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.

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.

Stability Enhancement of Supercavitating Projectiles by Unsteady Air Injection

2005 ◽  
Author(s):  
Yasmin Khakpour ◽  
Miad Yazdani
Keyword(s):  
Numerical Simulation ◽  
Drag Reduction ◽  
High Speed ◽  
The Body ◽  
Air Injection ◽  
Cavity Surface ◽  
The Stability ◽  
Stability Enhancement

In this work, numerical simulation is used to study the stability enhancement of high speed supercavitating hydrofoils. Although supercavitation is known as one of the most effective methods for drag reduction, producing the cavity, either by ventilation or by cavitator at front of the body, may cause some instabilities on cavity surface and thus on the projectile’s motion. Therefore removing these instabilities comes as an important point of discussion. First of all, we calculate the sources of instabilities and measure respective forces and then present some approaches that significantly reduce these instabilities. One of these methods that could produce more stable supercavities is injecting of the air into the cavity unsteadily which varies through the projectile’s surface. This approach is provided by arrays of slots distributed on the projectile’s surface and unsteady injection is modeled over the surface. Furthermore, the position of ventilation, dramatically affects the stability like those in aerodynamics. In all approaches it is assumed that the supercavity covers the whole of the body, however the forces caused by the wakes, formed behind the body are taken into account. The calculation is performed at three cavitation numbers with respective velocities of 40 m/s, 50 m/s, 60 m/s.

Wavelet Analysis on Eddy Structure in Micro Bubble Two Phase Flow Using PIV

2004 ◽  
Author(s):  
Ling Zhen ◽  
Claudia del Carmen Gutierrez-Torres
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Large Scale ◽  
Operating Conditions ◽  
Small Scale ◽  
Turbulent Structures ◽  
Two Phase ◽  
Multi Scale ◽  
Eddy Structures

The question of “where and how the turbulent drag arises” is one of the most fundamental problems unsolved in fluid mechanics. However, the physical mechanism responsible for the friction drag reduction is still not well understood. Over decades, it is found that the turbulence production and self-containment in a boundary layer are organized phenomena and not random processes as the turbulence looks like. The further study in the boundary layer should be able to help us know more about the mechanisms of drag reduction. The wavelet-based vector multi-resolution technique was proposed and applied to the two dimensional PIV velocities for identifying the multi-scale turbulent structures. The intermediate and small scale vortices embedded within the large-scale vortices were separated and visualized. By analyzing the fluctuating velocities at different scales, coherent eddy structures were obtained and this help us obtain the important information on the multi-scale flow structures in the turbulent flow. By comparing the eddy structures in different operating conditions, the mechanism to explain the drag reduction caused by micro bubbles in turbulent flow was proposed.

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.

Turbulence Structure Modification and Drag Reduction by Microbubble Injections in a Boundary Layer Channel Flow

2008 ◽  
Vol 130 (11) ◽  
Author(s):  
C. C. Gutiérrez-Torres ◽  
Y. A. Hassan ◽  
J. A. Jimenez-Bernal
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Channel Flow ◽  
Single Phase ◽  
Pure Water ◽  
Channel Height ◽  
Turbulence Structure ◽  
Two Phase ◽  
Flow Measurements ◽  
Phase Water

Turbulent boundary layer modification in a channel flow using injection of microbubbles as a means to achieve drag reduction was studied. The physical mechanism of this phenomenon is not yet fully understood. To obtain some information related to this phenomenon, single-phase (pure water) flow and two-phase (water and microbubbles) channel flow measurements are taken. The void fraction conditions were varied while maintaining a Reynolds number of 5128 based on the half channel height. The study indicates that the presence of microbubbles within the boundary layer modifies the turbulence structure such that variations in time and space turbulent scales are observed, as well as ejection and sweep phenomena.

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.

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

1994 ◽  
Vol 38 (02) ◽  
pp. 133-136
Author(s):  
Jason C. Reed
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Drag Reduction ◽  
Channel Flow ◽  
Air Injection ◽  
Surface Coatings ◽  
Laminar Channel Flow ◽  
Reduction Methods ◽  
Low Volume ◽  
Near Wall

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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