Geneviève Comte-Bellot

Simple Eulerian time correlation of full-and narrow-band velocity signals in grid-generated, ‘isotropic’ turbulence

Space-time correlation measurements in the roughly isotropic turbulence behind a regular grid spanning a uniform airstream give the simplest Eulerian time correlation if we choose for the upstream probe signal a time delay which just ‘cancels’ the mean flow displacement. The correlation coefficient of turbulent velocities passed through matched narrow-band niters shows a strong dependence on nominal filter frequency (∼ wave-number at these small turbulence levels). With plausible scaling of the time separations, a scaling dependent on both wave-number and time, it is possible to effect a good collapse of the correlation functions corresponding to wave-numbers from 0·5 cm −1 , the location of the peak in the three-dimensional spectrum, to 10 cm −1 , about half the Kolmogorov wave-number. The spectrally local time-scaling factor is a ‘parallel’ combination of the times characterizing (i) gross strain distortion by larger eddies, (ii) wrinkling distortion by smaller eddies, (iii) convection by larger eddies and (iv) gross rotation by larger eddies.

Intermittency of coherent structures in the core region of fully developed turbulent pipe flow

The present investigation is oriented towards a better understanding of the turbulent structure in the core region of fully developed and completely wall-bounded flows. In view of the already existing results concerning the bursting process in boundary layers (which are semi-bounded flows), an amplitude analysis of the Reynolds shear stress fluctuation u 1 u 2 , sorted into four quadrants of the u 1 , u 2 plane, was carried out in a turbulent pipe flow. For the wall side of the core region, in which the correlation coefficient u 1 u 2 / u ’ 1 u ’ 2 does not change appreciably with the distance from the wall, the structure of the Reynolds stress is found to be similar to that obtained in boundary layers: bursts, i.e. ejections of low speed fluid, make the dominant contribution to the Reynolds stress; the regions of violent Reynolds stress are small fractions of the overall flow; and the mean time interval between bursts is found to be almost constant across the flow. For the core region, the large cross-stream evolution of the correlation coefficient u 1 u 2 / u ’ 1 u ’ 2 is associated with a new structure of the Reynolds stress induced by the completely wall-bounded nature of the flow. Very large amplitudes of u 1 u 2 are still observed, but two distinct burst-like patterns are now identified and related to ejections originating from the two opposite halves of the flow. In addition to this interaction, a focusing effect caused by the circular section of the pipe is observed. As a result of these two effects, the mean time interval between the bursts decreases significantly in the core region and reaches a minimum on the pipe axis. Investigation of specific space-time velocity correlations reveals the possible existence of rotating structures similar to those observed at the outer edge of turbulent boundary layers. These coherent motions are found to have a scale noticeably larger than that of the bursts.

Changing lift and drag by jet oscillation : experiments on a circular cylinder with turbulent separation

Abstract Oscillating jet actuators have been implemented and tested on a circular cylinder. Their action on the separation of turbulent boundary layers is investigated using complementary approaches. Wall pressure distribution shows that a large lift is generated, at the expense, however, of a slightly increased drag. Particle image velocimetry measurements provide the mean and fluctuating velocity fields in the near-wake. The control jet deflects the mean flux lines towards the wall, illustrating that the separation is delayed. This effect appears more and more powerful as the pulsed jet velocity increases. Phase averaging of the PIV fields shows that periodic structures are generated by the control, and how these structures modify the aerodynamic forces by entraining the external flow towards the wall. Finally, a few comparisons are made with laminar boundary layers and some general mechanisms are presented for the lift increase.

Constant Voltage Anemometer Practice in Supersonic Flows

It is shown that the constant voltage anemometer (CVA) is well adapted for measurements in supersonic flows. Small hot-wire time constant, large bandwidth, high signal-to-noise ratio, variable hot-wire overheat, and independence of the cable capacitance effects of the hot wire were all observed with the CVA. Turbulence data were acquired using two methods. In the first method, signals were acquired with the CVA compensated with a fixed time constant of 0.10 ms at all test points. In situ hot-wire time constant and overheat were also acquired at each test point, which were then used in postprocessing of the turbulence signals. In the second method, full hardware compensation with the actual in situ time constant was set at each test point. The former method shows a larger bandwidth. The noise level was approximately the same in both methods. Theoretical estimates of the relative output levels of CVA, constant current anemometer, and constant temperature anemometer at different overheat ratios show that the CVA has larger output voltage level

