Measurements in a Transitional Boundary Layer With Görtler Vortices

Abstract

The laminar-turbulent transition process has been documented in a concave-wall boundary layer subject to low (0.6%) free-stream turbulence intensity. Transition began at a Reynolds number, Rex (based on distance from the leading edge of the test wall), of 3.5×105 and was completed by 4.7×105. The transition was strongly influenced by the presence of stationary, streamwise, Görtler vortices. Transition under similar conditions has been documented in previous studies, but because concave-wall transition tends to be rapid, measurements within the transition zone were sparse. In this study, emphasis is on measurements within the zone of intermittent flow. Twenty-five profiles of mean streamwise velocity, fluctuating streamwise velocity, and intermittency have been acquired at five values of Rex, and five spanwise locations relative to a Görtler vortex. The mean velocity profiles acquired near the vortex downwash sites exhibit inflection points and local minima. These minima, located in the outer part of the boundary layer, provide evidence of a “tilting” of the vortices in the spanwise direction. Profiles of fluctuating velocity and intermittency exhibit peaks near the locations of the minima in the mean velocity profiles. These peaks indicate that turbulence is generated in regions of high shear, which are relatively far from the wall. The transition mechanism in this flow is different from that on flat walls, where turbulence is produced in the near-wall region. The peak intermittency values in the profiles increase with Rex, but do not follow the “universal” distribution observed in most flat-wall, transitional boundary layers. The results have applications whenever strong concave curvature may result in the formation of Görtler vortices in otherwise 2-D flows. Because these cases were run with a low value of free-stream turbulence intensity, the flow is not a replication of a gas turbine flow. However, the results do provide a base case for further work on transition on the pressure side of gas turbine airfoils, where concave curvature effects are combined with the effects of high free-stream turbulence and strong streamwise pressure gradients, for they show the effects of embedded streamwise vorticity in a flow that is free of high-turbulence effects.

Development of boundary-layer flow in the presence of forced wavelength Görtler vortices

“…29 A tilting of the vortices that causes a local rise in intermittency above the near wall values is suspected to be the cause of the inflection point in the mean velocity profile and seems to be the source of turbulence production. 38 Volino and Simon 38 suggested that the instability which causes the tilting of the Görtler vortices by producing a region of high mean velocity gradient near the inflection point may be driving the transition process in this flow.…”

Transition to Turbulence Under Low‐Pressure Turbine Conditions

“…In order to take a fundamental look at turbomachinery transition, recent experiments have been conducted in somewhat simplified geometries which represent certain aspects of the low‐pressure turbine flows. These include the works of Blair40, Baughn et al 41, Qiu & Simon42, Simon et al 43, Sohn et al 44, Murawski et al 45, Jonas46, Funazaki & Kitazawa47, Matsubara et al 48, Boyle et al 49, Volino & Simon50–52, Boyle & Simon53, Volino & Hultgren54, Hatman & Wang55,56, Chakka & Schobeiri57, Funazaki et al 58,59 and Schobeiri et al 60. The Volino & Simon study was on a curved surface with various pressure gradients imposed.…”

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

“…At the second peak, more inflection point is found, that is, at approximately y = 0.6 mm. The more inflection points seem to trigger the breakdown of vortex structures [28]. Figure 6(c) shows that the maximum growth of disturbance amplitude ߢ ௨,௫ decreases at the flat entrance plate before slightly increases towards the first peak and decreases once more as the flow moves further downstream.…”