Sven Scharnowski

Parallax correction for precise near-wall flow investigations using particle imaging

For the basic understanding of turbulence generation in wall-bounded flows, precise measurements of the mean velocity profile and the mean velocity fluctuations very close to the wall are essential. Therefore, three techniques are established for high-resolution velocity profile measurements close to solid surfaces: (1) the nanoprobe sensor developed at Princeton University, which is a miniaturization of a classical hot-wire probe [Exp. Fluids 51, 1521 (2011)]; (2) the laser Doppler velocimetry (LDV) profile sensor, which allows measurement of the location of the particles inside the probe volume using a superposition of two fringe systems [Exp. Fluids 40, 473 (2006)]; and (3) the combination of particle image velocimetry and tracking techniques (PIV/PTV), which identify the location and velocity of submicrometer particles within the flow with digital imaging techniques [Exp. Fluids 52, 1641 (2006)]. The last technique is usually considered less accurate and precise than the other two. However, in addition to the measurement precision, the effect of the probe size, the position error, and errors due to vibrations of the model, test facility, or measurement equipment have to be considered. Taking these into account, the overall accuracy of the PTV technique can be superior, as all these effects can be compensated for. However, for very accurate PTV measurements close to walls, it is necessary to compensate the perspective error, which occurs for particles not located on the optical axis. In this paper, we outline a detailed analysis for this bias error and procedures for its compensation. To demonstrate the capability of the approach, we measured a turbulent boundary layer at Re(δ)=0.4×10(6) and applied the proposed methods.

Control of the Reattachment Length of a Transonic 2D Backward-Facing Step Flow

When a turbulent flow reaches a sudden expansion as in the case of a backward-facing step (BFS), a turbulent separated shear layer originates from the trailing edge. The reattachment location of this shear layer varies in time and space, causing high dynamic loads on the reattachment surface. This is an inherent problem on some of today’s cryogenic space launchers, where the main engine’s nozzle suffers from high buffeting loads during the transonic phase of the ascent, due to a turbulent reattaching shear layer, especially originating from a geometric discontinuity, similar to a BFS. For this reason the aim of the current research focuses on stabilizing and reducing the reattachment length of the separated shear layer on a BFS in transonic flow by the means of passive flow control devices. Several streamwise vortex generators were examined in the Trisonic Wind Tunnel Munich (TWM) with particle image velocimetry (PIV). The investigations were conducted on a 2D model at Mach 0.8 and a Reynolds number of \(1.8 \times 10^5\) with respect to the step height (h). The results show that streamwise vortex generators have a tremendous effect on reducing the size of the recirculation region, reducing the reattachment location by 75 % or more, when compared to the reattachment location of a BFS wake.

Extensive characterisation of a high Reynolds number decelerating boundary layer using advanced optical metrology

ABSTRACT Over the last years, the observation of large-scale structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations. Nevertheless, partly due to the lack of comprehensive experimental data at sufficiently high Reynolds number, our understanding of turbulence near walls, especially in decelerating situations, is still quite limited. The aim of the present contribution is to combine the equipment and skills of several teams to perform a detailed characterisation of a large-scale turbulent boundary layer under adverse pressure gradient. Extensive particle image velocimetry (PIV) measurements are performed, including a set-up with 16 sCMOS cameras allowing the characterisation of the boundary layer on 3.5 m, stereo PIV and high resolution near wall measurements. In this paper, detailed statistics are presented and discussed, boundary conditions are carefully characterised, making this experiment a challenging test case for numerical simulation.

Towards fully-resolved PIV measurements in high Reynolds number turbulent boundary layers with DSLR cameras

AbstractIn this study, we describe a multi-camera large field-of-view (FOV) planar-PIV experiment to capture the wide range of scales that coexist in high Reynolds number turbulent boundary layers. The proposed measurements are designed to capture spatial flow features over a greater range than current common practices, and at significantly lower cost. With this goal in mind, specialist PIV cameras are substituted with modern consumer full-frame digital cameras, which are typically available at a fraction of the cost, with higher resolution sensors. These cameras are configured to capture single-frame double-exposed images (DE-PIV), but at a much higher spatial resolution than what is available from specialist PIV cameras that capture double-frame single-exposure images (SE-PIV). This work discusses a set of simulations and experiments to quantitatively assess the quality of the PIV velocity fields from these two approaches for large field-of-view measurements. Our findings confirm that despite the known loss-of-accuracy associated with DE-PIV, the use of high-resolution cost-effective consumer cameras provides an economically feasible PIV solution with the necessary performance and accuracy for high spatial range measurements in wall-bounded turbulent flows.Graphical abstract