Dynamic visualization of complex flow fields using digital holographic interferometry
A new technique enables full-field, non-contact, and highly sensitive measurements.
Complex flow phenomena are common in our daily lives (e.g., in candle flames, automobile designs, movement of bubbles, and optimization of aircraft structures). The study of complex flow fields is therefore an important endeavor. The behavior of flow fields, however, is complex and changes constantly in time and space. It can thus be difficult to study the mechanisms and characteristics involved with theoretical analyses and numerical simulations alone. Instead, experimental methods are more suited to the characterization of complex flow fields.
In the past there have been many experimental approaches for making flow field measurements. These methods include particle image velocimetry, laser-induced fluorescence, laser Doppler velocimetry, and phase Doppler particle analysis. Unfortunately, most of these techniques exhibit consistency problems (i.e., between the flow velocity and the entrained particles). Flow visualization techniques (especially highly sensitive and non-contact optical measurement methods), however, can be used to measure complex flow fields without affecting their mechanical properties.1, 2 These techniques therefore have significant potential to be used in the exploration of the physical mechanisms underlying complex flow fields.
We have been investigating the potential of digital holographic interferometry (DHI) as a way to navigate the problems associated with studying complex flow fields. DHI is a non-contact, non-destructive, highly-sensitive, fast, and full-field measurement approach. Our newly developed DHI technique is different from traditional optical holographic interferometry methods because it allows a hologram to be recorded with the use of digital imaging devices (e.g., a CCD or CMOS). In addition, the holographic image can be reconstructed by numerically simulating the diffractive propagation of the object beam. From this simulation, the complex amplitude distribution (i.e., the phase and amplitude information) of the object field can be calculated directly. Here the phase is wrapped from −π to π, so we need to perform a phase unwrapping operation to acquire the real phase value. We obtain the phase information of object fields in different states by dynamically recording a series of holograms. We then calculate the corresponding phase difference between the initial and intermediate states. The characteristic parameters (e.g., velocity, density, concentration, temperature, and refractive index) of a complex flow field will affect the wavefront phase distribution of a beam that passes through it. We are able to ultimately determine the characteristic parameters of the flow field by establishing a physical model and then finding the relationship between the phase and the characteristic parameters.
To achieve dynamic visualization and measurement of different complex flow fields, we have designed several DHI experimental setups. We put together these different versions by introducing an infinity optical system design, a physical aberration correction, mechanical fine-tuning, telecentric lens structures, and a dual wavelength technique. We have used a Rayleigh-Bénard convection (RBC) measurement (used in many hydraulic and fluid mechanics models) to demonstrate our technique (see Figure 1). During our RBC experiment, the temperature difference between the top and bottom plates of the container created a density gradient. Once this gradient was established, gravity caused the cooler and denser liquid to be pulled from the top to the bottom. The temperature gradient also caused the density and refractive index of the liquid to be altered. The phase distribution of the beam wavefront passing through the liquid was therefore modulated.3
A time sequence of 2D wrapped phase maps that we obtained with our DHI technique, for the RBC example, is shown in Figure 1. The relationships between phase, refractive index, and temperature mean that these phase maps also reflect the isotherms in the convective rolls that are vertical to the optical axis. The system reaches a steady state—as shown in Figure 1(e)—after 133s. We find that the velocity of the liquid is low near to the sidewalls of the container. The temperature transmission speed in these regions therefore slows down and shows a stranded phenomenon even though the temperature gradient remains the same. The top panel of Figure 2 shows the normalized temperature difference distribution after we performed the unwrapping operation on the phase map shown in Figure 1(e) and used the phase-temperature relationship. The normalized temperature differences along two horizontal lines through the resultant convection pattern are also shown in Figure 2.
Similarly, we are able to use our DHI approach to visualize and measure a variety of other complex flow phenomena (see Figure 3). These include a Kármán vortex street in a liquid,4 airflow in wind channels,5 crystallization processes in protein-lysozyme solutions,6 thermocapillary action of droplets,7 acoustic standing waves,10 heat dissipation processes of a heat sink,11 ternary diffusion,12 laser ablation processes,8 and shock wave fields on liquid or solid surfaces.9 With the use of DHI to reconstruct the phase distribution of the flow fields, we are able to successfully demodulate the characteristic parameters of these complex systems.
We have developed a DHI technique for dynamic visualization and measurement of different complex flow fields. We have also created several sets of the associated experimental apparatus, the digital holographic interferometer, and a digital holographic microscope. We now intend to apply our technology and experimental equipment to a wider range of complex flow field measurements, including micro-flow fields or flow fields on micro-nano structure surfaces.
Jianlin Zhao is currently a professor in the School of Science. He received his PhD in optics from the Xián Institute of Optics and Precision Mechanics at the Chinese Academy of Sciences in 1998. He has published more than 380 journal and international conference papers in the fields of optical information processing, digital holography, micro-nano photonics, and optical fiber sensors.
Jianglei Di is currently an associate professor in the School of Science. He received his PhD in optical engineering from Northwestern Polytechnical University in 2012. He has published more than 40 journal and international conference papers in the field of digital holography.