Increase Of Gap Spacing Research Articles

A numerical investigation of inter-carriage gap configurations on the aerodynamic performance of a wind-tunnel train model

The influence of different inter-carriage gap configurations, including end wall geometries (3 cases) and gap spacings (0, 5, 8, 10, 15, 20, and 30 mm), on the aerodynamic characteristics of a wind-tunnel train was investigated. The shear stress transport (SST) k- ω turbulence model was employed to determine the airflow features of the train at Re = 2.25 × 106. For validation, the numerical drag force and pressure distributions on the streamlined heads were compared with the experimental benchmark of wind tunnel experiment. The numerical data show that substantial variations in the flow fields, pressure distributions and aerodynamic forces are observed between the trains with and without gap spacings, no matter which configuration is employed. As the gap spacing increases, the airflow along train body rushes into the gap easily, causing the formation of vortices at the gap between the internal and external windshields. The decreasing restriction of flow in the gap also contributes to the pressure differences on the end walls. With the increase of gap spacings, the pressure on both of the first and second inter-carriage gaps is decreased, and it on the first one is a little higher than that on the second at each gap spacing. The end wall geometry affects the flow structures around the train, especially in the region below half-height of the train. This results in a difference in the boundary layer thicknesses and drag contribution in all cases. The discrepancy of end wall geometry causes a substantial variation in the aerodynamic drag between different cases. As gap spacing increases, the aerodynamic drag of the head car decreases, while those of the middle and rear cars increase significantly. When the three cases are compared, the discrepancy of the total aerodynamic drag of Case 1 is the smallest when compared to the base case with a minimum of 0.03% at 10 mm gap spacing and followed by 0.05% at 8 mm. Therefore, to determine the aerodynamic forces for high-speed trains with fully enclosed inter-carriage configuration in wind tunnel test, having a high comparative value as the actual trains, the end wall geometry in Case 1 is recommended with a gap spacing of 10 mm or 8 mm.

Mixed Convective Transport around Counter Rotating Tandem Cylinders at Low Reynolds Number

Rotating an object in a flowing fluid brings stability to the flow around the object. Consequently, an inherently unsteady flow may be degenerated into a steady pattern. On the contrary, cross thermal buoyancy brings instability to the flow causing triggering of vortex shedding even at relatively low Reynolds numbers. Hence, rotation and cross buoyancy produces opposing effects in this context. Furthermore, multiple objects in a flowing fluid develop intriguing wake dynamics following their interference effects. When tandem cylinders are placed in a flowing stream, depending on the gap between them, the vortex shedding may get triggered. These physical behaviors are coupled in this work to understand the simultaneous effect of rotation, cross thermal buoyancy (Richardson number, 0–1) and cylinder gap spacing (0.2–3) on the wake dynamics around tandem circular cylinders at Reynolds number 100 and Prandtl number 0.71. It is observed that at a critical rotational speed, the unsteady periodic flow is transfigured to a steady pattern. At smaller gap spacings (0.2, 0.7), a higher rotational speed is required particularly at higher strengths of buoyancy (0.5, 1) to control the flow instabilities and suppress the vortex shedding. The critical rotational speed is maximum at gap spacing of 0.2 for the highest buoyancy strength. Furthermore, the critical rotational speed reduces as the gap spacing increases.

Numerical Study of Negative Corona Discharge Characteristics at Different Electrode Gap Spacing

Atmospheric pressure needle-to-plane negative corona discharges have been studied considering the effect of photoionization in electrode gap spacing from 2.9 to 3.9 mm. Trichel pulses, spatial distributions of electron density and electric field, pulse duration time, and pulse frequency under different gap spacing were investigated in this work. It is found that peak value and duration time of the first Trichel pulse decrease with the increase of gap spacing. The movement of charged particles is faster in narrow gap spacing, which leads to higher pulse frequency. Moreover, the effect of photoionization on negative corona discharge under different gap spacing is also studied by numerical simulation. It is found that electric field, as well as electron density, is weakened due to the effect of photoionization. Besides, the magnitude of Trichel pulses, duration time of the first pulse, and pulse frequency are relatively small when taking the effect of photoionization into consideration. These results are of great importance to fully understand the mechanism of negative corona discharge under different conditions.

