In this study, the influence of the discharge orifice shape of swirl liquid jets injected in transverse gas flow was investigated by high-speed photography and shadowgraphy techniques. Three shapes of discharge orifice, including circular, square, and elliptical were tested at different flow conditions. Due to the cross-sectional asymmetry in the ellipse, it is important to place it horizontally or vertically in airflow, and these two states are considered in this study. The main characteristics of liquid jets in transverse air, including trajectory, breakup point, jet width, breakup length, drop size distribution, and Sauter mean diameter, were obtained by image processing. Visualization of flow development revealed that the asymmetry of the swirl hollow cone in the transverse air causes an expanded new structure that cannot be seen in the circular cross section. This structure was called the conical bag and the inflated sheet in elliptical and square cross sections, respectively. The results indicate that the variations of momentum ratio are more effective in the path of non-circular swirl jets. The ellipse in the horizontal direction and square cross section have the breakup point's lowest transverse and longitudinal coordinates, respectively. Theoretical models for predicting swirl liquid jet trajectory and its breakup point were developed and presented. Gamma probability distribution function was fitted on the drop size according to the discrete distribution of the drop size. The results indicated that the distribution of elliptical shapes in the horizontal direction produces the smallest droplets among them while square cross section has a wider distribution.

A theoretical and experimental study was conducted to investigate the effect of injection angle on surface waves. Linear stability theory was utilized to obtain the analytical relation. In the experimental study, high-speed photography and shadowgraph techniques were used. Image processing codes were developed to extract information from photos. The results obtained from the theoretical relation were validated with the experimental results at different injection angles. In addition, at the injection angle of 90 ^∘ , the theoretical results were evaluated with the experimental results of other researchers. This evaluation showed that the theory results were in good agreement with the experimental data. The proper orthogonal decomposition (POD) and the power spectra density (PSD) analysis were also used to investigate the effect of the injection angle on the flow structures. The results obtained from the linear stability were used to determine the maximum waves’ growth rate, and a relation was presented for the breakup length of the liquid jet at different injection angles. The breakup length results were compared with theory and published experimental data. The presented relation is more consistent with experimental data than other theories due to considering the nature of waves. The results showed that the instability of the liquid jet is influenced by three forces: inertial, surface tension, and aerodynamic. Therefore, Rayleigh–Taylor, Kelvin–Helmholtz, Rayleigh–Plateau, and azimuthal instabilities occur in the process. Decreasing the injection angle changes the nature of waves and shifts from Rayleigh–Taylor to Kelvin–Helmholtz. That reduces the wavelength and increases the growth rate of the waves. Axial waves have a significant impact on the physics of the waves and influence parameters. If axial waves are not formed, the growth rate of the waves is independent of the injection angle. An increase in the gas Weber number causes a change in the type of dominant waves and a greater instability of the liquid jet. In contrast, an increase in the liquid Weber number causes an enhancement in the resistance of the liquid jet against the transverse flow without changing the type of the dominant waves. Decreasing the density ratio reduces the effect of Rayleigh–Taylor waves and strengthens the Kelvin–Helmholtz waves. It causes two trends to be observed for the growth rate of waves at low spray angles, while one trend occurs at high spray angles.

As far as we are aware, all the previous studies have focused on circular injectors. Here, the effect of non-circular orifice shape on pressure swirl injector was investigated experimentally. For this purpose, three injectors; circular, square and rectangular were manufactured. The nozzles had the same cross-sectional area. Water was used as the working fluid and tests were conducted at room temperature and pressure. The Weber number varied from 484 to 1936. Flow parameters of the swirl jets were obtained by an in-house image processing code. The measured parameters were: discharge coefficient, spray angle, breakup length, lateral spread, SMD of drops and wavelength of surface waves formed on the cone. The influence of nozzle shape on all these parameters are discussed.A new breakup regime morphology for non-circular pressure swirl nozzles was defined. At lower Weber numbers, the air core was trapped inside the liquid sheet and relatively large ligaments were formed and separated from the edges (fiber breakup regime). As the Weber number was increased Rayleigh-Taylor instability produced cell-like structures near the nozzle exit. These cell-like structures eventually broke up at the thickened edges of the cells and formed Christmas tree like structures (Christmas tree regime). We found that all the measured flow parameters can best be correlated as a function of Reynolds number and Weber number based on liquid sheet thickness at nozzle exit. All these correlations were reported. Breakup length and lateral spread were heavily influenced by the breakup regime. In fiber breakup regime the breakup length and lateral spread increased with increasing Weber number. However, in Christmas tree (cellular) breakup regime, breakup length and lateral spread decreased with increasing Weber number.

