Magnetic Field Effects On Backward-facing Step Flow of Ferrofluids

Backward-facing step (BFS) flow is a benchmark case study in fluid mechanics. Its control by means of electromagnetic actuation has attracted great interest in recent years. This paper focuses on the effects of a uniform stationary magnetic field on the laminar ferrofluid BFS flows for the Reynolds number range 0.1=Re=400 and different expansion ratios. The coupled ferrohydrodynamic equations, including the microscopically derived magnetization equation, for a two-dimensional domain are solved numerically by an Open FOAM solver after validation and a test of accuracy. The application of a magnetic field causes the corner vortices in the concave corner behind the step to be retracted compared with their positions in the absence of a magnetic field. The maximum percentage of the normalized decrease in length of these eddies reaches 41.23% in our simulations. For small Reynolds numbers (<10), the flow separation points on the convex corner are lowered in the presence of a magnetic field. Furthermore, the dimensionless total pressure drop between the channel inlet and outlet decreases almost linearly with Reynolds number Re, but the drop is greater when a magnetic field is applied. On the whole, the normalized recirculation length of the corner vortex increases nonlinearly with increasing magnetic Reynolds number Rem and Brownian Péclet number Pe, but it tends to constant values in the limits Re ≪ 1 and Re ≫ 1.

Profiling of sudden contraction of circular pipe by sharp-edged pipe insert

Measures to reduce energy losses in pipeline transport systems must be implemented already at the design stage. In particular, this also applies to local resistances of pipelines. For symmetrical sudden pipe contraction, one of such measure is profiling using insert. The paper considers a sharp-edged pipe insert, which in comparison with others has larger areas of flow separation. The diameter of the insert was calculated as the arithmetic mean between the diameters forming the sudden pipe contraction. Using the methodology described in the literature, the contraction length ratio as the areas of influence between two symmetrical sudden contractions of a circular pipe during the flow of a single-phase turbulent flow of Newtonian fluid was investigated. The coefficient of mutual influence of these local resistances was determined by formulae for direct-flowing and non-direct-flowing locking devices, which simulated symmetrical and asymmetrical fluid flow after the constriction plane, respectively. The contraction rate from 0.064 to 0.696 inclusive are considered. According to the results of calculations, no mutual influence of two sudden contractions of circular pipe was found. This indicates the unsuitability of using formulae for locking devices in this methodology for determining the contraction length ratio. Therefore, to study this problem on the sudden pipe contraction with different contraction rates, it is proposed to conduct a physical experiment. In a mathematical experiment, you can use formulae to determine the following values: upstream recirculation length and downstream recirculation length; inlet length of cylindrical pipes with turbulized the flow devices; length of the velocity profile stabilization after local resistances.

Interactions between Tandem Cylinders in an Open Channel: Impact on Mean and Turbulent Flow Characteristics

The flow hydrodynamics around a single cylinder differ significantly from the flow fields around two cylinders in a tandem or side-by-side arrangement. In this study, the experimental results on the mean and turbulence characteristics of flow generated by a pair of cylinders placed in tandem in an open-channel flume are presented. An acoustic Doppler velocimeter (ADV) was used to measure the instantaneous three-dimensional velocity components. This study investigated the effect of cylinder spacing at 3D, 6D, and 9D (center to center) distances on the mean and turbulent flow profiles and the distribution of near-bed shear stress behind the tandem cylinders in the plane of symmetry, where D is the cylinder diameter. The results revealed that the downstream cylinder influenced the flow development between cylinders (i.e., midstream) with 3D, 6D, and 9D spacing. However, the downstream cylinder controlled the flow recirculation length midstream for the 3D distance and showed zero interruption in the 6D and 9D distances. The peak of the turbulent metrics generally occurred near the end of the recirculation zone in all scenarios.

