Begell House Inc.
Journal of Flow Visualization and Image Processing
JFV
1065-3090
19
1
2012
THE PERFORMANCE OF HEXAGONALLY ARRANGED HYBRID SYNTHETIC JETS
1-13
Shu-Shen
Hsu
Institute of Applied Mechanics, National Taiwan University
Jozef
Kordik
Faculty of Mechanical Engineering, Czech Technical University in Prague, Institute of Thermomechanics AS CR, Czech Republic
Zdenek
Travnicek
Institute of Thermomechanics of the Czech Academy of Sciences
An-Bang
Wang
National Taiwan University, Institute of Applied Mechanics, No. 1, Roosevelt Rd., Sec. IV, 106, Taipei, Taiwan Republic of China
The double-acting operation of a reciprocating pump with hexagonally arranged hybrid synthetic jets (HSJ) was studied in this research. Unlike the typical synthetic jet (SJ) which is a zero-net-mass-flux flow, the hybrid synthetic jet is a nonzero-net-mass-flux flow which was measured by the hot-wire anemometer. The results show the higher extrusion to suction volume ratio is 1.69 or volumetric efficiency is 26% in hybrid synthetic jets by integrating the velocity profile comparing with 19% in trigonally arranged one. Although the velocity along the central SJ axis decreases quickly, the velocity is higher than that along the trigonally arranged one.
FLOW STRUCTURES AROUND A LARGE-DIAMETER CIRCULAR CYLINDER
15-35
Giancarlo
Alfonsi
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci 42b, 87036 Rende (Cosenza), Italy
Agostino
Lauria
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci 42b, 87036 Rende, Cosenza, Italy
Leonardo
Primavera
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci, Cubo 42b, 87036 Rende (Cosenza), Italy
In ocean and coastal engineering, a widely used tool for the investigation of wave phenomena is the velocity potential, in which both hypotheses of inviscid fluid and irrotational flow are incorporated. In some cases, a close-form analytical expression of the potential can be devised. In other cases, and especially in complex problems of contemporary engineering, no expressions of the potential exist, so that the system of the Euler equations - cast in terms of the potential - is solved numerically, usually by means of numerical techniques of integral nature. It can though be recognized that the assumptions of both inviscid fluid and irrotational flow are rather restrictive.
Within this class of problems, if one wants firstly (mainly due to limited computing power available) to give up one of the two previous hypothesis - namely, that of irrotational flow - an appropriate strategy for the investigation of wave-related phenomena is represented by the numerical integration of the Euler equations in the velocity-pressure formulation. Under this viewpoint it becomes of remarkable importance to investigate the differences that exist between a flow field derived from a velocity potential and one resulting from the numerical solution of the Euler equations in primitive variables, as related to the wave case at hand.
In this work these issues - relatively unexplored in the literature - are addressed, with reference to the case of the diffraction of water waves caused by a large-diameter, surface-piercing, vertical circular cylinder. The close-form velocity potential for this problem is first analyzed, as related to a number of strictly linear wave cases. Then, some of these cases are simulated numerically by solving the Euler equations in primitive variables, and the results are compared. For further investigation of the flow fields, the swirling-strength criterion for flow-structure extraction is applied to the velocity fields related to one of the wave cases examined. It is found that, in terms of flow structures, remarkable differences exist between the differently derived flow fields.
INTERFACIAL FILM DYNAMICS OF A MOVING MENISCUS INSIDE SQUARE CAPILLARIES: AN EXPERIMENTAL INVESTIGATION
37-56
Ajay
Tripathi
Department of Mechanical Engineering, Jaypee University of Engineering & Technology , A-B Road, Raghogarh, Guna (M.P.), India
S. K.
Agrawal
Mechanical Engineering Department, Motilal Nehru National Institute of Technology, Allahabad (U.P.), India
Mini/micro systems with single- as well as multiphase flows have many appealing pragmatic applications like pulsating heat pipes (PHP), microfluidics, fuel cells, etc. Understanding the interfacial film dynamics of such systems will help us in manipulating the performance parameters, such as bubble/slug velocity, length, shape, dynamic contact angle, thickness of the liquid film that surrounds the bubbles, mixing and flow circulation within the liquid slugs, and pressure drop along the flow, which will further improve the efficiency of multiphase microsystems. The present work aims at studying the interfacial contact line behavior of a single meniscus and liquid slug formed between different fluids. The experiments were performed using a high-speed camera to determine the dynamic contact angle of a moving meniscus. A single-syringe infusion pump along with a syringe is used, pushed with different speeds to get different flow rates. The interface was viewed and captured through a high-speed camera for four different fluids, viz., water, ethanol, glycerine, and silicone oil. Beside the dynamic contact angle information, present experimental observations were compared with the existing molecular and phenomenological models.
VISUALIZATION OF FLOW THROUGH THE TURBINE BLADE CASCADE WITH OPTIMIZED STREAMWISE BOUNDARY LAYER FENCE
57-80
K. N.
Kumar
Thermal Turbomachines Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
Mukka
Govardhan
Thermal Turbomachines Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras Chennai, India
The present study focuses on the study of critical points formed on the surfaces of a turbine cascade with and without streamwise endwall fences with the help of flow visualization. A fence whose height varies linearly from the leading edge to the trailing edge and is located in the middle of the flow passage produces the least Coefficient of Secondary Kinetic Energy (CSKE) and is the optimum fence. The reduction in CSKE by the optimum fence is 27% compared to the baseline case. The geometry of the fence is new and is reported for the first time. The objective of the fence is to block the passage vortex from crossing the passage and impinging on the suction surface of the blade. A saddle point is formed near the leading edge on the endwall for baseline and optimum fence cases. There is nearly no change in saddle point location. Distribution of critical points on the endwall near the trailing edge of the blade is symmetrical for the baseline case, while no symmetry exists for the optimum fence case. Based on skin friction line patterns, it is clear that the pressure-side leg of the horseshoe vortex is diverted by the optimum fence, and hence, impinged on the suction surface of the blade with reduced intensity. Skin friction lines on the suction surface show that with application of the optimum fence, the spanwise penetration of the passage vortex is reduced by 33%.
A STUDY OF VORTICAL STRUCTURES PAST THE LOWER PORTION OF THE AHMED CAR MODEL
81-95
Giancarlo
Alfonsi
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci 42b, 87036 Rende (Cosenza), Italy
Agostino
Lauria
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci 42b, 87036 Rende, Cosenza, Italy
Leonardo
Primavera
Fluid Dynamics Laboratory, Universita della Calabria, Via P. Bucci, Cubo 42b, 87036 Rende (Cosenza), Italy
Vortical structures that develop past the lower portion of the Ahmed car model (in the φ = 25° slant-angle configuration) have been studied numerically. Numerical simulations are executed (at ReL = 2.88 × 106) by means of a finite-volume computational code following the LES approach, and the results are compared with the experimental data obtained by other authors at the same value of the Reynolds number. For further investigation of the flow fields, the method of the imaginary part of the complex eigenvalue pair of the velocity-gradient tensor (λci or swirling-strength criterion) is applied to the velocity field to extract the turbulence vortical structures.
As a result we found that vortical structures of arch type generate from the lower portion of the body, in particular the plane-channel-type zone that is included between the floor of the computational domain and the external side of the body bottom characterized by alternatively high and low turbulent kinetic-energy contents.