Thomas Knast

Bio: Thomas Knast is an academic researcher from Monash University. The author has contributed to research in topics: Jet (fluid) & Field of view. The author has an hindex of 2, co-authored 4 publications receiving 23 citations.

Coupling Modes of an Underexpanded Twin Axisymmetric Jet

Abstract: An experimental investigation into the coupling behavior of screeching axisymmetric twin supersonic jets is presented. Acoustic measurements and schlieren photography are used to identify four dist...

A low-cost high-speed CMOS camera for scientific imaging

Abstract: Non-intrusive temporally and spatially resolved measurements of dynamic phenomena are heavily reliant on high-speed (>1 kHz) digital scientific cameras. The cost of these cameras is a major constraint on the operation of many experimental and educational research facilities. In this paper we present a performance analysis of a low-cost high-speed CMOS camera, the 1.4. Developed for consumer use, we investigate its potential as a scientific camera. It uses a 12 bit CMOS sensor with px at 6.6 pitch and 1 minimum global shutter. It is capable of recording at 1057 Hz at full frame and up to 38 kHz with a reduced field of view. It provides a number of features not typically found in low-cost consumer cameras, such as external triggering and shutter gating control, clock outputs, and raw binary data output. We test the linearity of the sensor response using a pulsed LED source and analyse the sensor performance in terms of noise, jitter and intensity lag. A quantitative demonstration of the camera's performance under realistic experimental conditions is demonstrated with an image correlation velocimetry measurement in a high-speed propellant spray. The camera compares favourably against several scientific high speed cameras from major manufacturers. The camera is well suited for high resolution forward-scattering and in-line imaging techniques such as schlieren, shadowgraphy, holography and bright-field microscopy. Spatial bias in the dark field noise floor makes it generally unsuitable for low-light measurement conditions. However, the small footprint and low cost make it ideal as an educational tool and for multi-camera experiments. These tests were conducted independently of the manufacturer and the authors have no conflicts of interest to disclose.

Low cost high speed digital cameras for experimental fluid mechanics

Abstract: Scientific high-speed digital cameras are essential for many experimental techniques in fluid mechanics. Their high cost is a major constraint on the capability and accessibility of flow diagnostic facilities. Advances in semiconductor manufacturing in recent decades have led to order of magnitude gains in the performance of the sensors used in these cameras. However, due to the small size of the market and the need for customisation and flexibility in design, the scientific camera market has not experienced the order-of-magnitude price decreases seen in the consumer sector. Recent supply side changes in the Chinese electronics market have led to an explosion of niche crowdfunded electronics. The impact of this shift in the electronics industry has not yet been fully appreciated, but it is now beginning to impact the scientific research community. In this paper, we present a technical overview and performance assessment of one of the first custom built crowd-funded scientific cameras, the Chronos 1.4. It is a low cost high-speed camera based on a Luxima LUX1310 CMOS sensor package. It has 1280 × 1024 pixels at 6.6 μm pitch with a 1.2 μs global shutter, a bandwidth of 1.4 Gpix/s, flexible external triggering, and an open source development toolchain. It is capable of over 1000 frames/s (full sensor) and 38,565 frames/s with reduced field of view (336 × 96 pixels). We assess the performance of the Chronos 1.4 with a well-characterised stroboscopic LED light source. Under the same conditions and with an identical optical setup, the Chronos 1.4 compares favourably to two other scientific highspeed cameras. The small pixel pitch makes it ideal for highly magnified measurements. The low cost (USD $3k) and relatively small footprint is also ideal for multi-camera experiments and as an educational aid. We demonstrate its application in two experiments. Themes: Experimental techniques & facilities

Aeroacoustic resonance and self-excitation in screeching and impinging supersonic jets – A review:

Abstract: Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by ...

The generation of screech tones by shock leakage

Abstract: The mechanism underpinning the generation of screech tones has remained an open question for many years. In this paper, direct experimental observations of the shock-leakage mechanism first proposed by Manning & Lele (AIAA Paper 1998, p. 282) are presented. Ultra-high-speed schlieren images are filtered to preserve only upstream-propagating components, with the upstream motion of the shock tip and subsequent emission of an acoustic wave visible for a number of operating conditions. The flow visualizations are supported by the ray-tracing model for shock leakage of Shariff & Manning (Phys. Fluids., vol. 25, issue 7, 2013, 076103), applied to velocity fields corresponding to a reconstructed screech cycle. The predictions of the model, when applied to real data, are in close agreement with the phenomena observed in the flow visualizations. It is demonstrated that shock leakage does not necessarily occur either at the point of maximum wave amplitude or maximum vorticity fluctuation. While the first point of shock leakage is shown to vary between cases, sound emission from multiple sources is observed for most cases considered. Finally, it is shown that variations in vortex strength captured in the velocity data are sufficient to explain variation in shock-leakage location observed in the flow visualization data.

