scholarly journals Turbulent Superstructures in Inert Jets and Diffusion Jet Flames

Fluids ◽  
2021 ◽  
Vol 6 (12) ◽  
pp. 459
Author(s):  
Vadim Lemanov ◽  
Vladimir Lukashov ◽  
Konstantin Sharov
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Near Field ◽  
Reynolds Numbers ◽  
Transverse Dimension ◽  
Diffusion Combustion ◽  
Measuring Instruments ◽  
Low Reynolds Numbers ◽  
Time Flow ◽  
Scale Motion

An experimental study of spatially localized very large-scale motion superstructures, propagating in a jet of carbon dioxide at low Reynolds numbers, was carried out. A hot-wire anemometer and a high-speed 2D PIV with a frequency of 7 kHz were used as measuring instruments. Such a puff-type superstructure in a jet with a longitudinal dimension of up to 20–30 nozzle diameters are initially formed in the jet source—a long tube in a laminar-turbulent transition mode (without artificial disturbances). It is shown that this regime with intermittency in time, when part of the time flow is laminar and the other part of time is turbulent, exists both at the exit from the nozzle and in the near field of the jet. Thus, the structural stability of such turbulent superstructures in the near field of the jet was found. Despite the large longitudinal scale, these formations have a transverse dimension of the order of several nozzle diameters. These structures have a complex internal topology, that is, superstructures are a conglomeration of vortices of different sizes from macroscale to microscale. Using the example of diffusion combustion of methane in air, it is demonstrated that in reacting jets, the existence of such large localized perturbations is a powerful physical mechanism for a global change in the flame topology. At the same time, the presence of a cascade of vortices of different sizes in the puff composition can lead to fractal deformation of the flame front.

Coherent structures in coflowing jets and wakes

1978 ◽  
Vol 88 (3) ◽  
pp. 451-463 ◽  
Author(s):  
A. E. Perry ◽  
T. T. Lim
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Glass Tube ◽  
Reynolds Numbers ◽  
Reynolds Number Flow ◽  
Naturally Occurring ◽  
Number Flow ◽  
High Reynolds ◽  
Scale Motion

By applying small lateral oscillations to a glass tube from which smoke was issuing, perfectly periodic coflowing jets and wake structures were produced at Reynolds numbers of order 300-1000. These structures remained coherent over long streamwise distances and appeared to be perfectly frozen when viewed under stroboscopic light which was synchronized with the disturbing oscillation. By the use of strobing laser beams, longitudinal sections of the structures were photographed and an account of the geometry of these structures is reported.When the tube was unforced, similar structures occurred but they modulated in scale and frequency, and their orientation was random.A classification of structures is presented and examples are demonstrated in naturally occurring situations such as smoke from a cigarette, the wake behind a three-dimensional blunt body, and the high Reynolds number flow in a plume from a chimney. It is suggested that an examination of these structures may give some insight into the large-scale motion in fully turbulent flow.

Wake Analysis of a Finite Width Gurney Flap

2015 ◽  
Author(s):  
D. Holst ◽  
A. B. Bach ◽  
C. N. Nayeri ◽  
C. O. Paschereit ◽  
G. Pechlivanoglou
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Tip Vortex ◽  
Finite Width ◽  
Low Reynolds Numbers ◽  
Gurney Flaps ◽  
Gurney Flap ◽  
Pitch System

The results of stereo Particle-Image-Velocimetry measurements are presented in this paper to gain further insight into the wake of a finite width Gurney flap. It is attached to an FX 63-137 airfoil which is known for a very good performance at low Reynolds numbers and is therefore used for small wind turbines and is most appropriate for tests in the low speed wind tunnel presented in this study. The Gurney flaps are a promising concept for load control on wind turbines but can have adverse side effects, e.g. shedding of additional vortices. The investigation focuses on frequencies and velocity distributions in the wake as well as on the structure of the induced tip vortices. Phase averaged velocity fields are derived of a Proper-Orthogonal-Decomposition based on the stereo PIV measurements. Additional hot-wire measurements were conducted to analyze the fluctuations downstream of the finite width Gurney flaps. Experiments indicate a general tip vortex structure that is independent from flap length but altered by the periodic shedding downstream of the flap. The influence of Gurney flaps on a small wind turbine is investigated by simulating a small 40 kW turbine in Q-Blade. They can serve as power control without the need of an active pitch system and the starting performance is additionally improved. The application of Gurney flaps imply tonal frequencies in the wake of the blade. Simulation results are used to estimate the resulting frequencies. However, the solution of Gurney flaps is a good candidate for large scale wind turbine implementation as well. A FAST simulation of the NREL 5MW turbine is used to generate realistic time series of the lift. The estimations of control capabilities predict a reduction in the standard deviation of the lift of up to 65%. Therefore finite width Gurney flaps are promising to extend the lifetime of future wind turbines.

