Showing papers on "Boundary layer published in 2014"

Low-Frequency Unsteadiness of Shock Wave/Turbulent Boundary Layer Interactions

Abstract: Shock wave/boundary layer interactions occur in a wide range of supersonic internal and external flows, and often these interactions are associated with turbulent boundary layer separation. The resulting separated flow is associated with large-scale, low-frequency unsteadiness whose cause has been the subject of much attention and debate. In particular, some researchers have concluded that the source of low-frequency motions is in the upstream boundary layer, whereas others have argued for a downstream instability as the driving mechanism. Owing to substantial recent activity, we are close to developing a comprehensive understanding, albeit only in simplified flow configurations. A plausible model is that the interaction responds as a dynamical system that is forced by external disturbances. The low-frequency dynamics seem to be adequately described by a recently proposed shear layer entrainment-recharge mechanism. Upstream boundary layer fluctuations seem to be an important source of disturbances, but the evidence suggests that their impact is reduced with increasing size of the separated flow.

Two-point statistics for turbulent boundary layers and channels at Reynolds numbers up to δ + ≈ 2000

Abstract: Two-point statistics are presented for a new direct simulation of the zero-pressure-gradient turbulent boundary layer in the range Re θ = 2780–6680, and compared with channels in the same range of Reynolds numbers, δ+ ≈ 1000–2000. Three-dimensional spatial correlations are investigated in very long domains to educe the average structure of the velocity and pressure fluctuations. The streamwise velocity component is found to be coherent over longer distances in channels than in boundary layers, especially in the direction of the flow. For weakly correlated structures, the maximum streamwise length is O ( 7 δ ) for boundary layers and O ( 18 δ ) for channels, attained at the logarithmic and outer regions, respectively. The corresponding lengths for the spanwise and wall-normal velocities and for the pressure are shorter, O ( δ -2δ). The correlations are shown to be inclined to the wall at angles that depend on the distance from the wall, on the variable being considered, and on the correlation level used to define them. All these features change little between the two types of flows. Most the above features are also approximately independent of the Reynolds number, except for the pressure, and for the streamwise velocity structures in the channel. Further insight into the flow is provided by correlations conditioned on the intensity of the perturbations at the reference point, or on their sign. The statistics of the new simulation are available in our website.

Paradigms for Tropical Cyclone Intensification

Abstract: : We review the four main paradigms of tropical cyclone intensification that have emerged over the past five decades, discussing the relationship between them and highlighting their strengths and weaknesses. A major focus is on a new paradigm articulated in a series of recent papers using observations and high- resolution, three-dimensional, numerical model simulations. Unlike the three previous paradigms, all of which assumed axial symmetry, the new one recognises the presence of localised, buoyant, rotating deep convection that grows in the rotation-rich environment of the incipient storm, thereby greatly amplifying the local vorticity. It exhibits also a degree of randomness that has implications for the predictability of local asymmetric features of the developing vortex. While surface moisture fluxes are required for intensification, the postulated evaporation-wind feedback process that forms the basis of an earlier paradigm is not. Differences between spin up in three-dimensional and axisymmetric numerical models are discussed also. In all paradigms, the tangential winds above the boundary layer are amplified by the convectively-induced inflow in the lower troposphere in conjunction with the approximate material conservation of absolute angular momentum. This process acts also to broaden the outer circulation. Azimuthally-averaged fields from high-resolution model simulations have highlighted a second mechanism for amplifying the mean tangential winds. This mechanism, which is coupled to the first via boundary-layer dynamics, involves the convergence of absolute angular momentum within the boundary layer, where this quantity is not materially conserved, but where air parcels are displaced much further radially in - wards than air parcels above the boundary layer. It explains why the maximum tangential winds occur in the boundary layer and accounts for the generation of supergradient wind speeds there.

