This article presents a composite guidance law comprising a three-stage proportional navigation (PN) structure for the simultaneous arrival of multiple missiles. Motivated by the fact that the ...selection of the navigation constant considerably influences the trajectory shape, the proposed composite law is designed to have different navigation constants at each stage to achieve the desired impact time. The three stages of the proposed law are configured to sequentially increase, maintain, and decrease the magnitude of the look angle by switching only the navigation constant, through which the seeker's field-of-view constraint, which is mainly determined by the look angle, can be satisfied without additional considerations. Furthermore, the simple PN-based structure of the proposed law makes it possible to obtain an exact closed-loop solution, providing various useful information for practical implementation, such as the expected trajectory, required guidance command, and range of achievable impact time. In addition, the relevant investigations based on the theoretical analysis as well as the demonstration results obtained through the numerical simulation are presented herein.
This paper proposes a guidance law that achieves the desired terminal impact time without violating a seeker's field-of-view (FOV) limits. In order to derive the guidance law, kinematic conditions ...for impact time control are defined, and the backstepping control technique is applied for the satisfaction of the conditions. As a virtual control input for the backstepping structure, the missile lead angle, which represents the angle between the line-of-sight (LOS) and missile velocity vector, is used and its magnitude is limited by a prescribed limit. Then, the seeker's look angle can also be confined within a specific range because the seeker look angle is mainly determined by the difference between the LOS and the velocity vector. This capability to confine the seeker's look angle with achieving the desired impact time is the main contribution of the paper. To evaluate the performance of the proposed law, numerical simulation is conducted. The result demonstrates that the proposed guidance law enables the missile to simultaneously achieve the interception and desired impact time without violating the prespecified FOV limits.
•Two-phase flow and pressure drop characteristics associated with slug flow are examined.•Flow visualization images and temporal records of pressure drop oscillation are presented.•Relevant slug flow ...parameters are measured and compared with previous correlations.•A new correlation for elongated bubble length in air-oil slug flow is developed.•A theoretical slug flow model for predicting total pressure gradient of slug unit is presented.
This study examines two-phase flow and pressure drop characteristics associated with slug flow in a tube with an inner diameter of 4 cm. Experiments are conducted using air and low viscosity mineral oil as working fluids at atmospheric pressure and two different oil temperatures, 25 and 45°C. The superficial gas and liquid velocities are in the range of 0.33–2.43 m/s and 0.20–1.20 m/s, respectively. Flow visualization images and temporal records of pressure drop oscillation are presented for different gas and liquid superficial velocities. The relevant flow parameters associated with slug flow, such as the elongated bubble velocity, elongated bubble length, liquid slug length, and liquid film thickness, are measured, and the relationship between the slug flow parameters and the superficial gas and liquid velocities is investigated. The measured pressure gradient data are also compared to predictions of previous homogeneous equilibrium models and semi-empirical correlations for two-phase frictional pressure gradient. A theoretical slug flow model for predicting the total pressure gradient of slug unit is presented using empirical correlations for the slug flow parameters. The new model accurately predicts the experimental pressure gradient data, evidenced by an overall mean absolute error of 19.3%.
•A comprehensive experimental flow boiling study is conducted on the detailed analysis of flow instability.•Dominant flow patterns showing multi-channel flow instability are classified based on the ...visualization results.•Effects of heat flux, mass velocity, pressure, and inlet subcooling on flow instability are investigated.•A new flow stability map is developed utilizing a mini-channel heat sink.
