Study of Drag Forces on a Designed Surface in Bubbly Water Lubrication Using Electrolysis

2006 ◽  
Vol 128 (6) ◽  
pp. 1383-1389 ◽  
Author(s):  
Haosheng Chen ◽  
Jiang Li ◽  
Darong Chen
Keyword(s):  
Drag Reduction ◽  
Side Wall ◽  
Water Electrolysis ◽  
Disk Surface ◽  
Viscous Drag ◽  
Fluid Viscosity ◽  
Bubble Behavior ◽  
Pressure Drag ◽  
Drag Forces ◽  
Reduction Effect

To study the drag reduction effect of a bubbly fluid, a pin-disk experiment is performed on the Universal Micro Tribotester system. Bubbles are generated by water electrolysis in holes that are specially designed on the disk surface. Experiment result shows that the drag force experiences a dynamic process, both drag reduction and drag increment effects appear in the process depending on the bubble behavior. This process is numerically simulated using computational fluid dynamics (CFD), and the explanations for the drag variation are given based on the analysis of drag forces on each wall of the disk surface. The drag reduction occurs when the bubble fills the hole, as the viscous drag on the air-liquid surface is small, and the pressure drag is reduced as the side wall of the hole is covered by the bubble. The drag increment is thought to be caused by the increment of the fluid viscosity when the bubble leaves the hole and flows in the fluid.

Numerical Analysis of Drag Reduction Characteristics of Biomimetic Puffer Skin: Effect of Spinal Height and Tilt Angle

2021 ◽  
Vol 21 (9) ◽  
pp. 4615-4624
Author(s):  
Hong-Gen Zhou ◽  
Chang-Feng Jia ◽  
Gui-Zhong Tian ◽  
Xiao-Ming Feng ◽  
Dong-Liang Fan
Keyword(s):  
Drag Reduction ◽  
Tilt Angle ◽  
Fluid Velocity ◽  
Viscous Drag ◽  
Total Drag ◽  
Pressure Drag ◽  
Reduction Efficiency ◽  
Distribution Parameters ◽  
Reduction Effect ◽  
Reduction Characteristics

Based on the migratory phenomenon of the puffer and the cone-shaped structures on its skin, the effects of spinal height and tilt angle on the drag reduction characteristics is presented by numerical simulation in this paper. The results show that the trend of total drag reduction efficiency changes from slow growth to a remarkable decline, while the viscous drag reduction efficiency changes from an obvious increase to steady growth. The total and viscous drag reduction efficiencies are 19.5% and 31.8%, respectively. In addition, with the increase in tilt angle, the total drag reduction efficiency decreases gradually; the viscous drag reduction efficiency first increases and then decreases, finally tending to be stable; and the total and viscous drag reduction efficiency reaches 20.7% and 26.7%, respectively. The flow field results indicate that the pressure drag mainly originates at the front row of the spines and that the total pressure drag can be effectively controlled by reducing the former pressure drag. With the increase in low-speed fluid and the reduction in the near-wall fluid velocity gradient, the viscous drag can be weakened. Nevertheless, the drag reduction effect is achieved only when the decrement of viscous drag is greater than the increment of pressure drag. This work can serve as a theoretical basis for optimizing the structure and distribution parameters of spines on bionic non-smooth surfaces.

On the Effects of Aero Boundary Layer Control on Pressure Drag Reduction in Supercavitating Bodies

2005 ◽  
Author(s):  
Yasmin Khakpour ◽  
Miad Yazdani
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Pressure Distribution ◽  
Constant Pressure ◽  
Viscous Drag ◽  
Cavity Surface ◽  
Gradient Flows ◽  
Supercavitating Vehicles ◽  
Pressure Drag ◽  
Layer Control

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we’ll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present some approaches by which this probable separation can be controlled. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to delay probable separation and so significant pressure drag reduction is achieved. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered. Moreover, electromagnetic forces cause to delay or even prevent separation in high pressure gradient flows and interesting results obtained in this regard shows significant drag reduction in supercavitating vehicles.

Turbulent Drag Reduction Effect by Hydrogen and Oxygen Microbubbles Made by Electrolysis

2006 ◽  
Author(s):  
Xinlin Lu ◽  
Hiroharu Kato ◽  
Takafumi Kawamura
Keyword(s):  
Drag Reduction ◽  
Void Fraction ◽  
Bubble Diameter ◽  
Water Electrolysis ◽  
Air Injection ◽  
Air Bubbles ◽  
Turbulent Drag Reduction ◽  
Circulating Water ◽  
Turbulent Drag ◽  
Reduction Effect

Turbulent drag reduction by very small hydrogen microbubbles was investigated experimentally. The method for generating microbubbles of 10–60 μm by water electrolysis was established firstly. Experiments were carried out using a circulating water tunnel, and it was observed that the small microbubbles generated by electrolysis can achieve the same drag reduction as the injected air bubbles at much lower void fraction. The distribution of microbubble was examined using the microscope photography. The peak of local void fraction was found to be very close to the wall, while no correlation was found between the average bubble diameter and the distance from the channel wall. The present experimental results suggest that the very small microbubbles produced by electrolysis are 10∼100 times more effective in terms of the drag reduction than large bubbles made by air injection. So it is considered that the diameters of microbubbles play an important role to drag reduction.

