Laminar Vortex Research Articles

Computational fluid dynamics simulations of unsteady mixing in spacer-filled direct contact membrane distillation channels

Direct contact membrane distillation (DCMD) is a promising means of concentrating brines to their saturation limit. During that process, membrane spacers play a key role in temperature polarization, concentration polarization, and mineral scaling. These interactions are not well understood, because they are difficult to study experimentally and numerically, and the flow regimes are not fully charted. We consequently develop a tailored in-house CFD code that simulates unsteady two-dimensional heat and mass transport in plate-and-frame DCMD systems with cylindrical spacers. The code uses a combination of finite-volume methods in space, projection methods in time, and recent advances in immersed boundary methods for the spacer surfaces. Using the code, we explore how the transition to unsteady laminar vortex shedding affects polarization and permeate production of DCMD systems. We show that the impact of spacers can be explained by examining the various steady and unsteady vortical flow structures generated in the bulk and near the membranes. Overall, we show that though unsteady vortex structures tend to mix temperature polarization layers with the bulk, they are not similarly able to mix the concentration layers. Rather, vortical structures tend to create regions of preferential salt accumulation. In the vortex shedding regime, the net result is that spacers often increase vapor production at the expense of increasing the risk of mineral scaling.

Controllable vortex shedding from dissipative exchange flows in ferromagnetic channels

Ferromagnetic channels subject to spin injection at one extremum sustain long-range coherent textures that carry spin currents known as dissipative exchange flows (DEFs). In the weak injection regime, spin currents carried by DEFs decay algebraically and extend through the length of the channel, a regime known as spin superfluidity. Similar to fluids, these structures are prone to phase-slips that manifest as vortex-antivortex pairs. Here, we numerically study vortex shedding from DEFs excited in a magnetic nanowire with a physical obstacle. Using micromagnetic simulations, we find regimes of laminar flow and vortex shedding as a function of obstacle position tunable by the and spin injection sign and magnitude. Vortex-antivortex pairs translate forward (VF regime) or backward (VB regime) with respect to the detector's extremum, resulting in well-defined spectral features. Qualitatively similar results are obtained when temperature, anisotropy, and weak non-local dipole fields are included in the simulations. These results provide clear features associated with DEFs that may be detected experimentally in devices with nominally identical boundary conditions. Furthermore, our results suggest that obstacles can be considered as DEF control gates, opening an avenue to manipulate DEFs via physical defects.

Lagrangian interpolation algorithm for PIV data

A new Lagrangian based interpolation scheme is proposed for recovering velocity field data from within masked regions in particle image velocimetry (PIV) experiments. As a first step, the mean field within the masked region is filled through an iterative convolution operation using an optimized kernel. Next, the Lagrangian interpolation scheme estimates velocity field data within the masked region using a weighted average of past and future snapshot data along a mean field streamline from outside of the masked region. The Lagrangian interpolation scheme is compared directly with a traditional bilinear interpolation approach for two datasets: the flow development over a circular cylinder within the laminar vortex shedding regime (ReD=150) and turbulent vortex shedding regime (ReD=3000). The results show that for both Reynolds numbers investigated, the Lagrangian approach provides up to a 5× improvement in average velocity reconstruction error. Application of the proposed interpolation scheme to the estimation of body forces was assessed. For both the momentum and impulse control volume approaches to estimate instantaneous forces, the Lagrangian interpolation scheme provides up to 10× improvement in the estimation of volume integral contributions to instantaneous drag and lift.

Some observations concerning "laminarization" in heated vertical tubes

For low "turbulent" Reynolds numbers in strongly-heated vertical gas flow, fundamental results of existing direct numerical simulation (DNS) databases have been examined to deduce differences between cases which "revert" to turbulent and those that yield integral parameters which correspond to laminar flow (henceforth laminarizing or laminarized). Objectives are (1) to examine the streamwise evolution of near-wall flow structures and (2) to determine which, if any, proposed laminarization parameters could be used to discriminate between turbulent and laminarizing flows resulting. Views of streamlines in r-θ cross sections showed evidence of a ring of irregular-shaped streamwise vortices near the wall at all locations. The transverse advection of these vortices is hypothesized to provide a path of least resistance to the transport of streamwise momentum in the wall-normal direction. It is demonstrated that a steady streamwise laminar vortex can provide apparent turbulent momentum transport (as a consequence of the Reynolds decomposition) such that a reasonable mean velocity profile is predicted near the wall. For the present cases, the values of the non-dimensional radius (yc,w+ = Reτ,w) and of the modified wall Reynolds number did discriminate whether turbulent or laminarizing flow resulted.

