Plume Deflection Angle Research Articles

Dispersion in an array of buildings in stable and convective atmospheric conditions

Wind tunnel experiments were conducted to study the impact of atmospheric stratification on flow and dispersion within and over a regular array of rectangular buildings. Three stable and two convective incoming boundary layers were tested with a Richardson number ranging from −1.5 to 0.29. Dispersion measurements were carried using a fast response flame ionisation detector. The results show that the stratification effect on the plume width is significantly lower than the effect on the vertical profiles. Stable stratification did not affect the plume central axis inside the canopy, but in the unstable case the axis appeared to deviate from the neutral case direction. Above the canopy both stratification types caused an increase in the plume deflection angle compared to the neutral case. Measured mean concentrations in stable stratification were up to two times larger in the canopy compared to the neutral case, while in convective conditions they were to three times smaller. The proportionality between the vertical turbulent fluxes and the vertical mean concentration gradient was also confirmed in the stratified cases. The high-quality experimental data produced during this work may help developing new mathematical models and parametrisation for non-neutral stratified conditions, as well as validating existing and future numerical simulations.

Analysis of pulsed laser deposited amorphous chalcogenide film thickness distribution: Plume deflection angle dependence

Pulsed laser deposition exploiting a KrF excimer laser was used to fabricate amorphous As-S thin films from bulk As2S3 glass target. Thickness profile of the film was extracted from variable angle spectroscopic ellipsometry data. The dependence of thickness distribution of prepared thin layer on laser beam plume deflection angle was evaluated and corresponding equations were suggested.

Comparison Between Large-Eddy Simulation and Reynolds-Averaged Navier–Stokes Computations for the MUST Field Experiment. Part II: Effects of Incident Wind Angle Deviation on the Mean Flow and Plume Dispersion

Large-eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) computations of pollutant dispersion are reported for the Mock Urban Setting Test (MUST) field experiment flow. In particular we address the effects of incident wind angle deviation on the mean velocity and on the mean concentration fields. Both computational fluid dynamical methods are assessed by comparing the simulation results with experimental field data. The comparative analysis proposes to relate the plume deflection with the flow channelling effects. The results show that the plume deflection angle varies with the altitude. As the ground is approached the plume is shown to be almost aligned with the street canyon direction and independent of the incident wind directions considered. At higher altitudes well above the obstacles, the plume direction is aligned with the mean wind direction as in dispersion over flat terrain. The near-ground plume deflection is the consequence of a strong channelling effect in the region near the ground. The mean concentration profiles predicted by LES and RANS are both in good qualitative agreement with experimental data but exhibit discrepancies that can be partly explained by the influence of small incident wind angle deviation effects. Compared to RANS, LES predicts a higher channelling and thus a higher deflection of the plume. Results on the fluctuating intensity of the concentration obtained from LES show a satisfactory agreement with experiments. This information is not available from RANS for which only the mean concentration modelling is considered.

Double-peak droplet mass distribution observed during sub-ps laser ablation of Si targets

The angular distribution of the ablated material was studied during sub-ps Si laser ablation deposition using a special hemicylindrical substrate holder and different laser fluences ranging between 0.4 and 1.7 J/cm2. Scanning electron microscopy analysis of the deposited films showed that, independent of the fluence, the distribution of the deposited droplets presents two maxima. The first maximum corresponds to the average plume deflection angle value due to the local surface orientation produced by the preferential etching process. The second maximum is observed approximately at 45° with respect to the normal of the target surface, and is related to the phase explosion products that expand along the incident laser beam direction. The investigation of the twofold distribution of the sub-μm size deposited droplets is important to improve the quality of the deposited coatings.

