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首页> 外文期刊>Experiments in Fluids: Experimental Methods and Their Applications to Fluid Flow >On the use of helium-filled soap bubbles for large-scale tomographic PIV in wind tunnel experiments
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On the use of helium-filled soap bubbles for large-scale tomographic PIV in wind tunnel experiments

机译：在风洞实验中使用氦填充的肥皂泡进行大型X线断层摄影

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摘要

The flow-tracing fidelity of sub-millimetre diameter helium-filled soap bubbles (HFSB) for low-speed aerodynamics is studied. The main interest of using HFSB in relation to micron-size droplets is the large amount of scattered light, enabling larger-scale three-dimensional experiments by tomographic PIV. The assessment of aerodynamic behaviour closely follows the method proposed in the early work of Kerho and Bragg (Exp Fluids 50:929-948, 1994) who evaluated the tracer trajectories around the stagnation region at the leading edge of an airfoil. The conclusions of the latter investigation differ from the present work, which concludes sub-millimetre HFSB do represent a valid alternative for quantitative velocimetry in wind tunnel aerodynamic experiments. The flow stagnating ahead of a circular cylinder of 25 mm diameter is considered at speeds up to 30 m/s. The tracers are injected in the free-stream and high-speed PIV, and PTV are used to obtain the velocity field distribution. A qualitative assessment based on streamlines is followed by acceleration and slip velocity measurements using PIV experiments with fog droplets as a term of reference. The tracing fidelity is controlled by the flow rates of helium, liquid soap and air in HFSB production. A characteristic time response, defined as the ratio of slip velocity and the fluid acceleration, is obtained. The feasibility of performing time-resolved tomographic PIV measurements over large volumes in aerodynamic wind tunnels is also studied. The flow past a 5-cm-diameter cylinder is measured over a volume of 20 x 20 x 12 cm(3) at a rate of 2 kHz. The achieved seeding density of <0.01 ppp enables resolving the Karman vortices, whereas turbulent substructures cannot be captured.
机译：研究了用于低速空气动力学的亚毫米直径充氦肥皂泡（HFSB）的流动追踪保真度。与微米大小的液滴相关的使用HFSB的主要兴趣是大量的散射光，从而可以通过层析PIV进行大规模的三维实验。空气动力学行为的评估紧紧遵循Kerho和Bragg的早期工作（Exp Fluids 50：929-948，1994）中提出的方法，该方法评估了机翼前缘停滞区域周围的示踪剂轨迹。后一项研究的结论与目前的工作有所不同，后者得出的结论是，毫米毫米HFSB确实代表了风洞空气动力学实验中定量测速的有效替代方法。在速度高达30 m / s的情况下，认为流量在直径25 mm的圆柱体之前停滞。示踪剂注入到自由流和高速PIV中，PTV用于获得速度场分布。在基于流线的定性评估之后，使用雾滴作为参考项的PIV实验对加速度和滑移速度进行测量。跟踪保真度由HFSB生产中的氦气，液体肥皂和空气的流速控制。获得特征时间响应，定义为滑移速度与流体加速度之比。还研究了在空气动力学风洞中对大体积进行时间分辨断层成像PIV测量的可行性。经过5厘米直径圆柱体的流量以20 kHz的速率在20 x 20 x 12 cm（3）的体积上进行测量。达到的<0.01 ppp的播种密度能够解决卡曼涡旋，而湍流的子结构无法捕获。

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