Olivier Cadot

CHARACTERIZATION OF THE LOW-PRESSURE FILAMENTS IN A THREE-DIMENSIONAL TURBULENT SHEAR FLOW

In this paper results on the low‐pressure filaments that appear spontaneously in three‐dimensional turbulent flows are presented. An individual characterization of the filaments is first obtained by studying the correlations between the flow visualization and local measurements of the pressure and the velocity. Then, a statistical study of the time recordings of the pressure that exhibits intermittent short and deep depressions is presented. It is shown that the pressure histograms depend only on the square of the injection velocity, and that the rate of production of strong depressions is independent of the Reynolds number. These results impose severe constraints on the possible mechanisms of formation of the filaments; they are consistent with a simple model, in which the formation of the filaments results primarily from the partial rollup of stretched shear layers. In this model there is a difference between the hierarchies of pressure and vorticity filaments: the filaments with the largest depression are the thickest (and the longest), while the filaments with the strongest vorticity are likely to be the thinnest (and shortest).

Drag reduction of a bluff body using adaptive control methods

A classical actuator is used to control the drag exerted on a bluff body at large Reynolds number (Re=20000). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. As shown by visualizations, the actuator delays the separation point. The size of the low-pressure region behind the body is decreased and the drag reduced. It is found that the faster the rotation of the cylinder, the lower the drag. In a first study, the goal of the control is for the system to reach a drag consign predetermined by the experimentalist. The control loop is closed with a proportional integral correction. This adaptive method is shown to be efficient and robust in spite of the large fluctuations of the drag. In the second method, the system finds itself its optimal set point. It is defined as the lowest cost of global energy consumption of the system (drag reduction versus...

Turbulent drag reduction in a closed flow system: Boundary layer versus bulk effects

We present an experimental study of drag reduction by polymer additives in a closed turbulent flow with a zero mean velocity at large Reynolds numbers. The study is performed using two different forcing schemes: a smooth forcing, for which the fluid is brought into movement through viscous boundary layers, and a very rough forcing where the fluid is stirred inertially by baffles. For a polymer solution of 30 wppm, a steady drag reduction is only observed for the smooth forcing. In the other case, absolutely no decrease of the turbulent energy dissipation is observed. When drag reduction is observed, the turbulent intensity in the bulk is unaltered upon addition of the polymer. This result demonstrates that, in our experimental system, the reduction of the energy dissipation does not only occur mainly in the boundary layers (as is well known for a long time), but takes place solely in the boundary layers. We also study heterogeneous drag reduction by local injection of a concentrated polymer solution. Duri...

Observation of wave turbulence in vibrating plates

The nonlinear interaction of waves in a driven medium may lead to wave turbulence, a state such that energy is transferred from large to small length scales. Here, wave turbulence is observed in experiments on a vibrating plate. The frequency power spectra of the normal velocity of the plate may be rescaled on a single curve, with power-law behaviors that are incompatible with the weak turbulence theory of Düring et al. [Phys. Rev. Lett. 97, 025503 (2006)10.1103/PhysRevLett.97.025503]. Alternative scenarios are suggested to account for this discrepancy -- in particular the occurrence of wave breaking at high frequencies. Finally, the statistics of velocity increments do not display an intermittent behavior.

On experimental sensitivity analysis of the turbulent wake from an axisymmetric blunt trailing edge

The sensitivity to local disturbances of the turbulent wake over a 3D blunt body with an axisymmetric detachment is investigated at Re = 2.1·10 4. The flow presents a favored m = 2 azimuthal symmetry set by two wings. The instantaneous wake is measured either above or below the plane containing the wings but leads a statistical symmetric wake. Topology shifts are random but occur mostly after a large number of global mode periods. The statistical symmetry is highly sensitive to any asymmetric disturbance. As a consequence, depending on its position a small control cylinder in the close wake fixes the wake to one asymmetric topology affecting shedding activity and drag. The effect of an axisymmetric perturbation (m = 0) on flow topology and dynamics is also studied ; it induces significant drag reductions and global mode modifications when acting on mixing layers. Whatever the disturbance, the sensitivity of the wake seems concentrated into the mixing layers and may depend more on their local turbulent characteristics than on the inviscid dynamics of vorticity.

