Using Grooved Surfaces to Improve the Efficiency of Air Injection Drag Reduction Methods in Hydrodynamic Flows

A summary of experiments using grooved surfaces to trap and hold (via surface tension forces) an injected airstream in a low-speed (1.25 to 5 m/s) water flow is presented. The purpose of creating a low-volume near-wall air sheet is to possibly enhance the efficiency of current air injection drag reduction methods in terms of unit gas volume per % drag reduction. Flow visualization and preliminary quantitative data are included for a laminar channel flow, a disturbed laminar channel flow, and a flat plate turbulent boundary-layer flow. A stable convecting low-volume, near-wall gas film is produced in several instances. Groove dimension and the presence of anti-wetting surface coatings are shown to greatly affect the formation and stability of the gas sheet. Deeper, narrower grooves, anti-wetting surface coatings, and shallow-angle gas injection increase the stability of the attached gas layer. Convected disturbances are shown to increase the interfacial instability of the attached sheet. It is not known if a gas sheet can be held under a turbulent boundary layer over 3 m/s, or if the groove sizes needed to do so would become too small to be of use in a practical high-speed hydrodynamic flow.

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Vorticity Modification in a Turbulent Channel Flow by Microbubble Injection

12th International Conference on Nuclear Engineering, Volume 1 ◽

10.1115/icone12-49582 ◽

2004 ◽

Cited By ~ 1

Author(s):

Claudia del C. Gutierrez-Torres ◽

Jose A. Jimenez-Bernal ◽

Elvis E. Dominguez-Ontiveros ◽

Yassin A. Hassan

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Drag Reduction ◽

Channel Flow ◽

Turbulent Channel Flow

Investigation of the drag reduction phenomenon has been carried out for several years. Several techniques to reduce the drag have been applied and researched for a number of years. Microbubbles injection within a turbulent boundary layer is one method utilized to achieve reduction of drag. In this work, the effects of the presence of microbubbles in the boundary layer of a turbulent channel flow are discussed.

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Drag Reduction of a Turbulent Boundary Layer over an Oscillating Wall and Its Variation with Reynolds Number

International Journal of Aerospace Engineering ◽

10.1155/2015/891037 ◽

2015 ◽

Vol 2015 ◽

pp. 1-9 ◽

Cited By ~ 4

Author(s):

Martin Skote ◽

Maneesh Mishra ◽

Yanhua Wu

Keyword(s):

Boundary Layer ◽

Reynolds Number ◽

Turbulent Boundary Layer ◽

Drag Reduction ◽

Channel Flow ◽

Downstream Distance ◽

The Past ◽

Layer Flow ◽

Reynolds Number Dependency ◽

Downstream Development

Spanwise oscillation applied on the wall under a spatially developing turbulent boundary layer flow is investigated using direct numerical simulation. The temporal wall forcing produces a considerable drag reduction over the region where oscillation occurs. Downstream development of drag reduction is investigated from Reynolds number dependency perspective. An alternative to the previously suggested power-law relation between Reynolds number and peak drag reduction values, which is valid for channel flow as well, is proposed. Considerable deviation in the variation of drag reduction with Reynolds number between different previous investigations of channel flow is found. The shift in velocity profile, which has been used in the past for explaining the diminishing drag reduction at higher Reynolds number for riblets, is investigated. A new predictive formula is derived, replacing the ones found in the literature. Furthermore, unlike for the case of riblets, the shift is varying downstream in the case of wall oscillations, which is a manifestation of the fact that the boundary layer has not reached a new equilibrium over the limited downstream distance in the simulations. Taking this into account, the predictive model agrees well with DNS data. On the other hand, the growth of the boundary layer does not influence the drag reduction prediction.

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Active control of a turbulent boundary layer based on local surface perturbation

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.261 ◽

2014 ◽

Vol 750 ◽

pp. 316-354 ◽

Cited By ~ 37

Author(s):

H. L. Bai ◽

Y. Zhou ◽

W. G. Zhang ◽

S. J. Xu ◽

Y. Wang ◽

...

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Drag Reduction ◽

Skin Friction ◽

Active Control ◽

Valuable Insight ◽

Friction Drag ◽

Local Skin ◽

Insight Into ◽

Near Wall

AbstractActive control of a turbulent boundary layer has been experimentally investigated with a view to reducing the skin-friction drag and gaining some insight into the mechanism that leads to drag reduction. A spanwise-aligned array of piezo-ceramic actuators was employed to generate a transverse travelling wave along the wall surface, with a specified phase shift between adjacent actuators. Local skin-friction drag exhibits a strong dependence on control parameters, including the wavelength, amplitude and frequency of the oscillation. A maximum drag reduction of 50 % has been achieved at 17 wall units downstream of the actuators. The near-wall flow structure under control, measured using smoke–wire flow visualization, hot-wire and particle image velocimetry techniques, is compared with that without control. The data have been carefully analysed using techniques such as streak detection, power spectra and conditional averaging based on the variable-interval time-average detection. All the results point to a pronounced change in the organization of the perturbed boundary layer. It is proposed that the actuation-induced wave generates a layer of highly regularized streamwise vortices, which acts as a barrier between the large-scale coherent structures and the wall, thus interfering with the turbulence production cycle and contributing partially to the drag reduction. Associated with the generation of regularized vortices is a significant increase, in the near-wall region, of the mean energy dissipation rate, as inferred from a substantial decrease in the Taylor microscale. This increase also contributes to the drag reduction. The scaling of the drag reduction is also examined empirically, providing valuable insight into the active control of drag reduction.

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