Circular Cylinder In Cross Flow Research Articles

Control of vortex shedding and acoustic resonance of a circular cylinder in cross-flow

This study experimentally investigates the effectiveness of a control rod in suppressing self-excited acoustic resonance within a range of Reynolds numbers (Re) spanning from 2.1 × 104 to 1.6 × 105. The investigation focuses on specific parameters, including diameter ratio (d/D) values of 0.1, 0.2, and 0.3; gap ratio (G/D) values of 0.05, 0.1, and 0.2; and angular positions (θ) ranging from 0 to 180 degrees. Comparative analyses are conducted between cases featuring the control rod and a reference case (base case) without it. The near-wake flow field is characterized using Particle Image Velocimetry (PIV), and aeroacoustic response measurements are employed to quantify the aeroacoustic noise emission, particularly during self-excited acoustic resonance. Simultaneous measurements of fluctuating lift force and aeroacoustic response measurements, facilitate the quantification of energy transfer from the flow field to the acoustic field during self-excited acoustic resonance. The results reveal that the control rod’s placement significantly impacts the Strouhal periodicity, with outcomes heavily dependent on the rod’s angular orientation. At certain angular positions, the control rod reduces the sound pressure level (SPL) generated during acoustic resonance excitation. However, at different angular positions, the rod exacerbates resonance excitation. This variability is attributed to the control rod’s profound influence on the vortex core formation and the energy transfer mechanism during acoustic resonance.

A Bottom-up Transient Theory for Concentration Polarization with Applicability to over-Limiting Current Conditions

Renewable power sources have drawn interest due to scarcity of fossil fuels and associated pollution during their combustion-based power production. Renewables are projected to produce approximately 60% of US electricity generation by 2050, with an expected contribution of 2000 TW-h from wind power alone. To overcome the mismatch between electricity demand and intermittent supply, such power plants require large-scale energy-storage device integrated with them. In contrast to other technologies, redox flow batteries (RFBs) offer independently scalable power and energy capacity that is demanded for such applications. Despite significant achievement in their designs through experimentation and modeling, limited understanding has been established to date concerning their transient response while operating under irregular charge/discharge scenarios in renewable-powered grids and micro-grids. Under such conditions pore-scale mass transport physics and different mechanisms of concentration polarization can occur, but the conventional film law of mass transfer (FLoMT) using a constant Sherwood number cannot capture time-varying diffusion layer thickness near electrode surfaces. Further, the mean solute advection rate through porous RFB electrodes can deviate from the value expected from product of superficial velocity and average concentration inside the electrode if pore-scale concentration polarization is substantial. Consideration of these effects in the present bottom-up theory makes it applicable to over-limiting conditions, while predicting concentration polarization in agreement with experimental observations. We accomplish this by creating theory with similarity to the theory of Ralph White and co-workers for solid-state, particle-scale diffusion in Li-ion batteries [J. Power Sources, 2012, Vols. 214 and 218], while our theory is instead applied to an arbitrary representative volume element of flowing liquid electrolyte inside electrode pores.Here, we present a theory to incorporate these transient effects into a bottom-up transient model (B-UTM) by using frequency-dependent transfer functions (TFs) that are derived from the Fourier transformed pore-scale mass conservation equations (MCEs). These TFs preserve pore-scale mass transfer physics during their embedding into an up-scaled model. One of these TFs is the spectral Sherwood number that extends the film law of mass transfer to transient conditions and that correlates reactive flux with concentration polarization through an output-input relationship in the frequency domain. Another TF named the advective-flux transfer function captures the acceleration/suppression of solute advection rate that arises from the inhomogeneity in pore-scale concentration distributions. We first present validation of this theory using transient RFB experiments where the introduced B-UTM predicts the time evolution of concentration polarization in good agreement. Meanwhile the conventional FLoMT model that uses time-independent Sherwood number and neglects the solute acceleration/suppression effect shows systematically larger polarization (up to 400%), which restricts such models from simulating charging/discharging with applied current near and above the limiting current.The utility of the introduced bottom-up theory is demonstrated by incorporating it into a multi-scale RFB model in which redox-active electrolyte flows through a porous electrode comprised of an array of circular cylinders in crossflow that mimics commonly used carbon felt. The frequency dependence of the introduced TFs obtained from numerical solution of pore-scale MCEs are analyzed for different electrode porosity and Péclet number, where both the TFs reach a non-zero finite value in the limit of vanishing frequency. The non-zero value of the spectral Sherwood number in this limit is identical to the Sherwood number obtained in the pseudo-steady limit (PSL) where transient effects are negligible. With increasing frequency both the TFs show a transition from lagless response to semi-infinite Warburg response with orders higher values compared to that of the PSL. Finally, these TFs are embedded into an up-scaled model to obtain the time-domain response of RFBs operating under a step impulse applied current with different step durations. Numerically obtained concentration polarization and the average reactant concentration inside electrodes is used to construct non-dimensional regime maps showing a regime of over-limiting current. The results show that fast current fluctuations are sustained even when current exceeds the limiting current, which provides a means to increase the utilization of charge capacity of existing flow-based electrochemical devices that are operated under transient conditions. The theory introduced here can find applicability in other flow-based electrochemical devices including electrochemical separations and CO2 capture. Figure 1

