Downstream Plate Research Articles

A rational method for determining intermittency in the transitional boundary layer

A new rational procedure is proposed for determining the intermittency in the streamwise direction. One of the key parameters for the intermittency determination is the selection of a threshold value, which often involves a certain level of subjectivity. Here, a reliable way of choosing the threshold value in a more objective manner is proposed. The proposed approach involves a single threshold value, equal to the magnitude of the maximum laminar perturbation in the transitional flow. The results obtained are validated with the widely used dual-slope method. In this paper, the measurements are carried out on an experimental arrangement, involving the interaction of an upstream aerofoil wake with a downstream flat plate boundary layer. As a by-product of the study, a scaling parameter has been identified which captures the length of the transition zone as the proximity of the aerofoil in the wall-normal direction is varied.Graphic abstract

LES of Flow Through and Around a Finite Patch of Thin Plates

AbstractLarge eddy simulations (LESs) are performed for turbulent flow through and around a porous patch of thin vertical plates at a plate Reynolds number of Rep=5,800. The plates are arranged in a staggered pattern, presenting an elliptical planform and mimicking streamwise‐oriented blades of emergent vegetation. The immersed boundary method is employed to explicitly resolve the interaction between flow and plates. Three flow cases, each with a different number of plates within the same planform area, that is, different patch density, are studied. The Reynolds number based on freestream velocity and plate length is the same in all cases. Inspection of the distribution of velocity and vorticity in the horizontal plane reveals that downstream plates are significantly impacted by the wakes from upstream plates. It is therefore proposed that the plates can be divided into two groups based on the local flow characteristics, which are a function of position within the patch: a free group and a wake group. This classification is subsequently used in the quantitative analysis of boundary layer development and drag force at plate scale. The thickness and character of the simulated boundary layers on the plates differ significantly from predictions based on analytical or empirical relationships, which is due to wake effects and the finite length of the plates. The simulations demonstrate the so‐called sheltering effect; that is, the drag forces acting on downstream plates (in the wake group) are significantly lower than those acting on upstream plates, a result of the lower approach flow speed. Although the front‐area‐to‐lateral‐area ratio of the plates is low (1/40), pressure drag is observed to be larger than friction drag for each plate. The ratio of pressure drag to the total drag at patch scale shows only very little dependence on the plate density of the patch.

On the couple dynamics of wall-mounted flexible plates in tandem

The dynamics of two rectangular, flexible plates of low aspect ratio $h/b$ (height/width = 4) was experimentally investigated in tandem arrangements under uniform flows at various Cauchy numbers $Ca=\unicode[STIX]{x1D70C}_{f}bU_{0}^{2}h^{3}/(EI)\in [15,77]$ (where $\unicode[STIX]{x1D70C}_{f}$ is the fluid density, $U_{0}$ is the incoming flow velocity, $E$ is the Young’s modulus and $I$ is the second moment of the area) and spacings $s_{x}=\unicode[STIX]{x0394}x/h=0.5$, 1 and 2. Planar particle image velocimetry (PIV) and particle tracking velocimetry (PTV) were used to characterize the surrounding flow field and oscillations of the wall-mounted structures. Results show that the oscillations of the upstream plate were dominated by its natural frequency. However, the motions of the downstream plate were significantly modulated by the induced flow and coherent motions shed from the upstream structure. Such modulation led to highly correlated motions between the plates with similar amplitude in the case $s_{x}=0.5$, to comparatively larger oscillation amplitude of the downstream plate in the intermediate separation $s_{x}=1$, and roughly decoupled interaction for $s_{x}=2$. Despite that the intensity of the oscillations of the upstream plate increased monotonically with $Ca$, this was not the case for the downstream plate at $s_{x}=1$ and 2 due to flow fluctuations, vortex shedding and large structure deformation; as a result, it exhibited a local minimum. Supported with measurements, a mathematical model was derived to quantitatively explain this behaviour.

