The on-wall structures in an attachment-line flow are discussed based on an exact solution of the Navier–Stokes (NS) equations, focusing on distinct features of skin friction, surface pressure, and boundary enstrophy flux (BEF) and intrinsic connections between them.

Two-dimensional flow over bluff bodies is studied in the unsteady laminar flow regime using numerical simulations. In previous investigations, lift and drag forces have been studied over different cross section shapes like circles, squares and ellipses. We aim to extend the previous research by studying the variation of hydrodynamic forces as the shape of the body changes from a circular cylinder to a more streamlined or a bluffer body. The different body shapes are created by modifying the downstream circular arc of a circular cylinder into an ellipse, hence elongating or compressing the rear part of the body. The precise geometry of the body is quantified by defining a shape factor. Two distinct ranges of shape factors with fundamentally different behavior of lift and drag are identified. The geometry constituting the limit is where the rear part ellipse has a semi minor axis of half the radius of the original circle, independent of the Reynolds number. On the other hand, the vortex shedding frequency decreases linearly over the whole range of shape factors. Furthermore, the variation of the forces and frequency with Reynolds number, and how the relations vary with shape factor are reported.

This article reports flow behaviour around a suspended cube obtained using three dimensional lattice Boltzmann method (LBM) based simulations. The Reynolds number (Re) range covered is from 84 to 770. Four different flow regimes are noted based on the flow structure in this range of Re: steady axisymmetric (84 < Re < 200), steady nonaxisymmetric (215 < Re < 250), unsteady nonaxisymmetric in one plane and axisymmetric in the other plane (276 < Re < 300), and unsteady non-axisymmetric in streamwise orthogonal planes (339 < Re < 770). Recirculation length and drag coefficient follow inverse trend in the steady flow regime. The unsteady flow regime shows hair pin vortices for Re < 300 and then it becomes structureless. The nature of force coefficients have been examined at various Reynolds numbers. Temporal behaviour of force coefficients are presented along with phase dependence of side force coefficients. The drag coefficient decreases with increase in Reynolds number in the steady flow regime and the side force coefficients are in phase. Drag coefficients are compared with established correlations for flow around a cube and a sphere. The side force coefficients are perfectly correlated at Re = 215 and they are anti correlated at Re = 250. At higher Reynolds numbers, side force coefficients are highly uncorrelated. This work adds to the flow dynamics reported at low and moderate Re and extends it to unsteady regime at higher Re.

This paper presents an experimental investigation of the pressure drop through valves in vertical upward flows. Experiments were carried out using a 1¼" (DN 32) ball and a gate valve. Five opening areas have been investigated from fully open to the nearly fully closed valve, using air with a superficial velocity of (0-3.5 m/s) and water (0.05-0.91 m/s). These ranges cover single-phase and the bubbly, slug and churn two-phase flow regimes. It was found that for the single-phase flow experiments, the valve coefficient increases with the valve opening and is the same, in both valves, for the openings smaller than 40%. The single-phase pressure drop increases with the liquid flowrate and decreases with the opening area. The two-phase flow pressure drop was found considerably increased by reducing the opening area for both valves. It reaches its maximum values at 20% opening for the ball valve and 19% opening for the gate valve. It was also inferred that at fully opening condition, the two-phase flow multiplier, for both valves, has been found close to unity for most of the tested flow conditions. For the 40 and 20 % valve openings the two-phase multiplier decreases in the power-law with liquid holdup for the studied flow conditions. Models proposed originally for evaluating the pressure drop through an orifice in single-phase and two-phase flows were also applied and assessed in the present experimental data.

