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.

Plunging jets from orifices of different geometry

Plunging jets are used in many industrial and civil applications, as for example in sewage and water treatment plants, in order to enhance aeration and mass transfer of volatile gases. They are also observed in natural processes as rivers self-purification, waterfalls and weirs. Many investigations dealt with the plunging jets in different configurations, but the dependence on Reynolds number and jet geometry were still not sufficiently addressed. For example, Mishra et al. (2020) studied an oblique submerged water impinging jet at different nozzle-to-plate distances and impingement angles, but only at a rather small Reynolds numbers (2600). On the other hand, different jet geometries have been extensively considered, but not for the plunging jet configuration (Mi, 2000; Hashiehbaf &Romano, 2013). In this work, plunging water jets issuing in air from orifices of different shape are considered. The aim of the work is to detail and compare jet behaviors in terms of velocity fields generated after impacting the air-water interface, as a function of Reynolds number and orifice geometry. However, air bubbles entrainment is mainly avoided in order to study the jet characteristics in a simpler case and use it as a reference starting point for future works.

Аerodynamic drag at small Reynolds numbers and the method of calculation of the air velocity in one- and many-row finned beams with an exhaust shaft

The periodic switching of fans at certain ambient air temperatures and a constant power is a promising method to enhance the energy operating efficiency of air-cooled heat exchangers. Equipping these heat exchangers with devices increasing the propulsion (for example, an exhaust shaft) facilitates the intensification of heat transfer due to strengthening the free movement of air by lifting forces. Meanwhile, the heat exchanger is used at the mixed convection regime. To make the thermal design of air-cooled heat exchangers with an exhaust shaft, we must have data on the aerodynamic drag of tube beams at small Reynolds numbers (Re < 1000) that permit to calculate the air flow velocity. However, at present, studies on the aerodynamic drag at mixed convection are virtually missing. Moreover, it is necessary to take into account the influence of external air flows on the gravitational pull created by the shaft since air-cooled heat exchangers are designed for outdoor use. Using the results of the experimental investigation, we obtained information about the mass-exchange processes in the finned beam and the exhaust shaft, developed a method for calculating the air velocity in one- and many-row finned beams with the exhaust shaft and determined their aerodynamic drag at small Reynolds numbers. We also established the influence of external air flows on the gravitational pull created by the shaft.

MODELING HEAT EXCHANGE DEPENDING ON THE PRANDTL NUMBER FOR VARIOUS GEOMETRIC AND REGIME PARAMETERS

Objectives. The aim is to study the dependency of the distribution of integral heat transfer during turbulent convective heat transfer in a pipe with a sequence of periodic protrusions of semicircular geometry on the Prandtl number using the calculation method based on a numerical solution of the system of Reynolds equations closed using the Menter’s shear stress transport model and the energy equation on different-sized intersecting structured grids.Method. A calculation was carried out on the basis of a theoretical method based on the solution of the Reynolds equations by factored finite-volume method closed with the help of the Menter shear stress transport model, as well as the energy equation on different-scaled intersecting structured grids (fast composite mesh method (FCOM)).Results. The calculations performed in the work showed that with an increase in the Prandtl number at small Reynolds numbers, there is an initial noticeable increase in the relative heat transfer. With additional increase in the Prandtl number, the relative heat transfer changes less: for small steps, it increases; for median steps it is almost stabilised, while for large steps it declines insignificantly. At large Reynolds numbers, the relative heat transfer decreases with an increase in the Prandtl number followed by its further stabilisation.Conclusion. The study analyses the calculated dependencies of the relative heat transfer on the Pr Prandtl number for various values of the relative h/D height of the turbulator, the relative t/D pitch between the turbulators and for various values of the Re Reynolds number. Qualitative and quantitative changes in calculated parameters are described all other things being equal. The analytical substantiation of the obtained calculation laws is that the height of the turbuliser is less for small Reynolds numbers, while for large Reynolds numbers, it is less than the height of the wall layer. Consequently, only the core of the flow is turbulised, which results in an increase in hydroresistance and a decrease in heat transfer. In the work on the basis of limited calculation material, a tangible decrease in the level of heat transfer intensification for small Prandtl numbers is theoretically confirmed. The obtained results of intensified heat transfer in the region of low Prandtl numbers substantiate the promising development of research in this direction. The theoretical data obtained in the work have determined the laws of relative heat transfer across a wide range of Prandtl numbers, including in those areas where experimental material does not currently exist.