On the spectrographic representation of cardiovascular flow instabilities

In the past decade, high-fidelity computational fluid dynamics (CFD) has uncovered the presence of high-frequency flow instabilities (on the order of 100 s of Hz) in a variety of cardiovascular applications. These fluctuations are typically reported as pulsatile velocity–time traces or fast-Fourier-transformed power-frequency spectra, often from a single point or at most a handful of points. Originally inspired by its use in spectral Doppler ultrasound, here we demonstrate the utility of the simplest form of time–frequency representation – the spectrogram – as a more comprehensive yet still-intuitive means of visualizing the potential harmonic complexity of pulsatile cardiovascular flows. After reviewing the basic theory behind spectrograms, notably the short-time Fourier transform (STFT), we discuss the choice of input parameters that inform the appearance and trade-offs of spectrograms. We show that spectrograms using STFT were able to highlight spectral features and were representative of those obtained from more complex methods such as the Continuous Wavelet transforms (CWT). While visualization properties (colourmap, filtering, smoothing/interpolation) are shown to affect the conspicuity of spectral features, the window properties (function, size, overlap) are shown to have the greatest impact on the resulting spectrogram appearance. Using a set of cerebral aneurysm CFD cases, we show that spectrograms can readily reveal the case-specific nature of the time-varying flow instabilities, whether broadband, suggesting intermittent turbulent-like flow, or narrowband, suggesting laminar vortex shedding, or some combination thereof.

Laminar Vortex Shedding in the Wake of a Cube

Abstract Flow around a cube is numerically studied in the laminar vortex shedding regime. The objective is to examine the three-dimensional vortex shedding mechanism and understand the temporal behavior of the wake. Vortices were identified using λ2 criterion for Re = 250–770. The wake of the cube sheds paired hairpin vortices, which moves in the streamwise direction and attains a constant shape with time. The analysis of separation distance and angular orientation of hairpin vortices for flow around a cube are presented here for the first time in the literature. The separation (d) between the paired hairpin vortices scales as t−1/2. The orientation of hairpin vortices changes with time and attains a near-normal orientation with respect to the axial direction. At Re ≥ 339, the hairpin twists with respect to axial direction losing the axisymmetry in one plane noted for 276 ≤ Re ≤ 300. The hairpin vortices disintegrate into smaller vortices at higher Re = 570 and 770. A quasi-periodic nature of the flow has been revealed by the phase plots. The drag and side forces generated due to the flow are studied with pressure force mostly contributing to the drag. One of the side force coefficients dominates owing to the asymmetry of the wake in one plane and symmetry in the other orthogonal streamwise plane. These results clearly bring out the asymmetric nature of flow in the shedding regime.

Effect of mixed convection on laminar vortex breakdown in a cylindrical enclosure with a rotating bottom plate

Vortex breakdown plays a central role in the performance of countless rotating machinery applications, many of which contain thermal gradients either inadvertently or by design. The effect of thermal gradients on vortex breakdown and further flow development in a cylindrical domain with a rotating bottom plate is examined using the Generalized Integral Transformation Technique (GITT) with a streamfunction-only formulation. A thermal gradient is imposed in the axial direction, such that the buoyancy forces oppose the base flow driven by the rotation of the lower plate, i.e. the temperature difference acts to stabilize the flow. The hybrid numerical-analytical approach is shown to accurately capture vortex breakdown phenomena for a variety of conditions involving single, double and triple recirculation bubbles. The buoyancy forces – expressed in terms of the Richardson number (Ri) – act to suppress vortex breakdown in all cases examined and led to a series of flow transitions with increasing Ri, characterized by the appearance of a stratified structure with multiple fluid layers. These flow transitions have a significant impact on the overall performance of the system. The torque coefficient decreases with Ri, compared to the base (isothermal) case following an empirical power law relationship, which is independent of Reynolds number, aspect ratio or number of fluid layers present. Flow stratification suppresses the transport of angular momentum; azimuthal velocity is shown to decline exponentially in the regions where layering occurs, accompanied by a sharp reduction in the Nusselt number, as fluid layers act to insulate the upper plate.

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

Effects of Streamlining a Bluff Body in the Laminar Vortex Shedding Regime

Abstract Two-dimensional flow over bluff bodies is studied in the unsteady laminar flow regime using numerical simulations. In previous investigations, lift and drag forces have been studied over different cross-sectional shapes like circles, squares, and ellipses. We aim to extend the previous research by studying the variation of hydrodynamic forces as the shape of the body changes from a circular cylinder to a more streamlined or a bluffer body. The different body shapes are created by modifying the downstream circular arc of a circular cylinder into an ellipse, hence elongating or compressing the rear part of the body. The precise geometry of the body is quantified by defining a shape factor. Two distinct ranges of shape factors with fundamentally different behavior of lift and drag are identified. The geometry constituting the limit is where the rear part ellipse has a semi-minor axis of half the radius of the original circle, independent of the Reynolds number. On the other hand, the vortex shedding frequency decreases linearly over the whole range of shape factors. Furthermore, the variation of the forces and frequency with Reynolds number, and how the relations vary with the shape factor are reported.

