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

Abstract

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

The Wall Pressure Spectrum of High Reynolds Number Rough-Wall Turbulent Boundary Layers

“…On the left, the smooth wall shape factor data is related to the momentum thickness based Reynolds number. The present study's findings were found to be in general agreement with the smooth wall data from Blake (1970), Fernholz and Finley (1995), and Brzek et al (2009).…”

“…Therefore, to make a more accurate comparison, the boundary layer shape factor has been plotted in relationship to the boundary layer thickness to roughness height ratio δ/kg for rough walls at the right of Figure 3.4. When presented in this way, the present study's findings align with the general trend of rough wall data as found by Blake (1970) and Brzek et al (2009).…”

The Turbulent Boundary Layer and Frictional Drag Characteristics of New Generation Marine Fouling Control Coatings

“…These values are relatively low compared to the 0.52 value normally expected on the smooth wall [38]. This behavior can be attributed to the relatively high freestream turbulence intensity [33]. The calculated local frictional resistance coefficients (cf) and friction velocities (uτ) are presented in Table 3 together with the Reynolds number ( 1 ) depending on the displacement thickness for all test cases.…”

“…Subsequently, the moments of the velocities were taken with the transit time averaging technique. In order to determine the friction velocity, velocity profile fitting method and total stress method were applied as in [32] and [33], respectively. There is a maximum of 2.3% difference between the results of the two methods, which is consistent with the literature [34].…”

Effect of Free-stream Turbulence on the Flow Around a S809 Wind Turbine Blade

“…The probability density function (PDF) for the height of the roughness peaks follows a normal distribution, as shown in figure 3b. Brzek et al (2009) provides further details of the rough surface used in this study.…”