Employing Particle Image Velocimetry Research Articles

Experimental heat transfer analysis of the effects of thermophysical properties on PCM at freezing conditions employing Particle Image Velocimetry

Cold chain logistics of fresh produce distribution are facing new challenges owing to the increasing consumer demand, often traveling long distances, and facing temperature variations during storage and distribution stages. Nowadays, solutions for improving food preservation by searching effective energy storage strategies, such as the use of PCM, are capturing more interest. Nevertheless, most studies are focused on organic PCM with ambient temperature melting points analyses. This leaves an important research gap in the experimentation of other types of eutectic materials with melting points below 0 °C, something of great interest for food preservation in the refrigeration industry. Velocity vector field and heat transfer via natural convection in a small cavity filled with two different PCM at below 0 °C conditions were experimentally investigated employing temperature measurements and Particle Image Velocimetry. In this regard, brine (18.8 wt%) and ethylene glycol-water (EGW) blend (30.5 wt%) PCM with equal melting temperature, but different thermophysical properties, were selected to compare their thermal behavior. It was investigated the effects of thermophysical properties such as density, specific heat capacity, thermal conductivity, and viscosity on melting front, heat transfer and flow patterns through the evaluation of liquid fraction, heat storage, Nu, Gr, Ra, and velocity field. The maximum Nu value for brine was almost the double of that of EGW. Pr values of EGW were in average 1.88 times higher than those of brine. Besides, Gr and Ra values of brine were in average 7.23 and 3.97 higher than those of EGW, respectively.

Experimental and numerical research on the viscous flow field induced by a swing plate

This study investigated the intricacies of the viscous flow field generated by a swing plate, with an emphasis on delineating vortex shedding and energy dissipation phenomena, employing particle image velocimetry (PIV) and large eddy simulation (LES). A resource-efficient, custom-built PIV system was harnessed for experimental data acquisition. The results unveiled a notable congruence between PIV and LES regarding vorticity and vortex localization during abbreviated rolling periods, thereby substantiating the credibility of LES. Conversely, for prolonged periods, LES manifested discrepancies attributed to its constraints in accurately representing elaborate turbulent structures. The investigation was extended to study the influence of varying rolling periods(4–8 s) on the vorticity field. A diminished rolling period culminated in a heightened peak rolling moment and augmented vorticity while preserving the structural integrity of the vortices, indicative of a linear energy transference schema. Furthermore, vortex shedding frequency was found to be directly proportional to the rolling period, with the shedding frequency being half the roll frequency, exhibiting regimented behavior corroborated by a stable Strouhal number. Additionally, an analysis at swing plate heights of 10–50 mm revealed that elevating the swing plate height amplified the vortex intensity up to a threshold, beyond which shear forces instigated vortex fragmentation. These revelations hold paramount significance in the optimization of design and operational strategies across domains including fluid manipulation, energy extraction, and biomimetic propulsion systems.

Generation and Observation of Long-Lasting and Self-Sustaining Marangoni Flow.

When solute molecules in a liquid evaporate at the surface, concentration gradients can lead to surface tension gradients and provoke fluid convection at the interface, a phenomenon commonly known as the Marangoni effect. Here, we demonstrate that minute quantities of ethanol in concentrated sodium hydroxide solution can induce pronounced and long-lasting Marangoni flow upon evaporation at room temperature. By employing particle image velocimetry and gravimetric analysis, we show that the mean interfacial speed of the evaporating solution sensitively increases with the evaporation rate for ethanol concentrations lower than 0.5 mol %. Placing impermeable objects near the liquid-gas interface enforces steady concentration gradients, thereby promoting the formation of stationary flows. This allows for contact-free control of the flow pattern as well as its modification by altering the objects shape. Analysis of bulk flows reveals that the energy of evaporation in the case of stationary flows is converted to kinetic fluid energy with high efficiency, but reducing the sodium hydroxide concentration drastically suppresses the observed effect to the point where flows become entirely absent. Investigating the properties of concentrated sodium hydroxide solution suggests that ethanol dissolution in the bulk is strongly limited. At the surface, however, the co-solvent is efficiently stored, enabling rapid adsorption or desorption of the alcohol depending on its concentration in the adjacent gas phase. This facilitates the generation of large surface tension gradients and, in combination with the perpetual replenishment of the surface ethanol concentration by bulk convection, to the generation of long-lasting, self-sustaining flows.

