Aerodynamic Boundary Layer Research Articles

A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer

Abstract. An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. In this paper we re-visit the canonical problem of a TC in vertical wind shear on an f-plane. A suite of numerical experiments is performed with intense TCs in moderate to strong vertical shear. We employ a set of simplified model physics – a simple bulk aerodynamic boundary layer scheme and "warm rain" microphysics – to foster better understanding of the dynamics and thermodynamics that govern the modification of TC intensity. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. The ventilation of the TC core with dry environmental air at mid-levels and the dilution of the upper-level warm core are two prevailing hypotheses for the adverse effect of vertical shear on storm intensity. Here we propose an alternative and arguably more effective mechanism how cooler and drier (lower θe) air – "anti-fuel" for the TC power machine – can enter the core region of the TC. Strong and persistent, shear-induced downdrafts flux low θe air into the boundary layer from above, significantly depressing the θe values in the storm's inflow layer. Air with lower θe values enters the eyewall updrafts, considerably reducing eyewall θe values in the azimuthal mean. When viewed from the perspective of an idealised Carnot-cycle heat engine a decrease of storm intensity can thus be expected. Although the Carnot cycle model is – if at all – only valid for stationary and axisymmetric TCs, a close association of the downward transport of low θe into the boundary layer and the intensity evolution offers further evidence in support of our hypothesis. The downdrafts that flush the boundary layer with low θe air are tied to a quasi-stationary, azimuthal wave number 1 convective asymmetry outside of the eyewall. This convective asymmetry and the associated downdraft pattern extends outwards to approximately 150 km. Downdrafts occur on the vortex scale and form when precipitation falls out from sloping updrafts and evaporates in the unsaturated air below. It is argued that, to zero order, the formation of the convective asymmetry is forced by frictional convergence associated with the azimuthal wave number 1 vortex Rossby wave structure of the outer-vortex tilt. This work points to an important connection between the thermodynamic impact in the near-core boundary layer and the asymmetric balanced dynamics governing the TC vortex evolution.

A simple interaction law for viscous–inviscid interaction

The viscous–inviscid interaction (VII) philosophy for modelling aerodynamic boundary layers is discussed. ‘Traditionally’ the shear-layer equations are solved with pressure prescribed by the inviscid flow, but then the solution breaks down in a singularity related to flow separation. In the quasi-simultaneous coupling approach this singularity is overcome by making use of an interaction law. A novel mathematical analysis is presented of the essential properties of such interaction laws, which is based on classical theory for non-negative matrices. The performance of a highly simplified interaction law is demonstrated for separated airfoil flow beyond maximum lift.

Vertical advection and nocturnal deposition of ozone over a boreal pine forest

Abstract. Night-time ozone deposition for a Scots pine forest in Southern Finland was studied at the SMEAR II measurement station by evaluating the turbulent eddy covariance (EC), storage change and vertical advection fluxes. Similarly to night-time carbon dioxide flux, the eddy-covariance flux of ozone was decreasing with turbulence intensity (friction velocity), and storage change of the compound did not compensate the reduction (well-known night-time measurement problem). Accounting for vertical advection resulted in invariance of ozone deposition rate on turbulence intensity. This was also demonstrated for carbon dioxide, verified by independent measurements of NEE by chamber systems. The result highlights the importance of advection when considering the exchange measurements of any scalar. Analysis of aerodynamic and laminar boundary layer resistances by the model approach indicated that the surface resistance and/or chemical sink strength was limiting ozone deposition. The possible aerial ozone sink by known fast chemical reactions with sesquiterpenes and NO explain only a minor fraction of ozone sink. Thus the deposition is controlled either by stomatal uptake or surface reactions or both of them, the mechanisms not affected by turbulence intensity. Therefore invariance of deposition flux on turbulence intensity is expected also from resistance and chemical sink analysis.

A Runback Criterion for Water Drops in a Turbulent Accelerated Boundary Layer

Predicting the runback threshold for liquid drops in aerodynamic boundary layers is a challenging problem with numerous applications including aircraft icing simulations. The critical parameters that govern drop runback are investigated in this experiment by using a wind tunnel that provides a turbulent accelerated flow similar to flows near an unswept wing’s leading edge. The experiments feature water drops on aluminum with a contact angle of 70±5deg. Results show that significant water∕air interface unsteadiness precedes drop runback. This is likely due to air-flow separation in the drop wakes. For displacement-thickness-scaled Reynolds numbers ranging from 348 to 429, a constant-Weber-number runback threshold We=3.45±0.09 is found to adequately correlate the runback results.

Laser reflection method for determination of shear stress in low density transitional flows

The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air–oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.