Morphological Changes and Plume Deflection Effect during Pulsed Laser Ablation Deposition of Al

The morphological changes and corresponding plume deflection effect during long laser (XeCl excimer laser, λ=308 nm, τ=30 ns) irradiation of Al target have been investigated,and results showed that, a the number of laser pulses per site increases, the target morphology changes and produces a visible deviation of the plasma plume up to 5º. Scanning electron spectroscopy analys of the target surface related the plume deflection angle to the target morphology and the number of laser pulses per site. Typically well-defined columnar structures oriented along the laser beam direction were observed on the target surface. The plume created during the laser ablation was clearly visible to the naked eye and was recorded by a digital camera. Detailed studies were also performed on the ablation rate at different laser fluences (4.6 J/cm2-12.5 J/cm2). The morphologies and the thickness of the Al thin films deposited on Si substrates during the present laser ablation experiments were also studied. Finally, the scenario explaining the formation of columnar structure on target surface responsible for the plume deflection effect will be discussed.

06/01241 Experimental study of the resulting flow of plume-thermosiphon interaction: application to chimney problems: Zinoubi, J. et al. Applied Thermal Engineering, 2005, 25, (4), 533–544

This paper describes the mean gas fraction distribution in the two-phase flow of a gas–liquid bubble plume set to develop adjacent either to a wall or to another bubble plume. When this happens, the plume exhibits a type of Coanda effect, bending either towards the wall or the other plume. The local mean gas fraction measurements are carried out using the electro-resistivity probe technique in an air–water system. The deflection angle of the plumes is shown to present a dependence on the modified Weber and Froude numbers of the bubbles. A Gaussian distribution for the mean gas fraction profile, observed to exist for axisymmetric single plumes, is shown not to occur in flow geometries where the Coanda effect is allowed to set in. The transition zone between the downstream flow where two Gaussian plumes are observed and the far upstream flow where the plumes are seen to merge onto a single plume is characterized. Photographs of the flow are shown to illustrate the phenomenon. Results are present for two- and three-dimensional mean gas fraction distributions. In addition, a simple theory based on integral methods is advanced for the prediction of the plume deflection angle; the theory considers a variable entrainment co-efficient.

Detailed studies of the plume deflection effect during sub-ps laser ablation of Si target

For the first time the plume deflection effect during the irradiation of silicon target with sub-ps KrF laser pulses was investigated. The morphological changes of the laser ablated target and the corresponding plume deflection angle have been studied in vacuum with different laser fluences and number of pulses. In accordance with earlier results obtained by nanosecond laser, we found a strong correlation between the morphological changes on the irradiated target and the observed plume deflection angle. As the number of laser pulses increases, the formation of well-defined silicon columnar microstructures oriented towards the laser beam has been observed on the targets ablated with fluences between 0.1 and 1.7 J/cm 2 (2.2 × 10 11 and 37.4 × 10 11 W/cm 2 of power density, respectively). It has also been observed that the plume deflection effect is present for laser fluences between 0.1 and 1.2 J/cm 2, with a maximum deflection angle always higher than 20°. Simultaneously, an array of silicon substrates placed on a hemi-cylindrical holder was used for deposition experiments, in order to study the influence of the plume deflection on the thickness distribution profile of the deposited films. Profilometric analyses of the deposited silicon films revealed a non-uniform material deposition. Comparison with the results obtained by nanosecond lasers is reported and discussed.

Laser ablation of silicon and copper targets. Experimental and finite elements studies

The ablation process induced by excimer lasers is a collective phenomenon that basically involves two phenomena: the laser radiation–matter interaction and the dynamic of the ablation plume. The laser parameters, the thermal and optical properties of the material, and the surface morphology are critical factors in the ablation mechanisms affecting the direction of the ablation plume expansion. In this study, the role of the surface roughness and the evolution of its morphology under the laser irradiation were investigated. Assuming a thermal ablation model, a theoretical study of the initial steps of the laser ablation process by a finite element method using ANSYS (6.1) was performed. Different ablation experiments were carried out on silicon and copper targets using a XeCl laser. The target surface morphology changes were observed by SEM and the plume deflection was recorded by a digital camera. An acceptable agreement between the experimental and simulated results was found. This study contributes to a better understanding of the physical processes involved in the laser ablation and the relations between the plume deflection angle and the surface roughness.

