Reduction Of Vorticity Research Articles

Mitigation of flow-induced vibrations in high-speed flows using triply periodic minimal surface structures

Flow-induced vibrations (FIVs) pose challenges and limit intended functionalities in many industrial sectors, especially in high-tech industries such as semiconductor manufacturing. Because of extreme precision requirements, any mechanical perturbation to the manufacturing process detrimentally affects production quality and process yield. The impact of FIV must, thus, be minimized. This study investigates the FIV reducing properties of triply periodic minimal surface (TPMS) inserts in industrial cooling systems by expanding the Darcy–Forchheimer model for flow through porous media to a turbulent regime. Using the expanded model, we performed full-scale finite-element simulations for high-speed flows in a pipe with an abrupt expansion of diameter and analyzed the effects of TPMS inserts on the reduction of induced turbulence and FIV. Our data revealed that the TPMS inserts exhibit promising characteristics for FIV mitigation revealing up to 97.6% reduction in turbulent kinetic energy and 32.7% reduction in vorticity. These results complement available data on using TPMS inserts to eliminate geometry-induced instabilities in internal pipe flows.

Fusiform versus Saccular Intracranial Aneurysms-Hemodynamic Evaluation of the Pre-Aneurysmal, Pathological, and Post-Interventional State.

Minimally-invasive therapies are well-established treatment methods for saccular intracranial aneurysms (SIAs). Knowledge concerning fusiform IAs (FIAs) is low, due to their wide and alternating lumen and their infrequent occurrence. However, FIAs carry risks like ischemia and thus require further in-depth investigation. Six patient-specific IAs, comprising three position-identical FIAs and SIAs, with the FIAs showing a non-typical FIA shape, were compared, respectively. For each model, a healthy counterpart and a treated version with a flow diverting stent were created. Eighteen time-dependent simulations were performed to analyze morphological and hemodynamic parameters focusing on the treatment effect (TE). The stent expansion is higher for FIAs than SIAs. For FIAs, the reduction in vorticity is higher (Δ35-75% case 2/3) and the reduction in the oscillatory velocity index is lower (Δ15-68% case 2/3). Velocity is reduced equally for FIAs and SIAs with a TE of 37-60% in FIAs and of 41-72% in SIAs. Time-averaged wall shear stress (TAWSS) is less reduced within FIAs than SIAs (Δ30-105%). Within this study, the positive TE of FDS deployed in FIAs is shown and a similarity in parameters found due to the non-typical FIA shape. Despite the higher stent expansion, velocity and vorticity are equally reduced compared to identically located SIAs.

Reduced systolic flow efficacy in left ventricular dyssynchrony evident by blood speckle imaging

Abstract Introduction Intraventricular vortices facilitate efficient blood flow in a healthy heart. Changes in the blood flow pattern can cause energy loss and inefficient filling or ejection. Thus changes, or reduction in vorticity may therefore be of importance for cardiac function. Blood speckle imaging (BSI) is a novel blood flow visualization tool based on echocardiography, partially overcoming the angel dependency of conventional colour Doppler. BSI tracks speckles generated by the moving blood cells, similar to myocardial speckle tracking, but requires a very high frame rate limiting penetration. Dyssynchrony in left bundle branch block (LBBB) affects the systolic functions but there is limited insight into how LBBB alters the blood flow vorticity. Purpose We investigated the effect of dyssynchrony on blood flow hemodynamics using blood speckle imaging. Methods In eight anesthetized canine mongrels, left ventricular flow was visualized using BSI. A qualitative visual assessment of vortices during the cardiac cycle, together with a quantitative assessment based on kinetic energy loss and vorticity were made. Regional myocardial function parameters were measured by sonomicrometry. Peak septal flash strain in isovolumetric contraction (IVC) was measured by sonomicrometry (figure) as an indicator of dyssynchrony. Assessments were made at baseline, under right ventricular free wall pacing (RV pacing) mimicking LBBB, after induced LBBB by ablation and under cardiac resynchronization therapy (CRT). Results During IVC at baseline all animals had a central ventricular vortex. However during RV pacing and LBBB it was distorted in various patterns. RV pacing significantly reduced the number of vortices in systole and disrupted the central vortex in IVC (1.0±0.0 vs 0.4±0.5, p=0.01). Energy loss (EL) in systole increased (40.2±12.5 vs 55.3±23.3 mW/m, p<.05). Septal end systolic strain was significantly reduced (-8.8±5.1 vs -1.9±7.3 %, p<0.01) and septal flash strain was significantly increased (1.0±1.4 vs 7.7±3.6 and 6.6±3.8 [ER1] [ER2] %, p<0.01) in RV pacing and LBBB. CRT caused a realignment of the central vortex (0.8±0.5 vs0.3±0.5, p=0.03). The change in energy loss was not significant, but numerically reduced (55.4±38.8 vs 76.4±48.9 mW/m, p=0.06). See table and figure 1. Conclusions The main systolic vortex in IVC was disrupted during septal flash by RV pacing and LBBB, altering the flow pattern compared to baseline. The resultant unstructured flow pattern had increased energy loss in systole, indicating a less efficient flow pattern and less preservation of energy. It should be further explored if this is of importance to understand the reduced LV function and LV remodelling during dyssynchrony.Figure 1Table 1

