scholarly journals Turbulent drag in a rotating frame

2016 ◽  
Vol 794 ◽  
Author(s):  
Antoine Campagne ◽  
Nathanaël Machicoane ◽  
Basile Gallet ◽  
Pierre-Philippe Cortet ◽  
Frédéric Moisy
Keyword(s):  
Large Scale ◽  
Scaling Law ◽  
Stereoscopic Particle Image Velocimetry ◽  
Turbulence Theory ◽  
Water Tank ◽  
Wave Interactions ◽  
Turbulent Drag ◽  
Image Velocimetry ◽  
Weak Turbulence Theory ◽  
Large Scale Flow

What is the turbulent drag force experienced by an object moving in a rotating fluid? This open and fundamental question can be addressed by measuring the torque needed to drive an impeller at a constant angular velocity ${\it\omega}$ in a water tank mounted on a platform rotating at a rate ${\it\Omega}$. We report a dramatic reduction in drag as ${\it\Omega}$ increases, down to values as low as 12 % of the non-rotating drag. At small Rossby number $Ro={\it\omega}/{\it\Omega}$, the decrease in the drag coefficient $K$ follows the approximate scaling law $K\sim Ro$, which is predicted in the framework of nonlinear inertial-wave interactions and weak-turbulence theory. However, stereoscopic particle image velocimetry measurements indicate that this drag reduction instead originates from a weakening of the turbulence intensity in line with the two-dimensionalization of the large-scale flow.

Effects of Squish Flow on Tangential Flow and Turbulence in a Diesel Engine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Yanzhe Sun ◽  
Kai Sun ◽  
Tianyou Wang ◽  
Yufeng Li ◽  
Zhen Lu
Keyword(s):  
Diesel Engine ◽  
Turbulent Flows ◽  
Large Scale ◽  
Radial Flow ◽  
Tangential Component ◽  
Tangential Flow ◽  
Image Velocimetry ◽  
Turbulence Production ◽  
Large Scale Flow ◽  
Main Effects

Emission and fuel consumption in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical effects of squish flow on the tangential flow and turbulence production are still far from well understood. To identify the effects of squish flow, Particle image velocimetry (PIV) experiments are performed in a motored optical diesel engine equipped with different bowls. By comparing and associating the large-scale flow and turbulent kinetic energy (k), the main effects of the squish flow are clarified. The effect of squish flow on the turbulence production in the r−θ plane lies in the axial-asymmetry of the annular distribution of radial flow and the deviation between the ensemble-averaged swirl field and rigid body swirl field. Larger squish flow could promote the swirl center to move to the cylinder axis and reduce the deformation of swirl center, which could decrease the axial-asymmetry of annular distribution of radial flow, further, that results in a lower turbulence production of the shear stress. Moreover, larger squish flow increases the radial fluctuation velocity which makes a similar contribution to k with the tangential component. The understanding of the squish flow and its correlations with tangential flow and turbulence obtained in this study is beneficial to design and optimize the in-cylinder turbulent flow.

Experimental Study of Large-Scale Flow Structures in an Aneurysm

2018 ◽  
Author(s):  
Paulo Yu ◽  
Vibhav Durgesh
Keyword(s):  
Large Scale ◽  
Flow Structures ◽  
Pump System ◽  
Vortical Structures ◽  
Abnormal Growth ◽  
Image Velocimetry ◽  
Large Scale Flow ◽  
The Impact ◽  
Inflow Conditions ◽  
Aneurysm Model

An aneurysm is an abnormal growth in the wall of a weakened blood vessel, and can often be fatal upon rupture. Studies have shown that aneurysm shape and hemodynamics, in conjunction with other parameters, play an important role in growth and rupture. The objective of this study was to investigate the impact of varying inflow conditions on flow structures in an aneurysm. An idealized rigid sidewall aneurysm model was prepared and the Womersley number (α) and Reynolds number (Re) values were varied from 2 to 5 and 50 to 250, respectively. A ViVitro Labs pump system was used for inflow control and Particle Image Velocimetry was used for conducting velocity measurements. The results showed that the primary vortex path varied with an increase in α, while an increase in Re was correlated to the vortex strength and formation of secondary vortical structures. The evolution and decay of vortical structures were also observed to be dependent on α and Re.

