Streamwise Distance Research Articles

Experimental investigation on the vortex-induced vibration of an inclined flexible pipe and the evaluation of the independence principle

This paper reports the experimental results of the vortex-induced vibration (VIV) response of an inclined flexible pipe with various oblique angles (θ) ranging from 0° to 60°. The flexible pipe with an aspect ratio of 75 was fixed at both ends. The VIV response was examined in the reduced velocity range of 4.02–17.55 to evaluate the IP (independence principle) validity. The experimental results illustrate that the spatial distribution of response amplitudes and associated modal weights varies with θ, and the differences are enlarged in the mode transition cases. With increasing the θ, the onset normal reduced velocity (Urn) of mode transition shifts to a lower value and the Urn range of mode transition is narrowed gradually. Additionally, the mode competition is enhanced, resulting in pronounced traveling waves with accelerated propagation speed. The vortex shedding pattern varies along the span, presenting the 2S (two vortices are shed per cycle) and P + S (a pair of vortices and a single vortex are shed from two sides of the cylinder in one cycle) patterns at the upstream and downstream pipe segments, respectively. The streamwise distance between two adjacent vortices is enlarged with the increase in θ. The pipe placed with different inclined angles presents different partitions of coupling pattern, and the Urn range of each sub-region is different. By comprehensive consideration of the spatial–temporal evolution of VIV response, the mode transition features, the coupling pattern, and vortex shedding characteristics, the IP is invalid in predicting the VIV of flexible pipe.

Solute transport characteristics in the streambed due to rigid non-submerged plants: Experiment and simulations

The presence of aquatic plants in natural and artificial rivers can cause substantial changes in the flow pattern. In this study, laboratory experiments and numerical simulations are conducted to explore the influences of rigid non-submerged plant densities on the transport of solutes in the hyporheic zone. The laboratory experiments and numerical modelling results show that solute enters into the streambed around plant, and solute in the overlying water experienced a sharp decrease at the beginning, followed by a gradual decline phase. The concentration in the shallow pore water increases rapidly, and increases at a decreasing rate with the increase of depth. The presence of a plant causes pressure gradients at the sediment–water interface before and after the plant. Specifically, the presence of plants induces the convection exchange of surface water and pore water, enables the solute to transport in the streambed through three circulation flows. Solute tends to transport to the root of the plant, or bypass the plant and return to the overlying water over time. The rate and quantity of solute transport in the streambed are related to the streamwise and spanwise distances of the plant. When the plant densities reach the distance that affect the flow structures near each other, smaller streamwise distances lead to the transport of fewer solutes into the streambed and prevent them from penetrating to the deeper part of the streambed, while larger spanwise distances have the same effect on solute transport in the streambed. The solute effective exchange depth for streamwise distance of 20 cm is 4.3 times that of 3 cm, while which for spanwise distance of 3 cm is 5.05 times that of 15 cm at time of 28 h. This study shows that the presence of aquatic plants leads to the circulation of solutes in the hyporheic zone, which promotes the transport of pollutants from overlying water to streambed through hyporheic exchange after the occurrence of a water pollution event. Further, plants densely in the streamwise direction and sparsely in the spanwise direction will reduce the range of pollutants transport in the streambed. This study is of great significance for water ecological restoration through rational arrangement density of aquatic plants.

Numerical study of turbulent flow and heat transfer in a novel design of serpentine channel coupled with D-shaped jaggedness using hybrid nanofluid

