Begell House Inc.
International Journal of Fluid Mechanics Research
FMR
2152-5102
38
4
2011
A Vortex Plate Theory of Hovering Animal Flight
291-311
Khaled M. S.
Faqih
Information Systems Department, Al al-Bayt University, Jordan
A mathematical model of vortex structure generated during animal hovering flight is adopted to determine the induced power requirement. The wake structure is modelled by a series of equispaced rigid rectangular vortex plates, positioned horizontally and moving vertically downwards with identical speeds; each plate is generated during powering of the functionally wing stroke. These plates are assumed to remain undistorted during their motion and act as rigid surfaces. The flow around the far wake is assumed to be identical to that of the flow about an infinite series of parallel vortex plates moving normal to themselves with constant speed. The vortex representation of the wake considered in the current theory allows a considerable loss of momentum to occur. This loss is approximated by Prandlt’s tip theory. The boundary conditions of this disjointed set of vortex plates determine their velocity potential which contribute to the calculation of the induced power requirement. The current theory is based on the assumption that the impulse associated with the vortex plate is adequate to support the animal’s weight for the duration of the wing stroke period. It is determined that the ratio of the initial vortex plate area Ai to wing swept area Ad is equal to the normal spacing parameter f; which can be related to the hovering parameter K by equating the impulse of the vortex sheet to the vortex plate impulse; K depends upon the animal’s morphology and the kinematics characteristic of the wing stroke. The classical model for induced power estimate is the actuator-disk model which renders the advantage of simplicity. This type of modelling does not precisely correspond to actual events that occur in animal flight. However, the current approach accords well with the nature of the wingbeat since it considers the unsteadiness in the wake as an important fluid dynamical characteristic. Induced power in hovering is calculated as the aerodynamic power required to generate the vortex wake system since this wake is primarily the main physical product of hovering action. Specific mean induced power to mean wing tip velocity ratio is determined by solely the normal spacing parameter f for a given wing stroke amplitude. The current theory gives much higher specific induced power estimate than anticipated by classical methods.
Marangoni Convection inWater-Isopropanol Mixtures in the Presence of Soret Effect
312-327
Mohammad A
Rahman
Adjunct Professor
M. Ziad
Saghir
Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria St., Toronto, ON M5B2K3, Canada
In the present study, the onset of Marangoni convection in a liquid layer overlaying a porous layer where the whole system is being laterally heated is investigated. The non-linear two-dimensional Navier Stokes equations, the energy equation, the mass balance equation and the continuity equation are solved for the liquid layer. The Brinkman model is used for the porous layer. The partial differential equations are solved numerically by using the finite element technique. Two different cases are analyzed in this paper. In the first case, the Marangoni convection in the absence of the Soret effect is studied. Results revealed when the liquid layer and the porous layer have identical thickness, the convective flow remains in the liquid layer and this is independent of changing the aspect ratios (length-to-height). However, the flow covers the entire cavity when the liquid layer thickness is very small (less than 1/10 of the porous layer). In the case of Marangoni convection in the presence of Soret effect, it has been found that the convective flow remains in the liquid layer even when the liquid layer thickness is very small.
Large Eddy Simulation of a Methane Diffusion Flame: The effect of the Chemical Mechanism on NOx Emissions
328-345
Balram
Panjwani
Department of Energy and Process Engineering, Norwegian University of Science and Technology, NTNU, Norway
Ivar S.
Ertesvag
Department of Energy and Process Engineering, Norwegian University of Science and Technology, N-7462 Trondheim, Norway
Kjell Erik
Rian
Computational Industry Technologies (ComputIT) N-7462 Trondheim, Norway
The accurate prediction of the mass fraction of NOx and OH in turbulent combustion is one of the challenging problems. A large eddy simulation (LES) of a CH4/H2/N2 diffusion flame "DLR Flame A" was carried out at a Reynolds number of 15200, and special emphasis was placed on NOx predictions. A steady state flamelet model was used for combustion closure model. However, the steady state flamelet model is not appropriate for the prediction of NOx. In the present study, a transport equation for NOx was solved, and the source term was estimated from the flamelet tables. In LES, the inflow boundary conditions influence the entire flow field, and the effects of the boundary conditions become more important during combustion. The effect of inflow boundary conditions was studied, and the results were quantified in terms of the nozzle diameter. NOx predictions are dependent on the chemical mechanism; thus, the GRI-Mech 3.0, GRI-Mech 2.11 and San Diego mechanism were studied. The results of the flamelet model were in good agreement for the temperature and major species for all the reaction mechanisms. However, for NOx, the San Diego mechanism performed better than the other reaction mechanisms. The results of the present study showed that the steady flamelet model could accurately predict kinetically controlled reactions, such as the formation of NOx.
A New Approach for Determining the Momentum Thickness in Laminar Boundary Layers
346-352
Hossein
Hamidifar
Department of Irrigation and Reclamation Engineering, University of Tehran, Karaj, Iran
In this paper, a new approach is proposed to determine the momentum thickness within laminar boundary layers over a flat plate. A new coefficient, Cm, is introduced in which relates the momentum thickness to the distance from the edge of the plate. This coefficient is independent from the distance from the edge and could be determined easily using a proposed graph which relates the Cm to the upstream flow velocity and the fluid temperature. The results of the sensitivity analysis showed that as the upstream flow velocity increases, the Cm coefficient is less dependent to the velocity. Also, an illustrative example is solved which indicates the benefits of the proposed approach in compare with the traditional methods as Blasius solution.
Explicit Solutions for Steady Three-Dimensional Problem of Condensation Film on Inclined Rotating Disk
353-365
Hamed
Shahmohamadi
University of California, Los Angeles
Mohammad Mehdi
Rashidi
Tongji University
The similarity transform for the steady three-dimensional problem of condensation film on an inclined rotating disk gives a system of nonlinear ordinary differential equations which is analytically solved by applying a newly developed method namely the variational iteration method (VIM). The velocity and temperature profiles are shown and the influence of Prandtl number on the heat transfer and Nusselt number is discussed in detail. The validity of our solutions is verified by the numerical results.
Wave Propagation in Turbulent Sea Water
366-386
D.
Leandri
Laboratoire des Systémes Navals Complexes, Toulon, France
Y.
Lacroix
Laboratoire des Systémes Navals Complexes, Toulon, France
V. I.
Nikishov
Institute of Hydromechanics of National Academy of Sciences of Ukraine 8/4, Zhelyabov St., 03680, Kyiv-180, MSP, Ukraine
Characteristics of light wave propagating in a turbulent sea water are determined by fluctuation spectrum of optical refractive index n. The behavior of the spectrum is controlled by fluctuations of temperature and salinity. Acoustic scattering from oceanic microstructure is due to sound speed and density fluctuations that in turn also depend on the temperature and salinity fluctuations. The work gives a short description of the spectrum model of fluctuations of optical refractive index depending on dissipation rates of component fluctuations, as well as the dissipation rates of turbulent energy, which describes both inertial-convective and viscous-convective subranges. Others values, like n (density, conductivity, sound speed), can be also considered. Some statistical characteristics of the light wave for real distributions of T and S in depth were calculated by the methods of geometric optics and smooth perturbations using the developed spectrum model. It was shown that the above spectrum peculiarities lead to considerable changes of statistical characteristics of propagating waves, in particular, level fluctuations, correlation function of the level fluctuation, structural function of the phase.