Applying an appropriate roughness model is one of the most effective ways to improve the accuracy of the numerical simulation for centrifugal compressors, and it is also the key to the study of surface deterioration such as compressor fouling and corrosion. The basis for using the roughness model is to determine the conversion relation between physical roughness and equivalent roughness. In this paper, numerical and experimental studies on the roughness conversion relation for the centrifugal compressors were performed by means of a unique self-developed "pseudo front stage" device. An equivalent roughness conversion coefficient of 8.5 was determined on the basis of fitting performance of the U-bend and return channel model. The validity of the conversion coefficient in improving the performance prediction accuracy of the centrifugal compressors was verified by three centrifugal compressor models. The concept of weighted average roughness was proposed to quantify the roughness sensitivity and roughness level of the centrifugal compressor. Quantitative research on the roughness effect of centrifugal compressors indicated that the peak efficiency degradation due to the roughness effect in the compressors was linearly associated with the logarithm of the relative roughness when the impeller of the centrifugal compressor was in the hydraulically rough regime. Doubling or halving the roughness value of all components of a centrifugal compressor in the hydraulically rough flow regime will result in a change in the peak efficiency of the compressor by about 1.72%.

The present paper reports a numerical study of fully-developed turbulent flow over a flat plate with a step change from a smooth to a rough surface. The Reynolds number based on momentum thickness for the smooth flow was ?Re?_?=5950. The focus of the study was to investigate the capability of the Reynolds Averaged Navier Stokes (RANS) equations to predict the internal boundary layer (IBL) created by the flow configuration. The numerical solution used the two-layer k-e model of [1] to implement the effects of surface roughness on the turbulence and mean flow fields via the use of a hydrodynamic roughness length y_0. The prediction for the mean velocity field revealed a development zone immediately downstream of the step in which the mean velocity profile included a lower region affected by the surface roughness below and an upper region with the characteristics of the smooth-wall boundary layer above. In this zone, both the turbulence kinetic energy and Reynolds shear stress profiles were characterized by a significant reduction in magnitude in the outer region of the flow that is unaffected by the rough surface. The turbulence kinetic energy profile was used to estimate the thickness of the IBL, and the resulting growth rate closely matched the experimental results. As such, the IBL is a promising test case for assessing the ability of RANS models to predict the discrete roughness configurations often encountered in industrial and environmental applications.

A previously presented overset mesh enabled hybridizable discontinuous Galerkin (HDG) finite element method is extended in this work to an isentropic compressible (pseudo-compressible) fluid. This formulation is a first-principles approach, and is complementary to the augmented Lagrangian approach that was utilized in previous HDG incompressible Navier-Stokes formulations which eliminate the global pressure field. \hl{This is the first original presentation combining overset meshes, HDG, and fluid flow, specifically isentropic flow for low mach number applications.} Verification of the code implementation of the proposed overset-HDG formulation is performed via the method of manufactured solutions on a successively refined overset mesh configuration containing five meshes, and for order $k=1,\ldots,4$ Lagrange polynomial elements in both two- and three-dimensions. Optimal order convergence, $k+1$, can be observed in all fields for both the two- and three-dimensional simulations\hl{, for each mesh}. A two-dimensional benchmark problem is also presented, to enable code-to-code comparison as a preliminary validation exercise.

Tip clearance is a crucial aspect of turbomachines in terms of aerodynamic and thermal performance. A gap between the blade tip surface and the stationary casing must be maintained to allow the relative motion of the blade. The leakage flow through the tip gap measurably reduces turbine performance and causes high thermal loads near the blade tip region. Several studies focused on the tip leakage flow to clarify the flow-physics in the past. The "squealer" design is one of the most common designs to reduce the adverse effects of tip leakage flow. In this paper, a genetic-algorithm-based optimization approach was applied to the conventional squeal tip design to enhance aerothermal performance. A multi-objective optimization method integrated with a meta-model was utilized to determine the optimum squealer geometry. Squealer height and width represent the design parameters which are aimed to be optimized. The objective functions for the genetic-algorithm-based optimization are the total pressure loss coefficient and Nusselt number calculated over the blade tip surface. The initial database is then enlarged iteratively using a coarse-to-fine approach to improve the prediction capability of the meta-models used. The procedure ends once the prediction errors are smaller than a prescribed level. The present study indicates that squealer height and width have complex effects on the aerothermal performance, and optimization study allows to determine the optimum squealer dimensions.

