Oscillatory Flow Research Articles

Coarse Sand Transport Processes in the Ripple Vortex Regime Under Asymmetric Nearshore Waves

AbstractLarge‐scale wave flume experiments are conducted in the ripple vortex regime to study near bed coarse sand transport processes below asymmetric surface waves typical of the coastal nearshore region. For this purpose, a set of complementary acoustic instruments were deployed under regular nearshore wave conditions. Time‐resolved velocity, sand concentration and sand flux profiles are measured across both the dense bedload and dilute suspension layers with an Acoustic Concentration and Velocity Profiler. The equilibrium 2D suborbital ripples are in good agreement in terms of dimensions, shape and onshore migration rate with Wang and Yuan (2018, https://doi.org/10.1029/2018jc013810, 2020, https://doi.org/10.1016/j.coastaleng.2019.103583). Stoss ripple vortex entrainment around the trough‐to‐crest flow reversal (FR+) is found to be more energetic in terms of sand pick‐up into suspension compared to the counter rotating lee side vortex around the FR‐ flow reversal, as a consequence of the onshore skewed wave acceleration. Ripple vortex driven nearbed velocity phase leads around both flow reversals exceed typical bed friction induced values found in turbulent Wave Boundary Layers. Intrawave sand erosion events can be distinguished locally at the two ripple vortex positions around the flow reversals and two events more uniformly distributed along the ripple profile at wave crest and trough. Spatial fields of sand flux reveal the origin of the net onshore directed suspended and bedload transport. Good agreement is found with the mechanism identified under asymmetric oscillatory flows in Wang and Yuan (2020, https://doi.org/10.1016/j.coastaleng.2019.103583). Differences with ripple vortex regime under skewed shoaling waves and symmetric oscillatory flows are highlighted.

Viscoelasticity of suspension of red blood cells under oscillatory shear flow

We present a numerical analysis of the rheology of a suspension of red blood cells (RBCs) for different volume fractions in a wall-bounded, effectively inertialess, small amplitude oscillatory shear (SAOS) flow for a wide range of applied frequencies. The RBCs are modeled as biconcave capsules, whose membrane is an isotropic and hyperelastic material following the Skalak constitutive law. The frequency-dependent viscoelasticity in the bulk suspension is quantified by the complex viscosity, defined by the amplitude of the particle shear stress and the phase difference between the stress and shear. SAOS flow basically impedes the deformation of individual RBCs as well as the magnitude of fluid-membrane interactions, resulting in a lower specific viscosity and first and second normal stress differences than in steady shear flow. Although it is known that the RBC deformation alone is sufficient to give rise to shear-thinning, our results show that the complex viscosity weakly depends on the frequency-modulated deformations or orientations of individual RBCs but rather depends on combinations of the frequency-dependent amplitude and phase difference. The effect of the viscosity ratio between the cytoplasm and plasma and of the capillary number is also assessed.

Effects of buoyancy on the dispersion of drugs released intrathecally in the spinal canal.

This paper investigates the transport of drugs delivered by direct injection into the cerebrospinal fluid (CSF) that fills the intrathecal space surrounding the spinal cord. Because of the small drug diffusivity, the dispersion of neutrally buoyant drugs has been shown in previous work to rely mainly on the mean Lagrangian flow associated with the CSF oscillatory motion. Attention is given here to effects of buoyancy, arising when the drug density differs from the CSF density. For the typical density differences found in applications, the associated Richardson number is shown to be of order unity, so that the Lagrangian drift includes a buoyancy-induced component that depends on the spatial distribution of the drug, resulting in a slowly evolving cycle-averaged flow problem that can be analysed with two-time scale methods. The asymptotic analysis leads to a nonlinear integro-differential equation for the spatiotemporal solute evolution that describes accurately drug dispersion at a fraction of the cost involved in direct numerical simulations of the oscillatory flow. The model equation is used to predict drug dispersion of positively and negatively buoyant drugs in an anatomically correct spinal canal, with separate attention given to drug delivery via bolus injection and constant infusion.