This study aimed to examine numerically the effects of a dimpled surface over a mini-channel heat exchangeron the flow characteristics and heat transfer across a serpentine channel with a uniformrectangular cross-section. The dimples were arranged in parallel with a spanwise (y/d) distance of 3.125 and streamwise (x/d) distance of 11.25 along just one side of the serpentine channel's surface.Turbulent flow regime with Reynolds number ranging from 5 × 103 to 20 × 103 in the channel with the surface modificationwas studied using water and various volume concentrations(φ = 0.1%, 0.33%, 0.75%, 1%) ofAl2O3-Cu/waterhybrid nanofluid as the coolant to achieve a three-step passive heat transfer enhancement. Applying the Finite Volume Method (FVM), RNG k-e turbulence model, and a constant heat flux of 50 kW/m2, simulations were run assuming the mixture of Al2O3-Cunanoparticles homogenous using ANSYS 2020 R1. The second-order upwind approach is used forapproximation of solution and discretizationwith SIMPLE pressure–velocity coupling. Taking heat transfer increment and pressure drop penalty into consideration,the dimpled serpentine channel provides a 1.47-times improvement in thermal efficiency using water as the coolant, andthe dimpled channel with 1% vol.Al2O3-Cu/water nanofluid enhanced thermal efficiency by a remarkable maximumof 2.67-times at Re 5 × 103. The study also indicates that thermal efficiency increased with an increasing volume concentration of the nanofluid and increment in thermal efficiency gradually decreased as the Re increased.

The effect of vertical arrangement on performance and wake characteristics of two tandem offshore wind turbines under various operating conditions

Apart from streamwise and lateral distances, a vertical distance may also be imposed for adjacent wind turbines to achieve performance improvement of downstream turbines. Previous research was primarily devoted to wind farm layout optimisation using simplified analytical wake models, while impacts of operating conditions were ignored for studies using more advanced Large-Eddy Simulation (LES) methods. In this paper, the effect of vertical arrangement on performance and wake characteristics of two tandem offshore wind turbines under various operating conditions is investigated using an LES approach coupled with an actuator line technique. Results show that vertically arranging turbines can improve downstream turbine performance, but its effects vary with the operating conditions. If turbines typically operate below or at the rated condition, due to deflection of upstream turbine wake associated with shaft tilt, raising the upstream turbine can result in greater improvement in power output, with a considerable rise of 50% for a relatively small height difference of 0.1 times rotor diameter. For above-rated operating conditions, the wind shear effect becomes important and it is preferable to increase the hub height of the downstream turbine to take advantage of higher wind speed. Wake characteristics of both turbines are analysed to support the results.

Parametric study of the effectiveness of active yaw control based on large eddy simulation

Previous studies on active yaw control (hereafter, AYC) have demonstrated its potential in wind farm power optimization. However, most focus are concentrated on the amount of relative power enhancement achieved by AYC, ignoring how various parameters affect the effectiveness of AYC, let alone the mechanism behind the variation of AYC efficiency. Evidently, this is not conducive to the practical implementation of AYC. In response, a parametric study of the effectiveness of AYC is pursued in this paper, by performing a series of large eddy simulations (hereafter, LES) for the NREL 5 MW turbine array under different operating conditions. The LES results reveal that AYC is more effective in the environment with lower incoming wind speed and smaller turbulence intensity level. What is more, the AYC efficiency is found to vary greatly with the streamwise distance between consecutive wind turbines, but is not sensitive to the change of column spacing within a typical range. A large power gain is achieved at wind angles corresponding to partial-wake operating conditions. The active yaw behavior of the last wind turbine in the array also shows the potential to increase the total power production but the corresponding power gain is small. Moreover, it is shown that the yaw direction of wind turbine has nonnegligible influence on AYC, especially in the stable boundary layer. From the power production perspective, positive yaw of wind turbine is recommended in the north hemisphere.

