Free Shear Layer Research Articles

Modification of response and suppression of vortex-shedding in vortex-induced vibrations of an elliptic cylinder

Numerical experiments are performed at a Reynolds number, Re of 100 to explore in two-dimensions, the undamped, transverse-only vortex-induced vibrations of a rigid elliptic cylinder of aspect ratio 0.5. With major axis oriented parallel to the flow, the cylinder behaves as a streamlined object. The mass ratio of the oscillator is 10. Re is based on the length of the major axis. The reduced speed, U* is varied from 1 to 8. The lock-in initiates at U*=3.5 and is soft in nature. The extent of lock-in gets severely truncated as compared to the one for a bluff oscillator; it spans just over an extremely narrow U* range of 3.5 to 4.2. The onset and closure of lock-in are marked with the oscillation frequency attaining its maximum and minimum, respectively. Streamlining of oscillator eliminates the whole of initial, quasi-periodic lower and desynchronization components of response. The response consists of a fragile periodic lower branch stretching over the range of lock-in and bracketed by a dominant pair of steady state regimes. The occurrence of peak response is noted at the onset of lock-in and the peak value is about 0.06 times the length of major axis. A closed standing wake characterizes the steady regimes while periodic vortex-shedding relates to the lower branch with weak vibrations. Interestingly, even in the steady regimes, the reduced speed is found to influence the surface pressure whereas global quantities like drag and lift forces remain invariant. In the lower branch, the mean surface pressure exhibits symmetry about base, thus mean lift disappears and wake mode is the basic anti-symmetric 2S. At the onset of lock-in, the instantaneous surface pressure is noticeably asymmetric along the upper and lower surfaces. Thus, vortex-shedding is strong and r.m.s. lift high. With rising U*, the strength of shedding decays, asymmetry of instantaneous pressure about the base weakens and consequently, r.m.s. lift falls. At U*=4.3, the free shear layers turn straight, shedding gets suppressed and the regime of steady state recovers.

Absolute/convective secondary instabilities and the role of confinement in free shear layers

Mixing layers can grow in time or space by vortex pairings that succeed each other in a nearly self-similar way. We use a point vortex model to study how confinement eventually limits this growth process, leading us to propose a wavelength selection mechanism for free shear layers with counterflow.

A Classification of Clay‐Rich Subaqueous Density Flow Structures

This study presents a classification for subaqueous clay‐laden sediment gravity flows. A series of laboratory flume experiments were performed using 9%, 15%, and 21% sediment mixture concentrations composed of sand, silt, clay, and tap water, on varying bed slopes of 6°, 8°, and 9.5°, and with discharge rates of 10 and 15 m3/hr. In addition to the characteristics of the boundary and plug layers, which have been previously used for the classification of open‐channel clay‐laden flows, the newly presented classification also incorporates the treatment of the free shear layer. The flow states within the boundary and free shear layers were established using calculation of the inner variable, self‐similarity considerations, and the magnitude of the apparent viscosity. Based on the experimental observations four flow types were recognized: (1) a clay‐rich plug flow with a laminar free shear layer, a plug layer, and a laminar boundary layer, (2) a top transitional plug flow containing a turbulent free shear layer, a plug layer, and a laminar boundary layer, (3) a transitional turbidity current with a turbulent free shear layer, no plug layer, and a laminar boundary layer, and (4) a fully turbulent turbidity current. A connection between the emplaced deposits and the relevant flow types is drawn and it is shown that a Froude number, two Reynolds numbers, and a dimensionless yield stress parameter are sufficient to associate an experimental flow type with a natural large‐scale density flow.