Vibration-Induced Flow and Streaming in Oscillatory Flow of Thermoacoustics

Induced acoustic streaming flow in thermoacoustic systems occurs due to acoustic vibrations, causing changes to the mean flow in the systems. This phenomenon creates a tendency to generate net fluid flow that can cause energy change within certain areas inside the system. However, the effects of the entire system’s vibrations on the flow streaming are not yet fully understood, yet it is important for a more effective operation. This study experimentally investigated the flow streaming resulting from vibration in a standing-wave thermoacoustic test rig with a flow frequency of 23.6 Hz. The existence of flow streaming due to vibration which was not counted in the theoretical formula is shown and indicated by experimental values. The result shows that there is a correlation between the amplitude of the drive ratio (DR) and the velocity of the oscillation of the flow. Upon examining both the theoretical and the practical evidence, it becomes clear that there exists a marginal flow velocity in directions other than the main flow due to the vibration of resonator’s wall as measured at the intake and outflow areas of the stack. This marginal flow velocity amplifies as the drive ratio of flow increases and it may potentially explain the observed difference between measured flow amplitude and the theoretical value.

Delayed Detached Eddy Simulation of Flow Separation Control Using Fluidic Oscillator

The investigation of flow separation control over a NACA 0015 airfoil model using a fluidic oscillator (FO) is conducted through delayed detached eddy simulation. First, the flowfield within and outside an FO operating in quiescent air is resolved simultaneously. The oscillation of the jet flow induced by the FO is attributed to the cyclic expansion and contraction of the recirculation bubbles located near the two Coanda surfaces in the mixing chamber. Significantly, the predicted jet oscillation frequency closely matches the experimental data, validating the accuracy of our findings. Next, the FO is integrated into an airfoil model to suppress the flow separation. The airfoil is under deep stall conditions, with angles of attack of 20 and 17° and a Reynolds number of Re=4.8×105. The key driving force for flow reattachment is the spanwise vortices induced by the oscillating jet, which substantially enhance the mixing between the separated flow and the external high-momentum flow. Consequently, the aerodynamic performance of the airfoil is notably improved. Additionally, both spectral and dynamic mode decomposition analyses indicate that the flow, under the influence of the FO forcing, becomes more orderly and well-organized and is effectively locked into the forcing frequency.

Pulsatile MHD Flow of Two Immiscible Nanofluid through a Porous Channel with Slip Effects

The present study is carried out to investigate the effects of shape factor nanoparticles on the oscillatory MHD flow of a nanofluid in two immiscible liquids in a horizontal porous channel with velocity and thermal slip on the walls. Thermal radiation, Joule heating, viscous and Darcy dissipations have been accounted for in the model. We have considered and as nanoparticles, in the lower region (Region-I) and upper region (Region-II) respectively, with water as a base fluid. The effective ratio of thermal conductivity of the nanofluid is evaluated using the Maxwell-Garnetts model. Graphical behavior of velocity, temperature, and rate of heat transfer distributions have been depicted for the cases of slip and no-slip effects. This study has been made to understand the impact of different nanoparticle shape factors on temperature and heat transfer rate. For various parameters, values of shear stress distribution at the walls and the mass flux are shown in tabular form. Our study asserts that with the increase of the strength of the magnetic field, the velocity of the liquid falls and enhances the temperature of the liquid. The influence of different combinations of nanoparticles, on the flow variables, have also been discussed. In order to validate the analytical results, the numerical evaluation of the closed-form results, for the velocity distribution, has been compared with those of the numerical method, by using the NDSolve command in MATHEMATICA, and a good agreement is observed.

