Two-color digital PIV employing a single CCD camera

 An extension of two color particle image velocimetry (PIV) is described where the color images are recorded onto a single high-resolution (3060×2036 pixel) color CCD sensor. Unlike mono-color CCD sensors, this system not only eliminates the processing time and the subsequent digitization time of film-based PIV but also resolves the directional ambiguity of the velocity vector without using conventional image-shifting techniques. For comparing the spatial resolutions of film and CCD data, a calibration experiment is conducted by recording the speckle pattern onto 35 mm color film and using a CCD sensor under identical conditions. This technique has been successfully implemented for simulated turbine film-cooling flows in order to obtain a more detailed characterization of the coolant-injection phenomenon and its interaction with freestream disturbances. Received: 20 November 1996/Accepted: 29 January 1998     

An algorithm for calculating water surface elevations from surface gradient image data

2-D water surface gradient data can be measured by a 2-D gradient detector (Zhang and Cox 1991; Zhang et al. 1993). An algorithm for the reconstruction of water surface elevation from measured surface gradient data is described in some detail. The boundary and noise effects are explored.This work has been supported by NSF and ONR. The author wishes to thank the anonymous reviewers for suggestions for improving the text.     

Directionally-unambiguous,digital particle image velocimetry studies using a image intensifier camera

The paper describes the implementation of a Digital Particle Image Velocimeter (in the Particle Tracking mode) using an advanced, ultra-high speed gated, image intensifier, monochrome video camera. The camera was controlled locally by an internal computer chip with which the image capture and analysis software suite could communicate. The software was mounted on a PC and the captured images were stored on a (PC) frame board, which was synchronised with the camera. The camera could be triggered by external events or by keyboard or software prompt. The frame grabber had on-board storage for four frames of 512 by 512 pixels, each having a resolution of 256 grey levels. This allowed the implementation of a flexible and cost effective DPIV system with a wide dynamic range.Examples are given of the use of the system at very low illumination levels and in the presence of flow directional ambiguity associated with gravity wave motion. Phase sampling in a periodic flow is also demonstrated. The image intensifier hardware and control software allowed multiple exposures, of different duration, within the same digital frame. The camera sensitivity could be adjusted to suit different levels of illumination. This feature is shown to allow the capture of DPIV images with embedded time signatures.     

Emission of dimers from a free surface of heated water

The emission rate of water dimers from a free surface and a wetted solid surface in various cases was calculated by a simplified Monte Carlo method with the use of the binding energy of water molecules. The binding energy of water molecules obtained numerically assuming equilibrium between the free surface of water and vapor in the temperature range of 298–438 K corresponds to the coordination number for liquid water equal to 4.956 and is close to the reference value. The calculation results show that as the water temperature increases, the free surface of water and the wetted solid surface become sources of free water dimers. At a temperature of 438 K, the proportion of dimers in the total flow of water molecules on its surface reaches 1%. It is found that in the film boiling mode, the emission rate of dimers decreases with decreasing saturation vapor. Two mechanisms of the emission are described.     

Determining local values of the heat transfer coefficient on a fin surface

The paper discusses methods of establishing the course of a boiling curve that base on measurements of temperature distribution on a fin surface. Two cases of a local correlation between the heat transfer coefficient and the wall superheat are considered. The first is the case of a smooth copper fin for which the boiling curve is defined using the power-law formula. It is essential to determine, however, how sensitive this method is to measurement errors. In the other case, the local values of the heat transfer coefficient are approximated by means of a power-law polynomial. The method efficiency is studied using the example of a fin surface modified with a laser beam.     

Determining parameters in nonequilibrium flow of a gas with homogeneous condensation

Analytic expressions are obtained for the parameters which determine the number of particles (clusters) and their mean radius and the supercooling at the starting point of active homogeneous condensation (the Wilson point). With a given law p=f(T), such parameters are the temperature T_s at the dew point and the mean cooling rate = dT/dt of the gas from the dew point to the Wilson point. In this context, with T_s = const and = const, the processes of nonequilibrium homogeneous condensation in the P-T diagram corresponding to the various initial states after the Wilson point may be described approximately by a single curve. So the parameters T_s and are determining parameters for the whole process of nonequilibrium homogeneous condensation. Equations are given which enable us to estimate approximately the cooling rate for flows in nozzles and jets. Computation results are given for steady flow in a Laval nozzle and unsteady spherically symmetric expansion of a gaseous sphere into a vacuum; these results demonstrate the noted regularities. Despite the great amount of literature, both periodical and monograph, on the nonequilibrium condensation of vapors, the important question of the determining parameters for the process of a homogeneous nonequilibrium condensation has not been studied to any great extent [1].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 80–87, May–June, 1985.     