In this study, the effect of aspect ratio of elliptical cross section for a pressure swirl nozzle injecting into stagnant air was investigated experimentally. Three elliptical nozzles with aspect ratios of 1, 1.5, and 2 were manufactured. Shadowgraphy technique was utilized to visualize flow characteristics of the pressure swirl injectors. An in-house image processing code was used to obtain cone angle, breakup length, lateral spread, drop size distribution, and Sauter mean diameter (SMD). It was found that discharge coefficient increased by increasing the elliptical cross section's aspect ratio. By increasing the aspect ratio from 1, the issuing jet's cone angle became different from different viewing angles and was out of symmetry. The cone angle from front view was always larger than from side view. The asymmetry on the elliptical surface caused the thickness of the liquid film at the edges to be higher than in the middle. This asymmetry was amplified as the aspect ratio was increased resulting in the edges having a longer breakup length than in the middle. It was found that the lateral spread decreased and the interaction of the air core with the liquid was reduced with increasing aspect ratio, resulting in an increase in breakup length and drop sizes. Benjamin's model for discharge coefficient was modified, and the influence of aspect ratio was added to the model. Models were developed based on Rizk and Lefebvre and Sivakumar's relations for spray cone angle and breakup length, respectively. Also, a correlation based on Wang and Lefebvre's model was presented for SMD. These developed models were compared with our measurements and other published data with good agreements.

In this study, the interaction of vortices generated from an oscillating airfoil with a hindfoil placed downstream of the oscillating forefoil at low-Reynolds-number flow was investigated numerically. The forefoil entered a deep dynamic stall induced by large-amplitude pitching oscillation. The dynamic stall process is characterized by unsteady separation and the formation of a strong clockwise vortex. A wall-resolved large-eddy simulation approach was applied to compute the flowfield. The numerical measurements were performed for an incompressible flow at a Reynolds number of Re = 30 000 based on chord length with a pitching reduced frequency of K= 0.5, and amplitude of A = 14.1° over Selig–Donovan 7003 airfoils. A single-airfoil case was validated against numerical and experimental measurements. In the present study, we investigated the flowfield and aerodynamic coefficients resulting from the deep dynamic stall of the pitching forefoil and the vortex interaction in tandem-airfoil configuration related to micro-air vehicle applications by employing large-eddy simulation approach. Large-eddy simulation was also compared to two-dimensional unsteady Reynolds-averaged Navier–Stokes simulation to determine the accuracy and validity of the low-fidelity approach in prediction of deep dynamic stall and vortex interaction at low-Reynolds-number flow.

An experimental study to investigate the flow of liquid jet issued from a high aspect ratio nozzle slit into an incoming airflow by changing the orientation angle from the incoming free-stream was performed. A two-dimensional liquid sheet emerged from the narrow slit into the subsonic air crossflow. Different orientation angles between 0 and 90 degrees were studied. High-speed photography and shadowgraphy techniques were utilized to visualize the flow physics. The influence of the slit orientation angle on the flow morphology and the flow regimes of liquid sheets was investigated. Some fluid flow parameters were obtained by analyzing the images. The changes in breakup height of different orientations were measured. A model was offered for the breakup height of the liquid sheet based on the liquid-to-gas momentum ratio, gas Weber number, and a new non-dimensional parameter as a representation of the angle of slit orientation. Also, the defined sheet trajectory for each orientation angle was obtained, and the variations were examined. Empirical correlations for the defined trajectory of the sheet in terms of liquid to gas momentum ratio and gas Weber number for each orientation angle were proposed.

Machine learning was used to optimize the geometric arrangement of a pair of unsteady actuators on flow separation over an efficient low Reynolds number airfoil in post-tall conditions. Large eddy simulation was used to validate the results. Two actuators: one with blowing and the other with suction openings were installed on the top surface of an airfoil at low Reynolds number of 60,000. An SD7003 airfoil at a post stall angle of attack of 13 degrees was utilized. The boundary layer flow of the top surface was manipulated by the actuators to control flow separation. The influence of several actuator parameters: frequency, energy input, opening area, location and orientation angle were considered in an optimization of the dual actuator configuration. A genetic algorithmbased optimization was implemented to find the most effective configuration of this coupling. Since the optimization process is time-consuming, machine learning was used to train artificial neural networks to be coupled with genetic algorithm to reduce the computational cost. The artificial neural networks and their training was constantly upgraded during the optimization cycle. Results for the optimal case indicated an increase in lift coefficient and the objective function in comparison to uncontrolled case by factors of 1.88 and 3.33 respectively. We also found a reduction in drag coefficient. It was also found that using a pair of actuators was more efficient than using a single actuator.

