Instantaneous Turbulent Structures Research Articles

Multi-Scale Flow Estimation Within Stereoscopic PIV Processing Based On Deep Learning

Classical methods of particle-image velocimetry (PIV) based on state-of-the-art cross-correlation algorithms constitute a spatial filtering operation that leads to a loss of spatial resolution of the estimated velocity fields. This is associated with numerous limitations, e.g., when it comes to aligning high-fidelity numerical simulations, since the detection of small turbulent scales is often precluded. An optical flow neural network adapted for PIV processing, called RAFT-PIV, can overcome the drawback of reduced spatial resolution while maintaining the reliability and robustness of the classical approach. The present work aims to extend the application of the aforementioned deep learning based method by integrating it within a stereoscopic PIV processing workflow. Experimental data from an optically accessible single-cylinder combustion engine the flow field of which is characterized by complex, three-dimensional flow structures and a broad range turbulent scales are processed using RAFT-PIV. The 2D-3C vector fields obtained from stereoscopic reconstruction are compared against a classical cross-correlation based PIV evaluation, providing insight into instantaneous quantities and turbulent structures. A qualitative comparison of the instantaneous in-plane and out-of-plane velocity components reveals that the reconstructed flow obtained from the deep learning based estimation includes a wide range of resolved turbulent length scales. It outperforms the classical method. Furthermore, the findings of the present study demonstrate that the pixel accuracy has an effect on both physical quantities and turbulence characteristics. In particular, an increase in turbulent kinetic energy is observed across the engine cycles of the investigated engine flow states. Moreover, a higher fraction of three-dimensional isotropic turbulent structures is derived from RAFT-PIV compared to the results from the cross-correlation based technique.

Large eddy simulation of film cooling for a forward expansion hole under main flow oscillation

This study conducted a large eddy simulation (LES) to predict the flow structure and effectiveness of film cooling in a gas turbine. The study focused on a forward expansion hole on a flat plate with an inclined injection angle of 35° and spanwise injection angles (orientation angles) of 0° and 30° under main flow oscillation. In gas turbines, the mainstream flow can exhibit unsteadiness owing to stator–rotor interactions or freestream turbulence. Here, sinusoidal functions are used to approximate the flow oscillations, and their effects are evaluated at frequencies of 0, 8, and 32 Hz, corresponding to Strouhal numbers of 0, 0.603, and 2.413, respectively. The study aims to characterize the film cooling at two time-averaged blowing ratios: 0.5 and 1.0. The range of Strouhal numbers considered in this study is relevant to actual transonic gas turbines. The computational results obtained through LES are validated using experimental data and compared with the Reynolds-averaged Navier–Stokes simulation results. The findings demonstrate the efficiency of LES to predict accurately the distribution of spanwise film cooling effectiveness and spanwise-averaged effectiveness for forward expansion holes on turbine blades under flow oscillations. Comprehensive analyses of various aspects such as the instantaneous turbulent structures of the film cooling flow fields, time-averaged temperature, and root-mean-squared values of the coolant temperature and a proper orthogonal decomposition analysis for velocity vectors are performed. The results show that when the oscillation frequency of the main flow varied from 0 Hz to 32 Hz for a time-averaged blowing ratio of 0.5, the film cooling effectiveness for the forward expansion hole reduced significantly. However, when the frequency increased under a time-averaged blowing ratio of 1.0, the film cooling effectiveness for the hole was only slightly affected by the oscillations. Moreover, an orientation angle of 30° enhanced the film-cooling performance even under flow oscillations. Thus, this paper reports the development of highly effective film-cooling strategies that can guide researchers aiming to mitigate the adverse effects of flow oscillations, thereby enhancing the reliability and efficiency of gas turbine engines.

