275 research outputs found

    Two-Scalar Turbulent Rayleigh-Benard Convection: Numerical Simulations and Unifying Theory

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    We conduct direct numerical simulations for turbulent Rayleigh-B\'{e}nard (RB) convection, driven simultaneously by two scalar components (say, temperature and salt concentration) with different molecular diffusivities, and measure the respective fluxes and the Reynolds number. To account for the results, we generalize the Grossmann-Lohse theory for traditional RB convections~(Grossmann and Lohse, J. Fluid Mech., 407, 27-56; Phys. Rev. Lett., 86, 3316-3319; Stevens et al., J. Fluid Mech., 730, 295-308) to this two-scalar turbulent convection. Our numerical results suggest that the generalized theory can successfully predict the overall trends for the fluxes of two scalars and the Reynolds number. In fact, for most of the parameters explored here, the theory can even predict the absolute values of the fluxes and the Reynolds number with good accuracy. The current study extends the generality of the Grossmann-Lohse theory in the area of the buoyancy-driven convection flows.Comment: 13 pages, 3 figures, and 1 tabl

    Flow induced dissolution of femtoliter surface droplet arrays

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    The dissolution of liquid nanodroplets is a crucial step in many applied processes, such as separation and dispersion in food industry, crystal formation of pharmaceutical products, concentrating and analysis in medical diagnosis, and drug delivery in aerosols. In this work, using both experiments and numerical simulations, we \textit{quantitatively} study the dissolution dynamics of femtoliter surface droplets in a highly ordered array under a uniform flow. Our results show that the dissolution of femoliter droplets strongly depends on their spatial positions relative to the flow direction, drop-to-drop spacing in the array, and the imposed flow rate. In some particular case, the droplet at the edge of the array can dissolve about 30% faster than the ones located near the centre. The dissolution rate of the droplet increases by 60% as the inter-droplet spacing is increased from 2.5 μ\mum to 20 μ\mum. Moreover, the droplets close to the front of flow commence to shrink earlier than those droplets in the center of the array. The average dissolution rate is faster for faster flow. As a result, the dissolution time TiT_{i} decreases with the Reynolds number Re of the flow as TiRe3/4T_{i}\propto Re^{-3/4}. The experimental results are in good agreement with numerical simulations where the advection-diffusion equation for the concentration field is solved and the concentration gradient on the surface of the drop is computed. The findings suggest potential approaches to manipulate nanodroplet sizes in droplet arrays simply by dissolution controlled by an external flow. The obtained droplets with varying curvatures may serve as templates for generating multifocal microlens in one array

    On powers of Hamilton cycles in Ramsey-Tur\'{a}n Theory

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    We prove that for rNr\in \mathbb{N} with r2r\geq 2 and μ>0\mu>0, there exist α>0\alpha>0 and n0n_{0} such that for every nn0n\geq n_{0}, every nn-vertex graph GG with δ(G)(11r+μ)n\delta(G)\geq \left(1-\frac{1}{r}+\mu\right)n and α(G)αn\alpha(G)\leq \alpha n contains an rr-th power of a Hamilton cycle. We also show that the minimum degree condition is asymptotically sharp for r=2,3r=2, 3 and the r=2r=2 case was recently conjectured by Staden and Treglown.Comment: 19 pages, 4 figure

    Factors associated with fatigue in acquired immunodeficiency syndrome patients with antiretroviral drug adverse reactions: a retrospective study

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    AbstractObjectiveTo retrospectively study the prevalence of fatigue and factors associated with fatigue among acquired immunodeficiency syndrome (AIDS) patients with antiretroviral drug adverse reactions.MethodsData were collected from case report forms (CRFs) for a project funded by the 11th National 5-year Special Science and Technology Program on Major Infectious Diseases. Fatigue was defined by patient self-report. The outcomes were the prevalence of fatigue and the potential risk factors of fatigue. Univariate and multivariate logistic regression analyses were conducted to identify the factors associated with fatigue.ResultsAmong the 228 subjects, the prevalence of fatigue was 86.8%. In univariate analysis, the significant differences in demographic characteristics between patients with and without fatigue were: gender [OR=2.29; 95% CI (1.05-4.98)], education level [OR=0.40; 95% CI (0.18-0.85)], anemia [OR=3.80; 95% CI (1.27-11.31)], time of HIV diagnosis [OR= 0.29; 95% CI (0.13-0.65)], and route of infection [OR=0.14; 95% CI (0.06-0.32)]. Abnormal taste and rapid pulse were more commonly seen in patients with fatigue (P<0.05), while abdominal distension and lumbar soreness were encountered less often in patients with fatigue (P<0.05). Multivariate analysis showed that the four main factors associated with fatigue were anemia [OR=3.50; 95% CI (1.01-12.15)], route of infection [OR=3.40; 95% CI (1.21-9.58); P=0.02<0.05], lumbar soreness [OR=0.06; 95% CI (0.02-0.18); P=0.000<0.05], and rapid pulse [OR=10.58; 95% CI (2.16-51.75); P=0.004<0.05].ConclusionThis study demonstrated that fatigue is common (86.8% prevalence) in AIDS patients with antiretroviral drug adverse reactions, and that anemia, route of infection (i.e., non-commercial blood donation) and rapid pulse were risk factors, while lumbar soreness was a protective factor related to fatigue. More attention should be paid to fatigue and more efforts should be made to find ways to prevent, control and eliminate this symptom in AIDS patients with antiretroviral drug adverse reactions

    Adaptive Excitation Control for the Underactuated Biped Robot

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    AbstractA control method to make the chaotic gait converge to a stable cycle gait is proposed for the biped robot with knees. This control method is called adaptive excitation control. It is based on the principle of bionics and the principle of self-excited. The control law is a combination of sinusoidal input and sensory feedback control. The control torque is only inputted into the robot's hip. Therefore, the walking process is low energy consuming. Simulation results show that the control method proposed in this paper is effective. It can enlarge the basin of attraction of limit cycle and increase the gait stability

    What rotation rate maximizes heat transport in rotating Rayleigh-B\'{e}nard convection with Prandtl number larger than one?

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    The heat transfer and flow structure in rotating Rayleigh-B\'enard convection are strongly influenced by the Rayleigh (RaRa), Prandtl (PrPr), and Rossby (RoRo) number. For Pr1Pr\gtrsim 1 and intermediate rotation rates, the heat transfer is increased compared to the non-rotating case. We find that the regime of increased heat transfer is subdivided into a low and a high RaRa number regime. For Ra5×108Ra\lesssim 5\times10^8 the heat transfer at a given RaRa and PrPr is highest at an optimal rotation rate, at which the thickness of the viscous and thermal boundary layer is about equal. From the scaling relations of the thermal and viscous boundary layer thicknesses, we derive that the optimal rotation rate scales as 1/Roopt0.12Pr1/2Ra1/61/Ro_\mathrm{opt} \approx 0.12 Pr^{1/2}Ra^{1/6}. In the low RaRa regime the heat transfer is similar in a periodic domain and cylindrical cells with different aspect ratios, i.e.\ the ratio of diameter to height. This is consistent with the view that the vertically aligned vortices are the dominant flow structure. For Ra5×108Ra\gtrsim 5\times10^8 the above scaling for the optimal rotation rate does not hold anymore. It turns out that in the high RaRa regime, the flow structures at the optimal rotation rate are very different than for lower RaRa. Surprisingly, the heat transfer in the high RaRa regime differs significantly for a periodic domain and cylindrical cells with different aspect ratios, which originates from the sidewall boundary layer dynamics and the corresponding secondary circulation.Comment: 16 pages, 11 figures, 15 table
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