Drag measurement and dynamic simulation of Martian wind-driven sensor platform concepts

The purpose of this work is to (a) determine the drag coefficient of three wind-driven systems (referred to as tumbleweeds) in a simulated Martian atmospheric boundary layer; two concepts from NASA Langley (LaRC) and one from Texas Tech University (TTU), and (b) perform a dynamic analysis of the TTU tumbleweed to establish the feasibility of operation in a simulated Martian environment. The TTU Wind Tunnel is used in order to determine the drag coefficient for the tumbleweeds in both the aerodynamic and atmospheric boundary layer (ABL) test sections. A comparison of the two mean drag coefficients for each tumbleweed model reveals the extent to which an ABL affects drag on the models. It appears that no transformation exists that can be used to transform aerodynamic-based drag coefficients into boundary-layer-based drag coefficients; therefore, reliance upon ABL tests is important. It is generally accepted that a complete ABL test under conditions of neutral atmospheric stability requires knowledge of the incoming (approach) mean velocity and turbulent intensity profile, spectral distribution, roughness height, and Reynolds number. Given the fact that limited data exists for the Martian flows, physical simulations of an atmospheric surface layer with knowledge of the mean velocity and general turbulence characteristics was developed in order to obtain drag coefficients for several tumbleweed wind platform designs. The tumbleweeds drag coefficients were effectively constant with the boundary layer affected coefficient less than the respective aerodynamically obtained coefficient. Of particular interest in this study is the TTU tumbleweed, which underwent extensive testing in order to obtain a force function to describe its aerodynamic characteristics in any orientation relative to the wind.

Experimental investigation of aerodynamic characteristics of the laser guided bomb model

Aerodynamic test results and boundary layer flow visualization on the wings and fins of laser guided bomb model are presented in this article. Test was performed in the T-38 wind tunnel. Influence of fin position relative to conic top of the model on aerodynamic characteristics was analyzed. Analysis was performed on aerodynamic characteristics of the model for two Mach numbers M∞ = 0,8 and 0,9 and fin deflection δ=15°. Normal forces present on the model and fin were measured by internal strain gauge balances. Obtained test results of normal force on the model and especially on the fin were associated with flow visualization results, which were performed by oil emulsion method. Presented images of flow visualization show changes of the flow around the model and confirm the results of the aerodynamic measurements. .

Advanced High Turning Compressor Airfoils for Low Reynolds Number Condition—Part I: Design and Optimization

High performance compressor airfoils at a low Reynolds number condition at Re=1.3×105 have been developed using evolutionary algorithms in order to improve the performance of the outlet guide vane (OGV), used in a single low pressure turbine (LPT) of a small turbofan engine for business jet aircrafts. Two different numerical optimization methods, the evolution strategy (ES) and the multi-objective genetic algorithm (MOGA), were adopted for the design process to minimize the total pressure loss and the deviation angle at the design point at low Reynolds number condition. Especially, with respect to the MOGA, robustness against changes of the incidence angle is considered. The optimization process includes the representation of the blade geometry, the generation of a numerical grid and a blade-to-blade analysis using a quasi-three-dimensional Navier-Stokes solver with a k-ω turbulence model including a newly implemented transition model to evaluate the performance. Overall aerodynamic performance and boundary layer properties for the two optimized blades are discussed numerically. The superior performance of the two optimized airfoils is demonstrated by a comparison with conventional controlled diffusion airfoils (CDA). The advantage in performance has been confirmed by detailed experimental investigations, which are presented in Part II of this paper.

Sensitivity of High-Resolution Simulations of Hurricane Bob (1991) to Planetary Boundary Layer Parameterizations

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model is used to simulate Hurricane Bob (1991) using grids nested to high resolution (4 km). Tests are conducted to determine the sensitivity of the simulation to the available planetary boundary layer parameterizations, including the bulk aerodynamic, Blackadar, Medium-Range Forecast (MRF) model, and Burk–Thompson boundary layer schemes. Significant sensitivity is seen, with minimum central pressures varying by up to 16 mb and maximum winds by 15 m s−1. The Burk–Thompson and bulk aerodynamic boundary layer schemes produced the strongest storms while the MRF scheme produced the weakest storm. Simulated horizontal precipitation structures varied substantially between the different PBL schemes, suggesting that accurate forecasts of precipitation in hurricanes can be just as sensitive to the formulation of the PBL as they are to the cloud microphysical parameterizations. Each PBL scheme is different in its formulation of the vertical mixing within the PBL and the surface fluxes, with the exception of the MRF and Blackadar schemes, which share essentially the same surface flux parameterization. Detailed analyses of the PBL schemes describe the key differences in the surface fluxes and how they impact storm intensity. In order to isolate the effects of vertical mixing and surfaces fluxes, simulations were conducted in which each of the surface flux schemes was used in conjunction with the same vertical mixing scheme, and vice versa. These experiments indicate that simulated intensity is largely determined by the surface fluxes rather than by the vertical mixing, with the exception of the MRF PBL case, in which excessively deep vertical mixing acts to dry the lower PBL and reduce hurricane intensity. Simulations that vary only the surface fluxes suggest that the intensity of the simulated hurricane increases with increasing values of the ratio of the exchange coefficients for enthalpy and momentum, Ck/CD. However, even for identical values of Ck/CD, the simulated intensity varies depending on the wind speed dependence of the surface roughness parameter z0.

