Body Drag Research Articles

Influence of geometrical defects on aerodynamic drag of non-watertight Ahmed body

Influence of geometrical defects on aerodynamic drag of non-watertight Ahmed body

Drag of Rankine bodies embedded in porous media at small Darcy numbers

This paper studies the flow over a solid inclusion embedded in a porous medium of low permeability. We study Rankine bodies which represent circular cylinders and spheres when the aspect ratio is one, and thin plates and almost uniform rods when the aspect ratio is small. Using boundary layer theory, the flow field and pressure distribution are determined. It is found that the pressure drag dominates the shear drag except for small aspect ratios. Thus, geometry plays a decisive role in the choice of using the Darcy equation or the Darcy-Brinkman equation. The results are important for the determination of flow resistance of a non-homogeneous porous medium which has solid inclusions.

Relationships Between Strength and Power With the 74.84-kg (165-lb) and 90.72-kg (200-lb) Body Drags.

Moreno, MR, Dawes, JJ, Orr, RM, Dulla, JM, and Lockie, RG. Relationships between strength and power with the 74.84-kg (165-lb) and 90.72-kg (200-lb) body drags. J Strength Cond Res 38(7): 1305-1313, 2024-A body drag is an essential job task in law enforcement. It is purported that strength and power should contribute to this task, but there has been limited analysis. This study examined strength (1-repetition maximum hexagonal bar deadlift [1RM HBD]) and power (vertical jump [VJ], standing broad jump [SBJ]) relationships with the 74.84-kg and 90.72-kg body drags. Forty-three civilians (24 men and 19 women) completed the SBJ, VJ, and 1RM HBD in 1 session. Peak anaerobic power measured in watts (PAPw) was derived from the VJ. Relative measures were calculated from the SBJ, PAPw, and HBD. In another 2 sessions, subjects completed 74.84-kg and 90.72-kg drags in a counterbalanced order using 2 techniques: standard and adapted (time taken to pick dummy up from ground included in time). Independent samples t-tests compared the sexes and subjects who could or could not complete all drags. Partial correlations and stepwise regression, both controlling for sex, examined relationships between strength and power and the drags. In general, men outperformed women in almost all tests (p ≤ 0.024). Thirty-five subjects completed all drags and were stronger and more powerful than subjects who could not (p ≤ 0.011). The SBJ correlated with the 74.84-kg standard (r = -0.355) and 90.72-kg adapted (r = -0.373) drags; the VJ correlated with the 74.84-kg standard drag (r = -0.356). Peak anaerobic power measured in watts and 1RM HBD correlated with all drags (r = -0.382 to -0.668); PAPw was involved in all significant predictive relationships (explained variance = 34-62%). Absolute strength and power training should be a focus for recruits to enhance their ability to perform dragging tasks, especially with heavier masses.

Reducing the Drag of a Prolate Body by Modification of Its Nose Shape

Reducing the Drag of a Prolate Body by Modification of Its Nose Shape

Numerical investigation and comparison of the aerodynamic characteristics of non-pneumatic tire and pneumatic tire

A non-pneumatic tire, in which the tire pressure is replaced by an elastic support body, is called an airless tire with elastic support (ALTES). In this study, the differences in the aerodynamic characteristics between ALTES and pneumatic tire (PT) were comparatively investigated using URANS simulations based on the DrivAer model. It was found that the vehicle body drag coefficient of the vehicle equipped with ALTES was reduced by 27% compared to that of the vehicle equipped with PT and had a significant drag-reduction effect. The drag coefficient and moment coefficient of the ALTES wheels are 2 and 3.5 times those of the PT wheels, respectively, which offset the drag-reduction benefits. The drag coefficient and aerodynamic power loss of vehicles equipped with ALTES were 97–102% and 108.5%–114% of those of vehicles equipped with PT. It is reasonable and necessary to evaluate the aerodynamic performance of ALTES based on the aerodynamic power loss. By suppressing the wheel ventilation flow and reducing the wheel drag, the aerodynamic wheel cover can reduce the aerodynamic power loss of a vehicle equipped with ALTES by 12%, and the vehicle aerodynamic performance is comparable to that of a PT.

Study on the steady state position of ocean towing cables

A study is conducted on the static equilibrium posture of the towing cable in water under a constant speed state in the towing system. Firstly, the underwater force state of the towing body is calculated and then the posture and tension of the towing cable are analyzed and calculated. As the cable is divided into two states which are above water and below water, a differential equation system is established as the boundary. The initial value of the equation can be transformed by the drag and gravity values of the towing body. The program solves the differential equations using the fourth-order Runge Kutta method and draws the attitude curve. Finally, it verifies the theoretical calculation results through experiments.

