Long-range critical wetting: observation of a critical end point

Alkanes deposited on aqueous substrates exhibit two different types of wetting behavior: alternatively to the usual first-order wetting transition, a sequential-wetting scenario of a long-range critical wetting transition preceded by a first-order thin-thick transition may be observed. Here, we present the first successful experimental attempt to locate the transition point between the standard first-order wetting and the long-range critical wetting: a critical end point, observed in a mixture of pentane and hexane which is deposited on an aqueous solution of glucose. Furthermore, we present the first direct measurement of the contact angle in the intermediate wetting state (frustrated-complete wetting) in the sequential-wetting scenario of hexane on brine and compare to theoretical predictions.     

Simultaneously suppressing frequency and intensity noise in a Nd:YAG nonplanar ring oscillator by means of the current-lock technique

We show that frequency and intensity noise in a Nd:YAG laser are correlated to a high degree and can be traced to the same underlying cause, namely, power fluctuations of the pump source. Because of this correlation, simultaneous suppression of frequency and intensity noise by 30 dB is achieved by means of a single actuator, the pump power.     

Quantitative, dynamic fuel distribution measurements in combustion-related devices using laser-induced fluorescence imaging of biacetyl in iso-octane

Knowledge of in-situ fuel distributions in practical combustion devices, such as internal combustion engines, is crucial for research and devlopment purposes. Numerous imaging techniques, mostly based on laser-induced fluorescence (LIF), have been developed and yield high levels of 2-D spatial information, but generally lack the temporal resolution (frame rates) necessary to resolve important timescales at sub-millisecond levels for sustained times. A planar LIF technique for quantitatively visualizing fuel distribution is presented which gives not only high spatial resolution, but also high temporal resolution. Using a high-speed CMOS camera, a lens-coupled image intensifier, and frequency-tripled diode-pumped Nd:YAG laser allows for capturing LIF images of biacetyl that is used as a fluorescence tracer at 12 kHz (one crank-angle resolution at 2000 RPM) for hundreds of consecutive engine cycles. The LIF signal strength of biacetyl doped in iso-octane is shown to vary substantially over a wide range of temperatures and pressures. The low absorption coefficient at 355 nm and a longpass filter in the detection path exclude bias errors due to laser beam attenuation and fluorescence trapping. An intensifier gate time of 350 ns is shown to suppress the detection of phosphorescence signals under practical conditions. An example for a quantitative high-speed measurement of fuel concentration at varying pressure and temperature conditions is presented. Quantitative equivalence ratio maps are shown for the fuel injection event within a single cycle in a spark-ignition direct-injected engine, showing the ability of the technique to not only reveal static fuel concentration maps, but also the motion of the fuel cloud along with very steep gradients. Spray velocities determined from the moving fuel cloud are in agreement with previous particle image velocimetry measurements.     

Dynamical SUSY breaking in heterotic M-theory

It is shown that four-dimensional N=1$N=1$ supersymmetric QCD with massive flavors in the fundamental representation of the gauge group can be realized in the hidden sector of E₈×E₈$E_8\times E_8$ heterotic string vacua. The number of flavors can be chosen to lie in the range of validity of the free-magnetic dual, using which one can demonstrate the existence of long-lived meta-stable non-supersymmetric vacua. This is shown explicitly for the gauge group Spin(10)$\mathit{Spin}(10)$, but the methods are applicable to Spin(Nc)$\mathit{Spin}(N_c)$, SU(Nc)$\mathit{SU}(N_c)$ and Sp(Nc)$\mathit{Sp}(N_c)$ for a wide range of color index Nc$N_c$. Hidden sectors of this type can potentially be used as a mechanism to break supersymmetry within the context of heterotic M-theory.     

An algebraic variational multiscale–multigrid method for large-eddy simulation of turbulent variable-density flow at low Mach number

An algebraic variational multiscale–multigrid method is proposed for large-eddy simulation of turbulent variable-density flow at low Mach number. Scale-separating operators generated by level-transfer operators from plain aggregation algebraic multigrid methods enable the application of modeling terms to selected scale groups (here, the smaller of the resolved scales) in a purely algebraic way. Thus, for scale separation, no additional discretization besides the basic one is required, in contrast to earlier approaches based on geometric multigrid methods. The proposed method is thoroughly validated via three numerical test cases of increasing complexity: a Rayleigh–Taylor instability, turbulent channel flow with a heated and a cooled wall, and turbulent flow past a backward-facing step with heating. Results obtained with the algebraic variational multiscale–multigrid method are compared to results obtained with residual-based variational multiscale methods as well as reference results from direct numerical simulation, experiments and LES published elsewhere. Particularly, mean and various second-order velocity and temperature results obtained for turbulent channel flow with a heated and a cooled wall indicate the higher prediction quality achievable when adding a small-scale subgrid-viscosity term within the algebraic multigrid framework instead of residual-based terms accounting for the subgrid-scale part of the non-linear convective term.