Experimental investigation of flow characteristics over a surface-modified structured torpedo model

The present work aims to improve the aerodynamic performance of navigating objects using bio-inspired surface modification. Inspired by the denticles present over sharkskin, this work investigates the study of a surface-modified structured torpedo model. Different modified surface structures (riblets) are used to study their effect on the drag and flow characteristics. The modified structure placed in the shear layer modifies the flow above as well as behind the torpedo. A detailed flow field is studied using PIV (Particle Image Velocimetry) and CTA (Constant Temperature Anemometry) over the torpedo model. The experiments are performed in a subsonic wind tunnel for the Reynolds numbers (Re) range of 1.02–5.08 × 105, thus covering the laminar-turbulent transition range. The study reveals that riblets structure enhances the momentum flux, thus enabling the surface to reduce drag by suppressing the vorticity generation and reducing the turbulent dissipation. Different turbulent and wake characteristics are measured in the work. The velocity and fluctuation analysis shows the variation in wake width and wake velocity for different cases. The result showed that riblet effectively reduces drag coefficient by 11.7% using an optimal structure. The maximum reduction in formation length of 4.76% is observed at Re 3.05 × 105. The decrease in the formation length reduces the adverse pressure gradient zone and decreases the power requirement. The wake half-width at the 2D downstream location in the streamwise direction shows a reduction of 9% for the drag-reducing case at Re 3.05 × 105. The vorticity reduction is observed in the wake zone of drag-reducing riblets and suggests less dissipation of the eddy. The results show the perspective to improve the current research status by using riblet surface for marine application. The study could effectively reduce drag, thus improving the operating performance by reducing the energy consumption and increasing the speed of the navigating vessel.

Velocity measurement of magnetic particles simultaneously affected by two-phase flow and an external magnetic field using dual-sided SPIM-µPIV

Micro-particle image velocimetry (µPIV) measurement in gas–liquid flow is a challenging task mainly due to the volume illumination of the laser light. A special solution frequently adopted is the use of a thin laser sheet with a reduced macro-scale 2D-PIV illumination method. In this study, a novel dual-sided single-plane illumination microscopy-micro particle image velocimetry (SPIM-µPIV) technique was developed and successfully applied to multi-phase flow. The velocity of magnetic particles in a power-generation unit using waste heat simultaneously affected by two-phase flow and an external magnetic field was measured. This technique illuminates a light source in a thin sheet form, thus reducing errors due to particle overlap. Both sides of the laser sheet were illuminated to prevent particle shadowing and amplify the fluorescence signal. The velocity distributions of the wall and center were measured simultaneously. The average magnetic particle diameter was 200 nm; the excitation and emission wavelengths were 530 and 650 nm, respectively. The central magnetic force of the external magnetic field was 0.23 T. The velocity of the magnetic particles was measured under bubbly and slug flows, where magnetic particles are disturbed and power can be generated. For the bubbly flow, the velocity and vorticity were reduced by 11.89% and 32.29%, respectively, and by 34.70% and 8%, respectively, for the slug flow. The bubbly and slug flows showed opposite results in terms of velocity and vorticity reduction.

