Body Drag Research Articles

Mooring systems with submerged buoys: influence of buoy geometry and modelling fidelity

Mooring systems often make use of submerged buoys (SBs) in order to make the moorings compliant. In this paper we present the dynamic effects of changing the buoy geometry or the buoy model fidelity on the mooring system response. Three cylindrical SBs with increasing slenderness (height/diameter) are studied for a mooring leg with two polyester ropes and a SB. The results show a large impact of SB geometry on the mooring dynamics. A larger height/diameter ratio (with preserved mass and buoyancy) is shown to be beneficial as it gives both smaller tension force magnitudes and, more importantly, avoids slack-snap occurrence in the upper cable. We further present a comparison between four numerical methods for SB dynamics: (i) a high-fidelity model using computational fluid dynamics (CFD); (ii) the Morison equation with slender body drag force approximation using numerical quadrature; (iii) the Morison equation with an independent evaluation of the fluid drag due to translation and rotation; and (iv) a translating Morison model which simulates a vertical cylinder in three degrees of freedom with no rotation. All methods are used together with a high-order finite element mooring dynamics solver. The results show that the translating method is inadequate to model this mooring configuration. The remaining three methods agree moderately well, but the Morison formulations give larger motions and higher tensions compared to the CFD results. We show that the quadrature drag model is better suited to model the drag moment on SBs than the independent model, and that the improvement increases with increasing slenderness of the buoy. The uncertainty, sensitivity and importance of the hydrodynamic coefficients of the buoy are discussed and examined by a regression analysis from the CFD data.

Between-Sex Differences in the Work Sample Test Battery Performed by Law Enforcement Recruits: Implications for Training and Potential Job Performance.

Lockie, RG, Beitzel, MM, Dulla, JM, Dawes, JJ, Orr, RM, and Hernandez, JA. Between-sex differences in the work sample test battery performed by law enforcement recruits: implications for training and potential job performance. J Strength Cond Res 36(5): 1310-1317, 2022-Law enforcement officers perform a range of demanding job-specific tasks, and these tasks are the same for all officers, regardless of sex. Female officers tend to be at a physical disadvantage compared with male officers, and this could affect their performance in job-specific tasks. This study investigated the between-sex differences in the work sample test battery (WSTB; duplicates what an officer encounters on-duty) in law enforcement recruits. A retrospective analysis was performed on 308 recruits (259 men and 49 women) from 5 training academy classes. The WSTB incorporated 5 tests: a 99-yard obstacle course (99OC), 165-pound body drag (BD), 6-foot chain link fence (CLF), solid wall (SW) climb, and 500-yard run (500R). These tests were typically performed in the last weeks of academy and must be completed to a state-mandated minimum standard for recruits to graduate. Independent-samples t tests (p < 0.05) and effect sizes (d) calculated between-sex differences. Noting that when individual data were considered, there were male recruits who were outperformed by female recruits. However, on average, female recruits were slower on all WSTB tasks than male recruits (p < 0.01). Large effects were present for the 99OC, CLF, and 500R differences (d = 1.26-1.69), a moderate effect for the BD (d = 0.85), and a small effect for the SW (d = 0.56). Slower performance in job-specific tests could translate to slower performance in tasks required on-duty. Training staff should develop the qualities necessary for WSTB performance in female recruits and underperforming male recruits to enhance future job performance.

TRAINING LOAD DEMANDS MEASURED BY SURFACE ELECTROMYOGRAPHY WEARABLE TECHNOLOGY WHEN PERFORMING LAW ENFORCEMENT-SPECIFIC BODY DRAGS

The use of surface electromyography (sEMG) wearable technology to measure training load (TL) during law enforcement-specific tasks (e.g. a body drag) requires investigation. This study determined muscle activation differences represented as TL during a 9.75-m drag with 74.84 kg and 90.72 kg dummies. Eight men and three women were fitted with a compression short or legging embedded with sEMG wearable technology to measure the quadriceps (QUAD; vastus medialis+vastus lateralis), biceps femoris (BF), and gluteus maximus (GM). After fitting on day one, participants completed maximal voluntary isometric contractions for each muscle to normalize the sEMG signal and calculate TL units. On days two and three, participants performed a 9.75 m body drag using either the 74.84 kg or the 90.72 kg dummy while wearing the technology. Participants lifted the dummy off the floor to a standing position and dragged it as quickly as possible over 9.75 m. Paired samples t-tests calculated between-drag differences for: time; QUAD, BF, GM, and total TL; and QUAD-BF, GM-BF, anterior-posterior (QUAD-GM+BF) ratios. QUAD TL was 9% greater (p=0.035), and GM TL was 8% lower (p=0.043), in the 90.72 kg body drag compared to the 74.84 kg drag. There were no between-mass differences in time, BF TL, total TL, or the ratios. QUAD TL increased while GM TL decreased when participants dragged a 90.72 kg dummy. As drag time was not different between the masses, drag mechanics may have changed leading to increased QUAD TL. sEMG wearable technology could be a useful method to measure TL in law enforcement-specific dragging tasks.

