A shallow water free-surface turbulent jet is investigated at Froude numbers variable between about 0.15 and 0.6 and a Reynolds number equal to 5000, with a jet height-to-width aspect ratio variable ...from 0.5 to 1.1. In these experimental conditions, the flow is unstable and develops local transverse instabilities, which are amplified to a global meandering motion, forcing the jet to oscillate orthogonally to its axis. Instantaneous and averaged velocity fields are obtained by means of high-density, correlation-based time-resolved Particle Tracking Velocimetry. From the average point of view, the present jet configuration resembles a confined jet condition similar to that of wall jets, but with additional relevant three-dimensional effects, retaining self-similar properties, with a net co-flow and some ambient fluid entrainment. For increasing Froude numbers, such configuration is moved downstream and the jet spreading is delayed. The high resolution in space and time of present measurements allows to locally detect and following perturbed patterns and to derive amplitude and frequency of oscillation of the global meandering motion, the former increasing with axial distance and decreasing with Froude number, the latter doing just the opposite. The velocity of propagation of perturbed patterns is also investigated in comparison to the jet mean velocity, the local celerity of propagation of fluctuating velocity fields being derived. Results indicate that axial fluctuations propagate downstream similarly to transverse fluctuations along the orthogonal direction, thus showing a close coupling among the two motions and the onset of a self-sustained mechanism driven by the mean flow, responsible for the generation of the macroscopic meandering motion. This effect is in agreement with numerical predictions and is emphasised as the Froude number increases.
Graphic abstract
In this contribution, three methodologies based on temperature-sensitive paint (TSP) data were further developed and applied for the optical determination of the critical locations of flow separation ...and reattachment in compressible, high Reynolds number flows. The methodologies rely on skin-friction extraction approaches developed for low-speed flows, which were adapted in this work to study flow separation and reattachment in the presence of shock-wave/boundary-layer interaction. In a first approach, skin-friction topological maps were obtained from time-averaged surface temperature distributions, thus enabling the identification of the critical lines as converging and diverging skin-friction lines. In the other two approaches, the critical lines were identified from the maps of the propagation celerity of temperature perturbations, which were determined from time-resolved TSP data. The experiments were conducted at a freestream Mach number of 0.72 and a chord Reynolds number of 9.7 million in the Transonic Wind Tunnel Göttingen on a VA-2 supercritical airfoil model, which was equipped with two exchangeable TSP modules specifically designed for transonic, high Reynolds number tests. The separation and reattachment lines identified via the three different TSP-based approaches were shown to be in mutual agreement, and were also found to be in agreement with reference experimental and numerical data.
•Relation between propagation celerity UU of u′ and friction velocity uτ.•Relation between propagation celerity UT of T′ and UU.•Extraction of UT from time-lag of correlation peaks ...occurrence.•Extraction of UT based on Taylor’s hypothesis.•Profiles and maps of friction quantities uτ and Cf.
We report about the feasibility of two criteria for the direct measurement of the skin friction τ which are based on the investigation of the passive transport of temperature fluctuations, as obtained from Temperature-Sensitive Paint (TSP) data. The first criterion represents a proof-of-concept about the reliability of the use of the passive transport of temperature fluctuations Tw for the estimation of uτ. It relies on the identification of the time lag corresponding to the correlation peak between temperature time histories taken at points separated by fixed streamwise distance from the investigated location. The second criterion expands the former to check the feasibility of the skin friction measurement by means of Tw propagation celerity in a wider range of flow conditions. It is derived by minimizing the deviation from the Taylor hypothesis of the equation of transport of temperature fluctuations, which corresponds to the energy equation for incompressible flows at the investigated conditions. Firstly, a common rule about the relationships between propagation celerity UT of the temperature disturbances at the wall beneath a turbulent boundary layer and friction velocity uτ is assessed from literature. Starting from this theoretical basis, the focus is placed on the flow over the suction side of a NACA 0015 hydrofoil model and in particular on the laminar separation bubble developing on this model surface, investigated experimentally at a chord Reynolds number of Re=1.8×105 and angles of attack AoA=1°,3°,5°,7°,10°. The profiles of time- and spanwise-averaged UT(x) and Cf(x) (friction coefficient) are proposed and critically analyzed. Time averaged maps of the same quantities are then reported and commented as well. Paper topics are focused on:•The relationship between the propagation celerity of the velocity disturbances UU and the friction velocity uτ•The relationship between the propagation celerity of the temperature disturbances UT and UU•The algorithms for the extraction of the propagation celerity of temperature perturbations UT based on both the time lag of the correlation peak occurrence and the minimization of the deviation of transport equation for temperature fluctuations from the Taylor’s hypothesis.•The resulting profiles and maps of friction quantities uτ and Cf.
An experimental investigation on a horizontal two-phase air-water jet has been conducted in order to develop a technique that can provide information on the interaction between the air bubbles and ...turbulent flow. In order to allow a measurement of the liquid phase flow field and of the bubbles size, the Particle Image Velocimetry (PIV) has been used, with the liquid phase seeded with a fluorescent tracer consisting of Rhodamine WT mixed with a commercial polyester resin. A phase discrimination algorithm has been developed and successfully used for the detection and separation of tracers and bubbles in distinct images. Three data sets have been recorded: one for a one-phase water jet and two for a two-phase water jet. The obtained results allow a valid characterization of the flow field and an estimation of the effect of the gaseous phase in the air-water jet flow. An analysis of the bubbles size and number has been also conducted and here reported.
