1,721,010 research outputs found
Wake transition and aerodynamics of a dragonfly-inspired airfoil
Get PDFWe investigate the dynamics and the stability of the incompressible flow past a corrugated dragonfly-inspired airfoil in the two-dimensional (2-D) α-Re parameter space, where α is the angle of attack and Re is the Reynolds number. The angle of attack is varied in the range of -5°≤ α≤ 10°, and Re (based on the free stream velocity and the airfoil chord) is increased up to Re =6000. The study relies on linear stability analyses and three-dimensional (3-D) nonlinear direct numerical simulations. For all α, the primary instability consists of a Hopf bifurcation towards a periodic regime. The linear stability analysis reveals that two distinct modes drive the flow bifurcation for positive and negative α, being characterised by a different frequency and a distinct triggering mechanism. The critical Re decreases as |α| increases, and scales as a power law for large positive/negative α. At intermediate Re, different limit cycles arise depending on α, each one characterised by a distinctive vortex interaction, leading thus to secondary instabilities of different nature. For intermediate positive/negative α, vortices are shed from both the top/bottom leading- and trailing-edge shear layers, and the two phenomena are frequency locked. By means of Floquet stability analysis, we show that the secondary instability consists of a 2-D subharmonic bifurcation for large negative α, of a 2-D Neimark-Sacker bifurcation for small negative α, of a 3-D pitchfork bifurcation for small positive α and of a 3-D subharmonic bifurcation for large positive α. The aerodynamic performance of the dragonfly-inspired airfoil is discussed in relation to the different flow regimes emerging in the α-Re space of parameters
Linear global and asymptotic stability analysis of the flow past rectangular cylinders moving along a wall
Get PDFThe primary instability of the steady two-dimensional flow past rectangular cylinders moving parallel to a solid wall is studied, as a function of the cylinder length-to-thickness aspect ratio AR = L/D and the dimensionless distance from the wall g = G/D. For all A, two kinds of primary instability are found: a Hopf bifurcation leading to an unsteady two-dimensional flow for g >= 0.5, and a regular bifurcation leading to a steady three-dimensional flow for g < 0.5. The critical Reynolds number Re-c,Re- (2-D) of the Hopf bifurcation (Re = U infinity D/nu, where U-infinity is the free stream velocity, D the cylinder thickness and. the kinematic viscosity) changes with the gap height and the aspect ratio. For AR <= 1, Re-c, (2-D) increases monotonically when the gap height is reduced. For AR > 1, Re-c, (2-D) decreases when the gap is reduced until g approximate to 1.5, and then it increases. The critical Reynolds number Re-c, (3-D) of the three-dimensional regular bifurcation decreases monotonically for all AR, when the gap height is reduced below g < 0.5. For small gaps, g < 0.5, the hyperbolic/elliptic/centrifugal character of the regular instability is investigated by means of a short-wavelength approximation considering pressureless inviscid modes. For elongated cylinders, AR > 3, the closed streamline related to the maximum growth rate is located within the top recirculating region of the wake, and includes the flow region with maximum structural sensitivity; the asymptotic analysis is in very good agreement with the global stability analysis, assessing the inviscid character of the instability. For cylinders with AR <= 3, however, the local analysis fails to predict the three-dimensional regular bifurcation
The Turbulent Flow over the BARC Rectangular Cylinder: A DNS Study
Get PDFA direct numerical simulation (DNS) of the incompressible flow around a rectangular cylinder with chord-to-thickness ratio 5:1 (also known as the BARC benchmark) is presented. The work replicates the first DNS of this kind recently presented by Cimarelli et al. (J Wind Eng Ind Aerodyn 174:39-495, 2018), and intends to contribute to a solid numerical benchmark, albeit at a relatively low value of the Reynolds number. The study differentiates from previous work by using an in-house finite-differences solver instead of the finite-volumes toolbox OpenFOAM, and by employing finer spatial discretization and longer temporal average. The main features of the flow are described, and quantitative differences with the existing results are highlighted. The complete set of terms appearing in the budget equation for the components of the Reynolds stress tensor is provided for the first time. The different regions of the flow where production, redistribution and dissipation of each component take place are identified, and the anisotropic and inhomogeneous nature of the flow is discussed. Such information is valuable for the verification and fine-tuning of turbulence models in this complex separating and reattaching flow
