Effects of Dissolved Gas on Unsteady Cavitating Flow Around a Clark Y-11.7% Hydrofoil

2015 ◽  
Author(s):  
Haruki Daido ◽  
Satoshi Watanabe ◽  
Shin-ichi Tsuda
Keyword(s):  
High Speed ◽  
Cavitating Flow ◽  
Gas Content ◽  
Dissolved Gas ◽  
Drag Forces ◽  
Cavitation Behavior ◽  
Lift And Drag ◽  
Super Cavity ◽  
Unsteady Cavitating Flow ◽  
Low Do

In the present study, the effects of dissolved gas content on the unsteady cavitating flow around a Clark Y-11.7% hydrofoil are investigated in a cavitation tunnel. Lift and drag forces in various cavitating conditions are directly measured by strain gauges attached on the cantilever supporting the hydrofoil. In addition, the cavitating flow is filmed from the top and the side simultaneously using two high speed video cameras. The high (roughly 6–8ppm) and low (1–2ppm) DO conditions are examined to obtain the qualitative tendencies of the effects of dissolved gas on unsteady cavitation behavior and lift/drag characteristics. It is found that that the relationship between the cavitation behavior and the lift/drag fluctuations does not qualitatively differ in the two different DO conditions, while the amplitude is slightly larger in the low DO condition. At transitional cavity oscillation, in the both DO conditions, the lift/drag coefficients increase during the growth stage of sheet/bubble cavities on the hydrofoil and they decrease when the developed super-cavity disappears. Moreover, it seems that the amplitude of the lift/drag forces in the low DO condition is larger than in the high DO condition but the frequency of lift force fluctuation is not very different.

Prediction of drag and lift forces of a high-speed vessel at full scale by CFD-based GEOSIM method

2022 ◽  
pp. 147509022110707
Author(s):  
Ugur Can ◽  
Sakir Bal
Keyword(s):  
High Speed ◽  
Full Scale ◽  
Navier Stokes ◽  
Drag Forces ◽  
Fluid Body ◽  
Unsteady Rans ◽  
Lift Forces ◽  
Drag And Lift ◽  
Lift And Drag ◽  
Model Family

In this study, it was aimed to obtain an accurate extrapolation method to compute lift and drag forces of high-speed vessels at full-scale by using CFD (Computational Fluid Dynamics) based GEOSIM (GEOmetrically SIMilar) method which is valid for both fully planing and semi-planing regimes. Athena R/V 5365 bare hull form with a skeg which is a semi-displacement type of high-speed vessel was selected with a model family for hydrodynamic analyses under captive and free to sinkage/trim conditions. Total drag and lift forces have been computed for a generated GEOSIM family of this form at three different model scales and full-scale for Fr = 0.8 by an unsteady RANS (Reynolds Averaged Navier–Stokes) solver. k–ε turbulence model was used to simulate the turbulent flow around the hulls, and both DFBI (Dynamic Fluid Body Interaction) and overset mesh technique were carried out to model the heave and pitch motions under free to sinkage/trim condition. The computational results of the model family were used to get “drag-lift ratio curve” for Athena hull at a fixed Fr number and so the corresponding results at full scale were predicted by extrapolating those of model scales in the form of a non-dimensional ratios of drag-lift forces. Then the extrapolated full-scale results calculated by modified GEOSIM method were compared with those of full-scale CFD and obtained by Froude extrapolation technique. The modified GEOSIM method has been found to be successful to compute the main forces (lift and drag) acting on high-speed vessels as a single coefficient at full scale. The method also works accurately both under fully and semi-planing conditions.

scholarly journals Water entry of spinning spheres

2009 ◽  
Vol 625 ◽  
pp. 135-165 ◽  
Author(s):  
TADD T. TRUSCOTT ◽  
ALEXANDRA H. TECHET
Keyword(s):  
High Speed ◽  
Water Entry ◽  
Video Sequences ◽  
Force Model ◽  
Translational Velocity ◽  
Oncoming Flow ◽  
Mass Ratio ◽  
Drag Forces ◽  
Lift And Drag ◽  
Force Data

