Speed Regime Research Articles

Tropical Cyclone Winds Retrieved From C-Band Cross-Polarized Synthetic Aperture Radar

This paper presents a geophysical model function (GMF) that has been developed to describe the relation of the ocean surface wind with the normalized radar cross section (NRCS) at C-band cross polarization (cross-pol). Synthetic aperture radar (SAR) images have been simultaneously collected at copolarization (co-pol) and cross-pol at moderate to high wind speeds. Using the SAR co-pol retrieved wind fields and an uncertainty estimate of the retrieved wind speeds, the cross-pol dependencies of the NRCS are investigated with respect to wind, incidence angle, and polarization pairs. For wind speeds above 10 m/s, there is a significant dependence of the NRCS on wind speed. However, the SAR cross-pol data are also significantly affected by the noise floor and crosstalk between the channels. Estimates of the noise floor are determined and removed from the NRCS. Three GMFs are developed: the first is for transmission at horizontal (H) polarization and the second at vertical (V) polarization. A third GMF accounts for wind direction dependence. Validation of the GMFs is conducted by comparison with collocated Stepped Frequency Microwave Radiometer (SFMR) data. The resulting bias of -0.7 m/s and standard deviation of 3.7 m/s demonstrate the excellent performance for these GMFs for wind speed retrieval between 10 and 35 m/s. Furthermore, comparisons show that SAR cross-pol retrieved wind speeds are of similar quality as those of SFMR and are significantly better in the moderate to high wind speed regime than SAR co-pol retrieved winds.

Technology and operational sensitivity assessment for hypersonic endurance flight vehicles

AbstractIn an effort to increase the air-breathing endurance capability of current hypersonic research aircraft (i.e. X-43, 7 seconds; X-51, 5 minutes), the authors have explored the technical and operational solution space for a 30 minute cruise endurance demonstrator operating in the Mach 6 to Mach 8 speed regime. The focus of this activity has been on exploration of the available solution space through a unique screening process to assess the implication and interplay between the (a) mission, (b) baseline vehicle, and (c) operational scenarios. This study concludes that an air-launched, liquid hydrogen fuelled, 30 minute duration Mach 6 demonstrator (with 10 min Mach 8 capability) provides the largest feasible solution space of the trades examined (i.e. largest design margins, lowest technical risk) when compared to a kerosene-powered equivalent.

Probability distributions of wind speed in the UAE

For the evaluation of wind energy potential, probability density functions (pdfs) are usually used to describe wind speed distributions. The selection of the appropriate pdf reduces the wind power estimation error. The most widely used pdf for wind energy applications is the 2-parameter Weibull probability density function. In this study, a selection of pdfs are used to model hourly wind speed data recorded at 9 stations in the United Arab Emirates (UAE). Models used include parametric models, mixture models and one non-parametric model using the kernel density concept. A detailed comparison between these three approaches is carried out in the present work. The suitability of a distribution to fit the wind speed data is evaluated based on the log-likelihood, the coefficient of determination R2, the Chi-square statistic and the Kolmogorov–Smirnov statistic. Results indicate that, among the one-component parametric distributions, the Kappa and Generalized Gamma distributions provide generally the best fit to the wind speed data at all heights and for all stations. The Weibull was identified as the best 2-parameter distribution and performs better than some 3-parameter distributions such as the Generalized Extreme Value and 3-parameter Lognormal. For stations presenting a bimodal wind speed regime, mixture models or non-parametric models were found to be necessary to model adequately wind speeds. The two-component mixture distributions give a very good fit and are generally superior to non-parametric distributions.

S1620103 ナノ厚さ潤滑膜を介した固体二面間摺動時の接触状態と凝着・摩擦特性に関する研究

To achieve ultra-high recording density magnetic disk drives, it is essential to understand contact conditions and adhesion and friction properties between two sliding solid surfaces mediated by nanometer-thick lubricant films. With the aim of gaining such an understanding, we designed and developed a coaxial vertical illumination type microscope and integrated it with a previously developed pin-on-disk type tribotester. Measurements were performed with a semi-spherical glass sliding pin loaded on a magnetic disk covered by a 1.4-nm-thick perfluoropolyether Z03 film as a function of sliding speed. At sliding speeds lower than 2.09 mm/s, we observed that a liquid bridge formed between the sliding pin and disk surfaces, and it grew with time particularly in the direction of disk rotation. With increasing sliding speed, adhesion force decreased monotonically, whereas friction force decreased initially and then increased. We suggest that the sharp decrease of friction force in low sliding speed regime is associated with the sharp decrease of adhesion force, which is one component of the normal loading force, and the sharp decrease of adhesion force is due to the decrease of meniscus force caused by the increased difficulty in forming liquid bridge with increasing sliding speed.

