Flight-crash events in superfluid turbulence

2019 ◽  
Vol 876 ◽  
Author(s):  
P. Švančara ◽  
M. La Mantia
Keyword(s):  
Turbulent Flows ◽  
Energy Transport ◽  
Quantized Vortices ◽  
Viscous Fluids ◽  
Superfluid Turbulence ◽  
Small Particles ◽  
Unique Type ◽  
Time Symmetry ◽  
Typical Particle ◽  
The Mean

We show experimentally that the mechanisms of energy transport in turbulent flows of superfluid $^{4}\text{He}$ are strikingly different from those occurring in turbulent flows of viscous fluids. We argue that the result can be related to the role played by quantized vortices in this unique type of turbulence. The flow-induced motions of relatively small particles suspended in the liquid reveal that, for scales of the order of the mean distance between the vortices, the particles do not tend on average to decelerate faster than they accelerate, whereas, at larger scales, a classical-like asymmetry is recovered. It follows that, in the range of investigated parameters, flight-crash events are less apparent than in classical turbulence. We specifically link the outcome to the time symmetry of quantized vortex reconnections observed at scales comparable to the typical particle size.

Flows of liquid4He due to oscillating grids

2017 ◽  
Vol 832 ◽  
pp. 578-599 ◽  
Author(s):  
P. Švančara ◽  
M. La Mantia
Keyword(s):  
Turbulent Flows ◽  
Particle Tracking Velocimetry ◽  
Reynolds Numbers ◽  
Quantized Vortices ◽  
Viscous Fluids ◽  
Statistical Distributions ◽  
Small Particles ◽  
Oscillating Grids ◽  
The Mean ◽  
Cryogenic Flows

We investigate cryogenic flows of liquid4He between two grids oscillating in phase, at temperatures ranging from approximately 1.3 to 2.5 K, resulting in suitably defined Reynolds numbers up to$10^{5}$. We specifically study the flow-induced motions of small particles suspended in the fluid by using the particle tracking velocimetry technique. We focus on turbulent flows of superfluid4He that occur below approximately 2.2 K and are known to display, in certain conditions, features different from those observed in flows of classical viscous fluids, such as water. We find that, at large enough length scales, larger than the mean distance between quantized vortices, representing the quantum length scale of the flow, the shapes of the velocity and velocity increment statistical distributions are very similar to those obtained in turbulent flows of viscous fluids. The experimental outcome strongly supports the view that, in the range of investigated parameters, particles probing flows of superfluid4He behave as if they were tracking classical flows.

Lagrangian accelerations of particles in superfluid turbulence

2013 ◽  
Vol 717 ◽  
Author(s):  
M. La Mantia ◽  
D. Duda ◽  
M. Rotter ◽  
L. Skrbek
Keyword(s):  
Quantum Turbulence ◽  
Fluid Model ◽  
Quantized Vortices ◽  
Superfluid Turbulence ◽  
Thermal Counterflow ◽  
Length Scales ◽  
Order Of Magnitude ◽  
The Mean ◽  
Two Fluid Model ◽  
Two Fluid

AbstractQuantum turbulence in thermal counterflow of superfluid ${\text{} }^{4} \mathrm{He} $ is studied at length scales comparable to the mean distance $\ell $ between quantized vortices. The Lagrangian dynamics of solid deuterium particles, of radius ${R}_{p} $ about one order of magnitude smaller than $\ell $, is analysed in a planar section of the experimental volume by using the particle tracking velocimetry technique. We show that the average amplitude of the acceleration of the particles seems to increase as the temperature decreases and applied heat flux increases and this can be explained by exploiting the two-fluid model of superfluid ${\text{} }^{4} \mathrm{He} $. We also report that, at the probed length scales, the normalized distribution of the acceleration of the particles appears to follow an unexpected classical-like behaviour.

scholarly journals Measurements and Characterization of Turbulence in the Tip Region of an Axial Compressor Rotor

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Yuanchao Li ◽  
Huang Chen ◽  
Joseph Katz
Keyword(s):  
Strain Rate ◽  
Shear Layer ◽  
Turbulent Flows ◽  
Reynolds Stress ◽  
Suction Side ◽  
Stress And Strain ◽  
Spatial And Temporal Variability ◽  
Mean Flow ◽  
The Mean ◽  
Stress And Strain Rate

