Experiments in Fluids最新文献

It is well known that the inadequate spatial resolution of hot-wire anemometry can lead to significant underestimation of measured quantities, such as the streamwise Reynolds stress, particularly at high Reynolds numbers. In this study, we propose a spatial resolution correction method based on a new scaling (the mean turbulent energy dissipation rate and the kinematic viscosity) introduced by Tang and Antonia (2022) for wall-bounded turbulent flows. This method is tested in a zero pressure gradient boundary layer at several Reynolds numbers ((Re_{tau}) = 2284, 3475, 4045, 4162, and 14000), where (Re_{tau}) is based on the friction velocity and the boundary layer thickness. By replacing the under-resolved small-scale portion of the energy spectra measured by hot-wires with (Re_{tau})-independent spectra obtained from direct numerical simulation (DNS) of wall-bounded turbulent flows, the proposed correction method provides reasonable estimates of the streamwise Reynolds stress in the near-wall region.

The present study focuses on the thermal characterization of a Rayleigh–Bénard (R–B) convection (Rayleigh number Ra = 2.20⋅10⁷ and Prandtl number Pr = 29.9) in the synthetic heat transfer oil Marlotherm LH (benzyltoluene) with a two-color laser-induced fluorescence measurement technique (2c-LIF). For this purpose, a compact convection chamber with unity aspect ratio was developed, which enables extreme temperature differences up to 120 K. The fluorescence signal is generated by doping the heat transfer oil with the fluorophore Nile red and its excitation by a pulsed Nd:YAG laser at 532 nm. First, the 2c-LIF technique is calibrated under homogeneous temperature conditions in the cell. Here, the relative thermal sensitivity decreases with increasing liquid temperatures. Second, the detachment and rise or fall of multiple thermal plumes in the R–B cell is analyzed, while the bottom wall was heated to 360 K, and the top wall was cooled to 240 K, resulting in a respective temperature field of the mixture in the range of 300–345 K. The time-resolved LIF measurements enable a characterization of the buoyancy-driven flow in terms of temperature field, heat flux density, thermal plume shape and plume velocity. The local heat flux density (11.5 kW/m²), heat transfer coefficient (311 W/m²⋅K) and Nusselt number (36.4) of the cold boundary were determined from the temperature profile. The highest plume velocities are in the range of 15 mm/s at the studied condition with large temperature stratification. No stationary large recirculation zones were detected in the cell, which are typical for such thermal R–B convection conditions.

This work demonstrates the use of high-speed, single-camera Background-Oriented Schlieren (BOS) as a robust diagnostic tool for characterizing reactive, high-temperature supersonic jets issued from a non-aluminized solid-propellant rocket motor (SRM). Tailored to handle strong optical deflections and intense plume luminosity exacerbated by after-burning, the BOS system yields relevant quantitative reconstructions of axisymmetric mean refractive index fields in the stable regime of the SRM, when corrected for deflection drifts induced by plume disturbances. The resulting dense, large-field measurements capture key quantitative features of two distinct SRM plumes, one in isolated operation and one with a supersonic co-flow. Supported by an uncertainty analysis, these reconstructed fields provide a valuable benchmark for assessing the performance of corresponding numerical simulations.

A cost-effective dual-color scanning PIV system is developed, experimentally demonstrated, and validated. The scanning PIV system has two CW DPSS lasers of different wavelengths (green: 532 nm and blue: 473 nm), which sweep through the region of interest to provide illumination. The illuminated region is captured by a conventional DSLR camera. Two different color lasers produce two illuminations, which are captured on a single frame. The single-frame color recording causes the phenomenon of color crosstalk, which is the leakage of light to neighboring pixels on the imaging sensor. Due to the color crosstalk, some unwanted particle images are observed in different color channels, referred to as ghost particles. This leads to inaccurate velocity measurements, and to mitigate the color crosstalk from images, a correction algorithm is proposed in this study. The captured images are corrected using the color crosstalk correction algorithm and processed further to obtain the velocity field. The scanning PIV system is tested by measuring the flow field downstream of a moving circular cylinder, and validated by measuring steady vortex flow generated using a magnetic stirrer. The applicability of the proposed scanning PIV system is also discussed.

An investigation into the influence of topographical shape and stratification profile on the kinetic energy of propagating internal waves generated by tidal flow in evanescent regions is accomplished using four different methods. Experiments, analytical modeling, and numerical modeling with two different analysis methods are each used to explore resulting propagating internal waves after an evanescent region. Due to varying stratification, just above the evanescent generation region, the waves are propagating and contribute to the internal wave energy available throughout the oceans. Each analysis method captures different dynamics best, and those dynamics are defined here, but general trends are found to be the same. As the relative length of the evanescent region above the topography increases or the average relative buoyancy frequency in this region decreases, the internal wave energy in the propagating region decreases due to enhanced decay distance or rate before reaching the propagating region. It is also found that the average stratification in each of the evanescent and propagating regions may be used instead of the entire profile to estimate propagating wave dynamics—a relevant simplification especially to increase computational speed. Finally, an equation to approximate propagating wave energy from an evanescent region as a function of stratification and topographic parameters is given, based on results from all four methodologies.

