  Turbulent characteristics within shear layer have been studied, recently,
within vegetative canopy, buildings, dunes. Kevin-Helmholtz instability
triggered hairpin vortex shedding has been widely concluded as the "signature"
of mixing layer analogy. However, convoluted roughness types complicate the
observing practice of turbulent evolving progress. To simplify that, spanwise
homogeneous roughness has been adopted in this work to capture the streamwise
flow alternations under different streamwise roughness element distances.
Streamwise successive object distance $\lambda_x$ is used to show the roughness
element distance. The transitional trends from turbulent channel flow to canopy
flow have been observed from Case 1 to Case 4. However, when $\lambda_x/h<1.0$,
as successive distance decreases, the evidences indicate turbulent flow is
transferring from canopy flow into channel flow from Case 4 to Case 6.
Instantaneous results verifies the enhancement of turbulent mixing and
decreasing turbulent coherency. The attached-eddy hypothesis (AEH) becomes
valid in $\lambda_x/h<1.0$ Cases. Meanwhile, Reynolds-averaged flow results
show the roughness shear "curve" effectiveness will decrease when $\lambda_x$
keeps decreasing after $\lambda_x/h<1.0$. The probability density function
(PDF) of three fluctuating variables on a sampling point over element display
the "increasing-decreasing" trend of decreasing spacing effect on upper region,
wherein the maximal effect emerges in Case 2 and 3. And the swirling structural
visualizations also demonstrate the progress of turbulent structure changing in
roughness sublayer at different streamwise successive object distances.
Finally, the instantaneous results show wall-normal momentum flux transports
and turbulence swirling structures are correlated to the element spacing.
