r. chen

Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.

Most hatcheries in China rely on live prey for successful rearing of fish and shrimp larvae. Besides the high costs, live prey with pathogenic bacteria also increases the risk of contracting foodborne diseases. More than 3.5 billion large yellow croaker (Larimichthys crocea) larvae (total length over 4-centimeters) were raised in 2016. However, no formulated diet is available for replacing live feed in early stages of this croaker so far. In the present study, a new microdiet (Yq) is investigated for rearing croaker larvae at early stages (15 days after hatching). A commercial import diet (Cd) and live prey (Lp) were fed as controls. About 20,000 larvae per tank were hatched from Ningde Fufa Fishery Company in May 4th, 2017. Quality parameters, such as growth performance, survival and three digestive enzyme activities were measured. The survival rate of Yq (58.5%), Cd (51.5%) and Lp (69.6%) exhibited no significant difference (p>0.05). Furthermore, the cost of Yq was only about 23.3% of the cost of Lp. Specific growth rate in Lp was about 8 times higher (p<0.05) than that in two other microdiet treatments. Tryptic activity was slightly higher than other enzymes’ activities, revealing that trypsin plays an important role in the degradation process of large yellow croaker larvae. In the Lp group, tryptic activity was 4 times higher than that of the other two groups. In addition, the activity of three diets on α-amylase and lipase activities were not significantly different (p>0.05). The high survival rate and low cost of Yq showed that it is suitable in rearing Larimichthys crocea larvae at the early stages.