From snapshots to manifolds – a tale of shear flows

We propose a novel nonlinear manifold learning from snapshot data and demonstrate its superiority over proper orthogonal decomposition (POD) for shedding-dominated shear flows. Key enablers are isometric feature mapping, Isomap, as encoder and,$K$-nearest neighbours ($K$NN) algorithm as decoder. The proposed technique is applied to numerical and experimental datasets including the fluidic pinball, a swirling jet and the wake behind a couple of tandem cylinders. Analysing the fluidic pinball, the manifold is able to describe the pitchfork bifurcation and the chaotic regime with only three feature coordinates. These coordinates are linked to the vortex-shedding phases and the force coefficients. The manifold coordinates of the swirling jet are comparable to the POD mode amplitudes, yet allow for a more distinct and less noise-sensitive manifold identification. A similar observation is made for the wake of two tandem cylinders. The tandem cylinders are aligned and located at a streamwise distance which corresponds to the transition between the single bluff body and the reattachment regimes of vortex shedding. Isomap unveils these two shedding regimes while the Lissajous plot of the first two POD mode amplitudes features a single circle. The reconstruction error of the manifold model is small compared with the fluctuation level, indicating that the low embedding dimensions contain the coherent structure dynamics. The proposed Isomap–$K$NN manifold learner is expected to be of great importance in estimation, dynamic modelling and control for a large range of configurations with dominant coherent structures.

Turbulent entrainment in finite-length wind farms

In this article, we present an entrainment-based model for predicting the flow and power output of finite-length wind farms. The model is an extension of the three-layer approach of Luzzatto-Fegiz & Caulfield (Phys. Rev. Fluids, vol. 3, 2018, 093802) for wind farms of infinite length, and assumes dependence of key flow quantities, such as the wind farm bulk velocity, on the streamwise distance from the farm entrance. To assist our analysis and validate the proposed model, we undertake a series of large-eddy simulations with different turbine spacing arrangements and layouts. Comparisons are also made with the top-down model with entrance effects of Meneveau (J. Turbul., vol. 13, 2012, N7) and data from the literature. The finite-length entrainment model is shown to be capable of capturing the power drop between contiguous rows of turbines as well as describing the advection and turbulent transport of kinetic energy in both the entrance and fully developed regions. The fully developed regime is approximated only deep in the wind farm, after approximately 15 rows of turbines. Our data suggest that for the cases considered in this study, the empirical coefficients that can be used to describe turbulent entrainment and transfers above the wind farm exhibit little dependence on the farm layout and may be considered constant for modelling purposes. However, the flow field within the wind farm layer can be strongly modulated by the turbine density (spacing) as well as the array layout, and to that extent it can be argued that they are both primary factors determining the wind farm power output.

Effect of coolant pulsation on film cooling performance on flat plate

• Large eddy simulation was used for investigating square-wave pulsating film cooling on flat plate. • The experiment of pulsating film cooling was carried out. • Evolution of coherent structures in pulsating film cooling was analyzed. Combined with infrared thermography tests, large eddy simulation was used to study the square-wave pulsating film cooling on flat plate at the time-averaged blowing ratios of 0.5 and 1.5. The effects of two pulsating parameters including frequency and amplitude were discussed in detail. At low blowing ratio, the introduction of coolant pulsation leads to the decrease of cooling effectiveness, and the effect of pulsating amplitude is much more pronounced than frequency. At high blowing ratio, cooling effectiveness can be enhanced by coolant pulsation at low amplitude, but reduced at high amplitude. As the typical vortex structure in pulsating film cooling, the starting vortex is formed as the jet is turned on. The interaction between hairpin vortex and starting vortex shows that, at high pulsating amplitude, the starting vortex can help coolant detach from the wall, and has an adverse influence on cooling performance. The statistical characteristics and power spectrum density of instantaneous velocity were investigated. It was found that the introduction of coolant pulsation can result in the increases of eddy energy and turbulent kinetic energy in the near filed region, but this effect becomes weaker and weaker as the streamwise distance increases.

Computational analysis on solar air heater with combination of alternate dimple protrusions and intrusions on absorber plate with one rounded corner triangular duct.