Turbulent spots in a channel: large-scale flow and self-sustainability

2013 ◽  
Vol 731 ◽  
Author(s):  
Grégoire Lemoult ◽  
Jean-Luc Aider ◽  
José Eduardo Wesfreid
Keyword(s):  
Large Scale ◽  
Large Time ◽  
Rectangular Channel ◽  
Small Scale ◽  
Turbulent Spot ◽  
Time Resolved ◽  
Image Velocimetry ◽  
Streamwise Streaks ◽  
Large Scale Flow ◽  
Scale Motion

AbstractUsing a large-time-resolved particle image velocimetry field of view, a developing turbulent spot is followed in space and time in a rectangular channel flow for more than 100 advective time units. We show that the flow can be decomposed into a large-scale motion consisting of an asymmetric quadrupole centred on the spot and a small-scale part consisting of streamwise streaks. From the temporal evolution of the energy of the streamwise and spanwise velocity perturbations, it is suggested that a self-sustaining process can occur in a turbulent spot above a given Reynolds number.

Measurements of Periodic Reynolds Stress Oscillations in a Forced Turbulent Premixed Swirling Flame

2018 ◽  
Author(s):  
Christopher Douglas ◽  
Jamie Lim ◽  
Travis Smith ◽  
Benjamin Emerson ◽  
Timothy Lieuwen ◽  
...  
Keyword(s):  
Reynolds Stress ◽  
High Speed ◽  
Large Scale ◽  
Turbulent Transport ◽  
Stereoscopic Particle Image Velocimetry ◽  
Thermoacoustic Instability ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Swirling Flame ◽  
Dynamics Simulations

This work is motivated by the thermoacoustic instability challenges associated with ultra-low emissions gas turbine combustors. It demonstrates the first use of high-speed dual-plane orthogonally-polarized stereoscopic-particle image velocimetry and synchronized OH planar laser-induced fluorescence in a premixed swirling flame. We use this technique to explore the effects of combustion and longitudinal acoustic forcing on the time- and phase-averaged flow field — particularly focusing on the behavior of the Reynolds stress in the presence of harmonic forcing. We observe significant differences between ensemble averaged and time averaged Reynolds stress. This implies that the large-scale motions are non-ergodic, due to coherent oscillations in Reynolds stress associated with the convection of periodic vortical structures. This result has important implications on hydrodynamic stability models and reduced order computational fluid dynamics simulations, which do show the importance of turbulent transport on the problem, but do not capture these coherent oscillations in their models.

Scaling Effect on Dynamic Impact Response of Structures in Fluid Fields

2013 ◽  
Author(s):  
Zhongheng Guo ◽  
Lingyu Sun ◽  
Taikun Wang ◽  
Junmin Du ◽  
Han Li ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Dimensional Analysis ◽  
Large Scale ◽  
Scaling Laws ◽  
Scaling Law ◽  
Fluid Pressure ◽  
Structural Safety ◽  
Scale Model ◽  
Small Scale ◽  
Water Tank

At the conceptual design phase of a large-scale underwater structure, a small-scale model in a water tank is often used for the experimental verification of kinematic principles and structural safety. However, a general scaling law for structure-fluid interaction (FSI) problems has not been established. In the present paper, the scaling laws for three typical FSI problems under the water, rigid body moves at a given kinematic equation or is driven by time-dependent fluids with given initial condition, as well as elastic-plastic body moves and then deforms subject to underwater impact loads, are investigated, respectively. First, the power laws for these three types of FSI problems were derived by dimensional analysis method. Then, the laws for the first two types were verified by numerical simulation. In addition, a multipurpose small-scale water sink test device was developed for numerical model updating. For the third type of problem, the dimensional analysis is no longer suitable due to its limitation on identifying the fluid pressure and structural stress, a simulation-based procedure for dynamics evaluation of large-scale structure was provided. The results show that, for some complex FSI problems, if small-scale prototype is tested safely, it doesn’t mean the full-scale product is also safe if both their pressure and stress are the main concerns, it needs further demonstration, at least by numerical simulation.

