Showing papers on "Starting vortex published in 2015"

RANS and LES computations of the tip-leakage vortex for different gap widths

Abstract: In hydraulic turbines, the tip-leakage vortex is responsible for flow instabilities and for promoting erosion due to cavitation. To better understand the tip vortex flow, Reynolds- averaged Navier–Stokes (RANS) and large eddy simulation (LES) computations are carried out to simulate the flow around a NACA0009 blade including the gap between the tip and the wall. The main focus of the study is to understand the influence of the gap width on the development of the tip vortex, as for instance its trajectory. The RANS computations are performed using the open source solver OpenFOAM 2.1.0, two incidences and five gaps are considered. The LESs are achieved using the YALES2 solver for one incidence and two gaps. The validation of the results is performed by comparisons with experimental data available downstream the trailing edge. The position of the vortex core, the mean velocity and the mean axial vorticity fields are compared at three different downstream locations. The results show that the mean behaviour of the tip vortex is well captured by the RANS and LES computations compared to the experiment. The LES results are also analysed to bring out the influence of the gap width on the development of the tip-leakage vortex. Finally, a law that matches the vortex trajectory from the leading edge to the mid-chord is proposed. Such a law can be helpful to determine, in case of cavitation, if the tip vortex will interact with the walls and cause erosion.

On the Unsteady Behavior of the Flow around NACA 0012 Airfoil with Steady External Conditions at Re=1000

Abstract: Even for the stationary airfoils, due to the boundary and shear layer interactions of upper and lower surface of the airfoils, alternating vortex patterns form and the flow becomes time dependent. In the current study, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases. The flow patterns are presented for wide range of angles of attack values. The vortex pattern generated is analyzed numerically for different angles of attack at Re=1000 around NACA 0012 airfoil. At this Reynolds number, the flow is laminar and boundary layers are quite thick. Flow separation and unsteady vortex shedding is observed even at low angles of attack. For NACA 0012 airfoil, the unsteady vortex pattern is observed at about 8° angle of attack for Re=1000. Spectral analysis is performed for angles of attack ranging from 0° to 90°. It is presented that amplitude spectrum of lift coefficient (C1) start to shows a peak at 8° for NACA 0012 and the aerodynamic forces presents oscillat...

Regimes of tonal noise on an airfoil at moderate Reynolds number

Abstract: Tonal noise generated by airfoils at low to moderate Reynolds number is relevant for applications in, for example, small-scale wind turbines, fans and unmanned aerial vehicles. Coherent and convected vortical structures scattering at the trailing edge from the pressure or suction sides of the airfoil have been identified to be responsible for such tonal noise generation. Controversy remains on the respective significance of pressure- and suction-side events, along with their interaction for tonal noise generation. The present study surveys the regimes of tonal noise generation for low to moderate chord-based Reynolds number between and and effective angle of attack between and for the NACA 0012 airfoil profile. Extensive acoustic measurements with smooth surface and with transition to turbulence forced by boundary layer tripping are presented. Results show that, at non-zero angle of attack, tonal noise generation is dominated by suction-side events at low Reynolds number and by pressure-side events at high Reynolds number. At smaller angle of attack, interaction between events on the two sides becomes increasingly important. Particle image velocimetry measurements complete the information on the flow field structure in the source region around the trailing edge. The influences of both angle of attack and Reynolds number on tonal noise generation are explained by changes in the mean flow topology, namely the presence and location of reverse flow regions on the two sides. Data gathered from experimental and numerical studies in the literature are reviewed and interpreted in view of the different regimes.

An experimental investigation on the surface water transport process over an airfoil by using a digital image projection technique

Abstract: In the present study, an experimental investigation was conducted to characterize the transient behavior of the surface water film and rivulet flows driven by boundary layer airflows over a NACA0012 airfoil in order to elucidate underlying physics of the important micro-physical processes pertinent to aircraft icing phenomena. A digital image projection (DIP) technique was developed to quantitatively measure the film thickness distribution of the surface water film/rivulet flows over the airfoil at different test conditions. The time-resolved DIP measurements reveal that micro-sized water droplets carried by the oncoming airflow impinged onto the airfoil surface, mainly in the region near the airfoil leading edge. After impingement, the water droplets formed thin water film that runs back over the airfoil surface, driven by the boundary layer airflow. As the water film advanced downstream, the contact line was found to bugle locally and developed into isolated water rivulets further downstream. The front lobes of the rivulets quickly advanced along the airfoil and then shed from the airfoil trailing edge, resulting in isolated water transport channels over the airfoil surface. The water channels were responsible for transporting the water mass impinging at the airfoil leading edge. Additionally, the transition location of the surface water transport process from film flows to rivulet flows was found to occur further upstream with increasing velocity of the oncoming airflow. The thickness of the water film/rivulet flows was found to increase monotonically with the increasing distance away from the airfoil leading edge. The runback velocity of the water rivulets was found to increase rapidly with the increasing airflow velocity, while the rivulet width and the gap between the neighboring rivulets decreased as the airflow velocity increased.

