No abstract

International audienceAn experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale. © 2004 American Institute of Physics

It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e., sufficiently large but not exceeding the critical amplitude for the inflectional instability, are able to reduce the growth of Tollmien-Schlichting ͑TS͒ waves up to their complete suppression. This investigation aims at experimentally verifying this stabilizing effect by generating stable and symmetric, close to sinusoidal, streaks of moderate amplitudes ͑ϳ12% of the free-stream velocity͒ by means of a spanwise array of cylindrical roughness elements. The three-dimensional ͑3D͒ streaky base flow is then subjected to a secondary instability generated through a spanwise slot in the plate by means of regulated blowing and suction. In this study the stabilizing role of the streaks on TS waves is unambiguously confirmed and by increasing the height of the roughness elements, thus inducing larger amplitude streaks, we are also able to show that the stabilizing action on the TS waves increases with the streak amplitude. These results are the first to confirm the numerical predictions reported in earlier works. The full cross-stream plane has been measured at different downstream positions allowing a complete evaluation and comparison of the different amplitude measures used in previous experimental works. Furthermore, theoretical impulse response analysis and stability calculations are applied to the present experimental streaky base flow enabling a qualitative comparison of the 3D modulated TS wave distribution.

This paper reports on near-wall two-component–two-dimensional (2C–2D) particle image velocimetry (PIV) measurements of a turbulent pipe flow at shear Reynolds numbers up to $Re_{\unicode[STIX]{x1D70F}}=40\,000$ acquired in the CICLoPE facility of the University of Bologna. The 111.5 m long pipe of 900 mm diameter offers a well-established turbulent flow with viscous length scales ranging from $85~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=5000$ down to $11~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=40\,000$. These length scales can be resolved with a high-speed PIV camera at image magnification near unity. Statistically converged velocity profiles were determined using multiple sequences of up to 70 000 PIV recordings acquired at sampling rates of 100 Hz up to 10 kHz. Analysis of the velocity statistics shows a well-resolved inner peak of the streamwise velocity fluctuations that grows with increasing Reynolds number and an outer peak that develops and moves away from the inner peak with increasing Reynolds number.