Dynamic Lift Research Articles

Experimental study of the self-excited resonance effect on the dynamic lift and flow structure around inline cylinders

The excitation of acoustic resonance by air flow around a row of inline cylinders is investigated experimentally using phase-locked particle image velocimetry and direct measurements of the dynamic lift forces. The experiments are performed during self-excited acoustic resonance for arrangements of two, three, four, and five inline cylinders with a spacing ratio L∕D=2.0, where L is the center-to-center distance between the cylinders and D is the cylinder diameter, to clarify the effect of increasing the number of cylinders on the resonance mechanism and the characteristics of the periodic flow cycle. The PIV measurements show that increasing the number of cylinders results in weaker and distorted periodic flow structures around the most downstream cylinders. However, the phase and strength of the flow structures within the most upstream gap are minimally affected. Intrinsic flow features, such as gap flow that is observed in some cases, lead to the disruption of the shear layer impingement on downstream cylinders. Moreover, direct measurements of the dynamic lift forces on all cylinders simultaneously identified the upstream gap as the location where most of the energy exchange between the flow and the acoustic fields occurs. Decomposition of the dynamic lift force with respect to the acoustic pressure revealed that the distorted flow structures were accompanied by weaker, sometimes negative, contribution to the acoustic resonance excitation. The findings presented in this article explain the observation that for inline configurations of cylinders, as well as inline tube bundles, increasing the number of cylinders in the flow direction does not result in stronger resonance, and demonstrate that countermeasures for resonance excitation should target the most upstream rows of cylinders.

How does LBP influence muscle activity during a cyclical dynamic lifting task?

How does LBP influence muscle activity during a cyclical dynamic lifting task?

Dynamic Analysis and Design of an Innovative Sit-to-Stand Gait Trainer

Abstract This paper illustrates three examples of how to implement dynamic lifting support, including the solution adopted in the Up n’ Free® (Easy-Walking Inc., Maple Glen, PA). The Up n' Free system can lift the user from a seated posture to a standing position through the employment of a four-bar mechanism and gas piston. When the user is standing, the mechanism can provide partial compensation from gravity to help the user regain mobility. The compensation can be provided while standing without taxing the arms and shoulders with the use of crutches. By using this system, users can sit and stand without assistance from another person and can also receive compensation for gravity loads while walking. This group is not involved with the design and commercialization of the Up n' Free product but uses that design as a reference of comparison for two additional actuation solutions, one of which involves a cam acting on a rubber band. The piston in the Up n' Free is positioned between the bottom part of the walker frame and the four-bar mechanism. We propose a more compact configuration where the piston is located within the four-bar mechanism. This work discusses the pros and cons of the two gas spring cylinder configurations. Common problems arising with both configurations are the difficulty of regulating the minimum and maximum force at the seat, as well as the inability to define a force profile for the transition from sitting to standing. Thus, a new actuation system to implement the dynamic lifting support is proposed, where the force profile is modulated using a cam acting on an elastic band. We were able to define a linear profile of the force during the transition from sit-to-stand, thereby avoiding sudden increases in force that are typical of actuation solutions encompassing gas pistons acting on a variable moment arm.

Testing to Identify Submaximal Effort: Lifting to a Perceived 50% Effort vs. an Assigned Submaximal Load.

Swift, MC, Townsend, R, Edwards, D, and Loudon, J. Testing to identify submaximal effort: Lifting to a perceived 50% effort vs. an assigned submaximal load. J Strength Cond Res 36(8): 2115-2120, 2022-The ability to accurately measure effort during postinjury functional testing allows for the validation of displayed physical limitations by injured workers. The Cross-Reference Testing System (XRTS) has been developed to identify submaximal efforts through distraction-based dynamic material handling testing. The XRTS material handling assessment compares dynamic lifts of weights in a crate and lifts using a lever arm device. The purpose of this study was to determine whether subjects lifting an assigned submaximal load influence test results compared with subjects lifting to but not exceeding a 50% perceived effort. Subjects in group A ( n = 35) were assigned the condition to attempt to lift to but not exceed a randomly assigned weight value for both the crate lift and XRTS at 3 lifting heights. Subjects in group B ( n = 32) were asked to lift to but not exceed 37.5 lb from the same 3 lifting heights. The reproducibility of effort was measured with current validity criteria for distraction-based material handling testing. Using the percent difference values, a 2 × 3 (group, lifting height) analysis of variance (ANOVA) was performed to test the hypothesis. The alpha level was set at 0.05. The mean percent change between comparative lifts was 31.13%, 95% CI (22.51-39.75) for group A and 29.26% 95% CI (21.91-36.61) for group B. The 2 × 3 ANOVA demonstrated no significant difference ( p = 0.751) between groups. The results indicate attempting to lift to a perceived 50% effort was not significantly different from attempting to lift to but not exceed an assigned submaximal load.

