scholarly journals Inertial particle acceleration in strained turbulence

2015 ◽  
Vol 785 ◽  
pp. 31-53 ◽  
Author(s):  
C.-M. Lee ◽  
Á. Gylfason ◽  
P. Perlekar ◽  
F. Toschi
Keyword(s):  
Particle Acceleration ◽  
Strain Rate ◽  
Probability Density ◽  
Energy Dissipation Rate ◽  
Distribution Functions ◽  
Inertial Particles ◽  
Inertial Particle ◽  
Mean Flow ◽  
The Mean ◽  
Mean Strain

The dynamics of inertial particles in turbulence is modelled and investigated by means of direct numerical simulation of an axisymmetrically expanding homogeneous turbulent strained flow. This flow can mimic the dynamics of particles close to stagnation points. The influence of mean straining flow is explored by varying the dimensionless strain rate parameter $Sk_{0}/{\it\epsilon}_{0}$ from 0.2 to 20, where $S$ is the mean strain rate, $k_{0}$ and ${\it\epsilon}_{0}$ are the turbulent kinetic energy and energy dissipation rate at the onset of straining. We report results relative to the acceleration variances and probability density functions for both passive and inertial particles. A high mean strain is found to have a significant effect on the acceleration variance both directly by an increase in the frequency of the turbulence and indirectly through the coupling of the fluctuating velocity and the mean flow field. The influence of the strain on the normalized particle acceleration probability distribution functions is more subtle. For the case of a passive particle we can approximate the acceleration variance with the aid of rapid-distortion theory and obtain good agreement with simulation data. For the case of inertial particles we can write a formal expression for the accelerations. The magnitude changes in the inertial particle acceleration variance and the effect on the probability density function are then discussed in a wider context for comparable flows, where the effects of the mean flow geometry and of the anisotropy at small scales are present.

scholarly journals The Influence of Specimen Geometry and Strain Rate on the Portevin-Le Chatelier Effect and Fracture in an Austenitic FeMnC TWIP Steel

Metals ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 1201
Author(s):  
Jidong Kang ◽  
Liting Shi ◽  
Jie Liang ◽  
Babak Shalchi-Amirkhiz ◽  
Colin Scott
Keyword(s):  
Strain Rate ◽  
High Speed ◽  
Twip Steel ◽  
Image Correlation ◽  
Specimen Geometry ◽  
Dynamic Strain ◽  
Mean Flow ◽  
Chatelier Effect ◽  
The Mean ◽  
Mean Strain

We studied the Portevin-Le Chatelier effect and fracture behavior of a FeMnC TWIP steel using high speed digital image correlation by varying the specimen geometry (flat vs. round) and test strain rate (0.001 vs. 0.1 s−1). The results show that the mean flow stress, the mean strain hardening rate and the mean strain rate sensitivity parameters are all independent of the specimen geometry and are uncorrelated with the presence or not of Portevin-Le Chatelier (PLC) bands, the type of PLC bands observed or the critical strain for band formation. However, both the fracture strains and stresses and the PLC behavior are highly geometry and/or strain rate dependent. Dynamic strain aging (DSA) and in particular the presence of PLC instabilities appears to play an important but as yet unclear role in promoting premature necking and final fracture.

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.

scholarly journals WKB theory for rapid distortion of inhomogeneous turbulence

1999 ◽  
Vol 390 ◽  
pp. 325-348 ◽  
Author(s):  
S. NAZARENKO ◽  
N. K.-R. KEVLAHAN ◽  
B. DUBRULLE
Keyword(s):  
Large Scale ◽  
Isotropic Turbulence ◽  
Two Dimensional ◽  
Turbulent Spot ◽  
Mean Flow ◽  
Mean Velocity ◽  
Inhomogeneous Turbulence ◽  
Large Scale Vortex ◽  
The Mean ◽  
Mean Strain

A WKB method is used to extend RDT (rapid distortion theory) to initially inhomogeneous turbulence and unsteady mean flows. The WKB equations describe turbulence wavepackets which are transported by the mean velocity and have wavenumbers which evolve due to the mean strain. The turbulence also modifies the mean flow and generates large-scale vorticity via the averaged Reynolds stress tensor. The theory is applied to Taylor's four-roller flow in order to explain the experimentally observed reduction in the mean strain. The strain reduction occurs due to the formation of a large-scale vortex quadrupole structure from the turbulent spot confined by the four rollers. Both turbulence inhomogeneity and three-dimensionality are shown to be important for this effect. If the initially isotropic turbulence is either homogeneous in space or two-dimensional, it has no effect on the large-scale strain. Furthermore, the turbulent kinetic energy is conserved in the two-dimensional case, which has important consequences for the theory of two-dimensional turbulence. The analytical and numerical results presented here are in good qualitative agreement with experiment.

