This study experimentally investigates the bleeding flow characteristics downstream of isotropic porous square cylinders as a function of permeability and pore configuration across a broad range of Darcy numbers ( $2.4 \times 10<^>{-5} \lt \textit{Da} \lt 2.9 \times 10<^>{-3}$ ). The porous cylinders, constructed with a simple cubic lattice design, were fabricated using a high-resolution three-dimensional printing technique. This novel design method, based on a periodic and scalable lattice structure, allows fine control over the number of lattice pores along the cylinder width, $D$ , and the corresponding permeability, independently of porosity. Permeability was carefully determined by measuring the pressure drop and superficial velocity for each porous structure considered in this study. High-resolution particle image velocimetry measurements were conducted in an open-loop wind tunnel to characterize the downstream flow structures. The results reveal that bleeding flow characteristics near the cylinder trailing edge are strongly influenced by both permeability and pore configuration. These structural behaviours are further explored using an analogy to multiple plane turbulent jets. This approach identifies three distinct flow regions downstream of porous square cylinders, determined by the structural pattern of the bleeding flow. Additionally, an analytical framework is developed to model the longitudinal extent of the merging region by integrating the momentum equation, incorporating the Darcy-Brinkman-Forchheimer model, with a boundary layer assumption. The analytical model is validated against experimental data, demonstrating its capability to predict the key dynamics of bleeding flow evolution. Our results provide new insights into the fluid dynamics of porous bluff bodies, establishing pore configuration and permeability as dominant parameters governing downstream flow structures.

Vortex gust interaction is a significant event in a multitude of systems, viz., interaction of propellers with wings, blade-vortex interaction in helicopter rotors, biological flyers, leading edge vortices on wings, etc. While this can result in beneficial effects such as increased lift, gust interaction also leads to detrimental effects like unsteady structural loading, vibrations and noise generation. In this article, the development of an experimental facility to study the effects of gust interaction on an airfoil is detailed. A NACA 0012 airfoil undergoing a combination of pitching and plunging motion is investigated using Particle Image Velocimetry (PIV) to generate a line vortex. The characteristics of the motion profile used to generate the vortex is described, and the characteristics of the resultant vortex generated, and its relationship to the kinematics of the vortex generator are discussed. This setup will enable the subsequent study of vortex gust interaction with airfoils.

The spatial organisation of a passive scalar plume originating from a point source in a turbulent boundary layer is studied to understand its meandering characteristics. We focus shortly downstream of the isokinetic injection ( $1.5\leqslant x/\delta \leqslant 3$ , $\delta$ being the boundary-layer thickness) where the scalar concentration is highly intermittent, the plume rapidly meanders and breaks up into concentrated scalar pockets due to the action of turbulent structures. Two injection locations were considered: the centre of the logarithmic region and the wake region of the boundary layer. Simultaneous quantitative acetone planar laser-induced fluorescence and particle image velocimetry were performed in a wind tunnel, to measure scalar mixture fraction and velocity fields. Single- and multi-point statistics were compared with established works to validate the diagnostic novelties. Additionally, the spatial characteristics of plume intermittency were quantified using 'blob' size, shape, orientation and mean concentration. It was observed that straining, breakup and spatial reorganisation were the primary plume-evolution modes in this region, with little small-scale homogenisation. Further, the dominant role of coherent vortex motions in plume meandering and breakup was evident. Their action is found to be the primary mechanism by which the injected scalar is transported away from the wall in high concentrations ('large meander events'). Strong spatial correlation was observed in both instantaneous and conditional fields between the high-concentration regions and individual vortex heads. This coherent transport was weaker for wake injection, where the plume only interacts with outer vortex motions. A coherent-structure-based mechanism is suggested to explain these transport mechanisms.

