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Some measurements of surface drag in urban-type boundary layers at various wind angles

Some measurements of surface drag in urban-type boundary layers at various wind angles
Some measurements of surface drag in urban-type boundary layers at various wind angles
Using experimental data obtained in naturally grown boundary layers over a generic urban-type roughness (height h) it is shown that the surface drag is strongly dependent on the flow direction with respect to the roughness orientation. The variations with wind direction are accompanied by corresponding changes in the parameters contained in the usual logarithmic description of the flow in the near-wall inertial layer, U/u?=1?ln[(z?d)/zo] , principally the roughness length z o, which can vary by a factor of around three. The maximum surface drag (and roughness length) occur when the flow direction is at an angle around 45° to the faces of the cubical roughness elements, consistent with the known fact that the drag of an isolated cube in a thick boundary layer is much larger at that orientation than for flow directions normal to the faces. An accurate electronic balance was used to determine the surface drag (and hence friction velocity u ? ) and pressure-tapped roughness elements allowed estimation of the zero plane displacement d. It is shown that the best logarithmic-law fits then generally require values of the von Kármán ‘constant’ ? significantly lower than its classical value of around 0.41. For a factor of six increase in the Reynolds number (from Urefh/??3,500 ), Reynolds number effects are shown to be very weak and, coupled with the form drag and total drag data, the results thus suggest that frictional contributions to the total surface drag are relatively small
0006-8314
407-422
Castro, Ian P.
66e6330d-d93a-439a-a69b-e061e660de61
Claus, J.
29dca742-3a5a-46cb-a274-a2e62900f3eb
Krogstad, P.-A.
ad6a4fa8-13e0-4a5a-907a-6515cbb85d42
Castro, Ian P.
66e6330d-d93a-439a-a69b-e061e660de61
Claus, J.
29dca742-3a5a-46cb-a274-a2e62900f3eb
Krogstad, P.-A.
ad6a4fa8-13e0-4a5a-907a-6515cbb85d42

Castro, Ian P., Claus, J. and Krogstad, P.-A. (2012) Some measurements of surface drag in urban-type boundary layers at various wind angles. Boundary-Layer Meteorology, 145 (3), 407-422. (doi:10.1007/s10546-012-9736-3).

Record type: Article

Abstract

Using experimental data obtained in naturally grown boundary layers over a generic urban-type roughness (height h) it is shown that the surface drag is strongly dependent on the flow direction with respect to the roughness orientation. The variations with wind direction are accompanied by corresponding changes in the parameters contained in the usual logarithmic description of the flow in the near-wall inertial layer, U/u?=1?ln[(z?d)/zo] , principally the roughness length z o, which can vary by a factor of around three. The maximum surface drag (and roughness length) occur when the flow direction is at an angle around 45° to the faces of the cubical roughness elements, consistent with the known fact that the drag of an isolated cube in a thick boundary layer is much larger at that orientation than for flow directions normal to the faces. An accurate electronic balance was used to determine the surface drag (and hence friction velocity u ? ) and pressure-tapped roughness elements allowed estimation of the zero plane displacement d. It is shown that the best logarithmic-law fits then generally require values of the von Kármán ‘constant’ ? significantly lower than its classical value of around 0.41. For a factor of six increase in the Reynolds number (from Urefh/??3,500 ), Reynolds number effects are shown to be very weak and, coupled with the form drag and total drag data, the results thus suggest that frictional contributions to the total surface drag are relatively small

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Published date: 2012
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 354300
URI: https://eprints.soton.ac.uk/id/eprint/354300
ISSN: 0006-8314
PURE UUID: 50d3f484-aa87-400f-8861-2f2a81387c80

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Date deposited: 08 Jul 2013 10:41
Last modified: 16 Jul 2019 21:29

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