Three-dimensional flow structures and unsteady forces on pitching and surging revolving flat plates

Abstract: Tomographic particle image velocimetry was used to explore the evolution of three-dimensional flow structures of revolving low-aspect-ratio flat plates in combination with force measurements at a Reynolds number of 10,000. Two motion kinematics are compared that result in the same terminal condition (revolution with constant angular velocity and 45? angle of attack) but differ in the motion during the buildup phase: pitching while revolving at a constant angular velocity; or surging with a constant acceleration at a fixed angle of attack. Comparison of force histories shows that the pitching wing generates considerably higher forces during the buildup phase which is also predicted by a quasi-steady model quite accurately. The difference in the buildup phases affects the force histories until six chords of travel after the end of buildup phase. In both cases, a vortex system that is comprised of a leading-edge vortex (LEV), a tip vortex and a trailing edge vortex is formed during the initial period of the motion. The LEV lifts off, forms an arch-shaped structure and bursts into substructures, which occur at slightly different phases of the motions, such that the revolving–surging wing flow evolution precedes that of the revolving–pitching wing. The delay is shown to be in accordance with the behavior of the spanwise flow which is affected by the interaction between the tip vortex and revolving dynamics. Further analysis shows that the enhanced force generation of the revolving–pitching wing during the pitch-up phase originates from: (1) increased magnitude and growth rate of the LEV circulation; (2) relatively favorable position and trajectory of the LEV and the starting vortex; and (3) generation of bound circulation during the pitching motion, whereas that of the revolving–surging wing is negligible in the acceleration phase.

Influence of structural flexibility on the wake vortex pattern of airfoils undergoing harmonic pitch oscillation

Abstract: Reported herein is an investigation of the influence of the structural flexibility of sinusoidally pitching airfoils on the pattern of vorticity shed into the wake. For rigid airfoils, it is well known that, depending on the oscillation frequency and amplitude, this pattern takes the form of the classical or reverse von Karman vortex street. The pattern may be characterized by the vortex circulation (Γ o ), vortex-to-vortex streamwise and cross-stream spacing (a and b, respectively), and vortex core radius (R). In the present work, these four parameters are obtained from particle image velocimetry measurements in the wake of airfoils consisting of a rigid “head” and flexible “tail” at chord Reynolds number of 2010 for different tail flexibilities. The results show that flexible airfoils exhibit the switch from classical to reverse von Karman vortex street (i.e., change in the sign of b) at a reduced frequency of oscillation lower than their rigid counterpart. At a given oscillation frequency, the Strouhal number at which this switch occurs is smallest for a given airfoil structural flexibility; which becomes stiffer with increasing frequency. Using Strouhal number based on the actual trailing edge oscillation amplitude, reasonable scaling is found of the dependence of not only b but also Γ o , a and R on the motion and structure parameters for all airfoils investigated. These results are complemented with analyses using a vortex array model, which together with the identified scaling of the wake vortex parameters, provide basis for the computation of the net thrust acting on the airfoil.

