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Numerical study of the indentation formation of a compound droplet in a constriction
Hoe D. Nguyen,Truong V. Vu,Phan H. Nguyen,Binh D. Pham,Nang X. Ho,Cuong T. Nguyen,Vinh T. Nguyen 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.4
A compound droplet deforming in a constricted tube widely appears in drug delivery and microfluidic devices. In such a constriction, an indentation can present at the trailing surface of the droplet. However, this aspect has not been fully investigated and understood so far. This study focuses on the effects of some dimensionless parameters on the negative curvature, i.e., indentation, at the trailing surface of a compound droplet moving through a constricted tube. The presence of the constriction at the middle of the tube length enhances the droplet indentation. Numerical results were obtained for the capillary number Ca (varied in range of 0.1 - 1.0), the inner-to-outer droplet radius ratio R 21 (varied in range of 0.2 - 0.9), the droplet-to-tube radius ratio R 10 (varied in range of 0.2 - 0.9), the inner-to-outer interfacial tension coefficient ratio σ 21 (varied in range of 0.1 - 6.4), and the normalized depth of the constriction d/R (varied in range of 0.0 - 0.8). The results reveal that the most influencing factor is Ca, increasing its value leads to the increment of the maximum indentation at the trailing surface of the inner and outer droplets. The indentation is also increased with increasing the value of R 10and d/R. In contrast, increasing R 21 results in a decrease in the indentation at the trailing surface of the outer droplet. When increasing σ 21 , the indentation at the trailing surface of the inner one is quickly suppressed, while the outer droplet is minorly affected. We also point out the patterns of the trailing surface of the inner and outer droplets and their transitions from one to the other patterns in the diagrams based on these parameters.
Numerical study of collision modes of multi-core compound droplets in simple shear flow
Binh D. Pham,Truong V. Vu,Cuong T. Nguyen,Hoe D. Nguyen,Vinh T. Nguyen 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.5
Collisions of multi-core compound droplets have generated substantial interest in recent years because of their applications in the industry and academia. This study uses the front-tracking method to simulate the transition between the two collision modes of multi-core compound droplets in a simple shear flow. Compound droplets initially assumed identical have two sub-droplets of equal size. Given the shear flow, the droplets collide with one another and behave in two main modes, namely, passing-over and reversing. In the passing-over mode, the droplets pass over one another after coming into contact. The reversing mode appears with two compound droplets returning to their initial sides after the collision. During collision, the subdroplets circulate approximately at the center of their enclosing outer droplets. Some parameters, including capillary number Ca, viscosity ratios μ io and μ mo , radius ratio R io of the subdroplets to the outer droplets, and sub-droplet angle a 0 , are investigated to determine their impact on these modes of collisions. We find that the transition from a reversing to passingover mode occurs when we increase the value of Ca from 0.01 to 0.63, R io from 0.20 to 0.475, and μ io and μ mo in the range of 0.16-6.3. However, an increase in the value of a 0 between −75 oand 90 o leads to a change from a passing-over to reversing mode. Diagrams of the colliding modes are also presented in this research.
Deformation of a compound droplet in a wavy constricted channel
Hung V. Vu,Truong V. Vu,Binh D. Pham,Hoe D. Nguyen,Vinh T. Nguyen,Hoa T. Phan,Cuong T. Nguyen 대한기계학회 2023 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.37 No.1
Controlling and adjusting the size and shape of compound droplets is of increasing interest in manufacturing applications using microfluidic channels of complicated geometry. Using numerical simulation in the evolution of computer science with the ability to expand the scope of research and optimize costs is a current research trend. We here provide a numerical simulation analysis of the dynamics of a compound droplet travelling in a circular and sinusoidal-wave tube. The simulations were performed with variations of the Reynolds number, capillary number, droplet size, and channel geometry. It follows that the capillary number strongly impacts the dynamics of the droplet, and the alternation of breakup and finite deformation modes. The deformation increases and the droplet is stretched along the centerline of the channel as the Reynolds number increases. Increasing the length of the wavy region makes the droplet more deformed and enhance its breakup. Regime diagrams based on some of these parameters are also plotted.
Optimal Pricing for Duopoly in Cognitive Radio Networks: Cooperate or not Cooperate?
