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A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes, and Reynolds numbers. The drag was reduced by as much as 35%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, and/or the Reynolds number. Here, we find that the key parameter is the acceleration, which greatly simplifies the complexity of the phenomenon. This result is shown to apply to channel flows with spanwise surface oscillation as well. This insight opens potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems. [ABSTRACT FROM AUTHOR]

A comprehensive characterization of the thermal response of nanoscale hot-wire probes is performed in both subsonic and supersonic flows. A constant current anemometer was designed for the measurement of the intrinsic thermal inertia of hot-wire probes. In particular, the nanoscale probe is considered with the effect of gold-plating on the supporting structure of the sensing element. Gold-plated nanoscale probes present a response time one order of magnitude smaller than conventional cylindrical hot-wire probes. Heat transfer simulations show that the temperature profile is considerably modified by the addition of a conductive metal layer, hence increasing the sensor's frequency response in both subsonic and supersonic flows. The increase of frequency response is finally exemplified by the numerical computation of the power spectral density of a turbulent flow signal without any electric compensation of the hot-wire signal. [ABSTRACT FROM AUTHOR]

A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour. [ABSTRACT FROM AUTHOR]

Highly resolved, two-component velocity measurements were made near the centreline of turbulent pipe flow for Reynolds numbers in the range $102 \le Re_\lambda \leq 411$ ($1800 \le Re_\tau \leq 24\,700$). These unique data were obtained with a nanoscale cross-wire probe and used to examine the inertial subrange scaling of the longitudinal and transverse velocity components. Classical dissipation rate estimates were made using both the integration of one-dimensional dissipation spectra for each velocity component and the third-order moment of the longitudinal structure function. Although the second-order moments and one-dimensional spectra for each component showed behaviour consistent with local isotropy, clear inertial range similarity and behaviour were not exhibited in the third-order structure functions at these Reynolds numbers. When corrected for the effects of radial inhomogeneities at the centreline following the generalized expression of Danaila et al. (J. Fluid Mech., vol. 430, 2001, pp. 87–109), re-derived for the pipe flow domain, the third-order moments of the longitudinal structure function exhibited a clearer plateau per the classical Kolmogorov 'four-fifths law'. Similar corrections described by Danaila et al. (J. Fluid Mech., vol. 430, 2001, pp. 87–109) applied to the analogous equation for the mixed structure functions (i.e. the 'four-thirds law') also yielded improvement over all ranges of scale, improving with increasing Reynolds number. The rate at which the 'four-fifths' law and 'four-thirds' law were approached by the third-order structure functions was found to be more gradual than decaying isotropic turbulence for the same Reynolds numbers. [ABSTRACT FROM AUTHOR]

A two-point, two-time similarity solution is derived for homogeneous decaying turbulence. This is the first known solution which includes the temporal decay at two-different times. It assumes that the turbulence is homogeneous in all three space dimensions, and finds that homogeneity holds across time. The solutions show that time is logarithmically "stretched" while the homogeneous spatial scales grow. This solution reduces to the two point, single time equation when the two times are set equal. The turbulence initially decays exponentially, then asymptotically as t-n where n ≥ 1 and equality is possible only if the initial energy is infinite. The methodology should be applicable to other non-equilibrium homogeneous turbulent flows. [ABSTRACT FROM AUTHOR]

Scaling of grid turbulence with a constant mean cross-stream temperature gradient is investigated using a combination of theoretical predictions and experimental data. A novel nanoscale temperature probe (T-NSTAP) was used to acquire temperature data. Conditions for self-similarity of the governing equations and the scalar spectrum are investigated, which reveals necessary conditions for the existence of a self-similar solution. These conditions provide a theoretical framework for scaling of the temperature spectrum as well as the temperature flux spectrum. One necessary condition, predicted by the theory, is that the characteristic length scale describing the scalar spectrum must vary as √1 in the case of a zero virtual origin for a self-similar solution to exist. As predicted by the similarity analysis, the data show the variance growing as a power law with streamwise position. When scaled with the similarity variable, as found through the theoretical analysis, the temperature spectra show a good collapse over all wavenumbers. A new method to determine the quality of the scaling was developed, comparing the coefficient of variation. The minimum coefficient of variation, and thus the best scaling, for the measured spectra agrees well with the similarity requirements. The theoretical work also reveals an additional requirement related to the scaling of the scalar flux spectrum. [ABSTRACT FROM AUTHOR]

A new wall shear stress model to be used as a wall boundary condition for large-eddy simulations of the atmospheric boundary layer is proposed. The new model computes the wall shear stress and the vertical derivatives of the streamwise velocity component by means of a modified, instantaneous, and local law-of-the-wall formulation. By formulating a correction for the modeled shear stress, using experimental findings of a logarithmic region in the streamwise turbulent fluctuations, the need for a filter is eliminated. This allows one to model the wall shear stress locally, and at the same time accurately recover the correct average value. The proposed model has been applied to both unique high Reynolds number experimental data and a suite of large-eddy simulations, and compared to previous models. It is shown that the proposed model performs equally well or better than the previous filtered models. A nonfiltered model, such as the one proposed, is an essential first step in developing a universal wall shear stress model that can be used for flow over heterogeneous surfaces, studies of diurnal cycles, or analyses of flow over complex terrain. [ABSTRACT FROM AUTHOR]

A new theory for the streamwise turbulent fluctuations in fully developed pipe flow is proposed. The theory extends the similarities between the mean flow and the streamwise turbulence fluctuations, as observed in experimental high Reynolds number data, to also include the theoretical derivation. Connecting the derivation of the fluctuations to that of the mean velocity at finite Reynolds number as introduced by Wosnik, Castillo & George (J. Fluid Mech., vol. 421, 2000, pp. 115–145) can explain the logarithmic behaviour as well as the coefficient of the logarithm. The slope of the logarithm, for the fluctuations, depends on the increase of the fluctuations with Reynolds number, which is shown to agree very well with the experimental data. A mesolayer, similar to that introduced by Wosnik et al., exists for the fluctuations for $300\gt {y}^{+ } \gt 800$, which coincides with the mesolayer for the mean velocities. In the mesolayer, the flow is still affected by viscosity, which shows up as a decrease in the fluctuations. [ABSTRACT FROM PUBLISHER]

A nanoscale thermal anemometry probe (NSTAP) has been developed to measure velocity fluctuations at ultra-small scales. The sensing element is a free-standing platinum nanoscale wire, 100 nm × 2 μm × 60 μm, suspended between two current-carrying contacts and the sensor is an order of magnitude smaller than presently available commercial hot wires. The probe is constructed using standard semiconductor and MEMS manufacturing methods, which enables many probes to be manufactured simultaneously. Measurements were performed in grid-generated turbulence and compared to conventional hot-wire probes with a range of sensor lengths. The results demonstrate that the NSTAP behaves similarly to conventional hot-wire probes but with better spatial resolution and faster temporal response. The results are used to investigate spatial filtering effects, including the impact of spatial filtering on the probability density of velocity and velocity increment statistics. [ABSTRACT FROM AUTHOR]