Normal Force Coefficient Research Articles

Impacts of the rudder profile on manoeuvring performance of ships

The profile of a ship rudder influences the forces it generates, which in turn influence the manoeuvring performance of a ship. Thus, rudder forces and moments should be calculated considering the profile. Instead of an empirical estimation of the rudder normal force coefficient, this paper applies a RANS method to determine the hydrodynamic characteristics of various profiles, i.e. lift and drag coefficients. The RANS method is validated with a classic NACA 0012 profile and applied to 9 profiles from the NACA series, the wedge-tail series, and the IFS series. Furthermore, the 2D open-water RANS results are corrected for the effects of the propeller slipstream and the rudder aspect ratio. New regression formulas of the normal force coefficients are proposed for the tested profiles. These formulas are then integrated into a standard MMG model. Taking the KVLCC2 tanker as a reference ship, the manoeuvring model is validated with free-running tests executed by MARIN. Finally, the manoeuvring performance of the reference ship with various rudder profiles are quantified with turning and zigzag manoeuvres. The simulation results confirm that the wedge-tail series is most effective (largest manoeuvring forces) while the NACA series is most efficient (highest lift to drag ratio) among the tested profiles. The IFS series achieves a balance of effectiveness and efficiency.

Numerical and experimental study of unsteady wind loads on panels of a radar aerial

This work experimentally and numerically analyzes the flow configurations and the dynamic wind loads on panels of rectangular L/h 5:1 cross section mounted on a structural frame of rectangular bars of L/h 0.5:1, corresponding to a radar structure. The fluid dynamic interaction between panels and frame wakes imposes dynamic loads on the panels, with particular frequencies and Strouhal numbers, different from those of isolated elements. The numerical scheme is validated by comparison with mean forces and velocity spectra of a panel wake obtained by wind tunnel tests. The flow configuration is analyzed through images of the numerical simulations. For a large number of panels, as in the radar array, their wakes couple in either phase or counter-phase configurations, changing the resultant forces on each panel. Instantaneous normal and tangential force coefficients are reported; their spectra show two distinct peaks, caused by the interaction of the wakes. Finally, a scaled model of a rectangular structure comprised of panels and frame elements is tested in the boundary layer wind tunnel in order to determine the influence of the velocity variation with height and the three-dimensionality of the bulk flow around the structure. Results show that the unsteady aerodynamic loads, being strongly influenced by the vortex shedding of the supporting elements and by the global 3-D geometry of the array, differ considerably on a panel in this array from loads acting on an isolated panel, not only in magnitude, but also in frequency.

Mechanistic modeling of micro-drilling cutting forces

This paper presents a mechanistic model for micro-drilling cutting forces that includes the cutting edge radius and the minimum chip thickness size effects. The proposed model considers three different cutting regions, i.e., ploughing-dominant, transition, and shearing-dominant, based on these size effects. Specific normal force and specific friction force coefficients have been determined through model calibration using micro-drilling experimental results. Model is validated with micro-drilling experimental results of different cutting conditions and of different machining environments. Comparisons of model simulated and experimental results show that ploughing force contributions are significant, especially at low feed rates. The proposed model has also been applied to characterize size effects in micro-drilling.

Molecular dynamics simulation of nanotribology properties of CuZr metallic glasses

The effects of scratch depth, scratch speed, and alloy composition on the mechanical deformation and nanotribology properties of CuZr metallic glasses are studied using molecular dynamics simulations based on the second-moment approximation of the many-body tight-binding potential. These effects are investigated in terms of atomic trajectories, slip vectors, friction force, normal force, and friction coefficient. The simulation results show that a few shear transformation zones independently develop at the contact area between the probe tip and the film. Pileup occurs in the nanoscratch process but not during nanoindentation at a depth of 2.4 nm. There are two areas on the surface where the atoms have high slip vector values during nanoscratching. These areas form due to the removal of atoms that piled up around the probe tip and those behind the probe tip, respectively. Both the friction force and the normal force increase with increasing scratch depth and scratch speed. Friction coefficients decrease with increasing scratch depth, scratch speed, and Zr content in films.

