# Imaging Flow Velocimetry with Laser Mie Scattering
## 5 Application Examples, Challenges and Perspectives  
Since the applications of ILDV as the imaging Doppler technique with the frequency-based signal are limited to slow convection flows and SFV as imaging time-of-flight techniques with time measurements are more focused on surface velocity measurements, the following consideration is focused on applications of DGV as an imaging Doppler technique with amplitude-based signal evaluation and PIV/PTV as imaging time-of-flight techniques with space measurements. An overview of the manifold PIV applications is given in the book [179]. A textbook that summarizes the manifold DGV applications does not exist, because the absolute number of groups working with DGV is very low. For a comparison between PIV and DGV, we refer the reader to the work of [PERSON] et al. [136], who applied both techniques in large-scale wind tunnels. Aerodynamic flow studies, e.g., in turbomachinery, as well as fundamental studies of boundary layers and turbulent flows usually require measurements close to walls. Boundary layer measurements close to walls with DGV and PTV are reported in [139; 180; 181], respectively. Under optimal conditions, PTV measurements allow flow velocity measurements with a minimal distance that is the radius of the scattering particle. DGV and PIV measurements inside a rotating machine are described in [182; 183; 184]. The occurrence of unavoidable light reflections causes measurement errors for PIV and DGV [185]. Error correction strategies based on a temporal filter to extract the moving particles [186] and based on the measurement with multiple observation directions to eliminate the influence of the incident direction [139] were developed for PIV and DGV, respectively. The investigation of flows in combustors and flames is an important application area. DGV measurements in flames were pioneered by [PERSON] et al. [187]. Combined DGV-PIV measurements in a combustor to obtain 3d velocity images with a single observation direction were introduced by [PERSON] et al. [36]. The single observation direction perpendicular to the window of the combustion chamber minimizes the errors due to image distortions. Furthermore, [PERSON] et al. reported time-resolved DGV measurements with a measurement rate of \(100\,\mathrm{kHz}\) to investigate flow velocity oscillations and thermoacoustic phenomena [188; 189]. High-speed PIV measurements with rates up to \(10\,\mathrm{kHz}\) were achieved by [PERSON] et al. [119; 190], whereas volumetric, single shot PIV measurements in flames were for instance reported by [PERSON] et al. [191]. Flow measurements in flames are challenging due to the flame light, which disturbs the optical flow velocity measurement. State-of-the-art techniques to cope with this influence are blocking the flame light in the detection unit with bandpass filters while illuminating with narrow-band lasers, applying high power lasers and/or pulsed lasers to surpass the flame light intensity and coding the scattered light signal for instance as a period signal with a high frequency. Furthermore, spatiotemporal variations of the refractive index field occur, which causes measurement errors for PIV and DGV. The effect is qualitatively described for PIV regarding flame flow measurements by [PERSON] et al. [192] and also quantitatively characterized regarding the shock wave in a shear layer and the different air density at an expansion fan by [PERSON] et al. [193]. [PERSON] et al. [194] performed simulations and experiments to compare the performance of PIV and DGV for refractive index changes in the imaging path by droplets condensed on a glass window and a flame. DGV seems to be more robust with respect to such disturbances. However, further studies are required to quantify the measurement error for PIV and DGV in flame flows. The challenge to measure in flows with a varying refractive index field also occurs for two-phase flows such as sprays. Spray flows are important, e.g., to optimize the injection of fuel. PIV spray measurements are described in [195; 196].