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Since laminar separation bubbles are neutrally shaped on the suction side of full-span wings in low Reynolds number flows, a roughness element can be used to improve the performance of micro aerial vehicles. The purpose of this article was to investigate the leading-edge roughness element’s effect and its location on upstream of the laminar separation bubble from phase portrait point of view. Therefore, passive control might have an acoustic side effect, especially when the bubble might burst and increase noise. Consequently, the effect of the leading-edge roughness element features on the bubble’s behavior is considered on the acoustic pressure field and the vortices behind the NASA-LS0417 cross-section. The consequences express that the distribution of roughness in the appropriate dimensions and location could contribute to increasing the performance of the airfoil and the interaction of vortices produced by roughness elements with shear layers on the suction side has increased the sound frequency in the relevant sound pressure level (SPL). The results have demonstrated that vortex shedding frequency was increased in the presence of roughness compared to the smooth airfoil. Also, more complexity of the phase portrait circuits was found, retrieved from velocity gradient limitation. Likewise, the highest SPL is related to the state where the separation bubble phenomenon is on the surface versus placing roughness elements on the leading edge leads to a negative amount of SPL.

We use the Carreau rheological model which properly accounts for the shear-thinning behaviour between the low and high shear rate Newtonian limits to investigate the problem of a semi-infinite hydraulic fracture propagating at a constant velocity in an impermeable linearly elastic material. We show that the solution depends on four dimensionless parameters: a dimensionless toughness (function of the fracture velocity, confining stress, material and fluid parameters), a dimensionless transition shear stress (related to both fluid and material behaviour), the fluid shear-thinning index and the ratio between the high and low shear rate viscosities. We solve the complete problem numerically combining a Gauss–Chebyshev method for the discretization of the elasticity equation, the quasi-static fracture propagation condition and a finite difference scheme for the width-averaged lubrication flow. The solution exhibits a complex structure with up to four distinct asymptotic regions as one moves away from the fracture tip: a region governed by the classical linear elastic fracture mechanics behaviour near the tip, a high shear rate viscosity asymptotic and power-law asymptotic region in the intermediate field and a low shear rate viscosity asymptotic far away from the fracture tip. The occurrence and order of magnitude of the extent of these different viscous asymptotic regions are estimated analytically. Our results also quantify how shear thinning drastically reduces the size of the fluid lag compared to a Newtonian fluid. We also investigate simpler rheological models (power law, Ellis) and establish the small domain where they can properly reproduce the response obtained with the complete rheology.

A numerical simulation method is employed to investigate the effect of the steady multiple plasma body forces on the flow field of stalled NACA 0015 airfoil. The plasma body forces created by multiple Dielectric Barrier Discharge (DBD) actuators are modeled with a phenomenological plasma method coupled with 2-dimensional compressible turbulent flow equations. The body force distribution is assumed to vary linearly in the triangular region around the actuator. The equations are solved using adual-timeimplicit finite volume method on unstructured grids. In this paper, the responses of the separated flow field to the effects of single and multiple DBD actuators over the broad range of angles of attack ( 9^0-〖30〗^0) are studied. The effects of the actuators positions on the flow field are also investigated. It is shown that the DBD have a significant effect on flow separation control in low Reynolds number aerodynamics.

At present, some researchers believe that Darcy’s law may not be valid, which may be the root cause of complex porous media problems. Porous media flow model is generally regarded as pipe flow, but in theory, it is necessary to consider it as bypass flow. However, there are few studies in this area. According to the seepage characteristics, we established the model of flowing around a single sphere in porous media and derived the resistance correlation of Darcy velocity (Re < 1). However, the Darcy experimental data analysis shows that the predicted value of the flow around the sphere is far lower than the experimental value, which proves that the assumption of the model of flowing around a single sphere is unreasonable. Even for the seepage of the extremely low Reynolds number, the flow interference around the sphere should be taken into account. Therefore, when the interference coefficient is 45, our formulas are superior to the classical Kozeny-Carman equation and Ergun equation in the prediction of all Darcy’s experimental data and Charles Ritter’s experimental data. Ergun equation has the largest error.