Study on extrudate swell of polypropylene in double-lumen micro profile extrusion

One of the basic problems in the design of polymer extrusion dies for multi-lumen micro profiles is the previously unknown extrudate swells, since they are more complex than those in axisymmetric extrusions. To investigate the three-dimensional extrudate swell of such geometries, numerical simulations and experiments are performed for the extrusion of a double-lumen micro profile using polypropylene. The selected profile exhibits typical non-axisymmetric geometry, uneven thickness and micro-scale dimension. A double convected pom–pom model considering the micro size effect is proposed to characterize viscoelastic properties of the polymer melt. Predictions are compared with experimental results as well as simulations using the power law model, and show the capacity of the proposed model for assessing the anisotropic swelling quantitatively. Influences of micro size effect, volumetric flow rate and material parameters on extrudate swell are investigated by parametric studies. It is found that swell ratios of various positions on the extrudate cross section are significantly different. The swelling anisotropy degree is enhanced with increasing volumetric flow rate as well as material parameters of second normal force coefficient and branching degree, while it is suppressed as the ratio of orientation to stretching relaxation time increases. The influence mechanisms of material parameters on extrudate swell are also discussed in detail. It is suggested that the anisotropic extrudate swell induced by micro size effect is significant and should be taken into account in high accuracy micro profile extrusions.

Analysis of time-varying friction excitations in helical gears with refined general formulation

Time-varying sliding friction force and friction torque are regarded as non-negligible excitation sources of vibration and noise in gears. The sliding friction force primarily excites the motion along the off-line-of-action direction, which transmits vibration to the housing through shafts and bearings and then radiates noise. Since the contact line intersects with the pitch line, and the directions of the friction forces are opposite on both sides of the pitch line, the calculation of the friction excitations in helical gears becomes more difficult, especially in the high contact ratio helical gears. However, there is no universal method for calculating the friction excitations in helical gears with different range of contact ratio. The changes of friction excitations in helical gears are highly dependent on the geometric parameters such as helix angle and face width among others. Yet, there exist very limited studies on these topics. In this study, a refined general formulation for the calculation of time-varying contact line and friction excitations is proposed by assuming uniform load distribution along the contact lines with time-varying normal force and friction coefficient. Key gear parameters such as modification coefficient, helix angle, and face width are analyzed to illustrate their effects on the time-varying contact line and friction excitations. The results demonstrate that the refined general formulation is effective for the calculation of the friction excitations in helical gears with different range of contact ratio, and the parametric analysis could supply some guidance for choosing gear parameters in the design of helical gears to reduce the friction excitations.

AERODYNAMIC CHARACTERISITICS OF A MISSILE COMPONENTS

A Missile is a self-propelled guided weapon system that travels through air or space. A powered, guided munitions that travels through the air or space is known as a missile (or guided missile). The Missile is defined as a space transversing unmanned vehicle that contains the means for controlling its flight path. The aerodynamic characteristics of a missile components such as body, wing and tail are calculated by using analytical methods to predict the drag and the normal forces of the missile. The total drag of the body is computed by using the parasite drag, wave drag, skin friction drag and base drag. The wing surface normal force coefficient (CN)Wing is a function of Mach number, local angle of attack, aspect ratio, and the wing surface plan form area (CN)Wing , based on the missile reference area, decreases with increasing supersonic Mach number and increases with angle of attack and the wing surface area. When the wing surface area is reduced the total weight of the missile and drag are reduced thereby increasing the lift and achieve excessive stability.

Fluidic Control of a 155 Millimeter Spin-Stabilized Projectile Using Coanda Effect

The present paper aims at investigating numerically the control of a 155 mm spin-stabilized projectile using the Coanda effect. The flight Mach number ranges from to . A tangentially blowing jet exits over a convex surface at the base of the projectile and creates flow asymmetry, which generates significant aerodynamic loads. First, Reynolds-averaged Navier–Stokes simulations are performed without projectile rotation. The jet extends 18 deg azimuthally, and numerous continuous blowing conditions are assessed. The variation of the normal force coefficient with respect to the jet momentum coefficient is presented for different freestream Mach numbers. The efficiency loss of the Coanda effect with supersonic jets is specifically discussed. Additionally, comparisons between overexpanded, adapted, and underexpanded supersonic jets are presented. In the second part, unsteady Reynolds-averaged Navier–Stokes simulations, accounting for a 400 Hz projectile rotation, are performed. Both continuous and periodic blowing are assessed. A pulsed jet is used in phase with the rotation to always act in a 90 deg sector centered in the lift plane. The resulting normal force benefits are more important than in the nonspinning case, and spin-averaged values lead to a nonzero normal force increment. Then, four pulsed jets equally distributed along the circumference of the projectile are simulated to further improve the control efficiency. The jet separation process is also investigated to determine the influence of the jet on the base pressure distribution.

