The rotary compressor is widely used in small air conditioners, and is the most important element in the system. It relies on eccentric rolling pistons that rotate at high speed to compress refrigerant in the cylinder. The lubricant oil in the rotary compressor is used for lubricating the bearing and sealing the clearance of sliding parts. However, the oil can experience complex and highly-coupled atomization processes when discharged from the cylinder, and part of oil droplets can exhaust from the rotary compressor by the refrigerant flow and reduce the efficiency and reliability of the compressor as a result. Thus, characterizing the behavior of oil droplets in the lower cavity of a rotary compressor where the atomization occurs is a major challenge for manufacturers who rely on CFD tools to predict the multiphase flow. By modifying a rotary compressor, the oil behavior in the lower cavity of a rotary compressor is observed and recorded by shadowgraphy. In the current phase, the number, size, and morphology of oil droplets are analyzed statistically with image processing method, which provides better understanding to the atomization mode in the lower cavity, the velocity of the mist of oil droplets is calculated with Optical Flow Velocimetry. The results can assist designers in improving the CFD analysis of compressors and ultimately reducing the Oil Discharge Rate (ODR).

In this paper, we investigate the effects of an imposed axial flow on hydrodynamic instabilities’ Couette-Taylor flow in the case where the wall of the inner cylinder of the system is grouved. Without imposed axial flow, the basic flow of a fluid between two coaxial cylinders known by Couette flow, which is characterized by several temporal and spatial symmetries. The increase in the rotation causes the breaking of these symmetries. In both cases where the surface of the inner cylinder is smooth and grooved, five different flow regimes can be determined: Taylor vortex flow (TVF), wavy vortex flow (WVF), and Modulated Wavy vortex flow (MWVF). Each time the flow passes from one hydrodynamic regime to another until it enters a state of turbulence, which is characterized by the destruction of all the symmetries that existed at the beginning. In addition, when an axial flow is imposed on a Taylor-Couette flow, new helical vortex structures are observed in both cases (with and without surface groove). The influence of surface structures (grooves) on the shear stress of the wall is discussed with and without axial base flow. A spatio-temporal description of several flow models was obtained using firstly, a visualization’s qualitative study using kalliroscope particles. Secondly, a quantitative study by polarography using simple probes have been used to characterize the impact of vortex structures on the Couette-Taylor flows without and with an axial flow on the transfer.

Hot-wire measurements were taken in the wake of ten finite length circular cylinders, six of which were also tapered, in a uniform flow in a low speed wind tunnel. The Reynolds number based on mean cylinder diameter ranged from 2100 ≤ Re ≤ 5500, the aspect ratio ( AR ) of the cylinders varied from 16 ≤ AR ≤ 64, and the taper ratio ( R T ) varied from 21.3 ≤ R T ≤ 96. The vortex shedding along the spans of the cylinders coalesced into discrete cells, the range of Strouhal numbers and the number of cells being a function of the cylinder aspect ratio and taper ratio. It was found that the number of discrete cells is linearly related to a cylinder geometry ratio ( CGR ) defined as CGR = AR (1 + AR / R T ).

Two-dimensional simulations are carried out for a heat exchanger to study the cross-flow behavior in a circular tube bank in a staggered configuration in case of bare cylinders and cylinders with splitter plate attachment. A considerable performance evaluation of the heat exchanger with splitter plate can be achieved by studying the heat transfer and the pressure drop of the flow. Numerical simulation results carried out from this study are compared to experimental results. The numerical investigation has been established to study the effect of splitter plate on the heat exchanger thermal performance as there were no previous studies performed on the optimization of the splitter thickness. The study also illustrates the effect of splitter plate thickness variation on pressure drop and heat transfer for different Reynolds number.

A new open-source CFD solver is being developed based on the vorticity transport equations for simulation of unsteady incompressible flow in complex geometries. This is a hybrid approach, which uses a compact high-order finite-difference method to predict the flow in the near-boundary region and a Lagrangian Vortex Particle Method (LVPM) for the off-boundary vorticity-containing region. This paper focuses on the latter, presenting the particular LVPM implemented in the package and demonstrating selected benchmarks from simulations of flow in 2D lid-driven cavity, flow around a rotating cylinder (both at Re = 1,000), and impulsively started flow over a sphere at Re = 25, 40, 60, 80, 100. In addition, novel ideas on the development of an efficient and lightweight Graphical User Interface (GUI), as well as new approaches to cross-platform hardware acceleration for Teraflop/s computing on a desktop — achieving over two orders of magnitude speedup vs. an optimized serial code — are discussed.

