說明 
259 p 
附註 
Source: Dissertation Abstracts International, Volume: 6404, Section: B, page: 1880 

Adviser: Donald O. Rockwell 

Thesis (Ph.D.)Lehigh University, 2003 

The nearwake of a cylinder with a surface nonuniformity is addressed. A helical wire pattern about the surface of a cylinder, whereby the wire has a diameter an order of magnitude smaller than the cylinder diameter, induces substantial alterations of both the quasitwodimensional (sectional) and the threedimensional wake structure. A technique of highimagedensity particle image velocimetry is employed in order to characterize the instantaneous and averaged flow patterns in three orthogonal planes in the nearwake 

In order to provide a basis for interpretation of the wake induced by the helical wire pattern, reference states of the wake from the plain cylinder are defined over a range of Reynolds number, based on cylinder diameter, up to 10,000 

For the case of the cylinder with the helical winding, extreme cases of quasilaminar and turbulent nearwakes from the corresponding plain (smooth) cylinder are considered. Quasitwodimensional (sectional) patterns of instantaneous or phasereferenced vorticity show large variations with the effective angle of incidence of the helical wire at a given spanwise location. Extreme types of patterns involve: (i) a relatively narrow wake with lack of largescale vorticity concentrations; and (ii) a wide wake with welldefined, largescale concentrations that are formed well downstream of the cylinder. The effective width of the nearwake can vary by as much as 50%, despite the very small diameter of the helical wire. A distinctive, additional feature at Re = 104 is the occurrence of coherent patterns of smallscale, shear layer vortices, which are evident at all values of incidence angle 

Corresponding patterns of the flow structure in planes oriented along the span of the cylinder show deterministic features of threedimensionality, which are linked to the aforementioned sectional patterns. For the quasilaminar wake at low Reynolds number, the threedimensional structure of the nearwake undergoes a welldefined transformation with increasing distance from the base. Spacetime volumes of vorticity and velocity evolve from a predominantly inline form to a staggered form. For the turbulent wake at higher Reynolds number, the threedimensional patterns are less deterministic, but they show a spanwise spatial periodicity that is correlated to the helical wire pattern 

School code: 0105 

DDC 
Host Item 
Dissertation Abstracts International 6404B

主題 
Engineering, Mechanical


Physics, Fluid and Plasma


0548


0759

Alt Author 
Lehigh University

