Vortical structures and development of laminar flow over convergent-divergent riblets

Abstract

In this work, the development of a laminar boundary layer over a rectangular convergent-divergent riblet section with a finite streamwise length is studied experimentally using dye visualization and particle image velocimetry in a water flume. The flow topology over this highly directional spanwise roughness is established from this study. It is shown that convergent-divergent riblets generate a spanwise flow above the riblets from the diverging line toward the adjacent converging line. This consequently leads to the formation of a weak recirculating secondary flow in cross-stream planes across the boundary layer that creates a downwash motion over the diverging line and an upwash motion over the converging line. It is found that the fluid inside the riblet valley follows a helicoidal path and it also interacts with the crossflow boundary layer hence playing a key role in determining the structure of the secondary flow across the boundary layer. The impact of riblet wavelength on vortical structures is also revealed for the first time. A larger riblet wavelength is seen to produce a stronger upwash/downwash and hence a more intense secondary flow as well as a stronger deceleration effect on the crossflow. Furthermore, the streamwise development of the flow over the riblet section can be divided into a developing stage followed by a developed stage. In the developing stage, the magnitude of induced streamwise velocity and vorticity over the converging line continues to increase, whereas in the developed stage the values of these parameters remain essentially unchanged.

Geometric optimization of riblet-textured surfaces for drag reduction in laminar boundary layer flows

“…With the assumption of no pressure gradient (i.e., dp/dz = 0), using Euler's equation for the inviscid flow outside the boundary layer we know that ρW∞(dW∞/dz) = −dp/dz = 0. Equation (29) can then be simplified further to relate the traction vector on the wall to the momentum area in the form of an ordinary differential equation as…”

“…This corresponded to an overall 1% reduction in the Physics of Fluids ARTICLE scitation.org/journal/phf total drag since skin friction accounted for only 80% of the total vehicle drag. 11,12 In recent years, new variations of riblet surfaces such as fractal riblets, 27 super-hydrophobic riblet surfaces, 28 as well as divergent-convergent riblet surfaces 29 have been explored in either channel flow or boundary layer flow conditions. In addition, a number of investigations have been expanding the use of riblet surfaces in radial turbine rotors, 30 or on the wings of a wind turbine.…”

Flow separation control over a rounded ramp with spanwise alternating wall actuation

“…As Townsend [28] proposed, the large-scale secondary motions generated as previously mentioned could be sustained due to the transverse gradients of the imposed stress on the bounding surface. Recently, Xu et al [29] investigated the development of a laminar boundary layer over a rectangular convergent-divergent riblet section in a water flume. They found that the fluid inside the riblet valley follows a helicoidal path and interacts with the crossflow playing an important role in determining the structure of the secondary flow.…”

“…Many experiments and numerical simulations have been devoted to spanwise heterogeneity control, but most of the research is limited to simple geometries, such as a flat plate boundary layer [8][9][10]16,17,24,26,30,31] or planar channel flow [22,25,29,[32][33][34]. The study of spanwise-heterogeneity small-scale control in separated flows has never been reported to the authors' knowledge.…”

Daralan ve Genişleyen Bir Kanal İçerisine Yerleştirilen Yarım Küreler Üzerinden Akış ve Isı Transferinin Sayısal Olarak Araştırılması

“…Daralan ve genişleyen kanalların sınır tabakasını önemli ölçüde değiştirdiğini ve Nusselt sayısının girdap oluşumu ve basınç düşüşü artışına karşın paralel plakalı kanallardan yaklaşık %400 daha fazla olduğunu belirlemişlerdir. Xu ve arkadaşları [13], sonlu bir akış uzunluğu ile dikdörtgen yakınsak ıraksak oluk kesiti üzerinde bir laminar sınır tabakasının geliştirilmesini bir su kanalında boya görselleştirme ve parçacık görüntü hızını kullanarak deneysel olarak incelenmiştir. Bu çok yönlü yayılma pürüzlülüğünün üzerindeki akış topolojisi bu çalışmadan belirlenmiştir.…”