Measurement of the Excitation Source of an Axisymmetric Shallow Cavity Shear Layer

The interaction of a cavity shear layer with the sound field of an acoustic mode can generate an aeroacoustic source which is capable of initiating and sustaining acoustic resonances in the duct housing the cavity. This aeroacoustic source is determined experimentally for an internal axisymmetric cavity exposed to high Reynolds number, fully developed turbulent pipe flow without the need to resolve the details of neither the unsteady flow field nor the flow-sound interaction process at the cavity. The experimental technique, referred to here as the standing wave method (SWM), employs six microphones distributed upstream and downstream of the cavity to evaluate the fluctuating pressure difference generated by the oscillating cavity shear layer in the presence of an externally imposed sound wave. The results of the aeroacoustic source are in good agreement with the concepts of free shear layer instability and the fluid-resonant oscillation behavior. The accuracy of the measurement technique is evaluated by means of sensitivity tests. In addition, the measured source is used to predict the self-excited acoustic resonance of a shallow cavity in a pipeline. Comparison of the predicted and measured results shows excellent prediction of the self-excited acoustic resonance, including the resonance frequency, the lock-in velocity range, and the amplitude of the self-generated acoustic resonance.

Robotic volumetric PIV of a full-scale cyclist

A novel approach to the measurement of large-scale complex aerodynamic flows is presented, based on the combination of coaxial volumetric velocimetry and robotics. Volumetric flow field measurements are obtained to determine the time-averaged properties of the velocity field developing around a three-dimensional full-scale reproduction of a professional cyclist. The working principles of robotic volumetric PIV are discussed on the basis of its main components: helium-filled soap bubbles as tracers; the compact coaxial volumetric velocimeter; a collaborative 6 degrees of freedom robot arm; particle image analysis based on Shake-the-Box algorithm and ensemble statistics to yield data on a Cartesian mesh in the physical domain. The spatial range covered by the robotic velocimeter and its aerodynamic invasiveness are characterised. The system has the potential to perform volumetric measurements in a domain of several cubic metres. The application to the very complex geometry of a full-scale cyclist in time-trial position is performed in a large aerodynamic wind tunnel at a flow speed of 14 m/s. The flow velocity in the near field of the cyclist body is gathered through 450 independent views encompassing a measurement volume of approximately 2 m3. The measurements include hidden regions between the arms and the legs, otherwise very difficult to access by conventional planar or tomographic PIV. The time-averaged velocity field depicts the main flow topology in terms of stagnation points and lines, separation and reattachment lines, trailing vortices and free shear layers. The wall boundary layers developing on the body surface hide below the level resolvable by the present measurements.

Physical effects of magnetic fields on the Kelvin-Helmholtz instability in a free shear layer

The Kelvin-Helmholtz instability of a parallel shear flow with a hyperbolic-tangent velocity profile has been simulated numerically at a high Reynolds number. The fluid is perfectly conducting with low viscosity, and the strength of the applied magnetic field varies from weak to strong. We found that the magnetic field parallel to the mainstream direction has a stabilizing effect on the shear flow. The magnetic field mainly stabilizes short-wave perturbations. Small viscosity and/or slight compressibility could introduce some instability even in the presence of a strong magnetic field in a certain circumstance. The suppressing effect of the magnetic field on the instability is accomplished by two parts: the separating effect of the transverse magnetic pressure and the anti-bending effect of magnetic tension pointing to the center of curvature. The former shows prevailingly stronger effect on the fluid interface than the latter does, which is different from the conventional opinion that magnetic tension dominates. Essentially it is mainly the Maxwell stress that weakens and balances the momentum transport conducted by the Reynolds stress, reducing the mixing degree of the upper fluid and the lower fluid.

Aerodynamics of a stay cable with helical fillets - Part I: Stability and load characteristics

The aerodynamic behaviour of a bridge stay cable with helical fillets in smooth flow at high Reynolds numbers is presented in this paper. The cable response and related sectional load characteristics were studied experimentally on a 1:1 scale cable section model. The studies showed that a cable with helical fillets inclined 60° to the flow could experience large amplitude wind induced vibrations and that the occurrence of vibrations were highly dependent on cable surface irregularities. The ambition is not to explain fully the excitation mechanism, but to present global and local influences of the helical fillets on the flow field. It was revealed that the flow field around the cable shifted between semi-stable transition states which took place when the transition from laminar to turbulent flow propagated from the free shear layers to the boundary layer. The transitions would form locally and spread along the cable axis. The helical fillet appeared to dominate the local flow structures when located at an angular position between 40° and 130° from the stagnation region. In the stagnation and base regions, the surface irregularities appeared to dominate. Furthermore, the helical fillets displaced the mean stagnation line. The application of quasi-steady theory with the measurement data available appeared not to be able to explain the vibrations.

