Body Drag Research Articles

Internal waves generated by a stratified wake: experiment and theory

This paper presents experimental and theoretical results on the internal waves emitted by a bluff body moving horizontally in a linearly stratified fluid. Three different bluff bodies (a sphere, a spheroid and a cylinder) have been used in order to study the effect of the shape of the bluff body, although most of the results are obtained for the sphere. Two types of internal waves have been observed experimentally: large wavelength lee waves generated by the bluff body itself and small wavelength coherent wake waves generated by the turbulent wake. First, the lee waves are separated from the wake waves by averaging the experimental measurements in the frame moving with the bluff body. The velocity amplitude of the lee waves scales as the inverse of the Froude number $F=2U_{B}/(ND)$ for $F>2$ (where $U_{B}$ is the towing velocity, $D$ the diameter and $N$ the buoyancy frequency). This scaling proves that the internal waves are related to the drag of the bluff body which is due to the separation of the flow behind the bluff body. This separation is usually not taken into account in the classical models which assume that the flow is dipolar. The drag can be modelled as a point force in the Navier–Stokes equations, which gives a correct prediction of the structure and the amplitude of the lee waves. Second, the wake waves have been separated from the lee waves by averaging the velocity fields in the frame moving at the phase velocity of the waves. The phase velocity and the wavelength scale as $F^{-2/3}$ and $F^{1/3}$ respectively which correspond to the velocity and distance between same sign vortices of the von Kármán vortex street. A simplified model is derived for the internal waves emitted by the double row of moving point vortices of the von Kármán street. The amplitude of the wake waves is measured experimentally and seems to depend on the Reynolds number.

Swimming performance and unique wake topology of the sea hare (Aplysia)

The Aplysia, also known as 'sea hare', is a marine mollusc that swims via large-amplitude flapping. With flow simulations we find a unique pattern in its wake of three vortex ring-like structures which overcome body drag. Swimming performance, however, appears to lag behind that of related species.

A Similarity Criterion for Supersonic Flow Past a Cylinder with a Frontal High-Porosity Cellular Insert

We have experimentally and numerically studied the influence of the ratio of the diameter of a cylinder with a frontal gas-permeable porous insert made of nickel sponge to the average pore diameter in the insert on the aerodynamic drag of this model body in supersonic airflow (M∞ = 4.85, 7, and 21). The analytical dependence of the normalized drag coefficient on a parameter involving the Mach number and the ratio of cylinder radius to average pore radius in the insert is obtained. It is suggested to use this parameter as a similarity criterion in the problem of supersonic airflow past a cylinder with a frontal high-porosity cellular insert.

Mechanical power curve measured in the wake of pied flycatchers indicates modulation of parasite power across flight speeds.

How aerodynamic power required for animal flight varies with flight speed determines optimal speeds during foraging and migratory flight. Despite its relevance, aerodynamic power provides an elusive quantity to measure directly in animal flight. Here, we determine the aerodynamic power from wake velocity fields, measured using tomographical particle image velocimetry, of pied flycatchers flying freely in a wind tunnel. We find a shallow U-shaped power curve, which is flatter than expected by theory. Based on how the birds vary body angle with speed, we speculate that the shallow curve results from increased body drag coefficient and body frontal area at lower flight speeds. Including modulation of body drag in the model results in a more reasonable fit with data than the traditional model. From the wake structure, we also find a single starting vortex generated from the two wings during the downstroke across flight speeds (1-9 m s-1). This is accomplished by the arm wings interacting at the beginning of the downstroke, generating a unified starting vortex above the body of the bird. We interpret this as a mechanism resulting in a rather uniform downwash and low induced power, which can help explain the higher aerodynamic performance in birds compared with bats.

Критерий подобия сверхзвукового обтекания цилиндра с передней высокопористой ячеистой вставкой

AbstractWe have experimentally and numerically studied the influence of the ratio of the diameter of a cylinder with a frontal gas-permeable porous insert made of nickel sponge to the average pore diameter in the insert on the aerodynamic drag of this model body in supersonic airflow ( M _∞ = 4.85, 7, and 21). The analytical dependence of the normalized drag coefficient on a parameter involving the Mach number and the ratio of cylinder radius to average pore radius in the insert is obtained. It is suggested to use this parameter as a similarity criterion in the problem of supersonic airflow past a cylinder with a frontal high-porosity cellular insert.

