Pressure Drag Force Research Articles

Flow induced vibrations, drag force, and pressure drop in conduits covered with biofilm

Biofilm was grown in closed conduit reactors under turbulent flow conditions. Structural development of the biofilm suggests that individual microcolonies behave like blunt bodies shedding vortices. The microcolonies assumed elongated forms, termed "streamers", possibly because of an exerted pressure drag force. The streamers when entrained in the water flow vibrated rapidly dissipating kinetic energy from the bulk liquid. The energy was transferred through the biofllm causing the underlying microcolonies to oscillate. The measured pressure drop was partially attributed to the loss of energy due to these flow induced vibrations and oscillations.

Drag and wake modification of axisymmetric bluff bodies using Coanda blowing

This work investigates the ability of Coanda jet blowing to modify the base pressure of a cylindrical body aligned axially in a flow, and thereby, produce overall drag reduction. It is found that blowing through one or two slot jets concentric to the outer body circumference can significantly influence the entire base flow region. The recirculating wake is eliminated and is replaced by freestream fluid entrained by the Coanda blowing. Base pressure rises significantly and leads to drag reduction of up to 30% beyond the thrusting action of the Coanda jet. A comparison between the power savings through drag reduction and the power requirement of the Coanda jet demonstrates that net benefits are attainable at certain body geometries and flow conditions. By judiciously selecting the jet blowing velocity, it is possible to produce a nearly flat wake velocity profile requiring little net power. RAG reduction of immersed bodies is a subject with a long history, leading to the early concept of streamlining. The net drag force on a body may be considered the sum of viscous drag and pressure drag forces. For streamlined aero- dynamic and hydrodynamic bodies, the pressure drag is small, and current research is directed towards the application of laminar flow control (e.g., suction), turbulent viscous drag reduction (e.g., riblets and large eddy breakup devices (LEBUs)), and the use of nonlinear aerodynamics for induced drag reduction (e.g., winglets or crescent wing planforms). For bluff bodies, on the other hand, streamlining is usually not an option for reducing drag because the bluff shape is often dictated by other constraints. This situation is particu- larly true for trucks, buses, and most automobiles. For bluff bodies then, where pressure drag dominates, drag reduction is primarily through base flow modification, including flow separation control using airfoils, the use of plates, cavities, base bleed, and suction/blowing. Bluff bodies may be considered of two general classifica- tions—high and low aspect ratio. The former may be modeled as bodies of large span relative to a characteristic height, and are generally two-dimensional in nature. Examples would be the classic cylinder in crossflow, or a symmetric airfoil with a thick, blunt trailing edge. Drag reduction for such bodies has been performed13 with drag reductions of up to 64% reported in the literature. Low aspect ratio bodies have also been studied. Such bodies are characterized by three-dimensionality or axisymmetry, with a sphere being the classic example. Other examples would be cylinders of rectangular or circular cross section aligned axially in the flow direction. Work on drag reduction of such bodies has been reported in Refs. 4-12. A variation of these studies would include inclined base regions such as fast-back auto- mobiles and cargo transport aircraft. During the energy crisis of the seventies, renewed interest in vehicle drag led to many important new findings which are summarized in the books

Buried piezoresistive sensors by means of MeV ion implantation

Abstract High energy (MeV) ion implantation was applied to fabricate a buried piezoresistor for microsensors. This method makes it possible to bury a p+ layer without a highly doped n+ overlayer. The buried piezoresistor has the advantage of stability because the influence of surface electrical field variation to the resistance is much smaller than that of the conventional piezoresistor. Differential pressure type and drag force type micro liquid flow sensors were fabricated using the buried piezoresistor. The buried piezoresistor formed by MeV ion implantation acts stably enough to detect the strain even when it is used in water without passivation.

Flame structure and stabilization in a divergent flow. Eddy structure behind a circular cylinder in a cold divergent flow.

In order to elucidate the effects of positive pressure gradient on flame structure and stabilization mechanism, it is first necessary to clarify those on turbulence characteristics and eddy structure not only in a cold shear layer, but also in a cold wake flow behind a bluff body. In this study, the influences of pressure gradient in the flow direction on the surface pressure around a circular cylinder and the eddy-shedding process are investigated in a two-dimensional cold air flow without combustion. Flow visualization and an FFT analysis of the oscillating surface pressure are made to examine time-averaged and fluctuating properties concerning the eddy-shedding process. It is found that increasing the free stream pressure gradient causes a transition of the wake flow pattern from an alternative eddy-shedding type to a symmetric one without Karman vortex. An increase in the pressure gradient extends the wake region, and simultaneously lowers the pressure drag force, which may be regarded as one of the most important characteristics of the divergent flow.