Extensive Studies of the Plume Deflection Angle During Laser Ablation of Si Target

The plume deflection effect during long laser irradiation of a Si target has been extensively studied. Two different laser ablation experiments were performed to study the plume deflection angle and the ablation rate versus the number of laser pulses/site. Scanning electron microscopy (SEM) analyses and profilometry of the Si target surface were performed to examine the continuous morphological changes produced by the laser irradiation. The roughening of the target surface strongly influenced the formation and expansion of the Si plume. Plume deflection angles higher than 30° have been observed by a digital camera. Complementary mass spectrometer measurements confirmed that the ion flux distribution is strongly oriented along the laser beam direction. In addition to laser ablation experiments, two other laser deposition experiments were performed: with the conventional configuration and with a new approach, termed frontal off-axis deposition. The new pulsed-laser deposition configuration provides much better control of uniformity and film thickness than the conventional deposition configuration.

High quality Hastelloy films deposited by XeCl pulsed laser ablation

Pulsed laser ablation has been used to deposit Hastelloy thin films on carbon steel substrates at room temperature. Depositions have been carried out by XeCl excimer laser ablation ( λ=308 nm, τ=30 ns) of a bulk Hastelloy target in a high vacuum system ( p=10 −5 Pa). In order to obtain as thick and uniform films as possible, we took in account the maximum plume deflection angle we observed in our experiments. Optical studies of the plume revealed a deviation of about 15° towards the laser beam. To minimize the effects of the plume deflection, the substrate was placed along the deflected plume axis instead of the normal to the target surface. The plume deflection angle and the ablation rate of Hastelloy target as a function of number of laser pulses per site have been also measured. Scanning electron microscopy (SEM), energy dispersion X-ray (EDX) spectroscopy and X-ray diffraction (XRD) have been performed to analyse, respectively, the surface morphology, the chemical composition and the crystallographic structure of the deposited films. In order to correlate the microstructure of the film surface with its corrosion properties, potentiodynamic curves and linear polarisation resistance analyses have been carried out.

Detailed studies of the plume deflection effect during long laser irradiation of solid targets

The changes of the laser ablated target morphology and the corresponding plume deflection angle have been studied. As the number of laser pulses/site increases, the target morphology changes and produces a visible deviation of the plasma plume. Scanning electron microscopy analyses of the target surface allowed us to correlate the plume deflection angle to the target roughness and, hence, to the number of laser pulses/site. The deflection effect has been observed for all the targets used (Ti, Si, Cu, Al–Sn alloy, graphite and magnetic compounds) and for whatever target surface morphology produced by laser irradiation. As earlier models explaining the origin of the plume deflection are not universal and less than ever satisfactory, we suggest a simple model that seems to be more realistic and general.

Bubble plumes and the Coanda effect

This paper describes the mean gas fraction distribution in the two-phase flow of a gas–liquid bubble plume set to develop adjacent either to a wall or to another bubble plume. When this happens, the plume exhibits a type of Coanda effect, bending either towards the wall or the other plume. The local mean gas fraction measurements are carried out using the electro-resistivity probe technique in an air–water system. The deflection angle of the plumes is shown to present a dependence on the modified Weber and Froude numbers of the bubbles. A Gaussian distribution for the mean gas fraction profile, observed to exist for axisymmetric single plumes, is shown not to occur in flow geometries where the Coanda effect is allowed to set in. The transition zone between the downstream flow where two Gaussian plumes are observed and the far upstream flow where the plumes are seen to merge onto a single plume is characterized. Photographs of the flow are shown to illustrate the phenomenon. Results are present for two- and three-dimensional mean gas fraction distributions. In addition, a simple theory based on integral methods is advanced for the prediction of the plume deflection angle; the theory considers a variable entrainment co-efficient.