Boundary Layer Separation Phenomena Research Articles

Airfoil Design and Flow Analysis of a Multi-Blade Centrifugal Fan: An Experimental and Simulation Study

To overcome the technical challenges of the multi-blade centrifugal fan, such as low efficiency and insufficient total pressure, the blades of the fan were optimally designed in this study. The flow field of the multi-blade centrifugal fan with a single-arc blade and an airfoil blade was simulated and compared using Computational Fluid Dynamics (CFDs). Under steady-state conditions, the total pressure, velocity field distribution, and aerodynamic performance of a multi-blade centrifugal fan were analyzed. The numerical results showed that there were vortices, secondary flows, and boundary layer separation phenomena in the flow passage of the single-arc multi-blade centrifugal fan. Based on the lift-to-drag ratio theory of airfoil in aerodynamics, four different airfoil blades were designed for the multi-blade centrifugal fan. The study found that the lift-to-drag ratio of the airfoil blades was positively correlated with the fan efficiency; among them, the A-type airfoil exhibited the highest lift-to-drag ratio within the 0–10 degree angle of attack range. The three-dimensional simulation results indicated that, except for the initial operating point B, the A-type airfoil showed higher fan efficiency under other operating conditions, and its total pressure curve was the most stable. In addition, the use of airfoil blades effectively suppressed the aforementioned adverse flow phenomena and improved the flow within the blade passage. Experimental verification further confirmed the effect of airfoil blades on improving fan performance: compared to single-arc blades, the efficiency of the multi-blade centrifugal fan increased by 3–7% after using airfoil blades, and the upper limit of high-efficiency flow increased from 450 m3/h to 650 m3/h. Meanwhile, the total pressure and power of the airfoil fan were also significantly improved. The results of this work are significant for guiding the optimal design of the fan.

Experimental Investigation of Simple-Built and Low-Speed Water Tunnel as a Platform for Studying Fluid Flow Using PIV Measurements

We present a thorough experimental investigation of a simple-built low-speed water tunnel that could facilitate the study of some of the primary fluid dynamic concepts based on quantitative flow measurements using Particle Image Velocimetry (PIV). The water tunnel system is designed and built using a simple arrangement of a double reservoir tank and a centrifugal pump that drives the fluid flow at low speed into a square cross-sectional and relatively short test section. The study shows that the system reasonably and steadily generates fully developed flows inside the test section with <20% of point velocity variations. Based on the fluid-structure tests, it is also observed that the system could produce moderately consistent boundary layer separation phenomena with the variance of the area mean vorticity within 25%. With this performance, we show that the system fulfills the essential requirement to be used as a simple, easy-to-operate, and easy-to-maintain educational platform to study fluid dynamic phenomena related to the interaction of a static sub-merged object and fluid flow.

Evaluation of separation point in a divergent channel in the presence of non-uniform magnetic field

Due to the pressure gradient increasing downstream, the boundary layer separation phenomena may occur on the divergent channel flow wall. Still, several methods have been applied to control or postponed the separation. Exerting a magnetic field on the fluid flow is one solution. In the present research, we aim to investigate the impact of magnetic fields on the separation location in a divergent channel numerically. Hence, the finite volume method using the OpenFOAM CFD toolbox is employed, and a code for 2-D divergent channel flow under a non-uniform magnetic field is developed. The obtained results have been validated in comparison with previous studies. The results are presented for a test case of the channel with divergence half-angle equals 2.5°, 30 cm length, and 2 cm inlet height. It is demonstrated when Ha increases from 0 to 2, the separation points for liquid lithium fluid flow with Re = 208.33 are postponed from 12.93 cm to 23.18 cm. Moreover, it is shown that the separation phenomena disappear entirely for the Ha value of more than 3.

Investigation on the pressure matching performance of the constant area supersonic-supersonic ejector

The pressure matching performance of the constant area supersonic-supersonic ejector has been studied by varying the primary and secondary Mach numbers. The effect of the primary fluid injection configurations in ejector, namely peripheral and central, has been investigated as well. Schlieren pictures of flow structure in the former part of the mixing duct with different stagnation pressure ratio of the primary and secondary flows have been taken. Pressure ratios of the primary and secondary flows at the limiting condition have been obtained from the results of pressure and optical measurements. Additionally, a computational fluid dynamics analysis has been performed to clarify the physical meaning of the pressure matching performance diagram of the ejector. The obtained results show that the pressure matching performance of the constant area supersonic-supersonic ejector increases with the increase of the secondary Mach number, and the performance decreases slightly with the increase of the primary Mach number. The phenomenon of boundary layer separation induced by shock wave results in weaker pressure matching performance of the central ejector than that of the peripheral one. Furthermore, based on the observations of the experiment, a simplified analytical model has been proposed to predict the limiting pressure ratio, and the predicted values obtained by this model agree well with the experimental data.