Hydrodynamic and rheological characteristics of a pseudoplastic fluid through a rotating cylinder

The fully developed turbulent flow of pseudoplastic ( ) and Newtonian fluids in an isothermal axially rotating cylinder has been carried out using a large eddy simulation (LES) with an extended Smagorinsky model. The simulation Reynolds number of the present predictions has been assumed to be at various rotation rates This investigation seeks to assess the influence of the centrifugal force induced by the swirl on the mean flow quantities, turbulent statistics, and instantaneous turbulence structure to describe the rheological behavior and the turbulence features. The predicted results indicate that with increasing rotation rate, the pseudoplastic fluid tends to behave like a liquid when approaching the pipe center due to the lower apparent fluid viscosity in the logarithmic region as the pipe wall rotates. Moreover, the reduction in the pseudoplastic apparent viscosity in the core region induces a pronounced increase in the axial velocity profile further away from the pipe wall toward the core region. It is interesting to note that the growth of the centrifugal force induced by the swirl driven by the rotating pipe wall results in an apparent attenuation in turbulence intensities of the axial velocity fluctuation and, consequently, in the kinetic energy of turbulent fluctuations and the turbulent Reynolds shear stress of the axial-radial velocity fluctuations, as the pipe wall rotates. Moreover, the increased rotation rate leads also to a noticeable increase in the Root mean square (RMS) of the radial and tangential fluctuations. It can be said that the transport mechanism of turbulence intensities from the axial components to the other ones exhibits a marked increase with increasing pipe wall rotation.

Large Eddy Simulation of Compound Open Channel Flows with Floodplain Vegetation

Floodplain vegetation is of great importance in velocity distribution and turbulent coherent structure within compound open channel flows. As the large eddy simulation (LES) technique can provide detailed instantaneous flow dynamics and coherent turbulent structure predictions, it is of great importance to perform LES simulations of compound open channel flows with floodplain vegetation. In the present study, a wall-modeled large eddy simulation (WMLES) method was employed to simulate the compound open channel flows with floodplain vegetation. The vegetation-induced resistance effect was modeled with the drag force method. The WMLES model, incorporating the drag force method, was verified against flume measurements and an analytical solution of vegetated open channel flows. Numerical simulations were conducted with a depth ratio of 0.5 and four different floodplain vegetation densities (frk = 0, 0.28 m−1, 1.13 m−1 and 2.26 m−1). The main flow velocity, secondary flow, bed shear stress and vortex coherent structure, based on the Q criterion, were obtained and analyzed. Based on the numerical results, the influences of floodplain vegetation density on the flow field and turbulent structure of compound open channel flows were summarized and discussed. Compared to the case without floodplain vegetation, the streamwise velocity in the main channel increased by 10.8%, 19.9% and 24.4% with the frk = 0.28 m−1, 1.13 m−1 and 2.26 m−1, respectively. The results also indicated that, when the floodplain vegetation density increased, the following occurred: the velocity increased in the main channel, while the velocity decreased in the floodplain; the transverse momentum exchange was enhanced; and the strip structures were more concentrated near the junction area of compound open channel flows.

Assessment on heat transfer deterioration to supercritical carbon dioxide in upward flows via large eddy simulation

The direct numerical simulation (DNS) of supercritical carbon dioxide flowing in heated circular tubes by Bae et al. [Phys. Fluids, 2005, 17: 105104] is reproduced by the present large eddy simulation (LES). Cases of forced convection and mixed convection of upward flows in different-diameter tubes are considered, which were also re-run by Nemati et al. [Int. J. Heat Mass Transfer, 2015, 83: 741–752] and Chu et al. [J. Nucl. Eng. Raidat. Sci., 2016, 2: 031019] using DNS technology. Heat transfer deterioration and recovery are well captured in the simulation. The wall temperature predicted by current work shows excellent consistency with those DNS data, even lower than deviations among them. Buoyancy effect, mean flow characteristics and turbulence statistics are comparatively analyzed for the mixed-convection cases. For upward flow in the 1-mm tube, the turbulence kinetic energy and Reynolds stresses are continuously reduced along the tube, where heat transfer is inhibited by buoyancy. For upward flow in the 2-mm tube, the mean velocity profile distorts to an M−shaped distribution in the downstream of the tube, corresponding to the regeneration of turbulence kinetic energy and Reynolds stresses. The instantaneous turbulent streaks and vortex structures are displayed to intuitively understand the development of turbulence along the tube. Considering the huge cost of DNS on computing resources and time consumption, LES technology is a reliable and feasible means to explore supercritical heat transfer.

LES of Turbulent Flow through Square Duct with Rounded Corners

In the present study, large eddy simulation (LES) for turbulent flows through square ducts with four rounded corners is performed using immersed boundary (IB) method. The geometrical shape of the duct is determined by an equation |y|α+ |z|α= Rα with a deformation parameter α. The distribution and magnitude of the mean secondary flow with α≧4 is almost close to those of the square-duct case. The center position of secondary circulation flows with α≦4 approaches the square-side bisector in contrast to the square-duct case. The profiles of the Reynolds stress and behaviour of the instantaneous turbulent structures in case of α≧4 are similar with those in square duct case.