A new finite/infinite scheme to model acoustic trim under realistic working conditions

The paper presents a finite-element scheme modeling all aspects of the behavior of acoustic trim: insulation, absorption, and damping. The models can combine different materials (viscoelastic, poroelastic, incompressible), acoustic fluid, and include complex effects like visco-thermal dissipation in thin air layers. Unbounded acoustic regions are modeled using stabilized conjugated infinite elements that present exceptional convergence properties. The structure can be subjected to a number of different excitations: kinematical (fixed motion), dynamical (discrete or distributed forces) or acoustical. An incident diffuse sound field or distributed pressure fluctuations corresponding to aerodynamic boundary layers can also excite the structure. The presentation will be organized as follows: (1) a brief review of the techniques involved: finite-element method for the structure and the acoustic near field, infinite elements for the acoustic far field; (2) material models considered: viscoelastic, poroelastic, incompressible, acoustic (including viscothermal effects); (3) excitation models: kinematical, dynamical, acoustical, aerodynamic; (4) validation of the approach; (5) three application cases: comparison of the dynamic behavior of a treated and untreated shell, acoustic transparency of an insulating sandwich, evaluation of the acoustic impedance of a porous layer.

Ferroelectric liquid crystals in aerodynamic testing

The process of simultaneous optical visualization and quantitative measurement of aerodynamic boundary layer parameters requires new concepts, materials and utilization methods. Measurement of shear stress in terms of the transmitted or the reflected light intensity from an aligned ferroelectric liquid crystal (FLC) thin (∼ 1μm) film deposited on a glass substrate has been the first step in this direction. In this paper, recent progress in utilization of FLC thin films for skin friction measurement and for studying the state of the boundary layer in a wind tunnel environment is reviewed. The switching characteristics of FLCs have been used to measure pressure from the newly devised system of partially exposed polymer dispersed ferroelectric liquid crystals (PEPDFLCs). In this configuration, a PEPDFLC thin film (∼10-25 μm) is sandwiched between two transparent conducting electrodes, one a rigid surface and the other a flexible sheet such as polyvinylidene fluoride or mylar. The switching characteristics of the film are a function of the pressure applied to the flexible transparent electrode and a predetermined bias voltage across the two electrodes. The resalts, considering the dielectrics of composite media, are discussed.

Tansley Review No. 59 Leaf boundary layers.

Studies of heat and mass exchange between leaves and their local environment are central to our understanding of plant-atmosphere interactions. The transfer across aerodynamic leaf boundary layers is generally described by non-dimensional expressions which reflect largely empirical adaptations of engineering models derived for flat plates. This paper reviews studies on leaves, and leaf models with varying degrees of abstraction, in free and forced convection. It discusses implecations of finding for leaf morphology as it affects - and is affected by - the local microclimate. Predictions of transfer from many leaves in plant communities are complicated by physical and physiological feedback mechanisms between leaves and their environment. Some common approaches, and the current challenge of integrating leaf-atmosphere interactions into models of global relevance, are also briefly addressed. Contents Summary 477 I. Introduction 478 II. Early studies 479 III. The formal description of leaf transfer 480 IV. Effects of turbulence on idealized shapes 484 V. Effects of aspect ratio and inclination 486 VI. Leaves and leaf models in forced convection 491 VII. Leaves and leaf models in mixed and free convection 493 VIII. Interpretation of leaf shape 494 IX. Leaves in plant canopies 499 X. Synopsis and conclusions 502 References 502.

A reflection-type oil-film skin-friction meter

Tanner has shown analytically that the slope characteristics of a thin oil-film lying in contact with a surface on which an aerodynamic boundary-layer is developing are explicitly related to the local skin-friction. Unlike many conventional methods of measuring skin-friction, the oil-film method is absolute in nature, requires no calibration and in principle, can be universally applied. In all existing forms of the meter, however, interferometry is used to make measurements of the oil film. In the present contribution, the technique has been simplified by completely eliminating interferometry. This has been achieved by making direct and dynamic measurements of the oil-film slope by directing a reflected beam of light off the top of the oil film to a small position sensing photodiode. The raw data now is very different from that in the interferometric method and consequently, new methods of determining skin-friction from measured oil-film slope histories have been developed. The reflection method has been verified in incompressible flat plate turbulent boundary-layers. The standard of deviation of the measurements is about 10% of the mean. The present version of the meter is compact and simple.