Sorting of Particles Using Inertial Focusing and Laminar Vortex Technology: A Review.

The capability of isolating and sorting specific types of cells is crucial in life science, particularly for the early diagnosis of lethal diseases and monitoring of medical treatments. Among all the micro-fluidics techniques for cell sorting, inertial focusing combined with the laminar vortex technology is a powerful method to isolate cells from flowing samples in an efficient manner. This label-free method does not require any external force to be applied, and allows high throughput and continuous sample separation, thus offering a high filtration efficiency over a wide range of particle sizes. Although rather recent, this technology and its applications are rapidly growing, thanks to the development of new chip designs, the employment of new materials and microfabrication technologies. In this review, a comprehensive overview is provided on the most relevant works which employ inertial focusing and laminar vortex technology to sort particles. After briefly summarizing the other cells sorting techniques, highlighting their limitations, the physical mechanisms involved in particle trapping and sorting are described. Then, the materials and microfabrication methods used to implement this technology on miniaturized devices are illustrated. The most relevant evolution steps in the chips design are discussed, and their performances critically analyzed to suggest future developments of this technology.

Effects of polymer/surfactant additives on forced convective heat transfer in vortex shedding flow past a circular cylinder

The unsteady incompressible flow of dilute polymer/surfactant solutions past an unconfined heated circular cylinder is studied to reveal the effects of these additives on convective heat transfer properties in the laminar and transitional vortex shedding regime. Both inelastic and elastic non-Newtonian features are considered and compared using two-dimensional numerical simulations. Inelastic power-law and purely viscoelastic Oldroyd-B as well as Giesekus and FENE-P constitutive models, exhibiting both shear thinning and elastic effects are adopted in the formulation. A finite element based software is used in simulations for the Reynolds number range 80≤Re≤300, power-law index range 0.6≤n≤1 and Weissenberg number Wi=1 for weakly elastic solutions. Heat transfer enhancement is observed under inelastic shear thinning effects through an increase of the local and average Nusselt numbers, however purely viscoelastic effects lead to a decrease in the rate of convective heat transfer for the studied range of parameters. The increase in the Giesekus model mobility factor which is associated with anisotropic Brownian motion effects on polymer/surfactant molecules increases the heat transfer rate, and shifts the location of the maximum local Nusselt number to the front of the cylinder. However, increasing the FENE-P model finite extensibility parameter related to the maximum polymer chain length, decreases the local Nusselt number while the maximum heat transfer rate location moves towards the rear region on the cylinder surface. Temperature gradients decrease in the wake region under elasticity whereas shear thinning leads to an increase in the gradients and to the formation of clustered isotherms.

Numerical investigation on flow control effects of dynamic hump for turbine cascade at different Reynolds number and hump oscillating frequency

From takeoff to cruise, the operating Reynolds number (Re) of the low-pressure turbine (LPT) in an aero-engine decreases significantly, making the boundary layer near the aft portion of the blade surface susceptible to separation. This paper discusses the feasibility of using local dynamic surfaces as an active flow control method to suppress the laminar flow separation on the suction surface in PAKB LPT cascade at low Re. A local dynamic surface, shaped as a half-sinusoidal hump with an oscillation amplitude of 1 mm, was positioned just upstream of the peak velocity point. Unsteady Reynolds-Averaged Navier–Stokes (RANS) simulations were performed to investigate the aerodynamic performance of the cascades with and without dynamic hump. At first, the effect of Re was studied with the hump oscillating frequency at 200 Hz. Compared with uncontrolled cascade at various Re from 25,000 to 150,000, the profile losses of the cascade controlled by dynamic hump reduced significantly at low Re condition where the separation bubble was very large, and increased slightly at high Re condition where the separation bubble was small. Then the effect of the hump oscillation frequency was investigated under the Re of 25,000. It was found that the controlled cascade loss reached a minimum in a certain hump oscillating frequency range. At low Re of 25,000, the loss of controlled cascade was mainly composed of low-loss continuous laminar vortices that were attached to the suction surface. Continuous vortices with a certain spacing confined the low-energy fluid near the suction surface by energy exchange between the free flow and near-wall flow, which suppressed the large-scale flow separation.