This study focuses on improving heat transfer by converting one of the corners of the duct to a rounded structure. To study the effect of dimpled shaped protrusions and intrusions on the rounded corner triangular duct with a constant radius of curvature by varying relative streamwise distance (z/e) with a constant transverse distance x'/e = 10,14 and 18. Steady-state, turbulent flow heat transfer under thermal boundary conditions is to be analyzed by varying different Reynolds numbers (5600 to 21000). The duct with dimple-shaped protrusions and intrusions is compared with a simple triangular duct. Optimization of relative horizontal distance (z'/e) by keeping constant protrusion to protrusion distance as z/e = 28 and relative transverse distance as x/e = 10, 14, and 18. It was noted that there was a significant loss in friction and a rise in heat transfer. The relationship between friction factor and Nusselt number was formulated using operating and roughness parameters, using the data collected from the numerical investigation. The friction factor increases significantly with roughness elements, and it is maximum for x'/e = 20 at a low Reynolds number. Nusselt number increases with roughness elements, and it is maximum for x'/e = 14 for all Reynolds numbers and all the models. Enhancement of Nusselt number is due to increase of local heat transfer because of local vortex neat heat transfer zone. The maximum outlet temperature is obtained at a low Reynolds number. The maximum temperature of the heated surface is obtained for Rc = 0.67h and the minimum for Rc = 0.33h.

A New Streamwise Scaling for Wind Turbine Wake Modeling in the Atmospheric Boundary Layer

In this study, we aim to investigate if there is a scaling of the streamwise distance from a wind turbine that leads to a collapse of the mean wake velocity deficit under different ambient turbulence levels. For this purpose, we perform large-eddy simulations of the wake of a wind turbine under neutral atmospheric conditions with various turbulence levels. Based on the observation that a higher atmospheric turbulence level leads to faster wake recovery and shorter near-wake length, we propose the use of the near-wake length as an appropriate normalization length scale. By normalizing the streamwise distance by the near-wake length, we obtain a collapse of the normalized wake velocity deficit profiles for different turbulence levels. We then explore the possibility of using the relationship obtained for the normalized maximum wake velocity deficit as a function of the normalized streamwise distance in the context of analytical wake modeling. Specifically, we investigate two approaches: (a) using the new relationship as a stand-alone model to calculate the maximum wake velocity deficit, and (b) using the new relationship to calculate the wake advection velocity within a physics-based wake expansion model. Large-eddy simulation of the wake of a wind turbine under neutral atmospheric conditions is used to evaluate the performance of both approaches. Overall, we observe good agreement between the simulation data and the model predictions, along with considerable savings in terms of the models’ computational costs.

Numerical study of the effect of potato cuboids separation in a convective drying

A numerical study (conjugate modeling) of potato cuboid separation in convective drying is performed with the Large Eddy Simulation approach. Any in-line potato array is represented by a single interacted zone through periodic boundary conditions. Numerical code is validated with four experimental studies focused on hydrodynamics (two cases), heat transfer (one case), and mass transfer (one case). Twelve case studies have been proposed to study the effect of streamwise and spanwise distances on heat and mass transfer. Results show that low streamwise separations (0.25) strongly affect the potato drying by the presence of a quasi-stationary wake that causes a blocking effect. The potato cuboid’s best thermal uniformity and mass transfer are presented at a spanwise distance (0.75) and streamwise separations (1.0). Results can be applied to get uniform heat and mass transfer in drying chambers.

Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics

AbstractNature‐based solutions to flood risk management, such as engineered logjams (ELJs), contribute to the reintroduction of wood in rivers. As part of stream restoration, and utilized in tributaries, ELJs increase upstream water levels, causing the flow to spill onto surrounding floodplains, resulting in the desynchronization of peak flows in a river network. To understand the effect of ELJs on local river hydrodynamics, we experimentally investigate the flow field upstream and downstream of six ELJs, using acoustic Doppler velocimetry and flow visualization. We consider channel‐spanning structures designed with a gap (b0) underneath, allowing unhindered baseflow. Our results revealed that upstream of the logjams, flow diverted toward the lower gap, creating a primary jet exiting underneath the structures, whose strength depends on the physical logjam design. Maximum jet velocities remained constant until a downstream distance of 4b0 for all logjams. The upper wake was structure‐dependent, with logjam structures allowing distinct internal flow paths generating secondary jets, which influenced near wake decay (x < 4b0) and turbulent mixing. The highest turbulence in the near wake was found for the non‐porous and short, porous logjam designs, while the upper wake of all long, porous logjams was characterized by low turbulent kinetic energy levels. Far wake decay (x > 4b0) was self‐similar for all logjams and resulted in near flow recovery at downstream streamwise distances greater than 35b0. ELJs are likely to enhance bed shear stress, increasing the risk of local scour and sediment mobilization. Our study expands the current knowledge of ELJ hydrodynamics and highlights potential implications for the riverine ecosystem.

Wind Tunnel Study on the Tip Speed Ratio’s Impact on a Wind Turbine Wake Development

We propose an experimental study on the influence of the tip speed ratio on the spatial development of a wind turbine wake. To accomplish this, a scaled wind turbine is tested in a wind tunnel, and its turbulent wake measured for streamwise distances between 1 and 30 diameters. Two different tip speed ratios (5.3 and 4.5) are tested by varying the pitch angle of the rotor blades between the optimal setting and one with an offset of +6∘. In addition, we test two Reynolds numbers for the optimal tip speed ratio, ReD=1.9×105 and ReD=2.9×105 (based on the turbine diameter and the freestream velocity). For all cases, the mean streamwise velocity deficit at the centerline evolves close to a power law in the far wake, and we check the validity of the Jensen and Bastankhah-Porté-Agel engineering wind turbine wake models and the Townsend-George wake model for free shear flows for this region. Lastly, we present radial profiles of the mean streamwise velocity and test different radial models. Our results show that the lateral profile of the wake is properly fitted by a super-Gaussian curve close to the rotor, while Gaussian-like profiles adapt better in the far wake.

The configuration effect of flapping foils for energy harvesting

The configuration of multiple flapping foils is studied via numerical simulations. We comprehensively consider the effects of the streamwise distance, vertical spacing, and phase difference on the energy harvesting performance of flapping foils. We divide flapping foil configurations into three categories: tandem, unaligned, and parallel. The foil tandem configuration is optimal for multiple foils if using the existing efficiency formula. However, tandem configurations expand the diffusion range of the wake turbulence. Wake diffusion has a critical effect on multiple foil configurations, and the utilization of the vertical spacing has been neglected. Here, the effective angle of attack and the effective velocity are proposed based on multiple flapping foils, which can well predict various rules of the lift coefficient and guide studies on the optimal configuration of multiple foils. The optimal phase difference for the simple parallel configuration system is 135°, and the energy harvesting efficiency of parallel foils decreases with increasing vertical spacing. The stepwise configuration maximizes the utilization of the vertical spacing, making it optimal for practical engineering applications. The stepwise configuration not only recycles the wake of the upstream foils but also utilizes it to enhance energy harvesting from flapping turbines. In the multiple foil configurations, the energy harvesting efficiency of the downstream foil over the upstream foil is achieved for the first time using a stepwise configuration.

Experimental and Numerical Investigations on the Mixing Process of Supercritical Jet Injected into a Supersonic Crossflow