Increasing vortex-induced vibration-based energy harvesting using a nature-inspired bluff body: An experimental study

The present study investigates the effect of changes on the cross-sectional geometry of a flow-induced vibration-based energy harvester. To do so, a bio-inspired geometry, which is adopted from the ‘Amaryllis’ flower is utilized. The fluid dynamic acting forces are numerically studied for the Reynolds numbers between 100 and 5000. Based on the attack angles of the so-called convex and concave petal cylinders are proposed, and three geometrical configurations with six, eight, and ten petals are considered. Furthermore, the concave and the convex cylinders’ vortex interaction and near wake flow are experimentally studied for two Reynolds numbers, employing Particle Image Velocimetry (PIV). A proportional relation was found between the enhancement of fluid dynamic forces and the shortening of vortex formation length that would improve Vortex-Induced Vibration (VIV). Finally, validated lift coefficients are employed in the analytically derived electromechanical equation of motion for the VIV-based energy harvester. It is shown that the newly proposed nature-inspired system can effectively increase the amount of harvested energy.

PIV measurements over a double bladed Darrieus-type vertical axis wind turbine: A validation benchmark

Vertical axis wind turbines (VAWTs) are very attractive for in-home power generation since they can be adopted even at low wind speeds and highly variable wind direction. Even if significant experimental research activity has been carried out to improve VAWTs performance, the ability to accurately reproduce flow field characteristics around turbine blades by CFD (computational fluid dynamics) techniques represents a powerful approach to further enhance wind turbines performance. Thanks to CFD, in fact, it is possible to reproduce flow characteristics with a detail level impossible to achieve by experiments. Nevertheless, in order to appropriately analyze the flow structure by CFD application, an accurate validation is essential, and high-quality measurements of some main flow characteristics are required. In recent publications the authors investigated, both experimentally and numerically, the performance of an innovative double bladed Darrieus-type VAWT, with the aim to define an optimal configuration also focusing on self-starting ability of the prototype by employing CFD technique. Nevertheless, comparison between experiments and numerical results was made only in terms of power and torque coefficients. To overcome such limitation, in this paper the authors propose an experimental benchmark case for CFD results validation, describing detailed flow field in correspondence of one pair of blades of the innovative Darrieus-type VAWT in static conditions. Measurements were performed employing Particle Image Velocimetry (PIV) technique on a scaled model of the turbine blades realized by 3D printing. An uncertainty analysis was also performed which showed a high accuracy of the obtained experimental results. The measurements of the main flow characteristics (bi-dimensional velocity components) were then used for a test case CFD validation of two different turbulence models.

A PIV investigation of laminar and turbulent viscoplastic fluid flow on axisymmetric abrupt contraction

The current work is devoted to analyzing viscoplastic fluid flow in a circular axisymmetric abrupt contraction with ratio . The velocity fields and static pressure along a hydraulic loop were measured employing Particle Image Velocimetry technique and a pressure transducers system, respectively. Four aqueous solutions of Carbopol 940 with different rheological parameters were used as viscoplastic fluids. Laminar and turbulent flow conditions are evaluated and compared with the generalized Hagen–Poiseuille law and the Malin friction factor correlations supported by turbulent fluctuations measurements. The formation of a plug core region is observed around the centerline of the pipe. The relationships between the velocity profiles and the shear stress distribution with the plug core were investigated supported via shear rate calculations. The results show that the yield stress highly influences the shape of the velocity profiles and the appearance of unyielded regions at the contraction edges. The shear stress maps show a layered distribution across the pipe diameter with higher values located at the contraction entrance. Instead, a constant shear stress distribution is found in the unyielded regions. The size of these unyielded regions increases with the yield stress but is reduced at regions with high shear rates due to increased inertial forces. Pressure drop measurements contrasted with the shear stress and velocity gradients maps illustrated that the pressure losses along the pipe are affected by the yield stress and the Reynolds number. However, a non-dependence of the pressure loss coefficient at the contraction plane with the rheological parameters is evidenced, as presented in earlier experimental results. This differs from many numerical studies since the contraction’s pressure loss coefficient for viscoplastic fluids is directly affected by the increase of inertial forces at this region.