Air-bubble entrainment by translating turbulent jets in stagnant water

This paper presents an experimental study on air-bubble entrainment in quiescent water by translating circular turbulent jets. The jet diameters, plunging heights, impact velocities, and translating velocities were varied during the experiments to investigate their relative effect on the bubble characteristics. The experimental observations reveal that the jet translation affects the air bubble entrainment mechanism and the distribution of bubble size. A rotating cavity forms around the plunging jet due to the translation of the jet. Depending on the translating velocity, the air bubble emanates from the cusp of the cavity and the downstream water surface meniscus with the jet. The bubble swarm produced by the translating jet exhibits vortex shedding with Strouhal numbers between 0.22 and 0.27, comparable to a circular cylinder in cross-flow. The peak of the bubble size distribution varies between 0.5 and 1.5 mm, while the Sauter mean diameter varies between 1.8 and 2.8 mm. The maximum penetration depth of bubbles is found to be a function of the jet impact to translating velocity ratio, and the Capillary number of the air–water interface. The spatial distribution of bubbles along the plume cross section exhibits Gaussian distributions. Finally, the terminal rising velocity of the bubbles shows no obvious effect of the jet translation.

On Velocity Spectra within the Wake behind a Circular Cylinder

The canonical case of a circular cylinder in crossflow is studied experimentally at Reynolds number 5k. The power-spectra of all velocity components in the turbulent wake are studied in detail. Special attention is paid to the appearance of peaks and spectra shape, i.e., to the slope of spectra parts. Signatures of presence forward energy cascade and inverse enstrophy cascade are detected. Three peaks corresponding to 1st, 2nd and 3rd harmonics of the Strouhal frequency are detected as well as local spectra slopes conforming to the Kolmogorov and Kraichnan theories.

Experimental Investigation of 3D Dynamical Effects in a Wake behind Circular Cylinder

A circular cylinder in crossflow is subjected to the study of 3D dynamical structure of its wake. The typical dynamics is characterized by quasi-periodic behaviour called Kármán – Bénard vortex street with the typical frequency in dimensionless form known as Strouhal number. The experimental study relies on stereo Particle Image Velocimetry method, the plane of measurement is perpendicular to the flow in the distance 3.8 cylinder diameters in streamwise direction. Reynolds number was around 5 thousand. The structure and dynamical behaviour of the wake along the cylinder axis is studied in details. On the top of the known streamwise velocity deficit definition of the wake, the streamwise oriented dynamically evolving vortices are detected. For the detailed dynamics examination, the Oscillation Pattern Decomposition method was used. Waves of various structures travelling along the cylinder axis as well as some pulsations were identified. The wake dynamics is characterised by the streamwise velocity component deficit and the streamwise vorticity concentration. Both structures are moving periodically in transversal direction with the same frequency, the velocity deficit is shifted by a quarter of period behind the vorticity concentration in the shedding process.

Lattice Boltzmann non-equilibrium extrapolation method for modeling hydrodynamic compatibility conditions at curved porous-fluid interfaces

PurposeThe lattice Boltzmann simulation of fluid flow in partial porous geometries with curved porous-fluid interfaces has not been investigated yet. It is mainly because of the lack of a method in the lattice Boltzmann framework to model the hydrodynamic compatibility conditions at curved porous-fluid interfaces, which is required for the two-domain approach. Therefore, the purpose of this study is to develop such a method.Design/methodology/approachThis research extends the non-equilibrium extrapolation lattice Boltzmann method for satisfying no-slip conditions at curved solid boundaries, to model hydrodynamic compatibility conditions at curved porous-fluid interfaces.FindingsThe proposed method is tested against the results available from conventional numerical methods via the problem of fluid flow through and around a porous circular cylinder in crossflow. As such, streamlines, geometrical characteristics of recirculating wakes and drag coefficient are validated for different Reynolds (5 ≤ Re ≤ 40) and Darcy (10−5 ≤ Da ≤ 5 × 10−1) numbers. It is also shown that without applying any compatibility conditions at the interface, the predicted flow structure is not satisfactory, even for a very fine mesh. This result highlights the importance of the two-domain approach for lattice Boltzmann simulation of the fluid flow in partial porous geometries with curved porous-fluid interfaces.Originality/valueNo research is found in the literature for applying the hydrodynamic compatibility conditions at curved porous-fluid interfaces in the lattice Boltzmann framework.

