1,721,045 research outputs found
Numerical study of transonic buffet on supercritical airfoil with different boundary layer states
Full text linkAccurate numerical simulations of flow over airfoils play an increasingly
important role in the design of aircraft major components such as wings and turbo-
machinery blades. These lifting devices often operate in demanding aerodynamic
conditions for optimum performances, and may experience the presence of shock
waves in operating conditions. Shocks may become unsteady under specific
conditions, undergoing a large-scale, low-frequency periodic motion, which affects
the entire flow-field. This unsteady phenomenon, named transonic buffet, is the
subject of the present numerical investigation, with an oscillating shock over the
suction side of the airfoil.
In this study, a range of transonic Mach numbers and angles of incidence are
considered, but the bulk of the analysis is carried out for flow conditions at free-
stream Mach number M∞ = 0.7 and angle of incidence α = 7°, which show
well established buffet. Large-eddy simulations (LES) with natural and forced
transition carried out at chord Reynolds number Re = 3000000 clearly highlight
the effects of the incoming boundary-layer state on the shock oscillations. While a
laminar upstream boundary layer yields weak oscillations of the shock, a turbulent
incoming boundary layer yields significant buffet. The LES database has been used
to establish veracity (or not) of suggested buffet pathways, mainly based on the
alleged existence of an acoustic feedback loop. This mechanism is actually found
to consist of two separate patterns: coherent pressure disturbances convected from
the shock to the trailing edge, and acoustic waves scattered at the trailing edge,
feeding the shock motion. Additional exploration of the pressure side role in the
unsteadiness reveals that is has but marginal effect on the phenomenon.
Direct numerical simulations (DNS) at lower Reynolds number (Re = 300000)
suggest a reversal in the previously observed trend. In this case, a laminar incoming
boundary layer yields stronger buffet as compared to its turbulent counterpart,
highlighting strong dependence of the buffet phenomenon on the Reynolds number
when natural transition is considered. In order to passively control buffet, we
consider devices whose design is similar to large-eddy break-up devices (LEBU),
consisting of a thin circular-arc airfoil placed between shock and trailing edge, with
the main goal of: i) breaking the eddies originating at the shock, responsible for
the acoustic scattering at the trailing edge; ii) manipulating the acoustic field in the
aft part of the airfoil. RANS simulations show potential for this kind of device for
complete stabilization of buffet. On the other hand, DNS shows that the device is
able to curtail the buffet, but not to eliminate it. Additional tests are needed in
order to assess the effectiveness of the control device, whose practical impact might
be very larg
A Novel Approach for Direct Numerical Simulation of Hydraulic Fracture Problems
No full textHydraulic fracturing is a non-linear and multi-physics problem involving the break up of a solid medium due to the action of hydrodynamic forces. Fluid and solid mechanics are involved at the same time together with fracture mechanics. Despite its relevance in many scientific and engineering fields, the theoretical and numerical description of hydraulic fracturing remains a challenging matter and the capabilities of existing models for applications are still limited. In this context, we propose a novel numerical approach to the Direct Numerical Simulation of hydraulic fracturing based on the Navier–Stokes equations coupled with peridynamic theory of solid mechanics through a multi-direct Immersed Boundary Method. The main advantage of this approach consists in the reliable crack-detection and tracking capabilities of peridynamics together with the capability of the Immersed Boundary Method of managing no-slip and no-penetration boundary conditions on complex and time-evolving interfaces. A massive-parallel solver based on this model has been implemented. We present a detailed theoretical description of the proposed methodology as well as the results of an extensive validation campaign for the new solver. Different benchmarking tests are provided together with the qualitative results of a simulation reproducing the fracture of a solid structure in a laminar, unsteady flow
Direct numerical simulation of the scouring of a brittle streambed in a turbulent channel flow
Full text linkThe natural processes involved in the scouring of submerged sediments are crucially relevant in geomorphology along with environmental, fluvial, and oceanographic engineering. Despite their relevance, the phenomena involved are far from being completely understood, in particular for what concerns cohesive or stony substrates with brittle bulk mechanical properties. In this frame, we address the investigation of the mechanisms that govern the scouring and pattern formation on an initially flattened bed of homogenous and brittle material in a turbulent channel flow, employing direct numerical simulation. The problem is numerically tackled in the frame of peridynamic theory, which has intrinsic capabilities of reliably reproducing crack formation, coupled with the Navier–Stokes equations by the immersed boundary method. The numerical approach is reported in detail here and in the references, where extensive and fully coupled benchmarks are provided. The present paper focuses on the role of turbulence in promoting the brittle fragmentation of a solid, brittle streambed. A detailed characterization of the bedforms that originate on the brittle substrate is provided, alongside an analysis of the correlation between bed shape and the turbulent structures of the flow. We find that turbulent fluctuations locally increase the intensity of the wall-stresses producing localized damages. The accumulation of damage drives the scouring of the solid bed via a turbulence-driven fatigue mechanism. The formation, propagation, and coalescence of scouring structures are observed. In turn, these affect both the small- and large-scale structures of the turbulent flow, producing an enhancement of turbulence intensity and wall-stresses. At the small length scales, this phenomenology is put in relation to the formation of vortical cells that persist over the peaks of the channel bed. Similarly, large-scale irregularities are found to promote the formation of stationary turbulent stripes and large-scale vortices that enhance the widening and deepening of scour holes. As a result, we observe a quadratic increment of the volumetric erosion rate of the streambed, as well as a widening of the probability density of high-intensity wall stress on the channel bed
A sharp-interface immersed boundary method for moving objects in compressible viscous flows
