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The linearised complex Ginzburg-Landau equation is a model for the evolution of small fluid perturbations, such as in a bluff body wake. By implementing actuators and sensors and designing an H 2 optimal controller, we control a supercritical, infinitedomain formulation of this system. We seek the optimal actuator and sensor placement that minimises the H 2 norm of the controlled system, from flow disturbances and sensor noise to a cost on the perturbation and input magnitudes. We formulate the gradient of the H 2 squared norm with respect to the actuator and sensor placements and iterate towards the optimal placement. When stochastic flow disturbances are present everywhere in the spatial domain, it is optimal to place the actuator just upstream of the origin and the sensor just downstream. With pairs of actuators and sensors, it is optimal to place each actuator slightly upstream of each corresponding sensor, and scatter the pairs throughout the spatial domain. When disturbances are only introduced upstream, the optimal placement shifts upstream as well. Global mode and Gramian analyses fail to predict the optimal placement; they produce H 2 norms about five times higher than at the true optimum. The wavemaker region is a better guess for the optimal placement.

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We show experimentally that a flow-induced, Reynolds number-dependent particle-capture mechanism in branching junctions can be enhanced or eliminated by varying the junction angle. In addition, numerical simulations are used to show that the features responsible for this capture have the signatures of classical vortex breakdown, including an approach flow aligned with the vortex axis and a pocket of subcriticality. We show how these recirculation regions originate and evolve and suggest a physical mechanism for their formation. Furthermore, comparing experiments and numerical simulations, the presence of vortex breakdown is found to be an excellent predictor of particle capture. These results inform the design of systems in which suspended particle accumulation can be eliminated or maximized. DOI: 10.1103/PhysRevLett.117.084501 Flows through branching junctions are common in everyday piping systems, industrial applications, and even physiological flows. Despite the prevalence of branching flows and the breadth of studies of these systems, recent discoveries demonstrate new features that have not been studied. For example, for flow through a T junction in which flow enters the base of the T and splits between the two symmetric outlets, it is natural to believe that suspended particles entering the system will find the junction to be a kinematically unstable stagnation region and be swept downstream through the outlets. However, it has been shown that, in fact, bubbles can be trapped in these regions within flow features that resemble vortex breakdown [1,2], which refers to a phenomenon where internal stagnation points develop, followed by regions of reversed flow with limited axial extent [3].Despite the fact that this capture mechanism depends strongly on the swirling motion of flow in the junction through the interplay of centrifugal, pressure gradient, and drag forces [1], the effect of varying the junction angle has not been explored. We introduce this geometric change, systematically varying the junction angle θ, which introduces significant changes in the secondary swirl velocities [4,5] (Fig. 1). As the Reynolds number Re is increased (while the flow remains laminar), the flow field undergoes qualitative changes involving the formation of internal stagnation regions with strong swirl. We use numerical simulations to show how these features originate and evolve as Re and θ are varied, and we show that the physical mechanism for their development is the same as for classical bubble-type vortex breakdown. Finally, we use experiments to demonstrate that the particle capture in two-phase flows is caused by these vortex breakdown features identified in our single-phase simulations.

A computational inquiry focuses on leading-edge vortex ͑LEV͒ growth and shedding during acceleration of a two-dimensional flat plate at a fixed 10°-60°angle of attack and low Reynolds number. The plate accelerates from rest with a velocity given by a power of time ranging from 0 to 5. During the initial LEV growth, subtraction of the added mass lift from the computed lift reveals an LEV-induced lift augmentation evident across all powers and angles of attack. For the range of Reynolds numbers considered, a universal time scale exists for the peak when ␣ Ն 30°, with augmentation lasting about four to five chord lengths of translation. This time scale matches well with the half-stroke of a flying insect. An oscillating pattern of leading-and trailing-edge vortex shedding follows the shedding of the initial LEV. The nondimensional frequency of shedding and lift coefficient minima and maxima closely match their values in the absence of acceleration. These observations support a quasisteady theory of vortex shedding, where dynamics are determined primarily by velocity and not acceleration. Finally, the nondimensional vortex formation time is found to be a function of the Reynolds number, but only weakly when the Reynolds number is high.

Category: Hindfoot, Sports, Trauma Introduction/Purpose: Achilles tendon rupture is a potentially devastating injury particularly for National Collegiate Athletic Association (NCAA) athletes. Little has been studied regarding the incidence and implications of Achilles tendon ruptures in this patient population. Better characterization of the factors commonly found in athletes who rupture their Achilles may provide clues to aid in their prevention. Methods: Achilles injuries across 16 sports among NCAA men and women during the 2004-05 to 2013-14 academic years were analyzed using the NCAA Injury Surveillance Program (NCAA-ISP). Achilles tendon rupture rates per 100,000 athlete-exposures (IR), operative rate, annual injury rate trends, re-injury rates, mechanism of injury, in-season status (pre/in/post-season) and time loss distributions were compiled and calculated. A sub-analysis of contact sports and sports played by both genders (C-BG) was performed to determine if there were significant differences in risks in patients who played in contact sports. Results: N=255 Achilles tendon injuries were identified over 10 academic years (IR: 2.17). The injury rate was higher in males compared to females (IR=2.33 vs. 1.89 respectively). Achilles injuries were most common in Men’s Basketball (IR=4.26), Soccer (IR=3.06), and Football (IR=2.69). The top three women’s sports with Achilles injury were Gymnastics (IR=16.73), Basketball (IR=3.32), and Soccer (IR=1.81). Thirty-three injuries were operative (13.1%) and 14.9% (N=38) were season-ending injuries. The average time loss was 10.65 days when excluding patients who had season/career ending injuries. Reinjury rate was 11.0% (N=28). 61.2% of all injuries occurred during the regular season (N=156) with 36.0% (N=92) and 2.7% (N=7) occurring in pre- and post- season, respectively. There was a significantly greater number of injuries in contact (N=198) versus non-contact sports (N=50) (p<0.001). Conclusion: Achilles tendon ruptures can be devastating injuries in professional and collegiate athletes. In our study, nearly 15% of all NCAA Achilles tendon ruptures resulted in season ending injuries or significant time loss and over 13% of injuries required operative management with a majority of injuries occurring during practice. In addition, a significantly higher proportion of athletes who played a contact sport had Achilles injuries. Better understanding of what circumstances more often tend to result in Achilles injuries can help establish prevention strategies.

In the last few years, many researchers have noted that regions of recirculating flow often exhibit particularly high sensitivity to spatially localized feedback. We explore the flow through a T-shaped pipe bifurcation-a simple and ubiquitous, but generally poorly understood flow configuration-and provide a complex example of the relation between recirculation and sensitivity. When Re ≥ 320, a phenomenon resembling vortex breakdown occurs in four locations in the junction, with internal stagnation points appearing on vortex axes and causing flow reversal. The structure of the recirculation is similar to the traditional bubble-type breakdown. These recirculation regions are highly sensitive to spatially localized feedback in the linearized Navier-Stokes operator. The flow separation at the corners of the "T," however, does not exhibit this kind of sensitivity. We focus our analysis on the Reynolds number of 560, near the first Hopf bifurcation of the flow. C 2015 AIP Publishing LLC.