Unsteady Flow Regimes Research Articles

Experimental study on the periodic pulsating ventilation by fluidic oscillator on pollutant dispersion and ventilation performance in enclosed environment

Ventilation plays an essential role in creating healthy and comfortable indoor environment, while adopting unsteady airflow field has the potential to improve the indoor air mixing effect, which is conducive to the dispersion and eventually the discharge of indoor pollutants. A fluidic oscillator capable of producing periodic pulsating airflow is employed to conduct a ventilation experiment to test the performance of such unsteady flow regime. The distribution of indoor pollutants at different source locations is compared under both steady and unsteady ventilation mode. The assessment of the fluidic oscillator ventilation system performance in eliminating pollutants involves the concentration exceedance rate, the decay of indoor pollutant concentration, and the resultant ventilation efficiency. At a relatively lower air supply rate, the fluidic oscillator device can enhance indoor air mixing, leading to a reduction of the peak indoor pollutant concentration. Additionally, the fluidic oscillator air terminal significantly prolongs the time needed to reach the pollutant concentration threshold, increases ventilation efficiency, and improves indoor air quality.

An investigation of flow regimes and mixing in a novel arrow-shaped jet reactor

Fluid mixing plays a vital role in the industry of chemical engineering. In this work, a novel arrow-shaped jet reactor by varying the angle of fluid impingement in the inlet channel was investigated. Flow field visualization experiments using Planar Laser-Induced Fluorescence (PLIF) technique were carried out to study various flow regimes and mixing performance. Results show that differentiated flow patterns of engulfment flow regimes appear within the arrow-shaped jet reactor. Particularly, vortex ring shedding was found in the unsteady engulfing flow regime, which promoted the mixing significantly. Besides, the mechanism of the variation of fluid impinging angle on mixing in an arrow-shaped jet reactor was investigated by numerical simulation. The results indicate that the transformation from double-leg vortex to single-leg vortex and the increase of the volume of vortex clusters caused by vortex ring shedding are the main reasons for the improvement of the mixing of engulfment flow regime.

A numerical study of natural convection through a vertical heated channel with a confined circular cylinder

This study examines thermal flow structures and heat transfer through a vertical heated channel with an adiabatic circular cylinder symmetrically positioned between lateral walls. A two-dimensional numerical simulation is conducted covering a range of parameters including Rayleigh numbers Ra = 8.9 × 106–8.9 × 108, cylinder positions relative to the channel height h = 0–0.50, and blockage ratios (the ratio of cylinder diameter to channel width) β = 0.25–0.75. Three distinct flow regimes are observed at different Rayleigh numbers and blockage ratios, including steady symmetric, unsteady periodic, and unsteady asymmetric flow regimes. The steady symmetric and unsteady periodic flows are observed at all blockage ratios, while the unsteady asymmetric flow is only observed at the highest blockage ratio of β = 0.75 for Rayleigh numbers above 8.9 × 107. It is found that the presence of the cylinder significantly enhances mixing and turbulence in the channel, which in turn enhances heat transfer through the channel. A 64.3% enhancement of heat transfer is achieved at β = 0.50 and h = 0.05 for Ra = 8.9 × 108.

An Investigation on the Steady and Unsteady Wake Flow Regimes and Aerodynamic Characteristics of the Trapezoidal Cylinders Using Immersed Boundary-Lattice Boltzmann Method

