Зовнішній опір мотогондоли газотурбінного двигуна з відсмоктуванням примежового шару

In 1904 Ludwig Prandtl led the foundation for what became known as boundary layer concept. Already before the 20th century there were more or less developed views about a frictional layer between the solid surface of an object in a stream and the free flow at some distance from the surface, but none of these were further developed. Prandtl aimed with his concept at a theory of flow detachment from solid surfaces. In its initial form, the concept was elaborated for laminar boundary layer flow by Prandtl’s doctoral student Heinrich Blasius. Turbulent boundary layer flow and detachment was observed at first in the Göttingen wind tunnel with experiments on the drag of spheres. In 1920, Prandtl’s “master pupil” Theodore von Kármán paved the ground for a theory of turbulent boundary layers that became subject of a fierce rivalry between Prandtl, Kármán and their “schools” in Göttingen and Aachen. By the same time, the transition from laminar to turbulent flow, with the focus on the boundary layer, became perceived as “the turbulence problem”. In 1924 hot-wire anemometry in a wind tunnel provided experimental evidence for the theoretically predicted laminar and turbulent flow profiles in the boundary layer. Both for the theory of the onset of turbulence and of fully developed turbulence the boundary layer concept proved crucial and fruitful.

A new correlation to determine the discharge coefficient of a critical Venturi nozzle with turbulent boundary layer

On the basis of advanced concentration and velocity profiles above a mobile seabed, an instantaneous analytical model is derived for sediment transport in asymmetric oscillatory flow. The applied concentration profile is obtained from the classical exponential law based on mass conservation, and asymmetric velocity profile is developed following the turbulent boundary layer theory and the asymmetric wave theory. The proposed model includes two parts: the basic part that consists of erosion depth and free stream velocity, and can be simplified to the total Shields parameter power 3/2 in accordance with the classical empirical models, and the extra vital part that consists of phase-lead, boundary layer thickness and erosion depth. The effects of suspended sediment, phase-lag and asymmetric boundary layer development are considered particularly in the model. The observed instantaneous transport rate proportional to different velocity exponents due to phase-lag is unified and summarised by the proposed model. Both instantaneous and half period empirical formulas are compared with the developed model, using extensive data on a wide range of flow and sediment conditions. The synchronous variation in instantaneous transport rate with free stream velocity and its decrement caused by increased sediment size are predicted correctly. Net transport rates, especially offshore transport rates with large phase-lag under velocity skewed flows, which existing instantaneous type formulas failed to predict, are predicted correctly in both direction and magnitude by the proposed model. Net sediment transport rates are affected not only by suspended sediment and phase-lag, but also by the boundary layer difference between onshore and offshore.

Influence of coupled boundary layer suction and bowed blade on flow field and performance of a diffusion cascade

<div class="htmlview paragraph">A region with floor boundary-layer suction upstream of the vehicle to remove the oncoming boundary layer is often used in automotive wind tunnels. These suction systems inevitably change the empty-tunnel pressure gradient. In this paper, the empty-tunnel pressure gradient created by the use of boundary layer suction and its effect on measured drag are investigated. By using excess suction - more suction than necessary to remove the floor boundary layer – it was possible to show experimentally that the major part of the drag increase due to boundary layer suction is created by unintended gradient effects. Only a minor part of the drag increase is due to the increased flow velocities at the lower parts of the vehicle, or in other words, due to the improved ground simulation. A theoretical model, using the concept of horizontal buoyancy to predict the gradient effect, is proposed. The model is compared to the experimental results as well as to CFD calculations. The use of the analytical correction method to correct for the unintended gradient effects is expected to be a significant improvement for results obtained using boundary layer suction. Alternative methods to control the boundary layer thickness, which do not introduce strong empty-tunnel gradients, are recommended.</div>

