Unsteady Hydrodynamic Forces Research Articles

Spatio–temporal analysis of hydrodynamic forces on the particle bed in an oscillatory flow environment

A numerical investigation of unsteady hydrodynamic forces on the particle bed in an oscillatory flow environment is performed by means of direct numerical simulations. Statistical descriptions of drag and lift forces for two particle sizes of diameter 372 and 125 in wall units in a very rough turbulent flow regime are reported. Characterization of unsteady forces in terms of spatial distribution, temporal autocorrelation, force spectrum as well as cross-correlations with measurable flow variables is carried out. Based on the concept of impulse, intermittency in the drag and lift forces is also investigated. Temporal correlations show drag and lift to be positively correlated with a time delay that is approximately equal to the Taylor micro-scale related to the drag/lift fluctuations. The force spectra for drag and lift reveal roughly two scaling regions,$-11/3$and$-7/3$; the former typically represents turbulence–mean-shear interactions, whereas the latter indicates dominance of turbulence–turbulence interactions. Particle forces are strongly correlated with streamwise velocity and pressure fluctuations in the near-bed region for both flow cases. In comparison to the large-diameter particle case, the spatial extent of these correlations is 2–3 times larger in homogeneous directions for the small sized particle, a feature that is reminiscent of longer near-bed structures. For both large- and small-particle cases, it is shown that the distributions of drag (lift) fluctuations, in particular, peakedness and long tails, match remarkably well with fourth-order Gram–Charlier distributions of velocity (pressure) fluctuations. Furthermore, it is demonstrated that the intermittency is larger in the case of the lift force compared to that for the drag in both flow cases. Distributions of impulse events are heavily and positively skewed and are well described by a generalized extreme value distribution.

2006A-G3-27 離散渦法による傾斜円柱に働く非定常流体力の数値シミュレーション法(一般講演(G3))

2006A-G3-27 離散渦法による傾斜円柱に働く非定常流体力の数値シミュレーション法(一般講演(G3))

Investigation of the correlation between noise & vibration characteristics and unsteady flow in a circulator pump

Circulator pumps have wide engineering applications but the acoustics, vibration and unsteady flow structures of the circulator pump are still not fully understood. We investigated the noise and vibration characteristics and unsteady flow structures in a circulator pump at different flow rates. Three-dimensional, unsteady RANS equations were solved on high-quality structured meshes with SST k-ω turbulence model numerically. Measurements were made in a semi-anechoic chamber to get an overview of noise and vibration level of a pump at different flow rates. The 1/3 octave-band filter technique was applied to obtain the explicit frequency spectra of sound, pressure fluctuations and vibration signals and their principal frequencies were identified successfully. The air-borne noise level of the designed condition is lower than that of the off-design conditions, and the highest sound pressure level is found at part-load condition. The acoustic emission from the pump is mainly caused by unsteady flow structures and pressure fluctuations. In addition, both the link between airborne noise and pressure fluctuation, and the correlation between vibration and unsteady hydrodynamic forces, were quantitatively examined and verified. This work offers good data to understand noise and vibration characteristics of circulator pumps and the relationships among the noise, vibration and unsteady flow structures.

2016S-GS1-5 複雑形状大型浮体に働く非定常流体力への潮流影響に関する時間領域計算

2016S-GS1-5 複雑形状大型浮体に働く非定常流体力への潮流影響に関する時間領域計算

The unsteady hydrodynamic force during the collision of two spheres in a viscous fluid

The time-dependent Stokes equations were solved in the vicinity of two spheres colliding in a viscous fluid with viscosity ν to determine the rate of change of the hydrodynamic forces during large accelerations associated with Hertzian mechanical contact of small duration \({\tau_{\rm c}}\). It was assumed that the gap clearance remains finite during contact and is approximately equal to the height σ of surface micro-asperities. The initial condition corresponds to the steady-state axisymmetric solution of Cooley and O’Neill (Mathematika 16:37–49, 1969), and the initial value problem for the time-dependent Stokes streamfunction was solved using Laplace transform methods. Assuming that σ is small compared to the sphere radius a, we used singular perturbation expansions and tangent-sphere coordinates to obtain an asymptotic solution for the viscous flow in the gap and around the moving sphere. The solution provides the dependence of the resistance, added mass and history forces on σ, the sphere velocity and acceleration, and the ratio of the sphere diameters. We found that the relative importance of viscous and mechanical forces during contact depends on a new Stokes number \({St_{\rm c}=\sigma^2/\nu \tau_{\rm c}}\). Integration of Newton’s equation for the motion of the sphere during mechanical contact showed that there is a critical \({St_{\rm c}=O(\sigma/a)}\) for which there is no rebound at the end of contact.

