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AuthorSamtaney, Ravi (3)Younis, Mohammad I. (3)Agambayev, Sumeyra (1)Alcheikh, Nouha (1)Bakhsh, Abeer (1)View MoreDepartmentMechanical Engineering Program (9)Physical Sciences and Engineering (PSE) Division (9)Computer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division (3)Clean Combustion Research Center (2)Advanced Membranes and Porous Materials Research Center (1)View MoreJournalPhysics of Fluids (3)AIP Advances (2)Applied Physics Letters (1)Biomicrofluidics (1)Journal of Applied Physics (1)View MoreKAUST Acknowledged Support UnitCompetitive Research Funds (1)KAUST Office of Sponsored Research (1)Shaheen (1)KAUST Grant NumberURF/1/1394-01 (1)URF/1/2162-01 (1)Publisher

AIP Publishing (9)

TypeArticle (9)Year (Issue Date)
2016 (9)

Item AvailabilityOpen Access (9)

Now showing items 1-9 of 9

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Reynolds number and geometry effects in laminar axisymmetric isothermal counterflows

Scribano, Gianfranco; Bisetti, Fabrizio (Physics of Fluids, AIP Publishing, 2016-12-29) [Article]

The counterflow configuration is a canonical stagnation flow, featuring two opposed impinging round jets and a mixing layer across the stagnation plane. Although counterflows are used extensively in the study of reactive mixtures and other applications where mixing of two streams is required, quantitative data on the scaling properties of the flow field are lacking. The aim of this work is to characterize the velocity and mixing fields in isothermal counterflows over a wide range of conditions. The study features both experimental data from particle image velocimetry and results from detailed axisymmetric simulations. The scaling laws for the nondimensional velocity and mixture fraction are obtained as a function of an appropriate Reynolds number and the ratio of the separation distance of the nozzles to their diameter. In the range of flow configurations investigated, the nondimensional fields are found to depend primarily on the separation ratio and, to a lesser extent, the Reynolds number. The marked dependence of the velocity field with respect to the separation ratio is linked to a high pressure region at the stagnation point. On the other hand, Reynolds number effects highlight the role played by the wall boundary layer on the interior of the nozzles, which becomes less important as the separation ratio decreases. The normalized strain rate and scalar dissipation rate at the stagnation plane are found to attain limiting values only for high values of the Reynolds number. These asymptotic values depend markedly on the separation ratio and differ significantly from the values produced by analytical models. The scaling of the mixing field does not show a limiting behavior as the separation ratio decreases to the smallest practical value considered.

Transient growth of a Vlasov plasma in a weakly inhomogeneous magnetic field

Ratushnaya, Valeria; Samtaney, Ravi (Physics of Plasmas, AIP Publishing, 2016-12-17) [Article]

We investigate the stability properties of a collisionless Vlasov plasma in a weakly inhomogeneous magnetic field using non-modal stability analysis. This is an important topic in a physics of tokamak plasma rich in various types of instabilities. We consider a thin tokamak plasma in a Maxwellian equilibrium, subjected to a small arbitrary perturbation. Within the framework of kinetic theory, we demonstrate the emergence of short time scale algebraic instabilities evolving in a stable magnetized plasma. We show that the linearized governing operator (Vlasov operator) is non-normal leading to the transient growth of the perturbations on the time scale of several plasma periods that is subsequently followed by Landau damping. We calculate the first-order distribution function and the electric field and study the dependence of the transient growth characteristics on the magnetic field strength and perturbation parameters of the system. We compare our results with uniformly magnetized plasma and field-free Vlasov plasma.

Low-dielectric layer increases nanosecond electric discharges in
distilled water

Hamdan, Ahmad; Cha, Min Suk (AIP Advances, AIP Publishing, 2016-10-24) [Article]

Electric discharge in liquids is an emerging field of research, and is involved into various environmental applications (water purification, fuel reforming, nanomaterial synthesis, etc.). Increasing the treatment efficiency with simultaneous decreasing of the energy consumption are the main goals of today’s research. Here we present an experimental study of nanosecond discharge in distilled water covered by a layer of dielectric material. We demonstrate through this paper that the discharge efficiency can be improved by changing the interface position regarding the anode tip. The efficiency increase is due to the increase of the discharge probability as well as the plasma volume. The understanding of the experimental results is brought and strengthened by simulating the electric field distribution, using Comsol Multiphysics software. Because the dielectric permittivity (ε) is discontinuous at the interface, the electric field is enhanced by a factor that depends on the relative value of ε of the two liquids. The present result is very promising in future: opportunities for potential applications as well as fundamental studies for discharges in liquid.

