Drop Height Research Articles

A comparative field study of free-fall cone and sphere penetrometers in soft sediment

This paper describes the field performance of two instrumented free-fall penetrometers (FFPs), which offer the possibility of low cost rapid strength profiling without requiring the equipment associated with conventional CPTs or drilling and sampling. The devices considered are a free-fall cone penetrometer (FFcP) and an instrumented free-fall sphere (IFFS). The performance of the FFPs in assessing soil undrained shear strength was investigated and compared through field trials in the soft sediments of the Swan River, Perth, Western Australia. The penetrometers achieved impact velocities of 1.4–5.8 m/s from drop heights of 0.7–3.0 m above the riverbed, and embedded to depths of up to 1.6 times the FFP length (1.45 m) and 4.5 times the IFFS diameter (0.89 m). The FFP data provided the shear strength indirectly using the acceleration measured from an onboard inertial measurement unit. The accuracy depends on the assumptions made regarding the magnitude of dynamic bearing and shaft frictional resistance. Uncertainty associated with dynamic bearing resistance causes a ±10% and ±13% variation in the calculated strength for the IFFS and the FFcP, respectively. However, the equivalent uncertainty in dynamic shaft resistance can cause a variation in the calculated strength of up to ±76% for the FFcP. The FFcP has higher embedment potential than the IFFS, but the IFFS has the advantage of a simpler interpretation without needing to account for dynamic shaft resistance.

Low-Velocity Impact Resistance of Glass Laminate Aluminium Reinforced Epoxy (GLARE) Composite

This study intends to determine the behavior of glass laminate aluminum-reinforced epoxy (GLARE) and glass fiber-reinforced polymer (GFRP) composites under a low-velocity impact test. Experimental tests and numerical simulations are considered for this investigation. All samples are made by the hand lay-up method. Moreover, specimens are produced with a 7075-T6 aluminium sheet with a 0.5 mm thickness, resin 3001, and E-glass fiber. The drop weight test performs the low-velocity impact at 6.7 J and 10 J impact energy levels and the heights of 1.0 m and 1.5 m. Numerical simulation is also conducted by ANSYS software to compare the results obtained by the experimental tests. Generally, results show that maximum deflections of the GLARE samples are significantly lower as compared to GFRP ones by 87% and 83.5% for 1.0 m and 1.5 m drop heights, respectively. Experimental results demonstrate that although aluminum sheets prevent damage to the fibers in GFRP, delamination and fractures between layers are observed in GFRP samples. An appropriate agreement is also obtained between the FE results and experimental data.

Investigation of Bruise Damage and Storage on Cucumber Quality

Bruise damage is one of the mechanical injuries that fresh produce can sustain during the postharvest supply chain. The study investigated the effect of drop impact levels, storage temperatures, and the storage period on the quality changes of cucumbers. A known mass ball was used to damage cucumbers once from three different drop heights (30, 60, and 90 cm) before they were stored for 24 days at 5 °C, 10 °C, and 22 °C. The data showed that the bruise area (BA), bruise susceptibility (BS), yellowness, and chroma* increased with the increase in the drop height and storage temperature. The study found that the bruise area (BA) and bruise susceptibility (BS) of the damaged cucumbers increased substantially (p < 0.05) with the increase in storage temperature and drop height. Due to the increment in drop height, storage temperature, and the storage period, the weight loss (Wl)% significantly increased after 24 days of storage. The storage period affects the firmness of damaged cucumbers stored in all storage conditions. The highest value of lightness (L*) was observed for the cucumbers bruised from the 60 cm drop height and stored at 22 °C with a value of 43.08 on day 24 of storage. Hue*, redness (a*), and total soluble solids (TSS) were all unaffected by the drop height. This study can serve as a resource for horticultural researchers and experts involved in the fresh fruit and vegetable supply chain. The study pays attention to the importance of postharvest supply chain activities, such as handling and storage to maintain the quality and prolong the shelf life of perishable produce, such as cucumbers.

