Steady Force Research Articles

Optimal Transient Growth in an Incompressible Flow past a Backward-Slanted Step

With the aim of providing a first step in the quest for a reduction of the aerodynamic drag on the rear-end of a car, we study the phenomena of separation and reattachment of an incompressible flow by focusing on a specific aerodynamic geometry, namely a backward-slanted step at 25 ∘ of inclination. The ensuing recirculation bubble provides the basis for an analytical and numerical investigation of streamwise-streak generation, lift-up effect, and turbulent-wake and Kelvin–Helmholtz instabilities. A linear stability analysis is performed, and an optimal control problem with a steady volumic forcing is tackled by means of a variational formulation, adjoint methods, penalization schemes, and an orthogonalization algorithm. Dealing with the transient growth of spanwise-periodic perturbations, and inspired by the need of physically-realizable disturbances, we finally provide a procedure attaining a kinetic-energy maximal gain on the order of 10 6 , with respect to the power introduced by the external forcing.

Control of oscillatory force tasks: Low-frequency oscillations in force and muscle activity

Force variability during steady force tasks is strongly related to low-frequency oscillations (<0.25 Hz) in force. However, it is unknown whether low-frequency oscillations also contribute to the variability of oscillatory force tasks. To address this, twelve healthy young participants (21.08 ± 2.99 years, 6 females) performed a sinusoidal force task at 15% MVC at two different frequencies (0.5 and 1 Hz) with isometric abduction of the index finger. We recorded the force from the index finger and surface EMG from the first dorsal interosseous muscle and quantified the following outcomes: 1) trajectory variability and accuracy; 2) power spectrum of force and EMG bursting below 2 Hz; 3) power spectrum of the interference EMG from 4 to 60 Hz. The trajectory variability and error significantly increased from 0.5 to 1 Hz task (P < 0.01). Increased force oscillations <0.25 Hz contributed to greater trajectory variability and error for both the 0.5 and 1 Hz oscillatory task (R2 > 0.33; P < 0.05). The <0.25 Hz oscillations in force were positively associated with greater power in the <0.25 Hz for EMG bursting (R2 > 0.52; P < 0.01). The modulation of the interference EMG from 35 to 60 Hz was a good predictor of the <0.25 Hz force oscillations for both the 0.5 Hz task and 1 Hz task (R2 > 0.66; P < 0.01). These results provide novel evidence that, similar to steady contractions, low-frequency oscillations of the motor neuron pool appear to be a significant mechanism that controls force during oscillatory force tasks.

Estimation of fluid forces on a spherical particle for two-way coupling simulation based on the volume averaging

The relationship between the flow field numerically obtained by the two-way coupling simulation and the fluid force on each particle is discussed. For estimating the fluid force on a particle, the effect of the disturbance by the particle needs to be considered. In the present study, for a particle located in a steady ambient flow, the fluid force is modelled as a function of the averaged velocity and pressure over an explicitly-defined averaging volume in the disturbed field. The gradients of the pressure and velocity induced by the particle are also modelled. It is found that the discretised gradients are almost independent of the distance between the reference points for sufficiently small distances. This fact makes the gradient models independent of the grid width of the two-way coupling simulation. The proposed steady viscous force model implicitly includes the correction terms due to the strain of the undisturbed flow and the history effect as the model is constructed with the disturbed flow. For the radius of the averaging volume smaller than 5 times the particle radius, the present force estimation method based on the disturbed flow particularly works well. The results suggest that the present method is effective in the two-way coupling simulation with the particles of comparable size to the minimum length scale of the background flow.

Quadratic quantities in acoustics: Scattering cross-section and radiation force

Time-harmonic acoustic waves interact with a scatterer: this interaction is calculated using linear, first-order theory. However, there are quadratic, second-order quantities that are of interest. These include the scattering cross-section and the steady radiation force; these quantities can be expressed as integrals of products of first-order quantities over a sphere. These integrals are evaluated exactly. The results are infinite series of products of the coefficients in the spherical multipole expansions of the incident and scattered fields; they do not depend on the radius of the spherical integration surface. For a specific scattering problem, the coefficients can be connected by an appropriate T-matrix. In most previous work, the spherical surface is moved to infinity so that far-field quantities can be introduced: it is shown that this process is not straightforward and it may introduce spurious difficulties.

