scholarly journals EN The wave model of secondary flows and coherent structures in pipes

2020 ◽  
Vol 55 (5-6) ◽  
pp. 273-281
Author(s):  
S. Surkov
Keyword(s):  
Wave Equation ◽  
Stream Function ◽  
Coherent Structures ◽  
Large Scale ◽  
Experimental Studies ◽  
Viscous Friction ◽  
Orthogonal Decomposition ◽  
Secondary Flows ◽  
Friction Forces ◽  
Special Cases

In this article, a theoretical analysis of the flows arising in the cross sections of fluid and gas flows is performed. Such flows are subdivided into secondary flows and coherent structures. From experimental studies it is known that both types of flows are long-lived large-scale movements (LSM) stretched along the flow. The relative stability of the vortices is traditionally explained by the fact that the viscous friction forces that inhibit the rotation are compensated by the intensification of the swirl when moving slowly rotating peripheral layers to the center of the vortex due to longitudinal tension. An analysis of this mechanism made it possible to develop a relatively simple model of vortex structures in which the viscous friction forces and axial expansion are considered to be infinitesimal. Under these assumptions, one can use the equations of motion of an ideal fluid in the variables “stream function - vorticity”. It is shown that under certain assumptions these equations take the form of a wave equation, and the boundary conditions are the condition that the stream function on the solid walls of the flow equals zero. The obtained solutions of the wave equation describe the following special cases: Goertler’s vortices between rotating cylinders, secondary flows in a pipe with a square cross section, swirling flow in a round pipe, paired vortex after bend of the pipe. The physical sense of more complex solutions of the wave equation has become clear relatively recently. Very similar structures were found in experimental studies using orthogonal decomposition (POD) of a turbulent pulsations field. This may mean that the eigenfunctions in the POD correspond to coherent structures that really arise in the flow. The results obtained confirm the hypothesis that secondary flows and coherent structures have a common nature. The solutions obtained in this paper can be used in processing the experiment as eigenfunctions for the orthogonal decomposition method. In addition, they can be used in direct numerical simulation (DNS) of turbulent flows

Improving turbine endwall cooling uniformity by controlling near-wall secondary flows

2016 ◽  
Vol 231 (14) ◽  
pp. 2689-2705 ◽  
Author(s):  
Mitra Thomas ◽  
Thomas Povey
Keyword(s):  
Mass Flux ◽  
Large Scale ◽  
Guide Vane ◽  
Pressure Ratio ◽  
Experimental Studies ◽  
Pressure Profile ◽  
Design Guidelines ◽  
Secondary Flows ◽  
The Impact ◽  
Endwall Cooling

In this paper, we propose a design philosophy for cooling high-pressure nozzle guide vane endwalls, which exploits the momentum of cooling jets to control vane secondary flows thereby improving endwall cooling uniformity. The impact of coolant-to-mainstream pressure ratio, hole inclination angle, hole diameter, vane potential field, and overall mass flux ratios are considered. Arguments are developed by examining detailed experimental studies conducted in a large-scale low-speed cascade tunnel with engine-realistic combustor geometry and turbulence profiles. Computational fluid dynamics predictions validated by the same are used to extend the parameter space. We show that the global flow field is highly sensitive to the inlet total pressure profile, which in turn can be modified by introducing relatively low mass flow rates of cooling gas at engine realistic coolant-to-mainstream pressure ratios and mass flux ratios. This interaction effect must be understood for successful design of optimised endwall cooling schemes, an effect which is not sufficiently emphasized in much of the literature on this topic. Design guidelines are given that we hope will be of use in industry.

scholarly journals Determining the flow separation near a small UAV by unsteady pressure sensors

2021 ◽  
Vol 2057 (1) ◽  
pp. 012008
Author(s):  
P A Polivanov ◽  
A A Sidorenko
Keyword(s):  
Wind Tunnel ◽  
Flow Separation ◽  
Coherent Structures ◽  
Large Scale ◽  
Experimental Studies ◽  
Pressure Sensors ◽  
Separation Flow ◽  
Pressure Pulsations ◽  
Small Uav ◽  
Aerial Vehicle

Abstract Experimental studies of pressure pulsations on the surface of a small unmanned aerial vehicle (SUAV) are carried out in a wind tunnel. The onset of the separation flow is determined on the basis of PIV and loads measurements. It is found that an increase of pressure pulsations does not always correspond to flow separation. The paper proposes to use correlation analysis to determine the flow separation by finding large-scale coherent structures generated by the separation.