Mechanics of bluff body drag reduction during transient near-wake reversals

A combination of passive and active methods is used to manipulate the symmetry of the turbulent wake of an Ahmed body. Dedicated experiments to study the wake reversals occurring between symmetry-breaking states are performed. We show how transient symmetric states explored during the reversals differ from static symmetry-breaking states in the flow organization they induce. In asymmetric states, a strong interaction and coupling between the recirculating flow from one side and the shear layer from opposite side triggers shear-layer instabilities and their amplification. The resulting large-scale flow engulfment in the recirculation region increases the recirculation intensity and thus increases base drag. By contrast, during the wake reversals the disorganization of the recirculating flow leads to a transient symmetric state with prevented shear-layer interaction and triggering mechanism with a concomitant drag reduction of ${\sim}8\,\%$ compared to symmetry-breaking states. Finally, recent experimental results for unbalanced wakes and methodologies addressing the control of wake asymmetries for drag reduction are discussed and interpreted based on the present findings. This leads us to provide new insights in control methods for wake asymmetries.

ON DETERMINATION OF INTERNAL DRAG WHEN TESTING MODELS OF INTEGRATED AIRCRAFT WITH DUCTS

The problem of determining the internal drag in ramjet integrated layouts is considered. It is shown that the standard technique for internal drag adjustment in tests of aircraft models with a duct developed for axisymmetric layouts can lead to differences in adjustment values when applied to the integral layout of the lifting body with an under-fuselage ramjet. It is proposed to include the aerodynamic drag of the external compression inlet braking body in the aerodynamic characteristics of the airframe, as well as to determine the internal drag only within the limits of the duct itself (i.e., from the mouth to the nozzle exit) and the need to measure the flow parameters in both the nozzle exit section and at the duct mouth. Calculation estimates of the internal drag of the integral layouts using the Navier-Stokes equations are given.

날갯짓 기능을 가진 도약 로봇의 종방향 안정성 해석

Due to safety concerns, it is difficult for human beings to access a disaster area to rescue survivors. Even a robot may fail to conduct a rescue mission if it can use only one mode of transportation. A robot with multiple modes of transportation may have a greater chance to successfully search for survivors within the golden time. In previous work to develop a dual-mode mobile robot, we introduced a jumping robot assisted by flapping wings called Jump-flapper. The robot was equipped with one driving motor and dual transmissions to operate a winching mechanism for elastic energy storage used for jumping and flapping mechanisms to provide additional thrust in the air immediately after jumping. In this paper, we analyzed the stability of Jump-flapper using a linearized equation of motion. With commercial software ANSYS-Fluent, computational fluid dynamics were used to compute stability derivatives included in the system matrix. The eigenvalue analysis finds that the robot eventually becomes unstable due to two unstable oscillatory modes. For a more accurate estimation of vertical jumping height, the drag of the robot body was included in the equation. Consequently, the equation of motion became non-linear and we used the Euler method to solve the non-linear equation. The estimated vertical jumping height agreed with the prediction by a simple model and with the measured height from our previous work.

Uncertainty and sensitivity study on blunt body's drag and heat reduction with combination of spike and opposing jet

Uncertainty and sensitivity study of three main parameters including length-to-diameter ratio, nozzle radius and total pressure ratio on the drag and heat reduction for the blunt body with spike and opposing jet is conducted. According to the simulation results obtained from the computational fluid dynamics, the relationship between input parameters and aerodynamic force/heat is established via the point-collocation non-intrusive polynomial chaos method. Uncertainties of pressure and heat flux caused by changes of input parameters are quantified. Sensitivity analysis is carried out to determine key parameters affecting pressure and heat flux changes. The results indicate that changes of peak pressure and peak heat flux are resulted from significant change in reattachment shock strength which varies with input parameters. Sensitivity analysis reveals that the length-to-diameter ratio has huge influence on the uncertainty in the recirculation zone, while the nozzle radius dominates the uncertainty in the reattachment zone. Moreover, it is found that the nozzle radius has an enormous effect on peak pressure and peak heat flux, and a significant negative correlation between them is identified.

