Experiments in Fluids最新文献

The development of micro-hot-wire anemometry probes for turbulence studies requires mitigating spatial filtering, end-conduction effects and probe intrusivity. Keeping these factors in mind, this work analytically and experimentally investigates the relevance of a micro-hot-wire probe design featuring elongated stubs, inspired by Wollaston-wire probes but fabricated using modern micro-fabrication techniques. The resulting probes are shown to be relatively easy to manufacture and capable of providing satisfactory velocity measurements in a zero-pressure-gradient turbulent boundary layer at (text {Re}_{tau }approx 1150) with (delta ={18}{hbox {mm}}). Different probes were tested, all featuring a micro-wire length sufficiently small to alleviate spatial filtering of near-wall small-scale turbulent structures. The investigation focuses on assessing end-conduction effects and probe intrusivity, with the latter still observable close to the wall for such micro-probes.

This study experimentally determines high-frequency aeroacoustic noises of the high-speed dual-impinging jet, focusing on the generation and evolution mechanism between impingement tones modes and far-field acoustic spectra. The variables of the high-speed impinging jet are Mach number (Ma = 0.9 and 1.1), fixed diameter (D) of nozzle, nozzle spacing (3.5D), and impingement distance (4D). A novel fast-responding pressure-sensitive paint (fast-PSP) with a significantly extended frequency response ability was designed to develop an accurate phase-resolving propagation process of aeroacoustic noises and handle high impingement momentum challenges posed by the high-speed impinging jet. The PSP raw data were enhanced by calibration, image restoration, and proper orthogonal decomposition filtering. Two distinct far-field spectral characteristics were identified based on synchronized acoustic measurements. The existence of the stagnation region affected by the fountain effect in the dual-impinging jet was determined using spatial–temporal cross-correlation analysis. Subsequently, the concurrent axisymmetric dual annulus mode and its coupling behavior at Ma = 0.9 and the non-axisymmetric helical mode and its periodic fragmentation-reconstruction patterns at Ma = 1.1 were identified by spectral proper orthogonal decomposition. Finally, the spatial–temporal evolution of the phase-locked first-order mode was extracted, and the transient variations of coherent structures and their mechanisms for discrete tone noise generation were quantitatively investigated. The expansion and coupling of the dual annulus modes promoted the dominance of single-tone peaks across the entire acoustic spectrum. The helical mode, exhibiting both rotational and expansion behaviors, enhanced the coherent vorticity at the periphery of the coherent structures, resulting in intense impingement tones sound sources.

Particle streak velocimetry (PSV) is a Lagrangian velocity measurement method based on streak imaging of moving particles and is regarded as the origin of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). Recently, the PSV technique has undergone further developments, realizing measurements of three velocity components in three dimensions (3D3C), by combining with stereoscopic observation, defocused imaging, light field photography and /or holography. Moreover, image processing algorithms based on deep learning have been successfully applied to PSV. Compared with PIV and PTV, the PSV technique can exhibit several advantages, including extending the upper limit of the velocity measurement range under the same equipment conditions, measuring with lower illumination intensity, often an overall lower equipment complexity and cost for the same measuring requirement, as well as avoiding the particle matching problems of PTV. However, the PSV method also has obstacles to overcome, such as directional ambiguity and the difficulty in identifying streak crossings. For the directional ambiguity problem, there are currently time-coding, color-coding, brightness-coding and determination methods using additional image frames that can be employed. The main application areas of PSV currently include microfluidics, high-velocity flows and large-scale flow field measurements. This review presents the state of the art of PSV and summarizes advantages, disadvantages, accuracy, complexity and application of various configurations. The configurations discussed are focused on those measuring three velocity components and several examples are described in which PSV can be advantageously applied. The review concludes with some future developments that can be anticipated.

