Turbulence Box Research Articles

Lidar configurations for wind turbine control

Lidar sensors have proved to be very beneficial in the wind energy industry. They can be used for yaw correction, feed-forward pitch control and load verification. However, the current lidars are expensive. One way to reduce the price is to use lidars with few measurement points. Finding the best configuration of an inexpensive lidar in terms of number of measurement points, the measurement distance and the opening angle is the subject of this study. In order to solve the problem, a lidar model is developed and used to measure wind speed in a turbulence box. The effective wind speed measured by the lidar is compared against the effective wind speed on a wind turbine rotor both theoretically and through simulations. The study provides some results to choose the best configuration of the lidar with few measurement points.

Balancing conflicting requirements for grid and particle decomposition in continuum-Lagrangian solvers

Abstract Load balancing strategies for hybrid solvers that involve grid based partial differential equation solution coupled with particle tracking are presented in this paper. A typical Message Passing Interface (MPI) based parallelization of grid based solves are done using a spatial domain decomposition while particle tracking is primarily done using either of the two techniques. One of the techniques is to distribute the particles to MPI ranks to whose grid they belong to while the other is to share the particles equally among all ranks, irrespective of their spatial location. The former technique provides spatial locality for field interpolation but cannot assure load balance in terms of number of particles, which is achieved by the latter. The two techniques are compared for a case of particle tracking in a homogeneous isotropic turbulence box as well as a turbulent jet case. A strong scaling study is performed to more than 32,000 cores, which results in particle densities representative of anticipated exascale machines. The use of alternative implementations of MPI collectives and efficient load equalization strategies are studied to reduce data communication overheads.

Large-eddy simulations of wind farm production and long distance wakes

The future development of offshore wind power will include many wind farms built in the same areas. It is known that wind farms produce long distance wakes, which means that we will see more occasions of farm to farm interaction, namely one wind farm operating in the wake of another wind farm. This study investigates how to perform accurate power predictions on large wind farms and how to assess the long distance wakes generated by these farms. The focus of this paper is the production's and wake's sensitivity to the extension of the grid as well as the turbulence when using Large-eddy simulations (LES) with pregenerated Mann turbulence. The aim is to determine an optimal grid which minimizes blockage effects and ensures constant resolution in the entire wake region at the lowest computational cost.The simulations are first performed in the absence of wind turbines in order to assess how the atmospheric turbulence and wind profile are evolving downstream (up to 12,000 m behind the position where the turbulence is imposed).In the second step, 10 turbines are added in the domain (using an actuator disc method) and their production is analyzed alongside the mean velocities in the domain. The blockage effects are tested using grids with different vertical extents. An equidistant region is used in order to ensure high resolution in the wake region. The importance of covering the entire wake structure inside the equidistant region is analyzed by decreasing the size of this region. In this step, the importance of the lateral size of the Mann turbulence box is also analyzed.In the results it can be seen that the flow is acceptably preserved through the empty domain if a larger turbulence box is used. The relative production is increased (due to blockage effects) for the last turbines using a smaller vertical domain, increased for a lower or narrower equidistant region (due to the smearing of the wake in the stretched area) and decreased when using a smaller turbulence box (due to decreased inmixing). The long distance wake behind the row is most impacted by the use of a smaller turbulence box, while the other simulation setups have less influence on these results.In summary, the results show the importance of having relatively large extensions of the domain, large extensions of the equidistant region and especially large extensions of the turbulence box.

Synthetic observations of simulated Pillars of Creation

We present synthetic observations of star-forming interstellar medium structures obtained by hydrodynamic calculations of a turbulent box under the influence of an ionising radiation field. The morphological appearance of the pillar-like structures in optical emission lines is found to be very similar to observations of nearby star forming regions. We calculate line profiles as a function of position along the pillars for collisionally excited [OIII]5007, [NII]6584 and [SII]6717, which show typical FWHM of 2--4 km/s. Spatially resolved emission line diagnostic diagrams are also presented which show values in general agreement with observations of similar regions. The diagrams, however, also highlight significant spatial variations in the line ratios, including values that would be classically interpreted as shocked regions based on one-dimensional photoionisation calculations. These values tend to be instead the result of lines of sight intersecting which intersect for large portions of their lenghts the ionised-to-neutral transition regions in the gas. We caution therefore against a straightforward application of classical diagnostic diagrams and one--dimensional photoionisation calculations to spatially resolved observations of complex three-dimensional star forming regions.

