A Study of Hydrodynamic Characteristics of Boundary Layer With Algae Roughness

Filamentous algae fouling such as Enteromorpha clathrata is a soft and hairy-like substance that can protrude even through a normal boundary layer. Typically, such fouling has been treated as traditional roughness functions to yield hydrodynamic characteristics [1]. This technique has been successfully used for thin fouling layer. However it may not be applicable on thicker layer since present study found substantial fluid flow within the layer. For such cases, the roughness cannot be treated simply as a passive geometric variable, but its kinematics and interactions with the flow have to be considered. The inner law (log law) dynamics may be abnormal to yield any meaningful roughness function if it is calculated in the traditional way as the departure of rough-wall log law profile over a smooth-wall log law profile. Moreover, measurement of velocity profile using LDV within the roughness is ambiguous because of the beam interference. In the present research velocity measurement of the Enteromorpha roughness boundary layer using pitot-static tube and laser Doppler velocimeter (LDV) were compared. Large discrepancies in the velocity profiles within and in the vicinity of the roughness layer were observed between the two methods. The pitot-static tube data showed significantly high velocities (60%–80% of the freestream) in the inner layer. On the other hand, LDV velocity measurements near and within the roughness layer was not reliable due to obstruction of the probe volume by the Enteromorpha filaments. The lack of good near wall data points led to inconsistencies in estimating the fluid dynamic characteristics such as skin friction coefficient and wall shear stress. Above the roughness, the pitot-static tube and LDV profiles showed relatively good agreement.