Efficient primary and parametric resonance excitation of bistable resonators

Ramini, Abdallah; Alcheikh, Nouha; Ilyas, Saad; Younis, Mohammad I. (AIP Advances, AIP Publishing, 2016-09-12) [Article]

We experimentally demonstrate an efficient approach to excite primary and parametric (up to the 4th) resonance of Microelectromechanical system MEMS arch resonators with large vibrational amplitudes. A single crystal silicon in-plane arch microbeam is fabricated such that it can be excited axially from one of its ends by a parallel-plate electrode. Its micro/nano scale vibrations are transduced using a high speed camera. Through the parallel-plate electrode, a time varying electrostatic force is applied, which is converted into a time varying axial force that modulates dynamically the stiffness of the arch resonator. Due to the initial curvature of the structure, not only parametric excitation is induced, but also primary resonance. Experimental investigation is conducted comparing the response of the arch near primary resonance using the axial excitation to that of a classical parallel-plate actuation where the arch itself forms an electrode. The results show that the axial excitation can be more efficient and requires less power for primary resonance excitation. Moreover, unlike the classical method where the structure is vulnerable to the dynamic pull-in instability, the axial excitation technique can provide large amplitude motion while protecting the structure from pull-in. In addition to primary resonance, parametrical resonances are demonstrated at twice, one-half, and two-thirds the primary resonance frequency. The ability to actuate primary and/or parametric resonances can serve various applications, such as for resonator based logic and memory devices. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Towards electromechanical computation: An alternative approach to realize complex logic circuits

Hafiz, Md Abdullah Al; Kosuru, Lakshmoji; Younis, Mohammad I. (Journal of Applied Physics, AIP Publishing, 2016-08-18) [Article]

Electromechanical computing based on micro/nano resonators has recently attracted significant attention. However, full implementation of this technology has been hindered by the difficulty in realizing complex logic circuits. We report here an alternative approach to realize complex logic circuits based on multiple MEMS resonators. As case studies, we report the construction of a single-bit binary comparator, a single-bit 4-to-2 encoder, and parallel XOR/XNOR and AND/NOT logic gates. Toward this, several microresonators are electrically connected and their resonance frequencies are tuned through an electrothermal modulation scheme. The microresonators operating in the linear regime do not require large excitation forces, and work at room temperature and at modest air pressure. This study demonstrates that by reconfiguring the same basic building block, tunable resonator, several essential complex logic functions can be achieved.

A smart microelectromechanical sensor and switch triggered by gas

Bouchaala, Adam M.; Jaber, Nizar; Shekhah, Osama; Chernikova, Valeriya; Eddaoudi, Mohamed; Younis, Mohammad I. (Applied Physics Letters, AIP Publishing, 2016-07-05) [Article]

There is an increasing interest to realize smarter sensors and actuators that can deliver a multitude of sophisticated functionalities while being compact in size and of low cost. We report here combining both sensing and actuation on the same device based on a single microstructure. Specifically, we demonstrate a smart resonant gas (mass) sensor, which in addition to being capable of quantifying the amount of absorbed gas, can be autonomously triggered as an electrical switch upon exceeding a preset threshold of absorbed gas. Toward this, an electrostatically actuated polymer microbeam is fabricated and is then functionalized with a metal-organic framework, namely, HKUST-1. The microbeam is demonstrated to absorb vapors up to a certain threshold, after which is shown to collapse through the dynamic pull-in instability. Upon pull-in, the microstructure can be made to act as an electrical switch to achieve desirable actions, such as alarming.

