This paper investigates the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination. Modern solid-state LED devices can provide averaged radiant power in excess of 10 W and by operating the LED with short high current pulses theoretical pulse energies up to several tens of mJ can be achieved. In the present work, a custom-built drive circuit is used to drive a Luminus PT-120 high-power LED at pulsed currents of up to 150 A and 1 μs duration. Volumetric illumination is achieved by directly projecting the LED into the flow to produce a measurement volume of ≈3–4 times the size of the LED die. The feasibility of the volumetric LED illumination is assessed by performing tomographic PIV of homogenous, grid-generated turbulence. Two types of LEDs are investigated, and the results are compared with measurements of the same flow using pulsed Nd:YAG laser illumination and DNS data of homogeneous isotropic turbulence. The quality of the results is similar for both investigated LEDs with no significant difference between the LED and Nd:YAG illumination. Compared with the DNS, some differences are observed in the power spectra and the probability distributions of the fluctuating velocity and velocity gradients. These differences are attributed to the limited spatial resolution of the experiments and noise introduced during the tomographic reconstruction (i.e. ghost particles). The uncertainty in the velocity measurements associated with the LED illumination is estimated to approximately 0.2–0.3 pixel for both LEDs, which compares favourably with similar tomographic PIV measurements of turbulent flows. In conclusion, the proposed high-power, pulsed LED volume illumination provides accurate and reliable tomographic PIV measurements in water and presents a promising technique for flow diagnostics and velocimetry.

/pdf/pulsed-high-power-led-illumination-for-tomographic-particle-56p91gj3ub.pdf

Pulsed, high-power LED illumination for tomographic particle image velocimetry

Trajectory Analysis of Fuel Injection into Supersonic Cross Flow Based on Schlieren Method

Schlieren and shadowgraph techniques are hundreds of years old, yet several important developments have occurred in the last decade, as summarized in this paper. Progress has been made in using the turbulent eddies of a refractive flow as “particle” tracers for seedless velocimetry. This approach will never supplant standard PIV, but “schlieren PIV” can be useful, for example, in cases where particles cannot be seeded in a turbulent flow under study. Background-oriented schlieren (BOS) has become very popular in just a few years. Given modern image-processing software for PIV or digital image correlation, a digital camera and a proper background, schlieren-like images of all sorts are easy to make without parabolic mirrors or even a knife-edge. “Rainbow schlieren” is the name for a quantitative schlieren instrument that uses color to make density, temperature, or species measurements in steady and unsteady planar or axisymmetric flows. Data acquisition and reduction are highly automated and the range of applications is very broad. Finally, shadowgraphy has not been forgotten: it is the simplest of all the optical flow visualization methods, but is often the best choice for imaging shock waves and turbulence. The addition of a retroreflective screen and a high-speed camera makes direct shadowgraphy a robust tool for the study of large-scale events in harsh environments.

Recent Developments in Schlieren and Shadowgraphy

Structured light-field focusing for flowfield diagnostics

Abstract The supersonic wake of a circular cylinder in Mach 3 flow was studied through high-speed, focussing schlieren photography. The mean and unsteady behaviour of the separated shear layers, the reattachment process, the recompression wave and the early wake are analysed, and discussed in detail. The fluctuations in the wake are stronger and more coherent than those within the approaching shear layers and the recirculation region. The recompression of the shear layers energises the finer scales in the flow which leads to a departure from a $-$1 spectral roll-off observed in the schlieren spectra further upstream. The recompression wave exhibits low-frequency unsteadiness and a ripple-type motion which occurs as it is perturbed by shocklets radiating from the coherent structures in the wake. The wake consists of coherent disturbances with the same characteristic frequency as that for an incompressible flow over a cylinder; however, this instability is suppressed as the wake accelerates, presumably due to increasing compressibility. The primary instability of the wake flow has a characteristic frequency nearly twice that of its incompressible counterpart and it is shown to be driven by the presence of aeroacoustic resonance in the wake. It is also shown that the resonance, which leads to the formation of broadband standing waves in the wake, is the result of an interaction between the wake instabilities and upstream propagating acoustic waves in the wake. The acoustic waves originate upstream of the reattachment region and are believed to be generated by the unsteady separation on the cylinder surface.

Supersonic cylinder wake dynamics

A focusing-schlieren optical system has been developed for performing velocity measurements in refractive turbulent flows using commercial particle image velocimetry (PIV) algorithms. Focusing-schlieren optics allows the visualization of refractive disturbances within a limited depth-of-focus, resulting in quasi-planar schlieren images. The schlieren “PIV” technique makes use of naturally-occurring refractive-turbulent eddies in a flow as PIV “particles” upon which velocimetry is performed. Current experiments are performed in a small supersonic wind tunnel to measure the Mach 3 turbulent boundary layer mean-velocity profile. Results from both focusing-schlieren PIV and shadowgraph PIV are compared to the velocity profile from a standard pitot-pressure survey. The natural intermittency of the outer part of the turbulent boundary layer plays a role in the schlieren PIV results, but useful measurements of the velocity profile can still be made. We also introduce an important improvement in schlieren “PIV”, the use of a pulsed LED light source in place of the twin pulsed lasers typically required for traditional PIV measurements. This comparatively-inexpensive white-light source eliminates the traditional problems of laser illumination in schlieren optical systems and improves the overall results.

http://mne.psu.edu/psgdl/pubs/2009-hargather-asm.pdf

J. Lawson

Papers

Focusing-Schlieren PIV Measurements of a Supersonic Turbulent Boundary Layer