Vortex Bubble Research Articles

Bubble interaction in low-viscosity liquids

An experimental study investigated how freely rising ellipsoidal bubbles approach each other, make contact and coalesce or breakup. Pulsed planar swarms of 10–20 bubbles with Eötvös numbers from 6.0 to 27.5 were released simultaneously in aqueous solutions of 0–48 wt% sugar with Morton numbers from 3.2 × 10 −11 to 3.7 × 10 −6. Bubble interaction was recorded by a video camera following the rising bubbles. Essentially, all coalescence and breakup events occurred after, not during, wake-induced collisions by a complex process related to the bubble vortex shedding cycle. This same process was also found in multi-bubble clusters and may account for excess turbulent kinetic energy generation in bubbly flow.

Water-Modeling Study of the Surface Disturbances in Continuous Slab Caster

The present work is based on four static molds using nozzles of different port diameter, port angle, and immersion depth. It has been observed that the meniscus is wavy. The wave amplitude shows a parabolic variation with the nozzle exit velocity. The dimensionless amplitude is found to vary linearly with the Froude number. Vortex formation and bubble entrainment by the wave occurs at the meniscus beyond a critical flow rate, depending upon the nozzle configuration, immersion depth, and the mold aspect ratio.

Aerodynamics of airfoils with vortex trapped by two spanwise fences

An airfoil configuration is presented whose lift is enhanced by a trapped-vortex flow field. Based on the research to date, it is recommended that two spanwise fences be used to enclose the trapped vortex and that the fence heights be adjusted so that the equilibrium condition can be achieved with little or no mass removal from the core region of the vortex. It is also shown that the vortex bubble can be located fore and aft on the airfoil to control aerodynamic parameters, such as the pitching moment. Applications of the high-lift concept presented here are briefly discussed.

A METHOD FOR ANALYSIS OF CAVITATING MARINE PROPELLERS IN NON- UNIFORM FLOW

The article presents a calculation method based on deformable lifting surface theo­ ry. This method enables hydrodynamic analysis of propellers of arbitrary geometry operating in three-dimensional non-uniform velocity field. In the course of analysis the time-dependent pressure distribution on the blades, the resulting unsteady hydrodynamic blade and shaft forces, the extent of unsteady sheet, bubble and tip vortex cavitation are calculated. Apart from description of the method the article includes the results of initial experimental verification based on cavitation tunnel tests with two propeller models. Analysis of different hydrodynamic aspects of marine propeller operation in the non-uniform flow behind ship stern has already become a standard element of the propeller design process. Application of theoretical methods for this purpose in­ stead of cavitation tunnel experiments has an obvious advantage in terms of time and cost, provided that their accuracy is acceptable. The method presented in this article is result of a compromise between as realistic a theoretical model as possible of the physical phenomena and limited computational resources available to the average propeller designer. The method is based on the unsteady lifting surface theory, modified especially to integrate the time-dependent cavitation phenomena into the lifting surface model. This approach is named the deformable lifting sur­ face theory as the dynamically changing cavities deform the initial geometry of the lifting surface. For each analysed blade position in the non-uniform flow the kin­ ematic boundary condition on the blade and dynamic boundary condition on the cavity surface are evaluated and the resulting distribution of velocity of cavity expansion (or contraction) is calculated. This leads to calculation of the cavity geometry for each analysed blade position. The geometry evaluated in the previous

Flow Visualization Experiments on Tethered Flying Green Lacewings Chrysopa Dasyptera

ABSTRACT Experiments on dust visualization of the flow around tethered flying green lacewings showed that, contrary to expectations based on the Weis-Fogh clap-and-fling mechanism, a leading edge separation bubble does not exist near either fore-or hindwings. At the beginning of the stroke cycle each wing operates as an independent generator of vorticity. The vortex bubbles of all the four wings then unite, producing a single U-shaped bubble. A hypothetical spatial structure for the vortex wake is derived from a series of registrated sections of the wake illuminated with a flat light beam. Some problems of wing functional morphology and insect flight aerodynamics are also discussed.

Interaction of a vortex ring and a laminar flame

The present paper reports the results of an experimental study of the interaction of a vortex ring (or more appropriately, a vortex bubble) with a laminar flame. Vortex bubbles of various diameters are generated in a vortex generator driven by a loudspeaker and propagate downwards in a vertical flame tube to interact “head on” with a laminar flame. Different combustible mixtures can be used in the flame and the vortex generator. Successful ignition of a combustible vortex bubble is only possible if the spark is located at the vortex core of the bubble and not elsewhere. This suggests that the tangential velocity gradient causes the flame to be quenched by the flame stretch mechanism as in a stagnation flow. In the penetration of the vortex bubble into the hot products behind the flame, ignition is never observed to occur at the bubble surface. Instead the hot products are entrained into the bubble and enter through the rear stagnation region. Ignition occurs when sufficient hot products have been entrained and mixed with the combustible gases in the bubble. Ignition is observed to originate near the bubble axis and spread radially outward to be entrained into the vortex core. Measurements of the entrainment time (time between the entrance of the bubble into the hot products and ignition) indicate that the entrainment time is very weakly dependent on the vortex circulation in accord with the observation of Maxworthy of non-reating vortex rings. Combustion time (period between ignition and complete burning of thevortex bubble) is found to be independent of the mixture composition and is of about the same, order of magnitude as the entrainment time.

Swirling flows in streamtubes of variable cross section.

The behavior of vortex flow at high swirls is investigated. The problem was first formulated in the method of weighted residuals, using Bessel functions as approximating functions. Convergent solutions require a large number of steps in the axial direction and do not seem to offer an advantage over traditional methods. As the results do not differ substantially from those of the zeroth approximation (the solution for 'cylindrical' flow), this approximation is then explored and discussed in more detail. It is used to derive a relationship predicting a critical radius for which stagnation on the axis (vortex breakdown) can be expected for a given swirl. Flow patterns with open and closed vortex bubbles are explored. At high swirls, even minute variations in outer streamtube radius have profound effects on the flow. Vortex breakdown bubbles in divergences, tubes, and free flows can be explained.

Some Experiments on the Reattachment of a Laminar Boundary Layer Separating From a Rearward Facing Step on a Flat Plate Aerofoil

Summary:The results of experiments on the reattachment of a laminar boundary layer, separating from a rearward facing step in a flat plate aerofoil, are correlated with the properties of the short leading edge bubble which forms on thin aerofoils near the stall.The experiments, comprising pressure measurements, Pitot explorations, liquid film and smoke studies, indicate that for all Reynolds numbers above the value given by the Owen-KIanfer criterion the reattachment is turbulent behind a stationary air reverse flow vortex bubble. It is also found that the reattachment is laminar for Reynolds numbers below the critical, which further supports Crabtree's interpretation of the Owen-KIanfer criterion in terms of the condition for the growth of turbulent bursts.