Direct numerical simulation of spiral turbulence

2010 ◽  
Vol 668 ◽  
pp. 150-173 ◽  
Author(s):  
S. DONG ◽  
X. ZHENG
Keyword(s):  
Outer Cylinder ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Cylinder Axis ◽  
Mean Velocity ◽  
Plane Normal ◽  
Concentric Cylinders ◽  
The 1960S

In this paper, we present results of three-dimensional direct numerical simulations of the spiral turbulence phenomenon in a range of moderate Reynolds numbers, in which alternating intertwined helical bands of turbulent and laminar fluids co-exist and propagate between two counter-rotating concentric cylinders. We show that the turbulent spiral is comprised of numerous small-scale azimuthally elongated vortices, which align into and collectively form the barber-pole-like pattern. The domain occupied by such vortices in a plane normal to the cylinder axis resembles a ‘crescent moon’, a shape made well known by Van Atta with his experiments in the 1960s. The time-averaged mean velocity of spiral turbulence is characterized in the radial–axial plane by two layers of axial flows of opposite directions. We also observe that, as the Reynolds number increases, the transition from spiral turbulence to featureless turbulence does not occur simultaneously in the whole domain, but progresses in succession from the inner cylinder towards the outer cylinder. Certain aspects pertaining to the dynamics and statistics of spiral turbulence and issues pertaining to the simulation are discussed.

Turbulent flow between counter-rotating concentric cylinders: a direct numerical simulation study

2008 ◽  
Vol 615 ◽  
pp. 371-399 ◽  
Author(s):  
S. DONG
Keyword(s):  
Turbulent Flow ◽  
Large Scale ◽  
Outer Cylinder ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Taylor Vortex ◽  
Small Scale ◽  
Mean Flow ◽  
Concentric Cylinders ◽  
High Reynolds

We report three-dimensional direct numerical simulations of the turbulent flow between counter-rotating concentric cylinders with a radius ratio 0.5. The inner- and outer-cylinder Reynolds numbers have the same magnitude, which ranges from 500 to 4000 in the simulations. We show that with the increase of Reynolds number, the prevailing structures in the flow are azimuthal vortices with scales much smaller than the cylinder gap. At high Reynolds numbers, while the instantaneous small-scale vortices permeate the entire domain, the large-scale Taylor vortex motions manifested by the time-averaged field do not penetrate a layer of fluid near the outer cylinder. Comparisons between the standard Taylor–Couette system (rotating inner cylinder, fixed outer cylinder) and the counter-rotating system demonstrate the profound effects of the Coriolis force on the mean flow and other statistical quantities. The dynamical and statistical features of the flow have been investigated in detail.

Direct Numerical Simulations of Transitional Separation–Bubble Development in Swept–Blade Flow Conditions

2012 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Pressure Field ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Flow Conditions ◽  
Crossflow Instability

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0° and 45°. The three-dimensional pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and most amplified disturbance frequency is observed in the separated-flow region of the swept configuration, and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

Three-dimensional structure and momentum transfer in a turbulent cylinder wake

1999 ◽  
Vol 394 ◽  
pp. 303-337 ◽  
Author(s):  
A. VERNET ◽  
G. A. KOPP ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Saddle Point ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Half Width ◽  
Small Scale ◽  
Spanwise Direction ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Simultaneous velocity and temperature measurements were made with rakes of sensors that sliced a slightly heated turbulent wake in the spanwise direction, at different lateral positions 150 diameters downstream of the cylinder. A pattern recognition analysis of hotter-to-colder transitions was performed on temperature data measured at the mean velocity half-width. The velocity data from the different ‘slices’ was then conditionally averaged based on the identified temperature events. This procedure yielded the topology of the average three-dimensional large-scale structure which was visualized with iso-surfaces of negative values of the second eigenvector of [S2+Ω2]. The results indicate that the average structure of the velocity fluctuations (using a triple decomposition of the velocity field) is found to be a shear-aligned ring-shaped vortex. This vortex ring has strong outward lateral velocities in its symmetry plane which are like Grant's mixing jets. The mixing jet region extends outside the ring-like vortex and is bounded by two foci separated in the spanwise direction and an upstream saddle point. The two foci correspond to what has been previously identified in the literature as the double rollers.The ring vortex extracts energy from the mean flow by stretching in the mixing jet region just upstream of the ring boundary. The production of the small-scale (incoherent) turbulence by the coherent field and one-component energy dissipation rate occur just downstream of the saddle point within the mixing jet region. Incoherent turbulence energy is extracted from the mean flow just outside the mixing jet region, but within the core of the structure. These processes are highly three-dimensional with a spanwise extent equal to the mean velocity half-width.When a double decomposition is used, the coherent structure is found to be a tube-shaped vortex with a spanwise extent of about 2.5l0. The double roller motions are integral to this vortex in spite of its shape. Spatial averages of the coherent velocity field indicate that the mixing jet region causes a deficit of mean streamwise momentum, while the region outside the foci of the double rollers has a relatively small excess of streamwise momentum.