This paper investigates the dynamic response of two freely rotatable rigid square cylinders to two-dimensional laminar flow in a microchannel. The square cylinders are laterally pinned side-by-side in the microchannel with a single freedom of rotation. Finite volume method coupled with a dynamic mesh technique is developed and validated to reveal the detailed motion characteristics of the cylinders and nearby flow structures. Under small Reynolds number ( Re = 50), both cylinders oscillate periodically. The oscillate curves (rotating angle v.s. time) are symmetrical with each other but with a certain phase difference. At Re = 150, both cylinders oscillate randomly. Under high Reynolds number ( Re = 300), the two cylinders both keep rotating in the opposite direction with the velocity magnitude fluctuating drastically around 1.75. Important motion details are presented to understand the Fluid-Structure interaction mechanism under different Reynolds number, including the time history of rotating angles and rotating velocities, lift and drag coefficients on the cylinders, distribution of pressure around the cylinder sides. Both pressure-induced torque and the shear induced one are obtained and their contributions to both cylinders’ rotation characteristics are quantitatively evaluated. Vortex structures and streamlines around the cylinders at specific moments are also revealed in this paper to help understanding the fluid-structure interaction phenomenon.

Simulating convective heat transfer using traditional numerical methods requires explicit definition of thermal boundary conditions on all boundaries of the domain, which is almost impossible to fulfill in real applications. Here, we address this ill-posed problem using machine learning techniques by assuming that we have some extra measurements of the temperature at a few locations in the domain, not necessarily located on the boundaries with the unknown thermal boundary condition. In particular, we employ physics-informed neural networks (PINNs) to represent the velocity and temperature fields while simultaneously enforce the Navier-Stokes and energy equations at random points in the domain. In PINNs, all differential operators are computed using automatic differentiation, hence avoiding discretization in either space or time. The loss function is composed of multiple terms, including the mismatch in the velocity and temperature data, the boundary and initial conditions, as well as the residuals of the Navier-Stokes and energy equations. Here, we develop a data-driven strategy based on PINNs to infer the temperature field in the prototypical problem of convective heat transfer in flow past a cylinder. We assume that we have just a couple of temperature measurements on the cylinder surface and a couple more temperature measurements in the wake region, but the thermal boundary condition on the cylinder surface is totally unknown. Upon training the PINN, we can discover the unknown boundary condition while simultaneously infer the temperature field everywhere in the domain with less than 5% error in the Nusselt number prediction. In order to assess the performance of PINN, we carried out a high fidelity simulation of the same heat transfer problem (with known thermal boundary conditions) by using the high-order spectral/hp-element method (SEM), and quantitatively evaluated the accuracy of PINN’s prediction with respect to SEM. We also propose a method to adaptively select the location of sensors in order to minimize the number of required temperature measurements while increasing the accuracy of the inference in heat transfer.

Particle size and settling speed determine the rate of particulate mass transfer from the ocean surface to the sea bed. Turbulent shear in the ocean can act on large, faster-settling flocculated particles to break them into slower-settling primary particles or sub-aggregates. However, it is difficult to understand the disruption behavior of aggregates and their response to varying shear forces due to the complex ocean environment. A study was conducted to simulate the disruption behavior of marine aggregates in the mixed layer of the ocean. The breakup process was investigated by aggregating and disrupting flocs of bentonite clay particles in a rotating and oscillating cylindrical tank 10 cm in diameter filled with salt water. This laboratory tank, which operated based on an extension of Stokes’ second problem inside a cylinder, created laminar oscillating flow superimposed on a constant rotation. This motion allowed the bentonite particles to aggregate near the center of the tank but also exposed large aggregates to high shear forces near the wall. A high-speed camera system was used, along with particle tracking measurements and image processing techniques, to capture the breakup of the large particle aggregates and locate their radial position. The breakup response of large aggregates and the sizes of their daughter particles after breakup were quantified using the facility. The disruption strength of the aggregated particles is presented and discussed relative to their exposure to varying amounts of laminar shear.

