The Onset of Bubble-Ring Cavitation on Hemispherical Headforms

1982 ◽  
Vol 104 (1) ◽  
pp. 115-122
Author(s):  
B. R. Parkin
Keyword(s):  
Pressure Coefficient ◽  
Reynolds Numbers ◽  
Present Theory ◽  
Cavitation Number ◽  
Approximate Theory ◽  
Freestream Velocity ◽  
Laminar Separation ◽  
Air Bubble ◽  
Air Content ◽  
Good Agreement

An approximate theory is developed to predict the onset of cavitation on hemispherical headforms for Reynolds numbers at which laminar separation is known to occur. Insofar as it is possible, the theory is based upon first principles. Fairly good agreement is obtained between the cavitation desinence trends recently measured by Holl and Carroll and the present theory. It is also found that the onset cavitation number should be less than the magnitude of the pressure coefficient at the laminar separation point and that the cavitation number increases with freestream velocity. As long as there is an appreciable concentration of dissolved air in the water, it is also found, in agreement with experiment, that the onset of bubblering cavitation is practically independent of air content. Moreover, the observed occurrence of a lowest speed for “bubble-ring” cavitation, which is the only cavitation form considered here, and the range of “cutoff” speeds predicted by the present asymptotic theory show very encouraging agreement. The present theory suggests that this cutoff speed and its accompanying cutoff cavitation number can also depend on the temperature of the water, provided that the initial size attributed to a “typical” spherical free-stream air bubble nucleus also varies with the temperature. At 80° F (26.6°C) it is estimated that the typical nucleus from which bubble-ring cavitation originates has a radius of about seven μm. At higher temperatures the nucleus radius decreases from this value while at lower temperatures the initial radius exceeds the value noted.

Occurrence of Bubbles in a Thin Wire at Low Reynolds Number

1992 ◽  
Vol 114 (2) ◽  
pp. 255-260 ◽  
Author(s):  
K. Sato
Keyword(s):  
Separation Zone ◽  
Reynolds Numbers ◽  
Cavitation Number ◽  
The Other ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Thin Wires ◽  
Different Types ◽  
Low Reynolds ◽  
Air Diffusion

Thin wires of various diameters from 0.07 to 0.7 mm are examined about appearances and characteristics of bubble occurrence behind them in the range of low Reynolds numbers. The appearance of bubbles is very dependent on diameters of wires. Two different types of bubbles can be observed in the present experiment. One is a streamer-type bubble for smaller wires and the other is a small unspherical bubble for larger wires. The incipient and the desinent values of cavitation number also change greatly with the bubble types. The streamer-type bubble is related to the presence of laminar separation zone and the growth due to air diffusion. The small unspherical bubble can be mainly attributed to the motion of rolled-up vortices and the growth due to vaporization.

An Effect of Air Content on the Occurrence of Cavitation

1960 ◽  
Vol 82 (4) ◽  
pp. 941-945 ◽  
Author(s):  
J. W. Holl
Keyword(s):  
Pressure Coefficient ◽  
Cavitation Number ◽  
Experimental Results ◽  
Simultaneous Occurrence ◽  
Minimum Pressure ◽  
Air Content ◽  
The Difference ◽  
Dissolved Air

The simultaneous occurrence of vaporous and gaseous cavitation on hydrofoils is considered. The experimental results show that gaseous cavitation occurs at much higher ambient pressures than that for the vaporous cavitation resulting in desinent-cavitation numbers twice the minimum-pressure coefficient of the hydrofoil. The analysis indicates that the difference between the desinent-cavitation number for the gaseous cavitation and that for the vaporous cavitation is proportional to the dissolved air content and inversely proportional to the square of the velocity.

