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In the present study, compressible low-Reynolds-number flow past a stationary isolated sphere was investigated by direct numerical simulations of the Navier–Stokes equations using a body-fitted grid with high-order schemes. The Reynolds number based on free-stream quantities and the diameter of the sphere was set to be between 250 and 1000, and the free-stream Mach number was set to be between 0.3 and 2.0. As a result, it was clarified that the wake of the sphere is significantly stabilized as the Mach number increases, particularly at the Mach number greater than or equal to 0.95, but turbulent kinetic energy at the higher Mach numbers conditions is higher than that at the lower Mach numbers conditions of similar flow regimes. A rapid extension of the length of the recirculation region was observed under the transitional condition between the steady and unsteady flows. The drag coefficient increases as the Mach number increases mainly in the transonic regime and its increment is almost due to the increment in the pressure component. In addition, the increment in the drag coefficient is approximately a function of the Mach number and independent of the Reynolds number in the continuum regime. Moreover, the effect of the Mach and Reynolds numbers on the flow properties such as the drag coefficient and flow regime can approximately be characterized by the position of the separation point.

In the present study, the flow control mechanism of SD7062 airfoil by a rod illustrated using Particle image velocimetry (PIV) technique at pre-stall angles of attack at Reynolds number of Re = 30000. The rod was installed on the suction surface of the airfoil at different chordwise locations. Diameter of the rod was normalized with the chord length of the airfoil and three diameter ratios (d / c = 0.017, 0.033 and 0.044) were examined at angles of attack of α = 6°, 8° and 10°. Formation of laminar separation bubble for the baseline airfoil and the effect of rod on the laminar separation bubble were investigated in detail. It is observed that the height of boundary layer was reduced up to 22% by proper rod location and diameter ratio. Moreover, the rod suppressed the unsteady vortices over the suction surface of airfoil significantly. Therefore, the peak magnitudes of turbulent statistics were also decreased up to 30% by the rod.

In this paper we study the self-induced low-Reynolds-number flow generated by a cylinder immersed in a stratified fluid. In the low Péclet limit, where the Péclet number is the ratio of the radius of the cylinder and the Phillips length scale, the flow is captured by a set of linear equations obtained by linearising the governing equations with respect to the prescribed far field conditions. We specifically focus on the low Péclet regime and develop a Green's function approach to solve the linearised equations governing the flow over the cylinder. We cross check our analytical solution against numerical solution of the nonlinear equations to obtain the range of the Péclet numbers for which the linear solution is valid. We then take advantage of the analytical solution to find explicit far-field decay rates of the flow. Our detailed analysis points out that the streamfunction and the velocity field decays algebraically in the far field. Intriguingly, this algebraic decay of the flow is much slower when compared with the exponential decay of the flow generated by a slow moving cylinder in the homogeneous Stokes regime, in the absence of stratification. Consequently, the flow generated by a cylinder in the stratified Stokes regime will have a larger domain of influence when compared with the flow generated by a cylinder in the homogeneous Stokes regime.

This study numerically examines the influences of transverse annulation around a cone surface on the characteristics of a flow over an orthocone. This work is inspired by Spyroceras, a fossilized genus of nautiloid cephalopods from the Paleozoic era, whose method of locomotion is understudied. As a baseline case, a flow over a smooth orthoconic model with a blunt cone end is investigated numerically at Reynolds numbers from 500 to 1500. As Reynolds increases, two different shedding mechanisms—hairpin-vortex wake and spiral-vortex wake—are captured. We notice that an introduction of annulation over the cone surface changes the critical Reynolds number for the transition of the shedding mechanism. The dominant shedding frequency increases with the Reynolds number for the smooth and annulated cone flows. Moreover, the annulation reduces the dominant frequency for the same Reynolds number and increases the time-averaged drag coefficient. Modal decompositions—Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD)—are used to capture the coherent structures and their oscillating frequencies. We have captured modes corresponding to the hairpin-vortex wake and spiral-vortex wake shedding mechanisms. Comparing the leading POD modes for the smooth and the annulated cone flows, we find that the annulation can reduce the twisting effects of the coherent structures in the wake. Additionally, we find that the SPOD analysis can identify modes presenting both hairpin-vortex wake and spiral-vortex wake in one flow condition as leading modes, while the POD leading modes only reveal one shedding mechanism in each flow.

Due to many restrictions applied by the necessity of fulfilling dimensional analysis in a numerical-experimental research and also the limits in experimental facilities a Low Reynolds Number simulation seems to be widespread. In this paper, effects of the diffuser angle on the aerodynamic behavior of the Ahmed body have been investigated for low Reynolds number flows. Numerical simulations were performed by solving the Reynolds Averaged Navier-Stokes (RANS) equations combined with different turbulence models. The Finite Volume Method (FVM) is used for simulations in Fluent 6.3.26 Software. The main objectives of the study are to improve the aerodynamic design of the body, analyzing the flow field to understand the nature of these improvements and reaching a suitable and reliable experimental-numerical setup for such a flow. Finally, it was concluded that the SST k-ω turbulence model with transitional flow corrections is the best choice. From the flow simulation and obtained experimental data, it was concluded that that drag coefficient is a function of three main phenomena. Results showed that the drag coefficient has its minimum value at a specific diffuser angle (8◦) and further increases in the angle lead to higher drag coefficient. On the other hand, the lift coefficient constantly decreases by increasing the diffuser angle. In order to show the validity of the numerical results, experimental data were obtained by measuring the drag and lift coefficients of scaled standard Ahmed body and a model with the diffuser angle of 8 degrees in a wind tunnel. Results confirmed that improvement of drag and lift coefficients occurs when diffuser region is considered for the Ahmed body. In addition, the flow field around the body was studied in detail to show the effects of the diffuser geometry on the aerodynamic characteristics of the body.

