Active Drag Research Articles

Reliability of estimating active drag in swimming using the assisted towing method with fluctuating speed

The reliability of active drag values was examined using a method that compared free swim speed with measurements taken by towing swimmers slightly faster than their maximum swim speed, while allowing their intra-stroke speed fluctuations. Twelve national age and open level swimmers were tested on two alternate days (Day 1 and Day 2). All participants completed four maximum swim speed, three passive drag and five active drag trials on each of the days. The reliability was determined using within-participant intra-class correlation coefficients (ICC) within each day and between the days. The ICCs for Day 1 and Day 2 were 0.82 and 0.85, respectively, while the comparison of the mean active drag values between days was 0.93. The data showed that the assisted towing method (ATM) with fluctuating speed was only moderately reliable within a single test. However, this method was more reliable when using the average value of active drag from both days (ICC = 0.93). This study identified that the ATM method with fluctuating speed had moderate reliability within-participant trials on values in a single day but high reliability for the average active drag values across different days.

AquaTrainer® Snorkel does not Increase Hydrodynamic Drag but Influences Turning Time.

Our purpose was to verify if the use of the new AquaTrainer(®) respiratory snorkel lead to an increase of front crawl hydrodynamic drag and whether the constraint of using an adapted turning technique influences its corresponding turning time. 12 swimmers performed 2 (without and with snorkel) 12×25 front crawl repetitions from low to maximal velocity on the measuring active drag system. Complementarily, 3 swimming turns were compared: open turn with snorkel, tumble turn and open turn without snorkel. Drag values were similar without vs. with snorkel at 0.9, 1.1, 1.3, 1.5 and 1.7 m.s(-1) velocities: 15.84 ±5.32 vs. 16.18±4.81, 25.60±6.69 vs. 26.03±6.17, 38.37±8.04 vs. 38.88±7.56, 54.64±10.06 vs. 55.08±9.55, 74.77±14.09 vs. 74.92±13.14 N, (respectively, p≥0.05), and high agreement between conditions was observed (p<0.01). Front crawl swimming with snorkel using the open turn implied an increase in turning time of 14.2 and 5.1% than the tumble turn and open turn without the apparatus (p<0.01). AquaTrainer(®) snorkel does not lead to an increase in active drag during front crawl performed at a large range of velocities and, consequently, the metabolic energy necessary to overcome total drag will not be affected. However, turning with it requires an additional time that should be taken into account in scientific research and training conditions.

Experimental and numerical investigation of transversal traveling surface waves for drag reduction

The active drag reduction impact of spanwise transversal surface waves on a turbulent boundary layer is investigated experimentally and numerically. The actuated surface in the experimental setup is an insert in a flat aluminum plate being excited by an electromagnetic actuator system. Particle-image velocimetry and μ-particle-tracking velocimetry measurements are conducted to determine the local drag reduction downstream of the actuated surface. Furthermore, large-eddy simulations are performed to corroborate that the overall friction decrease can be determined by the local skin friction measurements downstream of the actuated surface. The Reynolds number based on the momentum thickness located directly downstream of the actuated surface is Reθ=1300. The spanwise traveling wave has an amplitude of A+=9, a wave length of λ+=3862, and a period of T+=115. The experimental and numerical results of the velocity distributions show an excellent agreement upstream and downstream of the actuated surface. The low amplitude which is caused by the actuation constraints due to the technically relevant material aluminum leads to low drag reduction values of 1.27% and 0.89% in the simulation and the measurement. A dominant influence of the transition from the actuated wall to the non-actuated surface is excluded. Thus, a detection of the impact of the wall motion by local skin friction measurements downstream of the actuated surface is feasible.