The negative health impacts of extreme heat exposure can be mitigated by incorporating hyperlocal biometeorological observations into heat action planning, emergency responses, and heat-reducing urban design. A significant portion of outdoor human heat exposure is radiative, but it is often overlooked due to the absence of affordable, accurate, and user-friendly sensors. We developed a two cylinder anemometer and radiometer (CARla) consisting of unheated and heated gray components, which quantifies wind speed and the total radiation absorbed by the human body. The spectral properties of the gray coating match the standard short- and longwave absorptivity used in mean radiant temperature (MRT) calculations. We optimized the geometrical parameters of the cylinders, including height, wall thickness, and side-mounting, to minimize errors in MRT and wind speed measurements. Experiments were conducted across 15 outdoor sites in Tempe, Arizona, during the record-setting heat wave from August to October 2024. Results demonstrated that the MRT measured using CARla closely matched those measured using 3-way net radiometers. The average error in MRT using the new compact system was 1.3 ± 2.2 °C across a wide MRT range (20 to 75 °C). CARLa represents a significant improvement compared to other low-cost radiometers. The average difference between the CARla and ultrasonic anemometers for wind speed was − 0.05 ± 0.36 m·s−1 in the 0.25 to 3 m·s−1 range, comparable to standard low-cost anemometers. We integrated the CARla sensor with an Arduino-based logger, creating a cost-effective and accurate tool for broadly characterizing human exposure to extreme heat.

A novel experimental approach is proposed using a Convecting Rotating Cylinder (CRC) system to emulate the pressure perturbations imposed on a wall by an advecting vortex in a controlled and repeatable manner. The CRC system is constructed and characterized using Particle Image Velocimetry (PIV) and pressure measurements to understand the spatio-temporal characteristics of the imposed pressure. Stationary experiments were conducted to inform and finalize the design of the CRC. Designs with larger fins induced pressure perturbations with greater magnitudes and spread but the flow field deviated significantly from an ideal Rankine vortex. The final design uses a bare cylinder with a smooth surface for this reason. Translating experiments have been conducted with pressure and PIV measurements. The magnitude of the pressure perturbation measured at the wall is 42.5% that predicted by theory. An ensemble averaged PIV field around the translating cylinder illustrates a flow field resembling an ideal Rankine vortex. This work demonstrates the CRC's ability to impose a translating pressure perturbation on the wall by producing a Rankine vortex in a controllable and repeatable manner. Future work will entail fully characterizing the CRC temporally, and investigating the effects of pressure perturbations on the dynamics of turbulent boundary layers.

Turbulent/non-turbulent interfaces (TNTI) are evaluated for flows subject to the Rayleigh-Taylor instability (RTI). Experiments are conducted in a gas tunnel facility with air as heavy fluid and helium+nitrogen+air mixture as light fluid giving a low Atwood number of $ \approx 0.1 $ approximate to 0.1. Simultaneous velocity-density measurements are taken via particle image velocimetry and laser induced fluorescence. The nature of the TNTI on bubble front, as well as the change in mean quantities and turbulence statistics across this TNTI are investigated. The molecular mixing is studied relative to the TNTI. The TNTI shows a complex conditionally averaged volume fraction profile in its vicinity. In the external layer, the fluid is mostly pure heavy fluid, leading to no concentration gradients and a nearly zero measurement of the scalar dissipation. At the interface, there is a very large magnitude of scalar dissipation. In the adjustment layer, the scalar dissipation is nearly constant. These results challenge the conventional shape of profiles of turbulence statistics in RTI flows which are typically assumed parabolic. An alternate way to interpret the variation of turbulence statistics across the mixing width is to assume them to be nearly uniform in the core of the flow and be modulated by the location of the TNTI.

Experimental measurements investigating the turbulent boundary layer development over directional 2D roughness patterns, followed by separation behavior over a smooth contoured ramp will be discussed in the current work. The Particle Image Velocimetry (PIV) measurements were performed in the stream wise-wall-normal plane over a backward-facing hump. The 2D roughness pattern comprises directionally riblets in a converging and diverging pattern (C-D). This pattern introduces high- and low-momentum pathways (HMP's & LMP's), which alter the skin friction locally. From preliminary results, this was observed that this significantly affects the separation behavior over the trailing edge of the hump. The HMP regions were found to have weaker separation (smaller separation bubble, lower separation turbulent kinetic energy, TKE, etc.), while the LMP regions generally separated more strongly (and relative to the smooth-wall baseline flow). The separation TKE was found to vary by as much as 35% even though the bulk boundary layer was only found to differ by a few percent. This emphasizes the role of surface conditions in significantly altering the separation behavior of the boundary layer.

