Tomographic shadowgraphy of swirled non-reactive spray injection in a generic aero engine burner under realistic operating conditions

This contribution describes the application of tomographic shadowgraphy to measure instantaneous velocities of droplets undergoing airblast-atomization in the non-reactive flow of a generic aero engine burner model at Weber numbers of Weaero = 36

Feasibility of multi-aperture micro-PTV for wall shear stress measurements

We present a feasibility study on 3d-3c micro particle tracking velocimetry (µPTV) for boundary layer flows using a single high-speed camera with a microscope lens. Multi-aperture micro-PTV (MA-µPTV) relies on the "defocusing" concept (Willert & Gharib, 1992) and determines the wall distance of a particle from the size of projected particle image triplets generated by placing a triplet aperature on the entrance pupil of the microscope lens. Illumination with a high-speed pulsed laser is introduced through the same window used for imaging. The calibration of the orientation and size of the particle image triplets is described and accounts for variation of both lateral image position as well as wall distance. Geometrically constrained templates of 2d Gaussians are used to provide least square fitting of triple image intensities, thereby iteratively improving the initial centers and depth position of each particle. In addition, the convolution of a Gaussian and a Lorentzian is studied, that latter of which is considered promising for modeling defocused particle images at large distances from the focus. Lagrangian particle tracks are reconstructed from 3d particle positions using state-of-the-art tracking procedures. The measurement technique is demonstrated in a developing turbulent duct flow of a small wind tunnel up to Re_tau = 836 for a wall-bounded volume up to 1.7x1.3x1 mm^3 (in viscous units 52x40x31). Comparative profile-PIV measurements provide reference measurements including estimates of the wall shear stress. A comparison with flow statistics obtained with 2d-2c PIV is made possible by bin-averaging the velocities of the particle tracks in the wall-normal direction at a resolution of 10-16 µm. Mean wall shear rates obtained from MA-µPTV and single-line cross-correlation on the PIV data are in good agreement. The fluctuations in wall shear rate are consistent with correlations found in the literature. Statistics from bin-averaged velocities of all tracked particles indicate consistency with profile-PIV and are in agreement with DNS data up to wall distances of 28 viscous units

Event-based imaging for visualization and measurement of turbulent flows

Event-based vision (EBV), dynamic vision sensing (DVS) or neuromorphic imaging describe a rather new sub-field within computer vision, differing considerably from classical frame-based imaging (Gallego et al., 2022). Event cameras only record contrast changes ("events") within the scene, either going from dark to bright (positive event) or bright-to-dark (negative event). As the pixels "fire" independently an asynchronous stream of events results that consist of pixel coordinates, a time stamp and a binary contrast change signal. Static areas in the imaged scene provide no information; intensity data is essentially not available. At the same time, event cameras feature a very high dynamic range (>110 dB) and are considerably more sensitive than conventional CCD/CMOS cameras. In the context of particle imaging, narrow event streaks are produced in the space-time domain and can be processed to provide 3D-3C particle tracking velocimetry (PTV) data.The recently introduced event-based imaging velocimetry (EBIV) technique combines EBV and light sheet illumination to provide time-resolved, planar (2D-2C) velocity fields (Willert & Klinner 2022; Willert 2023). In this work we apply EBIV to obtain time-resolved velocity profiles of a turbulent boundary layer (TBL) in analogy to the profile-PIV technique (Willert, 2015). The latter has been used to simultaneously provide detailed velocity statistics and time-resolved data of turbulent flows (see eg. Willert et al. 2017). The field of view is generally illuminated by a high-speed pulsed laser that is collimated into a narrow light sheet. A second EBIV configuration captured the flow in the viscous sublayer of the TBL using a thin wall-parallel light sheet of <1 mm thickness and a set of three synchronized event-cameras. The ultimate aim of this setup is to estimate the unsteady wall shear stress field through triangulation of the recorded particle tracks which are addressed in this contribution. The dynamics of the near wall flow can already be visualized in the raw event data