A semi-analytical solution for the concentration of photosensitive suspension is developed in a hemispherical droplet illuminated with UV laser. A biharmonic equation in stream function is analytically solved using toroidal coordinates, which is used to solve the transport equation for concentration. Flow pattern and photosensitive material concentration are affected by the peak location of the UV light intensity, which corresponds to the surface tension profile. When the laser beam is moved from the droplet center to its edge, a rotationally symmetric flow pattern changes from a single counter clockwise circulation to a circulation pair and finally to a single clockwise circulation. This modulation in the orientation of circulation modifies the concentration distribution of the photosensitive material. The distribution depends on both diffusion from the droplet surface as well as Marangoni convection. The region beneath the droplet surface away from the UV light intensity peak has low concentration, while the region near the downward dividing streamline has the highest concentration. When the UV light peak reaches the droplet edge, the concentration is high everywhere in the droplet.

The parasitic drag losses incurred by wet clutches, used in transmission systems, can significantly affect vehicular powertrain efficiency. This paper presents a novel implicit solution for hydrodynamic parasitic drag losses of disengaged clutches. These are generated by conjunctional friction, taking into account lubricant film separation during co-directional and counter-directional disc pair rotations. Lubricant film rupture is considered through application of incipient reverse flow boundary condition, which is representative of lubricant film separation. The results point to the operating conditions at which significant power losses occur. In particular, the time efficient model is able to represent the small losses incurred during co-directional rotation of disc pairs.

A series of experiments were conducted to investigate the effect of the inclination angle of the cylinders on the wake flow characteristics for flow past two and three side-by-side inclined cylinders using the particle image velocimetry (PIV). Depending on the inclination angles, purely deflected gap flow, no-deflection gap flow, and flip-flop gap flow patterns are identified for both two and three cylinder cases. In both two and three cylinder cases, the flows through the gaps are found to be in purely deflected flow pattern at small inclination angles and flip-flop pattern at large inclination angles. For the three-cylinder case with flip-flop gap flow pattern, gap flows are predominantly in the outward deflection pattern (toward the two side cylinders) and are occasionally deflected inward (toward the middle cylinder). The gap flow deflection angles for all the tested inclination angles of the cylinders are quantified through statistical analysis, in addition to identifying the flow patterns. The deflection angle is found to decrease with increasing inclination angle for both two- and three-cylinder cases, and the outward deflection angle for the three cylinder cases is greater than the deflection angle of the two-cylinder case. The probability density distributions of the deflection angles approximately follow normal distribution. In the two-cylinder case, the mean flow field is asymmetrical about the x-axis when the possibility of the flow deflection toward one side of the gap is greater than that toward the other side.

In this study, natural convection of non-Newtonian power-law fluids around an array of elliptic cylinders has been investigated numerically. The governing equations have been solved using an in-house CFD code based on the well-known finite volume method. It is assumed that the flow and temperature fields are laminar, steady, and two-dimensional. Furthermore, due to the low-temperature difference between the tube walls and the surrounding fluid, the changes in the physical properties of the fluids are neglected. The numerical results are validated against the available experimental and numerical results. The results show that by increasing the non-Newtonian fluid power-law index, the ratio of average Nusselt number of the i-th cylinder to the average Nusselt number of a single cylinder under identical thermal conditions decreases. Moreover, it is found that the increase in the ratio of the distance between elliptic centers to the elliptic vertical diameter increases the ratio of the average Nusselt number of i-th cylinder to the average Nusselt number for a single cylinder. Finally, a mathematical expression is given for the array averaged Nusselt number.

