Delayed Detached Eddy Simulation Research Articles

Dynamic stall control of an offshore wind turbine airfoil using a leading-edge microcylinder

This study conducted a computational investigation to examine the impact of passive flow control using a microcylinder positioned near the leading edge of a dynamically stalled NACA0012 offshore wind turbine airfoil at a Reynolds number of 1×106. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations were used for parametric analyses to determine optimal control parameters, while Delayed Detached Eddy Simulation (DDES) provided insights into the transient vortex structures on the airfoil's suction surface, elucidating the control mechanisms of the microcylinder. The results reveal a strong correlation between the microcylinder’s effectiveness and its geometric configuration, particularly its diameter and distance from the airfoil surface. The microcylinder effectively modifies the flow field by generating vortices that interact with the boundary layer, thereby enhancing its resistance to adverse pressure gradients and delaying flow separation. The optimal configuration—characterized by a microcylinder with a diameter of 1% of the chord length (D/c = 0.01) positioned 1.5% of the chord length (L/c = 0.015) away from the airfoil surface—resulted in a 62% reduction in peak drag coefficient and a 50% decrease in aerodynamic hysteresis loop area. These improvements significantly enhance the dynamic aerodynamic performance and stability of the oscillating NACA0012 airfoil, effectively mitigating the detrimental effects of dynamic stall.

Passive Control of the Flow Around a Rectangular Cylinder with a Custom Rough Surface

Motivated by existing techniques for implementing roughness on cylinders to control flow disturbances, we performed delayed detached eddy simulations (DDES) at Re = 6×106 that generated unsteady turbulent flow around a rectangular cylinder with a controlled wrinkled surface and a 1:4 aspect ratio. A systematic study of the roughness effect was carried out by implementing different configurations of equally spaced grooves and bumps on the top-surface of the cylinder. Our results suggest that groove geometries reduce energy dissipation at higher rates than the smooth reference case, whereas bumped cylinders produce relative pressures characterized by a sawtooth pattern along the middle-upper part of the cylinder. Moreover, cylinders with triangular bumps increase mean drag and lift forces by up to 8% and 0.08 units, respectively, while circular bumps increase vorticity and pressure disturbances on the wrinkled surface. All of these effects impact energy dissipation, vorticity, pressure coefficients, and flow velocity along the wrinkled surface. Both the surface-manufactured cylinders and the proposed visualization techniques could be replicated in a variety of engineering developments involving flow characterization in the presence of roughness.

Study of transient pressure and fluctuation characteristics in a centrifugal pump using the delayed detached-eddy simulation method

This study employed OpenFOAM, the delayed detached-eddy simulation (DDES) turbulence model, and structured grids to develop numerical models for three centrifugal pumps with twisted blades. The internal pressure field, velocity field, forces, and fluctuation characteristics of the centrifugal pumps are comprehensively analyzed under various operating conditions. The findings indicate that the pressure is relatively higher in the flow passages near the volute tongue and the outlet within the impeller. Regions of high relative velocity (slip velocity) are mainly found on the suction side of the blades, indicating that the design of the blade suction side affects the fluid outward slip performance. As the flow rate increases, the forces and force fluctuation amplitudes of each pump component also rise. Conversely, as the rotational speed increases, the force on the blades or impeller gradually increases while the fluctuation amplitude decreases. In the stationary domain, the force on the volute gradually decreases while the fluctuation amplitude of this force increases. The shape of the volute tongue influences the rate at which pressure inside the volute is converted to outlet pressure. The power spectral density (PSD) of pressure fluctuations is smallest at the nominal flow rate, displaying a clear and distinct axial frequency pattern without complex low-frequency fluctuations. Under low flow and high-speed conditions, the PSD at the axial frequency is relatively small, whereas the pressure PSD at other low frequencies is relatively large. This indicates instability in the flow under these conditions.