An innovative method for prescribing turbulent thermal inflow information in spatially developing boundary layers under streamwise pressure gradients is introduced for attached flows. The approach is tested and validated in a suite of Direct Numerical Simulations (DNS) of thermal boundary layers for zero (ZPG) and adverse (APG) pressure gradients with momentum thickness Reynolds numbers (Reθ) up to 3000. The turbulent thermal data are generated based on the dynamic multi-scale approach proposed by Araya et al. [“A dynamic multi-scale approach for turbulent inflow boundary conditions in spatially evolving flows,” J. Fluid Mech. 670, 581–605 (2011)], which is extended to include thermal field simulations in the present article. The approach is based on the original rescaling-recycling method developed by Lund, Wu, and Squires [“Generation of turbulent inflow data for spatially developing boundary layer simulations,” J. Comput. Phys. 140, 233–258 (1998)] for ZPG flows. Isothermal walls are considered for the thermal field and the molecular Prandtl number is 0.71. In addition, only inlet momentum/thermal boundary layer thicknesses must be prescribed while other flow parameters such as the inlet friction velocity, uτ, and friction temperature, Θτ, are computed dynamically based on the flow solution obtained downstream by means of a test plane. This plane is located between the inlet and recycle stations. Based on the unique and extensive DNS results of heat transfer obtained in this investigation, the effects of Reynolds numbers and adverse pressure gradients on the flow and thermal parameters are also explored and visualized. The principal outcome of adverse pressure gradient on the flow parameters has been determined as a secondary peak, particularly on the streamwise velocity fluctuations in the outer region, which shows clear evidence of energy production in the outer flow and not only in the buffer layer as traditionally known. Nevertheless, this peak is not so obvious on the thermal fluctuations but it is hypothesized that the reason is mainly attributed to the absence of a freestream thermal gradient, as imposed in the velocity field. Furthermore, the high-speed streaks in the buffer layer are observed to be notably shorter and wider in a Strong APG than in the ZPG case. Finally, a significant decrease of the turbulent Prandtl number is attributed to the presence of a Strong APG.

Boundary layer ingestion (BLI) propulsion offers notable benefits for blended wing body (BWB) aircraft, and understanding the interrelated effects of BLI and propulsion–airframe integration (PAI) is critical for early-stage design decisions. This study numerically applies combined momentum- and energy-based analyses to a closely coupled but non-integrated BWB–turbofan configuration enabling a continuous transition from non-BLI to BLI conditions. By introducing an idealized capture streamtube–airframe interaction force, the drag of BLI layout is decomposed into additional and external components, enabling quantification of a lift-to-drag ratio improvement of 1.7–2.6, corresponding to a 7.14–8.27% gain in power saving coefficient (PSC). Additional drag reduction, the primary contributor to total drag savings, is analytically attributed to inlet total pressure loss. The resulting decrease in required thrust under BLI shows strong mathematical correlation with jet dissipation reduction, revealing an intrinsic link between drag reduction and power saving. PAI exerts a significant influence on the BLI benefits, including nacelle cowl drag penalties, significant variations in shock wave location and strength, and notable suppression of both boundary layer and wake dissipation for the portion of cowl immersed in the airframe wake. These findings inform the transition from podded to BLI engine layouts.

It is well known that elastic or compliant boundaries can have a stabilizing effect on boundary layer flow leading to a reduction in turbulence and frictional drag. This phenomenon has wide-ranging interdisciplinary applications from the study of energy-efficient propulsion to the study of blood flow through the cardiovascular system. While a substantial body of work exists on the theory of turbulent boundary layers and the transition of laminar to turbulent flow, it is equally important to measure in detail the flow near rigid and compliant boundaries to better understand the dynamics underlying the stabilizing effect and the reduction of turbulence. Recent advances in technology and computational resources have allowed the measurement and numerical simulation of boundary layer instabilities in unprecedented detail. We employ particle image velocimetry as well as high-frequency fiber-optics sensors to visualize and measure velocity and temperature fluctuations under various flow conditions: a laminar flow tank to study the development of Tollmien-Schlichting waves and the laboratory tank of the Simulated Turbulence and Turbidity Environment (SiTTE) to identify boundary layers streaks. The laboratory environments are complemented by computational fluid dynamics representations of the respective setups, implemented as high-resolution large-eddy simulation. The simulations provide spatial and temporal scales of boundary layer instabilities, allow the calculation of turbulence characteristics and add prediction capabilities. The combined approach allows the detailed characterization of boundary layer instabilities for a range of flow conditions, which is critical to improve our understanding of the impact of elastic boundaries, both active and passive, on boundary layer drag.