Numerical study of low-Reynolds number flow over rotating rigid helix: an investigation of the unsteady hydrodynamic force

A helical flagellum filament can be modeled as a rigid helix. Numerical simulations of the unsteady flow around a rigid, finite length rotating helix are performed to examine the characteristics of the hydrodynamic force exerted on the helix by the fluid medium. Newtonian and non-Newtonian fluid models are used. The Reynolds numbers of the flows simulated range between and Where appropriate, the calculated mean thrusts, obtained by using three different numerical solvers, are compared with that based on theoretical prediction. The simulated mean thrusts agree well with the theoretical predictions. Analyses show that hydrodynamic force variations, although of small amplitudes, are dominated by distinct discrete modes that are higher harmonics of the rotation frequency of the helix for the low-Reynolds number flows simulated. Simple correlations are developed for the dominant frequencies, where present, observed in the simulations of the helix with different sizes, rotation frequency, and fluid viscosity.

Component Analysis of Unsteady Hydrodynamic Force of Closed-Type Centrifugal Pump with Single Blades of Different Blade Outlet Angles

Single-blade centrifugal impellers for sewage systems undergo both unsteady radial and axial thrusts. Therefore, it is extremely important for the improvement of pump reliability to quantitatively grasp these fluctuating hydrodynamic forces and determine the generation mechanism behind them. In this study, we conducted component analyses of radial and axial thrusts of closed, single-blade centrifugal pumps with different blade outlet angles by numerical analysis while considering leakage flow. The results revealed the effect of the blade outlet angle on the components of radial and axial thrusts. For increased flow rates, the time-averaged values of the pressure component were similar for all impellers, although its fluctuating components were higher for impellers with larger blade outlet angles. Moreover, the fluctuating inertia component of the impeller with a blade outlet angle of 8° decreased as the flow rate increased, whereas those with 16° and 24° angles increased. Therefore, the radial thrust on the hydraulic part was significantly higher for impellers with high blade outlet angles.

Characterization of Oscillatory Boundary Layer over a Closely Packed Bed of Sediment Particles

Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport. To study pickup and transport of sediment, it is necessary to have a detailed knowledge of the small amplitude oscillatory flow over the sediment layer near the sea bed. Fully resolved direct numerical simulations are performed using fictitious domain approach (Apte & Finn, 2012) to investigate the effect of a sinusoidally oscillating flow field over a rough wall made of regular hexagonal pack of spherical particles. The flow arrangement is similar to the experimental data of Keiller & Sleath (1976). Transitional and turbulent flows at Reδ = 95, 150, 200 (based on the Stokes layer thickness, [Formula: see text] are studied. Turbulent flow is characterized in terms of coherent vortex structures, Reynolds stress variation and PDF distributions. The nature of unsteady hydrodynamic lift forces on sediment grains and their correlation to sweep and burst events is also reported. The dynamics of the oscillatory flow over the sediment bed is used to understand the mechanism of sediment pick-up.

Neural network-based adaptive control for a supercavitating vehicle in transition phase

A supercavitation is a modern technology that can reduce the frictional resistance of an underwater vehicle. To reach supercavity, which is when the cavity envelops the entire vehicle body, a vehicle passes through the transition phase from fully wetted to supercavitating operation where unsteady hydrodynamic forces and moments are created by partial cavity. This paper presents analytical and numerical investigations into the dynamics of a supercavitating vehicle in the transition phase as well as a neural network-based adaptive control method that guarantees a stable control performance with modeling uncertainty. The ventilated cavity model is used for the rapid supercavity condition, which is when the cavitation number is relatively high. The cavity profile decides the immersion depths of the fins and body to calculate the hydrodynamical effects on the body. The supercavitating system in the transition phase is replaced by linearized pitch motion dynamics to verify the performance of the design controller. In addition, a numerical simulation shows how the proposed control method can compensate for modeling uncertainty.

