Session L31: Experimental Techniques - Density Gradient & Free Surface

3D Imaging of Density Gradients Using Plenoptic BOS

The combination of background oriented schlieren (BOS) and a plenoptic camera, termed Plenoptic BOS, is explored through two proof-of-concept experiments. The motivation of this work is to provide a 3D technique capable of observing density disturbances. BOS uses the relationship between density and refractive index gradients to observe an apparent shift in a patterned background through image comparison. Conventional BOS systems acquire a single line-of-sight measurement, and require complex configurations to obtain 3D measurements, which are not always conducive to experimental facilities. Plenoptic BOS exploits the plenoptic camera's ability to generate multiple perspective views and refocused images from a single raw plenoptic image during post processing. Using such capabilities, with regards to BOS, provides multiple line-of-sight measurements of density disturbances, which can be collectively used to generate refocused BOS images. Such refocused images allow the position of density disturbances to be qualitatively and quantitatively determined. The image that provides the sharpest density gradient signature corresponds to a specific depth. These results offer motivation to advance Plenoptic BOS with an ultimate goal of reconstructing a 3D density field.   

Assessment of sources of error in Background Oriented Schlieren (BOS) measurements

Background Oriented Schlieren (BOS) is used to measure density gradients in a flow by tracking the apparent distortion of a target dot pattern. The quality of a BOS measurement depends on several factors such as the dot pattern, illumination, density gradients, optical system, cross-correlation algorithms and density reconstruction. To understand their contributions to the final error in the measurement and to develop an optimal set of design rules, we generate high fidelity synthetic images using ray tracing simulations. Past studies use ad-hoc models (or none) for simulating these effects and do not represent the issues introduced in a typical BOS setup, thereby limiting their utility. We have developed and implemented an image generation methodology based on ray tracing, where light rays emitted from a dot pattern are traced through the experimental setup including the density gradients, to generate high fidelity images representative of a real experiment. We apply this methodology to perform a comprehensive analysis of the various sources of error in the BOS technique and to better understand the issues involved in designing a successful experiment. The results of this study can guide future experiments and provide directions to improve the image analysis tools.   

Three-dimensional (3D) shadowgraph technique visualizes thermal convection

Shadowgraph technique has been widely used in thermal convection, and in other types of convection and advection processes in fluids. The technique reveals minute density differences in the fluid, which is otherwise transparent to the eyes and to light-sensitive devices. However, such technique normally integrates the fluid information along the depth of view and collapses the 3D density field onto a 2D plane. In this work, we introduce a stereoscopic shadowgraph technique that preserves the information of the fluid depth by using two cross-field shadowgraphs. The two shadowgraphs are coded with different and complementary colors, and each is seen by only one eye of the viewer. The two shadowgraphs can also be temporally modulated to achieve the same stereoscopic vision of the convective fluid. We further discuss ways to make use of this technique in order to extract useful information for research in fluids.   

High-speed schlieren imaging of rocket exhaust plumes

Experiments are conducted to examine the exhaust of a variety of rocket engines. The rocket engines are mounted in a schlieren system to allow high-speed imaging of the engine exhaust during startup, steady state, and shutdown. A variety of rocket engines are explored including a research-scale liquid rocket engine, consumer/amateur solid rocket motors, and water bottle rockets. Comparisons of the exhaust characteristics, thrust and cost for this range of rockets is presented. The variety of nozzle designs, target functions, and propellant type provides unique variations in the schlieren imaging.   

Studying the instantaneous velocity field in gas-sheared liquid films in a horizontal duct

In annular flow, the experimental validation of the basic assumptions on the liquid velocity profile is vital for developing theoretical models of the flow. However, the study of local velocity of liquid in gas-sheared films has proven to be a challenging task due to the highly curved and disturbed moving interface of the phases, small scale of the area of interrogation, high velocity gradients and irregular character of the flow. This study reports on different optical configurations and interface-tracking methods employed in a horizontal duct in order to obtain high-resolution particle image velocimetry (PIV) data in such types of complex flows. The experimental envelope includes successful measurements in 2D and 3D waves regimes, up to the disturbance wave regime. Preliminary data show the presence of complex structures in the liquid phase, which includes re-circulation areas below the liquid interface due to the gas-shearing action, together with non-uniform transverse movements of the liquid phase close to the wall due to the presence of 3D waves at the interface. With the aid of the moving interface-tracking, PIV, time-resolved particle-tracking velocimetry and vorticity measurements were performed.   