Bioengineering a cryogel-derived bioartificial liver using particle image velocimetry defined fluid dynamics

Bioartificial Liver (BAL) devices are extracorporeal systems designed to support or recover hepatic function in patients with liver failure. The design of an effective BAL remains an open challenge since it requires a complex co-optimisation of cell colonisation, biomaterial scaffold and BAL fluid dynamics. Building on previous evidence of suitability as a blood perfusion device for detoxification, the current study investigated the use of RGD-containing p(HEMA)-alginate cryogels as BAL scaffolds. Cryogels were modified with alginate to reduce protein fouling and functionalised with an RGD-containing peptide to increase hepatocyte adhesion. A novel approach for characterisation of the internal flow through the porous matrix was developed by employing Particle Image Velocimetry (PIV) to visualise flow inside cryogels. Based on PIV results, which showed the laminar nature of flow inside cryogel pores, a multi-layered bioreactor composed of spaced cryogel discs was designed to improve blood/hepatocyte mass exchange. The stacked bioreactor showed a significantly higher production of albumin and urea compared to the column version, with improved cell colonisation and proliferation over time. The cell-free cryogel-based device was tested for safety in a bile-duct ligation model of liver cirrhosis. Thus, a stacked bioreactor prototype was developed based on a surface-engineered cryogel design with optimised fluid dynamics for BAL use.

Effect of ground on flow characteristics and aerodynamic performance of a non-slender delta wing

The aerodynamic performance and the structure of vortical flow on a delta wing are affected by the influence of the ground in take-off and landing stages. In this context, the effect of the ground on a delta wing having a sweep angle of 40° was investigated by employing Particle Image Velocimetry (PIV), aerodynamic force measuring system and the dye flow visualization technique. Flow characteristics of delta wing were examined under two different angles of attack, α=8° and 11° and variation of the distance between trailing edge of the wing and ground, h normalized with the root chord length, c of the wing. It was observed that the existence of the ground attenuates the magnitude of peak values of primary and secondary vortices due to incomplete development of vortices. The ground effect caused the outboard movement of the leading-edge vortex in a spanwise direction as well as an increase in the size of vortices. Furthermore, the presence of the ground induced a decrease in Strouhal number, St due to the slowing down of vortex formation. Lift and drag coefficient, CL, and CD of the delta wing were observed to increase with descending from unbounded flight region into ground effect region. Finally, it was found that CL/CD increases by reducing the distance between ground and wing, h/c, and a rise of CL/CD is much more effective under the lower angles of attack, α.

Investigation of crossflow features of a slender delta wing

In the present work, the main features of primary vortices and the vorticity concentrations downstream of vortex bursting in crossflow plane of a delta wing with a sweep angle of Λ=70° were investigated under the variation of the sideslip angles, Β. For the pre-review of flow structures, dye visualization was conducted. In connection with a qualitative observation, a quantitative flow analysis was performed by employing Particle Image Velocimetry (PIV). The sideslip angles, Β were varied with four different angles, such as 0°, 4°, 12°, and 20° while angles of attack, α were altered between 25° and 35°. This study mainly focused on the instantaneous flow features sequentially located at different crossflow planes such as x/C=0.6, 0.8 and 1.0. As a summary, time-averaged and instantaneous non-uniformity of turbulent flow structures are altered considerably resulting in non-homogeneous delta wing surface loading as a function of the sideslip angle. The vortex bursting location on the windward side of the delta wing advances towards the leading-edge point of the delta wing. The trajectory of the primary vortex on the leeward side slides towards sideways along the span of the delta wing. Besides, the uniformity of the lift coefficient, CL over the delta wing plane was severely affected due to unbalanced distribution of buffet loading over the same plane caused by the variation of the sideslip angle, Β. Consequently, dissimilarities of the leading-edge vortices result in deterioration of the mean value of the lift coefficient, CL.

