Time-resolved Particle Image Velocimetry System Research Articles

Effects of aspect ratio on flow characteristics on free surface-mounted rectangular cylinders

Turbulent flow characteristics around free surface-mounted rectangular cylinders of different streamwise aspect ratios (AR=1, 2, 2.5, 3, 4, 5, and 8 denoted as AR1, AR2, AR2.5, AR3, AR4, AR5, and AR8, respectively) at a Reynolds number based on the freestream velocity and cylinder height, Reh = 11100, were investigated experimentally using a time-resolved particle image velocimetry system. The results reveal distinct flow behaviors for different aspect ratios. The separated shear layer is shed directly into the wake for AR1, while AR2 and AR2.5 exhibits intermittent reattachment on the cylinder and direct shedding into the wake. However, for AR≥3 cylinders, the separated shear layer reattaches onto the cylinder and is later shed into the wake after subsequent separation from the trailing edge of the cylinder. Comparative analysis with previous studies on rectangular cylinders in uniform flow and wall-mounted conditions demonstrates similarities in reattachment behavior on the cylinder to uniform flow cases and wake reattachment mechanism akin to wall-mounted cylinders subjected to a thin turbulent boundary layer. The Reynolds stresses, turbulence production, and vortex shedding patterns examined with frequency spectra and proper orthogonal decomposition of velocity fluctuations exhibit significant dependence on streamwise aspect ratio.

Analysis of Swirl Number Effects on Effusion Flow Behavior Using Time-Resolved Particle Image Velocimetry

Abstract The analysis of the interaction between the swirling and cooling flows, promoted by the liner film cooling system, is a fundamental task for the design of turbine combustion chambers since it influences different aspects such as emissions and cooling capability. In particular, high turbulence values, flow instabilities, and tangential velocity components induced by the swirling flow deeply affect the behavior of effusion cooling jets, demanding for dedicated time-resolved near-wall experimental analysis. The experimental setup of this work consists of a non-reactive single sector linear combustor test rig scaled up with respect to engine dimensions; the test section was equipped with an effusion plate with standard inclined cylindrical holes to simulate the liner cooling system. The rig was instrumented with a 2D time-resolved particle image velocimetry system, focused on different field of views. The degree of swirl for a swirling flow is usually characterized by the swirl number, Sn, defined as the ratio of the tangential momentum flux to axial momentum flux. To assess the impact of such parameter on the near-wall effusion behavior, a set of three different axial swirlers with swirl number equal to Sn = 0.6–0.8–1.0 were designed and tested in the experimental apparatus. An analysis of the main flow field by varying the Sn was first performed in terms of average velocity, root mean square, and Tu values, providing kinetic energy spectra and turbulence length scale information. In a second step, the analysis was focused on the near-wall regions: the strong effects of Sn on the coolant jets were quantified in terms of vorticity analysis and jet oscillation.

Characteristics of forced flow past a square cylinder with steady suction at leading-edge corners

We experimentally investigate the characteristics of a dynamic wake and of flow separation for a square cylinder with steady suction at its leading-edge corners. The wind tunnel experiments were conducted at a Reynolds number of 5946, and suction slots were manufactured symmetrically at the leading corners of the square cylinder. Steady suction was characterized with a suction momentum coefficient Cμ varying from 0.0227 to 0.3182. A time-resolved particle image velocimetry system was used to evaluate the control of leading-edge suction at different Cμ. Next, the measurements were analyzed by applying a proper orthogonal decomposition (POD) to study the control effectiveness. The POD results suggest that the first four modes of wake vortex shedding are transformed in controlled cases and that periodic Karman vortex shedding is suppressed. The results also show that, even with a very small momentum coefficient, the steady suction at the leading-edge corners stabilizes the cylinder wake. The wake region becomes longer and narrower in comparison with the baseline case. In addition, modifications of separation flow were visualized. At quite small Cμ, flow separation at the leading-edge corners is considerably suppressed. Upon increasing the suction momentum coefficient to 0.1364, flow separation at the leading edges is almost eliminated. Finally, we estimate the effect of drag reduction due to the leading-edge suction.

Turbulent Wake-Flow Characteristics in the Near Wake of Freely Flying Raptors: A Comparative Analysis Between an Owl and a Hawk

Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris's hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds' flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds' motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.

Drag reduction and flow structures of wing tip sails in ground effect

The hydrodynamics and flow structures of a base wing slotted with tip sails in proximity to the ground were studied experimentally in order to investigate the flow control efficiency of wing tip sails in ground effect. The experiment was conducted in a towing tank at a Reynolds number 1.5×105. The lift and drag forces were measured by a transducer, the velocity fields of the wing tip vortices were measured using a time-resolved particle image velocimetry system (TR-PIV). The tip-sails and ground clearance were both effective in reducing the total drag, the lift coefficients of the tip-sails wings were increased as compared with that of a base wing. The lift-drag ratios of the tip-sails wings were improved obviously in a range of angles of attack from 2° to stalling angle. The tip-sails played more important role in unwinding the concentrated wing tip vortices at higher angle of attack, the intensity of the tip vortices were much weaker than that of the base wing. The development of the wing tip vortices was suppressed as well due to the inhibition of the ground, the downwash speed was reduced and the induced drag was decreased.

