Boundary Layer Forcing Research Articles

Effect of typical arch structure on slipstream and wake flow of 600 km/h maglev train

PurposeThe aerodynamic differences between the head car (HC) and tail car (TC) of a high-speed maglev train are significant, resulting in control difficulties and safety challenges in operation. The arch structure has a significant effect on the improvement of the aerodynamic lift of the HC and TC of the maglev train. Therefore, this study aims to investigate the effect of a streamlined arch structure on the aerodynamic performance of a 600 km/h maglev train.Design/methodology/approachThree typical streamlined arch structures for maglev trains are selected, i.e. single-arch, double-arch and triple-arch maglev trains. The vortex structure, pressure of train surface, boundary layer, slipstream and aerodynamic forces of the maglev trains with different arch structures are compared by adopting improved delayed detached eddy simulation numerical calculation method. The effects of the arch structures on the aerodynamic performance of the maglev train are analyzed.FindingsThe dynamic topological structure of the wake flow shows that a change in arch structure can reduce the vortex size in the wake region; the vortex size with double-arch and triple-arch maglev trains is reduced by 15.9% and 23%, respectively, compared with a single-arch maglev train. The peak slipstream decreases with an increase in arch structures; double-arch and triple-arch maglev trains reduce it by 8.89% and 16.67%, respectively, compared with a single-arch maglev train. The aerodynamic force indicates that arch structures improve the lift imbalance between the HC and TC of a maglev train; double-arch and triple-arch maglev trains improve it by 22.4% and 36.8%, respectively, compared to a single-arch maglev train.Originality/valueThis study compares the effects of a streamlined arch structure on a maglev train and its surrounding flow field. The results of the study provide data support for the design and safe operation of high-speed maglev trains.

Unprecedented July Rainfall in North China in 2021: Combined Effect of Atlantic Warming and Arctic Sea‐Ice Loss

AbstractRecord‐breaking July‐mean precipitation hit North China in 2021, resulting in huge economic losses and human casualties. The prevailing easterlies over the southern flank of a remarkable anomalous anticyclone centered over the Kuril Islands transported sufficient water vapor into North China, resulting in persistent downpours on the windward side of the North China Plain. The anomalous anticyclone was a portion of the Eurasian Rossby wave trains. The record‐high Atlantic sea surface temperature anomalies induced the Rossby wave trains, which ended with an anomalous anticyclone over the Kuril Islands; meanwhile, the record‐low sea‐ice concentration over the Laptev–East Siberian Seas reinforced the anomalous anticyclone. Therefore, the combined effect of Atlantic and Arctic thermal forcings caused the record‐breaking July‐mean rainfall in North China in 2021. Using these two boundary layer forcings as independent predictors to run a multiple regression model produced encouraging hindcasting skills.

Wind-tunnel tests of synthetic jet control effects on airfoil flow separation

To find out the comprehensive principles of synthetic jet control effects on airfoil stall characteristics, fundamental contrastive wind-tunnel tests were conducted systematically. By using six-component balance, Particle Image Velocimetry (PIV) technology and boundary layer probe, the measurements of model aerodynamic forces, the whole velocity field over airfoil and velocity profiles in the boundary layer were conducted, respectively. Based upon the experimentally parametric analyses of synthetic jet control on airfoil, it is concluded that the incline angle of synthetic jet has an important impact on both the flow in boundary layer and aerodynamic forces of airfoil. A tangential jet inject energy and accelerate the velocity of flow in inner boundary layer thus has a better control effects on delay stall of airfoil when the momentum coefficient of jet is relatively large, and the normal jet helps to enlarge the thickness of boundary layer which is proved to have better control effects on flow separation control and improving the aerodynamic characteristics of airfoil when the momentum coefficient of a jet is relatively small.