Passive Flow Control for Drag Reduction on a Cylinder in Cross-Flow Using Leeward Partial Porous Coatings

This paper presents a numerical study on the impact of partial leeward porous coatings on the drag of circular cylinders in cross-flow. Porous coatings are receiving increasing attention for their potential in passive flow control. An unsteady Reynolds-averaged Navier–Stokes model was developed that agreed well with the numerical and experimental literature. Using the two-equation shear stress transport k−ω turbulence model, 2D flow around a circular cylinder was simulated at Re = 4.2×104 with five different angles of partial leeward porous coatings and a full porous coating. For coating angles below 130∘, the coating resulted in an increase in pressure on the leeward side of the cylinder. There was a significant reduction in the fluctuation of the pressure and aerodynamic forces and a damping effect on vortex shedding. Flow separation occurred earlier; the wake was widened; and there was a decrease in turbulence intensity at the outlet. A reduction of drag between 5 and 16% was measured, with the maximum at a 70∘ coating angle. The results differed greatly for a full porous coating and a 160∘ coating, which were found to cause an increase in drag of 42% and 43%, respectively. The results showed that leeward porous coatings have a clear drag-reducing potential, with possibilities for further research into the optimum configuration.

Investigation of the steady and unsteady forces acting on a pair of circular cylinders in crossflow up to ultra-high Reynolds numbers

Measurements of the steady and unsteady forces acting on a pair of circular cylinders in crossflow are performed from subcritical up to ultra-high Reynolds numbers. The two cylinders with equal diameters d are arranged inline at two centre-to-centre distances: S/d = 2.8 and 4. The trend of the drag curve for the upstream cylinder Cd_{1}(Re) at both distances is similar to that for a single circular cylinder. The development of the drag curves Cd_{2}(Re, S/d = 2.8, 4) of the downstream cylinder is inverse to that of the upstream cylinder. For both cylinder spacing values, the drag on the downstream cylinder is negative for subcritical Reynolds numbers, increases abruptly to positive values at the beginning of the supercritical regime, and shows a significant dip at transcritical Reynolds numbers. This drag inversion indicates that the critical distance Sc decreases sharply in the supercritical Reynolds number range. For S/d = 2.8 at Rerightarrow 10^{7}, the downstream cylinder experiences once more a thrust force. The curve of the Strouhal number St(Re) of the downstream cylinder for S/d = 4 is very close to that of a single cylinder. For Reynolds numbers of Reapprox 1times10^{6} - 7times10^{6}, the Strouhal numbers have nearly equal values of Stapprox 0.22 - 0.24 for both distances. This is followed by a branching. For Rerightarrow 10^{7} and the case S/d = 2.8, the Strouhal numbers dip at St = 0.17. However, for S/d = 4, they increase up to St = 0.27. In the supercritical range, two peaks occur in the power spectra for the large distance S/d = 4. Based on a wavelet analysis, we can conclude that the low-frequency mode, which does not occur for a single cylinder, is an interference effect.Graphic abstract

Efficient One-Shot Technique for Adjoint-Based Unsteady Optimization

A computationally efficient one-shot approach with a low memory footprint is presented for unsteady optimization. The proposed technique is based on a novel and unique approach that combines local-in-time and fixed-point iteration methods to advance the unconverged primal and adjoint solutions forward and backward in time to evaluate the sensitivity of the globally time-integrated objective function. This is in some ways similar to the piggyback iterations in which primal and adjoint solutions are evaluated at a frozen design. During each cycle, the primal, adjoint, and design update problems are solved to advance the optimization problem. This new coupled approach is shown to provide significant savings in the memory footprint while reducing the computational cost of primal and adjoint evaluations per design cycle. The method is first applied to an inverse design problem for the unsteady lid-driven cavity. Following this, vortex suppression and mean drag reduction for a circular cylinder in crossflow is considered. Both of these objectives are achieved by optimizing the rotational speeds for steady or periodically oscillating excitations. For all cases presented in this work, the proposed technique is shown to provide significant reductions in memory as well as computational time. It is also shown that the unsteady optimization problem converges to the same optimal solution obtained using a conventional approach.

