Sehoon Kim

Modeling and Performance Analysis of Address Allocation Schemes for Mobile Ad Hoc Networks

Address allocation is an essential part in maintaining a mobile ad hoc network (MANET) effectively, and several address allocation schemes have been proposed. In this paper, we present a set of analytical models to evaluate the efficiency of address allocation schemes. The derived models quantitatively characterize the efficiency of four popular address allocation schemes in terms of latency and communication overhead. Through the analysis, we achieve numerical results that show the impact of network parameters on the efficiency of these schemes. We also conduct simulations and compare with analytical results to validate our models. The analytical model developed in this paper is able to more accurately predict the performance of address allocation schemes over a various range of loss rates and would be useful in providing more insights for the study of an efficient address allocation scheme in MANETs. To our understanding, this is the first attempt in mathematically investigating the performance of addressing schemes in ad hoc networks.

Experimental Determination of Geometric Parameters for an Annular Injection Type Supersonic Ejector

The effects of four geometric parameters of an annular injection supersonic ejector, namely, the primary nozzle exit-to-throat area ratio, the contraction angle of the mixing chamber, the cross-sectional area and L/D ratio of the second-throat on the performance parameters including the secondary flow pressure, the starting pressure and unstarting pressure were investigated experimentally. The starting pressure exhibits linearly proportional dependence on the throat area ratio when the mixing chamber length is less than a certain critical value. For a longer mixing chamber, the starting pressure is proportional to the mixing chamber length while the unstarting pressure depends on the throat area ratio only. The geometric parameters of the second-throat do not affect the static pressure of the secondary flow. This implies that the secondary flow is aerodynamically choked in the mixing chamber and the static pressure of the secondary flow is determined by the choking condition since the mixing chamber of the annular injection ejector is relatively long. Based on the findings by the experiment, a simplified analytical model was proposed to predict the secondary flow pressure. The predicted secondary flow pressure agrees reasonably well with the measurement for a small contraction angle of the mixing chamber.

Starting Pressure and Hysteresis Behavior of an Annular Injection Supersonic Ejector

An analytical model to predict the primary stagnation pressure that starts an annular injection supersonic ejector is presented. If the length of the mixing chamber is longer than the critical length, the starting pressure increases proportionally to the mixing chamber length, which differs from conventional ejectors using central injection. To describe the dependency of the starting pressure on the mixing chamber length, we assume that the ejector starts when the exiting supersonic primary flow reaches the second throat. In the present model, we use a subsonic mixing model to calculate the secondary flow pressure in the mixing chamber. Applying the obtained pressure as a back pressure condition, the distance that the supersonic primary flow develops is calculated. Comparing the distance to the mixing chamber length, we derive a minimum pressure requirement diagram that accurately predicts the starting pressure and hysteresis for a given geometry.

Experimental Investigation of an Annular Injection Supersonic Ejector

E JECTORS have a wide variety of applications: thrust augmentation of jet engines, vacuuming systems, and thermocompressor of the desalination plant, to name a few [1–7]. Depending on the applications, different configurations of ejectors have been in use. Interestingly, however, most of the past works on ejectors reported central injection ejectors, where primary flow is injected along the centerline of the secondaryflow.When ejectors are used for pumping chemical lasers, central injection type ejectors cannot be used because the passage of primary flow is exposed to the secondary flow of hot burnt gas with temperatures well beyond 1200 K [8,9]. By injecting the primary flow annularly, direct contact between the hot secondary flow and primary flow passage can be avoided.Annular injection of the primaryflow is also used in a rocket based combined cycle engine, where the high-momentum of the secondary flow can be maintained by removing the protrusion of the primary flow passage [10,11]. As the annular injection ejectors make up an important part of the systems described above, the study of the annular injection ejectors have been device specific, which can explain the lack of the literature on ejectors with this injection arrangement. In the present study, we investigated the effect of shape of the primary nozzle and configuration of the flow passage downstream of the primary nozzle exit on the performances of an isolated annular injection ejector, namely, static pressure of the secondary pressure and the primary stagnation pressure at the starting and unstarting conditions. By doing so, we intend to understand the performance characteristics and provide a baseline data for ejector sizing. Figure 1 is a typical performance curve of an annular injection supersonic ejector. The normalized primary stagnation pressure was plotted against the normalized secondary pressure. As the stagnation pressure of the primary flow increases, the forepart of the diverging section of the primary nozzle becomes supersonic and the aftpart becomes subsonic with a normal shock demarcating these two flow regions. As a result, subsonicmixing occurs between the primary and secondary flows in the entire mixing chamber. This condition corresponds to region (1) in Fig. 1. As the stagnation pressure of the primary flow increases further, the shock wave is pushed outside of the primary nozzle. Therefore, supersonic mixing takes place in part of the mixing chamber as shown in region (2). When the primary stagnation pressure increases beyond the starting pressure in region (3), the whole mixing chamber is filled with supersonic primary flow, and the shock is swallowed by the second throat. At this condition, the design static pressure of the secondary flow is