Interactions of a streamwise-oriented vortex with a finite wing

Abstract: A canonical study is developed to investigate the unsteady interactions of a streamwise-oriented vortex impinging upon a finite surface using high-fidelity simulation. As a model problem, an analytically defined vortex superimposed on a free stream is convected towards an aspect-ratio-six ( ) plate oriented at an angle of and Reynolds number of in order to characterize the unsteady modes of interaction resulting from different spanwise positions of the incoming vortex. Outboard, tip-aligned and inboard positioning are shown to produce three distinct flow regimes: when the vortex is positioned outboard of, but in close proximity to, the wingtip, it pairs with the tip vortex to form a dipole that propels itself away from the plate through mutual induction, and also leads to an enhancement of the tip vortex. When the incoming vortex is aligned with the wingtip, the tip vortex is initially strengthened by the proximity of the incident vortex, but both structures attenuate into the wake as instabilities arise in the pair’s feeding sheets from the entrainment of opposite-signed vorticity into either structure. Finally, when the incident vortex is positioned inboard of the wingtip, the vortex bifurcates in the time-mean sense with portions convecting above and below the wing, and the tip vortex is mostly suppressed. The time-mean bifurcation is actually a result of an unsteady spiralling instability in the vortex core that reorients the vortex as it impacts the leading edge, pinches off, and alternately attaches to either side of the wing. The increased effective angle of attack inboard of impingement enhances the three-dimensional recirculation region created by the separated boundary layer off the leading edge which draws fluid from the incident vortex inboard and diminishes its impact on the outboard section of the wing. The slight but remaining downwash present outboard of impingement reduces the effective angle of attack in that region, resulting in a small separation bubble on either side of the wing in the time-mean solution, effectively unloading the tip outboard of impingement and suppressing the tip vortex. All incident vortex positions provide substantial increases in the wing’s lift-to-drag ratio; however, significant sustained rolling moments also result. As the vortex is brought inboard, the rolling moment diminishes and eventually switches sign as the reduced outboard loading balances the augmented sectional lift inboard of impingement.

Nonlinear Lift on a Triangular Airfoil in Low-Reynolds-Number Compressible Flow

Abstract: Numerical and experimental analyses of the aerodynamic performance of a triangular airfoil in low-Reynolds-number compressible flow are performed. This airfoil is one of the candidates for propeller blades on a possible future Martian air vehicle design. Based on past experimental studies conducted in the Mars Wind Tunnel at Tohoku University, this airfoil is known to exhibit nonlinear lift behavior. In the present study, direct numerical simulations of low-Reynolds-number compressible flow over a spanwise periodic triangular airfoil are conducted to identify the source of nonlinear lift. The numerical results reveal that the source of the nonlinear aerodynamic behavior is the lift enhancement provided by the large leading-edge vortex generated. For compressible low-Reynolds-number flow, the wake structure becomes elongated, causing the nonlinear lift enhancement to appear at higher angles of attack compared to the case of incompressible flow.

Origin and dynamics of vortex rings in drop splashing

Abstract: A vortex is a flow phenomenon that is very commonly observed in nature. More than a century, a vortex ring that forms during drop splashing has caught the attention of many scientists due to its importance in understanding fluid mixing and mass transport processes. However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we show that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along the drop wall, as demonstrated by a phase diagram established here, with different power-law dependencies of the angular velocities on the Reynolds number. These results provide important insight that allows understanding and modelling any type of vortex rings in nature, beyond just vortex rings during drop splashing.

Numerical investigation of vortex dynamics in an H-rotor vertical axis wind turbine

Abstract: We study the vortex dynamics of a two-dimensional H-rotor wind turbine using a Navier-Stokes solver. The turbulence model with the wall function is used as the turbulence closure. A sliding mesh technique is employed to handle the blade rotation. The vortex-blade interaction is systematically investigated and its influence on the force generation is discussed. Our simulations show that the vortex-blade interaction largely depends on the solidity and tip speed ratio. We further study the impact of solidity on the turbine performance. Our simulations show that the peak torque per blade decreases with the solidity while the peak torque azimuthal angle increases with the solidity. Our simulations also show that the increase in the azimuthal angle is more significant at low tip speed ratios than at high tip speed ratios. The impact of blade thickness is studied. Our simulations show that a thicker airfoil has a higher torque coefficient than a thinner airfoil. However, because for the thinner airfoil its peak ...

Unsteady lift for the Wagner problem in the presence of additional leading/trailing edge vortices

Abstract: This study amends the inviscid Wagner lift model for starting flow at relatively large angles of attack to account for the influence of additional leading edge and trailing edge vortices. Two methods are provided for starting flow of a flat plate. The first method is a modified Wagner function, which assumes a planar trajectory of the trailing edge vortex sheet accounting for a temporal offset from the original Wagner function given release of leading edge vortices and a concentrated starting point vortex at the initiation of motion. The second method idealizes the trailing edge sheet as a series of discrete vortices released sequentially. The models presented are shown to be in good agreement with high-fidelity simulations. Through the present theory, a vortex force line map is generated, which clearly indicates lift enhancing and reducing directions and, when coupled with streamlines, allows one to qualitatively interpret the effect of the sign and position of vortices on the lift and to identify the origins of lift oscillations and peaks. It is concluded that leading edge vortices close to the leading edge elevate the Wagner lift curve while a strong leading edge vortex convected to the trailing edge is detrimental to lift production by inducing a strong trailing edge vortex moving in the lift reducing direction. The vortex force line map can be employed to understand the effect of the different vortices in other situations and may be used to improve vortex control to enhance or reduce the lift.