Abstract In this study, the flow physics of the forward-facing (FF) and backward-facing (BF) trapezoidal cylinders (TC) subjected to two-dimensional, incompressible, and laminar flow is investigated using an in-house developed flexible forcing immersed boundary-lattice Boltzmann solver. The Reynolds number (Re) is defined based on the cylinder's characteristic length D. For the steady and unsteady flow regimes, Re is varied in the ranges of 10–40 and 75–125, respectively. The TCs shape is varied by modifying its nondimensional axial H/D and transverse Y/D length scales, between 0.5 to 2 and 0 to 1, respectively. Here, TCs horizontal central axis is always aligned along the incoming flow direction. It is observed that the flow separation points on the FF-TC and BF-TC are strongly influenced by the geometric (H/D and Y/D) and flow parameters (Re). Based on the boundary layer separation point, we have categorized the wake flow regimes behind the FF-TC and BF-TC into four types. In addition, the effect of the geometric and flow parameters on the drag coefficient (Cd), vortex shedding frequency, and steady and unsteady wake characteristics are thoroughly investigated here. Furthermore, by performing nonlinear regression analysis, we have proposed a set of correlation equations for the Cd and Strouhal number (St), using which the aerodynamic characteristics of differently shaped TC can be derived in the considered Re range without performing rigorous numerical simulations or experiments.

К вопросу о физико-математической модели высокотемпературного силицирования пористых углеродных изделий

We have developed an extended physical and mathematical model of the gaseous silicon transfer from a melt mirror to a porous carbon product at high temperatures under medium vacuum conditions. In this model, an assumption of the non-compressibility of the gas mixture is used that makes it possible to simplify the problem significantly and at the same time to obtain a reasonable result, confirmed in experiment. The model is simplified to the greatest extent possible, and in the future it can be used for numerical implementation of more complex physical formulations, including non-stationary three-dimensional silicification of arbitrary-shaped porous carbon products under the condition of a complex location of the silicon sources in the retort of the furnace. The model is described by a system of three differential equations making it possible to calculate the average mass flow rate of the gas mixture and the diffusive-convective transfer of silicon concentration to the product. In this paper, the unsteady flow regime of silicon vapor and remaining gas is considered. The approach presents a two-dimensional formulation of the problem in the case of a rectangular cavity with the conditions of gas slip and adherence to the upper and lower walls of the computational domain. For each formulation of the problem, we obtained a numerical solution of the system of differential equations that predicts the filling of the entire working space of the furnace with gaseous silicon in a very short time, which explains the abnormally large flow of silicon vapor deep into the porous carbon material that was observed in experiments. The numerical solution was obtained using the finite difference method; an explicit difference scheme was used for convenience and ease of implementation. The obtained results are expressed in terms of the known verified values of the material parameters of the gas mixture and confirm that the vapor-liquid-phase process of carbon materials silicification is possible in a reasonable time.

Intracycle Active Blade Pitch Control for Cross-Flow Tidal Turbines Using Embedded Electric Drive Systems

Cross-flow tidal turbines (CFTTs) have proven advantages over horizontal axis turbines in terms of high power density per unit area, simplicity of design, and operation independence from inflow direction. However, they suffer from an unsteady flow regime which can comprise dynamic blade stall and thus problems of material fatigue or even failure. Active pitch control mechanisms on blade level have been shown to provide a potential solution, when continuously adjusting the pitching angle of each individual blade during the whole rotational cycle of the turbine.
 As part of the research of the OPTIDE project, in this study, electric drive systems embedded in the blades of a CFTT flume model are proposed aiming to realize an active pitch control with high efficiency and fast response. The blade embedded actuation allows for reasonable flow conditions. For full scaled on-site applications this is required to reduce hydrodynamic losses and to protect the actuators and electronics from the harsh environmental and operation conditions. Based on the expected hydrodynamic loads from numerical flow simulations (CFD), several types of actuators are considered. The first type has brushless DC motors installed at both sides of each blade. Along with a gear box with proper reduction ratio, the actuators are able to provide required torque within expected cycle period. The second type of actuator drives the blade directly, which always results in faster pitching action, higher drive efficiency, more accurate positioning of blades, as well as simpler structure. Specifically, the shaft of each blade is designed as the primary of a limited angle torque motor, while the blades are used as the secondary with magnets inside. To mitigate the potential saturation effect on the iron of the primary­, the blade can also be used as the primary, where there always exists much larger space for windings. In this case, the magnets are now located at the shaft. By doing this, it is expected to output larger torque in a wider range of pitching angle as compared with the original one, while almost the same power is required. An experimental test bench for a single blade with both types of actuators is built to verify their ability of a fast and accurate pitching control. This also lays the foundation of identifying the optimal pitching angle for the control and inhibition of dynamic blade stall at various flow conditions and blade positions within the rotational cycle of the turbines. After successful optimization and testing of the model scaled mechatronical design, the actuators will be up scaled for realistic applications.