Do, Cuong T.,Tran, Nguyen H.,Zhu Han,Long Bao Le,Sungwon Lee,Choong Seon Hong IEEE 2014 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.13 No.5
<P>Pricing is an effective approach for spectrum access control in cognitive radio (CR) networks. In this paper, we study the pricing effect on the equilibrium behaviors of selfish secondary users' (SUs') data packets which are served by a CR base station (BS). From the SUs' point of view, a spectrum access decision on whether to join the queue of the BS or not is characterized through an individual optimal strategy that is joining the queue with a joining probability. This strategy also requires each SU to know the average queueing delay, which is a non-trivial problem. Toward this end, we provide queueing delay analysis by using the M/G/1 queue with breakdown. From the BS's point of view, we consider a duopoly market based on the two paradigms: the opportunistic dynamic spectrum access (O-DSA) and the mixed O-DSA & dedicated dynamic spectrum access (D-DSA). In the first paradigm, two co-located opportunistic-spectrum BSs utilize freely spectrum-holes to serve SUs. Then, we show the advantages of the cooperative scenario due to the unique solution that can be obtained in a distributed manner by using the dual decomposition algorithms. For the second paradigm, there are one opportunistic-spectrum BS and one dedicated-spectrum BS. We study a price competition between two BSs as a Stackelberg game. The cooperative behavior between two BSs is modeled as a bargaining game. In both paradigms, bargain revenues of the cooperation are always higher than those due to competition in both cases. Extensive numerical analysis is used to validate our derivation.</P>
Incentive Mechanisms for Economic and Emergency Demand Responses of Colocation Datacenters
Tran, Nguyen H.,Do, Cuong T.,Shaolei Ren,Zhu Han,Choong Seon Hong IEEE 2015 IEEE journal on selected areas in communications Vol.33 No.12
<P>Demand response programs have been considered critical for power grid reliability and efficiency. Especially, the demand response of datacenters has recently received encouraging efforts due to huge demands and flexible power control knobs of datacenters. However, most current efforts focus on owner-operated datacenters, omitting another critical segment of datacenter business: multitenant colocation. In colocation datacenters, while there exist multiple tenants who manage their own servers, the colocation operator only provides facilities such as cooling, reliable power, and network connectivity. Therefore, colocation has a unique feature that challenges any attempts to design a demand response program: uncoordinated power management among tenants. To tackle this challenge, two incentive mechanisms are proposed to coordinate tenant power consumption for demand response under two different scenarios. First, in the case of economic demand response where the operator can adjust an elastic energy reduction target, we show that there is an interaction between the operator and tenant strategies, where each side maximizes its own benefit. Hence, we apply a two-stage Stackelberg game to analyze this scenario and derive this game's equilibria. However, computing these equilibria can be intractable with exhaustive search; therefore, we propose an algorithm to find the Stackelberg equilibria with linear complexity. Second, in the case of emergency demand response where a fixed energy reduction target must be fulfilled, we devise two incentive schemes with the distributed algorithms that can achieve the same optimal social cost. While the first algorithm is based on the dual-decomposition method that is suitable for nonstrategic tenants, the second one is designed for strategic tenants to achieve a unique Nash equilibrium of a bidding game. Finally, trace-based simulations are also provided to illustrate the efficacy of our proposed incentive schemes.</P>
Spin Hall Conductivities of W-N Alloys
Quynh Anh T. Nguyen,D. D. Cuong,S. C. Hong,Sonny H. Rhim 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Motivated by recent reports on high spin Hall angle in W alloys [1-3], spin Hall conductivities (SHC) of W-N alloys are investigated theoretically using ab initio density functional calculations. Without N, SHC of α-W and β-W are -744 and 818 ℏ/S/cm, respectively. Among various combinations, we focus on W<sub>2</sub>N and WN. In particular, SHC of W<sub>2</sub>N is enhanced 18.7 % over β-W. High SHC of W<sub>2</sub>N is elucidated by large Berry curvature from 2/3rx. On the other hand, in WN case, three structures, NaCl-type, hexagonal and NbO-type, are considered. SHC of NaCl- and hexagonal types are -619 and –696 ℏ/S/cm, respectively, while NbO-type exhibits relatively low SHC of –194 ℏ/S/cm. However, considering energetics, W<sub>2</sub>N is more promising than WN, whose thermodynamic average is around –194 ℏ/S/cm.