High-performance Computing of Periodic Unsteady Flow Based on Time Spectral Method

The engineering application requires the solver of periodic unsteady flows to be of high efficiency and precision. Based on the developing Frequency-Domain methods, a fully implicit version of the Time Spectral Method (TSM) is established, the stability problem that occurs as the sampling number grows is improved. The TSM is extended toSST turbulence model, which improves the numerical accuracy and the ability of simulating complex flows. The periodic unsteady flows over a forced oscillating NACA0015 and Hypersonic Ballistic Shape (HBS) are numerically simulated. The computational results agree well with the experiment data at subsonic and hypersonic regime, validating the application of the TSM solver. Influences of the angle of attack and Ma on dynamic derivatives of the HBS are analyzed. Comparing with the Dual-Time-Step (DTS) method, the relative efficiency of the TSM improves as the Ma number increases. The TSM provides an order of magnitude improvement in computational efficiency over DTS without loss of generality at hypersonic speed regime.

Vehicle Routing Algorithms for Radially Escaping Targets

We introduce a novel dynamic vehicle routing problem termed the Radially Escaping Targets (RET) problem in which mobile targets appear uniformly randomly on a disk according to a stochastic process and move radially outward to escape the disk in a minimum amount of time. A single vehicle is assigned the task of intercepting the targets before they escape. We first obtain two fundamental upper bounds on the fraction of targets intercepted by the vehicle in the steady state---termed the capture fraction---for the RET problem. We then propose three policies to maximize the capture fraction for the RET problem, and identify parameter regimes in which they are suitable. All three policies are within a constant factor of the optimal in specific parameter regimes. For the asymptotic regime of low arrival rate, this factor is equal to one. For the asymptotic regime of high arrival rate, the factor is equal to 2.52 when the disk radius is greater than or equal to one. For moderate speed regimes, this factor is dep...

The Mechanochemistry of Damage and Terminal Ballistics

The asymmetric radial crack patterns that occur in brittle targets when impacted by high velocity projectiles are explained using a Phenomenological Mechanochemistry of Damage (PMD) engineering model. The developed model, which constitutes a simplification of the generalized PMD framework, reveals an energy instability during failure of brittle materials configured in a purely symmetric geometry and impact configuration. The underlying cause of the instability is due to the competition between stored elastic energy and the energy associated with new surface creation through broken chemical bonds. The instability manifests itself in the form of asymmetric radial cracking in the brittle target. The model is built upon the general PMD framework and assumes the target material is sufficiently brittle that strains are small and linear elasticity is applicable. Furthermore, the impact geometry is assumed to be purely symmetric, which leads to a reduction of the geometry to a one-dimensional radial configuration. The model is not restricted to any ballistic impact speed regime, provided the target material remains in the solid phase.

클러치 드래그 토크에 미치는 마찰재 면적 및 클리어런스의 영향

Dept. of Mechanical Engineering, Hannam University(Received October 12, 2014; Revised November 4, 2014; Accepted November 5, 2014)Abstract − The reduction of drag torque is an important research issue in terms of improving transmission effi-ciency. Drag torque in a wet clutch occurs because of the viscous drag generated by the transmission fluid in a narrowgap (clearance) between the friction plate and a separate plate. The objective of this paper is to observe the effects ofthe friction plate area and the clearance on the drag torque using finite element simulation. The two-phase flow of airand oil fluid is considered and modeled for the simulation. The simulation analysis reveals that as the rotational speedincreases, the drag torque generally increases to a critical point and then decreases sharply at a high speed regime. Theclearance between the two plates plays an important role in controlling drag torque peak. An increase in the clearancecauses a decrease in shear stress; thus, the drag torque also decreases according to Newton’s law of viscosity. An obser-vation of the effect of the area of contact between transmission fluid and friction plate shows that the drag torqueincreases with the contact area. The flow vectors inside the flow channel present clear evidence that the velocity of thefluid flows is faster with a larger friction plate, that is, in the case of a larger contact area. Therefore, the optimum sizeof the friction plate should be determined carefully, considering both the clutch performance and drag reduction. It isexpected that the results from this study can be very useful as a database for clutch design and to predict the drag torquefor the initial design with respect to various clutch parameters.Keywords − computational fluid dynamics (전산유체역학), drag torque (드래그 토크), two-phase flow (이상유동), volume of fluid method (유체 체적법), wet clutch (습식 클러치)