Numerical investigation on vortex-induced vibration of catenary riser conveying fluid under top-end heave excitation

ABSTRACT Due to the catenary shape of steel catenary risers (SCR), the heave motion of the floating platform can generate an equivalent oscillatory flow field and excite the vortex-induced vibration (VIV) of SCR. Internal flow conveyed by SCR further complicates the VIV response. A semi-empirical model is improved to simulate the VIV of SCR with internal flow under top-end heave excitation. VIV characteristics such as displacement temporal–spatial distribution and amplitude spectra spatial distribution are analysed and the fatigue damage is calculated. The effects of heave period, heave amplitude, internal flow volumetric flow rate and internal flow density on VIV of SCR are discussed. The heave excitation with short period or large amplitude tends to excite high-order-mode response and enlarge the fatigue damage. The VIV response becomes less regular and the fatigue damage is reduced when the internal flow volumetric flow rate or density increases.

Hydrodynamics and Mixing in Mesoscale Integrated Fluidic Oscillatory Flow Reactor (F-OFR)

Hydrodynamics and Mixing in Mesoscale Integrated Fluidic Oscillatory Flow Reactor (F-OFR)

Fluid-solid interaction simulations of an aeroelastic square prism in sinusoidal oscillatory flows

This study numerically investigates the aerodynamic and aeroelastic characteristics of a square prism (aeroelastic model) and wind field around it in sinusoidal oscillatory flows (SOFs). The reliability of the fluid-solid interaction (FSI) simulation is validated by a free vibration test and wind tunnel tests in smooth flow and SOF. The effects of the amplitude and frequency of SOFs are studied at the mean wind speed of vortex-induced resonance. The results show that increasing the amplitude and frequency of SOFs will amplify the root mean square (RMS) along-wind and across-wind base shear forces of the aeroelastic model but decrease the RMS across-wind displacement at the top of the aeroelastic model. The spectral analysis of the base shear forces indicates that the influence of vortex shedding on the across-wind base shear force is reduced by either increasing the amplitude or increasing the frequency of SOFs. The mean and instantaneous wind fields around the aeroelastic model in SOFs and smooth flow are compared, and the wake characteristics of the aeroelastic model in SOFs are analysed by dynamic mode decomposition. It is observed that when the frequency of SOFs is 1.5 times as large as the fundamental natural frequency of the aeroelastic model, the regular vortex shedding process is substantially affected.

Flow and thermal characteristics of Jeffrey nanofluid in the fixed and moving frames of reference with newly developed Reynolds model

Nanofluids with their excellent thermophysical properties strengthen the thermal performance of an ordinary fluid. Due to the efficacious thermophysical characteristics of nanofluids, the aim of the present study is to discuss the three distinct viscosity models for an incompressible Jeffrey nanofluid relative to its oscillatory oblique stagnation point flow. Three different viscosity models namely Brinkman model, Einstein model, and Reynolds model are discussed to scrutinize the thermophysical attributes of (TiO2-Kerosene oil) nanofluid. The unsteady magnetized flow problem of Jeffrey nanofluid is deliberated over an oscillatory medium corresponding to different frames of references namely fixed frame and moving frame. The thermal analysis is evaluated with the involvement of heat absorption/generation and radiation effects. In both frames of reference, the non-dimensional equations are numerically handled via an efficient technique of bvp4c. The fluid velocity and temperature distribution are graphically compared regarding three distinct viscosity models with numerous pertinent parameters. In the context of all viscosity models, both the distributions of velocity and temperature exhibit excellent behavior in the fixed and moving frame of reference. This study exhibits that the parameter that portrays the ratio of relaxation to retardation times deteriorates the flow field in the fixed frame for all viscosity models.

Design and construction of a single orifice oscillatory flow reactor for the continuous production of biodiesel from sunflower oil