Flow in the juncture of surface mounted stream-lined slabs of various aspect ratios was studied in a low speed wind tunnel for laminar as well as turbulent approach boundary layer conditions for Reynolds number of 70,000, 90,000 and 120,000. For a laminar approach boundary layer a wavy structure along the primary separation line was observed for an aspect ratio of 1 while at higher aspect ratios and in case of the turbulent boundary layer for all aspect ratios this pattern was not visible. Mean pressures measured along the plane of symmetry on the surface exhibited influence of aspect ratio as well as boundary layer condition. While the surface pressure gradient for the turbulent case continuously increased in the direction of the slab, the laminar boundary layer case had a distinct change in slope, the location of which was aspect ratio dependent. Additionally the pressure gradient for the turbulent boundary layer was lower in comparison to the laminar boundary layer. The pressure differential at the mean vortex location for the turbulent boundary layer was higher than the laminar boundary layer case and reduced with increasing aspect ratio for both cases. The mean vortex location moved away from the slab leading edge with increasing aspect ratio for the laminar case while movement in case of the turbulent boundary layer was not discernable. The mean vortex locations for the turbulent boundary layer were further upstream as compared to the laminar boundary layer cases

Energy integral equation for premixed flame-wall interaction in turbulent boundary layers and its application to turbulent burning velocity and wall flux evaluations

A distributed fan propulsion system can potentially achieve a 10%-20% fuel burn reduction compared with traditional podded engines. However, the ducted fan is continuously working under inflow distortion conditions caused by boundary layer ingestion, thus resulting in an increase of aerodynamic loss and decreasing the aircraft overall aerodynamic benefit achievement. To control the loss in the boundary layer ingestion ducted fan, a loss reduction design strategy based on the lip-installed bump is developed. The computational results show that the lip-installed bump can effectively suppress the intensity of the distorted inflow with a 9.7% reduction at the rotor inlet. The reason is that a high-pressure area will be presented on the mount of bump and a low-pressure on both sides of the bump compression surface which pushes the low-momentum flow in the boundary layer away from lip. In this way, the bump energizes the low-momentum flow on the downer wall of the lip and reduces the local boundary layer momentum loss thickness. In the meanwhile, as the distortion intensity is decreased near the downer wall at rotor inlet, the local blade load is also reduced, especially near the blade tip. Further, the decrease of the blade load near the tip leads to a reduction of the normal pressure difference between the pressure and suction surfaces which can eliminate the interaction of the main flow and the tip leakage flow, resulting in a 6% decrease of flow loss in the rotor blade tip region. In comparison to the baseline ducted fan, the fan with a lip-installed bump has achieved a 1.3% enhancement of adiabatic efficiency at the re-design point under inflow distortion condition, without a sacrifice of total pressure ratio.

Our objective was to determine whether blades of Macrocystis integrifolia from a wave‐sheltered site possess morphological features that “trip” velocity boundary layers (VBL) from laminar to turbulent in conditions of low water motion, compared to blades from a wave‐exposed site. The movement of seawater around morphologically distinct blades of M. integrifolia from wave‐sheltered and exposed sites was visualized in a recirculating flow tank in which seawater flow was initially laminar at velocities up to 8 cm s−1. Turbulent VBLs were observed for both blade morphologies at free‐stream seawater velocities of 2 cm s−1, and at a Reynolds numbers of between 102 and 104, which is lower than that predicted from boundary layer theory. Flow around blades from a wave‐sheltered site was dominated by separation induced by large edge undulations. For the wave‐exposed morphology, the flow pattern was similar to that along a smooth, flat plate, but the observed thickness of the VBL was up to a factor of 2 greater than that predicted from laminar or turbulent boundary layer theory. There were no obvious differences in how structural features such as marginal spines and bulbs affected seawater flow around the different blade morphologies.

