A novel potential/viscous flow coupling technique for computing helicopter flow fields

Because of the complexity of helicopter flow field, a zonal method of analysis of computational aerodynamics is required. Here, a new procedure for coupling potential and viscous flow is proposed. An overlapping, velocity coupling technique is to be developed with the unique feature that the potential flow surface singularity strengths are obtained directly from the Navier-Stokes at a smoother inner fluid boundary. The closed-loop iteration method proceeds until the velocity field is converged. This coupling should provide the means of more accurate viscous computations of the near-body and rotor flow fields with resultant improved analysis of such important performance parameters as helicopter fuselage drag and rotor airloads

Xenopus Drf1, a Regulator of Cdc7, Displays Checkpoint-dependent Accumulation on Chromatin during an S-phase Arrest

We have cloned a Xenopus Dbf4-related factor named Drf1 and characterized this protein by using Xenopus egg extracts. Drf1 forms an active complex with the kinase Cdc7. However, most of the Cdc7 in egg extracts is not associated with Drf1, which raises the possibility that some or all of the remaining Cdc7 is bound to another Dbf4-related protein. Immunodepletion of Drf1 does not prevent DNA replication in egg extracts. Consistent with this observation, Cdc45 can still associate with chromatin in Drf1-depleted extracts, albeit at significantly reduced levels. Nonetheless, Drf1 displays highly regulated binding to replicating chromatin. Treatment of egg extracts with aphidicolin results in a substantial accumulation of Drf1 on chromatin. This accumulation is blocked by addition of caffeine and by immunodepletion of either ATR or Claspin. These observations suggest that the increased binding of Drf1 to aphidicolin-treated chromatin is an active process that is mediated by a caffeine-sensitive checkpoint pathway containing ATR and Claspin. Abrogation of this pathway also leads to a large increase in the binding of Cdc45 to chromatin. This increase is substantially reduced in the absence of Drf1, which suggests that regulation of Drf1 might be involved in the suppression of Cdc45 loading during replication arrest. We also provide evidence that elimination of this checkpoint causes resumed initiation of DNA replication in both Xenopus tissue culture cells and egg extracts. Taken together, these observations argue that Drf1 is regulated by an intra-S-phase checkpoint mechanism that down-regulates the loading of Cdc45 onto chromatin containing DNA replication blocks

Toward a Unified Theory of Access to Local Telephone Systems

One of the most distinctive developments in telecommunications policy over the past few decades has been the increasingly broad array of access requirements regulatory authorities have imposed on local telephone providers. In so doing, policymakers did not fully consider whether the justifications for regulating telecommunications remained valid. They also allowed each access regime to be governed by its own pricing methodology and set access prices in a way that treated each network component as if it existed in isolation. The result was a regulatory regime that was internally inconsistent, vulnerable to regulatory arbitrage, and unable to capture the interactions among network elements that give networks their distinctive character. In this Article, Professors Daniel Spulber and Christopher Yoo trace the development of these access regimes and evaluate the extent to which the emergence of competition among local telephone providers has undercut the rationales traditionally invoked to justify regulating local telephone networks (e.g., natural monopoly, network economic effects, vertical exclusion, and ruinous competition). They then apply a five-part framework for classifying different types of access that models the interactions among different network components. This framework demonstrates the impact of different types of access on network configuration, capacity, reliability, and cost. The framework also demonstrates how mandated access can increase transaction costs by forcing local telephone providers to externalize functions that would be more efficiently provided within the boundaries of the firm