The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a supersonic crossflow were investigated by experimental and numerical methods, respectively. The schlieren system and acetone planar laser-induced fluorescence (PLIF) optical system were used to capture the flow-field structural characteristics and instantaneous plume. The mixture and real gas models were employed to calculate the interaction of a transverse jet and supersonic crossflow and reveal a good accuracy with the experimental results. The mixing efficiency and total pressure loss were analyzed based on the numerical results. The results indicate that the supercritical-state EHF directly changes to a gaseous state as it enters the supersonic crossflow from the injector. The EHF jet plume boundary increases with the increasing momentum flux ratio (q). As the streamwise and spanwise distance increases, the traverse heights and expand width increase, and the EHF jet plume presents a semicircle shape in the cross-sectional plane. With the increase in the traverse direction, the concentration distribution shows a fast and then slow power exponential decreasing law; the highest concentration point starts from the near-wall region and rises in the transverse direction with the flow distance increasing. For the same injection condition, the higher the inflow Mach number, the higher the mixing efficiency. For the same Ma, the mixing efficiency is better for the case with low injection pressure and high injection temperature. The total pressure loss is greater in the higher Ma, and high injection pressure conditions cause greater total pressure loss.

DNS Of the ignition process of n-heptane/air premixed combustion with low-temperature chemistry in turbulent boundary layer

In the present work, three-dimensional direct numerical simulation (DNS) of n-heptane/air premixed combustion in turbulent boundary layer was performed to explore the near-wall ignition process with low-temperature chemistry. A reduced chemical mechanism with 58 species and 387 elementary reactions for n-heptane combustion was used in the DNS. The general characteristics of the ignition process near the wall were examined. It was found that low-temperature ignition (LTI) dominates the upstream region, and high-temperature ignition (HTI) appears in the downstream region. The ignition process and the low-temperature chemistry pathways of the DNS are compared with those of a corresponding laminar case. It was found that the ignition process was affected by turbulence, which results in thickened reaction zones. However, the carbon flow analysis of low-temperature chemistry showed that turbulence rarely affects the low-temperature chemistry pathway. The combustion modes of various regions were scrutinized based on the budget terms of species transport equations and the chemical explosion mode analysis (CEMA). It was shown that the reaction term of RO2 is significant during the LTI process of the upstream region, and the reaction terms of CH2O and CO2 are evident in the downstream region, indicating the occurrence of HTI. It was also shown that auto-ignition is dominant in the upstream region. With increasing streamwise distance, the contribution of flame propagation increases, which takes over that of auto-ignition in the near-wall region.

Effect of Reynolds number on second-mode waves in transitional hypersonic boundary layers

The second-mode instability plays an important role in the natural transition of hypersonic boundary layer. In this paper, the Reynolds number effect on the second-mode waves has been experimentally investigated using the Nanoparticle-tracer based Planar Laser Scattering (NPLS) technique and PCB® fast response pressure sensors in Mach 6 quiet wind tunnel. The flow visualization results clearly capture the rope-like second-mode waves and the “quiet zone” before breakdown. The wavelength of second-mode waves is approximately twice the local boundary layer thickness, increasing with the streamwise distance and decreasing with the Reynolds number. The time delays between sensors indicate no significant Reynolds number effect on the propagation velocity of second-mode waves. The average velocity is about 0.82Ue. The characteristic frequencies are collated at different streamwise locations under different Reynolds numbers and show a linear proportion to Re/x. Based on the growth law of compressible conical boundary layer thickness, the characteristic frequency relation and wavelength relation are derived as a function of Reynolds number and streamwise distance. The theoretical estimation values agree well with the experimental results, which provides the potential feasibility to predict the characteristic frequency and wavelength in other tunnels.

Dynamics of tip vortex flow over three dimensional hydrofoils by LDV measurements