The complex hydrodynamics of water entry by a spinning sphere are investigated experimentally for low Froude numbers. Standard billiard balls are shot down at the free surface with controlled spin around one horizontal axis. High-speed digital video sequences reveal unique hydrodynamic phenomena which vary with spin rate and impact velocity. As anticipated, the spinning motion induces a lift force on the sphere and thus causes significant curvature in the trajectory of the object along its descent, similar to a curveball pitch in baseball. However, the splash and cavity dynamics are highly altered for the spinning case compared to impact of a sphere without spin. As spin rate increases, the splash curtain and cavity form and collapse asymmetrically with a persistent wedge of fluid emerging across the centre of the cavity. The wedge is formed as the sphere drags fluid along the surface, due to the no-slip condition; the wedge crosses the cavity in the same time it takes the sphere to rotate one-half a revolution. The spin rate relaxation time plateaus to a constant for tangential velocities above half the translational velocity of the sphere. Non-dimensional time to pinch off scales with Froude number as does the depth of pinch-off; however, a clear mass ratio dependence is noted in the depth to pinch off data. A force model is used to evaluate the lift and drag forces on the sphere after impact; resulting forces follow similar trends to those found for spinning spheres in oncoming flow, but are altered as a result of the subsurface air cavity. Images of the cavity and splash evolution, as well as force data, are presented for a range of spin rates and impact speeds; the influence of sphere density and diameter are also considered.

NUMERICAL AND EXPERIMENTAL STUDY ON UNSTEADY SHEDDING OF PARTIAL CAVITATION

2010 ◽  
Vol 24 (13) ◽  
pp. 1441-1444 ◽  
Author(s):  
BIN JI ◽  
XIANWU LUO ◽  
YULIN WU ◽  
XIAOXING PENG ◽  
HONGYUAN XU
Keyword(s):  
High Speed ◽  
Cavitation Erosion ◽  
Great Influence ◽  
Cavitating Flow ◽  
Incoming Flow ◽  
Partial Cavitation ◽  
Cavitation Tunnel ◽  
Cavity Shedding ◽  
Good Agreement ◽  
Unsteady Cavitating Flow

Periodically unsteady shedding of partial cavity and forming of cavitation cloud have a great influence on hydraulic performances and cavitation erosion for ship propellers and hydro machines. In the present study, the unsteady cavitating flow around a hydrofoil has been calculated by using the single fluid approach with a developed cavitation mass transfer expression based on the vaporization and condensation of the fluid. The numerical simulation depicted the unsteady shedding of partial cavity, such as the process of cavity developing, breaking off and collapsing in the downstream under the steady incoming flow condition. It is noted that good agreement between the numerical results and that of experiment conducted at a cavitation tunnel is achieved. The cavitating flow field indicates that the cavity shedding was mainly caused by the re-entrant jet near cavity trailing edge, which was also clearly recorded by high-speed photographing.

Large Eddy Simulation of Unsteady Cavitating Flow Around a Highly Skewed Propeller in Nonuniform Wake

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Chao Yu ◽  
Yiwei Wang ◽  
Chenguang Huang ◽  
Xiaocui Wu ◽  
Tezhuan Du
Keyword(s):  
Numerical Simulation ◽  
High Speed ◽  
Large Scale ◽  
Cavitating Flow ◽  
Ship Wake ◽  
Factors Affecting ◽  
Eddy Simulation ◽  
Subgrid Model ◽  
Large Eddy ◽  
Unsteady Cavitating Flow

Unsteady cavitating flows around propellers become increasingly prominent on large-scale and high-speed ships, but large eddy simulations (LES) are limited in the literature. In this study, numerical simulation of an unsteady cavitating flow around a highly skewed propeller in a nonuniform wake is performed based on an explicit LES approach with k−μ subgrid model. Kunz cavitation model, volume of fluid (VOF) method, and a moving mesh scheme are adopted. The predicted evolution of the unsteady cavitating flow around a highly skewed propeller in a nonuniform ship wake is in good agreement with experimental results. An analysis of the factors affecting the cavitation on the propeller is conducted based on numerical simulation. Furthermore, the influences between cavitation structures and vortex structures are also briefly analyzed.

The dynamics of partial cavity formation, shedding and the influence of dissolved and injected non-condensable gas

2017 ◽  
Vol 829 ◽  
pp. 420-458 ◽  
Author(s):  
Simo A. Mäkiharju ◽  
Harish Ganesh ◽  
Steven L. Ceccio
Keyword(s):  
Production Rate ◽  
Void Fraction ◽  
High Speed ◽  
Volume Fraction ◽  
Cavity Flow ◽  
Gas Content ◽  
Dissolved Gas ◽  
Cavity Shedding ◽  
Visual Imaging ◽  
Set Up

