Farhana Afroz

Multiple Radio Channel Assignement Utilizing Partially Overlapped Channels

Existing channel assignment algorithms designed for multi-radio multi-channel wireless mesh networks (MRMC-WMN) mainly deal with orthogonal or nonoverlapped channels. But in reality, the limited availability of orthogonal channel is a major issue where the network is very dense in terms of inter-nodal distances. On the other hand, partially overlapped channels (POC) are currently considered as a great potential for increasing the number of simultaneous transmissions and eventually upgrading the network capacity; especially in case of MRMC-WMN. In this paper, we address the limited orthogonal channel problem by exploring the usable POCs. The key technique lies in the fact that the interference between adjacent channels has to be considered intelligently in order to increase the overall capacity. Our contributions include a new interference model I-Matrix that helps selecting channels with less interference and a POC-based channel assignment algorithm. We evaluate the performance of our POC based algorithm in terms of capacity by comparing with the one using only orthogonal channels. Our results show capacity improvement as the increased link assignments at an average of more than 15 percent.

Call admission control: QoS issue for VoIP

One of the vital key elements for providing quality of service (QoS) for VoIP is the call admission control (CAC) capabilities of the session management/call session control function or gateway. Even though the network may be designed to meet a given performance and restoration objective for the engineered traffic loads, the actual traffic may be significantly higher. Without a CAC function in a VoIP network during overloads, links become congested and new calls keep getting admitted. All calls in progress, not just the new calls start dropping packets and experiencing longer delays. Contrast this to a circuit switched network, where new calls get blocked, but calls in progress experience good call quality. In the VoIP case, packet loss could become large enough that calls become unintelligible, callers hang-up their call, and most will reattempt. This paper gives an overview of potential CAC approaches, highlighting four basic alternatives; based on endpoint performance measurements, path-based bandwidth management, link-based bandwidth management, and per-call bandwidth reservation. The paper also recommends a link bandwidth management approach for its scalability and efficacy.

Separation control over a grooved surface inspired by dolphin skin.

Over many decades the biological surfaces of aquatic swimmers have been studied for their potential as drag reducing surfaces. The hydrodynamic benefit of riblets, or grooves embedded parallel to the flow which appear on surfaces such as shark skin, have been well documented. However the skin of dolphins is embedded with sinusoidal grooves that run perpendicular or transverse to the flow over their bodies. It is theorized that the transverse grooves present on dolphin skin trap vortices between them, creating a partial slip condition over the surface and inducing turbulence augmentation in the boundary layer, thus acting as a potential mechanism to reduce flow separation and thus pressure drag. In an attempt to test this hypothesis and study these effects, an adverse pressure gradient was induced above a flat plate resulting in a controlled region of flow separation occurring within a tripped, turbulent boundary layer. Small transverse grooves of both rectangular and sinusoidal shape were 3D printed and mounted to the plate to measure their effect on the boundary layer flow. The results were compared to a flat plate without grooves using digital particle image velocimetry (DPIV). The strength of the adverse pressure gradient was varied, and the observed control in flow separation and other effects upon the boundary layer are discussed.

Experimental study of laminar and turbulent boundary layer separation control of shark skin

The Shortfin Mako shark (Isurus oxyrinchus) is a fast swimmer and has incredible turning agility, and has flexible scales known to bristle up to 50° in the flank regions. It is purported that this bristling capability of the scales may result in a unique pass flow control method to control flow separation and reduce drag. It appears that the scales have evolved to be only actuated when the flow over the body is reversed; thereby inducing a method of inhibiting flow reversal close to the surface. In addition, bristled scales form cavities which could induce boundary layer mixing and further assist in delaying flow separation. To substantiate the hypothesis, samples of skin from the flank region of the mako have been tested in a water tunnel facility under various strengths of adverse pressure gradient (APG). Laminar and turbulent separation over the skin was studied experimentally using time-resolved digital particle image velocimetry, where the APG was generated and varied using a rotating cylinder. Shark skin results were compared with that of a smooth plate data for a given amount of APG. Both the instantaneous and time-averaged results reveal that shark skin is capable of controlling laminar as well as turbulent separation. Under laminar conditions, the shark skin also induces an early transition to turbulence and reduces the degree of laminar separation. For turbulent separation, the presence of the shark skin reduces the amount of backflow and size of the separation region. Under both flow conditions, the shark skin also delayed the point of separation as compared to a smooth wall.

Experimental and Computational Analysis of Natural Convection Over Flat Plate

Natural convection heat transfer over a flat plate with a heat source at bottom side of plate is studied experimentally and numerically. We consider the two-dimensional problem of both steady and unsteady natural convection over the flat plate at vertical, horizontal and inclined position. Experimental analysis is done for three different constant heat fluxes for each angle position. The Navier-Stokes and Energy equations with the Boussinesq approximation are written in Cartesian coordinate system. The problem is solved in the physical variables on the basis of a completely implicit Finite element Method order to examine the heat transfer characteristics. To see the effects of different angle position phenomena of natural convection over flat plate, the computational results presented in the form of streamlines for a wide range of Grashof number at different heat fluxes. The average Nusselt number of heated plate for different angle position has been observed.Copyright