Band-pass filtered velocity statistics in decaying turbulent box

For homogeneous isotropic turbulence study, the acquisition of band-pass filtered velocity increments (FVI) in a non-forced turbulent box is still a challenge both experimentally and numerically. Turbulence and associated physical processes, at a given instant, are permanently contaminated by a forcing process which can seldom be universal. The situation tends to be the origin of intermittency and the non-Gaussian probability density distribution for acceleration and velocity gradients. To reveal implied mechanism, grid turbulence is adapted to observe non-perturbed homogeneous isotropic turbulence. The velocity increments (VI) can be obtained following Comte-Bellot and Corrsin (GCBC) by means of two point-two time shifted velocity measurements. It is difficult to obtain decaying turbulence (DT) at large turbulent Reynolds number without pollution coming from walls. Nevertheless it is also significant to investigate DT in low Reynolds number regimes to determine non-polluted tendencies. The similarity of DT between particle image velocimetry (PIV) and hot wire anemometry measurements by GCBC are presented. Here we focus our tendency on VI and FVI probability density function (PDF) shapes in this letter. In conclusion, the tendency to Gaussian shape in inertial zone wavenumbers, demonstrates that there will be no intermittency if turbulent cascade is not perturbed.

The structure of the velocity field in a confined flow driven by an array of opposed jets

We investigate the confined flow in a new turbulence box configuration. Fluid is injected through two sets of 16 vertically opposed jets and outflows through two top/bottom porous planes. The resulting flow is generated by pairs of opposed round jets with backflow and their subsequent interactions. The research issue being addressed here is that of the dependence of the velocity field structure on two parameters: the injection Reynolds number based on jet diameter Reinj, which is varied between 6000 and 28000, and the flow geometry. The latter issue is addressed by investigating two kinds of flow geometries: (i) recirculating opposed jets (ROJ), for which the distance among two consecutive jet nozzles is 2.4 diameters and the nozzle-to-nozzle distance among each two opposed jets is 6 diameters, and (ii) simple opposed jets (SOJ), for which the distance among two consecutive jet nozzles is 4 diameters and the nozzle-to-nozzle distance among each two opposed jets is 10 diameters. The instantaneous aspect of the flow field is dominated by vortical structures and it is strongly dependent on the flow geometry. For both flow geometries, no coherence between each two consecutive pairs of jets is observed. All the statistics (dimensionless profiles of mean velocities and kinetic energy, derived quantities, and inner scales) do not depend on the Reynolds number and they are only the result of the flow geometry. The ROJ geometry leads to a flow which is characterized by strong interactions between opposed and neighboring jets which lead to both top-bottom and left-right instabilities in the central region. This leads to a strong energy and enstrophy injection, which imposes its signature on the two-dimensional kinetic energy spectra regime, characterized by a kxy−3 scaling, associated to the vortical structures present in the flow. The classical kxy−5/3 regime is very poorly represented, most likely because it is supposed to be present at scales smaller than the particle image velocimetry cut-off. Only a small volume of the chamber is characterized by local homogeneity and isotropy. The SOJ geometry is associated to flow which exhibits large regions that are locally homogeneous and isotropic. However, the top-bottom instabilities remain present. The mean pressure increase in the center of the chamber is of maximum 20% of the dynamic pressure at the inlet and decreases with increasing Reynolds number. The two-dimensional kinetic energy spectra clearly exhibit the classical kxy−5/3 scaling range; this is the signature of pure turbulence that finds enough space to develop. A fine-scale analysis is performed for both flow geometries in a particular point where the return flow is locally homogeneous, isotropic, and Gaussian. After an estimation of the mean energy dissipation rate according to various methods, we find that the Reynolds number based on the Taylor microscale scales as Reinj1/2, as in classical flows, and attains a maximum value of 350.