Current and density observations on a flow through a contraction and over a bottom elevation at the southern edge of the Cycladic Plateau in the Aegean Sea – East Mediterranean

A CTD/ADCP/surface-drifter survey in fall 2004 reveals the behaviour of a mesoscale unidirectional flow coming from the Cretan sea in the south with depths ∼1000 m and entering a channel-like area of the Cycladic shelf in the north, that forms a contraction which leads to a bottom elevation (sill depth ∼100 m), and finally returning into the Cretan Sea in the lee-side of the sill. The flow decelerates/accelerates upstream/downstream of the sill. The along-stream density contours near the sill bottom are raised prior to reaching the sill, while they deepen in the lee side of it indicating supercriticality. The long-wavelength internal wave speeds with realistic stratification and no-rotation are higher than the section averaged flow speeds and indicate subcriticality. A key element in this apparent paradox is the large height of the sill that potentially increases the body (drag) force exerted on the flow by the sill while flow blocking is also observed upstream of the sill.

Drag reduction in a spanwise-infinite boat-tailed body through multiple transverse grooves

Appropriately-designed spanwise-extruded small grooves, oriented transverse to the freestream flow, may delay flow separation in adverse pressure gradients by generating local and stable recirculation zones. In this study, we explore the potential of multiple grooves to reduce the drag on a boat-tailed bluff body characterized by vortex shedding. Numerical simulations reveal that the presence of two consecutive transverse grooves results in a maximum boat-tail drag reduction of 23.2%, further decreasing the drag by 15% when compared to the performance of the same boat-tailed bluff body with a single groove. This flow-control mechanism proves effective when the flow reattachment takes place downstream of both grooves throughout the entire vortex-shedding cycle, leading to the formation of two distinct local recirculation zones within the grooves. Lower frictional losses are observed along the streamline bounding the recirculation within the grooves as compared to the friction losses caused by the no-slip boundary conditions along the lateral walls of a plain boat tail (i.e., without grooves). Consequently, the boundary layer exhibits higher momentum downstream of the grooves, resulting in a delayed flow separation and an improved reduction in body drag for the boat tail.

Study of the spike-aerodisk-opposing jet on heat protection to both the spike-aerodisk and the blunt body and overall drag reduction in rarefied hypersonic flow in near space

The Direct Simulation Monte Carlo method (DSMC) is utilized in this study to investigate the flowfield surrounding the spike-aerodisk without and with an opposing jet under rarefied hypersonic conditions within near space (altitudes of 60∼90 km and Mach numbers 7∼20). As the flight altitude increases, the incoming flow gradually becomes rarefied, weakening the drag reduction effect of the spike-aerodisk model. At altitudes reaching 90 km, the spike-aerodisk loses its drag reduction effect due to the increasing proportion of the viscous drag coefficient. At a flight altitude of 70 km, as flight Mach numbers increase, the drag reduction and thermal protection performance of the spike-aerodisk remains unchanged, mainly demonstrating the excellent performance of the spike-aerodisk model at high Mach numbers. However, the peak wall heat flux coefficient on the aerodisk can be 4.8 times higher than on the blunt body. Therefore, an opposing jet is introduced to protect the spike-aerodisk from overheating and further improve the spike-aerodisk's drag and heat reduction efficiency. Within the investigated parameter range, when the opposing-jet pressure ratio is 0.02, the peak wall heat flux coefficient of the aerodisk can decrease from 0.58 to a level similar to that of the blunt body, approximately 0.12. To make the aerodisk more certain from overheating, choosing the opposing-jet pressure ratio of 0.08 can improve drag reduction and heat prevention on the blunt body by 60.9 % and 59.7 %, respectively, compared to the spike-aerodisk model without an opposing jet.

Active flow control for bluff body drag reduction using reinforcement learning with partial measurements

Active flow control for drag reduction with reinforcement learning (RL) is performed in the wake of a two-dimensional square bluff body at laminar regimes with vortex shedding. Controllers parametrised by neural networks are trained to drive two blowing and suction jets that manipulate the unsteady flow. The RL with full observability (sensors in the wake) discovers successfully a control policy that reduces the drag by suppressing the vortex shedding in the wake. However, a non-negligible performance degradation ( $\sim$ 50 % less drag reduction) is observed when the controller is trained with partial measurements (sensors on the body). To mitigate this effect, we propose an energy-efficient, dynamic, maximum entropy RL control scheme. First, an energy-efficiency-based reward function is proposed to optimise the energy consumption of the controller while maximising drag reduction. Second, the controller is trained with an augmented state consisting of both current and past measurements and actions, which can be formulated as a nonlinear autoregressive exogenous model, to alleviate the partial observability problem. Third, maximum entropy RL algorithms (soft actor critic and truncated quantile critics) that promote exploration and exploitation in a sample-efficient way are used, and discover near-optimal policies in the challenging case of partial measurements. Stabilisation of the vortex shedding is achieved in the near wake using only surface pressure measurements on the rear of the body, resulting in drag reduction similar to that in the case with wake sensors. The proposed approach opens new avenues for dynamic flow control using partial measurements for realistic configurations.