Simulation of Transonic Buffet Using a Time-Spectral Method

This paper extends the use of frequency-domain flow solvers to transonic buffet in turbulent flow regime. To do so, a time-spectral method with frequency adaptation is developed for the simulation of periodic flows of self-induced frequency. The time-spectral method is implemented into a cell-centered finite volume unsteady Reynolds-averaged Navier–Stokes (URANS) flow solver. This method uses the Fourier series to transform the URANS equations into a steady problem in the frequency domain, for a given frequency. This steady problem is recast in the time domain to allow the use of classic computational fluid dynamics techniques. Because transonic buffet is a self-induced phenomenon of unknown frequency before the computation, two algorithms are implemented in the time-spectral solver to determine its frequency. The first one relies on a gradient-based algorithm to minimize the residual of the system of equations. The second uses the phase shift per iteration of the frequency-domain solution. The accuracy of the time-spectral scheme for turbulent flows is first verified for the simulation of a pitching airfoil with a forced frequency. Then, the efficiency of the frequency-adaptation algorithms is assessed for the case of the laminar vortex shedding of a cylinder. Finally, the proposed method is applied to the solution of the transonic buffet over the OAT15A airfoil, and the results are compared to numerical and experimental results from the literature. It can be concluded that an accurate solution of the transonic buffet can be obtained with as few as five modes.

Numerical and Experimental Investigations of a Micromixer with Chicane Mixing Geometry

A micromixer is a new type of chemical engineering equipment used to intensify the mixing process. This article provides details on flow regimes in microchannels with a complex geometry, such as with chicane mixing geometry. Experiments involving water, ink, and a micro digital camera have determined both the micromixer’s initial mixing zone, and also the streamlines. Computational fluid dynamics (CFD) modelling helped identify the mechanism of stimulating effect; swirling and recirculation were identified as two special cases of the convective mixing process. To characterize the degree of mixing, a function of volume flow rate was proposed. A much higher degree of mixing in vortex flow compared to stratified flow was observed. The relationship between laminar flow and vortices shows a square-law dependence of pressure drop against the volume flow rate. The mixing cost and the mixing energy cost at Reynolds number of 50 are higher for the chicane micromixer than for micromixers without chicanes geometry.

Dynamic Stall Flow Structure and Forces on Symmetrical Airfoils at High Angles of Attack and Rotation Rates

This article describes a direct comparison between two symmetrical airfoils undergoing dynamic stall at high, unsteady reduced frequencies under otherwise identical conditions. Particle image velocimetry (PIV) was performed to distinguish the differences in flow structure between a NACA 0021 and a NACA 0012 airfoil undergoing dynamic stall. In addition, surface pressure measurements were performed to evaluate aerodynamic load and investigate the effect of laminar separation bubbles and vortex structures on the pressure fields surrounding the airfoils. Airfoil geometry is shown to have a significant effect on flow structure development and boundary layer separation, with separation occurring earlier for thinner airfoil sections undergoing constant pitch-rate motion. Inertial forces were identified to have a considerable impact on the overall force generation with increasing rotation rate. Force oscillation was observed to correlate with multiple vortex structures shedding at the trailing-edge during high rotation rates. The presence of laminar separation bubbles on the upper and lower surfaces was shown to dramatically influence the steady-state lift of both airfoils. Poststall characteristics are shown to be independent of airfoil geometry such that periodic vortex shedding was observed for all cases. However, the onset of deep stall is delayed with increased nondimensional pitch rate due to the delay in initial dynamic-stall vortex.

Axial mixing and vortex stability to in situ radial injection in Taylor–Couette laminar and turbulent flows

Taylor–Couette flows have been widely studied in part due to the enhanced mixing performance from the variety of hydrodynamic flow states accessible. These process improvements have been demonstrated despite the traditionally limited injection mechanisms from the complexity of the Taylor–Couette geometry. In this study, using a newly designed, modified Taylor–Couette cell, axial mass transport behaviour is experimentally determined over two orders of magnitude of Reynolds number. Four different flow states, including laminar and turbulent Taylor vortex flows and laminar and turbulent wavy vortex flows, were studied. Using flow visualization techniques, the measured dispersion coefficient was found to increase with increasing $Re$, and a single, unified regression is found for all vortices studied. In addition to mass transport, the vortex structures’ stability to radial injection is also quantified. A dimensionless stability criterion, the ratio of injection to diffusion time scales, was found to capture the conditions under which vortex structures are stable to injection. Using the stability criterion, global and transitional stability regions are identified as a function of Reynolds number, $Re$.