Vortex shedding of unsteady symmetric flow regime induced by secondary impinging in T-T jet reactors

Fluid mixing is widely employed in the chemical industry, and the mixing efficiency is intensely dependent on the flow characteristics in the reactors. In this work, flow regimes in a T-T jet reactor were investigated by Planar Laser-Induced Fluorescence (PLIF) at 27 ≤ Re ≤ 600. Segregated flow, vortex flow, steady and unsteady engulfment flow, and unsteady symmetric flow are generated in mixing channel with the increase of Re. Particularly, vortex shedding phenomenon was observed in the Reynolds number range of 400–600 after secondary impinging, which changed the flow topology and thus significantly improved the mixing of unsteady symmetric flow in the T-T jet reactor. Unlike the unsteady symmetric flow in a classical T jet reactor, the impinging cone with oscillatory behavior was formed in the mixing channel to generate vortex shedding. Velocity field of unsteady symmetric flow in a T-T jet reactor under the effect of secondary impinging was obtained by Particle Image Velocimetry (PIV). The fluid exhibits a large velocity distribution in the region of the impinging cone, followed by a gradual decay of velocity along the mixing channel and accompanied by vortex shedding. The numerical results reveal the mechanism of the improved mixing of unsteady symmetric flow under the influence of secondary impinging. It is found that the formation of additional leg vortices and the merging between different leg vortices by secondary impinging are the key reasons for the vortex shedding phenomenon and the improvement of mixing.

Evaluation of spacer-induced hydrodynamic mixing using particle image velocimetry: Impact on membrane distillation performance

Net-type spacers are used to promote hydrodynamic mixing in the feed channel of crossflow membrane filtration. The induced fluid flow mitigates concentration and temperature polarization effects on the membrane surface, leading to the enhancement of heat/mass transfer across the membrane. The objective of this research is to study the effect of spacer geometry on fluid flow and heat/mass transfer in the feed channel. The particle image velocimetry (PIV) technique was used to measure flow velocity in the feed channel with net-type spacers. Flow characteristics at a wide range of Reynolds numbers were described in terms of time-averaged mean velocity and turbulence kinetic energy (TKE). The PIV result revealed distinct profiles of mean velocity and TKE at different channel heights with bi-planar spacers. The spacer with the largest hydrodynamic angle of 120° (Net120) was found to be the most efficient in promoting flow unsteadiness because it led to the largest channel-averaged TKE among the selected spacers. The unsteady flow regime on Net120 was accompanied by the greatest pressure drop over the feed channel. Moreover, the effect of spacer-induced mixing on heat/mass transfer across the membrane was further evaluated via the flux measurement with vacuum membrane distillation experiments. In comparison with other spacers, Net120 resulted in a relatively higher flux because of the improved hydrodynamic mixing. These findings reveal the validity of the experimental approach in identifying the impact of key spacer design parameters on hydrodynamic mixing. Nusselt number was calculated to relate the hydrodynamic flow and thermal flow in the feed channel of membrane distillation, and a modified Nusselt number correlation was proposed to better predict convective heat transfer in the spacer-filled channel in VMD.