Controlled axisymmetric body-wake coupling in a pitching motion

The coupling between a pitching axisymmetric bluff body and its wake is modified in wind tunnel experiments using controlled interactions between fluidic actuators and the cross-flow over its aft end. The model trajectory is controlled using eight servo-controlled support wires with inline force transducers that are operated in closed loop with feedback from a motion analysis system which effects control authority of the wind tunnel model in all six degrees of freedom. The model dynamics studied in the present work is constrained to pitch. Actuation is effected by two integrated aft-facing synthetic jet actuators in the plane of the body pitch oscillation such that each actuator effects a time-dependent segment of local flow attachment over the aft surface. The present investigation focuses on the reciprocal relation between the response of the near and far wake to the actuation and the associated changes in the induced aerodynamic loads when the body executes nearly time-harmonic pitch over a range of reduced oscillation frequencies (up to k = 0.26) for ReD up to 2.3 × 105. The response of the wake to stabilizing and destabilizing actuations that effect reduction or enhancement of the aerodynamic loads is investigated using particle image velocimetry (PIV) and hotwire anemometry in the near and far wake, respectively. It is shown that the studied flow control approach induces aerodynamic loads that are comparable to the loads on the base flow pitching motions and therefore may be suitable for in-flight stabilization. The global unsteady aerodynamic loads on a pitching axisymmetric bluff body are controlled by modification of the coupling between the flow over body and its near wake using two synthetic jet actuators in the plane of motion on the body’s aft end. This actuation results in a time-dependent partial flow attachment of the nominally axisymmetric separating shear layer of the base flow over the aft end of the body, and the asymmetric changes have a profound effect on the evolution of the wake. These changes in the wake dynamics feed back to alter the global aerodynamic forces and moments, leading to about a 50% increase or decrease in the dynamic lift force or pitching moment, depending on the flow control strategy.

Sensitivity of musculoskeletal model-based lumbar spinal loading estimates to type of kinematic input and passive stiffness properties

The study investigated the potential for obtaining more accurate spine joint reaction force (JRF) estimates from musculoskeletal models by incorporating dynamic stereo X-ray imaging (DSX)-based in vivo lumbar vertebral rotational and translational kinematics compared to generic, rhythm (RHY)-based kinematics, while also observing the influence of accompanying inputs: intervertebral segment stiffness and neutral state. A full-body OpenSim® musculoskeletal model, constructed by combining existing lower- and upper-body models, was driven based on one volunteer’s (female; age 25; 60.8 kg; 176 cm) anthropometrics and kinematics from a series of upright standing and straight-legged dynamic lifting tasks. The lumbar spine portion was modified in a step-wise manner to observe effects of: (1) RHY vs. DSX lumbar kinematics; (2) No disc (bushing) stiffness (NBS); generic, linear bushing stiffness (LBS); subject-specific nonlinear bushing stiffness (NLBS); (3) Upright standing (UP) vs. Supine (SUP) neutral state; (4) Weight lifted: 4.5 kg vs. 13.6 kg. L4L5 JRF from 24 model variations based on combinations of aforementioned parameters were compared. Rhythm-based kinematics without translational components tends to over-predict JRF (31% and 39% for compression and shear, respectively) compared to DSX-based kinematics. Additionally, differences due to accompanying passive stiffness and neutral state choice combinations were even larger (>50%), indicating heightened demand on the quality of these accompanying inputs. The study not only highlights model sensitivity to choices made regarding the three primary inputs—kinematics, passive stiffness and neutral state— separately, but also how interactions between these choices can result in significant variability in joint loading estimates.