High heat flux and good axial temperature uniformity of channel flow boiling demonstrate that it is ideally suited for next-generation cooling systems. Such systems, however, are hampered by undesirable flow fluctuations, making channel flow boiling ineffective due to the rapid bubble growth within the channels. Furthermore, especially in the mini-channel heat sink, the interactions between multiple parallel channels and inlet/outlet plenum induce complex hydraulic and thermal oscillations. Therefore, lots of consistent and detailed experimental data are still needed to design a reliable flow boiling system. However, only limited studies have examined the effects of channel interaction on flow boiling instability. Thus, to provide a better understanding of the flow boiling instabilities, a mini-channel heat sink was tested with near-saturated inlet conditions using FC-72 as the working fluid. The mini-channel heat sink consisted of a 150 mm long and 70.4 mm wide base area, having 22 identical rectangular channels measuring 1.6 × 4.8 mm2. The flow instabilities were analyzed by observing the vapor backflow and flow fluctuation in mini-channels with variations in the mass velocity, pressure, and heat fluxes. The analysis is based on the detailed flow visualization in the parallel channels and inlet plenum. It was found that the rapid bubble generation in individual channels promotes vapor backflow and hydraulic oscillations. Further observation of the high-speed camera images shows that the vapor crossing over between parallel channels through the inlet plenum led to the synchronization of vapor oscillations, which is responsible for the parallel channel flow instability. This visualization study revealed six dominant flow patterns showing multi-channel flow instability. Based on the classification of dominant flow patterns, a better understanding of the operating conditions for stable flow boiling was obtained.
This paper presents an impact time control guidance law that does not perform explicit time-to-go estimation. In order to satisfy the interception and the desired impact time constraint ...simultaneously, a sliding surface variable formulated as a sum of the relative range and the desired time-to-go is defined, weighted by two nonzero weighting functions. Then, the achievement of the sliding mode satisfies the following equivalence: The relative range is zero if and only if the elapsed time equals to the desired impact time. It means that both the interception and impact time control can be satisfied at the same time in the sliding mode. The impact time control guidance law is derived to enforce the defined surface variable to the sliding mode. Because the law is designed based on the capture condition without separate time-to-go estimation process, the achievement of the sliding mode always guarantees the interception of the target at the desired impact time. In addition, the proposed law can be applied to an engagement considering nonstationary targets in a straightforward manner because the corresponding time-to-go estimation is not needed. Simulation results demonstrate that the proposed guidance law has better performance in comparison with the existing guidance law, in respect of satisfying the desired impact time constraint against a nonstationary target.
•Two-phase slug flow experiments in horizontal pipes are performed.•A consolidated database covering a wide range of operating conditions and fluid properties is constructed.•The parametric trends of ...the crucial flow parameters associated with slug flow are investigated.•Three empirical correlations are developed to predict slug length, bubble length, and bubble velocity.
A large number of studies have been conducted to understand the slug flow characteristics of air-liquid two-phase flow in horizontal pipes and to capture parametric trends of critical flow parameters associated with slug flow, such as the liquid slug length, elongated bubble length, and elongated bubble velocity. Based on these findings, many attempts have been made to develop empirical correlations to predict the slug flow parameters. However, the validity of these correlations is limited to a specific working fluid and narrow ranges of geometrical or operating conditions, thereby reducing their effectiveness as useful general predictive tools. This limitation has created a need to develop empirical correlations that offer high accuracy over a broad application range, i.e., a wide range of liquid viscosities, superficial liquid and gas velocities, and pipe diameters. In this study, two-phase slug flow experiments were performed to examine the hydrodynamic features of slug flow under two different pipe diameters (2 and 4 cm) and two different liquid temperatures (25 and 45°C). In addition, a new consolidated database consisting of 1563 slug flow data points was amassed from 16 sources, including the present experimental data, covering a wide range of operating conditions and fluid properties. In particular, this study includes various liquids with a wide range of viscosities, such as heavy oil, light oil, and water. The parametric study of air-liquid slug flow at various viscosity levels revealed that as the mixture velocity increases, the liquid slug length tends to decrease when the liquid viscosity is high and increase when the liquid viscosity is low. On the other hand, the elongated bubble length and elongated bubble velocity increase with increasing superficial gas velocity, regardless of the liquid viscosity. Based on these findings, three empirical correlations were developed to predict the liquid slug length, elongated bubble length, and elongated bubble velocity. These correlations provide excellent predictive capability against the consolidated database, with overall MAE values of 18.1%, 20.6%, and 9%, respectively.
