3,482 research outputs found
Skin-Friction and Forced Convection from Rough and Smooth Plates
The current models for skin-friction drag from a plate with a rough surface
are derived by analogy to flow within pipes having rough interiors.
But this analogy fails at low Reynolds number (Re) flow rates because the
boundary-layer must compress into the center of the pipe, while the plate
boundary-layer is unbounded. A significant discrepancy from the pipe analogy at
low Re was found by the rough plate experiments of Pimenta, Moffat, and Kays
(1975) and by experiments conducted by the present author (2019).
An additional problem is that the roughness parameter in pipe analogy
theories is tied to the drag measurements of flows inside Nikuradse's
assortment of sand-roughened pipes (1933). Prandtl and Schlichting (1934)
caution that their theory applies only to sand-roughness. More useful would be
a theory based on direct measurements of roughness profiles.
The present work derives the formula for a plate's skin-friction drag
coefficient given its root-mean-squared height-of-roughness, and Re bounds from
the roughness spatial frequency spectrum.
The present theory is in close agreement with the Mills-Hang (1983) theory,
the Pimenta et al measurements, and the experiments conducted by the author
over their respective Re ranges.
From boundary conditions of its rough plate analysis, the present work also
derives an exact formula for skin-friction from a smooth plate; this formula is
in very close agreement with measurements from Smith and Walker (1959) and
Spalding and Chi (1964) over 4 decades of Re.Comment: Rewritten to include skin-friction from smooth plates. 20 pages; 17
figures; 16 reference
Matching Dependencies with Arbitrary Attribute Values: Semantics, Query Answering and Integrity Constraints
Matching dependencies (MDs) were introduced to specify the identification or
matching of certain attribute values in pairs of database tuples when some
similarity conditions are satisfied. Their enforcement can be seen as a natural
generalization of entity resolution. In what we call the "pure case" of MDs,
any value from the underlying data domain can be used for the value in common
that does the matching. We investigate the semantics and properties of data
cleaning through the enforcement of matching dependencies for the pure case. We
characterize the intended clean instances and also the "clean answers" to
queries as those that are invariant under the cleaning process. The complexity
of computing clean instances and clean answers to queries is investigated.
Tractable and intractable cases depending on the MDs and queries are
identified. Finally, we establish connections with database "repairs" under
integrity constraints.Comment: 13 pages, double column, 2 figure
- …