Round Sudden-Expansion Flows

1986 ◽  
Vol 200 (6) ◽  
pp. 447-455 ◽  
Author(s):  
L Khezzar ◽  
J H Whitelaw ◽  
M Yianneskis
Keyword(s):  
Three Dimensional ◽  
Reynolds Numbers ◽  
Sudden Expansion ◽  
Vortex Rings ◽  
Mean Flow ◽  
Mean Velocity ◽  
Number Range ◽  
Laser Anemometry ◽  
Diameter Pipe ◽  
Inlet Geometry

This paper describes an experimental investigation of the water flows through one axisymmetric and two asymmetric round sudden expansions from a 48 mm to an 84 mm diameter pipe and eccentricities of the pipe axes of 0, 5 and 15 mm respectively. Flow visualization revealed the presence of vortex rings downstream of the plane of expansion for transitional Reynolds numbers (Re, based on the upstream pipe diameter and bulk flow velocity) and reattachment lengths were determined in the Reynolds number range 120–40 000 for all three cases. Detailed measurements of the three mean velocity components and corresponding fluctuations were obtained by laser anemometry for Re = 40000. Wall static pressure measurements are also presented. The results show that asymmetry of the inlet geometry strongly influences the distribution of mean and turbulence quantities downstream of the expansion and results in three-dimensional reattachment. In all three flows, the mean flow was nearly uniform and the turbulence nearly homogeneous at distances of seven diameters of the large pipe downstream of the expansion. Higher levels of turbulence were found in the asymmetric ducts with maxima twice those in the axisymmetric duct.

Flow Regime Characteristics in Porous Media Flows at High Reynolds Numbers

2012 ◽  
Author(s):  
Vishal A. Patil ◽  
James A. Liburdy
Keyword(s):  
Turbulent Flow ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Mean Velocity ◽  
Time Resolved ◽  
Vortical Structures ◽  
Large Eddy ◽  
High Reynolds ◽  
Media Flows

An experimental study on the turbulent flow characteristics in a randomly packed porous bed is presented and discussed. Time resolved PIV measurements, taken in specific pore spaces are used to evaluate transitional and developed turbulent flow statistics for pore Reynolds numbers from 54 to 3964. Three different regimes of steady laminar, transitional and turbulent flow are presented. Small scale coherent vortical structures are examined, using large eddy scale (LES) decomposition, for pore Reynolds number of greater than 1000. Integral length scales were found to reach asymptotic values of approximately 0.1 times the hydraulic diameter of the bed. The integral Eulerian time scales are found to reach an asymptotic value of approximately 0.3 times the convective time scale in the bed. Mean velocity vector maps show flattening of the velocity distribution due to increased momentum mixing. Turbulent stresses show increasing level of homogeneity at higher pore Reynolds numbers.

scholarly journals Peripheral mixing of passive scalar at small Reynolds number

2009 ◽  
Vol 624 ◽  
pp. 151-158 ◽  
Author(s):  
G. BOFFETTA ◽  
F. DE LILLO ◽  
A. MAZZINO
Keyword(s):  
Three Dimensional ◽  
Reynolds Numbers ◽  
Passive Scalar ◽  
Peripheral Region ◽  
Direct Numerical Simulations ◽  
Small Reynolds Number ◽  
Two Dimensional ◽  
Dispersion Modelling ◽  
Low Reynolds Numbers ◽  
Scalar Concentration

Mixing of a passive scalar in the peripheral region close to a wall is investigated by means of accurate direct numerical simulations of both a three-dimensional Couette channel flow at low Reynolds numbers and a two-dimensional synthetic flow. In both cases, the resulting phenomenology can be understood in terms of the theory recently developed by Lebedev & Turitsyn (Phys. Rev. E, vol. 69, 2004, 036301). Our results prove the robustness of the identified mechanisms responsible for the persistency of scalar concentration close to the wall with important consequences in completely different fields ranging from microfluidic applications to environmental dispersion modelling.

scholarly journals Transition to turbulence in an elliptic vortex

1996 ◽  
Vol 307 ◽  
pp. 43-62 ◽  
Author(s):  
T. S. Lundgren ◽  
N. N. Mansour
Keyword(s):  
Swirling Flow ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Periodic Wave ◽  
Transition To Turbulence ◽  
Energy Spectra ◽  
Small Scale ◽  
Two Dimensional ◽  
Secondary Instabilities ◽  
Vortex Tubes

Stability and transition to turbulence are studied in a simple incompressible two-dimensional bounded swirling flow with a rectangular planform – a vortex in a box. This flow is unstable to three-dimensional disturbances. The instability takes the form of counter-rotating swirls perpendicular to the axis which bend the vortex into a periodic wave. As these swirls grow in amplitude the primary vorticity is compressed into thin vortex layers. These develop secondary instabilities which roll up into vortex tubes. In this way the flow attains a turbulent state which is populated by intense elongated vortex tubes and weaker vortex layers which spiral around them. The flow was computed at two Reynolds numbers by spectral methods with up to 2563 resolution. At the higher Reynolds number broad three-dimensional shell-averaged energy spectra are found with nearly a decade of Kolmogorov k−5/3 law and small-scale isotropy.