Wind Profile Parameter Estimation Using Mathcad

AbstractWind profile parameters are required to estimate aerodynamic boundary layer resistances for heat, water vapor, and momentum transfer. Traditional methods used to compute these parameters take time to program, are slow to reach solutions, and often fail to achieve solution convergence. This study evaluated the applicability of a commercial mathematics software package, MathCAD, to estimate the three parameters—friction velocity (U*), zero‐plane displacement height (4, and the roughness length, Zo—of the adiabatic log‐law wind profile. The solutions were evaluated and compared to previous published solutions for several wind profiles using traditional methods. MathCAD was found to solve the wind profiles rapidly without convergence difficulty on the tested profiles. In all the comparisons, MathCAD produced almost exact duplications of previously published results. The ease, speed, and accuracy of the solution method employed in MathCAD appear to be superior to the more traditional methods.

Wind Tunnel Modeling of Small Particle Deposition

An inexpensive microsphere dispersion, deposition, and sampling system is described. This system was used to examine the transport and deposition of small particles (− 1 μm diameter) across the aerodynamic boundary layer which developed over a prototype deposition surface (smooth, flat, acrylic plate). Unit density, polystyrene latex microspheres (0.8 and 1.1 μm diameter) were deposited onto both oil-coated and dry, upper and lower surfaces. Observed deposition velocities were significantly larger than those reported in the literature, possibly because low relative humidities increased electrostatic charge on the experimental plate. The experimental results were greater than those predicted by a turbulent flow particle deposition model.

Whole-Plant Responses to Drought

The partitioning of carbon and interactions which cause limitations on gas exchange and growth under conditions of a limited supply of water and nutrients are discussed. Possible mechanisms of effects of air humidity on stomatal functioning and carbon assimilation are described. Also, it is shown that stomata respond to a signal from the root when the soil dries out prior to leaf wilting. Stomatal conductance determines canopy transpiration if the aerodynamic boundary layer resistance is low, such as in trees. Water shortage significantly affects extension growth and the root-shoot ratio at the whole- plant level. But experiments with xylem-tapping mistletoes show that stem growth can also be promoted by the presence of the mistletoe even when there is no apparent signal from the subtending shoot except the flow in the stem xylem. It appears that the internal plant water status may not affect gas exchange and carbon partitioning unless the plant fails to maintain a flow of water through the leaf epidermis and root tip.

Falkner-Skan Solution for Gravity-Driven Film Flow

It is demonstrated that the development of a viscous film flow down along a vertical wall can be described by the classical Falkner-Skan equation from aerodynamic boundary layer theory for the particular parameter-value m = 1/2. This leads to a well-known exact solution for the velocity field, as long as the viscous boundary layer can be considered not to interact with the free surface of the film. An exact reference solution for developing film flow is thus made available, against which approxiate solutions may be tested for accuracy.

An analysis of errors in the calculation of energy flux densities above vegetation by a Bowen-ratio profile method

The Bowen-ratio profile method for calculating total carbon dioxide, latent heat, and sensible heat flux density above a corn crop was used by measuring temperature, water vapor, and CO2 concentrations at several heights in the aerodynamic boundary layer of the crop. The ratio (α) of sensible heat flux density to carbon dioxide flux density as well as the Bowen ratio (β) were used in the computations. The two ratios, α and β were determined graphically from the slopes of linear plots of temperature vs CO2 concentration and vs water vapor. Each of the energy flux densities was computed from the two ratios and net radiation minus soil heat flux density. An analysis of probable error was performed on the Bowen-ratio profile method to evaluate the accuracy of the flux density estimates. Less than 10% error was found for latent heat flux density and less than 15% for carbon dioxide flux density under normal midday conditions for the instrumentation used. However, the carbon dioxide flux density error increased to over 40% when the sensible heat flux was small.

Aerodynamic Boundary Layer Effects on Flutter and Damping of Plates

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Derivation of Aerodynamic Kernel Functions

The method of Fourier transforms is used to determine the kernel function which relates the pressure on a lifting surface to the prescribed downwash within the framework of Dowell's (1971) shear flow model. This model is intended to improve upon the potential flow aerodynamic model by allowing for the aerodynamic boundary layer effects neglected in the potential flow model. For simplicity, incompressible, steady flow is considered. The proposed method is illustrated by deriving known results from potential flow theory.