Steady Force Research Articles

T-tail flutter simulations with regard to quadratic mode shape components

It is known that the dynamic aeroelastic stability of T-tails is dependent on the steady aerodynamic forces at aircraft trim condition. Accounting for this dependency in the flutter solution process involves correction methods for doublet lattice method (DLM) unsteady aerodynamics, enhanced DLM algorithms, unsteady vortex lattice methods (UVLM), or the use of CFD. However, the aerodynamic improvements along with a commonly applied modal approach with linear displacements results in spurious stiffness terms, which distort the flutter velocity prediction. Hence, a higher order structural approach with quadratic mode shape components is required for accurate flutter velocity prediction of T-tails. For the study of the effects of quadratic mode shape components on T-tail flutter, a generic tail configuration without sweep and taper is used. Euler based CFD simulations are applied involving a linearized frequency domain (LFD) approach to determine the generalized aerodynamic forces. These forces are obtained based on steady CFD computations at varying horizontal tail plane (HTP) incidence angles. The quadratic mode shape components of the fundamental structural modes for the vertical tail plane (VTP), i.e., out-of-plane bending and torsion, are received from nonlinear as well as linear finite element analyses. Modal coupling resulting solely from the extended modal representation of the structure and its influence on T-tail flutter is studied. The g-method is applied to solve for the flutter velocities and corresponding flutter mode shapes. The impact of the quadratic mode shape components is visualized in terms of flutter velocities in dependency of the HTP incidence angle and the static aerodynamic HTP loading.

Direct measurement of steady fluid forces upon a deformed cylinder in confined axial flow

This paper addresses the case of a cylinder array in axial flow at Reynolds numbers from 60000 to 110000. A comprehensive experimental and numerical study of the steady fluid forces arising from a geometrical perturbation in the array arrangement is carried out and compared with a semi-empirical model from the literature. The test rig consists of a 3x3 confined cylinder bundle with a pitch-to-diameter ratio equal to 1.33. The central cylinder can be rotated, translated or bent. Global forces and moments as well as pressure profiles at several sections are measured. Velocity profiles at both sides of the cylinder are also collected. Additionally, RANS simulations (Reynolds-averaged Navier–Stokes) are carried out. Pressure loss effects similar to the ones occurring in hydraulic pipes are found to play a major role on the velocity field. Rotation tests are in agreement with the literature: at low angles of incidence, the lift force is proportional to the angle. The supporting rod at the centre of the cylinder strongly disturbs the local lift force but does not change the global linear trend. In translation, the wake of the support generates a fluid stiffness effect. Bending tests allow to assess all terms of the semi-empirical model from the literature, which proves to be quite accurate at a sufficient distance from the ends and from the supports. In addition, the investigation provides refined quantitative measurements of local and global force coefficients of the statically deformed cylinder in rotation, translation and bending.

Application of time-domain methods for marine hydrodynamic and hydroelasticity analyses of floating systems

ABSTRACT The application of transient three-dimensional numerical code ITU-WAVE is presented for the prediction of hydrodynamic characteristics of floating bodies. The time histories of unsteady motions are directly presented with respect to Impulse Response Functions (IRFs). First-order steady forces of wave-resistance, sinkage force and trim moment are solved as the steady-state limit of surge radiation IRF. The numerical prediction of second-order mean force is presented using near-field method based on the direct pressure integration. The hydrodynamic and structural parts are fully coupled through modal analysis for the solution of hydroelastic analysis. A stiff structure is then studied assuming that contributions of rigid body modes are much bigger than elastic modes. A discrete control of latching is used to increase the bandwidth of the efficiency of Wave Energy Converters (WEC). ITU-WAVE numerical results for different floating bodies show satisfactory agreements compared to analytical, other numerical and experimental results.