A Preliminary Analysis of Relationships between a 1RM Hexagonal Bar Load and Peak Power with the Tactical Task of a Body Drag

A critical job task for law enforcement officers that should be influenced by strength is the body drag. This study analyzed relationships between absolute and relative strength measured by a one-repetition maximum hexagonal bar deadlift (1RM HBD), with body drags completed with 74.84 kg and 90.72 kg dummies. Twenty recreationally-trained individuals completed the 1RM HBD in one session, with peak power measured via a linear position transducer. Over two subsequent sessions, participants dragged the 74.84 kg and 90.72 kg dummies with two techniques. The first technique followed Californian standards, where participants wrapped their arms around the dummy and lifted it to standing before timing commenced. In the adapted technique, timing included the initial manipulation of the dummy. Participants dragged the dummy as quickly as possible over a 9.75 m distance. Partial correlations and linear regression (controlling for sex; p < 0.05) analyzed relationships between the HBD and body drags. The standard 74.84 kg body drag correlated with every HBD variable (r = -0.477 to -0.666), and was predicted by the absolute 1RM HBD (r2 = 0.467). The adapted 74.84 kg drag correlated with all HBD variables (r = -0.535 to - 0.754), and was predicted by peak power and the 1RM HBD (r2 = 0.758). Both 90.72 kg drags correlated with absolute and relative 1RM HBD (r = -0.517 to -0.670). Strength related to all body drags; peak power may be more important for drags with lighter loads. Strength training should be a focus in law enforcement to enhance drag performance.

Multi-objective design optimization of blunt body with spike and aerodisk in hypersonic flow

In order to reduce the aerodynamic drag and aerodynamic heating of the hypersonic blunt body, the aerodisk and spike are installed on the blunt body. In this paper, the influences of the aerodisk on the aerodynamic drag and aerodynamic heating of the hypersonic blunt body are studied by numerical method. The results show that the drag coefficient and maximum heat flux of the blunt body for the configuration with the aerodisk are reduced by 34.09% and 51.06% respectively compared with the configuration without the aerodisk, and the two configurations achieve the drag and heat reduction by reconstructing the flow field. On this basis, multi-objective design optimization of the hypersonic blunt body with the aerodisk and spike is performed by the weighting method and NSGA-II method. The results show that increasing the length of the spike and radius of the aerodisk can reduce total heat flux of the blunt body. The drag coefficient decreases with the increase of the length of the spike. However, with the increase of the radius of the aerodisk, the drag coefficient decreases first and then increases. In addition, the drag coefficient and total heat flux of the blunt body of typical Pareto optimal solution are reduced by 35.90% and 46.97% respectively compared with original design, and adjusting the weighting coefficient of the weighting method can obtain the Pareto frontier calculated by NSGA-II method.

Numerical Experiments for the Case of Acceleration of a Body of Revolution

Reducing the aerodynamic drag of bodies of revolution at supersonic speeds due to the creation of a bottom thrust is an important subject of aeromechanics. In the framework of this problem, it is proposed to perform numerical experiments on a model with a detachable bottom. It is assumed that the detachable bottom is affected by the main air flow jets separating from the surface of the body of revolution. The gas is reflected from the bottom and acts on the body of revolution, creating a bottom thrust and accelerating it.

Aerodisk effects on drag reduction for hypersonic blunt body with an ellipsoid nose

The paper systematically investigates the drag reduction of aerodisk fixed on the non-axisymmetric blunt body with an ellipsoid nose, different from conventional axisymmetric blunt body. The effects of spike length, aerodisk size and shape on drag reduction performance are numerically examined. Based on the analysis, a well-designed aerodisk configuration is determined and a maximum drag reduction of 41.9% is obtained for the whole blunt body. Meanwhile, some meaningful phenomena are discussed: (i) As the spike length increases, the drag of forebody and whole blunt body decreases first, then increases, and then decreases again. (ii) The total drag of the blunt body decreases first and then increases with the increase of aerodisk size. (iii) An elliptic disk with aspect ratio approaching that of the ellipsoid nose is superior to a round disk with the same size. This numerical study is expected to illustrate the significance of aerodisk to drag reduction over the non-axisymmetric blunt body and provide a reference for the design of aerodisk in hypersonic field.