Numerical Analysis of Protrusion Effect over an Airfoil at Reynolds Number -105

Need of micro aerial vehicles and Unmanned Air Vehicles is increasing due to military, defense and civilian requirements. These vehicles fly at very small Reynolds numbers and have to move in confined spaces with a bare minimum speed, to achieve high lift coefficient is the main concern. The main focus of this research paper is to carry out the computational analysis and study the unsteady flow over NACA0012 airfoil with right angle triangular protrusion at the Reynolds number 105 . The location of the protrusion is 0.05c, with three different heights of protrusion 0.005c, 0.01c, and 0.02c, normal to the surface of the airfoil. Geometric modeling and grid generation are created using the ICEM CFD software and numerical analysis carried out using commercial CFD Software at various angle of attacks ranging from 00 to 160 with 2 0 intervals. Numerical validation has done and compared. The results obtained from the research work is recommend that for smaller protrusions the lift and drag coefficients are unaffected in the low angle of attacks while the lift characteristic is significantly improved at the higher angle of attacks.

Experiments on flows in channels with spatially distributed heating

Flows in channels exposed to spatially distributed heating were investigated. Such flows are of interest as theoretical analyses suggest that heating leads to the reduction of pressure losses. A special apparatus providing the means for the creation of well-controlled spatially periodic heating with the desired intensity as well as precise control of the flow rate in flows with small Reynolds numbers was constructed. The apparatus works with air and provides optical access to the flow interior. The relevant theory has been generalized to handle the temperature fields measured in the experiments. The experiments were carried out for Reynolds numbers $Re<20$ and at a single Rayleigh number based on the peak-to-peak temperature difference and channel half-height of $Ra_{p}=3500$. Flow visualization and particle image velocimetry measurements demonstrated the formation of two-dimensional steady rolls whose size was dictated by $Re$, with the largest rolls observed for the smallest $Re$ and the roll size being gradually reduced as $Re$ increased until their complete elimination at the largest $Re$ used in the experiment. An excellent agreement between the theoretically and experimentally determined complex flow fields was found. Wall shear stresses extracted from the velocity measurements agree with their theoretical counterparts within the expected accuracy. The agreement between the experimental and theoretical velocity fields and their unique relation with the corresponding pressure fields indirectly verify the heating-induced pressure-gradient-reducing effect.

Dynamics of sedimentation of particle in a viscous fluid in the presence of two flat walls

The model problem of sedimentation of a solid spherical particle in a viscous fluid bordering two solid planar surfaces is considered. To find the solution of the hydrodynamic equations in the approximation of small Reynolds numbers with boundary conditions on a particle and on two planes, a procedure developed for numerical simulation of the dynamics of a large number of particles in a viscous fluid with one plane wall is used. The procedure involves usage of fictive particles located symmetrically to real ones with respect to the plane. To solve the problem of the real particle’s sedimentation in the presence of two planes, a system of fictive particles is introduced. An approximate solution was found using four fictive particles. Basing on this solution, numerical results are obtained on dynamics of particle deposition for the cases of planes oriented parallel and perpendicular to each other. In particular, the values of linear and angular velocities of a particle are found, depending on the distance to each plane and on the direction of gravity. In the limiting case, when one of the planes is infinitely far from the particle, we obtain known results on the dynamics of particle sedimentation along and perpendicular to one plane.

Dynamics of sedimentation of particle in a viscous fluid in the presence of two flat walls

The model problem of sedimentation of a solid spherical particle in a viscous fluid bordering two solid planar surfaces is considered. To find the solution of the hydrodynamic equations in the approximation of small Reynolds numbers with boundary conditions on a particle and on two planes, a procedure developed for numerical simulation of the dynamics of a large number of particles in a viscous fluid with one plane wall is used. The procedure involves usage of fictive particles located symmetrically to real ones with respect to the plane. To solve the problem of the real particle’s sedimentation in the presence of two planes, a system of fictive particles is introduced. An approximate solution was found using four fictive particles. Basing on this solution, numerical results are obtained on dynamics of particle deposition for the cases of planes oriented parallel and perpendicular to each other. In particular, the values of linear and angular velocities of a particle are found, depending on the distance to each plane and on the direction of gravity. In the limiting case, when one of the planes is infinitely far from the particle, we obtain known results on the dynamics of particle sedimentation along and perpendicular to one plane.