Flow field measurement around vortex cavitation

Abstract: Models for the center frequency of cavitating-vortex induced pressure-fluctuations, in a flow around propellers, require knowledge of the vortex strength and vapor cavity size. For this purpose, stereoscopic particle image velocimetry (PIV) measurements were taken downstream of a fixed half-wing model. A high spatial resolution is required and was obtained via correlation averaging. This reduces the interrogation area size by a factor of 2–8, with respect to conventional PIV measurements. Vortex wandering was accounted for by selecting PIV images for a given vortex position, yielding sufficient data to obtain statistically converged and accurate results, both with and without a vapor-filled vortex core. Based on these results, the low-order Proctor model was applied to describe the tip vortex velocity outside the viscous core, and the cavity size as a function of cavitation number. The flow field around the vortex cavity shows, in comparison with a flow field without cavitation, a region of retarded flow. This layer around the cavity interface is similar to the viscous core of a vortex without cavitation.

Vorticity transport and the leading-edge vortex of a plunging airfoil

Abstract: The three-dimensional flow field was experimentally characterized for a nominally two-dimensional flat-plate airfoil plunging at large amplitude and reduced frequencies, using three-dimensional reconstructions of planar PIV data at a chord-based Reynolds number of 10,000. Time-resolved, instantaneous PIV measurements reveal that secondary vorticity, of opposite sign to the primary vortex, is intermittently entrained into the leading-edge vortex (LEV) throughout the downstroke, with the rate of entrainment increasing toward the end of the stroke when the leading-edge shear layer weakens. A planar vorticity transport analysis around the LEV indicated that, during the downstroke, the surface vorticity flux due to the pressure gradient is consistently about half that due to the leading-edge shear layer for all parameter values investigated, demonstrating that production and entrainment of secondary vorticity is an important mechanism regulating LEV strength. A small but non-negligible vorticity source was also attributed to spanwise flow toward the end of the downstroke. Aggregate vortex tilting is notably more significant for higher plunge frequencies, suggesting that the vortex core is more three-dimensional.

Vortex flow structures and interactions for the optimum thrust efficiency of a heaving airfoil at different mean angles of attack

Abstract: The thrust efficiency of a two-dimensional heaving airfoil is studied computationally for a low Reynolds number using a vortex force decomposition. The auxiliary potentials that separate the total vortex force into lift and drag (or thrust) are obtained analytically by using an elliptic airfoil. With these auxiliary potentials, the added-mass components of the lift and drag (or thrust) coefficients are also obtained analytically for any heaving motion of the airfoil and for any value of the mean angle of attack α. The contributions of the leading- and trailing-edge vortices to the thrust during their down- and up-stroke evolutions are computed quantitatively with this formulation for different dimensionless frequencies and heave amplitudes (Stc and Sta) and for several values of α. Very different types of flows, periodic, quasi-periodic, and chaotic described as Stc, Sta, and α, are varied. The optimum values of these parameters for maximum thrust efficiency are obtained and explained in terms of the inte...

Vortex Shedding and Aerodynamic Performance of Airfoil with Multiscale Trailing-Edge Modifications

Abstract: An experimental investigation was conducted into the nature of the vortex shedding generated by truncated and nonflat serrated trailing edges of a NACA 0012 wing section as well as the aerodynamic performance of such trailing edges. In line with previous findings, the truncated trailing edge generates a significant amount of vortex shedding, while increasing both the maximum lift and drag coefficients, resulting in an overall reduction in the maximum lift-to-drag ratio compared to a plain NACA 0012 wing section. By decreasing the chevron angle φ of the nonflat trailing-edge serrations (i.e., by making them sharper), the energy of the vortex shedding significantly decreases, and the lift-to-drag ratios increase compared to a plain NACA 0012 wing section. Fractal/multiscale patterns made of scaled-down repetitions of these serrations were also investigated with a view to further improve performance. It was found that the energy of the vortex shedding increases with an increasing fractal iteration if the che...