Fluidization of Swelling Particles at Elevated Temperatures

Semicrystalline polymer (e.g., polyethylene and polypropylene) particles may swell in solvent at elevated temperatures, inducing flow instability in operation, reactor blockage, and undesired product. Understanding the fluidization behavior of swelling particles at elevated temperatures is therefore of great significance. This work conducts an experimental study on the fluidization of polyethylene particles in an elevated-temperature liquid–solid fluidized bed. With the increase in temperature, particle swelling develops over two phases, i.e., non-cohesive swelling and cohesive swelling, which is different from the fluidization of cohesive particles in gas–solid systems. Four regimes can be identified through static bed height and pressure drop measurements, i.e., stable fluidization, noncohesive-swelling fluidization, cohesive-swelling fluidization, and full defluidization. A phase diagram was then obtained to demarcate these regimes. Furthermore, an energy balance model was proposed to predict the initial agglomerating temperature Tag of swelling particles for early warning of abnormal fluidization in practical operation.

Approximation of aeration efficiency at sharp-crested weirs using metaheuristic regression approaches

Abstract This paper explores the ability of multivariate adaptive regression splines, decision trees, Gaussian processes, and multiple non-linear regression equation approaches to predict the aeration efficiency at various weirs and discusses their results. In total, 126 experimental observations were collected in the laboratory, of which 88 were arbitrarily selected for model training, and the rest were used for model validation. Various graphical presentations and goodness-of-fit parameters were used to assess the performance of the models. Performance evaluation results, Whisker plot, and Taylor's diagram indicated that the GP_rbf-based model was superior to other implemented models in predicting the aeration efficiency of weirs with CC (0.9961 and 0.9973), MAE (0.0079 and 0.0195), RMSE (0.0122 and 0.0251), scattering index (0.0594 and 0.1238), and Nash Sutcliffe model efficiency (0.9923 and 0.9564) values in the training and validating stages, respectively. The predicted values by GP_rbf lie within the ±30% error line in the training and validating stages, with most of it lying at/close to the line of agreement. The random forest model had better predictability than other decision tree models implied. The sensitivity analysis of parameters suggests shape factor and drop height as major influencing factors in predicting the aeration efficiency.

Experimental Analysis and Verification of the Influence on the Elastic Recovery Coefficient of Wheat

To establish a collision model of wheat grains impacting a force plate with a piezoelectric sensor, and to investigate the influence of the elastic recovery coefficient on the sensor’s detection accuracy during the collision process, this study employed object kinematic principles to construct a wheat elastic recovery coefficient measurement device. This device ascertains the elastic properties of wheat during collisions and determines the elastic recovery coefficient of the wheat collision model. The wheat variety Jinan No. 17 was selected for testing, and the effects of the contact material, grain drop height, material thickness, and grain moisture content on the collision recovery coefficient during the collision process were analyzed through single-factor and multi-factor experiments. The experimental results demonstrate that the collision recovery coefficient of wheat grains increases with the stiffness of the collision materials for different materials. The grain recovery coefficient of wheat exhibits a downward trend with increasing falling height and moisture content, while it tends to rise as the material thickness increases. Data analysis and comparison reveal that, given the determination of the collision material, the moisture content of wheat exerts the most significant effect on the elastic recovery coefficient, followed by material thickness, while the influence of falling height is less pronounced. The findings of this study can provide data support for simulation testing and product design of wheat combine harvester cleaning screen body mechanisms and wheat seeders.

Hydraulic investigations of standard and high performance trays in a DN1200 column

In the course of a more fluctuating power supply due to renewable energy sources, the need for tray columns with flexible operability increases. Accordingly, further research is required to better predict the behavior at load limits – especially for fixed and float valve trays. Within this work, the hydraulic behavior of numerous valve tray configurations was investigated employing a 1.2 m diameter gas/liquid (air/water) pilot test column. Fixed and float valve tray performance has been validated using sieve trays as a reference, which have already been extensively studied in the literature. In order to obtain significant data, weeping and entrainment limits in particular, as well as froth height and pressure drop, were measured quantitatively in the experiments. By varying the geometry parameters, numerous different cases were investigated. Based on all the data obtained, reliable statements can be made about the validity and applicability of calculation models known from the literature.

Cushioning properties and application of expanded polystyrene for a dynamic nonlinear system

AbstractTo explore the cushioning properties of expanded polystyrene (EPS), a novel two‐degree‐of‐freedom mathematical model is proposed with a dynamic nonlinear system and successfully implemented it. The maximum response acceleration of the product was first determined in the novel dynamic mathematical model and used as a parameter for evaluating product damage. Furthermore, a drop test was carried out to verify the theoretical model and showed good consistency. Based on the correct theoretical model, a number of simulations with different drop heights and thicknesses of the cushion were done to investigate cushioning properties of EPS. The results show that EPS can reduce the maximum response acceleration of the product by more than 20% of the excitation acceleration. Thus, EPS has good cushioning ability and can be used as packaging material to protect products. Finally, the theoretical model was applied to an example application. A new method for constructing the cushioning curve is recommended, which could replace the traditional ASTM D 1596 drop test method.