Small tubes and fins have long been used as methods to increase surface area for convective heat transfer in single-phase flow applications. As demands for high heat transfer effectiveness has increased, implementing evaporative phase-change heat transfer in conjunction with small fins, tubes, and surface structures in advanced heat exchanger and heat sink designs has become increasingly attractive. The complex two-phase flow that results from these configurations is poorly understood, particularly in how the gas phase interacts with the flow structure of the wake created by these bluff bodies. An experimental study of liquid-gas bubbly flow around a cylinder was performed to understand these complex flow physics. A 9.5 mm diameter cylinder was installed horizontally within a vertical water channel facility. A high-speed camera captured the movement of the liquid-gas mixture around the cylinder for a range of bubble sizes. Liquid Reynolds number, calculated based on the cylinder diameter, was varied approximately from 100 to 3000. Time-averaged probability of bubble presence was calculated to characterize the cylinder wake and its effects on the bubble motion. The influence of the liquid Reynolds number, superficial air velocity, and bubble size is discussed in the context of the observed two-phase flow patterns.

A series of numerical experiments have been carried out on the water entry problem of three-dimensional multi-degree-freedom cylinders. The circular cylinder was released above the water with a specified inclined angle and velocity at entry. The hydrodynamics of the water entry problem have been investigated numerically. The Piecewise Linear Interface Calculation (PLIC) schemes have been applied in conjunction with the Volume of Fluid (VOF) method to capture the interface. Overset meshes have been adopted to handle the moving object. The numerical model is built on the framework of OpenFOAM which is an open-source C++ toolbox. Numerical results have been obtained. Transient flow and pressure distributions have been generated. The presence of air entrapment which has been reported experimentally has also been confirmed in the numerical solution. The fluid physics of the oblique water entry problem such as the formation and development of the air entrapment has been explored. The transient positions and inclined angles of the moving circular cylinder have been found to be in good agreement with the experimental results. Parametric studies have been performed with major findings reported.

To investigate the effect of permeability on turbulent wake behind a cylinder in uniform flow, we conduct particle image velocimetry on turbulent wake behind permeable cylinders, which are made of mesh sheets, of different permeabilities and compare the results with those for a solid cylinder. For relatively lower permeability, turbulent wake is quite similar to the case for a solid cylinder except for a slight shift in the streamwise direction of the reversed flow region implying turbulent Kármán vortex shedding. On the other hand, for higher permeability, the structure of turbulence is qualitatively different. More concretely, turbulent Kármán vortices disappear. Interestingly, however, the momentum deficit for such flow is comparable with that of a solid cylinder. This considerable momentum deficit can be understood with isotropic turbulence caused by the flow penetrating through the mesh constructing the cylinders. These results imply that turbulent wake behind a permeable cylinder involves dynamics both of wake and grid turbulence and the latter one dominates when permeability is sufficiently high.

Vortex shedding in the flow field causes many kinds of problems such as increase of drag and noise. Especially, the von Kármán vortex street behind bluff bodies, e.g. a tire of an airplane and a pantograph of a train, greatly contributes to them. One of the effective methods to suppress the vortices is the use of plasma actuators (PAs). A PA can induce flow by applying a high-voltage and high-frequency AC voltage to its electrodes. In the present study, we use an opposed-type PA (O-PA), which consists of two PAs facing each other. The O-PA can induce a jet in the direction perpendicular to the surface because of a collision of flows induced by the two PAs. In this study, we investigate the control effect of an O-PA on the flow around a square cylinder using an O-PA by means of the PIV measurement. First, we measure the flow induced by an O-PA. It is confirmed that the velocity of the induced flow increases as the applied voltage V pp increases, and the O-PA induces the jet of about 1.5 m/s under V pp = 10kV. Next, we measure the flow around a square cylinder with no control. It is confirmed that the von Kármán vortex street occurs behind a square cylinder. Finally, we measure the flow around a square cylinder under the control by the O-PA attached on the rear surface. It is confirmed that the vortex shedding behind a square cylinder is suppressed by the O-PA under V pp = 10kV.