This work shows new insights on the application of two-color laser-induced-fluorescence (2c-LIF) thermometry in a droplet chain. A two-dye mixture is used in ethanol, water and ethanol/water mixtures in order to reach a high-temperature sensitivity and avoid the detection of lasing effects in the droplets. Various droplet sizes are recorded in regard to the limitation of the detection system for very small micrometric droplets. The breakup of a droplet chain is measured to assess the spectral detection system in applications with liquid structures of different sizes. Additionally, a proposal to expand the 2c-LIF application for studying ethanol/water droplets regarding mixture composition with a third color channel is presented. Forming two intensity ratios, the spectra can be used to obtain information on the mixture composition of the solvent. Measurements in different ethanol/water mixtures containing 0–100 vol% water are evaluated to show this possibility.

Droplet impact is a common phenomenon in daily life and various industrial applications. Previous research shows that surface geometry significantly influences impact outcomes. However, there is a gap in systematic research on how convex structures, similar in size to the droplet, influence impact behaviors. To address this, our study focused on producing various targets with different convexity to investigate the morphological evolution of droplet impact. Using high-speed imaging techniques, we examined these impacts with Weber numbers ranging from 5 to 346. The experimental results show that dry convex surfaces increase the maximum spreading diameter of droplets by altering liquid mass redistribution. Reduced air entrapment diminishes the circumferential instability of deformed droplets on these surfaces, as evidenced by fewer fingers formed. This study also proposes a hybrid model to predict the maximum spreading diameter on convex surfaces using the energy conservation method. Benefiting from models for flat surfaces, this new approach accounts for convex surface impacts, which alter the impact characteristics according to the convexity of the impact geometry. The model assumes that the droplet at its maximum spreading diameter resembles either a disc or a rim. Notably, the rim assumption was quite evident in several convex surface impacts, presenting a donut-shaped expansion. These results are combined through weighted summation The hybrid model’s predictions show a superior agreement with the experimental data compared to existing models. Additionally, the model’s weighting factors provide insights into the distribution of liquid mass between the central film and the surrounding rim.

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In the past years, volumetric velocimetry measurements with helium-filled soap bubbles as tracer particles have been introduced in wind tunnel experiments and performed at large-scale, enabling the study of complex body aerodynamics. A limiting factor is identified in the field of wind engineering, where the flow around ships is frequently investigated. Considering multiple wind directions, the optical access for illumination and 3D imaging rapidly erodes the measurement regions due to shadows and incomplete triangulation. This work formalizes the concepts of volumetric losses and camera redundancy, and examines the performance of multi-directional illumination and imaging for monolithic and partitioned modes. The work is corroborated by experiments around a representative ship model. The study shows that a redundant system of cameras yields the largest measurement volume when partitioned into subsystems. The 3D measurements employing two illumination directions and seven cameras, yield the time-averaged velocity field around the ship. Regions of flow separation and recirculation are revealed, as well as sets of counter-rotating vortices in several stations from the ship bow to the flight–deck. The unsteady regime at the flight–deck is examined by proper orthogonal decomposition, indicating that the technique is suited for the analysis of large-scale unsteady flow features.

Experimental and numerical simulation of launcher base flows are crucial for future launcher design. In experiments, exhaust plume simulation is often limited to cold or slightly heated gases. In numerical simulations, multi-species reactive flow is often neglected due to limited resources. The impact of these simplifications on the relevant flow features, compared to real flight scenarios, needs to be characterized in order to enhance the design process. Experimental and numerical investigations were carried out within the framework of the SFB/TRR 40 Collaborative Research Centre to study the impact of plume and wall temperature on the base flow of a generic small-scale launcher configuration. Wind tunnel tests were performed in the Hot Plume Testing Facility (HPTF) at DLR, Cologne, using subsonic ambient flow and pressurized air or hydrogen–oxygen combustion as exhaust gases. The tests were numerically rebuilt using the DLR TAU code employing a scale-resolved IDDES approach, including thermal coupling and detailed chemistry. The paper combines the experimental and numerical findings from the SFB/TRR 40 base flow studies and highlights the most prominent influences on the mean flow field, the pressure field, the dynamic wake flow motion, and the resulting aerodynamic forces on the nozzle. High-frequency pressure measurements, high-speed schlieren recordings, and time-resolved CFD results are evaluated using spectral and modal analysis of the one- and two-dimensional flow field data.

