Longitudinal Roll Research Articles

Linear and non-linear stability analysis of double-diffusive Hadley-Prats flow through horizontal porous layer with non-uniform thermal and solutal gradients

A scientific model is devised to understand the onset of double-diffusion Hadley-Prats flow through an infinite parallel permeable channel with non-uniform thermal, solutal gradients, and concentration based internal heat generation under the influence of convection conditions is considered. Darcy’s model is implemented in the context of a porous medium, which is characterized as isotropic and homogeneous. A linear and nonlinear stability analysis is conducted with the help of energy functional method and longitudinal roll perturbations are examined. The emerged dimensionless eigenvalue problem in the linear and nonlinear stability analysis is solved numerically by utilizing fourth order Runge-Kutta method (RK-4) method and shooting scheme. The critical values of wave and vertical thermal Rayleigh numbers are examined. A comprehensive analysis of solutions concerning the onset of convection mechanism is conducted. The produced results are discussed for the various values of emerged physical parameters. It is evident that, the presence of non-uniform thermal and solutal gradients and the internal heat generation mechanism causes the significant variations in the critical values of vertical thermal Rayleigh number in the channel regime.

Randomized self-controlled study comparing open-face vs. closed immobilization masks in fractionated cranial radiotherapy

PurposeTo compare patient discomfort and immobilisation performance of open-face and closed immobilization masks in cranial radiotherapy. Material and methodsThis was a single-center randomized self-controlled clinical trial. At CT simulation, an open-face and closed mask was made for each patient and treatment plans with identical dose prescription were generated for each mask. Patients were randomised to start treatment with an open-face or closed mask. Masks were switched halfway through the treatment course; every patient was their own control. Patients self-reported discomfort, anxiety and pain using the visual analogue scale (VAS). Inter- and intrafraction set-up variability was measured with planar kV imaging and a surface guided radiotherapy (SGRT) system for the open-face masks. Results30 patients with primary or metastatic brain tumors were randomized − 29 completed radiotherapy to a median total dose of 54 Gy (range 30–60 Gy). Mean discomfort VAS score was significantly lower with open-face masks (0.5, standard deviation 1.0) vs. closed masks (3.3, standard deviation 2.9), P < 0.0001. Anxiety and pain VAS scores were significantly lower with open-face masks (P < 0.0001). Closed masks caused more discomfort in infraorbital (P < 0.001) and maxillary (P = 0.02) areas. Two patients and 27 patients preferred closed or open-face masks, respectively. Interfraction longitudinal shifts and roll and yaw rotations were significantly smaller and lateral shifts were significantly larger with closed masks in combination with the laser system (P < 0.05) compared to open masks in combination with a SGRT system. Intrafraction variability did not differ between the masks. ConclusionsOpen-face masks are associated with decreased patient discomfort without compromising patient positioning and immobilisation accuracy.

Double-diffusive thermosolutal Hadley–Prats flow: stability analysis

ABSTRACT A mathematical model is formulated to study the onset of double-diffusion fluid flow through an infinite permeable channel with internal heat source and viscous dissipation effects under the influence of convection conditions. Numerically, the dimensionless emerging eigenvalue problem is tackled using the fourth-order Runge–Kutta scheme, specifically applied to longitudinal roll disturbances. Critical values of wave number and vertical thermal Rayleigh number are determined. A higher value of Gebhart number is observed to correlate significantly with the destabilizing phenomena in Hadley–Prats flow. Concentration-based internal heat generation also strongly modifies the critical thermal Rayleigh number. Also, it is found that the horizontal mass flow and viscous dissipation exert a substantial influence on the onset of instability in the flow regime. Linear instability analysis indicates that greater values of Lewis number in the porous medium stabilize the convection process for both values of mass diffusion parameter ( C z ) . Enhancing C z from negative to positive values diminishes the critical thermal Rayleigh ( R z ) value and consequently induces instability in the porous medium. Increased concentration-based internal heat generation generates the destabilization process in the flow region.