Time-resolved velocity profile measurement using event-based imaging

We describe the implementation of time-resolved velocity profile measurement using event-based vision (EBV) employing an event-camera in-place of a high-speed camera. A narrow light sheet provided by a pulsed laser is imaged by a narrow region of interest on the event camera sensor which allows the capture of a dense particle field. Subsequent multi-frame processing is employed in an off-line manner to retrieve time-resolved velocity profiles from which statistics and spectra can be obtained. The described system is capable of providing data quality on par with currently used, considerably more expensive, high-speed PIV hardware at equivalent frame rate of 10 kHz. The technique is demonstrated with measurements of a fully developed turbulent boundary layer in a wind tunnel at free-stream speeds up to 10 m/s. The data is directly compared to high-speed profile-PIV measurements obtained on the same facility at matching operating and regions of interest

Three-dimensional imaging of swirled spray injection in a generic aero engine burner under realistic operating conditions

Tomographic shadowgraph imaging is applied to reconstruct the instantaneous three-dimensional spray field immediately downstream of a generic aero engine fuel injector. Within the swirl passage of the injector model, a single kerosene jet undergoes air-blast atomization in a cross-flow configuration at Weber numbers of We=360-770, air pressures of p_a=4-7 bar and air temperatures of T_a=440-570K. High-speed, high magnification shadowgraphy is used to visualize the initial fuel atomization stages within the fuel injector before the spray enters the spray chamber. The 4-camera tomographic measurement setup is described in detail and includes a depth-of-field analysis with respect to droplet size based on Mie simulations and calibration data of the point-spread function. For a volume size of 16x13x10mm³ the smallest resolvable droplet diameter is estimated to be d=10µm within the focal plane and increases to approx. 20µm towards the edges of the volume. Droplet velocities above the resolution limit were retrieved by 3-d cross-correlation of two volumetric reconstructions recorded at two consecutive time-steps. This is accompanied by an error analysis on the random error dependency on the camera viewing geometry. The results indicate increasing motion and fluctuations of the spray tail with increasing temperature and Weber number. Validation against PDA data further downstream of the burner plate revealed consistency for size classes d=10µm and d=15µm. Deviations from PDA occur in regions with strong velocity gradients due to different spatial resolutions, the presence of reconstruction ambiguities (ghost particles), uncertainties inherent to the two-frame cross-correlation of spray volumes and the finite LED pulse duration

Event-Based Imaging Velocimetry - An Assessment Of Event-Based Cameras For The Measurement Of Fluid Flows

Event-based vision is a new upcoming field within the field of computer vision. Unlike conventional frame-based imaging, event-based vision (or dynamic vision sensing) asynchronously records binary signals of contrast changes at the pixel level with microsecond resolution. Event-based imaging of small particles illuminated by a laser or other continuous light source generates streaks of events in a space-time domain. From this data the particles' position in both space and time can be inferred. The present work explores the possibilities of harnessing the potentials of event-based vision for fluid flow measurement with focus on two-dimensional, two-component velocity field estimationat particle image densities corresponding to that of conventional PIV. Three different motion detection schemes are implemented. Measurements of a cylinder wake flow demonstrate that EBIV provides time resolved velocity data that is suitable for the retrieval of flow statistics as well as spectral and modal analysis. The present work is an extension of the recently published Exp. Fluids article by the authors on event-based imaging velocimetry (Willert & Klinner, 2022, DOI 10.1007/s00348-022-03441-6) and provides further details on current limitations related to event-based imaging

Event-based Imaging Velocimetry: An Assessment of Event-based Cameras for the Measurement of Fluid Flows

Contrary to conventional frame-based imaging, event-based vision (EBV) or dynamic vision sensing (DVS) asynchronously records binary signals of intensity changes for given pixels with microsecond resolution. The present work explores the possibilities of harnessing the potential of event-based vision for fluid flow measurement. The described implementations of event-based imaging velocimetry (EBIV) rely on the imaging small particles that are illuminated by a laser light sheet which is similar to classical two-dimensional, two-component (2d-2c) PIV with the difference that a continuously operating laser-light sheet is used without modulation of the laser or camera. The moving particles generate continuous time-stamped events on the detector that are later used to infer their velocity using a patch-wise processing schemes. Two flow estimation algorithms are proposed; one uses a "motion compensation" that maximizes the local contrast, the other is based on a sum-of-correlations approach. The underlying motion detection schemes along with the complete absence of background signal allows straightforward retrieval of the events associated with individual particles thereby allowing the reconstruction of individual particle tracks. Alternatively, the event data can be processed with conventional PIV algorithms using images reconstructed from the event data stream. The concepts are demonstrated on simple flows in water and air