Distribution characteristics of the flow around four flat plates in staggered at different arrangements

The three-dimensional turbulent flow around plate cascades in staggered is an important factor to induce hydro-mechanical vibration. Taking the upstream three plates in side-by-side configuration and the downstream single plate in tandem configuration to form the overall staggered layout of the cascades as the subject of investigation. Based on the Large-Eddy-Simulations (LES), the forces experienced by the plates, velocity distributions and three-dimensional vortex structures around the plates are analysed in detail. Some experiments were also carried out to validate the accuracy of the numerical results by LDA measurement. The mean velocity distributions from LES results agree fairly well with the experimental results. Results show that the existence of the downstream plate hinders the development of the upstream wake and has a significant inhibitory effect on the forces experienced by the upstream plates; When the gap spacing is smaller, the stream-wise length of the reverse flow region between the upstream and downstream plate becomes larger, and the minimum velocity in this region is lower than other reverse flow regions; The flow around the plate cascades exhibits high three-dimensional characteristics, and the amplitude of the fluctuation of the forces on the downstream plate under the interference of the upstream wakes is increased obviously. The downstream plate is located in the wake of the upstream cascade, and has a steady flow to the upstream wake. Analysis on the flow can provide theoretical support for the flow control in hydraulic machinery.

Flow-induced vibration of a square cylinder and downstream flat plate associated with micro-scale energy harvester

The phenomenon of flow-induced vibration (FIV) over a square cylinder at Reynolds numbers, Re = (3.6–12.5 × 103) is numerically studied. This current study provides a detailed explanation of the behaviour of transverse motion of square cylinder with the mass damping ratio, mζ* = 2.48. The computation of FIV is conducted by numerical simulation based on the Unsteady Reynolds Navier-Stokes (URANS) flow field using OpenFOAM software. The first part of the numerical simulation consists of an isolated square cylinder to validate the solution with previous studies. The computation of FIV with a total number of cells, N = 101,662 have shown a comparable pattern of amplitude curve. The coexistence of vortex-induced vibration (VIV) and galloping is observed for a single isolated cylinder. A downstream flat plate is introduced in the second part of the work. Different gaps separation between the cylinder and flat plate (0.1 ⩽G/D ⩽ 3) are simulated. Based on the amplitude curve against reduced velocities 4 ⩽UR⩽ 20, four regimes are identified. According to the power estimation, the optimum gap separation is G/D = 0.1. The harnessed power is higher than a single isolated square cylinder while preserving the robustness for the remote harvesting purpose.

Boundary layer instabilities in mixed convection and diffusion flames with an unheated starting length

The following study examines the role of streaklike coherent structures in mixed convection via a horizontal heated boundary layer possessing an unheated starting length. The three-dimensionality of flows in this configuration, which is regularly encountered in practical scenarios, has been experimentally probed using non-invasive detection methods. Experiments were conducted in a wind tunnel at the Missoula Fire Sciences Lab, and the wind speed was varied from 0.70 to 2.47 m/s. The buoyant source was varied significantly by either manipulating the surface temperature of a downstream hot plate or employing a diffusion flame. Streaks were visualized in the flow by means of infrared imaging or high speed video, and a novel detection algorithm was developed to quantify important properties and to spatially track these structures over time. Lognormal distributions of spacing were observed initially, and gradual deviations from this fit indicated a deviation from streaklike behavior. The onset of streaks was determined to be controlled by the pre-existing disturbances populating the incoming boundary layer. Further downstream, buoyant forces dominated the growth and deformation of these structures, whose length scale increased significantly. The width of structures was observed to asymptote to a stable value downstream, and this was determined to be a consequence of the finite distance over which heating was applied.

Downstream flat plate as the flow-induced vibration enhancer for energy harvesting

The prospect of harvesting energy from flow-induced vibration using an elastic square cylinder with a detached flat plate is experimentally investigated. The feasibility of flow-induced vibration to supply an adequate base excitation for micro-scale electrical power generation is assessed through a series of wind tunnel tests. The current test model of a single square cylinder is verified through a comparable pattern of vibration amplitude response with previous experimental study and two-dimensional numerical simulations based on the unsteady Reynolds averaged Navier–Stokes (URANS). In addition, a downstream flat plate is included in the wake of the square cylinder to study the effects of wake interference upon flow-induced vibration. A downstream flat plate is introduced as the passive vibration control to enhance the magnitude of flow-induced vibration and simultaneously increases the prospect of harvesting energy from the airflow. The study is conducted by varying the gap separation between the square cylinder and flat plate for 0.1≤ G/D ≤3. The highest peak amplitude is observed for the gap G/D = 1.2 with yrms/D = 0.46 at UR = 17, which is expected to harvest ten times more energy than the single square cylinder. The high amplitude vibration response is sustained within a relatively broader range of lock-in synchronization. Meanwhile, for G/D = 2 the vibration is suppressed, which leads to a lower magnitude of harvested energy. Contrarily, the amplitude response pattern for G/D = 3 is in agreement with the single square cylinder. Hence, the flat plate has no significance to the wake interference of the square cylinder when the gap separation is beyond 3D.