Eukaryotic swimming cells are shaped by hydrodynamic constraints

Eukaryotic swimming cells such as spermatozoa, algae or protozoa use flagella or cilia to move in viscous fluids. The motion of their flexible appendages in the surrounding fluid induces propulsive forces that balance viscous drag on the cells and lead to a directed swimming motion. Here, we use our recently built database of cell motility (BOSO-Micro) to investigate the extent to which the shapes of eukaryotic swimming cells may be optimal from a hydrodynamic standpoint. We first examine the morphology of flexible flagella undergoing waving deformation and show that their amplitude-to-wavelength ratio is near that predicted theoretically to optimise the propulsive efficiency of active filaments. Next, we consider ciliates, for which locomotion is induced by the collective beating of short cilia covering their surface. We show that the aspect ratios of ciliates are close to that predicted to minimise the viscous drag of the cell body. Both results strongly suggest a key role played by hydrodynamic constraints, in particular viscous drag, in shaping eukaryotic swimming cells.

Three-dimensional numerical study on reducing hypersonic blunt body drag and aeroheating with spike-aerodisk-bleed air channel

Shock drag and aeroheating are major problems faced by hypersonic vehicles. This study proposes a novel three-dimensional model combining spike-aerodisk with a bleed air channel featuring multiple lateral nozzles for drag and aeroheating reduction. The high-temperature and high-pressure air at the stagnation point of the aerodisk is introduced into the channel and ejected laterally through multiple nozzles to modify the flow field structure and reduce drag and aeroheating. In contrast to previous research employing a two-dimensional axisymmetric assumption, our innovation considers the structure of the exit nozzles and the spacing between nozzles. Due to multiple lateral nozzles, the connection thickness between nozzles can lead to asymmetric flow phenomena. Three-dimensional flow field simulation has been employed with the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the shear stress transport (SST) k-ω turbulence model. The effects of the convergence half-angle of the channel inlet, the divergent angle of the nozzle, and the nozzle position on modifying the characteristics of the flow field are studied. Increasing the convergence half-angle of the channel inlet can further push away the reattachment shock, but when it exceeds 60°, the drag reduction performance degrades. Reducing the divergent angle of the nozzle and increasing the number of nozzles can reduce the drag coefficient and the peak value of the Stanton number. However, when the divergent angle is reduced to 10°, the performance of reducing drag and aeroheating is weakened. In the range of research parameters, when the convergence half-angle is 60°, the divergent angle is 15°, and the nozzles are located in the middle of the spike, compared with the plain spike-aerodisk model, the drag coefficient and peak Stanton number of the combined model are further reduced by 17.8 % and 34.8 %, respectively. Our findings confirm that a multi-nozzle can effectively reduce drag and aeroheating while the channel structure is viable in practice.

Optimization of Co-Flow Jet Parameters for Ahmed Body Application

This study evaluates the drag reduction strategy of suction and blowing on idealize automotive vehicle, Ahmed Body. Optimization approach is adapted in order to analyse the effect of slot location, momentum coefficient and slot angle on the vehicle which experiencing drag. Despite all the efforts that have been done to reduce the Ahmed body drag using various active flow control system, most of the drag reduction were only less than 15%. A 25° Ahmed body with build in co-flow jet is modelled using a CAD software. The flow around the Ahmed body is simulated at Reynolds number based on length Re = 4.29 × 106. The governing equation were solve using an open source software package, which has been validated against experimental data. Pressure Implicit with Splitting of Operator (PISO) algorithm is applied to solve the equation. The outcome of the simulation are varies depending on the variables. Some show a decrease in drag while there are also that actually increase the drag of the system. This are caused by the suction and blowing slots that effect the surrounding air flow whether it is reducing or increasing the wake size downstream of the body. The result shows the momentum coefficient and location of both suction and blowing jet played an important role of manipulating the flow around the body and reducing the drag. The velocity contours indicated that the key to drag reduction is by using 40 m/s jet velocity, placement of suction and blowing away from each other.