Form drag coefficient quantification on rising bubbles using particle image velocimetry

Two-phase flow transport heavily depends on the generalized interfacial drag force term in the two-fluid model. The impact of accurate design and prediction associated with thermal energy systems is highly sensitive to multi-phase heat transfer characteristics. Because of this, the interfacial drag force has been studied with rigor for some time. The steady state drag force component in particular has been well characterized for rising single bubbles but has not been previously experimentally separated into its skin and form drag components. Historically, experimental studies were unable to measure the pressure distribution around a bubble to determine the form drag force along the bubble interface. This paper presents the outcomes of an experimental study wherein a new experimental method was developed which, for the first time, separates the form and skin drag coefficients on rising bubbles. Eleven air bubbles sizes representing spheroidal, ellipsoidal, and transition to spherical cap regimes (102<Re<104) were studied in a water test loop with velocity fields measured via particle image velocimetry; pressure fields were then synthesized from these velocity fields through the Queen2 algorithm. The skin and form drag coefficients were separated for single bubbles which showed a nominal trend of increasing form drag contribution with increasing Reynolds number. This work presents a new method and new outcomes for rising bubbles over several bubble regimes and includes a comprehensive uncertainty characterization of the resulting data.

The biomechanics of pollen release: new perspectives on the evolution of wind pollination in angiosperms.

Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence-induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal- and wind-pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.

Long-span Suspension Bridge Flutter Analysis with Drag Force Effects

The paper investigates the influence of the drag force onto the flutter velocity and frequency of the Akashi Kaikyo Bridge. Finite element analyses were run in ANSYS by combining unsteady lift and moment actions with: (a) unsteady drag, (b) steady drag, (c) no drag. The finite element results are compared to those obtained by an in-house MATLAB code based on a semi-analytic continuum model and with others from the literature. The continuum model includes flexural-torsional second-order effects induced by steady drag force into the bridge’s equations of motion, in addition to unsteady lift and moment actions. The results show that good predictions of the flutter velocity can be obtained by combining steady drag with unsteady lift and moment.

Estimation of wave-induced steady force using system identification, model tests, and numerical approach

This paper addresses the method for estimating maneuvering mathematical models in waves, and uses captive model tests to verify it. To obtain the wave force and moment, two kinds of captive model tests are performed. One is straight towing tests with different towing directions to consider various incident wave angles, while the other is circular motion tests at different regular wave frequency. These captive model tests provide added resistance and wave-induced steady force considering both different wave direction and ship speed, and different wave drift angles. The wave drift forces and moment obtained are validated by performing the maneuvering simulation in waves, and comparison with the results of free running model tests. This paper introduces the application of the Estimation-Before-Modeling (EBM) approach, one of the System Identification methods, to estimate mathematical models for ship maneuverability targeting KRISO Very Large Crude oil Carrier (KVLCC2). The most important force component for the prediction of the maneuverability of a ship is the accuracy of the wave-induced steady force acting on the ship. Here, the EBM approach is divided into two steps: First, the motion variables and forces are estimated by Extended Kalman Filter and Modified Bryson-Frazier smoother. Second, the wave-induced steady forces are determined by removing the hydrodynamic force acting on the ship during motion. Also, during the second step of the EBM approach, the database consisting of wave length and incident wave angles is constructed. All estimated variables are based on data obtained by free running model tests in regular waves. Finally, the estimated wave-induced steady force is interpolated, added to the hydrodynamic force component of the 3-DoF equations of motion, and the turning simulation of the ship in waves is then performed. Comparison of the results with the results of free running model tests confirms the validity of the established model.

Research on the dynamic characteristics of water hydraulic servo valves considering the influence of steady flow force

The International Thermonuclear Experimental Reactor remote-handling (ITER-RH) system powered by water hydraulics is a vision of the future. The 2D servo valve is a new type of servo valve, maybe suitable for the ITER-RH system. It has many advantages, including its fast response and strong anti-pollution ability. However, steady flow force is currently considered to be one factor that impacts the dynamic characteristics of servo valves. In this study, the influence of steady flow force on the dynamic characteristics was analyzed, and the structure was optimized accordingly. A mathematical model of the servo valve was built using theoretical analysis and numerical simulation. Results indicated that the amplitude bandwidth of the 2D water hydraulic servo valve was 169 Hz without considering the steady flow force, and it decreased to 162 Hz when the steady flow force was considered. Furthermore, a newly designed valve sleeve, to compensate for the axial steady flow force, was proposed by changing the angle of the inlet port designed in the valve sleeve to the spool axis. The maximum compensation capacity of the axial steady flow force was 35%, and the best angle of the inclined holes was approximately 120°. The optimized amplitude bandwidth of the 2D servo valve was 167 Hz. The results demonstrate that the optimized valve sleeve improved the dynamic response of the 2D servo valve.