Optimizing Jets for Active Control of Wake Refinement for Ground Vehicles

The present study investigates active drag reduction of an Ahmed body with a rear slant angle of 25 deg. The drag is reduced by implementing slot jets on the rear slant and rear surface of the Ahmed body. Transient numerical experiments were conducted using the improved delayed detached eddy simulation (IDDES) turbulence model. Jet velocity, position, size, and angle were parametrically varied, and time-averaged drag coefficients for various jet configurations were calculated. Reynolds numbers based on the length of the Ahmed body were varied, but special focus was given to the high-drag case when Re = 1.4 × 106. It was found that by using slot jets at the rear and rear slant, the drag coefficient was reduced by 22%. In order to investigate the physical mechanisms for the reduction in drag, proper orthogonal decomposition (POD) was used to visualize the turbulent coherent structures in the near wake of the Ahmed body.

Turbulent drag reduction by spanwise traveling ribbed surface waves

The combination of passive and active drag reduction techniques is investigated by particle-image velocimetry (PIV) and micro-particle tracking velocimetry (μ-PTV) measurements. To be more precise, the impact of a riblet-structured surface, which represents the passive control technique, undergoing spanwise transversal wave motion, which defines the active flow control method, on the wall-shear stress distribution of a zero-pressure gradient turbulent boundary layer is analyzed. The experimental setup consists of a flat plate equipped with an insert to generate a spanwise traveling wave on a riblet-structured aluminum sheet. The PIV and μ-PTV measurements are conducted downstream of the actuated riblet surface at two momentum thickness based Reynolds numbers Reθ=1200 and 2080. The surface deformation is generated by an electromagnetic actuator system at three amplitudes A= 0.25, 0.30, and 0.375 mm with a wavelength of λ=160 mm and a frequency of f=81 Hz. The nondimensional quantities in inner wall units defined by the friction velocity of the non-actuated flat plate flow at Reθ=1200 are the amplitudes A+=6, 7, and 9, the wavelength λ+=3862, and the frequency T+=110 and the corresponding quantities for Reθ=2800 are A+=11, 14 and 17, λ+=7170, and T+=380. The results are twofold. On the one hand, the combination of the riblet surface with the transversal wave motion increases the local drag reduction (DR). That is, compared to a boundary layer flow over a non-actuated smooth surface a local drag reduction of up to DR =9.4 % is determined at the chosen measurement position whereas the drag reduction due to the riblet surface is DR =4.7%. On the other hand, the dependence of the local drag reducing efficiency of the riblets on their geometric dimensions with respect to the flow field could be decreased. That is, the transversal wave motion extends the positive drag reducing efficiency range of the riblets.

Experiments on Active Drag Reduction on a Complex Outer Wing Model

The potential of active flow control by means of pulsed blowing to counter performance degradation at high angles of attack is demonstrated on a modern civil aircraft outer wing configuration provided by the European aeronautical industry. Flow control actuators are incorporated into the leading edge between the slat edge and the wingtip, where the wing’s slender shape and its high local curvature do not allow for the integration of mechanical leading-edge devices. Surface flow visualization identifies the highly three-dimensional topology of the uncontrolled flow, which is dominated by the slat edge vortex and leading-edge flow separation at high incidence angles. The influence of the varying-momentum coefficient on drag reduction, lift gain, and aerodynamic efficiency is studied, considering force and pressure measurements, as well as flow visualization. Results indicate that a momentum coefficient of suffices to reduce drag by up to 38%, compared to the uncontrolled baseline flow, and to offset aerodynamic performance degradation by 4 deg if the actuation effort is distributed efficiently along the span.

