Hydrodynamic characteristics of heated/non-heated and grooved/un-grooved spheres during free-surface water entry

摘要

It is well-known that using passive enhancement techniques (such as grooves, dimples, textures, and vortex-generators, and to name but a few) plays an important role in changing the hydrodynamic characteristics of falling or moving objects in a fluid. And this is while these methods are often known as simple and low-cost techniques. Hence, this paper focuses on finding the effect of horizontal and vertical surface grooves on hydrodynamic performances of falling heated/non-heated spheres into room temperature and calm water using a high-speed camera. The outputs include trajectory, descending velocity and acceleration, hydrodynamic force coefficient, and cavity formation characteristics. To focus more on the number of grooves and sphere temperature, all tests are performed in a constant impact velocity (U-0 = 0.31 m s(-1)) which are in the regime of Froude number (Fr = 1.0), Weber number (We = 13.6), Bond number (B = 13.6), and Impact Reynolds numbers (Re-0 = 7025.7). For the heated tests, the surface temperature of spheres is set to 100 degrees C and 200 degrees C. The results indicate that increasing the number of horizontal grooves has a remarkable effect on descent performances. For instance, the five-grooved sphere has the highest descent speed and acceleration due to the less upward resistance forces (fluid pressure and frictional drag). Besides, observations show that the cavity meniscus and the two-phase bubble behind the heated un-grooved sphere are almost axisymmetric and have a streamlined-shape. Further, an un-grooved sphere increases the descent velocity with increasing body temperature, whereas the opposite is true for a grooved sphere. (c) 2020 Elsevier Ltd. All rights reserved.