Surrounding Flow Field Research Articles

Active flow control on a 1:4 car model

Lift and drag of a passenger car are strongly influenced by the flow field around its rear end. The bluff body geometry produces a detached, transient flow which induces fluctuating forces on the body, affecting the rear axle, which may distress dynamic stability and comfort significantly. The investigations presented here deal with a 1:4 scale model of a simplified test car geometry that produces fluctuating lift and drag due to its strongly rounded rear geometry. To examine the influence of active flow control on this behavior, steady air jets were realized to exhaust from thin slots across the rear in three different configurations. Investigations were performed at \(Re = 2.1 \times 10^{6}\) and included the capturing of effective integral lift and drag, velocity measurements in the surrounding flow field with Laser Doppler Anemometry, surface pressure measurements and surface oil flow visualization on the rear. The flow field was found to be dominated by two longitudinal vortices, developing from the detachment of the flow at the upper C-pillar positions, and a recirculating, transverse vortex above the rear window. With an air jet emerging from a slot across the surface right below the rear window section, tangentially directed upstream toward the roof section, total lift could be reduced by more than 7 %, with rear axle lift reduction of about 5 % and negligible drag affection (\(<\)1 %).

Numerical Simulation of Human Thermal Comfort with Different Thermal Boundary Conditions in Buildings

How to use numerical simulation method to analyze human body heat transfer and human thermal comfort is introduced in this paper systematically. Under the same working conditions, numerical simulation of human body heat transfer has been finished based on three thermal boundary conditions, and then the results are compared. The results show that the third thermal boundary condition is better than the first and the second one, which have some problems in simulation and are not good at reflecting the fact on thermal comfort of human body. The third thermal boundary condition which is made to adapt the surrounding flow field automatically can get a more accurate result on calculating the heat transfer of different parts on human body and reflect hot or cool feeling preferably, which proves that the method put forward in this article to research the human body comfort is feasible.

The effect of cluster morphology on the turbulent flows over an armored gravel bed surface

Flume experiments are presented in which four distinct gravel and sand sediments were armored under three different flow rates. Following bed armoring, 134 clusters were identified on nine different armored beds. Clusters were separated into two categories, line and pebble, based on their morphology. Each cluster generated a recirculation cell characterized by increased turbulence that extended from just past the cluster crest in the downstream direction. The areas and orientations of the recirculation cells, together with analyses of the turbulent flow parameters and quadrant analyses, measured the influence of representative clusters on the surrounding flow field. The two basic types of cluster morphology produced very different effects on turbulent channel flows. Pebble clusters formed large recirculation cells and increased values of kinetic energy and momentum flux were measured vertically as well as longitudinally away from the cluster. Recirculation cells generated by line clusters were generally smaller and remained closer to the bed surface, restricting the impact of line clusters to flow at or below the cluster crest. The morphology of the line cluster induced turbulent processes similar to skimming flows. The difference in cluster influence over the immediate flow hydraulics was connected to the cluster morphology and orientation of the recirculation cell generated at the cluster. Line and pebble clusters formed as part of the armored bed surface structure at all the flow rates and grain size distributions tested without showing any preference. These results have potential implications for the general understanding and modeling of flows over armored gravel beds as the pebble and line clusters require morphology specific modeling.

Characteristics of the Train Wind and Analysis of Personnel Safety in Intercity Railway Station

Based on the 3D incompressibleequations and the standardturbulent model, taking numerical calculation as the main research method, the relative motion model of station-train was established, the air flow field environment that when a train (CRH2) ran in the intercity railway tunnel at a speed of 200km per hour was simulated. The changing process of the train wind on the platform when the train passed through the platform was analyzed, the distribution characteristics of the train wind on the platform were discussed, according to the train wind reference standards, the safety avoidance distance of people on the platform had been worked out. The results show that: the gradient of the boundary layer wind speed close to the train wall is very large; the train wind near the tail of the train is the most unfavorable position; in order to keep the personal safe, protective measures should be taken when necessary.