Methodological considerations for determining the volume and intensity of drop jump training. A systematic, critical and prepositive review.

passive or active control group during a plyometric training program. information on improvement with Drop Jump or Depth Jump, with other jumps, acceleration, sprint, strength, and power output. randomized controlled trials. We searched articles published in PubMed, SPORTDiscus, Web of Science, and Scopus. The search was conducted until 10 September 2022 for English-language articles only. The risk of bias was assessed using Grading of Recommendations, Assessment, Development and Evaluation (GRADE) for randomized controlled studies. We identified 31495 studies, of which only 22 were included. We found that six groups presented results with women, 15 presented results with men, and the remaining four presented mixed studies. Of the 686 people recruited, 329 participants aged 25.79 ± 4.76years were involved in training. Methodological problems in training intensity, volume distribution, and individualization were noted, but methodological recommendations for their solution are also provided. It is concluded that the drop height should not be understood as the intensity determinant of plyometric training. Intensity is determined by ground reaction forces, power output, and jump height, among other factors. Furthermore, the athletes' experience level selection should be based on the formulas recommended within this research. These results could be helpful for those who intend to conduct new plyometric training programs and research.

Vis-NIR and SWIR hyperspectral imaging method to detect bruises in pomegranate fruit.

Fresh pomegranate fruit is susceptible to bruising, a common type of mechanical damage during harvest and at all stages of postharvest handling. Accurate and early detection of such damages in pomegranate fruit plays an important role in fruit grading. This study investigated the detection of bruises in fresh pomegranate fruit using hyperspectral imaging technique. A total of 90 sample of pomegranate fruit were divided into three groups of 30 samples, each representing purposefully induced pre-scanning bruise by dropping samples from 100 cm and 60 cm height on a metal surface. The control has no pre-scanning bruise (no drop). Two hyperspectral imaging setups were examined: visible and near infrared (400 to 1000 nm) and short wavelength infrared (1000 to 2500 nm). Region of interest (ROI) averaged reflectance spectra was implemented to reduce the image data. For all hypercubes a principal components analysis (PCA) based background removal were done prior to segmenting the region of interest (ROI) using the Evince® multi-variate analysis software 2.4.0. Then the average spectrum of the ROI of each sample was computed and transferred to the MATLAB 2022a (The MathWorks, Inc., Mass., USA) for classification. A two-layer feed-forward artificial neural network (ANN) is used for classification. The accuracy of bruise severity classification ranged from 80 to 96.7%. When samples from both bruise severity (Bruise damage induced from a 100cm and 60 cm drop heights respectively) cases were merged, class recognition accuracy were 88.9% and 74.4% for the SWIR and Vis-NIR, respectively. This study implemented the method of selecting out informative bands and disregarding the redundant ones to decreases the data size and dimension. The study developed a more compact classification model by the data dimensionality reduction method. This study demonstrated the potential of using hyperspectral imaging technology in sensing and classification of bruise severity in pomegranate fruit. This work provides the foundation to build a compact and fast multispectral imaging-based device for practical farm and packhouse applications.

Estimation and comparison of gabion weir oxygen mass transfer by ensemble learnings of bagging, boosting, and stacking algorithms

ABSTRACT Gabion weir comprises porous materials packed with distinct shapes and sizes of gravel. The gabion weir is eco-friendlier than an impervious weir, as its opening enables water life and sediment materials to pass through it. Aeration is how natural processes or physical structures enhance the contact area and time between water and estranged air. This process improves the dissolved oxygen (D.O.) of water. The D.O. is one of the best determinants used for water quality measurement. This paper investigates the prediction of mass oxygen transfer over the gabion weir by ensemble models. The outputs of gabion weir oxygen mass transfer were estimated using bagging, boosting, and stacking by taking input parameters such as average size, porosity, the gabion weir height, discharge per unit width, and drop height. The dataset was taken by conducting experiments. By comparing these modeling ensembles, it was found that random forest-based bagging outperformed all proposed models. Nevertheless, all applied ensemble models were performing well, but published traditional equations were performing incredibly poorly. As sensitivity analysis suggests, the discharge per unit width was the most sensitive input. An uncertainty study was also carried out.

