Visualization of pre-set vortices in boundary layer flow over wavy surface in rectangular channel

Abstract

Smoke-wire flow visualization is used to study the development of pre-set counter-rotating streamwise vortices in boundary layer flow over a wavy surface in a rectangular channel. The formation of the vortices is indicated by the vortical structures on the cross-sectional plane normal to the wavy surface. To obtain uniform spanwise vortex wavelength which will result in uniform vortex size, two types of spanwise disturbances were used: a series of perturbation wires placed prior and normal to the leading edge of the wavy surface, and a jagged pattern in the form of uniform triangles cut at the leading edge. These perturbation wires and jagged pattern induce low-velocity streaks that result in the formation of counterrotating streamwise vortices that evolve downstream to form the mushroom-like structures on the crosssectional plane of the flow. The evolution of the most amplified disturbances can be attributed to the formation of these mushroom-like structures. It is also shown that the size of the mushroom-like structures depends on the channel entrance geometry, Reynolds number, and the channel gap.

Streamwise counter-rotating vortices generated by triangular leading edge pattern in flat plate boundary layer

“…The smoke-wire apparatus consisted of a Nickel-Chromium wire diameter of 0.193 mm, a pressurized container holding paraffin oil mounted on the top of the test section and a manual traversing mechanism as described by Nadesan et al (2014) and Budiman et al (2014). In practice, the oil container was fixed on the traversing mechanism movable in two axes.…”

“…Installing perturbation wires just before the leading edge may lead to the appearance of these vortices over concave surfaces amplified by centrifugal instability (Peerhossaini and Bahri 1998;Mitsudharmadi et al 2004). However, the leading edge pattern method is a more practical way to generate streamwise counter-rotating vortices (Budiman et al 2014;Hasheminejad et al 2014). In the present work, a triangular pattern (refer to Sec.2) is used at the flat plate leading edge to examine its effect on the development of such vortices.…”

“…It is also interesting that there are similarities between this type of streamwise vortices and those produced by some other passive devices like vortex generators (Velte et al 2007;Aider et al 2008;Nadesan et al 2014) or by centrifugal forces like Görtler vortices (Winoto and Low 1991;Winoto et al 2005;Tandiono et al 2009;Budiman et al 2014). Therefore, the purpose of this work is to study, by means of flow visualization, the development of streamwise counter-rotating vortices induced by triangular pattern on the leading edge of a flat plate, as shown in Fig 1, under zero-pressure gradient condition.…”

Development of pre-set counter-rotating streamwise vortices in wavy channel

“…This method was similar to that used by Hasheminejad et al [11] and the initial channel normal gap H is 35 mm, which gives the non-dimensionalized parameter S = a/H = 0.107. The channel width is 160 mm, which is sufficient to avoid the near wall effect from the [20]. During the experiments, the measured data were low-pass filtered at 3 kHz and sampled at 6 kHz for duration of 21 seconds.…”

“…The thicker part of the boundary layer is called the "upwash" region where low momentum fluid is ejected from the surface, while the thinner part is called the "downwash" region where high momentum outer fluid move towards the wall [19]. An earlier visualization study on laminar boundary layer flow by Budiman et al [20] showed that the presence of a wavy surface could delay the breakdown of such pre-set vortices until at least the first peak of corrugation, compared with the case for a flat plate.…”