We present a numerical investigation on flow around two square cylinders in side-by-side, tandem, and staggered arrangements at Reynolds number Re = 100 (which is based on the side length of the square cylinder). The dimensionless spacing between the two cylinders are T/D < 5.0 and L/D < 10.0 in the transverse and longitudinal directions, respectively. Numerical results reveal various flow patterns depending on the spacing, including the single bluff-body, flip-flopping, in-phase synchronized, and anti-phase synchronized patterns for the side-by-side arrangement, type-I and type-II patterns for the tandem arrangement, and nine complex flow patterns for the staggered arrangement. Moreover, these flow patterns can be divided into three categories: single bluff-body flow, small incidence angle flow, and large incidence angle flow. The flow structures, lift coefficient, drag coefficient, and Strouhal number (St) are presented to demonstrate the dependence of these parameters on the spacing, and the relationship between the flow pattern and lift and drag coefficients is also explored.

The effect of rotation on small-scale characteristics and scaling law in the mixing zone of the three-dimensional turbulent Rayleigh-Taylor instability (RTI) is investigated by the lattice Boltzmann method at small Atwood number. The mixing zone width $h\left(t\right)$, the root mean square of small scale fluctuation, the spectra, and the structure functions are obtained to analyze the rotating effect. We mainly focus on the process of the development of plumes and discuss the physical mechanism in the mixing zone in rotating and nonrotating systems. The variation of kinetic energy spectra $E_u$ and temperature energy spectra $E_{\theta }$ with the dimensionless rotation $\mathrm{\Omega }\tau$ demonstrate the suppression effect of rotation. Two scaling laws between the mixing layer width $h\left(t\right)$ and dimensionless time $t/\tau$ are obtained at various Coriolis forces($\sqrt{h\left(t\right)}\simeq t^{0.9}$ and $\sqrt{h\left(t\right)}\simeq t^{0.35}$). The rotation increasingly suppresses the growth of the mixing layer width $h\left(t\right)$. The velocity and temperature fluctuations are also suppressed by the rotation effect. The relation between the Nusselt number (Nu) and the Rayleigh number (Ra) indicates that the heat transfer is suppressed by the rotation effect in the rotating RT system. The width of the inertial subrange increasingly narrows with increasing $\mathrm{\Omega }\tau$.

In this paper, the characteristic of the internal flow structure and the spatiotemporal evolution of the rotating stall cell for a centrifugal compressor are investigated through the computational fluid dynamics approach at low-flow rate conditions. The qualitative study like generation mechanisms of the instable internal flow, the movement trend of the stall, and the spatiotemporal characteristics of fluctuation in the impeller are mainly focused in the present work. The stall cells in the impeller move along the impeller rotating direction, and significantly influenced by the volute tongue. The traveling mechanism of rotating stall is closely related to the volute tongue, which easily deteriorates the obvious distortion of the flow structure. Furthermore, the transition law of the rotating stall is quantificationally described through the density and velocity fluctuations. The stall evolution on the circumferential monitoring points is demonstrated by the pressure fluctuation both time and frequency scale, which verifies the influence of the volute tongue on the fluid structure in different operating conditions. In addition, the damage of the volute tongue that performed on the impeller passage and blade is analyzed specifically. The mean value of the fluctuation in the passage and the load on the blade decreases about 2.5% and 3.3%, respectively, after the blade passes through the volute tongue. The spatiotemporal evolution provides a profound understanding of the flow characteristics of stall evolution within the centrifugal compressors.

Ultra wide-band (UWB) communication holds great potential for a significantly improved data rate in future wireless systems. Accurate channel estimation and synchronization are critical for successful operation of a UWB system. We propose in this letter a completely blind channel estimation algorithm for UWB systems that employ pulse-position modulation, assuming perfect synchronization at the receiver. The algorithm exploits the first-order cyclostationarity in the received signal and performs certain circular deconvolution. The complexity is extremely low: Only some "overlap-add" operations and fast Fourier transform operations are needed. The algorithm is capable of simultaneously estimating multiple (say, more than 60) channel taps, and there is no ambiguity in either the amplitude or the phase of the estimated channel. It is shown that using estimated channel from 500 information symbols, the performance can approach that with a known channel within 3 dB in signal-to-noise ratio for a bit error rate of 0.001.

The effects of single-arc blade profile length on the performance of a forward multiblade fan are investigated in this paper by computational fluid dynamics and experimental measurement. The present work emphasizes that the use of a properly reduced blade inlet angle (β_1A) and properly improved blade outlet angle (β_2A) is to increase the length blade profile, which suggests a good physical understanding of internal complex flow characteristics and the aerodynamic performance of the fan. Numerical results indicate that the gradient of the absolute velocity among the blades in model-L (reducing the blade inlet angle and improving blade outlet angle) is clearly lower than that of the baseline model and model-S (improving the blade inlet angle and reducing blade outlet angle), where a number of secondary flows arise on the exit surface of baseline model and model-S. However, no secondary flow occurs in model-L, and the flow loss at the exit surface of the volute (scroll-shaped flow patterns) for model-L is obviously lower than that of the baseline model at the design point. The comparison of the test results further shows that to improve the blade profile length is to increase the static pressure and the efficiency of the static pressure, since the improved static pressure of the model-L rises as much as 22.5 Pa and 26.2%, and the improved static pressure efficiency of the model-L rises as much as 5 % at the design flow rates. It is further indicated that increasing the blade working area provides significant physical insight into increasing the static pressure, total pressure, the efficiency of the static pressure and the total pressure efficiency.