It is shown that the pressure signal measured at the outer edge of a jet mixing layer is entirely hydrodynamic in nature and provides a good measure of the large-scale structure of the turbulent flow. Measurement of the pressure signal provides a unique opportunity to utilize proper orthogonal decomposition (POD) to deduce the streamwise structure. Since pressure is a scalar, a significant reduction in the numerical and experimental complexity inherent in the analysis of velocity vector fields results.The POD streamwise eigenfunctions show that the structure associated with any frequency-azimuthal mode number combination displays the general characteristics of amplification-saturation-decay of an instability wave, all within about three wavelengths. High-frequency components saturate early in x and low-frequency components saturate further downstream, indicative of the inhomogeneous character of the flow in the streamwise direction. Application of the POD technique allows the phase velocity to be determined taking into account the inhomogeneity of the flow in the streamwise direction. The phase velocity of each instability wave (POD eigenvector) is constant and equal to 0.58U j , indicating that the jet structure is non-dispersive.Using the shot-noise decomposition, a characteristic event is constructed. This event is found to contain evidence of both pairings and triplings of vortex structures. The tripling results in a rapid increase in the first asymmetric (m l 1) component. On average, pairing occurs once every four U j \D while tripling occurs once every 13U j \D.

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A laboratory experiment was performed to study the dynamically rich interaction of a turbulent open channel flow with a bed-mounted axial-flow hydrokinetic turbine. An acoustic Doppler velocimeter and a torque transducer were used to simultaneously measure at high temporal resolution the three velocity components of the flow at various locations upstream of the turbine and in the wake region and turbine power, respectively. Results show that for sufficiently low frequencies the instantaneous power generated by the turbine is modulated by the turbulent structure of the approach flow. The critical frequency above which the response of the turbine is decoupled from the turbulent flow structure is shown to vary linearly with the angular frequency of the rotor. The measurements elucidate the structure of the turbulent turbine wake, which is shown to persist for at least fifteen rotor diameters downstream of the rotor, and a new approach is proposed to quantify the wake recovery, based on the growth of the largest scale motions in the flow. Spectral analysis is employed to demonstrate the dominant effect of the tip vortices in the energy distribution in the near-wake region and uncover meandering motions.

▪ Abstract  Cavitation in vortical structures is a common, albeit complex, problem in engineering applications. Cavitating vortical structures can be found on the blade surfaces, in the clearance passages, and at the hubs of various types of turbomachinery. Cavitating microvortices at the trailing edge of attached sheet cavitation can be highly erosive. Cavitating hub vortices in the draft tubes of hydroturbines can cause major surges and power swings. There is also mounting evidence that vortex cavitation is a dominant factor in the inception process in a broad range of turbulent flows. Most research has focused on the inception process, with limited attention paid to developed vortex cavitation. Wave-like disturbances on the surfaces of vapor cores are an important feature. Vortex core instabilities in microvortices are found to be important factors in the erosion mechanisms associated with sheet/cloud cavitation. Under certain circumstances, intense sound at discrete frequencies can result from a coupling between tip vortex disturbances and oscillating sheet cavitation. Vortex breakdown phenomena that have some commonalities are also noted, as are some differences with vortex breakdown in fully wetted flow. Simple vortex models can sometimes be used to describe the cavitation process in complex turbulent flows such as bluff body wakes and in plug valves. Although a vortex model for cavitation in jets does not exist, the mechanism of inception appears to be related to the process of vortex pairing. The pairing process can produce negative peaks in pressure that can exceed the rms value by a factor of ten, sometimes exceeding the dynamic pressure by a factor of two. A new and important issue is that cavitation is not only induced in vortical structures but is also a mechanism for vorticity generation.