Biodiesel, a renewable energy replacing fossil fuels, exhibits eco-friendly traits with beneficial lubrication properties. This research aims to design and construct a single-orifice oscillatory flow reactor to reduce the reaction time, improve the mixing process, and increase efficiency. The response surface method was used for the analysis of independent parameters like molar ratio (1:6–1:12), temperature (30–50 °C), reactor length (1–3 m), and catalyst concentration (0.75%-1.25%) on yield as the dependent variable. Elevated temperature up to 50 °C enhances conversion from 61.91% to 69.51%, peaking at 65.43% by 60 °C, adjusting the molar ratio to 1:9 boosts biodiesel conversion from 58.70% to 69.51%, dropping by 4% at 1:12 ratio. Catalyst concentration at 1% heightens conversion from 48.82% to 69.51%, but at 1.25%, it falls to 57.26%. Increasing reactor length to 2 m yields a 20% boost, while at 3 m, conversion drops by 4%. Optimal conditions, at 40 °C, 1:9 ratio, 1% catalyst, and 3 m reactor, resulted in a 91.98% conversion percentage. Experimental verification at the suggested point yields an 89% conversion, aligning acceptably with the model’s prediction. Biodiesel meets EN 14214 standards, positioning it as a viable diesel fuel alternative.

Analytical Investigations of the Performances and Effects of Heat Transfer Processes in Oscillatory Flow

Analytical Investigations of the Performances and Effects of Heat Transfer Processes in Oscillatory Flow

The identification of the generalised Maxwell fluid for n-hexadecane liquids via non-equilibrium molecular dynamics simulations

ABSTRACT Linear viscoelasticity is very important for chain-molecular fluids. The generalised Maxwell constitutive equation is classical in which the linear viscoelastic fluids consist of a series of spring and dashpot. It is significant to identify the generalised Maxwell fluid of linear viscoelasticity for n-hexadecane liquids through non-equilibrium molecular dynamics simulations of small-strain amplitude oscillatory shear flows. For describing a characteristic of material viscoelasticity, the predicted relaxation modulus curves at different temperatures exhibit three regions: transition, rubbery, and flow. Additionally, the Cox-Merz rule is verified in which the complex oscillation-shear viscosity almost equals the steady-state shear viscosity with respect to shear rates.

The state of microcirculation in patients with periodontal diseases and disorders of the occlusal relationships of the dentition

Background. The study of microcirculation disorders, as one of the key factors in the pathogenesis of periodontal diseases, is relevant in clinical dentistry. Premature contacts, acting as provoking factors for occlusal disorders, cause overload of individual teeth and aggravate the severity of inflammatory periodontal pathology. Objective registration of capillary blood flow and associated changes in the microvessels of the arteriolar and venular units in patients with periodontopathies and occlusal dis-orders will allow us to assess the degree and nature of tissue blood flow disorders in the gum tissue.Goal. Increasing the efficiency of diagnosing microcirculatory disorders in periodontal tissues in patients with chronic generalized mild periodontitis and disorders of the occlusal relationships of the dentition.Materials and methods. Using laser Doppler flowmetry (LDF), the state of mi-crocirculation in the periodontium was studied in 78 people aged 21–35 years with intact dentition and physiological occlusion, of which 33 people (group 1) had clinically healthy periodontium and the absence of occlusal disorders (group 1), with generalized chronic pulmonary periodontitis degree and premature occlusal contacts – 45 people (group 2). The microcirculation index (M), standard deviation (σ), and coefficient of variation (КV) were assessed. To obtain objective diagnostic data, amplitude-frequency analysis of the harmonic rhythms of LDF-grams was used with spectral decomposition into harmonic components of physiological oscillations of tissue blood flow (wavelet analysis).Results. In patients of group 2, in comparison with patients of group 1, a decrease in the level of periodontal blood perfusion, a reduction in the fluctuation of erythro-cyte flow and vasomotor activity of microvessels is determined, while a decrease in “active” and an increase in «passive» modulation of tissue blood flow contributes to an increase in the volume of circulating blood at arteriovenular anastomoses when the intensity of blood flow through the vessels of the capillary bed decreases.Conclusions. Patients of group 1 were diagnosed with a normoemic type of LDF-gram, which provides a relatively high degree of intensity of tissue blood flow, balance of regulatory mechanisms, and optimal trophism of periodontal tissues. In patients of group 2 with a spastic type of LDF-gram, characterized by a reduced level of perfusion, flux, vasomotor activity of microvessels, indicators of tissue blood flow and trophic function are significantly reduced.