Excitation of Free Shear-Layer Instabilities for High-Speed Flow Control

Free shear layers are building blocks of many flows of interest in applications, including jets, cavity flows, and separated flows. It was found several decades ago that free shear layers are unstable to small perturbations over a wide range of frequencies, that they are dominated by coherent large-scale structures (even at high Reynolds numbers), and that the dynamics of these structures dominate important processes: entrainment, mixing, momentum transport, and noise generation. These findings motivated extensive research activities to actively control their development using excitation of instabilities, but early experimental research focused primarily on the control of low-speed, low-Reynolds-number free shear layers. This extensive body of the literature is briefly reviewed. The authors’ recent work using localized arc filament plasma actuators in jets shows that free shear layers respond to the excitation over a large range of conditions that have been explored: jet Mach number (up to 1.65), convective Mach number (up to 1), and Reynolds number (up to ). However, the nature of large-scale structures, shear-layer growth rate, and generation of Mach waves all depend on the jet Mach number and compressibility level. The results clearly demonstrate the similarity of instability processes and development of large-scale structures in free shear layers, regardless of the Mach or Reynolds numbers.

Large eddy simulations of the all-speed turbulent jet flow using 3-D CFD code GASFLOW-MPI

Turbulent flow at a wide range of Mach numbers may occur during design based accidents (DBA) or severe accidents in nuclear power plants (NPPs), for instinct, incompressible gas mixing flow (subsonic), gas mixture critical flow (transonic) or detonation (supersonic). All of them need to be simulated in safety analysis of NPP containments, therefore we need the computer code to have the capability to simulate all-speed flow. GASFLOW-MPI is a well-developed parallel all-speed CFD software used to predict gas turbulent mixing, chemical combustion and other related thermal-hydraulic phenomena in containments of NPPs. In order to validate the all-speed turbulent flow capability of GASFLOW-MPI, four turbulent jet cases over a broad range of Mach number have been performed in this paper. The large eddy simulation (LES) turbulent model is employed to capture more details of turbulence and flow features in this study. The standard Smagorinsky sub-grid scale model is utilized to model the unresolved turbulent behaviors at sub-grid scale. Both the instantaneous and time-averaged flow fields are analyzed as well as turbulent statistics to compare with the experimental data sets. The simulation results are in good agreement with the experimental data sets available in the literature. The shear layer instability is observed in the instantaneous flow field which leads to the end of the potential core and then the centerline velocity begins to decay. The size of the potential core and the decay rate of the centerline velocity are consistent well with the experiments. The turbulence intensity in the fully developed region agree well with the experimental data and the velocity power spectrum reveals the -5/3 law. Simulation results show that the compressibility has a significant effect on a free shear layer growing rate which is consistent with the experiments. The complex shock wave structure is also captured exactly by the second-order total variation diminishing (TVD) scheme. GASFLOW-MPI code has been successfully validated using the experimental data of turbulent jet flow at the wide regime of Mach numbers.

Central recirculation zones and instability waves in internal swirling flows with an annular entry