Steady Flow Force Compensation and Test Research on Electrohydraulic Proportional Relief Valve

The pressure control accuracy of electrohydraulic proportional relief valve (EPRV) is affected by steady flow force. Thus, this paper investigates the method of designing a spool groove into the turbine bucket profile to compensate for the steady flow force maximally. The influence of steady flow force on the working pressure of the EPRV is analyzed. The calculation model of the steady flow force is deduced. Moreover, the computational fluid dynamics (CFD) model of the spool is established considering the fitting clearance between the spool and the valve body. The test bench is built to verify the CFD model. The turbine bucket profile is designed in the spool groove and optimized on the basis of the response surface method. Furthermore, the flow impact coefficient is defined and the compensation effect of the optimized spool to the steady flow force is tested. The results show that the absolute error value of the CFD simulation and test is smaller than 6.51%. When the working pressure is 0.5, 2, and 4 MPa, the flow impact coefficients of the optimized spool can be reduced by 100%, 59.15%, and 26.72%, respectively, compared with the original spool, indicating that the optimized spool can obviously compensate the steady flow force and improve the control accuracy of the EPRV.

Upgraded Force on a Wall from Reflected Surface Gravity Waves

During reflection from a solid vertical wall, surface gravity waves produce a steady force on the wall, which was calculated earlier by the momentum method. Recently the linear momentum of these waves has been estimated to be twice as large as deduced from the classical Stokes drift formula. Therefore, the magnitude of the reflected wave force has been doubled here and the algebraic form of the equation is converted to be more understandable physically. Wave force equals fluid density times the square of the particle’s orbital speed and a numerical constant. Observations are welcomed to compare with the theory.

Mechanical response and pore pressure generation in granular filters subjected to uniaxial cyclic loading

This paper presents results from a series of piping tests carried out on a selected range of granular filters under static and cyclic loading conditions. The mechanical response of filters subjected to cyclic loading could be characterized in three distinct phases; namely, (I) pre-shakedown, (II) post-shakedown, and (III) post-critical (i.e., the occurrence of internal erosion). All the permanent geomechanical changes such, as erosion, permeability variations, and axial strain developments, took place during phases I and III, while the specimen response remained purely elastic during phase II. The post-critical occurrence of erosion incurred significant settlement that may not be tolerable for high-speed railway substructures. The analysis revealed that a cyclic load would induce excess pore-water pressure, which, in corroboration with steady seepage forces and agitation due to dynamic loading, could then cause internal erosion of fines from the specimens. The resulting excess pore pressure is a direct function of the axial strain due to cyclic densification, as well as the loading frequency and reduction in permeability. A model based on strain energy is proposed to quantify the excess pore-water pressure, and subsequently validated using current and existing test results from published studies.

Contact Force Control on Soft Membrane for an Ear Surgical Device

Mechatronic and robotic systems are increasingly used in the health care and medical industry in recent years. Once the surgery is shifted from operating room to clinic or surgeon's office by using an office-based surgical device, the patient cannot be administered general anesthesia and thus the patient's motion can affect the performance of such a surgical device and even the success of the surgery. For an office-based ear surgical device designed to insert a tube on the tympanic membrane, the patient's head motion will affect the success rate of tube insertion. To address this issue without adding any extra equipment, a head motion compensation system based on contact force control is developed in this paper. A model describing the displacement–force relationship of the soft membrane is built based on viscoelastic behavior and contact effect. Based on the model, a proportional-integral-derivative controller obtained via constrained linear-quadratic optimization algorithm and a disturbance observer are designed. Finally, several experiments for system performance validation are conducted on a mock-up system, whose results show that the optimal force controller can perform a good set-point tracking ability. Meanwhile, the proposed control scheme can compensate the motion and achieve an accurate and steady contact force control.

Structural Optimization of a Water Hydraulic Pilot-Operated Pressure-Reducing Valve

The characteristics of a pilot-operated pressure-reducing valve for water hydraulics are analysed. The valve operates well with a primary pressure of up to 15MPa, a secondary pressure range of 10MPa to 14MPa, and a flow range of 20L/min to 40L/min. Optimizations are applied to reduce the cavitation noise of the valve and to improve the pressure regulation accuracy of secondary pressure. The first optimization is executed by reducing pressure drop and velocity through the throttle of main spool and the second optimization is carried out by lowering down the steady jet force on both main spool and the pilot spool. The dynamic characteristics of the secondary pressure are obtained by simulations. The results show the valve behaviours better with the improvement structure. The average variation of the secondary pressure is brought down from 0.308MPa to 0.175MPa under different primary pressure conditions and from 0.288MPa to 0.136MPa under different inlet flowrate conditions. The velocity through throttle of the main spool is 31.18% cut down on average.

