RANS feasibility study of using roughness to mimic transition strip effect on the crossflo

570? SpringerAvailable online at https://link.springer.com/joumal/42241

http://www.jhydrodynamics.comJournal of Hydrodynamics, 2019, 31(3): 570-581

https://doi.org/10.1007/s42241 -019-0005-5RANS feasibility study of using roughness to mimic transition strip effect on the crossflowseparation over a 6:1 prolate-spheroidMojtaba M. Amiri, Marcelo A. Vitola, Sergio H. Sphaier, Paulo T. EsperanzaOcean Technology Laboratory (LabOceano-COPPE), Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21941-907, Brazil(Received May 29, 2017, Revised September 26, 2017, Accepted September 28, 2017, Published online January 8, 2019)?China Ship Scientific Research Center 2019Abstract: An axisymmetric body at incidence experiences the three-dimensional crossflow separation. This separation is attributed to the adverse circumferential pressure gradient. However, the separation pattern is also dependent upon the structure of the boundary layer. In this regard, utilization of transition strip devices in experiments on axisymmetric bodies may modify this structure, and consequently the crossflow separation pattern. The main objective of the present research is to mimic numerically the transition strip effect on the crossflow separation over a 6:1 prolate-spheroid up to a = 30° incidence and ReL = 4.2 x 10&. However, to avoid direct modeling of the strip, which would increase the computational cost, an attempt was made to add roughness over the body surface. To estimate the roughness that simulates closely the transition strip effect, three different roughness values were considered. The numerical model is based on RANS and a Reynolds stress turbulence model implemented in STARCCM+. The simulations have been evaluated based on the local and global variables and validated against the available experimental data. The results demonstrate the effectiveness of using a proper roughness value to mimic the transition strip effect. They also show the importance of modeling the transition strip effect, which is normally not taken into account, to capture the crossflow separation pattern.Key words: Axisymmetric body, crossflow separation, transition strip device, CFD, RANS equations

IntroductionThe flow field around an axisymmetric body at incidence represents complex three-dimensional sepa? ration that in accordance with Refs. [1-2] is called crossflow separation. This separation is due to the adverse circumferential pressure gradient that develops from windward side to leeward side (Fig. 1). Moreover, the geometry lacks sharp edges, which makes it a challenge to determine the exact location of separation. Additionally, this separation results in the generation and detachment of vortices that modifies the pressure distribution on the leeward side and, therefore, produces significant forces and moments⑶.Several experimental researches have already been performed at Virginia Polytechnic Institute to examine the turbulent crossflow separation pattern over a prolate-spheroid, especially, 6:1 geometry, up* Biography: Mojtaba M. Amiri (1989-), Male, D. Sc., Naval Architect

Corresponding author: Mojtaba M. Amiri,E-mail: mojtabamaali@oceanica.ufij.br

to the angle of incidence 30° such as[4'8^. For instance, Wetzel and Simpson⑸ by using hot wire method demonstrated that the local minimum in circumfere tial skin friction distribution could be interpreted as an approximation of the separation location. Additionally, it was proved that separation location varies with axial location along the main axis and also is a function of incidence and Reynolds number151. Other researches such as Refs. [7?8] have demonstrated that the isotropic assumption of turbulence eddy-viscosity fails to capture the fluid characteristics for angles of inci-dence greater than ten degrees. Experimental flow-field visualization around axisymmetric bodies at incidence normally requires a detailed investigation of local variables. To measure each local variable, a separate set of apparatus such as hot-film sensors, laser Doppler velocimeter (LDV) or pressure probes are used. This can be costly and time-consuming. Accor- dingly, the obtained physical information from experiments is usually limited.On the other hand, the CFD provides a high level of flow detail that is difficult to obtain from experiment. Although the CFD demands much CPU time, considering how much additional information