Enhancing high-speed steering stability of wheel-legged vehicles by active roll control

Wheel-legged vehicles (WLVs) combine the speed of wheels with the active control of legs to traverse challenging terrain, which presents a new development possibility for enhancing the system’s mobility and stability. Most of the existing studies mainly focus on the stability of low-speed trajectory optimization or obstacle-surmounting by hybrid walking-driving. Without considering the stability of high-speed driving. To enhance the vehicle stability at high-speed steering, with the additional roll moment generated by the active roll motion taken into account, a 15-degree-of-freedom nonlinear yaw-roll coupled vehicle model is developed. Specifically, a fusion dynamic stability factor for skid steering is presented as the rollover threshold to determine the three-dimensional stability region of longitudinal speed, yaw rate and roll angle, based on which the vehicle’s ideal roll angle is obtained. Subsequently, a hierarchical parallel control scheme is proposed to decouple the yaw and roll motions of the wheel-legged vehicle. The fusion dynamic stability factor is regarded as the switching threshold of the upper-level controller, while the lower-level controller adopts the linear quadratic regulator and the sliding mode control to actively control additional roll moment and direct yaw moment, respectively. Furthermore, the studies for the dynamic model and the proposed controller are conducted through vehicle tests. Corresponding test results validate the advantages of the proposed control scheme over conventional schemes without active roll control, in which vehicle stability is effectively improved, thereby preventing vehicle rollover in the case of high-speed steering.

Accumulated Deformations of the Historical Development of St. Petersburg

An analysis of the deformations of the ordinary historical buildings of St. Petersburg, accumulated over the past 150–200 years, is presented. An effective way to assess the accumulated unevenness of sediments and rolls of historical buildings is proposed, which makes it possible to obtain a reliable idea of the shape of the deformation of the building. On the example of a representative sample of buildings from two central parts of the city, it is shown which forms of deformations prevail in the historical center. It was revealed that in half of the cases deformations occur in the form of bending in the longitudinal direction and roll of the facade walls outward in the cross section, which is most dangerous for buildings. The dependences of the relative unevenness of the settlement of historical buildings on the ratios of their length and width to height are given. The reasons for the formation of a characteristic type of deformations in the form of a bend are noted. The results of the assessment of the accumulated relative unevenness of sediments that buildings received in the course of their existence made it possible to establish that the accumulated deformations are more than an order of magnitude higher than the maximum permissible values for similar structures of new construction. At the same time, the operational suitability of buildings is maintained, which indicates their durability under the condition of constant maintenance and regular repairs. It is proposed to take into account the accumulated unevenness of settlement when assessing the category of technical condition of historical buildings and monuments. On the basis of on the conducted research, proposals are made to clarify the requirements of the current norms regarding the appointment of maximum additional deformations of brick historical buildings and cultural heritage sites.

Instabilities of rotating inclined buoyancy layers.

The boundary layer near a cooled inclined plate, which is immersed in a stably stratified fluid rotating about an axis parallel to the direction of gravity, is a model for katabatic flows at high latitudes. In this paper the base flow of such an inclined buoyancy layer is solved analytically for arbitrary Prandtl numbers. By applying linear stability analyses, five unstable modes are identified for both the fixed temperature and the isoflux boundary conditions, i.e., the stationary longitudinal roll (LR) mode, the oblique roll with low streamwise wave-number (OR-1) and high streamwise wave-number (OR-2) modes, and the Tolmien-Schlichting (TS) wave with low streamwise wave-number (TS-1) and high streamwise wave-number (TS-2) modes. It is indicated that the Coriolis effect induced by the rotation leads the critical modes to be three dimensional, and a larger tilt angle of the plate and stronger Coriolis effect cause both TS wave modes to be more unstable for both thermal boundary conditions. When the Coriolis effect is considered, the OR-1 and OR-2 modes are the most unstable mode at low and high tilt angles, respectively, but the TS-1 wave mode may be the most unstable one when the plate is nearly vertical. In addition, the spanwise phase velocities of the TS wave modes change directions as the tilt angle passes some threshold values for both thermal boundary conditions except for the TS-1 wave mode with a fixed temperature boundary condition, which propagates in the same spanwise direction for all explored tilt angles.

Pilot Command Implementation in Aircrafts: A New Approach Based on Dynamic Inversion

A new approach based on dynamic inversion is proposed in this paper for implementing pilot commands in an aircrafts. The command inputs from the pilot are assumed to be (i) the normal acceleration and forward velocity commands in the longitudinal mode and (ii) roll rate, height and forward velocity commands in the lateral mode. A major difference here is that the second derivatives of the velocities along body y and z directions are assumed to be zero, as opposed to the first derivatives (which is used in many published literature). The new approach leads to a significant reduction of tuning parameters in the control design process, which is a major advantage. Detailed derivations of the modified approach are presented with respect to the generic aircraft dynamics model available in [16]. Extensive Six-DOF simulation studies show that, besides the above advantage, the new approach leads to two other additional advantages; namely reduced oscillatory response and reduced control magnitude. In a comparison study with an existing method using the Six-DOF model for Boeing 747 (with the numerical data available in [16]), numerical results clearly show the improved performance of this new approach. Furthermore, in lateral mode a technique for obtaining a corresponding roll rate command from the desired bank angle command is also implemented. Comparison results show improved performance of the new approach with this modification as well.