A Starting Procedure of Supersonic Ejector to Minimize Primary Pressure Load

A N EJECTOR is a flow device in which the momentum of the primary flow is transferred to the secondary flow. The momentum transfer takes place as the primary flow is injected into the secondary flow that is stagnant or moving. Ejectors are classified as subsonic, transonic, and supersonic depending on the flow speed at the exit of the primary nozzle. Typically the flow channel of an ejector is axis symmetric. Depending on the configuration of the primary flow inlet, the ejectors are either a central injection type or an annular injection type. Primary flow is injected either annularly or centrally based on the nature of the secondary flow. Ejectors have many advantages over other devices in pumping fluids [1–9]. Although ejectors with central injection primary flow are widely used, annular injection of primary flow is indispensable to certain applications: pumping chemical lasers and high speed/high altitude test facilities. To pump a high power chemical laser, central injection of primary flow cannot be used, as the supply tubing of the primary flow is exposed in the stream of high temperature secondary flow [10]. For a test facility to simulate a high speed/high altitude environment, the central injection with protrusion of the primary supply into the secondary flow would cause significant loss in the momentum of the secondary flow. The starting behavior of a supersonic ejector with annular injection of primary flow is different from one with central injection [11,12]. In the present study, the starting behavior of a supersonic ejector with an annular injection of primary flow was investigated. To recover stagnation pressure, supersonic ejectors are generally equipped with a mixing tube downstream of the injection of the primary flow. A sequence of the starting operation of such a supersonic ejector with annular injection of primary flow is illustrated in Fig. 1. The plot begins at a point where both primary stagnation pressure and secondary static pressure are equal to ambient pressure. As the stagnation pressure of the primary flow injection increases, the secondary flow accelerates from a stagnant ambient state and its static pressure decreases as shown in region (1) of Fig. 1. In this region, the flow is supersonic only within the primary nozzle. The whole flowfield outside of the primary nozzle is still subsonic. As the primary pressure further increases, the shock wave moves out of the primary nozzle to form an oblique shock as in region (2) of Fig. 1. When the primary pressure increases beyond the starting point, the oblique shockwave is abruptly swallowed by themixing tube and the whole flowfield inside the ejector becomes supersonic; the ejector is started and the static pressure of the secondary flow drops abruptly in region (3). Once the ejector is started, the static pressure of the secondary flow is not sensitive to the variation of the stagnation pressure of the primary flow. The ejector maintains supersonic operation, even when the stagnation pressure of the primary flow decreases below the value at which the ejector started. The unstarting of the ejector occurswhen the stagnation pressure of the primaryflow is noticeably less than the starting pressure. The discrepancy between the starting and unstarting stagnation pressures of the primary flow is due to the hysteresis of the ejector operation. To take advantage of this hysteresis, the stagnation pressure of the primary flow is lowered to a value that is slightly higher than the unstarting pressure, once the ejector enters the supersonic operationmode [13,14]. In this way, the design requirement on the primary flow that drives the ejector can be reduced. In the present study, a new procedure to start a supersonic ejector is proposed to reduce the burden on the stagnation pressure of the primary flow further. The new starting procedure was tested and validated by varying the secondary inlet condition. It was discovered that a significantly less primary mass flow rate was required to start a supersonic ejector when the new starting procedure was used.

