A non-invasive airborne wave monitor

• A system is developed for remotely measuring local water surface fluctuations.
• An acoustic signal is reflected from the surface and the phase is monitored.
• The system is tested in the laboratory on gravity waves and flow induced waves.
• Accuracy is shown to be a function of acoustic wavelength and component setup.
• Errors are below 5% of acoustic wavelength and below 10% of water wave height.

This work presents a new acoustical method for remote measurement of the surface characteristics of the dynamic air–water interface in turbulent free-surface flows. The technique uses the reflection of a monochromatic ultrasonic wave by the dynamically rough air–water interface to measure the water surface position. It is found that with careful selection of the acoustical components and their configuration, the phase of the reflected signal responds to the local fluctuations in the fluid interface at the point of specular acoustic reflection. In order for the method to be applicable, three criteria must be satisfied: (1) the dominant wavelength of the surface under investigation must be greater than the first Fresnel zone corresponding to the wavelength and component geometry of the acoustical system; (2) the mean magnitude of the instantaneous local surface gradient must not exceed 0.025; and (3) the root-mean-square wave height must be greater than 1% of the acoustic wavelength. Under these conditions the mean error of the system is within 5% (and usually within 1%) of the acoustic wavelength, and is generally within 10% of the wave amplitude for turbulence generated waves, and 3% of the amplitude for gravity waves. This error may be reduced by optimising the acoustic wavelength for the surface of interest. For turbulent depth limited flows, the surface waves fall well within the criteria, and the absolute errors are independent of wave height, so for larger wave heights, the relative error can be considerably lower. The technique provides a robust system for monitoring the dynamics of free surface flows, which is non-invasive, low cost, and low power. The method has been tested on laboratory flows but should be applicable to remote sensing of free surface properties on a local scale in field environments where invasive techniques are difficult to implement such as might be found in coastal, river and wastewater environments.