Physical foundations of vehicle stability when moving uphill and at longitudinal roll back

Problem. The determination of the stages of stability for vehicles as technical systems when moving uphill on an inclined section of the road and their possible longitudinal roll back is considered. The considered stages ensure the safe operation of vehicles in various conditions and with various combinations of the values of the parameters that are included in the proposed equations characterizing the stability of the vehicle. Goal. The purpose of the paper is to improve the method of researching the physical foundations of the stability of vehicles when moving uphill on an inclined plane, the formation of mathematical models, which allows to significantly improve the mathematical support of the methods of calculating the stability of two-axle vehicles (cars, trucks, tractors, special and specialized vehicles, etc.) Methodology. The approaches adopted in the work to achieve the set goal are based on the classical laws of the mechanics of free-standing or moving objects, namely: the principle of possible movements, where calculated dynamic and mathematical models with their solutions are able to preserve the given conditions and parameters, and characterize the state of stability of the vehicle in case of its longitudinal roll back. Results. It was established and confirmed by a numerical examples that the longitudinal stability of a vehicle moving uphill depends on the longitudinal road slope, the vehicle design, the elasticity of the road surface, the critical overturning angle of the vehicle and its weight parameters. Originality. The proposed mathematical model of the longitudinal stability of the vehicle when moving uphill showed that its solution significantly depends on the values of the geometric parameters of the uphill and the vehicle, and also depends on the position of the center of gravity of the vehicle relative to the surface of the road surface. Practical value. The obtained equations for determining the values of the stability parameters of the vehicle when moving uphill on an inclined road section make it possible to specify the parameters for ensuring the logistics of vehicles in the conditions of their operation on uphill.

EFFECT OF VISCOUS DISSIPATION AND INTERNAL HEAT SOURCE ON MONO-DIFFUSIVE THERMOCONVECTIVE STABILITY IN A HORIZONTAL POROUS MEDIUM LAYER

A mathematical model is developed for studying the onset of mono-diffusive convective fluid flow in a horizontal porous layer with temperature gradient, internal heat generation, and viscous dissipation effects. Darcy's model is used for the porous medium, which is considered to be isotropic and homogenous. A linear instability analysis is conducted, and transverse or longitudinal roll disturbances are examined. The dimensionless emerging eigenvalue problem is solved numerically with the Runge-Kutta and shooting methods for both cases of disturbances, i.e,. longitudinal and transverse rolls. Critical wave number and critical vertical thermal Rayleigh number &lt;i&gt;R&lt;sub&gt;z&lt;/sub&gt;&lt;/i&gt; are identified. For higher values of Gebhart number, Ge, a significant destabilizing effect of Hadley-Prats flow is computed. Internal heat generation also strongly modifies the critical vertical Rayleigh number. Extensive interpretation of the solutions related to the onset of convection is provided. The study is relevant to geophysical flows and materials processing systems.

Patterning Behavior of Hybrid Buoyancy-Marangoni Convection in Inclined Layers Heated from Below

Alongside classical effects driven by gravity or surface tension in non-isothermal fluids, the present experimental study concentrates on other exotic (poorly known) modes of convection, which are enabled in a fluid layer delimited from below by a hot plate and unbounded from above when its container is inclined to the horizontal direction. By means of a concerted approach based on the application of a thermographic visualization technique, multiple temperature measurements at different points and a posteriori computer-based reconstruction of the spatial distribution of wavelengths, it is shown that this fluid-dynamic system is prone to develop a rich set of patterns. These include (but are not limited to), spatially localized (compact) cells, longitudinal wavy rolls, various defects produced by other instabilities and finger-like structures resulting from an interesting roll pinching mechanism (by which a single longitudinal roll can be split into two neighboring rolls with smaller wavelength). Through parametric variation of the tilt angle, the imposed temperature difference and the volume of liquid employed, it is inferred that the observable dynamics are driven by the ability of gravity-induced shear flow to break the in-plane isotropy of the system, the relative importance of surface-tension-driven and buoyancy effects, and the spatially varying depth of the layer. Some effort is provided to identify universality classes and similarities with other out-of-equilibrium thermal systems, which have attracted significant attention in the literature.