The turbulent boundary layer developing under a turbulence-laden free stream is numerically investigated using the temporal boundary layer framework. This study focuses on the interaction between the fully turbulent boundary layer and decaying free-stream turbulence. Previous experiments and simulations of this physical problem have considered a spatially evolving boundary layer beset by free-stream turbulence. The state of the boundary layer at any given downstream position in fact reflects the accumulated history of the co-evolution of boundary layer and free-stream turbulence. The central aim of the present work is to isolate the effect of local free-stream disturbances existing at the same time as the 'downstream' boundary layer. The temporal framework used here helps expose when and how disturbances directly above the boundary layer actively impart change upon it. The bulk of our simulations were completed by seeding the free stream above boundary layers that were 'pre-grown' to a desired thickness with homogeneous isotropic turbulence from a precursor simulation. Moreover, this strategy allowed us to test various combinations of the turbulence intensity and large-eddy length scale of the free-stream turbulence with respect to the corresponding scales of the boundary layer. The relative large-eddy turnover time scale between the free-stream turbulence and the boundary layer emerges as an important parameter in predicting if the free-stream turbulence and boundary layer interaction will be 'strong' or 'weak' before the free-stream turbulence eventually fades to a negligible level. If the large-eddy turnover time scale of the free-stream turbulence is much smaller than that of the boundary layer, the interaction will be 'weak', as the free-stream disturbances will markedly decay before the boundary layer is able be altered significantly as a result of the free-stream disturbances. For a 'strong' interaction, the injected free-stream turbulence causes increased spreading of the boundary layer away from the wall, permitting large incursions of free-stream fluid deep within it.

This paper considers the application of two types of performance accounting systems to a boundary layer ingesting (BLI) propulsion configuration to identify features that arise due to propulsion-airframe integration. To do this, a canonical, two-dimensional BLI configuration consisting of a fuselage element propelled by an integrated ducted propulsor has been developed, and aerodynamic forces and power for a range of angle of attack and propulsor mass flow has been calculated using MSES, a viscous-inviscid flow solver for multi-element airfoils. In a traditional thrust-drag accounting, BLI manifests as a negative “interference drag,” equal to 46% of the isolated fuselage element drag at zero angle of attack and design mass flow. The results also show that accounting for changes in vehicle angle of attack before changes in throttle setting reduces variation in the throttle-dependent force and power increments with respect to the angle of attack, suggesting that such an approach may be appropriate for BLI configurations. The second type of accounting system was the Power Balance Method, which has been suggested to be more suitable for the analysis of integrated geometries with BLI. The results show that the boundary layer, wake, and jet dissipation of the BLI configuration vary with configuration angle of attack and mass flow, a behavior that must be considered when using power-based accounting. The analysis also highlights the difficulty in defining separate contributions to viscous dissipation from adjacent surface wake and jet flows that are inherent to BLI configurations.

Heat, mass and momentum transfer in separated flows. The simple model of a backward‐facing step in flat plate boundary layers provides basic information on heat, mass and momentum transfer in local separated regions. These new investigations cover nearly the whole range of existence of incompressible separated flows with laminar as well as turbulent boundary layers at separation, including the two main parameters Reδ1 and Res: The Reynolds number Reδ1 represents the boundary layer at separation, the Reynolds number Res the shape of the body. It is demonstrated that independently of the state of the boundary layer at separation, there exist three types of local separated regions. Therefore a new general valid classification of local separated flows is introduced based on the actual state of the separated boundary (shear) layer from separation to reattachement. The new results presented are limited to the case where no temperature or concentration boundary layers exist at separation, i.e. to the case of unheated or inert starting length before separation: Only then can the analogy of heat, mass and momentum transfer give useful results over the whole range of existence of local separated regions. A new shear layer model is introduced to facilitate transfer of the results obtained for backward‐facing steps to arbitrary shaped bodies. A compilation and comparison of available literature values shows that this shear layer model for the step also permits the calculation of heat and mass transfer in separated areas on arbitrarily shaped bodies.