Coasting in live-bearing fish: the drag penalty of being pregnant.

Swimming performance of pregnant live-bearing fish is presumably constrained by the additional drag associated with the reproductive burden. Yet, it is still unclear how and to what extent the reproductive investment affects body drag of the females. We examined the effect of different levels of reproductive investment on body drag. The biggest measured increase in body volume due to pregnancy was about 43%, linked to a wetted area increase of about 16% and 69% for the frontal area. We printed three-dimensional models of live-bearing fish in a straight body posture representing different reproductive allocation (RA) levels. We measured the drag and visualized the flow around these models in a flow tunnel at different speeds. Drag grew in a power fashion with speed and exponentially with the increase of RA, thus drag penalty for becoming thicker was relatively low for low speeds compared to high ones. We show that the drag increase with increasing RA was most probably due to bigger regions of flow separation behind the enlarged belly. We suggest that the rising drag penalty with an increasing RA, possibly together with pregnancy-related negative effects on muscle- and abdominal bending performance, will reduce the maximum swimming speed.

Prediction of Main Regime Transition with Variations of Gas and Liquid Phases in a Bubble Column.

Industrial bubble columns mainly operate in a heterogeneous flow regime and identifying transition from homogeneous to heterogeneous flow is important. This work addresses the determination of flow regimes with gas–liquid systems in a bubble column. Various parameters of gas holdup, volumetric mass transfer coefficient, drift flux, and pressure standard deviations were investigated to precisely determine the superficial gas velocity at the transition regime. For a column aspect ratio (H/D: static liquid height to column diameter ratio) of 2.5 to 5, the transitional superficial gas velocity generally became higher with lower liquid height. However, the reverse trend was observed with a low density gas system due to the difference in force balance acting around the bubble. The weak body force and drag force in a lower axial position interrupted the mass transfer process. The experimental results showed similar ranges of the transitional superficial gas velocity regardless of the choice of parameter for detecting it. Because there is no precise correlation about transition regime properties, we proposed new correlations to predict both transition regime superficial gas velocity and gas holdup by taking the intersection of the two regimes. The correlations precisely captured the transition regime properties within 15% deviations even in gas–liquid systems that are not tested in this work.

Lower-body Power and its Relationship with Body Drag Velocity in Law Enforcement Recruits

Lower-body Power and its Relationship with Body Drag Velocity in Law Enforcement Recruits

Numerical investigation on drag and heat reduction mechanism of combined spike and rear opposing jet configuration

In order to reduce the hypersonic aerodynamic drag and heating, a combined spike and rear opposing jet configuration is proposed in this paper, and the CFD method is adopted to analyze the drag and heat reduction efficiency. The results show that the spike pushes the bow shock wave away from the blunt body, which translates the normal shock wave into the oblique shock wave and reduces the shock wave intensity. In addition, the low temperature jet gas is injected into the flow field, which reduces the temperature of the flow field after the shock wave. So the combined configuration reduces the aerodynamic drag and heating of the blunt body by the reconstruction of flow field, and the drag and heat reduction efficiency is better than the other configurations that already exist. The influences of the length of spike, total pressure of the opposing jet and jet gas on the drag and heat reduction efficiency are studied. The results show that increasing the length of the spike and the total pressure of the opposing jet can effectively improve the drag and heat reduction efficiency, and the decreasing rates of the aerodynamic drag and heating slow down gradually with the increase of above two parameters. In addition, the nitrogen has the best drag reduction efficiency and the carbon dioxide has the best heat reduction efficiency. The investigations in this paper verify the advantages and application in engineering of the combined configuration proposed in this paper.

Physical Fitness Characteristics That Relate to Work Sample Test Battery Performance in Law Enforcement Recruits.