Linear Parameter Varying Models-Based Gain-Scheduling Control for Lane Keeping System With Parameter Reduction

In this paper, a robust <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {H}_{2}$ </tex-math></inline-formula> controller based on Linear Parameter Varying (LPV) model with scheduling variable reduction is applied for a lane-keeping system. On a curved road section, varying longitudinal speed and roll motion lead to multiple parameter variations in the lateral vehicle dynamics. To trade-off between the complexity of the multiple scheduling variables and the accuracy of the LPV model, an order reducing method by Autoencoder (AE) is performed to obtain a reduced model and diminishes the conservatism of LPV-based controller design. Further, to ensure the continuous convex set membership of the reduced scheduling variables in online test scenarios, the Lipschitz constant of the offline trained neural network (NN) is tightly estimated by solving a Semidefinite Program (SDP). The convex set of local Linear Time-Invariant (LTI) controllers is designed according to the estimated Lipschitz bound of the trained NN. The LPV-based robust <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {H}_{2}$ </tex-math></inline-formula> feedback controller is then designed by solving a set of Linear Matrix Inequalities (LMIs). Numerical simulations with full vehicle dynamics from CarSim are given to demonstrate the performance of the proposed system on various test roads. The continuous finding of the reduced scheduling variable membership is guaranteed in the online tests, and the effectiveness of the proposed method is confirmed with the mean value of lateral offset error reduced by about 50% compared with LTI-based <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {H}_{2}$ </tex-math></inline-formula> controllers.

A comparative numerical study of bank effects on a cruise ship in crabbing motion based on URANS method

The crabbing motion of a cruise ship perpendicularly towards a quay wall is investigated by an unsteady Reynolds-averaged Navier-Stokes (URANS) method. In the numerical simulations, the cruise ship is approaching the quay wall at a constant lateral speed and stops at a rather small ship-quay distance. The crabbing motion with multiple degrees of freedom is simulated by the dynamic overset mesh technique, while the free surface elevation is simulated by the Volume of Fluid method. To obtain reliable temporal discretization for the crabbing process, the time-step dependence study is conducted first, from which a suitable time step is determined. From the computations, the hydrodynamic performances of the cruise ship including: longitudinal force, lateral force, roll moment and yaw moment, as well as surge, heave, roll, pitch and yaw motions are predicted. To evaluate the importance of numerical models used in the URANS method, a comparative investigation is carried out to evaluate the impacts of boundary condition, turbulence model and temporal discretization scheme on the numerical results. From the comparisons, notable influences by the turbulence model and the temporal discretization scheme on both hydrodynamic quantities and flow field features can be observed, especially when the bank effects tend to be pronounced; while the boundary conditions are shown to be less significant. The numerical results indicate remarkable variations of the hydrodynamic quantities in the crabbing process under the influences of ship speed and bank effects. Given that the surge motion is one of the most principal crabbing performances, the initial drift angle with a set of deviations is also computed, coupled with two typical ship speeds in terms of practical navigation. It suggests that more attentions should be paid to the low-speed crabbing to minimize the surge variations. The present study demonstrates promising abilities of CFD methods to deal with the hydrodynamic predictions of harbour manoeuvres in restricted waters.

Rayleigh–Bénard flow for a Carreau fluid in a parallelepiped cavity

A continuation method is used to study Rayleigh–Bénard convection in a non-Newtonian fluid inside a parallelepiped cavity. The cavity has its length equal to twice the side of the square cross-section. Shear-thinning and shear-thickening Carreau fluids are considered. The focus is put on the two stable branches which exist in the Newtonian case, a stable primary branch of transverse rolls $B_1$ and a primary branch of longitudinal roll $B_2$ , stabilized beyond a secondary bifurcation point $S_2$ . Although the primary bifurcation points are unchanged, the non-Newtonian properties strongly modify the bifurcation diagram. Indeed, for a shear-thinning fluid, the stable solutions can exist at much smaller Rayleigh numbers ${Ra}$ , on subcritical branches beyond saddle-node points $SN_1$ and $SN_2$ , and small perturbations can be sufficient to reach them. In agreement with Bouteraa et al. (J. Fluid Mech., vol. 767, 2015, pp. 696–734), the change of the primary bifurcations from supercritical to subcritical occurs at given values of what they define as the degree of shear-thinning parameter $\alpha$ . Moreover, the value of the Rayleigh number at the saddle-node points can be approximated by a simple expression, as proposed by Jenny et al. (J. Non-Newtonian Fluid Mech., vol. 219, 2015, pp. 19–34). In the case of a shear-thickening fluid, the branches remain supercritical, but the secondary point $S_2$ is strongly moved towards larger ${Ra}$ , making it more difficult to reach the longitudinal roll solution. Energy analyses at the bifurcations $SN_1$ , $SN_2$ and $S_2$ show that the changes of the corresponding critical thresholds ${Ra}_c$ are connected with the changes of the viscous properties, but also with changes of the buoyancy effect.