Numerical Investigation on the Vortex-Induced Vibration of a Flexible Plate behind a Circular Cylinder

Wang, H.; Zhai, Q.; Hou, J., and Xia, L., 2018. Numerical investigation on the vortex-induced vibration of a flexible plate behind a circular cylinder. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1326–1330. Coconut Creek (Florida), ISSN 0749-0208.Vortex-induced vibration (VIV) of piezoelectric cantilever plate can convert the kinetic energy of flow into electricity effectively, and this technique has become a new research focus in the field of ocean energy harvesting and utilization over the past few years. It is known that the lock-in effect existing in VIV phenomenon can significantly broaden the structural resonance region, and improve the flow energy harvesting efficiency. The present study aims to investigate the VIV responses of a flexible plate located in the wake of a circular cylinder using a strongly coupled finite element model. Numerical simulations are conducted for two typical plate lengths (L/D = 2.0, 4.0; D is the cylinder diameter) within a wide range of gap spacings 0.5 ≤ S/D ≤ 4.0. Generally, the results show that with the increase of gap spacing, the gap flow pattern changes from vortex suppression state to vortex shedding state. For L/D = 2.0, “soft” lock-in occurs all through the examined gap spacing range, among which the plate vibrates with significant amplitude. For L/D = 4.0, the lock-in region is found to be S/D ≤ 2.0, characterized by large-amplitude resonance and vortex suppression in the gap; however, with the appearance of the gap vortex shedding, the vibration becomes negligible at S/D ≥ 2.5. The pressure distribution of fluid along the upper and lower sides of the plate is analyzed to reveal the dynamic coupling mechanism between the flow and plate. This study will provide useful theoretical reference for optimum design of the VIV energy harvesting device.

Numerical investigation of flow characteristics around two side-by-side cylinders by immersed boundary-lattice Boltzmann flux solver

In this paper, to study the characteristics of the flow in a laminar regime, an immersed boundary-lattice Boltzmann flux solver (IB-LBFS) is applied to numerically simulate the unsteady viscous flows around two fixed and rotating circular cylinders in side-by-side arrangement. This method applies finite volume discretization to solve the macroscopic governing equations with the flow variables defined at cell centers. At the cell interface, numerical fluxes are physically evaluated by a local lattice Boltzmann solution. In addition, the no-slip boundary condition is accurately imposed by using the implicit boundary condition-enforced immersed boundary method. Due to the simplicity and high efficiency of IB-LBFS on non-uniform grids, it is suitable for simulating fluid flows with complex geometries and moving boundaries. Firstly, numerical simulations of laminar flow past two side-by-side cylinder are performed with different gap spacings at Reynolds numbers of 100 and 200. The simulation results show that a small gap spacing induces a biased flow and forms an irregular big wake behind two cylinders at a low Reynolds number. As the gap spacing increases, an in-phase or anti-phase flow is observed. Then, the effects of the main important parameters on flow characteristics are analyzed for flow past two side-by-side rotating cylinders, including the rotational speed, Reynolds number, and gap spacing. As the rotational speed is increased, the numerical results illustrate that unsteady wakes are suppressed and the flow becomes steady. As the gap spacing is increased, two separate vortex streets behind each cylinder are formed with a definite phase relationship and single shedding frequency.

The Impact of Landscape Fragmentation on Atmospheric Flow: A Wind-Tunnel Study

Landscape discontinuities such as forest edges play an important role in determining the characteristics of the atmospheric flow by generating increased turbulence and triggering the formation of coherent tree-scale structures. In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may lead to complex patterns of flow and turbulence that are potentially difficult to predict. Here, we investigate the effects of different levels of forest fragmentation on the airflow. Five gap spacings (of length approximately 5h, 10h, 15h, 20h, 30h, where h is the canopy height) between forest blocks of length 8.7h, as well as a reference case consisting of a continuous forest after a single edge, were investigated in a wind tunnel. The results reveal a consistent pattern downstream from the first edge of each simulated case, with the streamwise velocity component at tree top increasing and turbulent kinetic energy decreasing as gap size increases, but with overshoots in shear stress and turbulent kinetic energy observed at the forest edges. As the gap spacing increases, the flow appears to change monotonically from a flow over a single edge to a flow over isolated forest blocks. The apparent roughness of the different fragmented configurations also decreases with increasing gap size. No overall enhancement of turbulence is observed at any particular level of fragmentation.

Lattice Boltzmann Study of Wake Structure and Force Statistics for Various Gap Spacings Between a Square Cylinder with a Detached Flat Plate

A numerical study of uniform flow past a square cylinder in the presence of detached flat plate for various gap spacings using the multi-relaxation-time lattice Boltzmann method (MRT-LBM) has been undertaken. The Reynolds number is kept at 150 for all numerical calculations and the gap spacing (g) ranging from 0 to 11. Analyses of the vortex structure in the wake, the vortex shedding frequency, time-trace analysis of drag and lift coefficients and force statistics have been carried out. The existing results for flow past a square cylinder without detached flat plate are validated with the well-resolved results obtained experimentally and numerically. Numerical results reveal that as gap spacing increases, there exists a critical gap spacing at g = 2–2.25, where the mean drag coefficient, Strouhal number, root-mean-square values of the drag and lift coefficients reach either maxima or minima. Meanwhile, the shed vortices behind the detached flat plate are also affected. The observed results further indicate that there exists three different kinds of flow regimes: (i) extended-body flow regime (0 $${\leq}$$ g $${\leq}$$ 1.53); (ii) reattachment flow regime (1.9 $${\leq}$$ g $${\leq}$$ 4); and (iii) fully developed flow regime (4.8 $${\leq}$$ g $${\leq}$$ 11). The observed critical gap spacing and flow regimes are strongly dependent upon the gap spacings.

