Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
Currently Skimming:
Numerical Investigation on the Turbulent and Vortical Flows Beneath the Free Surface Around Struts
Numerical Investigation on the Turbulent and Vortical Flows Beneath the Free Surface Around Struts
Pages 794-811
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 794... ...
The proposed LES method can reveal the existence of the free surface turbulence which is observed in experiments prior to the overturning waves. ~ Introduction Free surface flow around a surface piercing blunt bow is one of the most complicated phenomena in the field of ship hydrodynamics because of various nonlinear characteristics.
|
From page 795... ...
The role of these disturbances might be a kind of a trigger for the transition to the turbulent flow and it might be assumed as a source that maintains the turbulence on the free surface. Main objective of the present study is to make clear the characteristics of the flow at the early stage of bow wave breaking around surface piercing struts, especially the effect of the vorticity generated on the free surface on the vertical and turbulent flows beneath the free surface.
|
From page 796... ...
, the wave front appears clearly and the position moves away from the bow(Fig.5~. At those Froude numbers, no significant features of turbulence appear on the free surface.
|
From page 797... ...
tie showed that the elliptic strut generated more intensive wrinkle, which was a kind of free surface instability, than that generated by the circular cylinder at the same speed although the necklace vortex of the circular cylinder was more intensive than that of the elliptic strut. He explained that the reason might be the different free surface curvatures in front of the bows.
|
From page 798... ...
where ti is unit tangential vector to the free surface. Finally, the following equations can be used as a dyn~c free surface boundary condition assuming that the normal component of the viscous and Reynolds stresses are negligibly small.
|
From page 799... ...
In the present study, nonslip condition is used for the velocity while the wave elevation on the body is linearly extrapolated using neighboring wave heights calculated by the kinematic free surface boundary condition. The pressure in this singular region is obtained by the dynamic free surface boundary condition directly.
|
From page 800... ...
no-shearing stress Fig.9 Computed velocity and vort~city~wy) d"tribut~ons for two different treatment of dynamic free surface boundary condition; NS05, Fn=O.8O, Rn=5000, T=15.0 (contour inter" val=10.OJ.
|
From page 801... ...
Although the wave Dont has a larger curvature than that of the concave surface, the vorticity is smaller at the wave front because the streamwise velocity becomes smaller Mound there. This is the reason why the counter-clockwise vorticity occupies the region in front of the bow.
|
From page 802... ...
generates the most intensive vorticity which is induced by the free surface curvature. The peaks of the vorticity are located beneath the free surface around the wave front for each models except NS24.
|
From page 803... ...
From these results, It can be concluded that the bow shape has strong relation with the free surface curvature, especially concave shape, which is responsible for the vertical flows around the bow. I.0 1.0 5.3 Grid dependency Fig.15 shows the effect of grid density in the vertical direction around the free surface for NS12 at Rn=lOs and Fn=0.25.
|
From page 804... ...
Although the situation is a little different in case of the free surface flow, there exists boundary layer on the free surface which may induce the free surface turbulence. As shown in Fig.6, the free surface flows of NS08 and NS12 are quite complicated.
|
From page 805... ...
At point D, the amplitude of the fluctuations of the velocity components becomes larger and the u-component ~ less than zero. This result indicates that the free surface flows at point D is turbulent and reverse flows exist there.
|
From page 806... ...
Fig.20 Computed time histories of velocity components at sin points en the free surface; NS12, Fn=O.
|
From page 807... ...
who measured the velocity components in front of a elliptic strut and a circular cylinder under the sub-breakirlg condition. Fig.22 shows the computed Reynolds stress componerlts at point D
|
From page 808... ...
Surface Shear Flow related to Bow Wave-Breaking of Full Ship Models, Journal of the Society of Naval Architects of Japan, Vol.149, pp.11-20, 1981. [73 Patel, V
|
From page 809... ...
t22] Hinatsu, M.: Numerical Simulation of Unsteady Viscous Nonlinear Waves Using Moving Grid System Fitted on a Free Surface, Journal of the Kansai Society of Naval Architects of Japan, No.217, pp.1-11, 1992.
|
From page 810... ...
Successful implementation of LES for a turbulent flow with no free boundary such as a solid wall is achieved through a special modification of SGS modeling near wall. In the present simulation for a free-surface turbulence, Smagorinsky's closure is applied without a 810
|
From page 811... ...
Of course, it is important to develop some modification of the SGS and MBL models for freesurface turbulence.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.