On the Tripped Rollovers and Lateral Skid in Three-Wheeled Vehicles and Their Mitigation

Active safety systems for three-wheeled vehicles seem to be in premature development; in particular, delta types, also known as tuk-tuks or sidecars, are sold with minimal protection against accidents. Unfortunately, the risk of wheel lifting and lateral and/or longitudinal vehicle roll is high. For instance, a tripped rollover occurs when a vehicle slides sideways, digging its tires into soft soil or striking an object. Unfortunately, research is mostly aimed at un-tripped rollovers while most of the rollovers are tripped. In this paper, models for lateral skid tripped and un-tripped rollover risks are presented. Later, independent braking and accelerating control actions are used to develop a dynamic stability control (DSC) to assist the driver in mitigating such risks, including holes/bumps road-scenarios. A common Lyapunov function and an LMI problem resolution ensure robust stability while optimization allows tuning the controller. Numerical and HIL tests are presented. Implementation on a three-wheeled vehicle requires an inertial measurement unit, and independent ABS and propulsion control as main components.

Presence of Longitudinal Roll Structures during Synoptic Forced Conditions in Complex Terrain

To investigate synoptic interactions with the San Andres Mountains in southern New Mexico, the Weather Research and Forecasting (WRF) model was used to simulate several days in the period 2018–2020. The study domain was centered on the U.S. Department of Agriculture (USDA) Agricultural Research Service’s Jornada Experimental Range (JER) and the emphasis was on synoptic conditions that favor strong to moderate winds aloft from the southwest, boundary layer shear, a lack of moisture (cloud coverage), and modest warming of the surface. The WRF simulations on these synoptic days revealed two distinct regimes: lee waves aloft and SW-to-NE oriented Longitudinal Roll Structures (LRS) that have typical length scales of the width of the mountain basin in the horizontal and the height of the boundary layer (BL) in the vertical. Analysis of the transitional periods indicate that the shift from the lee wave to LRS regime occurs when the surface heating and upwind flow characteristics reach a critical threshold. The existence of LRS is confirmed by satellite observations and the longitudinal streak patterns in the soil of the JER that indicate this is a climatologically present BL phenomenon.

Research on rolling process of bow parts with variable lateral roll spacing based on optimal bending angle distribution function

For roll-bending formation in bow-shaped sheet metal parts, the distance of the traditional roll-forming transverse unit is constant, which often causes corner wrinkles, corner cracks, tearing, springback, and peak longitudinal strain defects throughout the formation process. To solve these shortcomings, this paper proposes an optimized bending angle distribution function to synergize the variable lateral roll distance technique. By parameterizing the abstract function according to the function boundary trajectory, the bending angle of each roll pass is measured; furthermore, the optimal function curve can be obtained through finite element analysis. Then, the roll-bending simulation is conducted, and the forming angle of the sheet metal parts is redistributed to determine the most effective reduction of peak longitudinal strain and transverse roll spacing. The bending angle distribution function aids the variable transverse roll distance technology to execute roll-bending experiments on 3004Al arch parts, ultimately obtaining the optimal roll-bending angle and transverse roll distance. The results of the roll forming simulation, based on the optimized bending angle distribution function and variable transverse roll distance technology, corroborate the experimental results, and good roll forming efficiency and quality are achieved.