In the present study, the experimental set-up of Ganesh et al. (J. Fluid Mech., vol. 802, 2016, pp. 37–78) is used to examine the dynamics of a shedding cavity by examining the vapour production rate of the natural cavity and determining how minimal injection of non-condensable gas can substantially alter the vapour production rate, the resulting cavity flow and the cavity shedding process. The influence of the dissolved gas content on the shedding natural cavity flow is also examined. High-speed visual imaging and cinemagraphic X-ray densitometry were used to observe the void fraction dynamics of the cavity flow. Non-condensable gas is injected across the span of the cavity flow at two locations: immediately downstream of the cavity detachment location at the apex of the wedge or further downstream into mid-cavity. The gas injected near the apex is found to increase the pressure near the suction peak, which resulted in the suppression of vapour formation. Hence, the injection of gas could result in a substantial net reduction in the overall cavity void fraction. Injection at the mid-cavity did less to suppress the vapour production and resulted in less significant modification of both the mean cavity pressure and net volume fraction. Changes in the cavity void fraction, in turn, altered the dynamics of the bubbly shock formation. Variation of the dissolved gas content alone (i.e. without injection) did not significantly change the cavity dynamics.

Forces Due to a Magnetic Dipole Near a Sliding Conductor: Applications to Magnetic Levitation and Bearings

1994 ◽  
Vol 116 (4) ◽  
pp. 720-725 ◽  
Author(s):  
Michelle Simone ◽  
John Tichy
Keyword(s):  
Reynolds Number ◽  
Magnetic Induction ◽  
High Speed ◽  
Eddy Currents ◽  
Skin Depth ◽  
Induction Vector ◽  
Magnetic Induction Vector ◽  
Drag Forces ◽  
Conducting Body ◽  
Lift And Drag

A conducting body moving with respect to a magnet experiences lift and drag forces from the eddy currents induced in the conductor. The force on the conductor is dependent on the relative velocity between the conductor and the magnet. In this study, we investigate the force dependence on magnetic Reynolds number, a dimensionless indicator of velocity. The Lorentz equation is used to predict the force on the conductor, given the spatial dependence of the eddy currents and magnetic induction vector inside the conductor. Maxwell’s equations, which govern the electromagnetic quantities, are reduced to a single convection-diffusion equation for the magnetic induction vector inside the conducting body. An integral solution which satisfies the governing equation and boundary conditions is used to obtain the eddy currents and magnetic field. For our model, both lift and drag forces increase sharply with Reynolds number, reach a maximum, and decrease with increasing Reynolds number to an asymptotic limit. We also find that skin depth, the depth to which the eddy currents decay inside the conductor, decreases with increasing Reynolds number. The relevance to magnetically supported high-speed vehicles and magnetic bearings is discussed.

scholarly journals Pausing after clap reduces power required to fling wings apart at low Reynolds number

2020 ◽  
Author(s):  
Vishwa T. Kasoju ◽  
Arvind Santhanakrishnan
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
High Speed ◽  
Lift Coefficient ◽  
Pause Duration ◽  
Physical Models ◽  
Power Coefficient ◽  
Average Force ◽  
Drag Forces ◽  
Lift And Drag

AbstractThe smallest flying insects such as thrips (body length < 2 mm) are challenged with needing to move in air at chord-based Reynolds number (Rec) on the order of 10. Pronounced viscous dissipation at such low Rec requires considerable energetic expenditure for tiny insects to stay aloft. Free-flying thrips flap their densely bristled wings at large stroke amplitudes, bringing both wings in close proximity of each other at the end of upstroke (‘clap’) and moving their wings apart at the start of downstroke (‘fling’). From high-speed videos of free-flying thrips, we observed that their forewings remain clapped for approximately 10% of the wingbeat cycle before start of fling. We sought to examine if there are aerodynamic advantages associated with pausing wing motion after clap and before fling at Rec=10. A dynamically scaled robotic clap-and-fling platform was used to measure lift and drag forces generated by physical models of non-bristled (solid) and bristled wing pairs for pause times ranging between 0% to 41% of the cycle. In both solid and bristled wings, varying pause time showed no effect on average force coefficients generated within each half-stroke. This was supported by nearly identical time-variation of circulation of the leading and trailing edge vortices for different pause times. At smaller pause times, bristled wings showed larger reduction of cycle-averaged drag coefficient as compared to that of solid wings. For a given wing design (solid or bristled), the ratio of cycle-averaged lift coefficient to cycle-averaged drag coefficient was unchanged across different pause times. We observed 13.5% drop in cycle-averaged power coefficient and 3% drop in cycle-averaged lift coefficient when moving from 0% pause to 9% pause duration. Our results suggest that pausing at the end of clap can be beneficial for reducing the power required to fling, with a small reduction in lift.

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