Hydrodynamic Modeling of Turbulence Modulation by Particles in a Swirling Gas-Particle Two-Phase Flow.

Turbulence modulations by particles of a swirling gas–particle two-phase flow in an axisymmetric chamber are numerically simulated. To fully consider the preferential concentrations and the anisotropic dispersions of particles, a second-order moment model coupling particle–particle collision model was improved. Experimental validation for the proposed model, algorithm, and in-house codes by acceptable match was carried out. The effects of ultralight-expanded graphite and heavy copper particles with a large span of Stokes number on gas velocities and fluctuations, Reynolds shear stresses and tensor invariants, turbulence kinetic energies, and vortice structures are investigated. The results show that turbulent modulation exhibits strong anisotropic characteristics and remains in a close relationship with the flow structure. Modulation disturbances and vortex evolution are enforced by heavy-large particles with higher Stokes numbers. Preferential accumulations of ultralight particles in shear stress regions at lower vortices are weaker than those of heavy particles. For axial turbulence modulations, a heavy particle plays the primary role in the inhibition action because of larger inertia, and a light particle contributes to the enhancement effect due to excellent followability. The instantaneous flow information and coherent turbulent structure are failed to be acquired due to the limitation of the Reynolds time-averaged algorithm.

Effect of Bridge Abutment Length on Turbulence Structure and Flow through the Opening

The method of large eddy simulation (LES) was employed to investigate the flow and turbulence structure around bridge abutments of different lengths placed in a compound, asymmetric channel. The simulations were faithful representations of large-scale physical model experiments that were conducted in the hydraulics laboratory at the Georgia Institute of Technology. The experiments are considered idealized hydraulic models of the Towaliga River bridge at Macon, Georgia, consisting of flat horizontal floodplains on both sides of a parabolic main channel, two spill-through abutments with varying lengths [long-set back (LSB) and short-set back (SSB)], and a bridge spanning across the abutments. In the LES, a free flow scenario was simulated where the water surface was not perturbed by the bridge at any point. The Reynolds numbers, based on the bulk velocity and hydraulic radius, were 76,300 and 96,500 for LSB and SSB abutments, respectively. Validation of the simulation results using data from the complementary experiment is presented and agreement is found to be reasonably good. A thorough comparison of various flow variables between LSB and SSB scenarios to highlight the effect of flow contraction was carried out in terms of flow separation and instantaneous secondary flow, streamwise velocity, streamlines, stream traces, and turbulence structures. Further flow instability and vortex shedding generated in the shear layer downstream of the abutments were quantified by analyzing time series of the instantaneous velocity in the form of the probability density function, quadrant analysis, and power density spectra.

Special Issue: Direct and Large Eddy Simulation

This Special Issue contains extended versions of 16 papers, selected from a total of about 120 contributions presented at the ERCOFTAC workshop Direct and Large-Eddy Simulation (DLES11), held in Pisa, Italy on May 29–31, 2017. The DLES Workshop series, which started in 1994, focuses on modern techniques designed to simulate turbulent flows based on a partial or full resolution of the instantaneous turbulent flow structures, such as Direct Numerical Simulation (DNS), Large-Eddy Simulation (LES) and hybrid models combining LES and RANS approaches.

Numerical Study of the Mixing Inside a Jet Stirred Reactor using Large Eddy Simulations

Jet Stirred Reactors (JSR) have been extensively used in the last decades to investigate gas phase chemical kinetics. Inside the JSR efficient mixing through turbulent jets is required in order to obtain homogeneous compositions. One of the best ways to achieve the mixing of the gas phase is to use turbulent jets obtained from nozzles. In our research, Computational Fluid Dynamics (CFD) simulations were applied to predict the mixing and flow field characteristics inside a spherical reactor. Large-Eddy Simulations (LES) were used to compute the residence time distribution and the mixing inside the JSR for different flow rates. Our simulations concern a non-reacting mixture at ambient conditions. The results agree well with tracer-decay data, experimentally measured using laser absorption spectroscopy, and with a CFD analysis of the mixing rate based on the Reynolds-Averaged Navier-Stokes (RANS) approach. Our simulations enable us to provide detailed information concerning the instantaneous turbulent structures which effectuate mixing inside the JSR.

