Start-up Motion Research Articles

On the starting vortex generated by a translating and rotating flat plate

Abstract

Numerical simulation of ferrofluid-lubricated rough elliptical contact with start-up motion

As exotic magnetic colloids, ferromagnetic ferrofluids have excellent tribological characteristics for many applications in mechanical environments. The anti-wear issues of extremely unstable technological conditions such as frequent start-up motions is an intractable challenge. The strategy of employing ferrofluid lubrication in the start-up motion from ultra-low speed to operating speed is a potential approach to improving the severe wear of rough elliptical contact. This paper presents a revised mixed lubrication model that accounts for magnetic field circumstances. Considering Gaussian random roughness, time-varying effect, and thermal effect, the lubrication equations are numerically calculated. First, four start-up modes are evaluated and compared; then the differences between ferrofluids and base oil are quantitatively studied. Furthermore, the influences of ferromagnetic particle size and magnetic field intensity on the film formation and wear of rough elliptical contact are discussed. Results show that ferrofluid lubrication is of great significance in wear reduction compared with base oil. The start-up motion with high accelerated speed is advantageous in film lubrication and wear reduction. Additionally, decreases in ferromagnetic particle size and increases in magnetic field intensity satisfactorily enhance the anti-wear performance of rough elliptical contact.

The mixed regime and wear mechanism of elliptical contact in the start-up process considering interface material properties: A theoretical prediction

Abstracts The start-up motion is the significant process of many mechanical applications especially for high-frequency start-stop equipment; the formation of separating film and the main wear approximately occur in this process. In this paper, the improved mixed algorithm and a new methodology of describing Gaussian rough surface are developed and theoretically validated based on Stribeck curve. Furthermore, the transient film formation of smooth contact and rough contact with the start-up motion is analyzed and validated. Finally, the wear mechanism of different friction pairs is investigated, meanwhile, the applicability of Evans model and Archard model is evaluated. Results show that the contact ratio of smooth contact is considerably large but the transition time is much short in comparison with rough contact because the valleys of rough surface conserve lubricant avoiding solid contact. Combining the Evans equation and this mixed technique, the wear volume and wear rate were successfully calculated. The type of friction pair influences greatly the wear volume and wear rate of contact surfaces. The wear volume and wear rate of TiN–TiN the minimum in the start-up process, and alumina ceramics with satisfactory fracture toughness is a potential coating that can be applied to mechanical elements with a wide range of slide-roll ratio.

A numerical study on the aerodynamic performance and the self-starting characteristics of a Darrieus wind turbine considering its moment of inertia

Self-starting characteristics is a challenging issue in the field of Darrieus-type vertical axis wind turbines. Traditionally, the numerical simulations were performed at several constant rotational velocities for the turbine and the generated torques were reported for each rotational velocity, neglecting the effects of the turbine inertia on the transient start-up motion of the turbine. In the current study, a numerical method is proposed to study the self-starting characteristics of a Darrieus wind turbine, considering the turbine moment of inertia. The simulation starts from the initial stationary state and continues until the final steady-periodic condition. At each time step, the instantaneous rotational velocity of the turbine is computed based on the Newton’s second law, according to the instantaneous aerodynamic and mechanical forces acting on the turbine. Results indicate that as the rotor inertia increases, it takes a longer time for the turbine to reach its final velocity, in a manner that, the turbine might even fail to reach the final condition and the rotation halts. Results also show that as the rotor inertia decreases, the oscillations amplitude of the turbine rotational velocity increases. This can enable the turbine to pass higher resistant torques than those computed based on the traditional method.