Effects of Toroidal Forces in Current Loops Embedded in a Background Plasma: Erratum

Abstract : A study is made of dynamical properties of a three-dimensional semi-toroidal current loop embedded in a high-temperature stratified background plasma. The model loop carries a toroidal current density J sub t and poloidal current density J sub p, producing the magnetic field components B sub p and B sub t, respectively. Starting with a non-forces-free current loop in stable MHD equilibrium described by (i/c) J x B - div p O where the major radial forces as well as the minor radial forces are explicitly balanced, the dynamical properties resulting from major radial pertubations are investigated. The condition for instability is given in terms of a circuit parameter epsilon which is a measure the flux associated with the overall current distribution. The current loop and is due to the toroidal geometry of the pressure gradient and drag force due to the ambient gas. For the equilibrium loops studied, the motion is found to be subsonic. Time evolution of the loops and the magnetic energy converted via drag heating are presented. Results are also presented for loops with relatively strong current and magnetic fields which are not in equilibrium initially. It is found that for sufficiently large current, a current loop can be driven supersonically through the background gas, accompanied by shocks and rapid shock heating of the ambient gas. The wide range of dynamical behavior exhibited by current loops, and the pros and cons of the model are discussed in the context of current loops in the solar corona.

Effects of toroidal forces in current loops embedded in a background plasma

Abstract : A study is made of dynamical properties of a three-dimensional semi-toroidal current loop embedded in a high-temperature stratified background plasma. The model loop carries a toroidal current density J sub t and poloidal current density J sub p, producing the magnetic field components B sub p and B sub t, respectively. Starting with a non-forces-free current loop in stable MHD equilibrium described by (i/c) J x B - div p O where the major radial forces as well as the minor radial forces are explicitly balanced, the dynamical properties resulting from major radial pertubations are investigated. The condition for instability is given in terms of a circuit parameter epsilon which is a measure the flux associated with the overall current distribution. The current loop and is due to the toroidal geometry of the pressure gradient and drag force due to the ambient gas. For the equilibrium loops studied, the motion is found to be subsonic. Time evolution of the loops and the magnetic energy converted via drag heating are presented. Results are also presented for loops with relatively strong current and magnetic fields which are not in equilibrium initially. It is found that for sufficiently large current, a current loop can be driven supersonically through the background gas, accompanied by shocks and rapid shock heating of the ambient gas. The wide range of dynamical behavior exhibited by current loops, and the pros and cons of the model are discussed in the context of current loops in the solar corona.

Morphological adaptation of shape to flow: Microcurrents around lotic macroinvertebrates with known Reynolds numbers at quasi-natural flow conditions.

Using Laser Doppler Anemometry we measured current velocities in the median plane around dead lotic macroinvertebrates in a flume which reproduced natural near bottom hydraulics. We investigated specimens of the gastropods Ancylus, Acroloxus, and Potamopyrgus, the amphipod Gammarus, and the larval caddisflies Anabolia, Micrasema, and Silo of various size, various alignment to the flow or which were otherwise manipulated in order to clarify certain questions of adaptation of shape or case building style to flow, or the effects of flow on field distribution patterns. The steepest velocity gradients close to the animals were found near areas of their bodies protruding furthest into the flow. In such regions the rates of potential diffusive exchange processes, the potential corrasion (abrasion through suspended solids), and, for larger specimens, the lift forces (directed towards the water surface) must be highest. Posterior of these areas growing boundary layers formed above those species whose upper contour was approximately parallel to the upstream-downstream direction of the flow. All specimens removed momentum from the flow and thus experience a drag force (directed downstream). From the complete data set we derived the following general conclusions about the physical effects of potential morphological adaptations, taking into consideration diffusion through boundary layers, corrasion, lift forces, friction and pressure drag forces: The physical significance of these five factors generally depends on the Reynolds number of an animal and is largely affected by flow separation, which was significantly related to the ratio of body length to height and the slope of the posterior contour. A simultaneous effective morphological adaptation to all five factors is physically impossible and, in addition, would have to change from life at low (e.g. a young, small specimen of a species) to life at high (e.g. a fully grown specimen of the same species) Reynolds number.

On the drag coefficient of triangular profiles

Results of a laboratory study on the determination of the drag coefficient of triangular profiles simulating a train of sedimentary dunes are here presented. The study is carried out in a low speed wind tunnel by performing a direct measurement of the total drag force acting on one of the profiles and then subtracting the skin shear force to obtain the net pressure drag force. Drag coefficient behavior is studied as a function of the profile Reynolds number. Results are then discussed using some recent criteria available in literature.

