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Two preliminary concepts of propulsion based on a bacterial flagellum motor (BFM) were developed with the potential for use in micro-air-vehicles (MAV). The first uses the conventional turbojet without the turbine with a BFM to run the... more
Two preliminary concepts of propulsion based on a bacterial flagellum motor (BFM) were developed with the potential for use in micro-air-vehicles (MAV). The first uses the conventional turbojet without the turbine with a BFM to run the compressor. The second concept uses the ...
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Research Interests: Engineering and CRC
Facets and scope of large-scale disasters are briefly discussed in this forum. A universal quantitative metric is proposed that puts all natural and human-derived disasters on a common scale. Issues of prediction, control, and mitigation... more
Facets and scope of large-scale disasters are briefly discussed in this forum. A universal quantitative metric is proposed that puts all natural and human-derived disasters on a common scale. Issues of prediction, control, and mitigation of catastrophes are broadly introduced. The laws of nature govern the evolution of any disaster. In some cases, as for example weather-related disasters, those first-principles laws could be written in the form of field equations, but exact solutions of these often nonlinear differential equations are impossible to obtain, particularly for turbulent flows; heuristic models together with intensive use of supercomputers are necessary to proceed. In other cases, as for example earthquakes, the precise laws are not even known and prediction becomes somewhat problematic. Management of any type of disaster is more art than science. Nevertheless, much can be done by both social and physical scientists/engineers to alleviate the resulting pain and suffering.
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Manufacturing processes that can create extremely small machines have been developed in recent years. Microelectromechanical systems (MEMS) refer to devices that have characteristic length of less than 1 mm but more than 1 micron, that... more
Manufacturing processes that can create extremely small machines have been developed in recent years. Microelectromechanical systems (MEMS) refer to devices that have characteristic length of less than 1 mm but more than 1 micron, that combine electrical and ...
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Among the modem goals of external flow control are separation postponement, lift enhancement, transition delay or advancement, and drag reduction. These objectives are not necessarily mutually exclusive. For low-Reynolds-number lifting... more
Among the modem goals of external flow control are separation postponement, lift enhancement, transition delay or advancement, and drag reduction. These objectives are not necessarily mutually exclusive. For low-Reynolds-number lifting surfaces, where the formation of a laminar separation bubble may have a dominant effect on the flow field, the interrelation between the above goals is particularly salient, presenting an additional degree of complexity. The present article is an overview of passive and active techniques used to control a low-Reynolds-number boundary layer to achieve an improved performance. A unified view based on a vorticity framework will be presented to explain many of the available or contemplated control methods. Among the control techniques to be considered are wall suction/injection, shaping, heat transfer through the surface, introduction of a foreign substance into the boundary layer, fixed or moving geometric modifications, and turbulence manipulation. Among the practical considerations that will be reviewed for these devices are their cost of construction and operation, complexity, and potential trade-off’s or penalties associated with their use.
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Abstract The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance. In this article, methods of control to achieve transition delay, separation postponement, lift... more
Abstract The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance. In this article, methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence ...
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Research Interests: Engineering, Turbulence, Soft Computing, Energy Consumption, Flow Control, and 14 moreDrag Reduction, Turbulent boundary layer, Chaos Control, Drag, Turbulent Flow, Laminar Flow, Flow Regime, Control Strategy, Flow Separation, Negative Skin Friction, Coherent Structures, Reactive Control, Skin Friction, and Springer Ebooks
Uniform flow around a body consisting of a cylinder with a splitter plate attached to it is considered. The body is free to rotate about the axis of the cylinder. Numerical results show that on increasing the Reynolds number above a... more
Uniform flow around a body consisting of a cylinder with a splitter plate attached to it is considered. The body is free to rotate about the axis of the cylinder. Numerical results show that on increasing the Reynolds number above a critical value a symmetry-breaking bifurcation appears and the splitter plate migrates to an asymmetric equilibrium position, confirming previous high-Reynolds number experiments. The present results reveal that this phenomenon is due to the flow in the separation bubble behind the cylindrical part of the body.