Aspect ratio effect on flow past an elliptical cylinder near a moving wall

In this paper, flow over an elliptical cylinder located near a moving wall is investigated numerically to ascertain the effect of aspect ratio (AR) on flow characteristics in terms of wake pattern, separation and stagnation points, and hydrodynamic forces. The two-dimensional simulations span both steady and unsteady flow regimes, where the effect of AR is analyzed in conjunction with other important parameters, such as gap ratio (GR∈ [0.6, 1.2]), Reynolds number (Re ∈ [5, 150]), and angle of attack (AOA∈ [−45°, 45°]). At a low AR = 0.1, significant differences in the wake pattern and critical Re for steady to unsteady wake transition are observed in GR-Re space in comparison with previous results for high AR cylinders. This results from an influence of AR on vorticity convection into the wake. Additionally, the effect of variation in AR from a flat plate (AR = 0) to a circular cylinder (AR = 1) on the wake pattern is discussed and presented in AR-Re space at a low GR. The locations of separation and stagnation points are shown to result from an interplay of AR and wall suppression depicting a low sensitivity to Re with low AR or high wall suppression effect. Subsequently, the effect of AR in conjunction with AOA is analyzed where counterclockwise inclination is observed to reduce wall suppression, while a competing effect arising from the contrasting movements of cylinder bottom and rear side governs the wake pattern for clockwise inclination that is explicit at a certain AR. Eventually, the variations in drag and lift forces with AR for different AOA are characterized.

Numerical Analysis of Flow Characteristics in Impeller-Guide Vane Hydraulic Coupling Zone of an Axial-Flow Pump as Turbine Device

An axial-flow pump as a turbine (PAT), as compared to the conventional Francis turbine, has the advantages of not being restricted by the terrain and having lower cost to reverse the pump as a turbine for power generation. When an axial-flow pump is reversed as a turbine, the internal flow pattern is more complicated than when in the pump mode, which can cause instability in the whole system and result in degradation of the hydraulic performance and structural vibration. The impeller and guide vane are the core of the axial-flow PAT unit. This research compares the experimental and numerical simulation results in order to verify the energy performance and pressure pulsation signal of the axial-flow PAT. The unsteady flow regime, fluid force, and pressure pulsation characteristics of the impeller-guide vane hydraulic coupling zone are analyzed in detail. The findings demonstrate that both the dominant frequency of the fluid force pulsation signal and the flow field pressure pulsation signal appear at 3 times of the rotation frequency. The blade passing frequency (BPF) of the impeller is the dominant frequency, and other frequency components are also dominated by the harmonic frequency of the BPF. The impeller and guide vane are primarily subject to radial fluid force. Under partial working conditions, the pressure pulsation intensity in the flow field greatly increases, and the pressure pulsation amplitude at the guide vane outlet and impeller outlet appears to be more sensitive to the flow rate change.

Flow division under a steady flow mode

Many scientists have been involved in the division of open streams. Existing methods for calculating fission nodes do not allow choosing their optimal designs that create a favorable regime for dividing flows. Most of the available results of studies of fission nodes are scattered, non-systematic, and in some cases, contain data that do not coincide with each other. The conducted studies of division nodes were carried out mainly for the steady flow regime; the flow turbulence issues in the flow division section have been little studied. However, in practice, an unsteady flow regime and an increase in flow turbulence are often observed, which leads to complex channel processes in the water intake area. The aim of the work is to develop a refined method for the hydraulic calculation of flow division nodes with a calm flow regime. This goal is achieved by an analytical solution to the problem of determining the water depth in the nodes of flow division under a steady flow regime. The paper uses theoretical studies using the equation for changing the momentum, laboratory studies on a hydraulic model, field surveys of existing water intake units, and an analysis of the experimental data available in the literature on this issue. According to the theoretical studies, calculated dependencies were obtained to determine the depth of the main flow in front of the fission node. The equation is a cubic equation concerning the OX axis and a quadratic equation concerning the OY axis. These two equations are solved independently of each other and are intended to determine the flow depth h1, which is established before the fission node. Taking into account the simplicity of the solution for practical calculations, we recommend the first dependence, and the second dependence is proposed for performing control calculations.

