A novel method for three-dimensional three-component analysis of flows close to free water surfaces

Initial effort is made to establish a new technique for the measurement of three-dimensional three-component (3D3C) velocity fields close to free water surfaces. A fluid volume is illuminated by light emitting diodes (LEDs) perpendicularly to the surface. Small spherical particles are added to the fluid, functioning as a tracer. A monochromatic camera pointing to the water surface from above records the image sequences. The distance of the spheres to the surface is coded by means of a supplemented dye, which absorbs the light of the LEDs according to Beer–Lambert’s law. By applying LEDs with two different wavelengths, it is possible to use particles variable in size. The velocity vectors are obtained by using an extension of the method of optical flow. The vertical velocity component is computed from the temporal brightness change. The setup is validated with a laminar falling film, which serves as a reference flow. Moreover, the method is applied to buoyant convective turbulence as an example for a non stationary, inherently 3D flow.