Kinematics of Particles at Entrainment and Disentrainment

We address the issue of characterizing experimentally entrainment and disentrainment of sediment particles of cohesionless granular beds in turbulent open channel flows. Employing Particle Image Velocimetry, we identify episodes of entrainment and of disentrainment of bed particles by analysing the raw PIV images. We define a reference velocity for entrainment or disentrainment by space-averaging the flow field in the vicinity of the (entrained or disentrainned) particle and by time-averaging that space-average over a short duration encompassing the observed episode. All observations and measurements took place under generalized movement conditions and in non-controlled geometrical set-ups, resulting in unique databases of conditionally sampled turbulent flow kinematics associated with episodes of particle entrainment and of particle disentrainment. Exploring this database, the objective of this paper is to prove further insights on the dynamics of fluid-particle and particle-particle interactions at entrainment and disentrainment and to polemicize the use of a reference velocity to serve as a proxy for hydrodynamics actions responsible for entrainment or disentrainment. In particular, we quantify the reference velocity associated with entrainment and disentrainment episodes and discuss its potential to describe the observed motion vis-a-vis local bed micro-topography and the type of entrainment or disentrainment event. Entrainment may occur at a wide range of reference velocities, including smaller than mean (double-averaged) velocities. Anecdotal evidence was collected for some typologies of entrainment: (i) momentum transfer from flow to a single particle, (ii) momentum transfer from a perturbed local flow to a single particle, (iii) collective entrainment associated to momentum transfer between a moving and a resting particle and (iv) collective entrainment considered to be a dislodgment of several particles involving momentum transfer from other particles. In some of these cases, e.g., (ii) and (iii), the use of a reference velocity seems inadequate to characterize the entrainment episode. A word of caution about the use of entrainment models based on reference velocities is henceforth issued and contextualized. In the case of disentrainment, a reference velocity seems to constitute a better descriptor of the observed behaviour. The scatter in the observed values seems to express the contribution of bed micro-topography. All particles were found to experience frictional contacts with the resting bed surface particles, but some particles were stopped more abruptly due to the presence of an obstacle along their path. Most disentrainment of particles took place when the near-bed flow was featuring ejection events.

CFD modelling of a thermal chimney for air-cooled condenser

Thermal chimney driven/enhanced air-cooled condensers have increasingly found extensive applications in buildings, thermal and geothermal power plants. A small scale model of thermal chimney with rectangular cross-section of constant area was designed and one-row electrical heaters were installed to mimic the shell-and-tube heat exchanger, and simulations of conjugate heat transfer in the chimney were carried out by using computational fluid dynamics (CFD) software-ANSYS CFX at various heater nominal temperatures and 22.5 ℃ ambient temperature. The heat transfer models adopted include steady three-dimensional Reynolds-averaged Navier-Stokes equations and k-ω turbulence model as well as Boussinesq buoyancy assumption. The radiation effect from the heaters to the air was considered. The heater temperature profile was mapped by using forward-looking infrared camera and the air velocity in the chimney was measured by employing particle image velocimetry to validate CFD velocity fields. The measured temperature profile was modelled and involved into CFX as temperature boundary conditions. It was shown that the heaters can induce an air flow in the chimney to generate a cooling effect. As the heater nominal temperature increases from 80 ℃ to 170 ℃, the chimney energy gain coefficient rises from 0.40 to 0.60, but saturated beyond 130 ℃, the Reynolds number of the chimney is ranged in 2000–4000, while the Reynolds number of the heaters varies in 140–270, and the Nusselt number of the heaters is as low as 7.0-8.2. Flow separation can occur at lower than 130 ℃. The radiation from the heaters makes a slightly more 1/3 contribution in the heat transfer. It is suggested that the primary heat exchanger/heater should operate at a temperature above 130 ℃.

