Research Papers: Flows in Complex Systems

J. Fluids Eng. 2011;133(6):061101-061101-10. doi:10.1115/1.4004257.

A new type of cavitation triggered by boundary-layer turbulence-production has been identified from the turbines of the Three Gorges Power Station. Our previous studies point out that the addition of the ‘guide-plate’ is responsible for the increase of the turbulence level (in particular the low-frequency mode) in the free-stream which in turn promotes the boundary-layer transition through receptivity mechanism. A new vortex structure identified from our numerical studies is revealed in this paper. It explains well how the free-stream turbulence is influenced by this new vortex structure which is created by the addition of this ‘guide-plate’. These results support one of the claims in the hypothesis proposed in our previous studies. The other claims in the hypothesis about the triggering mechanism of this type of cavitation in the boundary-layer are being verified through our on-going (numerical and experimental) studies. The numerical studies reported here are performed by using CFD simulations of the entire flow passage of the prototype turbine.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061102-061102-12. doi:10.1115/1.4004084.

For inverted wings in close ground proximity, such as race car configurations, the aerodynamic ground effect can produce local velocities significantly greater than the freestream and the effects of compressibility may occur sooner than would be expected for a wing that is not close to a ground plane. A three-dimensional computational fluid dynamics study was conducted, involving a modified NASA GA(W)-2 LS [1]-0413 MOD inverted wing with an endplate, to investigate the onset and significance of compressibility for low subsonic Mach numbers. With the wing angle of incidence fixed, Mach numbers from 0.088 to 0.4 were investigated, at ground clearances ranging from infinite (free flight) to a height-to-chord clearance of 0.067. The freestream Mach number at which flow compressibility significantly affects the predicted aerodynamic coefficients was identified to be as low as 0.15. Beyond this point, as the compressible flow conditions around the wing result in changed pressure distribution and separation behavior, treating the flow as incompressible becomes inappropriate and leads to consistent underprediction of lift and drag. The influence on primary vortex behavior of density changes around the wing was found to be relatively inconsequential even at the higher end of the Mach scale investigated. By a freestream Mach number of 0.4 and at low clearances, local supersonic flow regions were established close to the suction peak of the lower wing surface in compressible simulations; the formation of a normal shock wave between the wing and the ground was shown to result in significant increases in separation and therefore overall drag, as well as a distinct loss of downforce.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061103-061103-8. doi:10.1115/1.4004085.

Liquid microjet arrays have received a lot of research attention in recent years due to its high heat flux cooling capability. The microjets are generated from a jet head cavity with a liquid inlet port on one wall and an array of micro-orifices on another wall. An important, yet relatively less studied aspect of the topic is the pressure (also frequently referred to as the pressure drop) necessary to generate the jets and maintain certain jet velocity. In this study we investigate the pressure drop for17 different array patterns of liquid jet issuing in a surrounding gas (air) medium, i.e., a free surface liquid jet. The number of jets varies from 1 to 126, while the jet diameter ranges from 99 to 208 μm. The current results show more than 200% deviation from the existing correlations in the literature. Through a systematic experimental study we identify the functional dependence of pressure drop on the various geometric parameters. The results uncover the reasons behind the widespread disagreement between the current data and the existing correlations. Pressure drop shows a weak, nonlinear dependence on the orifice wall thickness, compared to the linear dependence used in the existing correlations. Furthermore, the depth of the jet head cavity is shown to be an important parameter dictating pressure drop, unlike the previous studies that inherently assume the cavity to be an infinite reservoir. A new dimensionless pressure drop parameter is proposed and its variation with the jet Reynolds number is correlated. The new correlation predicts all the experimental data within a ± 10% range.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061104-061104-16. doi:10.1115/1.4004215.

The flow in the stern region of a fully appended hull is analyzed by both computational and experimental fluid dynamics. The study is focused on the velocity field induced by the rotating propellers. Measurements have been performed by laser Doppler velocimetry (LDV) on the vertical midplane of the rudder and in two transversal planes behind the propeller and behind the rudder. In the numerical approach, the real geometry of the propeller has been considered. To this purpose, a dynamic overlapping grids method has been used, which is implemented in the unsteady Reynolds averaged Navier–Stokes equations ( URANSE) solver developed at INSEAN. Uncertainty analysis has been performed on both data sets and the results from the two approaches are compared. The agreement between the two data sets is found to be good, the deviation in the velocity and vorticity fields lying within the evaluated uncertainties. Numerical data allowed the analysis of further details of the flow that could not be measured, like load conditions of the single blades, interaction of the propeller wake with the rudder, and pressure oscillations induced by the propeller on the vault of the stern.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061105-061105-9. doi:10.1115/1.4004080.

