Early Measurements of Turbulence in the Ocean: Motives and Techniques

Abstract

In the summer of 1950, the authors joined the staff of the Pacific Naval Laboratory (PNL) of the Defense Research Board of Canada. A research program was undertaken to see if the turbulent wake of a submarine could be detected for a useful distance behind the submarine. Initially almost nothing was known about either the nature of submarine wakes or the background turbulence in the ocean and the studies of both were undertaken. Initial attempts, starting in 1952, to adapt to the ocean the hot-wire anemometer technology used in studying laboratory turbulence, were total failures. The wires proved efficient plankton catchers, particularly of small gelatinous species. But by 1957 hot-films had been demonstrated to work in wind tunnels and the PNL team was expanded with additional physics and engineering support to develop a hot-film system specifically designed to work in seawater, which by this time was recognized to be an electrically conducting plankton soup. The research program was designed to include investigation of some fundamental aspects of the physics of high Reynolds number turbulence that were unresolved at the time. In 1941, A. N. Kolmogorov had published his seminal paper on the universal aspects of turbulence. The theory was widely respected, but was untested observationally because it applied to higher Reynolds numbers than were readily attainable in the laboratory. The required conditions did exist in coastal waters, and the authors set about confirming the predicted form of the energy spectrum using the very energetic turbulence found in tidal channels. Early results of this work were published in a letter to Nature in September 1959. Kolmogorov?s theory related the spectrum of turbulence to the rate of energy dissipation created by that turbulence. The only way to determine the dissipation was to measure the very small (millimeter) scales of the turbulence, and that required very small probes. Taking manufacturing difficulties into account, a probe scale of rather less than a millimeter was chosen. This scale permitted observation of the dissipation spectrum so that the constant in Kolmogorov dimensional analysis could be determined. Studies continued on the nature of turbulent wakes of ships and of the natural turbulence in the ocean. Knowing the form of the ?universal spectrum? including the ?Kolmogoroff constant? made it possible to determine the rate of energy dissipation?a quantity of interest in itself. By these means turbulence in the presence of wind wave motions from probe vibrations could be distinguished. In the early 1960s, probes were mounted on the bow of a submarine, which maneuvered so as to traverse its own wake. The wakes could be observed but were not clearly distinguishable from natural ocean turbulence, which, while very intermittent, appeared to be ubiquitous. These were among the first observations of turbulence in the thermocline.