Impulsfluss an vertikalen Wänden – Messung und Parametrisierung

<p>Der globale Klimawandel hat einen großen Einfluss auf das städtische Klima, wobei sich durch die hohe Bebauungsdichte und Versiegelung viele Effekte wie Hitzewellen zusätzlich verstärken. Damit unsere Städte auch in Zukunft lebenswerte Orte bleiben, müssen diese an die veränderten klimatischen Bedingungen angepasst werden. Um diese Anforderungen umzusetzen, können dank der gestiegenen Rechenkapazitäten vermehrt wirbel- und gebäudeauflösende Large-Eddy-Simulations-(LES) Modelle wie das PALM-4U (Maronga et al., 2015) in der Praxis zur Stadtplanung eingesetzt werden. Die in diesen Modellen verwendeten Annahmen und Parametrisierungen zum Windprofil sowie Impulsfluss an vertikalen Wänden von Gebäuden basieren jedoch mangels geeigneter Daten zumeist auf Grenzschichtmessungen über nahezu homogenen Flächen (Businger, 1971). Daher stellt sich die Frage, wie gut diese Annahmen an vertikalen Wänden zutreffen. Wie sehen das Windprofil und der Impulsfluss an einer realen Fassade aus?</p> <p>Zur Untersuchung dieser Fragestellungen wurden im Rahmen des „Stadtklima im Wandel [UC]<sup>2</sup>“ Projektes zwei 6 m lange Ausleger in etwa 42 m (10. Stock) und 64 m (16. Stock) Höhe an der Fassade eines insgesamt 85 m hohen Gebäudes im Zentrum von Hamburg installiert. Um detaillierte Informationen zur Turbulenz zu erhalten, werden an beiden Auslegern in 2 m, 4 m und 6 m Entfernung zur Fassade die drei Windkomponenten mit 20 Hz erfasst. Die Messungen werden seit August 2021 durchgeführt, sodass unterschiedlichste Anströmungsrichtungen des Gebäudes als auch zahlreiche synoptische Situationen von schwachem bis stärkeren Wind gemessen wurden.</p> <p>Der einzigartige Messdatensatz an einer realen Hochhausfassade liefert detaillierte Einblicke in das Windprofil sowie den Impulsfluss an Gebäuden in Städten. Dies ermöglicht die Untersuchung der aktuell in vielen LES Modellen genutzten Annahmen wie zum Beispiel des logarithmischen Windprofils an Fassaden. Darüber hinaus wird die im PALM‑4U Modell verwendete Parametrisierung für den Impulsfluss mit den Messungen verglichen. Die Form des Windprofils an der Fassade ist unter anderem von der Anströmungsrichtung, der Geometrie sowie der Messposition am Gebäude abhängig. Die Turbulenzintensität nimmt unabhängig der Anströmung in allen drei Komponenten mit größerem Abstand zur Fassade hin ab. Die Ergebnisse werden in Hinblick auf verbesserte Parametrisierungen in Modellen diskutiert.</p> <p> </p> <p><strong>Literatur:</strong></p> <p>Businger, J. A., Wyngaard, J. C., Izumi, Y., and Bradley, E. F.: 1971, ‘Flux-Profile Relationships in the Atmospheric Surface Layer’, <em>J. Atmos. Sci.</em> 28, 181–189.</p> <p>Maronga, B., Gryschka, M., Heinze, R., Hoffmann, F., Kanani-Sühring, F., Keck, M., Ketelsen, K., Letzel, M. O., Sühring, M., and Raasch, S. (2015): The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives, Geosci. Model Dev., 8, 1539-1637, DOI:10.5194/gmd-8-2515-2015.</p>

Design of a radiant panel test at reduced scale for the high throughput development of artificial turf structures

Artificial turf structures are increasingly used in closed areas and have to comply with the European fire standard for building products (EN ISO 13501-1). The main test to evaluate the fire performance of flooring products is the EN ISO 9239-1 radiant panel test. The test principle is to determine the critical heat flux of floorings exposed to a forced ignition and a specific heat flux profile. As large amounts of material are needed to perform the test, the development of a radiant panel test at reduced scale was considered. The experimental design methodology was implemented to mimic the heat flux profile. The fire performance of artificial turf structures was evaluated at both scales and the results were compared. The burnt lengths of the specimens and thus the critical heat flux are similar for both scales. Thus, the downscaled device could advantageously be used for high throughput development of artificial turf structures.

Sediment load determines the shape of rivers

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker [G. Parker, J. Fluid Mech. 89, 127–146 (1978)], that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid–sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge—a quantity notoriously difficult to measure in the field.