Numerical investigation of flow around an inline square cylinder array with different spacing ratios

Flow around an inline cylinder array consisting of six square cylinders at a Reynolds number of 100 is investigated numerically by using a second-order characteristic-based split finite element algorithm in this paper. The numerical method and the code for the solution of incompressible Navier–Stokes equations are validated for the flow past a single and two tandem square cylinders, and the numerical results show a good agreement with the available literatures. The study then focuses on the effect of spacing ratio (ratio of center-to-center distance s to cylinder width d, widely ranging in s/ d = 1.5–15.0) on flow characteristics by identifying flow patterns and extracting pressure distributions, force statistics as well as wake oscillation frequencies. Numerical results showed six different flow patterns, which appeared successively with the increase of gap spacing, namely, steady wake, non-fully developed vortex street in single row and double-row, fully developed vortex street in double-row, fully developed vortex street in partially recovered single-row and fully developed multiple vortex streets. A shielding effect of the first cylinder and reducing Bernoulli effect on the rear cylinder rows work in the pressure distribution even at very large gap spacing. In the vortex shedding regime, beyond the critical spacing of wake mode transition, force statistics show a periodic variation characteristic for the last four cylinders; moreover, multiple frequency components involve in the vortex shedding oscillation behind these cylinders and the dominant frequency jumps down with the increase of the spacing. Finally, the flow fields around the critical spacing range are comprehensively analyzed to reveal the crucial mechanism behind the observed aerodynamic characteristics.

Two-dimensional side-by-side circular cylinders at moderate Reynolds numbers

In a companion article [1], we described computer simulations of the flow around 2 two-dimensional, tandem circular cylinders in a flow for 1 ⩽ Re ⩽ 20 . In this article we adopt a similar approach to characterize the flow around side-by-side cylinders with surface-to-surface separation/diameter in the range 0.1 < s/ D < 30. The results revealed some distinct and interesting features of the flow, which are completely different than those observed at higher Reynolds numbers. At low Reynolds numbers, 1 ⩽ Re ⩽ 5 , for all gap spacings, the flow contains no regions of flow separation. At higher Re, four distinct flow behaviors were observed. For very small gap spacings, e.g. 0.1 < s/ D < 0.6 at Re = 20, two elongated “detached vortices” form downstream of the cylinders. The drag coefficient increases sharply with the gap spacing. For gap spacings 0.6 < s/ D < 0.7 at Re = 20, no vortices form anywhere in the flow. For gap spacings around s/ D ≈ 1 separation regions form only on the inside portions of the cylinders. For larger gap spacings s/ D > 1 the flow reverts to something similar to that around an isolated cylinder in the flow, i.e. two attached vortices on the rear side of each cylinder. In general, the drag coefficient increases as the gap spacing increases. At higher Reynolds number it is known that the cylinder lift coefficients decrease monotonically with gap spacing. In contrast, at these lower Reynolds number the lift coefficient curves rise to a maximum for 0.3 < s/ D < 3 and then decrease monotonically for larger s/ D.

Corona discharges and bubbling in liquid nitrogen

In liquid nitrogen, DC and AC corona discharge phenomena resemble each other. The corona inception voltage is smaller with the point negative than with the point positive, and increases as the gap spacing increases. Negative corona pulses are more frequent but smaller than positive ones. The height and repetition rate of the pulse with the point negative increase as the gap spacing decreases. The corona is accompanied by bubbling similar to a spray jet from the point tip. The bubbling is independent of the polarity of the applied voltage. Buoyancy has little influence on the corona.

Study of time lag of the needle gap

The study of high-voltage, steep-wave-front transients is difficult from the experimental standpoint because of the very short times involved. Due to the improvement which the cathode-ray oscillograph has enjoyed in recent years, a device is now available, by the use of which transients occuring in times as short as one-millionth of a second or less may be photographed. In the paper, the authors used an oscillograph developed by Dufour in France, with which a brief study was made of the time lag of needle gaps and of a needle to a plane. A description of the oscillograph is given including a discussion of the method of operation. The photographic film is placed inside the tube so that the electrons impinge directly on the film. The wave is drawn out along a time axis by the combined action of a sweeping motion and a perpendicular oscillating motion imparted to the electron stream by the action of proper electromagnetic fields. Tests were made with a wave which was nearly perpendicular, reaching its maximum in about one microsecond. Such a wave was obtained by the discharge of a condenser through a suitable circuit. An oscillogram which shows the wave front used is given, and attention is directed to the 20,800 kilocycle oscillation which appears superimposed on the wave front. The results of tests in which this wave front was applied to gaps are given and it is shown that with any given gap setting and sparking voltage that the time lags vary through wide limits. It is also shown that, for the same voltage, increased gap settings mean increased lag. The per cent over voltage, required to keep the lag to two microseconds or less, decreases as the gap spacing increases.