Neuromuscular Changes in Drop Jumps on Different Common Material Surfaces with Incremental Drop Heights

The purpose of this study was to compare changes in muscle pre-activation and short-latency responses in the lower limbs during drop jumps performed on different common soft and hard surfaces and at various platform heights. The study aimed to collect electromyography data from the rectus femoris, biceps femoris, tibialis anterior, gastrocnemius, and soleus of the dominant leg during drop jumps on sand, turf, polyurethane, and wood surfaces from platform heights of 30, 40, 50, and 60 cm. Muscle pre-activation refers to muscle activity 100 ms before ground contact during a drop jump, while short-latency responses refer to muscle activation occurring 30–60 ms after ground contact. These definitions were used to measure and analyze neuromuscular responses in the lower limb muscles during drop jumps using various surfaces and platform heights. Sand as a ground material and platform heights of 50 and 60 cm significantly enhanced pre-activation and activation in short-latency responses of the lower limb muscles (all p < 0.01). The difficulty of the drop jump task can enhance pre-activation and activation in the latency responses of lower limb muscles. It is recommended that athletes perform drop jumps on sandy surfaces or from platforms higher than 50 cm to induce muscle pre-activation of the lower limbs and to improve muscle activation levels in the latency responses after landing.

Determination of the critical drop height and critical flow velocity of aluminum alloy (AL-91% Mg-8% Fe-0.4% Zn-0.2%) in gravity sand casting

Casting is a manufacturing process in which molten metal is poured through the gating system to fill the mould cavity where it solidifies. Variations in casting parameters by different researchers have led to significant variations in casting guidelines, which have forced Foundry Engineers to carry out a number of trial-and-error runs to create guidelines based on their own experience. These variations in guidelines have led to defects occurring in casting during the mould filling process. This work aimed at determining the critical drop height and critical flow velocity of a certain molten aluminum alloy as it flow down the mould sprue in gravity sand casting. The continuity equation was used to describe the velocity distribution of the aluminum alloy as it flows down the sprue. The mathematical tool used in this research is the finite element method. It involves the discretization of the domain of interest into smaller finite elements. The weak form of the governing equation was obtained and integrated over the domain of interest. The results obtained, established the critical flow velocity of aluminum alloy, down the sprue, as 2.565 × 103 mm/s and the critical drop height as 377mm. Results obtained were compared with literature and were also used to produce various casts, it was observed that casts produced, using sprue height below the critical drop height obtained prevented casting defects, while at sprue height above the critical drop height, the danger of casting defects could not be avoided.

Experimental and numerical investigation of transient dynamic response on reinforced concrete tunnels against repeated impact loads

An attempt has been made to study the transient dynamic response of small scale reinforced concrete tunnels against repeated impact loads. A total of four semi-cylindrical tunnels of M25 grade concrete having length, base width and thickness as 1.2, 0.5 and 0.05 m were prepared and tested against the mass of the impactor of 104 kg. The reinforcement mesh of ∅6 mm with 60 mm c/c spacing was used. The specimens were kept at the earth's surface which defines the boundary conditions proximity to the actual tunnels. The yield stress of the steel bars was 415 MPa and the resulting percentage reinforcement was 0.4%. The RCC tunnel rests on a soil bed that has a specific weight of 1850 kg/m3. The drop height against samples 1 and 2 was 1.5 and 2.3 m, respectively, whereas samples 3 and 4 were studied against a 3 m drop height under repeated loading. The experimental results were recorded in terms of impact force, residual displacement and damage pattern. It was observed that sample 1 offered resistance up to five hits whereas samples 2 and 3 were found to offer resistance up to four and two hits, respectively. The numerical investigations were performed using the ABAQUS/EXPLICIT software and the concrete, reinforcement steel and the soil element were modelled by using the Concrete Damaged Plasticity model, Johnson-cook model, and Drucker-Prager model, respectively. The results obtained from the simulations were found in good agreement with the experimental results of samples 2 and 3. However, the numerical results of sample 1 were able to complete up to the fourth hit and simulation on the fifth hit was not performed due to the complete loss of concrete material stiffness at the fourth hit. Further, the simulations were conducted considering parameters such as varying length and base width of the tunnel in order to predict the damage intensity of the tunnel. The Mises stresses were found reduced with an increase of the tunnel length up to 3 m, thereafter the same was found to increase with the increase of tunnel length, however, the deformation in the tunnel was found to decrease with an increase in the length of the tunnel. It was also concluded that the tunnel base width is an important parameter that affects the Mises stress resulting in global stability to the structure and further restricting the stress concentration to a localized failure.

