A linear investigation on separation in laminar and turbulent boundary layers over low hills and valleys

Summary Laminar and turbulent linear models of stratified flow over small-slope topography have been compared and used to estimate separation features for a single two-dimensional hill and for a valley made by a pair of hills. As far as single symmetric hills are concerned, the importance of the obstacle shape have been evidenced. For the near field, the local slope characteristic of the terrain profile are of overwhelming importance. For the two-hill case, the dependence of the separation site on valley width and stratification is very different for the two models. Ventilation in the valley is modulated by the ratio between the valley width and the buoyancy wave-length only in the laminar case, whereas the turbulent model does not show these features for the realistic roughness values considered here.

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Riassunto Si fa un confronto tra modelli linearizzati di flussi turbolenti e laminari su ostacoli bidimensionali con pendenze piccole, in varie condizioni di stratificazione. Tali modelli sono usati per stimare le condizioni di separazione per una collina o per una valle compresa tra due colline. Nel primo caso si è notata l'importanza della forma dell'ostacolo scelto. Le caratteristiche locali di pendenza della forma del terreno risultano determinanti per il campo di perturbazioni prossimo all'ostacolo. Nel caso della valle si è studiata la collocazione del sito di prima separazione in funzione dell'ampiezza della valle e della stratificazione ed è stata riscontrata una notevole differenza tra i due modelli. Si ha ventilazione nella valle quando la sua ampiezza è un multiplo intero della lunghezza d'onda di galleggiamento, solo nel caso laminare; il modello turbolento non mostra queste caratteristiche per valori realistici di rugosità.

     

Laboratory simulation of rotating atmospheric boundary layer flows over obstacles

Summary The present study fits in the frame of a research program concerning in general the dynamics of airflow in the atmospheric boundary layer and in particular the influence of terrestrial rotation on the movements of air masses interacting with natural extended obstacles (mountains). The experiment has been performed by the method of hydraulic simulation, using schematic models at reduced scale in a channel placed on a rotating platform. We only considered the case of a neutral atmosphere and studied the wake of an obstacle with semi-circular section and the reciprocal interaction of two obstacles of this kind placed perpendicularly to the flow. In this last case we investigated the influence of the distance between the obstacles on their wake length and on the vorticity conditions inside the wakes. Among the various results we obtained, the modifications of the velocity profiles over the reliefs and their dependence on the rotation velocity are particularly interesting. We discuss here our results from two different points of view, namely the purely hydraulic one (which includes the effects of different rotation velocities) and the atmospheric one, according to which the model simulates—with given reduction scales—an actual situation characterized by a fixed value of the Coriolis parameter. As to the first approach to the problem, we found that: 1) The roll with horizontal axis, which is observed behind an obstacle, becomes narrower and narrower as the rotation velocity of the platform increases, while its stability in time and its definition in space increase. In general, it may be said that rotation plays a stabilizing role for vortex dimensions and velocity profiles. 2) The transversal velocity behind the obstacles may attain values about twice the mean longitudinal velocity of the flow. 3) When rotating, the roll is thicker at its left edge than on the right one, due to the transversal flux which provides fluid supply. 4) When the thickness of the boundary layer is increased by making the channel bottom rough, the above-mentioned phenomena are emphasized; moreover, with a second obstacle placed in the flow (and not too far from the first) the transversal velocity components increase, due to a canalization effect. 5) The accelerations of the low layers over the obstacles are strongly amplified by rotation, due to the action of Coriolis' force. As for the second approach, we checked the extent to which the simulation procedures adopted for our laboratory flows, and for their boundary conditions, can be representative of the features of atmospheric phenomena. In order to do that, we compared the dynamical structure of the low flow layers over the obstacle with the analogous structure observed in the field and in wind tunnel by other authors, as well as with the predictions of a few theoretical models. Inside the lowest part of the planetary boundary layer, where the overshooting due to the relief is confined, a good consistency was found among all these results, in particular for what concerns the maximum of overflow velocity. This work has been carried out during 1981–1982 on the rotating platform of Institut de Mécanique, Université de Grenoble, within the ambit of a research convention with Istituto di Cosmogeofisica of C.N.R., Torino.     

Acoustic sounder monitoring of windshear within the atmospheric boundary layer over Ile-Ife, Nigeria

Summary A tri-axial acoustic sounder with Doppler wind measuring capability has been used to measure boundary layer winds from which windshear fields have been evaluated over Ile-Ife (7.29° N, 4.34° E) Nigeria between April and June, 1988. The correlation between SODAR-derived windshear profile and a radiosonde-derived temperature profile is examined. Associated Richardson numbers have been used to categorize atmospheric layers into various stability types. A case study of wind fields and derived windshear zones associated with a rain event has been made. The authors of this paper have agreed to not receive the proofs for correction.     

