DDES of a supersonic coaxial helium jet

The mixing of a light gas in a scramjet combustion chamber is replicated through the simulation of a supersonic coaxial helium jet. Sensitivities of the RANS method to model constants is presented and the DDES method is employed to try and alleviate this modelling weakness. A significant delay in the shear layer transition is found to occur in the DDES simulation and it is suggested this may be due to the absence of inlet turbulence. The influence of the introduction of inflow turbulence is investigated and computational results are compared to experimental data. The influence of model constants on the DDES results are presented. © 2012 by Peter Cocks.

Transfer of a cold atmospheric pressure plasma jet through a long flexible plastic tube

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Inactivation of candida albicans by cold atmospheric pressure plasma jet

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Open-air direct current plasma jet : scaling up, uniformity, and cellular control

Atmospheric-pressure plasma jets are commonly used in many fields from medicine to nanotechnology, yet the issue of scaling the discharges up to larger areas without compromising the plasma uniformity remains a major challenge. In this paper, we demonstrate a homogenous cold air plasmaglow with a large cross-section generated by a direct current power supply. There is no risk of glow-to-arc transitions, and the plasmaglow appears uniform regardless of the gap between the nozzle and the surface being processed. Detailed studies show that both the position of the quartz tube and the gas flow rate can be used to control the plasma properties. Further investigation indicates that the residual charges trapped on the inner surface of the quartz tube may be responsible for the generation of the air plasma plume with a large cross-section. The spatially resolved optical emission spectroscopy reveals that the air plasma plume is uniform as it propagates out of the nozzle. The remarkable improvement of the plasma uniformity is used to improve the bio-compatibility of a glass coverslip over a reasonably large area. This improvement is demonstrated by a much more uniform and effective attachment and proliferation of human embryonic kidney 293 (HEK 293) cells on the plasma-treated surface.

Inactivation of a 25.5 µm Enterococcus faecalis biofilm by a room-temperature, battery-operated, handheld air plasma jet

Effective biofilm inactivation using a handheld, mobile plasma jet powered by a 12 V dc battery and operated in open air without any external gas supply is reported. This cold, room-temperature plasma is produced in self-repetitive nanosecond discharges with current pulses of ~100 ns duration, current peak amplitude of ~6 mA and repetition rate of ~20 kHz. It is shown that the reactive plasma species penetrate to the bottom layer of a 25.5 µm-thick Enterococcus faecalis biofilm and produce a strong bactericidal effect. This is the thickest reported biofilm inactivated using room-temperature air plasmas.

Pulsed dc- and sine-wave-excited cold atmospheric plasma plumes: A comparative analysis

Cold atmospheric-pressure plasma plumes are generated in the ambient air by a single-electrode plasma jet device powered by pulsed dc and ac sine-wave excitation sources. Comprehensive comparisons of the plasma characteristics, including electrical properties, optical emission spectra, gas temperatures, plasma dynamics, and bacterial inactivation ability of the two plasmas are carried out. It is shown that the dc pulse excited plasma features a much larger discharge current and stronger optical emission than the sine-wave excited plasma. The gas temperature in the former discharge remains very close to the room temperature across the entire plume length; the sine-wave driven discharge also shows a uniform temperature profile, which is 20-30 degrees higher than the room temperature. The dc pulse excited plasma also shows a better performance in the inactivation of gram-positive staphylococcus aureus bacteria. These results suggest that the pulsed dc electric field is more effective for the generation of nonequilibrium atmospheric pressure plasma plumes for advanced plasma health care applications.

Drag reduction by counterflow supersonic jet for a blunt cone in high enthalpy flows

Counterflow supersonic jet is used as a drag reduction device during the experiments in free piston driven shock tunnel, HST3. Accelerometer based force balance is employed to measure the drag force experienced by the 60-degree apex angle blunt cone model without and with the supersonic jet opposing the hypersonic flow. It is observed that the drag force decreases with increase in injection pressure ratio until the critical injection pressure is reached. Maximum reduction in drag force of 44 percent is recorded at the critical injection pressure ratio 22.36. Further increase in injection pressure ratio has reduced the percentage drag reduction. Change in nature of the flowfield around the model has also been observed across the critical injection pressure ratio.

Experimental and numerical investigation of jet injection in a wall bounded supersonic flow

Numerical and experimental studies of a supersonic jet (Helium) inclined at 45 degrees to a oncoming Mach 2 flow have been carried out. The numerical study has been used to arrive at a geometry that could reduce an oncoming Mach 5.75 flow to Mach 2 flow and in determining the jet parameters. Experiments are carried out in the IISc. hypersonic shock tunnel HST2 at similar conditions obtained from numerical studies. Flow visualization studies carried out using Schlieren technique clearly show the presence of the bow shock in front of the jet exposed to supersonic cross flow. The jet Mach number is experimentally found to be approximate to 3. Visual observations show that the jet has penetrated up to 60% of the total height of the chamber.