Effects of light propagation in middle intensity atmospheric turbulence

The purpose of this report is to present an experimental study of the effects of light propagation through atmospheric turbulence. Free space optical communication is a line-of-sight technology that transmits a modulated beam of visible light through the atmosphere for broadband communication. The fundamental limitations of free space optical communications arise from the environment through which it propagates. However these systems are vulnerable to atmospheric turbulence, such as attenuation and scintillation. Scintillation is due to the air index variation under the temperature effects. These factors cause an attenuated receiver signal and lead to higher bit error rate (BER). An experiment of laser propagation was carried out to characterize the light intensity through turbulent air in the laboratory environment. The experimental results agree with the calculation based on Rytov for the case of weak to intermediate turbulence. Also, we show the characteristics of irradiance scintillation, intensity distribution and atmospheric turbulence strength. By means of laboratory simulated turbulence, the turbulence box is constructed with the following measurements: 0.5 m wide, 2 m long and 0.5 m high. The simulation box consists of three electric heaters and is well described for understanding the experimental set up. The fans and heaters are used to increase the homogeneity of turbulence and to create different scintillation indices. The received intensity scintillation and atmosphere turbulence strength were obtained and the variation of refractive index, with its corresponding structure parameter, is calculated from the experimental results.

Energy spectra of quantum turbulence in He II and 3He-B: A unified view

Quantum turbulence in superfluid He II and in 3He-B that can be regarded as nearly isothermal, isotropic, and homogeneous is discussed within the two-fluid model. A general form of the 3D energy spectrum is proposed: at large length scales, where normal and superfluid eddies are locked together by the mutual friction force, the energy spectrum is essentially classical and includes an inertial range of a Kolmogorov K62 form. With increasing wavenumber k, the normal fluid part of the spectrum terminates due to finite viscosity, while the superfluid part of the spectral energy density changes towards k −3 and then back into Kolmogorov-like k −5/3 again. Agreement with computer simulations and experiments is claimed if account is taken of the turbulent box size and of the energy decay rate.

Turbulent scales of dilute particle-laden flows in microgravity

The work described in this paper attempts to characterize the effects of inertia, isolated from gravity, on the dispersion of solid particles in a turbulent air flow. The experiment consisted of releasing particles of various sizes in an enclosed box of fan-generated, near-homogeneous, isotropic, and stationary turbulent airflow and examining the particle behavior in a microgravity environment. The turbulence box was characterized in ground-based experiments using laser Doppler velocimetry techniques. Microgravity was established by free floating the experiment apparatus during the parabolic trajectory of NASA’s KC-135 reduced-gravity aircraft. The microgravity generally lasted about 20 s, with about 50 parabolas per flight and one flight per day over a testing period of four days. To cover a broad range of flow regimes of interest, particles with Stokes numbers St of about 1–100 were released in the turbulence box. The three-dimensional measurements of particle motion were made with a particle-tracking algorithm using a three-camera stereo imaging system. Digital photogrammetric techniques were used to determine the particle locations from the calibrated camera images. The epipolar geometry constraint identified matching particles from the three different camera views and a direct spatial intersection scheme determined the coordinates of particles in three-dimensional space. Since particle loadings were light, velocity and acceleration constraints allowed particles in a sequence of frames to be matched, resulting in particle tracks and dispersion measurements. The goal was to compare the dispersion of different Stokes number particles in zero gravity and thereby decouple the effects of gravity from inertia on the dispersion. Results show that higher inertia particles disperse less in isotropic, nondecaying turbulent flows under zero gravity, in agreement with current models. Measurements show that particles with St≈1 dispersed about ten times more than the St≈100 particles.

Evaluation of the equilibrium Eulerian approach for the evolution of particle concentration in isotropic turbulence

In the current work, the accuracy of the equilibrium Eulerian approach in evolving the particulate concentration field is evaluated by comparing it against the Lagrangian approach, for varying particle response time and terminal velocity. In particular, we compare the statistics of preferential accumulation and gravitational settling of particles in a cubic box of isotropic turbulence. Twelve simulations corresponding to four values of nondimensional particle response time, τ p=0.05, 0.1, 0.2, 0.4, and three values of nondimensional terminal velocity, | V s |=0.5,2,4 are considered. The equilibrium Eulerian approach obviates the need to solve additional governing equations for the particle velocity field. It, however, involves evolution of the particle concentration field using the equilibrium Eulerian velocity field. A spectral diffusion term is included in the particle concentration equation to provide an essentially non-oscillatory behavior to the solution. There is good agreement between the equilibrium Eulerian and Lagrangian statistics for small particles. With increasing particle size, the equilibrium Eulerian approach tends to somewhat overestimate particle preferential concentration in regions of excess strain-rate over rotation-rate compared to the Lagrangian approach. Over the entire range of parameters considered, the equilibrium approach provides a good approximation to the actual mean and rms fluctuating settling velocities of the particle.