A “twisted” microfluidic mixer suitable for a wide range of flow rate applications

Sivashankar, Shilpa; Agambayev, Sumeyra; Mashraei, Yousof; Li, Erqiang; Thoroddsen, Sigurdur T; Salama, Khaled N. (Biomicrofluidics, AIP Publishing, 2016-06-27) [Article]

This paper proposes a new “twisted” 3D microfluidic mixer fabricated by a laser writing/microfabrication technique. Effective and efficient mixing using the twisted micromixers can be obtained by combining two general chaotic mixing mechanisms: splitting/recombining and chaotic advection. The lamination of mixer units provides the splitting and recombination mechanism when the quadrant of circles is arranged in a two-layered serial arrangement of mixing units. The overall 3D path of the microchannel introduces the advection. An experimental investigation using chemical solutions revealed that these novel 3D passive microfluidic mixers were stable and could be operated at a wide range of flow rates. This micromixer finds application in the manipulation of tiny volumes of liquids that are crucial in diagnostics. The mixing performance was evaluated by dye visualization, and using a pH test that determined the chemical reaction of the solutions. A comparison of the tornado-mixer with this twisted micromixer was made to evaluate the efficiency of mixing. The efficiency of mixing was calculated within the channel by acquiring intensities using ImageJ software. Results suggested that efficient mixing can be obtained when more than 3 units were consecutively placed. The geometry of the device, which has a length of 30 mm, enables the device to be integrated with micro total analysis systems and other lab-on-chip devices.

BiGlobal linear stability analysis on low-Re flow past an airfoil at high angle of attack

Zhang, Wei; Samtaney, Ravi (Physics of Fluids, AIP Publishing, 2016-04-05) [Article]

We perform BiGlobal linear stability analysis on flow past a NACA0012 airfoil at 16° angle of attack and Reynolds number ranging from 400 to 1000. The steady-state two-dimensional base flows are computed using a well-tested finite difference code in combination with the selective frequency damping method. The base flow is characterized by two asymmetric recirculation bubbles downstream of the airfoil whose streamwise extent and the maximum reverse flow velocity increase with the Reynolds number. The stability analysis of the flow past the airfoil is carried out under very small spanwise wavenumber β = 10−4 to approximate the two-dimensional perturbation, and medium and large spanwise wavenumbers (β = 1–8) to account for the three-dimensional perturbation. Numerical results reveal that under small spanwise wavenumber, there are at most two oscillatory unstable modes corresponding to the near wake and far wake instabilities; the growth rate and frequency of the perturbation agree well with the two-dimensional direct numerical simulation results under all Reynolds numbers. For a larger spanwise wavenumber β = 1, there is only one oscillatory unstable mode associated with the wake instability at Re = 400 and 600, while at Re = 800 and 1000 there are two oscillatory unstable modes for the near wake and far wake instabilities, and one stationary unstable mode for the monotonically growing perturbation within the recirculation bubble via the centrifugal instability mechanism. All the unstable modes are weakened or even suppressed as the spanwise wavenumber further increases, among which the stationary mode persists until β = 4.

Linear simulations of the cylindrical Richtmyer-Meshkov instability in magnetohydrodynamics

Bakhsh, Abeer; Gao, Song; Samtaney, Ravi; Wheatley, V. (Physics of Fluids, AIP Publishing, 2016-03-09) [Article]

Numerical simulations and analysis indicate that the Richtmyer-Meshkov instability(RMI) is suppressed in ideal magnetohydrodynamics(MHD) in Cartesian slab geometry. Motivated by the presence of hydrodynamic instabilities in inertial confinement fusion and suppression by means of a magnetic field, we investigate the RMI via linear MHD simulations in cylindrical geometry. The physical setup is that of a Chisnell-type converging shock interacting with a density interface with either axial or azimuthal (2D) perturbations. The linear stability is examined in the context of an initial value problem (with a time-varying base state) wherein the linearized ideal MHD equations are solved with an upwind numerical method. Linear simulations in the absence of a magnetic field indicate that RMI growth rate during the early time period is similar to that observed in Cartesian geometry. However, this RMI phase is short-lived and followed by a Rayleigh-Taylor instability phase with an accompanied exponential increase in the perturbation amplitude. We examine several strengths of the magnetic field (characterized by β=2p/B^2_r) and observe a significant suppression of the instability for β ≤ 4. The suppression of the instability is attributed to the transport of vorticity away from the interface by Alfvén fronts.

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