High Frequency Case Research Articles

Large eddy simulation of turbulent jet controlled by two pulsed jets: Effect of forcing frequency

A turbulent jet with Reynolds number of 71000, forced by a pair of pulsed jets operating 180° out of phase, is investigated using large eddy simulation. The effect of the pulsing frequency on mixing efficiency and coherent structures is considered in detail through six cases: an unforced case and five cases with different frequencies (StD=0.05, 0.1, 0.15, 0.2 and 0.3, Strouhal number based on jet diameter). The mixing efficiency is increased when the pulsing frequency is low and conversely decreases at high frequency. In the present work, the optimal frequency is found to correspond to a Strouhal number of 0.15. The coherent structures observed in the forced case are complex and are found to be significantly different with the unforced case. These structures alternate in the forced plane with flapping motion and are symmetrical in the unforced plane. The spanwise structures form at the same frequency as the pulsed jets. After formation, they grow in azimuthal direction and become tilted when moving downstream. Vortex pairing occurs when the leading part of a structure catches up to the trailing part of its predecessor. The forcing frequency does not influence the vorticity magnitude of the coherent structures but does impact their space to develop. In lower frequency case, fewer structures that are larger size are formed. In higher frequency case, more structures are generated but they are limited in the radial direction and the size of them is smaller. Based on the results, it is clear that the optimal mixing enhancement case corresponds to an effective balance between the number and size of vortex structures.

Densities of Active species in N2/H2 RF and HF afterglows: application to surface nitriding of TiO2 nanocrystals

N2 /0–5% H2 flowing afterglows from Radio Frequency (RF) and High Frequency (HF) sources have been analyzed by optical emission spectroscopy. In similar conditions (pressure 5–6 Torr, flow rate 0.5 slm and power 100 W), it is found in pure N2 a nearly constant N-atom density from the pink to the late afterglow, which is higher in HF than in RF: (1–2) and 0.4 × 1015 cm−3 , respectively. With a N2 /2% H2 gas mixture, the early afterglows is changed to a late afterglow with about the same N-atom density for both RF and HF cases: (8–9) × 1014 cm−3 . Anatase TiO2 nanocrystals and Atomic Layer Deposition-grown films were exposed to the RF afterglows at room temperature. XPS analysis of the samples has shown that the highest N/Ti ratio of 0.24 can be achieved with the pure N2 late afterglow. In the HF pure N2 late afterglow, however, the N/Ti coverage was limited to 0.04 in spite of higher N-atom density. Such differences in the N content between the two RF and HF cases are attributed to the presence of a high O-atom impurity of 2 × 1013 cm−3 in HF as compared to that (8 × 1011 cm−3 ) in RF.

Seed electron production from O- detachment in high power microwave air breakdown

The existence of seed electrons is the precondition of air breakdown induced by high power microwave (HPM). Seed electrons are usually assumed to exist in background atmosphere when simulating the air breakdown triggered by HPM. However, this assumption may lead to some large errors especially in lower atmosphere where the number of electrons is very small. We establish a physical model of seed electron production from O- detachment collision with air molecules using the Monte Carlo method. A three-dimensional Monte Carlo program is developed to simulate this process. The average energies of O- and the average generation time of seed electrons under different electric intensities, frequencies, air pressures and breakdown volumes are obtained through simulation. The simulations show that the average generation time of seed electrons becomes longer with the increase of air pressure or the HPM frequency. The average seed electron generation time becomes shorter with the increase of electric intensity or breakdown volume. Finally, we simulate the processes of O- detachment collision with air molecules under the same experimental conditions. The comparative results show that the seed electron generation from O- detachment can explain the experimental results when the HPM frequency is low, while at higher frequencies, the average seed electron generation time becomes so long that it cannot correspond to the experimental value. Therefore some other mechanisms should be considered in the higher frequency case.

Oscillatory flow in a tube of time-dependent curvature. Part 1. Perturbation to flow in a stationary curved tube

Motivated by the study of blood flow in the coronary arteries, this paper examines the flow of an incompressible Newtonian fluid in a tube of time-dependent curvature. The flow is driven by an oscillatory pressure gradient with the same dimensionless frequency, α, as the curvature variation. The dimensionless governing parameters of the flow are α, the curvature ratio δ0, a secondary streaming Reynolds number Rs and a parameter Rt representing the time-dependence of curvature. We consider the parameter regime δ0<Rt<1 (Rs and α remain O(1) initially) in which the effect of introducing time-dependent curvature is to perturb the flow driven by an oscillatory pressure gradient in a fixed curved tube. Flows driven by low- and high-frequency pressure gradients are then considered. At low frequency (δ0<Rt<α<1) the flow is determined by using a sequence of power series expansions (Rs=O(1)). At high frequency (δ0<Rt<1/α2<1) the solution is obtained using matched asymptotic expansions for the region near the wall (Stokes layer) and the region away from the wall in the interior of the pipe. The behaviour of the flow in the interior is then determined at both small and intermediate values of Rs. For both the low and high frequency cases, we find the principal corrections introduced by the time-varying curvature to the primary and secondary flows, and hence to the wall shear stress. The physiological application to flow in the coronary arteries is discussed.

Neutral hydrogen in the solar wind acceleration region

Observation of solar Ly alpha radiation scattered by coronal neutral hydrogen atoms can be used to investigate the acceleration region of the solar wind. In this paper we focus on the use of these observations to study Alfven waves, which can accelerate the solar wind plasma to flow speeds observed in high-speed streams if their amplitude at the coronal base is 20 km/s or larger. The wave amplitude is then larger than the proton thermal speed in the outer corona, so that the mean proton speed (averaged over a wave period) is significantly larger than the proton thermal speed. For low-frequency wave the hydrogen atoms follow the proton motion in the waves, while for higher frequencies the protons move relative to the neutrals. Nevertheless, in the higher frequency case, the rates for charge exchange and recombination are high enough to broaden the velocity distribution function of neutral hydrogen. Both the wave motion of the hydrogen atoms in low-frequency Alfven waves and the 'heating' by higher frequency waves lead to a broadening of the scattered solar Ly alpha line. For coronal base amplitues of 20 km/s, the line broadening increases with heliocentric distance beyond 4-5 solar radii.

A study of recent data sets exhibiting wind‐induced ambient noise

The wind dependence of underwater ambient noise has recently received considerable attention, especially at low frequencies (1–500 Hz). In this paper, major findings are presented based on a study of several recent data sets exhibiting wind‐induced underwater ambient noise. The results are described separately for three frequency bands. At the frequency band 100–500 Hz, the wind dependence can be identified without difficulty. Wind‐dependent spray noise and other mechanisms are suggested in this frequency band. From about 10–100 Hz, the wind dependence can also be identified under special conditions. In comparison with the higher frequency case, different mechanisms of wind‐induced noise are indicated. In the band 1–10 Hz, the wind dependence can be observed in the data, but the basic mechanisms of noise sources within this band are not easily identified. Variations of ambient noise spectra depending on location, hydrophone depth, and sample size are discussed. A recommendation is made for future experime...

On the Dupree–Weinstock turbulence theory

We give a necessary criterion of validity for the Dupree–Weinstock theory of turbulence. In this connexion, we analyze two specific cases of high-frequency (HF) and low-frequency (LF) turbulence. In the HF case, we show that an appreciable modification of the growth rate of unstable modes cannot occur before trapping.