6,184 research outputs found
Logarithmic temperature profiles in the ultimate regime of thermal convection
We report on the theory of logarithmic temperature profiles in very strongly
developed thermal convection in the geometry of a Rayleigh-Benard cell with
aspect ratio one and discuss the degree of agreement with the recently measured
profiles in the ultimate state of very large Rayleigh number flow. The
parameters of the log-profile are calculated and compared with the measure
ones. Their physical interpretation as well as their dependence on the radial
position are discussed.Comment: 14 pages, no figur
Velocity profiles in strongly turbulent Taylor-Couette flow
We derive the velocity profiles in strongly turbulent Taylor-Couette flow for
the general case of independently rotating cylinders. The theory is based on
the Navier-Stokes equations in the appropriate (cylinder) geometry. In
particular, we derive the axial and the angular velocity profiles as functions
of distance from the cylinder walls and find that both follow a logarithmic
profile, with downwards-bending curvature corrections, which are more
pronounced for the angular velocity profile as compared to the axial velocity
profile, and which strongly increase with decreasing ratio between inner
and outer cylinder radius. In contrast, the azimuthal velocity does not follow
a log-law. We then compare the angular and azimuthal velocity profiles with the
recently measured profiles in the ultimate state of (very) large Taylor
numbers. Though the {\em qualitative} trends are the same -- down-bending for
large wall distances and (properly shifted and non-dimensionalized) angular
velocity profile being closer to a log-law than (properly shifted
and non-dimensionalized) azimuthal velocity profile -- {\em
quantitative} deviations are found for large wall distances. We attribute these
differences to the Taylor rolls and the height dependence of the profiles,
neither of which are considered in the theoretical approach
Motion and wake structure of spherical particles
This paper presents results from a flow visualization study of the wake
structures behind solid spheres rising or falling freely in liquids under the
action of gravity. These show remarkable differences to the wake structures
observed behind spheres held fixed. The two parameters controlling the rise or
fall velocity (i.e., the Reynolds number) are the density ratio between sphere
and liquid and the Galileo number.Comment: 9 pages, 8 figures. Higher resolution on demand. To appear in
Nonlinearity January 200
Extension Education About Healthy Weight: A Case Study Emphasizes Need to Find the Target Audience
Developing educational materials about weight management requires accurate content and sound phrasing. However, our recent experience with healthy weight education using traditional Extension recruitment strategies revealed that attention to reaching the target audience is vital when resource management and educational impact are considered. Delivery of a theory-based healthy weight educational program that missed its mark (reaching fewer than 30% of the intended learners) for Extension audiences serves as a basis to call for examining Extension recruitment practices and allocation of program resources for weight education
Fractal dimension crossovers in turbulent passive scalar signals
The fractal dimension of turbulent passive scalar signals is
calculated from the fluid dynamical equation. depends on the
scale. For small Prandtl (or Schmidt) number one gets two ranges,
for small scale r and =5/3 for large r, both
as expected. But for large one gets a third, intermediate range in
which the signal is extremely wrinkled and has . In that
range the passive scalar structure function has a plateau. We
calculate the -dependence of the crossovers. Comparison with a numerical
reduced wave vector set calculation gives good agreement with our predictions.Comment: 7 pages, Revtex, 3 figures (postscript file on request
Drop deformation by laser-pulse impact
A free-falling absorbing liquid drop hit by a nanosecond laser-pulse
experiences a strong recoil-pressure kick. As a consequence, the drop propels
forward and deforms into a thin sheet which eventually fragments. We study how
the drop deformation depends on the pulse shape and drop properties. We first
derive the velocity field inside the drop on the timescale of the pressure
pulse, when the drop is still spherical. This yields the kinetic-energy
partition inside the drop, which precisely measures the deformation rate with
respect to the propulsion rate, before surface tension comes into play. On the
timescale where surface tension is important the drop has evolved into a thin
sheet. Its expansion dynamics is described with a slender-slope model, which
uses the impulsive energy-partition as an initial condition. Completed with
boundary integral simulations, this two-stage model explains the entire drop
dynamics and its dependance on the pulse shape: for a given propulsion, a
tightly focused pulse results in a thin curved sheet which maximizes the
lateral expansion, while a uniform illumination yields a smaller expansion but
a flat symmetric sheet, in good agreement with experimental observations.Comment: submitted to J. Fluid Mec
Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow
We investigate the existence of multiple turbulent states in highly turbulent
Taylor-Couette flow in the range of to ,
by measuring the global torques and the local velocities while probing the
phase space spanned by the rotation rates of the inner and outer cylinder. The
multiple states are found to be very robust and are expected to persist beyond
. The rotation ratio is the parameter that most strongly
controls the transitions between the flow states; the transitional values only
weakly depend on the Taylor number. However, complex paths in the phase space
are necessary to unlock the full region of multiple states. Lastly, by mapping
the flow structures for various rotation ratios in a Taylor-Couette setup with
an equal radius ratio but a larger aspect ratio than before, multiple states
were again observed. Here, they are characterized by even richer roll structure
phenomena, including, for the first time observed in highly turbulent TC flow,
an antisymmetrical roll state.Comment: 9 pages, 7 figure
Multifocal Renal Cell Carcinoma: Clinicopathologic Features and Outcomes for Tumors ≤4 cm
A significant increase in the incidental detection of small renal tumors has been observed with the routine use of cross-sectional abdominal imaging. However, the proportion of small renal tumors associated with multifocal RCC has yet to be established. Here then, we report our experience with the treatment of multifocal RCC in which the primary tumor was ≤4 cm. In our series of 1113 RCC patients, 5.4% (60/1113) had multifocal disease at the time of nephrectomy. Discordant histology was present in 17% (10/60) of patients with multifocal RCC. Nephron sparing surgery was utilized more frequently in patients with solitary tumors. Overall, cancer-specific, and distant metastasis-free survival appeared to be similar between multifocal and solitary tumors. These findings are consistent with previous series which evaluated multifocal RCC with tumors >4 cm. With the known incidence of multifocality RCC, careful inspection of the entire renal unit should be performed when performing nephron sparing surgery
Nanometer-Resolved Collective Micromeniscus Oscillations through Optical Diffraction
We study the dynamics of periodic arrays of micrometer-sized liquid-gas
menisci formed at superhydrophobic surfaces immersed into water. By measuring
the intensity of optical diffraction peaks in real time we are able to resolve
nanometer scale oscillations of the menisci with sub-microsecond time
resolution. Upon driving the system with an ultrasound field at variable
frequency we observe a pronounced resonance at a few hundred kHz, depending on
the exact geometry. Modeling the system using the unsteady Stokes equation, we
find that this low resonance frequency is caused by a collective mode of the
acoustically coupled oscillating menisci.Comment: 4 pages, 5 figure
Drop Shaping by Laser-Pulse Impact
We show how the deposition of laser energy induces propulsion and strong
deformation of an absorbing liquid body. Combining high speed with stroboscopic
imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule
nanosecond laser pulse propels forward at several meters per second and deforms
until it eventually fragments. The drop motion results from the recoil momentum
imparted at the drop surface by water vaporization. We measure the propulsion
speed and the time-deformation law of the drop, complemented by
boundary-integral simulations. The drop propulsion and shaping are explained in
terms of the laser-pulse energy, the drop size, and the liquid properties.
These findings are, for instance, crucial for the generation of extreme
ultraviolet light in nanolithography machines.Comment: Submitted as research article to Physical Review Applied, 6 pages
with 6 figure
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