Phase separation has been proven to be beneficial to air-cooled parallel flow microchannel condensers for air conditioning systems. The inlet to the condenser with phase separation is at the middle of the condenser height. After the first pass, in the vertical second header of the condenser, refrigerant vapor separates from liquid mainly due to gravitational effects. In ideal case vapor should go to the top exit and liquid to the bottom exit, resulting in increased heat transfer. Due to interaction between vapor and liquid, separation is not perfect, expressed through the separation efficiency. This paper presents a parametric study of phase separation efficiency in the intermediate headers, with the target to improve separation efficiency. Header prototypes which have two exits are made with transparent PVC to simulate the real header and provide visual access. Using R-134a as a baseline, the measurement of separation efficiency and its general trend will be shown first. The experimental results are compared to the simulation of a mechanistic model based on flow regime and force balance analysis. Inlet mass flux in simulation is controlled at 87 - 311 kg·m-2·s-1 and inlet quality at 0.05 - 0.25. The observed flow patterns in header are compared with the modeling results. Then, the header diameter is increased, which effectively improves the separation efficiency due to reduction of vapor velocity in header. Finally, R245fa and R32 are modeled in comparison with R-134a to discuss the effect of fluid properties on separation efficiency.

In this study, air-water flow in a downward sloping pipe subsequent to the entrapping of an air pocket is investigated both numerically and experimentally. A transient, two-dimensional computational fluid dynamics model is applied to study the different possible flow regimes and their associated phenomena. The numerical model is based on the Reynolds averaged Navier Stokes (RANS) equations and the Volume of Fluid (VOF) method. Both numerical and experimental investigations provide visualization for the hydraulic jump, the blowback regime, and the full gas transport regime. The numerical results predict that the flow structure in the pipe downstream the toe of the hydraulic jump is subdivided into three distinct regions including the jet layer, the shear zone and the circulation region, which agrees qualitatively with the previous investigations of the hydraulic jump characteristics in open channel flow. Numerical results are in reasonable agreement with the experimental measurements of the circulation length and the hydraulic jump head loss.

Detailed laser Doppler velocimetry measurements of profiles of mean axial and tangential velocities and their corresponding r.m.s components in confined turbulent swirling pipe flow with and without the presence of a conical bluff body have been conducted. The working fluid was water and the value of Reynolds number based on the bulk velocity inside the pipe was 40,850. The main focus of the study was to document the interaction between turbulent swirling flow inside a pipe with the presence of a conical bluff body. The flow structure was found in all cases to be composed of a forced-free vortex. The swirl decay was exponential. The presence of the bluff body's effect on the flow was found to be limited to a short region upstream, so that the effect on the flow structure and strength of swirl upstream could be considered negligible. The flow around the bluff body adopted a different structure where the mean axial and tangential velocities tended to assume uniform profiles with turbulence activity limited to the region close to the surface of the bluff body.

Inducers show generally a positive influence on the performance of centrifugal pumps in the two-phase regime, since they produce more uniform mixtures and increase the pressure before the impeller. However, the effect is much more pronounced in part-load compared to overload conditions. In the present study, the air-water two-phase flow behaviour in a pump inducer was numerically investigated. The main objectives were to clarify the effect of the inducer, the effective operating range, and to examine flow mixing. Several flow conditions were studied, covering part-load, optimal, and overload pumping conditions, together with different, relevant gas volume fractions (1%, 3%, and 5%). The simulations were performed using a transient set-up and a moving-mesh approach. Two-phase air-water interactions were modeled by the Volume of Fluid (VOF) method. After checking the proper discretization in space and time, the model was validated against experimental results, revealing excellent agreement. The numerical analysis was able to explain the different effects of inducers in part-load and overload conditions. Under overload conditions, the flow separates, leading to the generation of axial vortices and to a negative pressure change across the inducer; additionally, the residence time is reduced, hindering mixing. These vortices are intensified as the gas volume fraction increases, reducing further the pressure downstream of the inducer. This is the reason why inducers can mainly be used in part-load and near optimal conditions in order to improve pumping of two-phase flows.

