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Footprints in micrometeorology and ecology
How to interpret meteorological measurements made at a given level over a surface with regard to characteristic properties such as roughness, albedo, heat, moisture, carbon dioxide, and other gases is an old question which goes back to the very beginnings of modern micrometeorology. It is made even more challenging when it is unclear whether these measurements are only valid for this point/region and precisely describe the conditions there, or if they are also influenced by surrounding areas. After 50 years of field experiments, it has become both apparent and problematic that meteorological measurements are influenced from surfaces on the windward side. As such, extending these measurements for inhomogeneous experimental sites requires a quantitative understanding of these influences. When combined with atmospheric transport models similar to air pollution models, the {u2018}footprint{u2019} concept {u2013} a fundamental approach introduced roughly 20 years ago {u2013} provides us with information on whether or not the condition of upwind site homogeneity is fulfilled. Since these first models, the development of more scientifically based versions, validation experiments and applications has advanced rapidly. The aim of this book is to provide an overview of these developments, to analyze present deficits, to describe applications and to advance this topic at the forefront of micrometeorological research.
Book
Reynolds stress scaling in the near-wall region of wall-bounded flows
A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour.
Journal Article
Reattachment streaks in hypersonic compression ramp flow: an input–output analysis
We employ global input–output analysis to quantify amplification of exogenous disturbances in compressible boundary layer flows. Using the spatial structure of the dominant response to time-periodic inputs, we explain the origin of steady reattachment streaks in a hypersonic flow over a compression ramp. Our analysis of the laminar shock–boundary layer interaction reveals that the streaks arise from a preferential amplification of upstream counter-rotating vortical perturbations with a specific spanwise wavelength. These streaks are associated with heat-flux striations at the wall near flow reattachment and they can trigger transition to turbulence. The streak wavelength predicted by our analysis compares favourably with observations from two different hypersonic compression ramp experiments. Furthermore, our analysis of inviscid transport equations demonstrates that base-flow deceleration contributes to the amplification of streamwise velocity and that the baroclinic effects are responsible for the production of streamwise vorticity. Finally, the appearance of the temperature streaks near reattachment is triggered by the growth of streamwise velocity and streamwise vorticity perturbations as well as by the amplification of upstream temperature perturbations by the reattachment shock.
Journal Article
Large-Eddy Simulation of the Atmospheric Boundary Layer
Over the last 50 years the large-eddy simulation (LES) technique has developed into one of the most prominent numerical tools used to study transport processes in the atmospheric boundary layer (ABL). This review examines development of the technique as a tool for ABL research, integration with state-of-the-art scientific computing resources, and some key application areas. Analysis of the published literature indicates that LES research across a broad range of applications accelerated starting around 1990. From that point in time, robust research using LES developed in several different application areas and based on a review of the papers published in this journal, we identify seven major areas of intensive ABL–LES research: convective boundary layers, stable boundary layers, transitional boundary layers, plant canopy flows, urban meteorology and dispersion, surface heterogeneity, and the testing and development of subgrid-scale (SGS) models. We begin with a general overview of LES and then proceed to examine the SGS models developed for use in ABL–LES. After this overview of the technique itself, we review the specific model developments tailored to the identified application areas and the scientific advancements realized using the LES technique in each area. We conclude by examining the computational trends in published ABL–LES research and identify some resource underutilization. Future directions and research needs are identified from a synthesis of the reviewed literature.
Journal Article
On the effect of free-stream turbulence on boundary-layer transition
Free-stream turbulence (FST) and its effect on boundary-layer transition is an intricate problem. Elongated unsteady streamwise streaks of low and high speed are created inside the boundary layer and their amplitude and spanwise wavelength are believed to be important for the onset of transition. The transitional Reynolds number is often simply correlated with the turbulence intensity (${Tu}$), and the characteristic length scales of the FST are often considered to have a small to negligible influence on the transition location. Here, we present new results from a large experimental measurement campaign, where both the ${Tu}$ and the integral length scale ($\\Lambda _x$) are varied ($1.8\\,\\% < {Tu}< 6.2\\,\\%$; $16\\ \\textrm {mm}< \\Lambda _x < 26\\ \\textrm {mm}$). In the current experiments it has been noted that on the one hand, for small $Tu$, an increase in $\\Lambda _x$ advances transition, which is in agreement with established results. On the other hand, for large $Tu$, an increase in $\\Lambda _x$ postpones transition. This trend can be explained by the fact that an optimal ratio between FST length scale and boundary-layer thickness at transition onset exists. Furthermore, our results strengthen the fact that the streaks play a key role in the transition process by showing a clear dependence of the FST characteristics on their spanwise scale. Our measurements show that the aspect ratio of the streaky structures correlates with an FST Reynolds number and that the aspect ratio can change by a factor of two at the location of transition. Finally, we derive a semi-empirical transition prediction model, which is able to predict the influence of $\\Lambda _x$ for both small and high values of ${Tu}$.
Journal Article
Transition Periods in the Diurnally-Varying Atmospheric Boundary Layer Over Land
The atmospheric boundary layer undergoes transitions between stable and convective states. Over land, in undisturbed conditions, these transitions occur daily in the morning and late afternoon or early evening. Though less well studied and presenting more challenges than the fully stable and fully convective states, such transitions have been the subject of growing interest over the last few decades. During transitions, all forcings are weak, and few simplifications are possible. Factors such as terrain, radiation, advection, and subsidence can seldom be safely neglected. Here, we review research on transitions over recent decades, with an emphasis on work published in Boundary-Layer Meteorology. The review is brief and inevitably reflects the interests and views of the authors.
Journal Article
Direct numerical simulation of a hypersonic transitional boundary layer at suborbital enthalpies
A Mach-10 hypersonic boundary layer of air overriding a cold, isothermal, non-catalytic flat wall, and with a stagnation enthalpy of $21.6\\ \\textrm {MJ}\\ \\textrm {kg}^{-1}$, is analysed using direct numerical simulations. The calculations include multicomponent transport, equilibrium vibrational excitation and chemical kinetics for air dissociation. The initially laminar boundary layer undergoes transition to turbulence by the resonance of a two-dimensional mode injected by a suction-and-blowing boundary condition imposed over a narrow spanwise porous strip. The ensuing turbulent boundary layer has a momentum Reynolds number of 3826 near the outflow of the computational domain. The relatively low temperature of the free stream renders the air chemically frozen there. However, the high temperatures generated within the boundary layer by viscous aerodynamic heating, peaking at a wall-normal distance $y^\\star \\simeq 10\\text {--}20$ in semi-local viscous units, lead to air dissociation in under-equilibrium amounts equivalent to 4 %–7 % on a molar basis of atomic oxygen, along with smaller concentrations of nitric oxide, which is mainly produced by the Zel'dovich mechanism, and of atomic nitrogen, the latter being mostly in steady state. A statistical analysis of the results is provided, including the streamwise evolution of (a) the skin friction coefficient and dimensionless wall heat flux; (b) the mean profiles of temperature, velocity, density, molar fractions, chemical production rates and chemical heat-release rate; (c) the Reynolds stresses and root-mean-squares of the fluctuations of temperature, density, pressure, molar fractions and chemical heat-release rate; and (d) the temperature/velocity and mass-fraction/velocity correlations.
Journal Article