Experimental and Computational Analysis of Bio-Inspired Winglets for Micro Air Vehicles

Micro Air Vehicles (MAVs) are increasingly being used for civil and military surveillance. As the surveillance requirements are increasing, improving the range of MAVs becomes imperative. The performance of MAVs can be improved if the induced drag due to wingtip vortices can be reduced. In the present study, we try to decrease the induced drag caused by the wingtip vortices, which makes up a major part of the total drag, by introducing a winglet. A unique yet simple design, which has not yet been studied thoroughly, is explored. Inspiration is taken from the feather structures of birds to design the proposed winglet. The performance of a fixed-wing MAV at a free stream velocity (U∞) of 20 m/s is studied. Multiple winglet configurations are used to compare the results with the baseline wing. An incompressible, steady three-dimensional simulation is carried out using the k-ω SST turbulence model. The experimental studies carried out for the baseline wing matched well with those obtained from CFD. Since the numerical model is valid, only computational study is done for the modified wing. The stall angle of the baseline wing is around 26°. Numerical results show that when the proposed winglet is used, the stall angle for the wing is increased to around 32°. The use of the winglet did not produce a considerable advantage at the lower Angle of Attack (AOA), but at higher AOA, the lift coefficient (CL) was considerably higher. The overall drag coefficient (CD) was higher at lower AOA when the winglet is used. But at AOAs greater than 5°, the CD reduced. Other effects of the winglet are addressed in terms of improvement in Lift-to-Drag ratio (L/D) and reduction in vorticity. The effect of the location and number of the feathers was studied to come up with an optimum winglet configuration. The experiments were carried for the wing with optimum winglet configuration and the results agreed fairly with the numerical results.

Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection: Vorticity with ambipolar and Hall effect

This paper presents the results of the analysis of three-dimensional simulations of solar magneto-convection that include the joint action of the ambipolar diffusion and the Hall effect. Three simulation runs are compared: One including both ambipolar diffusion and the Hall effect; one including only ambipolar diffusion and one without any of these two effects. The magnetic field is amplified from initial field to saturation level by the action of turbulent local dynamo. In each of these cases, we study 2 h of simulated solar time after the local dynamo reaches the saturation regime. We analyse the power spectra of vorticity, of magnetic field fluctuations and of the different components of the magnetic Poynting flux responsible for the transport of vertical or horizontal perturbations. Our preliminary results show that the ambipolar diffusion produces a strong reduction of vorticity in the upper chromospheric layers and that it dissipates the vortical perturbations converting them into thermal energy. The Hall effect acts in the opposite way, strongly enhancing the vorticity. When the Hall effect is included, the magnetic field in the simulations becomes, on average, more vertical and long-lived flux tube-like structures are produced. We trace a single magnetic structure to study its evolution pattern and the magnetic field intensification, and their possible relation to the Hall effect. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection.

This paper presents the results of the analysis of three-dimensional simulations of solar magneto-convection that include the joint action of the ambipolar diffusion and the Hall effect. Three simulation runs are compared: one including both ambipolar diffusion and the Hall effect; one including only ambipolar diffusion and one without any of these two effects. The magnetic field is amplified from initial field to saturation level by the action of turbulent local dynamo. In each of these cases, we study 2 h of simulated solar time after the local dynamo reaches the saturation regime. We analyse the power spectra of vorticity, of magnetic field fluctuations and of the different components of the magnetic Poynting flux responsible for the transport of vertical or horizontal perturbations. Our preliminary results show that the ambipolar diffusion produces a strong reduction of vorticity in the upper chromospheric layers and that it dissipates the vortical perturbations converting them into thermal energy. The Hall effect acts in the opposite way, strongly enhancing the vorticity. When the Hall effect is included, the magnetic field in the simulations becomes, on average, more vertical and long-lived flux tube-like structures are produced. We trace a single magnetic structure to study its evolution pattern and the magnetic field intensification, and their possible relation to the Hall effect. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

The Effect of Gas Nonideality on the Interface Reflectivity When Interacting With a Shock Wave

The phenomenon of shock wave refraction on a gaseous interface with plasma was analyzed for nonideal gas effects. The developed model was applied to nondissociating diatomic nitrogen gas at 3000 K. Comparatively to ideal gas, the interface reflectivity was found noticeably decreased resulting in 12% more intense shock transmission. The transition point determining the character of reflected wave was also found shifted with its coordinate being strongly dependent on incident shock strength and sort of gas. It was found that a delay in gas pressure drop and a sharp reduction of vorticity generated in the postshock flow are possible. The influence of real-gas effects on the shock instability locus, interface distortion, and wave drag reduction is discussed.

