Bubble Image Velocimetry Research Articles

An experimental model with passively variable stiffness to investigate the effect of body stiffness on the fish fast-start maneuver

We present an experimental model to investigate the influence of body flexibility on the fast-start response. The model is fabricated with a variable stiffness, characterized by the direction of applied force. When the fluid force is applied in one direction, the structure acts flexibly and bends, and when the force is applied in the opposite direction, it remains rigid. The structure is rotated from its initial straight position in its flexible direction to a given angle (Stage 1), and then it is rotated back in its rigid direction to an angle in the opposite direction (Stage 2). The wake patterns are studied with bubble image velocimetry (BIV) showing that the main propulsive jet, previously observed in the live fish fast-start, can be produced using this model. Based on our results, we determine that flexibility during Stage 1 and increased rigidity during Stage 2 predict velocity profiles and escape angles consistent with those observed in live fish. A parametric study on the effect of angular velocity and total swept angle on the produced power reveals that the magnitude of swept angle is critical to formation of the propulsive jet. The peak power transfer is found to be more sensitive to increasing the angular velocity for larger swept angles, where a complete jet is already formed.

Characterization of the bubble swarm trajectory in a jet bubbling reactor

AbstractThe bubble swarm trajectory in the jet bubbling reactor is measured through the bubble image velocimetry (BIV) technique. The result shows that the bubble swarm rises straightly when the jet Reynolds number is lower than 7,000. However, when the jet Reynolds number exceeds 14,000, the bubble swarm exhibits vortex‐like motion, and the bubble vortices oscillate periodically. The oscillating frequency of bubble vortices under the gas bubbling condition is lower than the flapping frequency of pure liquid jet. Moreover, the moving region and oscillating frequency of bubble vortices increase with the jet Reynolds number. The superficial gas velocity has little effect on the bubble swarm trajectory and the oscillating frequency. An empirical correlation between the oscillating frequency of bubble vortices and the jet Reynolds number is built based on the simple harmonic vibration theory.

Individual violent wave-overtopping events: behaviour and estimation

ABSTRACTTo better understand individual violent wave overtopping, of significance for coastal defence design, three breaking wave types (steep-fronted, plunging and broken) based on focused wave groups, were generated in laboratory and numerical models. High-speed video captured overtopping events and produced velocity vector maps by means of bubble image velocimetry (BIV). Results were compared with a numerical model based on a linear wave detection procedure and a two-phase incompressible Navier–Stokes-based solver. This novel approach revealed that the overtopping waves comprised an initial jet of 0.2 s duration, but dominated by quasi-steady flow. Whilst laboratory surface-elevation time-histories were highly repeatable, overtopping volume repeats were sensitive to the breaker type. Measured volumes were compared with: the numerical model (which over-predicted, but was reasonably accurate for steep-fronted waves); estimations based on BIV results (which provided very close agreement for the steep-fronted waves); and a weir-based analogy (which provided reasonable agreement, but always under-predicted).

EXPERIMENTAL MODELING OF TSUNAMI BORE IMPINGEMENT ON A SIMPLIFIED COASTAL BUILDING

Numerous laboratory efforts were devoted to improve our understanding to the process of tsunami wave-structure interaction and provide valuable data to validate numerical and analytical models. However, the highly turbulent and multiphase nature of tsunami bores makes the study of their impacts very challenging. Many experimental studies (e.g., Shafiei et al. 2016) employed wave gauges to measure the bore height and estimate the bore front velocity based on shallow water equations. Considering the complexity of the flow and its impact with structures, the conversion between the bore height and the bore velocity is far from straightforward. Therefore, this study attempts to apply the bubble image velocimetry (BIV, Ryu et al. 2005) technique to directly measure the flow velocities during the tsunami bore impact. A tsunami wave, that breaks on a sloping beach, propagates inland as a form of bore, and impinges on a rigid structure, is considered as the scenario of interest. The objective is to perform a comprehensive investigation on the borestructure interaction by examining the fluid velocity, impact pressure, and surge force during the impact event with various structure headings.