Sensor-Free Surface Density Detector

We have developed an optical-based method to measure the absolute air density on a wall surface in compressible turbulent boundary layers. The temporal resolution can be higher than 1MHz, and the spatial resolution can research 10 micron. For isothermal flows, our system can also be used to obtain the wall pressure distributions or volume-ratio of two-species gas. It is a powerful tool for observing turbulent fluctuations and flow separations in sub-, trans-, and supersonic airflows. The working principle of our method is to detect the air density by measuring the refractive index, which linearly depends on density and determines the transmission coefficient at the interface. For single- or multiple-point measurements, we do not need to install sensors on the wall surface, which is a big advantage compared to conventional methods. In 2D cases, a layer of anti-reflection coating is needed. The optical measurement range is not limited by the surface material or sensor. These advantages make our method a good complement or better alternative to the other approaches, such as focused laser differential interferometry technique, which provides density gradient, and pressure (temperature) sensitive paints, which depends significantly on the material properties.   

Free-surface tracking of submerged features to infer hydrodynamic flow characteristics

As sea level rise and stronger storm events threaten our coastlines, increased attention has been focused on coastal vegetation as a potentially resilient, financially viable tool to mitigate flooding and erosion. However, the actual effect of this “green infrastructure” on near-shore wave fields and flow patterns is not fully understood. For example, how do wave setup, wave nonlinearity, and canopy-generated instabilities change due to complex bottom roughness? Answering this question requires detailed knowledge of the free surface. We develop easy-to-use laboratory techniques to remotely measure physical processes by imaging the apparent distortion of the fixed features of a submerged cylinder array. Measurements of surface turbulence from a canopy-generated Kelvin-Helmholtz instability are possible with a single camera. A stereoscopic approach similar to Morris (2004) and Gomit et al. (2013) allows for measurement of waveform evolution and the effect of vegetation on wave steepness and nonlinearity.   

Time and space analysis of turbulence of gravity surface waves

Wave turbulence is a statistical state made of a very large number of nonlinearly interacting waves. The Weak Turbulence Theory was developed to describe such a situation in the weakly nonlinear regime. Although, oceanic data tend to be compatible with the theory, laboratory data fail to fulfill the theoretical predictions. A space-time resolved measurement of the waves have proven to be especially fruitful to identify the mechanism at play in turbulence of gravity-capillary waves [1]. We developed an image processing algorithm to measure the motion of the surface of water with both space and time resolution. We first seed the surface with slightly buoyant polystyrene particles and use 3 cameras to reconstruct the surface. Our stereoscopic algorithm is coupled to PIV so that to obtain both the surface deformation and the velocity of the water surface. Such a coupling is shown to improve the sensitivity of the measurement by one order of magnitude. We use this technique to probe the existence of weakly nonlinear turbulence excited by two small wedge wavemakers in a 13-m diameter wave flume. We observe a truly weakly nonlinear regime of isotropic wave turbulence. [1] Aubourg {\&} Mordant, \textit{Phys. Rev. Fluids} \textbf{1}, 2016   

Common-optical axis Fourier transform profilometry for water surface waves.

The Fourier transform profilometry -- a single-shot optical profilometric measurement of surface deformation -- has been widely used to visualize and measure water surface waves. This well-known method is based on an optical system composed of a video projector displaying a fringe pattern on the surface and a camera recording this pattern as the reference image. The deformed fringe pattern following deformation of the surface later is then recorded and compared to the reference image in order to produce a phase map, from which the height of the deformed surface is reconstructed through a phase-to-height relation. The biggest challenge encountered while applying this method for water surface is the light reflection which previously has been partially treated by enhancing the water light diffusivity with the addition of Titanium dioxide. As part of the effort to improve the accuracy and practical applicability of the method, in this talk we will present a new implementation of a common-optical axis geometry along with an appropriate phase-height relation. Furthermore, in the case of water surface waves, we introduce a proper light filtration, which removes all the reflections remaining after addition of Titanium dioxide. The proposed technique provides an order of magnitude improvement in the accuracy of detecting and reconstructing the surface deformation, which is crucial for studying small amplitude waves and bifurcation phenomena.