Investigation of effects of compressibility, geometric and flow parameters on the simulation of a synthetic jet behaviour

2016 ◽  
Vol 120 (1225) ◽  
pp. 521-546 ◽  
Author(s):  
F. Bazdidi-Tehrani ◽  
A. Abouata ◽  
M. Hatami ◽  
N. Bohlooli
Keyword(s):  
Reynolds Number ◽  
Turbulence Model ◽  
Mass Flow ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Instantaneous Velocity ◽  
Mean Velocity ◽  
Velocity Magnitude ◽  
Flow Parameters

ABSTRACTThe present paper focuses on a three-dimensional unsteady turbulent synthetic jet to assess the accuracy of a compressible simulation and some important parameters including the simulations of the actuator, cavity height and Reynolds number. The two-equationSST/k− ω turbulence model is used to predict the flow behaviour. Results show that the compressible simulation case is more accurate than the incompressible one and the dynamic mesh exhibits more reliable results than the mass flow inlet boundary in the compressible simulation. The compressible case displays a delay in the phase of instantaneous velocity for all three Reynolds numbers. Also, the maximum of mean velocity is less than the incompressible case. Moreover, an increase in the Reynolds number leads to an amplification of the peak of mean velocity magnitude. Finally, results demonstrate that a reduction in the cavity height regarding the compressible simulation case causes a reduction in the phase delay and rise in peak of instantaneous velocity magnitude.

scholarly journals The origin of turbulence in wall-bounded flows

2017 ◽  
Vol 21 (suppl. 3) ◽  
pp. 565-572 ◽  
Author(s):  
Jovan Jovanovic ◽  
Mina Nishi
Keyword(s):  
Three Dimensional ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Deterministic Theory ◽  
Sequence Of Events ◽  
Flow Configurations ◽  
High Reynolds ◽  
Special Circumstances ◽  
Anisotropy Decays ◽  
Universal Mechanism

The motion of liquids and gases can be either laminar, flowing slowly in orderly parallel and continuous layers of fluid that cannot mix, or turbulent in which motion exhibits disorder in time and space with the ability to promote mixing. Breakdown of ordered to disordered motion can follow different scenarios so that no universal mechanism can be identified even in similar flow configurations [1]. Only under very special circumstances can the mechanism associated with the appearance of turbulence be studied within the deterministic theory of hydrodynamic stability [2] or employing direct numerical simulations [3] which themselves cannot provide the necessary understanding [4]. Here we show that the representative mechanism responsible for the origin of turbulence in wallbounded flows is associated with large variations of anisotropy in the disturbances [5]. During the breakdown process, anisotropy decays from a maximum towards its minimum value, inducing the explosive production of the dissipation which logically leads to the appearance of small-scale three-dimensional motions. By projecting the sequence of events leading to turbulence in the space which emphasizes the anisotropic nature in the disturbances [6], we explain why, demonstrate how and present what can be achieved if the process is treated analytically using statistical techniques [7]. It is shown that the statistical approach provides not only predictions of the breakdown phenomena which are in fair agreement with available data but also requirements which ensure persistence of the laminar regime up to very high Reynolds numbers.

Instability and Heat Transfer in Mixed Convection Flow in a Horizontal Duct With Discrete Heat Sources

2001 ◽  
Author(s):  
Qinghua Wang ◽  
Yogesh Jaluria
Keyword(s):  
Mixed Convection ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Forced Flow ◽  
Bottom Surface ◽  
Convection Flow ◽  
Heat Sources ◽  
Mixed Convection Flow ◽  
Mean Velocity ◽  
Low Reynolds Numbers

Abstract Three dimensional mixed convection flow in a horizontal rectangular duct at low Reynolds numbers 5 ≤ Re ≤ 100 has been investigated numerically. Multiple strip heat sources are flushed-mounted on the bottom surface, modeling IC chips on PCBs. Two different flow patterns were observed. For Re ≥ 20, and Gr ≤ 1.5 × 104, only steady longitudinal rolls appear in the channel. The discontinuous deployment of heat sources on the bottom makes the longitudinal rolls expand and shrink periodically. The unsteady flow consists of steady longitudinal rolls near the side walls and traveling transverse rolls at the core region of the channel. The traveling velocity of the transverse rolls was found to be equal to the mean velocity of the forced flow. The implications of these observations to the cooling of electronic equipment are discussed.

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