scholarly journals Growth and breakdown of wave packets in a high-speed boundary layer

2016 ◽  
Vol 806 ◽  
pp. 1-4
Author(s):  
Aleksandr N. Shiplyuk
Keyword(s):  
High Speed ◽  
Wave Packets ◽  
Structural Evolution ◽  
Visualization Technique ◽  
High Enthalpy ◽  
Hypersonic Boundary Layers ◽  
Reflected Shock ◽  
Second Mode ◽  
Individual Wave ◽  
Short Time

The recent study of Laurence et al. (J. Fluid Mech., vol. 797, 2016, pp. 471–503) develops a new Schlieren-based technique for investigating instabilities and transition in hypersonic boundary layers. This method enables pioneering measurements in a reflected-shock wind tunnel of the characteristics of the second mode of instability on a slender cone, within very short time scales (approximately 1 ms). The visualization technique was shown to resolve the structural evolution of individual wave packets. It was revealed that the disturbance strength concentrates near the wall for high-enthalpy conditions.

Experimental study of second-mode instability growth and breakdown in a hypersonic boundary layer using high-speed schlieren visualization

2016 ◽  
Vol 797 ◽  
pp. 471-503 ◽  
Author(s):  
S. J. Laurence ◽  
A. Wagner ◽  
K. Hannemann
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Porous Ceramic ◽  
Propagation Speed ◽  
Mode Instability ◽  
Instability Waves ◽  
Local Maxima ◽  
High Enthalpy ◽  
Reflected Shock ◽  
Second Mode

Visualization experiments are performed to investigate the development of instability waves within the boundary layer on a slender cone under high Mach number conditions. The experimental facility is a reflected-shock wind tunnel, allowing both low (Mach-8 flight equivalent) and high-enthalpy conditions to be simulated. Second-mode instability waves are visualized using a high-speed schlieren set-up, with pulse bursting of the light source allowing the propagation speed of the wavepackets to be unambiguously resolved. This, in combination with wavelength information derived from the images, enables the calculation of the disturbance frequencies. At the lower-enthalpy conditions, we concentrate on the late laminar and transitional regions of the flow. General characteristics are revealed through time-resolved and ensemble-averaged spectra on both smooth and porous ceramic surfaces of the cone. Analysis of the development of individual wavepackets is then performed. It is found that the wavepacket structures evolve from a ‘rope-like’ appearance to become more interwoven as the disturbance nears breakdown. The wall-normal disturbance distributions of both the fundamental and first harmonic, which initially have local maxima at the wall and near $y/{\it\delta}=0.7$–0.75, exhibit an increase in signal energy close to the boundary-layer edge during this evolution. The structure angle of the disturbances also undergoes subtle changes as the wavepacket develops prior to breakdown. Experiments are also performed at high-enthalpy ($h_{0}\approx 12~\text{MJ}~\text{kg}^{-1}$) conditions in the laminar regime, and the visualization technique is shown to be capable of resolving wavepacket propagation speeds and frequencies at such conditions. The visualizations reveal a somewhat different wall-normal distribution to the low-enthalpy case, with the disturbance energy concentrated much more towards the wall. This is attributed to the highly cooled nature of the wall at high enthalpy.

Experimental analysis of shock stand-off distance over spherical bodies in high-enthalpy flows

2016 ◽  
Vol 231 (14) ◽  
pp. 2666-2676 ◽  
Author(s):  
Ruchi Thakur ◽  
G Jagadeesh
Keyword(s):  
High Speed ◽  
Computational Analysis ◽  
Visualization Technique ◽  
Flow Solver ◽  
High Enthalpy ◽  
Empirical Relations ◽  
Spherical Models ◽  
Quantitative Understanding ◽  
Hypersonic Shock ◽  
Shock Tunnels

The shock stand-off distance for basic spherical models was investigated in the flow regime of 1–2 km/s, a range for which data are unavailable in the open literature. Experiments were conducted on two hypersonic shock tunnels, at five different enthalpy conditions and two types of test gas, and the bow shock was captured using schlieren flow visualization technique. These results were compared with existing empirical relations and a good match was observed. To further corroborate the results, a computational analysis of the same problem was carried out using an in-house computational fluid dynamics code, HiFUN (High Resolution Flow Solver on UNstructured meshes), and the results were again seen to match well with the experiments. The shock stand-off distance is an important parameter for the design of high-speed airplanes, and therefore a solid quantitative understanding of this parameter is vital for all flow regimes. This paper aims to fill in the gap by investigating one of those regimes where available data are currently missing.

scholarly journals Neuro-Inspired Computing with Spin-VCSELs

2021 ◽  
Vol 11 (9) ◽  
pp. 4232
Author(s):  
Krishan Harkhoe ◽  
Guy Verschaffelt ◽  
Guy Van der Sande
Keyword(s):  
Processing Speed ◽  
Delay Time ◽  
High Speed ◽  
State Of The Art ◽  
Polarization Modulation ◽  
Time Interval ◽  
Computing Technique ◽  
High Modulation ◽  
Short Time ◽  
Modulation Speed