並列２長方形断面柱まわりの流れの可視化

In the previous papers, we measured pressure distributions, lift and drag forces and Strouhal numbers of two rectangular cylinders arranged side-by-side in a wind tunnels. Then, we observed flow-patterns around two cylinders by the aid of the visualization technique of a smoke-wire, changing the shape of a cross-section of models (B/H) and a gap distance (S/H) between two models. It can be clarified that a change of flow-patterns corresponds to one of aerodynamic forces measured. It was found that there occurs a biassed flow under the critical gap distance, and that bi-stable flow-patterns of the biassed and the non-biassed flows alternately appear just at the critical gap distance.

Radiation forces on small particles in the solar system

We present a new and more accurate expression for the radiation pressure and Poynting-Robertson drag forces; it is more complete than previous ones, which considered only perfectly absorbing particles or artificial scattering laws. Using a simple heuristic derivation, the equation of motion for a particle of mass m and geometrical cross section A, moving with velocity v through a radiation field of energy flux density S, is found to be (to terms of order v c ) m v ̇ = ( SA c )Q pr [(1 − r ̇ c ) S ̂ − v c ] , where Ŝ is a unit vector in the direction of the incident radiation, r ̇ is the particle's radial velocity, and c is the speed of light; the radiation pressure efficiency factor Q pr ≡ Q abs + Q sca(1 − 〈cos α〉), where Q abs and Q sca are the efficiency factors for absorption and scattering, and 〈cos α〉 accounts for the asymmetry of the scattered radiation. This result is confirmed by a new formal derivation applying special relativistic transformations for the incoming and outgoing energy and momentum as seen in the particle and solar frames of reference. Q pr is evaluated from Mie theory for small spherical particles with measured optical properties, irradiated by the actual solar spectrum. Of the eight materials studied, only for iron, magnetite , and graphite grains does the radiation pressure force exceed gravity and then just for sizes around 0.1 μm; very small particles are not easily blown out of the solar system nor are they rapidly dragged into the Sun by the Poynting-Robertson effect. The solar wind counterpart of the Poynting-Robertson drag may be effective, however, for these particles. The orbital consequences of these radiation forces-including ejection from the solar system by relatively small radiation pressures-and of the Poynting-Robertson drag are considered both for heliocentric and planetocentric orbiting particles. We discuss the coupling between the dynamics of particles and their sizes (which diminish due to sputtering and sublimation). A qualitative derivation is given for the differential Doppler effect, which occurs because the light received by an orbiting particle is slightly red-shifted by the solar rotation velocity when coming from the eastern hemisphere of the Sun but blue-shifted when from the western hemisphere; the ratio of this force to the Poynting-Robertson force is ( R ⊙ r ) 2[( w ⊙ n ) − 1] , where R ⊙ and w ⊙ are the solar radius and spin rate, and n is the particle's mean motion. The Yarkovsky effect, caused by the asymmetry in the reradiated thermal emission of a rotating body, is also developed relying on new physical arguments. Throughout the paper, representative calculations use the physical and orbital properties of interplanetary dust, as known from various recent measurements.

Mechanics of Escape Responses in Crayfish (Orconectes Virilis)

ABSTRACT Measurements of acceleration performance of crayfish (mean mass 0·018 kg) were made during lateral giant mediated tail flips (LG tail flips) and truncated tail flips at 15°C. The LG tail flip power stroke was composed of a lift-off phase, when crayfish accelerated vertically from the substrate, and a free swimming phase. The total duration of the power stroke was 44 ms, followed by a recovery stroke lasting 173 ms. Truncated tail flips were used in acceleration and swimming by crayfish free of the substrate. Power strokes had a mean duration of 36 ms, and recovery strokes 92 ms. Net velocities, acceleration rates, and distances travelled by the centre of mass were similar for both types of tail flips. Thrust was generated almost entirely by the uropods and telson. Velocities and angles of orientation to the horizontal of abdominal segments were similar for both types of tail flip. Angles of attack were large, varying from 30° to 90°. Pressure (drag) forces were considered negligible compared to inertial forces associated with the acceleration of added water mass. Thrust forces, energy and power were determined for exemplary tail flips. Thrust was 0·92 and 0·42 N for LG tail flip lift-off and swimming phases respectively, and 0·29 N for the swimming truncated tail flip. Rates of working were 0·39, 0·19, and 0·18 W respectively. The efficiency of converting muscle power to backward motion was estimated to be 0·5 for power strokes and o-68 for complete swimming cycles. Comparisons with fish performance suggested fish would be less efficient (0·1–0·2). The low efficiency is attributed to energy lost in lateral recoil movements.