Flow separation control on oscillating airfoils is crucial for enhancing the efficiency of turbine blades. In this study, a genetic algorithm was employed to optimize the configuration of a pure suction jet actuator on an oscillating airfoil at a Reynolds number of 1.35×105. Neural networks based on multilayer perceptrons were used to train the aerodynamic coefficients as functions of the control parameters and reduce the number of simulations. The objective function was the mean performance coefficient, defined as the ratio of the average lift to the average drag during an oscillation period. The control parameters were location, velocity, opening length, and suction jet angle relative to the airfoil surface. The optimal jet had the maximum velocity and opening length and was normal to the airfoil surface. The optimal jet location was near the leading edge vortex (LEV) (between 3% and 6% of the chord). The optimum jet can increase the average performance coefficient (average ratio of lift to drag during a period) by about 24 times. The major part of this improvement is related to reducing drag force. The average lift coefficient increases from about 0.58 to about 0.92 using this jet, while the average drag coefficient decreases from about 0.23 to about 0.02. The optimal jet suppressed the dynamic stall vortex, which resulted from the combination of two clockwise vortices: LEV and turbulent separation vortex. Suppressing this vortex prevented the counterclockwise trailing edge vortex from growing at the end of the airfoil.

An experimental study was performed to investigate the influence of injection angle on further details of breakup of a circular liquid jet injected into an airstream. Wavelength and frequency of axial oscillations caused by column waves, drops size distribution, orientation and aspect ratio of these drops were measured and reported. Injection angles between 30 and 90 degrees were considered. High-speed photography and shadowgraphy techniques were utilized to visualize and record the flow. Experimental data were obtained by processing images with the help of an in-house developed code. We found that gas Weber number and the injection angle are influential on wavelength and frequency of the column waves. The wavelength decreased and the frequency increased with increasing gas Weber number or decreasing the injection angle. Also, momentum ratio, gas Weber number and injection angle were affecting drop's average diameter. By reducing the injection angle, the atomization process was prolonged and the produced drops were larger. Although the injection angle affected the average drop size and number, as long as the regime was not changed, the probability distribution of drop size was independent of the injection angle. We extended theory of Ng et al. [1] to estimate wavelength, frequency of column waves formed on the liquid jet that was injected into an airstream to account for injection angle. A relative gas Weber number was defined by using the relative velocity of the gas to that of the liquid along the flow direction. The introduced relative gas Weber number was found useful in classifying the breakup regimes of the flow. A relation was proposed to estimate these wavelengths with good agreement with our data at different injection angles and other previously published data at normal injection of liquid jets. Two separate correlations for SMD estimation were proposed for before and after bag formation in the flow.

The dynamic stall is a common phenomenon in horizontal and vertical axis wind, reducing system efficiency. In order to enhance the aerodynamic performance (L/D) of a NACA0012 airfoil under the deep dynamic stall at Reynolds number of $$1.35 \times 10^{5}$$ , computational intelligence algorithms were utilized to find the best operational parameters of a continuous blowing jet. The airfoil undergoes a sinusoidal motion between − 5 and 25, and the rotation center is around a quarter of its chord. Unsteady Navier–Stokes equation (URANS) was used with $$k - \omega$$ SST turbulence model. Two types of computational intelligence algorithms, including neural networks and genetic algorithms, were coupled for this optimization. The average lift to drag ratio (L/D) in an oscillation period was considered as the objective function. The blowing jet parameters, which included location, opening length, velocity magnitude and angle of jet, were selected as design variables. Two neural networks have been utilized to find a relation between design variables and the mean lift and drag coefficients over a period to reduce the computational cost of the optimization. The optimization algorithm converged after almost 115 simulations. The ANNs in the last simulation were able to predict the input data with 92% and 93% regression coefficients for average values of drag and lift coefficient in terms of the operational parameters of the jet, respectively. The optimized jet enhanced the mean aerodynamic performance by reducing the drag coefficient and increasing the lift coefficient during a period of oscillation. For the optimal case, this parameter reached the value of 11.727 or 4.717 times the uncontrolled case. The most impact of the jet is in the downward movement. Significant improvement in aerodynamic performance was observed for the optimal blowing jet, which is due to the lack of formation leading edge vortex (LEV), dynamic stall vortex (DSV) and trailing edge vortex (TEV). The results indicated that about 2–5% of the chord is the best location for jet. This location is near the place where the leading edge vortex is formed. Aerodynamic performance improved better when the jet angle was in the range of 55°–70°. Although the jet momentum coefficient was not maximized, jet-opening length and blowing velocity magnitude converged to their maximum values quickly.