Flow Reversal of Unsteady Oscillatory Flow in Combined Convection Fully Developed Vertical Channel

Numerical simulations of flow and heat transfer in an unsteady oscillatory fully developed combined convection are conducted. The temperature on the right wall varies with sinusoidal rhythm over time within a locked mean temperature, whereas the temperature on the left wall remains constant. Both walls are also considered to be stationary. A numerical solution for the momentum and energy equations obtained using the Runga- Kutta method is used to analyze the effect of periodic oscillation on the velocity and temperature configurations, as well as the progression of heat generations. The combined convection process of heat transfer and its flow pattern in a vertical channel is important in many applications, including environmental, industrial, and engineering. This study is prompted by a problem with the heat transfer process, which is difficult, expensive, and time-consuming. The purpose of this study is to develop a mathematical model, find a numerical solution, and conduct parametric studies on the double-diffusive fully developed vertical channel with unsteady oscillatory flow, as well as to investigate the effect of various parameters on the velocity and temperature profiles. The effect of different dimensional parameters is tested and the flow reversal phenomenon is discussed. The dimensional equations are transformed into non-dimensional equations using the similarity technique. Then, the built-in program dsolve in Maple is used to numerically solve the boundary value problem (BVP). A validation study is conducted on a previously reported problem to confirm the validity of the present calculation. The flow and temperature numerical findings are represented visually. It is found that a large development of flow is caused by the plate temperature oscillation with high amplitude, therefore, the nature of the flow differs from the non-oscillating case.

Optimization of methyl ester synthesis using gas/liquid phase pulsed discharge plasma in a novel oscillatory slug flow reactor

In the present research, an innovative oscillatory slug flow reactor (OSFR) under the treatment of gas–liquid phase pulsed discharge plasma was developed for biodiesel production. The main goal was continuous production of high quality biodiesel at low temperature and pressure. Experimental tests were carried out under the influence of four main operating parameters including applied voltage, molar ratio, reactant flow rate, and catalyst concentration. The response surface method was employed to optimize experimental tests. The results showed that the proposed technology provided 94% production efficiency under the optimal conditions of voltage 19.4 kV, molar ratio 6.4, flow rate 2.7 ml/s, and catalyst 0.9 wt. %. According to the statistical analysis, increasing the applied voltage and reducing the flow rate have a strong effect on the Fatty Acid Methyl Ester yield, while the concentration of potassium hydroxide and methanol have less effect on the overall efficiency. In addition, the characteristics of the produced biodiesel were in accordance with ASTM D6751 standards. Surprisingly, the optimal energy consumption in this system was 95 kJ/l, which is more economically viable. In general, this study showed that the combined system of gas/liquid phase plasma in the OSFR reactor has a high synergistic potential for the transesterification reaction.

The role of fluid flow patterns in microbubble-mediated endothelial cell membrane permeabilization

Blood flow dynamics vary throughout the circulatory system, influencing the pathophysiology of vascular endothelium. We investigated endothelial cell response to ultrasound-stimulated microbubbles across diverse anatomical sites by mimicking assorted blood flow patterns. First, we examined the effect of culture condition on cell sensitivity to sonication by culturing HUVECs either statically or under pulsatile flow (8 or 16 dyn/cm2) for two days. Flow chambers were then co-perfused with microbubbles and propidium iodide under pulsatile flow (8 or 16 dyn/cm2) and sonicated (1MHz, 20 cycles, 1ms PRI, 300kPa) using an acoustically coupled microscope. Additionally, in a subset of studies we investigated ultrasound-assisted endothelial permeability under both pulsatile and oscillatory flow patterns. Compared to static, cells cultured under pulsatile flow resulted in 1.2 to 2.0-fold increase in the percentage of permeabilized endothelial cells (p < 0.0001). Next, compared to sonication under laminar flow (15–30 ml/min), endothelial permeability was significantly augmented under pulsatile flow, ranging from 1.3 to 2.1-fold. Additionally, compared to laminar flow, oscillatory flow at ∼16 ml/min with 0.5 Hz oscillation resulted in a 1.76-fold enhancement in cell perforation, yet yielded no observable permeability at slower flow rates (∼8 ml/min). These findings highlight the influence of local blood flow dynamics on ultrasound-mediated cell permeabilization.