Bubble image velocimetry with a field-deployable acoustic camera

This study explores performing bubble image velocimetry (BIV) analysis on images acquired from an acoustic camera (AC) to obtain measurements of the flow in the wake of a circular cylinder at two flowrates. The AC observes the returns from air micro-bubbles suspended in the flow which are used as tracer particles in PIV-style analysis. The size distribution of the micro-bubbles is measured with an acoustic bubble spectrometer (ABS). The AC BIV measurements of the wake agree well with the comparison measurements made by cross-stream arrays of acoustic Doppler velocimeters (ADVs) for the lower of the two measured flowrates. The AC BIV measurements of cross-stream profiles of the mean downstream velocities and the profiles of characteristics of the velocity fluctuations, such as the standard deviation of the downstream and cross-stream velocity fluctuations, the covariance of the horizontal velocity fluctuations, and the kinetic energy of the horizontal velocity fluctuations, all matched the ADV measurements well. However, the measurement technique is sensitive to bubble size distribution, as the higher flowrate AC BIV measurements struggled to replicate the ADV measurements, particularly the measurements of the velocity fluctuations. The higher flowrate experiment is revealed to have an order of magnitude more bubbles with size exceeding 15 present in the flow than the lower flowrate experiment. The increased bubble density caused increased noise in the AC images and a ‘flickering’ effect, with bubbles further away from the camera being intermittently obscured by those closer to the camera. The ability to make PIV-style measurements with an AC has ramifications for flow measurements in the field. ACs operate in turbid environments, do not require a laser and extensive optics, and do not require calibration with a grid plate to convert pixel space to physical space.

Application of SIM, HSPIV, BTM, and BIV Techniques for Evaluations of a Two-Phase Air–Water Chute Aerator Flow

Four image-based techniques—i.e., shadowgraphic image method (SIM), high-speed particle image velocimetry (HSPIV), bubble tracking method (BTM), and bubble image velocimetry (BIV)—are employed to investigate an aerator flow on a chute with a 17° inclination angle. The study focuses on their applications to the following issues: (1) to explore the characteristic positions of three water–air interfaces; (2) to interpret the evolution process of air bubbles shed from the wedged tip of the air cavity; (3) to identify the probabilistic means for characteristic positions near the fluctuating free surface; (4) to explore the probability distribution of intermittent appearance of air bubbles in the flow; (5) to obtain the mean streamwise and transverse velocity distributions of the water stream; (6) to acquire velocity fields, both instantaneous and mean, of air bubbles; (7) to construct a two-phase mean velocity field of both water flow and air-bubbles; and (8) to correlate the relationship among the probability distribution of air bubbles, the mean streamwise and transverse velocity profiles of air bubbles, and water stream. The combination of these techniques contributes to a better understanding of two-phase flow characteristics of the chute aerator.

An analytical model of final average droplet size prediction of wave spray cloud

This study introduces an analytical model to predict the final average droplet size within a spray cloud that is formed due to wave interactions with marine objects. The process of spray cloud formation from wave impact involves several complicated processes, and each of these phenomena should be taken into account when considering an analytical approach. Following this process-based approach, the model first considers the wave characteristics and relates wave properties to the maximum impact pressure also considering the effect of air entrapment occurrence. Secondly, the maximum impact pressure is correlated empirically to the maximum wave run-up velocity. Finally, wave run-up velocity is linked with water sheet breakup models, which leads to the prediction of the final average diameter of ligaments and droplets. The prediction model is compared with experimental measurements from experiments conducted in a tow tank to form a spray cloud due to wave impact with two lab-scale models. Several measuring techniques such as image processing, bubble image velocimetry (BIV), and digital particle image velocimetry (DPIV) methods were implemented to measure the physical quantity of each phenomenon during the event of wave interaction. The effects of initial wave characteristics, the geometrical parameters of the impact condition and water sheet thickness at the moment of impact, on the final average droplet diameter were investigated. A sensitivity analysis of most of the principal parameters was performed to show the high veracity of the prediction model.