Delay-based reservoir computing (RC), a neuromorphic computing technique, has gathered lots of interest, as it promises compact and high-speed RC implementations. To further boost the computing speeds, we introduce and study an RC setup based on spin-VCSELs, thereby exploiting the high polarization modulation speed inherent to these lasers. Based on numerical simulations, we benchmarked this setup against state-of-the-art delay-based RC systems and its parameter space was analyzed for optimal performance. The high modulation speed enabled us to have more virtual nodes in a shorter time interval. However, we found that at these short time scales, the delay time and feedback rate heavily influence the nonlinear dynamics. Therefore, and contrary to other laser-based RC systems, the delay time has to be optimized in order to obtain good RC performances. We achieved state-of-the-art performances on a benchmark timeseries prediction task. This spin-VCSEL-based RC system shows a ten-fold improvement in processing speed, which can further be enhanced in a straightforward way by increasing the birefringence of the VCSEL chip.

scholarly journals High-enthalpy hypersonic flows

2020 ◽  
Vol 2 (1) ◽  
Author(s):  
Joseph J. S. Shang ◽  
Hong Yan
Keyword(s):  
Hypersonic Flow ◽  
High Speed ◽  
Quantum Physics ◽  
Nonequilibrium Thermodynamic ◽  
Strong Shock ◽  
Hypersonic Flows ◽  
Electrically Conducting ◽  
High Enthalpy ◽  
Speed Range ◽  
Flow Theories

Abstract Nearly all illuminating classic hypersonic flow theories address aerodynamic phenomena as a perfect gas in the high-speed range and at the upper limit of continuum gas domain. The hypersonic flow is quantitatively defined by the Mach number independent principle, which is derived from the asymptotes of the Rankine-Hugoniot relationship. However, most hypersonic flows encounter strong shock-wave compressions resulting in a high enthalpy gas environment that always associates with nonequilibrium thermodynamic and quantum chemical-physics phenomena. Under this circumstance, the theoretic linkage between the microscopic particle dynamics and macroscopic thermodynamics properties of gas is lost. When the air mixture is ionized to become an electrically conducting medium, the governing physics now ventures into the regimes of quantum physics and electromagnetics. Therefore, the hypersonic flows are no longer a pure aerodynamics subject but a multidisciplinary science. In order to better understand the realistic hypersonic flows, all pertaining disciplines such as the nonequilibrium chemical kinetics, quantum physics, radiative heat transfer, and electromagnetics need to bring forth.

Liquid Behavior in Containers with a Liquid-Packing Bag for Liquid Products Subjected to Drop Impact

2006 ◽  
Vol 321-323 ◽  
pp. 1280-1283 ◽  
Author(s):  
Eisaku Umezaki ◽  
Yuuma Shinoda ◽  
Katsunori Futase
Keyword(s):  
High Speed ◽  
Free Fall ◽  
The Other ◽  
Impact Loads ◽  
Visualization Technique ◽  
Transparent Plastic ◽  
Plastic Films ◽  
Liquid Behavior ◽  
Liquid Products ◽  
Polyethylene Particles

The behavior of liquid in containers subjected to impact loads due to free fall was investigated using a visualization technique. Two types of containers were used. One consisted of a case made of transparent plastic plates and a liquid-packing bag made of transparent plastic films. The bag contained about 1,000 ml of liquid. The other was a case made only of transparent plastic plates. The case contained about 1,000 ml of water. The liquid consisted of water and ethanol. Polyethylene particles of about 3 mm in diameter were included in the liquid to visualize the movement of liquid in the containers. The containers were subjected to impact due to free fall, and photographs of the containers were taken using a high-speed camera. Results indicated that the behavior of liquid in the container with a liquid-packing bag is different from that of the container without a liquid-packing bag.

Micro-Expression Recognition using 3D - CNN

2020 ◽  
pp. 5-13
Author(s):  
Vishal Dubey ◽  
◽  
◽  
◽  
Bhavya Takkar ◽  
...  
Keyword(s):  
Emotional Response ◽  
High Speed ◽  
Actual State ◽  
Time Interval ◽  
Short Time Interval ◽  
Expression Recognition ◽  
Specific Emotion ◽  
3D Cnn ◽  
Short Time ◽  
Micro Expression

Micro-expression comes under nonverbal communication, and for a matter of fact, it appears for minute fractions of a second. One cannot control micro-expression as it tells about our actual state emotionally, even if we try to hide or conceal our genuine emotions. As we know that micro-expressions are very rapid due to which it becomes challenging for any human being to detect it with bare eyes. This subtle-expression is spontaneous, and involuntary gives the emotional response. It happens when a person wants to conceal the specific emotion, but the brain is reacting appropriately to what that person is feeling then. Due to which the person displays their true feelings very briefly and later tries to make a false emotional response. Human emotions tend to last about 0.5 - 4.0 seconds, whereas micro-expression can last less than 1/2 of a second. On comparing micro-expression with regular facial expressions, it is found that for micro-expression, it is complicated to hide responses of a particular situation. Micro-expressions cannot be controlled because of the short time interval, but with a high-speed camera, we can capture one's expressions and replay them at a slow speed. Over the last ten years, researchers from all over the globe are researching automatic micro-expression recognition in the fields of computer science, security, psychology, and many more. The objective of this paper is to provide insight regarding micro-expression analysis using 3D CNN. A lot of datasets of micro-expression have been released in the last decade, we have performed this experiment on SMIC micro-expression dataset and compared the results after applying two different activation functions.

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