Early turbulence and pulsatile flows enhance diodicity of Tesla\u2019s macrofluidic valve

Microfluidics has enabled a revolution in the manipulation of small volumes of fluids. Controlling flows at larger scales and faster rates, or macrofluidics, has broad applications but involves the unique complexities of inertial flow physics. We show how such effects are exploited in a device proposed by Nikola Tesla that acts as a diode or valve whose asymmetric internal geometry leads to direction-dependent fluidic resistance. Systematic tests for steady forcing conditions reveal that diodicity turns on abruptly at Reynolds number {rm{Re}}approx 200 and is accompanied by nonlinear pressure-flux scaling and flow instabilities, suggesting a laminar-to-turbulent transition that is triggered at unusually low {rm{Re}}. To assess performance for unsteady forcing, we devise a circuit that functions as an AC-to-DC converter, rectifier, or pump in which diodes transform imposed oscillations into directed flow. Our results confirm Tesla’s conjecture that diodic performance is boosted for pulsatile flows. The connections between diodicity, early turbulence and pulsatility uncovered here can inform applications in fluidic mixing and pumping.

A digital light processing 3D printed magnetic bioreactor system using silk magnetic bioink

Among various bioreactors used in the field of tissue engineering and regenerative medicine, a magnetic bioreactor is more capable of providing steady force to the cells while avoiding direct manipulation of the materials. However, most of them are complex and difficult to fabricate, with drawbacks in terms of consistency and biocompatibility. In this study, a magnetic bioreactor system and a magnetic hydrogel were manufactured by single-stage three-dimensional (3D) printing with digital light processing (DLP) technique for differentiation of myoblast cells. The hydrogel was composed of a magnetic part containing iron oxide and glycidyl-methacrylated silk fibroin, and a cellular part printed by adding mouse myoblast cell (C2C12) to gelatin glycidyl methacrylate, that was placed in the magnetic bioreactor system to stimulate the cells in the hydrogel. The composite hydrogel was steadily printed by a one-stage layering technique using a DLP printer. The magnetic bioreactor offered mechanical stretching of the cells in the hydrogel in 3D ways, so that the cellular differentiation could be executed in three dimensions just like the human environment. Cell viability, as well as gene expression using quantitative reverse transcription-polymerase chain reaction, were assessed after magneto-mechanical stimulation of the myoblast cell-embedded hydrogel in the magnetic bioreactor system. Comparison with the control group revealed that the magnetic bioreactor system accelerated differentiation of mouse myoblast cells in the hydrogel and increased myotube diameter and length in vitro. The DLP-printed magnetic bioreactor and the hydrogel were simply manufactured and easy-to-use, providing an efficient environment for applying noninvasive mechanical force via FDA-approved silk fibroin and iron oxide biocomposite hydrogel, to stimulate cells without any evidence of cytotoxicity, demonstrating the potential for application in muscle tissue engineering.

Parameter computation for a Lagrangian mechanical system model of a submerged vessel moving near a free surface

This paper describes the computation of parameters for a Lagrangian mechanical system model of a submerged vessel moving near an otherwise calm free surface using a medium-fidelity potential flow code. The software uses the boundary element method to solve for the flow potential on the body and the free surface. The model, a system of integro-differential equations involving functions of the velocity potential, is the “maneuvering” component of a Lagrangian nonlinear maneuvering and seakeeping model that was introduced in earlier work. Here, this nonlinear maneuvering model is reformulated and the potential flow software is modified to support parameter computations. Parameters are computed for a prolate spheroid moving parallel to a calm free surface at a constant forward speed. This motion induces a steady flow with respect to the body, which results in a steady surge force, heave force and pitch moment. These longitudinal forces and moment are computed using the reformulated Lagrangian nonlinear maneuvering model and the results are compared with basic panel code solutions for various depths and Froude numbers and for various computational parameter values.

