scholarly journals Formation mechanism of hairpin vortices in the wake of a truncated square cylinder in a duct

2010 ◽  
Vol 653 ◽  
pp. 519-536 ◽  
Author(s):  
VINCENT DOUSSET ◽  
ALBAN POTHÉRAT
Keyword(s):  
Formation Mechanism ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Critical Threshold ◽  
Square Cylinder ◽  
Hairpin Vortices ◽  
Bottom Part ◽  
Steady Regime ◽  
Aspect Ratios ◽  
Near Wall Turbulence

We investigate the laminar shedding of hairpin vortices in the wake of a truncated square cylinder placed in a duct, for Reynolds numbers around the critical threshold of the onset of vortex shedding. We single out the formation mechanism of the hairpin vortices by means of a detailed analysis of the flow patterns in the steady regime. We show that unlike in previous studies of similar structures, the dynamics of the hairpin vortices are entwined with that of the counter-rotating pair of streamwise vortices, which we found to be generated in the bottom part of the near wake (these are usually referred to as ‘base vortices’). In particular, once the hairpin structure is released, the base vortices attach to it, forming its legs, so these are streamwise, and not spanwise as previously observed in unconfined wakes or behind cylinders of lower aspect ratios. We also single out a trail of Ω-shaped vortices, generated between successive hairpin vortices through a mechanism that is analogous to that active in near-wall turbulence. Finally, we show how the dynamics of the structures we identified determine the evolution of the drag coefficients and Strouhal numbers when the Reynolds number varies.

The autonomous cycle of near-wall turbulence

1999 ◽  
Vol 389 ◽  
pp. 335-359 ◽  
Author(s):  
JAVIER JIMÉNEZ ◽  
ALFREDO PINELLI
Keyword(s):  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Wall Region ◽  
Streamwise Vortices ◽  
Outer Flow ◽  
Turbulent Fluctuations ◽  
The Mean ◽  
Turbulence Generation ◽  
Near Wall Turbulence ◽  
Near Wall

Numerical experiments on modified turbulent channels at moderate Reynolds numbers are used to differentiate between several possible regeneration cycles for the turbulent fluctuations in wall-bounded flows. It is shown that a cycle exists which is local to the near-wall region and does not depend on the outer flow. It involves the formation of velocity streaks from the advection of the mean profile by streamwise vortices, and the generation of the vortices from the instability of the streaks. Interrupting any of those processes leads to laminarization. The presence of the wall seems to be only necessary to maintain the mean shear. The generation of secondary vorticity at the wall is shown to be of little importance in turbulence generation under natural circumstances. Inhibiting its production increases turbulence intensity and drag.

Effect of Reynolds Number and Surface Roughness on the Efficiency of Centrifugal Pumps

2003 ◽  
Vol 125 (4) ◽  
pp. 670-679 ◽  
Author(s):  
J. F. Gu¨lich
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Physical Models ◽  
Wall Turbulence ◽  
Pump Efficiency ◽  
Hydraulic Efficiency ◽  
Centrifugal Pumps ◽  
Roughness Effects ◽  
Specific Speed ◽  
Near Wall Turbulence

A procedure has been developed to predict the effects of roughness and Reynolds number on the change in efficiency from a model or baseline to a prototype pump (“efficiency scaling”). The analysis of individual losses takes into account different roughnesses of impeller, diffuser/volute, impeller side disks, and casing walls in the impeller side rooms. The method also allows to predict the effect of roughness and Reynolds number on the hydraulic efficiency. The calculations are based on physical models but the weighting of impeller versus diffuser/volute roughness and the fraction of scalable losses within impeller and diffuser/volute are determined empirically from the analysis of tests with industrial pumps. The fraction of scalable impeller/diffuser/volute losses is found to decrease with growing specific speed. Roughness effects in the diffuser/volute are stronger than in the impeller, but the dominance of the stator over the rotor decreases with increasing specific speed. The procedure includes all flow regimes from laminar to turbulent and from hydraulically smooth to fully rough. It is validated by tests with viscosities between 0.2 to 3000 cSt and Reynolds numbers between 1500 and 108. The hydraulic losses depend on the patterns of roughness, near-wall turbulence, and the actual velocity distribution in the hydraulic passages. These effects—which are as yet not amenable to analysis—limit the accuracy of any efficiency prediction procedure for decelerated flows.

Scaling of near-wall turbulence in pipe flow

2010 ◽  
Vol 649 ◽  
pp. 103-113 ◽  
Author(s):  
MARCUS HULTMARK ◽  
SEAN C. C. BAILEY ◽  
ALEXANDER J. SMITS
Keyword(s):  
Reynolds Number ◽  
Boundary Layers ◽  
Pipe Flow ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Flow Data ◽  
Pipe Flows ◽  
Near Wall Turbulence ◽  
Reynolds Number Dependence ◽  
Near Wall

New measurements of the streamwise component of the turbulence intensity in a fully developed pipe flow at Reynolds numbers up to 145 000 indicate that the magnitude of the near-wall peak is invariant with Reynolds number in location and magnitude. The results agree with previous pipe flow data that have sufficient spatial resolution to avoid spatial filtering effects, but stand in contrast to similar results obtained in boundary layers, where the magnitude of the peak displays a prominent Reynolds number dependence, although its position is fixed at the same location as in pipe flow. This indicates that the interaction between the inner and outer regions is different in pipe flows and boundary layers.

