Wind energy is the fastest growing alternate source of energy in the world since its purely economic potential is complemented by its great positive environmental impact. The wind turbine, whether it may be a Horizontal-Axis Wind Turbine (HAWT) or a Vertical-Axis Wind Turbine (VAWT), offers a practical way to convert the wind energy into electrical or mechanical energy. Although this book focuses on the aerodynamic design and performance of VAWTs based on the Darrieus concept, it also discusses the comparison between HAWTs and VAWTs, future trends in design and the inherent socio-economic and environmental friendly aspects of wind energy as an alternate source of energy.

Wind turbine design with emphasis on Darrieus concept Ion Paraschivoiu

Measurements in a zero-pressure-gradient turbulent boundary layer over a mesh-screen rough wall indicate several differences, in both inner and outer regions, in comparison to a smooth-wall boundary layer. The mean velocity distribution indicates that, apart from the expected k-type roughness function shift in the inner region, the strength of the rough-wall outer region ‘wake’ is larger than on a smooth wall. Normalizing on the wall shear stress, there is a significant increase in the normal turbulence intensity and a moderate increase in the Reynolds shear stress over the rough wall. The longitudinal turbulence intensity distribution is essentially the same for both surfaces. Normalized contributions to the Reynolds shear stress from the second (Q2) and fourth (Q4) quadrants are greater over the rough wall. The data indicate that not only are Q2 and Q4 events stronger on the rough wall but their frequency of occurrence is nearly twice as large for the rough wall as for the smooth wall. Comparison between smooth- and rough-wall spectra of the normal velocity fluctuation suggests that the strength of the active motion may depend on the nature of the surface.

Comparison between rough- and smooth-wall turbulent boundary layers

One-point statistics are presented for new direct simulations of the zero-pressure-gradient turbulent boundary layer in the range Reθ = 2780–6680, matching channels and pipes at δ+ ≈ 1000–2000. For tripped boundary layers, it is found that the eddy-turnover length is a better criterion than the Reynolds number for the recovery of the largest flow scales after an artificial inflow. Beyond that limit, the integral parameters, mean velocities, Reynolds stresses, and pressure fluctuations of the new simulations agree very well with the available numerical and experimental data, but show clear differences with internal flows when expressed in wall units at the same wall distance and Reynolds number. Those differences are largest in the outer layer, independent of the Reynolds number, and apply to the three velocity components. The logarithmic increase with the Reynolds number of the maximum of the streamwise velocity and pressure fluctuations is confirmed to apply to experimental and numerical internal and ext...

One-point statistics for turbulent wall-bounded flows at Reynolds numbers up to δ+ ≈ 2000

http://iopscience.iop.org/article/10.1088/0169-5983/41/2/021404/pdf

Criteria for assessing experiments in zero pressure gradient boundary layers

The structure of turbulent boundary layers which develop with zero pressure gradient on a smooth wall and a k-type rough wall was examined using arrays of X-wires. Although the data were obtained only on two orthogonal planes, the technique provides some information on the three-dimensionality of the large-scale structures. The major effect of the roughness is to tilt the inclination of the structures towards the wall-normal direction. This is caused by the reduced damping of the wall-normal velocity fluctuations close to the rough surface and the break-up of structures whose scales are comparable to the size of the roughness elements. Both effects cause a reduction in the streamwise lengthscales, as suggested by all the measured two-point correlations. The correlations also show that the roughness tends to reduce the overall anisotropy of the large-scale motion. There is evidence to suggest that the magnitude of the vorticity field is larger over the rough wall.

Structure of turbulent boundary layers on smooth and rough walls

Measurement of the local skin friction coefficient and re-examination of the Reynolds-number-effect on the mean flow quantities were made in a zero-pressure gradient, smooth wall, turbulent boundary layer. In order to do this fairly, the two-dimensionality of the flow field was carefully adjusted. And, the wall shear stress rw was determined by direct measurement using a floating element device. The Reynolds number based on the momentum thickness R θ =U 1 θ/ν ranges from 840 to 6 220. The present experimental results of c f propose a new empirical formula which well represents the present experimental data and confirms that the Karman constant x= 0.41, which implies a slope of the logarithmic linear layer, is independent of R θ down to R θ =860, whereas the additive constant varies with R θ . The magnitude of the wake parameter takes an asymptotic value of 0.62 if the Reynolds number is sufficiently high. The effect of R θ on all the turbulent intensities is evident even within the viscous sublayer. The constant stress layer does exist in the Reynolds shear stress profiles despite that the constant value varies with R θ

Re-examination of the Reynolds-Number-Effect on the Mean Flow Quantities in a Smooth Wall Turbulent Boundary Layer.

Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller

Numerical-based theoretical analysis on effects of weak fluid acceleration of free-stream due to wind-tunnel blockage on grid-generated turbulence

This paper proposes a validation scheme for the effect of wind tunnel blockage on decaying grid-generated turbulence. This validation scheme was derived from the governing equations of the k–ε model. Analytical solutions for the validation scheme were derived by introducing a model of the difference between the rate of change of the effect of fluid acceleration on the turbulent kinetic energy and that of the effect on its dissipation. The derived solutions include a decay exponent that excludes the acceleration effect, a parameter characterizing the acceleration, the initial anisotropy, and the model coefficient of the k–ε model, and can be quantified by parameters which can be known. The physical meaning of the model was clarified. The derived solutions and model were confirmed to be accurate through numerical simulation. An equation for the decay exponent, which is also affected by the fluid acceleration, was developed using the derived solutions. This scheme was applied to the examination of the reduced fluid acceleration effect in a moderate-sized wind tunnel to measure the grid-generated turbulence. The fluid acceleration effect in the wind tunnel was confirmed to be small using the derived equations. The decay characteristics of the grid-generated turbulence in the wind tunnel were measured and were found to agree with those obtained in previous experiments.

Validation scheme for small effect of wind tunnel blockage on decaying grid-generated turbulence

The Reynolds number effect on the turbulent structure of a d-type rough wall turbulent boundary layer was studid experimentally in terms of the coherent structure. Mean bursting frequency, conditional averaged velocity signals and fractional contribution of the stress-generating motions were obtained with both the VITA method and the four-quadrant analysis, respectively. The effect of the three parameters, namely, bulk Reynolds number Rθ, roughness Reynolds number and friction parameter uτ/U1, were considered in the present study. Even scaled with the inner, outer and mixed variables, the nondimensional bursting frequency depends on the Reynolds number. The relative magnitude of fractional stress contribution of ejection to that of sweep increases with decreasing Reynolds number.

Shinsuke Mochizuki

Papers

Re-examination of the Reynolds-Number-Effect on the Mean Flow Quantities in a Smooth Wall Turbulent Boundary Layer.

Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller

Numerical-based theoretical analysis on effects of weak fluid acceleration of free-stream due to wind-tunnel blockage on grid-generated turbulence

Validation scheme for small effect of wind tunnel blockage on decaying grid-generated turbulence

Coherent structure of a D-type rough wall boundary layer