Effects of Outlier Flow Field on the Characteristics of In-Cylinder Coherent Structures Identified by Proper Orthogonal Decomposition-Based Conditional Averaging and Quadruple Proper Orthogonal Decomposition

2019 ◽  
Vol 141 (8) ◽  
Author(s):  
Rui Gao ◽  
Li Shen ◽  
Kwee-Yan Teh ◽  
Penghui Ge ◽  
Fengnian Zhao ◽  
...  
Keyword(s):  
Flow Field ◽  
Proper Orthogonal Decomposition ◽  
Coherent Structures ◽  
Large Scale ◽  
Flow Fields ◽  
Orthogonal Decomposition ◽  
Engine Cylinder ◽  
Conditional Averaging ◽  
Proper Orthogonal ◽  
Engine Cycle

Proper orthogonal decomposition (POD) offers an approach to quantify cycle-to-cycle variation (CCV) of the flow field inside the internal combustion engine cylinder. POD decomposes instantaneous flow fields (also called snapshots) into a series of orthonormal flow patterns (called POD modes) and the corresponding mode coefficients. The POD modes are rank-ordered by decreasing kinetic energy content, and the low-order, high-energy modes are interpreted as constituting the large-scale coherent flow structure that varies from engine cycle to engine cycle. Various POD-based analysis techniques have thus been proposed to characterize engine flow field CCV using these low-order modes. The validity of such POD-based analyses rests, as a matter of course, on the reliability of the underlying POD results (modes and coefficients). Yet a POD mode can be disproportionately skewed by a single outlier snapshot within a large data set, and an algorithm exists to define and identify such outliers. In this paper, the effects of a candidate outlier snapshot on the results of POD-based conditional averaging and quadruple POD analyses are examined for two sets of crank angle-resolved flow fields on the midtumble plane of an optical engine cylinder recorded by high-speed particle image velocimetry (PIV). The results with and without the candidate outlier are compared and contrasted. In the case of POD-based conditional averaging, the presence of the outlier scrambles the composition of snapshot subsets that define large-scale flow pattern variations, and thus substantially alters the coherent flow structures that are identified; for quadruple POD, the shape of coherent structures and the number of modes to define them are not significantly affected by the outlier.

Endwall Boundary Layer Flows and Losses in an Axial Turbine Stage

1982 ◽  
Vol 104 (1) ◽  
pp. 184-193 ◽  
Author(s):  
I. H. Hunter
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Experimental Studies ◽  
Axial Flow ◽  
Turbine Rotor ◽  
Secondary Flows ◽  
Boundary Layer Flows ◽  
Wall Boundary Layer ◽  
Instantaneous Position ◽  
Insight Into

In order to gain an insight into boundary layer phenomena in axial flow turbines, experimental studies were carried out on a large-scale, low speed, single stage machine. Low and high frequency instrumentation supplemented by flow visualizations were used to determine the details of the flows and losses in the stator and rotor blade rows. Measurements of the turbine stator flows, which were obtained at two different inlet wall boundary layer thicknesses, were generally consistent with typical linear cascade observations. A major difference concerned the presence of appreciable radially directed flow. Hot-wire anemometry measurements at the turbine rotor exit, revealed strong secondary flows at the high turning hub section. The flow pattern was seen to be influenced by the spacing between the two rows of blades and by the rotor’s instantaneous position with respect to the periodic flow field arising from the upstream stator row.

Reconstruction of the global velocity field in the axisymmetric mixing layer utilizing the proper orthogonal decomposition

2000 ◽  
Vol 418 ◽  
pp. 137-166 ◽  
Author(s):  
J. H. CITRINITI ◽  
W. K. GEORGE
Keyword(s):  
Velocity Field ◽  
Coherent Structures ◽  
Time Scale ◽  
Large Scale ◽  
Large Scale Structure ◽  
Ring Structure ◽  
Scale Structure ◽  
Orthogonal Decomposition ◽  
Potential Core ◽  
Proper Orthogonal

Experimental data are presented from 138 synchronized channels of hot-wire anemometry in an investigation of the large-scale, or coherent, structures in a high Reynolds number fully developed, turbulent axisymmetric shear layer. The dynamics of the structures are obtained from instantaneous realizations of the streamwise velocity field in a single plane, x/D = 3, downstream of a round jet nozzle. The Proper Orthogonal Decomposition (POD) technique is applied to an ensemble of these realizations to determine optimal representations of the velocity field, in a mean-square sense, in terms of an orthogonal basis. The coefficients of the orthogonal functions, which describe the temporal evolution of the POD eigenfunctions, are determined by projecting instantaneous realizations of the velocity field onto the basis.Evidence is presented to show that with a partial reconstruction of the velocity field, using only the first radial POD mode, the large-scale structure is objectively educed from the turbulent field. Further, it is shown that only five azimuthal Fourier modes (0,3,4,5,6) are necessary to represent the evolution of the large-scale structure. The results of the velocity reconstruction using the POD provide evidence for azimuthally coherent structures that exist near the potential core. In addition to the azimuthal structures near the potential core, evidence is also found for the presence of counter-rotating, streamwise vortex pairs (or ribs) in the region between successive azimuthally coherent structures as well as coexisting for short periods with them. The large-scale structure cycle, which includes the appearance of the ring structure, the advection of fluid by the ribs in the braid region and their advection toward the outside of the layer by a following ring structure, repeats approximately every one integral time scale. One surprising result was that the most spatially correlated structure in the flow, the coherent ring near the potential core which ejects fluid in the streamwise direction in a volcano-like eruption, is also the one with the shortest time scale.

scholarly journals Leisure Time Sports Activities and Life Satisfaction: Deeper Insights Based on a Representative Survey from Germany