Relationships between coordination, active drag and propelling efficiency in crawl

This study examines the relationships between the index of coordination (IdC) and active drag (D) assuming that at constant average speed, average drag equals average propulsion. The relationship between IdC and propulsive efficiency (ep) was also investigated at maximal speed. Twenty national swimmers completed two incremental speed tests swimming front crawl with arms only in free condition and using a measurement of active drag system. Each test was composed of eight 25-m bouts from 60% to 100% of maximal intensity whereby each lap was swum at constant speed. Different regression models were tested to analyse IdC–D relationship. Correlation between IdC and ep was calculated. IdC was linked to D by linear regression (IdC=0.246·D−27.06; R2=0.88, P<.05); swimmers switched from catch-up to superposition coordination mode at a speed of ∼1.55ms−1 where average D is ∼110N. No correlation between IdC and ep at maximal speed was found. The intra-individual analysis revealed that coordination plays an important role in scaling propulsive forces with higher speed levels such that these are adapted to aquatic resistance. Inter-individual analysis showed that high IdC did not relate to a high ep suggesting an individual optimization of force and power generation is at play to reach high speeds.

Planimetric frontal area in the four swimming strokes: Implications for drag, energetics and speed

The purpose of this study was to use the planimetric method to determine frontal area (Ap) throughout the stroke cycle in the four swimming strokes as well as during “streamlined leg kicking”. The minimum Ap values in all strokes are similar to those assessed during “streamlined leg kicking” (about 0.13m2). Active drag (Da=1/2ρ Cd Ap v2) was then calculated/estimated based on the average Ap values, as calculated for a full cycle in each condition. Da is the lowest in the “streamlined leg kicking” condition (Da=19.5v2, e.g., similar to the values of passive drag reported in the literature), is similar in front crawl (Da=30.0v2), backstroke (Da=26.9v2) and butterfly (Da=28.5v2) and is the largest in the breaststroke (Da=37.5v2).Based on the C vs. v relationships reported in the literature for the four strokes it is then possible to estimate drag efficiency: for a speed of 1.5ms−1, it ranges from 0.035–0.038 (breaststroke and backstroke, respectively) to 0.052–0.058 (butterfly and front crawl, respectively). This study is the first to establish Ap values throughout the swimming cycle for all swimming strokes and these findings have implications for active drag estimates, for the energetics of swimming and for swimming speed.

Passive drag reduction of square back road vehicles

Bluff body vehicles such as trucks and buses do not have a streamlined shapes and hence have high drag which can be reduced to make great savings in operational cost. While rectangular flaps have been widely studied as both passive add-ons and in active drag reducing systems for bluff bodies, changing the basic geometry of the flap has not been explored in literature. In this work, a baseline drag value is obtained for a simplified MAN TGX series truck in a CFD software, and the drag reduction of a proposed elliptically shaped flap is compared to aerodynamically equivalent rectangular flaps. The optimal mounting angle for both flaps is found to be 50°. A parametric study of changing the ellipse semi-major axis is carried out to find the optimal length for drag reduction. A maximum drag reduction of 11.1% is achieved using an elliptical flap with 0.12m semi-major axis; compared to 6.37% for a length equivalent rectangular flap, and 6.84% for a surface area equivalent rectangular flap. Results of the pressure distribution and velocity flow behind the rear of the truck are also given and analyzed.

Longitudinal modeling in sports: Young swimmers’ performance and biomechanics profile

New theories about dynamical systems highlight the multi-factorial interplay between determinant factors to achieve higher sports performances, including in swimming. Longitudinal research does provide useful information on the sportsmen’s changes and how training help him to excel. These questions may be addressed in one single procedure such as latent growth modeling. The aim of the study was to model a latent growth curve of young swimmers’ performance and biomechanics over a season. Fourteen boys (12.33±0.65years-old) and 16 girls (11.15±0.55years-old) were evaluated. Performance, stroke frequency, speed fluctuation, arm’s propelling efficiency, active drag, active drag coefficient and power to overcome drag were collected in four different moments of the season. Latent growth curve modeling was computed to understand the longitudinal variation of performance (endogenous variables) over the season according to the biomechanics (exogenous variables). Latent growth curve modeling showed a high inter- and intra-subject variability in the performance growth. Gender had a significant effect at the baseline and during the performance growth. In each evaluation moment, different variables had a meaningful effect on performance (M1: Da, β=−0.62; M2: Da, β=−0.53; M3: ηp, β=0.59; M4: SF, β=−0.57; all P<.001). The models’ goodness-of-fit was 1.40⩽χ2/df⩽3.74 (good-reasonable). Latent modeling is a comprehensive way to gather insight about young swimmers’ performance over time. Different variables were the main responsible for the performance improvement. A gender gap, intra- and inter-subject variability was verified.