Self-excited gas–solid two-phase flow using non-uniform soft fins

The purpose of this study is to reduce the conveying velocity and power consumption in gas–solid two-phase flows. Four pieces of soft fins with different (non-uniform) lengths, which are fixed on the horizontal central plane of the inlet, are employed to self-excite the gas–solid flow. A horizontal acrylic pipe, approximately 5 m long and with an inside diameter of 80 mm, is selected as the test pipeline. The test solid materials are spherical polyethylene particles with average diameter and density of 2.3 mm and 978 kg/m3, respectively. The solid mass flow rate and average gas velocity are 0.10–0.47 kg/s and 10–17 m/s, respectively. Compared with non-fin and previous uniform soft fin gas–solid flows, the minimum gas velocity, pressure drop, additional pressure drop, and power consumption are reduced when a lower gas velocity is achieved with the use of non-uniform soft fins. The highest reduction rates in power consumption and minimum velocity are approximately 5.87% and 10.04%, respectively. The particle concentration in flows with non-uniform soft fins is higher than that in the non-fin flow in the upper part of the pipe of the acceleration region and lower than those in non-fin and uniform fin flows near the pipe bottom. By employing particle image velocimetry measurement, the time-averaged axial particle velocity and fluctuating particle velocity in all non-uniform fin flows are found to be higher than those in non-fin and uniform fin flows, especially near the upper part of the pipe.

Increasing the Aerodynamic Performance of a Formula Student Race Car by Means of Active Flow Control

<div class="section abstract"><div class="htmlview paragraph">This article involves an experimental study regarding the capability of fluidic actuators to increase the aerodynamic performance of a four-element race car rear wing. Sweeping jet actuators are integrated in the upper flap, of which the angle of attack is increased by up to Δ<i>α</i><sub>F3</sub> = 40<sup>°</sup> with reference to a passively optimized setup. Different velocities of the emitted sweeping jets are applied to study the influence of momentum coefficients <i>c<sub>μ</sub></i> = 0.04 … 0.98%. To prove the feasibility of the approach, flow control is first applied to a stand-alone rear wing tested in a small wind tunnel. Subsequently, a realistic race car model featuring the controlled rear wing is investigated in a larger-scale wind tunnel. Employing particle image velocimetry, flow visualization techniques as well as pressure and force measurements, we show that the velocity field on the suction side of the upper flap is characterized by flow separation of different degrees when the angle of attack is increased beyond Δ<i>α</i><sub>F3</sub> = 20<sup>°</sup> (rear wing only) and Δ<i>α</i><sub>F3</sub> = 30<sup>°</sup> (complete race car). Generally, the employed fluidic devices have a positive impact in terms of the aerodynamic performance for all investigated angles of attack as overall downforce increases by up to 22 % are achieved, however accompanied by larger drag. Nonetheless, an in-house simulation indicates a potential lap time reduction of 0.17 % at the Formula Student endurance race circuit at Hockenheim under permanent actuation. Even greater gains are to be expected for a selective employment during cornering only or for skidpad and acceleration competitions.</div></div>

Characterisation of flow behaviour and velocity induced by ultrasound using particle image velocimetry (PIV): Effect of fluid rheology, acoustic intensity and transducer tip size

Acoustic streaming phenomena of ultrasound propagation through liquid media was investigated experimentally employing particle image velocimetry (PIV). Parameters associated with the ultrasonic processor of ultrasonic amplitude (i.e., acoustic power) and transducer tip diameter (i.e., surface area), as well as, fluid rheology (i.e., water, glycerol solution and CMC solution), were studied for their effects on overall flow behaviour and fluid velocity. PIV yielded velocity gradient maps, demonstrating the acoustic streaming phenomena of ultrasound and its associated flow behaviour as a function of ultrasonic amplitude and fluid rheology, whereby increasing amplitude allowed for greater penetration of the acoustic-beam through the bulk of the fluid, and increasing fluid rheology yielded the converse effect. Moreover, upon impingement of the acoustic-beam with the base of vessel, vortex formation occurred, yielding a recirculation pattern. The maximum observed fluid velocities for water, glycerol solution and CMC solution were 0.329 m s−1, 0.423 m s−1, and 0.304 m s−1, respectively (large diameter sonotrode tip for an ultrasonic amplitude of 80%). Furthermore, shear rates were attained (maximum values of 24.25 s−1), and Reynolds numbers were determined in order to assess the degree of turbulence as a function of investigated parameters.