Reducing Drag of Body by Adding a Plate

This paper focuses on theoretical investigations of the location of a separation point laminar boundary layer on the surface of a circular cylinder. The investigations were carried out by CFD software FlowSimulation. The displacement of the separation point on the surface of the circular cylinder is achieved by installing a flat plate in front of the cylinder parallel to the flow. It was found that the greatest displacement of the separation point to the back of the cylinder is possible when the chord of the plate equals to a quarter of the diameter of the circular cylinder. The flat plate allows not only to change the position of the separation point but also to reduce the drag by about 25%.

NUMERICAL ANALYSIS OF DRAG ON BLUNT BODIES WITH THE USE OF DIFFERENT CONICAL SPIKES AT SUPERSONIC SPEED

Aerospace vehicles such as re-entry vehicles, satellite launching vehicles, missiles, rockets move at very greater speed with variations in properties around the flow pattern. Greater amount of heat and drag are associated with these vehicles. Different spikes, bleeding devices, trailing flatters have been designed to reduce drag formed on aero-vehicles due to pressure, wave, skin friction factors. In present study, the blunt body drag is computed numerically by using Spallart-allmaras turbulence model for different angle of attack i.e 0,2,4,8 degrees at supersonic speed of Mach number 2 and conical spiked blunt body for L/D ratio of 1, 1.5, 2 at zero angle of attack has been computed. The front facing conical spike reduces the strong bow shock at front part of blunt body by converting it into oblique shock and shifting the reattachment point for different L/D ratio.

Parametric study on the drag and heat flux reduction mechanism of forward-facing cavity on a blunt body in supersonic flows

Drag and heat flux reduction is very important for the design of the hypersonic vehicle, and many novel strategies have been proposed recently. As one of the valid drag and heat flux reduction schemes, the forward-facing cavity has attracted an increasing attention for the scholars. In the current study, the influence of the cavity configuration on the drag and heat flux reduction mechanism of a blunt body has been investigated numerically by the two-dimensional axisymmetric Reynolds-averaged Navier–Stokes (RANS) equations coupled with the SST k–ω turbulence model, and the effects of the depth of the cavity and the angle of its trailing edge have been taken into consideration. At the same time, the numerical approaches employed in this paper have been validated against the available experimental data in the open literature, as well as the grid independency analysis. The obtained results shows that the cavity configuration has only a slight impact on the drag reduction of the blunt body in the supersonic flow with the freestream Mach number being 3.98, and the maximum drag reduction coefficient is within 1.0%. The forward-facing cavity with L/D=4.0 and θ=45° is the most beneficial configuration for the heat flux reduction of the blunt body in the range considered in the current study, and it should be validated by the wind tunnel test in the near future.

Body lift, drag and power are relatively higher in large-eared than in small-eared bat species.

Bats navigate the dark using echolocation. Echolocation is enhanced by external ears, but external ears increase the projected frontal area and reduce the streamlining of the animal. External ears are thus expected to compromise flight efficiency, but research suggests that very large ears may mitigate the cost by producing aerodynamic lift. Here we compare quantitative aerodynamic measures of flight efficiency of two bat species, one large-eared (Plecotus auritus) and one small-eared (Glossophaga soricina), flying freely in a wind tunnel. We find that the body drag of both species is higher than previously assumed and that the large-eared species has a higher body drag coefficient, but also produces relatively more ear/body lift than the small-eared species, in line with prior studies on model bats. The measured aerodynamic power of P. auritus was higher than predicted from the aerodynamic model, while the small-eared species aligned with predictions. The relatively higher power of the large-eared species results in lower optimal flight speeds and our findings support the notion of a trade-off between the acoustic benefits of large external ears and aerodynamic performance. The result of this trade-off would be the eco-morphological correlation in bat flight, with large-eared bats generally adopting slow-flight feeding strategies.