scholarly journals Analysis of the forward and backward in time pair-separation probability density functions for inertial particles in isotropic turbulence

2017 ◽  
Vol 830 ◽  
pp. 63-92 ◽  
Author(s):  
Andrew D. Bragg
Keyword(s):  
Probability Density ◽  
Isotropic Turbulence ◽  
Probability Density Functions ◽  
Density Functions ◽  
Inertial Particles ◽  
Mean Square ◽  
Inertial Particle ◽  
Particle Pair ◽  
Pair Separation ◽  
Dissipation Range

In this paper we investigate, using theory and direct numerical simulations (DNS), the forward in time (FIT) and backward in time (BIT) probability density functions (PDFs) of the separation of inertial particle pairs in isotropic turbulence. In agreement with our earlier study (Bragg et al., Phys. Fluids, vol. 28, 2016, 013305), where we compared the FIT and BIT mean-square separations, we find that inertial particles separate much faster BIT than FIT, with the strength of the irreversibility depending upon the final/initial separation of the particle pair and their Stokes number $St$. However, we also find that the irreversibility shows up in subtle ways in the behaviour of the full PDF that it does not in the mean-square separation. In the theory, we derive new predictions, including a prediction for the BIT/FIT PDF for $St\geqslant O(1)$, and for final/initial separations in the dissipation regime. The prediction shows how caustics in the particle relative velocities in the dissipation range affect the scaling of the pair-separation PDF, leading to a PDF with an algebraically decaying tail. The predicted functional behaviour of the PDFs is universal, in that it does not depend upon the level of intermittency in the underlying turbulence. We also analyse the pair-separation PDFs for fluid particles at short times, and construct theoretical predictions using the multifractal formalism to describe the fluid relative velocity distributions. The theoretical and numerical results both suggest that the extreme events in the inertial particle-pair dispersion at the small scales are dominated by their non-local interaction with the turbulent velocity field, rather than due to the strong dissipation range intermittency of the turbulence itself. In fact, our theoretical results predict that for final/initial separations in the dissipation range, when $St\gtrsim 1$, the tails of the pair-separation PDFs decay faster as the Taylor Reynolds number $Re_{\unicode[STIX]{x1D706}}$ is increased, the opposite of what would be expected for fluid particles.

Orientation and rotation of inertial disk particles in wall turbulence

2015 ◽  
Vol 766 ◽  
Author(s):  
Niranjan Reddy Challabotla ◽  
Lihao Zhao ◽  
Helge I. Andersson
Keyword(s):  
Translational Motion ◽  
Turbulent Channel Flow ◽  
Stokes Number ◽  
Wall Turbulence ◽  
Rotational Inertia ◽  
Inertial Particles ◽  
Mean Flow ◽  
Flow Vorticity ◽  
Oblate Spheroids ◽  
The Mean

AbstractThe translational and rotational dynamics of oblate spheroidal particles suspended in a directly simulated turbulent channel flow have been examined. Inertial disk-like particles exhibited a significant preferential orientation in the plane of the mean shear. The rotational inertia about the symmetry axis of the disk-like particles hampered the spin-up of the flattest particles to match the mean flow vorticity. The influence of the particle shape on the orientation and rotation diminished as the translational inertia increased from Stokes number 1 to 30. An isotropization of both orientation and rotation could be observed in the core region of the channel. The translational motion of the oblate spheroids had a weak dependence on the aspect ratio. We therefore concluded that inertial particles sample nearly the same flow field irrespective of shape. Nevertheless, the orientation and rotation of disk-like particles turned out to be qualitatively different from the dynamics of fibre-like particles.