Software corrected hot wire thermal lag for the constant voltage anemometer featuring a constant bandwidth at the selected compensation setting

Software compensation correction for thermal lag of a hot wire in the application of a constant voltage anemometer (CVA) for turbulence measurements in the boundary layer of a supersonic wind tunnel has been demonstrated. The CVA was used with a fixed compensation setting while measuring the in situ thermal lag (time constant) of the hot wire. Using the measured time constant, corrections are applied to the fixed compensation output of the CVA in postprocessing of the data. To demonstrate the flexibility of the approach it was used for two compensation settings at a test point to obtain the same results from both settings. A unique advantage of this approach is shown to be that for a given compensation setting in the CVA the bandwidth of the measurements for the test remains constant for all of the different test conditions and yields higher productivity. The results of the turbulence levels measured with this method agree with earlier research using other anemometers. Spectral plots of the mass flux and temperature and the measured in situ time constant responses under different conditions of the hot wire have been presented.

Automated constant voltage anemometer for measurements with fluid temperature drifts

An automated procedure has been developed for a constant voltage anemometer to obtain in situ heated and cold resistances of a hot wire in each measurement cycle. From these parameters mean velocity can be deduced in the presence of ambient flow temperature variations, using a calibration curve based on, e.g., King’s law with the assumption of constant coefficients over a given temperature range. The method requires only one hot wire. This is a definite advantage over previous methods established for constant current or constant temperature anemometers where an additional temperature sensor is needed, with a separate temperature calibration of the hot wire. Results from a typical test are presented to demonstrate the procedure. The prototype tested was a 400 kHz bandwidth unit and the measurement cycle can be changed through software commands to suit each test requirements.

Detection of Intermittent Events Maintaining Reynolds Stress

Over the last decade great attention has been paid to the intermittent maintenance of the Reynolds shear stress — ρuv and the possible existence of coherent structures in wall turbulent shear flows.

Transmission of acoustic waves through mixing layers and 2D isotropic turbulence

Ray tracing and parabolic equation methods have been used to study the properties of acoustic waves transmitted through turbulent velocity fields. A numerical simulation permits individual realizations of the turbulent field, which then allow, if desired, an ensemble averaging of the fields. Two flows have been considered, 2D isotropic turbulence, and a 2D mixing layer.The following complementary aspects are developed: the occurrence of caustics, the reinforced or weakened zones of the acoustic field, the eigenrays between a source and a receiver, and the associated travel times, variances, and scintillation index.

On the use of hot-wire anemometry in pulsating flows. A comment on ‘A critical review on advanced velocity measurement techniques in pulsating flows’

In their recent topical review, Nabavi and Siddiqui (Meas. Sci. Technol. 2010 21 042002) recommended the use of hot-wire anemometry for velocity measurements in pulsating flows, especially at high frequency. This recommendation is misleading. The procedures invoked by these authors are valid only for small-amplitude fluctuations, which are of little interest for pulsating flows. When large-amplitude velocity changes occur without flow reversal, new procedures for the correction of the nonlinearities of the hot wire are required. This case was thoroughly investigated for a constant-voltage anemometer by Berson et al (Rev. Sci. Instrum. 2009 80 045102). In addition, we show that when flow reversal occurs—a case most relevant to pulsating flows—accurate hot-wire velocity measurements are not possible because heat transfer is not well defined when velocity passes through zero and changes direction.

Lead-resistance effects in a constant voltage anemometer

Two effects of the lead resistances connecting the hot wire to a constant voltage anemometer (CVA) were analyzed and tested: one concerns the change in the sensitivity coefficient relating the anemometer output to velocity or temperature fluctuations, and the other the time constant of the hot wire determined by an in situ square-wave test technique. Small perturbations were assumed in both cases. The CVA output sensitivity was found to be reduced and the time constant increased with the lead resistance. Explicit formulas which involve the lead resistance, the cold wire resistance, and the wire overheat, as well as some characteristics of the CVA circuit, were established to take into account these effects. In the ranges tested, each effect can individually introduce as much as 10% error. Product of the two governs the overall response for the CVA. However, because the two effects change in opposite directions, interestingly, variation in the net response from their product is minimized. This feature may ...