Active flow control was applied to a tandem configuration of two SD7003 airfoils. The tandem configuration consisted of an upstream airfoil (forefoil) with a pitching motion at a fixed frequency and a downstream airfoil (hindfoil) that was not moving. Synthetic jet actuators (SJAs) were applied on both airfoils to control the flow fields at the low Reynolds number of 30 000. The flow physics inherently involved three different frequencies: frequency of the pitching forefoil and two actuation frequencies of the two of SJAs. In this study, we kept all three frequencies fixed at 5 Hz. However, we allowed for phase differences between them. An optimization study was conducted in order to improve total aerodynamic performance defined as the combined total time-averaged value of lift-to-drag ratio of both airfoils (L/D)tot. Injection angle of the two SJAs, phase differences between each SJA frequency, and frequency of the pitching motion in addition to vertical spacing between the airfoils were considered as design variables of the optimization study. Optimization algorithm was coupled with a machine learning method to reduce computational cost. We found that lift coefficients were enhanced, and drag coefficients were reduced for the optimum controlled case in comparison with the uncontrolled case, which led to an aerodynamic performance improvement of 304%. However, drag force was the dominant parameter in determining final performance value. For all design variables, drag force determined the final optimum values.

In the current study, the role of phase-difference between signals of two adjacent synthetic jet actuators (SJAs) in active control of flow over a rounded ramp geometry has been investigated. In order to accurately predict the separation and reattachment locations, wall-resolved large eddy simulation has been utilized to capture the locations of separation and reattachment. The two adjacent SJAs were placed upstream of the separation point. Six phase-differences between the two SJAs were considered, and two momentum coefficients were applied. First, the role of phase-difference in active flow control of a separation bubble behind a ramp-down region was investigated. Furthermore, the impact of an increased momentum ratio on the size and length of the separation zone was investigated to assess the effectiveness of phase-difference with respect to a higher velocity ratio. The effect of increased momentum ratio on the wall pressure fluctuations was also explored. As the second objective of this study, the flow and turbulent features were discussed to unveil the SJA actuation impact on the downstream flow. The time-averaged velocity and turbulent kinetic energy profiles and the turbulent production were examined and compared to the uncontrolled baseline case. It was found that a higher velocity ratio tremendously increased the turbulent energy before the separation point, while further downstream, the level of turbulent energy was uncoupled from the SJA momentum coefficient. Our study showed that by increasing the momentum ratio, the role of phase-difference in reducing the separation thickness lessened. Nevertheless, applying either a positive or a negative phase-difference of pi/2 still postponed the separation point.

The influence of changing the injection angle of a liquid jet injected into airstream in crossflow was investigated. The injection angles from 90 to 30 degrees were considered. Based on liquid velocity and air speed, liquid and gas Weber numbers from 8 to 600 and 0.2 to 27 were considered, respectively. Many flow parameters such as: Rayleigh-Taylor wavelengths, liquid jet trajectories, breakup lengths and penetration lengths by means of high-speed photography and shadowgraph technique were measured and reported. Based on the relative importance of the aerodynamic forces of the crossflow to the liquid momentum and surface tension forces different breakup regimes occurred. Using gas Weber number, the following liquid jet breakup regimes: column breakup, arc breakup, bag breakup and multimode breakup were observed at various injection angles. It was found that at lower injection angles the required gas Weber number to transition between regimes was slightly increased. At lower gas Weber number flows, increasing the injection angle caused the liquid jet to penetrate farther into the airstream and this process was gradual. However, at higher gas Weber number flows, there was a bifurcation in behavior: At the lower injection angles, the behavior was the same, but at the higher injection angles, the penetration behavior reversed and the penetration of liquid jet into airstream decreased as the injection angle was increased. We believe this phenomenon was due to switching of the breakup mechanism from Rayleigh-Taylor instability to shear breakup mechanisms at higher injection angles. Theoretical models for prediction of liquid jet trajectory and its breakup point taking the effect of injection angle into account were developed and presented. The models’ predictions were compared with our measurements and other published data resulting in good agreements.