Unsteady mixed convective heat transfer flow past an inclined square cylinder is numerically investigated using a local multi-quadric radial basis function (MQRBF) interpolation. The blockage parameter (ratio of square cylinder length d to height of the computational domain H) varies from 0.025 to 0.2. Air is considered as the working fluid and Prandtl number is fixed at 0.71. Richardson number generally affects the heat transfer efficiency ranging from 0 to 20. Inclined angle of square cylinder ranges from 0° to 45°. The inlet flow is assumed to be laminar and uniform. At the outlet of the computational domain, a convective boundary condition is compared with a traditional Neumann condition. A study of the shape parameter of MQRBF which is sensitive to the distribution of inhomogeneous supporting nodes is provided. The representative streamlines, vortex structures and isotherm patterns are presented and discussed. In addition, the overall lift and drag coefficients, average Nusselt number and Strouhal number for unsteady flow are analyzed for various Reynolds number and Richardson number.

Interaction between the wakes of two cylinders in side-by-side configuration creates interesting flow phenomena. The nature of the wake depends on the Reynolds number and the transverse pitch distance between the cylinders. The flow over two side-by-side cylinders of equal diameter is simulated in 3D at Reynolds number 5000 using Large Eddy Simulation (LES). The centre-to-centre transverse pitch ratio is varied and the flow behind the cylinders is classified into either a bi-stable flow regime with biased gap flow or a regime with parallel vortex streets. Furthermore, representative instantaneous flow fields, Strouhal number and the time varying drag coefficient C′ D are presented.

An experimental wind tunnel study was performed to assess the effect of aspect ratio and rotational speed of circular cylinders of varying diameter on the flow patterns behind the cylinders in the presence of a uniform upstream crossflow. Six circular cylinders of constant length but different diameters, producing aspect ratios 6 ≤ AR ≤ 32 were examined at a single upstream velocity such that the Reynolds number varied between 1920 ≤ Re ≤ 10240. Rotational speeds from stationary up to 3600 rpm were applied to the cylinders, so that the maximum relative velocity α = πfD/U ∞ = 0.80. Mean velocity profiles were measured three diameters downstream of the cylinder axis at 6 equidistant locations, and PSD power spectral density were generated for 26 equidistant locations along the cylinder, to create a comprehensive record of spanwise variations under all rotational conditions. For the highest aspect ratio tested, the wake velocity profiles were independent of rotational speed at all spanwise locations, whereas at lower aspect ratios, the maximum velocity defect diminished with increasing rotational speed along most of the span and became asymmetric near the free end. Two distinct shedding cells were found only for a cylinder with an aspect ratio of twelve at three relative spin rates of 0.067, 0.27, and 0.4. In cases where only a single cell existed, increased rotational speed produced a higher vortex shedding frequency on a given aspect ratio cylinder.

Complex flow structures arise as fluids are forced to flow across cylinder rows at moderate Reynolds numbers. In this study a numerical heat transfer analysis of 12 cylinders in an inline configuration is performed using Large Eddy Simulation (LES). The LES is conducted to get a better understanding of changes in the time averaged Nusselt number, 〈Nu〉, and local time averaged Nusselt number, 〈Nu θ 〉, for each cylinder in the cylinder row. The simulations are performed at Re = UD/v = 10,000 and Pr = 0.71 with isothermal cylinders and a constant and uniform inflow temperature. The results show that the time averaged Nusselt number increases slightly between the first and second cylinder due to increased turbulent velocity fluctuations. Beyond the second cylinder, the time averaged Nusselt number decreases until it reaches a near constant value after the fifth cylinder. For all 12 cylinders the local time averaged Nusselt number around the surface is highest at the stagnation point. The first cylinder in the row has the same distribution as the reference simulation conducted for a single cylinder. From the second cylinder and onwards a larger part of the overall heat transfer is in the spanwise direction compared to the first- and reference cylinder.

