Uncertainty of flow velocity measurements due to refractive index fluctuations

•Refractive index fluctuations increase the measurement uncertainty of DGV and PIV.•Systematic errors result from slow and large scale fluctuations.•Random errors for DGV and PIV are caused by fast fluctuations.•Regarding PIV small scale fluctuations cause random errors too.

The measurement uncertainty of optical flow velocity measurements suffers from distortions by refractive index fluctuations in the imaging path. These fluctuations can occur, e.g. due to a contamination of the optical access or temperature and pressure gradients within the flow.In this paper, the increase of the measurement uncertainty of particle image velocimetry (PIV) and Doppler global velocimetry (DGV) due to refractive index fluctuations in the imaging path is analyzed. Therefore flow measurements under the influence of refractive index fluctuations are performed. The results are validated and expanded by ray tracing simulations. Both experimental and numerical results confirm the different emergence of the measurement uncertainties of PIV and DGV.