scholarly journals Cavitation Number As a Function of Disk Cavitator Radius: A Numerical Analysis of Natural Supercavitation

2019 ◽  
Author(s):  
Reid Prichard ◽  
Wayne Strasser ◽  
Thomas Eldredge
Keyword(s):  
Numerical Analysis ◽  
Drag Coefficient ◽  
Skin Friction ◽  
Cavitation Number ◽  
Maximum Speed ◽  
Friction Drag ◽  
Disk Cavitator ◽  
Density Of Water

Abstract Due to the greater viscosity and density of water compared to air, the maximum speed of underwater travel is severely limited compared to other methods of transportation. However, a technology called supercavitation — which uses a disk-shaped cavitator to envelop a vehicle in a bubble of steam — promises to greatly decrease skin friction drag. While a large cavitator enables the occurrence of supercavitation at low velocities, it adds substantial drag at higher speeds. Based on CFD results, we propose a new relationship between drag coefficient and disk cavitator radius, and we predict the optimum cavitator radius for a particular torpedo design.

Log Boom Drag Measurement at Sea

1994 ◽  
Vol 31 (02) ◽  
pp. 145-148
Author(s):  
Sheldon I. Green ◽  
John R. Garfitt ◽  
G. Glenn Young
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Skin Friction ◽  
Wave Drag ◽  
Operating Conditions ◽  
Small Scale ◽  
Friction Drag ◽  
Drag Measurement

Measurements of the drag on a typical (52-section) log boom were made under calm conditions at sea. The drag coefficient based on planform area is essentially independent of Reynolds number over typical operating conditions, cD = 0.0083 + 0.0006. Engineering calculations suggest that the log boom drag is caused primarily by skin friction drag and that wake drag is slightly less important; wave drag is entirely insignificant. A small-scale (4-section) log boom was constructed to test the influence of log arrangement within the boom on boom drag. Neither aligning the front row of bundles transversely nor covering boom sections with an underwater shroud had a significant impact on the boom drag.

The Optimum Design of a Cavitator for High-Speed Axisymmetric Bodies in Partially Cavitating Flows

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
I. Rashidi ◽  
Mo. Passandideh-Fard ◽  
Ma. Pasandideh-Fard
Keyword(s):  
Boundary Element Method ◽  
Numerical Simulations ◽  
Drag Coefficient ◽  
Boundary Element ◽  
Cavitation Number ◽  
The Body ◽  
Total Drag ◽  
Conical Section ◽  
Disk Cavitator ◽  
Element Method

In this paper, the partially cavitating flow over an axisymmetric projectile is studied in order to obtain the optimum cavitator such that, at a given cavitation number, the total drag coefficient of the projectile is minimum. For this purpose, the boundary element method and numerical simulations are used. A large number of cavitator profiles are produced using a parabolic expression with three geometric parameters. The potential flow around these cavitators is then solved using the boundary element method. In order to examine the optimization results, several cavitators with a total drag coefficient close to that of the optimum cavitators are also numerically simulated. Eventually, the optimum cavitator is selected using both the boundary element method and numerical simulations. The effects of the body radius and the length of the conical section of the projectile on the shape of the optimized cavitator are also investigated. The results show that for all cavitation numbers, the cavitator that creates a cavity covering the entire conical section of the projectile with a minimum total drag coefficient is optimal. It can be seen that increasing the cavitation number causes the optimum cavitator to approach the disk cavitator. The results also show that at a fixed cavitation number, the increase in both the radius and length of the conical section causes the cavitator shape to approach that of the disk cavitator.

INVESTIGATIONS ON THE DRAG REDUCTION OF HIGH-SPEED NATURAL SUPERCAVITATION BODIES

2009 ◽  
Vol 23 (03) ◽  
pp. 405-408 ◽  
Author(s):  
WENJUN YI ◽  
JUNJIE TAN ◽  
TIANHONG XIONG
Keyword(s):  
Aspect Ratio ◽  
Drag Reduction ◽  
Drag Coefficient ◽  
High Speed ◽  
Cavitation Number ◽  
Drag Coefficients ◽  
Supercavitating Flow ◽  
Disk Cavitator ◽  
Reduction Characteristics ◽  
Flow Experiments

In order to investigate the drag reduction characteristics of a high-speed body with supercavitation shape, four types of typical disk cavitator models with different parameters were designed and tested. By measuring the velocity decrease histories during supercavitating flow experiments, the average drag coefficients were determined, which allows analysis and comparison of the influence of cavitator diameter, projectile aspect ratio, and cavitation number on the drag reduction. Based on the experimental results, numerical simulation of the drag reduction of supercavitation body was also carried out using a commercial software FLUENT6.2, and the results obtained agree well with the experimental data. Moreover, it is shown that the drag coefficients of the four bodies are in inverse ratio to the head area of cavitator when operating under natural supercavitating flow condition, and the smaller drag coefficient can be obtained by increasing the slender ratio of the bodies. Therefore, higher aspect ratio reduces drag coefficient, with the reduction of more than 95% under certain condition of cavitation number and supercaviation shape.

scholarly journals Reduction of turbulent skin-friction drag by passively rotating discs

2021 ◽  
Vol 923 ◽  
Author(s):  
Paolo Olivucci ◽  
Daniel J. Wise ◽  
Pierre Ricco
Keyword(s):  
Skin Friction ◽  
Friction Drag ◽  
Rotating Discs