Measurements of Periodic Reynolds Stress Oscillations in a Forced Turbulent Premixed Swirling Flame

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Christopher Douglas ◽  
Jamie Lim ◽  
Travis Smith ◽  
Benjamin Emerson ◽  
Timothy Lieuwen ◽  
...  
Keyword(s):  
Reynolds Stress ◽  
High Speed ◽  
Large Scale ◽  
Turbulent Transport ◽  
Stereoscopic Particle Image Velocimetry ◽  
Thermoacoustic Instability ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Swirling Flame ◽  
Dynamics Simulations

This work is motivated by the thermoacoustic instability challenges associated with ultra-low emissions gas turbine (GT) combustors. It demonstrates the first use of high-speed dual-plane orthogonally-polarized stereoscopic-particle image velocimetry (PIV) and synchronized OH planar laser-induced fluorescence in a premixed swirling flame. We use this technique to explore the effects of combustion and longitudinal acoustic forcing on the time- and phase-averaged flow field—particularly focusing on the behavior of the Reynolds stress in the presence of harmonic forcing. We observe significant differences between ensemble-averaged and time-averaged Reynolds stress. This implies that the large-scale motions are nonergodic, due to coherent oscillations in Reynolds stress associated with the convection of periodic vortical structures. This result has important implications on hydrodynamic stability models and reduced-order computational fluid dynamics simulations, which do show the importance of turbulent transport on the problem, but do not capture these coherent oscillations in their models.

Experimental investigation of flow development and gap vortex street in an eccentric annular channel. Part 1. Overview of the flow structure

2014 ◽  
Vol 752 ◽  
pp. 521-542 ◽  
Author(s):  
George H. Choueiri ◽  
Stavros Tavoularis
Keyword(s):  
Laser Doppler Velocimetry ◽  
Large Scale ◽  
Particle Image ◽  
Annular Channel ◽  
Stereoscopic Particle Image Velocimetry ◽  
Entrance Region ◽  
Narrow Gap ◽  
Velocity Fluctuations ◽  
Flow Development ◽  
Image Velocimetry

AbstractFlow visualization, laser Doppler velocimetry and planar and stereoscopic particle image velocimetry were used to investigate the isothermal velocity field along an eccentric annular channel with a diameter ratio of 0.5 and an eccentricity of 0.8 for a Reynolds number of 7300. Observation of the flow development has identified three distinct regions: the entrance region, the fluctuation-growth (FG) region and the rapid-mixing (RM) region. Weak quasi-periodic velocity fluctuations were first detected in the downstream part of the entrance region, and grew into very strong ones, reaching peak-to-peak amplitudes in the narrow gap that were nearly 60 % of the bulk velocity. Two mixing layers were identified on either side of the gap, which generated a street of counter rotating vortices and thorough large-scale mixing of the fluid in the channel.

scholarly journals Experimental Investigation of the Unsteady Stator/Rotor Wake Characteristics Downstream of an Axial Air Turbine

2021 ◽  
Vol 6 (3) ◽  
pp. 22
Author(s):  
Daniel Duda ◽  
Tomáš Jelínek ◽  
Petr Milčák ◽  
Martin Němec ◽  
Václav Uruba ◽  
...  
Keyword(s):  
Cross Sections ◽  
Large Scale ◽  
Field Measurements ◽  
Secondary Flows ◽  
Rotor Blades ◽  
Spatial Correlation Function ◽  
Image Velocimetry ◽  
Air Turbine ◽  
Large Scale Flow ◽  
End Wall

