Particle Image Velocimetry (PIV)

Particle Image Velocimetry" is an imaging measurement method used to determine the velocity field in a flow. A typical measurement setup consists of a pulsed laser with light sheet optics, a camera and a computer, which is used both to synchronize the individual components and to evaluate the measurement results in a computer-intensive manner. Since the light scattered by the flow is absorbed by the camera, it is usually necessary to add particles to the flow. The particles must have as good a flow sequence as possible and must not influence the flow itself so that the measurement results are not distorted. As a result, small particles with an average diameter of approx. 1 µm are used, which requires the use of a powerful laser. By exact synchronization of laser and camera it is achieved that two pictures with a short time interval Δt can be taken one after the other. In the next step, the displacement of the particles from one exposure to the next is determined from these image pairs using a suitable correlation method. Together with the known time interval Δt between the individual exposures, the flow velocity is finally calculated from this.

The described measurement setup is suitable for the investigation of the flow field in one plane and thus allows the determination of two velocity components. However, if the measuring system is extended by an additional camera, the third speed component within the light section can also be detected if the cameras are appropriately arranged. This measurement setup is usually referred to as stereoscopic PIV.

Besides the fact that this method allows a contactless and thus interference-free measurement of the flow velocity, its biggest advantage is the information content.  While most of the available measuring methods only allow a punctual measurement of the flow velocity, in "Particle Image Velocimetry" an entire velocity field is determined from only one double image acquisition.This makes the method particularly suitable for the verification of flow simulations or the generation of boundary conditions.

Fields of application:

  • Qualitative and quantitative determination of the flow in intake and exhaust systems, air intakes, pumps and fans
  • Determination of boundary conditions for flow simulations
  • Determination of time-averaged values in transient flows
  • Verification of flow simulations

 

Single exposure of a torus vortex. The light scattered by the seeding particles is clearly visible.
Temporally averaged velocity field of a torus vortex. The flow velocity is shown in false colours.