Sweeping Motion Research Articles

The study of a turbulent air flow over capillary–gravity water surface waves: Characteristics of coherent vortical structures

This study is concerned with the characteristics of coherent vortices within the turbulent air flow undulated by gravity and capillary–gravity water waves. The flow fields from the direct numerical simulations are analyzed. A formal scheme is developed to detect and classify the vortical structures. The scheme uses local analysis of the velocity-gradient tensor to define the vortex swirling core, and adopts the topological geometry of the swirling core to classify the vortical structures. Three types of vortical structures are identified, including quasi-streamwise vortex, reversed and forward (head pointing upstream and downstream, respectively) horseshoe vortices. Quasi-streamwise vortices are the dominant structure in the flow; their distribution initiates near the wave trough, extends downstream along the windward face, lifts up and terminates above the wave crest. More reversed horseshoe vortices are observed than the forward horseshoe vortices; both cluster around the region above wave trough. Reversed and forward horseshoe vortices, however, contribute the most to the production of Reynolds stress associated with sweep and ejection motions, respectively. The quasi-streamwise vortices contribute more to producing Reynolds stress associated with ejection event, indicating that the elongated vortices bend inward at the downstream end and can be considered as degenerate forward horseshoe vortices. The intensified distributions of horseshoe vortices near the wave trough are attributed to the combined effect of vortex turning from the spanwise turbulent component to the streamwise vortex and the vortex stretching of the streamwise vortex. The strong stretching of streamwise vortex above the windward face contributes to the continuous development of quasi-streamwise vortices. Accordingly, the presence of capillary ripples on the windward face mitigates effectively the occurrence of coherent vortices, in particular the quasi-streamwise vortices. In contrast, the presence of capillary ripples on the leeward surface presents a minor effect on the mitigation of coherent vortices.

Experimental Investigation of Sweeping Jet Film Cooling in a Transonic Turbine Cascade

Abstract The sweeping jet (SJ) film cooling hole has shown promising cooling performance compared to the standard shaped hole in low-speed conditions. The present work demonstrates the first attempt of SJ film cooling at an engine relevant Mach number. An experimental investigation was conducted to study the SJ film cooling on a nozzle guide vane suction surface. A well-established additive manufacturing technique commonly known as stereolithography (SLA) was utilized to design a transonic, engine representative vane geometry in which a row of SJ holes was used on the vane suction surface. Experiments were performed in a linear transonic cascade at an exit Mach number of 0.8 and blowing ratios of BR = 0.25–2.23. The measurement of heat transfer was conducted with the transient IR method, and the convective heat transfer coefficient (HTC) and adiabatic film cooling effectiveness were estimated using a dual linear regression technique (DLRT). Aerodynamic loss measurements were also performed with a total pressure Kiel probe at 0.25Cax downstream of the exit plane of the vane cascade. Experiments were also conducted for a baseline-shaped hole (777-hole) for a direct comparison. Results showed that the SJ hole has a wider coolant spreading in the lateral direction near the hole exit due to its sweeping motion that improves the overall cooling performance particularly at high blowing ratios (BR > 1). Aerodynamic loss measurement suggested that the SJ hole has a comparable total pressure loss to the 777-shaped hole.

Evaluation of the Effects of 660-nm and 810-nm Low-Level Diode Lasers on the Treatment of Dentin Hypersensitivity.

Introduction: Dentin hypersensitivity is a common oral problem that occurs as a short and sharp pain. There are many techniques to treat this condition, the latest of which is laser treatment. The aim of this study was to evaluate the effect of two types of low-power diode lasers (660 nm and 810 nm) on dentin hypersensitivity in order to achieve an acceptable clinical application by adjusting the effective parameters. Methods: In this randomized, double-blind clinical trial, sensitive teeth of 7 patients were divided into three groups with a randomized matching method: group I, treated with 660-nm diode laser irradiation, group II, treated with diode laser 810-nm, and group III, the control group. Irradiation parameters for 660-nm and 810-nm diode lasers were the power of 30 mW and 100 mW respectively, in contact and continuous modes, perpendicular to the tooth surface with a sweeping motion. Treatments were carried out in four sessions at weekly intervals. The data obtained were analyzed with SPSS 22, using one-way repeated measures ANOVA and the LSD (least significant difference) test. The significance level was considered as P≤0.05. Results: There were no significant differences in visual analogue scale (VAS) score changes between the two laser groups after the intervention in the first, second and third weeks compared to the baseline (P>0.05). These changes in the fourth week were significantly higher in the 810-nm laser group compared to the 660-nm laser group (P=0.04), and in the 660-nm laser group, they were more than the control group (P=0.02). The mean VAS scores at 1-week, 1-month and 2-month postoperative intervals were significantly lower in the 810-nm laser group than in the 660-nm laser group, and in the 660-nm laser group, they were less than the control group (P<0.001). Conclusion: The use of 660-nm and 810-nm diode lasers with the power of 30 and 100 mW respectively for 120 seconds was effective in reducing pain in patients with dentin hypersensitivity. However, the effect of the 810-nm laser on reducing the dentin hypersensitivity was more long-lasting than that of the 660-nm laser.

