Session A02: Turbulence: Measurements

An experimental investigation of airfoils and cylinders interacting with turbulent inflows.

Turbulence interaction with solid objects (such as airfoils and cylinders) significantly impacts the aerodynamic and vibroacoustic performance of engineering systems. Hence, it is desirable to understand these turbulence interactions and the underlying physical mechanisms controlling them. Here, we examine this flow configuration using an experimental framework designed to measure both the flow field using particle image velocimetry and the unsteady surface pressure using a remote microphone technique. Experiments are conducted on a NACA 0012 airfoil immersed in isotropic turbulent inflow generated by passive turbulence grids—similarly, cylinders placed in a tandem configuration are employed where an upstream cylinder is used to generate the turbulent inflow. Preliminary results on characterising the turbulent inflow conditions for each configuration will be presented. Further, results on the distortion of the turbulent flow structure are investigated in the near vicinity of the leading edge of the airfoil and the downstream cylinder.   

Understanding Phonetic Variations in Expiratory Turbulent Puffs through 3D Particle Tracking Velocimetry

We use high-speed 3D particle tracking velocimetry (PTV) to measure turbulent puffs created by articulating various explosive consonants: p, t, and k at a distance of one meter from the subject's mouth along the puff's streamwise direction. The energy dissipation rate (ε) was calculated via five methodologies: the second-order structure function, energy spectrum, Samgorinsky method, filter space technique, and dimensional analysis. Our results indicate a clear correlation between the ε estimated by different methods and the articulated consonants. Specifically, the average ε for p and k were consistently smaller than the ε for t. This disparity may be attributable to the unique articulatory movements associated with t, such as airflow obstruction by the tongue. In addition, we reveal that different pronunciations occupy distinct regimes of feature space spanned by the mean and turbulent characteristics, suggesting that phonetic complexities can generate puffs with distinct mean and turbulent characteristics. Thus, studies of airborne disease transmission among individuals should consider phonetic complexities.   

Lagrangian Particle Tracking at Large Reynolds Numbers

Particle tracking in turbulent flows is fundamental to the study of the transport of tracers, inertial particles or even active objects in space and time, i.e. the Lagrangian reference frame. It provides experimental tests of theoretical predictionscite{Toschi2009} (e.g. for the statistics of fluid accelerations and particle dispersion) and helps to understand important natural processes where the inertia of particles is important (e.g. cloud microphysicscite{Bertens2021}). While the spatial (Eulerian) features of turbulent flows have been studied for high, atmospheric Reynolds numbers ($R_lambda > 10^4$)cite{Tsuji2004}, the profound difficulties in accurately tracking particles in turbulent flows have limited the Reynolds numbers in the Lagrangian reference frame to Taylor scale Reynolds number $R_lambda lesssim 10^3$. Here we describe a setup that offers Lagrangian particle tracking at $R_lambda$ between 100 and 6000 in the Max Planck Variable Density Turbulence Tunnel (VDTT). We describe the imaging setup within the pressurised facility, the laser illumination, the particles and the particle dispersion mechanism. We verify that the particles are monodisperse, carry negligible charges and are good tracers for the whole range of experimental conditions. We detail the challenges and the constraints of the setup and we present typical data from the experiment.   

Fine-scale characterization of urban atmospheric turbulence for reproduction in multi-fan wind facility

The rapid development of drone activity demands that devices be tested in conditions that reflect real-world phenomena. Multi-fan wind facilities (also called windshaper) are increasingly used to test free-flying drones. Through control of individual fans, a windshaper can generate non-uniform multi-directional flows over an open jet area while actively controlling turbulence characteristics. Furthermore, previous studies have shown that multi-fan wind facilities can recreate atmospheric wind statistical properties and 3D turbulence spectra. These studies were performed at one spatial location, with all the fans operating under the same control command. Therefore, we collected three simultaneous high-frequency (10 kHz) point measurements of atmospheric flow at different spatial locations to broaden the studied capabilities from a single point to a plane.

The setup presented here consists of three synchronized three-component hotwire anemometers, which were deployed collocatedly with a sonic anemometer on an actuated rotating rack. The rack automatically aligned the hotwires with the mean flow and enabled rotation of over a 360° range. The setup was stationed on a high rooftop building in Geneva (CH) to capture the spatio-temporal characteristics of atmospheric wind and turbulence at the drone operating altitudes in an urban environment.   

Evolution of turbulent circulation through a smooth contraction

We study the statistical properties and evolution of circulation in a turbulent flow advected through a smooth 2.5:1, 2-D contraction. The turbulence is generated by active grids in a constant-head vertical water tunnel. The grid is operated in synchronous and random modes, yielding a Taylor-Reynolds number, Re_λ ~ 220 at the inlet to the contraction. We employ time-resolved tomographic particle image velocimetry with the shake-the-box algorithm to obtain volumetric velocity fields used to characterize the circulation in three perpendicular planes, computed over square loops of different sizes which mostly lie in the inertial range.  Circulation computed over closed loops can characterize the vortical coherent structures in a more representative way than the local vorticity. The mean stretching strengthens the streamwise circulation, whereas the transverse compression weakens the transverse circulation. Integral scales based on spatial correlation of streamwise circulation, were the largest compared to the other two directions. The probability density function of circulation transitioned from non-Gaussian to Gaussian behavior as the loop size increased from the dissipative scales to large scales and this transition persisted even under the influence of straining. The moments of these distribution exhibited a sub-Kolmogorov scaling exponents.   

Rotating Effects on Energy Transfer Mechanism of Isotropic Turbulence

Turbulent flows in rotating systems have significant implications across diverse fields, spanning from climate systems, and ocean currents, to turbomachinery. While previous research has primarily focused on homogenous turbulence and its impact on mean flow under rotation, the energy transfer mechanisms between scales and energy budget in rotating turbulence remain undefined, particularly analyzed using experimental measurements taken in non-inertial systems. In this study, we developed a novel rotating platform capable of achieving a wide range of experimental conditions. With an onboard 4-camera tomographic PIV system, we investigate the 3D flow structure of isotropic turbulence under different rotation conditions in the non-inertial system. The presence of centrifugal force and Coriolis force induces distinct asymmetry in mean flow and isotropy. Additionally, the energy transfer mechanism, analyzed by velocity spectra, is also affected by these forces in different directions. The velocity spectra along different directions in the rotating setting shows the difference and deviates from the Kolmogorov -5/3 power law observed in non-rotating isotropic turbulence.   

Adjoint-enhanced positioning algorithm for sensor network in turbulent environments

Accurate and swift identification of pollution source locations by remote measurements is challenging given the uncertainty in the background turbulent dispersion and the diffusion process. A multi-sensor algorithm based on the forward-adjoint duality relation is built for quick detection of a localized, continuous pollution source with unknown location and intensity in turbulence. According to the duality relation, the ratio of the observations of the scalar concentration at the sensor locations is mathematically related to the ratio of the corresponding adjoint fields at the source location, which provides important information to determine the source location. With a known velocity field, only four sensor measurements are required to locate a three-dimensional source. Having more measurement data will increase the robustness of the algorithm. When the background turbulence is unknown, the adjoint field of a sensor could be represented as a probability distribution, and the source location can be estimated through MCMC. As a result, the algorithm has the potential to quickly identify the probability of source locations according to the remote measurements of a sensor network. Preliminary results are demonstrated in the channel flow as well as isotropic turbulence setups.