Sonoluminescence and its neighbor cavitation bubble luminescence

Abstract: The fielcl of sonoluminescence

Portable 512 MEMS-Microphone-Array for 3D-Intensity- and Beamforming-Measurements using a FPGA based Data-Acquisition-System

In this paper a portable MEMS-Microphone-Array with integrated Data-Acquisition-System is presented. 512 digital MEMS-Microphones are located in a rectangular box of 17 x 15 x 20 cm. The microphone positions are chosen for sound intensity measurements, but are capable of beamforming as well. Depending on the frequency, these microphones can be used as an array of hundreds of 3D - intensity probes. The acoustic velocity is estimated using a high order three dimensional finite difference stencil to overcome the upper frequency limit of common pp-intensity probes. At low frequencies, pairs with larger spacing are used to reduce the requirement of accurate phase match of the microphone sensors. Additionally a procedure is shown for amplitude and phase calibration of MEMS-Sensors. All microphone data is collected by an FPGA and send via the UDP-Protocol to any host system for beamforming and intensity calculations in real time or for storing the data to disk

Experimental Investigations of Vortex Flow Phenomena on the DLR-F23 Combat Aircraft Configuration at Transonic Speeds

Modern high-agility aircraft are designed to cover a wide range of flight maneuvers, from transonic and supersonic performance to high maneuverability at subsonic speeds. The required flight envelope and mission spectrum of future multi-role combat aircraft lead to the development of new configurations with low aspect-ratio hybrid-delta wing planforms, consisting of multiple swept leading edges (LE). The corresponding flow field is dominated by complex vortex flows even at small angles of attack (AOA), vortex-vortex interactions and vortex-shock interactions, as well as vortex breakdown. These unsteady flow phenomena interact with the aircraft structure. The structural dynamics excitation, which is called buffeting, affects the maneuverability of the aircraft and reduces the lifespan of structural components. The design of the wing planform in terms of different leading edges with different LE sweep angles and additional leading-edge extensions (LEX) has, for this reason, a significant impact on the complexity of the vortex-dominated flow field and the occurring unsteady flow phenomena

Development of new methods for analysis of LPT data and application to high-subsonic jet measurements

Lagrangian particle tracking enables the accurate measurement of the position, velocity and acceleration of particles moving within fluid flows. Beyond the particle positions in each time step, an actual track of the individual particle is available over time. This can be exploited for new analysis methods that use this continuous nature of the tracks to improve binned statistics of the flow. Instead of considering just particle positions at each sampling time instance the movement of the tracked particle between those time steps can provide additional information. In the case of binning based averaging methods this results in better convergence of the statistics for a given amount of data. We apply such methods to a multi-pulse Shake-The-Box (STB) measurement of a Mach 0.84 jet in air and generate high resolution two-point correlation maps of the flow. Coherent structures in the shear layer are further investigated using event orientated conditional averaging based on quadrant analysis. As a novel approach for using the Lagrangian nature of the measurement data, we additionally show a control volume balance based calculation approach, which together with bin based statistics could provide a pathway towards investigating balance equations of flow quantitie

3D flow and deformation measurements of rigid and flexible wings under combined pitching and plunging motions using Lagrangian particle tracking

A dynamic test rig based on a hypocycloid gear has been designed for at DLR for underwater flapping wing applications. Three NACA0012 wings with different flexibility, to be attached to the end plate, have been constructed (half span: 150 mm, chord: 50 mm ). The wings perform a two-dimensional motion, i.e. within the x,z-plane, whereas the pitch axis is constant pointing in the y-direction. Markers with approx. 0.5mm diameter were printed onto the wings with pseudo-random distribution to allow detection of the wing motion and deformation. Lagrangian particle tracking of the markers, together with particles in the flow was performe

Full-volume investigation of moving rigid and flexible wings using Lagrangian Particle Tracking of surface markers and tracer particles

A dynamic test rig based on a hypocycloid gear has been designed at DLR for underwater flapping wing applications. Three NACA0012 wings with different flexibility, to be attached to the end plate, have been constructed (half span: 150 mm, chord: 50 mm). The wings perform a two-dimensional motion, i.e. within the x,z-plane, whereas the pitch axis is constantly pointing in the y-direction. Markers with approx. 0.5mm diameter were printed onto the wings with pseudo-random distribution to allow detection of the wing motion and deformation. A system of eight high-speed cameras captures the flow all around the wings and simulaneously captures the wing deformation by tracking the surface markers

Multi-Resolution, Time-Resolved PIV Measurements of a Decelerating Turbulent Boundary Layer near Separation