Numerical study of MHD boundary layer flow of a Maxwell fluid past a stretching sheet in the presence of nanoparticles

Abstract: In the present article, two dimensional boundary-layer flows and the heat transfer of a Maxwell fluid past a stretching sheet are studied numerically The effects of magnetohydrodynamics (MHD) and elasticity on the flow are considered Moreover, the effects of nanoparticles are also investigated Similarity transformations are presented to convert the governing nonlinear partial differential equation into coupled ordinary differential equations The reduced boundary layer equations of the Maxwell nanofluid model are solved numerically The effects of the emerging parameters, namely, the magnetic parameter M, the elastic parameter K, the Prandtl parameter Pr, the Brownian motion Nb, the thermophoresis parameter Nt and the Lewis number Le on the temperature and the concentration profile are discussed Interesting results are shown graphically The skin friction coefficient, the dimensionless heat transfer rate and the concentration rate are also plotted against the flow control parameters

Synthetic Turbulence Generators for RANS-LES Interfaces in Zonal Simulations of Aerodynamic and Aeroacoustic Problems

Abstract: The paper presents the detailed formulation and validation results of simple and robust procedures for the generation of synthetic turbulence aimed at providing artificial turbulent content at the RANS-to-LES interface within a zonal Wall Modelled LES of attached and mildly separated wall-bounded flows. There are two versions of the procedure. The aerodynamic version amounts to a minor modification of a synthetic turbulence generator developed by the authors previously, but the acoustically adapted version is new and includes an internal damping layer, where the pressure field is computed by “weighting” of the instantaneous pressure fields from LES and RANS. This is motivated by the need to avoid creating spurious noise as part of the turbulence generation. In terms of pure aerodynamics, the validation includes canonical shear flows (developed channel flow, zero pressure gradient boundary layer, and plane mixing layer), as well as a more complex flow over the wall-mounted hump with non-fixed separation and reattachment, with emphasis on a rapid conversion from modeled to resolved Reynolds stresses. The aeroacoustic applications include the flow past a trailing edge and over a two-element airfoil configuration. In all cases the methodology ensures a very acceptable accuracy for the mean flow, turbulent statistics and, also, the near- and far-field noise.

Influence of Magnetohydrodynamic and Thermal Radiation Boundary Layer Flow of a Nanofluid Past a Stretching Sheet

Abstract: The problem of laminar fluid flow which results from a permeable stretching of a flat surface in a nanofluid with the effects of heat radiation, magnetic field, velocity slip and convective boundary conditions have been investigated. The transport equations used in the analysis took into account the effect of Brownian motion and thermophoresis parameters. The solution for the velocity, temperature and nanoparticle concentration depends on parameters viz. thermal radiation parameter

On the Inviscid Limit Problem of the Vorticity Equations for Viscous Incompressible Flows in the Half-Plane

Abstract: We consider the Navier-Stokes equations for viscous incompressible flows in the half-plane under the no-slip boundary condition. By using the vorticity formulation we prove the local-in-time convergence of the Navier-Stokes flows to the Euler flows outside a boundary layer and to the Prandtl flows in the boundary layer in the inviscid limit when the initial vorticity is located away from the boundary. © 2014 Wiley Periodicals, Inc.

Seamless Stratocumulus Simulation across the Turbulent Gray Zone

Abstract: A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested The method blends a conventional boundary layer parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation The key parameter for blending the schemes is the ratio of grid length to boundary layer depth The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced

An improved dynamic non-equilibrium wall-model for large eddy simulation

Abstract: A non-equilibrium wall-model based on unsteady 3D Reynolds-averaged Navier-Stokes (RANS) equations has been implemented in an unstructured mesh environment. The method is similar to that of the wall-model for structured mesh described by Wang and Moin [Phys. Fluids 14, 2043–2051 (2002)], but is supplemented by a new dynamic eddy viscosity/conductivity model that corrects the effect of the resolved Reynolds stress (resolved turbulent heat flux) on the skin friction (wall heat flux). This correction is crucial in predicting the correct level of the skin friction. Unlike earlier models, this eddy viscosity/conductivity model does not have a stress-matching procedure or a tunable free parameter, and it shows consistent performance over a wide range of Reynolds numbers. The wall-model is validated against canonical (attached) transitional and fully turbulent flows at moderate to very high Reynolds numbers: a turbulent channel flow at Reτ = 2000, an H-type transitional boundary layer up to Reθ = 3300, and a hig...

