PIV Analysis of Ludwig Prandtl's Historic Flow Visualization Films

Around 1930 Ludwig Prandtl and his colleagues O. Tietjens and W. M\"uller published two films with visualizations of flows around surface piercing obstacles to illustrate the unsteady process of flow separation. These visualizations were achieved by recording the motion of fine particles sprinkled onto the water surface in water channels. The resulting images meet the relevant criteria of properly seeded recordings for particle image velocimetry (PIV). Processing these image sequences with modern PIV algorithms allows the visualization of flow quantities (e.g. vorticity) that were unavailable prior to the development of the PIV technique. The accompanying fluid dynamics video consists of selected original film sequences overlaid with visualizations obtained through PIV processing.Comment: Contribution to the "Gallery of Fluid Motion", 63rd Annual APS-DFD Meeting 2010, Long Beach (CA

The interaction of spatially modulated vortex pairs with free surfaces

Spatially modulated vortex pairs were generated below a free surface by two counter-rotating flaps whose edges approximate a sinusoid. The surface interactions of the vertically approaching vortex pairs were visualized by the shadowgraph technique. Two limiting cases were investigated in detail: the interaction with a surfactant-rich (contaminated) surface and with a surfactant-poor (‘clean’) surface. In the latter case shadowgraph images showed that the underlying vortex core formed a line of circular surface depressions. Subsequent measurements of the temporally evolving velocity fields using digital particle image velocimetry (DPIV) of the vortex pair cross-sections and the subsurface plane confirmed the connection process of the main vortex core with the surface. As a result of the connection the initially modulated vortex tube was broken into a line of U-vortices. In the presence of surfactants this connection could not be observed; rather a Reynolds ridge (or stagnation line) was formed and a very weak connection of the secondary separation vortex could be seen in the shadowgraphs as well as measured with the time-resolved DPIV technique. A prerequisite for connection of the vortex with the surface is that the flow's kinematics force the vortex core, that is, regions of concentrated vorticity, toward the surface. The ensuing locally concentrated viscous flux of surface-parallel vorticity through the surface is balanced by a local surface deceleration. Surface-normal vorticity appears on each side of the decelerated region whose gradually increasing circulation is directly balanced by the loss of circulation of the surface-parallel vortex. However, the shear forces caused by small amounts of surface contamination and its associated subsurface boundary layer inhibit the connection process by preventing the essential viscous flux of parallel vorticity through the surface. Instead, the subsurface boundary layer is associated with a flux of parallel vorticity into the surface which then concentrates into the observable secondary separation vortex

Megahertz Schlieren Imaging of Shock Structure and Sound Waves in Under-Expanded, Impinging Jets

The accompanying fluid dynamics videos visualize the temporal evolution of shock structures and sound waves in and around an under-expanded jet that is impinging on a rigid surface at varying pressure ratios. The recordings were obtained at frame rates of 500 kHz to 1 Mhz using a novel pulsed illumination source based on a high power light emitting diode (LED) which is operated in pulsed current mode synchronized to the camera frame rate.Comment: Contribution to "Gallery of Fluid Motion", 63rd Annual APS-DFD Meeting, Long Beach (CA

The flow field downstream of a hydraulic jump

A control-volume analysis of a hydraulic jump is used to obtain the mean vorticity downstream of the jump as a function of the Froude number. To do this it is necessary to include the conservation of angular momentum. The mean vorticity increases from zero as the cube of Froude number minus one, and, in dimensionless form, approaches a constant at large Froude number. Digital particle imaging velocimetry was applied to travelling hydraulic jumps giving centre-plane velocity field images at a frequency of 15 Hz over a Froude number range of 2–6. The mean vorticity determined from these images confirms the control-volume prediction to within the accuracy of the experiment. The flow field measurements show that a strong shear layer is formed at the toe of the wave, and extends almost horizontally downstream, separating from the free surface at the toe. Various vorticity generation mechanisms are discussed

