Karen Mulleners

Critical parameters for the partial coalescence of a droplet

The partial coalescence of a droplet onto a planar liquid-liquid interface is investigated experimentally by tuning the viscosities of both liquids. The problem mainly depends on four dimensionless parameters: The Bond number (gravity vs surface tension), the Ohnesorge numbers (viscosity in both fluids vs surface tension), and the density relative difference. The ratio between the daughter droplet size and the mother droplet size is investigated as a function of these dimensionless numbers. Global quantities such as the available surface energy of the droplet have been measured during the coalescence. The capillary waves propagation and damping are studied in detail. The relation between these waves and the partial coalescence is discussed. Additional viscous mechanisms are proposed in order to explain the asymmetric role played by both viscosities.

Dynamic stall development

Dynamic stall on an oscillating airfoil was investigated by a combination of surface pressure measurements and time-resolved particle image velocimetry. Following up on previous work on the onset of dynamic stall (Mulleners and Raffel in Exp Fluids 52(3):779–793, 2012), we combined time-resolved imaging with an extensive coherent structure analysis to study various aspects of stall development. The formation of the primary dynamic stall vortex was identified as the growth of a recirculation region and the ensuing instability of the associated shear layer. The stall development can be subdivided into two stages of primary and secondary instability with the latter being the effective vortex formation stage. The characteristic time scales associated with the primary instability stage revealed an overall decrease in dynamic stall delay with increasing effective unsteadiness of the pitching airfoil. The vortex formation stage was found to be largely unaffected by variations of the airfoil’s dynamics.

Dynamic Stall Control by Passive Disturbance Generators

Passive cylindrical disturbance generators mounted near the leading edge of an airfoil significantly improved its performance under dynamic stall conditions. Time-resolved particle image velocimetry and simultaneous pressure measurements were conducted at the midchord of a pitching airfoil equipped with passive disturbance generators. The disturbance generators were effective in reducing the strength of the dynamic stall vortex and therefore the negative pitching moment peak and hysteresis effects. When the disturbance generators were applied, the flow separation type was altered from leading- to trailing-edge stall. In contrast to the clean case, reattachment was initiated immediately after the separation reached the leading-edge region. In addition to the circular shape, also backward- and forward-wedge-shaped disturbance generators were investigated. Although the backward wedge also showed favorable results, the forward wedge was less successful. The shape of the disturbance generators appears to have ...

Flow Development on a Flat-Plate Wing Subjected to a Streamwise Acceleration

Reference EPFL-ARTICLE-228256doi:10.2514/1.J055497View record in Web of Science Record created on 2017-05-16, modified on 2017-07-19

Investigation of the unsteady flow development over a pitching airfoil by means of TR-PIV

The flow over an OA209 airfoil subjected to a sinusoidal pitching motion under dynamic stall conditions is investigated experimentally by means of time resolved particle image velocimetry (TR-PIV) and surface pressure measurements. Dynamic stall is distinguished by the formation and convection of large scale coherent structures and a delay in massive flow separation. A vortex detection scheme based on an identification function derived directly from the velocity fields is adopted to identify vortex cores. The combination of global time resolved imaging and an automated vortex identification algorithm allows for the investigation of the spatial and temporal evolution of vortical structures within a single oscillation. Furthermore, the mechanisms associated with the dynamic stall delay are considered.

Impact of surface roughness on the turbulent wake flow of a turbine blade

Roughened aeroengine blade surfaces lead to increased friction losses and reduced efficiency of the individual blades. The surface roughness also affects the wake flow of the blade and thus the inflow conditions for the subsequent compressor or turbine stage. To investigate the impact of surface roughness on a turbulent blade wake, we conducted velocity field measurements by means of stereo particle image velocimetry in the wake of a roughened turbine blade in a linear cascade wind tunnel. The turbine blade was roughened at different chordwise locations. The influence of the chordwise location of the added surface roughness was examined by comparing their impact on the width and depth of the wake and, the positions and distribution of vortical structures in the wake. Additionally, the friction loss coefficients for different surface roughness positions were estimated directly from the velocity field.

Dynamic stall of an experimental wind turbine blade

To understand the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5 MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. Proper orthogonal decomposition (POD) and vortex detection techniques are applied to the PIV fields to quantify relevant flow characteristics such as vortex size, separation angle, and separation point throughout a dynamic pitching cycle. The behavior of the POD coefficients provides time scales for the transitional stages which are quantified and compared, revealing that transition from attached flow to full stall is delayed to higher angles of attack and occurs at a higher rate than the transition from full stall to attached flow. The instantaneous flow fields are then reconstructed using the first four POD modes to demonstrate their prominent roles throughout the stall cycle and their ability to capture the general separation behavior over the blade surface

Flow field and force evolution for a symmetric hovering flat plate wing

The unsteady flow around a hovering flat-plate wing has been investigated experimentally using particle image velocimetry and direct force measurements. The measurements are conducted on a wing tha...

Resulting Aerodynamic Losses of Combinations of Localized Roughness Patches on Turbine Blades

Reference EPFL-ARTICLE-220867doi:10.2514/1.J054602View record in Web of Science Record created on 2016-08-17, modified on 2016-10-18

The role of surface vorticity during unsteady separation

Unsteady flow separation in rotationally augmented flow fields plays a significant role in a variety of fundamental flows. Through the use of time-resolved particle image velocimetry, vorticity accumulation and vortex shedding during unsteady separation over a three-dimensional airfoil are examined. The results of the study describe the critical role of surface vorticity accumulation during unsteady separation and reattachment. Through evaluation of the unsteady characteristics of the shear layer, it is demonstrated that the buildup and shedding of surface vorticity directly influence the dynamic changes of the separation point location. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic designs and has a broad impact within the field of unsteady fluid dynamics.Unsteady flow separation in rotationally augmented flow fields plays a significant role in a variety of fundamental flows. Through the use of time-resolved particle image velocimetry, vorticity accumulation and vortex shedding during unsteady separation over a three-dimensional airfoil are examined. The results of the study describe the critical role of surface vorticity accumulation during unsteady separation and reattachment. Through evaluation of the unsteady characteristics of the shear layer, it is demonstrated that the buildup and shedding of surface vorticity directly influence the dynamic changes of the separation point location. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic designs and has a broad impact within the field of unsteady fluid dynamics.