Kinematics and dynamics of green water on a fixed platform in a large wave basin in focusing wave and random wave conditions

Green water kinematics and dynamics due to wave impingements on a simplified geometry, fixed platform were experimentally investigated in a large, deep-water wave basin. Both plane focusing waves and random waves were employed in the generation of green water. The focusing wave condition was designed to create two consecutive plunging breaking waves with one impinging on the frontal vertical wall of the fixed platform, referred as wall impingement, and the other directly impinging on the deck surface, referred as deck impingement. The random wave condition was generated using the JONSWAP spectrum with a significant wave height approximately equal to the freeboard. A total of 179 green water events were collected in the random wave condition. By examining the green water events in random waves, three different flow types are categorized: collapse of overtopping wave, fall of bulk water, and breaking wave crest. The aerated flow velocity was measured using bubble image velocimetry, while the void fraction was measured using fiber optic reflectometry. For the plane focusing wave condition, measurements of impact pressure were synchronized with the flow velocity and void fraction measurements. The relationship between the peak pressures and the pressure rise times is examined. For the high-intensity impact in the deck impingement events, the peak pressures are observed to be proportional to the aeration levels. The maximum horizontal velocities in the green water events in random waves are well represented by the lognormal distribution. Ritter’s solution is shown to quantitatively describe the green water velocity distributions under both the focusing wave condition and the random wave condition. A prediction equation for green water velocity distribution under random waves is proposed.

Experimental Study on Variations in Behavior of Green Water and Flow Kinematics on Deck with Various Flare Angles

In this study, a series of experiments were performed to investigate the variations in the behavior of green water generation and the flow kinematics of bubbly flow on deck with various flare angles. The experiments were conducted in a 2-D wave flume using a simplified model of a BW Pioneer FPSO operating in the Gulf of Mexico, with a 100-year return period wave condition. The green water phenomena were captured with a high speed CCD camera. The variations in the behavior of the green water generation were investigated with various flare angles, and the horizontal mean velocity profiles of bubbly flow on deck obtained using bubble image velocimetry (BIV) were provided. The differences in flow kinematics of bubbly flow on deck were analyzed with various flare angles.

Large-scale laboratory observation of flow properties in plunging breaking waves

A plunging breaking wave of 1-m height was generated in a very large wave tank of 5 m in width, 5 m in depth, and 300 m in length filled with freshwater. The surface velocities in the highly aerated region of the breaking wave were measured using bubble image velocimetry (BIV), while the void fraction profiles were measured using fiber optic reflectometers (FOR). The internal velocities below the aerated region were also measured using an array of acoustic Doppler velocimeters (ADV). A wavelet-based technique was used to detect vortical structures at the free surface and estimate their length scales. The measured surface velocity fields were decomposed into wave induced and turbulence induced components to investigate the temporal and spatial evolution of mean kinetic energy and turbulent kinetic energy. It was found that turbulence is advected and diffused mainly following the phase speed of the breaking wave, rather than from the wave group velocity during the first splash-up process. The internal velocity measurements below the aerated regions show that turbulent kinetic energy decreases exponentially as the depth increases. Since scale effects under breaking waves with turbulence and air entrainment are less understood, results in flow kinematics, turbulence, and void fraction in the present study were compared with that in Lim et al. (2015) which investigated small scale plunging breaking waves with a 0.2-m wave height. It was found that flow kinematics and some dynamic properties such as void fraction and turbulent kinetic energy can be well represented between different physical scales if the traditional Froude scaling law is applied. Other dynamic properties, including bubble number and size distributions, seem to be significantly affected by the physical scales.