The performance evaluation of a single-body typed TDLAS sensor was experimentally conducted using a scramjet ground test facility. The scramjet ground test facility includes model scramjet isolator and combustor. The model scramjet isolator of the test facility can simulate the air flow condition of total temperature of 1,220 K, total pressure of 862 kPa, and Mach number of 2.43 which are representative of the internal flow condition of the scramjet isolator. To evaluate the performance of the single-body typed TDLAS sensor, the TDLAS sensor was flush-mounted on the model scramjet isolator wall of the test facility during the ground test, and the structural integrity and operability of the TDLAS sensor were analyzed based on the robustness of the TDLAS’s components, the stability of signal acquisition, and an accuracy of the measured data. The experimental ground test results demonstrated that the single-body typed TDLAS sensor in this study can withstand and operate well under the harsh mechanical and thermal environments of the model scramjet isolator.

A novel setup using a modular test bench with independent control of gas-phase velocity, temperature, injection pressure, and nozzle geometry was employed to perform a comprehensive parametric analysis of commercial ethanol and gasoline sprays under non-reactive conditions. High-speed imaging and Phase Doppler Interferometry quantified integral and pointwise spray characteristics across divergent and convergent nozzles, varying pressures (50–70 bar) and gas-phase temperatures (25–40 °C). Divergent nozzles produced narrow and stable plumes with rapid momentum decay, whereas convergent nozzles yielded wider sprays with delayed velocity peaks and sustained dispersion. Elevated temperatures and pressures strongly influence spray characteristics, markedly reducing smaller diameter class populations and shifting secondary breakup downstream. Ethanol sprays exhibited higher values of the Ohnesorge numbers than gasoline and a more constant projected area variance (PAV), resulting in consistent spray formation across all tested conditions. Fuel volatility governed the evolution of droplet size distribution throughout the sprays, with gasoline sprays displaying bimodal size distributions and ethanol maintaining its size distribution pattern. Dimensionless parameter analysis (Weber and Ohnesorge numbers) highlighted the transition from aerodynamic to oscillation-dominated breakup regimes and their influence in the formation of new droplets and consequently the rate of droplet size reduction between measurement points. These findings provide valuable insights for injector design and commercial fuel spray applications, highlighting the potential of ethanol (a renewable fuel in Brazil) due to its stable and regular spray structure. This characteristic makes it particularly suitable for use in narrow operational windows, potentially enhancing overall process efficiency

This study focuses on the hydrodynamic effects generated by a displacement of ship model in a confined channel, based on the simultaneous measurement of the flow field velocity and free surface deformation. To this end, an experimental set-up combining stereoscopic particle image velocimetry (stereo-PIV) and stereo-refraction measurements has been developed. The simultaneous application of these two optical methods presents significant experimental challenges, particularly in terms of optical alignment, calibration, and synchronization of multi-camera systems. The paper provides a detailed analysis of the measurement uncertainties of each method, extending this analysis to the global wake reconstruction. The influence of lateral confinement was studied by carrying out experiments at six sailing speeds. These speeds corresponded to Froude numbers ranging from 0.20 (subcritical) to 0.80 (transcritical). The experiments were repeated for three channel widths (0.5 m, 1.0 m and 1.5 m). The results highlight key hydrodynamic phenomena, such as rip currents and jet-like wake structures, as well as changes in vertical fluid velocities. These phenomena are all modulated by sailing speed and confinement.

A robust pairing algorithm with outlier removal is introduced in the context of two-pulse 3D particle tracking velocimetry at high seeding densities, with high concentrations of ghost particles. Integrating the vector field consensus approach from Ma et al. (IEEE Trans Image Process 23:1706–1721, 2014), the algorithm, its underlying hypotheses, and its relevant input parameters are investigated in the context of turbulent flow measurements. 2D synthetic tests are first carried out to quantify the algorithm’s performance and derive simple guidelines for optimal parameter tuning strategies based on experimental quantities. It is found that 2D vector fields with up to 90% outliers can be handled by our algorithm. 3D synthetic tests are then implemented to test the tracking strategy robustness to increasing image densities and ghost particle concentrations. We show that our algorithm can be used for particle pairing in particle clouds with up to 50% of ghost particles. Results submitted on the two-pulse dataset of the first LPT challenge, using the associated data portal with automatic evaluation, also showcase the overall excellent performances of the method. Finally, the method is used successfully on experimental data from our Giant Von Kármán setup (characterized by up to 65% of ghost particles), as evidenced by comparisons of its output with respect to results provided by the Shake-The-Box algorithm and with results provided by a pairing approach using a 3D cross-correlation predictor.

Wall shear stress ((tau _w)) quantification is fundamental in fluid dynamics but remains challenging in wind-tunnel experiments. Sensor-based methods offer high accuracy but lack spatial resolution for capturing complex three-dimensional effects. Conversely, oil-film visualization is a simple method to obtain high-resolution surface flow topology by processing a sequence of images using optical flow (OF) techniques. However, leveraging this approach for quantitative analysis suffers from noise and systematic biases. This study introduces SENSE (Sensor-Enhanced Neural Shear Stress Estimation), a data-driven approach that leverages a neural network to enhance OF-based (tau _w) estimation through the integration of sparse, high-fidelity sensor measurements via a multi-objective loss function. SENSE processes oil-film image sequences directly, inherently mitigating temporal noise without explicit averaging. The method is validated in a turbulent separated flow on a one-sided diffuser. Results demonstrate SENSE’s robustness to sequence length and spatial resolution compared to classical optical flow algorithms. Crucially, incorporating sparse sensor data significantly improves quantitative accuracy, achieving over 30% reduction in root-mean-squared error on validation sensors with only 8 strategically distributed sensors. The sensor data provide a global regularization effect, improving estimates far from sensor locations. SENSE offers a promising approach to elevate oil-film visualization to a reliable quantitative measurement technique by combining image sequences and sparse sensor data.