Observations of the Various Types of Limited Cavitation on Axisymmetric Bodies

1981 ◽  
Vol 103 (3) ◽  
pp. 415-424 ◽  
Author(s):  
J. W. Holl ◽  
J. A. Carroll
Keyword(s):  
Cavitation Number ◽  
Pressure Data ◽  
Roughness Effects ◽  
Laminar Separation ◽  
Fluctuating Pressure ◽  
Air Content ◽  
Speed Range ◽  
All Speeds ◽  
Axisymmetric Bodies ◽  
Flow States

Observations are presented of various types of limited cavitation on 5.1 cm diameter Schiebe, hemispherical, and DTNSRDC noses. These three noses were selected to give various flow states over the range of test speeds 6.1–21.3 m/s ranging from laminar separation for all speeds (hemispherical nose) to nonseparated flow for all speeds (Schiebe nose) with the DTNSRDC nose experiencing both separated and nonseparated flow over the speed range. The types of cavitation observed fall into two broad classes, namely transient and attached. Transient cavitation was observed in three forms, namely travelling-bubble, travelling-patch and bubble-ring. Three types of attached cavitation were observed, namely band, fixed-patch and developed. Bubble-ring and band cavitation are observed only on a body with laminar separation as noted previously by Arakeri and Acosta. Bubble-ring cavitation has been observed only on hemispherical and eighth caliber ogive noses. The desinent cavitation numbers for band and bubble-ring cavitation on the hemispherical nose are related to measured mean and fluctuating pressure data. Travelling cavitation was the most prevalent type of cavitation and the cavitation number decreased with speed and increased with air content. An analysis of travelling cavitation data from several investigations is presented. The cavitation number for fixed-patch cavitation increases with speed which suggests that surface roughness effects are involved.

scholarly journals Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

2020 ◽  
Vol 10 (20) ◽  
pp. 7350
Author(s):  
Florent Ravelet ◽  
Amélie Danlos ◽  
Farid Bakir ◽  
Kilian Croci ◽  
Sofiane Khelladi ◽  
...  
Keyword(s):  
Ambient Pressure ◽  
Reynolds Numbers ◽  
Maximum Amount ◽  
Cavitation Number ◽  
Steady Regime ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Silicon Oil ◽  
Dynamical Regimes ◽  
Dissolved Air

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from Re=800 to Re=2000. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between Re=1200 and Re=1400 for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between Re=1400 and Re=2000.

Numerical Simulations of Flow Around Wall-Mounted Square and Trapezoidal Structures at High Reynolds Numbers

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Martin Andersen ◽  
Guang Yin ◽  
Muk Chen Ong
Keyword(s):  
Turbulence Model ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Freestream Velocity ◽  
High Reynolds Numbers ◽  
Sst Turbulence Model ◽  
Layer Flow ◽  
High Reynolds ◽  
Two Sides ◽  
Good Agreement

Abstract In the present study, flow around symmetric trapezoidal wall-mounted structures with different slope angles of the two sides subjected to a boundary layer flow at Reynolds numbers of 1.19 × 105 and 1 × 106 (based on the height of the structures and the freestream velocity) is investigated using two-dimensional (2D) Reynolds-averaged Navier–Stokes (RANS) equations combined with the k − ω shear stress transport (SST) turbulence model. It is found that the drag coefficient of the wall-mounted square structures using the k − ω SST turbulence model is in good agreement with the available published experimental data. The effects of slope angles of the two sides on the hydrodynamic quantities and the flow fields around the structures have been investigated.

Viscous Effects in the Inception of Cavitation

1981 ◽  
Vol 103 (2) ◽  
pp. 280-287 ◽  
Author(s):  
V. H. Arakeri ◽  
Allan Acosta
Keyword(s):  
Fluid Flow ◽  
Pressure Coefficient ◽  
Reynolds Numbers ◽  
Viscous Effects ◽  
Laminar Separation ◽  
Wake Flows ◽  
Cavitation Inception ◽  
High Reynolds Numbers ◽  
Real Fluid ◽  
High Reynolds

The inception of cavitation in the steady flow of liquids around bodies is seen to depend upon the real fluid flow around the bodies as well as the supply of nucleating cavitation sources—or nuclei—within the fluid. A primary distinction is made between bodies having a laminar separation or not having a laminar separation. The former group is relatively insensitive to the nuclei concentration whereas the latter is much more sensitive. Except for the case of fully separated wake flows and for gaseous cavitation by diffusion the cavitation inception index tends always to be less than the magnitude of the minimum pressure coefficient and only approaches that value for high Reynolds numbers in flows well supplied with nuclei.