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} -[30]} ) 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.

This paper discusses details of design and fabrication of a simple open-loop modular mini wind tunnel for studying fluid microstructure interaction under low Reynolds number. A simple open-loop modular mini wind tunnel was designed and built after examining various design possibilities. The mini wind tunnel consists of five basic sections, namely wide-angle diffuser, settling chamber, contraction section, test section and exit diffuser. The settling chamber consists of flow straighter made from array of circular tubes. Mini wind tunnel is powered by incorporating a fan which provides air flow with Re < 1500. Flow behavior inside the mini wind tunnel, particularly test section, was investigated by performing flow simulation using COMSOL finite element modeling. Additionally, smoke experiments confirm streamline flow inside the test section. Fluid interaction of microcantilevers was investigated using the designed mini wind tunnel. Polydimethylsiloxane microcantilever beam of different aspect ratios was fabricated and tested for airflow sensing applications. Microcantilevers are placed normal to fluid flow and steady tip deflection.

The long-time viscous flow about two identical rotating circular cylinders in a side-by-side arrangement is investigated using an adaptive numerical scheme based on the vortex method. The Stokes solution of the steady flow about the two-cylinder cluster produces a uniform stream in the far field, which is the so-called Jeffery’s paradox. The present work first addresses the validation of the vortex method for a low-Reynolds-number computation. The unsteady flow past an abruptly started purely rotating circular cylinder is therefore computed and compared with an exact solution to the Navier–Stokes equations. The steady state is then found to be obtained for $t\\gg 1$ with ${\\mathit{Re}}_{\\omega } {r}^{2} \\ll t$ , where the characteristic length and velocity are respectively normalized with the radius ${a}_{1} $ of the circular cylinder and the circumferential velocity ${\\Omega }_{1} {a}_{1} $ . Then, the influence of the Reynolds number ${\\mathit{Re}}_{\\omega } = { a}_{1}^{2} {\\Omega }_{1} / \\nu $ about the two-cylinder cluster is investigated in the range $0. 125\\leqslant {\\mathit{Re}}_{\\omega } \\leqslant 40$ . The convection influence forms a pair of circulations (called self-induced closed streamlines) ahead of the cylinders to alter the symmetry of the streamline whereas the low-Reynolds-number computation ( ${\\mathit{Re}}_{\\omega } = 0. 125$ ) reaches the steady regime in a proper inner domain. The self-induced closed streamline is formed at far field due to the boundary condition being zero at infinity. When the two-cylinder cluster is immersed in a uniform flow, which is equivalent to Jeffery’s solution, the streamline behaves like excellent Jeffery’s flow at ${\\mathit{Re}}_{\\omega } = 1. 25$ (although the drag force is almost zero). On the other hand, the influence of the gap spacing between the cylinders is also investigated and it is shown that there are two kinds of flow regimes including Jeffery’s flow. At a proper distance from the cylinders, the self-induced far-field velocity, which is almost equivalent to Jeffery’s solution, is successfully observed in a two-cylinder arrangement.

The swimming characteristics of gyrotactic microorganisms are significant to understand the ecological activities in lakes, rivers and oceans. The swimming velocity of a typical motile microorganism, Chlamydomonas reinhardtii , was measured for both still water and low-Reynolds-number flow, based on a microfluidic system. Results show that the swimming speed is subject to Gaussian distribution for the still water, and corresponding mean swimming speed is 41 μm/s. The streamwise mean swimming velocity, 35 μm/s, in the moving water is slightly less than that in the still water. It is also shown that the swimming direction in the horizontal plane is dominated by cell randomness for the still water, and 80% of the cells are aligned with the ambient flow when the flow velocity exceeds 333 μm/s. The standard deviation of swimming direction and the percent of swimming direction in the streamwise direction can reach a stable status with the increase of the flow velocity.

The three-dimensional (3-D) transition of the leading-edge vortex (LEV) and the force characteristics of the plunging airfoil are investigated in the chord-based Strouhal number $St_c$ range of 0.10 to 1.0 by means of experimental measurements, numerical simulations and linear stability analysis in order to understand the spanwise instabilities and the effects on the force. We find that the interaction pattern of the LEV, the LEV from a previous cycle (pLEV) and the trailing-edge vortex (TEV) is the primary mechanism that affects the 3-D transition and associated force characteristics. For $St_c \\leq 0.16$, the 3-D transition is dominated by the LEV–TEV interaction. For $0.16 < St_c \\leq 0.44$, the TEV lies in the middle of the LEV and the pLEV and therefore vortex interaction between them is relatively weak; as a result, the LEV remains two-dimensional up to a relatively high Reynolds number of $Re = 4000$ at $St_c = 0.32$. For $0.44 < St_{c} \\leq 0.54$, and at relatively low Reynolds numbers, the pLEV and the TEV tend to form a clockwise vortex pair, which is beneficial for the high lift and stability of the LEV. For $0.49 \\leq St_c$, the pLEV and TEV tend to form an anticlockwise vortex pair, which is detrimental to the lift and flow stability. In the last $St_c$ range, vortex interaction involving the LEV, the TEV and the pLEV results in an unstable period-doubling mode which has a wavelength of about two chord-lengths and the 3-D transition enhances the lift.