Computational aeroacoustics of low-pressure axial fans installed in parallel

Abstract Ducted rotor-only Low-Pressure Axial (LPA) fans play an integral role in automotive thermal management of electric vehicles and are the primary source of noise from the underhood region. Multiple LPA fans are often placed in parallel in cooling packages of electric vehicles. There is little scientific work concerning aeroacoustics of ducted LPA fans operating in parallel. This work aims to address this gap through hybrid computational aeroacoustic simulations. Three-dimensional, full-annulus, transient simulations are done using the Delayed Detached Eddy Simulation (DDES) turbulence model. First, a numerical validation study is presented, where aerodynamic and aeroacoustic results from this work are compared to experimental results for a LPA fan issued by the European Acoustics Association (EAA). In the second part, aeroacoustic performance of two-fans placed in parallel is presented. A local diffusion zone is observed in the region where the two-fans are closest to one another. A previously unidentified vortex which envelopes the local diffusion zone is observed. For two-fans in parallel, the amplification of the acoustic spectrum scales in accordance to having two identical equally strong sound sources, i.e, by 6 dB. The scaling of the acoustic spectrum for two-fans in parallel in comparison to a single-fan suggests limited interaction between the acoustic field of the two-fans.

Influence of internal flow structure on flow energy losses in a centrifugal pump with splitter blades using entropy generation theory

Centrifugal pumps are essential in various industrial applications, including renewable energy. To maximize the pump’s overall performance, estimating flow energy losses (FEL) is crucial. However, due to internal flow complexity, it is necessary to employ new methods and approaches to better understand FEL mechanisms. This study uses entropy generation theory to investigate the relationship between FEL and various flow patterns in all pump hydraulic components. Numerical simulations were conducted using a 3-dimensional incompressible unsteady flow at four different flow coefficients. The delayed detached eddy simulation (DDES) model was used, and the results showed good agreement with experimental data compared to the SST k-w model. Emphasis was given to the flow energy losses and the relative and absolute velocity distribution in the impeller and diffuser components at various flow coefficients. Flow energy losses primarily occur in the impeller (70%) followed by the diffuser (23%). Impeller losses are concentrated at the outlet region due to the wake-jet phenomena (28.6%), splitter blades region (15.3%), and impeller’s leading edge (LE) (10.7%). Vaned diffuser losses occur in the vanless zone (stator-rotor interaction) and near leading/trailing edges.Moreover, wall shear stress and the significant relative velocity gradient near the walls of the impeller and diffuser blades are the main contributors to the FEL in this region. This study provides insights into a better understanding of FEL mechanisms and highlights areas for improving pump performance.

Numerical Simulation of the Unsteady Airwake of the Liaoning Carrier Based on the DDES Model Coupled with Overset Grid

The wake behind an aircraft carrier under heavy wind condition is a key concern in ship design. The Chinese Liaoning ship’s upturned bow and the island on the deck could cause serious flow separation in the landing and take-off area. The flow separation induces strong velocity gradients and intense pulsations in the flow field. In addition, the sway of the aircraft carrier caused by waves could also intensify the flow separation. The complex flow field poses a significant risk to the shipboard aircraft take-off and landing operation. Therefore, accurately predicting the wake of an aircraft carrier during wave action motion is of great interest for design optimization and recovery aircraft control. In this research, the aerodynamic around an aircraft carrier (i.e., Liaoning) was analyzed using the computational fluid dynamics technique. The validity of two turbulence models was verified through comparison with the existing data from the literature. The upturned bow take-off deck and the right-hand island were the main areas where flow separation occurred. Delayed detached eddy simulation (DDES), which combines the advantages of LES and RANS, was adopted to capture the full-scale spatial and temporal flow information. The DDES was also coupled with the overset grid to calculate the flow field characteristics under the effect of hull sway. The downwash area at 15° starboard wind became shorter when the hull was stationary, while the upwash area and turbulence intensity increased. The respective characteristics of the wake flow field in the stationary and swaying state of the ship were investigated, and the flow separation showed a clear periodic when the ship was swaying. Comprehensive analysis of the time-dependent flow characteristic of the approach line for fixed-wing naval aircraft is also presented.

Experimental and numerical study of H[formula omitted] enrichment on swirl/bluff-body stabilized lean premixed n-butane/air flame