Advanced Disjoint Address Allocation for Mobile Ad Hoc Networks

Regarding to address allocation, a mobile ad hoc network (MANET) may suffer from the lack of address and network partition/merging due to mobility of nodes. In this paper, we propose a new address allocation protocol for dealing with those problems. The proposed protocol is developed based on disjoint address set distribution with binary splitting for scalability, and provides special treatments for resolving the lack of addresses. We also present an effective technique for handling network partition and merging. Through simulation, we show that the proposed protocol is effective to allocate addresses in a MANET with reasonable latency and communication overhead.

Neighbor-Aware Adaptive Retry Limit for IEEE 802.11-Based Mobile Ad Hoc Networks

In a mobile ad hoc network (MANET), it has been addressed that packet losses due to collision are often misinterpreted as routing failures, and cause unnecessary overhead for routing maintenance. There have been several attempts to avoid the unnecessary overhead through reducing collision losses. They are effective in a static topology where most losses are due to collision. In a dynamic topology, however, packets are lost due to actual routing failures (induced by mobility) as well as due to collision, and efforts for reducing collision are not enough. In this paper, we propose a new scheme for adjusting the limit of RTS retransmissions. In the proposed scheme, we treat packet losses differently as follows: (a) upon collisions, we increase the limit to reduce collision losses; and (b) upon routing failures, we decrease the limit to avoid unnecessary retransmissions. Through extensive simulations, it is shown that the proposed scheme effectively improves throughput in various scenarios and outperforms other comparable schemes.

Transitional behavior of a supersonic flow in a two-dimensional diffuser

Two-dimensional blow-down type supersonic wind tunnel was designed and built to investigate the transient behavior of the startup of a supersonic flow from rest. The contour of the divergent part of the nozzle was determined by the MOC calculation. The converging part of the nozzle, upstream of the throat was contoured to make the flow profile uniform at the throat. The flow characteristics of the steady supersonic condition were visualized using the highspeed schlieren photography. The Mach number was evaluated from the oblique shock wave angle on a sharp wedge with half angle of 5 degree. The measured Mach number was 2.4 and was slightly less than the value predicted by the design calculation. The initial transient behavior of the nozzle was recorded by a high-speed digital video camera with schlieren technique. The measured transition time from standstill to a steady supersonic flow was estimated by analyzing the serial images. Typical transition time was approximately O.1sec.

Development of a rational design procedure for a pressure recovery system for HPCL

From the geometric parameter study, an optimal ejector design procedure of pressure recovery system for chemical lasers was acquired. For given primary flow reservoir conditions, an up-scaled ejector was designed and manufactured. In the performance test, secondary mass flow rate of 200g/s air was entrained satisfying the design secondary pressure, 40 ~ 50torr. Performance validation of a supersonic ejector system along with an investigation of effects of supersonic diffuser was conducted. Placement of the diffuser at the secondary inlet further reduced diffuser upstream pressure to 7torr. Lastly, the duplicate of apparatus (air 100 g/s secondary mass flow rate each) was built and connected in parallel to assess proportionality behavior on a system to handle larger mass flow rate. Test and comparison of the parallel unit demonstrated the secondary mass flow rate was proportional to the number of individual units that were brought together maintaining the lasing pressure.

THE ELECTRONIC ENERGY LOSS OF 100 KEV HEAVY IONS IN MEDIUM ENERGY ION SCATTERING ANALYSIS OF A TA2O5 ULTRATHIN FILM

Abstract To optimize the depth resolution of Medium Energy Ion Scattering Spectroscopy (MEIS), a 10 nm Ta 2 O 5 thin film on a Si(100) substrate was analyzed by MEIS using H + and heavy ions such as Li + , N + and Ne + ions. The use of heavy ions such as Li + and N + increased the electronic stopping powers 2–3 times but it also increased the electronic straggling compared to H + ions. For Ne + ions, the ion neutralization problem was so severe that the scattering ion intensity from the subsurface layer was attenuated very rapidly and a strong doubly ionized Ne ++ peak was observed. For 100 keV N + and Ne + ions, multiple scattering peaks were observed.