A field experiment was carried out to study the unsteady behavior of an instrumented full-scale 2.5 MW wind turbine under neutral conditions. The analysis focused on the structure of the instantaneous turbine power and strain at its foundation. A meteorological tower located 1.6 rotor diameters upstream of the turbine was used to characterize the turbulent flow. Mean velocity and temperature were steady during the 1 h period selected. The results suggest that the turbine power and foundation strain are modulated by atmospheric turbulence in a complex way. The spectral characteristics of both quantities exhibited three distinctive regions. Within the first region, defined by subrotor length scales, the turbine power was insensitive to the flow turbulence. In the intermediate region, with length scales up to those on the order of the atmospheric boundary layer thickness, the spectral contents of the power fluctuationsˆP and flowˆU exhibit a non-linear relationship of the formˆP D G.f /ˆU , where G.f / / . /f 2 is a transfer/damping function. In the third region, dominated by the very large scales of motions, the power fluctuations are found to be directly influenced by the flow. The strain also showed three regions, similar to the power fluctuations. However, it follows the structure of the inertial subrange of the turbulence at subrotor scales. Intermittent gusts were able to induce intermittent behavior on the turbine power. Finally, the flow and power correlation showed that the velocity at the hub height is the best descriptor of the flow turbulence within the rotor area.

Spectral models for turbulent pressure fluctuations are developed by directly Fourier transforming the integral solution to the Poisson equation for a homogeneous constantmean-shear flow. The turbulence-turbulence interaction is seen to possess the well-known k−7/3 inertial subrange and to dominate the high-wavenumber region. The turbulence–mean-shear contribution is seen to be dominant in the energy-containing range and falls off as $k^{-\frac{11}{3}}$ in the inertial subrange. The subrange constants and the mean-square pressure fluctuation are evaluated using a spectral model for the velocity. A spectral analysis of the velocity contamination of a pressure probe is also presented. Results are compared with spectral measurements with a static-pressure probe in the mixing layer of an axisymmetric jet.

A wind tunnel experiment has been performed to quantify the Reynolds number dependence of turbulence statistics in the wake of a model wind turbine. A wind turbine was placed in a boundary layer flow developed over a smooth surface under thermally neutral conditions. Experiments considered Reynolds numbers on the basis of the turbine rotor diameter and the velocity at hub height, ranging from Re D 1:66 10 4 to 1:73 10 5 . Results suggest that main flow statistics (mean velocity, turbulence intensity, kinematic shear stress and velocity skewness) become independent of Reynolds number starting from Re 9:3 10 4 . In general, stronger Reynolds number dependence was observed in the near wake region where the flow is strongly affected by the aerodynamics of the wind turbine blades. In contrast, in the far wake region, where the boundary layer flow starts to modulate the dynamics of the wake, main statistics showed weak Reynolds dependence.These results will allow us to extrapolate wind tunnel and computational fluid dynamic simulations, which often are conducted at lower Reynolds numbers, to full-scale conditions. In particular, these findings motivates us to improve existing parameterizations for wind turbine wakes (e.g. velocity deficit, wake expansion, turbulence intensity) under neutral conditions and the predictive capabilities of atmospheric large eddy simulation models.

Large Eddy Simulation and theoretical investigations of the transient cavitating vortical flow structure around a NACA66 hydrofoil, International Journal of Multiphase Flow (2014), doi: http://dx.Abstract: Compared to non-cavitating flow, cavitating flow is much complex owing to the numerical difficulties caused by cavity generation and collapse. In this paper, the cavitating flow around a NACA66 hydrofoil is studied numerically with particular emphasis on understanding the cavitation structures and the shedding dynamics. Large Eddy Simulation (LES) was coupled with a homogeneous cavitation model to calculate the pressure, velocity, vapor volume fraction and vorticity around the hydrofoil. The predicted cavitation shedding dynamics behavior, including the cavity growth, break-off and collapse downstream, agrees fairly well with experiment. Some fundamental issues such as the transition of a cavitating flow structure from 2D to 3D associated with cavitation-vortex interaction are discussed using the vorticity transport equation for variable density flow. A simplified one-dimensional model for the present configuration is adopted and calibrated against the LES results to better clarify the physical mechanism for the cavitation induced pressure fluctuations. The results verify the relationship between pressure fluctuations and the cavity shedding process (e.g. the variations of the flow rate and cavity volume) and demonstrate that the cavity volume acceleration is the main source of the pressure fluctuations around the cavitating hydrofoil. This research provides a better understanding of the mechanism driving the cavitation excited pressure pulsations, which will facilitate development of engineering designs to control these vibrations.