A feasibility study of velocity field measurements using the Particle Image Velocimetry (PIV) method in an axial air turbine model is presented. The wakes past the blades of the rotor wheel were observed using the PIV technique. Data acquisition was synchronized with the shaft rotation; thus, the wakes were phase averaged for statistical analysis. The interaction of the rotor blade wakes with the stator ones was investigated by changing the stator wheel’s angle. The measurement planes were located just behind the rotor blades, covering approximately 3 cm × 3 cm in axial × tangential directions. The spatial correlation function suggests that the resolution used is sufficient for the large-scale flow-patterns only, but not for the small ones. The scales of fluctuations correspond to the shear layer thickness at the mid-span plane but, close to the end-wall, they contain larger structures caused by the secondary flows. The length-scales of the fluctuations under off-design conditions display a dependence on the area of the stator and rotor wakes cross-sections, which, in turn, depend on their angle. The obtained experimental data are to be used for the validation of mathematical simulation results in the future.

Proper Orthogonal Decomposition Analysis of Non-Swirling Turbulent Stratified and Premixed Methane/Air Flames

2014 ◽  
Author(s):  
M. Mustafa Kamal ◽  
Christophe Duwig ◽  
Saravanan Balusamy ◽  
Ruigang Zhou ◽  
Simone Hochgreb
Keyword(s):  
Flow Field ◽  
Proper Orthogonal Decomposition ◽  
High Speed ◽  
Large Scale ◽  
Bluff Body ◽  
Decomposition Analysis ◽  
Orthogonal Decomposition ◽  
Stereoscopic Particle Image Velocimetry ◽  
Large Scale Flow ◽  
Proper Orthogonal

This paper reports proper orthogonal decomposition (POD) analyses for the velocity fields measured in a test burner. The Cambridge/Sandia Stratified Swirl Burner has been used in various studies as a benchmark for high resolution scalar and velocity measurements, for comparison with numerical model prediction. Flow field data was collected for a series of bluff-body stabilized premixed and stratified methane/air flames at turbulent, globally lean conditions (ϕ = 0.75) using high speed stereoscopic particle image velocimetry (HS-SPIV). In this paper, a modal analysis was performed to identify the large scale flow structures and their impact on the flame dynamics. The high speed PIV system was operated at 3 kHz to acquire a series of 4096 sequential flow field images both for reactive and non-reactive cases, sufficient to follow the large-scale spatial and temporal evolution of flame and flow dynamics. The POD analysis allows identification of vortical structures, created by the bluff body, and in the shear layers surrounding the stabilization point. In addition, the analysis reveals that dominant structures are a strong function of the mixture stratification in the flow field. The dominant energetic modes of reactive and non-reactive flows are very different, as the expansion of gases and the high temperatures alter the unstable modes and their survival in the flow.

scholarly journals Visualizing large-scale flow using synthetic aperture PIV

2019 ◽  
Vol 61 (1) ◽  
Author(s):  
Josje van Houwelingen ◽  
Ad P. C. Holten ◽  
Herman J. H. Clercx ◽  
Rudie P. J. Kunnen ◽  
Jaap Molenaar ◽  
...  
Keyword(s):  
Large Scale ◽  
Particle Image ◽  
Synthetic Aperture ◽  
Vortex Rings ◽  
Two Dimensional ◽  
Vorticity Field ◽  
Image Velocimetry ◽  
Large Scale Flow ◽  
Small Bubbles ◽  
Proof Of Principle

Abstract We discuss the application of synthetic aperture particle image velocimetry for measuring the flow around human swimmers using small bubbles as tracer. We quantify the two-dimensional projection of the velocity field in planes perpendicular to the viewing direction of an array of six cameras. With help of simulations, modelled after the experiment, we address questions about depth selectivity and occlusion in dense bubble fields. Using vortex rings in the swimming pool, we provide a proof of principle of the method. It is further illustrated by the vorticity field produced by a human swimmer. Graphic abstract

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