Effect of a Directionally Porous Wing Tip on Tip Vortex

This paper presents an experimental study of the effect of a directionally porous wing tip on the tip vortex using particle image velocimetry (PIV) on a half wing model with NACA 653218 as its airfoil section. Four different configurations of the directionally porous wing tip are tested. The vortex generated by the wing tips are examined at four different measuring planes downstream perpendicular to the flow axis. The flow field over the porous wing tip surface along the streamwise direction is obtained as well to understand the effects of the porosity on the flow which in the end affects the vortex downstream. Furthermore, the aerodynamic performance of all different configurations is compared to study their effects on the aerodynamic coefficients of the wing. The results show a high reduction in vorticity, up to 90%; tangential velocity reduction up to 67% and a significant reduction in vortex circulation in the near-far field. Effect on the lift to drag ratio is up to 20 %.

Flow Characteristics Around Step-Up Street Canyons with Various Building Aspect Ratios

We investigate the flow characteristics around step-up street canyons with various building aspect ratios (ratio of along-canyon building length to street-canyon width, and upwind building height to downwind building height) using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations as those in the wind-tunnel experiments. The CFD model reproduces the measured in-canyon vortex, rooftop recirculation zone above the downwind building, and stagnation point position reasonably well. We analyze the flow characteristics, focusing on the structural change of the in-canyon flows and the interaction between the in- and around-canyon flows with the increase of building-length ratio. The in-canyon flows undergo development and mature stages as the building-length ratio increases. In the development stage (i.e., small building-length ratios), the position of the primary vortex wanders, and the incoming flow closely follows both the upstream and downstream building sidewalls. As a result, increasing momentum transfer from the upper layer contributes to a momentum increase in the in-canyon region, and the vorticity in the in-canyon region also increases. In the mature stage (i.e., large building-length ratios), the primary vortex stabilizes in position, and the incoming flow no longer follows the building sidewalls. This causes momentum loss through the street-canyon lateral boundaries. As the building-length ratio increases, momentum transfer from the upper layer slightly decreases, and the reverse flow, updraft, and streamwise flow in the in-canyon region also slightly decrease, resulting in vorticity reduction.

Large eddy simulation of the tip-leakage cavitating flow with an insight on how cavitation influences vorticity and turbulence

• The tip-leakage cavitating flow is simulated by LES with Cartesian cut-cell mesh. • Three types of cavitating vortical flows, including TLV, TSV and IV, are analyzed from Euler/Lagrangian viewpoints. • Three stages for the spatial–temporal evolution of tip-leakage cavitating flow are discussed. • The physical mechanism for cavitation-turbulence interaction is clarified. Cavitation within a tip leakage flow remains a challenging issue in a variety of axial hydraulic machines. It is still not possible nowadays to predict cavitation occurrence in such a flow with acceptable accuracy. In the present study, we have carried out numerical simulations of a tip leakage cavitating flow, generated by a straight NACA0009 hydrofoil. We have used the LES method combined with the Schnerr–Sauer cavitation model. The numerical results agree well with experimental data. The evolution of the tip leakage cavitating flow, involving tip-leakage vortex (TLV), tip-separation vortex (TSV) and induced vortex (IV), is analyzed from Eulerian and Lagrangian viewpoints. The results show that the spatial evolution of the tip leakage cavitating flow can be divided into three stages: Stage Ⅰ, Independent development of the TLV and TSV; Stage Ⅱ, Fusion of the TLV and TSV; and Stage Ⅲ, Development of the IV and dissipation of the TLV. The Lagrangian coherent structures (LCSs) obtained from the numerical results indicate that the TLV cavitation significantly influence the local flow patterns. The vorticity transport equation was then used to further analyze the influence of the cavitation on the vortices. The results demonstrate that the stretching term dominates the TLV evolution and the dilatation term is responsible for the vorticity reduction inside the TLV cavity. The results also show how the cavitation influences the local turbulence and that the transport term in the turbulent kinetic energy equation influences the turbulence distribution near the TLV cavity.

