Digital imaging anemometry involves photographing particles suspended in the flow. The measurement method requires the walls of the area where the model is placed to be transparent. As a result of the measurement, we obtain a quantitative, spatial image of the flow (velocity vectors determining the values of coordinates along with their direction). The measurement method is non-invasive because it does not require the introduction of flow-disturbing sensors. The only disturbance to the flow is the introduction of a suspension chosen so that it moves at the speed of the medium being studied. In the case of flows in liquids, after introducing liquid crystals into the flow, the method allows for the measurement of temperature fields (Particle Image Thermometry – PIT).
The measurement tool used in these studies is digital cameras (CCD), which allow for capturing two images at a known time interval. The photographed surface of the flow is illuminated by a plane of light (known as a light sheet). Image registration takes place in a plane perpendicular to the plane of light. The measurement method requires the use of pulsed object illumination. Additionally, the light source must be synchronized with the camera, more precisely with the moment of capturing the images.
The essence of the measurement method will be presented in the case of flat flow. To determine the local displacement of markers from images I1 and I2 (I – pixel intensity of the image), sub-images Is1 and Is2, called a mask, are selected. For each mask position, a two-dimensional cross-correlation function is calculated. The position of the maximum of the function indicates the displacement vector in the xy plane. It should be noted that the displacement determined in this way is the average displacement within the mask. Therefore, the method is statistical in nature, and it is required that an appropriate number of markers be present within the mask. Knowing the displacement and the time between images, the flow velocity field can be calculated.