Optimization was used to find the best configuration of two airfoils in tandem placed into an incoming flow. Upstream airfoil (forefoil) was pitching at a fixed frequency, while the downstream airfoil (hindfoil) was kept at a fixed angle of attack. Study was performed at a low Reynolds number of 30,0 0 0 based on chord length. Selig-Donovan 7003 (SD7003) was used for both airfoils, which is a high-performance airfoil specially designed for low Reynolds number flows. The optimization studies were conducted using a genetic algorithm (GA) to maximize aerodynamic performance. The design variables in this study were: horizontal and vertical spacing between the airfoils and hindfoil's angle of attack. Since the optimization process is time-consuming, machine learning was used to train four artificial neural networks (ANNs) to be coupled with genetic algorithm to reduce the computational cost. Two separate optimization cases were considered at two different orders of magnitude in pitching amplitudes of the forefoil, while the pitching frequency was kept at constant value. We found that in both cases, optimum tandem configurations had a smaller combined drag coefficient in comparison with the addition of two separate airfoils. The case with high pitching amplitude produced higher magnitude of lift, while the low amplitude case resulted in a significant improvement in aerodynamic performance. (c) 2021 Elsevier Inc. All rights reserved.

Photo Gallery Experimental Visualization of Liquid-Gas Interactions M. Tadjfar, M. Tadjfar Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology 1Corresponding author. Email: mtadjfar@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar A. Jaberi, A. Jaberi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: aminjaberi3000@gmail.com Search for other works by this author on: This Site PubMed Google Scholar S. Najafi, S. Najafi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: arsamnajafi@gmail.com Search for other works by this author on: This Site PubMed Google Scholar A. Hatami, A. Hatami Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: afsnhtm@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar M.H. Aliyoldashi, M.H. Aliyoldashi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: mohammadhosein.aliyoldashi@mail.polimi.it Search for other works by this author on: This Site PubMed Google Scholar Y. Rezaei, Y. Rezaei Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: yrezaei@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar M. Mokhtari, M. Mokhtari Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: mehran.mokhtari@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar K. Asadollahbeiki, K. Asadollahbeiki Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: ksrbk23@gmail.com Search for other works by this author on: This Site PubMed Google Scholar M. Ebrahimi, M. Ebrahimi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: m.ebrahimi888@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar B. Khazaei B. Khazaei Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Email: bahmankhazaei@aut.ac.ir Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information M. Tadjfar Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology A. Jaberi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology S. Najafi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology A. Hatami Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology M.H. Aliyoldashi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology Y. Rezaei Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology M. Mokhtari Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology K. Asadollahbeiki Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology M. Ebrahimi Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology B. Khazaei Turbulence and Multiphase Flow Laboratory, Amirkabir University of Technology 1Corresponding author. Email: mtadjfar@aut.ac.ir Email: aminjaberi3000@gmail.com Email: arsamnajafi@gmail.com Email: afsnhtm@aut.ac.ir Email: mohammadhosein.aliyoldashi@mail.polimi.it Email: yrezaei@aut.ac.ir Email: mehran.mokhtari@aut.ac.ir Email: ksrbk23@gmail.com Email: m.ebrahimi888@aut.ac.ir Email: bahmankhazaei@aut.ac.ir J. Fluids Eng. Apr 2022, 144(4): 040908 (1 pages) Paper No: FE-22-1014 https://doi.org/10.1115/1.4053695 Published Online: January 29, 2022 Article history Received: January 9, 2022 Revised: January 25, 2022 Accepted: January 25, 2022

For a tandem airfoil configuration, an airfoil is placed in the wake of an upstream airfoil. This interaction affects the aerodynamic forces of the airfoils, especially the downstream one. In the present study a tandem configuration consists of an upstream pitching airfoil and a downstream stationary airfoil is investigated. This study aims to investigate the role of reduced frequency and pitch amplitude of the upstream airfoil’s motion on lift and drag coefficients of two airfoils. These two parameters play an important role in the formation of vortices. The investigation is done for Selig-Donovan 7003 (SD7003) airfoils at low Reynolds number of 30,000 using a computational fluid dynamics. Incompressible URANS equations were employed for solving the flow field. It was found that for a fixed reduced frequency of 0.5 thrust is produced on the hindfoil for a part of cycle for different pitch amplitudes from light to deep stall while for a fixed pitch amplitude at different reduced frequencies high level of thrust or drag can be produced. The reason is related to the type and intensity of vortex-blade interaction.