Experimental and numerical investigation of turbulent isothermal and reacting flows in a non-premixed swirl burner

This paper focuses on the experimental and numerical investigation of CH4/air non-premixed flame stabilized over a swirler burner with radial fuel injectors. The flame operates under a global equivalence ratio Φ = 0.8, a high swirl number Sn = 1.4 and at atmospheric pressure. Reynolds averaged Navier-Stokes (RANS) calculations, Delayed-Detached Eddy Simulation (DDES) and experimental measurements are performed for both cases, non-reacting and reacting swirling flows. Numerical flow fields are compared with detailed Stereoscopic Particle Image Velocimetry (Stereo-PIV) fields under non-reactive and reactive conditions. Temperature measurements are also performed and compared to the computed ones in the reacting flow. The analysis of averaged results reveals the presence of a central recirculation zone (CRZ), a swirling jet region (SJ) and shear layers (SL) for both flows. The instantaneous turbulent structures at the burner exit, visualized by the Q-criterion, display different instability modes. The main instabilities are the vortex rings due to the Kelvin–Helmholtz instability, and finger structures generated by the swirling instability. The presence of the flame leads to increase the jet angle compared to the non-reacting flow. The main flame front is found highly wrinkled and rolled up around the vortex ring structures. A small flame ring is present near the fuel injector; it is formed due to the presence of a recirculation bubble (RB) at this region.

Dynamics of plumes in turbulent Rayleigh–Bénard convection

Direct Numerical Simulation of turbulent Rayleigh–Bénard convection of air (Pr=0.7) in an infinite horizontal layer is performed in the range 7×104≤Ra≤2×106. The incompressible Navier–Stokes equations with Boussinesq approximation are solved in a 6:6:1 horizontally periodic box using finite-difference method with a high resolution convective scheme and 2nd-order Adams–Bashforth Crank–Nicolson (ABCN) time-stepping. Instantaneous turbulent flow structures suggest the formation of thin two-dimensional thermal plumes with filament-like cross sections that form disorganized networks of randomly oriented cells near the solid walls, and which gradually expand to form broader plumes in the bulk region. The second invariant technique and the second largest negative eigenvalue method yield identical flake-like vortical structures in the flow. The primary source of plume formation is observed to be boundary layer instabilities. Strong evidence of background large-scale convection cells with horizontal movement is found. Horizontal velocity components near the opposite walls are in opposite phase. The sustained opposite phase motion is disturbed by boundary layer instabilities causing bursts of vertical motion which act as the source of plume formation. The large scale motion yields a quasi-periodic state whose periodicity is of the order of the diffusion time-scale of the flow.

Turbulence dynamics in separated flows: the generalised Kolmogorov equation for inhomogeneous anisotropic conditions

The generalised Kolmogorov equation is used to describe the scale-by-scale turbulence dynamics in the shear layer and in the separation bubble generated by a bulge at one of the walls in a turbulent channel flow. The second-order structure function, which is the basis of such an equation, is used as a proxy to define a scale-energy content, that is an interpretation of the energy associated with a given scale. Production and dissipation regions and the flux interchange between them, in both physical and separation space, are identified. Results show how the generalised Kolmogorov equation, a five-dimensional equation in our anisotropic and strongly inhomogeneous flow, can describe the turbulent flow behaviour and related energy mechanisms. Such complex statistical observables are linked to a visual inspection of instantaneous turbulent structures detected by means of the Q-criterion. Part of these turbulent structures are trapped in the recirculation where they undergo a pseudo-cyclic process of disruption and reformation. The rest are convected downstream, grow and tend to larger streamwise scales in an inverse cascade. The classical picture of homogeneous isotropic turbulence in which energy is fed at large scales and transferred to dissipate at small scales does not simply apply to this flow where the energy dynamics strongly depends on position, orientation and length scale.