2020 ◽  
Author(s):  
Michael Mutz ◽  
Anne K. Reimers ◽  
Yolanda Demetriou
Keyword(s):  
Life Satisfaction ◽  
Large Scale ◽  
Leisure Time ◽  
Organizational Context ◽  
Experimental Studies ◽  
Well Being ◽  
Activity Level ◽  
Leisure Satisfaction ◽  
General Life Satisfaction ◽  
Sports Activities

Abstract Observational and experimental studies show that leisure time sporting activity (LTSA) is associated with higher well-being. However, scholars often seem to assume that 1) LTSA fosters “general” life satisfaction, thereby ignoring effects on domain satisfaction; 2) the effect of LTSA on well-being is linear and independent of a person’s general activity level; 3) the amount of LTSA is more important than the repertoire of LTSA, i.e. the number of different activities; 4) all kinds of LTSA are equal in their effects, irrespective of spatial and organisational context conditions. Using data from the German SALLSA-Study (“Sport, Active Lifestyle and Life Satisfaction”), a large-scale CAWI-Survey (N = 1008) representing the population ≥ 14 years, the paper takes a closer look on these assumptions. Findings demonstrate that LTSA is associated with general life satisfaction and domain-specific satisfaction (concerning relationships, appearance, leisure, work and health), but that the relationship is most pronounced for leisure satisfaction. Associations of sport with life satisfaction, leisure satisfaction and subjective health are non-linear, approaching an injection point from which on additional LTSA is no longer beneficial. Moreover, findings lend support to the notion that diversity in LTSA matters, as individuals with higher variation in sports activities are more satisfied. Finally, results with regard to spatial and organizational context suggest that outdoor sports and club-organized sports have additional benefits.

scholarly journals Interaction of Very Large Scale Motion of Coherent Structures with Sediment Particle Exposure

Water ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 248
Author(s):  
Sencer Yücesan ◽  
Daniel Wildt ◽  
Philipp Gmeiner ◽  
Johannes Schobesberger ◽  
Christoph Hauer ◽  
...  
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Numerical Study ◽  
Local Maximum ◽  
Wave Number ◽  
Exposure Level ◽  
Sediment Particle ◽  
High Wave Number ◽  
Large Scale Motion ◽  
Scale Motion

A systematic variation of the exposure level of a spherical particle in an array of multiple spheres in a high Reynolds number turbulent open-channel flow regime was investigated while using the Large Eddy Simulation method. Our numerical study analysed hydrodynamic conditions of a sediment particle based on three different channel configurations, from full exposure to zero exposure level. Premultiplied spectrum analysis revealed that the effect of very-large-scale motion of coherent structures on the lift force on a fully exposed particle resulted in a bi-modal distribution with a weak low wave number and a local maximum of a high wave number. Lower exposure levels were found to exhibit a uni-modal distribution.

scholarly journals Why does the spread of COVID-19 vary greatly in different countries? Revealing the efficacy of face masks in epidemic prevention

2021 ◽  
Vol 149 ◽  
Author(s):  
Jincheng Wei ◽  
Shurui Guo ◽  
Enshen Long ◽  
Li Zhang ◽  
Bizhen Shu ◽  
...  
Keyword(s):  
Social Isolation ◽  
Large Scale ◽  
Experimental Studies ◽  
Weighted Average ◽  
Filtration Efficiency ◽  
Volume Distribution ◽  
Particle Sizes ◽  
Economic Activities ◽  
Protection Measure ◽  
Source Of Infection

Abstract The severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) is highly contagious, and the coronavirus disease 2019 (COVID-19) pandemic caused by it has forced many countries to adopt ‘lockdown’ measures to prevent the spread of the epidemic through social isolation of citizens. Some countries proposed universal mask wearing as a protection measure of public health to strengthen national prevention efforts and to limit the wider spread of the epidemic. In order to reveal the epidemic prevention efficacy of masks, this paper systematically evaluates the experimental studies of various masks and filter materials, summarises the general characteristics of the filtration efficiency of isolation masks with particle size, and reveals the actual efficacy of masks by combining the volume distribution characteristics of human exhaled droplets with different particle sizes and the SARS-CoV-2 virus load of nasopharynx and throat swabs from patients. The existing measured data show that the filtration efficiency of all kinds of masks for large particles and extra-large droplets is close to 100%. From the perspective of filtering the total number of pathogens discharged in the environment and protecting vulnerable individuals from breathing live viruses, the mask has a higher protective effect. If considering the weighted average filtration efficiency with different particle sizes, the filtration efficiencies of the N95 mask and the ordinary mask are 99.4% and 98.5%, respectively. The mask can avoid releasing active viruses to the environment from the source of infection, thus maximising the protection of vulnerable individuals by reducing the probability of inhaling a virus. Therefore, if the whole society strictly implements the policy of publicly wearing masks, the risk of large-scale spread of the epidemic can be greatly reduced. Compared with the overall cost of social isolation, limited personal freedoms and forced suspension of economic activities, the inconvenience for citizens caused by wearing masks is perfectly acceptable.

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