Dynamic stall process on a finite span model and its control via synthetic jet actuators

Abstract: An experimental study of the process by which dynamic stall occurs on a finite span S809 airfoil was conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. Understanding the flow field around a dynamically pitching airfoil helped in controlling the dynamic stall process through active flow control via synthetic jet actuators. The three component, two dimensional flow fields were measured with a stereoscopic particle image velocimetry system. This study demonstrated that, through the introduction of periodic momentum near the leading edge of this model, the evolution of the dynamic stall vortex, which forms and convects downstream under dynamic conditions, could be delayed or suppressed in favor of the preservation of a trailing edge vortex that arises due to trailing edge separation and recirculation in the time averaged sense. This process seems to be the result of changing how the flow field transitions from trailing edge separation to a fully separated flow. In a phase-averaged sense, absent of flow control, this process is defined by the creation of a phase averaged leading edge recirculation region, which interacts with the trailing edge separation. Through the introduction of momentum near the leading edge, this process can be altered, such that the phase averaged trailing edge separation region is the dominant structure present in the flow. Additionally, a cursory investigation into the instantaneous flow fields was conducted, and a comparison between the phase averaged flow field and instantaneous fields demonstrated that while similar effects can be observed, there is a significant difference in the flow field observed in the instantaneous fields versus the phase averaged sense. This would imply that a different method of analyzing dynamic stall from PIV measurements may be necessary.

Oscillatory flow regimes around four cylinders in a square arrangement under small and conditions

Abstract: Sinusoidally oscillatory flow around four circular cylinders in an in-line square arrangement is numerically investigated at Keulegan–Carpenter numbers ( ) ranging from 1 to 12 and at Reynolds numbers ( ) from 20 to 200. A set of flow patterns is observed and classified based on known oscillatory flow regimes around a single cylinder. These include six types of reflection symmetry regimes to the axis of flow oscillation, two types of spatio-temporal symmetry regimes and a series of symmetry-breaking flow patterns. In general, at small gap distances, the four structures behave more like a single body, and the flow fields therefore resemble those around a single cylinder with a large effective cylinder diameter. With increasing gap distance, flow structures around each individual cylinder in the array start to influence the overall flow patterns, and the flow field shows a variety of symmetry and asymmetry patterns as a result of vortex and shear layer interactions. The characteristics of hydrodynamic forces on individual cylinders as well as on the cylinder group are also examined. It is found that the hydrodynamic forces respond in a similar manner to the flow field to the cylinder proximity and wake interference.

The singing vortex.

Abstract: Marine propellers display several forms of cavitation. Of these, propeller-tip vortex cavitation is one of the important factors in propeller design. The dynamic behaviour of the tip vortex is responsible for hull vibration and noise. Thus, cavitation in the vortices trailing from tips of propeller blades has been studied extensively. Under certain circumstances cavitating vortices have been observed to have wave-like disturbances on the surfaces of vapour cores. Intense sound at discrete frequencies can result from a coupling between tip vortex disturbances and oscillating sheet cavitation on the surfaces of the propeller blades. This research article focuses on the dynamics of vortex cavitation and more in particular on the energy and frequency content of the radiated pressures.

Theoretical and experimental study on the vortex at hydraulic intakes

Abstract: To study the characteristics of the vortex at a hydraulic intake, an experiment with a free surface vortex was conducted in a cylindrical tank. The vortex flow field was measured using particle image velocimetry (PIV) and extensive experimental data describing the vortex characteristics (radial and tangential velocity distributions, vortex core radius variation, water surface profile and circulation distribution) have been obtained and analysed. Based on the detailed experimental data, an empirical model describing the key vortex characteristics is developed. In addition, using the pressure distributions inside and outside the vortex core, a dimensionless equation for the critical submergence requiring only the Froude number and the circulation number is derived. Results using this equation are in agreement with the experimental data.

Flow in vortex diodes

Abstract: A vortex diode is used as a cavitation device for treatment of industrial waste water and also, as a leaky non-return valve in nuclear applications. It consists of a vortex chamber with an axial and tangential port. When the fluid is injected through the tangential port, a strong vortex flow is set up in the diode chamber. This flow is characterized by phenomena such as vortex transition, precessing vortex core, toroidal recirculation zone, reverse flow core and recirculation zone in the axial port. Although studies have been conducted on the “confined vortex” class of flows (and some of them on vortex diodes), none of them provides a collective account of key nuances of the flow in a vortex diode. The flow in the diode differs from other confined-vortex flows on account of the axial-velocity deficit, due to which, direct correlations from other confined vortex flows cannot be applied to the diode. This work attempts to address the aforementioned flow characteristics in the diode using results primarily from CFD simulations. The reported methodology, computational model and results will be useful to gain better understanding of flows in vortex diodes and to optimize designs of vortex diodes for variety of applications.

