Particle Image Velocimetry (PIV) systems

Particle Image Velocimetry (PIV) is a very common and useful tool to investigate the flow phenomena in models of blood vessels, heart valves or artificial organs. A 2D high-resolution PIV system has been developed, setup and successfully tested. The system contains a high-speed video camera (512 × 480 pixel at 250 fps, 512 × 240 pixel at 500 fps) for the image capture and a continuous laser light source (λ = 682 nm, 75 mW) with an adapted light sheet optics for the illumination of the flow. After recording the images are stored to the hard disk drive of a PC and can be analyzed using the cross correlation technique. The software used for the analysis is Davis by LaVision, Göttingen. The system is easy to set up since no synchronisation between the light source and the camera is necessary. The system is very useful for hydromechanics, where the velocities are small compared to aerodynamics.

Some results are shown below. Please, click on the image to pop-up enlarged images and movies.

Figure 1: Flow in blood chamber of new system. The flow was analyzed in a blood chamber of a new heart assist system. A movie of three images shows the diastole (inflow), the flow between diastole and systole and systole (outflow). More details of the heart assist sytem can be found here.

Figure 2: Flow in a carotid artery – More information about the carotic artery can be found here. Due to illumination reasons, the flow in the left branch could not be analyzed.

Figure 3: This is a result of the flow in a new designed valve. More details about the new S-shape valve can be found here.

A new method for the spatial and temporal assessment of the wall shear stress is currently being developed at the Biofluid Mechanics Lab. This method can be considered a special development of the classical PIV. It permits to look selectively at the flow close to the wall. The selection is made by using a fluid, which does not permit the light to penetrate deeply into the flow. The transparent flow model is illuminated by a monochromatic diffuse light source. Due to the limited penetration depth of the light only the particles moving close to the wall are lighted. Within the illuminated layer, the particles appear more or less bright, depending of their distance dp to the wall. A gray value analysis with a special image processing program permits to determine this distance, which is necessary for the calculation of the wall shear stress.

The method was tested to assess the laminar flow in a rectangular U-Duct with a backward-facing step at Re = 50. The following image shows the velocity field immediately after the step at a distance d_p = 0.2 mm from the wall duct.

Figure 4: Flow at the wall in a rectangular U-Duct with a backward-facing step