A new partially averaged Navier–Stokes (PANS) closure is derived based on the k − k L (KSKL) model. The aim of this new model is to incorporate the desirable features of the KSKL model, compared to the k − ω shear stress transport model, into the PANS framework. These features include reduced eddy-viscosity levels, a lower dependency on the cell height at the wall, well-defined boundary conditions, and improved iterative convergence. As well as the new model derivation, the paper demonstrates that these desirable features are indeed maintained, for a range of modeled-to-total turbulence kinetic energy ratios (f k ), and even for multiphase flow.

The primary cause of mechanical-related fish injury and mortality in turbomachinery is blade strike. Fish contained in the flow may strike with the rotor blades and the fixed diffuser vanes, the latter being a non-negligible factor causing fish damage in the pump system. In this study, an experiment-based correlation of fish mutilation ratio acts as critical strike velocity. The relation between strike damage in a vaned diffuser and the theoretical pump head is presented as a function of specific speed. As an example, a vaned diffuser is designed for a single-bladed, mixed-flow impeller with the purpose of improving fish friendliness. This pump can be scaled to operate with a head up to 14 m at peak efficiency, without fish damage in the diffuser. Subsequently, experiments are conducted to show the retained pump performance as well as the great improvement of fish friendliness.

Cyclone separators are an integral part of many industrial processes. A good understanding of the flow features is paramount to efficiently use them. The turbulent fluid flow characteristics are modeled using unsteady Reynolds-averaged Navier–Stokes (URANS), large eddy simulations (LES), and hybrid LES/Reynolds–averaged Navier–Stokes (RANS) turbulent models. The hybrid LES/RANS approaches, namely, detached eddy simulation (DES), delayed detached eddy simulation (DDES), and improved delayed detached eddy simulation (IDDES) based on the k − ω SST RANS approaches are explored. The study is carried out for three different inlet velocities (v = 8, 16.1, and 32 m/s). The results from hybrid LES/RANS models are shown to be in good agreement with the experimental data available in the literature. Reduction in computational time and mesh size are the two main benefits of using hybrid LES/RANS models over the traditional LES methods. The Reynolds stresses are observed in order to understand the redistribution of turbulent energy in the flow field. The velocity profiles and vorticity quantities are explored to obtain a better understanding of the behavior of fluid flow in cyclone separators.

Computational fluid dynamics (CFD) has become a popular tool compared to experimental measurement for thermal management in data centers. However, it is very time-consuming and resource-intensive when used to model large-scale data centers, and often unrealistic for real-time thermal analyses. In addition, it is prohibitive to use CFD for the optimization process where thousands of designs need to be generated. Floor tile airflow distribution control is a key technique for maintaining a sufficient cold air delivery to variable thermal loadings of server cabinets. Regular practice of deploying a set of identical floor tiles may not result in the best solution for airflow uniformity through tiles. In this paper, an optimization procedure based on response surface methodology (RSM) is proposed to find the best arrangement of mixed-porosity floor tiles for different targeted tile airflow distributions. Fast-approximation RSM based on radial basis function (RBF) allows thousands of designs to be generated for the optimization process which uses a genetic algorithm as its main solver. The method shows proven success in maximizing floor tile airflow uniformity, and in the inverse design optimization where various tile airflow distribution topologies, i.e., linear, parabolic, and sinusoidal shapes are targeted. For the considered data center and aisle configuration, the improvement over the all-uniform-tile design is 55% in terms of standard deviation to the average tile airflow rate, whereas 90%, 91%, and 94% in root-mean-square error (RMSE) for the linear, parabolic, and sinusoidal floor tile airflow distribution objectives, respectively.