Matrix‐free modified extended BDF applied to the discontinuous Galerkin solution of unsteady compressible viscous flows

SummaryIn this work, a time‐accurate integration of the discontinuous Galerkin space‐discretized Navier‐Stokes equations is performed exploiting the matrix‐free (MF) approach to speed up the solution process of the modified extended backward differentiation formulae (MEBDF) schemes. MEBDF are high‐order accurate implicit multistep schemes composed by three nonlinear stages. The proposed algorithm consists in solving the resulting nonlinear system of each stage with a preconditioned MF Newton/Krylov method using a frozen preconditioner strategy to improve its efficiency. Numerical results for compressible inviscid and viscous test cases, both with a known analytical solution, aim at assessing the performance of the proposed MF‐MEBDF algorithm, by comparing it with the one obtained by using its matrix‐explicit counterpart or the explicit strong stability preserving Runge‐Kutta scheme. In particular, the influence of some relevant physical (low‐speed flows) and discretization (aspect ratio, polynomial degree) aspects on the performance of the different time integration schemes are investigated, highlighting the pros and cons of the proposed algorithm and its effectiveness in solving nonstiff and stiff systems. Furthermore, the scalability in parallel computations of the proposed algorithm is investigated and, finally, its potential for efficient long‐time simulations is demonstrated computing a laminar vortex shedding behind a circular cylinder at different Reynolds numbers and by comparing its effectiveness with that of the strong stability preserving Runge‐Kutta scheme.

Electrokinetic manipulation of the von Kármán vortex street in the wake of a confined cylinder. I. DC electric field

The present study is concerned with the numerical simulation of the pressure-driven flow around a confined cylinder subjected to a DC electric field. The flow situation differs from the conventional von Kármán vortex street flow in terms of confinement. Additionally, the DC electric field induces a very small but finite electrokinetic velocity at the cylinder surface in contrast to the no-slip velocity in the conventional case. Various numerical simulations are performed in the Laminar Vortex Shedding Regime to investigate the influence of the confinement and the direction and strength of the electric field. For flows without electrokinetic manipulation, the blockage ratio shifts the critical Reynolds number to higher values. Likewise, the dimensionless shedding frequency (Strouhal number) at a given Reynolds number increases with increasing blockage ratio. For flows with electrokinetic manipulation, the time that is required to obtain a steady Laminar Vortex Shedding Regime is reduced compared to the corresponding pure pressure-driven flow. Steady electrokinetic manipulation does not influence the dimensionless shedding frequency. The reduction of the transient is related to differences in the flow topologies around the cylinder. The electrokinetic velocity breaks the axial flow symmetry of the Laminar Steady Regime and therefore accelerates the onset of the Laminar Vortex Shedding Regime.

Intensification of Heat Exchange in Laminar Vortex Air Flow in a Narrow Channel with a Row of Inclined Oval Trenches

With the use of multiblock computational technologies, the laminar flow and heat exchange in the modules arranged in a row at regular intervals in a plane-parallel channel, at the centers of which oval trenches oriented at an angle of 45o to the flow are positioned, were calculated at Re = 103. On the basis of the calculation data obtained, it was established that an increase in the maximum velocity of the flow in such a channel by one and a half times leads to the intensification of the heat exchange in it and an increase in the heat transfer from the surface region on its wall with an inclined trench by 80% with an increase in the hydraulic losses by 25%, as compared to those of an analogous smooth channel.

Vortex interaction with a rough wall formed by a hexagonal lattice of posts

An experimental study is reported which investigates the head-on collision of a laminar vortex ring of diameter D (ReΓ= 3000) on a fakir-like surface composed of slender circular posts protruding out of a planar layer. Lattices of the posts in hexagonal and random distribution (average porosity of ϵ = 0.94 in the layer) are compared to each other with respect to the plain wall. Prior to impact, the vortex ring develops the early state of natural azimuthal instabilities of different mode numbers N = 5-7 competing with each other. While impacting with the wall, the hexagonal lattice causes the rapid growth of secondary vortex structures in a regular mode number N = 6 arrangement at the outer edge of the primary ring in the form of six lobes which are aligned with the orientations of preferential “pathways” in the lattice. At the outer tip of the lobes, radial wall-jets are generated. Rotating the layer with the hexagonal lattice results in the same rotation of the secondary flow pattern with the jets’ orientation lock-in with the orientation of the lattice. The layer with random distribution of the posts at the same number density is not able to repeat this observation and no regular secondary flow pattern is seen. The results show that a tailored arrangement of such posts can be used for near-wall flow control such as in impacting jet flows or in boundary layer flows when instability modes in the flow lock-in with the wall pattern consisting of preferred pathways in the posts’ layer.