Flow behaviors of a downstream object can be affected significantly by an upstream object in close proximity. This concept is used for flow control in this study to maximize the lift/drag ratio on a NACA0012 airfoil. A cylinder with cross-flow translational motion is placed upstream of the airfoil. Numerical simulations are carried out with an immersed-boundary method to solve the incompressible, viscous flow at the Reynolds number of 2000. Control parameters that influence the dynamics of flow around the airfoil are systematically investigated, including the oscillating frequency and amplitude of the upstream cylinder, the distances between the cylinder and the airfoil, and the diameter of the cylinder. To obtain sample data properly and efficiently for carrying out the neural network study, the idea of the orthogonal test method is used to set the control parameters in the numerical simulation. The combination of the back-propagation neural network algorithm and the genetic algorithm is applied to find the optimal value of the lift/drag ratio and the corresponding control parameters. The results show that when the cylinder oscillating frequency increases, the ratio increases until negative coefficients occur; when the distance between the cylinder and the airfoil increases or the amplitude of oscillating cylinder increases, the ratio decreases first and then increases; and when the cylinder diameter increases, the ratio increases. Compared to the reference case, the optimized lift/drag ratio increases 178%.

In this study, the wake structure around freestream-aligned cylinder is investigated and its aerodynamic characteristics are discussed. A magnetic suspension and balance system (MSBS) was used to support a model without interference from a mechanical support device. Seven models with the fineness ratio (length to diameter, L/D ) of 0.5, 1.0, 1.25, 1.5, 1.75, 2.0, and 2.25 were used. Reynolds number based on the cylinder diameter were 3.2 × 10 4 and 6.3 × 10 4 . The velocity field was obtained by particle image velocimetry (PIV) in the center plane of the cylinder. In the case of fineness ratio over 1.5, the reattachment of shear layer was observed from the mean velocity field. The characteristic fluctuation of velocity was confirmed in power spectral density of streamwise component and vertical component. The length of the recirculation region is different depending on fineness ratio. The characteristic frequencies of the velocity fluctuation which seems to be due to recirculation bubble pumping and large-scale structure are observed from power spectrum density.

Under-expanded jets have wide range of application from fuel injection to rocket propulsion. In the present work, a numerical model was generated to investigate the fluid mechanics behavior of under-expanded jet formation and wall interaction of a jet produced by exhausting a high pressure cylinder through a narrow tube into a low pressure cylinder. Axisymmectic, Reynolds Averaged Navier Stokes simulations were conducted employing the ANSYS FLUENT explicit, Coupled Pressure-Velocity solver to determine the stagnation pressure at the wall downstream of the orifice. Transient cases were conducted using timestep sizes of 1.0 × 10 −8 s and 5.0 × 10 −9 s. Various gases were investigated with Hydrogen being the primary working fluid with pressure ratios ranging from 10 to 100. This paper will focus primarily on the Hydrogen jets for pressure ratios of 10, 20, and 70. Numerical results were validated from both experimental results and higher fidelity Large Eddy Simulation results specifically analyzing the jet formation. Error between Mach disk height, Mach disk width, and Prandtl-Meyer expansion fan angles of the jet for pressure ratios of 10 and 70 were kept below 5%. The peak stagnation pressures at the center of the far wall for pressure ratios of 10, 20, and 70 were observed to be 86,843 Pa, 127,786 Pa, and 315,843 Pa, respectively. The predicted peak pressures show a linear relationship with respect to the initial pressure ratio existing between the high pressure and low pressure regions when the ratios are bounded between 10 and 70.

A rotary compressor relies on an eccentric rolling piston, which rotates at high speed, to compress gas in the compression chamber. The oil in the rotary compressor is used for lubricating the bearing and sealing the clearance of sliding parts. However, the oil can exhaust from the rotary compressor by the refrigerant flow and reduce the reliability of the compressor as a result. Thus, studying the behavior of oil droplets distribution in a rotary compressor is a major challenge for manufacturers who rely on CFD tools to predict the multiphase flow. By modifying a rotary compressor, the oil behavior inside the cylinder is observed and recorded by a high-speed imaging system. In the current phase, multiple targeted locations, including the space between the bearing housing and the stator, and the space above the stator are measured in different conditions. The number, size, velocity, and morphology of oil droplets are analyzed based on multiple snapshots. The result can assist designers in improving the CFD analysis of compressors and ultimately reducing the oil discharge rate (ODR).