The impact of streamwise convex curvature on the supersonic turbulent boundary layer

With an inflow Mach number of 2.95, the isolated impacts of the convex curvature and the streamwise favorable pressure gradient on the supersonic turbulent boundary layer are experimentally investigated by employing particle image velocimetry. An experiment on the convex turbulent boundary layer with a zero streamwise pressure gradient is carefully arranged to investigate the impact of the streamline curvature. The results are compared with the favorable-pressure-gradient case (with pressure gradient parameter β = −0.60). For both of the zero-pressure-gradient and the favorable-pressure-gradient convex boundary layers, the log law is found to be well preserved in the streamwise velocity profile with inner scaling. The streamline convex curvature and the streamwise favorable pressure gradient are found to have similar impacts on the distribution of mean streamwise velocity. They both weaken the boundary layer’s wake strength, as well as the principal strain rate in the near wall region. While both of the convex streamline curvature and the favorable pressure gradient are observed to dampen the streamwise turbulent fluctuation, their effects on the normal fluctuation are opposite. The convex curvature reinforces the normal fluctuation, and its strengthening effect is much stronger than the relaxation effect of the favorable pressure gradient. Retrograde vortices are notably weakened by the favorable pressure gradient, which contributes to the relaxation of turbulence. The impacts of the streamline convex curvature and the streamwise favorable pressure gradient are more notable in the outer layer, while in the near wall region their contributions are weakened.

Experimental study on flow behavior of breathing activity produced by a thermal manikin

Breathing is the most common but essential activity of human body. People can't leave without breathing. The breathable air quality around the occupant should be maintained high to avoid the respiratory diseases transmission. This paper reports an experimental study on the flow behavior of breathing activity produced by a thermal manikin. Measurements were performed in the breathing zone of the manikin by employing particle image velocimetry (PIV) system, and the temporal and spatial distributions of breathing airflows both with and without the convective boundary layer (CBL) were characterized by use of phase averaging approach. In addition, a quadruple decomposition methodology extended from proper orthogonal decomposition (POD) was adopted to divide the instantaneous flow fields of breathing airflows into four parts, the flow characteristics as well as cycle-to-cycle variations upon each part studied respectively. The results showed that the breathing flow at different phases presented different flow behaviors during breathing process. CBL had a larger influencing area and played a key role in promoting the upward spread and transmission of exhaled contaminants, which cause the inhaled flow mainly come from more the lower jaw region and block the contaminants inhaled from other directions, thus reducing the risk of infection. It was found through the POD results that CBL enlarged the scale and magnified the energy of coherent structures at the transition stage between expiration and inspiration, and the large scale coherent structures were dominant and made more contribution to the contaminant dispersion in the entire breathing process.

An experimental investigation of the supersonic turbulent boundary layer subjected to convex curvature

By employing particle image velocimetry, the impact of convex curvature on the supersonic turbulent boundary layer with inflow Mach number of 2.95 is experimentally investigated. The test models are carefully designed to have a zero-pressure-gradient convex boundary layer to isolate the influence of the streamwise pressure gradient while looking at the impact of convex curvature only. The results are compared with the favorable-pressure-gradient case (with pressure gradient parameter β=-0.70). For both of the zero-pressure-gradient and favorable-pressure-gradient convex boundary layers, the log law is found to be well preserved in the profile of streamwise velocity with inner scaling. Both of the mean streamwise velocity and the principal strain rate of the zero-pressure-gradient case collapse after scaling. By accumulating the streamwise velocity along the wall normal direction, the convex curvature is found to have the potential of diminishing the organized motion of the large scale vortices, which cont...