Hybrid electric vehicle car body drag analysis using computational fluid dynamics

Aerodynamics drag was considered for giving better efficiency for a car on the road. The development of drag on a vehicle body was studied. The main objective was to determine the maximum drag occurring on the car body drag of the hybrid electric vehicle (HEV). The analysis was conducted on the car speed ranging between 40 km/h to 110 km/h using pressure points at various parts of the car. The car body was modelled in CAD using the mass and momentum equations discretized for an analysis. The technique to estimate the drag used was input of car body-drag (CFD) and exporting it to finite element analysis (FEA) to find the value of aerodynamics drag in terms of drag forces and drag coefficient. The values were validated with actual pressure reading from pressure probes on the car body. The result showed that contour and trajectories plots were also used to analyse the characteristics of streamlines flow or boundary layer occurred on the body of this model, especially for the forebody, upper body and rear body. The maximum rise of aerodynamics drag occurred between velocities of 80 km/h and 90 km/h which was about 5.04 % increase of drag coefficient. Further analysis is required with wind tunnels.

Computational analysis of vortex dynamics and performance enhancement due to body–fin and fin–fin interactions in fish-like locomotion

Numerical simulations are used to investigate the hydrodynamic benefits of body–fin and fin–fin interactions in a fish model in carangiform swimming. The geometry and kinematics of the model are reconstructed in three-dimensions from high-speed videos of a live fish, Crevalle Jack (Caranx hippos), during steady swimming. The simulations employ an immersed-boundary-method-based incompressible Navier–Stokes flow solver that allows us to quantitatively characterize the propulsive performance of the fish median fins (the dorsal and the anal fins) and the caudal fin using three-dimensional full body simulations. This includes a detailed analysis of associated performance enhancement mechanisms and their connection to the vortex dynamics. Comparisons are made using three different models containing different combinations of the fish body and fins to provide insights into the force production. The results indicate that the fish produces high performance propulsion by utilizing complex interactions among the fins and the body. By connecting the vortex dynamics and surface force distribution, it is found that the leading-edge vortices produced by the caudal fin are associated with most of the thrust production in this fish model. These vortices could be strengthened by the vorticity capture from the vortices generated by the posterior body during undulatory motion. Meanwhile, the pressure difference between the two sides of posterior body resulting from the posterior body vortices (PBVs) helps with the alleviation of the body drag. The appearance of the median fins in the posterior region further strengthens the PBVs and caudal-fin wake capture mechanism. This work provides new physical insights into how body–fin and fin–fin interactions enhance thrust production in swimming fishes, and emphasizes that movements of both the body and fins contribute to overall swimming performance in fish locomotion.

Drag and heat flux reduction mechanism of blunted cone with aerodisks

The major challenge among a number of design requirements for hypersonic vehicles is the reduction of drag and aerodynamic heating. Of all these techniques of drag and heat flux reduction, application of forward facing aerospike conceived in 1950s is an effective and simpler technique to reduce the drag as well as the heat transfer rate for blunt nosed bodies at hypersonic Mach numbers. In this paper, the flow fields around a blunt cone with and without aerodisk flying at hypersonic Mach numbers are computed numerically, and the numerical simulations are conducted by specifying the freestream velocity, static pressure and static temperatures at the inlet of the computational domain with a three-dimensional, steady, Reynolds-averaged Navier-Stokes equation. An aerodisk is attached to the tip of the rod to reduce the drag and heat flux further. The influences of the length of rod and the diameter of aerodisk on the drag and heat flux reduction mechanism are analyzed comprehensively, and eight configurations are taken into consideration in the current study. The obtained results show that for all aerodisks, the reduction in drag of the blunt body is proportional to the extent of the recirculation dead air region. For long rods, the aerodisk is found not that beneficial in reducing the drag, and an aerodisk is more effective than an aerospike. The spike produces a region of recirculation separated flow that shields the blunt-nosed body from the incoming flow, and the recirculation region is formed around the root of the spike up to the reattachment point of the flow at the shoulder of the blunt body. The dynamic pressure in the recirculation area is highly reduced and thus leads to the decrease in drag and heat load on the surface of the blunt body. Because of the reattachment of the shear layer on the shoulder of the blunt body, the pressure near that point becomes large.

Oral cavity hydrodynamics and drag production in Balaenid whale suspension feeding.