Superhydrophobic turbulent drag reduction as a function of surface grating parameters

Abstract: © 2014 Cambridge University Press. Despite the confirmation of slip flows and successful drag reduction (DR) in small-scaled laminar flows, the full impact of superhydrophobic (SHPo) DR remained questionable because of the sporadic and inconsistent experimental results in turbulent flows. Here we report a systematic set of bias-free reduction data obtained by measuring the skin-friction drags on a SHPo surface and a smooth surface at the same time and location in a turbulent boundary layer (TBL) flow. Each monolithic sample consists of a SHPo surface and a smooth surface suspended by flexure springs, all carved out from a 2.7 × 2.7 mm2 silicon chip by photolithographic microfabrication. The flow tests allow continuous monitoring of the plastron on the SHPo surfaces, so that the DR data are genuine and consistent. A family of SHPo samples with precise profiles reveals the effects of grating parameters on turbulent DR, which was measured to be as much as ∼75.

The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence

Abstract: Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.

A Scale-Consistent Terrestrial Systems Modeling Platform Based on COSMO, CLM, and ParFlow

Abstract: A highly modular and scale-consistent Terrestrial Systems Modeling Platform (TerrSysMP) is presented. The modeling platform consists of an atmospheric model (Consortium for Small-Scale Modeling; COSMO), a land surface model (the NCARCommunityLand Model,version3.5; CLM3.5), anda 3D variablysaturated groundwater flow model (ParFlow). An external coupler (Ocean Atmosphere Sea Ice Soil, version 3.0; OASIS3) with multiple executable approaches is employed to couple the three independently developed component models, which intrinsically allows for a separation of temporal‐spatial modeling scales and the coupling frequencies between the component models. IdealizedTerrSysMPsimulations arepresented,whichfocuson theinteractionofkey hydrologic processes, like runoff production (excess rainfall and saturation) at different hydrological modeling scales and the drawdown of the water table through groundwater pumping, with processes in the atmospheric boundary layer. The results show a strong linkage between integrated surface‐groundwater dynamics, biogeophysical processes, and boundary layer evolution. The use of the mosaic approach for the hydrological component model (to resolve subgrid-scale topography) impacts simulated runoff production, soil moisture redistribution, and boundary layer evolution, which demonstrates the importance of hydrological modeling scales and thus the advantages of the coupling approach used in this study. Real data simulations were carried out with TerrSysMP over the Rur catchment in Germany. The inclusion oftheintegratedsurface‐groundwaterflowmodelresultsin systematicpatternsin therootzonesoilmoisture, which influence exchange flux distributions and the ensuing atmospheric boundary layer development. In a first comparison to observations, the 3D model compared to the 1D model shows slightly improved predictions of surface fluxes and a strong sensitivity to the initial soil moisture content.

MHD Three-Dimensional Boundary Layer Flow of Casson Nanofluid Past a Linearly Stretching Sheet With Convective Boundary Condition

Abstract: Steady flow of a Casson fluid in the presence of a nanoparticle is studied. It is considered that the sheet is stretched in both the direction along the xy-plane. Moreover, we have considered the magnetohydrodynamics effect within the fluid and convective condition along the surface. Similarity transformation is used to convert the governing partial differential equations to a set of coupled nonlinear ordinary differential equations which are solved numerically. The behavior of emerging parameters are presented graphically and discussed for velocity, temperature, and nanoparticles fraction. Variation of the reduced Nusselt and Sherwood number against physical parameters are presented graphically. It is found that the reduced Nusselt number is the decreasing function and the reduced Sherwood number is the increasing function of Brownian parameter Nb and thermophoresis parameter Nt.

