves, on the other hand, exist due to the stratification of the ocean’s interior, which is smoothly distributed over the vertical, and so energy is carried from one depth level to the other. The internal tide generated over the continental slope propagates into the deep ocean, following a diagonal path, in a beam-like manner.
A special technique must be used for trapping the energy of these waves. The idea is that propellers or turbines should be placed under the sea, which would transform the energy of wave movement into a mechanical energy. This source of energy is thought to be a much cleaner and safer source of power.
The potential of using these waves to generate electric power is enormous. Several factors make the use of this source of renewable energy more appealing than the use of other sources:
- predictability
- little environmental impact
- practical feasibility
- less physical damage for the equipment
Although this technique is not widely used at present, the potential of using the energy of internal waves for future electricity generation is huge. This source of energy is much more predictable than solar power or wind energy, although it may cost more and be limited to particular sites where the energy can be obtained. However, modern technological improvements and developments (in turbine technology and design) suggest that it might become a lot easier and cheaper to get energy from this source.