Membrane sweeping for induction of labour.

Membrane sweeping for induction of labour.

Novel jet impingement atomization by synchronizing the sweeping motion of the fluidic oscillators

Novel jet impingement atomization by synchronizing the sweeping motion of the fluidic oscillators

Line-of-Sight Pursuit in Monotone and Scallop Polygons

We study a turn-based game in a simply connected polygonal environment [Formula: see text] between a pursuer [Formula: see text] and an adversarial evader [Formula: see text]. Both players can move in a straight line to any point within unit distance during their turn. The pursuer [Formula: see text] wins by capturing the evader, meaning that their distance satisfies [Formula: see text], while the evader wins by eluding capture forever. Both players have a map of the environment, but they have different sensing capabilities. The evader [Formula: see text] always knows the location of [Formula: see text]. Meanwhile, [Formula: see text] only has line-of-sight visibility: [Formula: see text] observes the evader’s position only when the line segment connecting them lies entirely within the polygon. Therefore [Formula: see text] must search for [Formula: see text] when the evader is hidden from view. We provide a winning strategy for [Formula: see text] in two families of polygons: monotone polygons and scallop polygons. In both families, a straight line [Formula: see text] can be moved continuously over [Formula: see text] so that (1) [Formula: see text] is a line segment and (2) every point on the boundary [Formula: see text] is swept exactly once. These are both subfamilies of strictly sweepable polygons. The sweeping motion for a monotone polygon is a single translation, and the sweeping motion for a scallop polygon is a single rotation. Our algorithms use rook’s strategy during its pursuit phase, rather than the well-known lion’s strategy. The rook’s strategy is crucial for obtaining a capture time that is linear in the area of [Formula: see text]. For both monotone and scallop polygons, our algorithm has a capture time of [Formula: see text], where [Formula: see text] is the number of polygon vertices.

Dynamics and wall collision of inertial particles in a solid–liquid turbulent channel flow

The dynamics and wall collision of inertial particles were investigated in non-isotropic turbulence of a horizontal liquid channel flow. The inertial particles were $125~\unicode[STIX]{x03BC}\text{m}$ glass beads at a volumetric concentration of 0.03 %. The bead-laden flow and the unladen base case had the same volumetric flow rates, with a shear Reynolds number, $Re_{\unicode[STIX]{x1D70F}}$, of the unladen flow equal to 410 based on the half-channel height and friction velocity. Lagrangian measurements of three-dimensional trajectories of both fluid tracers and glass beads were obtained using time-resolved particle tracking velocimetry based on the shake-the-box algorithm of Schanz et al. (Exp. Fluids, vol. 57, no. 5, 2016, pp. 1–27). The analysis showed that on average the near-wall glass beads decelerate in the streamwise direction, while farther away from the wall, the streamwise acceleration of the glass beads became positive. The ejection motions provided a local maximum streamwise acceleration above the buffer layer by transporting glass beads to high velocity layers and exposing them to a high drag force in the streamwise direction. Conversely, the sweep motion made the maximum contribution to the average streamwise deceleration of glass beads in the near-wall region. The wall-normal acceleration of the beads was positive in the vicinity of the wall, and it became negative farther from the wall. The investigation showed that the glass beads with sweeping motion had the maximum momentum, streamwise deceleration, and wall-normal acceleration among all the beads close to the wall and these values increased with increasing their trajectory angle. The investigation of the beads that collided with the wall showed that those with shallow impact angles (less than $1.5^{\circ }$) typically slide along the wall. The sliding beads had a small streamwise momentum exchange of ${\sim}5\,\%$ during these events. The duration of their sliding motion could be as much as five times the inner time scale of the unladen flow. The wall-normal velocity of these beads after sliding was greater than their wall-normal velocity before sliding, and was associated with the rotation induced lift force. Beads with impact angles greater than $1.5^{\circ }$ had shorter interaction times with the wall and smaller streamwise and wall-normal restitution ratios.