Wiping jet impingement pressure is important in controlling the coating mass (thickness) and influencing the smoothness of the thin metallic coating produced in continuous galvanizing lines (CGLs). However, the fluctuation of the impingement pressure profile that directly impacts the coating smoothness has not been adequately understood. To study key features of the impingement pressure fluctuation, the instantaneous impingement pressure profiles obtained from large eddy simulations (LES) were analyzed using proper orthogonal decomposition (POD). Dominant fluctuation modes of pressure profiles can be differentiated based on the energy contents of the modes corresponding to different jet types, namely, mixing, nonmixing, and transitional mixing jet. The dominant modes of mixing jets in the wiping region contain comparable strength of all modes (flapping, pulsing, and out-of-phase multipulsing). Nonmixing jets do not show discernable fluctuation modes, and transitional mixing jets show pulsing and flapping modes only. Additionally, instantaneous maximum pressure gradients and their locations were determined from the reduced-order reconstructions of the pressure profiles. From the analysis, frequency spectra of the magnitude and location fluctuations of the maximum pressure gradients associated with each of the jet types can be clearly distinguished. This is the knowledge that may be helpful for CGL operators in the operation of wiping jets.

This present study analyzes the Reynolds stress anisotropy in the nonuniform sediment beds under the condition of no seepage and downward seepage flow. The results show the estimation of the deviation measure from the isotropic turbulence in view of Reynolds stress tensor for turbulent flow in the presence of seepage through the channel bed. The investigation presents the Lumley triangle for flow turbulence, Eigen values, and the invariant functions for the whole flow depth subjected to no seepage and seepage beds. The longitudinal profile of anisotropy tensor within the near-bed zone for seepage flow provides the higher anisotropic stream than those of no seepage flow, while the remaining (transverse and vertical) profiles of anisotropy tensor in the vicinity of bed for seepage flows provide lower anisotropic stream. The anisotropic invariant maps show the near bed anisotropy inclining to be a two-component isotropy subjected to no seepage and seepage flow. With the increase in vertical distance from bed surface that is close to the water surface, the data sets of anisotropic invariant maps for no seepage and seepage flows show a trend of one-component isotropy, while it has an affinity to develop a three-component isotropy in the vicinity of midzone of the flow depth. Invariant function data sets present a well two-component isotropy in the near bed region of flow and a quasi-three component isotropy in the outer region of flow for seepage flows as compared to no seepage flow.

Tomographic particle image velocimetry (Tomo-PIV) has become a standard tool for capturing a three-dimensional (3D) velocity fields in nonreacting flows. However, the diagnostic approach can become costly and challenging to implement when extended to applications which require high-speed cameras. This limitation has led to the use of fiber wound bundles to allow for multiple views to be captured on a single camera sensor. Additionally, employing this diagnostic approach on reacting flow-fields becomes more complex as the introduction of the flame causes additional luminosity and optical distortion which impacts the particle field reconstruction. This work seeks to validate and determine the limitations when utilizing a single sensor fiber-coupled approach for capturing Tomo-PIV data on a reacting flow-field. A premixed propane (C 3 H 8 ) and air Bunsen burner flame is utilized to examine if the single sensor approach can meet the parameters for acceptable reconstruction based on previous research. The resulting velocity fields are then compared to a traditional PIV measurement to assess the deviation of the single sensor approach from a standard velocimetry measurement approach. It is demonstrated that there is strong agreement between the velocity and vorticity for the average flow-fields; however, when comparing the Reynolds shear stresses, a significant deviation is revealed. The deviation is attributed to strong velocity fluctuations occurring within the instantaneous Tomo-PIV data, which creates a significant divergence between the measurement techniques on an instantaneous basis. This demonstrates that while the approach can obtain reliable velocity and vorticity statistics, there are significant limitations in calculating second-order turbulence statistics. Thus, revealing that there is a tradeoff between the ability to extract the full velocity gradient tensor and the extent of the turbulence-related analysis which can be reliably performed.