Balaenid whales feed on large aggregates of small and slow-moving prey (predominantly copepods) through a filtration process enabled by baleen. These whales exhibit continuous filtration, namely, with the mouth kept partially opened and the baleen exposed to oncoming prey-laden waters while fluking. The process is an example of crossflow filtration (CFF) in which most of the particulates (prey) are separated from the substrate (water) without ever coming into contact with the filtering surface (baleen). This paper discusses the simulation of baleen filtration hydrodynamics based on a type of hydraulic circuit modeling commonly used in microfluidics, but adapted to the much higher Reynolds number flows typical of whale hydrodynamics. This so-called Baleen Hydraulic Circuit (BHC) model uses as input the basic characteristics of the flows moving through a section of baleen observed in a previous flume study by the authors. The model has low-spatial resolution but incorporates the effects of fluid viscosity, which doubles or more a whale’s total body drag in comparison to non-feeding travel. Modeling viscous friction is crucial here since exposing the baleen system to the open ocean ends up tripling a whale’s total wetted surface area. Among other findings, the BHC shows how CFF is enhanced by a large filtration surface and hence large body size; how it is carried out via the establishment of rapid anteroposterior flows transporting most of the prey-water slurry towards the oropharyngeal wall; how slower intra-baleen flows manage to transfer most of the substrate out of the mouth, all the while contributing only a fraction to overall oral cavity drag; and how these anteroposterior and intra-baleen flows lose speed as they approach the oropharyngeal wall.

On the problem of a free drag of a rigid body with a tapered front in a resisting medium

The author constructs a nonlinear mathematical model of a plane-parallel impact of the medium on a rigid body with a front part of the outer surface shaped as a circular cone. Multivariable analysis of the dynamic equations of motion was performed. A new family of phase patterns on the phase cylinder of quasi-velocities has been obtained. This family consists of infinitely numerous topologically inequivalent phase patterns. The sufficient conditions for the stability of an important mode of motion, i.e., rectilinear translational drag, have been obtained, as well as conditions for the presence of the self-oscillatory modes in the system.

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.

Effect of vortex generators on hydrodynamic behavior of an underwater axisymmetric hull at high angles of attack

Underwater vehicles when yawed to free stream flow, like submarines in a turning maneuver, produce large vortical separated regions. This vortical flow affects acoustic, body drag, control effectiveness and maneuverability. A suitable way to reduce the effects of this separated flow is to use vortex generators. The main goal of this study is to investigate the effect of the vortex generator on the flow field around a standard underwater model employing the vortex generators by using the oil flow visualization method in wind tunnel only in yaw direction (0° ≤ β ≤ 30° angles of attack, due to the experimental cost) and the CFD method in the pitch direction (0° ≤ α ≤ 30° angles of attack) and the yaw direction (0° ≤ β ≤ 30° angles of attack). Numerical simulations are performed for the investigation of the DARPA SUBOFF bare hull geometry with the vortex generators placed on it. The cross-flow interaction between the pressure side and the suction side is studied in the presence of angle of attack. For this study, the standard k-e model of turbulence is used for solving the Reynolds averaged Navier–Stokes equations (RANS). The effect of angle of attack on the flow structure, the force coefficients and the wall related flow variables are discussed in detail. The numerical simulations show that the strength of the separation is significantly reduced for the model with the vortex generators. The experimental method used for this study is the application of the oil flow visualizing method which can help one to precisely study the location of the flow separation and the cross-flow vortices around the SUBOFF submarine model. The results show that the vortex generators placed along the submarine do indeed significantly reduce the cross-flow separation on the submarine in a high incident angle.