Investigation of self-starting and high rotor solidity on the performance of a three S1210 blade H-type Darrieus rotor

Energy is an essential ingredient for socio-economic development and economic growth of a country. Energy is available in two different forms, fast depleting or non-renewable (coal, fuel, natural gas) and renewable (solar, wind, hydro etc). Wind is one of the potential renewable energy sources due to its abundance in the atmosphere in different scales of high, medium and low ranges. Vertical axis wind turbine (VAWT) can be installed in low wind speed regime for performing various small-scale functions ranging from electrifying a built environment to pumping water especially in remote places where grid-connected electricity is a scarce. Amongst various VAWT rotors, H-type Darrieus rotor has become more popular in the built environment for their straight blade designs and simpler construction features. However, the major problem facing such VAWT rotor is their non-self-starting characteristics due to symmetrical blade designs. Replacing VAWT's conventional blades with unsymmetrical blades and increasing rotor solidity could make potential solution to the above problem. However, there is still hardly any quantitative measure of the self-starting, torque, power coefficient etc. with increased rotor solidity so as to obtain some performance insights of high solidity unsymmetrical blade H-Darrieus rotor in low wind speed condition. In this paper a three-bladed H-type Darrieus rotor equipped with unsymmetrical S1210 blades is investigated first for its self-starting characteristics with different rotor solidities (from 0.8 to 1.2) at various azimuthal positions. Then the power coefficients (Cp) are evaluated for these solidities at various wind speeds. It will be shown that high blade solidity is in fact desirable for overall better performance of the rotor. There is an optimum rotor solidity at which power coefficient is the highest. And the maximum Cp of 0.32 is obtained for rotor solidity 1.0 and wind speed 5.7 m/s. The results are compared with some other symmetrical/unsymmetrical blade H-Darrieus rotors. Though the operating range is reduced but, for higher static and dynamic torque and comparable power coefficient with respect to existing rotors, the present rotor could be used for various small-scale applications especially that require high torque like pumping, grinding etc.

Computational analysis of flow physics of a combined three bladed Darrieus Savonius wind rotor

A 3D unsteady computational Fluid Dynamics (CFD) model is utilized to investigate the aerodynamics of a combined three-bladed Darrieus Savonius wind rotor. Unsteady Reynolds-averaged Navier–Stokes (RANS) equations of the CFD software are modelled to obtain the fluid flow distributions. In this, initial unsteady effects of the flow interactions are covered to reach to the steady state condition at which the flow physics is analysed. The fluid zone has a large stationary domain and rotating sub-domain connected by a sliding mesh interface. The 3D wake structure of the combined rotor is analysed to understand the physical processes responsible for its power production in low wind speed regime. Steep Coanda like vortices are observed in the rotor wake that migrates from the downstream of Savonius rotor on to the downstream of Darrieus rotor. This effect results into compression of vortices on the downstream of Darrieus rotor thereby pushing the rotor in its rotation, hence augment power production. Presence of attached stall vortices on the Darrieus blades and increased concentration of vortices on the downstream of Darrieus rotor justifies the design of Darrieus rotor on top of Savonius rotor, which could be reflected in high value of power coefficient of the rotor. The outcome of this research would improve the viability of such combined rotor for its use in the built environment.

Bubble evolution and properties in homogeneous nucleation simulations.

We analyze the properties of naturally formed nanobubbles in Lennard-Jones molecular dynamics simulations of liquid-to-vapor nucleation in the boiling and the cavitation regimes. The large computational volumes provide a realistic environment at unchanging average temperature and liquid pressure, which allows us to accurately measure properties of bubbles from their inception as stable, critically sized bubbles, to their continued growth into the constant speed regime. Bubble gas densities are up to 50% lower than the equilibrium vapor densities at the liquid temperature, yet quite close to the gas equilibrium density at the lower gas temperatures measured in the simulations: The latent heat of transformation results in bubble gas temperatures up to 25% below those of the surrounding bulk liquid. In the case of rapid bubble growth-typical for the cavitation regime-compression of the liquid outside the bubble leads to local temperature increases of up to 5%, likely significant enough to alter the surface tension as well as the local viscosity. The liquid-vapor bubble interface is thinner than expected from planar coexistence simulations by up to 50%. Bubbles near the critical size are extremely nonspherical, yet they quickly become spherical as they grow. The Rayleigh-Plesset description of bubble-growth gives good agreement in the cavitation regime.