Dynamic pressure measurements are indispensable in the field of fluid mechanics. Attaching tubing as a transmission line to the pressure transducer is often unavoidable but significantly reduces the usable bandwidth of the measurement system. Complex fluid-wall interactions and potential outgassing of air are present within systems with water-filled tubes. Comprehensive studies aiding researchers in selecting suitable transmission line parameters (i.e., material, length, and diameter) are not available. A simple calibration apparatus is designed for the frequency response characterization of multiple pressure transducers simultaneously applying a pressure step. The setup is thoroughly characterized and a detailed description is provided to optimize the bandwidth. A piezoresistive pressure transducer attached to water-filled tubes, as commonly used in hydrodynamic experiments, is characterized in the low-frequency range (i.e., (f le {300}) Hz). Tube-related effects, such as length, diameter, and material are investigated. The impact of entrapped air within the tubing is analyzed. The feasibility of substituting water with silicone oil to fill the tubes is explored. To optimize the usable bandwidth of the pressure measurement system for dynamic applications, it is essential to maintain short tubing that is as rigid as possible and free from entrapped air. Pressure wave propagation speed is reduced by two orders of magnitude in elastic transmission lines made of silicone. Pressure corrections through dynamic calibration are challenging due to the system’s sensitivity to various parameters affecting the dynamic response.

High spatial resolution and high accuracy estimation of 3D velocity fields are important for tomographic particle image velocimetry (Tomo-PIV), especially when measuring complex flow fields with delicate 3D structures. However, the widely used cross-correlation-based methods have limited spatial resolution, while the recently developed optical flow-based methods have low robustness and are sensitive to particle volume reconstruction errors. Therefore, 3D velocity estimation methods that simultaneously exhibit high resolution and robustness must be developed. In this study, we propose a novel velocity estimation method for Tomo-PIV measurement using the guided filter-based 3D hybrid variational optical flow (GF-HVOF) method to achieve high spatial resolution and highly accurate measurement of 3D flow field structure. First, we propose a novel L1-norm regularization term based on the Helmholtz decomposition theorem to preserve the divergence and vorticity of the fluid flow. Second, we propose a guided-filter-based constraint term using the result of the cross-correlation-based method as the guided flow field to improve the robustness of the optical flow method. Third, we propose a hybrid constraint term based on particle tracking velocimetry (PTV) method and a spatially weighted data term to reduce the effect of ghost particles and discrete errors generated during the reconstruction of particle volumes. The newly proposed hybrid method combines the advantages of optical-flow-based and cross-correlation-based methods and corrects the flow field using the PTV method. Velocity fields are estimated over synthetic and experimental particle volumes. The results show that the newly proposed GF-HVOF method achieves better performance and greater measurement accuracy than existing 3D fluid motion estimation methods.

The intermittency characteristics in transitional and turbulent flows can provide critical information on the underlying mechanisms and dynamics. While time–frequency (TF) analysis serves as a valuable tool for assessing intermittency, existing methods suffer from resolution issues and interference artifacts in the TF representation. As a result, no suitable or accepted methods currently exist for assessing intermittency. In this work, we address this gap by presenting a novel TF method—a Fourier-decomposed wavelet-based transform—which yields improved spatial and temporal resolution by leveraging the advantages of both integral transforms and data-driven mode decomposition-based TF methods. Specifically, our method combines a Fourier-windowing component with wavelet-based transforms such as the continuous wavelet transform (CWT) and superlet transform, a super-resolution version of the CWT. Using a peak-detection algorithm, we extract the first, second, and third most dominant instantaneous frequency (IF) components of a signal. We compared the accuracy of our method to traditional TF methods using analytical signals as well as an experimental particle image velocimetry (PIV) dataset capturing transition to turbulence in pulsatile pipe flows. Error analysis with the analytical signals demonstrated that our method maintained superior resolution, accuracy, and, as a result, specificity of the instantaneous frequencies. Additionally, with the pulsatile flow dataset, we demonstrate that IF components of the fluctuating velocities extracted by our method decompose energy cascade components in the flow. Additional investigations into corresponding spatial frequency structures resulted in detailed observations of the inherent scaling mechanisms of transition in pulsatile flows.