МОДЕЛИРОВАНИЕ ЦВЕТЕНИЯ ВОДЫ В КУЙБЫШЕВСКОМ ВОДОХРАНИЛИЩЕ В ГОДЫ С РАЗЛИЧНЫМ РЕЖИМОМ РЕГУЛИРОВАНИЯ СТОКА

The article studies the influence that the dynamics of currents have on algal blooms in the Kuibyshev Reservoir in the conditions of an unsteady flow regime. One of the ways to improve the ecological condition of the reservoir is to reduce cyanobacteria blooms by increasing the flow rate during the maximum warming of the water. The hydrodynamics of the aquatic environment is considered as a regulatory factor that reduces the negative effects of eutrophication and improves the quality of water in the reservoir. The author performed full-scale hydroecological observations in the summer periods of 2016 and 2017 and made model calculations of the dynamics of currents and spatial distribution of phytoplankton in the water area of the Kuibyshev Reservoir. As a result, the dependence of the chlorophyll a index (Chl a) on the flow regulation regime was established. The proposed model is based on the equations of long-wave hydrodynamics, known as the shallow-water equations, the equations of convective transport and development of the components of the ecosystem, which is implemented in the author’s software environment VOLNA. The model is constructed for the near-dam water area on a regular rectangular grid with a step of 200 m. The model was verified on the basis of data from field observations, and the quality of the calculations performed was evaluated. It has been found that the phytoplankton biomass index (Chl a) is related to the water flow velocity non-linearly and varies according to a power law. The conducted studies give reason to believe that the intensity of cyanobacteria blooms largely depends not on the water content of the year, but on the intra-annual redistribution of runoff.

A numerical study on the drag reduction and wake regime control of the tandem circular cylinders using splitter plates

The present study focuses on the control of an unsteady vortex flow regime and flow-induced forces on the tandem circular cylinder using the splitter plate-based technique at a Reynolds number (Re) of 100. The in-house developed flexible forcing immersed boundary lattice Boltzmann method (FFIBLBM) solver is employed for these two-dimensional numerical simulations. Three distinct tandem cylinder spacing is considered: small spacing (G/D = 2.0), medium spacing (G/D = 3.5), and large spacing (G/D = 5.0), where G is the center-to-center distance between the cylinders having diameter D. For each spacing, the effectiveness of two different configurations of splitter plates are examined: splitter plate attached to the upstream cylinder (TC-SPU) only and splitter plate attached to the downstream cylinder (TC-SPD) only. The arrangement and length of the splitter plate have a significant effect on the suppression of vortex shedding and reduction of flow-induced force on the cylinders. The result shows that the TC-SPD is beneficial for small spacing, yielding a drag reduction of 0.58 % and 0.05 % for the upstream and downstream cylinders. At the same time, both the TC-SPU and TC-SPD configurations are useful for suppressing the flow unsteadiness for medium spacing. However, the drag reduction in TC-SPD is more. In this case, the drag of the upstream and downstream cylinders is reduced by 1.4 % and 247 %, respectively. At large spacing, the TC-SPU arrangement is suitable for the suppression of vortex shedding from both cylinders. The corresponding drag reduction for the upstream and downstream cylinders is 13.7 % and 89.2 %, respectively.

Flow past a sphere: Numerical predictions of thixo-viscoelastoplastic wormlike micellar solutions

