Research Article

Experiments in Fluids

, Volume 50, Issue 4, pp 825-838

Open Access This content is freely available online to anyone, anywhere at any time.

On the velocity of ghost particles and the bias errors in Tomographic-PIV

  • G. E. ElsingaAffiliated withLaboratory for Aero and Hydrodynamics, Department of Mechanical Engineering, Delft University of Technology Email author 
  • , J. WesterweelAffiliated withLaboratory for Aero and Hydrodynamics, Department of Mechanical Engineering, Delft University of Technology
  • , F. ScaranoAffiliated withAerodynamics Group, Department of Aerospace Engineering, Delft University of Technology
  • , M. NovaraAffiliated withAerodynamics Group, Department of Aerospace Engineering, Delft University of Technology

Abstract

The paper discusses bias errors introduced in Tomographic-PIV velocity measurements by the coherent motion of ghost particles under some circumstances. It occurs when a ghost particle is formed from the same set of actual particles in both reconstructed volumes used in the cross-correlation analysis. The displacement of the resulting ghost particle pair is approximately the average displacement of the set of associated actual particles. The effect is further quantified in a theoretical analysis and in numerical simulations and illustrated in an actual experiment. It is shown that the bias error does not significantly affect the measured flow topology as deduced in an evaluation of the local velocity gradients. Instead, it leads to a systematic underestimation of the measured particle displacement gradient magnitude. This phenomenon is alleviated when the difference between particles displacement along the volume depth is increased beyond a particle image diameter, or when the reconstruction quality is increased or when the accuracy of the tomographic reconstruction is improved. Furthermore, guidelines to detect and avoid such bias errors are proposed.