Experimental investigation of turbulent flow in a rotating straight channel with continuous ribs

We experimentally study the combined effects of continuous ribs and rotations constructed in a square duct on the turbulent flows and flow separation. The ribs obstruct the channel by 10% of its height and are arranged in three different pitch-to-height ratios (P/e) of 10, 12, and 15. The Reynolds number (Re = ρU0D/μ) is fixed at 10 000, and the rotation number (Ro = ΩD/U0) varies from 0 to 0.52. A time-resolved particle image velocimetry system is applied to provide insights into the main flow and turbulence mechanism. Results show that rotation significantly changes main flow and turbulent characteristics. In particular, a main flow phenomenon has been found: on account of the secondary flow near the ribs, velocity profile deflects to the leading side under a low rotation number, and when Ro rises to 0.48 (critical value), velocity profile deflects to the trailing side. It gives an insight into main flow in a ribbed channel. Reattachment law has been investigated, which can optimize heat transfer by optimize rib arrangement. A proper orthogonal decomposition analysis is also considered to identify the spatial characteristics of the superimposed flow fields. Based on the experimental data, the existence of ribs with different P/e ratios and Coriolis forces play significant roles in rib-generated vortices as well as their turbulent activities.

Digital particle image velocimetry study on parameter influence on the behavior of impinging synthetic jets

This study conducts an experimental investigation of the effects of the stroke length and the Reynolds number on synthetic jet vortex rings impinging onto a solid wall. A two-dimensional time-resolved particle image velocimetry system is employed to measure the planar velocity field across the jet centerline. All the experiments have been performed at a constant orifice-to-wall distance (H0/D0 = 8.0), whereas three different stroke lengths (L0/D0 = 1.8, 3.6, and 7.2) and two Reynolds numbers (Resj = 111 and 333) are selected for comparison. Time-mean characteristics are first presented to reveal the basic flow feature of an impinging synthetic jet, and phase-averaged vorticity fields provide the information of the vortex evolutions during the interaction between synthetic jet vortex rings and the wall, i.e., the unsteady behavior of the impinging synthetic jet. With the help of the quantitative particle image velocimetry data, instantaneous wall pressure and wall shear stress distributions are evaluated simultaneously to link the dynamic vortical events in the vicinity of the wall with the wall fields. Finally, a proper orthogonal decomposition analysis is applied to extract dominant coherent structures in the flow field of the impinging synthetic jet and to highlight the combined effects of the stroke length and the Reynolds number. It is found that the stroke length effect on an impinging synthetic jet is mainly reflected in the vortex ring coherence before impacting the wall, whereas the effect of the Reynolds number on the flow behavior for a small stroke length is more significant than that for a large one. In particular, the previously impinged vortex ring is beneficial in suppressing the flow separation as well as the development of the secondary vortex ring. This validates the substantial difference between the impingement of consecutive vortex rings of a synthetic jet and that of a single vortex ring.

Design of an optically accessible turbulent combustion system

In order to design the next generation of gas turbine combustors and rocket engines, understanding the flame structure at high-intensity turbulent flows is necessary. Many experimental studies have focused on flame structures at relatively low Reynolds and Damköhler numbers, which are useful but do not help to provide a deep understanding of flame behavior at gas turbine and rocket engine operating conditions. The current work is focused on the presentation of the design and development of a high-intensity (Tu = 15–30%) turbulent combustion system, which is operated at compressible flow regime from Mach numbers of 0.3 to 0.5, preheated temperatures up to 500 K, and premixed conditions in order to investigate the flame structure at high Reynolds and Damköhler numbers in the so-called thickened flame regime. The design of an optically accessible backward-facing step stabilized combustor was designed for a maximum operating pressure of 0.6 MPa. Turbulence generator grid was introduced with different blockage ratios from 54 to 67% to generate turbulence inside the combustor. Optical access was provided via quartz windows on three sides of the combustion chamber. Extensive finite element analysis was performed to verify the structural integrity of the combustor at rated conditions. In order to increase the inlet temperature of the air, a heating section is designed and presented in this paper. Separate cooling subsystem designs are also presented. A 10 kHz time-resolved particle image velocimetry system and a 3 kHz planer laser-induced fluorescence system are integrated with the system to diagnose the flow field and the flame, respectively. The combustor utilizes a UNS 316 stainless steel with a minimum wall thickness of 12.5 mm. Quartz windows were designed with a maximum thickness of 25.4 mm resulting in an overall factor of safety of 3.5.

First International Conference on Optical and Laser Diagnostics (ICOLAD 2002)

This paper describes the development of time resolved particle image velocimetry (PIV) within the Department of Mechanical Engineering at the University of Strathclyde. The Department's first PIV systems were developed on a limited budget and used existing and second hand equipment. The original technique which, employed 16mm high speed cinematography, is described. The introduction and development of low cost systems employing high speed digital video (HSDV) is discussed and, finally, the Department's new time resolved PIV system, supplied by Dantec Dynamics, is introduced. For each of the PIV systems that have been developed a critical analysis of their functionality is given and samples of the data that they have been produced are shown. Data are presented from systems such as de-rotated centrifugal impellers, air bubbles growing in columns of water, pulsatile jets and vortex shedding.