Three‐dimensional transient rip currents: Bathymetric excitation of low‐frequency intrinsic variability

AbstractThe ROMS‐WEC model [Uchiyama et al., 2010] based on an Eulerian wave‐averaged vortex‐force asymptotic theory of McWilliams et al. (2004) is applied to analyze 3‐D transient wave‐driven rip currents and associated intrinsic very low‐frequency (VLF) variability in the surf zone on a surveyed bathymetry under spatiotemporally uniform offshore incident waves. The 3‐D rip currents are substantially depth‐dependent due to the vertical recirculation, composed of pairs of counter‐rotating longitudinal overturning roll cells that promote surface convergence. The vortex force plays an important role in vorticity budget, preconditioning overall vorticity reduction. These rip currents are intrinsically unstable and contribute about 70% to kinetic energy (KE) as eddy kinetic energy (EKE), consistent with observations. The dominant fluctuation period fits the VLF band, at about 18 min. The current effect on waves (CEW) alters not only the mean rip structure, but also the associated turbulence as the modified cross‐shore EKE profile with considerable accentuation in the inner surf zone. Increased alongshore bathymetric variability proportionally intensifies KE and intrinsic EKE, whereas it reduces the VLF period. With a guide of a pseudo 2D model, we reveal that vortex tilting effect due to the horizontal vorticity inherent in the 3‐D rip currents promotes collapse of the 3‐D eddies through an enhanced forward kinetic energy cascade, leading to short‐lived, laterally‐stretched 3‐D eddies resulting in elongated filaments that decay more quickly than coherent, long‐lived, circular 2‐D eddies.

Enhancement of vorticity reduction by floor splitter in pump sump to improve pump efficiency

The effort to reduce swirls and vortices in pump intake which originated from high vorticity regions in the pumped fluid has been a major task in pump station design because it affects the pump performance and ultimately the energy consumption. In this paper, a numerical study on the influence of cross sectional shape of floor splitter on the effectiveness of vorticity reduction in pump intake was carried out with the aid of computational fluid dynamics (CFD) software. A simulation model setup consisting of a rectangular channel and a column pipe positioned at the downstream of the channel was developed with the incorporation of Reynolds-Averaged Navier–Stokes (RANS) k-∊ turbulence model. The model was tested with the installation of floor splitter at the bottom of the channel directly beneath the column pipe. Four floor splitters with different cross sectional shapes were examined in the simulation. The results showed that the use of floor splitter is effective in reducing flow vorticity. Two of the floor splitter types, which have square and trapezoidal cross sections, produced results similar to the findings in the experiment by Kang et al. (2014) and thus gives some certainties to simulation. Triangular cross section, which was derived from the trapezoidal cross section, exhibited the best overall performance in vortex reduction. On the other hand, another shape derived from the trapezoid, which was a trapezoidal cross section with rounded edges, displayed superior vortex suppression than triangular cross section near pipe entrance but limited to areas without interference from counter flows such as the flow near the back wall. Triangular cross section displayed better performance in the region with counter flows thanks to its sharp apex. These results are helpful to determine our ongoing experimental study as well as actual site installation.

Damping insert materials for settling chambers of supersonic wind tunnels

This study describes the application of a novel damping insert material for reducing the flow fluctuations in a tandem nozzle supersonic wind tunnel. This new damping material is composed of multi-layer stainless steel wired meshes. The influences of the multi-layer mesh, such as the quantity of the mesh layer and the installed location in the settling chamber, to the freestream quality have been investigated. A Pitot probe instrumented with a Kulite pressure sensor and a hot-wire probe are employed to monitor the flow fluctuation in the test section of the wind tunnel. Thereafter, a combined modal analysis is applied for the disturbance qualification. Additionally, the transient Mach number in the test section is measured. The disturbance qualification indicates that the multi-layer mesh performs well in providing reduction of vorticity reduction and acoustic fluctuations. Comparable flow quality of the freestream was also obtained using a combination of flexible damping materials. However, the life-span of the new damping materials is much longer. The time transient of the Mach number measured in the test section indicates that the mean flow is rather constant over run time. Furthermore, the time-averaged pressure along the settling chamber is recorded and it shows the distribution of pressure drop by settling chamber inserts.

Why did the storm ex-Gaston (2010) fail to redevelop during the PREDICT experiment?

Abstract. An analysis is presented of the failed re-development of ex-Gaston during the 2010 PREDICT field campaign based on the European Centre for Medium Range Weather Forecast (ECMWF) analyses. We analyze the dynamics and kinematics of ex-Gaston to investigate the role of dry, environmental air in the failed redevelopment. The flow topology defined by the calculation of particle trajectories shows that ex-Gaston's pouch was vulnerable to dry, environmental air on all days of observations. As early as 12:00 UTC 2 September 2010, a dry layer at and above 600 hPa results in a decrease in the vertical mass flux and vertical relative vorticity. These findings support the hypothesis that entrained, dry air near 600 hPa thwarted convective updraughts and vertical mass flux, which in turn led to a reduction in vorticity and a compromised pouch at these middle levels. A compromised pouch allows further intrusion of dry air and quenching of subsequent convection, therefore hindering vorticity amplification through vortex tube stretching. This study supports recent work investigating the role of dry air in moist convection during tropical cyclogenesis.