Mean Flow Characteristics of a Turbulent Offset Jet

1986 ◽  
Vol 108 (1) ◽  
pp. 82-88 ◽  
Author(s):  
J. R. R. Pelfrey ◽  
J. A. Liburdy
Keyword(s):  
Strain Rate ◽  
Flat Surface ◽  
Flow Characteristics ◽  
Wall Jet ◽  
Longitudinal Variation ◽  
Mean Flow ◽  
Region Flow ◽  
The Mean ◽  
Plane Jet ◽  
Offset Jet

A detailed study of the mean flow characteristics of a turbulent offset jet is presented. The flow is characterized by a longitudinal variation of curvature, skewed impingement onto a flat surface, a recirculating region, and the development of a wall jet region. Flow structure is described in the preimpingement, recirculation and impingement regions. An interdependence is shown among the pressure differential across the jet, jet curvature and entrainment. The magnitude of the curvature strain rate is found to be significant and implies that this flow cannot be accurately modelled as a thin shear layer. The jet decay and spread rates are similar to those of a plane jet if appropriate curved coordinates are used. The extent of the impingement region is approximately 20 nozzle widths downstream, in agreement with previous studies.

On the Constitutive Relation for the Reynolds Stresses and the Prandtl-Kolmogorov Hypothesis of Effective Viscosity in Axisymmetric Strained Turbulence

1999 ◽  
Vol 122 (1) ◽  
pp. 48-50 ◽  
Author(s):  
J. Jovanovic´ ◽  
I. Otic´
Keyword(s):  
Strain Rate ◽  
Stress Tensor ◽  
Reynolds Stress ◽  
Constitutive Relation ◽  
Effective Viscosity ◽  
Constitutive Relations ◽  
Reynolds Stresses ◽  
Reynolds Stress Tensor ◽  
The Mean ◽  
Mean Strain

The constitutive relation for the Reynolds stress tensor is considered for turbulence developing in axisymmetric strain fields. It is confirmed that the Reynolds stress tensor is aligned linearly with the mean strain rate. In contrast to the Prandtl-Kolmogorov, hypothesis, the effective viscosity is found to grow in proportion to the anisotropy of turbulence and the length scale based on the magnitude of the mean strain rate. Using invariant theory the effective viscosity is determined for the limiting states of turbulence. Additional analysis of the constitutive relations is supplemented for the dissipation and pressure-strain correlations. It is shown that analytical derivations are in excellent agreement with the data obtained from direct numerical simulations. [S0098-2202(00)02801-7]

scholarly journals The amplification of large-scale motion in a supersonic concave turbulent boundary layer and its impact on the mean and statistical properties

2019 ◽  
Vol 863 ◽  
pp. 454-493 ◽  
Author(s):  
Qian-Cheng Wang ◽  
Zhen-Guo Wang ◽  
Ming-Bo Sun ◽  
Rui Yang ◽  
Yu-Xin Zhao ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Strain Rate ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Principal Strain ◽  
Mean Flow ◽  
Turbulence Statistics ◽  
Concave Boundary ◽  
The Mean

Direct numerical simulation is conducted to uncover the response of a supersonic turbulent boundary layer to streamwise concave curvature and the related physical mechanisms at a Mach number of 2.95. Streamwise variations of mean flow properties, turbulence statistics and turbulent structures are analysed. A method to define the boundary layer thickness based on the principal strain rate is proposed, which is applicable for boundary layers subjected to wall-normal pressure and velocity gradients. While the wall friction grows with the wall turning, the friction velocity decreases. A logarithmic region with constant slope exists in the concave boundary layer. However, with smaller slope, it is located lower than that of the flat boundary layer. Streamwise varying trends of the velocity and the principal strain rate within different wall-normal regions are different. The turbulence level is promoted by the concave curvature. Due to the increased turbulence generation in the outer layer, secondary bumps are noted in the profiles of streamwise and spanwise turbulence intensity. Peak positions in profiles of wall-normal turbulence intensity and Reynolds shear stress are pushed outward because of the same reason. Attributed to the Görtler instability, the streamwise extended vortices within the hairpin packets are intensified and more vortices are generated. Through accumulations of these vortices with a similar sense of rotation, large-scale streamwise roll cells are formed. Originated from the very large-scale motions and by promoting the ejection, sweep and spanwise events, the formation of large-scale streamwise roll cells is the physical cause of the alterations of the mean properties and turbulence statistics. The roll cells further give rise to the vortex generation. The large number of hairpin vortices formed in the near-wall region lead to the improved wall-normal correlation of turbulence in the concave boundary layer.

Sign in / Sign up

Export Citation Format

Share Document