The deformation of air bubbles in a liquid flow field is of relevant interest in phenomena such as cavitation, air entrainment, and foaming. In complex situations, this problem cannot be addressed ...theoretically, while the accuracy of an approach based on Computational Fluid Dynamics (CFD) is often unsatisfactory. In this study, a novel approach to the problem is proposed, based on the combined use of a shadowgraph technique, to obtain experimental data, and some machine learning algorithms to build prediction models. Three models were developed to predict the equivalent diameter and aspect ratio of air bubbles moving near a plunging jet. The models were different in terms of their input variables. Five variants of each model were built, changing the implemented machine learning algorithm: Additive Regression of Decision Stump, Bagging, K-Star, Random Forest and Support Vector Regression. In relation to the prediction of the equivalent diameter, two models provided satisfactory predictions, assessed on the basis of four different evaluation metrics. The third model was slightly less accurate in all its variants. Regarding the forecast of the bubble’s aspect ratio, the difference in the input variables of the prediction models shows a greater influence on the accuracy of the results. However, the proposed approach proves to be promising to address complex problems in the study of multi-phase flows.
A dilute fiber suspension in a turbulent channel with a backward-facing step is investigated by means of Feature Tracking. Its combination with a phase-discrimination methodology, which is described ...in detail, allows simultaneous and separate measurement of carrier and dispersed phases velocity fields, the orientation and rotation rate of fibers as well as the fiber–fluid translational and rotational slip velocities. The patterns of fibers concentration, angular velocity and the probability distribution of fibers velocity appear to be dominated by the mechanical interactions with the wall and the local high shear rather than by near-wall turbulent structures. The translational slip velocity obtained from instantaneous data shows that fibers move faster than the surrounding fluid inside the buffer layer, the velocity gap reducing gradually when approaching the channel centerline. On the other hand, the rotational slip profile suggests a gradual decoupling of the translational and rotational dynamics. Downstream of the step, the excess of streamwise velocity displayed by fibers is still observed and extends in the free-shear region, whereas the rotation rate slip decreases at a relatively short distance from the step, as the effect of the wall presence fades away.
We study the dynamics of a sandy bed around a slender vertical cylinder forced by progressive, non-linear water waves. The seabed evolves continuously under the effects of the up-welling, ...down-welling and rolling events induced by vortical coherent structures. In turn, these are closely connected to the shape of the seabed, which is modified by the scouring and/or the deposition of the sand. Starting from a flat seabed, progressive waves induce a rapid and transient modification of the bottom morphology towards a dynamically stable equilibrium state, which is the focus of this work. The dynamical equilibrium state is a function of the wave period and is reached when the seabed morphology is not substantially altered. We describe such a state by an Eulerian in-phase analysis of the sand particle motion, inferred from Lagrangian data collected over a large number of wave passages. This analysis relies on the use of the defocusing digital PIV technique (DDPIV), for the first time applied to the specific flow of interest here. On the basis of the Eulerian analysis, the triggering of the key-events (up- and down-welling, rolling) over the wave phase is captured by identifying, through the Q > 0 criterion, the coherent flow structures responsible for the events. This analysis is coupled with the description of the sediment trajectories, analyzed in a Lagrangian manner and effectively assessing how and where the solid phase is transported during the key-events. Five main mobilization/transport mechanisms have been identified, three during the onshore flow and two during the offshore flow: (i) generation of a coherent structure reminiscent of a horseshoe vortex at the toe, (ii) intense scouring at the top of the flatbed region, (iii) vortex shedding in the wake during direct (onshore) flow, (iv) shear crossflow on the lee-side of the cylinder and (v) large vertical shearing in the flatbed region during the reverse (offshore) flow. At flow reversal, this shearing mechanism impacts on a significant area of sediments in the incoming region of the flow.
Incipient stall characterization from skin-friction maps Miozzi, Massimo; Capone, Alessandro; Klein, Christian ...
International journal of numerical methods for heat & fluid flow,
03/2021, Letnik:
31, Številka:
2
Journal Article
Recenzirano
Purpose
The purpose of this study is the characterization of the dramatic variation in the flow scenario occurring at incipient stall conditions on a NACA0015 hydrofoil at moderate Reynolds numbers ...via the experimental analysis of time- and space-resolved skin-friction maps. The examined flow conditions are relevant for a variety of applications, including renewable energy production and unmanned and micro-aerial vehicles.
Design/methodology/approach
Grounding on the global temperature data acquired via temperature-sensitive paint, the proposed methodology adopts two approaches: one to obtain time-resolved, relative skin-friction vector fields by means of an optical-flow-based algorithm and the other one to extract quantitative, time-averaged skin-friction maps after minimization of the dissimilarity between the observed passive transport of temperature fluctuations and that suggested by the Taylor hypothesis.
Findings
Through the synergistic application of the proposed methods, the time-dependent evolution of the incipient stall over the hydrofoil suction side is globally described by firstly identifying the trailing edge separation at an angle of attack (AoA) AoA = 11.5°, and then by capturing the onset of upstream oriented, mushroom-like structures at AoA = 13°. The concomitant occurrence of both scenarios is found at the intermediate incidence AoA = 12.2°.
Originality/value
The qualitative, time-resolved skin-friction topology, combined with the quantitative, time-averaged distribution of the streamwise friction velocity, enables to establish a portrait of the complex, three-dimensional, unsteady scenario occurring at the examined flow conditions, thus providing new, fundamental information for a deeper understanding of the incipient stall development and for its control.
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