An Analysis of the Wake of Pedalling Cyclists in a Tandem Formation

Aerodynamics is a critical factor in the performance of cyclists, with drag being the dominant resistive force at racing speeds. Many cycling events involve athletes riding in close proximity. Previous studies have shown that flow interactions result in significant drag reductions for tandem cyclists, inline parallel to the flow [1–5]. However, the flow mechanisms associated with these changes have not been well characterised to date.This paper reports the results of wake surveys of an athlete in single and tandem formations in the Monash Wind Tunnel. Flow measurements were phase averaged over segments of the crank cycle to investigate the dynamic evolution of the wake of a pedalling cyclist. Results show that the dominant hip vortices previously identified in the wake of a single cyclist remain dominant in the trailing cyclist wake, with minimal reduction in vorticity. Wake profiles indicate the large drag saving experienced by a trailing cyclist are not due to disruption of the wake vortices or significant streamwise momentum recovery.

Three-dimensional hemodynamic design optimization of stents for cerebral aneurysms

Flow-diverting stents occlude aneurysms by diverting the blood flow from entering the aneurysm sac. Their effectiveness is determined by the thrombus formation rate, which depends greatly on stent design. The aim of this study was to provide a general framework for efficient stent design using design optimization methods, with a focus on stent hemodynamics as the starting point. Kriging method was used for completing design optimization. Three different cases of idealized stents were considered, and 40-60 samples from each case were evaluated using computational fluid dynamics. Using maximum velocity and vorticity reduction as objective functions, the optimized designs were identified from the samples. A number of optimized stent designs have been found from optimization, which revealed that a combination of high pore density and thin struts is desired. Additionally, distributing struts near the proximal end of aneurysm neck was found to be effective. The success of the methods and framework devised in this study offers a future possibility of incorporating other disciplines to carry out multidisciplinary design optimization.

Study of parameters for evaluating flow reduction with stents in a sidewall aneurysm phantom model.

The effect of stent design parameters such as porosity, pore density, number of strands, and strut angle to the artery were studied in vitro using particle image velocimetry (PIV). Five mesh stents were implanted into a sidewall aneurysm model. The flow features in a sidewall aneurysm silicone phantom model were investigated at a Reynolds number of 300. It was found that the lowest porosity stent had the best value for velocity and vorticity reduction in an aneurysm pocket. The stent with higher pore density had a tendency to decrease the mean and maximum velocities, but it was not superior to the effects of porosity. In addition, investigation of the evaluation system confirmed that higher stent strut angles to the parent artery had a tendency to lower mean velocity, as shown by PIV and CFD. However, this effect was relatively smaller compared to porosity and pore density. Our evaluation system suggested the best combinations of parameters for the development of an ideal stent would be lower porosity, higher pore density, and higher strut angle. The results obtained in this study indicated that our evaluation system could be useful for various purposes related to evaluation of endovascular interventional devices.

Spectral nudging sensitivity experiments in a regional climate model

In this study, the scale selective bias correction (SSBC) method described by Kanamitsu et al. (2010) is further examined by considering the full wind nudging and the vertically weighted damping coefficient. The 2001 June–July–August RSM simulation over a relatively large domain covering much of the Asian continent, the northern part of Australia, and the Indian and western Pacific oceans was the main focus. The full wind nudging shows wind fields closer to the driving global analysis. However, it leads to significantly distorted fields (e.g., temperature and geopotential height) aloft, accompanying excessive precipitation over the western Pacific. The gradual reduction of vorticity nudging from the model top to the ground surface improves rainfall patterns without a discernible distortion of large-scale fields. Further evaluation of a 10-year-summer simulation over East Asia confirmed that this revised SSBC method improves the monsoonal rainfall against the method of Kanamitsu et al. It is therefore concluded that vorticity nudging alleviates largescale errors by maintaining the near geostrophic balance between mass and winds. The reduction of this nudging factor in the lower troposphere allows the ageostrophic component of wind to develop as in nature, which leads to the improvement of precipitation.