Coherent structures, uniform momentum zones and the streamwise energy spectrum in wall-bounded turbulent flows

Large-scale motions (LSMs) in wall-bounded turbulent flows have well-characterised instantaneous structural features (Kovasznay et al., J. Fluid Mech., vol. 41 (2), 1970, pp. 283–325; Meinhart & Adrian, Phys. Fluids, vol. 7 (4), 1995, pp. 694–696) and a known spectral signature (Monty et al., J. Fluid Mech., vol. 632, 2009, pp. 431–442). This work aims to connect these previous observations through the development and analysis of a representative model for LSMs. The model is constructed to be consistent with the streamwise energy spectrum (Monty et al. 2009) and amplification characteristics of the Navier–Stokes equations (McKeon & Sharma, J. Fluid Mech., vol. 658, 2010, pp. 336–382), and is found to naturally recreate characteristics of instantaneous turbulent structures, including a bulge shape (Kovasznay et al. 1970) and the presence of uniform momentum zones (Meinhart & Adrian 1995) in the streamwise velocity field. The observed structural similarity between the LSM representative model and instantaneous experimental data supports the use of travelling wave models to connect statistical and instantaneous descriptions of coherent structures, and clarifies a simple general equivalency between symmetry in a Reynolds-decomposed velocity field and asymmetry in the laboratory frame.

Proper-Orthogonal-Decomposition Study of Turbulent Near Wake of S805 Airfoil in Deep Stall

A proper-orthogonal-decomposition analysis was performed on the turbulent flow in the near-wake region of an S805 airfoil in deep stall at an angle of attack of 30 deg. The flow was measured using tomographic particle image velocimetry at a Reynolds number of 4600. Instantaneous turbulence structures, which were significant contributors to the first two proper-orthogonal-decomposition modes, were studied. These structures included the large-scale Kármán vortex and small-scale shear-layer vortices that were interacting with the Kármán vortex. An interesting correspondence was observed between the rotational vectors in proper-orthogonal-decomposition modes 4 and 5 and the locations of the leading-edge shear-layer vortices in the flowfields, which were major contributors to these two modes. Similarity was found in the autospectral functions computed from the velocity fields, which were significant contributors to the first five proper-orthogonal-decomposition modes. The wavelengths corresponding to the peak values of autospectral functions could be related to the size of the induced flows by the Kármán vortex and the streamwise spacing of the shear-layer vortices.

Experimental assessment of characteristic turbulent scales in two-phase flow of hydraulic jump: from bottom to free surface

A hydraulic jump is a turbulent shear flow with a free-surface roller. The turbulent flow pattern is characterised by the development of instantaneous three-dimensional turbulent structures throughout the air–water column up to the free surface. The length and time scales of the turbulent structures are key information to describe the turbulent processes, which is of significant importance for the improvement of numerical models and physical measurement techniques. However, few physical data are available so far due to the complexity of the measurement. This paper presents an investigation of a series of characteristic turbulent scales for hydraulic jumps, covering the length and time scales of turbulent flow structures in bubbly flow, on free surface and at the impingement point. The bubbly-flow turbulent scales are obtained for Fr = 7.5 with 3.4 × 104 < Re < 1.4 × 105 in both longitudinal and transverse directions, and are compared with the free-surface scales. The results highlight three-dimensional flow patterns with anisotropic turbulence field. The turbulent structures are observed with different length and time scales respectively in the shear flow region and free-surface recirculation region. The bubbly structures next to the roller surface and the free-surface fluctuation structures show comparable length and time scales, both larger than the scales of vortical structures in the shear flow and smaller than the scales of impingement perimeter at the jump toe. A decomposition of physical signals indicates that the large turbulent scales are related to the unsteady motion of the flow in the upper part of the roller, while the high-frequency velocity turbulence dominates in the lower part of the roller. Scale effects cannot be ignored for Reynolds number smaller than 4 × 104, mainly linked to the formation of large eddies in the shear layer. The present study provides a comprehensive assessment of turbulent scales in hydraulic jump, including the analyses of first data set of longitudinal bubbly-flow integral scales and transverse jump toe perimeter integral scales.