Residual force enhancement is reduced in permeabilized fiber bundles from mdm muscles

Residual Force enhancement (RFE) is defined as the increase in steady state force after active stretch compared with force after isometric contraction at the same final length. Recent studies have demonstrated a pivotal role for titin in RFE. In myofibrils stretched beyond overlap of thick and thin filaments, Ca2+ activated myofibrils had a higher titin-based force and a steeper increase with force with stretch than passively stretched myofibrils at the same average sarcomere length. These results suggest higher titin-based stiffness in active myofibrils, perhaps due to N2A titin-actin interactions in active wild type (WT) muscle. Mdm mice carry a small deletion in titin that includes 53 amino acids in Ig83 in the N2A region. It has been proposed that the mdm mutation prevents N2A titin-actin interactions so that active mdm muscles are more compliant than WT muscles. This decrease in active muscle stiffness should be associated with a reduction in RFE. Based on this rationale, we investigated RFE in permeabilized soleus (SOL) and extensor digitorum longus (EDL) fiber bundles from 20-30 day old mdm mice (n=5), harvested at a young age to diminish the effects of degeneration and regeneration, and 30-60 day old WT mice (n=5). Contractile stress and RFE of EDL (mdm n=19 and WT n=20) and SOL fibers (WT n=19 and mdm n=16) was analyzed using JMP and Graph pad (PRISM) software. Statistics included Shapiro-Wilk (normality), Levene's and Bartlett (homogeneity of variances), Welch's one-way ANOVA (unequal variances), Dunnett's T3 (multiple comparisons), and T-tests with Welch's correction (unpaired). To investigate differences in RFE between genotypes, we performed isometric contractions, as well as active and passive stretch from an average sarcomere length of 2.6 - 3.0 µm on each fiber. This stretch amplitude, within the physiological range, resulted in no observable breakage or slipping of fibers during experiments. We predicted that mdm fiber bundles would show reduced RFE compared to WT fibers. Contractile stress when activated at 2.6 µm was significantly greater in WT EDL (Welch's one-way ANOVA; P<0.05) bundle and SOL (Welch's one-way ANOVA; P<0.05) compared to fiber bundles. Our results showed that mdm muscles exhibit significantly lower RFE than WT for both SOL (Welch's one-way ANOVA; P<0.0001) and EDL (Welch's one-way ANOVA; P<0.0001). This result is consistent with previous observations in single myofibrils and intact muscles. The data support the hypothesis that titin contributes to RFE. We suggest that RFE is reduced in mdm muscles due to impaired Ca2+ dependent titin-actin interactions, resulting from the small deletion in titin that includes 53 amino acids in Ig83 in the N2A region.

Symmetry properties of nonlinear hydrodynamic interactions between responsive particles.

Two identical particles driven by the same steady force through a viscous fluid may move relative to one another due to hydrodynamic interactions. The presence or absence of this relative translation has a profound effect on the dynamics of a driven suspension consisting of many particles. We consider a pair of particles which, to linear order in the force, do not interact hydrodynamically. If the system possesses an intrinsic property (such as the shape of the particles, their position with respect to a boundary, or the shape of the boundary) which is affected by the external forcing, hydrodynamic interactions that depend nonlinearly on the force may emerge. We study the general properties of such nonlinear response. Analysis of the symmetries under particle exchange and under force reversal leads to general conclusions concerning the appearance of relative translation and the motion's time reversibility. We demonstrate the applicability of the conclusions in three specific examples: (a) two spheres driven parallel to a wall; (b) two deformable objects driven parallel to their connecting line; and (c) two spheres driven along a curved path. The breaking of time reversibility suggests a possible use of nonlinear hydrodynamic interactions to disperse or assemble particles by an alternating force.

Stabilization of the fluidic pinball with gradient-enriched machine learning control

Abstract

Acoustic levitation of a rigid nano-sphere at non-continuum conditions

Abstract

Steady lift-force generation on a circular cylinder utilising a longitudinal vortex: Influence of geometrical parameters

A novel technique for producing a steady lift force on a circular cylinder in a uniform flow driven by the longitudinal vortex (LV) is developed. The LV forms behind the cylinder when a strip plate is located downstream in a cruciform arrangement. The upstream cylinder can stably move by itself via an aerodynamic effect of the LV-produced steady lift force. However, the effects of geometrical parameters on system performance have not been investigated. In this study, the effective parameters of the presented mechanism—the width ratio of the strip plate, W / d , and the exposed length ratio of the circular cylinder over the strip plate, l e / d —are numerically examined. An unsteady Reynolds-averaged Navier–Stokes simulation using a shear-stress-transport k–ω turbulence model is employed for this analysis. The experimental investigation in our wind tunnel laboratory is used to primarily validate the computational predictions. The distribution of the aerodynamic forces acting on the cylinder surface is continuously evaluated along the length span. Flow fields around the system are also visualised for discussion of the vortical effects. The results indicate that the proper parameters of the system needed to generate the available driving force for the moving cylinder are ( i ) a strip-plate width of 1.5 ​< ​ W/d ​< ​2.5, with W/d ​= ​2 being considered as the most suitable, and ( ii ) an optimal exposed length of the cylinder of l e ​= ​1.5 d . • Numerical results were validated with experimental data of the wind tunnel test. • Flow fields were numerically visualised providing vortical structures. • Aerodynamic-force distributions along the cylinder length were continuously investigated.