This paper presents a model to describe the behavior of sloshing in a general tank with a uniform fluid depth. An equivalent linearized mechanical model is developed for a tuned liquid damper (TLD) with arbitrary tank geometry. The finite element method is employed to determine the mode shapes of the sloshing fluid. In general, the mode shapes of arbitrary tanks will have response components in the x- and y-directions. The mode shapes enable the generalized properties of the sloshing fluid to be determined; these properties are subsequently used to establish equivalent mechanical properties. The nonlinear damping of slat-type damping screens is linearized, permitting it to be included in the model as amplitude-dependent viscous damping. The proposed model is in excellent agreement with existing linearized models for the special cases of rectangular and circular tanks. Sinusoidal shake table tests are conducted on tanks with chamfers placed in selected corners. In the literature, no experimental testing has focused on tanks of arbitrary shape with a constant fluid depth. The proposed model is in good agreement with the experimental results for the mode dominated by motion in the direction of excitation. However, the model is found to underestimate the response of the mode which is dominated by motion perpendicular to the excitation direction. The linearized mechanical model developed can serve as a useful preliminary TLD design tool.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2011;133(6):061201-061201-7. doi:10.1115/1.4004300.

In this work, the homotopy analysis method is applied to enable discussion of the three-dimensional shrinking flow of Jeffrey fluid in a rotating system. The fluid is electrically conducting in the presence of a uniform applied magnetic field, and the induced magnetic field is neglected. The similarity transformations reduce the nonlinear partial differential equations into ordinary differential equations. The convergence of the obtained solutions is checked. Graphs are plotted and discussed for various parameters of interest.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061202-061202-8. doi:10.1115/1.4004086.

The flow field in a cylindrical container driven by a flat bladed impeller was investigated using particle image velocimetry (PIV). Three Reynolds numbers (0.02, 8, 108) were investigated for different impeller locations within the cylinder. The results showed that vortices were formed at the tips of the blades and rotated with the blades. As the blades were placed closer to the wall the vortices interacted with the induced boundary layer on the wall to enhance both regions of vorticity. Finite time lyapunov exponents (FTLE) were used to determine the lagrangian coherent structure (LCS) fields for the flow. These structures highlighted the regions where mixing occurred as well as barriers to fluid transport. Mixing was estimated using zero mass particles convected by numeric integration of the experimentally derived velocity fields. The mixing data confirmed the location of high mixing regions and barriers shown by the LCS analysis. The results indicated that mixing was enhanced within the region described by the blade motion as the blade was positioned closed to the cylinder wall. The mixing average within the entire tank was found to be largely independent of the blade location and flow Reynolds number.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061203-061203-10. doi:10.1115/1.4004093.

The objective of the present research is to develop new fundamental knowledge of the entropy generation process in laminar flow with significant fluctuations (called pre-transition) and during transition prematurely induced by strong freestream turbulence (bypass transition). Results of direct numerical simulations are employed. In the pre-transitional boundary layer, the perturbations by the streaky structures modify the mean velocity profile and induce a “quasi-turbulent” contribution to indirect dissipation. Application of classical laminar theory leads to underprediction of the entropy generated. In the transition region the pointwise entropy generation rate (S′′′)+ initially increases near the wall and then decreases to correspond to the distribution predicted for a fully-turbulent boundary layer as the flow progresses downstream. In contrast to a developed turbulent flow, the term for turbulent convection in the turbulence kinetic energy balance is significant and can play an important role in some regions of the transitioning boundary layer. More turbulent energy is produced than dissipated and the excess is convected downstream as the boundary layer grows. Since it is difficult to measure and predict true turbulent dissipation rates (and hence, entropy generation rates) exactly other than by expensive direct numerical simulations, a motivation for this research is to evaluate approximate methods for possible use in experiments and design. These new results demonstrate that an approximate technique, used by many investigators, overestimates the dissipation coefficient Cd by up to seventeen per cent. For better predictions and measurements, an integral approach accounting for the important turbulent energy flux is proposed and validated for the case studied.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061204-061204-13. doi:10.1115/1.4004090.