At present, pumped-storage power technology is the only available and effective way for the load balancing and energy storage in the grid network scale. During the frequent switch back and forth conditions, there are severe pressure pulsation and cavitation in pump-turbines. However, their generation mechanism has not been determined yet. This work contributes to the numerical simulation of the transient behaviors in a prototype pump-turbine during the load rejection process with special emphasis on cavitation effect. In this study, the two-dimensional dynamic remesh and variable speed slide mesh methodologies were employed to perform the simulation of the transient single-phase flow and cavitation flow in a pump-turbine. The simulation results of single-phase flow and cavitation flow were both consistent with the experimental data except in local regions based on the experimental validation of prototype tests. However, the numerical results considering cavitation effects have a better behavior than those of single-phase flow in the predictions of pressure pulsation and rotational speed. Then, the cavitation flow simulation results were analyzed deeply, especially in pressure pulsation and cavitation flow field. Analysis revealed that three typical complex frequency components of pressure were captured in the cavitation flow, which significantly affect the axial hydraulic thrust on the runner. And it is validated that they are primarily induced by the cavity collapse near the trailing edges of the runner blades in reverse pump mode and the interaction between cavitation and vortex rope in draft-tube in turbine mode.

Phase separation has been proven to increase performance of condensers of energy conversion systems (in vapor compression systems). Instead of conventional design, the inlet to a microchannel condenser prototype is in the middle of the height. After the first pass, in the vertical second header of the condenser, vapor phase separates from liquid phase due to gravity and sometimes other effects. In ideal case vapor should go to the top and liquid to the bottom, resulting in increased heat transfer. Due to interaction between vapor and liquid, separation is not perfect, expressed through the separation efficiency. A mechanistic model presented in the paper is built to predict the phase separation efficiency based on force balance analysis for the liquid phase and correlations for the two-phase pressure drop. For the force balance criteria, liquid phase is divided into droplets and film and treated separately. Initial agreement with experiment is achieved to be within ±15%. For a second header at one inlet condition of the flow, the liquid separation efficiency is a function of the vapor separation efficiency. Liquid separation efficiency decreases with increasing inlet mass flux.

Time-average velocity distribution in steady and uniform channel flows is important for fundamental research and practical application as it is always three-dimensional (3D), regardless of channel geometry. However, its determination has predominantly been carried out by using complex numerical software, even for the simplest geometry such as rectangular channels. The log-law was developed initially for circular pipe flows, where a single shear velocity is used to normalize the velocity (u + ) and its distance (y + ). Tracy and Lester found that the performance of the log-law can be extended to express velocity profiles in rectangular channels when the global shear velocities (gRS) 0.5 and (ghS) 0.5 are used to normalize the measured velocity u and its distance y. This study extends this discovery from the channel central line to the corner regions, and its general form of log-law was found to be valid even in trapezoidal or triangular open channels or closed ducts. This modified log-law can produce good agreement with the measured velocity with an average error of less than 5%. Therefore, this study provides a simple and reliable tool for engineers and researchers to estimate the velocity contours in straight and smooth channel flows.

For seamless integration of growing electricity production from intermittent renewable energy sources, Francis turbines are under increasing demand to extend their operating range. This requires Francis turbines to operate under off-design conditions, where various types of cavitation are induced. At deep part load condition, an interblade cavitation vortex observed in a runner blade channel is a typical cavitation phenomenon causing pressure fluctuations and erosion, which prevent a reliable operation of Francis turbines at deep part load. The underlying mechanisms of its development are, however, yet to be understood. In an objective of revealing its developing mechanisms, the present study is aimed at investigating flow structures inside runner blade channels by comparison of three different operating conditions at deep part load using numerical simulation results. After demonstrating interblade vortex structures are successfully simulated by performed computations, it is shown that flow inside the runner at deep part load operation is characterized by a remarkable development of recirculating flow on the hub near the runner outlet. This recirculating flow is concluded to be closely associated with interblade vortex development. The skin-friction analyses applied to the hub identify the flow separation caused by a nonuniform distribution of flow, which describes the underlying physical mechanism of interblade vortex development. Investigations are further extended to include a quantitative evaluation of the specific energy loss induced by interblade vortex development. The integration of energy flux defined by rothalpy evidences the energy loss due to the presence of strong interblade vortex structures.