Simulation of Turbulent Flow with High Reynolds Number over an Elliptical Cylinder

A complex turbulent flow with high Reynolds number (Re= 5 × 105, 1 × 106, 2 × 106 and 3.6 × 106) over an elliptical cylinder, is investigated using 2D URANS equations with a standard k-ε turbulence model. The present study compares the effect of various upstream velocity profiles on the variations in flow characteristics along the surface of an elliptical cylinder having variable axis ratios. The comparison of results with the published data for a circular cylinder in cross flow shows the importance of axis ratio and other velocity profiles to reduce the drag force and variation in pressure distribution. Drag force over an elliptical cylinder is found to be lower than that of a circular cylinder. The drag force is seen to increase with increasing slenderness ratio. The drag coefficient of an elliptical cylinder with an axis ratio 0.6 is found to reduce by half as compared to that of a circular cylinder. The influence of slenderness ratio and velocity profile over back pressure recovery, flow separation and shear stress distribution is observed for an elliptical cylinder.

Numerical study of fluid flow and heat transfer characteristics of an oscillating porous circular cylinder in crossflow

In this paper, numerical simulation of fluid flow and heat transfer characteristics of a porous cylinder subjected to a transverse oscillation in subcritical crossflow are studied for the first time. As such, the effects of Darcy number, 10−6 ≤ Da ≤ 10−2, reduced frequency, 0.2688 ≤ f* ≤ 1.075, dimensionless amplitude, A/d = 0.5 and 1, and Reynolds number, 5 ≤ Re ≤ 40, on the problem are investigated. It is revealed by the results that for an oscillating porous cylinder even at the subcritical Reynolds number of 40, the vortex shedding surprisingly develops behind the cylinder for cases with Da ≤ 10−4, f* = 1.075, and A/d = 1. Furthermore, it is shown that this subcritical vortex shedding always happens at the lock-in situation. The oscillation of the cylinder is shown to always increase the lift and drag coefficients compared to the stationary cylinder. According to the results, interestingly, the average drag coefficient increases with increasing Darcy number at intermediate Darcy numbers (10−4 ≤ Da ≤ 10−3). It is concluded that two mechanisms boost the heat transfer rate, namely, the vortex shedding, which is the case for the low Darcy zone at the highest frequency and amplitude of the oscillation, and the flow penetration, which is of more importance to the high Darcy zone. In conclusion, the maximum increase in the average Nusselt number is achieved at the highest values of the frequency and amplitude, which provide 18%, 28%, 51%, and 81% heat transfer enhancement compared to the stationary cylinder for Da = 10−6, 10−4, 10−3, and 10−2, respectively.

POD Spectrum of the Wake behind a Circular Cylinder

The wake dynamics behind a long circular cylinder in cross-flow was studied using the POD method. The temporal parts of POD modes, Chronoses, are subjected to frequency analysis. Five groups of modes are distinguished according to the frequency contents. The low order high energy modes contain the vortex shedding frequency or its harmonics up to 3rd order plus Kolmogorov spectrum. The higher order modes are characterized by combination of Kolmogorov spectrum with the white noise spectrum, its importance grows with the mode order. The very high order modes are characterised by the white noise spectrum only.

On the effect of temporal error in high-order simulations of unsteady flows

The present paper focuses on the effect that temporal error has on unsteady solutions modeled with a high-spatial-order, finite-element, streamline upwind/Petrov-Galerkin solver. Six L-stable, first- through fourth-order time-integration methods are discussed and exercised on two canonical test cases: the convecting isentropic vortex and the two-dimensional circular cylinder in crossflow. All results are shown on grids composed of P2 (third-order) elements. The isentropic vortex case is used to verify temporal order of accuracy, assess the relative split between dissipative and dispersive errors for each method with various time steps, and quantify the relative computational cost of each method. The results from the two-dimensional circular cylinder in crossflow simulations at various Reynolds numbers are compared to experimental results and accepted numerical results for vortex shedding frequency (Strouhal number), mean drag coefficient, and RMS lift and drag coefficient. The convergence of those parameters with respect to time step size is shown. These results highlight the benefits of high-order (greater than second order) time-integration methods and the need for time-step convergence studies on unsteady simulations.