Manipulation of the flow induced by afterbody vortices using sweeping jets

In this study, the impact of oscillatory fluids on the flow induced by afterbody vortices, resembling those of a high-speed train, was investigated in wind tunnel experiments. The afterbody vortices were generated by a cylindrical model with a slanted base at Reynolds numbers ranging from 87 000 to 200 000. A fluidic oscillator was utilized to produce sweeping jets (SWJ) acting on the vortical flow, which is similar to those observed over the rear of a typical high-speed train. Our particle image velocimetry measurements on cross-stream planes reveal that the oscillatory flow produced by the sweeping jet significantly reduces the magnitude of induced flow, resulting in a 17.7% and 15.9% decrease in the total velocity and horizontal slipstream, respectively, at a velocity ratio of 6.4. This is attributed to the injection of the kinetic energy of sweeping jets into the core of the afterbody vortices, which leads to a higher dispersion of vorticity and incoherent energy distributions. The iso-surfaces of resultant velocities reveal that the fluid injection by the sweeping jet increased local velocities, resulting in a spanwise shrinkage of the induced velocity structure downstream. The study demonstrates the capability of sweeping jets in decreasing induced flow by manipulating afterbody vortices and their potential for reducing the slipstream produced by high-speed trains at the tracksides.

Heat transfer analysis of MHD oscillatory SWCNT/MWCNT‐H2O hybrid nanofluid flow in a channel

AbstractRegulating the thermal conductivity of the fluid in various heat exchange systems plays an important role. In this regard, the carbon nanotubes (CNTs), with their higher thermal conductivity, can enhance the heat transfer potential. The main objective of the present study is to explore the impact of suction and injection on the oscillatory flow of CNT‐based hybrid nanofluid through a channel under the influence of the magnetic field for analysing heat transfer perspectives. While developing a mathematical model of the problem, we also considered other relevant elements, such as thermal radiation for optically thin fluids and porous mediums. The behaviour of a water‐SWCNT + MWCNT hybrid nanofluid is studied concerning the volume fraction of CNTs. The governing flow equations have been solved to obtain the correct solutions for the distribution of velocities and temperatures. Graphs and tables are used to assess the impact of relevant parameters on the flow and derived quantities. Adding CNTs to a fluid reduces its temperature and velocity, making it ideal for cooling systems. However, plates exhibit a maximum heat transfer of 35.35% and 25.35%, respectively, for and . The magnetic and constant pressure parameters greatly diminish the fluid's velocity, while the thermal buoyancy forces strengthen the flow. The effectiveness of the current study is confirmed by comparing its results to those from previous investigations.

Effect of turbine's torque and speed variation on hydraulic vibration analysis during transient processes

AbstractWith the rapid development of large‐capacity hydraulic turbines and pumped‐storage power stations, hydraulic vibration analysis attracts increasing attention nowadays. To explore the influence of torque and speed variation on hydraulic vibration characteristics during transient processes and ensure the stability of hydropower stations, an improved analytical formula of turbine impedance involving the torque and speed variation was then derived. The free vibration analysis and forced vibration analysis using the transfer matrix method and the hydraulic impedance method were carried out. According to the numerical simulations, the obtained attenuation factors show differences in magnitude which illustrates that the torque variation and speed variation in transients do affect the damping rate of the vibration. In addition, the results indicated the natural angular frequencies of hydropower system were mainly dependent on the length of water conveyance system and pipe characteristics. It can be inferred form vibration mode shapes that for some specific frequencies, the magnitude of oscillatory pressure head and flow rate along the downstream tail tunnel are influenced by the hydraulic turbine. The correlation between the turbine impedance and guide vane openings tells that the hydraulic impedance is getting smaller as the opening increases. What's more, the corresponding free vibration analysis shows that the absolute value of the damping rate is closely related to the value of the flow rate.