Flow Characterization of Bluff Bodies: A Two-Dimensional Transformation From Square to Triangular Cylinder

In the present study, two-dimensional unsteady, incompressible flow around a square body that is being transformed into a vertex oriented towards the flow configuration of a triangular body is numerically investigated at Re =100 using ANSYS FLUENT 19.0 software. The purpose is to explore the effect of this transformation on the wake characteristics of a square body with l/d = 1 to a triangular body with l/d = 0; where l is the length of lateral and front surface, and d is the body height. The effect on the flow behavior caused by the leading-edge transformation from the prospect of wake width, recirculation length and stagnation pressure difference is discussed. It is seen that as the l/d ratio decreases, the vortex strength increases which is attributed to the higher stagnation pressure difference value resulting in more intense rolling of the shedding vortex and a smaller wake width. For lower l/d, the fluid traverses a longer distance along the lateral surfaces resulting in greater loss of momentum and hence the lower vortex formation length. The mean drag coefficient is found to be minimum for l/d = 0.75 with stagnation pressure difference and recirculation length being the more dominating factor on this variation. The flow in all the cases separates at the rear surface and the general trend of decrease in drag coefficient with decrease in wake width is not followed. However, such modification leads to better aerodynamic outcome by weakening the periodic drag and lift forces.

A Numerical Study of Correlation Between Recirculation Length and Shedding Frequency in Vortex Shedding Phenomena

The purpose of this paper is to characterize and to estimate the recirculating length behind an aerodynamic profile in ground effect with Gurney Flap. The flow characterization at high Reynolds numbers was performed by means of numerical analysis. A correlation between the size of the recirculation length and the frequency of vortex shedding was studied. The vortex shedding has a characteristic frequency, which, in this work, is correlated to the size of a recirculation length defined by the authors. The numerical investigation methodology applied to the profile with Gurney Flap, was previously developed on the well-documented test case of the flow around a cylinder at high Reynolds. The case was chosen to investigate and to validate the numerical approach with experimental data.

Simulation of vortex characteristics in a planar ψ-shaped branching microchannel with lattice Boltzmann method

The vortex in the branching microchannel enhances the mixing and heat transfer performance. To investigate the vortex intensity quantitatively, a lattice Boltzmann model for incompressible power-law fluid is developed by setting the range of lattice viscosity (0.001 [Formula: see text] 1). The validation of the current model is carried out by modeling the vortex in a T-shaped branching channel and the Poiseuille flow of power-law fluids. Then the vortex intensity in the [Formula: see text]-shaped microchannel is numerically studied in terms of Reynolds number, branching angle and power-law index. The result indicates that both the recirculation length and height increase with the increase of the Reynolds number. The branching angle has a negative impact on the recirculation length, and it has little effect on the recirculation height. The influence of the power-law index on recirculation length and height depends on the Reynolds number.

Influence of near-wall PIV data on recirculation hemodynamics in a patient-specific moderate stenosis: Experimental-numerical comparison

BACKGROUND: Recirculation zones within the blood vessels are known to influence the initiation and progression of atherosclerotic lesions. Quantification of recirculation parameters with accuracy remains subjective due to uncertainties in measurement of velocity and derived wall shear stress (WSS). OBJECTIVE: The primary aim is to determine recirculation height and length from PIV experiments while validating with two different numerical methods: finite-element (FE) and -volume (FV). Secondary aim is to analyze how FE and FV compare within themselves. METHODS: PIV measurements were performed to obtain velocity profiles at eight cross sections downstream of stenosis at flow rate of 200 ml/min. WSS was obtained by linear/quadratic interpolation of experimental velocity measurements close to wall. RESULTS: Recirculation length obtained from PIV technique was 1.47 cm and was within 2.2% of previously reported in-vitro measurements. Derived recirculation length from PIV agreed within 6.8% and 8.2% of the FE and FV calculations, respectively. For lower shear rate, linear interpolation with five data points results in least error. For higher shear rate either higher order (quadratic) interpolation with five data points or lower order (linear) with lesser (three) data points leads to better results. CONCLUSION: Accuracy of the recirculation parameters is dependent on number of near wall PIV data points and the type of interpolation algorithm used.