This study focused on the stabilization of lean premixed flames of n-butane/H2/air with swirl/bluff-body. The research explored four different hydrogen fractions (0, 20%, 40%, and 80%) at a constant equivalence ratio and Reynolds number. The turbulent reacting flow was resolved using a combination of Delayed Detached Eddy Simulation (DDES) and Flamelet Generated Manifold (FGM) model. The velocity fields and reaction zones were obtained through Particle Image Velocimetry (PIV) and OH* chemiluminescence measurements respectively. H2 addition to n-butane significantly enhanced flame characteristics. The chemiluminescence imaging showed that the flame base became stronger with increasing hydrogen addition, reducing the risk of flame lift-off. Hydrogen addition not only increased the overall reaction rate but also changed the combustion intensity at the nozzle exit from weak to strong, which is crucial for flame stabilization. Interestingly, no flashback was observed even at 80% H2 addition to n-butane at the same level of power rating. The flow strain rate analysis of the PIV measurements showed that the inclusion of hydrogen skewed the strain rate probability density functions towards the positive side, indicating an increase in the burning rate. The results demonstrate a progressive increase in preferential diffusion of H2 from reactants to products as the H2 enrichment in the fuel rises and resulted in a more intense flame. Also, the present simulation results were validated with the PIV data, and they showed good agreement. Specifically, an 80% H2 blend exhibited a 16% peak enhancement in flame surface density compared to pure n-butane while concurrently reducing CO2 emissions by 23.2%. The incorporation of H2 also led to the increased vortex strength and strain rate within the flame.

Correction of Aero-Optical Effect with Blow–Suction Control for Hypersonic Vehicles

High-speed turbulence induces significant aero-optical effects that severely disrupt the functionality of imaging systems of hypersonic vehicles. In this study, the aero-optical correction of various jet cooling modes is investigated using a Terminal High Altitude Area Defense (THAAD)-like seeker model and the imaging impact of high-speed flow field and flow control on the optical window is analyzed by the Delayed Detached Eddy Simulation (DDES) method. The findings reveal that a jet mode parallel to the window exhibits better cooling effectiveness compared to a perpendicular jet mode along the body axis; however, it introduces additional wavefront distortion, leading to degraded imaging quality. Although micro-vortex generators (MVGs) can reduce density fluctuations near the window from a refractive index perspective, they do not effectively mitigate wavefront distortion or improve window cooling efficiency. Finally, incorporating suction control, a comprehensive flow control solution, significantly improves the flow field structure near the window, resulting in a more uniform temperature distribution and reduced wavefront distortion. Applying this flow control method results in a 14.7% reduction in wavefront distortion at 3 Ma and an approximately 20% maximum value reduction at 5 Ma. This study proposes a novel and comprehensive flow control method to effectively mitigate the aero-optical effect in hypersonic flows, providing a new avenue for subsequent researchers in this field.

KHz Rate Fs/Ps-CARS Thermometry For Supersonic H_2/Air Combustion Studies

Accurate flow field measurements are essential to probe the phenomena involved in aeronautical engines undergoing high temperature and high turbulences. Indeed, reliable experimental datasets are needed to compare and validate numerical models. Among other laser diagnostics, Coherent Anti-stokes Raman Scattering (CARS) is a well-established non-linear spectroscopic technique used to probe harsh environments, since it provides non-intrusive local point measurements of chemical composition and temperature. We report here hybrid fs/ps Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) thermometry campaign performed on N 2 in a supersonic H2 /air combustion. We used a mobile and compact laser architecture that shapes, synchronize and generates the laser beams necessary for CARS spectroscopy. The instrument was moved to a large-scale facility representative of a scramjet combustor (ONERA LAERTE/LAPCAT-II bench). H2 /air was injected during several burst at 0,12 and 0,15 equivalent ratios. Using kHz single-shot measurements combined with motorized translation stages, spatial and temporal resolution were obtained to probe the flow and the combustion at various positions upstream and downstream the hydrogen injectors. At these positions, horizontal and vertical temperature profiles were retrieved and compared with a numerical unsteady fluid dynamic simulation using delayed detached eddy simulations (DDES) developed at ONERA (CEDRE). The position of the combustion inside the combustion chamber could be identified through several horizontal and vertical profile temperature measurements. The signal-to-noise ratio of the detection was optimized by empirically adjusting the number of averaged laser shots based on the level of flow turbulence. For instance, far downstream the H2 injection, averaging over 10 to 100 pulses was possible as the heated air flow was steady. This was confirmed by the good agreement with numerical simulations although some distortion could still be observed and will be used to adjust the numerical simulation algorithm. However, averaging was no longer possible close to the injectors, where a highly turbulent H2 /air combustion was observed. Nevertheless, taking advantages of the high rate single-shot capacity of the spectroscopic technique, temperature could still be retrieved inside this area. These measurements provided a valuable experimental database for comparing and refining computational fluid dynamics modeling.