Numerical simulation of buckling and asymmetric behavior of flexible filament using temporal second-order immersed boundary method

PurposeThe purpose of this paper is to perform two-dimensional numerical simulation involving fluid-structure interaction of flexible filament. The filament is tethered to the bottom of a rectangular channel with oscillating fluid flow inlet conditions at low Reynolds number. The simulations are performed using a temporal second-order finite volume-based immersed boundary method (IBM). Further, to understand the relation between different aspect ratios i.e. ratio of filament length to channel height (Len/H) and fixed channel geometry ratio, i.e. ratio of channel height to channel length (H/Lc) on mixing and pumping capabilities.Design/methodology/approachThe discretization of governing continuity and Navier–Stokes equation is done by finite-volume method on a staggered Cartesian grid. SIMPLE algorithm is used to solve fluid velocity and pressure terms. Two cases of oscillatory flow conditions are used with the flexible filament tethered at the center of bottom channel wall. The first case is sinusoidal oscillatory flow with phase shift (SOFPS) and second case is sinusoidal oscillatory flow without phase shift (SOF). The simulation results are validated with filament dynamics studies of previous researchers. Further, parametric analysis is carried to study the effect of filament length (aspect ratio), filament bending rigidity and Reynolds number on the complex deformation and behavior of flexible filament interacting with nearby oscillating fluid motion.FindingsIt is found that selection of right filament length and bending rigidity is crucial for fluid mixing scenarios. The phase shift in fluid motion is also found to critically effect filament displacement dynamics, especially for rigid filaments. Aspect ratio, suitable for mixing applications is dependent on channel geometry ratio. Symmetric deformation is observed for filaments subjected to SOFPS condition irrespective of bending rigidity, whereas medium and low rigidity filaments placed in SOF condition show severe asymmetric behavior. Two key findings of this study are: symmetric filament conformity without appreciable bending produces sweeping motion in fluid flow, which is highly suited for mixing application; and asymmetric behavior shown by the filament depicts antiplectic metachronism commonly found in beating cilia. As a result, it is possible to pin point the type of fluid motion governing fluid mixing and fluid pumping. The developed computational model can, thus, successfully demonstrate filament-fluid interaction for a wide variety of similar problems.Originality/valueThe present study uses a temporal second-order finite volume-based IBM to examine flexible filament dynamics for various applications such as fluid mixing. Also, it highlights the relationship between channel geometry ratio and filament aspect ratio and its effect on filament sweep patterns. The study further reports the effect of filament displacement dynamics with or without phase shift for inlet oscillating fluid flow condition.

Effects of Mobilized Wall Friction Angle on Resultant Seismic Earth Pressure on Shallow Foundation

AbstractTo investigate the seismic earth pressure on a shallow foundation, a dynamic centrifuge test using a sinusoidal sweep motion was performed on a superstructure–footing model embedded in dry ...

Near-wall motion of inertial particles in a drag-reduced non-Newtonian turbulent flow

The kinematics of inertial particles suspended in the near-wall region of Newtonian and non-Newtonian turbulent channel flows was experimentally investigated. The non-Newtonian fluid was a homogeneous solution of 90 part per million of a polyacrylamide polymer in water with 66% drag reduction. All the experiments were performed at the same volumetric flow rate with Reynolds number of 34,300 based on bulk velocity, channel height, and the kinematic viscosity of water. The inertial particles were 250-μm glass beads with St of 35 (in water) at a volumetric concentration of 0.05%. A time-resolved two-dimensional particle tracking velocimetry was used to record particle images at acquisition frequency of 17.6 kHz and detect trajectory of flow tracers and the glass beads. The recorded data were processed using a two-dimensional particle tracking algorithm to obtain the Lagrangian kinematics of the beads. The comparison between laden flows of water and polymer solution showed reduction of number density of the beads and their momentum in the vicinity of the wall in the polymeric flow. The polymer solution remarkably reduced the wall-normal and shear Reynolds stresses of the beads, but had a negligible effect on their streamwise Reynolds stress. The wall-normal fluctuation of the beads reduced in the polymeric flow and their trajectories became parallel with the channel wall. Results also showed that the ejection and sweep motions were not the major mechanism for wall-normal distribution of the beads in the polymeric flow. Outcomes suggest that drag-reducing polymer solutions have the potential of reducing erosive wear in particle-laden pipelines.