A Finite Cavity Flow about a Slender Body of Revolution in a Tube

In order to estimate a pressure drag force acting on an axisymmetric body of revolution located on the axis of a tube, a slender cone where it is easy to find a separation point is used as a body. Both the case where the blockage ratio (the ratio of maximum cross sectional area of body to cross sectional area of tube) is small and it is large are analyzed with a cavity model of open type by a slender body theory. Drag force coefficients and cavitation numbers are calculated. According to the result, when the blockage ratio is large, the drag force coefficients are hardly influenced by the forms of the cavity, and they become very large as the blockage ratio increases.

1. Radiation Pressure and Poynting-Robertson Drag for Small Spherical Particles

A new heuristic derivation of the radiation pressure and Poynting-Robertson drag forces is presented for particles with general optical properties; previous derivations considered only perfectly absorbing materials. The equation of motion for a particle of mass m and geometrical cross-section A, moving with velocity v through a radiation field of energy flux F, is (to terms of order v/c)where r̂ is the radial unit vector, ṙ is the radial velocity, and c is the speed of light. The radiation pressure efficiency factor Qpr includes both scattering and absorption, it is evaluated using Mie theory for small spherical particles with measured optical properties that are irradiated by the actual solar spectrum. Very small particles (<0.01 μm) are not substantially affected by radiation forces.

Time development of the flow about an impulsively started cylinder

A method is developed to determine the time-dependent flowfield about an impulsively started circular cylinder. An outer potential flow model is interfaced with an inner viscous flow region. The wake is described by a set of elementary point vortices. The position at which the point vortices are shed from the cylinder is obtained from a solution to the unsteady incompressible laminar boundary-layer equations. A rear shear-layer is postulated to account for backflow induced vorticity. Wake development is detailed from the initial formation of the two symmetric vortices to subsequent asymmetry and eventual alternate shedding. Unsteady pressure distributions, lift and drag forces, and Strouhal number are calculated and compared with experiment.

Measurement of the Mean Pressure Distribution About a Double-wedge Strut in Turbulent Shear Flow

SummarySurface pressure distributions have been measured on a cylinder with a double-wedge cross section in a two-dimensional channel flow. The flow is sheared in the direction of the cylinder axis, using a grid of parallel wires with variable spacing, in order to simulate the type of geometry and velocity variation which occur for annulus support struts in turbomachines. The strut had a 38° included angle at the leading and trailing edges and an aspect ratio of 1.33 based on the maximum thickness. This was fixed between the parallel walls of a rectangular wind tunnel with end leakage eliminated. Surface pressure distributions were measured over a Reynolds number range of 0.5 × 105 to 1.5 × 105 for three different shear flow conditions. Despite the presence of sharp edges at the mid-chord position, delayed boundary-layer separation beyond the plane of maximum thickness has been found for sufficiently high spanwise shear in the upstream flow. Surprisingly, therefore, some similarity with previous results for struts of circular cross section appears to exist. The influence of the shear flow conditions on the pressure drag force has been determined by numerical integration of the data. Only a slight dependence on the upstream flow conditions is found, although the effect of secondary velocity components may still be observed if the spanwise variations in the local drag force are examined.

Dynamic Response of Suspended Underwater Systems

A method is presented for analyzing the steady-state or stationary random heave response of suspended underwater systems. The nonlinear continuous system is replaced by a discretized pseudolinear system by means of an equation difference minimization technique. The resulting pseudolinear system is solved by an iterative scheme. An example of the application of the proposed method of analysis to a suspended hydrophone system is given. The example shows that the nonlinear pressure drag forces have a very significant effect on the over-all response of the system. The nature of the response is discussed and some practical conclusions are given.

The drift-flux asymptotic limit of barotropic two-phase two-pressure models

We study the asymptotic behavior of the solutions of barotropic two-phase two-pressure models, with pressure relaxation, drag force and external forces. Using Chapman-Enskog expansions close to the expected equilibrium, a drift-flux model with a Darcy type closure law is obtained. Also, restricting this closure law to permanent flows (defined as steady flows in some Lagrangian frame), we can obtain a drift-flux model with an algebraic closure law, in the spirit of Zuber-Findlay models. The example of a two-phase flow in a vertical pipe is described.