The 13th Symposium on Boundary Layers and Turbulence

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STRATIFIED FLOW OVER AN ISOLATED TOPOGRAPHY OF VARIABLE WIDTH

F A. Castro, Univ. do Porto, Porto, Portugal; and J. M. L. M. Palma
Introduction and objectives

The great majority of the published work on the stratied ow around circular topography for the non-

linear regime (Nh0 =U0 > 1) neglects the in uence of the obstacle width, NLh=U0 . N is the Brunt-V ais al a

frequency, U0 is the uniform upstream velocity, and h0 and Lh are the maximum height and width of

the circular topography whose shape is given by h(x; y) = h0 =f[1 +(x=Lh)2+ (y=Lh)2]3 = 2g.

The linear theory [3] enables us a crude estimation on the behaviour of ows over topography with variable ratio

between the transversal and longitudinal characteristic lengths (r = ay=ax). Ref. [3] also shows that for

topography with r < 1 , the most visible feature is the occurrence of ow splitting, which is replaced by

wave breaking for r > 1. If r = 1, [3] indicates that the two phenomena (wave breaking and ow splitting)

occur almost simultaneously, although the theory [3] could not be used to predict the ow characteristics

when the width of a circular topography is changed, within the hydrostatic regime.

Previouly published work (e.g.: [4], [1], [2]) indicates that if Nh0 =U0 =1.5 there is a high pressure drag,

whereas accordingly with [4], [1], [2] and [5], for Nh0 =U0 =4.5 the mountain drag is much reduced and

regions of separated ow arise both upstream and downstream of the topography.

The main objectives of our work is studying the in uence of the obstacle width, the parameter NLh=U0 .

Two typical regimes based on the Nh0 =U0 (1.5 and 4.5) were studied, i.e. while Nh0 =U0 was kept

constant, NLh=U0 was changed between 5 and 60, uncovering dierent ow regimes and characteristics.

Results and conclusions

Our results led to the conclusion that apart from Nh0 =U0 , NLh=U0 is a parameter also relevant and

may aect the ow regime. For instance, when Nh0 =U0 =1.5 and NLh=U0 changes from 10 to 20, the

nondimensional drag along the ow direction (Dx=Dlin) is reduced; that is related with a reduction of

amplitude of the gravity waves and the occurrence of a well dened region of separated ow downstream

of the topography

The 13th Symposium on Boundary Layers and Turbulence