Most existing impact angle control guidance (IACG) laws that consider the reduced seeker field-of-view (FOV) focus only on a stationary or a nonmaneuvering moving target. In actual warfare, however, ...a number of targets are capable of maneuvering with lateral acceleration, which prohibits the existing FOV-constrained IACG laws from guaranteeing the reliable performance. In this article, we develop a guidance law that ensures the accurate IACG with obeying the FOV constraint against a maneuvering target. The proposed guidance law is based on the engagement kinematics against a maneuvering target and structured as a sliding mode controller to cope with unknown disturbance such as the normal acceleration of the target. The introduced sliding surface structure includes a sigmoid function of which output is limited in magnitude in order to restrict the missile look angle within the preset limits. In consequence, the proposed law has the capability to fulfill IACG without violating the FOV constraint against a maneuvering moving target. Numerical simulations that support the effectiveness of the proposed guidance law are also included in this article.
•A novel multistage porous structure for higher heat flux electronic application is proposed.•Multistage stage channels and micropores enhance surface wettability.•The novel design induces a ...breathing phenomenon, resulting in a significant improvement in boiling heat transfer performance.•A 6.5-fold enhancement in critical heat flux and 300–700% improvement in heat transfer coefficient were obtained.
An experimental pool boiling study on multistage cross-flow porous structure was conducted under atmospheric pressure using saturated FC-72 as the working fluid. The novel design comprises two stages: each stage contains cross-flow rectangular channels of 1.5 mm in height and 1 mm wide connected by thorough circular pores 0.325 mm in diameter. The dimensions of the pores and channels in both stages were kept identical. Tests were conducted in three positions, enhanced surface horizontal position (EH), enhanced surface vertical position (EV-a), where upper stage channels were parallel to the buoyancy force, and enhanced surface vertical position (EV-b), where lower stage channels were parallel to the buoyancy force. The critical heat flux was significantly enhanced with an improved boiling heat transfer rate in all three positions using the multistage cross-flow porous structure. The highest heat flux of 107 W/cm2 was recorded in EV-b, followed by EV-a and EH with a wall superheat of 42 °C. The heat transfer coefficient was improved by 300% to 700%, and the critical heat flux was enhanced by 6.5 times compared to the plain surface. Improved performance is attributed to the cross-flow of multistage porous structure and orientation of the test section, which induced a breathing phenomenon and prevented the drying out of the test section by resisting the merging of large vapor bubbles for a longer period and decreasing its lingering time over the surface. Bubble behaviors were investigated utilizing a high-speed camera. Two separate modes of the boiling mechanism were observed. At low heat fluxes below 50 W/cm2, bubble discharge and liquid intake co-occurred at random pores and channels, whereas at high heat fluxes, volatile bubbles exhibited a breathing phenomenon.
This paper presents an impact angle control guidance law that confines the missile look angle during homing in order to not exceed a seeker's field-of-view limit. A sliding surface variable whose ...regulation guarantees the interception of a stationary target at the desired impact angle is designed, and the guidance law is derived to make the surface variable go to the sliding mode. Using a magnitude-limited sigmoid function in the surface variable, the proposed law prohibits the look angle from exceeding the specified limit during the entire homing. This capability to confine the missile look angle is valuable when a seeker's field-of-view is restricted, since imposing the terminal impact angle constraint demands the missile to fly a curved trajectory. Furthermore, the proposed law can be implemented under bearings-only measurements because the command does not involve any information of the relative range and line-of-sight rate. Numerical simulations are conducted to demonstrate the validity of the proposed law. The result shows that the proposed guidance law accomplishes the impact angle constraint without violating the prescribed look angle limit although it only uses the information of bearing angles.
Alzheimer's disease (AD) is characterized by the accumulation of amyloid plaques and neurofibrillary tangles accompanied by cognitive dysfunction. The aim of the present study was to elucidate ...preventive and therapeutic potential of stem cells for AD. Among stem cells, autologous human adipose-derived stem cells (hASCs) elicit no immune rejection responses, tumorigenesis, or ethical problems. We found that intravenously transplanted hASCs passed through the BBB and migrated into the brain. The learning, memory and pathology in an AD mouse model (Tg2576) mice greatly improved for at least 4 months after intravenous injection of hASC. The number of amyloid plaques and Aβ levels decreased significantly in the brains of hASC-injected Tg mice compared to those of Tg-sham mice. Here, we first report that intravenously or intracerebrally transplanted hASCs significantly rescues memory deficit and neuropathology, in the brains of Tg mice by up-regulating IL-10 and VEGF and be a possible use for the prevention and treatment of AD.
PDF