Results are presented for the explosive blast-driven instability of a stratified interface in a diverging cylindrical geometry using high-speed optical diagnostics. Specifically, the perturbation growth was studied using the qualitative Mie scattering technique, and the velocity, mixing transition, and vorticity characteristics are studied to investigate the behavior of the interface using planar particle image velocimetry (PIV). The role of density contrast is studied using CO2-air (low Atwood number, A) and SF6-air (high-A) interfaces. The perturbation growth h t theta was studied, where the decay parameter theta was found for A 0.22 and A 0.68 to be 0.34 and 0.50, respectively. Using velocity information from PIV, it was found that the initial vorticity deposition from the pressure impulse can accurately be estimated using known models previously developed for Richtmyer-Meshkov instability. There is a significant addition to the interface circulation by the long pressure decay phase of the blast wave, which gives rise to a variable acceleration Rayleigh-Taylor instability contribution. The high-A case showed a prolonged increase in circulation, plausibly due to higher inertia. This trend was also supported by variations in peak turbulence intensity, where a sharp increase at early times quickly decays with increased mixing between the two fluids. Finally, the interface Reynolds number of the higher A was found to exceed all criteria for the mixing transition (also indicated by the qualitative observations of the mixing in spike-structures in the Mie scattering images).

Sweat evaporation is critical to human thermoregulation, but current understanding of the process on 20 μm to 2 cm scale is limited. To this end, we introduce a wind-tunnel-shaped ventilated capsule with an infrared window for simultaneous infrared sweat imaging and evaporation rate measurement. Implementing the capsule in pilot human subject tests suggests that the common assumption of sweat being an isothermal film is only valid when the evaporation rate is low and sweat forms puddles on the skin. Before transitioning to this filmwise mode, sweating occurs in cyclic dropwise mode, displaying a 3x higher mass transfer coefficient in the same conditions. Imaging highlighted distinct phenomena occurring during and between these modes including out-of-duct evaporation, pulsating droplets, temporary and eventually lasting crevice filling, and individual drop-to-film spreading. In all, sweat evaporation is an impactful area that our results show is ripe for exploration, which can be achieved quantitatively using the introduced platform.

Extreme heat is a current and growing global health concern. Current heat exposure models include meteorological and human factors that dictate heat stress, comfort, and risk of illness. However, radiation models simplify the human body to a cylinder, while convection ones provide conflicting predictions. To address these issues, we introduce a new method to characterize human exposure to extreme heat with unprecedented detail. We measure heat loads on 35 body surface zones using an outdoor thermal manikin (“ANDI”) alongside an ultrasonic anemometer array and integral radiation measurements (IRM). We show that regardless of body orientation, IRM and ANDI agree even under high solar conditions. Further, body parts can be treated as cylinders, even in highly turbulent flow. This geometry-rooted insight yields a whole-body convection correlation that resolves prior conflicts and is valid for diverse indoor and outdoor wind flows. Results will inform decision-making around heat protection, adaptation, and mitigation.

As populations and temperatures of urban areas swell, more people face extreme heat and are at increasing risk of adverse health outcomes. Radiation accounts for much of human heat exposure but is rarely used as heat metric due to a lack of cost-effective and accurate sensors. To this end, we fuse the concepts of a three-globe radiometer-anemometer with a cylindrical human body shape representation, which is more realistic than a spherical representation. Using cost-effective and readily available materials, we fabricated two combinations of three cylinders with varying surface properties. These simple devices measure the convection coefficient and the shortwave and longwave radiative fluxes. We tested the devices in a wind tunnel and at fourteen outdoor sites during July 2023’s record-setting heat wave in Tempe, Arizona. The average difference between pedestrian-level mean radiant temperature (MRT) measured using research-grade 3-way net radiometers and the three-cylinder setup was 0.4 ± 3.0 °C ( ± 1 SD). At most, we observed a 10 °C MRT difference on a white roof site with extreme MRT values (70 °C to 80 °C), which will be addressed through discussed design changes to the system. The measured heat transfer coefficient can be used to calculate wind speed below 2 m·s−1; thus, the three cylinders combined also serve as a low-speed anemometer. The novel setup could be used in affordable biometeorological stations and deployed across urban landscapes to build human-relevant heat sensing networks.