Stochastic disturbances, induced by plasma actuator in a flat plate boundary layer

Mechanism of hydrodynamic noise generation in a subsonic flat plate boundary layer by a barrier discharge plasma actuator is described. The origin of the pulsations is an inscintric unsteadiness of the discharge structure caused by wandering of the microdischarges. Statistics of discharge wandering is obtained from discharge light emission. Propagation of the disturbances in a slightly unstable Blasius boundary layer is studied both experimentally and numerically. It is demonstrated that the discharge-induced noise can be modeled as a sum of delta-correlated localized boundary layer forcing events, with each event represented by the region of longitudinal and transversal force. Discharge-induced disturbances in the boundary layer undergo three main stages as they move downstream: streak-like structures in the near field, oblique wave fans, and eventually a plane Tollmien–Shllichting wave. A simple statistical model, describing the dependency of the pulsations power on actuator driving frequency and voltage, is proposed.

Causality in the shock wave/turbulent boundary layer interaction

What drives flow unsteadiness in the shock wave/turbulent boundary layer interaction? Two mechanisms are widely recognized to play a role in this problem: upstream boundary layer forcing and downstream feedback. In this work, through the combined use of system identification, spectral decomposition, and linear stability analysis, we demonstrate that the downstream feedback, driven by upstream traveling acoustic waves, is the dominant mechanism behind the low-frequency shock motion. The tools described herein enable an accurate reconstruction of the shock fluctuations and could find potential applications in active flow control.

A Cartesian grid method with improvement of resolving the boundary layer structure for two‐dimensional incompressible flows

AbstractThe Cartesian gird has its unique advantages in computational fluid dynamics, especially for complicated boundary cases. However, the boundary layer structures can not be resolved efficiently and effectively using the Cartesian mesh. To overcome such a problem, a new boundary layer structure resolving (BLSR) algorithm is proposed on the basis of the boundary layer physics and force balance analysis. For the present two‐dimensional test cases, numerical results justify that the surface friction and drag force can be more accurately calculated without refining the near‐wall resolution. In principle this BLSR algorithm is easy to implement with negligible increase of the computational cost.

Stirring of Sea‐Ice Meltwater Enhances Submesoscale Fronts in the Southern Ocean

AbstractIn the sea‐ice‐impacted Southern Ocean, the spring sea‐ice melt and its impact on physical processes set the rate of surface water mass modification. These modified waters will eventually subduct near the polar front and enter the global overturning circulation. Submesoscale processes modulate the stratification of the mixed layer (ML) and ML properties. Sparse observations in polar regions mean that the role of submesoscale motions in the exchange of properties across the base of the ML is not well understood. The goal of this study is to determine the interplay between sea‐ice melt, surface boundary layer forcing, and submesoscale flows in setting properties of the surface ML in the Antarctic marginal ice zone. High‐resolution observations suggest that fine‐scale lateral fronts arise from either/both mesoscale and submesoscale stirring of sea‐ice meltwater anomalies. The strong salinity‐driven stratification at the base of the ML confines these fronts to the upper ocean, limiting submesoscale vertical fluxes across the ML base. This strong stratification prevents the local subduction of modified waters by submesoscale flows, suggesting that the subduction site that links to the global overturning circulation does not correspond with the location of sea‐ice melt. However, surface‐enhanced fronts increase the potential for Ekman‐driven cross‐frontal flow to modulate the stability of the ML and ML properties. The parameterization of submesoscale processes in coupled‐climate models, particularly those contributing to the Ekman buoyancy flux, may improve the representation of ML heat and freshwater transport in the ice‐impacted Southern Ocean during summer.