Continuous blowing and synthetic jet actuators were implemented to investigate their effects on a fully stalled airfoil. An opening tangential to the boundary layer configuration was installed over the suction surface of the Selig-Donovan airfoil at the angle of attack of 16° and Reynolds number of 60,000. An optimization analysis was carried out to look for the optimum operational design point. Genetic algorithm, artificial neural network, and computational fluid dynamic simulations were combined to perform the optimization. Inserting location, opening diameter, velocity amplitude, and synthetic jet frequency were considered as design variables. Results indicated a significant improvement in aerodynamic characteristics, performance, and lift and drag coefficients. Using unsteady actuation caused a better improvement in aerodynamic characteristics compared to the steady case and also led to a remarkable reduction in the applied momentum coefficient. Contours of different flow field parameters were depicted for both cases and their similarities and dissimilarities were identified. Moreover, the synthetic jet actuator displayed a lower increase in the friction coefficient than the continuous blowing actuator. Therefore, it showed a higher performance improvement in comparison with the continuous blowing jet.

One of the most common ways to obtain mixing between liquid and air, is by injecting the liquid jet into an incoming gaseous crossflow. The physics of this mixing flow is very complicated due to the presence of many flow interfacial instabilities. Usually, a perpendicular liquid jet into the cross flow airstream is used as the standard method of mixing. In the present work, the effect of the injection angle of the liquid flow emanated from a circular nozzle into airstream was experimentally investigated. The flow characteristics of the liquid jet were visualized by diffused backlight shadowgraph technique and high-speed photography. Water was used as the working liquid and tests were conducted into an incoming airstream at room temperature and pressure. A circular nozzle with 1.5 mm in diameter was used. The injection angles of the 30, 45, 60 and 90 degrees of the liquid jet into the airstream were considered. Different parameters of liquid jet flow such as breakup length and trajectory were measured. It was found that at low angles the path was independent from the momentum ratio.

An experimental study was carried out to investigate the effects of entrance length on the main characteristics of rectangular liquid jets discharged into the stagnant atmosphere. Six rectangular nozzles, all with the same aspect ratio of 3 but with different entrance length ratios ranging from 3.3 to 60 were constructed. The physics of the fluid flows was visualized by the aid of backlight shadowgraph technique and high speed photography. Flow visualizations revealed that in the mid-range of Weber numbers, the perturbations induced over the liquid surface remarkably depended on the entrance length ratio. Moreover, the characteristics of the axis-switching instability of rectangular liquid jets were measured. It was found that axis-switching wavelength was independent of the entrance length, while the amplitude of axis-switching was directly influenced. For entrance length ratios smaller than 10, the amplitude was increased with increase of entrance length, whereas for entrance length ratios higher than 10, this trend was reversed. Measurements of breakup length also showed that the transition of flow regimes was not perceptibly affected by the entrance length.

Flow dynamics of a two-dimensional liquid sheet injected transversely into a subsonic airstream was comprehensively investigated. An injector with an aspect ratio of 90 and a thickness of 0.35 mm was used to produce the two-dimensional liquid flow. Experimental visualizations were used to identify the main features of the liquid sheet in crossflow. Tests were conducted for a wide range of flow conditions to achieve a complete understanding of the flow physics. Experiments revealed that the sheet flow developed an expanded shape that was absent in conventional liquid jets in crossflow. This newly found structure was named as inflated sheet. Regimes of two-dimensional liquid sheet flows were categorized into: biconvex, enclosed inflated sheet, open inflated sheet, bag breakup/sheet rupture, and multimode breakup. A mapping of the flow was suggested to identify the transition from one regime into another. Furthermore, the main parameters of the liquid sheets including height, trajectory, and breakup point were measured and their dependency on the momentum ratio and Weber number was investigated. Results showed that different stages of inflated sheet development directly influenced the trajectory and height of the liquid sheet. A power-law empirical correlation was suggested to predict the trajectory of the liquid sheet. Also, a finite difference scheme was used to calculate the growth rate of the liquid sheet height. It was demonstrated that the sheet height increased more rapidly with the decrease of momentum ratio. Proper orthogonal decomposition method was applied to the flow visualizations and dominant frequencies of bags were identified and compared at different flow conditions.