Evolution of the atmospheric convective boundary layer monitored by the metric RASS

Summary The automatic metric RASS (Radio Acoustic Sounding System), operating at an acoustic wavelength ∼1 m, proved to be able to measure continuously the thermal vertical profiles as far as about 1 km in 50% of soundings with an accuracy generally within a few tenths ofK degrees and with an altitude resolution of (30÷40) m. A set of triennal data of the diurnal evolution of the convective boundary layer (CBL) during Summer, as monitored by a metric RASS operating in the Po Valley (Italy) is presented and discussed. Although atmospheric turbulence, pertinent to the considered unstable conditions, is one of the main factors limiting the radio-acoustic sounding range, by means of the metric RASS it is possible to follow the growth of the mixing layer and to estimate the multilayered structure of the CBL. In addition to the RASS data, the measurements of wind speed, temperature, radiation, humidity and so on, performed by conventional equipments near ground level, are considered.

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Riassunto Il RASS (Radio Acoustic Sounding System) operante in modo automatico alla lunghezza d'onda acustica ∼1 m, ha dimostrato di poter misurare con continuità il profilo termico verticale dell'atmosfera fino a ∼1 km nel 50%, dei sondaggi, con una precisione entro alcuni decimi di gradoK ed una risoluzione verticale di (30÷40) m. Vengono qui presentati ed analizzati i dati triennali (1984–86) sull'evoluzione dello strato limite convettivo durante l'estate, misurati da un RASS metrico operante in Valle Padana. Sebbene la turbolenza atmosferica, associata alle condizioni instabili considerate, sia uno dei principali fattori che limitano la quota di sondaggio del RASS, con il sistema metrico è possibile seguire la crescita dello strato rimescolato ed analizzare la struttura termica inomogenea dello strato limite convettivo. Oltre alle misure RASS sono stati considerati i dati di sistemi tradizionali per la misura della velocità del vento, della radiazione solare, dell'umidità, ecc. in prossimità del suolo.

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Резюме В исследованиях атмосферного граничиого слоя используется разнообразная аппаратура и опорные платформы. Однако имеется мало информации об относительно недоступных областях выше поверхностного слоя. Для измерений в более высоких областях используются летательные аппараты и болышие аэростаты, но они не могут обеспечить непрерывные наблюдения. Автоматическая метрическая радио-акустическая зондирующая система, действующая на длине волны ∼1 м, обеспечивает непрерывные измерения температурных вертикальных профилей до высоты 1 км с точностью до нескольких десятых кельвина и разрешением по высоте (30÷40) м. Приводятся и обсуждаются данные за трехлетний период для суточной эволюции конвективного граничного слоя втечение лета, которая контролировалавсь с помощью метрической радио-акустической зондирующей системы в долине По (Италия). Эта область характерется преобладающими слабыми ветрами. Хотя атмосферная турбулентность является основным фактором, ограничивающим применение радио-акустической зондирующей системы, с помощью метрической радио-акустической зондирующей системы можно проследить перемешивание слоев и оценить многослойную структуру конвективного граничного слоя. Кроме данных радио-акустической зондирующей системы, рассматриваются измерения скорости ветра, температуры, излучения, влажности и т.д., выполненные с помощью обычной аппаратуры.

     

A large-eddy simulation of buoyant plumes in a convective boundary layer

Summary By means of a large-eddy simulation technique, the time-dependent large-scale flow field of a convective atmospheric boundary layer has been claculated. Using a conservation equation for the concentration dispersion we applied this simulation technique to dispersion from both passive and buoyant sources. The results were found to be in good agreement with laboratory and field experiments. It was possible to separate the mean plume motion into a part induced by convective turbulence and a part induced by plume buoyancy. We found that the part of the plume motion caused by convective turbulence was strongly influenced by plume buoyancy. On the other hand, it appears that the large-scale motions of convective turbulence have a strong influence on the entrainment processes governing the motion due to buoyancy. Paper presented at the GNFAO/EURASAP Meeting, Turin, September 1989. To speed up publication, proofs were not sent to the authors and were supervised by the Scientific Committee.     