Effect of Wing Tip Vortex on Dynamic Lift Generation

Effect of Wing Tip Vortex on Dynamic Lift Generation

Research on two-stroke compression release braking performance of a variable mode valve actuation system

In order to enhance the braking safety and improve the fuel economy and emission of heavy-duty engines, a variable mode valve actuation system which can switch operation modes flexibly among four-stroke driving, two-stroke compression release braking, and cylinder deactivation modes on a four-stroke engine was developed in this article. The switching was controlled by the four-stroke driving module and the two-stroke braking module, both of which have two states: effective state and failure state. Besides, a full-cycle numerical model of a six-cylinder turbocharged engine was established for the performance analysis of two-stroke compression release braking mode. The orthogonal design method was introduced in the present study to obtain the optimum valve parameters which can result in high braking power and maintain the maximum cylinder pressure at a lower level at the same time. Then, the two-stroke compression release brake braking power with the optimum valve parameters was compared with four-stroke compression release braking power. Meanwhile, the two-stroke braking cam profile of the variable mode valve actuation system was designed according to the optimum valve parameters, and the two-stroke braking performance with the dynamic valve lift of the variable mode valve actuation system was validated by the numerical model including the hydraulic system. The results indicated that a higher engine speed leads to higher braking power at the same valve lift. Besides, two-stroke compression release brake braking power of the variable mode valve actuation system achieved 525.3 kW at 2600 r/min and 358.1 kW at 1900 r/min, 52.9% and 71.3% higher than four-stroke compression release braking power, respectively. Although the two-stroke compression release brake braking power with dynamic valve lift is slightly less than that with the optimum valve parameters, it is still much higher than that of the four-stroke compression release braking power. Therefore, it has a good application prospect for heavy-duty engines.

Development of a novel harness system to aid in rehabilitation of horses

Development of a rehabilitation harness to aid in recovery from musculoskeletal injuries is needed because serious complications can arise from long-term use of rescue slings. This study’s objective was to determine the anatomical structures of the horse that can bear significant weight, the potential complications that could arise if a horse is not properly supported by the harness and the % weight compensation achievable with the newly developed harness when used together with a dynamic rehabilitation lift. This dynamic lift can reduce the load the limbs carry, either withers-to-pelvis or left-to-right when used in combination with the rehabilitation harness under development. The rehabilitation harness prototype described here was made of cotton/nylon with sheepskin inserts, forming a blanket with high-strength strapping supporting the load-bearing structures of the horse. This prototype was load tested up to 600 kg, for safety, with no sign of failure. In an adult horse, the harness allowed for 40% load reduction from both front (125 of 303 kg [60% of 506 kg]) and hind (80 of 203 kg [40% of 506 kg]) legs before complications (abnormal posture) occurred. Pressure was measured to determine areas of high pressure which lead to the addition of an H-frame and a figure-eight pattern of strapping to the forelimb support reducing pressure, improving posture and achieving greater load reduction (46% [140 of 301.2 kg]). Abnormalities in respiratory rate or pattern were not observed. Future research will include testing the harness longer term (up to six weeks) with the incorporation of an air-pressurised breastplate to detect high-pressure, high-temperature, high-moisture areas, modifying the design further for improved horse-comfort reducing the risk of complications and enabling long-term use of the harness during rehabilitation.

Initial testing of a computer-integrated weight compensation system for rehabilitation of horses