No full textSharp-interface Immersed Boundary Methods for moving solids in compressible viscous flows often exhibit spurious noise propagating from the moving boundary. In compressible flows, filtering or upwinding discretization techniques are often used to overcome the problem. In the present work, we show that using a conservative energy-preserving finite difference method for convection in combination with a Ghost-Point-Forcing-Method (GPFM) is able to keep controlled the pressure-velocity spurious noise. In order to deal with compressible flow in a wide range of Mach numbers the shock-dynamics appearing in high-speed flows conditions has been addressed hybridising the latter scheme with a fifth-order weighted-essentially-non-oscillatory (WENO) scheme. The latter, in the path of keeping the numerical dissipation minimal, has been confined around the shock locations using a proper detector. In this respect, the method appears a suitable alternative for direct and large-eddy simulation of moving objects in compressible flows. The entire methodology was found to be robust ranging from weakly compressible to highly supersonic flows including shocks. In order to prove the cleanliness and the robustness of the method, several well-documented benchmarks and test cases, in a wide range of both Reynolds and Mach numbers, are presented
Maximizing Vanadium Redox Flow Battery Efficiency: Strategies of Flow Rate Control
Full text linkVanadium redox flow batteries (VRFBs) are one of the most promising technologies for large-scale energy storage due to their flexible energy and power capacity configurations. The energy losses evaluation assumes a very important rule on the VRFB characterization in order increase the efficiency of the battery. Very few papers describe the relations between hydraulic, electrical and chemical contributions to the system energy losses, especially in a large size VRFB system. In the first part a fluid dynamics characterization of a 9kW / 27 kWh VRFB test facility has been conducted. In particular, we will consider the internal resistance as the sum of an ohmic and a transport resistance. Secondly, an overall loss assessment based on both numerical and experimental results has been carried out. Finally, some improvements in the battery management strategy and in stack engineering are proposed, that results from this work and can help the future designer to develop more efficient VRFB stack with a compact design
Sedimentation of large particles in turbulent environments
No full textThe aim of the present study is to investigate the sedimentation of large non-colloidal spherical particles in both quiescent and turbulent environments. To this aim, Direct Numerical Simulations are performed using an Immersed Boundary Method to account for the dispersed phase. The solid volume fractions considered are in the range 0.5%-1.0%, while the solid to fluid density ratio is set equal to 1.02. The particle diameter is chosen to be approximately 12 Komlogorov lengthscales in nominal conditions. The results show that the mean settling velocities decrease in the turbulent cases. The overall drag is increased both by the non-linear finite Reynolds number behavior and by unsteady effects, which are negligible in quiescent cases
A semi-stochastic approach for point-particle dispersion in Wall-Modeled Large Eddy Simulation of particle-laden turbulent flows
Full text linkThis study investigates the integration of a semi-stochastic model, based on the Langevin equations with drift-correction, into the Wall-Modeled Large-Eddy Simulation (WM-LES) framework for simulating particle-laden turbulent channel flows in one-way coupling conditions. Various velocity statistics are presented for both the carrier and dispersed phases. The results confirm that the proposed approach accurately reproduces particle dynamics across a range of Reynolds and Stokes numbers when compared with reference data from Direct Numerical Simulations. The model effectively captures key aspects of particle behavior, including velocity fluctuations, accurate near-wall particle velocities and the expected logarithmic profile of the axial velocity in the logarithmic region. Additionally, it successfully replicates the trends in particle concentration both near the wall and throughout the bulk region of the flow. The findings highlight the potential of the proposed semi-stochastic WM-LES framework to enhance the accuracy and efficiency of particle-laden flow simulations. The proposed approach provides valuable insights into turbulent particulate transport modeling and may benefit applications in engineering and environmental sciences, such as sediment transport, pollutant dispersion, and industrial mixing
Near-wall turbulence modulation by small inertial particles
No full textWe use interface-resolved simulations to study near-wall turbulence modulation by small inertial particles, much denser than the fluid, in dilute/semi-dilute conditions. We considered three bulk solid mass fractions, and, with only the latter two showing turbulence modulation. The increase of the drag is strong at, but mild in the densest case. Two distinct regimes of turbulence modulation emerge: for smaller mass fractions, the turbulence statistics are weakly affected and the near-wall particle accumulation increases the drag so the flow appears as a single-phase flow at slightly higher Reynolds number. Conversely, at higher mass fractions, the particles modulate the turbulent dynamics over the entire flow, and the interphase coupling becomes more complex. In this case, fluid Reynolds stresses are attenuated, but the inertial particle dynamics near the wall increases the drag via correlated velocity fluctuations, leading to an overall drag increase. Hence, we conclude that, although particles at high mass fractions reduce the fluid turbulent drag, the solid phase inertial dynamics still increases the overall drag. However, inspection of the streamwise momentum budget in the two-way coupling limit of vanishing volume fraction, but finite mass fraction, indicates that this trend could reverse at even higher particle load
High-fidelity simulation and low-order analysis for planetary descent investigation of capsule-parachute interaction
No full textThe project focuses on characterizing the unsteady dynamics of the parachute-capsule system during the descent phase of planetary entry in a supersonic flow regime. Currently, LargeEddy Simulation, coupled with an Immersed-Boundary Method, is utilized to examine the timeevolving flow behavior of a rigid supersonic parachute trailing behind a reentry capsule as it descends through the Martian atmosphere. The flow is simulated at Ma=2 and Re=10^6. A massive GPU parallelization has been utilized to enable a high-fidelity resolution of the turbulent structures in the flow, essential for capturing its dynamic behavior. We demonstrate through low-order modeling of the unsteady turbulent wake of the capsule that low-frequency fluctuations within the wake are the primary trigger for flow instability in front of the canopy volume. Proper-Orthogonal Decomposition is utilized to investigate the system dynamics and analyze how various turbulence contributions influence the phenomenon
- …