The objective of this paper is to observe and investigate the early evolution of the shock wave, induced by a nanosecond pulsed laser in still water. A numerical method is performed to calculate the propagation of the shock wave within 1 μ s, after optical breakdown, based on the Gilmore model and the Kirkwood–Bethe hypothesis. The input parameters of the numerical method include the laser pulse duration, the size of the plasma, and the maximally extended cavitation bubble, which are measured utilizing a high time-resolved shadowgraph system. The calculation results are verified by shock wave observation experiments at the cavitation bubble expansion stage. The relative errors of the radiuses and the velocity of the shock wave front reach the maximum value of 45% at 5 ns after breakdown and decrease to less than 20% within 20 ns. The high attenuation characteristics of the shock wave after the optical breakdown are predicted by the numerical method. The quick time and space evolution of the shock wave are carefully analyzed. The normalized shock wave width is found to be independent of the laser energy and duration, and the energy partition ratio is around 2.0 using the nanosecond pulsed laser.

Computational fluid dynamics (CFD) simulation of flow over a three-dimensional (3D) axisymmetric hill presents a unique set of challenges for turbulence modeling. The flow past the crest of the hill is characterized by boundary layer separation, complex vortical structures, and unsteady wake flow. As a result, traditional eddy-viscosity Reynolds-averaged Navier–Stokes (RANS) models have been found to perform poorly for this benchmark test case. Recent studies have focused on the use of large eddy simulation (LES) and hybrid RANS–LES (HRL) methods to improve accuracy. In this study, several different HRL models are investigated, and results from the different models are evaluated relative to each other, to an eddy-viscosity RANS model, and to previously documented high-fidelity LESs and experimental data. Results obtained from the simulations in terms of mean flow statistics, surface pressure distribution, and turbulence characteristics are presented and discussed in detail. Results indicate that eddy-viscosity based HRL models can improve predictions over comparable eddy-viscosity based RANS models such as the shear stress transport (SST) k– ω model used in this study, but only when the development of turbulent velocity fluctuations in the separated shear layer and recirculation region are well resolved.

Temporally developing direct numerical simulations (T-DNS) are performed for freestream turbulence induced bypass transition flow with zero-pressure gradient over a flat-plate boundary layer for a range of freestream turbulence intensities ( Tu ) of 1.4 % to 6 % . The objective is to understand the role of pressure–strain terms on bypass transition onset and to propose and validate a phenomenological hypothesis for the identification of a robust transition onset marker for use in transition-sensitive Reynolds-averaged Navier–Stokes (RANS) simulations. Results show that transition initiates at a location where the slow pressure–strain term becomes more dominant than the rapid term in the pretransitional boundary layer region. A simple transition onset marker based on one-point statistical quantities is derived from the scaling of the ratio of the slow and rapid pressure fluctuation source terms. The critical value of the marker is found to vary within a narrow range ( ± 2.8 % ) and satisfies previously identified criteria for a robust transition onset marker.

Reynolds-averaged Navier–Stokes (RANS) approach is used to simulate the steady-state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the baseline explicit algebraic Reynolds stress model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer are generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for cases LB and MB. For case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k − ω shear-stress transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the “simplest” case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

The swirling secondary flow in curved pipes is studied in three-space dimensions using a weakly compressible smoothed particle hydrodynamics (WCSPH) formulation coupled to new nonreflecting outflow boundary conditions. A large eddy simulation (LES) model for turbulence is benchmarked with existing experimental data. After validation of the present model against experimental results for a 90 deg pipe bend, a detailed numerical study aimed at reproducing experimental flow measurements for a wide range of Reynolds numbers has been performed for different pipe geometries, including U pipe bends, S-shaped pipes, and helically coiled pipes. In all cases, the SPH calculated behavior shows reasonably good agreement with the measurements across and downstream the bend in terms of streamwise velocity profiles and cross-sectional contours. Maximum mean-root-square deviations from the experimentally obtained profiles are always less than ∼ 1.8 %. This combined with the very good matching between the SPH and the experimental cross-sectional contours shows the uprising capabilities of the present scheme for handling engineering applications with streamline curvature, such as flows in bends and manifolds.