The characteristics of the central recirculation zone and the induced instability waves of a swirling flow in a cylindrical chamber with a slip head end have been numerically investigated using the Galerkin finite element method. The effects of Reynolds number as well as swirl level adjusted by the injection angle were examined systematically. The results indicate that at a high swirl level the flow is characterized by an axisymmetric central recirculation zone (CRZ). The fluid in the CRZ takes on a solid-body rotation driven by the outer main flow through a free shear layer. Both the solid-body rotating central flow and the free shear layer provide the potential for the development of instability waves. When the injection angle increases beyond a critical value, the basic axisymmetric flow loses stability, and instability waves develop. In the range of Reynolds numbers considered in this study, three kinds of instability were identified: inertial waves in the central flow, and azimuthal and longitudinal Kelvin-Helmholtz waves in the free shear layer. These three types of waves interact with each other and mix together. The mode selection of the azimuthal waves depends strongly on the injection angle, through the perimeter of the free shear layer. Compared with the injection angle, the Reynolds number plays a minor role in mode selection. The flow topologies and characteristics of different flow states are analyzed in detail, and the dependence of flow states on the injection angle and Reynolds number is summarized. Finally, a linear analysis of azimuthal instabilities is carried out; it confirms the mode selection mechanisms demonstrated by the numerical simulation.

Evolution and transition mechanisms of internal swirling flows with tangential entry

The characteristics and transition mechanisms of different states of swirling flow in a cylindrical chamber have been numerically investigated using the Galerkin finite element method. The effects of the Reynolds number and swirl level were examined, and a unified theory connecting different flow states was established. The development of each flow state is considered as a result of the interaction and competition between basic mechanisms: (1) the centrifugal effect, which drives an axisymmetric central recirculation zone (CRZ); (2) flow instabilities, which develop at the free shear layer and the central solid-body rotating flow; (3) the bouncing and restoring effects of the injected flow, which facilitate the convergence of flow on the centerline and the formation of bubble-type vortex breakdown; and (4) the damping effect of the end-induced flow, which suppresses the development of the instability waves. The results show that the CRZ, together with the free shear layer on its surface, composes the basic structure of swirling flow. The development of instability waves produces a number of discrete vortex cores enclosing the CRZ. The azimuthal wave number is primarily determined by the injection angle. Generally, the wave number is smaller at a higher injection angle, due to the reduction of the perimeter of the free shear layer. At the same time, the increase in the Reynolds number facilitates the growth of the wave number. The end-induced flow tends to reduce the wave number near the head end and causes a change in wave number from the head end to the downstream region. Spiral-type vortex breakdown can be considered as a limiting case at a high injection angle, with a wave number equal to 0 near the head end and equal to 1 downstream. At lower Reynolds numbers, the bouncing and restoring effect of the injected flow generates bubble-type vortex breakdown.

Numerical study of supersonic turbulent free shear layer mixing and combustion

 Abstract. Numerical study of two-dimensional supersonic hydrogen-air mixing and combustion in free shear layer is performed. The system of Favre-Averaged Naveir-Stokes equations for multispecies reacting flow is solved using ENO scheme of third-order in accuracy. The k-ε two-equation turbulence models with compressibility correction are applied to calculate the eddy viscosity coefficient. In order to produce the roll-up and pairing of vortex rings, an unsteady boundary condition is applied at the inlet plane. At the outflow, the non-reflecting boundary condition is taken. The influence of different Mach numbers on the formation of vorticity structures and shear layer growth rate are studied. The obtained results are compared with available experimental data and the numerical results of other authors. For the description of reaction pathways of hydrogen, a seven species chemical reaction model by Jachimowski is adopted. The influence of Mach numbers on turbulent mixture and combustion is reported.       G M T   Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский   Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский                 Звуковая функция ограничена 200 символами     Настройки : История : Обратная связь : Donate Закрыть

Nose-Tip Transition Control by Surface Roughness on a Hypersonic Sphere

The surface heat flux on a 100 mm diameter hypersonic sphere was reduced through surface roughness on its forebody. The test model was subjected to a hypersonic freestream of Mach 8.8 and Reynolds number 1.98 million/m, in a shock tunnel. Forebody surface heat transfer rates measured on smooth and rough spheres, under the same free-stream conditions, were compared. The comparison of heat flux indicated an overall reduction in surface heating rates on the rough model, which could be attributed to the delayed nose tip transition. The surface roughness on the forebody of the model generated miniature cavities. Stability of the free shear layer over the miniature cavities and entrapment of the destabilizing vortices in the cavities, make the flow over the rough test model more stable than the attached boundary layer over the smooth model, under transitional conditions.