Stability and dynamics of the laminar flow past rectangular prisms
Get PDFThe laminar flow past rectangular prisms is studied in the space of length-to-height ratio , width-to-height ratio and Reynolds number; and are the streamwise and cross-flow dimensions of the prisms. The primary bifurcation is investigated with linear stability analysis. For large, an oscillating mode breaks the top/bottom planar symmetry. For smaller, the flow becomes unstable to stationary perturbations and the wake experiences a static deflection, vertical for intermediate and horizontal for small. Weakly nonlinear analysis and nonlinear direct numerical simulations are used for and larger. For and 2.25, the flow recovers the top/bottom planar symmetry but loses the left/right one, via supercritical and subcritical pitchfork bifurcations, respectively. For even larger, the flow becomes unsteady and oscillates around either the deflected (small) or the non-deflected (intermediate) wake. For intermediate and, a fully symmetric periodic regime is detected, with hairpin vortices shed from the top and bottom leading-edge (LE) shear layers; its triggering mechanism is discussed. At large and for all, the flow approaches a chaotic state characterised by the superposition of different modes: shedding of hairpin vortices from the LE shear layers, and wake oscillations in the horizontal and vertical directions. In some portions of the parameter space the different modes synchronise, giving rise to periodic regimes also at relatively large
Finite-size inertial spherical particles in turbulence
Get PDFWe investigate by direct numerical simulations the fluid-solid interaction of non-dilute suspensions of spherical particles moving in triperiodic turbulence, at the relatively large Reynolds number of. The solid-to-fluid density ratio is varied between and, the particle diameter is in the range (is the Kolmogorov scale) and the volume fraction of the suspension is. Turbulence is sustained using the Arnold-Beltrami-Childress cellular-flow forcing. The influence of the solid phase on the largest and energetic scales of the flow changes with the size and density of the particles. Light and large particles modulate all scales in an isotropic way, while heavier and smaller particles modulate the largest scales of the flow towards an anisotropic state. Smaller scales are isotropic and homogeneous for all cases. The mechanism driving the energy transfer across scales changes with the size and the density of the particles. For large and light particles the energy transfer is only marginally influenced by the fluid-solid interaction. For small and heavy particles, instead, the classical energy cascade is subdominant at all scales, and the energy transfer is essentially driven by the fluid-solid coupling. The influence of the solid phase on the flow intermittency is also discussed. Besides, the collective motion of the particles and their preferential location in relation to properties of the carrier flow are analysed. The solid phase exhibits moderate clustering; for large particles the level of clustering decreases with their density, while for small particles it is maximum for intermediate values
Anisotropic Mean Flow Enhancement and Anomalous Transport of Finite-Size Spherical Particles in Turbulent Flows
Get PDFWe investigate the influence of dispersed solid spherical particles on the largest scales of the turbulent Arnold-Beltrami-Childress (ABC) flow. The ABC flow is an ideal instance of a complex flow: it does not have solid boundaries, but possesses an inhomogeneous and three-dimensional mean shear. By tuning the parameters of the suspension, we show that particles modulate the largest scales of the flow toward an anisotropic, quasi-two-dimensional and more energetic state. In this regime, particles move along quasistraight trajectories and exhibit anomalous transport
Dynamics and applications of finite-size fibre-like objects in turbulent flows
Get PDFThis review delves into the dynamics of fibre-laden turbulent flows, a field that has garnered substantial attention due to its relevance in both natural and engineering contexts. The focus here is mainly on finite-size fibres, those exceeding the Kolmogorov scale, diverging from the commonly studied smaller ones. The study synthesises current understanding of the behaviour and organisation of both rigid and flexible finite-size fibres within turbulent flows, underscoring the added complexity these anisotropic particles introduce compared to their spherical counterparts. The influence of the length, the curvature and the inertia on the dynamics of rigid and flexible fibres is addressed. Fibre-based novel experimental methods, such as Fibre Tracking Velocimetry, are highlighted. Ultimately, this paper seeks to provide a clearer picture of the intricate dynamics at play in fibre-laden turbulent flows and their practical implications in various fields