We report on measurements of the time-evolving velocity profile of a turbulent boundary layer subjected to a strong adverse pressure gradient (APG) at Reynolds numbers up to Re_theta ~55000 with an upstream friction Reynolds number exceeding Re_tau ~10000. Near the point of flow separation high-resolution imaging at high camera frame rates captured the time evolving velocity profile using the so-called "profile-PIV" technique in a nested imaging configuration of two cameras operating at different image magnifications. One camera used an image magnification better than unity to resolve the viscous scales directly at the wall while the remainder of the roughly 200 mm thick boundary layer is simultaneous captured by the second camera. In the APG the variance of the stream-wise velocity exhibits no "inner peak" commonly found in turbulent boundary layers without pressure gradient influence. Spectral analysis further shows that the peak energy within the boundary layer shifts away from the wall toward lower frequencies. The overlap between the simultaneously imaged areas allows to assess and, to first order, correct for the effect of spatial smoothing on statistical quantities, spectra and related quantities. A multi-frame cross-correlation algorithm was used to process the extensive database. In addition, a newly developed 2D-2C "Shake-The-Box" algorithm(STB) provided highly resolved particle tracking data beyond the reach of conventional PIV processing

Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB

Exhalation of small aerosol droplets and their transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathway for indirect or airborne respiratory virus transmission from person to person, e.g. for SARS-CoV 2 or measles (Morawska and Cao 2020). Understanding airborne transport mechanisms of viruses via small bio-aerosol particles inside closed populated rooms is an important key factor for optimizing various mitigation strategies (Morawska et al. 2020), which can play an important role for damping the infection dynamics of any future and the ongoing present pandemic scenario, which unfortunately, is still threatening due to the spreading of several SARS-CoV2 variants of concern, e.g. delta (Kupferschmidt and Wadman 2021). Therefore, a large-scale 3D Lagrangian Particle Tracking experiment using up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of ~ 370 µm as passive tracers in a 12 m³ generic test room has been performed, which allows to fully resolve the Lagrangian transport properties and flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs have been used and the Shake-The-Box (STB) (Schanz et al. 2016) Lagrangian particle tracking algorithm has been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around seated breathing persons

Large-scale volumetric flow studies on transport of aerosol particles using a breathing human model with and without face protections

Exhalation of small aerosol particle droplets and their airborne transport, dispersion and (local) accumulation in closed rooms have been identified as the main pathways for direct and indirect respiratory virus transmission from person to person, e.g. for SARS-CoV-2 or measles (Morawska and Cao 2020) (Chen et al. 2021). Therefore, understanding airborne transport mechanisms of aerosol particles inside closed populated rooms is an important key factor for assessing and optimizing various mitigation strategies (Morawska et al. 2020) (Morawska et al. 2013). Unsteady flow features, which are typically evolving in such mixed convection flow scenarios, govern the respective particle transport properties. Experimental and numerical methods are required which enable capturing the related broad range of scales in such internal flows over many cubic meters in order to provide reliable data for the adaptation of proper mitigation measures (distances, masks, shields, air purifiers, ventilation systems etc.). In the present work we show results of a large-scale 3D Lagrangian Particle Tracking (LPT) experiment which has been performed in a 12 m³ generic test room capturing up to 3 million long lived and nearly neutrally buoyant helium-filled soap bubbles (HFSB) with a mean diameter of dHFSB~ 370 µm as (almost) passive tracers. HFSB are used as fluid mechanical replacements for small aerosol particles dP < 5µm and allow to resolve the Lagrangian transport properties and related unsteady flow field inside the whole room around a cyclically breathing thermal manikin (Lange et al. 2012) with and without mouth-nose-masks and shields applied. Six high-resolution CMOS streaming cameras, a large array of powerful pulsed LEDs and the variable-time-step Shake-The-Box (VT-STB) (Schanz et al. 2016, Schanz et al. 2021) LPT algorithm have been applied in this experimental study of internal flows in order to gain insight into the complex transient and turbulent aerosol particle transport and dispersion processes around a seated and breathing human model

Dynamic-stall measurements using time-resolved pressure-sensitive paint on double-swept rotor blades

The study presents an optimized pressure-sensitive paint (PSP) measurement system that was applied to investigate unsteady surface pressures on recently developed double-swept rotor blades in the rotor test facility at the German Aerospace Center (DLR) in Göttingen. The measurement system featured an improved version of a double-shutter camera that was designed to reduce image blur in PSP measurements on fast rotating blades. It also comprised DLR's PSP sensor, developed to capture transient flow phenomena (iPSP). Unsteady surface pressures were acquired across the outer 65% of the rotor blade with iPSP and at several radial blade sections by fast-response pressure transducers at blade-tip Mach and Reynolds numbers of M_{tip} = 0.282 - 0.285 and Re_{tip}= 5.84 - 5.95 * 10^5. The unique experimental setup allowed for scanning surface pressures across the entire pitch cycle at a phase resolution of 0.225 deg azimuth for different collective and cyclic-pitch settings. Experimental results of both investigated cyclic-pitch settings are compared in detail to a delayed detached eddy simulation using the flow solver FLOWer and to flow visualizations from unsteady Reynolds-averaged Navier Stokes (URANS) computations with DLR's TAU code. The findings reveal a detailed and yet unseen insight into the pressure footprint of double-swept rotor blades undergoing dynamic stall and allow for deducing "stall maps", where confined areas of stalled flow on the blade are identifiable as a function of the pitch phase