In this work, we present numerical solutions to the benchmark flow past a sphere problem considering the rheology of semi-diluted and concentrated wormlike micellar viscoelastic solutions using the most recent model-variant of the Bautista–Manero–Puig (BMP) family of fluids, i.e. the BMP+ _ τ p model (López-Aguilar et al., [1] ). This study is performed using our in-house finite volume/element algorithm, stabilised to attain highly-elastic non-linear solutions with two novel constitutive and boundary-condition implementations, i.e. the ABS- f (ABSolute f -functional) and the VGR (Velocity GRadient) corrections (López-Aguilar et al., [2] ). The structuring level of various wormlike micellar fluids with different solute concentrations (from semi-diluted to extremely concentrated solutions) and degrees of strain-hardening is studied with a dynamic dimensionless fluidity that connects the internal structure construction-destruction dynamics with viscoelasticity. In complex flow, a viscoelastic flow-regime unveils the prediction of a flow transition from a stable steady flow (related to a steady sphere settling) to an unsteady quasi-periodic flow regime, in which transient cycles of vortex formation-disappearance behind the sphere are present, resembling experimental findings on negative-wake instabilities and oscillations in the settling velocity for the descend of particles in wormlike micellar solutions. This instability is predicted here for semi-diluted to concentrated wormlike micellar solutions and it is located in flow-rate ranges corresponding to extensional Deborah numbers in the interval of around 26 . 7 ≤ D e e x t ≤ 40 . 3 , as reported experimentally. This behaviour is correlated with a chaotic flow-structure behind the obstacle, for which strong fluctuation in the fluid’s internal-structure is recorded. In the steady viscoelastic flow regime, correlation between the BMP+ _ τ p fluidity structure-variable and its normal-stress response around and downstream of the sphere (in the form of localised extrema) is detected, where interaction between the rheological response of the material and mixed shear-to-extensional deformation provide insights into the forces the fluid elements experiment in the sphere wake. Here, a monotonic decline in the drag-correction factor is observed, this being one of the main experimental manifestations of the steady sphere-settling phase in these materials. These findings may serve to further understand flow transitions and instabilities related to negative wakes in this kind of shear-thinning extensional-hardening fluids (Rothstein and Mohammadigoushki, [3] ). In a plastic regime, complex thixo-viscoelastoplastic BMP+ _ τ p features manifest through the prediction of fore-aft asymmetries promoted by the thixotropic structure construction-destruction and viscoelastic properties of the model under extremely low solute-concentrations. Predicted yield-fronts resemble those reported experimentally by Holenberg et al. [4] for carbopol solutions. • Negative-wake instability predictions in the De -range recorded experimentally. • Correlation between extensional viscosity and normal stresses in the sphere wake. • Localised N 1 and internal-structure f -functional in the sphere wake. • First normal-stress activity in the sphere wake follows extensional viscosity response. • Asymmetric yield-fronts under highly-concentrated gel-like wormlike micellar solutions.

Study of flow through and around a pair of porous cylinders covering steady and unsteady regimes

The present article focuses on the incompressible flow around two identical porous cylinders for a side-by-side configuration in a closed channel. The formation of various flow patterns behind permeable cylinders is more intriguing and further compelling to assimilate the underlying flow physics. The effects of three critical parameters, gap ratio (s/d), Reynolds number (Re), and the Darcy number (Da), on the flow behavior are investigated for the ranges of s/d = 1.5–6, Re = 5–100, and Da = 10−6–10−2. Both attached standing and detached vortices are observed in a steady flow regime. One secondary wake structure is also observed for s/d = 1.5, whose size gradually reduces with increased permeability. In an unsteady flow regime, the jet-like flow in the gap section mainly governs the unsteady wake patterns. In the low range of Darcy numbers (10−6–10−3), asymmetric flip-flopping patterns are observed for s/d = 1.5 and 2; and synchronized wake patterns either in anti-phase or in-phase mode are observed for higher gap ratios. The velocity profiles in the gap and free sides of the cylinders and pressure distribution along the porous surface are also discussed to facilitate the understanding of different wake patterns. Surprisingly, a case of pattern shifting from anti-phase to in-phase mode is observed when permeability is altered for the same flow-time. Symmetric and clustered strands of vorticity near the centerline are observed for all cases of s/d at Da = 10−2. The effects of Re, s/d, and Da on the drag coefficient and critical Reynolds number are also discussed. A jump in the drag values, a maximum of 13.9% for s/d = 3.5, is witnessed for the mid-range of Da at higher Re.