Anomalous ionization loss of high-energy e+e− pairs in thin targets

The problem of ionization loss of a high-energy electron-positron pair in a system of two plates – thick upstream, in which the pair is created, and thin downstream – is considered. It is shown that the process of transition radiation formation after the pair emission from the upstream plate significantly influences upon its ionization loss in the downstream plate. The consideration is made without the use of approximation of parallel electron and positron velocities which is widely applied for calculation of pair ionization loss. It is demonstrated that such approach reveals new peculiar features of asymptotical behavior of ionization loss in this process at large distance between the plates.

A high-efficiency tunable TEM-TE11 mode converter for high-power microwave applications

The tunable high power microwave source (HPM’s) is considered to be an important research direction. However, the corresponding mode converter has been researched little. In this paper, a high-efficiency tunable mode converter (HETMC) is investigated for high-power microwave applications. The HETMC that is consisted of coaxial inner and outer conductors, with four metal plates arranged radially, at 90° in the coaxial gap, and matching rods can transform coaxial transverse electromagnetic (TEM) mode to TE11 coaxial waveguide mode. The results show that adjusting the length of the downstream plate, and the distance between the rods installed upstream and the closest edges of the plates, can improve the conversion efficiency and bandwidth remarkably. Moreover, when the frequency ranges from 1.63 GHz to 2.12 GHz, the conversion efficiency is above 95% between 1.63 GHz and 2.12 GHz with a bandwidth of 26.1%. Besides, the unwished reflection and transmission can be eliminated effectively in the HETMC.

Numerical investigation of fluid flow past a square cylinder using upstream, downstream and dual splitter plates

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.

Experimental and numerical evaluation of the performance of a novel compound demister

A novel compound demister that combines an upstream tube bank and downstream wave plates was proposed in this work for application in multistage flash (MSF) desalination process. Its performance was evaluated by experimental and numerical methods. Compared with the individual tube-bank and wave-plate demisters, the compound demister is found to have the highest separation efficiency (>95%) with much less fluctuation for a wide range of gas velocities. At low gas velocities (≤4m/s) or for removing small droplets (<20μm), the separation efficiency of the compound demister is much higher than that of the wave-plate demister mainly because of the large separation capability of the tube bank. At high gas velocities (>4m/s), the compound demister shows higher resistance to droplet re-entrainment that occurs at inlet gas velocity of approximately 7m/s compared with the tube-bank demister. This is due to the compensation from the wave plates in the compound demister that separate secondary droplets generated by tubes. The compound demister possesses higher dry pressure drops than either the tube-bank or wave-plate demister, but is acceptable for industrial application. All these advantages make the compound demister a promising candidate for droplet removal in the desalination process.

Impinging nonpremixed coflow methane–air flames with unity Lewis number

The present work extends earlier theoretical models in order to analyze nonpremixed flames stabilized between an axisymmetric coflow burner and a downstream plate that produces a stagnating flow field. This unusual configuration (i.e., it is not a coflow nor a counterflow flame but an impinging nonpremixed flame) is analyzed with two approaches previously developed as extensions to the Burke–Schumann theory: the Extended Roper Model and the Stream Function as a Coordinate Model. The flow field is obtained through the solution of a stagnation point viscous flow for incompressible fluids between stationary porous plates. In order to simplify the analysis, unity Lewis number is considered. Infinitely fast conversion of the reactants into products is assumed together with the Shvab–Zel’dovich coupling function approach, which allows the solution of a reduced number of conservation equations free of source terms. The influence of the distance between the impinging plate and the burner tip on a methane–air flame is analyzed and the semi-analytical results are compared with experimental data for several vertical positions of the stagnation plate with respect to the burner port. The model is able to effectively capture general aspects of the flame such as its shape, standoff distance, and separation distance at which flame tip opening is observed.