The effect of the free shear layer instability of jet by the plasma induced flow.

The effect of the free shear layer instability of jet by the plasma induced flow.

Detached-Eddy Simulations for Active Flow Control

The present study conducted improved delayed detached-eddy simulations of actively controlled flows with a harmonic actuation on a backward-facing step and pulsed blowing on a pitching airfoil for reattachment and separation controls. Spectra analyses based on the unexcited flowfield data were carried out by using dynamic mode decomposition to identify spatial structures corresponding to the different Strouhal frequencies. The characteristic physical modes have been extracted for both cases. With better understanding of unsteady flow features, effective control practices were illustrated. For the backward-facing step case, the optimal excitation frequency was found to be identical with the step-mode frequency of the baseline flow. Such harmonic actuation enhanced the pairing process, forming the largest-scale spanwise coherent structures in the free shear layer, resulting in a 40% maximum reduction of the bubble length. For the airfoil case, with the optimum excitation frequency determined by the vortex-shedding mode of the baseline flow, a 194% increase of the lift-to-drag ratio was obtained; the pulsed blowing with the jet orifices in an antiphased manner was found to be very effective to damp the fluctuations of the lift around its mean value, though it led to a small decrease in mean lift.

Control of a three-dimensional turbulent shear layer by means of oblique vortices

The effect of local forcing on the separated, three-dimensional shear layer downstream of a backward-facing step is investigated by means of large-eddy simulation for a Reynolds number based on the step height of 10,700. The step edge is either oriented normal to the approaching turbulent boundary layer or swept at an angle of $$40^\circ $$ . Oblique vortices with different orientation and spacing are generated by wavelike suction and blowing of fluid through an edge parallel slot. The vortices exhibit a complex three-dimensional structure, but they can be characterized by a wavevector in a horizontal section plane. In order to determine the step-normal component of the wavevector, a method is developed based on phase averages. The dependence of the wavevector on the forcing parameters can be described in terms of a dispersion relation, the structure of which indicates that the disturbances are mainly convected through the fluid. The introduced vortices reduce the size of the recirculation region by up to 38%. In both the planar and the swept case, the most efficient of the studied forcings consists of vortices which propagate in a direction that deviates by more than $$50^\circ $$ from the step normal. These vortices exhibit a spacing in the order of 2.5 step heights. The upstream shift of the reattachment line can be explained by increased mixing and momentum transport inside the shear layer which is reflected in high levels of the Reynolds shear stress $$-\rho \overline{u'v'}$$ . The position of the maximum of the coherent shear stress is found to depend linearly on the wavelength, similar to two-dimensional free shear layers.

Developing three dimensional potential solver for investigation of propulsion performance of rigid and flexible oscillating foils

Heave and pitch motion of an oscillating airfoil in uniform flow will cause generation of forwarding thrust. Applying a combination of these two motions on flexible foil, one can increase thrust and therefore the efficiency. This is the way that most fishes and other flying animals uses to consume less energy. In this paper, hydrodynamic forces and efficiency of an oscillating airfoil is investigated. A code is developed based on potential flow formulation in combination with Time Stepping Method (TSM) with nonlinear free shear layer dynamic approach to predict the wake behind the lifting bodies. A linear Morino type Kutta condition has been implemented on panels adjacent to trailing edge. In this methodology, there is no need to define the wake shape before starting the simulation and it is an important feature in treating the hydrodynamics of submerged bodies. To validate the presented algorithm, some test cases were investigated. Comparison of propulsion performance of rigid oscillating foil with flexible undulating in different Strouhal number is the novelty of our research. We observe that in a wide range of frequencies, undulation movements is more efficient than rigid oscillations. The numerical results show a good agreement related to analytical and experimental measurements.

