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In our experiment a vortical flow behind a traveling plate turns into turbulence. By exactly repeating this experiment 42 times with a robot, we study the statistics of this transition. In each realization the fate of the flow is followed over 1.7 s when the plate travels with a constant velocity. It suddenly turns turbulent at a scaled traveled distance of x∗≈5.5. We register the vorticity in a plane that divides the plate perpendicularly. We introduce an original Lagrangian measure of variability between the experiment realizations. The finite-time Lyapunov exponent field of a single experiment predicts this variability; thus we confirm ergodicity. Apart from pointwise measures, yielding a distribution over the field of view, we study the statistics of the circulation computed over the upper and lower half of the domain. The almost perfect symmetry both of the mean and of the fluctuations points to their origin as the fluctuating vortex ring trailing behind the plate. During the initial phase long-time correlations exist in the flow, but they cease once the flow turns turbulent. By ordering our repeated experiments we find that extreme circulations are preceded by circulations that are larger than the median., Fluid Mechanics

Measurements were conducted in the fully developed turbulent flow in a pipe with internal diameter D at a Reynolds number of Re D= 1.6 × 10 5 . The pipe walls were equipped with regularly spaced square ribs of relative height h/ D= 0.154 , while the pitch-to-roughness height was varied between p/ h= 1.67 and p/ h= 6.67 . The measurements include mean velocity components, Reynolds shear and normal stresses and pressure losses. It is investigated whether the effects of the large roughness on the (time and axially averaged) velocity profile can be described by the classical rough-wall formulation by allowing the value of the von Kármán constant to deviate from its standard value of 0.41., Fluid Mechanics, Energy Technology

We report results on the instantaneous drag force on plates that are accelerated in a direction normal to the plate surface, which show that this force scales with the square root of the acceleration. This is associated with the generation and advection of vorticity at the plate surface. A new scaling law is presented for the drag force on accelerating plates, based on the history force for unsteady flow. This scaling avoids previous inconsistencies in using added mass forces in the description of forces on accelerating plates., Fluid Mechanics

Abstract: A novel experimental imaging-based method is presented for the non-intrusive determination of shock wave characteristics (i.e. shock wave speed and magnitude, and shock-induced liquid velocity) in a bubbly flow solely from gas bubble velocities. Shock wave speeds are estimated by the relative motion between gas bubbles at two locations by splitting the camera field-of-view using a mirror construction, increasing the dynamic spatial range of the measurement system. Although gas bubbles have in general poor tracing properties of the local fluid velocity, capturing the relative dynamics provides accurate estimates for the shock wave properties. This proposed imaging-based method does not require pressure transducers, the addition of tracer particles, or volumetric reconstruction of the gas bubbles. The shock wave magnitude and shock-induced liquid velocity are computed with a hydrodynamic model, which only requires non-intrusively measured variables as input. Two reference measurements, based on pressure transducers and the liquid velocity field by particle image velocimetry, show that the proposed method provides reliable estimates for the shock wave front speed and the shock-induced liquid velocity within the experimental range of 70 < Us< 400 m/s. Graphical abstract: [Figure not available: see fulltext.]., Multi Phase Systems, Fluid Mechanics, Process and Energy

The principal aim of the work presented here is to investigate and demonstrate that a forward tilted rowing blade would result in a more efficient and effective motion of the blade through the water that would result in a higher boat speed when an equal input power is provided. A 1:5 scaled rowing boat is used to determine the performance of rowing blades with different sizes and blade angles. This is used to validate the results of a previous study where the optimal blade angle of 15 (Formula presented.) with respect to the oar shaft was determined (1). The input power and speed of the rowing boat can be compared between original and modified oar blades. Measurements in a towing tank demonstrate that a modified rowing blade result in faster rowing by 0.4% at the same input power. Maintaining the same stroke rate, the improvement of the blade efficiency is compensated by using a 4–6% increased blade area to yield the same input power., Fluid Mechanics