Cluster stability as a new method to assess changes in performance and its determinant factors over a season in young swimmers.

To apply a new method to identify, classify, and follow up young swimmers based on their performance and its determinant factors over a season and analyze the swimmers' stability over a competitive season with that method. Fifteen boys and 18 girls (11.8±0.7 y) part of a national talent-identification scheme were evaluated at 3 different moments of a competitive season. Performance (ie, official 100-m freestyle race time), arm span, chest perimeter, stroke length, swimming velocity, speed fluctuation, coefficient of active drag, propelling efficiency, and stroke index were selected as variables. Hierarchical and k-means cluster analysis were computed. Data suggested a 3-cluster solution, splitting the swimmers according to their performance in all 3 moments. Cluster 1 was related to better performances (talented swimmers), cluster 2 to poor performances (nonproficient swimmers), and cluster 3 to average performance (proficient swimmers) in all moments. Stepwise discriminant analysis revealed that 100%, 94%, and 85% of original groups were correctly classified for the 1st, 2nd, and 3rd evaluation moments, respectively (0.11≤Λ≤0.80; 5.64≤χ2≤63.40; 0.001<P≤.68). Membership of clusters was moderately stable over the season (stability range 46.1-75% for the 2 clusters with most subjects). Cluster stability is a feasible, comprehensive, and informative method to gain insight into changes in performance and its determinant factors in young swimmers. Talented swimmers were characterized by anthropometrics and kinematic features.

Hydrodynamic profile of young swimmers: changes over a competitive season.

The aim of this study was to analyze the changes in the hydrodynamic profile of young swimmers over a competitive season and to compare the variations according to a well-designed training periodization. Twenty-five swimmers (13 boys and 12 girls) were evaluated in (a) October (M1); (b) March (M2); and (c) June (M3). Inertial and anthropometrical measures included body mass, swimmer's added water mass, height, and trunk transverse surface area. Swimming efficiency was estimated by the speed fluctuation, stroke index, and approximate entropy. Active drag was estimated with the velocity perturbation method and the passive drag with the gliding decay method. Hydrodynamic dimensionless numbers (Froude and Reynolds numbers) and hull velocity (i.e., speed at Froude number = 0.42) were also calculated. No variable presented a significant gender effect. Anthropometrics and inertial parameters plus dimensionless numbers increased over time. Swimming efficiency improved between M1 and M3. There was a trend for both passive and active drag increase from M1 to M2, but being lower at M3 than at M1. Intra-individual changes between evaluation moments suggest high between- and within-subject variations. Therefore, hydrodynamic changes over a season occur in a non-linear fashion way, where the interplay between growth and training periodization explain the unique path flow selected by each young swimmer.

Can 8 Weeks of Training in Female Swimmers Affect Active Drag?

Hydrodynamic drag is the force that a swimmer has to overcome in order to maintain his movement through water and is influenced by velocity, shape, size and the frontal surface area Thus, the aim of this study was to assess the effects of 8 weeks of training on active drag in young female swimmers. 8 female age group swimmers belonging to the same swimming club participated in this study. Active drag measurements were conducted in two different trials: at the beginning of the season and after 8 weeks of training. The velocity perturbation method was used to determine active drag in front crawl swimming. After 8 weeks of training, mean active drag decreased, although no significant differences were found between the two trials. No significant differences were observed in swimming velocity between the two trials. It seems that 8 weeks of swimming training were not enough to allow significant improvements on swimming technique. One can recommend that specific training sets concerning technique correction and improvement in young swimmers should be a main aim during training planning.