Experimental characterization of viscoelastic effects on two- and three-dimensional shear instabilities

Instabilities of a wake produced by a circular cylinder in a uniform water flow are studied experimentally when viscoelastic solutions are injected through holes pierced in the cylinder. It is shown that the viscoelastic solutions fill the shear regions and drastically modify the instabilities. The two-dimensional instability giving rise to the Karman street is found to be inhibited: the roll-up process appears to be delayed and the wavelength of the street increases. The wavelength increase obeys an exponential law and depends on the elasticity number, which provides a ratio of elastic forces to inertial forces. The three-dimensional instability leading to the A mode is generally found to be suppressed. In the rare case where the A mode is observed, its wavelength is shown to be proportional to the wavelength of the Karman street and the streamwise stretching appears to be inhibited. Injection of viscoelastic solutions also decreases the aspect ratio of the two-dimensional wake, and this is correlated with stabilization of the A mode and with changes in the shape of the Karman vortices. The observations of this work are consistent with recent numerical simulations of viscoelastic mixing layers. The results suggest mechanisms through which polymers inhibit the formation of high-vorticity coherent structures and reduce drag in turbulent flows.

Bi-stability in the turbulent wake past parallelepiped bodies with various aspect ratios and wall effects

The turbulent wake past parallelepiped bodies with a rectangular blunt trailing edge of height H and width W is investigated in wall proximity: various aspect ratios H* = H/W ∈ [0.51, 1.63] and ground clearances C* = C/W ∈ [0, 1.00] are explored at a Reynolds number of 4.5 × 104 based on the body width W. Base pressure measurements and particle image velocimetry show that the close wake often undergoes antisymmetric instabilities that can be either in the lateral direction (parallel to the wall) or in the vertical direction (normal to the wall). The instantaneous wake presents preferred states with high degrees of asymmetry; in some configurations, topology shifts are observed after long time scales Tl ∼ 103W/U0 leading to bistable behaviors. The effect of ground proximity is thoroughly studied for H* = 0.74 corresponding to the reference Ahmed geometry and for a case with a height dimension larger than its width H* = 1/0.74 = 1.34. When C* > 0.08, it is found that the Ahmed body is bistable in the latera...

Turbulent drag reduction by polymers

The reduction of turbulent energy dissipation by addition of polymers is studied experimentally. We first address the question of where the action of the polymers is taking place. Subsequently, we show that there is a direct correlation of drag reduction with the elongational viscosity of the polymers. For this, the reduction of turbulent energy dissipation by addition of the biopolymer DNA is studied. These results open the way for a direct visualization study of the polymer conformation in a turbulent boundary layer.

Experimental observation of resonances in modulated turbulence

The response of turbulence to a periodic forcing of the Reynolds number is studied. The turbulent flow is produced in a closed geometry between two counter-rotating disks. The time series of both the global injected power and the local velocity are measured. The injected power fluctuations exhibit resonances for well-determined values of the modulation frequency, making it possible to estimate a turbulent cascade time. For modulation periods larger than the measured cascade time, the response of the velocity fluctuations is simply proportional to the modulation of the forcing. For high-frequency modulations, the velocity fluctuations are strongly damped: their amplitude decreases as the inverse of the modulation frequency

Shear instability inhibition in a cylinder wake by local injection of a viscoelastic fluid

The wake behind a circular cylinder in water is visualized when colored solutions are injected through rows of holes pierced in the cylinder. When these solutions are viscoelastic, a drastic change of the shape of the wake is observed compared to that observed for water injections. The aspect ratio of the wake is decreased, the wavelength of the vortices is increased, and a large region of slow fluid motion is developed behind the cylinder. It is shown that these observations are consistent to a shear instability inhibition due to a local elasticity.