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

2020 ◽  
Vol 21 (6) ◽  
pp. 621
Author(s):  
Veerapathiran Thangaraj Gopinathan ◽  
John Bruce Ralphin Rose ◽  
Mohanram Surya
Keyword(s):  
Leading Edge ◽  
Reynolds Numbers ◽  
Sweep Angle ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Flow Energy ◽  
Airplane Wing ◽  
Low Reynolds ◽  
Naca 0015 ◽  
Wing Performance

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

Simulation of Unsteady Flow Around a Cylinder

2015 ◽  
Vol 3 (2) ◽  
pp. 28-49
Author(s):  
Ridha Alwan Ahmed
Keyword(s):  
Stokes Equations ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Mean Value ◽  
Navier Stokes ◽  
Cylinder Surface ◽  
Navier Stokes Equations ◽  
Flow Around A Cylinder ◽  
Numerical Grid ◽  
Good Agreement

       In this paper, the phenomena of vortex shedding from the circular cylinder surface has been studied at several Reynolds Numbers (40≤Re≤ 300).The 2D, unsteady, incompressible, Laminar flow, continuity and Navier Stokes equations have been solved numerically by using CFD Package FLUENT. In this package PISO algorithm is used in the pressure-velocity coupling.        The numerical grid is generated by using Gambit program. The velocity and pressure fields are obtained upstream and downstream of the cylinder at each time and it is also calculated the mean value of drag coefficient and value of lift coefficient .The results showed that the flow is strongly unsteady and unsymmetrical at Re>60. The results have been compared with the available experiments and a good agreement has been found between them

Heat Transfer in a Surfactant Drag-Reducing Solution—A Comparison With Predictions for Laminar Flow

2005 ◽  
Vol 128 (6) ◽  
pp. 557-563 ◽  
Author(s):  
Paul L. Sears ◽  
Libing Yang
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Reynolds Numbers ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Nusselt Numbers ◽  
Drag Reducing Additive ◽  
Good Agreement

Heat transfer coefficients were measured for a solution of surfactant drag-reducing additive in the entrance region of a uniformly heated horizontal cylindrical pipe with Reynolds numbers from 25,000 to 140,000 and temperatures from 30to70°C. In the absence of circumferential buoyancy effects, the measured Nusselt numbers were found to be in good agreement with theoretical results for laminar flow. Buoyancy effects, manifested as substantially higher Nusselt numbers, were seen in experiments carried out at high heat flux.

Predictions of the Effect of Roughness on Heat Transfer From Turbine Airfoils

1998 ◽  
Author(s):  
Anil K. Tolpadi ◽  
Michael E. Crawford
Keyword(s):  
Heat Transfer ◽  
Side Surface ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Surface Finishing ◽  
Transfer Coefficients ◽  
Average Roughness ◽  
Wide Range ◽  
Turbine Airfoils ◽  
Good Agreement

The heat transfer and aerodynamic performance of turbine airfoils are greatly influenced by the gas side surface finish. In order to operate at higher efficiencies and to have reduced cooling requirements, airfoil designs require better surface finishing processes to create smoother surfaces. In this paper, three different cast airfoils were analyzed: the first airfoil was grit blasted and codep coated, the second airfoil was tumbled and aluminide coated, and the third airfoil was polished further. Each of these airfoils had different levels of roughness. The TEXSTAN boundary layer code was used to make predictions of the heat transfer along both the pressure and suction sides of all three airfoils. These predictions have been compared to corresponding heat transfer data reported earlier by Abuaf et al. (1997). The data were obtained over a wide range of Reynolds numbers simulating typical aircraft engine conditions. A three-parameter full-cone based roughness model was implemented in TEXSTAN and used for the predictions. The three parameters were the centerline average roughness, the cone height and the cone-to-cone pitch. The heat transfer coefficient predictions indicated good agreement with the data over most Reynolds numbers and for all airfoils-both pressure and suction sides. The transition location on the pressure side was well predicted for all airfoils; on the suction side, transition was well predicted at the higher Reynolds numbers but was computed to be somewhat early at the lower Reynolds numbers. Also, at lower Reynolds numbers, the heat transfer coefficients were not in very good agreement with the data on the suction side.

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