Management of severely injured or neurologic horses is challenging, with ambulatory support limited to static lifts and rescue slings. The aim of this study was initial testing and adjustment of a novel computer-integrated dynamic lift system, including measuring effects of increasing weight compensation (i.e. load reduction) and time attached to the lift. This automated system was developed to improve outcomes and reduce complications in horses with ambulatory difficulties, allowing for controlled mobility and varying load carried by the horse with independent front and hind limb support. Two healthy Thoroughbred horses were studied using the Anderson rescue sling. The lift was programmed to respond to weight and movement of horses. Weight compensation (% bodyweight) was incrementally increased, for front and hind limbs, to maximum percent tolerated, based on heart/respiratory rates and behavioural scoring. The time attached to the lift was then incrementally increased at maximum tolerated weight compensation previously determined. Measures included heart/respiratory rates, behavioural scoring, muscle enzyme activity and blood flow to distal limbs. Results were analysed descriptively. Avoidance behaviour was observed at front and hind end weight compensation of 18 and 4%, respectively. Average maximum time attached to the lift was 2.25 hours. After 60 minutes, respiratory rate increased >20 breaths (b)/minute, reaching 60 b/minute in one horse and 36 b/minute in the other, with shallow breathing. Other measures remained normal. In conclusion, lift programming was successful for weight compensation and mobility during lift support. Complications included avoidance behaviour and respiratory distress at >20% weight compensation, likely caused by the Anderson rescue sling. To address these limitations, a new rehabilitation harness better suited for long-term use is under development.

Troubleshooting of Von Karman Induced Penstock Protection Valve Resonance at FMHL+

The FMHL power plant was originally a 240 MW pumped-storage power plant in Switzerland, put in operation in 1971 which installed capacity was extended to 480 MW in 2016 with maximal output power set to 420 MW including 60 MW as reserve. The significant increase of the total discharge of the power plant in generating mode from 32 m3/s at 240 MW to 57 m3/s at 420 MW might be critical due to the higher flow velocity throughout the different safety valves of the power plant. During commissioning, the discharge was increased stepwise from 32 to 57 m3/s while monitoring the vibration of the safety valves. Significant vibration amplitudes have been recorded at a frequency of 62.5 Hz on the penstock protection valve for a discharge of 43 m3/s. Von Karman vortex shedding induced resonance on the stiffener of the bi-plan valve obturator was then suspected. The paper presents troubleshooting of this resonance problem, where CFD and FEM investigations have been performed to define appropriate trailing edge modifications of the obturator stiffener plates able to significantly reduce the Von Karman dynamic lift coefficient and thus reduce drastically vibration amplitudes.

Effects of a passive back exoskeleton on the mechanical loading of the low-back during symmetric lifting

Low-back pain is the number one cause of disability in the world, with mechanical loading as one of the major risk factors. Exoskeletons have been introduced in the workplace to reduce low back loading. During static forward bending, exoskeletons have been shown to reduce back muscle activity by 10% to 40%. However, effects during dynamic lifting are not well documented. Relative support of the exoskeleton might be smaller in lifting compared to static bending due to higher peak loads. In addition, exoskeletons might also result in changes in lifting behavior, which in turn could affect low back loading.The present study investigated the effect of a passive exoskeleton on peak compression forces, moments, muscle activity and kinematics during symmetric lifting. Two types (LOW and HIGH) of the device, which generate peak support moments at large and moderate flexion angles, respectively, were tested during lifts from knee and ankle height from a near and far horizontal position, with a load of 10 kg.Both types of the trunk exoskeleton tested here reduced the peak L5S1 compression force by around 5–10% for lifts from the FAR position from both KNEE and ANKLE height. Subjects did adjust their lifting style when wearing the device with a 17% reduced peak trunk angular velocity and 5 degrees increased lumbar flexion, especially during ANKLE height lifts.In conclusion, the exoskeleton had a minor and varying effect on the peak L5S1 compression force with only significant differences in the FAR lifts.

