A mechanism stimulating sound production from air bubbles released from a nozzle

Abstract: Gas bubbles in water act as oscillators with a natural frequency inversely proportional to their radius and a quality factor determined by thermal, radiation, and viscous losses. The linear dynamics of spherical bubbles are well understood, but the excitation mechanism leading to sound production at the moment of bubble creation has been the subject of speculation. Experiments and models presented here show that sound from bubbles released from a nozzle can be excited by the rapid decrease in volume accompanying the collapse of the neck of gas which joins the bubble to its parent

Automated processing of oceanic bubble images for measuring bubble size distributions underneath breaking waves

Accurate in situ measurements of oceanic bubble size distributions beneath breaking waves are needed for a better understanding of air–sea gas transfer and aerosol production processes. To achieve this goal, a novel high-resolution optical instrument for imaging oceanic bubbles was designed and built in 2013 for the High Wind Gas Exchange Study (HiWinGS) campaign in the North Atlantic Ocean. The instrument is able to operate autonomously and can continuously capture high-resolution images at 15 frames per second over an 8-h deployment. The large number of images means that it is essential to use an automated processing algorithm to process these images. This paper describes an automated algorithm for processing oceanic images based on a robust feature extraction technique. The main advantages of this robust algorithm are it is significantly less sensitive to the noise and insusceptible to the background changes in illumination, can extract circular bubbles as small as one pixel (approximately 20 μm) in radius accurately, has low computing time (approximately 5 seconds per image), and is simple to implement. The algorithm was successfully used to analyze a large number of images (850 000 images) from deployment in the North Atlantic Ocean as part of the HiWinGS campaign in 2013

Notes on meteorological balloon mission planning

In the 21st century the high altitude gas balloon remains an indispensable tool in atmospheric science, meteorology and other applications requiring stratospheric observations. A prerequisite of the effectiveness of many types of balloon operations is an accurate trajectory forecasting capability, complete with appropriate error estimates. This is particularly important in targeted flights, sample return missions or flights of expensive instruments, whose recovery is essential. The ASTRA (Atmospheric Science Through Robotic Aircraft) initiative led to the development of such a forecast model, which is at the centre of the present paper. A key source of error in such models is our incomplete understanding of the drag opposing the rise of balloons in the free atmosphere – here we propose a new, stochastic model based on empirical data derived from thousands of radiosonde flights. We also examine other sources of prediction error affecting the accuracy of the flight path forecast, such as uncertainties in the wind profile and balloon envelope manufacturing variability. A Monte Carlo framework is used to provide probabilistic touchdown point estimates taking these error sources into account. The above elements have been integrated into a web service, which can be used as a flight planning tool – here we review the key features of its architecture

A candidate mechanism for exciting sound during bubble coalescence

Coalescing bubbles are known to produce a pulse of soundat the moment of coalescence, but the mechanism driving the sound production is uncertain. A candidate mechanism for the acoustic forcing is the rapid increase in the bubble volume, as the neck of air joining the two parent bubbles expands. A simple model is presented here for the volume forcing caused by the coalescence dynamics, and its predictions are tested against the available data. The model predicts the right order of magnitude for the acoustic amplitude, and the predicted amplitudes also scale correctly with the radius of the smaller parent bubble

A mechanism stimulating sound production from air bubbles released from a nozzle

Gas bubbles in water act as oscillators with a natural frequency inversely proportional to their radius and a quality factor determined by thermal, radiation, and viscous losses. The linear dynamics of spherical bubbles are well understood, but the excitation mechanism leading to sound production at the moment of bubble creation has been the subject of speculation. Experiments and models presented here show that sound from bubbles released from a nozzle can be excited by the rapid decrease in volume accompanying the collapse of the neck of gas which joins the bubble to its paren

Ignition of HMX and RDX

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The effect of coupling on bubble fragmentation acoustics