Flux profile at focal area of concentrating solar dishes

We analytically, experimentally and computationally explore the solar radiation flux distribution in the interior region of a spherical mirror and compare it to that of a paraboloidal one with the same aperture area. Our investigation has been performed in the framework of geometrical optics. It is shown that despite one can assign a quasi focus, at half the radius, to a spherical mirror, the light concentration occurs as well on an extended line region which starts at half-radius on the optical axis. In contrast to a paraboloidal concentrator, a spherical mirror can concentrate the radiation parallel to its optical axis both in a point-focus and in a line-focus manner. The envelope of the reflected rays is also obtained. It is shown that the flux distribution has an axial symmetry. The radial dependence of the flux on a flat circular receiver is obtained. The flux longitudinal dependence is shown to exhibit three distinctive regions in the interval [0, R] (R is mirror radius). We obtain the radiational (optical) concentration ratio characteristics and find the optimal location of the flat receiver of a given size at which the concentration ratio is maximised. In contrast to a parabolic mirror, it is shown that this location depends on the receiver size. Our findings offers that in spherical mirrors one can alternatively use a line receiver and gains a considerable thermal energy harvest. Our results are supported by Monte Carlo ray tracing performed by Zemax optical software. Experimental validation has been performed in lab with a silver-coated lens as the spherical mirror.

Representing Low-Intensity Fire Sensible Heat Output in a Mesoscale Atmospheric Model with a Canopy Submodel: A Case Study with ARPS-CANOPY (version 5.2.12)

Mesoscale models are a class of atmospheric numerical model designed to simulate atmospheric phenomena with horizontal scales of about 2–200 km, although they are also applied to microscale phenomena, with horizontal scales less than about 2 km. Mesoscale models are capable of simulating wildland fire impacts on atmospheric flows if combustion by-products (e.g., heat, smoke) are properly represented in the model. One of the primary challenges encountered in applying a mesoscale model to studies of fire-perturbed flows is the representation of the fire sensible heat source in the model. Two primary methods have been implemented previously: turbulent sensible heat flux, either in the form of an exponentially-decaying vertical heat flux profile or surface heat flux; and soil temperature perturbation. In this study, the ARPS-CANOPY model, a version of the Advanced Regional Prediction System (ARPS) model with a canopy submodel, is utilized to simulate the turbulent atmosphere during a low-intensity operational prescribed fire in the New Jersey Pine Barrens. The study takes place in two phases: model assessment and model sensitivity. In the model assessment phase, analysis is limited to a single control simulation in which the fire sensible heat source is represented as an exponentially-decaying vertical profile of turbulent sensible heat flux. In the model sensitivity phase, a series of simulations are conducted to explore the sensitivity of model-observation agreement to (i) the method used to represent the fire sensible heat source in the model and (ii) parameters controlling the magnitude and vertical distribution of the sensible heat source. In both phases, momentum and scalar fields are compared between the model simulations and data obtained from six flux towers located within and adjacent to the burn unit. The multi-dimensional model assessment confirms that the model reproduces the background and fire-perturbed atmosphere as depicted by the tower observations, although the model underestimates the turbulent kinetic energy at the top of the canopy at several towers. The model sensitivity tests reveal that the best agreement with observations occurs when the fire sensible heat source is represented as a turbulent sensible heat flux profile, with surface heat flux magnitude corresponding to the peak 1-min mean observed heat flux averaged across the flux towers, and an e-folding extinction depth corresponding to the average canopy height in the burn unit. The study findings provide useful guidance for improving the representation of the sensible heat released from low-intensity prescribed fires in mesoscale models.

ECR discharge sustained by millimeter waves as a source of dense plasma flux

The results of the investigation of the dense ECR discharge hydrogen plasma flux formation in the single solenoid magnetic field are presented in this work. The transversal flux profile obtained at the optimal system parameters is shown. The possibility of the formation of homogeneous plasma fluxes with density of 750 mA/cm2 and total current of 5 A is demonstrated. The results of the first experiments of the hydrogen ion beam extraction from the ECR discharge plasma in the single magnetic coil are presented. The record values of the ion current density higher than 1.5 A/cm2 were obtained. The results of the research presented in this paper show the prospects of the proposed system for applications of the neutral beam injector development for the plasma heating in the controlled thermonuclear fusion installations.

Flux–Profile Relationships in the Stable Boundary Layer—A Critical Discussion

Flux–profile relationships are crucial for parametrizing surface fluxes of momentum and heat, that are of central relevance for applications such as climate modelling and weather forecast. Nevertheless, their functional forms are still under discussion, and a generally accepted formulation does not exist yet. We reviewed the four main formulations proposed in the literature so far and assessed how they affect the theoretical behaviour of the kinematic heat flux (H0) and the temperature scale (T*) in the stable boundary layer, as well as their consequences on the existence of critical values for both the gradient and the flux Richardson numbers. None of them turned out to be fully consistent with the literature published so far, with two of them leading to very unreliable expressions for both H0 and T*. All considered, a convincing description of flux–profile relationships still needs to be found and seems to represents a considerable challenge.