Analysis on the Main Design Parameters Influencing the Impact Efficiency of Dual-Chamber-Controlled Hydraulic Drifter

Previous investigators demonstrate that the impact performance of the drifter is affected by the forces such as viscous friction resistance and axial thrust of liquid. In this research, some other forces including hydraulic sticking force, sealing resistance, steady flow force and transient flow force are also taken into the consideration to develop a non-linear mathematical model of the dualchamber-controlled hydraulic drifter. We take piston mass, supply flow rate, supply pressure and the front and back work areas of the piston as the main design parameters to predict the impact efficiency of the drifter using the controlling variable method. The result shows that the impact efficiency decrease as the increasing of the piston mass or supply flow rate. The impact efficiency reaches a peak value 0.48 when the supply pressure P ranges from 150 to 230 bars. The decrease of the front work area A1 of the piston, as well as the increase of the back work area A2, would improve the impact efficiency. With the combination of the design parameters P, A1 and A2, there is a high efficiency area 162.5 < P < 230 bars and 0.48 < A1/A2 < 0.86 with the impact efficiency value 0.46–0.495.

Comparison of pilot-scale floating offshore wind farms with shared moorings

As floating wind turbine technology matures and wind farms are considered at larger scales in deeper waters, moorings may take a larger share of the costs. In response, floating wind farms could be designed such that the moorings are shared between turbines, using interconnecting mooring lines between adjacent floating offshore wind turbine platforms and also possibly connecting multiple mooring lines to the same anchor. This may reduce the total length of mooring line, the wind farm footprint, and the number of anchors; worthwhile advantages if not overcome by increases in mooring strength requirements.A design algorithm is presented for preliminary sizing of shared mooring systems for pilot-scale floating offshore wind farms. Quasi-static modelling is used to analyze the performance of shared-mooring systems produced by the algorithm. The model accounts for steady wind thrust forces and nonlinear mooring line reactions. A four-turbine floating wind farm is designed and analyzed with three alternative shared-mooring configurations and four water depths. Results are presented in terms of platform displacements, mooring tensions, and cost, providing an initial comparison of shared mooring options and their potential for cost savings over conventional individually-moored configurations.

Numerical investigation of lift-force generation on a moving circular cylinder in a uniform flow driven by longitudinal vortex

A new concept for generating a steady lift force on a circular cylinder in a uniform flow is presented. A strip-plate is placed behind the upstream cylinder in a cruciform arrangement with a suitable gap distance, s, between them to employ the longitudinal vortex. The circular cylinder is driven by the aerodynamic effects of the longitudinal vortex induced steady lift force. In this paper, the system was examined using the unsteady Reynolds-averaged Navier–Stokes (URANS) approach with the shear-stress-transport (SST) k-ω model and adaptive wall function in the commercial CFD software, SC/Tetra. Two conditions were simulated for comparisons; i.e., the stationary-isolated cylinder and the parallel moving cylinder with a downstream strip-plate. Two effective parameters of the presented mechanism were investigated as the velocity ratio, VR, and the gap distance ratio, s/d. The wind-tunnel investigation in our laboratory was used for validation as well as the experiments, and other numerical results in the literature of the stationary-isolated cylinder, and the similar cases of flow over a free-end cylinder were employed for discussion. Three-dimensional structure of the longitudinal vortex and the limiting streamlines around the cylinder surfaces were numerically visualized. Distributions of the circumferential pressure and driving force coefficients were continuously evaluated along the cylinder axis.

Individual preferences in motor coordination seen across the two hands: relations to movement stability and optimality.

The framework of the uncontrolled manifold (UCM) hypothesis was used to explore variables related to stability of task performance in the two hands of young healthy individuals. Fourteen young adults performed four-finger accurate constant force production tasks interrupted by a voluntary quick force pulse production and by an externally imposed displacement of all fingers. Three groups of variables were used to quantify stability of steady force production: (1) indices of the inter-trial variance were computed within the UCM and orthogonal to the UCM; (2) indices of motor equivalence were computed between steady-state intervals separated by the force pulse and by thefinger-lifting episode; and (3) referent coordinate and apparent stiffness were computed using the data during the ascending phase of the finger-lifting episode. In another task, the subjects performed accurate constant force production with visual feedback removal after the 8th second, and the drop in the total force after the removal was computed. There were differences between the right and left hand in some outcome variables such as variance within the UCM, and the timing of anticipatory synergy adjustments prior to the force pulse, consistent with the dynamic dominance hypothesis. There were significant correlations between the two hands for indices that were unrelated to accuracy of performance: variance within the UCM, index of motor equivalence, referent coordinate, apparent stiffness, and the drop of total force after visual feedback removal. We interpret these findings within the concept of stability-optimality trade-off. In particular, we conclude that individual subjects select particular, person-specific solutions within the spectrum allowed by the explicit task constraints, and this choice is consistent between the two hands. We conclude with a hypothesis that selecting specific solutions within the stability-optimality trade-off may represent an individual's personal preference consistent between the two hands.