This study determined relationships between an agency-specific fitness test battery (PT500), and a work sample test battery (WSTB) in law enforcement recruits. Retrospective analysis on 219 males and 34 females from one agency was conducted. The PT500 comprised: push-ups, sit-ups, and mountain climbers in 120 s; pull-ups; and 201 m and 2.4 km runs. The WSTB comprised: 99 yard (90.53 m) obstacle course (99OC); body drag (BD) with a 165 pound (75 kg) dummy; 6 foot (1.83 m) chain link fence (CLF) and solid wall (SW) climb; and 500 yard (457.2 m) run (500R). Partial correlations, controlling for sex, calculated PT500 and WSTB relationships (p < 0.05). Stepwise regression determined whether fitness predicted WSTB performance. The 500R related to all PT500 assessments (r range = −0.127–0.574), 99OC related to all bar push-ups and mountain climbers, and BD related to none. The CLF related to sit-ups, pull-ups, and 2.4 km run; SW related to mountain climbers, pull-ups, and 2.4 km run (r range = −0.127–−0.315). Push-ups, pull-ups, and 2.4 km run were involved in predictive relationships for 99OC, CLF, SW, and 500R (r2 range = 0.217–0.500). To perform better in the WSTB and job-specific tasks, developing upper-body strength and aerobic fitness may be beneficial.

Analysis of influence of cavity content on flow pulsations

Assumptions on the cavity content have an insignificant influence on predictions of time-average characteristics of sheet cavitation. However, cavity pulsation can substantially depend on this content because of compressibility of gas-liquid mixtures within the cavities. Compressibility is predetermined by the local sound speed that sharply depends on void fraction and pressure within the cavities. This speed can be indeed very small, whereas used in many CFD solvers definitions of the mixture density as a linear combination of component densities can lead to enormous overestimation of this speed and, as a result, to underestimation of pulsations of cavity volume and of body lift and drag. This paper provides numerical analysis of the influence of cavity content on pulsations and compares the results with experimental data for a specially designed hydrofoil.

Energetic assessment of an embedded aircraft propulsion: an analytic approach

This paper investigates the energetic advantage of the embedded propulsion compared to a state-of-the-art propulsion of an aircraft. Hereby, the integral method of boundary layer theory together with the potential theory is applied to analyze the boundary layer thickness and the impact of the flow acceleration due to the embedded propulsion. The aircraft body is treated as a flat plate and the engine as a momentum disk. For an embedded propulsion, there is a trade-off of the engine size, since the propulsion efficiency is affected by the boundary layer. On the one hand, the propulsion inlet momentum is noticeably reduced for a small engine size and the viscous friction is reduced due to boundary layer ingestion. On the other hand, a slow jet speed, i.e., a large engine size, increases the propulsion efficiency as known. The outcome of the energetic assessment is the following: the propulsion efficiency is increased and the drag of the aircraft body is reduced by the embedded propulsion compared to a conventional propulsion. The optimized aircraft engine size depending on Reynolds number is given as well as the dimensionless cost function.

Calculation the total resistance and squat of underwater towed vehicle based on CFD

CFD method is used to calculate drag and sinking force of underwater towed body. Using cubic spline curve interpolation and finite difference method are used to calculate the tangential force and normal force of towing cable. Finally, the squat of underwater towed vehicle in the water and the total force at the end of towing cable are obtained.

Optimal design and energy harvesting performance of carangiform fish-like robotic system

Aquatic unmanned vehicles (AUVs) are making a shift to more efficient swimming designs inspired from nature that utilize more flexible materials to make the system more mimetic. Creating more mimetic body caudal fin swimming AUV designs means that aquatic locomotion is achieved through an oscillatory body undulation. This oscillatory motion can be modeled as a spatio-temporal function by replicating the behavior of spinal flexion. A computational fluid dynamic analysis determined that the frequency which produces enough thrust to overcome body drag is greater than 2.5 Hz. The spatio-temporal function and the frequency range (2.5–5 Hz) are then defined for two aquatic AUVs of lengths 0.4 and 1 m. Because these systems express an oscillatory body undulation, they have energy harvesting potential where piezoelectric patches can be attached along the fish length. Investigations into impacts of piezoelectric material length, placement, and frequency of oscillation for the two respective fish lengths are carried out using a gradient decent method to determine the optimal system arrangement for both systems that generate maximum power. The undulatory motion for 0.4 m is scaled from the 1 m length but the optimal attachment location of the piezoelectric layer always at the tip of the tail where the largest flexural change occurs. The ratio of the piezoelectric length to fish length for the two considered fish lengths is found to be 0.237. This analysis is very beneficial for the design of efficient carangiform fish-like robots.