Abstract

Dynamics of a Supported Cylinder in Axial Flow

2011 ◽  
Vol 99-100 ◽  
pp. 1059-1062
Author(s):  
Ji Duo Jin ◽  
Ning Li ◽  
Zhao Hong Qin
Keyword(s):  
Nonlinear Dynamics ◽  
Numerical Analysis ◽  
Numerical Simulations ◽  
Nonlinear Model ◽  
Dynamical Behavior ◽  
Axial Flow ◽  
Viscous Force ◽  
Friction Drag ◽  
Flutter Instability ◽  
Nonlinear Force

The nonlinear dynamics are studied for a supported cylinder subjected to axial flow. A nonlinear model is presented for dynamics of the cylinder supported at both ends. The nonlinear terms considered here are the quadratic viscous force and the structural nonlinear force induced by the lateral motions of the cylinder. Using two-mode discretized equation, numerical simulations are carried out for the dynamical behavior of the cylinder to explain the flutter instability found in the experiment. The results of numerical analysis show that at certain value of flow velocity the system loses stability by divergence, and the new equilibrium (the buckled configuration) becomes unstable at higher flow leading to post-divergence flutter. The effect of the friction drag coefficients on the behavior of the system is investigated.

Effects of the air layer of an idealized superhydrophobic surface on the slip length and skin-friction drag

2016 ◽  
Vol 790 ◽  
Author(s):  
Taeyong Jung ◽  
Haecheon Choi ◽  
John Kim
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Superhydrophobic Surface ◽  
Slip Length ◽  
Joint Probability ◽  
Turbulent Channel Flow ◽  
Water Interface ◽  
Friction Drag ◽  
Recirculating Flow ◽  
Air Water Interface

The anisotropy of the slip length and its effect on the skin-friction drag are numerically investigated for a turbulent channel flow with an idealized superhydrophobic surface having an air layer, where the idealized air–water interface is flat and does not contain the surface-tension effect. Inside the air layer, both the shear-driven flow and recirculating flow with zero net mass flow rate are considered. With increasing air-layer thickness, the slip length, slip velocity and percentage of drag reduction increase. It is shown that the slip length is independent of the water flow and depends only on the air-layer geometry. The amount of drag reduction obtained is in between those by the empirical formulae from the streamwise slip only and isotropic slip, indicating that the present air–water interface generates an anisotropic slip, and the streamwise slip length ($b_{x}$) is larger than the spanwise one ($b_{z}$). From the joint probability density function of the slip velocities and velocity gradients at the interface, we confirm the anisotropy of the slip lengths and obtain their relative magnitude ($b_{x}/b_{z}=4$) for the present idealized superhydrophobic surface. It is also shown that the Navier slip model is valid only in the mean sense, and it is generally not applicable to fluctuating quantities.

scholarly journals Aviation Impacts on Fuel Efficiency of a Future More Viscous Atmosphere

2020 ◽  
Vol 101 (10) ◽  
pp. E1761-E1780
Author(s):  
Diandong Ren ◽  
Rong Fu ◽  
Robert E. Dickinson ◽  
Lance M. Leslie ◽  
Xingbao Wang
Keyword(s):  
Skin Friction ◽  
Climate Models ◽  
Climate Model ◽  
Fuel Efficiency ◽  
Aviation Fuel ◽  
Vertical Shear ◽  
Friction Drag ◽  
Dominant Term ◽  
Warming Climate ◽  
Additional Fuel

AbstractAircraft cruising near the tropopause currently benefit from the highest thermal efficiency and the least viscous (sticky) air, within the lowest 50 km of Earth’s atmosphere. Both advantages wane in a warming climate, because atmospheric dynamic viscosity increases with temperature, in synergy with the simultaneous engine efficiency reduction. Here, skin friction drag, the dominant term for extra aviation fuel consumption in a future warming climate, is quantified by 34 climate models under a strong emissions scenario. Since 1950, the viscosity increase at cruising altitudes (∼200 hPa) reaches ∼1.5% century‒1, corresponding to a total drag increment of ∼0.22% century‒1 for commercial aircraft. Meridional gradients and regional disparities exist, with low to midlatitudes experiencing greater increases in skin friction drag. The North Atlantic corridor (NAC) is moderately affected, but its high traffic volume generates additional fuel cost of ∼3.8 × 107 gallons annually by 2100, compared to 2010. Globally, a normal year after 2100 would consume an extra ∼4 × 106 barrels per year. Intermodel spread is <5% of the ensemble mean, due to high inter–climate model consensus for warming trends at cruising altitudes in the tropics and subtropics. Because temperature is a well-simulated parameter in the IPCC archive, with only a moderate intermodel spread, the conclusions drawn here are statistically robust. Notably, additional fuel costs are likely from the increased vertical shear and related turbulence at NAC cruising altitudes. Increased flight log availability is required to confirm this apparent increasing turbulence trend.

The Effect of Freshwater Biofilms on Skin Friction Drag

2009 ◽  
pp. 1940-1945
Author(s):  
Jessica Andrewartha ◽  
Jane Sargison ◽  
Alan Henderson ◽  
Kate Perkins ◽  
Greg Walker
Keyword(s):  
Skin Friction ◽  
Friction Drag ◽  
Freshwater Biofilms
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