Three-dimensional computational fluid dynamics (volume of fluid) modelling coupled with a stochastic discrete phase model for the performance analysis of an invert trap experimentally validated using field sewer solids

Invert traps are used to trap sewer solids flowing into a sewer drainage system. The performance of the invert trap in an open rectangular channel was experimentally and numerically analysed using field sewer solids collected from a sewer drain. Experiments showed that the free water surface rises over the central opening (slot) of the invert trap, which reduces the velocity near the slot and allows more sediment to be trapped in comparison with the case for the fixed-lid model (assuming closed conduit flow with a shear-free top wall) used by earlier investigators. This phenomenon cannot be modelled using a closed conduit model as no extra space is provided for the fluctuation of the water surface, whereas this space is provided in the volume of fluid (VOF) model in the form of air space in ANSYS Fluent 14.0 software. Additionally, the zero atmospheric pressure at the free water surface cannot be modelled in a fixed-lid model. In the present study, experimental trap efficiencies of the invert trap using field sewer solids were fairly validated using a three-dimensional computational fluid dynamics model (VOF model) coupled with a stochastic discrete phase model. The flow field (i.e., velocities) predicted by the VOF model were compared with experimental velocities obtained employing particle image velocimetry. The water surface profile above the invert trap predicted by the VOF model was found to be in good agreement with the experimentally measured profile. The present study thus showed that the VOF model can be used with the stochastic discrete phase model to well predict the performance of invert traps.

An experimental investigation of the supersonic turbulent boundary layer subjected to concave curvature

By employing particle image velocimetry, the response of a Mach 2.95 turbulent boundary layer to the concave curvature is experimentally investigated. The radius of the concave wall is 350 mm, and the turning angle is 20∘. Logarithmic law is well preserved in the profile of streamwise velocity at all streamwise positions despite the impact of curvature. The varying trend of principal strain rate is found to be different at different heights within the boundary layer, which cannot be explained by the suggestion given by former researchers. Based on the three-layer model proposed in this paper, distribution of the principal strain rate is carefully analyzed. The streamwise increase of wall friction is suggested to be brought by the increase of velocity gradient in the thin subsonic layer. Increases of the static temperature and the related sound speed are responsible for that. Larger correlated turbulent motions could be introduced by the concave curvature. The probability density histograms of streamwise velocity reveal that the large scale hairpin packets are statistically well organized. The concave curvature is found to have the potential of reinforcing the organization, which explains the increase of turbulent level in the supersonic concave boundary layer.

Vortex street suppression of a circular cylinder using perforated semi-circular fairing in shallow water

In this study, the effect of perforated fairing on vortex street suppression of a circular cylinder was investigated experimentally in shallow water. In order to investigate the effect of arc angle, α and porosity, β which are the main parameters of the study, three different arc angles (α=120°, 150° and 180°) and six different porosities (β=0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were examined. Perforated fairing was concentrically located with respect to the circular cylinder along its downstream direction. Turbulent statistics (turbulent kinetic energy, TKE and Reynolds shear stress, 〈u′v′〉) in the wake region were obtained by employing particle image velocimetry (PIV) technique at a Reynolds number of ReD=5×103 based on the circular cylinder diameter, D. The results depicted that the flow structure downstream of bare cylinder was significantly affected by the presence of perforated fairing for the porosity, β values, in the range of β=0.3–0.6. It is found that the wake region of the cylinder was elongated substantially along the main flow direction and the vortex shedding frequency, was reduced substantially. Moreover, opposing shear layers lost their strength considerably compared with the bare cylinder case. The peak magnitude of Reynolds shear stress, 〈u′v′〉 was reduced up to 75% for the arc angle of α=180° and the location of peak magnitude of Reynolds shear stress, 〈u′v′〉 moved further downstream regions for all cases. Compared to the bare cylinder case, the most effective flow control was obtained for the case having β=0.6 porosity and α=180° arc angle.