Rainer Hain

A simple single camera 3C3D velocity measurement technique without errors due to depth of correlation and spatial averaging for microfluidics

A method to determine the three components (3C) of the velocity field in a micro volume (3D) using a single camera is proposed. The technique is based on tracking the motion of individual particles to exclude errors due to depth of correlation (DOC) and spatial averaging as in µPIV (micro particle image velocimetry). The depth position of the particles is coded by optical distortions initiated by a cylindrical lens in the optical setup. To estimate the particle positions, a processing algorithm was developed based on continuous wavelet analysis and autocorrelation. This algorithm works robustly and gives accurate results comparable to multi-camera systems (tomographic PIV, V3V). Particle tracking was applied to determine the full 3C velocity vector in the volume without the error due to spatial averaging and DOC, which are inherent limitations in µPIV due to the interrogation windows size and volume illumination. To prove the applicability, measurements were performed in a straight channel with a cross section of 500 × 500 µm2. The depth of the measurement volume in the viewing direction was chosen to be 90 µm in order to resolve the near-wall gradients. The three-dimensional velocity distribution of the whole channel could be resolved clearly by using wave front deformation particle tracking velocimetry.

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Reθ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.

Principles of a Volumetric Velocity Measurement Technique Based on Optical Aberrations

In this contribution, a simple and robust three dimensional measurement technique for the determination of all velocity components is presented. As opposed to other techniques, only a single camera is required in order to calculate the particle positions in physical space. This is possible because the depth position of the particles is encoded using an optical aberration or wavefront distortion, called astigmatism. The astigmatism causes the particle images to have ellipse-like shapes. The length of the semi-major axis and the semi-minor axis depends on the depth wise position of the particle. It will be shown that this effect is well suited for extracting the particle positions. In the first section, an introduction is given and the measurement principle is shown in detail. Subsequently, the validation of the technique is illustrated by means of synthetically generated images. Finally, experimental results are presented and a conclusion is drawn.

Simulation of Wing and Nacelle Stall

Numerical stall simulations are challenging in terms of physical models involved, overall computation effort, and the needed efforts for validation. The present paper describes coordinated, fundamental research into new simulation methodologies and their validation for wing and nacelle stall that also include the effects of atmospheric gusts. The research is carried out by the DFG funded Research Unit FOR 1066, which is composed of German Universities and the German Aerospace Center, DLR. The Research Unit investigates advanced models of turbulence, advanced physics-based gust models, and new numerical approaches for gust simulation. These modeling and computational activities are supplemented by new validation experiments, that aim at providing stall data on wings and engine nacelles with well defined, generic distortions of the onset flow.

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.

In situ calibrated defocusing PTV for wall-bounded measurement volumes

In many situations, 3D velocity measurements in thin (~1 mm) but wide (~100 × 100 mm2) flow channels is an important task. To resolve the in-plane and out-of-plane velocity gradients properly, a precise calibration is required, since 3D measurement approaches rely strongly on the accuracy of the calibration procedure. It is likely that calibration targets do not fit domains with small depths, due to their size. Furthermore, in fields where such measurements are of interest, the accessibility of the measurement volume is often limited or even impossible. To overcome these drawbacks, this paper introduces an in situ calibrated defocusing particle tracking velocimetry approach for wall-bounded measurement domains with depths in the low millimeter range. The calibration function for the particle depth location is directly derived from the particle image geometries and their displacements between two frames. Employing only a single camera, this defocusing approach is capable of measuring the air flow between two parallel glass plates at a distance of 1 mm with an average uncertainty of 2.43% for each track, relative to the maximum velocity. A tomographic particle tracking velocimetry measurement, serving as a benchmark for the single camera technique, reaches an average uncertainty of 1.59%. Altogether, with its straightforward set-up and without requiring a calibration target, this in situ calibrated defocusing approach opens new areas of application for optical flow velocimetry. In particular, for measurement domains with small optical windows and a lack of accessibility.

Flow Investigations in a Stalling Nacelle Inlet Under Disturbed Inflow

A generic, cold flow-through nacelle under atmospheric disturbed inflow was experimentally investigated. The boundary layer development of the nacelle resembles that of a powered engine at take-off conditions and high mass flow rates. A vortex generator was positioned \({{l}/{c_\text {n}}=3.3}\) in front of the nacelle’s leading edge in order to generate an inhomogeneous inflow. This vortex generator is a motor driven airfoil, which pitches from angles of attack \({\alpha _{\text {vg}}=-11^{\circ }}\) to \({\alpha _{\text {vg}}=11^{\circ }}\) within approximately \({58\,\text {ms}}\). This way, vortices are generated in the wind tunnel. These vortices induce an upward velocity component which is comparable to a realistic gust. The investigations are performed for attached, as well as separated, flow in the nacelle. It is shown that for the attached flow the low speed near-wall area and the turbulent fluctuation velocities are increased. For the higher angle of attack of the nacelle, the interaction with the generic gust increases the size of the turbulent separation bubble as well as the amplitude of the fluctuation velocities. In both cases the changes might lead to problems on the compressor of a powered jet engine. The results indicate crucial distortions due to the interaction of the relatively small generic gust with the flow through the nacelle.

Experimental Investigation of a Turbulent Boundary Layer Subject to an Adverse Pressure Gradient at \(Re_{\theta }\) up to 10000 Using Large-Scale and Long-Range Microscopic Particle Imaging

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient for two Reynolds numbers \(Re_\theta =6200\) and \(Re_\theta =8000\). We perform detailed multi-resolution measurements by combining large-scale and long-range microscopic particle imaging. We investigate scaling laws for the mean velocity and for the total shear stress in the inner layer. In the inner part of the inner layer the mean velocity can be fitted by a log-law. In the outer part a modified log-law provides a good fit, which depends on the pressure gradient parameter and on a parameter for the mean inertial effects. Emphasis is on the Reynolds number effects on the mean velocity and shear stress.

Investigation of the Law-of-the-Wall for a Turbulent Boundary Layer Flow Subject to an Adverse Pressure Gradient Using Particle Imaging

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient for two Reynolds numbers \(Re_\theta =6200\) and \(Re_\theta =8000\). We perform detailed multi-resolution measurements by combining large-scale and long-range microscopic particle imaging. The flow is designed to be close to equilibrium in the sense that characteristic integral flow parameters evolve slowly in streamwise direction to study scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part of the inner layer the mean velocity can be fitted by a log-law. We observe larger values for the log-law slope than in zero-pressure gradient flows and study possible history effects. The outer part can be described by a modified log-law, which depends on the pressure gradient parameter and on a parameter for the effect of the mean inertial terms. Finally we present a new composite profile for the mean velocity.

Experimental Investigation of the Flow in a Stalling Engine Inlet

A flow-through nacelle was investigated at the Institute of Fluid Mechanics and Aerodynamics at the Universitat der Bundeswehr in Munich. The boundary layer loading of the nacelle resembles that of a powered engine at take-off conditions and high mass flow rates. The aim of the project is to generate an experimental validation database in order to develop a physically based numerical code to calculate the flow at the edge of the flight envelope. The investigations concentrated on the flow under an inhomogeneous onstream. Therefore, a vortex generator was positioned 1.74 m in front of the nacelle’s leading edge. This vortex generator is a motor driven airfoil, which pitches from angles of attack of αvg = -11° to αvg = 11° within approximately 60 ms.