Experiments in Fluids最新文献

This letter presents an adaptation of a PIV-based pressure reconstruction for non-Newtonian fluid flows. This adapted method is applied to the free-surface flow of a viscoplastic fluid (featuring a yield stress) on a slope and interacting with an obstacle positioned over the channel bed and submerged in respect of the flow. For such a yield stress fluid, the viscosity is not constant but varies locally, which requires appropriate consideration in the momentum conservation equation. In addition, flow is characterized by regions where the fluid behaves like a solid, i.e., where the shear stress does not surpass the fluid yield stress. Then, the flow is characterized by the presence of non-sheared regions which require specific treatment in the pressure reconstruction process. This letter presents the principles of the adapted pressure reconstruction method, validates its implementation using two-dimensional numerical data, and demonstrates its feasibility by applying it to experimental 2D-PIV data from the literature.

The global drive toward carbon–neutral marine propulsion has positioned ammonia as a compelling alternative fuel due to its zero-carbon emissions. However, ammonia flash-boils easily, and there is no consensus in the literature on which optical techniques to use. Hence, this study aims to identify suitable optical methods to characterize flashing liquid ammonia sprays. Ammonia sprays were investigated in a constant volume chamber under ambient pressures (0.2–20 bar), corresponding to pressure ratio (R_P) values from 42.85 to 0.43 at a fuel temperature of 20 °C. Four optical diagnostics: Diffused Back Illumination (DBI), Mie scattering, shadowgraphy, and schlieren, were applied to compare their suitability. A single-hole injector and high-speed imaging were used to measure spray tip penetration (STP) and spray width (SW). All methods produced comparable STP values; however, differences were observed under flash boiling. DBI captured the liquid core, while shadowgraphy and schlieren detected a wider spray, indicating that they detected ammonia vapor and dispersed fine droplets; however, schlieren's additional detection of shock waves at high pressures made data processing more difficult. Mie scattering lost accuracy in wide plumes due to scattering-induced signal loss. It was concluded that a combination of DBI and shadowgraphy is most suitable to detect the dense liquid core, the vapor, and any dispersed atomized droplets.

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.