Variational fluid flow measurements from image sequences: synopsis and perspectives

Variational approaches to image motion segmentation has been an active field of study in image processing and computer vision for two decades. We present a short overview over basic estimation schemes and report in more detail recent modifications and applications to fluid flow estimation. Key properties of these approaches are illustrated by numerical examples. We outline promising research directions and point out the potential of variational techniques in combination with correlation-based PIV methods, for improving the consistency of fluid flow estimation and simulation.     

Determining the shear modulus of water in experiments with a floating disk

The problem of decaying rotation of a disk floating on the surface of a viscoelastic fluid in a cylindrical container is solved by numerical methods. The motion is found to have the form of decaying oscillations observed previously for water. In addition to the viscosity coefficient, the constructed mathematical model of the viscoelastic fluid has two more independent parameters: shear modulus and time of relaxation of elastic stresses. Elastic parameters of water are determined through comparisons with experimental data. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 1, pp. 100–103, January–February, 2008.     

Measurement of surface velocities and shears at a wavy air–water interface using particle image velocimetry

 Measurements have been accomplished within the aqueous surface viscous sublayer of wind-forced microscale breaking waves using particle image velocimetry. These measurements have been used to derive surface velocities and tangential stresses. Assessment is made of the performance of the experimental equipment and techniques developed to accomplish these measurements. Comparison is made with previous studies of viscous sublayer behaviour at the air–water interface and at smooth solid surfaces. Received: 7 June 1996/Accepted: 30 April 1997     

An experimental investigation on the surface water transport process over an airfoil by using a digital image projection technique

In the present study, an experimental investigation was conducted to characterize the transient behavior of the surface water film and rivulet flows driven by boundary layer airflows over a NACA0012 airfoil in order to elucidate underlying physics of the important micro-physical processes pertinent to aircraft icing phenomena. A digital image projection (DIP) technique was developed to quantitatively measure the film thickness distribution of the surface water film/rivulet flows over the airfoil at different test conditions. The time-resolved DIP measurements reveal that micro-sized water droplets carried by the oncoming airflow impinged onto the airfoil surface, mainly in the region near the airfoil leading edge. After impingement, the water droplets formed thin water film that runs back over the airfoil surface, driven by the boundary layer airflow. As the water film advanced downstream, the contact line was found to bugle locally and developed into isolated water rivulets further downstream. The front lobes of the rivulets quickly advanced along the airfoil and then shed from the airfoil trailing edge, resulting in isolated water transport channels over the airfoil surface. The water channels were responsible for transporting the water mass impinging at the airfoil leading edge. Additionally, the transition location of the surface water transport process from film flows to rivulet flows was found to occur further upstream with increasing velocity of the oncoming airflow. The thickness of the water film/rivulet flows was found to increase monotonically with the increasing distance away from the airfoil leading edge. The runback velocity of the water rivulets was found to increase rapidly with the increasing airflow velocity, while the rivulet width and the gap between the neighboring rivulets decreased as the airflow velocity increased.     

Collision of water drops with a plane water surface

The dependence of the outcome of the collision of uncharged water drops with a plane water surface on the impact angle , the velocity v₁ and the radius r₁ of the drops has been investigated experimentally. The impact parameters were varied over the intervals: v₁=0.40–1.05 m/sec, r₁=75–150m, and =16-85°. The method employed made it possible to avoid having to monitor the individual high-speed impact process. A stream of drops, produced in a vibrating reed type monodisperse droplet generator, was directed at the target. The impact parameters were measured by means of pulsed illumination. The results are expressed in the form of the dependence of the rebound probability and the coalescence coefficient E_S on the impact parameters. The existence of alternating conditional rebound-coalescence-rebound zones for different impact angles is established, together with a decrease in E_S with increase in r₁ and v₁. The data obtained generalize the results of previous experiments.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 165–168, May–June, 1990.     