Improvements for the DDES model with consideration of grid aspect ratio and coherent vortex structures

The delayed detached eddy simulation (DDES) model has achieved great success in simulations of massive separations, but has been found to significantly delay the Kelvin-Helmholtz (K-H) instability in weak separations due to the “grey area” problem. A new DDES model was proposed by considering the grid aspect ratio and local coherent vortex structures, named the DDES with an adaptive grid-scale and adaptive coefficient (DDES-AGAC). The DDES-AGAC model was examined in detail by computing four typical types of turbulent flows: fully attached turbulent boundary layer, massive separation, weakly unstable flow with a fixed separation point, and weak separation over a smooth surface due to pressure gradient. The results confirm the advantages of the new DDES-AGAC model over the standard DDES. It exhibits high efficiency and insensitivity to spanwise or circumferential grid spacing in flow simulations of quasi-three-dimensional flows. For the weakly unsteady separation, it helps to accelerate the rapid switch from Reynolds-averaged Navier-Stokes equations to large eddy simulation, unlock the K-H instability in the shear layer, and hence accurately resolve the turbulent structures, profiles of velocity and turbulent kinetic energy, and spectra in the separation regions. Furthermore, it does not result in a noticeable drag or skin friction reduction in the fully attached flow simulations. The DDES-AGAC model is expected to be widely used in simulations of engineering flows with separations.

Investigating 3-D Flow Characteristics of Wind Turbine Airfoils using Delayed Detached Eddy Simulations at High Reynolds Numbers

This numerical study investigates the 3-D flow field and aerodynamic characteristics of DU 00-W-212 wind turbine airfoil at high Reynolds numbers. The Computational Fluid Dynamics (CFD) simulations are performed at Reynolds number of 3 million at three different angles of attack for the pre-stall, stall, and post-stall conditions, by using an open source CFD solver SU2. The Delayed Detached Eddy Simulations (DDES) with the Spalart-Allmaras (SA) turbulence model and Langtry-Menter (LM) transition model are performed to address the challenge of predicting unsteady flow characteristics with transition, particularly on the suction side of the airfoil. The 3-D DDES with SA results with and without LM transition model are compared with the 2-D RANS with SA simulations and available experimental data.

Numerical study of separation flows in a U-duct using DDES method

Separation flow in a curved duct is a common phenomenon in engineering applications, and it highly contributes to the performance of fluid machinery. Accurate prediction of curved duct flows using the computational fluid dynamics method remains a challenge due to the limitations of turbulence modeling. Hence, the high-fidelity method of the delayed detached eddy simulation (DDES) approach is employed to simulate the U-duct flow with a Reynolds number of 105. The DDES results are compared with experimental data from the study by Monson et al. (1990) and analyzed in detail. The Q-criterion is defined to analyze the vortex structures and study the mechanism in the flow separation region. Discussions are made on turbulence characteristics, including turbulence energy spectra, helicity density, and turbulence anisotropy in the U-duct flow. Results indicate that the regions near the wall and within flow separation are highly anisotropic. The turbulence near the wall region is in a two-dimensional state, and the turbulence within the flow separation region is in a “rod-like” state.

Numerical Simulation of Unsteady Cavitation at the Tongue of a Centrifugal Pump

Abstract The flow separation at the tongue of the centrifugal pump will cause a sharp reduction of local pressure, thus inducing the cavitation phenomenon, which hurts the stable operation of the pump. This article focuses on the centrifugal pump as the research object. The Delayed Detached Eddy Simulation (DDES) method based on the SST turbulence model was employed to numerically simulate the cavitation at the tongue of the centrifugal pump. The typical characteristics and unsteady changes of cavitation were observed and studied, and the pressure at the tongue position was monitored for flow rates of 1.36 Q 0 and 1.54 Q 0. The results show that, in the cloud cavitation stage, the area of the cavitation bubble presents periodic changes, the main frequency of the cavitation area pulsation is twice the axial frequency, the periodic shedding of the cavitation bubble is caused by the attached vortex (re-entrant jet) and the pressure gradient, and the wall attached vortex will periodically shed and break. The high intensity vortices generated by flow separation at the tongue will change periodically with the rotation of the blade. With the increase of inlet flow, the pressure pulsation at the tongue increases, and the shedding and collapse of the cavitation bubble will enhance the pressure pulsation.

