Instability and Transition due to Near-Critical Roughness in a Hypersonic Laminar Boundary Layer

Measurements of instability and transition were obtained in the wake of a cylindrical roughness within the laminar nozzle-wall boundary layer of the Purdue Mach-6 Quiet Tunnel. Using wall-mounted pressure transducers along the wake centerline, the root-meansquare pressure and power spectra were computed to find evidence of instabilities within the roughness wake. The roughness height was adjusted to explore the case of incipient transition on the nozzle wall. It appeared that small variations in the experimental parameters could have a large effect on transition for the near-critical case. Several dominant disturbance frequencies were identified for a range of conditions. These disturbances appear to be due to instabilities developing within the wake of the roughness. The streamwise evolution of these disturbances are reported, as well as the spanwise distribution at one streamwise location within the wake. These results can be used as a test case to continue to develop methods for computing the stability of roughness wakes

Risk measures for direct real estate investments with non-normal or unknown return distributions

The volatility of returns is probably the most widely used risk measure for real estate. This is rather surprising since a number of studies have cast doubts on the view that volatility can capture the manifold risks attached to properties and corresponds to the risk attitude of investors. A central issue in this discussion is the statistical properties of real estate returns—in contrast to neoclassical capital market theory they are mostly non-normal and often unknown, which render many statistical measures useless. Based on a literature review and an analysis of data from Germany we provide evidence that volatility alone is inappropriate for measuring the risk of direct real estate. We use a unique data sample by IPD, which includes the total returns of 939 properties across different usage types (56% office, 20% retail, 8% others and 16% residential properties) from 1996 to 2009, the German IPD Index, and the German Property Index. The analysis of the distributional characteristics shows that German real estate returns in this period were not normally distributed and that a logistic distribution would have been a better fit. This is in line with most of the current literature on this subject and leads to the question which indicators are more appropriate to measure real estate risks. We suggest that a combination of quantitative and qualitative risk measures more adequately captures real estate risks and conforms better with investor attitudes to risk. Furthermore, we present criteria for the purpose of risk classification

Roughness-induced instability in a laminar boundary layer at Mach 6

Roughness can cause a boundary layer to become turbulent, increasing aeroheating from the laminar rate. Empirical correlations are currently used to predict the onset of roughness-induced transition but do not take into account the flow physics that cause it. More accurate physics-based prediction methods must be developed, based on the growth of instabilities within the wake of the roughness. An isolated roughness element was used to introduce instabilities into a laminar nozzle-wall boundary layer in the Boeing/AFOSR Mach-6 Quiet Tunnel. Qualitative temperature-sensitive paint measurements show several hot streaks within the wake of the roughness. Pitot and hot-wire probes were used to measure an instability in the wake of the roughness. These are believed to be the first such measurements at hypersonic speeds. The instability was observed to grow downstream of the roughness and was strongest off the wake centerline, at a height near the roughness height. Mean-flow pitot pressures in the wake of the roughness were recorded for comparison to future computations of the wake. Further characterization of this instability can assist development and validation of a physics-based transition prediction method for roughness-induced transition

Roughness-induced instabilities in a Mach-6 laminar boundary layer

To develop improved methods of transition prediction for isolated roughness, based on the growth of disturbances in the roughness wake, the underlying insta- bility mechanisms must first be characterized. A cylindrical roughness element was used to introduce instabilities into the laminar nozzle-wall boundary layer in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Instabilities were detected in the roughness wake using flush-mounted pressure sensors, at both near-effective and near-critical conditions. These are the first such instabilities measured at hypersonic speeds. Experimentally-observed instabilities were compared to computations performed by others for a large roughness with a height of 1.2 times the boundary-layer thickness. Direct numerical simulations allowed a detailed analysis of the entire flow field, while experimental measurements discovered the real flow physics and confirmed the findings of the computations. For a large roughness height of 10.2 mm, the dominant mechanism for transition was identified. An instability with a frequency near 21 kHz was detected upstream of the roughness, as predicted by the computations, suggesting that the instability originates within the separation region. Unstable shear layers and horseshoe vortices appeared to cause transition downstream of the roughness for this case. As the roughness height was reduced, there appeared to be a change in the dominant instability mechanism. Several possible instabilities were identified for smaller, near-critical roughness heights that caused incipient transition on the nozzle wall