EXPERIMENTS WITH STEPPED PLANING HULLS FOR SPECIAL OPERATIONS CRAFT

Many of today’s special operations craft and high-speed patrol boats can operate at volume Froude numbers high enough to justify the use of a stepped planing hull; however most of these boats are un-stepped. This paper describes experiments to determine the effect of adding a single step into the bottom of a planing hull with loading parameters consistent with modern special operations craft. In contrast to multi-step pleasure craft, which can operate at volumetric Froude numbers of 10, special operations craft often operate at volumetric Froude numbers of around 5, owing to the increased payload and decreased speed. Previous towing tests on a variety of stepped hull configurations indicated that a promising configuration for special operations craft may be a single step located near the transom (alternatively known as a hydrodynamic transom forward of the stern). The present experiments investigate this configuration further, by testing a single-step hull, with step located at 25% of the length forward of the transom. The step height is systematically varied to observe the effect on resistance, trim, wetted surface area and porpoising stability over a wide range of speeds. Of the configurations tested, the best reduced model resistance by 25% at the highest speeds tested, while increasing resistance by 10% in the hump speed regime. The stepped hulls tested had porpoising limits similar to conventional planing monohulls. A short method is provided to illustrate when a stepped hull may be advantageous for a given design.

Crack Branching Characteristics at Different Propagation Speeds: From Quasi-Static to Supersonic Regime

Classical dynamic fracture mechanics predicts that the crack branching occurs when crack propagation speed exceeds a subsonic critical velocity. In this paper, we performed simulations on the dynamic fracture behaviors of idealized discrete mass–spring systems. It is interesting to note that a crack does not branch when traveling at supersonic speed, which is consistent with others' experimental observations. The mechanism for the characteristics of crack branching at different propagation speeds is studied by numerical and theoretical analysis. It is found that for all different speed regimes, the maximum circumferential stress near the crack tip determines the crack branching behaviors.

Continuous robust control for aeroelastic vibration control of a 2-D airfoil under unsteady flow

In this paper, a particular class of 2-D aeroelastic systems accounting for structural nonlinearities in pitching displacement and operating in an unsteady aerodynamic incompressible flowfield description is considered. By using the flap hinge torque of a trailing-edge flap surface as the control input, a continuous controller is proposed to suppress the aeroelastic vibrations of the wing section model. The control design based on the choice of the pitching angle as the output variable yields a semi-global asymptotic stability result. Furthermore, the system is theoretically shown to be robust to external disturbances. Numerical simulation results verify the efficacy of the proposed control strategy toward suppressing aeroelastic vibration in both pre- and post-flutter flight speed regimes under a multitude of external disturbances.

Response Time to Sudden Changes in Speed and Load Regimes for Turbocharged Diesel Engine

The paper includes results from experimental determinations of parameters for turbocharged MB836Db diesel engine, which functioning at dynamic conditions. The studied functioning modes resulting from sudden changes in acceleration and/or loading. The authors used for experimental determinations an railway Diesel engine turbocharged, MB836Db. The results are obtained based on measurements made in the department of Thermal Systems and Environmental Engineering from University “Dunarea de Jos” of Galati. Authors used the equipment which contains PLC module CBM 500, which is put on the turbocharged MB836Db engine. Experimental measurements were performed for dynamic regimes (regimes varying in time) , in case of four changes for engine's acceleration and load.In conditions of acceleration (fast increase of rotation and/or load) only one part of exhaust gasses's energy is transformed in mechanical work for turbocompressor; the rest of the energy is used for the acceleration of different parts which are in rotation movement, to overcome the inertia of the turbocharger with free rotation. That is why, the pressure delivered by the turbocharger in case of functioning on unsteady conditions, at any moment is decreased than the supercharger pressure in steady working conditions, at the same speed and load regimes.The experimental results obtained for unsteady working conditions shows that the performances of engine and turbocompressor are reduced compared to the performances in stationary operating conditions, and this because the turbocompressor inertia is greater than the inertia of the injection system. Because of this issue is modified significantly the combustible mixture quality (decrease in the ratio of air and fuel ), which determines the increase of chemical pollution and decrease of engine performances.The authors presents some conclusions on the influence of the turbocharger’s response on the performances of engine in case of regimes varying in time, such as changes in acceleration and load.

