The turbulence structure and turbulence kinetic energy transport in multiscale/fractal-generated turbulence in a wind tunnel are investigated. A low-blockage, space-filling square-type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. On the basis of the thickness of the biggest grid bar, t0, and the inflow velocity U∞, the Reynolds numbers (Re0) are set to 5900 and 11 400; these values are the same as those considered in previous experiments [D. Hurst and J. C. Vassilicos, “Scalings and decay of fractal-generated turbulence,” Phys. Fluids 19, 035103 (2007)10.1063/1.2676448; N. Mazellier and J. C. Vassilicos, “Turbulence without Richardson-Kolmogorov cascade,” Phys. Fluids 22, 075101 (2010)10.1063/1.3453708]. The turbulence characteristics are measured using hot-wire anemometry with I- and X-type probes. Generally, good agreements are observed despite the difference in the size of the test sections used: The longitudinal integral length-scale Lu and the Taylor microscale λ, and their ratio Lu/λ, are approximately constant during decay and are independent of the turbulent Reynolds number Reλ. Centerline statistical results support the finding of Mazellier and Vassilicos, namely, that the classical scaling of Lu/λ ∼ Reλ and the Richardson–Kolmogorov cascade are not universal to all boundary-free weakly sheared/strained turbulence. The cross-sectional profiles show that in the entire cross section of the tunnel, Lu/λ hardly changes in the decay region of the rms velocity, which implies that the turbulent field is self-similar. The production and transport of turbulence kinetic energy K in fractal grid turbulence are also investigated from cross-sectional profiles of the advection \documentclass[12pt]{minimal}\begin{document}${\cal A}^*$\end{document}A*, production \documentclass[12pt]{minimal}\begin{document}${\cal P}^*$\end{document}P*, triple-correlation transport \documentclass[12pt]{minimal}\begin{document}${\cal T}^*$\end{document}T*, pressure transport Π*, viscous diffusion \documentclass[12pt]{minimal}\begin{document}${\cal D}^*$\end{document}D*, and dissipation ɛ terms in the K transport equation. In the upstream region, turbulence produced by the biggest grid bar is transported to the central and outward regions by \documentclass[12pt]{minimal}\begin{document}${\cal T}^*$\end{document}T*. In the decay region, there is low turbulence production downstream of the interior of the biggest grid bar; turbulence energy in this region is mainly transported outward rather than toward the central region by \documentclass[12pt]{minimal}\begin{document}${\cal T}(=$\end{document}T(=\documentclass[12pt]{minimal}\begin{document}${\cal T}^*/\varepsilon )$\end{document}T*/ɛ). This characteristic of \documentclass[12pt]{minimal}\begin{document}${\cal T}$\end{document}T may cause a faster decay of K in the central region, as observed by Valente and Vassilicos [“The decay of turbulence generated by a class of multiscale grids,” J. Fluid Mech. 687, 300 (2011)10.1017/jfm.2011.353]. The advection term \documentclass[12pt]{minimal}\begin{document}${\cal A}^*$\end{document}A* is high and positive in the decay region, whereas Π* and \documentclass[12pt]{minimal}\begin{document}${\cal D}^*$\end{document}D* are low.

Free-standing films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) (PEDOT/PSS) were prepared by casting water dispersion of its colloidal particles. Morphology, water vapor sorption, and electro-active polymer actuating behavior of the resulting films were investigated by means of atomic force microscopy, sorption isotherm, thermal mechanical analysis, and electromechanical analysis. It was found that the PEDOT/PSS film sorbed 60% of moisture at relative water vapor pressure of 0.95. Upon application of 10 V, the film underwent contraction of 2.4% in air at 50% relative humidity (RH) which significantly increased to 4.5% at 90% RH. The principle lay in desorption of water vapor sorbed in the film due to Joule heating, where electric field was capable of controlling the equilibrium of water vapor sorption. The film generated contractile stress as high as 17 MPa under isometric conditions and work capacity attained 174 kJ m(-3), where Young's modulus of the film increased from 1.8 to 2.6 GPa by application of 6 V at 50% RH.

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This paper proposes a validation scheme for the effect of wind tunnel blockage on decaying grid-generated turbulence. This validation scheme was derived from the governing equations of the k-ϵ model. Analytical solutions for the validation scheme were derived by introducing a model of the difference between the rate of change of the effect of fluid acceleration on the turbulent kinetic energy and that of the effect on its dissipation. The derived solutions include a decay exponent that excludes the acceleration effect, a parameter characterizing the acceleration, the initial anisotropy, and the model coefficient of the k-ϵ model, and can be quantified by parameters which can be known. The physical meaning of the model was clarified. The derived solutions and model were confirmed to be accurate through numerical simulation. An equation for the decay exponent, which is also affected by the fluid acceleration, was developed using the derived solutions. This scheme was applied to the examination of the reduced fluid acceleration effect in a moderate-sized wind tunnel to measure the grid-generated turbulence. The fluid acceleration effect in the wind tunnel was confirmed to be small using the derived equations. The decay characteristics of the grid-generated turbulence in the wind tunnel were measured and were found to agree with those obtained in previous experiments.

In this paper, direct numerical simulations are carried out to study single-square grid-generated turbulence at a Reynolds number \documentclass[12pt]{minimal}\begin{document}$Re_{L_0}$\end{document}ReL0 = 20 000 (based on the inlet velocity Uin and the length of grid bar L0). Different from the regular grid and the multiscale/fractal grid, here only single large square grid is placed at the center near the inlet. First, we investigate the evolutions of turbulence characteristics (e.g., mean streamwise velocity, turbulence intensity, Taylor microscale, etc.) along the centerline. The common characteristics possessed by turbulent flows generated by the single square grid and by the fractal square grid are presented. We confirm the hypothesis proposed by Mazellier and Vassilicos [“Turbulence without Richardson-Kolmogorov cascade,” Phys. Fluids 22, 075101 (2010)] that for the fractal square grid, the location of turbulence intensity peak along the centerline is mainly determined by large-scale wake interactions. Current numerical results show that in turbulence generated by the single square grid, wake interactions occur close to the grid and cause extreme/intense events. Then, the spatial development of invariants of the velocity gradient tensor is studied. For example, the (QW, −QS) maps are analyzed to show how turbulence generated by a single square grid obtains large scale vortices along the centerline.

We simulate fractal-generated turbulence (Hurst and Vassilicos 2007 Phys. Fluids 19 035103)) by means of a direct numerical simulation and address its fundamental characteristics. We examine whether the fractal-generated turbulence in the upstream region has a nature similar to that of a wake. We propose an equation for predicting peak values of the velocity fluctuation intensity and devise a method for formulating the functional form of the quantity of interest by focusing on the time scale of decaying turbulence, and we examine those forms for the turbulent kinetic energy and rms of pressure fluctuation through this method. By using the method, both of these functional forms are found to be power-law functions in the downstream region, even though these profiles follow exponential functions around these peaks. In addition, decay exponents of these quantities are estimated. The integral length scales of velocity fluctuations for transverse as well as streamwise directions are essentially constant in the downstream direction. Decaying turbulence having both these characteristics conflicts with decaying turbulence described by the theory predicting exponential decay. We discuss a factor causing the difference by focusing on the functional form of the transfer function of homogeneous, isotropic turbulence.