Minimization of time-averaged and unsteady aerodynamic forces on a thick flat plate using synthetic jets

This article reports an experimental investigation on the reduction of the aerodynamic forces on a model consisting of an elongated blunt trailing edge plate using an active control system based on an open-loop, periodic actuation using synthetic jets. Measurements using PIV were conducted at two Reynolds numbers Reh, based on the thickness of the plate h, of about 7200 and 14400. The control arrangement consisted of two synthetic jet actuators coupled to two slots placed symmetrically along the model base. The synthetic jets were operated either in-phase or anti-phase (180° out-of-phase) with actuation frequencies of 50Hz and 100Hz. Both in-phase and anti-phase actuations were found to be effective in vortex shedding suppression and achieved considerable reduction of the time-averaged drag coefficient, C¯D. For Re=7200 and actuation frequency of 50Hz, both in-phase and anti-phase actuations resulted in a negative C¯D, suggesting that the synthetic jets acted as a propulsive device. The effectiveness of both control strategies was further evaluated by their capability of reducing the amplitudes of the oscillating drag and lift (normal force) coefficients C˜D and C˜N respectively. The experimental results showed that the in-phase actuation decreased the amplitude of the oscillating drag coefficient by about 25% compared to that of the natural wake but C˜D remained periodic with a frequency of the actuation. More importantly, in-phase actuation was found to result in a considerable decrease (nearly by one order of magnitude) in the amplitude of the oscillating normal force coefficient, C˜N. In contrast, the anti-phase actuation increased the amplitudes of both the oscillating lift and drag coefficients by nearly 95% and 30% respectively compared to the natural wake.

Simulation of a 5MW wind turbine in an atmospheric boundary layer

This article presents detached eddy simulation (DES) results of a 5MW wind turbine in an unsteady atmospheric boundary layer. The evaluation performed in this article focuses on turbine blade loads as well as on the influence of atmospheric turbulence and tower on blade loads. Therefore, the turbulence transport of the atmospheric boundary layer to the turbine position is analyzed. To determine the influence of atmospheric turbulence on wind turbines the blade load spectrum is evaluated and compared to wind turbine simulation results with uniform inflow. Moreover, the influences of different frequency regimes and the tower on the blade loads are discussed. Finally, the normal force coefficient spectrum is analyzed at three different radial positions and the influence of tower and atmospheric turbulence is shown.

Effects of head deformation on aerodynamic characteristics of spinning vehicle

Slight deformation of head influences the aerodynamic characteristics of head, after-body, and tail fins of spinning vehicle. The influence of head deformation was analyzed by the DES method. The results reveal that the frequency of interference on the after-body and fins caused by head deformation is different with the spin frequency of vehicle. It is more important that the interference caused by the head deformation has a lag phenomenon, which is related to the flow velocity and the spin rate of the vehicle. The normal force coefficient and side force coefficient of spinning vehicle with deformed head rise as the rotational speed of vehicle increases, and lag behind that in low spin rate conditions. In high Mach number conditions, the fluctuation magnitude of aerodynamic characteristics of spinning vehicle with deformed head is larger than those in low Mach number conditions.

Impact of parameter settings on normal force and gap height during tribological measurements

There has been increased interest over the past decade to use soft tribology to mimic conditions in the oral cavity. Tribological measurements in these studies have been performed on a variety of instruments, both commercial and custom-made. While the effects of tribological surface characteristics (e.g. roughness, modulus, hydrophobicity) on tribological measurements have been examined, there has been little study of measurement variability due to differences in tribometer settings. Therefore, this study examined the effect of different tribometer measurement parameters on the magnitude and fluctuation of normal force, gap height, and friction coefficient. The instantaneous values of normal force and gap height, and the extent of fluctuation of those measurements, were affected by multiple instrument parameters. Friction coefficient values were not significantly affected by instrumental parameters; however, fluctuations in normal force and gap height can contribute to data variation, reducing measurement precision and adding confounding factors to data.

