13,022 research outputs found
The effect of wall cooling on a compressible turbulent boundary layer
Experimental results are presented for two turbulent boundary-layer experiments conducted at a free-stream Mach number of 4 with wall cooling. The first experiment examines a constant-temperature cold-wall boundary layer subjected to adverse and favourable pressure gradients. It is shown that the boundary-layer data display good agreement with Coles’ general composite boundary-layer profile using Van Driest's transformation. Further, the pressure-gradient parameter β_K found in previous studies to correlate adiabatic high-speed data with low-speed data also correlates the present cooled-wall high-speed data. The second experiment treats the response of a constant-pressure high-speed boundary layer to a near step change in wall temperature. It is found that the growth rate of the thermal boundary layer within the existing turbulent boundary layer varies considerably depending upon the direction of the wall temperature change. For the case of an initially cooled boundary layer flowing onto a wall near the recovery temperature, it is found that δ_T ~ x whereas the case of an adiabatic boundary layer flowing onto a cooled wall gives δ_T ~ x^½. The apparent origin of the thermal boundary layer also changes considerably, which is accounted for by the variation in sublayer thicknesses and growth rates within the sublayer
Finite disturbance effect on the stability of a laminar incompressible wake behind a flat plate
An integral method is used to investigate the interaction between a two-dimensional, single frequency finite amplitude disturbance in a laminar, incompressible wake behind a flat plate at zero incidence. The mean flow is assumed to be a non-parallel flow characterized by a few shape parameters. Distribution of the fluctuation across the wake is obtained as functions of those mean flow parameters by solving the inviscid Rayleigh equation using the local mean flow. The variations of the fluctuation amplitude and of the shape parameters for the mean flow are then obtained by solving a set of ordinary differential equations derived from the momentum and energy integral equations. The interaction between the mean flow and the fluctuation through Reynolds stresses plays an important role in the present formulation, and the theoretical results show good agreement with the measurements of Sato & Kuriki (1961)
An experiment on the adiabatic compressible turbulent boundary layer in adverse and favourable pressure gradients
A wind-tunnel model was developed to study the two-dimensional turbulent boundary layer in adverse and favourable pressure gradients with out the effects of streamwise surface curvature. Experiments were performed at Mach 4 with an adiabatic wall, and mean flow measurements within the boundary layer were obtained. The data, when viewed in the velocity transformation suggested by Van Driest, show good general agreement with the composite boundary-layer profile developed for the low-speed turbulent boundary layer. Moreover, the pressure gradient parameter suggested by Alber & Coats was found to correlate the data with low-speed results
Demonstration of a Lightguide Detector for Liquid Argon TPCs
We report demonstration of light detection in liquid argon using an acrylic
lightguide detector system. This opens the opportunity for development of an
inexpensive, large-area light collection system for large liquid argon time
projection chambers. The guides are constructed of acrylic, with TPB embedded
in a surface coating with a matching index of refraction. We study the response
to early scintillation light produced by a 5.3 MeV alpha. We measure coating
responses from 7 to 8 PE on average, compared to an ideal expectation of 10 PE
on average. We estimate the attenuation length of light along the lightguide
bar to be greater than 0.5 m. The coating response and the attenuation length
can be improved; we show, however, that these results are already sufficient
for triggering in a large detector
Novel stripe-type charge ordering in the metallic A-type antiferromagnet Pr{0.5}Sr{0.5}MnO{3}
We demonstrate that an A-type antiferromagnetic (AFM) state of
Pr{0.5}Sr{0.5}MnO{3} exhibits a novel charge ordering which governs the
transport property. This charge ordering is stripe-like, being characterized by
a wave vector q ~ (0,0,0.3) with very anisotropic correlation parallel and
perpendicular to the stripe direction. This charge ordering is specific to the
manganites with relatively wide one-electron band width (W) which often exhibit
a metallic A-type AFM state, and should be strictly distinguished from the
CE-type checkerboard-like charge ordering which is commonly observed in
manganites with narrower W such as La{1-x}Ca{x}MnO{3} and Pr{1-x}Ca{x}MnO{3}.Comment: REVTeX4, 5 pages, 4 figure
Stability Theory for Cross-Hatching. Part II. An Experiment on Turbulent Boundary Layer over a Wavy Wall
An experimental investigation of turbulent boundary layer flow over wavy surfaces was conducted at low speed. Two models with the ratio of the amplitude to the wave length a/λ = 0.03 and wave lengths λ = 6" and 12" were tested in an open-circuit wind tunnel. The free stream velocity was 15.4 m/sec, giving Reynolds number Re = 2.54 X 10^4 per inch. Boundary-layer thickness varied from δ = 1.5" to δ = 4.1" by means of boundary-layer trips of various height, in order to change the ratio λ/δ. The fol- lowing measurements were taken: wall pressure distribution, average velocity and turbulence level, wall stress distribution, static and total pressures, and shear stress distribution across the layer.