Viscous constraints on microorganism approach and interaction

2018 ◽  
Vol 851 ◽  
pp. 715-738 ◽  
Author(s):  
Mehdi Jabbarzadeh ◽  
Henry Chien Fu
Keyword(s):  
Reynolds Number ◽  
Near Field ◽  
Reynolds Numbers ◽  
Suspended Particles ◽  
The Other ◽  
Direct Approach ◽  
Low Reynolds Numbers ◽  
Physical Constraints ◽  
Low Reynolds ◽  
Viscous Hydrodynamics

Microorganisms must approach other suspended organisms or particles in order to interact with them during a host of life processes including feeding and mating. Microorganisms live at low Reynolds number where viscosity dominates and strongly affects the hydrodynamics of swimmer and nearby cells and objects. Viscous hydrodynamics makes it difficult for two surfaces to approach closely at low Reynolds numbers. Nonetheless, it is observed that microorganisms in fluid are still able to approach closely enough to interact with each other or suspended particles. Here, we study how the physical constraints provided by viscous hydrodynamics affects the feasibility of direct approach of flagellated and ciliated microorganisms to targets of different sizes. We find that it is feasible for singly flagellated swimmers to approach targets that are the same size or bigger. On the other hand, for squirmers, the feasibility of approach depends on near-field flows that can be controlled by the details of their swimming strokes.

Separation and Transition Control on an Aft-Loaded Ultra-High-Lift LP Turbine Blade at Low Reynolds Numbers: High-Speed Validation

2006 ◽  
Vol 129 (2) ◽  
pp. 340-347 ◽  
Author(s):  
Maria Vera ◽  
Xue Feng Zhang ◽  
Howard Hodson ◽  
Neil Harvey
Keyword(s):  
Surface Roughness ◽  
Mach Number ◽  
Surface Finish ◽  
High Speed ◽  
Reynolds Numbers ◽  
High Lift ◽  
Low Reynolds Numbers ◽  
Low Speed ◽  
Transition Mechanism ◽  
Low Reynolds

This paper presents the second part of an investigation of the combined effects of unsteadiness and surface roughness on an aft-loaded ultra-high-lift low-pressure turbine (LPT) profile at low Reynolds numbers. The investigation has been performed using low- and high-speed cascade facilities. The low- and high-speed profiles have been designed to have the same normalized isentropic Mach number distribution. The low-speed results have been presented in the first part (Zhang, Vera, Hodson, and Harvey, 2006, ASME J. Turbomach., 128, pp. 517–527). The current paper examines the effect of different surface finishes on an aft-loaded ultra-high-lift LPT profile at Mach and Reynolds numbers representative of LPT engine conditions. The surface roughness values are presented along with the profile losses under steady and unsteady inflow conditions. The results show that the use of a rough surface finish can be used to reduce the profile loss. In addition, the results show that the same quantitative values of losses are obtained at high- and low-speed flow conditions. The latter proves the validity of the low-speed approach for ultra-high-lift profiles for the case of an exit Mach number of the order of 0.64. Hot-wire measurements were carried out to explain the effect of the surface finish on the wake-induced transition mechanism.

Mobile Payment Development in China

2013 ◽  
Vol 464 ◽  
pp. 416-419
Author(s):  
Li Zhao ◽  
Hua Ying Shu
Keyword(s):  
Financial Institutions ◽  
High Speed ◽  
Large Scale ◽  
Rapid Development ◽  
Near Field ◽  
Mobile Internet ◽  
Mobile Payment ◽  
Industry Chain ◽  
3G Network ◽  
Internet Economy

The thesis first analyzes the concept of mobile payment, and then it makes a distinction between remote payment and near field payment. In addition to that, it infers the development of global mobile payment, which involves the users and the market size. Secondly it probes into Chinas rapid development of mobile internet economy, whose 3G network and smart phones provides high-speed mobile Internet and visual, convenient interface for mobile payment. Thirdly, harmonization of standards of mobile payment will help create industry chain parties openness, cooperation and win-win situation and promote mobile payment intensive and large-scale development process. In the end, it probes into the win-win business model for the telecom operators and financial institutions: Telecom operators will lease SIM card space to obtain revenue and financial institutions will get more fee income.