Modeling of turbulent flows in axial turbomachines is challenging due to the high spatial and temporal variability in the distribution of the strain rate components, especially in the tip region of rotor blades. High-resolution stereo-particle image velocimetry (SPIV) measurements performed in a refractive index-matched facility in a series of closely spaced planes provide a comprehensive database for determining all the terms in the Reynolds stress and strain rate tensors. Results are also used for calculating the turbulent kinetic energy (TKE) production rate and transport terms by mean flow and turbulence. They elucidate some but not all of the observed phenomena, such as the high anisotropy, high turbulence levels in the vicinity of the tip leakage vortex (TLV) center, and in the shear layer connecting it to the blade suction side (SS) tip corner. The applicability of popular Reynolds stress models based on eddy viscosity is also evaluated by calculating it from the ratio between stress and strain rate components. Results vary substantially, depending on which components are involved, ranging from very large positive to negative values. In some areas, e.g., in the tip gap and around the TLV, the local stresses and strain rates do not appear to be correlated at all. In terms of effect on the mean flow, for most of the tip region, the mean advection terms are much higher than the Reynolds stress spatial gradients, i.e., the flow dynamics is dominated by pressure-driven transport. However, they are of similar magnitude in the shear layer, where modeling would be particularly challenging.

Experimental and Numerical Investigation of Centrifugal Pump Performance When Handling Viscous Fluids

2005 ◽  
Author(s):  
M. H. Shojaee Fard ◽  
M. B. Ehghaghi ◽  
F. A. Boyaghchi
Keyword(s):  
Soviet Union ◽  
Centrifugal Pump ◽  
Turbulent Flows ◽  
Characteristic Curve ◽  
Three Dimensional ◽  
Operating Conditions ◽  
Viscous Fluids ◽  
Test Bed ◽  
Flow Process ◽  
Pump Performance

On the test bed of centrifugal pump, the centrifugal pump performance has been investigated using water and viscous oil as Newtonian fluids, whose kinematic viscosities are 1 × 10−6, 43 × 10−6 and 62 × 10−6 m2/s, respectively. Also, the finite volume method is used to model the three dimensional viscous fluids for different operating conditions. For these numerical simulations the SIMPLEC algorithm is used for solving governing equations of incompressible viscous/turbulent flows through the pump. The κ-ε turbulence model is adopted to describe the turbulent flow process. These simulations have been made with a steady calculation and using the multiple reference frame (MRF) technique to take into account the impeller-volute interaction. Numerical results are compared with the experimental characteristic curve for each viscous fluid. The data obtained allow the analysis of the main phenomena existent in this pump, such as: head, efficiency, power and pressure field changes for different operating conditions. Also, the correction factors for oils are obtained from the experimental for part loading (PL), best efficiency point (BEP) and over loading (OL) and the results are compared with proposed factors by American Hydraulic Institute (HIS) and Soviet Union (USSR). The comparisons between the numerical and experimental results show a good agreement.

The Estimation of mean shape and mean particle number in overlapping particle systems in the plane

1995 ◽  
Vol 27 (01) ◽  
pp. 102-119 ◽  
Author(s):  
Wolfgang Weil
Keyword(s):  
Particle Number ◽  
Convex Set ◽  
Boolean Model ◽  
Particle Systems ◽  
Typical Particle ◽  
The Mean ◽  
Convexity Assumptions ◽  
Particle Process ◽  
Simply Connected ◽  
Number Of Particles

A stationary (but not necessarily isotropic) Boolean model Y in the plane is considered as a model for overlapping particle systems. The primary grain (i.e. the typical particle) is assumed to be simply connected, but no convexity assumptions are made. A new method is presented to estimate the intensity y of the underlying Poisson process (i.e. the mean number of particles per unit area) from measurements on the union set Y. The method is based mainly on the concept of convexification of a non-convex set, it also produces an unbiased estimator for a (suitably defined) mean body of Y, which in turn makes it possible to estimate the mean grain of the particle process.

scholarly journals Natural logarithms

2013 ◽  
Vol 718 ◽  
pp. 1-4 ◽  
Author(s):  
B. J. McKeon
Keyword(s):  
Turbulent Flows ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Wall Turbulence ◽  
Significant Advance ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Logarithmic Scaling ◽  
The Mean ◽  
High Reynolds

AbstractMarusic et al. (J. Fluid Mech., vol. 716, 2013, R3) show the first clear evidence of universal logarithmic scaling emerging naturally (and simultaneously) in the mean velocity and the intensity of the streamwise velocity fluctuations about that mean in canonical turbulent flows near walls. These observations represent a significant advance in understanding of the behaviour of wall turbulence at high Reynolds number, but perhaps the most exciting implication of the experimental results lies in the agreement with the predictions of such scaling from a model introduced by Townsend (J. Fluid Mech., vol. 11, 1961, pp. 97–120), commonly termed the attached eddy hypothesis. The elegantly simple, yet powerful, study by Marusic et al. should spark further investigation of the behaviour of all fluctuating velocity components at high Reynolds numbers and the outstanding predictions of the attached eddy hypothesis.