Kolmogorov-size particles in homogeneous and isotropic turbulence
Get PDFWe investigate the fluid-solid interaction of suspensions of Kolmogorov-size spherical particles moving in homogeneous isotropic turbulence at a microscale Reynolds number of Reλ≈140. Two volume fractions are considered, 10-5 and 10-3, and the solid-to-fluid density ratio is set to 5 and 100.We present a comparison between interface-resolved (PRDNS) and one-way-coupled point-particle (PP-DNS) direct numerical simulations. We find that the modulated energy spectrum shows the classical -5/3 Kolmogorov scaling in the inertial range of scales and a -4 scaling at smaller scales, with the latter resulting from a balance between the energy injected by the particles and the viscous dissipation, in an otherwise smooth flow. An analysis of the small-scale flow topology shows that the particles mainly favour events with axial strain and vortex compression. The dynamics of the particles and their collective motion studied for PR-DNS are used to assess the validity of the PP-DNS. We find that the PP-DNS predicts fairly well both the Lagrangian and Eulerian statistics of the particle motion for the low-density case, while some discrepancies are observed for the high-density case. Also, the PP-DNS is found to underpredict the level of clustering of the suspension compared with the PR-DNS, with a larger difference for the high-density case
Structure function tensor equations with triple decomposition
Get PDFExact budget equations are derived for the coherent and stochastic
contributions to the second-order structure function tensor. They extend the
anisotropic generalised Kolmogorov equations (AGKE) by considering the coherent
and stochastic parts of the Reynolds stress tensor, and are useful for the
statistical description of turbulent flows with periodic or quasi-periodic
features, like e.g. the alternate shedding after a bluff body. While the
original AGKE describe production, transport, inter-component redistribution
and dissipation of the Reynolds stresses in the combined space of scales and
positions, the new equations, called AGKE, contain the phase
as an additional independent variable, and describe the interplay among the
mean, coherent and stochastic fields at the various phases. The newly derived
AGKE are then applied to a case where an exactly periodic external
forcing drives the flow: a turbulent plane channel flow modified by harmonic
spanwise oscillations of the wall to reduce drag. The phase-by-phase action of
the oscillating transversal Stokes layer generated by the forcing on the
near-wall turbulent structures is observed, and a detailed description of the
scale-space interaction among mean, coherent and stochastic fields is provided
thanks to the AGKE
Ascending–descending and direct–inverse cascades of Reynolds stresses in turbulent Couette flow
Get PDFThe interaction between small-and large-scale structures and the coexisting bottom-up and top-down processes are studied in a turbulent plane Couette flow, where space-filling longitudinal rolls appear at relatively low values of the Reynolds number. A direct numerical simulation database at is built to replicate the highest considered in recent experimental work by Kawata & Alfredsson (Phys. Rev. Lett., vol. 120, 2018, 244501). Our study is based on the exact budget equations for the second-order structure function tensor, i.e. the anisotropic generalized Kolmogorov equations (AGKE). The AGKE study production, redistribution, transport and dissipation of every Reynolds stress tensor component, considering simultaneously the physical space and the space of scales, and properly define the concept of scale in the inhomogeneous wall-normal direction. We show how the large-scale energy-containing motions are involved in the production and redistribution of the turbulent fluctuations. Both bottom-up and top-down interactions occur, and the same is true for direct and inverse cascading. The wall-parallel components and show that both small and large near-wall scales feed the large scales away from the wall. The wall-normal component is different, and shows a dominant top-down dynamics, being produced via pressure-strain redistribution away from the wall and transferred towards near-wall larger scales via an inverse cascade. The off-diagonal component shows a top-down interaction, with both direct and inverse cascades, albeit the latter takes place within a limited range of scales
- …