The effect of transient conditions on synovial fluid protein aggregation lubrication

Little is known about the prevailing lubrication mechanisms in artificial articular joints and the way in which these mechanisms determine implant performance. The authors propose that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins within the contact inlet region. A direct relationship between contact film thickness and size of the protein aggregation within the inlet region has been observed.In this paper the latest experimental observations of the protein aggregation mechanism are presented for conditions which more closely mimic joint kinematics and loading. Lubricant films were measured for a series of bovine calf serum solutions for CoCrMo femoral component sliding against a glass disc. An optical interferometric apparatus was employed to study the effects of transient motion on lubricant film formation. Central film thickness was measured as a function of time for a series of transient entrainment conditions; start-up motion, steady-state and non-steady-state uni-directional sliding, and bi-directional sliding. The size of the inlet aggregations was found to be dependent upon the type of transient condition. Thick protective protein films were observed to build up within the main contact region for all uni-directional tests. In contrast the inlet aggregation was not observed for bi-directional tests. Contact film thickness and wear was found to be directly proportional to the presence of the inlet protein phase. The inlet phase and contact films were found to be fragile when disrupted by surface scratches or subjected to reversal of the sliding direction.

21113 滑り/転がり潤滑が混在する条件下での凹凸路面と弾性体間の運動開始時の挙動(GS-24【振動(1)】)

21113 滑り/転がり潤滑が混在する条件下での凹凸路面と弾性体間の運動開始時の挙動(GS-24【振動(1)】)

Sound generation due to unsteady motion of a cylinder

A two-dimensional model problem of sound generation due to prescribed unsteady starting and stopping motion of a circular cylinder is studied by the numerical solution of two-dimensional compressible Navier–Stokes equations. The unsteady flow near the cylinder surface and the sound generation and propagation are analyzed with respect to three parameters: the time scale of the startup and stopping motion, the peak Mach number (based on peak velocity and ambient speed of sound), and the Reynolds number (based on peak velocity, cylinder diameter, ambient density, and dynamic viscosity). The flow behavior is studied for fast (time scale of unsteady motion similar to acoustic time scale) or very slow startup motion. The Mach number is varied between 0.1 and 0.4 and the Reynolds number between 150 and 9500. The accuracy of unsteady flow solution is demonstrated by comparison to the experimental data. For fast startup motion, a sharp peak is observed in the drag curve during the acceleration phase of motion. We find that the dominant contribution to total drag comes from the pressure drag and the peak drag force scales with peak velocity. The startup process introduces weak shock waves in the flow, which contributes to the far-field noise. For slow startup motion, the drag curve is smoother and the drag force scales with the square of the peak velocity. For fast startup motion, the acoustic energy that propagates to the far-field scales with the square of the peak velocity, whereas for slow startup, it scales with the fourth power of the peak velocity. The amount of sound energy radiated to the far-field is found to be a small percentage of the total energy input to set up the entire motion of cylinder. An interpretation for the observed scaling relations in the numerical data is presented using different asymptotically valid reduced forms of the governing equations.

Start-up flow of a viscoelastic fluid in a pipe with a fractional Maxwell’s model

Unidirectional start-up flow of a viscoelastic fluid in a pipe with a fractional Maxwell’s model is studied. The flow starting from rest is driven by a constant pressure gradient in an infinite long straight pipe. By employing the method of variable separations and Heaviside operational calculus, we obtain the exact solution, from which the flow characteristics are investigated. It is found that the start-up motion of a fractional Maxwell’s fluid with parameters α and β, tends to be at rest as time goes to infinity, except the case of β=1. This observation, which also can be predicted from the mechanics analogue of fractional Maxwell’s model, agrees with the classical work of Friedrich and it indicates that a fractional Maxwell’s fluid presents solid-like behavior if β≠1 and fluid-like behavior if β=1. For an arbitrary viscoelastic model, a conjecture is proposed to give an intuitive way judging whether it presents fluid-like or solid-like behavior. Also oscillations may occur before the fluid tends to the asymptotic behavior stated above, which is a common phenomenon for viscoelastic fluids.

