Sheilla Torres-Nieves

Effects of free-stream turbulence on rough surface turbulent boundary layers

Several effects of nearly isotropic free-stream turbulence in transitionally rough turbulent boundary layers are studied using data obtained from laser Doppler anemometry measurements. The free-stream turbulence is generated with the use of an active grid, resulting in free-stream turbulence levels of up to 6.2 %. The rough surface is characterized by a roughness parameter k + ≈ 53, and measurements are performed at Reynolds numbers of up to Re θ = 11 300. It is confirmed that the free-stream turbulence significantly alters the mean velocity deficit profiles in the outer region of the boundary layer. Consequently, the previously observed ability of the Zagarola & Smits (J. Fluid Mech., vol. 373, 1998, p. 33) velocity scale U ∞ δ*/δ to collapse results from both smooth and rough surface boundary layers, no longer applies in this boundary layer subjected to high free-stream turbulence. In inner variables, the wake region is significantly reduced with increasing free-stream turbulence, leading to decreased mean velocity gradient and production of Reynolds stress components. The effects of free-stream turbulence are clearly identifiable and significant augmentation of the streamwise Reynolds stress profiles throughout the entire boundary layer are observed, all the way down to the inner region. In contrast, the Reynolds wall-normal and shear stress profiles increase due to free-stream turbulence only in the outer part of the boundary layer due to the blocking effect of the wall. As a consequence, there is a significant portion of the boundary layer in which the addition of nearly isotropic turbulence in the free-stream, results in significant increases in anisotropy of the turbulence. To quantify which turbulence length scales contribute to this trend, second-order structure functions are examined at various distances from the wall. Results show that the anisotropy created by adding nearly isotropic turbulence in the free-stream resides mostly in the larger scales of the flow. Furthermore, by analysing the streamwise Reynolds stress equation, it can be predicted that it is the wall-normal gradient of (u 2 v) term that is responsible for the increase in (u 2 ) profiles throughout the boundary layer (i.e. an efficient turbulent transport of turbulence away from the wall). Furthermore, a noticeable difference between the triple correlations for smooth and rough surfaces exists in the inner region, but no significant differences are seen due to free-stream turbulence. In addition, the boundary layer parameters δ*/δ 95 , H and c f are also evaluated from the experimental data. The flow parameters δ*/δ 95 and H are found to increase due to roughness, but decrease due to free-stream turbulence, which has significance for flow control, particularly in delaying separation. Increases in c f due to high free-stream turbulence are also observed, associated with increased momentum flux towards the wall.

Free-Stream Turbulence Effects on the Flow around an S809 Wind Turbine Airfoil

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence on the flow around a smooth and rough surface airfoil, specifically under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Re c ≈ 182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (from L/D ≈ 1.696 to L/D ≈ 1.787). Analysis of the mean flow over the suction surface shows that, contrary to what is expected, free-stream turbulence is actually advancing separation, particularly when the turbulent scales in the free-stream are of the same order as the chord. This is a result of the complex dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.

Isotropic Free-stream Turbulence Promotes Anisotropy in a Turbulent Boundary Layer

The study of how external conditions affect turbulent boundary layers is important since such effects are often present in common engineering applications. Earlier investigations on surface roughness have shown its effect on the mean velocity and Reynolds stress profiles. Similarly, the effects of free-stream turbulence have been well documented [1, 2, 3]. However, the results available until now are limited to low Reynolds numbers. Hence, the aim of this investigation is to study the effects of high free-stream turbulence on rough surface turbulent boundary layers, at relatively high Reynolds numbers. This investigation focused on the penetration mechanisms of free-stream turbulence into the boundary layer, identifying the length scales that dominate these mechanisms and studying the effects on the resulting turbulence anisotropy [4]. These effects will also be studied in turbulent boundary layers subject to favorable pressure gradients.

The Role of Large Scales of Turbulence in Wind Turbine Blades at Various Angles of Attack

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence (FST) on the flow around a smooth and rough surface airfoil, specifically under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Rec ≈182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (from L/D ≈ 1.696 to L/D ≈ 1.787). Analysis of the mean flow over the suction surface shows that, contrary to what is expected, freestream turbulence is actually advancing separation, at stall conditions, particularly when the turbulent scales in the free-stream are of the same order as the chord. This is a result of the complex dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.

Effect of Isotropic Free-stream Turbulence in Favorable Pressure Gradient Turbulent Boundary Layers over a Rough Surface

Laser Doppler and Hotwire anemometry measurements were performed to study the effect of various conditions, namely free-stream turbulence (FST), favorable pressure gradient, and surface roughness, on turbulent boundary layers. Measurements were carried out at Re θ ≤ 4,300 and free-stream turbulence levels of up to 7%, generated using an active grid. Results show that with the addition of FST, classical scaling laws are not able to collapse the profiles of mean velocity. Moreover, boundary layer parameters, including skin friction coefficient, confirm a complex interaction between the external conditions and the inner/outer flow. The discrepancy in the behavior of the stream-wise and wall-normal variances due to the presence of free-stream turbulence suggests that the addition of nearly isotropic free-stream turbulence promotes anisotropy in the body of the boundary layer. Second-order structure functions are examined to identify and quantify which turbulence length-scales contribute mostly to creating this discrepancy. The analysis demonstrates that the effect of FST resides in a wide range of length scales, and is not limited to the largest scales of the flow as in the case of ZPG flows.

Effect of Isotropic Free-stream Turbulence on the Anisotropy of Favorable Pressure Gradient Turbulent Boundary Layers

up to 76. The free-stream turbulence was generated by means of an active grid. Results show that classical scaling laws are not able to collapse the mean velocity profiles when additional levels of free-stream turbulence are present. The differences in the influence of free-stream turbulence on the streamwise and wall-normal components of the Reynolds normal stresses show that there is a portion of the boundary layer where the presence of free-stream turbulence results in a significant increase in anisotropy. This was also observed for zero pressure gradient (ZPG) boundary layers in a prior paper. Second order structure functions are examined, at various distances from the wall, to identify and quantify the length scales that dominate this process. It is shown that for favorable pressure gradient flows the effect of free-stream turbulence resides in a much wider range of turbulent length scales as compared to the ZPG case.

High Free-stream Turbulence Over a Rough Favorable Pressure Gradient Turbulent Boundary Layer

The study of external conditions over turbulent boundary layers is of great importance for the understanding of common engineering problems. Earlier investigations on surface roughness and external pressure gradient have shown their effect on the mean velocity and Reynolds stress profiles. However, the role of the free-stream turbulence as an external condition is not completely understood, even for the simplest of turbulent boundary layer flows (i.e., the smooth, zero pressure gradient turbulent boundary layer). Specifically, the joint effect of these conditions is still uncertain. Hence, wind-tunnel experiments have been performed in order to study the joint effects of free-stream turbulence, surface roughness (k + , turbulence intensity affects the inner and outer regions, while and only change in the outer flow. Also, profiles show that high free-stream turbulence does not overcome the destruction of the viscous regions near the wall caused by surface roughness. Furthermore, a 20% increase in the skin friction is reported, mainly due to surface roughness and free-stream turbulence. The behavior of the boundary layer parameters confirms the complex interaction between free-stream turbulence, surface roughness and favorable pressure gradient.