Cost-Effective Hybrid RANS-NLES Method for Jet Turbulence and Noise Prediction

2012 ◽  
Author(s):  
M. Mahak ◽  
Paul G. Tucker ◽  
Prasun K. Ray
Keyword(s):  
Boundary Layer ◽  
Cost Effective ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Navier Stokes ◽  
High Concentration ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Layer Region ◽  
Near Wall Turbulence

Jets at higher Reynolds numbers have a high concentration of energy in the small scales in the nozzle vicinity. This is challenging for LES, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well known Reynolds number (Re) independence of jets. We reduce the jet Re whilst rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid largeeddy simulation type of approach which is incorporated by using near wall turbulence model with modified properties. No Subgrid Scale (SGS) model is used in these simulations. Hence, they effectively become Numerical Large Eddy Simulation (NLES) with Reynolds-averaged Navier-Stokes (RANS) covering the full boundary layer region. The noise post processing is carried out using Ffowcs-Williams-Hawking (FWH) approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875. The results for Overall Sound Pressure Level (OASPL) are observed to be within 2–3% accuracy range and directivity of sound is also captured accurately for both the cases. The low Re simulations hence, can be more beneficial in saving time and cost of the simulation while providing reasonably accurate results.

scholarly journals Particle Accumulation Structures in a 5 cSt Silicone Oil Liquid Bridge: New Data for the Preparation of the JEREMI Experiment

2021 ◽  
Vol 33 (2) ◽  
Author(s):  
Paolo Capobianchi ◽  
Marcello Lappa
Keyword(s):  
Liquid Bridge ◽  
Silicone Oil ◽  
Fluid Dynamic ◽  
Solid Particles ◽  
Critical Threshold ◽  
Marangoni Flow ◽  
Disk Radius ◽  
Carrier Fluid ◽  
Aspect Ratios ◽  
Time Periodic

AbstractSystems of solid particles in suspension driven by a time-periodic flow tend to create structures in the carrier fluid that are reminiscent of highly regular geometrical items. Within such a line of inquiry, the present study provides numerical results in support of the space experiments JEREMI (Japanese and European Research Experiment on Marangoni flow Instabilities) planned for execution onboard the International Space Station. The problem is tackled by solving the unsteady non-linear governing equations for the same conditions that will be established in space (microgravity, 5 cSt silicone oil and different aspect ratios of the liquid bridge). The results reveal that for a fixed supporting disk radius, the dynamics are deeply influenced by the height of the liquid column. In addition to its expected link with the critical threshold for the onset of instability (which makes Marangoni flow time-periodic), this geometrical parameter can have a significant impact on the emerging waveform and therefore the topology of particle structures. While for shallow liquid bridges, pulsating flows are the preferred mode of convection, for tall floating columns the dominant outcome is represented by rotating fluid-dynamic disturbance. In the former situation, particles self-organize in circular sectors bounded internally by regions of particle depletion, whereas in the latter case, particles are forced to accumulate in a spiral-like structure. The properties of some of these particle attractors have rarely been observed in earlier studies concerned with fluids characterized by smaller values of the Prandtl number.

scholarly journals Turbulence Intensity Modulation by Micropolar Fluids

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 195
Author(s):  
George Sofiadis ◽  
Ioannis Sarris
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Fluid Flows ◽  
Turbulent Channel Flow ◽  
High Volume ◽  
Reynolds Numbers ◽  
Intensity Modulation ◽  
Wall Turbulence ◽  
Volume Fractions ◽  
Physical Phenomena

Fluid microstructure nature has a direct effect on turbulence enhancement or attenuation. Certain classes of fluids, such as polymers, tend to reduce turbulence intensity, while others, like dense suspensions, present the opposite results. In this article, we take into consideration the micropolar class of fluids and investigate turbulence intensity modulation for three different Reynolds numbers, as well as different volume fractions of the micropolar density, in a turbulent channel flow. Our findings support that, for low micropolar volume fractions, turbulence presents a monotonic enhancement as the Reynolds number increases. However, on the other hand, for sufficiently high volume fractions, turbulence intensity drops, along with Reynolds number increment. This result is considered to be due to the effect of the micropolar force term on the flow, suppressing near-wall turbulence and enforcing turbulence activity to move further away from the wall. This is the first time that such an observation is made for the class of micropolar fluid flows, and can further assist our understanding of physical phenomena in the more general non-Newtonian flow regime.

Mechanism of control of the near-wall turbulence using a micro-cavity array

2021 ◽  
Vol 33 (7) ◽  
pp. 075114
Author(s):  
S. S. Bhat ◽  
A. Silvestri ◽  
B. S. Cazzolato ◽  
M. Arjomandi
Keyword(s):  
Wall Turbulence ◽  
Micro Cavity ◽  
Near Wall Turbulence ◽  
Cavity Array ◽  
Near Wall
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