Pierre Paranthoen

The control of vortex shedding behind heated circular cylinders at low Reynolds numbers

We report some experiments undertaken to investigate the control of vortex shedding behind electrically heated cylinders at low Reynolds numbers owing to the heat input brought to the cylinder. In airflow, depending on the Reynolds number value, complete suppression or modification of the vortex shedding phenomenon can be achieved with increase of heat input. Experimental results suggest that this control could result of a slight change of the separation point location due to the increase of the dynamic viscosity of the fluid.

A tomographic study of measurements in a V-shaped H2air flame and a lagrangian interpretation of the turbulent flame brush evolution

Abstract Flamelet structures and the evolution of flame brush thickness of H 2 air V-shaped flames are studied by laser tomography. The flames have a lean equivalence ratio of φ = 0.2. The flow velocity is 5 m/s and the turbulence intensity at the flame holder generated by a grid varies from 3.5% to 5%. The flames are within the wrinkled laminar flame regime. A sheet of pulsed laser light (Ruby rod) of 75 × 80 mm illuminates an oil aerosol seeded in the H 2 air flow, and the instantaneous flame structures are obtained when the oil droplets evaporate on passage through the flame front. The flames are imaged onto a CCD camera. The camera is interfaced with a matrix memory of 512 × 512 pixels with a dynamic range of 256 gray scales, and the tomographic records are analyzed by a digital image processing technique. The gray level corresponding to the flame front is determined by a filtering and thresholding operation. The first and second moments and correlations of flame front positions are deduced from the measurement of 240 instantaneous, statistically independent flames. The results show that the turbulence characteristics and equivalence ratio have an important effect on the shape and the dynamics of the flame. In particular, the growth of turbulence flame brush, δ T , is linear for a distance that can be correlated with the Lagrangian time scale of the turbulence. Furthermore, the slope of this evolution is proportional to the turbulence intensity at the flame holder. The linear growth is consistent with results observed by others for wrinkled laminar flames. The changes in the local flame surface area with downstream distance are estimated by the length of the flamelet segment within a vertical window of 10 pixels. The flame area increases are found to be accompanied by increases in the mean flame angle as shown by the width of the burned zone. These results confirm that the wrinkled laminar flame model is valid for the flames studied here. The flame front fluctuation results are interpreted using a Lagrangian diffusion model developed by Karlovitz. From this model, the value of the rms displacement of a flame element is calculated based on the approach flow turbulence characteristics (in comparison with the propagation speed of the flame fronts). Model predictions and experimental results are in good agreement. This indicates that the concept of a diffusion process for describing flame movement is important for study of premixed turbulent flame propagation.

Frequency response of fine wire thermocouple

A theoretical and experimental study of the frequency response of fine wire thermocouples is presented. Influence of sensor geometry and flow parameters on the behaviour of transfer functions is given. Comparison between time constants obtained from internal heating technique (IHT) and external heating technique (EHT) shows significant differences due to the nonhomogeneous heating of the wire by both Joule and Peltier effects when IHT is used.

Dispersion measurements in turbulent flows (boundary layer and plane jet)

Temperature measurements concerning dispersion of heat downstream of a steady line source located successively in a boundary layer and in a plane jet are presented and discussed Temperature statistics obtained in these two turbulent flows are compared in a satisfactory way using a rescaling scheme based on the value of the temporal integral Lagrangian scale of the vertical velocity fluctuation at the source location A simple diffusion model has been developed and is qualitatively in agreement with experimental results in the first phase of the diffusion process

Correction of temperature fluctuation measurements using cold wires

It has been shown previously that the frequency response of cold wires used to measure a turbulent temperature field can be noticeably attenuated because of thermal inertia of the wire, end conduction heat loss between wire and prongs, and spatial resolution. The correction of temperature signals is, therefore, indispensable. The transfer functions of cold wires have been determined by using a home-made dynamic calibration system. A theoretical analysis is used to take into account spatial resolution. The proposed method of correction for temperature fluctuation spectra and RMS intensity accounting for these three effects is applied to the determination of temperature variance profile in the heated wake of a line source in a turbulent boundary layer.

Influence of Wollaston wire and prongs on the response of cold wires at low frequencies

Abstract This paper deals with the theoretical and experimental study of heat conduction loss for a platinum wire used as a temperature sensor when the influence of Wollaston wire is taken into account. In this case the transfer function at low frequencies can be noticeably modified. An experimental investigation of this phenomenon has been undertaken using a procedure which allows temperature fluctuations to be produced in a larger range of frequencies than has been usually made until now.

On the alignment dynamics of a passive scalar gradient in a two-dimensional flow

A Lagrangian study of the statistical properties of the orientation of a passive scalar gradient is performed using experimental data and a simple, numerical analysis. It is shown that, in a low-Reynolds number Benard-von Karman street, the temperature gradient downstream of a heated line source does not align with the asymptotic orientation predicted by the Lapeyre et al. model [Phys. Fluids 11, 3729 (1999)] in the hyperbolic regions. This result is ascribed to fluctuations of strain persistence along Lagrangian trajectories. A numerical analysis of the scalar gradient alignment properties shows that these fluctuations, together with a low level of the rate of strain, may lead to preferential orientations that are different from the theoretical one predicted by an asymptotic model.

Influence de la conduction sur la fonction de transfert des fils froids dans les tres basses frequences; resultats experimentaux

Abstract This paper is devoted to the experimental study of the influence of conduction between a wire used as a sensor of temperature and its prongs on measurements of temperature variances and spectra. The difference in the behaviour of a wire of aspect ratio l /d between static conditions (calibration) and dynamical conditions (measurements in a turbulent temperature can be noticeable. This can involve an important underestimation of variances and spectra if no correction is made.

Nonstationary aspects of passive scalar gradient behaviour

The dynamics of a passive scalar gradient experiencing fluctuating velocity gradient through the Lagrangian variations of strain persistence is studied. To this end, a systematic, numerical analysis based on the equation for the orientation of the gradient of a nondiffusive scalar in two-dimensional flow is performed. When the gradient responds weakly its orientation properties are determined by the mean value of strain persistence. Statistical alignment of the scalar gradient with the direction defined by the opposed actions of strain and rotation, by contrast, requires the gradient to keep up with strain persistence fluctuations. These results have been obtained for both strain- and effective-rotation-dominated regimes and are supported by relevant experimental data. Consequences of the unsteady behaviour of the scalar gradient on mixing properties are also analysed.

Mesure de la fonction de transfert d'un fil utilise comme capteur de temperature; Influence de la conduction

Abstract This paper deals with an experimental set-up allowing to determine frequency response of wires used as sensors of temperature fluctuations. Some spectra concerning such wires with different aspect ratios l/d are also given and influence of conduction is discussed.