A Physically Based Stochastic Boundary Layer Perturbation Scheme. Part I: Formulation and Evaluation in a Convection-Permitting Model

AbstractWe present a simple, physically consistent stochastic boundary layer scheme implemented in the Met Office’s Unified Model. It is expressed as temporally correlated multiplicative Poisson noise with a distribution that depends on physical scales. The distribution can be highly skewed at convection-permitting scales (horizontal grid lengths around 1 km) when temporal correlation is far more important than spatial. The scheme is evaluated using small ensemble forecasts of two case studies of severe convective storms over the United Kingdom. Perturbations are temporally correlated over an eddy-turnover time scale, and may be similar in magnitude to or larger than the mean boundary layer forcing. However, their mean is zero and hence they, in practice, they have very little impact on the energetics of the forecast, so overall domain-averaged precipitation, for example, is essentially unchanged. Differences between ensemble members grow; after around 12 h they appear to be roughly saturated; this represents the time scale to achieve a balance between addition of new perturbations, perturbation growth, and dissipation, not just saturation of initial perturbations. The scheme takes into account the area chosen to average over, and results are insensitive to this area at least where this remains within an order of magnitude of the grid scale.

Wind Tunnel Investigations of Aeroelastic Electricity Transmission Tower under Synoptic and Typhoon Winds

Wind tunnel tests of an aeroelastic tower model at a length scale of 1∶40 were conducted to investigate the dynamic responses of an electricity transmission tower under synoptic and typhoon winds. The aeroelastic tower model was successively subjected to boundary layer and typhoon wind forcing in a single wind tunnel. The purpose of this study is to assess differences of the dynamic structural responses under two types of wind forcing and understand the underlying mechanism of transmission tower failures due to strong typhoons. The results indicated that the acceleration responses of the aeroelastic tower model were composed of resonant and background parts, while dominated by the resonant counterpart, and the dynamic responses of the transmission tower could be increased by up to 30% in the along-wind and crosswind directions under typhoon wind forcing. The wind load factors determined from the experimental results under typhoon winds were generally larger than those obtained from Chinese codes and standards. Suggestions on the wind-resistant design of an overhead transmission tower in typhoon-prone areas were proposed. The dynamic wind loads should be substituted by the most conservative values among different Chinese standards in estimating the equivalent static wind loadings of overhead towers under typhoon wind forcing.

Influence of various factors on the spreading kinetics

It was found that at the stage of fast spreading (τ≤ 0,02c) at constant driving force within the wetting perimeter, the following exerts the main impact on the spreading kinetics: inertial forces that prevent spreading and change their sign when the speed slows, viscous resistance forces, the influence of which increases in proportion to the growth of the thickness of the boundary layer and vertical forces, which begin to appear when the spreading velocity slows down.

Diurnal cycle of precipitation over the Maritime Continent simulated by a spectral cumulus parameterization

The validity of a spectral cumulus parameterization (spectral scheme) for simulating a diurnal cycle of precipitation over the Maritime Continent (MC) was examined using a regional atmospheric model. The impacts of entrainment parameterization and each type of convective closure, i.e., non-equilibrium (or equilibrium) closure for deep convection, mid-level, and shallow convective closures, were also examined. When vertically variable entrainment and appropriate convective closures were employed, the model adequately simulated a diurnal cycle of precipitation over both land and ocean as compared to the observation. Analysis regarding the entrainment parameterization revealed that variable entrainment parameterization was needed not only for simulating better mean patterns of precipitation, but also for more realistic phases of diurnal cycles. The impacts of convective closures appeared in the differences in the precipitation amplitude. Analysis on diurnal cycles of convective properties and tendencies revealed that the cycles between boundary layer forcing and convective heating determined convection strength and were affected by each type of convective closure. It can be concluded that the spectral scheme with appropriate convective closures is able to simulate a realistic diurnal cycle over the MC.

Generalized Polynomial Chaos With Optimized Quadrature Applied to a Turbulent Boundary Layer Forced Plate

We explore the use of generalized polynomial chaos (GPC) expansion with stochastic collocation (SC) for modeling the uncertainty in the noise radiated by a plate subject to turbulent boundary layer (TBL) forcing. The SC form of polynomial chaos permits re-use of existing computational models, while drastically reducing the number of evaluations of the deterministic code compared to Monte Carlo (MC) sampling, for instance. Further efficiency is attained through the application of new, efficient, quadrature rules to compute the GPC expansion coefficients. We demonstrate that our approach accurately reconstructs the statistics of the radiated sound power by propagating the input uncertainty through the computational physics model. The use of optimized quadrature rules permits these results to be obtained using far fewer quadrature nodes than with traditional methods, such as tensor product quadrature and Smolyak sparse grid methods. As each quadrature node corresponds to an expensive deterministic model evaluation, the computational cost of the analysis is seen to be greatly reduced.