740 ヒアルロン酸の分子量が粘度と潤滑膜形成能に与える影響(J02-5 生物の機能/運動/機構/模倣(5),ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Influence of molecular weight of hyaluronic acid on its viscosity was investigated through cone-plate viscometer. Water solution of hyaluronic acid had non-Newtonian flow, and the higher molecular mass showed higher viscosity. Lubricating ablility of hyaluronic acid at start-up motion was investigated by using a reciprocating friction tester, in which polyvinyl formal cylindrical surface slid on glass plate. High molecular mass of hyaluronic acid had an excellent film formation property in boundary lubrication mode at start-up, even if static loading duration was applied for invalid state of squeeze film.

Role of the tip vortex in the force generation of low-aspect-ratio normal flat plates

We investigate experimentally the force generated by the unsteady vortex formation of low-aspect-ratio normal flat plates with one end free. The objective of this study is to determine the role of the free end, or tip, vortex. Understanding this simple case provides insight into flapping-wing propulsion, which involves the unsteady motion of low-aspect-ratio appendages. As a simple model of a propulsive half-stroke, we consider a rectangular normal flat plate undergoing a translating start-up motion in a towing tank. Digital particle image velocimetry is used to measure multiple perpendicular sections of the flow velocity and vorticity, in order to correlate vortex circulation with the measured plate force. The three-dimensional wake structure is captured using flow visualization. We show that the tip vortex produces a significant maximum in the plate force. Suppressing its formation results in a force minimum. Comparing plates of aspect ratio six and two, the flow is similar in terms of absolute distance from the tip, but evolves faster for aspect ratio two. The plate drag coefficient increases with decreasing aspect ratio.

Flow past a sphere undergoing unsteady rectilinear motion and unsteady drag at small Reynolds number

The flow induced by a sphere which undergoes unsteady motion in a Newtonian fluid at small Reynolds number is considered at distances large compared with sphere radius a. Previous solutions of the unsteady Oseen equations (Ockendon 1968; Lovalenti & Brady 1993b) for rectilinear motion are refined. Three-dimensional Fourier transforms of the disturbance field are integrated over Fourier space to derive new concise equations for the velocity field and history force in terms of single history integrals.Various slip-velocity profiles are classified by the ratio A of the particle relative displacement, z′p(t′) − z′p(τ′), to the diffusion length, l′D = 2[v(t′ − τ′)]1/2, where v is the kinematic viscosity of the fluid. Most previous studies are concerned with large-displacement motions for which the ratio is large in the long-time limit. It is shown using asymptotic calculations that the flow at any point at large distance z past a sphere for arbitrary large-displacement and non-reversing motion is the same as the steady-state laminar wake if z is expressed in terms of the time elapsed since the particle was at that point in the laboratory frame. The point source solution for the remainder of the far flow is also valid for the unsteady case.A start-up motion with slip velocity V′p = γ′(t′)−1/2, t′ > 0, is investigated for which A is finite. A self-similar solution for the flow field is obtained in terms of space coordinates scaled by the diffusion length, u′ = auss(η)/t′ where η = r′/2(vt′)1/2. The unsteady Oseen correction to the drag is inversely proportional to time.When A is small in the long-time limit (a small-displacement motion) the flow field also depends on the space coordinates in terms of η. The distribution of the streamwise velocity uz is symmetrical in z.

A Hierarchical Forecast Control of a Dynamica Biped Locomotion System

A hierarchical structure for the control of a dynamical biped locomotion robot is presented. High gain local feedbacks at the articular joints for making the biped locomotion robust are implemented as the control in the lower level. In the upper level, the reference signal to the local feedback is generated by using a new low order model which is connected to the actual biped locomotion and which gives a good approximation for almost all walking. The anguler momentum is controlled in the hierarchical control structure, since it must be manipulated in many walking motions such as the start-up motion, the stop motion and the recovery motion from the disturbance. The system in our study can talk dynamically and continuously at the speed of 0.8 m/sec. The experiment which is carried out in real-time control shows satisfactory results.