Turbulent channel flow over an anisotropic porous wall – drag increase and reduction

The effect of the variations of the permeability tensor on the close-to-the-wall behaviour of a turbulent channel flow bounded by porous walls is explored using a set of direct numerical simulations. It is found that the total drag can be either reduced or increased by more than 20 % by adjusting the permeability directional properties. Drag reduction is achieved for the case of materials with permeability in the vertical direction lower than the one in the wall-parallel planes. This configuration limits the wall-normal velocity at the interface while promoting an increase of the tangential slip velocity leading to an almost ‘one-component’ turbulence where the low- and high-speed streak coherence is strongly enhanced. On the other hand, strong drag increase is found when high wall-normal and low wall-parallel permeabilities are prescribed. In this condition, the enhancement of the wall-normal fluctuations due to the reduced wall-blocking effect triggers the onset of structures which are strongly correlated in the spanwise direction, a phenomenon observed by other authors in flows over isotropic porous layers or over ribletted walls with large protrusion heights. The use of anisotropic porous walls for drag reduction is particularly attractive since equal gains can be achieved at different Reynolds numbers by rescaling the magnitude of the permeability only.

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Numerical simulation of turbulent channel flow over a viscous hyper-elastic wall

Journal of Fluid Mechanics ◽

10.1017/jfm.2017.617 ◽

2017 ◽

Vol 830 ◽

pp. 708-735 ◽

Cited By ~ 39

Author(s):

Marco E. Rosti ◽

Luca Brandt

Keyword(s):

Channel Flow ◽

Solid Phase ◽

Turbulent Channel Flow ◽

Inertial Range ◽

Spanwise Direction ◽

Logarithmic Scale ◽

Normal Velocity ◽

Mean Velocity ◽

Elastic Wall ◽

Hyper Elastic

We perform numerical simulations of a turbulent channel flow over an hyper-elastic wall. In the fluid region the flow is governed by the incompressible Navier–Stokes (NS) equations, while the solid is a neo-Hookean material satisfying the incompressible Mooney–Rivlin law. The multiphase flow is solved with a one-continuum formulation, using a monolithic velocity field for both the fluid and solid phase, which allows the use of a fully Eulerian formulation. The simulations are carried out at Reynolds bulk $Re=2800$ and examine the effect of different elasticity and viscosity of the deformable wall. We show that the skin friction increases monotonically with the material elastic modulus. The turbulent flow in the channel is affected by the moving wall even at low values of elasticity since non-zero fluctuations of vertical velocity at the interface influence the flow dynamics. The near-wall streaks and the associated quasi-streamwise vortices are strongly reduced near a highly elastic wall while the flow becomes more correlated in the spanwise direction, similarly to what happens for flows over rough and porous walls. As a consequence, the mean velocity profile in wall units is shifted downwards when shown in logarithmic scale, and the slope of the inertial range increases in comparison to that for the flow over a rigid wall. We propose a correlation between the downward shift of the inertial range, its slope and the wall-normal velocity fluctuations at the wall, extending results for the flow over rough walls. We finally show that the interface deformation is determined by the fluid fluctuations when the viscosity of the elastic layer is low, while when this is high the deformation is limited by the solid properties.

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DRAG REDUCTION IN TURBULENT CHANNEL FLOW WITH TWO-TIERED SUPERHYDROPHOBIC WALLS

10.1615/tfec2017.bev.018117 ◽

2017 ◽

Author(s):

Richard Perkins ◽

Julie Crockett ◽

Daniel Maynes

Keyword(s):

Drag Reduction ◽

Channel Flow ◽

Turbulent Channel Flow

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Drag reduction and transient growth of a streak in a spanwise wall-oscillatory turbulent channel flow

Physics of Fluids ◽

10.1063/5.0050547 ◽

2021 ◽

Vol 33 (6) ◽

pp. 065122

Author(s):

A. Yakeno

Keyword(s):

Drag Reduction ◽

Channel Flow ◽

Turbulent Channel Flow ◽

Transient Growth

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Frictional Drag Reduction: Review and Numerical Simulation of Microbubble Drag Reduction in a Channel Flow

10.3940/rina.ijme.2018.a2.460 ◽

2018 ◽

Vol Vol 160 (A2) ◽

Author(s):

S Sindagi ◽

R Vijayakumar ◽

B K Saxena

Keyword(s):

Numerical Simulation ◽

Drag Reduction ◽

Channel Flow ◽

Void Fraction ◽

Rectangular Channel ◽

Air Injection ◽

Spanwise Direction ◽

Flat Plates ◽

Frictional Drag ◽

Injection Point

The reduction of ship’s resistance is one of the most effective way to reduce emissions, operating costs and to improve EEDI. It is reported that, for slow moving vessels, the frictional drag accounts for as much as 80% of the total drag, thus there is a strong demand for the reduction in the frictional drag. The use of air as a lubricant, known as Micro Bubble Drag Reduction, to reduce that frictional drag is an active research topic. The main focus of authors is to present the current scenario of research carried out worldwide along with numerical simulation of air injection in a rectangular channel. Latest developments in this field suggests that, there is a potential reduction of 80% & 30% reduction in frictional drag in case of flat plates and ships respectively. Review suggests that, MBDR depends on Gas or Air Diffusion which depends on, Bubble size distributions and coalescence and surface tension of liquid, which in turn depends on salinity of water, void fraction, location of injection points, depth of water in which bubbles are injected. Authors are of opinion that, Microbubbles affect the performance of Propeller, which in turn decides net savings in power considering power required to inject Microbubbles. Moreover, 3D numerical investigations into frictional drag reduction by microbubbles were carried out in Star CCM+ on a channel for different flow velocities, different void fraction and for different cross sections of flow at the injection point. This study is the first of its kind in which, variation of coefficient of friction both in longitudinal as well as spanwise direction were studied along with actual localised variation of void fraction at these points. From the study, it is concluded that, since it is a channel flow and as the flow is restricted in confined region, effect of air injection is limited to smaller area in spanwise direction as bubbles were not escaping in spanwise direction.

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