Growth influences biomechanical profile of talented swimmers during the summer break

This study aimed to analyse the effect of growth during a summer break on biomechanical profile of talented swimmers. Twenty-five young swimmers (12 boys and 13 girls) undertook several anthropometric and biomechanical tests at the end of the 2011–2012 season (pre-test) and 10 weeks later at the beginning of the 2012–2013 season (post-test). Height, arm span, hand surface area, and foot surface area were collected as anthropometric parameters, while stroke frequency, stroke length, stroke index, propelling efficiency, active drag, and active drag coefficient were considered as biomechanical variables. The mean swimming velocity during an all-out 25 m front crawl effort was used as the performance outcome. After the 10-week break, the swimmers were taller with an increased arm span, hand, and foot areas. Increases in stroke length, stroke index, propelling efficiency, and performance were also observed. Conversely, the stroke frequency, active drag, and drag coefficient remained unchanged. When controlling the effect of growth, no significant variation was determined on the biomechanical variables. The performance presented high associations with biomechanical and anthropometric parameters at pre-test and post-test, respectively. The results show that young talented swimmers still present biomechanical improvements after a 10-week break, which are mainly explained by their normal growth.

Bacterial nucleoid structure probed by active drag and resistive pulse sensing

Recent biophysical approaches have provided key insights into the enthalpic and entropic forces that compact the nucleoid in the cell. Our biophysical approach combines two complementary, non-invasive and label-free techniques: a precisely timed steerable optical trap and a high throughput microcapillary Coulter counter. We demonstrate the ability of the latter technique to probe the physical properties and size of many purified nucleoids, at the individual nucleoid level. The DNA-binding protein H-NS is central to the organization of the bacterial genome. Our results show that nucleoids purified from the Δhns strain in the stationary phase expand approximately five fold more than the form observed in WT bacteria. This compaction is consistent with the role played by H-NS in regulating the nucleoid structure and the significant organizational changes that occur as the cell adapts to the stationary phase. We also study the permeability to the flow of ions and find that in the experiment nucleoids behave as solid colloids.

Characterization of speed fluctuation and drag force in young swimmers: A gender comparison

The aim of this study was to compare the speed fluctuation and the drag force in young swimmers between genders. Twenty-three young pubertal swimmers (12 boys and 11 girls) volunteered as subjects. Speed fluctuation was measured using a kinematical mechanical method (i.e., speedo-meter) during a maximal 25-m front crawl bout. Active drag, active drag coefficient and power needed to overcome drag were measured with the velocity perturbation method for another two maximal 25m front crawl bouts with and without the perturbation device. Passive drag and the passive drag coefficient were estimated using the gliding decay velocity method after a maximal push-off from the wall while being fully immersed. The technique drag index was also assessed as a ratio between active and passive drag. Boys presented meaningfully higher speed fluctuation, active drag, power needed to overcome drag and technique drag index than the girls. There were no significant gender differences for active drag coefficient, passive drag and passive drag coefficient. There were positive and moderate-strong associations between active drag and speed fluctuation when controlling the effects of swim velocity. So, increasing speed fluctuation leads to higher drag force values and those are even higher for boys than for girls.

Young Swimmers’ Classification Based on Kinematics, Hydrodynamics, and Anthropometrics

The aim of this article has been to classify swimmers based on kinematics, hydrodynamics, and anthropometrics. Sixty-seven young swimmers made a maximal 25 m front-crawl to measure with a speedometer the swimming velocity (v), speed-fluctuation (dv) and dv normalized to v (dv/v). Another two 25 m bouts with and without carrying a perturbation device were made to estimate active drag coefficient (CDa). Trunk transverse surface area (S) was measured with photogrammetric technique on land and in the hydrodynamic position. Cluster 1 was related to swimmers with a high speed fluctuation (ie, dv and dv/v), cluster 2 with anthropometrics (ie, S) and cluster 3 with a high hydrodynamic profile (ie, CDa). The variable that seems to discriminate better the clusters was the dv/v (F=53.680; P<.001), followed by the dv (F=28.506; P<.001), CDa (F=21.025; P<.001), S (F=6.297; P<.01) and v (F=5.375; P=.01). Stepwise discriminant analysis extracted 2 functions: Function 1 was mainly defined by dv/v and S (74.3% of variance), whereas function 2 was mainly defined by CDa (25.7% of variance). It can be concluded that kinematics, hydrodynamics and anthropometrics are determinant domains in which to classify and characterize young swimmers' profiles.