Explosively driven particle fields imaged using a high speed framing camera and particle image velocimetry

A high speed framing camera and a particle image velocimetry instrument were used to determine the properties of explosively driven particle fields in early microsecond and later millisecond times. Test items were configured in a two inch long cylindrical shape with a half inch diameter core of organic explosive. The core was surrounded by a particle bed of aluminum or tungsten powder of a specific particle size distribution. Position data from the leading edge of the particle fronts for each charge was recorded with a high speed framing camera at early time and with a particle image velocimetry (PIV) instrument at later time to determine particle velocity. Using a PIV image, a velocity gradient along the length of the particle field was established by using the mean particle velocity value determined from three separate horizontal bands that transverse the particle field. The results showed slower particles at the beginning of the particle field closest to the source and faster ones at the end. Differences in particle dispersal, luminescence, and agglomeration were seen when changes in the initial particle size and material type were made. The aluminum powders showed extensive luminescence with agglomeration forming large particle structures while the tungsten powder showed little luminescence, agglomeration and no particle structures. Combining velocity data from the high speed framing camera and PIV, the average drag coefficient for each powder type was determined. The particle field velocities and drag coefficients at one meter showed good agreement with the numerical data produced from a computational fluid dynamics code that takes advantage of both Eulerian and Lagrangian solvers to track individual particles after a set post detonation time interval.     

Determining the parameters of the fractional exponential heredity kernels of linear viscoelastic materials

The parameters of the fractional exponential creep and relaxation kernels of linear viscoelastic materials are determined. Methods that approximate the kernel by using the Mittag-Leffler function, the Laplace-Carson transform, and direct approximation of the creep function by the original equation are analyzed. The parameters of fractional exponential kernels are determined for aramid fibers, parapolyamide fibers, glass-reinforced plastic, and polymer concrete. It is shown that the kernel parameters calculated through the direct approximation of the creep function provide the best agreement between theory and experiment. The methods are experimentally validated for constant-stress and variable-stress loading in the modes of additional loading and complete unloading Translated from Prikladnaya Mekhanika, Vol. 44, No. 9, pp. 12–25, September 2008.     

Minimizing sequences of variational problems with small parameters

A class of variational problems with small parameters is studied. Their zeroth-order asymptotic solutions are constructed. It is shown that the zeroth-order asymptotic solution is just the minimizing sequence of variational problems as the small parameter approaches to zero.     

一种新的闪光照相CCD图像的扩散滤波方法

为了提高闪光CCD图像质量,提出了基于偏微分方程（partial differential equation, PDE）的智能中值-非线性扩散滤波(intelligent median-nonlinear diffusion filtering, IMNDF)方法,充分发挥了智能中值滤波和非线性扩散滤波的优势。数值实验结果证明,该方法对于两类最具代表性的噪声（高斯和脉冲）的混合问题有很好的消噪效果,其信噪比改善因子（promotion of signal-to-noise ratio, PSNR）较传统方法提高了3~5倍。对闪光照相CCD图像取得了很好的消噪声结果。     

Analytical calculation of the parameters of a molecular gas on a surface in the Smoluchowski problem

An analytical solution of the classical Smoluchowski problem on the temperature jump in molecular (monatomic, diatomic, and polyatomic) gases is presented. The gas occupies a half–space above a flat wall, with a constant temperature gradient and evaporation rate from the gas—condensed phase interface set far from this wall. The distribution function is explicitly constructed both in the half–space and at its boundary. Formulas for the concentration and temperature at the interface are derived; in the case of diatomic and polyatomic gases, formulas for temperatures determined by translational and rotational degrees of freedom of molecules are obtained. Numerical calculations are performed.     

Deformation of the water surface behind a hydrofoil of finite span

On the basis of the results of [1], a study is made of the variation of the water level behind a horizontal hydrofoil, and an approximate method is proposed for computing the fall (or rise) of the water level at high speeds.