Development of a boundary layer model for evaluation of dispersion in complex terrain

Summary Trajectories in the turbulent boundary layer have been computed using a dynamic model, for a separating and a nonseparating case. The turbulent kinetic energy, the eddy diffusion coefficient and the hill factor have been evaluated along the trajectories, to get some insight into the different dispersion properties of such flows. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

DNS of compressible turbulent boundary layer over a blunt wedge

Wall turbulence is more complicated than free turbulence, and the direct numerical simulation (DNS) of wall turbulence is more difficult. In recent years, most of DNS cases for wall turbulence are simplified by using temporal mode, where streamwise pe- riodic boundary condition is imposed. In temporal mode, spatial transition will be con- sidered as an analogue of time-evolving transition. For the channel turbulence, an equivalent body force can substitute for the mean gradient of pressure, …     

Noise of a turbulent boundary layer flow over smooth and rough plates at low mach numbers

The spectral levels of the quadrupole noise generated by a boundary layer flow over a smooth surface are calculated. Explicit dependences of the noise levels on the Reynolds number are obtained for the low-frequency and high-frequency ranges. It is shown that the logarithmic zone of the velocity profile is responsible for the region of the quadrupole noise spectrum with a hyperbolic dependence on frequency. A method of calculating the dipole noise of a boundary layer flow over a rough surface is developed. The method is based on the use of the combined probability density for the turbulent velocity fluctuations and the random dimensions of protuberances of the rough surface. The two constants involved in this theory are determined from a special experiment. It is shown that the surface roughness noticeably increases the radiation levels of a boundary layer flow in a certain frequency range.     

The scaling of the wall pressure fluctuations in polymer-modified turbulent boundary layer flow

Wall pressure fluctuations and integrated skin friction were measured beneath a turbulent boundary layer that was modified by adding drag-reducing polymer to the pure water flow. The measurements were performed on an axisymmetric model, equipped with an isolated cylindrical drag balance section, and placed in the test section of the 0.3048-m-diam water tunnel at ARL Penn State. Data were acquired at a free-stream velocity of 10.7 m/s with pure water and with polymer added to the water at concentrations of 1, 5, 10, and 20 weight parts per million. Nondimensionalization of the wall pressure fluctuation frequency spectra with traditional outer, inner, and mixed flow variables failed to adequately collapse the data. The mean square wall pressure fluctuations were found to scale linearly with the wall shear stress. Polymer addition had little effect on the characteristic time scale of the flow. These properties were used to develop a novel form of the nondimensional wall pressure fluctuation spectrum that provided the best collapse of the measured data.     

On the analytical solutions of unstable surface layer similarity equations

Summary An approximate relationship between the gradient, Ri, and the bulk Richardson number, Ri_b, for unstable atmospheric conditions, is suggested. The proposed relation shows much better agreement with the estimation provided by a numerical iterative method than the usual approximations suggested by Louis and Byun.     

LES of spatially developing turbulent boundary layer over a concave surface

We revisit the problem of a spatially developing turbulent boundary layer over a concave surface. Unlike previous investigations, we simulate the combined effects of streamline curvature as well as curvature-induced pressure gradients on the turbulence. Our focus is on investigating the response of the turbulent boundary layer to the sudden onset of curvature and the destabilising influence of concave surface in the presence of pressure gradients. This is of interest for evaluating the turbulence closure models. At the beginning of the curve, the momentum thickness Reynolds number is 1520 and the ratio of the boundary layer thickness to the radius of curvature is δ₀/R = 0.055. The radial profiles of the mean velocity and turbulence statistics at different locations along the concave surface are presented. Our recently proposed curvature-corrected Reynolds Averaged Navier-Stokes (RANS) model is assessed in an a posteriori sense and the improvements obtained over the base model are reported. From the large Eddy simulation (LES) results, it was found that the maximum influence of concave curvature is on the wall-normal component of the Reynolds stress. The budgets of wall-normal Reynolds stress also confirmed this observation. At the onset of curvature, the effect of adverse pressure gradient is found to be predominant. This decreases the skin friction levels below that in the flat section.     

Dependence of the wind profile power law exponent on the Pasquill stability classes and height interval in a stably stratified surface boundary layer.

Summary Using the logarithmic wind profile under stable stratification, a classification scheme was obtained for the Monin-Obukhov stability length and for the wind profile power law exponent in terms of the surface roughness length and the PasquillD-G stability classes, anywhere in the surface layer.     

Constrained large-eddy simulation of supersonic turbulent boundary layer over a compression ramp

A supersonic turbulent boundary layer over a compression ramp is numerically investigated using the constrained large-eddy simulation (CLES) method. The compression corner is characterised by a deflection angle of 24°. The free-stream Mach number is Ma_∞ = 2.9, and the Reynolds number based on the momentum thickness of inlet boundary layer is Re_θ = 2300. The mean and statistical quantities, such as mean velocity, wall pressure and Reynolds stresses, are thoroughly analysed and compared with those from traditional large-eddy simulation (LES), experimental measurement and direct numerical simulation (DNS). It turns out that CLES can predict the friction coefficient, wall-pressure distribution, size of separation bubble, Reynolds stresses, etc. more accurately than traditional LES, and the results are in reasonable agreement with the experimental and/or DNS data. Also discussed are the effects of specific parameterisations of the Reynolds constraint and interfacial positions separating the constrained and unconstrained regions on the performance of the CLES method.     

