The Experts below are selected from a list of 15474 Experts worldwide ranked by ideXlab platform
Tamer A Zaki - One of the best experts on this subject based on the ideXlab platform.
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optimal heat flux for delaying transition to turbulence in a high speed boundary layer
Journal of Fluid Mechanics, 2021Co-Authors: Reza Jahanbakhshi, Tamer A ZakiAbstract:The optimal steady wall heat flux is sought to delay transition to turbulence in a Mach 4.5 boundary layer, when the free stream disturbance is the nonlinearly most dangerous condition that causes the earliest breakdown to turbulence in the adiabatic flow. The transition-delay strategy is pursued using direct numerical simulations, and the problem is formulated as a constrained optimization where the objective is to minimize the viscous drag force and the control vector is the heat flux at the wall. The optimization is performed using an ensemble-variational approach. The algorithm identifies regions of the flow that are most sensitive to thermal treatment: most of the heating and cooling is applied upstream of the Synchronization Point of the slow and the fast modes. Attenuation of key instability waves by the optimal heat flux mitigates the nonlinear interactions that preceded breakdown to turbulence in the reference flow, and hence transition is appreciably delayed. Detailed analysis of the acoustic, entropic and solenoidal components of the momentum density vector reveals the stabilizing effects of the optimal heat flux. In addition, the influence of the thermal treatment is not limited to the pretransitional flow where it is concentrated, but rather it persists downstream even within the newly formed fully turbulent boundary layer.
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sensitivity of high speed boundary layer stability to base flow distortion
Journal of Fluid Mechanics, 2019Co-Authors: Junho Park, Tamer A ZakiAbstract:The linear stability of high-speed boundary layers can be altered by distortions to the base velocity and temperature profiles. An analytic expression for the sensitivity is derived for parallel and spatially developing boundary layers, the latter using linear parabolized stability equations and their adjoint. Both the slow mode, S, and the fast mode, F, are investigated at Mach number 4.5. The mode S is more sensitive with respect to distortion in base velocity than in base temperature. The sensitivity is largest within the boundary layer away from the wall. Near the critical layer, where the phase speed of the mode equals the base streamwise velocity, the sensitivity to the base streamwise velocity is negative. For the mode F, there is a discontinuous jump in the sensitivity when the phase speed is below unity, and a critical layer is established. The sensitivity of the two modes increases with the Reynolds number, but there is a sudden drop and a jump in the sensitivities of the modes S and F, respectively, near the Synchronization Point where the phase speeds of the two modes are equal. Furthermore, the maximum uncertainty bounds are obtained for the distorted base state that maximizes the destabilization or stabilization of the modes by solving the Lagrangian optimization problem for the sensitivity. The sensitivity of the flow stability to surface heating is then studied, and changes in growth rate and the $N$ -factor are evaluated. The formulation provides a clear physical interpretation of these changes, and establishes uncertainty bounds for stability predictions for a given level of uncertainty in wall temperature.
Filippo Capolino - One of the best experts on this subject based on the ideXlab platform.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
IEEE Transactions on Electron Devices, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive an exceptional Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward-wave oscillators (BWOs) operating at millimeter-wave and terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of exceptional Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results show that BWO operating at an EPD regime is capable of generating output power exceeding 3 kW with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
arXiv: Applied Physics, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive a degenerate Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward wave oscillators (BWOs) operating at millimeter-wave and Terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of degenerate Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results shows that BWO operating at an EPD regime is capable of generating output power exceeding 3 kwatts with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
Tarek Mealy - One of the best experts on this subject based on the ideXlab platform.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
IEEE Transactions on Electron Devices, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive an exceptional Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward-wave oscillators (BWOs) operating at millimeter-wave and terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of exceptional Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results show that BWO operating at an EPD regime is capable of generating output power exceeding 3 kW with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
arXiv: Applied Physics, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive a degenerate Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward wave oscillators (BWOs) operating at millimeter-wave and Terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of degenerate Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results shows that BWO operating at an EPD regime is capable of generating output power exceeding 3 kwatts with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
Jeroen Hoebeke - One of the best experts on this subject based on the ideXlab platform.
