Oceanographical Measurements
The laboratory course "Oceanographical Measurements" aims at training experimental skills and deepening existing or rather gaining new knowledge about physical and oceanographical phenomena by means of experimental demonstration and verification. The students are guided through a number of experiments in small groups. They set-up test rigs, carry out measurements and subsequently analyse their aquired data using mathematical and graphical methods. A course assessment is based on detailed lab reports of all experiments performed. Topics covered by the experiments are (amongst others)
- ocean surface waves, internal waves acoustic waves in water
- instabilities in geophysical flows
Surface, internal and acoustic waves
A set of experiments focusses on principal wave characteristics. The restoring forces driving ocean surface waves are the surface tension and the gravity. At very short wavelengths (&lambda < 1,7cm), surface tension outweighs gravity. The propagation velocity of so-called capillary waves or ripples decreases with increasing wavelength. Gravity is the dominant driving force for waves with larger wavelengths (gravity waves). The propagation speed of waves travelling through deep water (defined as water whose depth is greater than the depth of the wave motion) is a function of the wavelength (speed increases as wavelength increases) while in shallow water and close to the banks the main variable controlling the wave speed is the water depth.
The relationship between wavelength, propagation speed and water depth is demonstrated using a small acrylic glass tank (see upper left picture below). To perform the experiment, standing waves are generated by gently lifting and setting down one end of the tank. The wavelength of the emerging wave is twice the tank length. Several wave oscillations are video recorded and the wave period is then determined from the film footage (see upper right picture below). The cuboid tank shape allows for studying different wavelengths and water depths. The experimental results are compared to wave theory.
A common phenomena observed in oceans is the formation of internal waves at density boundaries. Sharp differences of density occur e.g. due to rapid changes in salinity or temperature with depth. Internal waves are demonstated using a two-layer acrylic glass tank filled with two fluids of different density and colour (see lower left picture above). The wave propagation speed is determined similarly to the surface waves and then used to derive the densities of the two liquids.
Acoustic methods play a major role for probing fluids. The techniques benefit from non-intrusive measurements that can easily be carried out from moving vessels. They are deployed for instance for probing flow velocity (Acoustic Doppler Current Profiler), temperature (Ocean Acoustic Tomography) and water depth (Sonar). The acoustic wave lab experiment serves the study of possible influences on the sound propagation velocity due to changeable oceanic qualities such as temperature or salinity (see lower left picture above).
Rotating tank experiments
Rotating tanks are used to demonstrate coriolis effects on wind-driven ocean currents and to study their underlying mechanisms. The applied demonstration in a rotating tank helps to understand fundamental dynamical processes which are otherwise described by elaborate mathematical equations and numerical models.
Detailed descriptions of several rotating tank experiments can be found in WEATHER in a TANK
Two rotating tank rigs (see pictures to the left) are available at the Institute of Oceanography to support teaching and lab courses. During the simulations, dyed water and ice cubes are used to visualise flow patterns and flow instabilities. Footage from a co-rotating video camera allows to view and study the flow phenomena in the rotating reference system.
The lab experiment shown in the movie illustrates the formation of Rossby waves. The ocean surface signature of Rossby waves amounts to a mere few centimeters. However, Rossby waves play an important role in ocean dynamics with significant impacts on weather and climate
Rossby waves form as a result of the conservation of angular momentum, or more precisely, potential vorticity (tendency of the fluid to "spin"): the ratio between absolute (planetary + relative) vorticity and vertical depth of the fluid column (i.e. ocean depth) is constant at all times. The planetary vorticity varies with latitude due to earth's spherical shape. Consequently, the relative vorticity of a fluid parcel which is displaced from its latitude circle must change to keep the absolute vorticity constant.
In the rotating tank experiments Rossby waves are generated by varying the depth of the rotating tank. The shallow area represents the north pole and the deeper area (video: tank bottom) represents the equator. A dyed ice cube is carefully placed into the "north-eastern" corner of a cyclonically rotating tank. Cold water from the ice cube sinks and in return circumfluent water is drawn towards the ice cube. Conservation of angular momentum initiates a cyclonic ice cube rotation relative to the tank rotation which then pushes water westwards of the ice cube down the slope. The water column is stretched and cyclonic vorticity is induced. The fluid parcel travels along the cyclonic path until it features a northwards flow component and returns to its initial latitude circle. Eastwards of the ice cube water is pushed up the slope sqashing the water column and thus inducing anticyclonic vorticity. The restoring force is proportional to the meridional displacement and a once displaced fluid parcel will ocillate around its initial latitude circle. A meridional wave originates which propagates westwards.
Wind-Wave-Tank
The wind-wave tank (WWK, German only) of Hamburg University is situated on the premises of the Federal Waterways Engineering and Research Institute in HH-Rissen. Experiments in the wind-wave tank focus on aspects of wind-wave interactions at the ocean surface and subsequent impacts on the flow field close to the water-air interface.
The wind-wave tank in 24 m long, 1 m wide with a mean water depth of 0.5 m. The air space is 1 m high. A radial blower provides wind speeds up to 15 m/s, generating waves with characteristical wavelengths extending to 50 cm and amplitudes up to 2 cm. Long waves with wavelenghts ranging between 0.25 m and 3 m and amplitudes up to 3 cm can be generated with a mechanical wave flap. The available instrumentation allows for measurements on either side of the water-air interface.
Wave surface elevation are measured using a thin tungsten wire probe (Fig.below, right) and compared to wave slopes measured with a laser slope gauge. Experiments are carried out at different wind speeds to study wind-wave interactions as well as the generation and propagation of capillary and gravity waves and the phenomena of micro-breaking at wave crests.
An Acoustic Doppler-Velocimeter (Fig. below, left) is available to measure all three water flow velocity components in high temporal resolution. Vertical profiles of ADV measurements are used to study the wind generated flow characteristics as well as the orbital motion of fluid particles in waves.
Vertical profiles of the mean wind speed are measured with a pitot static tube (Fig. below) for several reference wind speeds. The measurements are used to derive the wind speed dependency of the roughness layer above the water surface and thus explore the energy transfer from the flow into the wave field.
Available measurement equipment at the wind-wave tank laboratory include oscilloscopes, measurement amplifiers, Ad converters and several computers running the software packages which are required to carry out the measurements and to analyse the aquired data. The students are actively involved in the experimental design and some preliminary analysis (e.g. wire probe and wave flap calibrations) are carried out in situ to integrate their results into the following measurement programme.