Experiments on the Flow around two tandem circular cylinders from sub- up to transcritical Reynolds numbers

Measurements were taken on two tandem circular cylinders in cross flow for Reynolds numbers from subcritical up to transcritical values (2 ⋅ 105≤ Re ≤ 6 ⋅ 106). The tandem arrangement with small spacing S = 1.56 d (d: diameter of the cylinders) represented a twin cable assembly of the main cable of the Messina Bridge. The tests were carried out in a special facility for high Reynolds numbers, the DNW High Pressure Wind tunnel in Göttingen. The steady forces on both cylinders were measured using a two-sided strain gauge balance. The wake profiles were taken by a pressure rake.At the angle of incidence α = 0°the appearance of the drag-curve Cd1 depending on Re for the front cylinder is similar to the behavior of a single smooth circular cylinder. The shape of the corresponding curve Cd2 of the downstream cylinder resembles the inverse of the upstream one with negative drag in the subcritical and transcritical Re-range. The critical Reynolds number regime is around Rec≈ 4⋅ 105. Here, the drag on the second cylinder changes sign from Cd2≈ -0.4 to +0.4. Further, asymmetric flow, reflected in steady lift Cl2≈ 1, was observed.The supercritical regime is followed by the upper transition, after that, the transcritical range begins at Re ≈ 3.5⋅ 106. In that case a second drag inversion occurs, that is coupled with a change of the topological structure of the flow and thus can be a source of instability.In all relevant Reynolds number ranges for both cylinders curves of the force/moment coefficients were created, i.e. the dependence of these coefficients upon α. These curves for the sub-, super-, and upper transition as well as for transcritical Reynolds numbers are strongly nonlinear in nature with a curved character, pronounced extrema, and zero-crossings. In the subcritical case there are, in addition, hysteretic jumps of lift and drag. For all Reynolds number ranges investigated, we found that for the rigidly coupled tandem cylinders, the derivative of the global lift (around α = 0°) is negative and the corresponding Den Hartog criterion indicates instability.During the experiments at transcritical Reynolds numbers there were undesired vibrations, in particular for the front cylinder. The observed vibration phenomena are likely caused by vortex resonance and they are coupled with the mentioned drag-inversion at very high Reynolds numbers.

An investigation on VIV of a single 2D elastically-mounted cylinder with different mass ratios

The effect of the mass of a 2D elastically-mounted circular cylinder in cross-flow on its vortex-induced vibrations and on the related vortex shedding lift forces is analyzed via a single-degree-of-freedom multi-frequency model (sdof-mf). The mechanical system in question is characterized by low mass ratio, low structural damping and Reynolds number of order $$10^4$$. The proposed sdof-mf model relies on the decomposition of the total hydrodynamic force in a inertia/drag force, conventionally associated with the cylinder motion in still fluid, and an additional lift force associated to pure vortex shedding. The lift force is assumed to be composed by not-Fourier-dependent harmonics; this constitutes the key point of the proposed sdof-mf model. The parameters of this model are determined via a parameter identification method based, in this case, on VIV data obtained via CFD. The simulations are carried out changing systematically the values of the mass ratio, within the range of engineering practice, and covering a wide range of flow regimes including lock-in conditions. The results from the application of the sdof-mf model highlight the large influence of the mass ratio on the response of the cylinder and on the vortex shedding lift force. The effects are clearly visible on the maximum amplitude at lock-in, on the range of incident flow velocity over which synchronization occurs, on ultra/sub harmonic behavior and phase lag of the cylinder motion, and finally on the magnitude and harmonic content of the lift force induced by pure vortex shedding.

A comprehensive investigation of vortex induced vibration effects on the heat transfer from a circular cylinder

In this paper, the effect of vortex induced vibration (VIV) on convective heat transfer from an elastically mounted rigid circular cylinder in cross-flow is investigated numerically. The motion of cylinder is modeled by using a mass-spring-damping system. The effect of reduced velocity and damping ratio on the vortex formation, vortex shedding process, cylinder displacement amplitude, Nusselt number and the position of maximum local Nusselt number is studied. In this study the Reynolds Number and the mass ratio are 150 and 2, respectively. Furthermore, different values of reduced velocity and damping ratio are investigated (Ur= 3, 4, 5, 6, 7, 8 and ζ= 0. 0.01, 0.05, 0.1). The numerical results demonstrate that the reduced velocity and damping ratio can affect the heat transfer considerably. The beating phenomenon is occurs at Ur= 4 and ζ= 0.05 which leads to changes in the displacement amplitude and Nusselt number widely with respect to the time. In the beating phenomenon the total Nusselt number decreases in comparison with a stationary cylinder. The maximum heat transfer enhancement is obtained at Ur=4, ζ=0.