Experimental and analytical study of the high-strain-rate compressive behavior of SFRC

The compressive behavior of steel-fiber-reinforced-concrete (SFRC) is dependent on the loading rate. This research investigates, experimentally and analytically, the effect of loading rate on the compressive behavior of SFRC designed to be used in prefabricated urban protective furniture. For this purpose, cylinder SFRC specimens were subjected to modified instrumented-drop-weight-impact tests at four dropping heights and quasi-static tests with four different strain rates. The inertia force was analytically obtained and also experimentally measured. The results demonstrate that by increasing the strain rate, elastic modulus, compressive strength, and energy dissipation capacity have increased. Three different models were proposed for predicting each mechanical characteristic, one in the range of quasi-static and the others in the range of impact corresponding to the split Hopkinson pressure bar and drop-weight-impact tests. The experimental dynamic to static ratios obtained for SFRC properties were discussed and compared with those proposed by present study and from other researchers. Three proposed models significantly improve the prediction the dynamic increase factor values in terms of compressive strength, modulus of elasticity and toughness.

Experimental studies on rock failure mechanisms under impact load by single polycrystalline diamond compact cutter

Percussive drilling shows excellent potential for promoting the rate of penetration (ROP) in drilling hard formations. Polycrystalline diamond compact (PDC) bits account for most of the footage drilled in the oil and gas fields. To reveal the rock failure mechanisms under the impact load by PDC bits, a series of drop tests with a single PDC cutter were conducted to four kinds of rocks at different back rake angles, drop heights, drop mass, and drop times. Then the morphology characteristics of the craters were obtained and quantified by using a three-dimensional profilometer. The fracture micrographs can be observed by using scanning electron microscope (SEM). The distribution and propagation process of subsurface cracks were captured in rock-like silica glass by a high-speed photography system. The results can indicate that percussive drilling has a higher efficiency and ROP when the rock fractures in brittle mode. The failure mode of rock is related with the type of rock, the impact speed, and the back rake angle of the cutter. Both the penetration depth and fragmentation volume get the maximum values at a back rake angle of about 45°. Increasing the weight and speed of falling hammer is beneficial to improving the rock breaking effects and efficiency. The subsurface cracks under the impact load by a single PDC cutter is shaped like a clamshell, and its size is much larger than the crater volume. These findings can help to shed light on the rock failure mechanisms under the impact of load by a single PDC cutter and provide a theoretical explanation for better field application of percussive drilling.

An experimental study of the saturated impulse for metal plates under slamming

Engineering structures can suffer severe local structural damage when subjected to extreme dynamic pulse loadings such as slamming impact. However, there is paucity of information on how the loading pulses affect the large plastic deformations. This paper presents information on an experimental investigation into the dynamic pressure pulse generated by slamming and the plastic response of metal plate. In our experiments, plates made of aluminum and steel alloy, were dropped from various heights ranging from 0.1 m to 1.3 m, onto the calm water in a tank. Based on the experimental results, the effects of structural material and water impact velocity on the slamming pressures pulse are investigated in detail. The results indicate that not only the peak pressure but also the pressure pulse duration play important roles in governing the dynamic response of the impacted structures. It is shown that the maximum strain and deflection at the plate center increase almost linearly with the drop height of the drop model. In view of the lack of previous experimental evidence for the saturation phenomenon of structures under slamming, we compared the dynamic response at the center of the plate with the experimentally measured slamming pressure under various drop heights, so that we have evidently verified the saturation phenomenon of flat plates for the first time under slamming in drop tests. It is also revealed that the dynamic response of the flat plates under slamming becomes more prone to the saturation phenomenon with the increase of drop height.