Flow dynamics of sweeping jet impingement upon a large convex cylinder

Flow dynamics associated with sweeping jets impinging upon a convex cylinder are experimentally examined in a water tank using time-resolved particle image velocimetry (TR-PIV). The cylinder has a large cylinder-jet diameter ratio (D/Dh = 15) and is placed 4 Dh from the jet exit, where Dh is the hydrodynamic diameter of the jet exit. The sweeping jets with two spreading angles as a result of different Reynolds numbers are considered, and a steady impinging jet is examined for comparison. As the baseline case, the steady impinging jet can only cover a small area along the convex surface. At both Reynolds numbers, Re = 3.3 × 103 and Re = 10 × 103, the jet has a strong impact on the convex surface by direct impingement only within a small region between angular angles of θ ≈ ±8° away from the centerline. However, the sweeping jet has a spreading angle of about 30° at the lower Re = 3.3 × 103. As a result, the jet exerts a more even distribution of the impingement in a larger region along the convex surface. Near-wall vortices induced by the impingement are identified to cause early separation of the wall jet flow from the convex surface. The sweeping jet has a much larger spreading angle of about 80° at the higher Re = 10 × 103. As such, the sweeping jet impinges upon the lateral sides of the cylinder at about θ ≈ ±45°, resulting in a weak impingement around the centerline. In addition, the jet column breaks during the sweeping motion: the jet close to the exit switches to one side of the cylinder, while the remaining jet develops on the other side. Proper orthogonal decomposition (POD) analysis reveals that the remaining jet mainly contributes to the impingement at the higher Reynolds number. In general, compared with a steady jet, a sweeping jet can cover a much larger area, even half of a big convex cylinder, with strong impingement.

Non-uniform flow over cobble bed with submerged vegetation strip

To better understand the influence of non-uniform flow in coarse-bed streams with a submerged vegetation strip on flow characteristics, data were collected from experiments in a laboratory flume and field investigations in a cobble-bed river reach in central Iran. The results of the experiments and field observations showed that the velocity profiles for non-uniform flows in cobble-bed streams with vegetation strips can be divided into either two or three sub-zones. The velocity profiles of non-uniform flows in the laboratory flume and cobble-bed stream deviated from the log law in the wake and mixing zones, but fitted approximately in the log-law layer zone. The distribution of Reynolds stress for non-uniform flows was influenced by the vegetation strip and showed a non-concave shape. Near the channel bed, sweep motion occurred more frequently than the ejection process. Close to the top of vegetation strip, the correlation coefficient (ru′w′) was negative, indicating downward momentum transport due to sweep and ejection processes. However, ru′w′was positive near the water surface, indicating upward momentum by the vegetation strip due to inward and outward interactions. The study shows that laboratory results cannot be easily applied to natural rivers without considerable assumptions.

Measurement of two-dimensional heat transfer and flow characteristics of an impinging sweeping jet

This study experimentally explores the two-dimensional heat transfer characteristics of an impinging sweeping jet generated from a feedback–channel-type fluidic oscillator. The results were compared to those of a steady square jet. A phosphor thermometry technique was used to measure the surface temperature field on an aluminum plate for different Reynolds numbers and jet-to-wall spacing. A two-dimensional planar Particle Image Velocimetry (PIV) measurement of the sweeping impinging jet was also conducted to couple the flow characteristics and heat transfer performance. The local Nusselt number distributions on the flat plate were evaluated by converting the temperature fields. The heat transfer performance of the sweeping jet is much higher than that of the steady jet because the sweeping motion of the jet induces significant turbulent flow and clearly improves the thermal transport near the impingement plate. However, when the jet-wall spacing becomes greater than 5, the heat transfer performance of the sweeping jet drastically decreases due to the reduced impinging velocity, and the heat transfer of the steady square jet surpasses that of the sweeping jet. In the ensemble averaged velocity fields, two main jet streams are ejected to the plate like oblique impinging jets near the sidewalls of the exit because the jet flow is attached to sidewall one or the other for more than 70% of an oscillation period. There was high similarity between the ensemble averaged velocity profiles and the Nusselt number profiles.

