Near-wall coherent structures in the turbulent channel flow of a dilute polymer solution

A large eddy simulation (LES) of a plane turbulent channel flow is performed at a Reynolds number Re? = 590 based on the channel half width, ? and wall shear velocity, u? by approximating the near wall region using differential equation wall model (DEWM). The simulation is performed in a computational domain of 2?? x 2? x ??. The computational domain is discretized by staggered grid system with 32 x 30 x 32 grid points. In this domain the governing equations of LES are discretized spatially by second order finite difference formulation, and for temporal discretization the third order low-storage Runge-Kutta method is used. Essential turbulence statistics of the computed flow field based on this LES approach are calculated and compared with the available Direct Numerical Simulation (DNS) and LES data where no wall model was used. Comparing the results throughout the calculation domain we have found that the LES results based on DEWM show closer agreement with the DNS data, especially at the near wall region. That is, the LES approach based on DEWM can capture the effects of near wall structures more accurately. Flow structures in the computed flow field in the 3D turbulent channel have also been discussed and compared with LES data using no wall model.

Download Full-text

Coherent structures in the linearized impulse response of turbulent channel flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.15 ◽

2019 ◽

Vol 863 ◽

pp. 1190-1203 ◽

Cited By ~ 9

Author(s):

Sabarish B. Vadarevu ◽

Sean Symon ◽

Simon J. Illingworth ◽

Ivan Marusic

Keyword(s):

Channel Flow ◽

Coherent Structures ◽

Large Scale ◽

Body Force ◽

Stokes Equations ◽

Turbulent Channel Flow ◽

Kinetic Energy Density ◽

Reynolds Stresses ◽

Mean Velocity ◽

Velocity Fluctuations

We study the evolution of velocity fluctuations due to an isolated spatio-temporal impulse using the linearized Navier–Stokes equations. The impulse is introduced as an external body force in incompressible channel flow at $Re_{\unicode[STIX]{x1D70F}}=10\,000$. Velocity fluctuations are defined about the turbulent mean velocity profile. A turbulent eddy viscosity is added to the equations to fix the mean velocity as an exact solution, which also serves to model the dissipative effects of the background turbulence on large-scale fluctuations. An impulsive body force produces flow fields that evolve into coherent structures containing long streamwise velocity streaks that are flanked by quasi-streamwise vortices; some of these impulses produce hairpin vortices. As these vortex–streak structures evolve, they grow in size to be nominally self-similar geometrically with an aspect ratio (streamwise to wall-normal) of approximately 10, while their kinetic energy density decays monotonically. The topology of the vortex–streak structures is not sensitive to the location of the impulse, but is dependent on the direction of the impulsive body force. All of these vortex–streak structures are attached to the wall, and their Reynolds stresses collapse when scaled by distance from the wall, consistent with Townsend’s attached-eddy hypothesis.

Download Full-text

Exact coherent states of attached eddies in channel flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.1017 ◽

2019 ◽

Vol 862 ◽

pp. 1029-1059 ◽

Cited By ~ 10

Author(s):

Qiang Yang ◽

Ashley P. Willis ◽

Yongyun Hwang

Keyword(s):

Reynolds Number ◽

Channel Flow ◽

Coherent Structures ◽

Time Scale ◽

Coherent States ◽

Plane Channel ◽

Reynolds Numbers ◽

Wall Region ◽

Self Similar ◽

Near Wall

A new set of exact coherent states in the form of a travelling wave is reported in plane channel flow. They are continued over a range in $Re$ from approximately $2600$ up to $30\,000$, an order of magnitude higher than those discovered in the transitional regime. This particular type of exact coherent states is found to be gradually more localised in the near-wall region on increasing the Reynolds number. As larger spanwise sizes $L_{z}^{+}$ are considered, these exact coherent states appear via a saddle-node bifurcation with a spanwise size of $L_{z}^{+}\simeq 50$ and their phase speed is found to be $c^{+}\simeq 11$ at all the Reynolds numbers considered. Computation of the eigenspectra shows that the time scale of the exact coherent states is given by $h/U_{cl}$ in channel flow at all Reynolds numbers, and it becomes equivalent to the viscous inner time scale for the exact coherent states in the limit of $Re\rightarrow \infty$. The exact coherent states at several different spanwise sizes are further continued to a higher Reynolds number, $Re=55\,000$, using the eddy-viscosity approach (Hwang & Cossu, Phys. Rev. Lett., vol. 105, 2010, 044505). It is found that the continued exact coherent states at different sizes are self-similar at the given Reynolds number. These observations suggest that, on increasing Reynolds number, new sets of self-sustaining coherent structures are born in the near-wall region. Near this onset, these structures scale in inner units, forming the near-wall self-sustaining structures. With further increase of Reynolds number, the structures that emerged at lower Reynolds numbers subsequently evolve into the self-sustaining structures in the logarithmic region at different length scales, forming a hierarchy of self-similar coherent structures as hypothesised by Townsend (i.e. attached eddy hypothesis). Finally, the energetics of turbulent flow is discussed for a consistent extension of these dynamical systems notions to high Reynolds numbers.

Download Full-text

Predictability Study of Viscoelastic Turbulent Channel Flow Using Deep Learning

Volume 2: Computational Fluid Dynamics ◽

10.1115/ajkfluids2019-5261 ◽

2019 ◽

Author(s):

Atsushi Nagamachi ◽

Takahiro Tsukahara

Keyword(s):

Channel Flow ◽

Viscoelastic Fluid ◽

Coherent Structures ◽

Turbulent Channel Flow ◽

Wall Region ◽

Nonlinear Relationship ◽

Multi Layer Perceptron ◽

Numerical Instability ◽

Z Score ◽

Score Normalization

Abstract We tested Artificial Neural Networks (ANNs) to predict a fully-developed turbulent channel flow of a viscoelastic fluid in preparation for elucidating flow phenomenon and solving the difficulty in DNS (Direct Numerical Simulation) due to numerical instability of the viscoelastic fluid. Two kinds of ANNs (multi-layer perceptron (MLP) and U-Net) were trained using DNS data to predict conformation stress from given instantaneous field. The MLP showed accurate predictions and predictions got better with z-score normalization. ANN predicted accurately in near-wall region having coherent structures. In addition, we demonstrated that ANN get the nonlinear relationship between velocity gradient and viscoelastic stress partially.

Download Full-text

Assessing the Effects of Near Wall Corrections of the Force Acting on Particles in Gas-Solid Channel Flows

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2005-77304 ◽

2005 ◽

Author(s):

Boris Arcen ◽

Anne Tanie`re ◽

Benoiˆt Oesterle´

Keyword(s):

Channel Flow ◽

Lift Force ◽

Particle Concentration ◽

Turbulent Channel Flow ◽

Shear Velocity ◽

Solid Particles ◽

Wall Region ◽

Wall Effects ◽

Strong Shear ◽

Near Wall

The importance of using the lift force and wall-corrections of the drag coefficient for modeling the motion of solid particles in a fully-developed channel flow is investigated by means of direct numerical simulation (DNS). The turbulent channel flow is computed at a Reynolds number based on the wall-shear velocity and channel half-width of 185. Contrary to most of the numerical simulations, we consider in the present study a lift force formulation that accounts for the weak and strong shear as well as for the wall effects (hereinafter referred to as optimum lift force), and the wall-corrections of the drag force. The DNS results show that the optimum lift force and the wall-corrections of the drag together have little influence on most of the statistics (particle concentration, mean velocities, and mean relative and drift velocities), even in the near wall region.

Download Full-text