Evaluation of a Trunk Supporting Exoskeleton for reducing Muscle Fatigue

Although the effect of wearing a back-support exoskeleton during lifting has been demonstrated to reduce overall muscle activation, less is known about how wearing exoskeletons affect muscular fatigue and oxygen consumption. The purpose of this study is to evaluate the effect of wearing a back-support exoskeleton (backX) on muscle fatigue during repetitive lifting by assessing whether wearing backX increases endurance time relative to lifting unassisted. A secondary objective of this study is to quantify changes in oxygen consumption rate while performing a repetitive lifting task with and without backX to address a common industry concern. The UC Ergonomics Lab evaluated backX on twelve male subjects by measuring bilateral muscle activity of the erector spinae and oxygen consumption rate. Summary measures of muscle activity for 50 and 90 percent of the repetitive lifting session were used to characterize peak and mean muscle activity. Oxygen consumption rate was collected continuously during the repetitive lifting session. Compared to the unassisted condition, wearing backX reduced peak lumbar erector spinae activation by 16.5% and 21.8% (p < 0.05). The time subjects could hold a back-straining posture after the repetitive lifting session increased by 52% after wearing backX during the lifting task. The was no significant negative change in oxygen consumption rate. This study confirms that wearing a backX reduces muscle activation in the lower back for this specific dynamic lifting task. Additionally, we find that wearing a backX may reduce the risk of low back injuries by reducing muscle activity and increasing endurance time to fatigue

Roles of land-surface properties and terrains on Maritime Continent rainfall and its seasonal evolution

The Maritime Continent (MC) is characterized by a seasonal evolution of rainfall distinct from other regions, due to its unique land–sea distribution and topography. In this study, the roles of surface properties and terrains in controlling the regional climatological rainfall were investigated, based on general circulation model experiments. Results show that the existence of terrain can increase the MC land (MCL) rainfall mainly through its dynamical lifting effect, but otherwise has only moderate influence on rainfall over the MC ocean (MCO). On the other hand, the impact of MC land–sea distribution on the regional rainfall is more seasonally dependent. When replacing the MC flat-land with ocean, rainfall is significantly increased over both MCL and MCO during boreal summer-to-fall, but not in the winter-to-spring season. Further inspection showed that by eliminating the MC flat-land, there is enhanced atmospheric water vapor and convective instability in the summer-to-fall period, contributing to a dramatic increase in precipitation. On the other hand, changes in convective instability and atmospheric water vapor over the MCL act to counteract each other, leading to an only moderate change in rainfall during boreal winter-to-spring. The model-based results suggest that this seasonally dependent influence of the MC flat-land on regional climate mean rainfall is not determined by its modulation on the diurnal cycle. Our results also suggest a larger sensitivity of model bias in representing land–sea distribution/fraction over the MC region during the dry season (i.e. boreal summer) than in the wet season (i.e. boreal winter).

Numerical Simulation of Seakeeping Performance on the Preliminary Design of a Semi-Planing Craft

This study established a seakeeping program to evaluate the motion responses of a high speed semi-planing craft and to develop a database for future route planning. A series 62 mono-hull was chosen for the test cases, comparing seakeeping performances with full-scale on-board measurements. The statistical results were obtained using spectral analysis, which combines the International Towing Tank Conference (ITTC) spectrum with the response amplitude operator (RAO) responses of each wave heading for a given sailing speed. The speed polar diagram was made to illustrate five degree-of-freedom (DOF) motion responses between sailing speeds and wave heading angles in a particular sea state. Although the craft has different trim angles at high speeds (because of dynamic lift) under various loading and draft conditions, this study only investigated the trim angles of 0° (even keel), 1° by the stern, and 2° by the stern, to understand the difference between their seakeeping performances. The results in this study provide a useful guideline for evaluating operational regulations and safety for high speed semi-planing crafts in the future.

Influence of experimental pain on the spatio-temporal activity of upper trapezius during dynamic lifting – An investigation using Bayesian spatio-temporal ANOVA

High-density surface electromyography (HDsEMG) provides a detailed analysis of a muscle’s spatial distribution of activity. We applied a Bayesian spatio-temporal statistical method to quantify how acute nociception and task repetition alters the upper trapezius instantaneous spatial distribution of activity during dynamic muscular contractions. Ten male adults performed repeated lifting of a 1 kg box between shelves positioned at hip and shoulder heights with a cycle time of 3 s for 50 cycles under four conditions: baseline, isotonic and hypertonic saline injections (nociception) to the right upper trapezius, and 15 min post injection. Activity of the right upper trapezius was measured using a 64-channel surface electrode grid. Statistical inference was performed using Integrated Nested Laplace Approximations (INLA), and significance was determined by a non-zero crossing of the Bayesian 95% credible intervals (CrI). The maximal decrease in activity after nociception was −38.1 µV [95% CrI −40.9 to −35.3] at 30% of the lift cycle when compared to baseline. The maximal reduction in muscle activity between the early and later phases of lifting in the presence of nociception was by 10.4 µV [95% CrI 8.2–12.6]. A more holistic understanding of muscle behaviour is achieved using spatio-temporal inference than traditional reductionist methods.