An experimental investigation of coupled underexpanded supersonic twin-jets

Abstract: High-resolution particle image velocimetry measurements of coupled underexpanded twin-jets are presented. Two nozzle pressure ratios are examined, which are selected due to a change in coupled plume mode indicated by a discontinuous jump in screech frequency. Estimates of the turbulent flow statistics, shear-layer thickness, merge point, inter-nozzle mixing, and integral length scales are provided. The higher nozzle pressure ratio case shows a strong standing-wave present in the velocity fluctuation amplitude and integral length scale. The ratios of standing, acoustic, and hydrodynamic wavelength are compared and find a close fit to Panda’s relation for screech. This indicates that screech in the twin-jet system operates with similar length-scale and frequency characteristics to single jets and provides evidence to suggest screech is an integral part of the twin-jet coupling process. Second-order spatial velocity correlation maps reveal the larger modal structure. A symmetric mode is found for the higher pressure ratio and a weakly symmetric mode for the lower. Comparison is made between where the standing-wave is present and where it is not. It is found that the standing-wave, not the shock structure, is the driver of turbulence coherence modulation near the jet. In regions that are affected only by the standing-wave, it is found that it contributes to both the turbulence intensity and coherence modulation.

Investigation of supersonic twin-jet coupling using spatial linear stability analysis

Abstract: The present work focuses on the study of the resonance and coupling of an underexpanded circular twin-jet system operating at a nozzle pressure ratio of . Particle image velocimetry data from previous work were revisited, and a symmetry-imposed proper orthogonal decomposition (POD) was performed. It is shown that the system is dominated by a single POD mode pair symmetric about the internozzle plane, and the resonance loop is modulated by a third POD mode related to shear thickness modulation. A spatial Fourier transform of the leading POD mode pair leads to the identification of the peak wavenumbers and radial shapes of the different waves at play in the screech phenomenon. Locally parallel linear stability analysis around the experimental mean flow is also performed, in order to provide clarification of the mode ‘locking’ mechanism, i.e. the selection of the global mode associated with screech. It is shown that the characteristics of the Kelvin–Helmholtz wavepackets alone are not sufficient to explain the coupling observed in the experimental data. A consideration of the upstream-travelling guided jet mode offers an explanation; only specific symmetries of upstream modes can be supported in the frequency range at which resonance occurs. Results from stability analysis point to structures at frequencies and wavenumbers close to those found experimentally, and their spatial structures show excellent agreement with the POD modes. The present results suggest that the resonance loop is closed by an upstream-travelling guided jet mode for the twin-jet system at high nozzle pressure ratio.

A unifying theory of jet screech

Abstract: Abstract This paper describes the mechanism underpinning modal staging behaviour in screeching jets. An upstream-propagating subsonic guided-jet mode is shown to be active in all stages of screech. Axial variation of shock-cell spacing manifests in the spectral domain as a series of suboptimal peaks. It is demonstrated that the guided-jet mode may be energized by interactions of the Kelvin–Helmholtz wavepacket with not only the primary shock wavenumber peak, but also suboptimal peaks; interaction with suboptimals is shown to be responsible for closing the resonance loop in multiple stages of jet screech. A consideration of the full spectral representation of the shocks reconciles several of the classical models and results for jet screech that had heretofore been paradoxical. It is demonstrated that there are multiple standing waves present in the near field of screeching jets, corresponding to the superposition of the various waves active in these jets. Multimodal behaviour is explored for jets in a range of conditions, demonstrating that multiple peaks in the frequency spectra can be due to either changes in which peak of the shock spectra the Kelvin–Helmholtz wavepacket is interacting with, or a change in azimuthal mode, or both. The absence of modal staging in high-aspect-ratio non-axisymmetric jets is also explained in the context of the aforementioned mechanism. The paper closes with a new proposed theory for frequency selection in screeching jets, based on the observation that these triadic interactions appear to underpin selection of the guided-jet mode wavelength in all measured cases.