The effect of different inter-pad distances on the determination of active drag using the Measuring Active Drag system

The Measuring Active Drag (MAD) system was developed to determine active drag in swimming by measuring the push-off force exerted at fixed pads placed below the waterline. The imposed inter-pad distance, which to date has been kept constant while using the MAD system, could affect the active drag because it requires the use of different stroke frequencies. The aim of the present study was therefore to determine the effect of inter-pad distance on active drag at a given speed. In particular, drag-velocity curves at three different inter-pad distances (1.25m, 1.35m and 1.45m) were determined using the MAD system for eleven competitive swimmers. Variation of 16% in inter-pad distance (14% change in stroke frequency) revealed no significant difference in calculated active drag between different inter-pad distances and a low (<5%) average coefficient of variation over different inter-pad distances was found. In addition, inter-test reliability, which was determined for the two 1.35m conditions only, was high (ICC>0.90) for measurements on two consecutive days. The results suggest that it may not be necessary to adapt the inter-pad distance of the MAD system based on anthropometric characteristics of the subject or the velocity-related stroke length in free swimming.

A unified view of energetic efficiency in active drag reduction, thrust generation and self-propulsion through a loss coefficient with some applications

An analysis of the energy budget for the general case of a body translating in a stationary fluid under the action of an external force is used to define a power loss coefficient. This universal definition of power loss coefficient gives a measure of the energy lost in the wake of the translating body and, in general, is applicable to a variety of flow configurations including active drag reduction, self-propulsion and thrust generation. The utility of the power loss coefficient is demonstrated on a model bluff body flow problem concerning a two-dimensional elliptical cylinder in a uniform cross-flow. The upper and lower boundaries of the elliptic cylinder undergo continuous motion due to a prescribed reflectionally symmetric constant tangential surface velocity. It is shown that a decrease in drag resulting from an increase in the strength of tangential surface velocity leads to an initial reduction and eventual rise in the power loss coefficient. A maximum in energetic efficiency is attained for a drag reducing tangential surface velocity which minimizes the power loss coefficient. The effect of the tangential surface velocity on drag reduction and self-propulsion of both bluff and streamlined bodies is explored through a variation in the thickness ratio (ratio of the minor and major axes) of the elliptical cylinders.

Linking Selected Kinematic, Anthropometric and Hydrodynamic Variables to Young Swimmer Performance

The aim of this study was to develop a structural equation model (i.e., a confirmatory technique that analyzes relationships among observed variables) for young swimmer performance based on selected kinematic, anthropometric and hydrodynamic variables. A total of 114 subjects (73 boys and 41 girls of mean age of 12.31 ± 1.09 years; 47.91 ± 10.81 kg body mass; 156.57 ± 10.90 cm height and Tanner stages 1-2) were evaluated. The variables assessed were the: (i) 100 [m] freestyle performance; (ii) stroke index; (iii) speed fluctuation; (iv) stroke distance; (v) active drag; (vi) arm span and; (vii) hand surface area. All paths were significant (p < .05). However, in deleting the path between the hand surface area and the stroke index, the model goodness-of-fit significantly improved. Swimming performance in young swimmers appeared to be dependent on swimming efficiency (i.e., stroke index), which is determined by the remaining variables assessed, except for the hand surface area. Therefore, young swimmer coaches and practitioners should design training programs with a focus on technical training enhancement (i.e., improving swimming efficiency).