Turbulence in the Upper-Ocean Mixed Layer

Abstract: Nearly all operational models of upper-ocean mixing assume that the turbulence responsible for this mixing is driven by the atmospheric fluxes of momentum, heat, and moisture and the shear imposed by the ocean circulation. This idealization is supported by historical measurements of dissipation rate within the boundary layer. Detailed measurements made recently by many investigators and supported by theoretical and numerical results have found significant deviations from this classical view attributable to the influence of surface waves. Although a review of these measurements finds strong support for the influence of waves-and, in particular, for the predictions of large-eddy simulations, including the Craik-Leibovich vortex force-there are insufficient data to give definitive support to a new paradigm.

Solutal-convection regimes in a two-dimensional porous medium

Abstract: We numerically characterize the temporal regimes for solutal convection from almost first contact to high dissolved solute concentration in a two-dimensional ideal porous layer for Rayleigh numbers -dependent shut-down regime. For the constant-flux and shut-down regimes, we rigourously derive upscaled equations connecting the horizontally averaged concentration, vertical advective flux and plume widths. These are partially complete; a universal expression for the plume width remains elusive. We complement these governing equations with phenomenological boundary conditions based on a marginally stable diffusive boundary layer at the top and zero advective flux at the bottom. Making appropriate approximations in each regime, we find good agreement between predictions from this model and simulated results for both solutal and thermal convection. In the partially permeable upper boundary case, fluid from the convecting layer can penetrate an overlying separate-phase-solute bearing layer where it immediately saturates. The regime diagram remains almost the same as for the impermeable case, but the dissolution flux is significantly augmented. Our work is motivated by dissolution of carbon dioxide relevant to geological storage, and we conclude with a simple flux parameterization for inclusion in gravity current models and suggest that the upscaled equations could lay the foundation for accurate inclusion of dissolution in reservoir simulators.

The turbulent/non-turbulent interface and entrainment in a boundary layer

Abstract: The turbulent/non-turbulent interface in a zero-pressure-gradient turbulent boundary layer at high Reynolds number (Re = 14500) is examined using particle image velocimetry. An experimental set-up is utilized that employs multiple high-resolution cameras to capture a large field of view that extends 2δ× 1:1δ in the streamwise/wallnormal plane with an unprecedented dynamic range. The interface is detected using a criteria of local turbulent kinetic energy and proves to be an effective method for boundary layers. The presence of a turbulent/non-turbulent superlayer is corroborated by the presence of a jump for the conditionally averaged streamwise velocity across the interface. The steep change in velocity is accompanied by a discontinuity in vorticity and a sharp rise in the Reynolds shear stress. The conditional statistics at the interface are in quantitative agreement with the superlayer equations outlined by Reynolds (J. Fluid Mech., vol. 54, 1972, pp. 481-488). Further analysis introduces the mass flux as a physically relevant parameter that provides a direct quantitative insight into the entrainment. Consistency of this approach is first established via the equality of mean entrainment calculations obtained using three different methods, namely, conditional, instantaneous and mean equations of motion. By means of 'mass-flux spectra' it is shown that the boundary-layer entrainment is characterized by two distinctive length scales which appear to be associated with a two-stage entrainment process and have a substantial scale separation. © 2014 Cambridge University Press. ;

Flow and heat transfer analysis of Williamson nanofluid

Abstract: In the present paper, we have examined the two-dimensional flow of Williamson fluid over a stretching sheet under the effects of nano-sized particle also described as nano Williamson fluid. The boundary layer equations of nano Williamson fluid model along with energy and nanoparticle volume fraction are presented and simplified with the help of useful transformations. Governing equations are somewhat different from the ones present in literature (reason is explained in the introduction section). The expressions for coefficients of skin friction and Nusselt number have been computed. The physical features of non-dimensional Williamson parameter, Lewis number, Schmidt number and nano particle parameters (diffusivity ratio and heat capacities ratio) have been discussed by plotting the graphs of velocity, temperature and nanoparticle volume fraction.