Numerical simulation on evolution of subharmonic low-speed streaks in minimal channel turbulent flow

The evolution of low-speed streaks in the turbulent boundary layer of the minimum channel flow unit at a low Reynolds number is simulated by the direct numerical simulation (DNS) based on the standard Fourier-Chebyshev spectral method. The subharmonic sinuous (SS) mode for two spanwise-aligned low-speed streaks is excited by imposing the initial perturbations. The possibilities and the physical realities of the turbulent sustaining in the minimal channel unit are examined. Based on such a flow field environment, the evolution of the low-speed streaks during a cycle of turbulent sustaining, including lift-up, oscillation, and breakdown, is investigated. The development of streamwise vortices and the dynamics of vortex structures are examined. The results show that the vortices generated from the same streak are staggered along the streamwise direction, while the vortices induced by different streaks tilt toward the normal direction due to the mutual induction effect. It is the spatial variations of the streamwise vortices that cause the lift-up of the streaks. By resolving the transport dynamics of enstrophy, the strength of the vortices is found to continuously grow in the logarithmic layer through the vortex stretching mechanism during the evolution of streaks. The enhancement of the vortices contributes to the spanwise oscillation and the following breakdown of the low-speed streaks.

Coupling CFD with detailed microkinetic modeling in heterogeneous catalysis

The atomic-scale understanding of a catalytic process is crucial for the rational development of catalytic technologies. It requires the identification of the dominant reaction mechanism, that is an intrinsic multiscale property of the system. In this respect, it is of utmost importance to obtain a fundamental understanding about the interactions of the catalyst reactivity with the surrounding flow field in the reactor. Here, we propose a new solver (catalyticFOAM), that allows for the solution of Navier–Stokes equations for complex and general geometries for reacting flows at surfaces, based on a detailed microkinetic description of the surface reactivity. The catalyticFOAM solver exploits the operator-splitting technique, based on the separation of transport and reaction terms. The proposed numerical algorithm makes possible the simulation of multidimensional systems with complex and detailed kinetic mechanisms, overcoming the unfeasible computational effort that would be required by fully-coupled algorithms. Examples concerning the H2 fuel rich combustion on Rh are presented as showcases in structured and randomly packed reactors. The proposed approach represents an essential step for the first-principles based multiscale analysis of catalytic processes and paves the way toward the rational understanding and development of new reaction/reactor concepts.

Investigation of the Physical Phenomena Associated with Rain Impacts on Supersonic and Hypersonic Flight Vehicles

The U.S. Army AMRDEC has been performing research focusing on both analytical modeling and obtaining validation data through experimentation. The focus of this research is to understand both the impact event, and the associated material damage mechanisms. A key aspect of weather encounter modeling is that the local vehicle flow field environment alters the impact boundary conditions with time. Therefore, an analytical model must be able to include not only the vehicle flow field, but also the time accurate embedded flow field surrounding the droplets as they alter shape prior to impact. The impact event is highly dependent on the actual droplet shape at impact, and therefore extensive modeling and testing was required to obtain data suitable for code validation. The empirical testing effort consisted of single droplet impacts, high-speed rocket sled testing, and testing in a ballistics range. The data presented focuses on the data collected at the ballistics range for Mach numbers ranging from 2 to 7 for various shapes. The core analytical effort was developed by extensive modifications of the Smoothed Particle Hydrodynamics C-Code (SPHC) from Stellingwerf Consulting, augmented with an improved water equation of state that included the supercooled regime to augment the fidelity of the impact physics for rain drops at high altitudes. The analytical model demonstrates that the temporal distortion of water in a post-shock environment and the impact events for various projectiles can be captured with reasonable accuracy. The next step is to obtain data within the shock layers of relevant vehicle shapes in order to obtain the detailed demise and coupled flow field physics in order to validate the model in a realistic environment. The data presented reveals the droplet shape change and demise characteristics inside the shock structures for missile domes and sphere-cone designs, along with the complex embedded shock structure and impact physics associated with these events. This data forms a subset of empirical data that can be used for the fully-coupled full-scale impact events on flight vehicles.

Optimization of Microchannel Heat Sinks Using Genetic Algorithm

In this study, a genetic algorithm is employed to minimize the entropy generation rate in microchannel heat sinks. The entropy generation rate allows the combined effects of thermal performance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. Previously developed models for the heat transfer, pressure drop and entropy generation rate are used in the optimization procedure. The results of optimization are compared with existing results obtained by the Newton–Raphson method. It is observed that the GA gives better overall performance of the microchannel heat sinks.

Experimental Investigation on Scour under a Vibrating Catenary Riser

This paper investigates the impact of a vibrating steel catenary riser on scour in a steady current, motivated by the fact that pipeline scour studies are traditionally based on a horizontal stationary pipe. Laboratory experiments were conducted using a truncated riser subjected to a vertical sinusoidal motion of varying amplitude and frequency. By including a stationary test case for comparison, the results show that the scouring profiles for a stationary and a vibrating riser are fundamentally different. Using an acoustic Doppler velocimeter profiler to measure the flow velocities, the surrounding flow field was shown to be markedly modified by the riser vibration. The velocities fluctuated considerably at the vibration frequency, thereby promoting scour. The maximum scour depth increases significantly with both vibration amplitude and frequency, but frequency has a more profound influence. An explanation for this phenomenon is proposed based on various observed and measured results.

