Raupach's model, however. In my model, the ob-

In the limit of large p

stacle's aerodynamic properties likely change as

λ(Φ)V 

 -λ(Φ)V 

p



its orientation with the wind changes. Other-

1-

≈ exp

.

(17)



phA(Φ)  

hA(Φ) 

wise, the concept of streamlining would be







meaningless. Thus, CR and A must depend on Φ.

But because I could find no study in the literature

Finally, following R92, for m and n greater than 1,

that reported the aerodynamic properties of an

I estimate the sheltered volume to be

obstacle like that depicted in Figure 3, I assume

that the drag contributions from the various fac-

U

1

c2L(Φ)h2   h

V=

es are additive. Mathematically, my hypothesis is

(18)

u

2

*

FD (Φ) = ρUh ∑  CRi (Φ)Ai (Φ)

2

(13)

where c2 is another empirical constant. Hence,

i

using eq 5, 17 and 18 in eq 16, we get

where CRi is the drag coefficient for a particular

face of the sastrugi-like obstacle, and Ai is the sil-

A (Φ)  

    2

τR (Φ) = λ(Φ)ρUh ∑  CRi (Φ) i



houette area of that face. The sum is over all faces

A(Φ) 



presented to the wind at angle Φ.





i





The obstacle in Figure 3 presents only three

⋅

Uh

exp-c2 λ(Φ)

faces. When 0 ≤ Φ ≤ 90, the wind can see the

(19)

.



u 

front triangular face; this has form drag coeffi-



*

cient CR1. When β ≤ Φ ≤ 180 β, the wind can see

the side ridge; this has drag coefficient CR2.

In eq 19, define

When 180 β ≤ Φ ≤ 180, the wind can see only

A (Φ)

the rear silhouette; this has drag coefficient CR3.

CR (Φ) ≡ ∑  CRi (Φ) i

^

.

(20)

A(Φ)

As I mentioned, I found no study that evaluat-

i