Mixing of fluids in a coaxial jet is studied under four distinct viscosity ratios, $m=1$ , $10$ , $20$ and $40$ , using highly resolved large-eddy simulations (LES), particle image velocimetry and planar laser-induced fluorescence. The accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. For the highest and lowest viscosity ratios, standard RANS models with unclosed terms pertaining to viscosity variations are employed. We show that the standard Reynolds-averaged Navier–Stokes (RANS) approach with no explicit modelling for variable-viscosity terms is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying mixing physics and sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, momentum budget analysis with emphasis on variable-viscosity terms, analysis of the turbulent activity and decay of turbulence are investigated using highly resolved LES data. The mixing dynamics is found to be quite distinct in each mixing mode. Variable viscosity manifests multiple effects that are working against each other. Viscosity gradients induce additional instabilities while increasing overall viscosity decreases the effective Reynolds number leading to laminarization of the turbulent jet, explaining the lack of dispersion and turbulent diffusion. Momentum budget analysis reveals that variable-viscosity terms are significant to be neglected. The scaling of the energy spectrum cascade suggests that in the TLL mode the unsteady laminar shedding is responsible for the eddies observed.

Dynamic inline spectroscopic measurements and Large Eddy Simulation (LES) observe the products of a competitive-consecutive test reaction system within a well-characterized coaxial flow. The test reaction is a competitive consecutive chemistry which produces two spectrophotometrically measurable products. For the first time, these products are measured spatially throughout the reactor. Experimental and computational results map the reaction progress and product distribution. The evolution of this chemical process is explained in terms of the inlet, jet, and pipe flow regimes of the confined jet. The inlet and jet regimes were demonstrated to produce the majority of the desired product, whereas the undesired product is predominantly generated in the pipe regime. CO 2022 Published by Elsevier Ltd.

In this study, ensembles of experimental data are presented and utilized to compare and validate two models used in the simulation of variable density, compressible turbulent mixing. Though models of this kind (Reynolds Averaged Navier-Stokes and Large-Eddy Simulations) have been validated extensively with more canonical flows in previous studies, the present approach offers novelty in the complexity of the geometry, the ensemble based validation, and the uniformity of the computational framework on which the models are tested. Moreover, all experimental and computational tasks were completed by the authors which has led to a tightly coupled experimental configuration with its "digital twin.” The experimental divergent-shock-tube facility and its data acquisition methods are described and replicated in simulation space. A 2D Euler model which neglects the turbulent mixing at the interface is optimized to experimental data using a Gaussian process. This model then serves as the basis for both the 2D RANS and 3D LES studies that make comparisons to the mixing layer data from the experiment. RANS is shown to produce good agreement with experimental data only at late flow development times. The LES ensembles generally show good agreement with experimental data, but display sensitivity to the characterization of initial conditions. Resolution dependent behavior is also observed for certain higher-order statistics of interest. Overall, the LES model successfully captures the effects of divergent geometry, compressibility, and combined non-linear instabilities inherent to the problem. The successful prediction of mixing width and its growth rate highlight the existence of three distinct regimes in the development of the instability, each with similarities to previously studied instabilities.