Local investigation of the dynamic and thermal fields to characterize the transition onset from steady to unsteady laminar flow in a complex geometry

The present work investigated numerically the onset of the transition from steady to unsteady laminar flow regime in a three dimensional complex geometry. The behavior of the dynamic and thermal fields of the chaotic fluid flow has been analyzed for numerous Reynolds numbers ranging from 190 to 550. Several parameters were used during this study to verify the fluid pattern such as; velocity and temperature profiles, secondary flow, flow intensity, helicity and the Nusselt number. It is found that at Re = 400, the symmetry of the flow is destroyed in the second and the third geometric periods whereas it still remains present in the first period which indicates the first step towards the transition. While at Re = 445, the symmetry is still preserved in the first period of the geometry, but the velocity begins to be unsteady with periodic variation at a single frequency where a clear impression can be given on the transition from the steady laminar regime to the unsteady regime. Results illustrate that the flow goes through intermediate steps to reach the unsteady domain before exiting the laminar zone. Moreover, the flow in the geometry of interest is not of the same nature so it can be totally or partially unsteady where different stages can be cited as; stable and symmetric flow, stable asymmetric flow, unstable flow with one or more frequencies and finally erratic flow.

Effect of an end-clearance width on the gap cavitation structure: Experiments on a wall-bounded axis-equipped hydrofoil

The paper deals with an experimental study of tip-clearance cavitation inception and development and its vortical structure coupled with dynamics of the main attached cavity on the suction side of a two-dimensional symmetric hydrofoil equipped with a rotation axis. The gap is formed by the end face of the model and a transparent sidewall of the test channel. The experiments were performed for attack angles of 3° and 9° and 0.4-, 0.8-, 1.75- and 3.75-mm gaps (or 1.8%–17% relative thickness) under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation occurrence, high-speed imaging was applied. The leakage flow velocity was measured inside the clearance by a modified Particle Tracking Velocimetry technique. It is shown that the mean velocity field of the leaking flow is split into eight distinctive zones where the flow direction and velocity magnitude substantially differ. Positions and extents of these zones are practically independent of the primary flow regime but are affected by the attack angle. Local velocity values of the leakage flow are unexpectedly found to be about 20% higher in the region of a gap cavity than the mean bulk velocity of the incoming flow. Cavitating cores of various vortices manifest themselves in the recorded images, showing that the vortex structure of the leakage flow associated with the tip-clearance cavitation is very complicated and the hydrofoil axis makes it even more complex by inducing new vortices and cavities. For the gaps considered, an increase of its size causes the tip-clearance cavitation to be initiated at higher cavitation numbers, i.e., this is favorable for its occurrence, while the development of the main cavity is hindered. In unsteady flow regimes, dynamics of the primary cavity on the hydrofoil suction side significantly influences the leakage flow direction and the tip-clearance cavitation evolution. Periods of oscillation cycles of the main and gap cavities coincide but the maximum size of the gap cavity is reached with a phase lag relative to the main one. At the small incidence angle, thicker gaps and unsteady flow conditions, extremely transient pressure waves are registered in the clearance, with their velocities ranging from 41 to 81 m/s.

Influence of the hydrofoil trailing-edge shape on the temporal-spatial features of vortex shedding

Ocean energy is a kind of clean and valuable renewable resource, which can be efficient utilized through turbomachinery. The turbulent pulsation effect and unsteady flow regime will cause pressure pulsation which can destroy the stability of the unit, especially at the tailing edge of blade. This paper compares pressure field change in the downstream region with different trailing edge geometry by numerical simulation and the pulsation tracking network method. The reasons for weakening downstream pressure pulsation with the Donaldson modification and the propagation law of the pressure pulsation are clarified. By Donaldson modification, the main frequency of pressure pulsation becomes the frequency of vortex shedding (fvs) and the amplitude is reduced to 10% of the original value. The propagation law of pressure pulsation has changed and the scope of influence has been expanded. The position of vortex generation can be judged by the phase variation. The weakening of the fvs amplitude is due to the stable low-pressure region and the phase difference variation. The inhibition of the convergence effect of vortex causes the weakening of the 2fvs amplitude. This study provides an effective method and reference for understanding the downstream pressure pulsation variation of shedding vortex.