Interactions between two flat plates at different positions in a current

Two-dimensional flow over two inclined flat plates in a staggered arrangement at low Reynolds numbers has been investigated by numerical simulation. The two flat plates with the same length d and thickness are modeled using the immersed boundary method. The spacing between the centers of the two plates in streamwise and transverse direction is in the range of s/d≤6.0 and t/d≤0.8, respectively, with one plate always located in the wake interference area of the other plate. The lift, drag, and moment coefficients are investigated systematically based on the variation of the spacing and the inclination angles of the two plates. The frequency domain was obtained through fast Fourier transform of the velocity component. The results show that, for cases with s/d<2.5, both of the plates are suffering significant effect due to the existing of the other plate. The drag and lift forces on the upstream plate decrease with 17% and 20%, while the drag and lift forces on the downstream plate decrease with 52% and 66%. For cases with s/d>2.5, the interaction to the upstream plate is smaller compared to the downstream plate, and new frequencies are exited in the wake of the second plate.

Sound from high-Reynolds number flow over bluff bodies

Purpose – This paper aims to investigate the aerodynamic sound generated from flow over bluff bodies at a high Reynolds number. By taking circular and square cylinders as two representative geometries for the cross-section of bluff bodies, this study aims to clarify the difference in flow formation and sound generation between the two types of bluff bodies. Furthermore, the possibility for a downstream flat plate to be used as sound cancellation passive mechanism is also discussed in this study. Design/methodology/approach – Sound source from the near field is numerically solved by using the Unsteady Reynolds-Averaged Navier Stokes equations. While for the sound at far-field, the compact sound theory of Curle’s analogy is used. Findings – Magnitude of the generated sound is dominant by the aerodynamic forcer fluctuations, i.e. lift and drag, where the lift fluctuation gives the strongest influence on the sound generation. The square cylinder emits 4.7 dB higher than the sound emitted from flow over the ci...

Integrated Air Preheater and Anode Off-Gas Oxidizer

The performance of a cathode air preheater with internal off-gas oxidation is presented. This component is a plate heat exchanger, handling both injection and oxidation of depleted anode gas in the hot cathode flow. The gas mixture oxidizes in the heat exchanger, and the reaction heat is used to preheat cathode air. An initial integration effort using two separate components is reported. The system consists of a close-coupled injector and a downstream plate heat exchanger, which was catalytically coated. The performance was constrained by reaction kinetics: measurements showed that the off-gas and cathode mixture typically ignited within 5 – 10 ms at mixture temperatures above 750°C. CFD on this system shows that part of the mixture takes more than 15 ms before reaching the inlet port of the heat exchanger, leading to early ignition. Both CFD and SEM analysis showed that the walls of the heat exchanger overheat to 1000 – 1100 °C. The SEM analysis reveal the effect of this overheating on scale growth and evaporation of Cr VI in the heat exchanger, and its subsequent condensation on the cool, catalytically coated walls. The integrated HEX and oxidizer consists of an internal injector, which delivers the anode gas to each cathode flow path between pairs of heat exchanging plates. The injection occurs adjacent to a catalytically coated zone in the heat exchanger. The CFD effort covers the combined effects of injection, mixing, generation of reaction heat and heat exchange. The results show that the injection and subsequent mixing is uniform. The mixture reaches the cooled sections of the heat exchanger within 3 ms, and thus below the auto ignition time. These calculations are in line with experimental performance data on the integrated component. Figure 1

Control of vortex shedding and drag reduction through dual splitter plates attached to a square cylinder

In this paper we have made a numerical study on the control of vortex shedding and drag reduction of a cylinder by attaching thin splitter plates. The wake structure of the cylinder of square cross-section with attached splitter plates is analyzed for a range of Reynolds number, based on the incident stream and height of the cylinder, in the laminar range. The Navier-Stokes equations governing the flow are solved by the control volume method over a staggered grid arrangement. We have used the semi-implicit method for pressure-linked equation (SIMPLE) algorithm for computation. Our results show that the presence of a splitter plate upstream of the cylinder reduces the drag, but it has a small impact on the vortex shedding frequency when the plate length is beyond 1.5 time the height of the cylinder. The presence of a downstream splitter plate dampens the vortex shedding frequency. The entrainment of fluid into the inner side of the separated shear layers is obstructed by the downstream splitter plate. Our results suggest that by attaching in-line splitter plates both upstream and downstream of the cylinder, the vortex shedding can be suppressed, as well as a reduction in drag be obtained. We made a parametric study to determine the optimal length of these splitter plates so as to achieve low drag and low vortex shedding frequency.

