Particle Image Velocimetry Flow Visualization Research Articles

Effects of rotor separation on the axial descent performance of dual-rotor configurations

Rotorcraft can encounter highly unsteady flow when descending at a steep angle, leading to a flow condition called vortex ring state, which is associated with strong oscillatory airloads and substantial losses in mean rotor thrust. This study examines the aerodynamic coupling between closely arranged rotors in vertical flight and assesses the extent to which rotor–rotor interactions affect the rotor performance in this flight stage. Wind tunnel experiments were performed on a small-scale, dual-rotor set-up with adjustable rotor spacing, and the effect of rotor separation on thrust generation was quantified. Pairs of 4 in., 5 in. and 6 in. rotors ( $3.0 \times 10^4< Re<8.1 \times 10^4$ ) were investigated, with load cell measurements showing significant thrust losses and concomitantly increased thrust oscillations as descent rate increased. Peak losses and fluctuations were consistently recorded at descent rates of 1.2–1.3 times the hover induced velocity for all rotor sizes and separations. While tests showed that the mean aerodynamic performance of dual-rotor systems is generally similar to that of single rotors, appreciable changes to the descent characteristics could be observed at low rotor separations. Particle image velocimetry flow visualization suggests considerable changes to the flow field as rotor separation decreases, where individual vortex ring systems merge into a single vortex ring structure.

Parametric study of small-scale rotors in axial descent

Despite extensive research in multirotor aerodynamics in the recent past, axial descent, specifically the vortex ring state, still poses great challenges for multirotor configurations as this flight stage is typically accompanied by severe losses in rotor thrust and strong thrust fluctuations. This paper presents a parametric study to investigate the influence of relevant geometric parameters of a small-scale rotor blade on the rotor performance in axial descent. Design variables subject to variation were the collective pitch, chord length, taper ratio, number of blades, as well as the tip geometry. Custom rotors for each parameter modification were manufactured and experimentally evaluated in wind tunnel tests with mean thrust recordings and measurements of the thrust fluctuations serving as performance metrics. Results indicated that rotor blades with larger aspect ratio and higher blade loading coefficient are less affected by the adverse aerodynamics in the vortex ring state, experiencing lower thrust losses and vibrational loads. Particle image velocimetry flow visualization confirmed that the aerodynamic losses in the vortex ring state can be attributed to blade vortex interactions. Comparison of the rotor flow structure in hover of all investigated rotor designs suggested that improvements in the descent performance of a rotor stem from a combination of reduced tip vortex strength and increased axial tip vortex convection rate. Using the experimental findings of this study, a predictive model for approximating the maximum extent of mean thrust losses in axial descent for a given blade geometry and hover thrust coefficient could be established.

Flow-Excited Resonance of Diametral Acoustic Modes in Ducted Rectangular Cavities

The excitation of trapped diametral acoustic modes within a rectangular cavity attached to a cylindrical duct is investigated numerically and experimentally. Because the excited acoustic modes are highly three-dimensional, their interaction with the cavity shear layer results in complex patterns of flow oscillations. These are investigated for the special case of a square cavity, which exhibits three unique behaviors of the excited acoustic modes, including both stationary and spinning mode patterns. Phase-locked particle image velocimetry flow visualization accompanied with numerical simulation of the resonant sound fields indicate that the formation of disturbances is nonuniform and akin to the radial acoustic particle velocity distribution. In the instance of two acoustic modes being excited simultaneously, separate circumferential segments of the cylindrical shear layer do oscillate independently and at different frequencies, corresponding to the frequencies of the excited modes. Further, because of the symmetry inherent within the square geometry, two degenerate modes (i.e., orthogonal mode shapes with the same resonance frequency) generate a spinning mode when they are excited. The interaction of the spinning acoustic mode with the cavity shear layer leads to the formation of a three-dimensional helical vortex structure. These results underline the fact that a uniform excitation over the whole shear-layer circumference is not necessary for the generation of orderly vortical structures that maintain their coherence over the whole length of the cavity and sustain strong acoustic resonances.

Vortex Shedding and its Nonlinear Acoustic Effect Occurring at a Slit

This paper presents a theoretical study of the nonlinear acoustic properties of a slit in a thin plate subjected to highintensity sound excitation. In the present model, the conventional discrete vortexmethod is employed to simulate the near-field two-dimensional unsteady flow in order to capture the mechanism of the sound–vortex interaction, which is in combination with a spanwise-averaged three-dimensional Green’s function method used to connect the nearfield flow quantities with the far-field sound pressure. This model is compared with an existing particle image velocimetry flow visualization and a direct numerical simulation model, showing good qualitative agreement. It is revealed that the oscillatory slitflow is dominated by a pair of spiral-like counter-rotating vorticesmoving away from the slit and eventually colliding into each other. Because of the vortical flow effect, increases in the sound pressure level result in significant increases in acoustic resistance and modest decreases in acoustic reactance. For the parametric range in this study, increases in the aspect ratio of the slit result in slight increases in acoustic resistance but unnegligible reductions in acoustic reactance. However, the influence of the aspect ratio on acoustic impedance tends to be less important when the sound pressure level exceeds a certain high value.

Flow Reversals in Thermally Driven Turbulence

We analyze the reversals of the large-scale flow in Rayleigh-Bénard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large-scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner-flow rolls play a crucial role for the large-scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large-scale diagonal flow, thus leading to reversal. The Rayleigh vs Prandtl number space is mapped out. The occurrence of reversals sensitively depends on these parameters.

Development of a high speed particle image velocimetry technique using fluorescent tracers to study steam bubble collapse

Use of the particle image velocimetry flow visualization techique, with digital cameras for data acquisition, to study high speed fluid flows is usually limited by the camera frame acquisition rate. The velocity of the fluid under study must be limited to insure that particles suspended in the flow field remain in the camera's focal plane in successive images. However, the use of digital cameras for data acquisition is desirable to simplify and expedite the data analysis. A method is presented which will measure changes in the flow field that occur at different framing rates. This method was tested by acquiring the velocity field during experiments involving the injection of saturated steam into subcooled water. Firstly, changes in the position of tracer particles at a framing rate of 53.8 ms per frame were measured. White polystyrene tracer particles were used to follow the flow over an area of 18.9 × 16.5 mm 2. Since the area under study is wide, the beam had to be expanded. This step lowers the available energy and highly-scattering tracer particles were necessary to obtain a sufficient exposure of the cameras. Secondly, changes at the relatively fast framing rate of 100 to 320 μs per frame were measured. In this case the view area was smaller (7.3 × 5.7 mm 2), so the beam did not have to be expanded vertically and the energy was sufficient to allow the use of fluorescent particles. In order to achieve this fast framing rate, the laser had to be operated in a double pulse mode with one camera exposed twice. The directional ambiguity presented by the double exposure can be resolved by capturing a single exposure of the second pulse with a second digital camera. The study of collapsing steam bubbles also presents the problem of distinguishing between the light reflected by the liquid-gas interface and the light refracted by the surrounding tracer particles. This problem was resolved through the use of fluorescent seeds and appropriate filters. The filter blocked most of the green light reflected by the bubbles and passed the red light emitted by the fluorescent seeds.