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991.
The majority of aquatic vertebrates are suction feeders: by rapidly expanding the mouth cavity they generate a fluid flow
outside of their head in order to draw prey into their mouth. In addition to the biological relevance, the generated flow
field is interesting fluid mechanically as it incorporates high velocities, is localized in front of the mouth, and is unsteady,
typically lasting between 10 and 50 ms. Using manometry and high-speed particle image velocimetry, this is the first study
to quantify pressure within and outside the mouth of a feeding fish while simultaneously measuring the velocity field outside
the mouth. Measurements with a high temporal (2 ms) and spatial (<1 mm) resolution were made for several feeding events of
a single largemouth bass (Micropterus salmoides). General properties of the flow were evaluated, including the transient velocity field, its relationship to pressure within
the mouth and pressure at the prey. We find that throughout the feeding event a relationship exists for the magnitude of fluid
speed as a function of distance from the predator mouth that is based on scaling the velocity field according to the size
of the mouth opening and the magnitude of fluid speed at the mouth. The velocity field is concentrated within an area extending
approximately one mouth diameter from the fish and the generated pressure field is even more local to the mouth aperture.
Although peak suction pressures measured inside the mouth were slightly larger than those that were predicted using the equations
of motion, we find that these equations give a very accurate prediction of the timing of peak pressure, so long as the unsteady
nature of the flow is included. 相似文献
992.
The interface thermal contact resistance between the copper and stainless steel thin disk is investigated using the non-contact
photo-thermal technique. The diffusivity and the interface thermal contact resistance were estimated using the phase sensitivity
measurement over a temperature range from 20 to 300 K, at 1.20, 2.38 and 4.28 MPa contact pressure, respectively. Results
show that the interface thermal contact resistance decreases with the rise of the temperature and contact pressure. 相似文献
993.
994.
A one-dimensional approximate analytical model, which preserves the main features of soil-crop-atmospheric hydrodynamics,
has been suggested for plant roots of low soil-root conductivity ratio (SRCR). The proposed approach involves physically based
concepts, such as mass balance equation, Darcy’s law, and related water uptake and plant transpiration functions. Two main
assumptions have been made to derive the analytical solution: (1) gravitational flow is adopted and (2) the uniform soil moisture
distribution within the root water activity zone is supposed. The mass balance equation in its integral form is solved by
the method of characteristics. This leads to the two functional equations for soil pressure head and root potential, which
can be solved simultaneously by using common software. The model has been further verified against the numerical one. The
model represents a reasonable compromise between the complicated mechanism of unsaturated water flow with root water uptake
(RWU) and still insufficient knowledge of the soil-plant-atmospheric continuum. It is able to account for temporal fluctuations
in root activity zone and provides a relatively simple algorithm for investigation of RWU-mechanism. Besides the theoretical
and applicative importance, this flow model yields water and velocity distributions within soil profile, and, thereby, constitutes
a preliminary step toward solution of contaminant transport problems in vadose zone.
Deceased 相似文献
995.
Ratnesh K. Shukla Jeff. D. Eldredge 《Theoretical and Computational Fluid Dynamics》2007,21(5):343-368
An inviscid vortex sheet model is developed in order to study the unsteady separated flow past a two-dimensional deforming
body which moves with a prescribed motion in an otherwise quiescent fluid. Following Jones (J Fluid Mech 496, 405–441, 2003)
the flow is assumed to comprise of a bound vortex sheet attached to the body and two separate vortex sheets originating at
the edges. The complex conjugate velocity potential is expressed explicitly in terms of the bound vortex sheet strength and
the edge circulations through a boundary integral representation. It is shown that Kelvin’s circulation theorem, along with
the conditions of continuity of the normal velocity across the body and the boundedness of the velocity field, yields a coupled
system of equations for the unknown bound vortex sheet strength and the edge circulations. A general numerical treatment is
developed for the singular principal value integrals arising in the solution procedure. The model is validated against the
results of Jones (J Fluid Mech 496, 405–441, 2003) for computations involving a rigid flat plate and is subsequently applied
to the flapping foil experiments of Heathcote et al. (AIAA J, 42, 2196–2204, 2004) in order to predict the thrust coefficient.
The utility of the model in simulating aquatic locomotion is also demonstrated, with vortex shedding suppressed at the leading
edge of the swimming body.
相似文献
996.