The Impact of Boundary Layer Separation on Local Exhaust Design and Worker Exposure

Abstract The phenomenon of boundary layer separation is an important factor in determining a worker's breathing zone concentration. This article presents the results of flow visualization and tracer gas studies, conducted in a wind tunnel with a mannequin, designed to examine this phenomenon. A simple conceptual model, based on mass transport by vortex shedding, provides a reasonable estimate of the mannequin's breathing zone concentration. An empirical model is developed which relates the measured concentration to the distance from the source to the breathing zone for the situation when the contaminant is released downstream in a uniform flow. Applications of the results are discussed.

Detailed study of the rotor-stator interaction phenomenon in a moving cascade of airfoils

In turbomachinery the Rotor-Stator Interaction (RSI) is an important phenomenon that has a strong influence on the machine behavior. These interactions can have a significant impact on the vibrational and acoustical characteristics of the machine. Unsteadiness and turbulence play a fundamental role in complex flow structure and the use of Computational Fluid Dynamics (CFD) is becoming a usual requirement in design in turbomachinery due to the difficulties and high cost of the necessary experiments needed to identify RSI phenomena. The flow inside a turbomachinery working under design condition is complex but apparently, when working under off-design conditions, it becomes more complex due to the boundary layer separation phenomena. Therefore, the choice of an appropriate turbulence model is far from trivial and a suitable turbulence modeling plays a very important role for successful CFD results. In this work the RSI generated between a moving cascade of blades and fixed flat plate located downstream were studied by means of CFD modeling and compared against experimental results. Design and off-design conditions were modeled and a detailed comparison between them has been made. To analyze in detail the flow pattern, mean velocities in the boundary layer were obtained and compared against experimental results. Furthermore, results concerning to turbulence intensity were compared against an experimental database. It was observed that for each operating condition, the flow in the cascade show special features. For flow inside the turbomachine under design conditions there is no separation, the wake is thin and the characteristic length of the eddies is small. For off-design conditions, there is a large separation and the wake is thick with large eddies. The results obtained can be used to obtain a deeper insight into the RSI phenomena.

Analysis of boundary layer separation phenomena by infrared thermography Use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects

Aerodynamic bodies operating at low Reynolds numbers are normally subjected to local boundary layer separation phenomena; these strongly modify airfoils behavior. The body geometry and the incoming flow angle of attack define the pressure pattern over the surface in a way that there will be sections with a favorable pressure gradient and others with a negative one, the latter causing the flow to slow down and sometimes to separate. A laminar separation, followed by a turbulent transition in the separated shear layer and a subsequent turbulent reattachment, causes the Laminar Separation Bubble (LSB) phenomenon. The LSB presence induces an aerodynamic drag increase and an efficiency decrease; this problem is of interest in many application fields including wind turbine energy production, where a LSB may induce less energy output and occasionally mechanical problems due to pulsating pressure variations (bubble bursting phenomena). This paper illustrates an infrared measurement approach, useful for the instantaneous observation of the boundary layer pattern on an airfoil surface, as well as an analysis of the effects of both an acoustic and a mechanical system on the behavior of the LSB. These latter induce disturbances in the developing boundary layer in order to promote the turbulent transition and contrasting the LSB presence. The first one involves the use of a subwoofer and the analysis is performed on an Eppler 205 airfoil; the second one makes use a Micro Electro Mechanical System (MEMS) and the IR observation is carried out on a WT01 airfoil developed for small wind turbines (about 1 kW). The results show a pronounced effectiveness for both the methodologies particularly when using MEMS.