During sedimentation in a miscible, viscous liquid, polymeric drops self-assemble into a toroidal-spiral structure, which can be solidified by photoinitiated cross-linking. With a suitable arrangement three drops can be encapsulated, possibly enabling controlled release of a therapeutic drug cocktail.

The thickness of boundary layer increases rapidly in supersonic low Reynolds number flows. As a result, there will be shock wave-boundary layer interference. The disturbances of downstream can spread upstream through in boundary layer because the flow in boundary layer is subsonic, which may decrease flow field uniformity and stability. The pressure of this type of flow is always very low. For the sake of flow continuity, the increase of flow pressure is necessary. A kind of supersonic low Reynolds number flow control technique was proposed. The high pressure supersonic flow jet-ted from plates which located in the flow channel blew away boundary layer, cut off the disturbances spread from downstream to upstream, and eliminated the shock wave-boundary layer interference. The pressure of primary flow was also increased by mixing with the high pressure jetting flow. The calculation model of this flow control technique was established. A group of standard inputs were selected to calculate the parameters of flow and the preliminary law of flow was also obtained.

Low Reynolds number flow of a viscous fluid through bundles in non-crimp stitched fabrics is considered for different arrangements of the fibres within the bundles. Using a previous derived model, two-dimensional Navier–Stokes solutions are sampled for the flow between a few fibres with a subsequent minimisation of the dissipation rate in the total system of fibres. Based on the detailed geometry of the fabrics flow induced elastic deformations of the fibre bundles are then derived and the overall permeability is computed for different pressure gradients. The permeability of random arrays of a large number of as well mono-dispersed as poly-dispersed fibres increases as the flow-induced deformation increases and despite the relative shift of the fibres is small the overall change in permeability is essential. For a system with gaps between bundles the change in permeability depends on the orientation of the flow field with respect to the geometry studied while for a regular packing the alteration in permeability is negligible.

Random packed beds have more complex interior structure than structured beds and are widely used in industry and engineering. CFD simulation was carried out to investigate and analyze the local flow and heat transfer in a 120-sphere random packed bed. 3D Navier–Stokes equation was solved with a finite volume formulation based on the Chimera meshing technique. Investigation was focused on low Reynolds number flow (Re = 4.6–56.2), which typically occurs in packed bed reactors in bio-chemical fields. Detailed temperature field information was obtained. Inhomogeneity of flow and heat transfer due to the non-uniform distribution of void fraction was discussed and analyzed.

International audience; Pressure-imposed rheometry is used to study the rheological properties of suspensions of non-colloidal spheres in yield-stress fluids. Accurate measurements for both the shear stress and the particle normal stress are obtained in the dense regime. The rheological measurements are favourably compared with a model based on scaling arguments and homogenisation methods.

A method for generating mixing in an electroosmotic flow of an electrolytic solution in a three-dimensional channel is proposed. When the width-to-height aspect ratio of the channel cross-section is large, mixing of a blob of a solute in a slip-driven three-dimensional flow in a rectangular channel can be used to model and assess the effectiveness of this method. It is demonstrated through numerical simulations that under certain operating conditions, rapid and efficient mixing can be achieved. Future investigation will include the solution of the exact equations and experimentation.

Various micropumps are devised, researched and developed for the liquid supply systems in the fields of micro machine, medical engineering and biotechnology and so on. In the present study, a new viscous micropump with a cylindrical rotor in a duct was presented. A gate is devised and installed in the wall over the cylindrical rotor to control the pump characteristics, i.e. the flow field and the pressure performance. The model experiment and the numerical simulation are performed. In the model experiment, the low Reynolds number flow is realized by using glycerin as a working fluid for a centimeter-scale (non-micrometer-scale) pump model. In the numerical simulation, the commercial software STAR-CD is used. Qualitative and quantitative agreements between experimental and numerical results are obtained with respect to the flow field and the pressure performance. The installed gata changes the flow field and accordingly the pressure performance. Therefore, it can be said that the pump characteristics can be controlled by using the gate.