Event-based imaging velocimetry using pulsed illumination

The paper addresses the shortcoming of current event-based vision (EBV) sensors in the context of particle imaging. Latency is introduced both on the pixel level as well as during read-out from the array and results in systemic timing errors when processing the recorded event data. Using pulsed illumination, the overall latency can be quantified and indicates an upper bound on the frequency response on the order of 10-20 kHz for the specific EBV sensor. In particle-based flow measurement applications, particles scattering the light from a pulsed light source operating below this upper frequency can be reliably tracked in time. Through the combination of event-based vision and pulsed illumination, flow field measurements are demonstrated at light pulsing rates up to 10 kHz in both water and air flows by providing turbulence statistics and velocity spectra. The described EBV-based velocimetry system consists of only an EBV camera and a (low-cost) laser that can be directly modulated by the camera, making the system compact, portable and cost effective

Event-based imaging velocimetry - An introduction

The presentation provides an introduction to the concepts of event-based imaging (EBI), also known as dynamic vision sensing or neuromorphic imaging. EBI constitutes a pradigm shift in the field of imaging since it does not record typical frame-based image data. Rather, the EBI sensor provides an asynchronous stream of contrast-change events on the pixel level. In the present context the focus is to demonstrate the potentials of EBI in the field of flow visualization and measurement, in particular, particle tracking velocimetry and flow field reconstruction akin to the established particle image velocimetry technique (PIV)

Event-based imaging velocimetry using pulsed illumination

The paper addresses the shortcoming of current event-based vision (EBV) sensors in the context of particle imaging. Latency is introduced both on the pixel level as well as during read-out from the array and results in systemic timing errors when processing the recorded event data. Using pulsed illumination, the overall latency can be quantified and indicates an upper bound on the frequency response on the order of 10-20 kHz for the specific EBV sensor. In particle-based flow measurement applications, particles scattering the light from a pulsed light source operating below this upper frequency can be reliably tracked in time. Through the combination of event-based vision and pulsed illumination, flow field measurements are demonstrated at light pulsing rates up to 10 kHz in both water and air flows by providing turbulence statistics and velocity spectra. The described EBV-based velocimetry system consists of only an EBV camera and a (low-cost) laser that can be directly modulated by the camera, making the system compact, portable and cost-effective

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

Characterization of the flow field inside a Ranque-Hilsch vortex tube using filtered Rayleigh scattering, Laser-2-Focus velocimetry and numerical methods

The design process of aero engines as well as stationary gas turbines is largely dominated by the cost and time efficient methods of Computational Fluid Dynamics (CFD). Over the past decade the CFD solver TRACE for Favre-averaged compressible Navier-Stokes equations has been developed at the Institute of Propulsion Technology and has been adopted for research as well as industrial applications. In the context of turbomachinery design, reliable modeling of the turbulent flow phenomena involved is a crucial aspect and one of the major subjects of numerical research in fluid dynamics. Novel approaches accounting for the anisotropy of the Reynolds stress tensor promise an improved accuracy in the simulation of industrially relevant configurations. One key aspect in the development strategy of turbulence models is the direct comparison of computational results with validation data produced from appropriate experimental setups with well-defined geometries and boundary conditions. The Ranque-Hilsch vortex tube (RHVT) was chosen in this respect due to its simple geometry with no moving parts on the one hand and its nevertheless complex 3D flow features on the other hand. To provide suitable experimental data the filtered Rayleigh scattering technique extended by the method of frequency scanning (FSM-FRS) was chosen to characterize the RHVT's averaged flow field, since it is capable of simultaneously providing planar information on temperature, pressure and flow field velocity (through the Doppler shift). As the reconstruction of a three component velocity field from FSM-FRS data would require the measurement plane to be observed from three independent directions, the point-wise Laser-2-Focus (L2F) technique is applied to provide 2C velocity profiles at discrete positions downstream from the cold exit

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