Near-wall Lagrangian particle tracking velocimetry using event-based imaging

We describe the implementation of a particle tracking velocimetry (PTV) system based on event-based vision (EBV) and demonstrate its application for the characterization of a turbulent boundary layer (TBL) in air. One configuration uses a single event-camera to image a narrow light sheet extending wall-normal from the wall to obtain the time-resolved velocity profile. In the other configuration a thin light sheet grazes the surface of a glass window illuminating only the viscous sublayer of the turbulent boundary layer. The data simultaneously captured by three synchronized event-cameras is used to reconstruct the 3d particle tracks within a few 100 µm of the wall. Particle movement within the viscous sub-layer permit the estimation of the local, instantaneous wall shear stress under the assumption of linearity between particle velocity and wall shear stress. Thereby 2d distributions of the instantaneous wall shear stress are obtained. PTV tracking algorithms that are orders magnitude faster than correlation-based schemes provide data quality on par with currently used, considerably more expensive, high-speed PIV hardware

Investigation of shock-induced flow separation over a transonic compressor blade by conditionally averaged PIV and high-speed shadowgraphs

The impact of separation control has been investigated in a highly loaded transonic compressor cascade at an inlet Mach number of 1.21 and a chord based Reynolds number of 1.4 × 106. Applied control devices are air jet vortex generators (AJVG) and a surface roughness patch. Comparative flows without transition control imply a variation of the upstream turbulence level from 0.5% to 2.5%. Above the suction side, velocities of the unsteady separation region have been captured by particle image velocimetry (PIV). The aerodynamic load alternation due to shock motion results in flexure of the blade surface which has been measured and compensated prior to PIV processing. Single PIV shots indicate shape variations of both the lambda shock system and the associated separation region while the shock foot position is fluctuating within a range of up to 23% of chord. Large sets of statistically independent PIV samples are conditionally averaged upon instantaneous passage shock positions at a resolution of 1% of chord length to quantify the size of flow separation. Large bubble separation occurs if the turbulence of the incoming flow is low. The separation region becomes smaller when AJVGs are applied but still exhibits bubble separation at rear shock positions. The size of the separation region is significantly reduced either if a roughness patch is applied or if the turbulence level of the incoming flow is high. The frequency range of shock motion is analyzed by shock tracking on the basis of high speed shadowgraphs. A Fourier analysis of shock motion in the low frequency range (<1khz) indicates the highest spectral densities for the turbulent case and the lowest densities if roughness patch are applied. Joined probability density distributions of blade displacements and amplitudes of shock movement revealed that upward transverse blade deflections are more frequent at shock positions downstream of the mean shock position while downward deflections are more frequent at frontal shock position

Stereoscopic Measurements of Blade Deformation on a Shrouded Prop-Fan with Boundary Layer Ingestion by means of Image Pattern Correlation Technique

The material presented herein describes the implementation of optical blade deformation measurements based on the image pattern correlation technique (IPCT) in a stereoscopic imaging configuration applied to the first rotor of a counter-rotating shrouded fan stage (CRISPMulti) which has a diameter of about 1m. The correlation-based stereoscopic image analysis is discussed, as well as the difficulties with regard to the correlation-based stereomatching of two camera views for the highly curved blade surface involving pattern displacements up to 12% of the image height. Cross-correlation of speckle images at reference positionwith images of the displaced positions recovered the full 3c displacement field in the optically assessable area of the suction side. The three-dimensional displacements could be evaluated with triangulation errors below 0.5mm at blade tip velocities exceeding 250m/s (4830rpm). Comparative measurements using a third camera imaging the blade tip match the stereo IPCT results within deviations of 0.5mm for the axial component whereby the circumference component matches closely