scholarly journals Effect of fluctuations on mean-field dynamos

2018 ◽  
Vol 84 (4) ◽  
Author(s):  
A. Alexakis ◽  
S. Fauve ◽  
C. Gissinger ◽  
F. Pétrélis
Keyword(s):  
Turbulent Flows ◽  
Mean Field ◽  
Liquid Sodium ◽  
Mean Flow ◽  
Dynamo Action ◽  
Scale Separation ◽  
Turbulent Fluctuations ◽  
The Mean ◽  
The One ◽  
The Magnetic Field

We discuss the effect of different types of fluctuations on dynamos generated in the limit of scale separation. We first recall that the magnetic field observed in the VKS (von Karman flow of liquid sodium) experiment is not the one that would be generated by the mean flow alone and that smaller scale turbulent fluctuations therefore play an important role. We then consider how velocity fluctuations affect the dynamo threshold in the framework of mean-field magnetohydrodynamics. We show that the detrimental effect of turbulent fluctuations observed with many flows disappears in the case of helical flows with scale separation. We also find that fluctuations of the electrical conductivity of the fluid, for instance related to temperature fluctuations in convective flows, provide an efficient mechanism for dynamo action. Finally, we conclude by describing an experimental configuration that could be used to test the validity of mean-field magnetohydrodynamics in strongly turbulent flows.

The Effects of Upstream Wall Roughness on Separated and Reattached Flows Over a Forward-Facing Step

2020 ◽  
Author(s):  
Sedem Kumahor ◽  
Xingjun Fang ◽  
William Ediger ◽  
Mark F. Tachie
Keyword(s):  
Correlation Coefficient ◽  
Turbulent Flows ◽  
Eddy Viscosity ◽  
Leading Edge ◽  
Step Height ◽  
Stream Velocity ◽  
Reynolds Stresses ◽  
Third Order ◽  
The Mean ◽  
Forward Facing Step

Abstract Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers developed over smooth and rough walls were investigated using time-resolved particle image velocimetry. Both smooth and fully rough upstream bottom wall conditions were examined and the resultant oncoming boundary layer thickness were 4.3 and 6.7 times the step height, respectively. The Reynolds number based on the step height and free-stream velocity was 7800. The mean velocities, Reynolds stresses analyzed in both Cartesian and curvilinear coordinate systems, eddy viscosity, correlation coefficient and third order moments are discussed. The results indicate that, due to the enhanced turbulence intensity and shear rate in the fully rough case, distinct elevated regions of vertical and shear Reynolds stresses are consistent upstream of the leading edge of the step while the magnitude of the Reynolds stresses are consistently higher than observed in the smooth case. The correlation coefficient, eddy viscosity and third order moments also show distinct elevated regions upstream of the leading edge of the step in the fully rough case. Above the step, distinct elevated regions of the Reynolds stresses, eddy viscosity and correlation coefficient are observed in both cases with the peak values at a vertical location corresponding to the maximum elevation of the mean separating streamline.

Deposition of Small Particles From Turbulent Flows

2003 ◽  
Author(s):  
Z. Wu ◽  
J. B. Young
Keyword(s):  
Turbulent Flow ◽  
Turbulent Flows ◽  
Channel Flow ◽  
Gas Turbines ◽  
Particle Deposition ◽  
Numerical Study ◽  
Turbulent Channel Flow ◽  
Brownian Diffusion ◽  
Small Particles ◽  
Two Fluid

This paper deals with particle deposition onto solid walls from turbulent flows. The aim of the study is to model particle deposition in industrial flows, such as the one in gas turbines. The numerical study has been carried out with a two fluid approach. The possible contribution to the deposition from Brownian diffusion, turbulent diffusion and shear-induced lift force are considered in the study. Three types of turbulent two-phase flows have been studied: turbulent channel flow, turbulent flow in a bent duct and turbulent flow in a turbine blade cascade. In the turbulent channel flow case, the numerical results from a two-dimensional code show good agreement with numerical and experimental results from other resources. Deposition problem in a bent duct flow is introduced to study the effect of curvature. Finally, the deposition of small particles on a cascade of turbine blades is simulated. The results show that the current two fluid models are capable of predicting particle deposition rates in complex industrial flows.

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