Inertial forces for particle manipulation near oscillating interfaces

Oscillating microscale interfaces give rise not only to steady flows, but also steady inertial forces on particles. Our efficient theoretical description of these forces, which can be attractive or repulsive, provides a toolbox for separating and sorting microscale objects like biological cells.

Evaluation of an energy consistent entrainment model for volumetrically forced jets using large eddy simulations

We examine an existing energy-consistent approach for modeling entrainment rate coefficient, α, in the context of a volumetrically forced jet. The steady buoyancy forcing, with a Gaussian radial profile, is externally imposed within a prescribed height range. Large Eddy Simulations (LESs) of the forced jet are performed to generate the relevant flow statistics. We vary the forcing Richardson number RiFZ (ranging from 0 to 2.12) and the ratio of the forcing region to the jet width, λFZ (ranging from 0.33 to 1), over a set of 11 cases. We also construct a Gaussian one-dimensional model for evolving the forced jet, based on mixing length model. LES statistics shows that the mixing length constant, Cmix, can reduce significantly in the presence of volumetric forcing. Large differences exist between the results from LESs and 1-D model if a constant value of Cmix is used in the model. Using the value of Cmix obtained from LESs, the error between the 1-D model and LESs is reduced, especially for cases with λFZ = 1. For low values of λFZ, the velocity profile of the jet in the forcing zone departs from a Gaussian shape. The 1-D model is nevertheless able to successfully predict the qualitative trends for α, jet width, and centerline velocity and yields a good estimate for the buoyancy flux. The LES statistics also show reasonably good agreement between the exact closure for α and its measured value.

A Two-Dimensional Dynamical Model for the Subseasonal Variability of the Asian Monsoon Anticyclone

The Asian monsoon anticyclone, which develops in the upper troposphere and lower stratosphere during boreal summer, exhibits significant subseasonal variability with a characteristic spatial structure. The dynamics of this variability is investigated using a nonlinear β-plane shallow-water model. The equivalent depth is estimated using reanalysis data to relate the three-dimensional dynamics in isentropic coordinates to the shallow-water model. Composite analysis reveals the resemblance of the horizontal structures between the Montgomery streamfunction and thickness on the 360-K level. However, the coefficients of the linear regressions between those two variables are strongly dependent on latitude. The estimated equivalent depths of the northern region are more than 2 times greater than those of the southern region. This is attributable to the background thermal structure around the tropopause. Based on this, a latitude-dependent mean depth is incorporated into the shallow-water model to numerically investigate responses to a steady localized forcing in the subtropics. With the inclusion of the latitudinal dependence of the mean depth, the vortex shedding state is able to have a longitudinally confined structure, which differs from the conventional case of constant mean depth. The spatial structure of this numerical solution corresponds to the observed structure, in which low-PV air is largely confined to finite longitudes within the Asian monsoon anticyclone. This suggests the possible role of dynamical instability and the interaction with the subtropical jet in determining the characteristic structure of the Asian monsoon anticyclone.

Self-induced velocity correction for improved drag estimation in Euler–Lagrange point-particle simulations

In Euler–Lagrange (EL) simulations the force on each particle is obtained from a point-particle model, which is then coupled back to the fluid momentum. The feedback force modifies the flow at the particle location and it is important to evaluate the resulting self-induced velocity disturbance, since the point-particle models are based on the undisturbed flow. An exact solution of the Oseen's equation for flow generated by a steady Gaussian feedback force was obtained, which along with the corresponding finite Reynolds number numerical simulations, provided a steady model for the self-induced velocity disturbance. The unsteady problem of a time dependent Gaussian feedback force was then theoretically investigated in the zero Reynolds number limit. The corresponding finite Reynolds number unsteady results were obtained using companion numerical simulations. Based on these results an unsteady model for predicting the self-induced velocity disturbance was developed. The two main non-dimensional quantities affecting the self-induced velocity disturbance are the Reynolds number based on Gaussian width Reσ and the non-dimensional feedback force F˜. The resulting self-induced velocity correction model is general and can be applied in a variety of EL point-particle simulations, with the time history of Reσ and F˜ as input. The quasi-steady and unsteady versions of the model were tested in the context of a freely settling particle. The unsteady model was shown to predict the self-induced velocity disturbance to reasonable accuracy for a wide range of Reynolds and Stokes numbers. Issues pertaining to practical implementation and limitations are discussed.