Mean and turbulence dynamics in unsteady Ekman boundary layers

Unsteady pressure gradients in turbulent flows not only influence the mean, but also affect the higher statistical moments of turbulence. In these flows, it is important to understand if and when turbulence is in quasi-equilibrium with the mean in order to better capture the dynamics and develop effective closure models. Therefore, this study aims to elucidate how turbulence decays or develops relative to a time-varying mean flow, and how the turbulent kinetic energy (TKE) production, transport and dissipation respond to changes in the imposed pressure forcing. The focus is on the neutral unsteady Ekman boundary layer, where pressure-gradient, Coriolis and turbulent friction forces interact, and the analyses are based on a suite of large-eddy simulations with unsteady pressure forcing. The results indicate that the dynamics is primarily controlled by the relative magnitudes of three time scales: the inertial time scale (characterized by Coriolis frequency${\sim}12$ hours at mid-latitudes), the turbulent time scale (${\sim}2$ hours for the largest eddies in the present simulations) and the forcing variability time scale (which is varied to reflect different (sub)meso to synoptic scale dynamics). When the forcing time scale is comparable to the turbulence time scale, the quasi-equilibrium condition becomes invalid due to highly complex interactions between the mean and turbulence, the velocity profiles manifestly depart from the log-law and the normalized TKE budget terms vary strongly in time. However, for longer, and surprisingly for shorter, forcing times, quasi-equilibrium is reasonably maintained. The analyses elucidate the physical mechanisms that trigger these dynamics, and investigate the implications on turbulence closure models.

Identification of forcing mechanisms of convective initiation over mountains through high-resolution numerical simulations

Convection and its ensuing severe weather, such as heavy rainfall, hail, tornado, and high wind, have significant impacts on our society and economy (e.g., Cao et al., 2004; Fritsch and Carbone, 2004; Verbout et al., 2006; Ashley and Black, 2008; Cao, 2008; Cao and Ma, 2009; Zhang et al., 2014). Due to its localized and transient nature, the initiation of convection or convective initiation remains one of the least understood aspects of convection in the scientific communities, and it is a significant challenge to accurately predict the exact timing and location of convective initiation (e.g., Cai et al., 2006; Wilson and Roberts, 2006; Xue and Martin, 2006; Cao and Zhang, 2016). Convective initiation can be attributed to various forcing mechanisms, including, but not limited to, upper-level forcing, boundary-layer forcing, and a combination of them (e.g., Bennett et al., 2006; Lopez, 2007; Marsham et al., 2011). For surface-based convective initiation, boundary-layer forcing often determines the precise location where convection is triggered within large areas of potential instability (e.g., Pielke, 2001). The most common boundary-layer triggering mechanisms include surface fronts, drylines, gust fronts, sea/lake land breezes, orographic circulations, and boundary layer horizontal convective rolls. In addition, inhomogeneity in land surface characteristics can also produce sufficient vertical motion to force convection, particularly under strong surface heating conditions (Trier et al., 2004). Various possible mechanisms for convective initiation are generally recognized, but the exact forcings responsible for a specific case are often difficult to ascertain, especially when multiple physical processes and their interactions are involved. In this issue (Page 1120–1136), Wang et al. (2016) devote themselves to the understanding of the physical mechanisms responsible for convective initiation over the Dabie Mountains located in eastern China. By performing numerical simulations and sensitivity experiments for a real case associated with weak synoptic flows of Meiyu front, the