Numerical investigation of flow dynamics and scalar transport in a wall-bounded turbulent jet

We present the results of Large Eddy Simulation of a turbulent jet discharging into a confined slot with the wall spacing to jet inlet width ratio = 0.1 and Re = 104. The jet exhibits a meandering motion accompanied by formation of checkerboard pattern of large vortices in the mixing layer. A passive scalar transport was simulated with uniform inlet distribution of the scalar. It was found that the fluctuations of spanwise velocity component grow downstream and their maximum location is migrating from the lateral boundaries of the jet (free-shear layers) to the wall-boundary layers. Some evidence of counter-gradient turbulent scalar transport was found in the mixing layers of the jet, which may be attributed to the influence of observed large-scale checkerboard-type vortices.

Direct numerical simulation of a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation

The statistical properties are presented for the direct numerical simulation of a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) at the verge of separation. The APG TBL has a momentum thickness-based Reynolds number range from $Re_{\unicode[STIX]{x1D6FF}_{2}}=570$ to 13 800, with a self-similar region from $Re_{\unicode[STIX]{x1D6FF}_{2}}=10\,000$ to 12 300. Within this domain the average non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}=39$, where for a unit density $\unicode[STIX]{x1D6FD}=\unicode[STIX]{x1D6FF}_{1}P_{\!e}^{\prime }/\unicode[STIX]{x1D70F}_{w}$, with $\unicode[STIX]{x1D6FF}_{1}$ the displacement thickness, $\unicode[STIX]{x1D70F}_{w}$ the mean shear stress at the wall and $P_{\!e}^{\prime }$ the far-field pressure gradient. This flow is compared with previous zero pressure gradient and mild APG TBL ($\unicode[STIX]{x1D6FD}=1$) results of similar Reynolds number. All flows are generated via the direct numerical simulation of a TBL on a flat surface with far-field boundary conditions tailored to apply the desired pressure gradient. The conditions for self-similarity, and the appropriate length and velocity scales, are derived. The mean and Reynolds stress profiles are shown to collapse when non-dimensionalised on the basis of these length and velocity scales. As the pressure gradient increases, the extent of the wake region in the mean streamwise velocity profiles increases, whilst the extent of the log-layer and viscous sublayer decreases. The Reynolds stress, production and dissipation profiles of the APG TBL cases exhibit a second outer peak, which becomes more pronounced and more spatially localised with increasing pressure gradient. This outer peak is located at the point of inflection of the mean velocity profiles, and is suggestive of the presence of a shear flow instability. The maximum streamwise velocity variance is located at a wall normal position of $\unicode[STIX]{x1D6FF}_{1}$ of spanwise wavelength of $2\unicode[STIX]{x1D6FF}_{1}$. In summary as the pressure gradient increases the flow has properties less like a zero pressure gradient TBL and more akin to a free shear layer.

Large eddy simulation of turbulent flow using the parallel computational fluid dynamics code GASFLOW-MPI

Abstract GASFLOW-MPI is a widely used scalable computational fluid dynamics numerical tool to simulate the fluid turbulence behavior, combustion dynamics, and other related thermal–hydraulic phenomena in nuclear power plant containment. An efficient scalable linear solver for the large-scale pressure equation is one of the key issues to ensure the computational efficiency of GASFLOW-MPI. Several advanced Krylov subspace methods and scalable preconditioning methods are compared and analyzed to improve the computational performance. With the help of the powerful computational capability, the large eddy simulation turbulent model is used to resolve more detailed turbulent behaviors. A backward-facing step flow is performed to study the free shear layer, the recirculation region, and the boundary layer, which is widespread in many scientific and engineering applications. Numerical results are compared with the experimental data in the literature and the direct numerical simulation results by GASFLOW-MPI. Both time-averaged velocity profile and turbulent intensity are well consistent with the experimental data and direct numerical simulation result. Furthermore, the frequency spectrum is presented and a –5/3 energy decay is observed for a wide range of frequencies, satisfying the turbulent energy spectrum theory. Parallel scaling tests are also implemented on the KIT/IKET cluster and a linear scaling is realized for GASFLOW-MPI.