Visualisation of results from LES of combustion

We examine the role of visualisation in the context of LES simulations of premixed turbulent combustion. The physical processes involved in premixed turbulent combustion are extremely complex, and the modelling of both the turbulence (via LES) and the combustion (via flame-wrinkling models) is difficult. Appropriate visualisation is required to understand the behaviour of the models, and ultimately to understand better the flow processes which are important in many industrial applications. We examine visualisations of two specific cases; simple flame kernel growth in a box of turbulence, and combustion behind a backward-facing step. A number of visualisation techniques are used to produce results that are similar to experimentally determined Schlieren and Mie photography for the flame kernel. In addition, isosurfaces of the reaction regress variable coloured by the laminar flame speed and sub-grid wrinkling are also plotted in an attempt to gain deeper insight into the physics of turbulent combustion in the context of these particular cases. Finally we discuss the role of the WWW in the continuing development of scientific visualisation techniques.

Variation of nitric oxide concentration during inspiration.

To evaluate the pattern of inspiratory nitric oxide concentration in a simple, constant flow delivery system during the use of two phasic-flow ventilatory modes. Laboratory study in a lung model. University experimental laboratory. Nitric oxide (800 ppm in nitrogen) was administered continuously into the inspiratory circuit to deliver a nitric oxide concentration of 10 and 40 ppm to a test lung during volume-controlled (constant flow) and pressure-controlled (decelerating flow) ventilation, with an FIO2 of 1.0. In each mode, minute ventilation of 7, 14, and 21 L/min and installation of mixing chambers (none, 1-L, 2-L, and 3.2-L turbulence boxes) were studied, respectively. Nitric oxide and nitric dioxide were monitored by chemiluminescence. Since the nitric oxide/nitrogen gas is the only nitrogen source in the system during ventilation with an FIO2 of 1.0, we evaluated the fluctuation in the inspiratory nitric oxide (NOx) concentration by measuring nitrogen with a fast-response analyzer. To test the effect of the measurement site, we measured nitric oxide concentrations using chemiluminescence at different positions in the inspiratory and expiratory limbs, with and without the mixing chambers, with a minute ventilation of 14 L/min and a nitric oxide concentration of 40 ppm. Nitrogen dioxide production was not influenced by the flow pattern. During a nitric oxide concentration of 10 ppm, nitrogen dioxide was always < 0.6 ppm. During a nitric oxide concentration of 40 ppm, the highest nitrogen dioxide (4.47 ppm) concentration was found at the lowest minute ventilation and the largest inspiratory circuit volume. Nitric oxide values displayed by chemiluminescence indicated stable concentrations at all settings. However, without mixing chambers, NOx concentration calculated from nitrogen measurements demonstrated marked inspiratory fluctuations and was highest with a minute ventilation of 21 L/min and higher during pressure-controlled ventilation compared with volume-controlled ventilation (nitric oxide concentration of 40 ppm, pressure-controlled ventilation: 14.5 to 130.5 ppm; volume-controlled ventilation: 21.6 to 104.7 ppm; nitric oxide concentration of 10 ppm, pressure-controlled ventilation: 3.2 to 30.9 ppm; volume-controlled ventilation: 4.5 to 27.1 ppm). NOx concentration fluctuation decreased with an increasing mixing chamber, and was negligible at all settings with the 3.2-L turbulence box. Nitric oxide concentration fluctuation influenced chemiluminescence measurements. The displayed nitric oxide values varied, depending on the sampling site, and did not accurately reflect mean inspiratory nitric oxide concentration. Incorporation of a mixing chamber eradicated this sampling site influence. Continuous flow delivery of nitric oxide into the circuit of a phasic-flow ventilator results in marked inspiratory nitric oxide concentration fluctuation that is not detected by a slow-response chemiluminescence analyzer. Moreover, nitric oxide concentration fluctuation can influence the accuracy of the chemiluminescence measurements. These effects can be diminished by using additional mixing chambers to facilitate a stable gas concentration. As these mixing volumes increase the contact time of nitric oxide with oxygen, an increase of nitrogen dioxide has to be taken into account.

Mathematicians Open the Black Box of Turbulence

Mathematicians Open the Black Box of Turbulence