Study on the effects of towed array on the resistance and flow characteristics of unmanned underwater vehicle

Unmanned underwater vehicle (UUV) towing a number of trailing bodies carrying detectors as a UUV-towed array system is widely used in fields such as submarine topography exploration, submarine pipeline laying, and submarine energy survey due to its simple structure. This study employs the Computational Fluid Dynamics (CFD) method, using the general submarine models Joubert BB2 and Persia 110 as tug and towing models, to calculate the hydrodynamic performance of the towed system under full control constraints, linear control constraints, and plane control constraints respectively, under the condition of infinite depth and fixed speed. The surrounding flow field velocity under the above three conditions is also analyzed. The study gradually explores the drag and lift of the towed body, the pressure distribution on the surface of the hull, and the force on the catenary, as well as the influence of the towing array on the main UUV and the surrounding flow field. The findings demonstrate that the existence of the towed array system will increase the resistance of the main UUV, with drag movement showing periodic reciprocating motion. Furthermore, the control degree of the towed cable becomes lighter the farther away it is from the main UUV, resulting in a larger range of motion.

Numerical and Experimental Characterization of Rotational Floating Body Drag

Hydrodynamic drag plays a significant role in the motions and response of floating bodies – whether it be a wave energy converter, floating wind structure, or offshore oil & gas platform. Existing literature provides significant overview of the methodologies (both experimental and numerical) to characterize translational drag, however, there is limited research on the contributions (and methods of application) for rotational drag.
 
 This paper will detail both numerical modelling and a physical experimental campaign to assess how rotational drag impacts floating body dynamics, and best practices for numerical model inclusion. Specific focus will be on 1) the variety of methods used to input rotational drag into numerical models; 2) processes and lessons learnt from the experimental derivation of rotational drag coefficients; and 3) how does weakly non-linear wave stretching methods influence rotational drag.
 
 The experimental campaign is currently underway to classify the significance of rotational drag coefficients in characterizing floating body behavior. Translational and rotational drag coefficients of a simplified, inertial property matched, 1:50 floating body is being determined through a series of calibration tests. Both traditional free decay tests and forced oscillation tests will be implemented to evaluate these coefficients across multiple degrees-of-freedom. The final paper will present an overview of the experimental campaign, the results and lesson learnt.
 
 On the numerical side, the floating body will be modelled in the open-source wave energy converter modelling tool, WEC-Sim, and validated against the experimental results. Numerical results will be presented to review general body responses, with and without rotational drag, and generic wave conditions plus those expected at the PacWave wave energy test site in Oregon, USA.
 
 The inclusion of rotational drag coefficients and weakly nonlinear hydrodynamics are expected to improve computational model results, especially in the nonlinear wave excitation range, providing a better understanding of floating body behavior.

Drag Assessment for Boundary Layer Control Schemes with Mass Injection

The present study considers uniform blowing in turbulent boundary layers as active flow control scheme for drag reduction on airfoils. The focus lies on the important question of how to quantify the drag reduction potential of this control scheme correctly. It is demonstrated that mass injection causes the body drag (the drag resulting from the stresses on the body) to differ from the wake survey drag (the momentum deficit in the wake of an airfoil), which is classically used in experiments as a surrogate for the former. This difference is related to the boundary layer control (BLC) penalty, an unavoidable drag portion which reflects the effort of a mass-injecting boundary layer control scheme. This is independent of how the control is implemented. With an integral momentum budget, we show that for the present control scheme, the wake survey drag contains the BLC penalty and is thus a measure for the inclusive drag of the airfoil, i.e. the one required to determine net drag reduction. The concept of the inclusive drag is extended also to boundary layers using the von Kàrmàn equation. This means that with mass injection the friction drag only is not sufficient to assess drag reduction also in canonical flows. Large Eddy Simulations and Reynolds-averaged Navier–Stokes simulations of the flow around airfoils are utilized to demonstrate the significance of this distinction for the scheme of uniform blowing. When the inclusive drag is properly accounted for, control scenarios previously considered to yield drag reduction actually show drag increase.