Research on the excitation force and vortex dynamics characteristics of pump-jet propulsor induced by shafting whirling vibration: Non-uniform blade tip clearance

The propulsion shafting whirling vibration causes non-uniform dynamic changes in the rotor tip clearance, which directly have a significant influence on the excitation force and vortex dynamic characteristics of the pump-jet propulsor. In the current study, based on improved delay detached eddy simulation, the influence of non-uniform blade tip clearance on the excitation force and vortex dynamics characteristics of the pump-jet propulsor is studied under design conditions. The results show that the application of propulsion shafting whirling vibration induces significant changes in the excitation force of the pump-jet propulsor. The rotor blades modulate the excitation forces of the stator blades and duct. The transverse and vertical excitation forces are more significant than the longitudinal excitation force. The magnitude change in the circular orbit shows a linear relationship with the excitation force magnitude. The characteristic frequency of the transverse and vertical excitation forces of each component is the shaft rotation frequency. In contrast, the characteristic frequency of the longitudinal excitation force is twice the shaft rotation frequency. In the elliptical orbit, the excitation force of each component is compressed or stretched in the time domain, and the dominant frequency is shifted in the frequency domain; there is no longer a linear relationship between the vibration magnitude change and the excitation force magnitude. Furthermore, an energy generation mechanism in the wake field of the pump-jet propulsor induces vortex frequency due to the whirling vibration of the propulsion shafting system.

Numerical Investigation of Rotor and Stator Matching Mode on the Complex Flow Field and Pressure Pulsation of a Vaned Centrifugal Pump

The match of rotor and stator blades significantly affects the flow field structure and flow-induced pressure pulsation characteristics inside the pump. In order to study the effects of the rotor and stator matching mode on the complex flow field and pressure pulsation of a centrifugal pump with a vaned diffuser, this paper designs three different vaned diffusers (DY5, DY8 and DY9) and uses the DDES (Delayed Detached Eddy Simulation) numerical method combined with structured grids to simulate the unsteady flow phenomena of the model pump under rated conditions. The results show that, under different rotor and stator matching modes, the pressure pulsation spectrum is dominated by the blade passing frequency and its harmonics. The matching mode of the rotor and stator significantly affects the time–frequency domain characteristics of the pressure pulsation inside the pump, and it is observed that the pressure pulsation energy of vaned diffusers with more blades is significantly smaller than that of fewer-blade vaned diffusers in comparison to the energy of the pressure pulsation at the blade passing frequency and within the 10–1500 Hz frequency band. Combined with the distribution characteristics of the complex flow field inside the pump, it can be found that increasing the number of vaned diffuser blades can reduce the energy of flow-induced pressure pulsation, improve the distribution of high-energy vortices in the interaction zone and stabilize the flow inside the centrifugal pump effectively.

Numerical studies on shock wave and boundary layer interaction in the high-load turbine rotor cascades

Design of transonic high-load turbine is the essential approach to improve thrust-to-weight ratio of gas turbine engines. The shock wave and boundary layer interaction (SWBLI) in high-load turbine is one of the important unsteady sources and the main source of loss in turbine stages. In order to study mechanism of SWBLI in high-load turbine, rotor blades with loading coefficients range from 1.6 to 2 were designed and Delayed Detached Eddy Simulations (DDES) were carried out. The influences of loading coefficient, exit isentropic Mach number and incidence angle on characteristics of shock wave, SWBLI as well as flow details in the blade passages were compared. Results indicated the shock wave was generated and enhanced as exit isentropic Mach numbers increased leading to the increase of 2.8% in the total pressure loss coefficient. There was not significant difference in the total pressure loss coefficient as loading coefficient increased, however the shock wave was stronger and the separation bubble was longer at the loading coefficient of 1.6. Meanwhile, the influence of strong separation near the leading edge of suction surface induced by variation of incidence angles on the characteristics of the shock wave and the loss was also obvious.

Experimental investigations on cooling characteristics of different trailing-edge cutback geometries

One important method for enhancing the performance of modern engines is to increase the turbine inlet temperature. Due to the high velocity near the throat, the heat transfer to the blade trailing edge is high, particularly on the pressure side near the trailing edge. Therefore, the design methodology for cooling structures becomes critical for the thinner trailing edge compared to other parts of the blade. Trailing-edge cutback is a common cooling structure that guides cold air to form an effective film near the blade pressure side. However, the cutback lip induces strong and unsteady shedding vortexes, which significantly reduce the downstream coolant effectiveness.Previous studies have explored the film cooling performances of cutback structures and the interaction between coolant and mainstream using experimental and numerical methods. Experimental results were obtained using Pressure-Sensitive Paint (PSP) and Particle Image Velocimetry (PIV) in a large-scale wind tunnel, while the Delayed Detached Eddy Simulation (DDES) with the Shear Stress Transport (SST) turbulence model was employed for capturing the unsteady flow field.In this paper, further investigation will be conducted on the lip profiles of the fillet structure to improve the effectiveness of the film coolant. The rounded lip design delays boundary layer separation, thereby reducing the scale and intensity of shedding vortices. This improvement in the effectiveness of the rounded lip nozzle model is observed across all blowing ratios. Furthermore, at low or medium blowing ratios, the rounded lip design significantly reduces momentum exchange, inhibiting the mixing between the mainstream flow and cooling air. This leads to a more pronounced enhancement in cooling effectiveness.