Unsteady hydrodynamic forces acting on a hand and its flow field during sculling motion

The goal of this research is to clarify the mechanism by which unsteady forces are generated during sculling by a skilled swimmer and thereby to contribute to improving propulsive techniques. We used particle image velocimetry (PIV) to acquire data on the kinematics of the hand during sculling, such as fluid forces and flow field. By investigating the correlations between these data, we expected to find a new propulsion mechanism. The experiment was performed in a flow-controlled water channel. The participant executed sculling motions to remain at a fixed position despite constant water flow. PIV was used to visualize the flow-field cross-section in the plane of hand motion. Moreover, the fluid forces acting on the hand were estimated from pressure distribution measurements performed on the hand and simultaneous three-dimensional motion analysis. By executing the sculling motion, a skilled swimmer produces large unsteady fluid forces when the leading-edge vortex occurs on the dorsal side of the hand and wake capture occurs on the palm side. By using a new approach, we observed interesting unsteady fluid phenomena similar to those of flying insects. The study indicates that it is essential for swimmers to fully exploit vortices. A better understanding of these phenomena might lead to an improvement in sculling techniques.

Time-Accurate Analysis of the Viscous Flow Around Puller Podded Drive Using Sliding Mesh Method

In this paper a computational method is presented for predicting the unsteady hydrodynamic forces acting on podded drive components. These numerical simulations are performed with the aim of accurately studying the interaction between the propeller, the pod, and the strut. In order to simulate the unsteady viscous flow around a puller type podded drive, a Reynolds-Averaged Navier–Stokes (RANS) solver is used. The time-accurate calculations are made by applying the sliding mesh method. Structured and unstructured mesh techniques are used for the propeller and podded drive. The method is applied in the case of the straight condition. The unsteady propeller thrust and torque coefficient fluctuations are predicted for advance velocity ratios ranging from 0.2 to 1.0. The time averaged forces of the podded drive obtained by an unsteady analysis are compared to and verified by the steady result and the experimental data. Finally, discrepancies between the simulation results and the experimental data have been quantitatively evaluated in terms of the relative percentage error for the propulsive characteristics.

Numerical prediction of hydrodynamic forces on a ship passing through a lock

While passing through a lock, a ship usually undergoes a steady forward motion at low speed. Owing to the size restriction of lock chamber, the shallow water and bank effects on the hydrodynamic forces acting on the ship may be remarkable, which may have an adverse effect on navigation safety. However, the complicated hydrodynamics is not yet fully understood. This paper focuses on the hydrodynamic forces acting on a ship passing through a lock. The unsteady viscous flow and hydrodynamic forces are calculated by applying an unsteady RANS code with a RNG k-ɛ turbulence model. User-defined function (UDF) is compiled to define the ship motion. Meanwhile, the grid regeneration is dealt with by using the dynamic mesh method and sliding interface technique. Numerical study is carried out for a bulk carrier ship passing through the Pierre Vandamme Lock in Zeebrugge at the model scale. The proposed method is validated by comparing the numerical results with the data of captive model tests. By analyzing the numerical results obtained at different speeds, water depths and eccentricities, the influences of speed, water depth and eccentricity on the hydrodynamic forces are illustrated. The numerical method proposed in this paper can qualitatively predict the ship-lock hydrodynamic interaction. It can provide certain guidance on the manoeuvring and control of ships passing through a lock.