Effects of viscoelasticity on the onset of vortex shedding and forces applied on a cylinder in unsteady flow regime

The present paper aims to investigate the effect of viscoelasticity on the onset of vortex shedding of a high concentration polymer solution over a cylinder using the finite volume method for the first time. To describe the behavior of the viscoelastic fluid, mathematically, the Phan–Thien–Tanner (PTT) model is employed. The convergence problems are resolved using the rheoFoam solver developed by previous researchers based on the log-conformation method. The exact critical Reynolds number (Recr), which corresponds to the onset of vortex shedding, is estimated by implementing numerous unsteady simulations at each elasticity number (El). The Recr is also calculated at every retardation ratio (β) and elongational viscosity. The results revealed a significant impact of viscoelasticity on Recr so that the flow of a high viscosity ratio PTT becomes unstable at higher Re (at very low El) or lower Re (at higher El), compared to a Newtonian fluid. In addition, Recr decreases linearly with β according to Recr=−34.5β+46.525 and increases with extensional viscosity. It is also found that β plays a vital role in the effect of viscoelasticity on the flow parameters. The averaged drag coefficient (CD¯) and the amplitude of lift coefficient (CLmax) do not have similar behaviors for low and high β. Moreover, viscoelasticity enlarges the vortices and increases the shedding frequency. A comprehensive physical analysis of flow structures is carried out using the distribution of time-averaged stress components and pressure over the cylinder. The numerical results demonstrated the three regimes of drag reduction at El < 0.015, drag enhancement at 0.015 < E1 < 1, and a Newtonian behavior at El > 1 that is an opposite trend compared to a steady regime. The variations of CLmax with El are also similar to CD¯, but at different critical elasticity numbers (El = 0.005 and 2). It is found that the normal stress changes the drag force by the variation of pressure distribution over the cylinder, while the shear stress directly affects the drag and lift forces. In addition, the viscoelasticity decreases the size of the vortices behind the cylinder and increases their vorticity, and changes the position of maximum normal stress, which leads to drag variations. It was also concluded that the higher the elongational viscosities, the lower the shedding frequency.

Screening analysis and unconstrained optimization of a small-scale vertical axis wind turbine

The demand for alternative and renewable energy sources has been substantially growing in recent years, mainly steered by economic and environmental inconveniences of conventional energy sources, such as oil and its derivatives. In this context, wind energy has emerged as an attractive renewable source, envisioning possibilities of developing more efficient equipment to meet the ever-growing energy demand. In this work, we coupled Computational Fluid Dynamics (CFD) with an optimization based on response surface (RS) methodologies to find an optimal design for a small-scale NACA 0021 Darrieus vertical axis wind turbine (VAWT) operating at a tip speed ratio of 2.63. For that, we investigated four geometric parameters: number of blades (N), rotor diameter (D), chord length (c), and pitch angle (β). For the numerical model, we considered a two-dimensional, incompressible, turbulent, and unsteady flow regime. A sensitivity analysis (SA) via Morris’ method was performed to identify the influence of the four geometric parameters on the turbine aerodynamic performance. Our results reveal that the pitch angle (β) contributes the most (58%) to the turbine performance. The resulting optimized turbine design increased the conversion efficiency by 40%. Additionally, we also present a detailed discussion on the flow phenomenology considering the impact of each one of the four geometric parameters on the power coefficient. Finally, the strategy adopted here, in which a qualitative sensitivity analysis combined to the response surface and unconstrained optimization, was shown to be robust and can be applied to high-dimensional and computational-expensive CFD models to reduce costs with adequate results regarding fluid flow phenomena.