Effect of arrangement of inline splitter plate on flow past square cylinder

Two-dimensional numerical study has been carried out to investigate the effect of the presence of a single splitter plate in upstream location, and presence of two splitter plates, in upstream as well as downstream locations, for low Reynolds number incompressible flow past a square cylinder. Using finite difference discretisation and MAC algorithm, an indigenously developed computational code has been used for solution of the governing flow equations. The length of the plate and its position do play good role in controlling the flow past the cylinder. The difficulty lies in identifying an optimum configuration of these parameters. The length of the attached splitter plate as well as separation of the detached splitter plate of fixed length are varied for both the upstream and downstream configurations up to maximum Reynolds number of 200. To begin with, the effect of upstream splitter plate alone in attached as well as detached configuration is studied with an aim to identify the optimum configuration associated with reduced drag as well as delayed onset of vortex shedding. Then, for the optimum configuration of the upstream plate, the effect of downstream plate on flow characteristics and critical Reynolds number for onset of vortex shedding has been studied.

Development of ion source for simulation of edge localized mode in divertor plasma

A helium ion beam is injected into a linear plasma device for the development of an ion beam source simulating high energy particle flux in divertor plasma. Beam current density more than 10 mA/cm(2) is extracted. Measurement of beam currents indicates that the beam is transported along the linear device and reaches to the downstream end plate.

Development and utilization of ``Plate Changer'' system for neutrino interaction locations in OPERA emulsion target

In the OPERA experiment, so-called Scan Back method isused to locate neutrino interaction vertices in the emulsion target namedECC (Emulsion Cloud Chamber). In Scan Back method, tracks detected in themost downstream emulsion plate in ECCs are followed to upstream plate byplate until it reaches to the interaction points. In order to treat a numberof neutrino interactions recorded in OPERA, dedicated systems called PlateChanger has been developed and utilized to neutrino event location in Japan.The details of the system have been described in this report. Until the endof November 2012, 6223 Scan Back trials have been performed using thissystem, and strongly contributed to the detection of tau neutrinoappearance.

VIVs of a circular cylinder with a downstream strip-plate in cruciform arrangement

Earlier wind tunnel experiments by the present authors on the transverse vibration of a circular cylinder with a downstream counterpart in cruciform arrangement showed that either of the two longitudinal vortices – trailing or necklace vortices, depending on the gap-to-diameter ratio s/d – induces a resonant oscillation, like the Kármán-vortex-induced vibration (KVIV). In this study, the downstream cylinder is replaced by a strip plate with a width w comparable in magnitude with the cylinder diameter d, expecting that interference of a plate would be stronger than that of a circular cylinder. The characteristics of longitudinal vortices and VIV of the upstream cylinder are investigated through wind tunnel experiments over the range of Reynolds number 3000−22000, as well as a visualization experiment in a water tunnel at around Re=1700. When the system is fixed, the two longitudinal vortices (trailing and necklace) shed periodically over respective ranges of s/d for plates with 0.5≤w/d≤0.62, while the trailing vortex sheds periodically but the necklace vortex is not observed for plates with 0.75≤w/d≤1.0. When the cylinder is elastically supported for transverse vibration, the plate effectively suppresses KVIV, at distances further downstream for larger w/d plates. An unexpected effect is that KVIV is enhanced by the plate with 0.5≤w/d≤1.0 over a certain range of s/d. The two longitudinal vortices induce resonant vibrations under conditions of w/d and s/d correlating with their characteristics for the fixed system. The velocity ranges of the trailing vortices are much wider than for the case of the two-circular-cylinder system. The effects of the downstream plate on VIV can be utilized for oscillation control, i.e., not only to suppress but also to induce oscillations for beneficial purposes.