Prandtl-number dependence of heat transport in turbulent Rayleigh-Bénard convection.

We present measurements of the Nusselt number N as a function of the Rayleigh number R and the Prandtl number sigma in cylindrical cells with aspect ratios gamma = 0.5 and 1.0. We used acetone, methanol, ethanol, and 2-propanol with Prandtl numbers sigma = 4.0, 6.5, 14.2, and 34.1, respectively, in the range 3x10(7) less, similarR less, similar10(11). At constant R, N(R,sigma) varies with sigma by only about 2%. This result disagrees with the extrapolation of the Grossmann and Lohse theory beyond its range of validity, which implies a decrease by 20% over our sigma range, but agrees with their recent extension of the theory to small Reynolds numbers.

Effect of sidewall conductance on heat-transport measurements for turbulent Rayleigh-Bénard convection.

For measurements of turbulent heat transport in Rayleigh-Bénard convection the correction for the sidewall conductance is usually neglected or based on measurements or estimates for the empty cell. It is argued that the lateral thermal coupling between the fluid and the wall can invalidate these approaches, and that corrections based on calculations of the two-dimensional temperature fields are required in some cases. These corrections can increase gamma obtained from fits of N=N(0)Rgamma (R is the Rayleigh number) to the Nusselt number N(R) by 0.02 or more, yielding values in the range 0.30 to 0.33, which are larger than most theoretical predictions.

Deviations from linear theory for fluctuations below the supercritical primary bifurcation to electroconvection

Over two decades ago it was predicted that nonlinear interactions between thermally driven fluctuations in dissipative nonlinear nonequilibrium systems lead to deviations from mean-field theory. Here we report experimental observations of such deviations as a supercritical primary bifurcation is approached. We measured the mean-square director-angle fluctuations below the bifurcation to electroconvection of two different nematic liquid crystals. For epsilon(mf) identical withV2/V(2)(c,mf)-1 less, similar-0.1 ( V is the applied voltage) we find approximately |epsilon(mf)|(-gamma) with gamma given by linear theory (LT). Closer to the bifurcation there are deviations from LT with a smaller gamma and with V(2)(c)>V(2)(c,mf).

Heat transport in turbulent rayleigh-Benard convection

We present measurements of the Nusselt number N as a function of the Rayleigh number R in cylindrical cells with aspect ratios 0. 510(7) they are consistent with N = asigma-1/12R1/4+bsigma-1/7R3/7 as proposed by Grossmann and Lohse for sigma greater, similar2.

Convection in the presence of a first-order phase change.

We report experimental and theoretical results for two-phase convection in a thin horizontal layer of a fluid with a first-order phase change and heated from below. A top layer of the nematic phase of a liquid crystal is located above the bottom layer of the isotropic phase of the same substance. A horizontal field of 1000 G is applied in order to align the director of the nematic phase. Over some ranges of the thickness of the isotropic phase, and in sufficiently large thermal gradients, the more dense nematic phase can be stably stratified above the less dense isotropic one, with a stable interface between them. Based on the equations of motion derived for this problem by Busse and Schubert [J. Fluid Mech. 46, 801 (1971)], we evaluate the bifurcation lines between the quiescent and convecting states and the corresponding critical wave vectors as a function of the interface position. We report experimental measurements based on Nusselt-number determinations for the locations of the bifurcation lines. They are in good agreement with the theoretical results. We also report approximate determinations of the critical wave numbers which are semiquantitatively consistent with the theory. A great diversity of patterns is observed in the convecting states, including normal and parallel rolls, rolls with defects and disorder, target patterns and spirals, and cellular flow with upflow or downflow at the cell center. These patterns are discussed in terms of the breaking of the mirror symmetry at the horizontal midplane by the interface, and in terms of the orienting effects of the magnetic field.

Bifurcation to worms in electroconvection.