Drone Scale Coaxial Rotor Aerodynamic Interactions Investigation

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Dhwanil Shukla ◽  
Narayan Komerath
Keyword(s):  
High Speed ◽  
Vortex Sheet ◽  
Reynolds Numbers ◽  
Thrust Coefficient ◽  
Low Reynolds Numbers ◽  
Rotor Aerodynamics ◽  
Induced Drag ◽  
Coaxial Rotor ◽  
Simplifying Assumptions ◽  
Uninhabited Aerial Vehicles

Coaxial rotor uninhabited aerial vehicles (UAVs) are compact compared to single rotor UAVs of comparable capacity. At the low Reynolds numbers (Re) where they operate, the simplifying assumptions from high Re rotor aerodynamics are not valid. The low Re coaxial rotor flowfield is studied including aerodynamic interactions and their effect on performance. The evolution of the wake is captured using high-speed stereo particle image velocimetry (SPIV). Improvement of upper rotor performance due to viscous swirl recovery from the lower rotor is discovered and then verified by analyzing PIV data. Interesting vortex–vortex sheet interactions are observed under the coaxial rotor affecting wake structure spatially and temporally. A qualitative model explaining the observed wake interaction phenomena is presented. Comparison with the performance of high Re rotors shows higher profile and induced drag at low Re for the same thrust coefficient.

PIV Study of the Near-Field Region of a Turbulent Round Jet

2010 ◽  
Author(s):  
Ivana M. Milanovic ◽  
Khaled J. Hammad
Keyword(s):  
Large Scale ◽  
Near Field ◽  
Flow Characteristics ◽  
Turbulent Jets ◽  
Reynolds Numbers ◽  
Field Region ◽  
Initial Development ◽  
Number Range ◽  
Developed Turbulence ◽  
Fully Developed Turbulent Flow

Turbulent jets have been extensively studied in the past due to their fundamental importance and wide spread usage in numerous industrial processes to enhance momentum, heat and mass transfer. Most previous work focused on the far-field or self-similar region of the flow. However, the initial development region, where the flow is dominated by streamwise and large-scale, Kelvin-Helmholtz-type, structures, received far less attention. In the current study, Particle Image Velocimetry (PIV) was used to obtain reliable statistics in the near-field region of a turbulent submerged jet. The jet issued from an 84 diameter, D, long pipe which ensured fully-developed turbulent flow conditions at the outlet. The two-dimensional flow field in the plane containing the jet axis was measured in the initial 8D region, for three Reynolds numbers: 14,602, 19,135, and 24,685. The selected Reynolds numbers overlap with the previously identified critical Reynolds number range, 10,000–20,000, where flow characteristics of a jet undergo a dramatic transition to a much more chaotic and well-mixed state or fully developed turbulence.

Thin-Filament Pyrometry: A Novel Thermometry Technique for Combusting Flows

1989 ◽  
Vol 111 (1) ◽  
pp. 46-52 ◽  
Author(s):  
L. P. Goss ◽  
V. Vilimpoc ◽  
B. Sarka ◽  
W. F. Lynn
Keyword(s):  
Temperature Distribution ◽  
Flow Field ◽  
Diffusion Flame ◽  
Thin Filament ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Temporal Response ◽  
Ambient Conditions ◽  
Low Reynolds Numbers ◽  
Novel Technique

A novel technique is described for making temperature measurements in a combusting flow. The technique, Thin-Filament Pyrometry, is based on the blackbody emission of a small ceramic filament (15 μm), which is introduced into the flow field under study. Because the emission along the entire length of the filament is recorded, the complete spatial temperature distribution is measured. The temporal response of the filament is ∼ 700 Hz under ambient conditions. To demonstrate the capabilities of the technique in a combusting flow, a H2-N2 jet diffusion flame was studied. The evolution of the large-scale buoyancy-driven structures at low Reynolds numbers was followed by this technique.

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