IUTAM Symposium on Variable Density Low-Speed Turbulent Flows

The instability of the three-dimensional high-shear layer associated with a near-wall low-speed streak is investigated experimentally. A single low-speed streak, not unlike the near-wall low-speed streaks in transitional and turbulent flows, is produced in a laminar boundary layer by using a small piece of screen set normal to the wall. In order to excite symmetric and anti-symmetric modes separately, well-controlled external disturbances are introduced into the laminar low-speed streak through small holes drilled behind the screen. The growth of the excited symmetric varicose mode is essentially governed by the Kelvin–Helmholtz instability of the in ectional velocity profiles across the streak in the normal-to-wall direction and it can occur when the streak width is larger than the shear-layer thickness. The spatial growth rate of the symmetric mode is very sensitive to the streak width and is rapidly reduced as the velocity defect decreases flowing to the momentum transfer by viscous stresses. By contrast, the anti-symmetric sinuous mode that causes the streak meandering is dominated by the wake-type instability of spanwise velocity distributions across the streak. As far as the linear instability is concerned, the growth rate of the anti-symmetric mode is not so strongly affected by the decrease in the streak width, and its exponential growth may continue further downstream than that of the symmetric mode. As for the mode competition, it is important to note that when the streak width is narrow and comparable with the shear-layer thickness, the low-speed streak becomes more unstable to the anti-symmetric modes than to the symmetric modes. It is clearly demonstrated that the growth of the symmetric mode leads to the formation of hairpin vortices with a pair of counter-rotating streamwise vortices, while the anti-symmetric mode evolves into a train of quasi-streamwise vortices with vorticity of alternate sign.

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IUTAM Symposium on The Physics of Wall-Bounded Turbulent Flows on Rough Walls

10.1007/978-90-481-9631-9 ◽

2010 ◽

Cited By ~ 7

Keyword(s):

Turbulent Flows ◽

Rough Walls ◽

Iutam Symposium

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Variable Density Speed Hump

International Journal of Students Research in Technology & Management ◽

10.18510/ijsrtm.2016.423 ◽

2016 ◽

Vol 4 (2) ◽

pp. 35-37

Author(s):

Rajenderpal Singh Bhamrah

Keyword(s):

Shear Thickening ◽

Variable Density ◽

Great Resistance ◽

Shear Thickening Fluid ◽

Low Speed ◽

Technical Paper ◽

Rigid Obstacle ◽

Low Viscosity ◽

Flexible Material ◽

Resistance To Deformation

This technical paper relates to a device that reduces the speed of any overspeeding vehicles travelling on a roadway.It is formed by at least one hollow strip of flexible material, made up of several receptacles located in the shell body. Each receptacle is impregnated with a dilatant shear-thickening fluid. The material is placed under compression during impact when the vehicle strikes it and the fluid itself acts as means for controlling the resistance to deformation of the strip.Thus, if the vehicle travels at a low speed the fluid has a low viscosity and the strip is easily deformed, whereas if the speed of the vehicle is high the viscosity of the fluid is high and as a result has great resistance to deformation, thus forming a rigid obstacle to the passage of the vehicle. Drivers must always slow down when driving over the conventional speed bumps to prevent damage to their vehicle. However, the Variable Density Speed Hump is sensitive to the speed of the vehicle.The vehicle needs to slow down only if it is overspeeding.

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A numerical study of a variable-density low-speed turbulent mixing layer

Journal of Fluid Mechanics ◽

10.1017/jfm.2017.583 ◽

2017 ◽

Vol 830 ◽

pp. 569-601 ◽

Cited By ~ 16

Author(s):

Antonio Almagro ◽

Manuel García-Villalba ◽

Oscar Flores

Keyword(s):

Growth Rate ◽

Mach Number ◽

Incompressible Flows ◽

Numerical Study ◽

Density Ratio ◽

Mixing Layer ◽

Variable Density ◽

The Self ◽

Low Speed ◽

Self Similar

Direct numerical simulations of a temporally developing, low-speed, variable-density, turbulent, plane mixing layer are performed. The Navier–Stokes equations in the low-Mach-number approximation are solved using a novel algorithm based on an extended version of the velocity–vorticity formulation used by Kim et al. (J. Fluid Mech., vol 177, 1987, 133–166) for incompressible flows. Four cases with density ratios $s=1,2,4$ and 8 are considered. The simulations are run with a Prandtl number of 0.7, and achieve a $Re_{\unicode[STIX]{x1D706}}$ up to 150 during the self-similar evolution of the mixing layer. It is found that the growth rate of the mixing layer decreases with increasing density ratio, in agreement with theoretical models of this phenomenon. Comparison with high-speed data shows that the reduction of the growth rates with increasing density ratio has a weak dependence with the Mach number. In addition, the shifting of the mixing layer to the low-density stream has been characterized by analysing one-point statistics within the self-similar interval. This shifting has been quantified, and related to the growth rate of the mixing layer under the assumption that the shape of the mean velocity and density profiles do not change with the density ratio. This leads to a predictive model for the reduction of the growth rate of the momentum thickness, which agrees reasonably well with the available data. Finally, the effect of the density ratio on the turbulent structure has been analysed using flow visualizations and spectra. It is found that with increasing density ratio the longest scales in the high-density side are gradually inhibited. A gradual reduction of the energy in small scales with increasing density ratio is also observed.

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A Time-Accurate Projection Method for Low-Mach Number Variable Density Flows in Curvilinear Grids

Volume 1: Symposia, Parts A, B and C ◽

10.1115/fedsm2009-78327 ◽

2009 ◽

Author(s):

F. N. Fard ◽

B. Lessani

Keyword(s):

Mach Number ◽

Turbulent Flows ◽

Projection Method ◽

Time Integration ◽

Variable Density ◽

Integration Scheme ◽

Coordinate Systems ◽

Low Mach Number ◽

Curvilinear Coordinate ◽

Time Integration Scheme

A time-accurate numerical algorithm is proposed for low Mach number variable density flows in curvilinear coordinate systems. In order to increase the stability of the method, a predictor-corrector time integration scheme, coupled with the projection method, is employed. The projection method results in a constant-coefficient Poisson equation for the pressure in both the predictor and corrector steps. The continuity equation is fully satisfied at each step. To prevent the pressure odd-even decoupling typically encountered in collocated grids, a flux interpolation technique is developed. The spatial discretization method offers computational simplicity and straightforward extension to 3D curvilinear coordinate systems, which are essential in the simulation of turbulent flows in complex geometries. The accuracy and stability of the algorithm are tested with a series of numerical experiments, and the results are validated against the available data in the literature.

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