Spatially evolving regular water wave under the action of steady wind forcing

The spectra plotted in this work demonstrate suppression of random wind-generated waves by a deterministic long wavemaker-generated wave in a moderately sized wind-wave facility. Experiments performed for a wide range of experimental conditions show that the effect of a regular wave on the local wind-wave field excited by steady wind remains essential as long as the local dominant frequency of random wind waves remains below that of the regular waves.

Salvesen’s Method for Added Resistance Revisited

Abstract Almost 4 years after the appearance of Salvesen–Tuck–Faltinsen (STF) strip theory (Salvesen et al., 1970, “Ship Motions and Sea Loads,” Annual Meeting of the Society of Naval Architecture and Marine Engineers (SNAME), New York, Nov. 12–13), Salvesen in 1974 published his popular method for calculation of added resistance (Salvesen, 1974, “Second-Order Steady State Forces and Moments on Surface Ships in Oblique Regular Waves,” Vol. 22; Salvesen, 1978, “Added Resistance of Ships in Waves,” J. Hydronautics, 12(1), pp. 24–34). His method is based on an exact near-field formulation; however, he applied the long-wave and the weak-scatterer assumptions to present his approximate method using the integrated quantities (hydrodynamic and geometrical coefficients). Considering the available computational powers in the 1970s, both of these assumptions were absolutely justifiable. The intention of this paper is to disseminate the results of a recent study at the Technical University of Denmark, whereby the Salvesen’s formulation has been revisited and the added resistance is computed from the original exact equation without invoking the weak-scatterer or the long-wave assumptions. This is performed using the solutions of the radiation and the scattering problems, obtained by a low-order boundary element method and the two-dimensional free-surface Green function inside our in-house STF theory implementation (Bingham and Amini-Afshar, 2020, DTU_Strip Theory Solver). The weak-scatterer assumption is then removed through a direct calculation of the x-derivatives of the velocity potentials and the normal vectors along the body. Knowing the velocity potentials over each panel, the long-wave assumption is also avoided by a piece-wise analytical integration of sectional Kochin Function (Kochin, 1936, “On the Wave Resistance and Lift of Bodies Submerged in Fluid,” Transactions of the Conference on the Theory of Wave Resistance, Moscow.). The presented results for five ship geometries testify that the correct treatment of the original equation is achieved only after both of the above-mentioned assumptions are removed. Implemented in this manner, Salvesen’s method proves to be relatively more accurate and robust than has been generally perceived during all these years.

Size-dependent stability analysis of a functionally graded cylindrical shell subjected to swirling annular flow including the fluid viscosity

In this paper, the stability and dynamics of a functionally graded materials (FGMs) cylindrical micro-shell subjected to swirling annular fluid through the annulus between the inner shell and the outer shell are firstly investigated. The fluid perturbation pressure related to the shell vibration is determined by the potential flow theory. The steady viscous forces are derived based on the fully developed turbulent flow. The shell motion equations are established on the basis of the Hamilton's principle along with the Donnell shell theory. To capture the size effect of micro-shell, the zero-level contour method is adopted to obtain the solutions of shell motion equations. First, the system subjected to an ideal annular flow is studied. The results show effects of the fluid rotation, material length-scale parameter, and material properties on the fluid-micro-shell system stability. Furthermore, the coupling effect of the fluid rotation and the material length-scale parameter on the dynamical stability of the system is evaluated. Second, the system subjected to a viscous annular flow is further studied. A more impressive result is observed that the fluid viscosity effect renders the system subjected to an annular flow more unstable, and the physical explanation for the phenomenon is presented.

Aerodynamic response of a bristled wing in gusty flow

Abstract