Understanding the formation and evolution of bubble populations is important in a wide range of situations, including industrial processes, medical applications, and ocean science. Passive acoustical techniques can be used to track changes in the population, since each bubble formation or fragmentation event is likely to produce sound. This sound potentially contains a wealth of information about the fragmentation process and the products, but to fully exploit these data it is necessary to understand the physical processes that determine its characteristics. The focus of this paper is binary fragmentation, when turbulence causes one bubble to split into two. Specifically, the effect that bubble-bubble coupling has on the sound produced is examined. A numerical simulation of the acoustical excitation of fragmenting bubbles is used to generate model acoustic signals, which are compared with experimental data. A frequency range with a suppressed acoustic output which is observed in the experimental data can be explained when coupling is taken into account. In addition, although the driving mechanism of neck collapse is always consistent with the data for the larger bubble of the newly formed pair, a different mechanism must be driving the smaller bubble in some situations

The relationship between shock sensitivity and morphology in granular RDX

It is known that batches of the secondary explosive RDX from different manufacturers show their significant variation in shock sensitivity. No obvious correlation between shock sensitivity and either chemical composition or morphology has previous been identified which explains this. We use a range of techniques to study the microstructure of RDX crystals and the bulk morphology of granular beds in order to assess which hotspot mechanisms tend to be dominant. Crystals were characterized using mercury porosimetry, environmental scanning electron microscopy (ESEM) and optical microscopy.This range of methods yields quantitative and qualitative data on internal void size and number and surface structure. Shock sensitivity is quantified using small-scale gap tests, and this demonstrates the clear differences in sensitivity between batches from different manufacturers. The samples used are from three manufacturers, produced by both the Woolwich and Bachmann processes, and all have an average particle size of approximately 1200 ?RDXm

High Altitude Gas Balloon Trajectory Prediction - a Monte Carlo Model

The high altitude gas balloon is an indispensable tool in atmospheric science, meteorology and other applications requiring stratospheric observations. A pre-requisite ofthe effectiveness of many types of balloon operations is an accurate trajectory forecasting capability. In particular, targeted flights, sample return missions or flights of expensive instruments (whose recovery is essential) rely on such models. In this paper we describea new balloon flight simulation model, which takes into account a range of environmental, physical and operational uncertainties to generate a predicted trajectory equipped with landing site location error estimates. A key source of error in such models is our incomplete understanding of the drag opposing the rise of balloons in the free atmosphere - here we propose a new, stochastic drag model based on empirical data derived from thousands of radiosonde flights. We also examine other sources of prediction error affecting the accuracy of the flight path forecast, such as uncertainties in the wind profile and balloon envelope manufacturing variability. We show how integrating these elements into a process that generates Monte Carlo ensembles of simulated trajectories has yielded a practically useful flight planning tool, which we have made available to the ballooning community as a free online service

Improvements to the methods used to measure bubble attenuation using an underwater acoustical resonator

Active acoustic techniques are commonly used to measure oceanic bubble size distributions, by inverting the bulk acoustical properties of the water (usually the attenuation) to infer the bubble population. Acoustical resonators have previously been used to determine attenuation over a wide range of frequencies (10–200 kHz) in a single measurement, corresponding to the simultaneous measurement of a wide range of bubble sizes (20–300 μm radii). However, there is now also considerable interest in acquiring measurements of bubbles with radii smaller than l6 micrometres, since these are thought to be important for ocean optics and as tracers for near-surface flow. To extend the bubble population measurement to smaller radii, it is necessary to extend the attenuation measurements to higher frequencies. Although the principles of resonator operation do not change as the frequency increases, the assumptions previously made during the spectral analysis may no longer be valid. In order to improve the methods used to calculate attenuation from acoustical resonator out-puts, a more complete analysis of the resonator operation is presented here than has been published previously. This approach allows for robust attenuation measurements over a much wider frequency range and enables accurate measurements from lower-quality spectral peaks