Removing fences from sweep motion videos using global 3D reconstruction and fence-aware light field rendering

Diminishing the appearance of a fence in an image is a challenging research area due to the characteristics of fences (thinness, lack of texture, etc.) and the need for occluded background restoration. In this paper, we describe a fence removal method for an image sequence captured by a user making a sweep motion, in which occluded background is potentially observed. To make use of geometric and appearance information such as consecutive images, we use two well-known approaches: structure from motion and light field rendering. Results using real image sequences show that our method can stably segment fences and preserve background details for various fence and background combinations. A new video without the fence, with frame coherence, can be successfully provided.

Supporting dynamic conceptions of area and its measure

ABSTRACTDesign research was conducted to investigate the development of dynamic conceptions of area. Students used simple physical instruments, squeegees, and paint, to generate rectangular areas continuously as a sweeping motion of one length through another. Then, they dissected the resulting swept space into discrete units of measure by coordinating measures of the lengths of the sides, including side lengths measured in different units, such as “squeegee” and inch. Analysis of classroom discourse, formative assessments, and flexible interviews conducted 2 months after instruction illustrated that students developed dynamic images and meanings of area and its measure. These expanded their understanding of multiplicative product to include referent-transforming quantity and provided means for re-envisioning the meaning of the commutative and distributive properties of multiplication. Students employed these properties to relate fractional and whole number area products. Dynamic generation and dissection in tandem helped students bridge discrete and continuous conceptions of area and its measure.

Dissimilarity Between Dust, Heat, and Momentum Turbulent Transports During Aeolian Soil Erosion

AbstractMeasuring accurately size‐resolved dust flux near the surface is crucial for better quantifying dust losses by semiarid soils. Dust fluxes have been usually estimated from the flux‐gradient approach, assuming similarity between dust and momentum turbulent transport. This similarity has, however, never been verified. Here we investigate the similarity between dust (0.3 to 6.0 μm in diameter), momentum, and heat fluxes during aeolian erosion events. These three fluxes were measured by the Eddy Covariance technique during the WIND‐O‐V (WIND erOsion in presence of sparse Vegetation's) 2017 field experiment over an isolated erodible bare plot in South Tunisia. Our measurements confirm the prevalence of ejection and sweep motions in transporting dust as for heat and momentum. However, our measurements also reveal a different partition of the dust flux between ejection and sweep motions and between eddy time scales compared to that of momentum and heat fluxes. This dissimilarity results from the intermittency of the dust emission compared to the more continuous emission (absorption) of heat (momentum) at the surface. Unlike heat emission and momentum absorption, dust release is conditioned by the wind intensity to initiate sandblasting. Consequently, ejection motions do not carry dust as often as heat and low momentum from the surface. This dissimilarity diminishes with increasing wind intensity as saltation patterns, and thus dust emission through sandblasting, become spatially more frequent. Overall, these findings may have implications on the evaluation of dust flux from techniques based on similarity with momentum or heat turbulent transport.

Reynolds number dependence of turbulent structures associated with high-amplitude wall pressure peaks in channel flow

We investigate the turbulent structures associated with positive and negative high-amplitude wall pressure peaks in fully developed turbulent channel flows. The analysis is performed by conditional sampling of a channel flow dataset computed in direct numerical simulations. The Reynolds number was varied from 180 to 4000 based on the channel half-depth h and the friction velocity uτ. Positive and negative wall pressure events were associated with small-scale vortex structures identified by Q-criterion. When scaled in wall units, the overall size of the structure was independent of Reynolds number. Positive and negative wall pressure events were also associated with a large-scale sweep motion (of ) from the outer layer, which constitutes a shear layer in the near-wall region. In a statistical analysis of the instantaneous pressure field, a sequence of negative and positive pressure peaks was found to predominate at the wall. The relative contributions from the large- and small-scale structures were qualitatively assessed by investigating the rapid and slow pressure terms from Poisson’s equation. Both terms contributed nearly equally to the positive wall pressure peaks, but the slow term dominated the negative pressure peaks (with a relative contribution of approximately 60%).