Complicated flow in tip flow field of a compressor tandem cascade using delayed detached eddy simulation

The complex flow characteristics in the tip region of a tandem cascade with tip clearance have been calculated and analyzed using Delayed Detached Eddy Simulation (DDES). The coherent mechanism of the vortex structures near the blade tip was discussed, and the unsteady behaviors and features in the tip flow field were analyzed. Additionally, the interaction between the tip leakage flow and the gap jet was revealed. The results show that, compared to the datum cascade, the blade tip load of the rear blade increases while that of the front blade decreases. Unsteady fluctuations of the tandem cascade are mainly caused by the interaction between the tip leakage flow and gap jet, and by the mixing of the vortex structures, but there is no essential change in the spectrum feature of the tip leakage flow. Finally, a detailed analysis of the development of vortices in the tip region is conducted by the topological structures of the flow field. Combined with the three-dimensional vortex structures, the schematic diagram of the vortex system of the datum single-row cascade and tandem cascade is summarized.

Study on the Tip Leakage Loss Mechanism of a Compressor Cascade Using the Enhanced Delay Detached Eddy Simulation Method.

The leakage flow has a significant impact on the aerodynamic losses and efficiency of the compressor. This paper investigates the loss mechanism in the tip region based on a high-load cantilevered stator cascade. Firstly, a high-fidelity flow field structure was obtained based on the Enhanced Delay Detached Eddy Simulation (EDDES) method. Subsequently, the Liutex method was employed to study the vortex structures in the tip region. The results indicate the presence of a tip leakage vortex (TLV), passage vortex (PV), and induced vortex (IV) in the tip region. At i=4°,8°, the induced vortex interacts with the PV and low-energy fluid, forming a "three-shape" mixed vortex. Finally, a qualitative and quantitative analysis of the loss sources in the tip flow field was conducted based on the entropy generation rate, and the impact of the incidence on the losses was explored. The loss sources in the tip flow field included endwall loss, blade profile loss, wake loss, and secondary flow loss. At i=0°, the loss primarily originated from the endwall and blade profile, accounting for 40% and 39%, respectively. As the incidence increased, the absolute value of losses increased, and the proportion of loss caused by secondary flow significantly increased. At i=8°, the proportion of secondary flow loss reached 47%, indicating the most significant impact.

A computational fluid dynamics study of the influence of sleeper shape and ballast depth on ballast flight during passage of a simplified train

The paper assesses the effect on the air flow regime underneath a simplified high-speed train of changing the ballast depth and the sleeper shape, with regard to its potential for causing ballast flight or pickup. The study was carried out numerically using the commercial Computational Fluid Dynamics (CFD) software AnSys Fluent. The flow profile beneath the underbody of the train was generated by means of a moving wall above the track. The Delayed Detached Eddy Simulation (DDES) with the SST [Formula: see text] turbulence model was used to simulate turbulent flow, and the ballast bed roughness was applied parametrically using the wall roughness feature when resolving the boundary layer. CFD simulations were validated for flow over a cube, showing good agreement with experimental results. Up to three different depths to the ballast surface and three different sleeper profiles were investigated. Velocity profiles and aerodynamic forces on cubes placed between or on top of the sleeper blocks were used to assess the propensity of individual ballast grains for movement. For a standard G44 sleeper, increasing the ballast depth and/or the ballast bed roughness was found to reduce aerodynamic loads on an individual ballast grain. A ballast grain on top of the sleeper is more prone to uplift than a grain on the surface of the ballast bed in the crib. A curved upper surface to the sleeper is beneficial in that it prevents ballast from settling on top, the most vulnerable position. However, the reduced flow separation associated with the curved top may increase the likelihood of ballast pickup from the crib. Hence new sleeper shapes intended to reduce the potential for ballast flight should not only prevent ballast from settling on top, but also increase flow separation through the provision of a sharp surface. A prismatic sleeper shape that achieves both is suggested.