Horizontal flow fields observed in Hinode G-band images

The flare-prolific active region NOAA 10930 offered both a developing delta-spot and a decaying satellite sunspot of opposite polarity. The objective of this study is to characterize the photometric decay of the satellite sunspot and the evolution of photospheric and chromospheric horizontal proper motions in its surroundings. We apply the local correlation tracking technique to a 16-hour time-series of Hinode G-band and CaIIH images and study the horizontal proper motions in the vicinity of the satellite sunspot on 2006 December 7. Decorrelation times were computed to measure the lifetime of solar features in intensity and flow maps. We observed shear flows in the dominant umbral cores of the satellite sunspot. These flows vanished once the penumbra had disappeared. This slow penumbral decay had an average rate of 152Mm2/day over an 11-hour period. Typical lifetimes of intensity features derived from an autocorrelation analysis are 3-5min for granulation, 25-35min for G-band bright points, and up to 200-235min for penumbrae, umbrae, and pores. Long-lived intensity features (i.e., the dominant umbral cores) are not related to long-lived flow features in the northern part of the sunspot, where flux removal, slowly decaying penumbrae, and persistent horizontal flows of up to 1 km/s contribute to the erosion of the sunspot. Finally, the restructuring of magnetic field topology was responsible for a homologous M2.0 flare, which shared many characteristics with an X6.5 flare on the previous day. Notwithstanding the prominent role of delta-spots in flaring, we conclude based on the decomposition of the satellite sunspot, the evolution of the surrounding flow fields, and the timing of the M2.0 flare that the vanishing magnetic flux in the decaying satellite sunspot played an instrumental role in triggering the homologous M2.0 flare and the eruption of a small Halpha filament.

Normalized Lift: An Energy Interpretation of the Lift Coefficient Simplifies Comparisons of the Lifting Ability of Rotating and Flapping Surfaces

For a century, researchers have used the standard lift coefficient CL to evaluate the lift, L, generated by fixed wings over an area S against dynamic pressure, ½ρv 2, where v is the effective velocity of the wing. Because the lift coefficient was developed initially for fixed wings in steady flow, its application to other lifting systems requires either simplifying assumptions or complex adjustments as is the case for flapping wings and rotating cylinders.This paper interprets the standard lift coefficient of a fixed wing slightly differently, as the work exerted by the wing on the surrounding flow field (L/ρ·S), compared against the total kinetic energy required for generating said lift, ½v2. This reinterpreted coefficient, the normalized lift, is derived from the work-energy theorem and compares the lifting capabilities of dissimilar lift systems on a similar energy footing. The normalized lift is the same as the standard lift coefficient for fixed wings, but differs for wings with more complex motions; it also accounts for such complex motions explicitly and without complex modifications or adjustments. We compare the normalized lift with the previously-reported values of lift coefficient for a rotating cylinder in Magnus effect, a bat during hovering and forward flight, and a hovering dipteran.The maximum standard lift coefficient for a fixed wing without flaps in steady flow is around 1.5, yet for a rotating cylinder it may exceed 9.0, a value that implies that a rotating cylinder generates nearly 6 times the maximum lift of a wing. The maximum normalized lift for a rotating cylinder is 1.5. We suggest that the normalized lift can be used to evaluate propellers, rotors, flapping wings of animals and micro air vehicles, and underwater thrust-generating fins in the same way the lift coefficient is currently used to evaluate fixed wings.

Real-time fluid dynamics investigation and physiological response for erythromycin fermentation scale-up from 50 L to 132 m3 fermenter

The physiological response of erythromycin fermentation scale-up from 50 L to 132 m(3) scale was investigated. A relatively high oxygen uptake rate (OUR) in early phase of fermentation was beneficial for erythromycin biosynthesis. Correspondingly, the maximal consistency coefficient (K) reflecting non-Newtonian fluid characteristics in 50 L and 132 m(3) fermenter also appeared in same phase. Fluid dynamics in different scale bioreactor was further investigated by real-time computational fluid dynamics modeling. The results of simulation showed that the impeller combination in 50 L fermenter could provide more modest flow field environment compared with that in 132 m(3) fermenter. The decrease of oxygen transfer rate (OTR) in 132 m(3) fermenter was the main cause for impairing cell physiological metabolism and erythromycin biosynthesis. These results were helpful for understanding the relationship between hydrodynamic environment and physiological response of cells in bioreactor during the scale-up of fermentation process.