On the turbulent flow in piston engines: Coupling of statistical theory quantities and instantaneous turbulence

Planar particle image velocimetry (PIV) and tomographic PIV (TPIV) measurements are utilized to analyze turbulent statistical theory quantities and the instantaneous turbulence within a single-cylinder optical engine. Measurements are performed during the intake and mid-compression stroke at 800 and 1500 RPM. TPIV facilitates the evaluation of spatially resolved Reynolds stress tensor (RST) distributions, anisotropic Reynolds stress invariants, and instantaneous turbulent vortical structures. The RST analysis describes distributions of individual velocity fluctuation components that arise from unsteady turbulent flow behavior as well as cycle-to-cycle variability (CCV). A conditional analysis, for which instantaneous PIV images are sampled by their tumble center location, reveals that CCV and turbulence have similar contributions to RST distributions at the mean tumble center, but turbulence is dominant in regions peripheral to the tumble center. Analysis of the anisotropic Reynolds stress invariants reveals the spatial distribution of axisymmetric expansion, axisymmetric contraction, and 3D isotropy within the cylinder. Findings indicate that the mid-compression flow exhibits a higher tendency toward 3D isotropy than the intake flow. A novel post-processing algorithm is utilized to classify the geometry of instantaneous turbulent vortical structures and evaluate their frequency of occurrence within the cylinder. Findings are coupled with statistical theory quantities to provide a comprehensive understanding of the distribution of turbulent velocity components, the distribution of anisotropic states of turbulence, and compare the turbulent vortical flow distribution that is theoretically expected to what is experimentally observed. The analyses reveal requisites of important turbulent flow quantities and discern their sensitivity to the local flow topography and engine operation.

Effects of oblique and transverse injection on the characteristics of jet in supersonic crossflow

The effect of oblique and transverse injection on the transverse jet was investigated by the hybrid RANS/LES simulation with recycling–rescaling procedure. The streamwise velocity distribution and instantaneous fine turbulent structures of transverse jet obtained by the particle image velocimetry (PIV) and nanoparticle-based planar laser-scattering (NPLS) are applied to validate the accuracy of simulation approach. Statistics obtained from the hybrid RANS/LES simulation with fine mesh shown good agreement with the experimental results. Four kinds of vortex structures are observed in the flow field, namely leading edge vortices, hanging vortices, counter-rotating vortex pairs (CVPs) and horseshoes vortex. The instantaneous results and spatial correlation analysis show that the size and interval of large-scale structures in windward shear layer of 45o jet are smaller and shorter than that of 90o jet, respectively. Compared with the 45o jet, the stronger shear between injectant and crossflow leads to the early breakup of CVPs in 90o jet, so the length of CVPs in 90o jet is shorter than that of 45o jet in time-averaged results. The simulation also shows that the rotating number of CVPs in 45o jet is more than in 90o jet, but the mixing of 45o jet is worse than that of 90o jet, which indicates that the large-scale structures in the shear layer between the crossflow and injectant make more contribution to the mixing process.

Free-Surface versus Rigid-Lid LES Computations for Bridge-Abutment Flow

Two separate large eddy simulations (LES) are carried out to investigate the effects of accurate computation of the curvilinear water-surface deformation of the flow through a bridge contraction. One LES employs the rigid-lid boundary condition at the water surface, while the other uses a highly accurate level-set method (LSM), which allows the water surface to adjust itself freely in response to the flow. The simulation with the LSM is validated with data from complementary physical model tests under analogous geometrical and flow conditions. Streamwise velocity, bed-shear stress and second-order turbulence statistics obtained from both simulations are compared, and it is shown that the turbulence structure of this flow is influenced strongly by the water-surface deformation. While bed-shear stresses and first-order statistics are very similar for both cases, the instantaneous turbulence structure and consequently, the second-order statistics, are distinctly different. The correct prediction of the water-surface deformation of such flows is deemed important for the accuracy of their simulation. Read More: http://ascelibrary.org/doi/10.1061/%28ASCE%29HY.1943-7900.0001028

Particle Transport and Deposition in a Turbulent Square Duct Flow With an Imposed Magnetic Field

In this paper, we study particle transport and deposition in a turbulent square duct flow with an imposed magnetic field using direct numerical simulations (DNS) of the continuous flow and Lagrangian tracking of particles. The magnetic field and the velocity induce a current and the interaction of this current with the magnetic field generates a Lorentz force that brakes the flow and modifies the flow structure. A second-order accurate finite volume method is used to integrate the coupled Navier–Stokes and magnetohydrodynamic (MHD) equations and the solution procedure is implemented on a graphics processing unit (GPU). Magnetically nonconducting particles of different Stokes numbers are continuously injected at random locations in the inlet cross section of the duct and their rates of deposition on the duct walls are studied with and without a magnetic field. Because of the modified instantaneous turbulent flow structures as a result of the magnetic field, the deposition rates and patterns on the walls perpendicular to the magnetic field are lower than those on the walls parallel to the magnetic field.