The current work presents simultaneous, high-speed measurements at 60,000 fields per second of velocity and mole fraction using particle image velocimetry (PIV) and planar laser induced acetone-fluorescence in a Richtmyer-Meshkov instability of an inclined interface (Atwood number, At = 0.22). Specifically, around 2 ms of temporal evolution of the vortex structures and their associated scalar modes immediately following the interface-reshock interaction is presented. Two initial interface conditions are discussed-(a) a sharp, inclined 'single mode' interface and (b) a 'multi-mode' interface where small perturbations are imposed on the single mode case. A 2D wavelet decomposition of the scalar flow field shows a highly intermittent distribution of small-scale variance throughout the interface even at late times. These are correlated strongly with the vortex structures and local turbulence intensity, where each small-scale scalar mode is sandwiched between two co-rotating vortex structures. This indicates that the interstitial regions between the vortices are significant hotspots of entrainment, which is then dispersed by the induced, counter-flow velocity fields. The multimode case demonstrates similar organization at large scales, while the scalar field is much more homogeneous at smaller scales. These observations highlight the importance of capturing the early time vortex evolution to accurately estimate any late time intermittency, especially where deposition of intense vorticity on sharp interfaces is present.

Received 27 July 2022DOI:https://doi.org/10.1103/PhysRevFluids.7.110001©2022 American Physical Society

The dynamics of molecular mixing and the energy transfer process in the Rayleigh-Taylor instability (RTI) are studied through the collection of simultaneous velocity-density measurements using particle image velocimetry (PIV) and laser induced fluorescence (LIF). Statistically stationary experiments are performed in a convective-type gas tunnel facility which allows long experimental times and enables collection of statistically important turbulence data. The data and analyses presented in this paper are expected to help validate variable-density turbulence models and further our understanding of instability-driven flows.

Turbulent shear-driven mixing in a coaxial and co-flowing configuration is studied using experiments and computations to understand and model the effect of viscosity gradients in the flow field. Two liquids with a large disparity in dynamic viscosity are mixed, with a low viscosity, high momentum jet directed into a high viscosity, low momentum co-flow in a pipe. Simultaneous experimental measurements of the velocity and concentration fields are made using high-resolution PIV and PLIF to obtain their turbulent cross-correlation statistics for viscosity ratios of 1 and 40. LES simulations are also performed using dynamic mixing sub-grid model to investigate the three dimensional mixing structure of the flow for the two cases. The overall structure of turbulent mixing in the coaxial confined jet configuration is studied to identify the mixing regions of the flow and the effect of viscosity gradients on the dynamics of the same. Besides the effect of Reynolds number between the two cases that manifests as reduced mixing, it was noted that the transport of turbulent kinetic energy and scalar concentration variance shows significant asymmetries that arise from the viscosity gradients in the field. The scalar mixing structure between the two cases is studied in detail with relevance to complex mixing-limited reactions frequently encountered in such environments. It was found that turbulence production and associated scalar mixing is highly skewed in variable viscosity flows, where the low viscosity regions show enhanced turbulence activity. The implications of such turbulence skewness on the chemistry of reaction systems involving variable viscosity environments are discussed in further detail.

The current work presents a study of qualitative relationships between the flow unsteadiness, induced by nozzle geometry variation, and non-equilibrium condensation behavior at high temporal scales for supercritical carbon dioxide (sCO2) flow. A closed sCO2 loop facility is used to drive a supercritical carbon dioxide flow through a rectangular converging–diverging channel. As the flow traverses through the channel, it experiences a local pressure reduction in a stalled region, eventually leading to non-equilibrium condensation of the working fluid. The flow is visualized using high-speed shadowgraphy and schlieren techniques to characterize the unsteady flow dynamics at the diverging section of the nozzle. Spectral information of the condensation behavior is retrieved from the optical diagnostics using power spectral density calculations, revealing multiple dominant frequencies in the diverging section of the nozzle. Three different nozzles are studied to understand how these dominant frequencies of condensation phenomenon change with respect to the nozzle geometry. Each nozzle design is distinguished with a unique dominant frequency, which is significantly impacted by the effect of flow throat velocity and the degree of adverse pressure gradient. Nozzles with divergence of 15° shows a dominant frequency in the range of 5 – 6.25 kHz in condensing behavior while nozzles with 6° divergence exhibited half the frequency of the phase change. These effects resulted from the extent of adverse pressure gradient imposed by the divergence angle of the nozzle. Additionally, these dominant frequencies are also compared to the wall-pressure signals acquired from high-frequency pressure transducers. Spectral analysis of wall-pressure signals, which are directly linked to nozzle designs, reveals a strong coupling between the condensation behavior and the wall-pressures.