Separation Control of Aero Boundary Layer in Supercavitating Bodies and Its Effect on Pressure Drag Reduction

2005 ◽  
Author(s):  
Yasmin Khakpour ◽  
Miad Yazdani
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Constant Pressure ◽  
Flow Region ◽  
Air Injection ◽  
Viscous Drag ◽  
Cavity Surface ◽  
Cavitation Flow ◽  
Two Phase ◽  
Pressure Drag

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. In fact such methodologies consider cavitation flow statically, rather than taking dynamic effects of overall flow into account. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Thereby, in order to configure the system conditions which may be cause of cavity perturbation and so system oscillation, we need to use proper methodologies in which turbulence shear stress effects and role of their distribution, are suitably come into account. Numerical simulation of supercavitating flows is pursued in this paper. The effect of air injection in the cavity as a means of stabilization is examined. A k-epsilon model is employed for the liquid flow region while a single-fluid two phase model is applied in the cavity region. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to prevent the probable separation and so significant pressure drag reduction up to 70% is observed. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered.

An Examination of Trapped Bubbles for Viscous Drag Reduction on Submerged Surfaces

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Kelly A. Stephani ◽  
David B. Goldstein
Keyword(s):  
Drag Reduction ◽  
Flat Plate ◽  
Tap Water ◽  
Water Environment ◽  
Film Coating ◽  
Force Balance ◽  
Viscous Drag ◽  
Vertical Orientation ◽  
Drag Forces ◽  
Horizontal Orientation

Viscous drag reduction on a submerged surface can be obtained both in the limit of an unbroken gas film coating the solid and in the nanobubble or perhaps microbubble coating regime when an air layer is created with superhydrophobic coatings. We examine an intermediate bubble size regime with a trapped-bubble array (TBA) formed in a tap water environment using electrolysis to grow and maintain bubbles in thousands of millimeter-sized holes on a solid surface. We show that even though surface tension is sufficient to stabilize bubbles in a TBA against hydrostatic and shear forces beneath a turbulent boundary layer, no drag reduction is obtained. Drag measurements were acquired over Reynolds numbers based on plate length ranging from 7.2×104<ReL<3.1×105 using either a force balance for plates mounted in a vertical orientation, or by performing a momentum integral balance using a wake survey for a flat plate mounted in either vertical or horizontal orientation. In that the drag forces were small, emphasis was placed on minimizing experimental uncertainty. For comparison, the flow over a flat plate covered on one side by a large uninterrupted gas film was examined and found to produce large drag reductions of up to 32%.

scholarly journals Numerical Study of Effect of Sawtooth Riblets on Low-Reynolds-Number Airfoil Flow Characteristic and Aerodynamic Performance

Processes ◽  
2021 ◽  
Vol 9 (12) ◽  
pp. 2102
Author(s):  
Xiaopei Yang ◽  
Jun Wang ◽  
Boyan Jiang ◽  
Zhi’ang Li ◽  
Qianhao Xiao
Keyword(s):  
Reynolds Number ◽  
Drag Reduction ◽  
Numerical Study ◽  
Low Reynolds Number ◽  
Aerodynamic Performance ◽  
Operating Conditions ◽  
Viscous Drag ◽  
Airfoil Flow ◽  
Low Reynolds ◽  
Reduction Effect

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

Experimental Study of Air Layer Sustainability for Frictional Drag Reduction

2014 ◽  
Vol 58 (01) ◽  
pp. 30-42 ◽  
Author(s):  
Bhat Nikhil Jagdish ◽  
Tay Zhi Xian Brandon ◽  
Tiaw Joo Kwee ◽  
Arun Kr. Dev
Keyword(s):  
Drag Reduction ◽  
Experimental Studies ◽  
Hydrophobic Surface ◽  
Viscous Drag ◽  
Total Resistance ◽  
Frictional Drag ◽  
Form Drag ◽  
Friction Resistance ◽  
Slow Steaming ◽  
Pressure Drag

Frictional drag reduction by microbubbles is a promising engineering method for reducing ship fuel consumption, especially for large, slow steaming vessels. Total resistance can be broken down into frictional drag and form drag (also known as pressure drag or profile drag). Ship's hull form optimization is commonly to reduce the form drag of a ship. Another technique would be required to deal with the frictional (viscous) portion of the total resistance. One such technique that reduces the friction resistance is the air lubrication technique. This research looks at possible enhancement for the microbubbles drag reduction technique with the use of hydrophobic plates to trap and retain an air layer. The hydrophobic surface cannot sustain bubbles by itself. Laser-machined microstructure coupled with hydrophobic coatings allows the rapid formation of air layer rapidly and sustainability of the air layer is recorded. With extensive experimental studies, we have shown that an air layer can be entrained around a moving flat plate thereby reducing friction. This could pave the way for applying this technique around the wall of moving ship hulls thereby minimizing the viscous drag and reducing the shipping costs.

Research on Automobile Aerodynamic Drag Reduction Based on Isobaric Surface of a Blunt Body

2013 ◽  
Vol 328 ◽  
pp. 634-638
Author(s):  
Xing Jun Hu ◽  
Lei Liao ◽  
Xiu Cheng Li ◽  
Chang Hai Yang ◽  
Peng Guo ◽  
...  
Keyword(s):  
Drag Reduction ◽  
Blunt Body ◽  
Aerodynamic Drag ◽  
Simulation Method ◽  
The Body ◽  
Viscous Drag ◽  
Isobaric Surface ◽  
Pressure Drag ◽  
Aerodynamic Drag Reduction ◽  
The Relationship

This paper focuses on a new method of aerodynamic drag reduction. In this paper numerical simulation method is adopted to investigate the relationship between the aerodynamic drag characteristics of a blunt body and the distribution of total pressure around the body. The study shows that when the shape of a blunt body is modified to be close to its isobaric surface, the pressure drag of the body can be reduced largely while the viscous drag increases slightly, and the summary of the drag gets lower as a result. This conclusion will have profound guiding significance in the aerodynamic shape designing and the aerodynamic drag reduction of an automobile.

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