CFD Based Design Optimizations of the Diffuser of a Gas Dynamically Driver Laser Cavity

Based on the an earlier CFD analysis of the performance of the gas-dynamically controlled laser cavity [1]it was found that there is possibility of optimizing the geometry of the diffuser that can bring about reductions in both size and cost of the system by examining the critical dimensional requirements of the diffuser. Consequently,an extensive CFD analysis has been carried out for a range of diffuser configurations by simulating the supersonic flow through the arrangement including the laser cavity driven by a bank of converging – diverging nozzles and the diffuser. The numerical investigations with 3D-RANS code are carried out to capture the flow patterns through diffusers past the cavity that has multiple supersonic jet interactions with shocks leading to complex flow pattern. Varying length of the diffuser plates is made to be the basic parameter of the study. The analysis reveals that the pressure recovery pattern during the flow through the diffuser from the simulation, being critical for the performance of the laser device shows its dependence on the diffuser length is weaker beyond a critical lower limit and this evaluation of this limit would provide a design guideline for a more efficient system configuration.The observation based on the parametric study shows that the pressure recovery transients in the near vicinity of the cavity is not affected for the reduction in the length of the diffuser plates up to its 10% of the initial size, indicating the design in the first configuration that was tested experimentally has a large factor of margin. The flow stability in the laser cavity is found to be unaffected since a strong and stable shock is located at the leading edge of the diffuser plates while the downstream shock and flow patterns are changed, as one would expect. Results of the study for the different lengths of diffusers in the range of 10% to its full length are presented, keeping the experimentally tested configuration used in the earlier study [1] as the reference length. The conclusions drawn from the analysis is found to be of significance since it provides new design considerations based on the understanding of the intricacies of the flow, allowing for a hardware optimization that can lead to substantial size reduction of the device with no loss of performance.

Conformation-Changing Aggregation in Hydroxyacetone: A Combined Low-Temperature FTIR, Jet, and Crystallographic Study

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dinners and oligomers. The O-H center dot center dot center dot O=C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H center dot center dot center dot O-H hydrogen bonds in competition with isolated O-H center dot center dot center dot O=C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from similar to 0 degrees in the gas phase to similar to 180 degrees in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.

Strongly magnetized cold degenerate electron gas: Mass-radius relation of the magnetized white dwarf

We consider a relativistic, degenerate electron gas at zero temperature under the influence of a strong, uniform, static magnetic field, neglecting any form of interactions. Since the density of states for the electrons changes due to the presence of the magnetic field (which gives rise to Landau quantization), the corresponding equation of state also gets modified. In order to investigate the effect of very strong magnetic field, we focus only on systems in which a maximum of either one, two, or three Landau level(s) is/are occupied. This is important since, if a very large number of Landau levels are filled, it implies a very low magnetic field strength which yields back Chandrasekhar's celebrated nonmagnetic results. The maximum number of occupied Landau levels is fixed by the correct choice of two parameters, namely, the magnetic field strength and the maximum Fermi energy of the system. We study the equations of state of these one-level, two-level, and three-level systems and compare them by taking three different maximum Fermi energies. We also find the effect of the strong magnetic field on the mass-radius relation of the underlying star composed of the gas stated above. We obtain an exciting result that it is possible to have an electron-degenerate static star, namely, magnetized white dwarfs, with a mass significantly greater than the Chandrasekhar limit in the range 2.3-2.6M(circle dot), provided it has an appropriate magnetic field strength and central density. In fact, recent observations of peculiar type Ia supernovae-SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg-seem to suggest super-Chandrasekhar-mass white dwarfs with masses up to 2.4-2.8M(circle dot) as their most likely progenitors. Interestingly, our results seem to lie within these observational limits.

Reply to ``Comment on `Strongly magnetized cold degenerate electron gas: Mass-radius relation of the magnetized white dwarf'''

We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar Phys. Rev. D 89, 103017 (2014)] and in the concerned comment, by the same authors, against our work U. Das and B. Mukhopadhyay, Phys. Rev. D 86, 042001 (2012)] is erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude that the arguments put forth by Nityananda and Konar are misleading.

Studies on the effects of varying secondary gas properties in a low entrainment ratio supersonic ejector

A dilution cum purge ejector for application in fuel cells represents a domain of ejector operation involving low entrainment ratio with differing secondary and primary gas; which is hardly investigated and a cohesive design framework is not readily available. We comprehensively study a constant area ejector using analytical, experimental and numerical tools at low entrainment ratio (0.004-0.065) with Air, Helium and Argon as secondary gas while the primary gas is Air. For the first time, limits of operating parameters used in control volume method to design the ejector are found to be highly dependent on the secondary molecular weight. The entrainment ratio in the ejector (low for Helium and high for Argon) is affected by the molecular weight and the static pressure within the ejector (low for Air and high for Argon & Helium) by the gamma of the secondary gas. Sufficient suction pressure (0.3-0.55 bar) is generated by the ejector thereby preventing any backflow of secondary gas at all primary stagnation pressures (1.5, 2.2 and 3.1 bar). Numerical results agree well with experimental results. The ejector is shown to completely dilute and purge the secondary flow, meeting all key design requirements. (C) 2014 Elsevier Ltd. All rights reserved.

A model supersonic buried-nozzle jet: instability and acoustic wave scattering and the far-field sound

We consider sound source mechanisms involving the acoustic and instability modes of dual-stream isothermal supersonic jets with the inner nozzle buried within an outer shroud-like nozzle. A particular focus is scattering into radiating sound waves at the shroud lip. For such jets, several families of acoustically coupled instability waves exist, beyond the regular vortical Kelvin-Helmholtz mode, with different shapes and propagation characteristics, which can therefore affect the character of the radiated sound. In our model, the coaxial shear layers are vortex sheets while the incident acoustic disturbances are the propagating shroud modes. The Wiener-Hopf method is used to compute their scattering at the sharp shroud edge to obtain the far-field radiation. The resulting far-field directivity quantifies the acoustic efficiency of different mechanisms, which is particularly important in the upstream direction, where the results show that the scattered sound is more intense than that radiated directly by the shear-layer modes.