Measurement of spray-cloud characteristics with bubble image velocimetry for braking wave impact

The process of spray-cloud production and flow kinematics arising from breaking wave impact on a fixed flat-shaped plate and a bow-shaped plate models were investigated based on the image velocimetry method. During wave impact and water sheet formation on a flat surface, an enormous amount of aerated flow is created in front of the surface. This process produces a significant number of bubbles in the resulting flow. The Bubble Image Velocimetry (BIV) was used to measure the wave run-up velocity. The BIV method uses these bubbles as a tracer and measures bubble velocity by correlating the texture in the bubble images. A high-speed camera was employed to record images for calculating fluid velocity. Instead of using a laser light sheet, a diffused LED backlight illuminated the bubble images. Two perpendicular plane views were used to capture the footages: a side and a front view. The wave run-up velocity measurements at the moment of impact are compared with the results of several published experimental results. Aside from the BIV method, spray characteristics were examined based on the Digital Particle Image Velocimetry (DPIV) method. This method was used for the area that the concentration of bubbles is low or no bubbles are present. Measurements of droplet size, and velocity, as well as wave run-up velocity, were major concerns in this study. Results of velocimetry from both views were compared, and a satisfactory agreement was achieved among >90% of the data. The highly transient process of spray formation from the liquid sheet breakup was discussed, and the drag coefficients and drag forces were calculated in two different stages of spray formation. Several pressure sensors were used to measure the impact pressure and three capacitive wave probes to measure the free surface profile. It was observed that higher impact pressure leads to the production of more dense and finer spray with a higher transition velocity. Moreover, it was illustrated that smaller size droplets have a higher velocity in compare with the medium and large size droplets, however, because of the effect of drag force these small size droplets cannot reach the maximum spray heights.

Bubble size and bubble velocity distribution in bubble columns under industrial conditions

Bubble column reactors are widely used in many industrial applications due to their simplicity and safety of operation. Despite these advantages, the design and scale‐up of bubble column reactors is still challenging especially for industrial conditions at elevated pressure and temperature. One reason is the uncertainties concerning the specific interfacial area which is directly dependent on the bubble size distribution, bubble velocity, and gas hold‐up. All these parameters are difficult to measure under industrial conditions due to the opaqueness of the bubbly flow and the safety risks of using organic solvents at elevated pressures and temperatures. This article introduces endoscopic bubble image velocimetry, a new measuring method that enables the detection of bubble sizes and bubble velocities in organic solvents at elevated pressure and temperature (pmax = 1.85 MPa and Tmax = 70 °C) for maximal gas hold‐ups of 16 %. For this system it becomes evident that the bubble size distribution for low superficial gas velocities is almost unaffected by pressure and temperature, whereas the bubble velocity decreases slightly.

Alternating skimming flow over a stepped spillway

The study of stepped spillways in laboratory scales has been essentially focused on two separated sub-regimes within skimming flow. In this paper we investigate the appearance of an unclassified alternating skimming flow regime in a 0.5 m wide stepped spillway which does not fit on these earlier definitions, and which does not occur in a 0.3 m wide spillway. Our aim is to explain the genesis of this unclassified flow which is visualised in the physical stepped spillway, by using 3D numerical modelling. Flow depths and velocities are measured using an ultrasonic sensor and Bubble Image Velocimetry in the wider flume (0.5 m). The numerical model is validated with the experimental data from the 0.5 m wide spillway. After validation, the channel width of the same numerical model is reduced to 0.3 m wide spillway in order to characterise (compare) the case without (with) alternating skimming flow. Both cases are solved using Reynolds-Averaged Navier–Stokes equations together with the Volume-of-Fluid technique and SST k-\(\omega\) turbulence model. The experimental results reveal that the alternating skimming flow regime is characterised by an evident seesaw pattern of flow properties over consecutive steps. In turn, the numerical modelling clarified that this seesaw pattern is due to the presence of a complex system of cross waves along the spillway. These cross waves are also responsible for a mass and momentum exchange in the transversal direction and for the formation of the alternating skimming flow in the spillway.

