Sergiy Gerashchenko

Lagrangian measurements of inertial particle accelerations in a turbulent boundary layer

Two-dimensional Lagrangian acceleration statistics of inertial particles in a turbulent boundary layer with free-stream turbulence are determined by means of a particle tracking technique using a high-speed camera moving along the side of the wind tunnel at the mean flow speed. The boundary layer is formed above a flat plate placed horizontally in the tunnel, and water droplets are fed into the flow using two different methods: sprays placed downstream from an active grid, and tubes fed into the boundary layer from humidifiers. For the flow conditions studied, the sprays produce Stokes numbers varying from 0.47 to 1.2, and the humidifiers produce Stokes numbers varying from 0.035 to 0.25, where the low and high values refer to the outer boundary layer edge and the near-wall region, respectively. The Froude number is approximately 1.0 for the sprays and 0.25 for the humidifiers, with a small variation within the boundary layer. The free-stream turbulence is varied by operating the grid in the active mode as well as a passive mode (the latter behaves as a conventional grid). The boundary layer momentum-thickness Reynolds numbers are 840 and 725 for the active and passive grid respectively. At the outer edge of the boundary layer, where the shear is weak, the acceleration probability density functions are similar to those previously observed in isotropic turbulence for inertial particles. As the boundary layer plate is approached, the tails of the probability density functions narrow, become negatively skewed, and their peak occurs at negative accelerations (decelerations in the streamwise direction). The mean deceleration and its root mean square (r.m.s.) increase to large values close to the plate. These effects are more pronounced at higher Stokes number. In the vertical direction, there is a slight downward mean deceleration and its r.m.s., which is lower in magnitude than that of the streamwise component, peaks in the buffer region. Although there are free-stream turbulence effects, and the complex boundary layer structure plays an important role, a simple model suggests that the acceleration behaviour is dominated by shear, gravity and inertia. The results are contrasted with inertial particles in isotropic turbulence and with fluid particle acceleration statistics in a boundary layer. The background velocity field is documented by means of hot-wire anemometry and laser Doppler velocimetry measurements. These appear to be the first Lagrangian acceleration measurements of inertial particles in a shear flow.

Step Excrescence Effects for Manufacturing Tolerances on Laminar Flow Wings

Manufacturing tolerances for laminar flow wings can be significantly tighter than those of conventional aircraft. The tighter tolerances can significantly affect the assessment of the practicality of designing for laminar flow. However, existing data on the effects of excrescences typical of manufacturing process are limited. Further, information on the effects—often beneficial—of pressure gradient present on the laminar flow wings is not generally available. To address these concerns, a series of experiments has been undertaken to examine the effects of surface steps in the presence of pressure gradients. The step geometries were selected to represent those that result from actual aircraft manufacturing processes. The range of pressure gradients correspond to those typical of laminar flow wings. Initial experiments were conducted in a low-speed wind tunnel. Later experiments used a novel propelled-model test facility. The results of these studies show that the allowable sizes of surface excrescences for laminar flow wings may be significantly greater than has conventionally been assumed. This could significantly influence the more widespread use of laminar flow for drag reduction, resulting in more efficient aircraft.

On the role of gravity and shear on inertial particle accelerations in near-wall turbulence

Recent experiments in a turbulent boundary layer by Gerashchenko et al . ( J. Fluid Mech. , vol. 617, 2008, pp. 255–281) showed that the variance of inertial particle accelerations in the near-wall region increased with increasing particle inertia, contrary to the trend found in homogeneous and isotropic turbulence. This behaviour was attributed to the non-trivial interaction of the inertial particles with both the mean shear and gravity. To investigate this issue, we perform direct numerical simulations of channel flow with suspended inertial particles that are tracked in the Lagrangian frame of reference. Three simulations have been carried out considering (i) fluid particles, (ii) inertial particles with gravity and (iii) inertial particles without gravity. For each set of simulations, three particle response times were examined, corresponding to particle Stokes numbers (in wall units) of 0.9, 1.8 and 11.8. Mean and r.m.s. profiles of particle acceleration computed in the simulation are in qualitative (and in several cases quantitative) agreement with the experimental results, supporting the assumptions made in the simulations. Furthermore, by comparing results from simulations with and without gravity, we are able to isolate and quantify the significant effect of gravitational settling on the phenomenon.

An Approach to Measuring Step Excrescence Effects in the Presence of a Pressure Gradient

An experimental study was undertaken to determine the effects of step excrescences on boundary layer transition using a unique ground test facility in which the test model was propelled though still air. The models used were designed to have a nominally constant pressure gradient so that the results would be relevant to laminar flow aircraft whose wings often have long runs of mildly favorable pressure gradient. The models had an integrated continuously adjustable two-dimensional step, which could be adjusted to be forward-facing or aft-facing. The large model was used to increase the Reynolds numbers examined so that the results are applicable to laminar flow flight vehicles. Multiple measurement methods, including Preston tubes, hot wires, accelerometers, a boundary layer traverse, and static pressure taps were used to provide comparison data, and to add to the physical understanding of the results. The propelled-model test approach required that the instrumentation be self-contained and ride along with the model as the carrier vehicle moved down the test track. Due to the relatively short times available for data-taking (approximately 15-30 seconds per run), the initialization and data analysis techniques had to be tailored for this application.

Hot-Wire Measurements of the Influence of Surface Steps on Transition in Favorable Pressure Gradient Boundary Layers

An examination of the effects of surface step excrescences on boundary layer transition was performed, using a unique experimental facility. The objective of the work was to characterize the variation of transition Reynolds numbers with measurable step size and boundary layer parameters, with the specific goal of specifying new tolerance criteria for laminar flow airfoils, alongside a fundamental investigation of boundary layer transition mechanisms. This paper focuses on interpretation of hot-wire measurements, including supporting stability calculations, undertaken as part of the study. The results for both forward and aft-facing steps indicated a substantial stabilizing effect of favorable pressure gradient on excrescence-induced boundary layer transition. These findings suggest that manufacturing tolerances for laminar flow aircraft could be loosened in areas where even mild favorable pressure gradients exist.

Entrainment and mixing of water droplets across a shearless turbulent interface with and without gravitational effects

We describe experiments of the entrainment and mixing of water (sub-Kolmogorov scale) droplets across a turbulent―non-turbulent interface (TNI) as well a turbulent― turbulent interface (TTI) in shearless grid turbulence, over a time scale in which evaporation is insignificant. The flow is produced by means of a splitter plate with an active grid and water sprays on one side and screens or an active grid on the other side. The Taylor microscale Re λ on the turbulent side is 275 and the average dissipation scale Stokes number, St η ≈ 0.2, and based on the integral scale, St l ≈ 0.003. By changing the orientation of the grid system, gravitational effects may be excluded or included. We show that in the absence of gravity, for the Stokes number range studied (0.06 ≤ St η ≤ 1.33), the droplet distribution does not change across the interface. With gravity, the larger drops are selectively mixed and this is more pronounced for the TNI than for the TTI. The particle concentration distribution is an error function for the TTI but departs significantly for the TNI due to the intermittency in the flow. In terms of particle concentration, the entrainment is most efficient for the TTI with gravity. The results are related to droplet entrainment in clouds.