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seamless roaming and guaranteed communication using a synchronized single hop multi gateway 802 15 4e tsch network
Ad Hoc Networks, 2019Co-Authors: Jetmir Haxhibeqiri, Abdulkadir Karaagac, Ingrid Moerman, Jeroen HoebekeAbstract:Abstract Industrial wireless sensor and actuator networks (WSANs) are being used to improve the efficiency, productivity and safety of industrial processes. One open standard that is commonly used in such cases is IEEE 802.15.4e. Its Time-Slotted Channel Hopping (TSCH) mode employs a time synchronized based medium access control (MAC) scheme together with channel hopping to alleviate the impact of channel fading and interference. Until now, most of the industrial WSANs have been designed to only support static nodes and are not able to deal with mobility. In this paper, we show how a single-hop, multi-gateway IEEE 802.15.4e TSCH network architecture can tackle the mobility problem. We introduce the Virtual Grand Master (VGM) concept that moves the Synchronization Point from separated Backbone Border Routers (BBRs) towards the backbone network. With time Synchronization of all BBRs, mobile nodes can roam from one BBR to another without time deSynchronization. In addition to time Synchronization, we introduce a mechanism to synchronize the schedules between BBRs to support fast handover of mobile nodes. We show that with the proposed network architecture handovers happen instantly without any packet losses, while the handover time can be up to tens of seconds without any time Synchronization between BBRs. The solution is evaluated in a testbed setting as well as in a real industrial environment.
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seamless roaming and guaranteed communication using a synchronized single hop multi gateway 802 15 4e tsch network
arXiv: Networking and Internet Architecture, 2018Co-Authors: Jetmir Haxhibeqiri, Abdulkadir Karaagac, Ingrid Moerman, Jeroen HoebekeAbstract:Industrial wireless sensor networks (WSNs) are being used to improve the efficiency, productivity and safety of industrial processes. An open standard that is commonly used in such cases is IEEE 802.15.4e. Its TSCH mode employs a time synchronized based MAC scheme together with channel hopping to alleviate the impact of channel fading. Until now, most of the industrial WSNs have been designed to only support static nodes and are not able to deal with mobility. In this paper, we show how a single-hop, multi-gateway IEEE 802.15.4e TSCH network architecture can tackle the mobility problem. We introduce the Virtual Grand Master (VGM) concept that moves the Synchronization Point from separated Backbone Border Routers (BBRs) towards the backbone network. With time Synchronization of all BBRs, mobile nodes can roam from one BBR to another without time deSynchronization. In addition to time Synchronization, we introduce a mechanism to synchronize the schedules between BBRs to support fast handover of mobile nodes.
Ahmed F Abdelshafy - One of the best experts on this subject based on the ideXlab platform.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
IEEE Transactions on Electron Devices, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive an exceptional Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward-wave oscillators (BWOs) operating at millimeter-wave and terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of exceptional Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results show that BWO operating at an EPD regime is capable of generating output power exceeding 3 kW with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
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high power backward wave oscillator using folded waveguide with distributed power extraction operating at an exceptional Point
arXiv: Applied Physics, 2021Co-Authors: Tarek Mealy, Ahmed F Abdelshafy, Filippo CapolinoAbstract:The concept of exceptional Point of degeneracy (EPD) is used to conceive a degenerate Synchronization regime that is able to enhance the level of output power and power conversion efficiency for backward wave oscillators (BWOs) operating at millimeter-wave and Terahertz frequencies. Standard BWOs operating at such high frequency ranges typically generate output power not exceeding tens of watts with very poor power conversion efficiency in the order of 1%. The novel concept of degenerate Synchronization for the BWO based on a folded waveguide is implemented by engineering distributed gain and power extraction along the slow-wave waveguide. The distributed power extraction along the folded waveguide is useful to satisfy the necessary conditions to have an EPD at the Synchronization Point. Particle-in-cell (PIC) simulation results shows that BWO operating at an EPD regime is capable of generating output power exceeding 3 kwatts with conversion efficiency of exceeding 20% at frequency of 88.5 GHz.