Application of Image Technique and Optical Fiber Sensor for Air-water Mixture Flow

Measurements of multiphase flows containing bubbles have been limited because most existing methods target one phase flows. Especially, multiphase flows with a high void ratio have been rarely successful in measurements due to the sudden change of density and thick interfaces between air and water. This study introduces two methods that are capable of measuring flow fields regardless of bubble void ratio, named bubble image velocimetry and bundle fiber optic flow meter. The calculation of the depth of field is suggested to reduce and estimate errors by perspective image velocimetry. The bundle fiber optic flow meter is designed to increase a measurement rate using many optical fibers with a thin diameter. The two methods measured bubble plumes to test reliability and the velocity measurements show good agreement. In addition a hydraulic jump, one of the multiple flows in rivers was measured to test applicability of the methods.

Green water velocity due to breaking wave impingement on a tension leg platform

The present study employed the image-based bubble image velocimetry (BIV) technique to investigate the flow kinematics of a plunging breaking wave impinging on a geometry-simplified, unrestrained tension leg platform (TLP). A high-speed camera was used to record images for the BIV velocity determination for both fluid and structure velocities. The plunging breaker was generated using a wave focusing method, and repeated measurements were acquired to calculate the mean flow and turbulence intensity using ensemble averaging. BIV measurements were performed on two perpendicular planes: side view and top view. The flow measurements were compared with those of a similar experiment on a fixed structure by Ryu et al. (Exp Fluids 43(4):555–567, 2007a). The maximum velocity occurred in the run-up stage with a magnitude reaching 2.8C with C being the phase speed of the breaking wave. The dominant velocities for three distinct phases—impingement, run-up, and overtopping—are very close to those found on the fixed structure. Turbulence intensity was also examined, and the ratio among the three components was quantified. Ryu et al. (Appl Ocean Res 29(3):128–136, 2007b) reported that Ritter’s dam-break flow solution agrees surprisingly well with the measured green water velocity on the fixed structure to a certain degree. The present study followed the same approach and confirmed that Ritter’s solution is also in excellent agreement with the green water velocity on the unrestrained TLP model. Based on the self-similar behavior, the prediction equation proposed by Ryu et al. (2007a) was used to model the green water velocity distribution. The results show that the prediction equation is applicable to not only a fixed structure, but also the unrestrained TLP for green water velocity caused by extreme waves.

Surface velocity and impact pressure of green water flow on a fixed model structure in a large wave basin

The present study investigated green water velocities and impact pressures caused by the impact of overtopping waves on a fixed deck structure in a large-scale, three-dimensional deep-water wave basin. Using the bubble image velocimetry technique, detailed two-dimensional surface flow structures on a horizontal plane, including the temporal and spatial distributions of the maximum horizontal velocities, were successfully obtained. Pressure measurements were also obtained along four different vertical positions at three different locations on the horizontal plane. Based on the mean velocity distributions on the deck surface, the most significant spatial variability of the propagating green water flow is the protruding wave front near the center of the deck during the early stages of the wave overtopping. The maximum front speed of 1.5C was first observed near the midpoint of the deck along the deck centerline with C being the wave phase speed. The flow velocities decreased to below 1C once the wave front passed the rear edge of the deck. Most of the measured pressures showed impulsive impacts characterized by a sudden rise of the pressure peak. The highest pressure was observed as 1.65ρC2 at a midpoint and a rear edge of the deck with ρ being the water density. Correlations between wave kinetic energy and dynamic pressure were examined to determine the impact coefficients. The phase speed based impact coefficient was found to vary within a narrow range between 0.29 and 1.69 and a practical value of 1.5 may be used in applications. It appears that the impact pressures on the structures were strongly affected by the changing front shape of the broken wave and the impulsiveness of the impinging wave that contains a considerable amount of air entrainment.