A numerical Euler–Lagrange method for bubble tower CO2 dissolution modeling

The present paper proposes an application for an Euler–Lagrangian one-way coupling method for CO2 in water bubble tower reactor design. As a first step to model a large-scale bubble reactor a single bubble while rising and dissolving can be modeled as a spherical body submitted to punctual forces. Combined with a description for the decreasing mass and size of a dissolving bubble in an aqueous solution this will be basis for bubble swarm modeling. A link between mass transfer boundary layer and bubble force is established through the mass time derivative of the dissolving bubble as function of the local Reynolds, Schmidt and Sherwood numbers. Furthermore, the turbulent mixer fluid environment of a stirred reactor is presented as the basis of this research work for a slice of fluid and a rotating mesh model is presented as the basis for the simulation. A turbulent flow interface (k–ɛ turbulence) from a commercial software is used. Finally, bubble swarm displacement is introduced and bubbles are tracked individually using a Lagrangian spherical particle approach. Aqueous CO2 concentration as function of time and position is studied. Results are of relevance for CO2 mineralization processes, specifically for the bubble reactor used in the so-called Slag2PCC process being scaled up in Finland.

Investigations of boundary layer structure, cloud characteristics and vertical mixing of aerosols at Barbados with large eddy simulations

Abstract. Large eddy simulations (LESs) are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, we use the cell perturbation method based on finite amplitude potential temperature perturbations. In this work, this method is for the first time tested and validated for moist boundary layer simulations with open lateral boundary conditions. Observational data obtained from the SALTRACE field campaign is used for both model initialization and a comparison with Doppler wind and Raman lidar data. Several numerical sensitivity tests are carried out to demonstrate the problems related to “gray zone modeling” when using coarser spatial grid spacings beyond the inertial subrange of three-dimensional turbulence or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties in the downwind area west of Barbados are markedly affected in these kinds of simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and location. Saharan dust layers that reach Barbados via long-range transport over the North Atlantic are included as passive tracers in the model. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom at z ≈ 1800 m become apparent. The exact position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability (especially inversion strength) and wind shear. Comparisons of LES model output with wind lidar data show similarities in the downwind vertical wind structure. Additionally, the model results accurately reproduce the development of the daytime convective boundary layer measured by the Raman lidar.

Experimental Facility Development Toward Sound-Excitation Effects on Forced Convection Heat Transfer

In light of the background in periodic boundary-layer forcing as employed for flow control applications, the present work discusses the role of standing acoustic waves toward convective heat transfer modification in a turbulent straight channel flow. To this end, a dedicated low-speed wind-tunnel facility is designed, analyzed, and built, and it is specifically geared toward comprehensive investigation of sound-excited heat transfer for different aerodynamic and acoustic boundary conditions. To identify frequency ranges conducive toward exciting acoustic resonances, both in the transverse and longitudinal directions, a detailed numerical analysis of the acoustic resonance behavior is carried out. Resulting mode patterns and arising pressure nodes and antinodes are discussed in detail. On selected transverse and complex coupled modes, experimental surface temperature measurements are carried out by steady wideband liquid crystal thermometry. Findings indicate that, depending on the matched eigenfrequency and standing wave pattern, a slight elevation or reduction of heat transfer (locally and globally) is possible, whereas no change is observed for the pure traveling wave forcing. Although the acoustically induced changes are small in magnitude, the dependence on the excitation frequency is clearly quantifiable beyond the measurement error.