DNS analysis of boundary layer flashback in turbulent flow with wall-normal pressure gradient

The presence of swirl in combustion systems produces a marked change in their boundary layer flashback behaviour. Two aspects of swirling flow are investigated in this study: the effect of the swirl-generated wall-normal pressure gradient, and the effect of misalignment between the mean flow direction and the direction of flame propagation. The analysis employs Direct Numerical Simulation (DNS) of fuel-lean premixed hydrogen-air flames in turbulent planar channel flow with friction Reynolds number of 180. The effect of swirl on the flashback process is investigated by imposing a wall-normal pressure gradient profile. Analysis of the DNS data shows how the resulting differences in flow field and flame topology contribute to the differences in the overall flashback speed. Misalignment of the flow and propagation directions leads to asymmetry in the flame shape statistics as streaks of high velocity fluid in the boundary layer cleave into the flame front at an angle, yielding an increase in flame surface density away from the wall. Swirl has a stabilising effect on the turbulent flame front during flashback along the centre-body of a swirling annular flow due to the density stratification across the flame front, and produces a reduction in turbulent consumption speed. However the swirl also sets up a hydrostatic pressure difference that drives the flame forward, and the net effect is that the flashback speed is increased. The dominance of hydrostatic effects motivates development of relatively simple modelling for the effect of swirl on flashback speed. A model accounting for the inviscid momentum balance and for confinement effects is presented which adequately describes the effect of swirl on flashback speed observed in previous experimental studies.     

Sound from boundary layer flow over steps and gaps

This study is concerned with the radiated sound from boundary layer flows over small forward and backward steps and gap configurations of similar dimension. These measurements were performed in the Virginia Tech Anechoic Wall Jet Facility for step heights that ranged from approximately 10 percent to 100 percent of the incoming boundary layer height. The results show the influence of step height and boundary layer edge velocity on the far-field sound from forward and backward steps. Neither source shows clear dipole directivity and at least the larger step heights considered in this study are shown to not be acoustically compact. A new mixed scaling normalization is proposed for the far-field spectra from both types of step. Backward steps are shown to be much weaker producers of far-field sound than similarly sized forward steps. The implications of this behavior are discussed with respect to the far-field sound measured from various gap flows.     

A diagnostic study of turbulence features in wind tunnel flow over simple topographic shapes

Summary The turbulence features observed in wind tunnel experiments of flow over isolated simple-shaped two-dimensional hills and valleys are analysed in the light of transport equations of Reynolds stresses in the streamline coordinate system. The analysis has shown that near the surface the streamwise pressure gradient is the driving parameter for all the stresses. At higher levels, the effects of streamline curvature and streamwise pressure gradient act differently on the source terms of stresses, showing that the shear stress is the most sensitive to streamline curvature, whereas the streamwise stress is the most sensitive to streamwise pressure gradient. Although two of the considered topographies are specular images, the turbulence response is not so, since it depends on the sequence of curvatures to which it is subjected. The information and analysis presented will constitute a valuable resource to those seeking an understanding of atmospheric boundary layer development over complex terrain. It will also provide a support for the development and validation of more rigorous turbulence models.     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

Study of local scaling in the stable atmospheric boundary layer for the evaluation of turbulence parameters

Summary A renewed interest in the dynamic and thermal structure of the atmospheric boundary layer (ABL) has recently revived, due to the contribution that this layer can provide to the improvement of skill of weather prediction and climate simulation in the most advanced general circulation models. The attention is particularly focused on non-stationary and non-homogeneous turbulent structures of this transition layer, for which many parametrization schemes, based on local-and non-local-closure assumptions, have been proposed. However, also stationary, non-homogeneous, continous-turbulence conditions can give rise to some complication in the ABL parametrization when air stability becomes stronger and stronger. In this situation, the governing scales of the classical non-local Monin-Obukhov similarity theory are no more representative, and new local scales must be derived from local forcings of the flux. This paper summarizes a few relevant points of the local-similarity theory and shows the results of a local-scaling analysis obtained by observation data of wind and temperature profiles in the ABL in the northern Po Valley Paper presented at the IX Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 8–10, 1992, Rome.