초공동 수중운동체의 천이구간 특성을 고려한 동역학 모델링 및 심도제어 연구

A supercavitation is modern technology that can be used to reduce the frictional resistance of the underwater vehicle. In the process of reaching the supercavity condition which cavity envelops whole vehicle body, a vehicle passes through transition phase from fully-wetted to supercaviting operation. During this phase of flight, unsteady hydrodynamic forces and moments are created by partial cavity. In this paper, analytical and numerical investigations into the dynamics of supercavitating vehicle in transition phase are presented. The ventilated cavity model is used to lead rapid supercavity condition, when the cavitation number is relatively high. Immersion depth of fins and body, which is decided by the cavity profile, is calculated to determine hydrodynamical effects on the body. Additionally, the frictional drag reduction associated by the downstream flow is considered. Numerical simulation for depth tracking control is performed to verify modeling quality using PID controller. Depth command is transformed to attitude control using double loop control structure.

Unsteady hydrodynamic forces acting on a robotic arm and its flow field: Application to the crawl stroke

This study aims to clarify the mechanisms by which unsteady hydrodynamic forces act on the hand of a swimmer during a crawl stroke. Measurements were performed for a hand attached to a robotic arm with five degrees of freedom independently controlled by a computer. The computer was programmed so the hand and arm mimicked a human performing the stroke. We directly measured forces on the hand and pressure distributions around it at 200Hz; flow fields underwater near the hand were obtained via 2D particle image velocimetry (PIV). The data revealed two mechanisms that generate unsteady forces during a crawl stroke. One is the unsteady lift force generated when hand movement changes direction during the stroke, leading to vortex shedding and bound vortex created around it. This bound vortex circulation results in a lift that contributes to the thrust. The other occurs when the hand moves linearly with a large angle of attack, creating a Kármán vortex street. This street alternatively sheds clockwise and counterclockwise vortices, resulting in a quasi-steady drag contributing to the thrust. We presume that professional swimmers benefit from both mechanisms. Further studies are necessary in which 3D flow fields are measured using a 3D PIV system and a human swimmer.

Effect of Blade Outlet Angle on Unsteady Hydrodynamic Force of Closed-Type Centrifugal Pump with Single Blade

Geometrically, the single-blade centrifugal impeller, commonly used today as a sewage pump, is not axially symmetric. For this reason, the static pressure around the impeller fluctuates greatly when the impeller is rotating, and not only the radial thrust but also the axial thrust shows large fluctuations. Therefore, it is extremely important for the improvement of pump reliability to quantitatively grasp these fluctuating hydrodynamic forces. In this study, we investigated the unsteady hydrodynamic forces in a closed-type centrifugal pump with a single blade for different blade outlet angles using a numerical analysis that takes into account both experiment and the leakage flow. The results clearly showed the effect of the blade outlet angle on that act on the impeller. The root-mean-square value of the fluctuating component of the total radial thrust was roughly the same for whichever impeller at low flow rate, but at high flow rates, the value increased for impellers with larger blade outlet angles. Moreover, when the leakage flow rate increased with increasing static pressure around the impeller, such that the rear and front shroud parts were subject to high pressure, the absolute value of the axial thrust on both these parts increased.

Unsteady hydrodynamic forces acting on a robotic hand and its flow field

This study aims to clarify the mechanism of generating unsteady hydrodynamic forces acting on a hand during swimming in order to directly measure the forces, pressure distribution, and flow field around the hand by using a robotic arm and particle image velocimetry (PIV). The robotic arm consisted of the trunk, shoulder, upper arm, forearm, and hand, and it was independently computer controllable in five degrees of freedom. The elbow-joint angle of the robotic arm was fixed at 90°, and the arm was moved in semicircles around the shoulder joint in a plane perpendicular to the water surface. Two-component PIV was used for flow visualization around the hand. The data of the forces and pressure acting on the hand were sampled at 200Hz and stored on a PC.When the maximum resultant force acting on the hand was observed, a pair of counter-rotating vortices appeared on the dorsal surface of the hand. A vortex attached to the hand increased the flow velocity, which led to decreased surface pressure, increasing the hydrodynamic forces. This phenomenon is known as the unsteady mechanism of force generation. We found that the drag force was 72% greater and the lift force was 4.8 times greater than the values estimated under steady flow conditions. Therefore, it is presumable that swimmers receive the benefits of this unsteady hydrodynamic force.

