Pressure fluctuations are an important ingredient in turbulence, e.g. in the pressure strain terms which redistribute turbulence among the different fluctuating velocity components. The variation of the pressure fluctuations inside a turbulent boundary layer has hitherto been out of reach of experimental determination. The mechanisms of non-local pressure-related coupling between the different regions of the boundary layer have therefore remained poorly understood. One reason for this is the difficulty inherent in measuring the fluctuating pressure. We have developed a new technique to measure pressure fluctuations. In the present study, both mean and fluctuating pressure, wall pressure, and streamwise velocity have been measured simultaneously in turbulent boundary layers up to Reynolds numbers based on the momentum thickness Rθ ≃ 20000. Results on mean and fluctuation distributions, spectra, Reynolds number dependence, and correlation functions are reported. Also, an attempt is made to test, for the first time, the existence of Kolmogorov's -7/3 power-law scaling of the pressure spectrum in the limit of high Reynolds numbers in a turbulent boundary layer.

Transverse structure functions are obtained at high Reynolds numbers in atmospheric turbulence ͑Taylor microscale Reynolds numbers between 10 000 and 15 000͒. These measurements confirm that their scaling exponents are different from those for longitudinal structure functions. Implications of this conclusion are discussed briefly. ͓S1063-651X͑97͒50511-3͔ PACS number͑s͒: 47.27. Ak, 47.27.Jv Anomalous scaling in turbulence has been studied traditionally in terms of the so-called longitudinal structure functions ͑LSF's͒, which are moments of velocity increments ⌬u r ϵu(xϩr)Ϫu(x), where u is the velocity component in a certain direction x and the separation distance r is measured also in the same direction. For most flows, experimental convenience necessitates that the direction x be that of the mean flow. Several attempts ͓1-10͔ have been made recently to obtain the so-called transverse structure functions ͑TSF's͒, which are moments of velocity increments for which the separation distance is transverse to the direction of the velocity component considered. A few of these measurements ͑e.g., Refs. ͓1,2,6͔͒ suggest ͑or imply͒ that the scaling exponents for TSF are equal, to within experimental uncertainties, to those for LSF. If the two sets of exponents are indeed equal, the hierarchy of models built up on the basis of LSF ͑see, e.g., Ref. ͓11͔͒ remains essentially intact. On the other hand, there exist measurements ͓3-5,8-10͔ purporting to show that the transverse exponents of order greater than 2 are measurably smaller than the longitudinal exponents. If true, this observation calls for additional complexity in smallscale phenomenology-and might even suggest the absence of strict scaling in the problem.To make a convincing case that high-order TSF exponents are smaller than those of LSF, it must first be shown that the inertial-range scales are isotropic. A minimum condition for local isotropy to exist is that the second-order exponents in the inertial range should be equal for LSF and TSF. It is known ͑e.g., Ref. ͓12͔, Fig. 5͒ that this requires, in shear flows, a Taylor microscale Reynolds number of the order of 1000 and higher. All the results cited above have been obtained at modest Reynolds numbers. Some of them have been made in shear flows. One might therefore wonder if the observed differences between the two sets of exponents are due perhaps to the lack of isotropy in the inertial range. Further, the scaling range at moderate Reynolds number is modest at best.In this context, we have made a series of measurements in atmospheric turbulence at Taylor microscale Reynolds numbers ranging between 10 000 and 15 000. These Reynolds numbers are comparable to the highest ever used for studies of small-scale turbulence ͑e.g., ͓13,14͔͒. Here, we examine the velocity data solely to address the following issue: Are there genuine differences between the longitudinal and transverse exponents? As already remarked, this question is important for the theory of small-scale turbulence.The velocity data were acquired by mea...

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The large-scale circulation, often called "wind," in the confined thermal turbulence of mercury is studied experimentally. The instantaneous velocity profile at 128 points is directly measured using ultrasonic velocimetry. The periodic velocity oscillation is observed in the case of the aspect-ratio Gamma = 1,2 but not in Gamma = 0.5. Its peak frequency is scaled by f(c) proportional Ra(gamma(c)), where Ra is the Rayleigh number and gamma(c) = 0.43,0.45 for Gamma = 1,2. f(c) is close to the wind circulation frequency f(p), and has the same order of transit time from the bottom to the top of the convection cell. A single roll circulation is expected in Gamma = 1; however, axisymmetric toroidal rings may exist near the upper and lower plate for Gamma = 0.5, which are stable up to Ra = 7 x 10 (10).

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