Evaluation of unsteady pressure fields and forces in rotating airfoils from time-resolved PIV

The instantaneous pressure fields and aerodynamic loads are obtained for rotating airfoils from time-resolved particle image velocimetry (TR-PIV) measurements. These allowed evaluating the contribution from the local acceleration (unsteady acceleration) to the instantaneous forces. Traditionally, this term has been neglected for wind turbines with quasi-steady flows, but results show that it is a dominant term in the wake where high temporal variations in the flow field are present due to vortex shedding. Briefly, time-resolved particle image velocimetry TR-PIV measurements are used to calculate flow velocity fields and corresponding spatial and temporal derivatives. These derivatives are then used in the Poisson equation to solve for the pressure field and later used in the integral momentum equation to solve for the instantaneous forces. The robustness of the measurements is analyzed by calculating the PIV uncertainty and the independence of the calculated forces. The experimental mean aerodynamic forces are compared with theoretical predictions from the blade element momentum theory showing good agreement. The instantaneous pressure field showed dependence with time in the wake due to vortex shedding. The contribution to the instantaneous forces from each term in the integral momentum equation is evaluated. The analysis shows that the larger contributions to the normal force coefficient are from the unsteady and the pressure terms, and the larger contribution to the tangential force coefficient is from the convective term.

Model improvements for evaluating the effect of tower tilting on the aerodynamics of a vertical axis wind turbine

ABSTRACTIf a vertical axis wind turbine is mounted offshore on a semi‐submersible, the pitch motion of the platform will dominate the static pitch and dynamic motion of the platform and wind turbine such that the effect of tower tilting on the aerodynamics of the vertical axis wind turbine should be investigated to more accurately predict the aerodynamic loads. This paper proposes certain modifications to the double multiple‐streamtube (DMS) model to include the component of wind speed parallel to the rotating shaft. The model is validated against experimental data collected on an H‐Darrieus wind turbine in skewed flow conditions. Three different dynamic stall models are also integrated into the DMS model: Gormont's model with the adaptation of Strickland, Gormont's model with the modification of Berg and the Beddoes–Leishman dynamic stall model. Both the small Sandia 17 m wind turbine and the large DeepWind 5 MW are modelled. According to the experimental data, the DMS model with the inclusion of the dynamic stall model is also well validated. On the basis of the assumption that the velocity component parallel to the rotor shaft is small in the downstream part of the rotor, the effect of tower tilting is quantified with respect to power, rotor torque, thrust force and the normal force and tangential force coefficients on the blades. Additionally, applications of Glauert momentum theory and pure axial momentum theory are compared to evaluate the effect of the velocity component parallel to the rotor shaft on the accuracy of the model. Copyright © 2013 John Wiley & Sons, Ltd.

Rotor Design and Performance Study of a Vertical-Axis Wind Turbine Based on DMS

Specifically address the design of a 5KW H-type vertical-axis wind turbine (H-VAWT) with NACA 0018 airfoil considering the factors that affect wind turbine power. The double-multiple streamtube (DMS) theoretical model is analyzed and summarized and calculated by Matlab. The 5KW H-type vertical axis wind turbines aerodynamic performance is calculated by the model written in Matlab. The curve of the power coefficient as a function of the tip-speed ratio and the curve of the normal force coefficient and the tangential farce as a function of the blade position is given by Matlab. From the curves we can see that upwind rotor aerodynamic load is larger, downwind rotor aerodynamic load is smaller and there is a serious flow retarding effect in the rotor downwind area.

