Lowest Twist Research Articles

Turbulent heat transfer enhancement in tubular heat exchangers with different twisted tape inserts

Experimental and numerical analyses are carried out to investigate the influence of twisted tape inserts on the heat transfer and the flow behavior in double tube heat exchangers. First, all the performance factors, namely the Nusselt number, friction factor, and thermal performance factor, were studied for a basic heat exchanger (BHE). Afterwards, twisted tapes with three different twist ratios (7.5, 6, and 4.5) were inserted inside the inner tube of the BHE, which resulted in three different modified heat exchangers (MHEs). For the numerical study, a 3D numerical model is developed with the k-ε RNG turbulent model to visualize the flow and the heat transfer behavior inside the heat exchangers. In both studies, turbulent flow field is maintained, ranging Reynolds number from 15000 to 50000. From the experimental result, an enhanced heat transfer, characterized by the performance factors, is found for all the MHEs compared to the BHE. The most enhanced thermal performance factor is achieved for the MHE with the lowest twist ratio. Finally, a good agreement between obtained numerical and experimental results reveals that the present numerical model can reliably predict the flow and heat transfer behavior in double tube heat exchangers.

Renormalization group flows, the a-theorem and conformal bootstrap

Every renormalization group flow in d spacetime dimensions can be equivalently described as spectral deformations of a generalized free CFT in (d − 1) spacetime dimensions. This can be achieved by studying the effective action of the Nambu-Goldstone boson of broken conformal symmetry in anti-de Sitter space and then taking the flat space limit. This approach is particularly useful in even spacetime dimension where the change in the Euler anomaly aUV− aIR can be related to anomalous dimensions of lowest twist multi-trace operators in the dual CFT. As an application, we provide a simple proof of the 4d a-theorem using the dual description. Furthermore, we reinterpret the statement of the a-theorem in 6d as a conformal bootstrap problem in 5d.

Application and evaluation of the neural network in gearbox

We developed old designed of a Back-Propagation neural network (BPNN), which it was designed by other researchers, and we made modification in their structure. The 1 st velocity ratio was discriminated by lowest speed, and highest twist. The 6 th velocity ratio was discriminated by highest speed, and lowest twist. The aim of this paper is to design neural structure get best performance to control an electrical automotive transportation six-speed gearbox of the vehicle. We focus on the evaluation of the BPNN to select the suitable number of layers and neurons. Experimentally, the structure of the proposed BPNN are constructed from four layers: eight input nodes in the first layer that received data in binary number, 45 neurons in 1 st hidden-layer, 25 neurons in 2 nd hidden-layer, and 6 neurons in the fourth layer. The MSE and number of Epochs are the main factors used for the evaluation of the proposed structure, and compared with the other structures which was designed by other researchers. Experimentally, we discovered that the best value of Epoch and MSE was chosen when the BPNN consisted of two hidden-layers, 45, and 25 neurons in the 1 st and 2 nd hidden-layer respectively. The implementation was applied using MATLAB software.

Leading multi-stress tensors and conformal bootstrap

Near lightcone correlators are dominated by operators with the lowest twist. We consider the contributions of such leading lowest twist multi-stress tensor operators to a heavy-heavy-light-light correlator in a CFT of any even dimensionality with a large central charge. An infinite number of such operators contribute, but their sum is described by a simple ansatz. We show that the coefficients in this ansatz can be determined recursively, thereby providing an operational procedure to compute them. This is achieved by bootstrapping the corresponding near lightcone correlator: conformal data for any minimal­ twist determines that for the higher minimal-twist and so on. To illustrate this procedure in four spacetime dimensions we determine the contributions of double- and triple-stress tensors. We compute the OPE coefficients; whenever results are available in the literature, we observe complete agreement. We also compute the contributions of double-stress tensors in six spacetime dimensions and determine the corresponding OPE coefficients. In all cases the results are consistent with the exponentiation of the near lightcone correlator. This is similar to the situation in two spacetime dimensions for the Virasoro vacuum block.