A predetermined flow pattern in a magnetorheological damper providing continuously variable resistance to flow is required for efficient damping of a given load. The required predetermined flow pattern rests on the a priori determination of the constitutive properties of the magnetorheological (MR) fluid determined to generate variable resistance to flow. The inverse problem of constructing the predetermined response of the damper with a specific displacement pattern of the piston in the damper for efficient damping of a given load is solved. The MR fluid in the damper is modeled as a Bingham phase change material with time-dependent yield stress offering continuously variable resistance to the flow in the piston to achieve the required specific displacement pattern. The governing equations are solved for any time history of the dimensionless yield stress of the fluid which in turn is determined from the imposed response of the damper. Analytical tools developed can be used in optimizing damper performance. The application of the method to resonance mitigation is illustrated.

A detailed numerical study of harmonically pitching airfoils of NACA00 series is presented here. Based on the analysis of the computational fluid dynamics (CFD) results, a hypothesis is made that a simple data model can capture the dynamics of the airfoils in pitch. The data model is based on the c l − α (lift coefficient–angle of attack) hysteresis loops that retain generic geometrical characteristics for a wide range of reduced frequency, k, encountered in flutter in transonic flows for all the NACA00 airfoils considered. The model was trained on a subset of the considered NACA00 airfoils and then tested on the remaining NACA00 set, for a subset of the reduced frequencies. The model predictions of the c l − α hysteresis loops for the test set are shown to be in excellent agreement with the CFD results for the range of k typical of transonic flutter. The data model offers a paradigm shift in the prediction of transonic flow dynamics of pitching airfoils and will guide the development of a new transfer function that will be incorporated in a new aeroelastic framework leading to an appropriate transonic flutter model for use in the development of future aircraft.

A compressible flow with wall friction has been predicted in a constant cross section duct by means of a barotropic modeling approach, and new analytical formulas have been proposed that also allow any possible heat transfer to the walls to be taken into account. A comparison between the distributions of the steady-state flow properties, pertaining to the new formulas, and those of a classic Fanno analysis has been performed. In order to better understand the limits of the polytropic approach in nearly chocked flow applications, a numerical code, which adopts a variable polytropic coefficient along the duct, has been developed. The steady-state numerical distributions along the pipe, obtained for either a viscous adiabatic or an inviscid diabatic flow by means of this approach, coincide with those of the Fanno and Rayleigh models for Mach numbers up to 1. A constant polytropic exponent can be adopted for a Fanno flow that is far from choking conditions, while it cannot be adopted for the simulation of a Rayleigh flow, even when the flow is not close to choking conditions. Finally, under the assumption of diabatic flows with wall friction, the polytropic approach, with a constant polytropic exponent, is shown to be able to accurately approximate cases in which no local maximum is present for the temperature along the duct. The Mach number value at the location where the local maximum temperature possibly occurs has been obtained by means of a new analytical formula.

The objective of this paper is to investigate the dynamic characteristics of transient cavitating flow over a twisted NACA0009 hydrofoil. The large eddy simulation (LES) approach is selected for the computation of fluid flow and the Zwart model is used for the mass transfer due to cavitation. Moreover, the skin-friction coefficient and boundary-vorticity flux (BVF) are used to study the flow separation. Numerical results show that the attached shear layer separates from the boundary layer and then squeezes to form the separation line under the obstruction of the reentrant jet. The analysis based on the terms of vorticity transport equation demonstrates that vortex stretching and vortex dilatation terms dominate the evolution of the multiscale vortex. Moreover, the secondary shedding induced by the side-entrant jet enhances the instability of partial cavities and the underlying mechanism is comprehensively revealed. Furthermore, the feature of the pressure fluctuation indicates that high pressure generated by the cavity collapse at the tail simultaneously propagates to the leading edge and downstream of the hydrofoil. This enhances the intensity of the reentrant jet and side-entrant jet, promoting occurrences of flow separation near the suction surface and cavity shedding to a certain extent.

