Numerical Experiment on Drag Reduction of Turbulent Flow by Near-Wall Dumping for Spanwise Oscillation in Specific Wave-Number Range

Stability curves are computed for both spatially and temporally growing disturbances in a stratified mixing layer between two uniform streams. The low Froude number limit, in which the effects of buoyancy predominate, and the high Froude number limit, in which the effects of density variation are manifested by the inertial terms of the vorticity equation, are considered as limiting cases. For the buoyant case, although the spatial growth rates can be predicted reasonably well by suitable use of the results for temporal growth, spatially growing disturbances appear to have high group velocities near the lower cutoff wave-number. For the inertial case, it is demonstrated that density variations can be destabilizing. More precisely, when the stream with the higher velocity has the lower density, both the wave-number range of unstable disturbances and the maximum spatial growth rate are increased relative to the case of homogeneous flow. Finally, it is shown how the growth rate of the most unstable wave in the inertial case diminishes as buoyancy becomes important.

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Hydromagnetic Stability of Two Rivlin-Ericksen Elastico-Viscous Superposed Conducting Fluids

Zeitschrift für Naturforschung A ◽

10.1515/zna-1997-6-711 ◽

1997 ◽

Vol 52 (6-7) ◽

pp. 528-532

Author(s):

R. C. Sharma ◽

P. Kumar

Keyword(s):

Magnetic Field ◽

Wave Number ◽

Horizontal Magnetic Field ◽

Number Range ◽

Wave Number Range ◽

Unstable Configuration ◽

The Stability ◽

Superposed Fluids ◽

The Magnetic Field ◽

Conducting Fluids

Abstract The stability of the plane interface separating two Rivlin-Ericksen elastico-viscous superposed fluids of uniform densities when the whole system is immersed in a uniform horizontal magnetic field has been studied. The stability analysis has been carried out, for mathematical simplicity, for two highly viscous fluids of equal kinematic viscosities and equal kinematic viscoelasticities. It is found that the stability criterion is independent of the effects of viscosity and viscoelasticity and is dependent on the orientation and magnitude of the magnetic field. The magnetic field is found to stabilize a certain wave-number range of the unstable configuration. The behaviour of growth rates with respect to kinematic viscosity and kinematic viscoelasticity parameters are examined numerically.

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Simplified theory of the near-wall turbulent layer of Newtonian and drag-reducing fluids

Journal of Fluid Mechanics ◽

10.1017/s0022112082001426 ◽

1982 ◽

Vol 119 ◽

pp. 423-441 ◽

Cited By ~ 9

Author(s):

M. A. Goldshtik ◽

V. V. Zametalin ◽

V. N. Shtern

Keyword(s):

Velocity Profile ◽

Turbulent Flow ◽

Drag Reduction ◽

Outer Boundary ◽

Viscous Sublayer ◽

Viscous Layer ◽

Mean Velocity ◽

Turbulent Layer ◽

The Mean ◽

Near Wall

We propose a simplified theory of a viscous layer in near-wall turbulent flow that determines the mean-velocity profile and integral characteristics of velocity fluctuations. The theory is based on the concepts resulting from the experimental data implying a relatively simple almost-ordered structure of fluctuations in close proximity to the wall. On the basis of data on the greatest contribution to transfer processes made by the part of the spectrum associated with the main size of the observed structures, the turbulent fluctuations are simulated by a three-dimensional running wave whose parameters are found from the problem solution. Mathematically the problem reduces to the solution of linearized Navier-Stokes equations. The no-slip condition is satisfied on the wall, whereas on the outer boundary of a viscous layer the conditions of smooth conjunction with the asymptotic shape of velocity and fluctuation-energy profiles resulting from the dimensional analysis are satisfied. The formulation of the problem is completed by the requirement of maximum curvature of the mean-velocity profile on the outer boundary applied from stability considerations.The solution of the problem does not require any quantitative empirical data, although the conditions of conjunction were formulated according to the well-known concepts obtained experimentally. As a result, the near-wall law for the averaged velocity has been calculated theoretically and is in good agreement with experiment, and the characteristic scales for fluctuations have also been determined. The developed theory is applied to turbulent-flow calculations in Maxwell and Oldroyd media. The elastic properties of fluids are shown to lead to near-wall region reconstruction and its associated drag reduction, as is the case in turbulent flows of dilute polymer solutions. This theory accounts for several features typical of the Toms effect, such as the threshold character of the effect and the decrease in the normal fluctuating velocity. The analysis of the near-wall Oldroyd fluid flow permits us to elucidate several new aspects of the drag-reduction effect. It has been established that the Toms effect does not always result in thickening of the viscous sublayer; on the contrary, the most intense drag reduction takes place without thickening in the viscous sublayer.