Sensorimotor memory for object weight is based on previous experience during lifting, not holding

To allow skilled object manipulation, the brain must generate a motor command specifically tailored to the object properties. For instance, in object lifting, the forces applied by the fingertips must be scaled to the object's weight. When lifting a series of objects, forces are usually scaled according to recent experience from previously lifted objects, an effect often referred to as sensorimotor memory. In this study, we investigated the specific time period during which stored information from previous object manipulation is used to mediate sensorimotor memory. More specifically, we examined whether sensorimotor memory was based on weight information obtained between object contact and lift completion (lifting phase) or during stable holding (holding phase). Participants lifted light and heavy objects in a randomised order in virtual reality that could increase or decrease in weight after the object was lifted and held in the air. In this way, we could distinguish whether the force planning in the next lift was scaled depending on weight information gathered from either the dynamic lifting or static holding period. We found that force planning was based on the previous object weight experienced during the lifting, but not holding, phase. This suggest that the lifting phase is a key time period for building up sensorimotor memory for planning future hand-object interactions.

Effects of leading edge defects on aerodynamic performance of the [formula omitted] airfoil

Defects at the leading edge of blades are an important source of power loss for wind turbines. In the present work, a systematic study is carried out on two typical types of defects, i.e., the surface concaved deep defects and surface distributed shallow defects on the S809 airfoil. Different defect shapes, ranges, equivalent depths and their influence mechanisms are investigated via CFD. For deep defects, an enclosed vortex is formed in the defect cavity, which suppresses the momentum exchange between the external flow and internal flow, so that the airfoil aerodynamic performance is highly sensitive to the defect opening range and is little affected by the defect shape and equivalent depth. Flow around the leading edge is strongly hindered by the deep defects and an elongated leading-edge separation bubble is formed at the suction side of the airfoil. At large angles of attack, flow at the suction side is dominated by flow separation at both the leading edge and trailing edge. In contrast, for shallow defects, all the defect equivalent depth, opening range and shape can significantly influence the airfoil aerodynamic performance; and quantitatively, the effect of defect equivalent depth is the most significant. For the present simulation cases with defects, the maximum lift coefficient is notably decreased (by 35% to 61%) accompanied by a sharp increase in drag coefficient (by 131% to 217%). Under dynamic pitching motions, the opening of the dynamic lift (drag)-coefficient hysteresis curve is effectively enlarged. The present work aims to provide an important reference for the maintenance and management of turbine blade defects.

The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

AbstractThis paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.

Wake structures and acoustic resonance excitation of a single finned cylinder in cross-flow

This paper presents an experimental and numerical investigation to clarify the effect of circular fins on the wake structures, the dynamic loading, as well as the flow–sound interaction mechanism downstream of a single finned cylinder in cross-flow. It is revealed that adding circular fins to the cylinder enhances the flow coherence along the cylinder’s span, strengthens the intensity of the vortex shedding process downstream of the cylinder, reduces the vortex formation length, and increases the dynamic lift force acting on the cylinder. Thus, adding circular fins to the cylinder makes the flow more susceptible to acoustic resonance excitation, compared to an equivalent bare cylinder. Three-dimensional numerical simulations of the flow field around the cylinder show that the flow is entrained between the fins which results in the generation of an accelerated jet-like flow in the developing shear layer region. The jet-like flow emanating from the fins leads to a closer and stronger shear layer roll-up, and hence a shorter vortex formation region. This, in turn, increases the fluctuating lift force on the cylinder, as the strong oscillating shear layers occur closer to base of the cylinder. Additionally, with the reduction of the vortex formation length, the base pressure coefficient decreases and therefore the drag force on the cylinder increases. These intrinsic flow features explain the global effect of circular fins on the vortex shedding process and there by on the excitation mechanism of acoustic resonance.