Numerical Characterisation of Active Drag and Lift Control for a Circular Cylinder in Cross-Flow

Synthetic jet actuators have shown promise to control drag and lift for a bluff body in cross-flow. Using unsteady RANS CFD modelling, a significant modification of the drag coefficient for a circular cylinder in cross-flow at R e = 3900 is achieved by varying the actuation frequency. The variation in actuation frequency corresponds to a range in Stokes number of 2.4 < S t o < 6.4. The trends in drag coefficient modification largely agree with the findings of past publications, achieving a maximum drag reduction at S t o = 4.9 for a fixed jet Reynolds number of the synthetic jet of R e U ¯ o = 12. A decrease in the adverse pressure gradient near the jet orifice correlated with a momentum increase in the viscous sublayer and stronger vortical structures at the rear of the cylinder. In these same conditions, a decrease in turbulence intensity was observed in the far field wake, which is a relevant finding in the context of wind and tidal turbine arrays.

HF DBD plasma actuators for reduction of cylinder noise in flow

Surface high frequency dielectric barrier discharge (HF DBD) was used to reduce flow-induced noise, radiated by circular cylinder in cross flow. Effect of HF DBD actuators is studied for flow velocity up to 80 m s−1 (Reynolds numbers up to 2.18 · 105), corresponding to the typical aircraft landing approach speed. Noise measurements were performed by microphone array in anechoic chamber; averaged flow parameters were studied by particle image velocimetry (PIV). Actuator was powered by high-frequency voltage in hundreds kHz range in steady or modulated mode with the modulation frequency of 0.3–20 kHz (Strouhal number St of 0.4 to 20). It is demonstrated that upstream directed plasma actuators are able to reduce the vortex noise of a cylinder by 10 dB. Noise reduction is accompanied by significant reorganization of the wake behind a cylinder, decreasing both wake width and turbulence level. The physical mechanism related to broadband noise control by HF DBD actuator is also discussed.

Turbulence intensity effects on heat transfer and fluid-flow for a circular cylinder in crossflow

The intrinsically unsteady heat transfer on the surface of a cylinder in crossflow has been investigated in detail by numerical simulation as a function of the freestream turbulence intensity and the Reynolds number. After a brief startup transient, a periodic steady state is established at all circumferential locations. The resulting timewise fluctuations were seen to be of different phase depending on where on the circumference they occur. On one side of the cylinder, maxima occurred at the same moment in time as minima occurred on the other side. This finding and comparisons of the magnitudes of the local heat transfer coefficients showed that side-to-side symmetry does not prevail in the presence of the unsteadiness. The fluctuation frequencies were found to be virtually uniform over the entire circumference of the cylinder and varied only slightly with the Reynolds number and the turbulence intensity. The full slate of results included: (a) timewise and circumferential variations of the local heat transfer coefficient, (b) timewise variations of the all-angle spatial-averaged heat transfer coefficient, (c) spatial variations of the timewise-averaged heat transfer coefficient, (d) spatial- and timewise-averaged heat transfer coefficients as a function of turbulence intensity and Reynolds number, (e) timewise fluctuation frequencies, (f) comparisons with the experimental literature, and (g) effect of the selected turbulence model. As expected, the magnitude of the heat transfer coefficient increases as the turbulence intensity increases.

Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation Closure for Separated Transitional Flows

The numerical prediction of transition from laminar to turbulent flow has proven to be an arduous challenge for computational fluid dynamics, with few approaches providing routine accurate results within the cost confines of engineering applications. The recently proposed transition model shows promise for predicting attached and mildly separated boundary layers in the transitional regime, but its accuracy diminishes for massively separated flows. In this effort, a new turbulence closure is proposed that combines the strengths of the local dynamic kinetic energy model and the widely adopted transition model using an additive hybrid filtering approach. This method has the potential for accurately capturing massively separated boundary layers in the transitional Reynolds number range at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, NACA 63-415 wing, and circular cylinder in crossflow. The new closure captures the physics associated with a separated wake (circular cylinder) across a range of Reynolds numbers from 10 to 2 million () and performed significantly better in capturing performance and flowfield features of engineering interest than existing turbulence models. The transitional hybrid approach is numerically robust and requires less than 2% extra computational work per iteration as compared with the baseline Langtry–Menter transition model.