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Absorption coefficient of Dupont Teflon FEP in the 20–130 wave-number range

Applied Optics ◽

10.1364/ao.24.002746 ◽

1985 ◽

Vol 24 (17) ◽

pp. 2746 ◽

Cited By ~ 9

Author(s):

Mark A. Ordal ◽

Robert J. Bell ◽

Ralph W. Alexander ◽

Raymond E. Paul

Keyword(s):

Absorption Coefficient ◽

Wave Number ◽

Number Range ◽

Wave Number Range

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Liquid Turbulence Spectra in Two-Phase Bubbly Flow Under Turbulence Augmentation and Suppression Conditions

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2006-98335 ◽

2006 ◽

Author(s):

Mohamed E. Shawkat ◽

Chan Y. Ching ◽

Mamdouh Shoukri

Keyword(s):

Void Fraction ◽

Bubbly Flow ◽

Bubble Diameter ◽

Optical Probe ◽

Wave Number ◽

Two Phase ◽

Number Range ◽

Turbulence Spectra ◽

Turbulence Suppression ◽

Wave Number Range

An experimental investigation was performed in air-water bubbly flow to study the liquid turbulence spectra in a 200mm diameter vertical pipe. A dual optical probe was used to measure the local void fraction and bubble diameter while the liquid velocities were measured using hot-film anemometry. Experiments were performed at two liquid superficial velocities of 0.2 and 0.68m/s for gas superficial velocities in the range of 0 to 0.18m/s. Generally, as the void fraction increases there is a turbulence augmentation. However, a turbulence suppression was observed near the pipe wall at the higher liquid flow rate for low void fraction. In the augmentation case, the turbulence spectra showed a significant increase in the energy at the wave number range comparable to the bubble diameter. In the suppression case, the spectra showed that suppression initially occurs at the low wave number range and then extends to higher wave numbers as suppression increased.

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Summer planetary-scale oscillations: aura MLS temperature compared with ground-based radar wind

Annales Geophysicae ◽

10.5194/angeo-27-1763-2009 ◽

2009 ◽

Vol 27 (4) ◽

pp. 1763-1774 ◽

Cited By ~ 10

Author(s):

C. E. Meek ◽

A. H. Manson

Keyword(s):

Temperature Data ◽

Wave Number ◽

Medium Frequency ◽

Period Range ◽

Number Range ◽

Data Presentation ◽

Continuous Sampling ◽

Planetary Scale ◽

Mesospheric Temperature ◽

Wave Number Range

Abstract. The advent of satellite based sampling brings with it the opportunity to examine virtually any part of the globe. Aura MLS mesospheric temperature data are analysed in a wavelet format for easy identification of possible planetary waves (PW) and aliases masquerading as PW. A calendar year, 2005, of eastward, stationary, and westward waves at a selected latitude is shown in separate panels for wave number range −3 to +3 for period range 8 h to 30 days (d). Such a wavelet analysis is made possible by Aura's continuous sampling at all latitudes 82° S–82° N. The data presentation is suitable for examination of years of data. However this paper focuses on the striking feature of a "dish-shaped" upper limit to periods near 2 d in mid-summer, with longer periods appearing towards spring and fall, a feature also commonly seen in radar winds. The most probable cause is suggested to be filtering by the summer jet at 70–80 km, the latter being available from ground based medium frequency radar (MFR). Classically, the phase velocity of a wave must be greater than that of the jet in order to propagate through it. As an attempt to directly relate satellite and ground based sampling, a PW event of period 8d and wave number 2, which appears to be the original rather than an alias, is compared with ground based radar wind data. An appendix discusses characteristics of satellite data aliases with regard to their periods and amplitudes.

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Large eddy simulation of near-wall turbulent flow over streamlined riblet-structured surface for drag reduction in a rectangular channel

Thermal Science ◽

10.2298/tsci190909059a ◽

2020 ◽

Vol 24 (5 Part A) ◽

pp. 2793-2808

Author(s):

Hussain Al-Kayiem ◽

Desmond Lim ◽

Jundika Kurnia

Keyword(s):

Large Eddy Simulation ◽

Turbulent Flow ◽

Drag Reduction ◽

Rectangular Channel ◽

Wall Region ◽

Mean Velocity ◽

Structured Surface ◽

Eddy Simulation ◽

Large Eddy ◽

Near Wall

Sharkskin-inspired riblets are widely adopted as a passive method for drag reduc?tion of flow over surfaces. In this research, large eddy simulation of turbulent flow over riblet-structured surface in a rectangular channel domain were performed at various Reynolds numbers, ranging from 4200-10000, to probe the resultant drag change, compared to smooth surface. The changes of mean streamwise velocity gradient in wall-normal direction at varied locations around riblet structures were also investigated to reduce mechanisms of streamlined riblet in reducing drag. The computational model is validated by comparing the simulation results against analytical and experimental data, for both smooth and riblet surfaces. Results in?dicating that the performance of the proposed streamlined riblet shows 7% drag reduction, as maximum, which is higher than the performance of L-shaped riblet with higher wetted surface area. The mean velocity profile analysis indicates that the streamlined riblet structures help to reduce longitudinal averaged velocity component rate in the normal to surface direction of near-wall region which leads to laminarization process as fluid-flows over riblet structures.

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