Experimental and Computational Modeling of the Kinematics and Aerodynamics of Flapping Wing

High-speed videography is used in measuring the kinematic and deformation parameters of the flapping wing. Based on these data, a theoretical analysis of the underlying physics is performed using computational fluid dynamics simulations. The time varying of the pitching angle in the chordwise directions exhibits a significant second harmonic. Results suggest the mechanics of membrane deformations during a flapping cycle is analogous to the buckling of a bistable structure. Noticeably, with an increase in the freestream speed, the downstroke duration increases. The solution to the three-dimensional fluid dynamics problem is constructed using two-dimensional solutions obtained for several sections of the wing by the improved discrete vortex method. The inertial component is dominant in the normal force coefficient, and hence, added mass is the main mechanism in aerodynamic force production for the studied problem. A normal component of the acceleration of the wing’s trailing edge taken with a negative sign is introduced as a kinematic parameter that is essential in flapping-wing aerodynamics. The results show a satisfactory agreement in trends of the acceleration and force coefficients. From the analysis of kinematical changes, it follows that synchronization of acceleration and of the pitching angle is important for achieving maximum values of the vertical force coefficients.

Study of nanoscratching of polymers by using molecular dynamics simulations

Molecular dynamic simulations are performed to study the nanoscratching behavior of polymers. The effects of scratching depth, scratching velocity and indenter/polymer interaction strength are investigated. It is found that polymer material in the scratching zone around the indenter can be removed in a ductile manner as the local temperature in the scratching zone exceeds glass transition temperature Tg. The recovery of polymer can be more significant when the temperature approaches or exceeds Tg. The tangential force, normal force and friction coefficient increase as the scratching depth increases. A larger scratching velocity leads to more material deformation and higher pile-up. The tangential force and normal force are larger for a larger scratching velocity whereas the friction coefficient is almost independent of the scratching velocities studied. It is also found that stronger indenter/polymer interaction strength results in a larger tangential force and friction coefficient.

Bifurcations and catastrophes in aerodynamic characteristics

The results of an experimental investigation of the longitudinal stability of a model maneuvering aircraft are presented. The results for a wide angle-of-attack range are obtained in a wind tunnel flow on an aerodynamic setup of free oscillations with a single degree of freedom. It is shown that the static aerodynamic dependences of the normal force and pitch moment coefficients on the angle of attack include catastrophic transitions from one steady state into another. The salient features of these transitions are established. It is experimentally found that the loss of the longitudinal stability of the model aircraft in a flow with variation in the deflection angles of stabilizers is softly realized via the Hopf bifurcation. At high angles of attack the flow regimes are found to exist in which steady motion represents a strange attractor.

A New Method for Horizontal Axis Wind Turbine Angle of Attack Determination

This paper proposes a new method for horizontal axis wind turbine (HAWT) angle of attack (AOA) determination. Despite common circular planes for data extraction, here, data is extracted on rectangular planes. NREL Phase VI is used for validation of the method. Results show that even in a high velocity wind the proposed plane can be an appropriate choice. Furthermore, the average radial distributions of axial and tangential induction factors are calculated based on the velocity values at the planes. Moreover, local normal force coefficients are calculated and then, the local AOA are compared with 2D results and other 3D values for different wind speeds.

A numeric investigation of friction behaviors along tool/chip interface in nanometric machining of a single crystal copper structure

The interaction between the tool rake face and the chip is critical to chip morphology, cutting forces, surface quality, and other phenomena in machining. A large body of existing literature on nanometric machining or nano-scratching only considers the overall friction behavior by simply regarding the total force along tool movement direction as the friction force, which is not suitable for describing the intriguing friction phenomena along the tool/chip interface. In this study, the molecular dynamic (MD) simulation is used to model the nanometric machining process of single crystal copper with diamond tools. The effects of three factors, namely, tool rake angle, depth of cut, and tool travel distance, are considered. The simulation results reveal that the normal force and friction force distributions along tool/chip interface for all cases investigated are similarly shaped. It is found that the normal force consistently increases along the entire tool/chip interface when a more negative rake angle tool is used. However, the friction force increases as the rake angle becomes more negative only in the contact area close to the tool tip, and it reverses the trend in the middle of tool/chip interface. Meanwhile, the increase of depth of cut overall increases the normal force along the tool/chip interface, but the friction force does not necessarily increase. Also, the progress of tool into the work material does not change the patterns of normal force, friction force, or friction coefficient distributions to a great extent. More importantly, it is discovered that the traditional sliding model with a constant friction coefficient can be used to approximate the later section of friction distributions. However, no friction model for traditional machining is appropriate to describe the first section of friction distributions obtained from the MD simulation.