Wall pressure perturbation is much lower than predicted by uniform, inviscid theory and is slightly non-symmetric. Wall stress distribution has a peak with C_f/C_f_o = 1.2 upstream of the crest and a dip of C_f/C_f_o = 0.6 upstream of the trough.
The turbulence intensities and shear stress distributions near the wall show oscillatory modulation superimposed on the reference flat plate pro- files. The amplitude of the oscillations decays exponentially toward the edge of the layer, so that in the outer part of the layer the turbulence quantities are practically independent of the longitudinal position
Stripes Induced by Orbital Ordering in Layered Manganites
Spin-charge-orbital ordered structures in doped layered manganites are
investigated using an orbital-degenerate double-exchange model tightly coupled
to Jahn-Teller distortions. In the ferromagnetic phase, unexpected diagonal
stripes at = (=integer) are observed, as in recent experiments.
These stripes are induced by the orbital degree of freedom, which forms a
staggered pattern in the background. A -shift in the orbital order across
stripes is identified, analogous to the -shift in spin order across
stripes in cuprates. At =1/4 and 1/3, another non-magnetic phase with
diagonal static charge stripes is stabilized at intermediate values of the
-spins exchange coupling.Comment: reordering of figure
Stability Theory for Cross-Hatching. Part II. An Experiment on Turbulent Boundary Layer over a Wavy Wall
An experimental investigation of turbulent boundary layer flow over wavy surfaces was conducted at low speed. Two models with the ratio of the amplitude to the wave length a/λ = 0.03 and wave lengths λ = 6" and 12" were tested in an open-circuit wind tunnel. The free stream velocity was 15.4 m/sec, giving Reynolds number Re = 2.54 X 10^4 per inch. Boundary-layer thickness varied from δ = 1.5" to δ = 4.1" by means of boundary-layer trips of various height, in order to change the ratio λ/δ. The fol- lowing measurements were taken: wall pressure distribution, average velocity and turbulence level, wall stress distribution, static and total pressures, and shear stress distribution across the layer.
Wall pressure perturbation is much lower than predicted by uniform, inviscid theory and is slightly non-symmetric. Wall stress distribution has a peak with C_f/C_f_o = 1.2 upstream of the crest and a dip of C_f/C_f_o = 0.6 upstream of the trough.
The turbulence intensities and shear stress distributions near the wall show oscillatory modulation superimposed on the reference flat plate pro- files. The amplitude of the oscillations decays exponentially toward the edge of the layer, so that in the outer part of the layer the turbulence quantities are practically independent of the longitudinal position
Reentrant charge ordering caused by polaron formation
Based on a two-dimensional extended Hubbard model with electron-phonon
interaction, we have studied the effect of polaron formation on the charge
ordering (CO) transition. It is found that for fully ferromagnetically ordered
spins the CO state may go through a process of appearance, collapse and
reappearance with decreasing temperature. This is entirely due to a
emperature-dependent polaron bandwidth. On the other hand, when a paramagnetic
spin state is considered, only a simple reentrant behavior of the CO transition
is found, which is only partly due to polaron effect. This model is proposed as
an explanation of the observed reentrant behavior of the CO transition in the
layered manganite LaSrMnO.Comment: 4 pages, 2 eps figures, revised version accepted by Phys. Rev. Let
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