0429 クローズド形二枚羽根遠心ポンプの変動流体力に及ぼすケーシングスロート面積の影響(OS4-6 流体機械の研究開発とそれに関連した複雑流動現象,OS4 流体機械の研究開発とそれに関連した複雑流動現象)

0429 クローズド形二枚羽根遠心ポンプの変動流体力に及ぼすケーシングスロート面積の影響(OS4-6 流体機械の研究開発とそれに関連した複雑流動現象,OS4 流体機械の研究開発とそれに関連した複雑流動現象)

Error Estimation for Neglecting Unsteady Hydrodynamic Forces in the Equation of Motion for an Immersed Pendulum

ABSTRACTThe unsteady motion of a fully immersed solid sphere at low to moderate particle Reynolds number, from 1 to about 1600, has been modeled by considering the quasi-steady viscous drag, the added mass force, and the history force. The last force results from unsteady boundary layer growth and is often neglected in multiphase flow applications due to its complex formula. However, it has been shown that this unsteady viscous force is more crucial than the widely employed added mass force to describe an unsteady sphere motion. Thus, targeting the flow regime when the quasi-steady viscous drag is dominating, this work proposes two simple formulae to approximate the errors of neglecting either the history force or the added mass force in describing a fully immersed spherical pendulum motion. The proposed formulae are shown to capture the actual error when the numerical solution of a partial model that omits one force component is compared to the full model prediction when the sphere density is not too close to the ambient liquid.

Challenging zebrafish escape responses by increasing water viscosity

Escape responses of fishes have long been studied as a model locomotor behavior in which hypothesized maximal or near-maximal muscle power output is used to generate rapid body bending. In this paper we present the results of experiments that challenged zebrafish (Danio rerio) to perform escape responses in water of altered viscosity, to better understand the effects that the fluid mechanical environment exerts on kinematics. We quantified escape kinematics using 1000 frames s(-1) high-speed video, and compared escape response kinematics of fish in three media that differed in viscosity: 1 mPa s (normal water), 10 mPa s and 20 mPa s (20 times normal water viscosity). We hypothesized that because viscosity is increased but not density there will be a different effect on kinematic variables resulting from unsteady (acceleration-dependent) hydrodynamic forces and steady (velocity-dependent) ones. Similarly, we hypothesized that the kinematics of stage 1 will be less affected by viscosity than those of stage 2, as higher angular velocities are reached during stage 1 resulting in higher Reynolds numbers. Our results showed a significant overall effect of viscosity on escape response kinematics but the effect was not in accordance with our predictions. Statistical tests showed that increasing viscosity significantly decreased displacement of the center of mass during stage 1 and after 30 ms, and decreased maximum velocity of the center of mass, maximum angular velocity and acceleration during stage 1, but increased time to maximum angular acceleration and time to maximum linear velocity of the center of mass. Remarkably, increasing water viscosity 20 times did not significantly affect the duration of stage 1 or stage 2.

Generalized two-dimensional Lagally theorem with free vortices and its application to fluid–body interaction problems

AbstractThe Lagally theorem describes the unsteady hydrodynamic force on a rigid body exhibiting arbitrary motion in an inviscid and incompressible fluid by the properties of the singularities employed to generate the flow and the body motion and to meet the boundary condition. So far, only sources and dipoles have been considered, and the present work extends the theorem to include free vortices in a two-dimensional flow. The present extension is validated by reproducing the system dynamics or the force evolution of three literature problems: (i) a free cylinder interacting with a free vortex; (ii) the moving Föppl problem; and (iii) a cylinder in constant normal approach to a fixed identical cylinder. This work further extends the bifurcation analysis on the moving Föppl problem by including the solid-to-liquid density ratio as a new parameter, in addition to the system total impulse and the vortex strength. We then apply the theorem to the problem where a moving Föppl system is made to approach a fixed or a free neutrally buoyant target cylinder of identical size from far away. The force developed on each cylinder is examined with respect to the vortex pair configuration and the target mobility. When approaching a fixed target, a greater force is developed if the vortex pair has stronger circulation and larger structure. The mobility of the target cylinder, however, can modify the hydrodynamic force by reducing its magnitude and reversing the force ordering with respect to the vortex pair configuration found for the case with fixed target. Possible mechanisms for such a change of force nature are given based on the currently derived equation of motion.