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ABSTRACT Tripping devices are usually installed at the entrance of laboratory-scale pipe test sections to obtain a fully developed turbulent flow sooner. The tripping of laminar flow to induce turbulence can be carried out in different... more
ABSTRACT Tripping devices are usually installed at the entrance of laboratory-scale pipe test sections to obtain a fully developed turbulent flow sooner. The tripping of laminar flow to induce turbulence can be carried out in different ways, such as using cylindrical wires, sand papers, well-organized tape letters, fences, etc. Claims of tripping effects have been made periodically since the classical experiments of Nikuradse (1932), which covered a significant range of Reynolds numbers. NikuradseÂ's data have become the metric by which theories are established, and have also been the subject of intense scrutiny. Several subsequent experiments reported friction factors as much as 5% lower than those measured by Nikuradse, and the authors of those reports attributed the difference to tripping effects. In the present study, measurements with and without ring tripping devices of different blocking areas of 10%, 20%, 30% and 40% have been carried out to determine the effect of entrance condition on the developing flow field in pipes. Along with pressure drop measurements to compute the skin friction, both Pitot tube and hot-wire anemometry measurements have been used to accurately determine the mean velocity profile over the working test section at different Reynolds numbers in the range of 1× 10^5--4.5 × 10^5. The results we obtained suggest that the tripping technique has an insignificant effect on the wall friction factor, in agreement with Nikuradse's original data.
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ABSTRACT Thermal flow sensors with a wide dynamic range are presently not available in spite of the strong demand for such sensors in practical fluid flow measurements. The present innovation involves the use of a... more
ABSTRACT Thermal flow sensors with a wide dynamic range are presently not available in spite of the strong demand for such sensors in practical fluid flow measurements. The present innovation involves the use of a ``pulsed-wire'' anemometer together with a novel signal processing approach. A minute wire is heated using sinusoidal alternating current, and two sensing wires---acting as resistance thermometers---are set parallel to, and at a small distance on either side of, the pulsed wire. The thermal wake of the pulsed wire is convected downstream to one of the two receiving wires which detect its delayed arrival. By appropriate signal processing, the present sensor can be operated such that the phase shift between the periodic current that drives the central wire and the detected signal by either the upstream or downstream wire takes into account a combination of convection, diffusion and the finite thermal response time of both the pulsed wire and the receiving wire. Because the time constants increase as the flow speed decreases, the time lag due to thermal inertia supplements the time lag due to the true time of flight, thus yielding an effective operating range of 0.05 m/s
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ABSTRACT Thermal flow sensors with a wide dynamic range approaching 1:1000 are presently not available in spite of the large demand for such sensors in practical fluid flow measurements. During the last meeting (paper JG4, Bul. Am. Phys.... more
ABSTRACT Thermal flow sensors with a wide dynamic range approaching 1:1000 are presently not available in spite of the large demand for such sensors in practical fluid flow measurements. During the last meeting (paper JG4, Bul. Am. Phys. Soc. 45, no. 9, p. 141, 2000), we described such a probe consisting of a minute wire heated using sinusoidal alternating current and two sensing wires acting as resistance thermometers and set parallel to, and at a small distance on either side of, the pulsed wire. Herein we detail the development of a single wire heated using square waves of electrical current. The elimination of the sensing wires reduces the complexity as well as the cost of the sensor and improves its spatial resolution. Unlike time-of-flight sensors, however, the present single-element sensor is sensitive to the physical properties and temperature of the ambient fluid. The present device is suited for measuring slowly-varying unidirectional flows over a very wide dynamic range. For a given current amplitude and frequency, the nominal output of the single sensor is the increase in wire temperature (or resistance) between times just before the leading edge of the current pulse and just after the trailing edge of the pulse. In practice, an integral of the resistance over the pulse duration is computed and averaged over several pulses. This output is a function of the wire's time constant or thermal inertia and thus of the flow speed as well as the heat convected from the heated wire to the flow. We exploit the fact that the time constant decreases as the flow speed increases while the rate of heat transfer increases. At very low flow speeds, the response is determined almost entirely by the time constant whereas at high speeds the device acts almost like a constant-current hot-wire anemometer. At low speeds, therefore, the wire thermal inertia augments the output signal of the basic hot wire increasing its speed range and sensitivity above that of a conventional hot-wire flowmeter. We demonstrate that a wire of diameter 12.5 microns excited using a square wave of 17-msec duration and 30-Hz frequency is usable in the speed range of 0.01 to 25 m/s, yielding a dynamic range of 1:2500, almost two orders of magnitude broader range than that for traditional time-of-flight pulsed-wire anemometers.