Frictional drag reduction by bubble injection

Abstract: The injection of gas bubbles into a turbulent boundary layer of a liquid phase has multiple different impacts on the original flow structure. Frictional drag reduction is a phenomenon resulting from their combined effects. This explains why a number of different void–drag reduction relationships have been reported to date, while early works pursued a simple universal mechanism. In the last 15 years, a series of precisely designed experimentations has led to the conclusion that the frictional drag reduction by bubble injection has multiple manifestations dependent on bubble size and flow speed. The phenomena are classified into several regimes of two-phase interaction mechanisms. Each regime has inherent physics of bubbly liquid, highlighted by keywords such as bubbly mixture rheology, the spectral response of bubbles in turbulence, buoyancy-dominated bubble behavior, and gas cavity breakup. Among the regimes, bubbles in some selected situations lose the drag reduction effect owing to extra momentum transfer promoted by their active motions. This separates engineers into two communities: those studying small bubbles for high-speed flow applications and those studying large bubbles for low-speed flow applications. This article reviews the roles of bubbles in drag reduction, which have been revealed from fundamental studies of simplified flow geometries and from development of measurement techniques that resolve the inner layer structure of bubble-mixed turbulent boundary layers.

A dynamic slip boundary condition for wall-modeled large-eddy simulation

Abstract: Wall models for large-eddy simulation (LES) are a necessity to remove the prohibitive resolution requirements of near-wall turbulence in high Reynolds turbulent flows. Traditional wall models often rely on assumptions about the local state of the boundary layer (e.g., logarithmic velocity profiles) and require a priori prescription of tunable model coefficients. In the present study, a slip velocity boundary condition for the filtered velocity field is obtained from the derivation of the LES equations using a differential filter. A dynamic procedure for the local slip length is additionally formulated making the slip velocity wall model free of any a priori specified coefficients. The accuracy of the dynamic slip velocity wall model is tested in a series of turbulent channel flows at varying Reynolds numbers and in the LES of a National Advisory Committee for Aeronautics (NACA) 4412 airfoil at near-stall conditions. The wall-modeled simulations are able to accurately predict mean flow characteristics, including the formation of a trailing-edge separation bubble in NACA 4412 configuration. The validation cases demonstrate the effectiveness of this wall-modeling approach in both attached and separated flow regimes.

Confinement effects in shock wave/turbulent boundary layer interactions through wall-modelled large-eddy simulations

Abstract: We present wall-modelled large-eddy simulations (WLES) of oblique shock waves interacting with the turbulent boundary layers (TBLs) (nominal and ) developed inside a duct with an almost-square cross-section ( ) to investigate three-dimensional effects imposed by the lateral confinement of the flow. Three increasing strengths of the incident shock are considered, for a constant Mach number of the incoming air stream , by varying the height (1.1, 3 and 5 mm) of a compression wedge located at a constant streamwise location that spans the top wall of the duct at a 20° angle. Simulation results are first validated with particle image velocimetry (PIV) experimental data obtained at several vertical planes (one near the centre of the duct and three near one of the sidewalls) for the 1.1 and 3 mm-high wedge cases. The instantaneous and time-averaged structure of the flow for the stronger-interaction case (5 mm-high wedge), which shows mean flow reversal, is then investigated. Additional spanwise-periodic simulations are performed to elucidate the influence of the sidewalls, and it is found that the structure and location of the shock system, as well as the size of the separation bubble, are significantly modified by the lateral confinement. A Mach stem at the first reflected interaction is present in the simulation with sidewalls, whereas a regular shock intersection results for the spanwise-periodic case. Low-frequency unsteadiness is observed in all interactions, being stronger for the secondary shock reflections of the shock train developed inside the duct. The downstream evolution of secondary turbulent flows developed near the corners of the duct as they traverse the shock system is also studied.