Reducing Friction Drag on Flat Plates

There are in existence several methods of boundary layer control which have been developed for the purpose of reducing friction drag of streamlined bodies. These include such elements as acceleration of the boundary layer (blowing), injection of a different gas, cooling of the wall, etc. All of them are described in some detail in scientific literature. In this paper, a new method for the reduction of friction drag, which has been insufficiently studied. The proposed method is based on the use of a liquid film on the surface of the streamlined body. One solution to the problem of the flow over a flat semi-infinite plate set at an angle of deflection to the horizon, and having a thin liquid film on its surface by external airflow is presented. The film is formed at the plate in the form of raindrops. Liquid moves by gravity and friction on the outer surface of the film. Influence on the boundary layer raindrops and thermal effects are ignored. In general, the problem is conjugate, including the problem of a film flow (internal problem) and the problem of the boundary layer of incoming air (external problem). For the solution of the dual problem, a method of successive approximations was used. This method assumes the fact that the external and internal problems can be solved separately, either logically or iteratively. At each new approximation, the inner problem is solved with regard to the friction resulting from the external problem; the solution of which, in turn, takes into account the speed at the boundary surface, obtained from the previous approximation of the interior problem. Thus, the iterative process continues until the speed and, consequently, the friction at the phase interface change little from iteration to iteration. For the interior problem, the method of asymptotic expansion in a small parameter was used. This paper gives null and first approximation. The external problem was solved using the numerical method of finite differences. In this paper we present some results showing the reduction of friction drag on the flat plate.

Aerodynamics of Ascending Flight in Fruit Flies

As a very basic flight mode, ascending flight is obviously of great importance to all kinds of manmade and natural fliers. Yet, for the most commonly seen fliers – insects, researches on this flight mode are rare. In this paper, we combined both experimental measurements and numerical simulations to investigate the kinematical characteristics, aerodynamic performance and power requirement of ascending flight in fruit flies (Drosophila virilis). The flies ascend at an advance ratio of about 0.12. The most significant characteristic of ascending flight is larger stroke amplitude compared to hovering, while the other kinematics is very similar. From an aerodynamics point of view, this increased stroke amplitude is needed to overcome the negative effects of “downwash flow”, caused by the upward motion of the fly. Same as hovering, the ascending fruit flies utilize delayed stall and fast pitching-up mechanisms to generate the majority of the lift required for balancing the weight and body drag. By using a larger stroke-amplitude to overcome the negative effects of “downwash flow”, larger energy cost (about 20%) than that of equivalent hovering is required.

Extremum seeking to control the amplitude and frequency of a pulsed jet for bluff body drag reduction

Feedback control of fluid flows presents a challenging problem due to nonlinear dynamics and unknown optimal operating conditions. Extremum seeking control presents a suitable method for many flow control situations but involves its own challenges. In this paper, we provide a brief analysis of the extremum seeking method, with attention to modifications that we find to be advantageous. In particular, we present an adaptation for optimisation of the frequency of a harmonic input signal, a common scenario for open-loop flow control systems. We then present results from the experimental implementation of our modified method to the open-loop control system of Oxlade et al. (J Fluid Mech 770:305–318, 2015), an axisymmetric bluff-body wake, forced by a pulsed jet. We find that the system is able to achieve optimal operating conditions in both the amplitude and frequency of the harmonic input signal, and is able to largely reject the disturbances arising from measurements of a highly turbulent flow. We finally show the ability of the extremum seeking system to adapt to changing conditions.

Bluff body drag manipulation using pulsed jets and Coanda effect

The impact of fluidic actuation on the wake and drag of a three-dimensional blunt body is investigated experimentally. Jets blowing tangentially to the main flow force the wake with variable frequency and amplitude. Depending on the forcing conditions, two flow regimes can be distinguished. First, in the case of broadband actuation with frequencies comprising the natural wake time scale, the convection of the jet structures enhances wake entrainment, shortens the length of the recirculating flow and increases drag. Secondly, at higher actuation frequencies, shear-layer deviation leads to fluidic boat tailing of the wake. It additionally lowers its turbulent kinetic energy thus reducing the entrainment of momentum towards the recirculating flow. The combination of both mechanisms produces a rise in the base pressure and reduces the drag of the model. Both actuation regimes are characterized by complementary velocity, pressure and drag measurements at several upstream conditions and control parameters. By adding curved surfaces to deviate the jets by the Coanda effect, periodic actuation is reinforced and drag reductions of approximately 20 % are achieved. The unsteady Coanda blowing not only intensifies the flow deviation and the base pressure recovery but also preserves the unsteady high-frequency forcing effect on the turbulent field. The present results encourage further development of fluidic control to improve the aerodynamics of road vehicles and provide a complementary insight into the relation between wake dynamics and drag.