Vortex Shedding from Airfoils in Reverse Flow

Abstract: The vortex shedding characteristics of three airfoils held at static angles of attack through 360 deg are presented with a focus on reverse flow (150≤α≤180 deg). Wind tunnel testing was performed on one airfoil with a sharp trailing edge (NACA 0012) and two airfoils featuring a blunt trailing edge (ellipse and DBLN-526). Time-resolved particle image velocimetry and smoke flow visualization were used to identify three reverse flow wake regimes: slender body vortex shedding, turbulent, and deep stall vortex shedding. The slender body regime is present for low angles of attack and low Reynolds numbers. In the turbulent regime, separation occurs in reverse flow at the sharp aerodynamic leading edge of a NACA 0012, whereas flow separation occurs further down the chord of airfoils with a blunt geometric trailing edge. The deep stall vortex shedding frequency was measured using unsteady force balance measurements. The Strouhal number Std (based on the projected diameter d of the airfoils) was found to be 0.145–...

Flow Reattachment Using Synthetic Jet Actuation on a Low-Reynolds-Number Airfoil

Abstract: Wind-tunnel experiments are used to study the effect of momentum coefficient and excitation frequency on flow separation using synthetic jet actuation. Experiments are conducted on a NACA 0025 airfoil at a chord-based Reynolds number of 100,000 and angle of attack of 10 deg. The actuator is located near the leading edge, downstream of the mean separation location. High-frequency excitation is able to reattach the flow and eliminate the large-scale vortex shedding in the wake, leading to a decrease in drag of approximately 45%. Low-frequency excitation is employed to target the instabilities associated with the separated shear layer and vortex shedding in the wake. Excitation of the wake instability also causes the flow to reattach; however, it leads to organization of the large-scale vortex shedding. By forcing the boundary layer at the frequency of the shear-layer instability, the threshold momentum required to reattach the flow is an order of magnitude smaller as compared with high-frequency excitation,...

On hairpin vortex generation from near-wall streamwise vortices

Abstract: The generation of a hairpin vortex from near-wall streamwise vortices is studied via the direct numerical simulation (DNS) of the streak transient growth in the minimal channel flow at $$Re_\tau =400$$ . The streak profile is obtained by conditionally averaging the DNS data of the fully developed turbulent channel flow at the same Reynolds number. The near-wall streamwise vortices are produced by the transient growth of the streak which is initially subjected to the sinuous perturbation of the spanwise velocity. It is shown that the arch head of the hairpin vortex first grows from the downstream end of the stronger streamwise vortex and then connects with the weaker, opposite-signed streamwise vortex in their overlap region, forming a complete individual hairpin structure. The vorticity transport along the vortex lines indicates that the strength increase and the spatial expansion of the arch head are due to the stretching and the turning of the vorticity vector, respectively. The hairpin packets could be further produced from the generated individual hairpin vortex following the parent-offspring process. Generation of hairpin vortex from a pair of counter-rotating streamwise vortices due to stretching and turning effects. (a1)(b1)(c1) perspective view of vortex and strain vectors; (a2)(b2)(c2) distribution of stretching and turning terms along the vortex line projected to y - z plane. Meshed surface: $$\lambda _{ci} = 10$$ ; black vector: strain vector; red solid line: vortex line; dashed line: turning; dash-dotted line: stretching.

Flow development and structural loading on dual step cylinders in laminar shedding regime

Abstract: The flow development over a dual step cylinder is investigated numerically at a Reynolds number (ReD) of 150 for a range of aspect ratios, 0.2 ≤ L/D ≤ 5, and diameter ratios, 1.1 ≤ D/d ≤ 4. The results reveal the following four distinct types of wake topology downstream of the larger diameter cylinder: (i) shedding of hairpin vortices, (ii) transient asymmetric shedding, (iii) primarily spanwise shedding, and (iv) no vortex shedding. Dominant vortex interactions are reconstructed for each regime. These interactions, involving half-loop vortex connections, vortex merging, and direct vortex connections are shown to occur periodically as the large and small cylinder structures undergo vortex dislocations. Topological schematics are introduced to relate the characteristic frequencies to the periodic vortex interactions. The observed types of wake topology are shown to produce distinctly different mean and fluctuating forces on the dual step cylinder. For lower aspect and diameter ratios (L/D ∼ 1 and D/d ∼ 1.5...