Visual information processing in older adults: reaction time and motor unit pool modulation.

Presently, there is no evidence that magnification of visual feedback has motor implications beyond impairments in force control during a visuomotor task. We hypothesized that magnification of visual feedback would increase visual information processing, alter the muscle activation, and exacerbate the response time in older adults. To test this hypothesis, we examined whether magnification of visual feedback during a reaction time task alters the premotor time and the motor unit pool activation of older adults. Participants responded as fast as possible to a visual stimulus while they maintained a steady ankle dorsiflexion force (15% maximum) either with low-gain or high-gain visual feedback of force. We quantified the following: 1) response time and its components (premotor and motor time), 2) force variability, and 3) motor unit pool activity of the tibialis anterior muscle. Older adults exhibited longer premotor time and greater force variability than young adults. Only in older adults, magnification of visual feedback lengthened the premotor time and exacerbated force variability. The slower premotor time in older adults with high-gain visual feedback was associated with increased force variability and an altered modulation of the motor unit pool. In conclusion, our findings provide novel evidence that magnification of visual feedback also exacerbates premotor time during a reaction time task in older adults, which is correlated with force variability and an altered modulation of motor unit pool. Thus these findings suggest that visual information processing deficiencies in older adults could result in force control and reaction time impairments. NEW & NOTEWORTHY It is unknown whether magnification of visual feedback has motor implications beyond impairments in force control for older adults. We examined whether it impairs reaction time and motor unit pool activation. The findings provide novel evidence that magnification of visual feedback exacerbates reaction time by lengthening premotor time, which implicates time for information processing in older adults, which is correlated with force variability and an altered modulation of motor unit pool.

Aeromechanics Analysis of a High-Advance-Ratio Lift-Offset Coaxial Rotor System

The aeromechanics of a model-scale, lift-offset rotor system designed for high-advance-ratio forward flight were investigated by means of a numerical comprehensive analysis as well as hover and wind-tunnel testing. The 2-m-diam rotor system was tested in single-rotor as well as in coaxial counter-rotating rotor configuration. The focus was on the development of the computational model and its validation for advance ratios of up to 0.5, at lift offsets up to 20%. Blade structural characteristics were modeled according to the measured elastic properties, and reduced-order aerodynamics modeling using a free-vortex wake method was used to ensure computational efficiency. Numerical predictions of coaxial rotor performance, cyclic controls, steady hub forces, and blade clearance at multiple advance ratios and varying lift offsets in general correlated well with the measurements and particularly well for medium to high advance ratios (). Discrepancies at high lift offsets may be related to an underprediction of the cyclic controls. Vibratory hub loads correlated well with the measurements in both trends and magnitudes for the single rotors. For the coaxial configuration, good agreement was found in the vibratory thrust and drag, whereas the side forces captured the trends but underpredicted the magnitudes seen in the experiments, which included significant higher-frequency content.

Mathematical Modeling Of The Hydrodynamic Forces On A Low Aspect-ratio Curved Otter Board

As an important accessory of single trawl, otter board has the effect of providing net horizontal expansion. Hydrodynamic performance of otter board directly affects net horizontal expansion, affects further the catching and fishing efficiency[1-2].For improving the fishing efficiency and technology level and solving the world energy crisis in some way, carrying out the research of trawl door hydrodynamic performance in order to design and optimize new energy-efficient with high expansion and low drag force otter board becomes a new researchers’ focus [2].This paper selects a low aspect-ratio curved otter board as research object, presents a method based on the flume experiment results and for mathematical modeling of the hydrodynamic forces on the otter board. These forces have been found as a function of angles of attack and slip. The coefficients are parameterized for smoothing and computational performance.A method is summarized for extending to the evaluation of arbitrary shape otter board and outside the normal operating.A verified by experiment results computational efficient model of the steady state hydrodynamic forces is finally proposed, suitable for trawl control system design and analysis.