Aerodynamic analysis of a Mars exploration manned capsule

The paper deals with the aerodynamic analysis of a manned braking system entering the Mars atmosphere, with the aim to support planetary entry system design studies. The capsule configuration is an axisymmetric blunt body close to the Apollo capsule. Several fully three-dimensional Computational Fluid Dynamics analyses have been performed to assess the flowfield environment around the vehicle to address the aerodynamic performance of the entry capsule within mission exploration to Mars. To this end, a wide range of flow conditions including reacting and non-reacting flow, different angles of attack, and Mach numbers have been investigated and compared. Moreover, non-equilibrium effects on the flowfield around the capsule have been also investigated. Results show that real-gas effects, for all the angles of attach considered, increase both the aerodynamic drag and pitching moment, whereas the lift is only slighted affected. Finally, comparison of the results highlights that experimental and CFD aerodynamic findings available for the Apollo capsule in air adequately represent the static coefficients of the capsule in the Mars atmosphere.

Simulation of self‐propelled anguilliform swimming by local domain‐free discretization method

SUMMARYThe local domain‐free discretization method is extended in this work to simulate fluid–structure interaction problems, the class of which is exemplified by the self‐propelled anguilliform swimming of deforming bodies in a fluid medium. Given the deformation of the fish body in its own reference frame, the translational and rotational motions of the body governed by Newton's Law are solved together with the surrounding flow field governed by Navier–Stokes equations. When the body is deforming and moving, no mesh regeneration is required in the computation. The loose coupling strategy is employed to simulate the fluid–structure interaction involved in the self‐propelled swimming. The local domain‐free discretization method and an efficient algorithm for classifying the Eulerian mesh points are described in brief. To validate the fluid–structure interaction solver, we simulate the ‘lock‐in’ phenomena associated with the vortex‐induced vibrations of an elastically mounted cylinder. Finally, we demonstrate applications of the method to two‐dimensional and three‐dimensional anguilliform‐swimming fish. The kinematics and dynamics associated with the center of mass are shown and the rotational movement is also presented via the angular position of the body axis. The wake structure is visualized in terms of vorticity contours. All the obtained numerical results show good agreement with available data in the literature. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Numerical Study on Heat Reduction of Various Counterflowing Jets over Highly Blunt Cone in Hypersonic Flow

In this paper, the effectiveness of counterflowing jets as heat-reduction devices for large-angle blunt cones flying at hypersonic Mach numbers is numerically simulated with various coolant jets. Different jet conditions have been chosen to investigate the effect of the counterflow jet on the surrounding flow field of nose cone. The compressible, unsteady, axisymmetric Navier-Stokes equations are solved with SST turbulence model for free stream Mach number of 5.75 at 0° angle of attack with and without gas injection. The coolant gas (air, Carbon Dioxide, and helium) is chosen to inject into the hypersonic flow at the nose of the model. The numerical results presented the surface heat reduction for different coolant jets. According to the investigation of various conditions of opposing jets, important phenomena of flow field and some effective jet conditions are found.

Foundation-based flow acceleration structures for marine current energy converters

This study presents a preliminary experimental study investigating the potential benefits of foundation-based flow acceleration structures for marine current energy converters (MCECs). Such structures would provide multiple benefits, including; increased device power output, increased foundation footprint and scour protection. Two phases of experimental testing have been conducted. The first series of tests investigated the flow acceleration caused by a ramp foundation without the presence of a MCEC. They were scaled from a shallow tidal flow site and provide evidence that these structures could give power benefits of 12–25% depending on ramp size and flow depth. An optimum ramp size was established based on the suitability of the vertical velocity profiles for energy extraction. The second phase of testing modelled a commercial marine current energy convertor with a gravity-based ramp foundation. The device was modelled using actuator plates to represent the interaction between the device and the surrounding flow field. A proposed single-operation installation method using a concrete ramp foundation is proposed and preliminary stability calculations are presented.