ЭКСПЕРИМЕНТАЛЬНОЕ МОДЕЛИРОВАНИЕ ПЛЮМА, ОБРАЗУЮЩЕГОСЯ В НАКЛОННОМ ПЛОСКОМ КОРОВОМ СЛОЕ СУБДУЦИРУЮЩЕЙ ОКЕАНИЧЕСКОЙ ПЛИТЫ

The results of experimental studies of the thermal and hydrodynamic structure of the plume conduit melted out in a flat inclined layer above a local heat source are presented. For different thermal powers on the plume base, the evolution of the plume conduit structure and the plume breakthrough on the surface are presented. It was found that for heater power N = 5.6 W and N = 6.4 W and relative power Ka = 1.18, the plume does not reach the surface of the inclined layer. For N = 7.9 W (Ka = 1.42) and N = 8.9 W (Ka = 1.53), the plume reaches the surface, and the first plume cell (plume head) has a mushroom shape. Depending on the value of the Ka criterion (relative thermal power), the upper cell structure and the regime of plume breakthrough on the surface change. There is an ascending flow at the upper bounding surface of the inclined layer and a descending flow along the rest of the perimeter of the plume conduit. Longitudinal roll flows are also observed. The roll axes are oriented in the direction of the ascending flow. The roll flows transfer heat from the ascending flow to the descending one and hence reduce the unstable stratification existing in the layer. Due to these flows, the turbulent viscosity of the melt increases, which also reduces unstable stratification. In the conduit portion, where the rolls are weak, unstable stratification is additionally compensated by the plume conduit widening. Laboratory experiments have shown that the regime of plume breakthrough on the surface of an inclined flat layer depends on the Ka criterion, and this dependence differs from that of plume breakthrough on the surface in an unbroken solid massif

Flight Dynamics of Transport Aircraft Equipped with Flared-Hinge Folding Wingtips

This paper studies the influence of flared-hinge folding wingtips on the aerodynamic derivatives and flight dynamics of a narrow-body transport aircraft. In addition, the influence of fold angle, hinge-line angle (flare angle), and wingtip size on wing loading is assessed. The Tornado vortex lattice method is used for aerodynamic predictions and to estimate the stability and control derivatives. First, the aircraft is assumed rigid, and different hinge angles, fold angles, and wingtip sizes are considered. Then, structural flexibility is considered by coupling Tornado with a linear structural finite element solver to facilitate quasi-static aeroelastic analysis. The quasi-static deflections method is used to study the flight dynamics of the flexible aircraft. The effect of the flared-hinge folding wingtips on the longitudinal and lateral-directional flight dynamic modes of the rigid and the flexible aircraft is compared and contrasted. Furthermore, the variations of longitudinal trim settings and roll response with flared-hinge folding wingtips are assessed.

An experimental investigation of the bicycle motion during a hands-on shimmy

This paper presents original data records of linear accelerations and angular velocities of six points of a racing bicycle collected during a ‘genuine’ hands-on shimmy. The records, together with GPS, forward speed data, and action cam videos are thoroughly analysed to understand the actual motion of the bicycle and its parts, at lower (weave) and higher (wobble or shimmy) frequencies. The main goal is to assess how and to what extent each part, and what compliances, are involved in the shimmy mechanism. In particular, the motions of the front and rear frames, and thus of the whole bicycle, during shimmy are carefully rebuilt. At higher frequencies, the rear frame rotates rigidly about an axis parallel to the yaw axis, while it undergoes a relative torsion between the head tube and the dropouts in the longitudinal (roll) direction. Likewise, the front frame rotates rigidly about the steering axis (in the entire frequency range), while it bends laterally consistently with the rear frame torsion. Although the overall motion is very complex, these results validate and extend models, conjectures, and theoretical results of the scientific literature about shimmy.

사각 채널 내의 볼텍스 유동에 관한 실험적 연구(2)

Mixed convective vortex flow in the 3-D horizontal rectangular channel filled with high viscous fluid is investigated for various aspect ratio experimentally. The particle image velocimetry technique with thermo-sensitive liquid crystal tracers (PIT) is used for visualizing and analysis. This method allows simultaneous measurement of velocity and temperature. Quantitative data of temperature and velocity are obtained by applying the color-image processing to a visualized image, and neural network is applied to the color-to-temperature calibration. In this study, the working fluid is silicon oil (Pr = 909), the aspect ratio (channel width to heigh) is varied from 2 to 6 at constant Reynolds number (Re = 4×10<SUP>-3</SUP>). From the present study, the temperature and velocity of mixed convective vortex flow can be visualized simultaneously, and it is found that the longitudinal roll number in the channel is increased with increasing aspect ratio, and the experimental results agree well with numerical results.