A simple expression is derived of the componential contributions that different dynamical effects make to the frictional drag in turbulent channel, pipe and plane boundary layer flows. The local skin friction can be decomposed into four parts, i.e., laminar, turbulent, inhomogeneous and transient components, the second of which is a weighted integral of the Reynolds stress distribution. It is reconfirmed that the near-wall Reynolds stress is primarily important for the prediction and control of wall turbulence. As an example, the derived expression is used for an analysis of the drag modification by the opposition control and by the uniform wall blowing/suction.

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Key pointsr Luminal lipid in the duodenum modulates gastroduodenal functions via the release of gut hormones and mediators such as cholecystokinin and 5-HT. Abstract Intraduodenal fatty acids (FA) and bacterial overgrowth, which generate short-chain FAs (SCFAs), have been implicated in the generation of functional dyspepsia symptoms. We studied the mechanisms by which luminal SCFA perfusion affects duodenal HCO 3 − secretion (DBS), a measure of mucosal neurohumoral activation. Free fatty acid receptor (FFAR) 1 (FFA1), which binds long-chain FA (LCFA), and SCFA receptors FFA2 and FFA3 were immunolocalised to duodenal enteroendocrine cells. FFA3 colocalised with glucagon-like peptide (GLP)-1, whereas FFA2 colocalised with 5-HT. Luminal perfusion of the SCFA acetate or propionate increased DBS, enhanced by dipeptidyl peptidase-IV (DPPIV) inhibition, at the same time as increasing GLP-2 portal blood concentrations. Acetate-induced DBS was partially inhibited by monocarboxylate/HCO 3 − exchanger inhibition without affecting GLP-2 release, implicating acetate absorption in the partial mediation of DBS. A selective FFA2 agonist dose-dependently increased DBS, unaffected by DPPIV inhibition or by cholecystokinin or 5-HT 3 receptor antagonists, but was inhibited by atropine and a 5-HT 4 antagonist. By contrast, a selective FFA1 agonist increased DBS accompanied by GLP-2 release, enhanced by DPPIV inhibition and inhibited by a GLP-2 receptor antagonist. Activation of FFA1 by LCFA and presumably FFA3 by SCFA increased DBS via GLP-2 release, whereas FFA2 activation stimulated DBS via muscarinic and 5-HT 4 receptor activation. SCFA/HCO 3 − exchange also appears to be present in the duodenum. The presence of duodenal fatty acid sensing receptors that signal hormone

A series of infrared-active optical phonons have been detected in type-I clathrate Ba8Ga16Ge30 by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba 2+ (2) with T1u symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.PACS numbers: 63.20. Ry, During the past decade, thermoelectric materials such as clathrates and filled-skutterudites have renewed an interest in phonons [1]. These conductors are formed by polyhedral building blocks, where each polyhedral cage accommodates a guest ion like an alkali-earth or rareearth element. If the cage is oversized, the guest ions can vibrate with large amplitude around an on-center or off-center site in the cage potential. These Einsteinlike local modes have been called rattling phonons. The renewed interest above has been paid to interactions of these rattling phonons with acoustic phonons propagating through the cage network and carrying heat entropy, and, equally or more interestingly, with conduction electrons. However, these issues are still far from being well understood that even the charge dynamics of a single ion in rattling vibration have been little known.This paper reports on the optical conductivity of rattling phonons detected in a type-I clathrate Ba 8 Ga 16 Ge 30 (hereafter abbreviated as BGG), featuring the anharmonicity effects on the conductivity spectra in comparison with model calculations.This compound belongs to a family of ternary type-I clathrates having cubic crystal structure with space group P m3n [2]. The unit cell of the host framework consists of 6 tetrakaidecahedra and 2 dodecahedra. The latter smaller cages around 2a sites occupy the bodycentered-cubic sites, while the former oversized cages around 6d sites line up along the principal axes by sharing both the pentagonal faces with the dodecahedral cages and the hexagonal faces with neighboring tetrakaidecahedra. Every cage encapsulates one Ba 2+ ion, and the guest ions in the smaller (2a) and oversized (6d) cages are defined as Ba 2+ (1) and (2), respectively. These guest ions satisfy the Zintl rule for charge compensations that the more electro-positive guest atom donates electrons to the more electro-negative cage; 16 electrons are transferred from 8 Ba atoms to 16 Ga atoms of the cage. With finely tuned Ga/Ge concentration, therefore, the system can be a heavily doped semiconductor, in general, having both the charge-carrier's sign and density controlled.So far the electric...

We present optical conductivity studies of the type-I clathrate Ba8Ga16Sn30, using a terahertz time-domain spectrometer (0.3-3.0 THz). The lowest-lying spectral peak at 0.72 THz due to the Ba(2) ion's off-center vibration in the oversized cage shows a drastic and anomalous temperature dependence. Below about 100 K, the single broad peak splits into two subpeaks, and with further lowering of the temperature, the spectral shape of this so-called rattling phonon shows non-Boltzmann broadening to the point that the linewidth becomes comparable to the peak frequency. Whereas the initial splitting can be understood by assuming a multiwell anharmonic potential, the strong linewidth broadening toward low temperature cannot, since the Boltzmann factor generally sharpens the low-temperature spectra. The observed behavior suggests strong interaction between the local anharmonic phonons and other excitations.