Influence of sinusoidal riblets on the coherent structures in turbulent boundary layer studied by time-resolved particle image velocimetry

Drag reduction by riblets has drawn the attention of many researchers because of its low production cost and easy maintenance. But due to the fact that the rather low drag reduction riblets can offered, an easy modification to the structure of riblets to improve the performance would be more than necessary. In this work, an investigation of the influences on coherent structure of straight riblets and sinusoidal riblets (s-riblets) in a turbulent boundary layer (TBL) at various Reynolds numbers is carried out experimentally by using the time-resolved particle image velocimetry (TR-PIV). It is found that the skin friction of the turbulent boundary layer is reduced close to the wall, and the logarithmic velocity profile shifts upwards over riblets and s-riblets. The turbulence intensity and Reynolds shear stress are also reduced in the near wall region compared with the scenario of the smooth case, and a better performance on drag reduction is obtained over s-riblets. Coherent structures including hairpin vortex and low speed streaks are extracted over test plates by using the correlation coefficient and &lt;inline-formula&gt;&lt;tex-math id="M2"&gt;\begin{document}$\lambda_{ci}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; vortex identification method, to study the mechanism of drag reduction caused by riblets. It is shown that the spatial scale of coherent structure in streamwise and wall-normal direction decrease over riblets and s-riblets to various degrees, the inclination angle between the mainstream and coherent structure also decreases, meaning that the wall-normal movement and upwash motion are suppressed over riblets and s-riblets. Results from the conditional sampling method demonstrate that the induction of ejection and sweep motions by hairpin vortex are inhibited over riblets and hence the exchange of energy and momentum and the self-sustaining mechanism in turbulence are influenced. Furthermore, at the same &lt;inline-formula&gt;&lt;tex-math id="M3"&gt;\begin{document}$Re_{\tau}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M3.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M3.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the spanwise spacing of low speed streaks turns wider with wall-normal position increasing. At the same &lt;inline-formula&gt;&lt;tex-math id="M4"&gt;\begin{document}$ y^{+} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M4.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181875_M4.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, a larger spacing is seen over riblets and s-riblets, implying that spanwise movement of the streaks is restrained and hence becomes more stable.

Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer

Coherent structures of a three-dimensional (3D) separation due to an adverse pressure gradient are investigated experimentally. The flow set-up consists of a flat plate to develop a turbulent boundary layer upstream of an asymmetric two-dimensional diffuser with one diverging surface. The diffuser surface has an initial mild curvature followed by a flat section where flow separation occurs. The top and the two sidewalls of the diffuser are not equipped with any flow control mechanism to form a 3D separation. Planar particle image velocimetry (PIV) using four side-by-side cameras is applied to characterize the flow with high spatial resolution over a large streamwise-wall-normal field of view (FOV). Tomographic PIV (tomo-PIV) is also applied for volumetric measurement in a domain flush with the flat surface of the diffuser. The mean flow obtained from averaging instantaneous velocity fields of this intermittent unsteady flow appears as a vortex with an elliptical cross-section. The major axis of the ellipse is tilted with respect to the streamwise direction. As a result, the average velocity in the mid-span of the diffuser has an upstream forward flow and a downstream backward flow, separated by a point of zero wall shear stress. Sweep motions mainly carry out transport of turbulent kinetic energy upstream of this point, while ejections dominate at the downstream region. In the instantaneous flow fields, forward and backward flows have equivalent strength, and the separation front is extended in the spanwise direction. The conditional average of the separation instants forms a saddle-point structure with streamlines converging in the spanwise direction. Proper orthogonal decomposition (POD) of the tomo-PIV data demonstrates that about 42 % of the turbulent kinetic energy is present in the first pair of modes, with a strong spanwise component. The spatial modes of POD also show focus, node and saddle-point structures. The average of the coefficients of the dominant POD modes during the separation events is used to develop a reduced-order model (ROM). Based on the ROM, the instantaneous 3D separation over the diffuser is a saddle-point structure interacting with focus-type structures.

A Friction Model-Based Frequency Response Analysis for Frictional Servo Systems

This paper presents a friction model-based frequency response analysis (FRA) method, which gives a precise linear mechanical dynamics model to design effective controllers and analyze accurate control characteristics for frictional servo systems. As well known, frequency-domain identification approaches using a sine sweep are widely used to obtain linear dynamics. However, nonlinear friction in the mechanism varies the apparent frequency-domain characteristic of the linear dynamics due to the nonlinearity. The proposed FRA estimates effective excitation thrust for actual linear dynamics in the sine sweep movement, by means of a friction model as well as a phase delay model. Theoretical analyses show that the proposed FRA can identify the correct linear dynamics, preventing influence of nonlinear friction as well as phase delay properties included in a plant system. The effectiveness of the proposed FRA is verified through theoretical analyses and experiments both in frequency and time domains, in comparison to two conventional FRA methods.