Experimental and numerical investigations on laminar-turbulent transition behind roughness with free-stream turbulence

粗糙度对边界层稳定性的影响是流体力学中一个已知但尚未完全理解的问题。 新的理论方法现在可以计算出一个整体的临界雷诺数，但同时也认识到，这并不对应于实验中的过渡雷诺数。实验中的背景干扰被怀疑是一种解释。 因此，这也意味着转捩雷诺数取决于湍流的程度。本研究项目的目的是研究边界层中粗糙元后面的流动不稳定性实际上在多大程度上取决于自由流湍流。 为此，首先在未扰动边界层中确定转捩雷诺数。 然后，湍流度缓慢增加，导致过渡雷诺数根据我们的预期下降。通过对不同粗糙度直径的参数研究，确定并检验了这种关系的普适性。这些实验得到了直接数值模拟的支持，以分析全球流场中的现象，并更好地理解潜在的不稳定机制。 此外，我们或我们的合作伙伴使用线性稳定性理论进行了计算，并与实验和直接数值模拟进行了比较，实验和数值结果具有很高的科学和实用价值。 一方面，我们提供了急需的实验数据，这是理论方法进一步发展所必需的。 为此，我们调整我们的设置的理论研究。此外，我们的实验提供了精确和可靠的数据，可用于已知湍流水平的实际应用中。

The influence of roughness on the stability of boundary layers is a known but not yet fully understood problem in fluid mechanics. New theoretical methods can now calculate a global critical Reynolds number, but at the same time it was recognized that this does not correspond to the transition Reynolds number in the experiment. Background disturbances in the experiment were suspected as an explanation. Consequently, this also means that the transition Reynolds number depends on the degree of turbulence.The aim of this research project is to investigate the extent to which flow instabilities behind roughness elements in the boundary layer actually depend on free-stream turbulence. For this purpose, the transition Reynolds number is first determined in the undisturbed boundary layer. The degree of turbulence is then slowly increased, causing the transition Reynolds number to drop according to our expectations. This relationship is determined and tested for universality by parameter studies for different roughness diameters.These experiments are supported by direct numerical simulations to analyze phenomena in the global flow field and to better understand the underlying instability mechanisms. In addition, calculations using linear stability theory are performed by us or our cooperating colleagues and compared with the experiment and direct numerical simulations.The experimental and numerical results are of high scientific and practical relevance. On the one hand, we provide urgently needed experimental data, which are necessary for the further development of theoretical methods. To this end, we adapt our setups to the theoretical investigations. In addition, our experiments provide precise and robust data that can be used in practical applications with known turbulence levels.