Stereoscopic particle image velocimetry is used to characterize the variation of the turbocharger compressor inlet velocity field as a function of rotational speed, with an emphasis on surge. While the velocity magnitudes at choke or mild surge increased with rotational speed, the velocity profiles remained qualitatively similar. The variation in deep surge flow field with shaft speed, however, was more substantial. At 80 krpm, the overall flow field was comparable at different time instances (at different points on the surge cycle): the core flow near the duct center was always directed into the impeller, whereas reversed flow occupied an annular region near the periphery in nearly all time instances. However, at 140 krpm, while the negative flow rate (cross-sectional average flow is directed out of the inducer back into the inlet duct) portion of the surge cycle was still similar to the overall surge flow field at 80 krpm, over a substantial part of the positive flow rate (cross-sectional average flow is directed into the impeller) portion of the surge cycle, there was no sign of reversed flow within the visualization domain. As the rotational speed was increased, the surge loop (obtained by combining the particle image velocimetry (PIV) and pressure transducer data) extended over a wider portion of the compressor map with higher maximum (positive) and minimum (negative) flow rates, along with higher amplitude pressure fluctuations. The mean amplitude of mass flow rate and pressure ratio fluctuations at deep surge increased in nearly a quadratic fashion with rotational speed.

Based on the study of leakage characteristics of labyrinth seal structure (LSS), a new type of combined seal structure (CSS) consisting of the labyrinth structure and the nozzle structure has been proposed. The sealing characteristics of CSS and LSS are compared by means of numerical simulation and experiments, and the effects of the internal resistance of the device, structural geometric parameters and other factors on the leakage characteristics of CSS are studied. The results illustrate the following conclusions: (a) When the inlet flow is 12 m 3 /h and the internal resistance of the device is 2000–4000 Pa, the leakage rate of CSS decreases by 30%–40% in comparison with that of LSS, which indicates that the performance of CSS is much better than that of LSS. (b) The leakage rate increases as the internal resistance of the device increases. When the internal resistance of the device increases from 2000 Pa to 8000 Pa, the leakage rate increases from 26% to 72%. (c) When the internal resistance of the device is constant, the larger the inlet flow, the smaller the leakage rate. (d) The choice of nozzle radius in structural geometric parameters is more important for the leakage rate than the tooth height and teeth numbers. When the nozzle radius decreases, Δ P A B (pressure difference between the labyrinth structure and the nozzle structure) and the leakage rate decrease accordingly.

This work quantifies the uncertainty in Stevens Mark VII Perceived Level of sonic booms due to uncertainties in atmospheric profiles. The influence of temperature, humidity, and wind profiles, at six cities around the globe, on the sonic boom loudness is calculated. The flow field around an aircraft was obtained by solving the three-dimensional (3D), compressible Euler equations using the UNS3D solver. The near-field pressure signature is then propagated through the atmosphere by solving the augmented Burgers equation using the sBOOM solver. The uncertainty is modeled using a nonintrusive polynomial chaos approach. A sensitivity analysis is performed to identify the altitude range and the atmospheric variable to which the Perceived Level is most sensitive. It was shown that the Perceived Level is not sensitive to any particular altitude but rather the atmospheric profiles. It was also seen that the Perceived Level is highly sensitive to humidity and temperature profiles and less sensitive to the wind profiles.

This paper presents a method for a cost- and time-effective calibration procedure for five-hole probes for the transonic flow regime based on additive manufacturing and a numerical calibration routine. The computational setup and calibration routine are described in detail. The calibration procedure is tested on a custom-built L-shaped conical probe of 30 deg half-angle with a flat tip and an outer diameter of 2.4 mm. The probe tip is manufactured in stainless steel using direct metal laser sintering. Numerical calibration is carried out over a Mach number range of 0.2–1.4 and pitch and yaw angles of ± 45 deg . The numerical calibration charts are validated with wind tunnel tests across the entire Mach number range and the expected accuracy of the numerical calibration method is quantified. Exemplary results of area traverses up- and downstream of a linear transonic turbine cascade with tip clearance are presented and discussed briefly.