Ultra thin silicon films directly bonded onto silicon wafers

Ultra thin film of silicon bonded on 4 in. (001) silicon wafers have been obtained by combining a direct hydrophobic silicon bonded technique with a layer transfer. The twist angle between the ultra thin Si film and the Si substrate was varied from 0 to 15°. X-ray reflectivity measured the thickness and the roughness of the ultra thin films. Complementary results concerning the interface structure were obtained with high resolution transmission electronic microscopy. It is shown that an ultra thin film (a few nm) can be reproductively prepared upon the full 4 in. wafers. Moreover, this process gives very small thickness fluctuations and a small surface roughness. The bonding interface has a low concentration of oxide precipitates and presents two arrays dislocations respectively, due to the twist (screw dislocations) and a residual tilt angle (mixed dislocations) of the crystals. Dissociation of the screw dislocations is also observed on the lowest twist angle sample.

Conformational analysis of methyl phenyl sulphoxides containing fluorine substituents in the phenyl ring based on 1H, 13C and 17O NMR chemical shifts and long‐range nJ(HF) and nJ(CF) coupling constants

AbstractThe preferred conformations of the methylsulphinyl group in a number of phenyl methyl sulphoxides substituted in the phenyl ring with one, two or three fluorine atoms have been studied by employing a multinuclear NMR approach. From the 1H, 13C and 17O chemical shifts of the methylsulphinyl group, and the torsional angles obtained either from MO calculations or experimental determinations for a number of the compounds, empirical correlations have been derived which enabled us to extract the preferred orientation of the SO bond or of the methyl group relative to the phenyl ring in all the molecules examined. For a limited number of the compounds, the structure of the preferred conformer is satisfactorily close to the solid‐state structure found in an independent study carried out with x‐ray diffraction. The twist of the SO bond relative to the phenyl ring changes as a function of the number of ortho substituents, being highest when two substituents are present. The lowest twist, torsional angle nearly zero, was found in ortho‐monosubstituted compounds and the SO bond is oriented towards the opposite side with respect to the substituent. Analyses were also carried out of the conformational dependence of long‐range nJ(HF) and nJ(CF) coupling constants, where the first nucleus in the parentheses refers to the methylsulphinyl group. The behaviour of these coupling constants as a function of internal rotation around the exocyclic CS bond was derived from semi‐empirical INDO MO FPT calculations, and scaled interpolating equations were derived in order to reproduce the experimental results. It thus emerges that 5J(HF), 4J(CF), 5J(CF) and 6J(CF) are conformationally dependent and show, experimentally, significant changes. These parameters can be usefully employed for the conformational analysis of methyl phenyl sulphoxides.

Polarized parton distributions of the nucleon

Spin-dependent (polarized) parton distributions are presented satisfying the available experimental and theoretical constraints. In particular we present radiatively generated predictions for ΔG(x, Q)2, compatible with recent data, and for the gluon asymmetry ΔG(x, Q2)G(x, Q2). It is furthermore shown that the data on g1p(x, Q02) do not provide unique information on ΔG(x, Q02) in contrast to its unambiguous first moment ∫01 ΔG(x, Q02) dx. In addition, a prediction for the structure function g1n(x, Q02 ⋍ 10 GeV2) is given for the case of lowest twist dominance.

QCD sum rules and pion wave functions

We consider light cone sum rules for a vertex function involving a pion state. These incorporate radiative corrections, continuum effects and power corrections; the latter depend on the non-asymptotic form of a higher twist component of the pion wave function. We derive restrictions on this component from sum rules for two-point functions and propose a model wave function for it. Finally we analyse our vertex sum rules in the light of this information and find results for the lowest twist component in good agreement with those already obtained from two-point function sum rules with derivative currents.

Effect of Mercerization on the Tensile Properties of Rotor Spun Yarn

Rotor spun 14s cotton yarns were twisted over a range of twist multipliers from 4.25 to 5.5. The yarns were mercerized with sodium hydroxide and stretched to their original length. Tenacity of mercerized yarns was significantly greater than the grey yarns at all twist levels. A substantial improvement of 52.8% in tenacity was observed at the lowest twist. Even slack mercerization improved yarn tenacity at lower twist levels (up to 4.7) close to that for tension mercerized yam. Maximum increase in yarn tenacity by slack mercerization was 46% at the lowest twist. Tension mercerization increases yarn tenacity primarily by improvement of fiber packing at lower twist levels, but at higher twist levels fiber orientation, fiber packing, and improvement in fiber tenacity all affect yarn tenacity.