The primary bifurcation to electroconvection of the liquid crystal 4-ethyl-2-fluoro-4'-[2-(trans-4-pentylcyclohexyl)ethyl]-biphenyl (I52) with planar alignment leads to localized structures of convection rolls known as "worms" when the conductivity of the fluid is relatively small. Worms coexist with the conduction state. They have a unique small width in the direction perpendicular to the director and a varying, usually much greater, length parallel to the director. Previous experiments had not determined whether the bifurcation to worms is supercritical or subcritical. We estimated the voltage V(c) corresponding to the stability limit of the conduction state by measuring the mean-square amplitude of the thermally induced fluctuations below onset and extrapolating to V(c). We found that worms appear already well below V(c). Thus the bifurcation is subcritical. Measurements of the lifetime of the conduction state below V(c) gave information about the voltage V(s) corresponding to the saddle node below which no worms form. We measured V(c) and epsilon(s)=V(s)2/V(c)2-1 as a function of the conductance sigma for a cell of thickness 24 microm and found for our sample that epsilon(s) approaches zero from negative values near sigma approximately 1.2 x 10(-8) Omega(-1) m(-1) as sigma increases. For larger sigma we found the bifurcation to be supercritical. We have been unable to determine so far whether the experimentally observed transition with decreasing sigma from a supercritical to a subcritical bifurcation occurs via a tricritical bifurcation, or whether the worm saddle node is disconnected from the primary supercritical bifurcation line as suggested by theory.

Milk-responsive atopic dermatitis is associated with a casein-specific lymphocyte response in adolescent and adult patients.

BACKGROUND: Specific T-cell responses to food antigens have been described in children with atopic dermatitis (AD). However, a subgroup of adolescent and adult patients still experiences food-responsive AD. OBJECTIVE: This study was designed to evaluate lymphocyte responses to bovine casein in adult patients with AD. METHODS: The stimulatory capacity of lipopolysaccharide-depleted bovine casein was tested in proliferation assays and in limiting dilution assays. Casein-specific T-cell clones (TCCs) were generated and characterized. RESULTS: Higher proliferative responses to casein and modulation of cytokine receptors were observed in patients with milk-responsive AD compared with individuals without clinical reactions on milk ingestion. The results did not correlate with the amount of casein-specific IgE in the serum. The frequencies of T cells that grew in the presence of casein or house dust mite antigens were similar. Only 27% of CD4+ casein-specific TCCs from these patients, but the majority of house dust mite-specific TCCs, produced IL-4 on mitogen stimulation. Interferon-gamma, on the other hand, was produced by the majority of TCCs with both specificities. CONCLUSION: A specific T-cell-mediated immune response to casein can be found in the blood of adolescent and adult patients with milk-related exacerbation of AD. In contrast to house dust mite-specific T cells, casein-specific T cells of adult patients who respond to cow's milk with worsening of AD produce little or no IL-4.

Detection of a kappa-casein-specific lymphocyte response in milk-responsive atopic dermatitis.

BACKGROUND: Bovine casein leads to an expansion of lymphocytes expressing the cutaneous lymphocyte antigen and to specific lymphocyte proliferation in a subgroup of patients with milk-responsive atopic dermatitis (AD). The casein fraction is composed of different proteins with defined and completely different sequences. OBJECTIVE: To define the stimulatory capacity of the major casein protein (alpha, beta and kappa) in lymphocyte proliferation assays with cells from milk-allergic and non-allergic individuals. METHODS: Proliferative responses of peripheral blood mononuclear cells to lipopolysaccharide-depleted casein subfractions were measured by thymidine incorporation. Lymphocytes from milk-responsive patients with AD were compared with cells from non-responsive patients with AD and to non-atopic individuals. Atopic individuals with immediate symptoms following consumption of cow's milk were included as positive controls. Casein-specific T-cell clones (TCC) from four patients with milk-responsive AD were restimulated with unfractionated casein and kappa-casein. RESULTS: Higher proliferative responses to unfractionated casein and alpha-, beta- and kappa-casein were observed in milk-responsive patients compared with non-responders. Unfractionated casein and kappa-casein discriminated best between the milk-responsive patients with AD and non-responders. Twenty-five of 31 TCG from patients with milk-responsive AD reacted to the mixed casein preparation and kappa-casein. CONCLUSION: A pronounced kappa-casein-specific T-cell-mediated immune response is found in the blood of many patients with a history of milk-related exacerbation of AD.

Excitation of spirals and chiral symmetry breaking in rayleigh-benard convection.

Spiral-defect populations in low-Prandtl number Rayleigh-Bénard convection with slow rotation about a vertical axis were measured in carbon dioxide at high pressure. The results indicate that spirals act like "thermally excited" defects and that the winding direction of a spiral is analogous to a magnetic spin. Rotation about a vertical axis, the spiral analog of the magnetic field, breaks the zero-rotation chiral symmetry between clockwise and counterclockwise spiral defects. Many properties of spiral-defect statistics are well described by an effective statistical-mechanical model.