The accuracy of turbulent kinetic energy (TKE) dissipation rate measured by PIV is studied. The critical issue for PIV-based
dissipation measurements is the strong dependency on the spatial resolution, Δx, as reported by Saarenrinne and Piirto (Exp Fluids Suppl:S300–S307, 2000). When the PIV spacing is larger than the Kolmogorov scale, η, the dissipation is underestimated because the small scale
fluctuations are filtered. For the case of Δx smaller than the Kolmogorov scale, the error rapidly increases due to noise. We introduce a correction method to eliminate
the dominant error for the small Δx case. The correction method is validated by using a novel PIV benchmark, random Oseen vortices synthetic image test (ROST),
in which quasi-turbulence is generated by randomly superposing multiple Oseen vortices. The error of the measured dissipation
can be more than 1,000% of the analytical dissipation for the small Δx case, while the dissipation rate is underestimated for the large Δx case. Though the correction method does not correct the underestimate due to the low resolution, the dissipation was accurately
obtained within a few percent of the true value by using the correction method for the optimal resolution of η/10 < Δx < η/2. 相似文献
997.
The boundary layer structure of oscillatory shallow open channel flows has been studied in a wide flume. Fluorescence solution
was released at a porous rough bed through a diffuser covered by gravel of 0.5 cm grain size. A planar laser-induced fluorescence
(PLIF) system was used to visualise the dye plumes in both vertical and horizontal planes for a qualitative understanding
of the roles of large-scale flow structures in mass transport. A variety of tests were conducted for a range of oscillatory
periods (30–240 s), water depths (3–16 cm) and velocity amplitudes (0.027–0.325 m/s), which cover a wide range of oscillatory
flows with Reynolds numbers Re
a
varied from 0.3 × 104 (laminar) to 2.1 × 106 (fully turbulent). For quantitative investigation, a novel technique, namely combined laser-induced fluorescence (LIF) and
2D laser Doppler velocimetry (LDV) (LIF/LDV), was developed and used to measure the velocity and solute concentration simultaneously
in a vertical plane over 50 cycles. From the dye plumes revealed by the PLIF in transitional flows, there are different patterns
of flow structure and solute transport with three representative stages of acceleration, deceleration and flow reversal. In
the acceleration stage, turbulence was suppressed with dye layers adhering to the surface with little vertical mass transport.
In the deceleration stage, flame-like turbulent structures occurred when turbulence generation was prominent. This was investigated
quantitatively by recording the percentage occurrence of the adhered smooth layers per cycle. For those smooth bed cases with
Re
a
< 1.8 × 105, the adhered smooth dye layers type of boundary layer occupied 100% of the oscillation period. Over a sufficiently high Re
a
, a rough bed can generate fully turbulent oscillatory flows without the appearance of adhering dye layers. Between these
two extremes, a transitional flow regime occurs in a wide range of flow conditions: Re
a
> 2.7 × 104 over the rough bed and Re
a
> 8.3 × 106 over a smooth bed. 相似文献
998.
CFD Calculation of Turbulent Flow with Arbitrary Wall Roughness 总被引:1,自引:0,他引:1
David Apsley 《Flow, Turbulence and Combustion》2007,78(2):153-175
999.
Gérard L. Vignoles Pierre Charrier Christophe Preux Bruno Dubroca 《Transport in Porous Media》2008,75(3):295-317
Viscous flow, effusion, and thermal transpiration are the main gas transport modalities for a rarefied gas in a macro-porous
medium. They have been well quantified only in the case of simple geometries. This paper presents a numerical method based
on the homogenization of kinetic equations producing effective transport properties (permeability, Knudsen diffusivity, thermal
transpiration ratio) in any porous medium sample, as described, e.g. by a digitized 3D image. The homogenization procedure—neglecting
the effect of gas density gradients on heat transfer through the solid—leads to closure problems in for the obtention of effective properties; they are then simplified using a Galerkin method based on a 21-element basis set.
The kinetic equations are then discretized in space with a finite- volume scheme. The method is validated against experimental data in the case of a closed test tube.
It shows to be coherent with past approaches of thermal transpiration. Then, it is applied to several 3D images of increasing
complexity. Another validation is brought by comparison with other distinct numerical approaches for the evaluation of the
Darcian permeability tensor and of the Knudsen diffusion tensor. Results show that thermal transpiration has to be described
by an effective transport tensor which is distinct from the other tensors. 相似文献
1000.
Juan Dávila Manuel del Pino Monica Musso 《Archive for Rational Mechanics and Analysis》2011,200(1):89-140
In a bounded domain \({\Omega \subset \mathbb R^2}\) with smooth boundary we consider the problemwhere ν is the unit normal exterior vector, ε > 0 is a small parameter and f is a bistable nonlinearity such as f(u) = sin(π u) or f(u) = (1 ? u 2)u. We construct solutions that develop multiple transitions from ?1 to 1 and vice-versa along a connected component of the boundary ?Ω. We also construct an explicit solution when Ω is a disk and f(u) = sin(π u).
相似文献
$\Delta u = 0 \quad {\rm{in }}\, \Omega, \qquad \frac{\partial u}{\partial \nu} = \frac1\varepsilon f(u) \quad {\rm{on }}\,\partial\Omega,$