Due to low Reynolds number flow, it is always challenging to generate vortex in a microfluidic structure. This paper presents an experimental study on vortex generation under a fluid flow in a resonator-shaped microfluidic chamber incorporated with PZT actuations. The vortex development is visualized by tracing fluorescent dye in the flow and characterized under various experimental conditions, including the actuation voltage, actuation frequency and flowrate. It is demonstrated that given an actuation frequency and flowrate, large scale vortices can be generated inside the microfluidic chamber and fully controlled by the actuation voltage. A frequency window is identified, in which the vortex generation is possible.

In this study, we examine a steady two-dimensional slow flow past a rigid cylinder coated with a thin layer of immiscible fluid. The Reynolds number for the external bulk flow is assumed small and flow within the film is driven by the action of the bulk fluid’s tangential viscous stress acting at the interface. Using double asymptotic expansions based on the bulk fluid’s Reynolds number and the aspect ratio of the film thickness to the cylinder’s radius, we derive the leading- and first-order equations governing the steady-state film dynamics, and obtain analytical solutions, in terms of the film thickness, for the bulk flow. We solve the governing film equations, finding that solutions feature a drained region. We briefly discuss the influence of the Capillary number and fluid viscosities, and conclude by showing how the presence of the film affects the drag on the film-coated cylinder.

Using computer simulations, we investigate the behavior of an immiscible binary AB fluid that is driven to flow through a microchannel, which is decorated with a checkerboard pattern of chemically distinct A- and B-like patches on the top and bottom walls. We isolate conditions where the coupling between the imposed flow field and thermodynamic interactions yields a rich interfacial behavior between the A and B fluids and complex velocity patterns over the checkerboard domains. In effect, the A and B fluids undergo extensive mixing in specific regions of the channel, even for low Reynolds number flow. Decreasing the height-to-width ratio of the patterned microchannel enhances the extent of mixing between these two components. On the other hand, the length of the A/B interfaces is optimized in microchannels that have a square cross section. The results provide guidelines for designing microfluidic devices that can be used to effectively intermix multicomponent fluids.

The effect of shear rate on the inception of gaseous cavitation in a low Reynolds number flow is studied. An experiment is carried out using a two-cylinder apparatus and paraffinic base oil with air dissolved in it as a test fluid. With a low shear rate, the observed inception time is considerably later than the time obtained in a calculation considering only the floating bubbles in the fluid. This fact can be explained by taking into account the slower growth speed of small nucleus attached to the wall, which results from the resistance encountered when the air passes across the interface of the bubbles. It is supposed that when the shear rate of flow becomes high, this resistance suddenly decreases. By considering the behavior of absorbed substances at the interface, the author arrives at a nondimensional parameter determining the shear rate and the size of the bubbles when the sudden change in resistance occurs.

Axial velocity measurements in the die entry region of a capillary rheometer using a laser Doppler anemometer and a data logging system are reported for a Newtonian silicone oil and a highly elastic polymer solution under low Reynolds number flow conditions (NRe

Entrance pressure drop of extruder dies increased with a decrease in the ratio of barrel-to-die diameter during the extrusion of corn. For the low Reynolds number flow studied (1.77 x 10−3≤ Re ≤ 9 x 10−5) the viscous component, obtained from Weissberg's analysis, contributed 2% to 40% of the entrance pressure drop and increased as the barrel-to-die diameter ratio decreased. The friction factor for laminar fully developed flow through the die followed the expression f = 16/Re'. Fully developed flow within the die was achieved at a die length-to-diameter ratio of 9.

A Laser Doppler Anemometer and a high speed data logging system are used to study the details of flow in the die land region. An abrupt entry cylindrical glass die, 6 mm I.D. and 210 mm long, is used. A 9.7 poise Newtonian Silicone Oil and a highly elastic polymer solution are investigated under low Reynolds number flow conditions (NRe < 1).

(1986). Low Reynolds Number Flow Heat Exchangers. Nuclear Technology: Vol. 73, No. 1, pp. 129-130.