Some aspects of the problem of secondary eyewall formation in idealized three‐dimensional nonlinear simulations

AbstractSome aspects of the problem of secondary eyewall formation (SEF) are investigated with the aid of an idealized model. A series of experiments are conducted, starting with a strong annular vortex embedded in a quiescent background flow and forced by the sustained heating associated with a spiral rainband (control experiment). Following this, two experiments are configured to assess the impact of vertical wind shear (VWS) in the SEF process. The importance of the boundary layer force imbalance is finally investigated in a number of simulations in which surface and boundary layer physics are included. From the control experiment, it is found that in the absence of background environmental flow, the sustained latent heating associated with a spiral rainband can form a secondary eyewall even in the absence of a frictional boundary layer. The presence of VWS acts negatively in the SEF process by disrupting the organization of the potential vorticity induced by the rainband. When boundary layer physics is included, some similarities with previous studies are seen, but there is no SEF. These results suggest that the boundary layer most likely contributes to, rather than initiate, a secondary eyewall.

Representing Equilibrium and Nonequilibrium Convection in Large-Scale Models

AbstractA new diagnostic convective closure, which is dependent on convective available potential energy (CAPE), is derived under the quasi-equilibrium assumption for the free troposphere subject to boundary layer forcing. The closure involves a convective adjustment time scale for the free troposphere and a coupling coefficient between the free troposphere and the boundary layer based on different time scales over land and ocean. Earlier studies with the ECMWF Integrated Forecasting System (IFS) have already demonstrated the model’s ability to realistically represent tropical convectively coupled waves and synoptic variability with use of the “standard” CAPE closure, given realistic entrainment rates.A comparison of low-resolution seasonal integrations and high-resolution short-range forecasts against complementary satellite and radar data shows that with the extended CAPE closure it is also possible, independent of model resolution and time step, to realistically represent nonequilibrium convection such as the diurnal cycle of convection and the convection tied to advective boundary layers, although representing the late night convection over land remains a challenge. A more in-depth regional analysis of the diurnal cycle and the closure is provided for the continental United States and particularly Africa, including comparison with data from satellites and a cloud-resolving model (CRM). Consequences for global numerical weather prediction (NWP) are not only a better phase representation of convection, but also better forecasts of its spatial distribution and local intensity.

On continuous spectra of the Orr–Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances

AbstractSmall-amplitude perturbations are governed by the linearized Navier–Stokes equations, which are, for a parallel or nearly parallel shear flow, customarily reduced to the Orr–Sommerfeld (O-S) and Squire equations. In this paper, we consider continuous spectra (CS) of the O-S and Squire operators for the Blasius and asymptotic suction boundary layers, and address the issue of whether and when continuous modes can represent free-stream vortical disturbances and their entrainment into the shear layer. For the Blasius boundary layer, we highlight two particular properties of the CS: (i) the eigenfunction of a continuous mode simultaneously consists of two components with wall-normal wavenumbers $\pm {k}_{2} $, a phenomenon which we refer to as ‘entanglement of Fourier components’; and (ii) for low-frequency disturbances the presence of the boundary layer forces the streamwise velocity in the free stream to take a much larger amplitude than those of the transverse velocities. Both features appear to be non-physical, and cast some doubt about the appropriateness of using CS to characterize free-stream vortical disturbances and their entrainment into the boundary layer, a practice that has been adopted in some recent studies of bypass transition. A high-Reynolds-number asymptotic description of continuous modes and entrainment is present, and it shows that the entanglement is a result of neglecting non-parallelism, which has a leading-order effect on the entrainment. When this effect is included, entanglement disappears, and moreover the streamwise velocity is significantly amplified in the edge layer when ${R}^{- 1} \ll \omega \ll 1$, where $R$ is the Reynolds number based on the local boundary-layer thickness. For the asymptotic suction boundary layer, which is an exactly parallel flow, both temporal and spatial CS may be defined mathematically. However, at a finite $R$ neither of them represents the physical process of free-stream vortical disturbances penetrating into the boundary layer. The latter must instead be characterized by a peculiar type of continuous modes whose eigenfunctions increase exponentially with the distance from the wall. In the limit $R\gg 1$, all three types of CS are identical at leading order, and hence can be used to represent free-stream vortical disturbances and their entrainment. Low-frequency disturbances are found to generate a large-amplitude streamwise velocity in the boundary layer, which is reminiscent of longitudinal streaks.