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124 Abstract The spiraling and straight gas-filled vortices formed by a rotating mixture of a gas and a liquid flowing through a finite-length cylinder are the subject of this paper. The working fluids considered are primarily helium and... more
124 Abstract The spiraling and straight gas-filled vortices formed by a rotating mixture of a gas and a liquid flowing through a finite-length cylinder are the subject of this paper. The working fluids considered are primarily helium and water. The bubbly liquid enters and leaves the cyclone-type separator tangentially. A gas-core vortex forms due to the resulting swirling motion of the mixture and, ideally, most of the gas leaves through an opening centered in the inlet end-wall of the vertical cylinder. The sensitivity of the gas-core configurations to the relative angle between the tangential inlet and outlet (~) and to the length-to-diameter ratio of the cylinder (L/D) are investigated experimentally. Direct observations of the flow field are made using video and still cameras. The various gas-core vortex configurations are classified in a stability diagram. Although, as many as eight different types of core patterns have been identified, they are of two basic modes: straight an...
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Research Interests: Physics and Turbulence
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The evolution of multi-mode instabilities in a hypersonic boundary layer and their effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization,... more
The evolution of multi-mode instabilities in a hypersonic boundary layer and their effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization, fast-response pressure sensors, fluorescent temperature-sensitive paint (TSP), and particle image velocimetry (PIV). Calculations are also performed based on both parabolized stability equations (PSE) and direct numerical simulations (DNS). It is found that second-mode dilatational waves, accompanied by high-frequency alternating fluid compression and expansion, produce intense aerodynamic heating in a small region that rapidly heats the fluid passing through it. As a result, the surface temperature rapidly increases and results in an overshoot over the nominal transitional value. When the dilatation waves decay downstream, the surface temperature decreases gradually until transition is completed. A theoretical analysis is provided to interpret the temperatur...
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Submitted for the DFD08 Meeting of The American Physical Society Flow control by combining radial pulsation and rotation of a cylinder in uniform flow H. OUALLI, S. HANCHI, École Militaire Polytechnique, Algiers, Algeria, A. BOUABDALLAH,... more
Submitted for the DFD08 Meeting of The American Physical Society Flow control by combining radial pulsation and rotation of a cylinder in uniform flow H. OUALLI, S. HANCHI, École Militaire Polytechnique, Algiers, Algeria, A. BOUABDALLAH, Université des Sciences et de la Technologie Houari Boumedienne, Algiers, Algeria, M. GAD-EL-HAK, Virginia Commonwealth University, Richmond, USA — Flow visualizations and hot-wire measurements are carried out to study a circular cylinder undergoing simultaneous radial pulsation and rotation and placed in a uniform flow. The Reynolds number is in the range of 1,000–22,000, for which transition in the shear layers and near wake is expected. Our previous experimental and numerical investigations in this subcritical flow regime have established the existence of an important energy transfer mechanism from the mean flow to the fluctuations. Radial pulsations cause and enhance that energy transfer. Certain values of the amplitude and frequency of the puls...
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The current research utilizes Molecular Dynamics Simulations to model and predict the PTFE glassy transition temperature using OPLS-AA PTFE force-field parameters. Achieving the aforementioned objective involved performing two major... more
The current research utilizes Molecular Dynamics Simulations to model and predict the PTFE glassy transition temperature using OPLS-AA PTFE force-field parameters. Achieving the aforementioned objective involved performing two major tasks. First, building PTFE amorphous structure using Material Studio®. Second, performing Molecular Dynamics simulations using NAMD®. The latter task involves a polymer relaxation process, which was started with NVT followed by NPT ensemble simulations to predict PTFE glassy transition temperature. The results of our simulations were in good agreement with experimental findings.