Experimental study on plunging breaking waves in deep water

AbstractThis study presents a unique data set that combines measurements of velocities and void fraction under an unsteady deep water plunging breaker in a laboratory. Flow properties in the aerated crest region of the breaking wave were measured using modified particle image velocimetry (PIV) and bubble image velocimetry (BIV). Results show that the maximum velocity in the plunging breaker reached 1.68C at the first impingement of the overturning water jet with C being the phase speed of the primary breaking wave, while the maximum velocity reached 2.14C at the beginning of the first splash‐up. A similarity profile of void fraction was found in the successive impinging and splash‐up rollers. In the highly foamy splashing roller, the increase of turbulent level and vorticity level were strongly correlated with the increase of void fraction when the range of void fraction was between 0 and 0.4 (from the trough level to approximately the center of the roller). The levels became constant when void fraction was greater than 0.5. The mass flux, momentum flux, kinetic energy, potential energy, and total energy were computed and compared with and without the void fraction being accounted for. The results show that all the mean and turbulence properties related to the air‐water mixture are considerably overestimated unless void fraction is considered. When including the density variation due to the air bubbles, the wave energy dissipated exponentially a short distance after breaking; about 54% and 85% of the total energy dissipated within one and two wavelengths beyond the breaking wave impingement point, respectively.

Measuring void fraction and velocity fields of a stepped spillway for skimming flow using non-intrusive methods

Stepped spillways have higher energy dissipation than smoother hydraulic structures used to divert flood discharges. The inception point related to air entrainment is, however, located further upstream causing an undesired bulking of the flow depth. For large discharge rates and for straight stepped spillways, the skimming flow regime may be assumed two dimensional; this is an attractive feature for the application of non-intrusive flow visualization techniques because these methods measure the flow characteristics in the vicinity of the sidewalls which are likely to correlate with the flow at the centre of the flume. This paper tests the hypothesis that such techniques can be used to measure the flow inside the flume. The hypothesis is tested against measurements taken with an intrusive probe. Void fraction contour lines and velocity fields are obtained in 12 different stepped spillway configurations using the image processing procedure and the bubble image velocimetry, respectively. The void fraction and velocity results are overall consistent with the probe measurements. The velocity fields show a persistent underestimation of the probe measurements which can at least be partially explained by sidewall effects and possible probe’s overestimation.

Experimental study on flow kinematics and impact pressure in liquid sloshing

This paper experimentally studied flow kinematics and impact pressure of a partially filled liquid sloshing flow produced by the periodic motion of a rectangular tank. The study focused on quantifying the flow velocities and impact pressures induced by the flow. Filled with water at a 30 % filling ratio, the tank oscillated at a resonant frequency and generated the violent sloshing flow. The flow propagated like breaking waves that plunged on both side walls and formed up-rushing jets that impacted on the top wall. Velocities of the multiphase flow were measured using the bubble image velocimetry technique. A total of 15 pressure sensors were mounted on the top wall and a side wall to measure the impact pressures. The local kinetic energy obtained by the measured local velocities was used to correlate with the corresponding pressures and determine the impact coefficient. In the sloshing flow, the flow direction was dominantly horizontal in the same direction of the tank motion before the wave crest broke and impinged on a side wall. At this stage, the maximum flow velocities reached 1.6C with C being the wave phase speed. After the wave impingement, the uprising jet moved in the vertical direction with a maximum velocity reached 3.6C before it impacted on the top wall. It was observed that the impact coefficients differed by almost one order of magnitude between the side wall impact and the top wall impact, mainly due to the large difference between the local velocities. A nearly constant impact coefficient was found for both side wall and top wall impacts if the impact pressures were directly correlated with the flow kinetic energy calculated using C instead of the local velocities.

LABORATORY MEASUREMENTS OF A STEADY BREAKER USING PIV AND BIV

Image-based particle image velocimetry (PIV) and bubble image velocimetry (BIV) techniques were employed to measure the flow field in both the non-aerated and aerated regions in a steady hydraulic jump. The jump has a Froude number of 4.62 with a large amount of air entrainment. The mean velocities and turbulence properties were obtained by ensemble averaging the repeated measurements. The spatial variations of mean velocity profiles, turbulent intensity, and Reynolds stress were discussed in details. The results show that the ratio between the maximum bubble velocity and the maximum water velocity is almost constant with a value about 0.6. The turbulence intensity of bubbles is very high in the aerated region and reaches about 0.4, while the turbulence velocity of water below the roller is significantly lower. The Reynolds stress is mostly negative and increases as elevation increases. The maximum Reynolds stress occurs at the free surface.