Numerical Analysis of Drag Force Acting on 2D Cylinder Immersed in Accelerated Flow

In this study, the drag exerted by an accelerating fluid on a stationary 2D circular cylinder is numerically investigated using Fluent 19.2 based on the finite-volume method. The SST k–ω model is chosen as the turbulence model because of its superiority in treating the viscous near-wall region. The results are compared to literature, and the numerical methods are validated. The acceleration of the inflow is analyzed for the range of 0.0981–9.81 m/s2, and the drag for each acceleration is compared. Additionally, the effect of the initial velocity on the drag acting on the circular cylinder is investigated at two initial velocities. As a result, a supercritical region, typically found under steady state conditions, is observed. Furthermore, vortex shedding is observed at a high initial velocity. This flow characteristic is explained via comparison with respect to the recirculation length and separation angle.

Can Large Precipitating Cloud Hydrometeors Generate Secondary Cloud Droplets in its Wake?

<p>Atmospheric clouds play a very important role in the evolution of global atmosphere and climate through various interactive physical processes dynamically active over a huge range of scales [Devenish et al. QJRMS 2012, Grabowski and Wang. ARFM 2013]. However, many of such processed are yet to be understood; and in such context, we attempt to understand such a scientific question: whether large precipitating cloud drops can generate secondary droplets in it’s wake. Motivated by experimental investigation of large sedimenting cloud droplets [∼ mm radius] which showed presence of secondary cloud droplets in it’s wake [Prabhakaran et al. PRL 2017, ArXiv 2019]; we conduct direct numerical simulations of such precipitating hydrometeors using Lattice-Boltzmann method (LBM) to simulate cloud like ambient solving the evolution of the supersaturation field in the wake of the hydrometeor, and to investigate it’s impact on the nucleation of cloud aerosols. In our simulation results, we found various flow regimes based on the Reynolds number (Re = Droplet Diameter * Droplet Velocity / Kinematic Viscosity) in compliance with past researches. Steady axisymmetric wake for Re up to ∼ 220, after that steady oblique wake up to Re ∼ 280, then a transient oscillating nature of the wake up to Re ∼ 350, and beyond that Re, the wake is observed to become chaotic and turbulent. Comparison of drag coefficient, recirculation length and separation angles for fluid velocity at various Re shows good agreement with existing numerical and experimental simulations. The temperature profiles also fit well with other researches for similar Prandtl number (ratio of kinematic viscosity to thermal diffusivity). Evolution of the density of water vapor is similar to the temperature field, since both the equations show similar structure and the mass diffusivity of water vapor is almost same to the thermal diffusivity for atmospheric clouds. Distribution of the supersaturation field is computed using Clausius-Clapeyron Equation which gives saturation vapor pressure depending on temperature. In such simulations with background flow at -15<sup>o</sup> C temperature with 60% relative humidity (RH) and with the hydrometeor as a warm cloud droplet at 4<sup>o</sup> C temperature and 100% RH at it’s surface, the wake shows symmetric regions of supersaturation in the near vicinity of the hydrometeor at Re = 200. Whereas, at Re = 273, the wake is observed to become oblique, so the supersaturated region. Small pockets of supersaturated warm air parcels are observed to travel in the downstream direction when the hydrometeor started shedding vortices at higher Re. However, while traveling downstream, such supersaturated pockets also lost its’ excess of water vapor depending on the ambient cloud conditions. Due to higher supersaturation at the near vicinity of the warm hydrometeor, the cloud aerosols trapped inside the wake can be activated. However, whether such activated aerosols can become a drizzle drop, or may evaporate its liquid water content in subsaturated region, is to be understood by Lagrangian tracking of such aerosol tracers.</p>