Analysis of boundary layer separation phenomena by infrared thermography. Use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects

Analysis of boundary layer separation phenomena by infrared thermography. Use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects

A quantitative IR thermographic method to study the laminar separation bubble phenomenon

The boundary layer separation phenomena are present in many application fields: for example the sailplanes, the micro-vehicles, the small wind turbines, the airplanes and the cars. On aerodynamic bodies operating at low Reynolds numbers, lesser than one million, it may happen a boundary layer local separation defined as: laminar separation bubble. This phenomenon induces a body drag increase and an eventual lift decrease; in some situations the cyclical bubble formation and detachment may induce pressure pulses and consequent vibration phenomena. In a previous research work [Internat. J. Thermal Sci. 43 (2004) 315] was verified the possibility to show qualitatively the presence of a laminar bubble by means of a thermographic observation of the body surface. In this work is verified the possibility to carry out a quantitative study of the laminar bubble phenomenon by using the same IR technique. Three characteristic points of the bubble are particularly studied: the laminar separation point, the transition point and the turbulent reattachment point. The laminar bubble behaviour is analysed on a RR3823HL airfoil by varying the angle of attack and the Reynolds number; the adimensional Stanton number, based on the airfoil chord, is obtained in order to individuate the requested points in a more simple and objective way. This adimensional number is carried out by means of a finite numerical difference approach that makes a balance among the heat fluxes on the airfoil surface.

A thermographic method to evaluate the local boundary layer separation phenomena on aerodynamic bodies operating at low Reynolds number

This study uses infrared thermography to focus on local separation phenomena affecting the laminar boundary layer on aerodynamic bodies operating at low Reynolds numbers, and particularly on the laminar separation bubble that occurs, for instance, in applications such as gliders, microplanes and small windfarm turbines. The work was organized into several stages. First the reliability of the wind tunnel available at the Energetics Department of the “Università Politecnica delle Marche” was tested by comparing the results obtained on an Eppler 387 profile with data provided by other, similar tunnels for the same profile. Then the presence of a laminar bubble was ascertained using classic measuring methods, e.g., the spring balance and pressure distribution analysis. The thermographic apparatus was set up and recordings were made and compared with the trend of the pressure coefficient: coupling the data showed an excellent consistency in the identification of the bubble and confirmed the feasibility of using thermography for the non-invasive measurement of this phenomenon. The final phase of the study involved a preliminary attempt to obtain quantitative information from the thermographic images too: two quantities, L S and B E , were defined for this purpose and enabled a preliminary characterization of the behavior of the laminar bubble according to the angle of attack.

MODELING A WORKER'S EXPOSURE FROM A HAND-HELD SOURCE IN A UNIFORM FREESTREAM

The phenomenon of boundary layer separation can be an important factor in determining a worker's exposure to toxic airborne pollutants. A conceptual model was developed to understand this phenomenon and to predict the average concentration in the reverse flow region downstream of a worker in a uniform freestream. Subsequently, the assumptions of this model were tested experimentally in wind tunnel studies. On the basis of these results, a revised model is presented and validated by using a tracer gas method. The revised model provides a reasonable estimate of the average concentration in the reverse flow region of the mannequin. Empirical models are presented that relate both the average concentration in the reverse flow region and the breathing zone concentration to the body dimensions and the freestream air velocity. Applications and limitations of the results are discussed.

Entry flow into a circular tube of slowly varying cross-section

The entry flow into a circular tube of slowly varying cross-section is studied by considering the flow field to consist of two regions—a core region and a boundary layer region. As in classical boundary layer problem, the equations in the two regions are solved separately using a matching condition at the edge of the boundary layer. For different wall geometries, the velocity, shearing stress and vorticity are computed and the phenomenon of boundary layer separation is discussed. It is found that the shape of the wall geometry plays an important role on the fluid dynamic characteristics. The model is quite versatile and can be applied to a variety of engineering problems involving slowly varying, converging or diverging tubes.

Observations on the radial flow of water between fixed parallel plates

Dye filament techniques were employed to investigate streamline configurations for both converging and diverging radial flow. Inherent characteristic of diverging flow are the boundary layer separation phenomena and the formation of axially symmetric vortex sheets. The onset of boundary layer separation is predicted from a dimensionsless stability parameter which is derived from a semi-empirical expression found to be in approximate agreement with wall pressure data. For flows significancyly in excess of the critical rate, disturbance oscillations were observed to amplify and generate eddies. At very high rates of diverging flow, both steady state and transient cavitation were observed. These phenomena are closely linked with the vortex formations. In contrast, converging flow retained a laminar character up to the highest rate of flow attainable.