Ralph J. Volino

Sink flow boundary layers on smooth and rough walls were studied experimentally. In all cases a turbulent, zero-pressure-gradient boundary layer was subject to acceleration with K = 3.2 × 10^–6, which suppressed the turbulence in the outer region and produced conditions similar to those in turbulent sink flow cases with lower K. In the smooth-wall case, after the momentum thickness Reynolds number had dropped to about 600, the near-wall turbulence then dropped, resulting in relaminarisation. In the rough-wall cases, the near-wall turbulence was sustained in spite of the strong favourable pressure gradient, and relaminarisation did not occur. A temporary equilibrium appears to occur that is similar to that seen with lower K, in spite of the ratio of the boundary-layer thickness to the roughness height dropping to less than 5. Mean velocity and Reynolds stress profiles, quadrant analysis and turbulence spectra are used to show the development of the boundary layer in response to the pressure gradient and the differences between the rough- and smooth-wall cases. This is believed to be the first study to consider the spatial evolution of constant-K rough-wall boundary layers with K large enough to cause relaminarisation in the smooth-wall case.

The evolution of a turbulent boundary layer over truncated cones in staggered and random configurations is investigated using direct numerical simulations. Three random planform densities and one staggered, which closely match the configuration in the recent experiment by Womack et al. (J. Fluid Mech., vol. 933, 2022, A38), are considered. The experiment is also used to validate the present computations which are found to reproduce the flow measurements very accurately. Spanwise heterogeneity in the mean streamwise velocity in the form of high and low momentum pathways was detected in both the staggered and random arrangements, indicating the presence of secondary flows in the cross-stream plane. The momentum pathways found in the staggered arrangements are much weaker and confined near the roughness crest, while in the random arrangements they approach the edge of the boundary layer. We found a clear correlation of the momentum pathways with the leading edge of the roughness, while the correlation to the local topography was very weak for all the cases studied. Vorticity transport analysis is also employed to explore the nature of the secondary flows formed, highlighting the strong presence of vortex stretching near the leading edge of the roughness. The impact of the momentum pathways on the flow statistics, especially in the random arrangements, is demonstrated by the breakdown of outer-layer similarity.

Measurements were made in rough-wall boundary layers subject to favourable, zero and adverse pressure gradients. Profiles of mean velocity and turbulence quantities were acquired and velocity fields were measured in multiple planes to document flow structure. Comparisons were made to equivalent smooth-wall cases with the same free stream velocity distributions. Outer layer similarity was observed between the rough- and smooth-wall cases in all quantities in the favourable and zero pressure gradient regions, but large differences were observed with adverse pressure gradients. In both the smooth- and rough-wall cases, the favourable pressure gradient reduced the turbulence in the boundary layer, and increased the size of turbulence structures relative to the boundary layer thickness in both the streamwise and spanwise directions, while lowering their inclination angle with respect to the wall. When the boundary layer was returned to a zero pressure gradient following the favourable pressure gradient region, the turbulence level and the size and inclination of the structures returned to their canonical zero pressure gradient condition. The response of the boundary layer was somewhat faster in the rough-wall case, causing it to reach equilibrium in a shorter streamwise distance after the changes in pressure gradient than in the smooth-wall case. The adverse pressure gradient increased turbulence levels relative to the wall friction velocity, reduced the size of turbulence structures relative to the boundary layer thickness and increased their inclination angle. The changes with the adverse pressure gradient were significantly larger with the rough wall than the smooth. The results suggest that similarity might be achieved with adverse pressure gradients if smooth- and rough-wall cases with the same Clauser pressure gradient parameter history are compared.

Film cooling experiments were conducted on a flat wall subject to favorable and adverse pressure gradients with a constant acceleration parameter, K. The test wall included a single row of five round holes in line with the flow direction and inclined at 35 deg to the surface. The hole spacing was 3 diameters. The wall opposite the test wall was moveable and was set to angles with respect to the test wall that produced K values of −0.5 × 10−6, 0, 1 × 10−6, 2 × 10−6, 2.5 × 10−6, and 3 × 10−6. Blowing ratios of 0, 0.5, 1, and 1.5 were tested at each acceleration. The test wall was equipped with constant flux surface heaters, and data were acquired for each flow condition with the wall both heated and unheated. An infrared camera was used to record wall temperature in a region spanning the three center holes and extending 20D downstream of the holes. From these measurements, local film cooling effectiveness and heat transfer coefficients were determined. In the flow, velocity and temperature profiles were acquired using hot-wire anemometry and a traversing thermocouple probe. Particle image velocimetry was used to acquire velocity fields in a plane perpendicular to the flow direction and 10D downstream of the holes. The pressure gradient had a noticeable effect on the flow, with the favorable pressure gradient reducing liftoff and moving the film cooling jets closer to the wall and reducing turbulence levels in the boundary layer. The adverse pressure gradient had the opposite effect. Near the wall, however, the effects of the pressure gradient on the film cooling effectiveness and heat transfer were more subtle.