In this study, it is shown how the equivalent sand roughness required in the Moody chart can be calculated for arbitrarily shaped wall roughnesses. After a discussion of how to define the wall location and roughness height in the most reasonable way, a numerical approach based on the determination of entropy production in rough pipes and channels is presented. As test cases, three different two-dimensional roughness types have been chosen which are representative of regular roughnesses on machined surfaces. In the turbulent range, skin friction results with these test roughnesses can be linked to Nikuradse's sand roughness results by a constant factor. For laminar flows, a significant effect of wall roughness is identified which in most other studies is neglected completely. The dissipation model of this study is validated with experimental data for laminar and turbulent flows.

The three-dimensional inhomogeneous flow in the exhaust hoods of low pressure steam turbines is a major cause of losses and the design of low-loss exhaust hoods remains a challenge, particularly in retrofit units. This paper examines the sensitivity of certain geometrical exhaust hood parameters on the pressure recovery of the whole exhaust system of low pressure steam turbines. The experimental investigations are carried out in a scaled exhaust system test rig operating at full-scale Mach numbers and near design flow conditions. The measurements for all exhaust hood configurations have been performed on two axial-radial diffuser geometries at two different load points, which represent the outflow in the design point of a last stage rotor with and without shrouds. The flow measurements make use of pneumatic probes and wall pressure taps. The influence of the exhaust hood area, the flow area in the horizontal joint plane and the location of the steam inlet are examined. The sensitivity of the pressure recovery on these parameters is evaluated. The flow area in the horizontal joint plane is identified as the most sensitive geometrical parameter in the exhaust hood of low pressure steam turbines.

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In our study we determine the influence of wall roughness on friction for pipe and channel flows by numerically calculating the entropy production in the flow. It turns out that there is an appreciable influence of wall roughness in laminar flows, though this effect often is neglected completely. In addition to the friction factor results, we gain an understanding of the physics since we have access to the dissipation distribution in the flow field close to the roughness elements. For a concise description of flows over rough walls there should be a reasonable choice of the wall location as well as the roughness parameter. Various options are discussed and assessed.

In this study, the influence of wall roughness on laminar gas flow in small channels is investigated experimentally, comparing results to CFD solutions including the calculation of entropy production. In the experimental set-up, air flows between two parallel disks that create a channel of continuously adjustable height, ranging from macro to micro size. The roughness is created by a processed silicon wafer with homogeneous surface elements. The experimental results after a careful error analysis are compared to numerical models in order to validate these models. In rough micro channels, a crucial problem is the actual location of the wall. This location strongly influences the corresponding theoretically determined pressure drop of the flow. This influence, as well as entropy production considerations are discussed. The effects found in the experiments are in good agreement with the corresponding CFD-models and hence strongly indicate an influence of wall roughness on laminar flow.