Formation of protein and protein-gold nanoparticle stabilized microbubbles by pressurized gyration.

A one-pot single-step novel process has been developed to form microbubbles up to 250 μm in diameter using a pressurized rotating device. The microbubble diameter is shown to be a function of rotational speed and working pressure of the processing system, and a modified Rayleigh-Plesset equation has been derived to explain the bubble-forming mechanism. A parametric plot is constructed to identify a rotating speed and working pressure regime, which allows for continuous bubbling. Bare protein (lysozyme) microbubbles generated in this way exhibit a morphological change, resulting in microcapsules over a period of time. Microbubbles prepared with gold nanoparticles at the bubble surface showed greater stability over a time period and retained the same morphology. The functionalization of microbubbles with gold nanoparticles also rendered optical tunability and has promising applications in imaging, biosensing, and diagnostics.

A flow pattern that sustains an edge flame in a straining mixing layer with finite thermal expansion

The role of thermal expansion in diffusion edge flames is investigated numerically in the strained mixing layer configuration. We are able to access both the positive as well as the negative edge flame speed regime when density variation, and therefore full hydrodynamic coupling, is present. The computational approach employs a homotopy method to gradually map the solutions from the computationally simpler constant-density flow to the more challenging variable-density case. Particular attention is paid to the role of boundary conditions and how they, in turn, can induce an undesirable streamwise pressure gradient in the trailing diffusion flame that affects the edge flame speed. A new approach is designed to eliminate this adverse pressure gradient. Previous studies observe that the ratio of the edge flame speed to the premixed stoichiometric laminar flame velocity scales approximately as the square root of the ratio of the cold stream density to the stoichiometric density (which is lower). At least for the small set of parameters investigated here, it is found that the speedup of the normalized edge flame velocity might be superlinear on the density ratio, a fact we attribute to the lack of pressure gradient behind the edge flame. This result is new and complements previous results, for different boundary conditions, which strongly suggest that the edge flame speed is a strong function of the particular hydrodynamic boundary conditions employed in the simulations.

Experimental Investigation of Trimaran Models in Shallow Water

Experiments were conducted at the U.S. Naval Academy's Hydromechanics Laboratory to determine the effect of finite water depth on the resistance, heave, and trim of two different trimaran models. The models were tested at the same length to water depth ratios over a range of Froude numbers in the displacement speed regime. The models were also towed in deep water for comparison. Additionally, the side hulls were adjusted to two different longitudinal positions to investigate possible differences resulting from position. Near critical speed, a large increase in resistance and sinkage was observed, consistent with observations of conventional displacement hulls. The data from the two models are scaled up to a notional 125-m length to illustrate the effects that would be observed for actual ships similar in size to the U.S. Navy's Independence Class Littoral Combat Ship. Faired plots are developed to allow for rapid estimation of shallow water effect on trimaran resistance and under keel clearance. An example is provided.

Experimental Investigation of Trimaran Models in Shallow Water

Experiments were conducted at the U.S. Naval Academy's Hydromechanics Laboratory to determine the effect of finite water depth on the resistance, heave, and trim of two different trimaran models. The models were tested at the same length to water depth ratios over a range of Froude numbers in the displacement speed regime. The models were also towed in deep water for comparison. Additionally, the side hulls were adjusted to two different longitudinal positions to investigate possible differences resulting from position. Near critical speed, a large increase in resistance and sinkage was observed, consistent with observations of conventional displacement hulls. The data from the two models are scaled up to a notional 125-m length to illustrate the effects that would be observed for actual ships similar in size to the U.S. Navy's Independence Class Littoral Combat Ship. Faired plots are developed to allow for rapid estimation of shallow water effect on trimaran resistance and under keel clearance. An example is provided.

재진입 비행체 외부 열유동장의 고온반응기체 효과에 관한 전산해석

Aerothermodynamic characteristics of re-entry vehicles in hypersonic speed regimes are investigated by applying CFD methods based on the Navier-Stokes-Fourier equations. A special emphasis is placed on the effects of high temperature chemically reacting gases on shock stand-off distance and thermal characteristics of the flowfields. A ten species model is used for describing the kinetic mechanism for high temperature air. In particular, the hypersonic flows around a cylinder are computed with and without chemically reacting effects. It is shown that, when the chemically reacting effects are taken into account, the shock stand-off distance and temperature are significantly reduced.