Tip vortices are observed downstream of the flow past the three-dimensional hydrofoils used in propeller, wind and tidal turbines and they largely determine the forces exerted on such structures. In this study, we present the Laser Doppler Velocimetry (LDV) measurements in the flow past three-dimensional hydrofoils namely NACA 4412, NACA 16020, NACA 66(2)415 inclined at an angle of 10° with respect to the free-stream velocity, U∞. The three-dimensional hydrofoils have a semi-span, s = 60 mm, chord lengths, c = 48 mm, 60 mm & 80 mm which correspond to three aspect ratios, ∧ = 2.5, 2.0 & 1.5, respectively, at the chord length based Reynolds number, Re ranging from Re = 1.92 × 105 − 6.39 × 105. LDV measurements were performed at several coordinate points in zy−plane perpendicular to the stream-wise, x direction, measuring the mean stream-wise, u(z, y) and mean velocity component along the z-axis, w(z, y) together with the root-mean square of their fluctuations in the plane located at a stream-wise distance, x/c = 1.0. The swirling strength of the tip vortices are qualitatively evident from the measurements of z− velocity component field, w(z, y) for all the hydrofoils, at various aspect ratios and Reynolds numbers. It is observed that the magnitude of normalized axial velocity deficit, (u(z, y) − U∞)/U∞ and normalized turbulent kinetic energy, TKE(z, y) are dependent on the type of hydrofoil, their aspect ratio, ∧ and the Reynolds number, Re. TKE within the region of the wake and vortex contain about 99% of the peak magnitude of TKE observed in the x/c = 1.0 plane. The vortex center, vortex position and vortex radius are shown to be dependant on the hydrofoil shape, aspect ratio and Reynolds number.

Investigating the flow field dynamics of transonic shock buffet using particle image velocimetry

An experimental investigation was performed in order to better understand the transonic shock buffet phenomenon and determine the dominant flow interactions at specific flow conditions. A rigid wing in the shape of an OAT15A airfoil was placed in the Trisonic Wind Tunnel Munich, where both the Mach number and the angle of attack were varied between 0.65<M∞<0.77\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.65< M_\\infty < 0.77$$\\end{document} and 3.8<α<6.3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.8< \\alpha < 6.3^\\circ$$\\end{document} respectively. With the use of high-speed imaging, high-quality optics and state-of-the-art laser equipment, highly resolved velocity field measurements were obtained via particle image velocimetry, where the streamwise and vertical velocity components were computed over the suction side of the wing center plane. It was shown that sustained buffet first occurs at M∞≥0.74\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_\\infty \\ge 0.74$$\\end{document} when maintaining the angle of attack constant at α=5.8∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha =5.8^\\circ$$\\end{document}. Similarly, an increase in α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} for a fixed M∞=0.74\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_\\infty =0.74$$\\end{document} also led to the development of shock buffet. Instantaneous snapshots confirmed the presence of a recirculation region downstream of the moving shock, where an increase in the wake size was confirmed when the shock was located most upstream. Streamwise correlations were also computed near the airfoil’s upper surface in order to extract the characteristic convective velocity of flow structures. The convective velocity appeared to increase with streamwise distance, ranging on average between 50 and 150 m/s. Overall, these time-resolved velocity field measurements allow for the investigation of the flow dynamics during shock buffet and highlight the independent effect of Mach number and angle of attack on this complex phenomenon.

Investigation of onset of velocity transition in free convection over an inclined flat plate by PIV

In this work, the experimental investigation of onset of velocity transition in free convection boundary layer over a 45° inclined plate has been carried out. Experiments were performed on a 150 mm long flat plate with uniform heat flux ranging from 100 W/m2 to 5000 W/m2. Particle image velocimetry (PIV) technique was used to determine the flow characteristics over an inclined plate. The impact of various hydrodynamic parameters such as shear stress and velocity boundary layer thickness for defining or identifying the onset of velocity transition has been discussed. Also, the influence of various values of the heat fluxes on the onset of transition has been examined. It has been observed that the initiation of transition is first manifested as an increase in the overall velocity boundary layer thickness. Therefore, the thickness of the velocity boundary layer has been used as a hydrodynamic measure to determine the streamwise distance from the leading edge where the transition begins. This distance decreases with an increase of heat flux. The critical Rayleigh number for the onset of the transition under various heating conditions has been reported. For example, the critical Rayleigh numbers for the beginning of transition at heat fluxes of 100 W/m2 and 5000 W/m2 are 6.07 × 108 and 2.73 × 109 respectively.