Dynamic analysis of small droplets at constant pressure and analytical model for drip systems and very low pressure drop-on-demand applications

We study the evolution of the growing volume of a droplet formed out of a capillary tube driven at a constant applied pressure by optical volume detection. A particular application aims at modeling the dynamic of the drop in the case of the injection of drugs in the middle-ear cavity using a small target drug delivery system. Various dispense tips such as a steel needle or silicone catheters and different t-BuOH aqueous solutions were used. A physical model based on the simplified Laplace–Young equation for a spherical droplet was developed to interpret the experimental results and model the observed deviation of the mean flow rate from the Poiseuille law when the drop radius is smaller than the capillary length. We emphasize the large influence of the surface tension which introduces a highly nonlinear effect at low applied pressure just above the capillary threshold pressure. The model fits well the evolution of the growing volume at various applied pressure up to a maximum diameter corresponding to the drop size close to the capillary length of the fluid, lc=γ/ρg, based on the liquid density ρ, gravity g and surface tension γ. We demonstrate that the model can also be applied to non spherical pendant droplets providing the apex elongation over the drop height of a spherical cap of the same volume is kept in a 30% range.

Analysis of damage caused by drop impacts for mulberry fruits: An dropping experiment and numerical simulation

AbstractIn the process of mechanized production of mulberry, falling collision damage is one of the main damage forms, which causes serious damage and accelerates fruit decay. Through the drop collision test and simulation analysis of mulberry fruit, the relationship between drop conditions and fruit damage degree was established, so as to provide reference for reducing fruit falling damage. The drop test is carried out. The relationship between factors and indicators is established. The drop height, fruit plate angle, fruit plate material, and drop direction were taken as factors, and then the energy loss was selected as indicator. The drop process of mulberry fruit is analyzed by numerical simulation, and the maximum stress and deformation of mulberry fruit are analyzed. With the increase of the drop height, the kinetic energy of the fruit increases. When the fruit plate material is pearl cotton, the drop height changes from 200 to 1000 mm, the energy loss of fruit falling increases from 4.42 × 10−0.3 J to 2.14 × 10−0.2 J. When the drop height of fruit is 200 mm, the angle of fruit plate is 30°, and the material of fruit plate is pearl cotton, the minimum energy loss of fruit is 3.39 × 10−0.3 J, the fruit damage was the least. This study can provide a reference to reduce the damage of mulberry fruit in the process of mechanical harvest and transportation.Practical ApplicationsThe existing mulberry harvesting methods are manual harvesting, while the vibration harvesting methods have high harvesting efficiency and serious damage. When the harvesting parts hit the tree trunk, the mulberry fruit will leave the tree and falls onto the fruit‐receiving device in the process of mechanical harvesting. There will be a collision and falling of the mulberry, which may cause serious damage. Therefore, in the process of mulberry fruit harvest, transportation, the falling, and collision damage is the main damage form.At present, there is few research on the drop impact damage test of mulberry fruit. In this study, the effects of mulberry falling direction, fruit plate material and drop height on mulberry damage were discussed by drop experiment. Then, the optimization results were carried out. The change and distribution of stress and strain in the process of collision were analyzed by numerical simulation, and the relationship between falling conditions and damage level was established. It is important to analyze the dynamic characteristics of mulberry in the process of drop. This study can be applied as a reference to reduce mulberry fruit impact damage, and provide a theoretical reference for the design of mulberry vibration harvesting machinery.

Study of Dam-Break Flow Over a Vegetated Channel With and Without a Drop

The effects of a drop in a vegetated channel on the flow characteristics during a dam failure were investigated using a numerical approach. A CFD package was used for all dam failure simulations. After performing validation tests, the model was used to predict the flow characteristics during a dam failure over a vegetated channel with positive, negative, or no drop. A positive drop resulted in a backflow into the reservoir with a negative discharge of Qbackflow = − 0.01 m3/s. Even in the channel with no drop, backflow into the reservoir occurred in the last stage due to the drag effect of the vegetation. The maximum discharge occurred at the peak time tpeak = 2.5 s above the drop for all models. The peak discharge decreased from Qpeak = 0.037 m3/s to Qpeak = 0.032 m3/s with increasing positive drop height. However, a negative drop had no significant effect on peak discharge values. In most cases, the maximum flow velocity was around um = 2 m/s. The maximum eddy values decreased by Δωmax = 500 1/s for the positive drops and increased by Δωmax = 700 1/s for negative drops at the middle stage.