Thrust generation and propulsive efficiency in dolphin-like swimming propulsion

Given growing interest in emulating dolphin morphology and kinematics to design high-performance underwater vehicles, the current research effort is dedicated to studying the hydrodynamics of dolphin-like oscillatory kinematics in forward propulsion. A computational fluid dynamics method is used. A realistic three-dimentional surface model of a dolphin is made with swimming kinematics reconstructed from video recording. The oscillation of the dolphin is found to enhance the attachment of the boundary layer to the posterior body, which then leads to body drag reduction. The flapping motion of the flukes is found to generate high thrust forces in both the downstroke and the upstroke, during which vortex rings are shed to produce strong thrust jets. The downstroke jets are found to be on average stronger than the upstroke jet, which then leads to net positive lift production. The flexion of the peduncle and flukes is found to be a crucial feature of dolphin-like swimming kinematics. Dolphin-inspired swimming kinematics were created by varying the flexion angle of the peduncle and flukes, which then resulted in significant performance variation. The thrust benefits and propulsive efficiency benefits are associated with a slight decrease and slight increase of the flexion of the peduncle and flukes, respectively.

Prepositive Synergistic Bulge Design for Improving Aerodynamic Performance of Submerged Inlet

A submerged inlet has good stealth characteristics and a low external drag, but it also has the disadvantage of low internal flow efficiency. In view of this, a new efficiency enhancement method based on the prepositive synergistic bulge of the inlet’s anterior lip is proposed. Taking the submerged inlet of an aircraft as the baseline configuration, a miniature bulge with a square bottom and an outer convex form is designed in front of the inlet’s anterior lip. Through the convex shape of the bulge, part of the low-energy boundary layer airflow is diverted away from the inlet’s entrance, so that the airflow greatly reduces the flow separation after entering the inlet, and the internal flow performance of the entire submerged inlet is improved. Taking the flow field of an aircraft in the classic cruise state as an example, the simulation analysis results show that the flow field characteristics of the entire submerged inlet are obviously improved after adding the synergistic bulge. The total pressure recovery coefficient of the new inlet configuration increased by 1.36%, the total pressure distortion index decreased by 10.86%, and the body drag only increased by 0.37% compared with the baseline case. According to calculations of synergistic bulge inlet configurations with different design parameters, the effect of this configuration is relatively stable, whereby the aspect ratio of the bulge has the greatest impact on the performance, and its value should not be less than 0.75. In addition to the advantages of not requiring additional components or occupying space and being easy to manufacture, the method of adding a synergistic bulge can improve the aerodynamic performance of the baseline inlet under most cruise flight conditions, and its additional drag is small, which gives it a wide applicability range.

Deep dives and high tissue density increase mean dive costs in California sea lions (Zalophus californianus).

Diving is central to the foraging strategies of many marine mammals and seabirds. Still, the effect of dive depth on foraging cost remains elusive because energy expenditure is difficult to measure at fine temporal scales in wild animals. We used depth and acceleration data from eight lactating California sea lions (Zalophus californianus) to model body density and investigate the effect of dive depth and tissue density on rates of energy expenditure. We calculated body density in 5 s intervals from the rate of gliding descent. We modeled body density across depth in each dive, revealing high tissue densities and diving lung volumes (DLVs). DLV increased with dive depth in four individuals. We used the buoyancy calculated from dive-specific body-density models and drag calculated from swim speed to estimate metabolic power and cost of transport in 5 s intervals during descents and ascents. Deeper dives required greater mean power for round-trip vertical transit, especially in individuals with higher tissue density. These trends likely follow from increased mean swim speed and buoyant hinderance that increasingly outweighs buoyant aid in deeper dives. This suggests that deep diving is either a 'high-cost, high-reward' strategy or an energetically expensive option to access prey when prey in shallow waters are limited, and that poor body condition may increase the energetic costs of deep diving. These results add to our mechanistic understanding of how foraging strategy and body condition affect energy expenditure in wild breath-hold divers.

Base suction, entrainment flux, and wake modes in the vortex formation region at the rear of a three-dimensional blunt bluff body.

A slitted base cavity of constant depth with a varying filling ratio 0≤R_{f}≤100% is experimentally investigated to reduce the form drag of a three-dimensional blunt body (the so-called squareback Ahmed body) at a Reynolds number Re=2.89×10^{5}. The drag reduction is achieved by a decrease of base suction (or, equivalently, the increase of pressure at the base). The plain cavity (R_{f}=100%) reduces the base suction by 22% compared to the case with no cavity (R_{f}=0). All intermediate filling ratio are obtained by the enlargement of the slits, initially having a zero width for the plain cavity case. It is shown that the gradual base suction change can be related to the level of the entrainment flux of the free shear layers developing from the rear separation and to the suppression of the transverse steady asymmetric instability of the wake. The model of the vortex formation region length of Gerrard [J. Fluid Mech. 25, 401 (1966)0022-112010.1017/S0022112066001721] is shown to provide an insightful interpretation of the drag reduction mechanism using ventilated base cavities.