In this study, we are interested in the hydrodynamics of impinging plane jets. Plane jets are widely used in ambience separation in HVAC, fire safety, food process engineering, cooling of electronic components etc. Despite their important industrial applications, plane jets have not been studied as extensively as axisymmetric jets. Plane jets exhibit different kind of instabilities stemming from either streamlines with strong curvature in the impingement region or inflection points in the transverse profile of the streamwise component velocity in the lateral mixing layers. Previous works in the GEPEA laboratory were performed on these flows. These works and the majority of the studies reported in the literature deal with turbulent air jets in various configurations. Very little studies have been done on water impinging jets. Taking into account the fact that the viscosity of water is smaller than air, at the same Reynolds number, it is easier to detect phenomena such as vortices. Phenomena can be observed at lower velocities making it possible to record signals with standard frequency bandwidths. This makes it easier also to do a Lagrangian tracking of vortices. We specially focused our study on the impinging zone of the jet. The dynamics of the impinging zone has not formed the subject of numerous studies. There were no studies that characterize the vortices at the impinging region of water jets in terms of size, centre position, vortex intensity, convection velocities, eccentricity, statistical distribution and turbulent length and time scales. Consequently, a confined water plane jet impinging a flat plate was studied using standard and high speed PIV (Particle Image Velocimetry). We used POD decomposition for filtering PIV data. Then, we applied the λ2 criterion to the recorded velocity fields to detect and characterize the vortices at the impingement. A statistical analysis was then performed. Turbulent length scales, time scales and convection velocities of eddies occurring at the impingement were determined using two point space time correlations. The obtained results were correlated to the dynamics and geometric properties of the jet. A wide range of Reynolds numbers is considered: 3000, 6000, 11000 and 16000. The corresponding results are presented in this paper.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2011;133(6):061205-061205-8. doi:10.1115/1.4004150.

Scalar, eddy viscosity models are widely used for predicting engineering turbulent flows. System rotation, or streamline curvature, can enhance or reduce the intensity of turbulence. Methods to incorporate the effects of rotation and streamline curvature consist of introducing parametric variation of model coefficients, such that either the growth rate of turbulent energy is altered; or such that the equilibrium solution bifurcates from healthy to decaying solution branches. For general use, parameters must be developed in coordinate invariant forms. Effects of rotation and of curvature can be unified by introducing the convective derivative of the rate of strain eigenvectors as their measure.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2011;133(6):061301-061301-7. doi:10.1115/1.4004022.

The relationship between the thermodynamic effect and subsynchronous rotating cavitation was investigated with a focus on cavity fluctuations. Experiments on a three-bladed inducer were conducted with liquid nitrogen at different temperatures (74, 78, and 83 K) to confirm the dependence of the thermodynamic effects. Subsynchronous rotating cavitation appeared at lower cavitation numbers in liquid nitrogen at 74 K, the same as in cold water. In contrast, in liquid nitrogen at 83 K the occurrence of subsynchronous rotating cavitation was suppressed because of the increase of the thermodynamic effect due to the rising temperature. Furthermore, unevenness of cavity length under synchronous rotating cavitation at 83 K was also decreased by the thermodynamic effect. However, surge mode oscillation occurred simultaneously under this weakened synchronous rotating cavitation. Cavity lengths on the blades oscillated with the same phase and maintained the uneven cavity pattern. It was inferred that the thermodynamic effect weakened peripheral cavitation instability, i.e., synchronous rotating cavitation, and thus axial cavitation instability, i.e., surge mode oscillation, was easily induced due to the synchronization of the cavity fluctuation with an acoustic resonance in the present experimental inlet-pipe system.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Fluids Eng. 2011;133(6):064501-064501-3. doi:10.1115/1.4004216.

The present state-of-the-art ocean models use an eddy viscosity that depends on structure parameter (Cμ ). In this paper we use a Reynolds stress anisotropy based formulation for the eddy viscosity because in addition to the value of turbulent kinetic energy, it also depends on the degree of anisotropy. The formulation is incorporated into the General Ocean Turbulence Model (GOTM) and simulated using the famous test case of Ocean Weather Station (OWS) Papa experiment. Even if there is not much of an improvement in terms of results with this model, it can be very easily incorporated into the ocean models removing cumbersome equations for structure parameters.

Commentary by Dr. Valentin Fuster

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