SPRAY CHARACTERISTICS OF A PRESSURE-SWIRL FUEL INJECTOR SUBJECTED TO A CROSSFLOW AND A COFLOW

An analysis of fuel spray was conducted using flow visualization and phase Doppler particle anemometry (PDPA) in an experimental facility housed at the Propulsion Directorate of the US Air Force Research Laboratory (AFRL). In order to model typical real-world aircraft engine conditions, this test facility was equipped with a pressure-swirl injector in combination with heated crossflow air normal to the fuel (JP-8) injection and, optionally, with coflow air parallel to the primary direction of fuel injection. Results are presented for fixed fuel rate (Reynolds number for the liquid flow of 9200 and a liquid jet Weber number of 7100) and varied crossflow velocity for four momentum flux ratios. Flow visualization and coherent structure velocimetry clearly demonstrated how the crossflow reoriented the conical spray pattern and led to large spatial variation in the measured Sauter mean diameter. At locations downstream of the injector, the PDPA data demonstrated that the vertical component of velocity was relatively low for small droplets and high for large drop sizes. This bimodal behavior may be rationalized by considering the initial propagation direction of the spray, the residence time of the fuel droplets, and the shear of the surrounding flow field. The addition of a coflow stream of air to the system led to significant reductions in Sauter mean diameter of the droplets in the region of the spray cone downstream of the fuel injector. The coflow also tended to disrupt the bimodality of the drop size distribution.

Fluid-particle interaction simulations of the interception of red blood cells in shear flow

Numerical simulations of the motion of red blood cells (RBCs) freely suspended in shear flow have been successfully performed to investigate the nature of pairwise interception of RBCs using a fluid-particle interaction method based on the arbitrary Lagrangian-Eulerian (ALE) method and a dynamic mesh method. The applicability of the interaction method that we used was validated by comparing the simulation results with an analytical solution for an elliptical particle in shear flow. We found that positive and negative transverse shifts of the RBCs take place during the interceptions, yielding a non-zero RBC self-diffusivity, and that a phase shift occurs during the rotating behavior and lasts even after the separation. The behaviors of the approaching RBCs are adjusted by interactions with the surrounding flow fields during interception. The pressure between a pair of RBCs causes either an attractive or repulsive force. The nature of the pairwise interception is influenced not only by the flow fields but also by kinematic characteristics (i.e., instantaneous translational and rotational behavior) of the two RBCs.

A versatile implicit iterative approach for fully resolved simulation of self-propulsion

We present a computational approach for fully resolved simulation of self-propulsion of organisms through a fluid. A new implicit iterative algorithm is developed that solves for the swimming velocities of the organism with prescribed deformation kinematics. A solution for the surrounding flow field is also obtained. This approach uses a constraint-based formulation of the problem of self-propulsion developed by Shirgaonkar et al. [1]. The approach in this paper is unlike the previous work [1] where a fractional time stepping scheme was used. Fractional time stepping schemes, while efficient for moderate to high Reynolds number problems, are not suitable for zero or low Reynolds number problems where the inertia term in the governing equation is absent or negligible. In such cases the implicit iterative algorithm presented here is more appropriate. We validate the method by simulating self-propulsion of bacterial flagellum, jellyfish ( Aurelia aurita), and larval zebrafish ( Danio rerio). Comparison of the computational results with theoretical and experimental results for the test cases is found to be very good.

Intravascular flow fields shape intussusceptive pillars in the chick chorioallantoic membrane

Intussusceptive (nonsprouting) angiogenesis is an essential process in the development of the chick chorioallantoic membrane (CAM). The influence of intravascular flow fields on intussusceptive pillar formation in the CAM, however, remains largely unknown. Using fluorescent vascular tracers and digital time‐series video reconstructions, intussusceptive pillars and the surrounding flow fields were identified during the development days 12–16. The mechanical forces within the CAM vessels, including wall shear stress, velocity, and blood pressure, were mapped using 3D finite element models and computational flow simulations. The pillars demonstrated a quasi‐circular circumference (shape factor 0.41+0.13) and comprised 1.7+1.2% of the bifurcation cross‐sectional area within 2nd and 3rd order vessels. The presence of pillars was independent of vessel diameter (R=0.32) or bifurcation angle (R=0.48) (p&gt;0.4), but blood flow surrounding the pillars demonstrated a narrow range of velocities. More importantly, the location and geometry of the pillar correlated closely with the streamlines of the dominant vessel flow (p=0.02). We conclude that these spatial dynamics reflect a responsiveness of blood vessels to intravascular flow fields and indicate the importance of mechanical forces in shaping both intussusceptive angiogenesis and vessel structure in the microcirculation.Grant Funding Source: NIH Grant HL47078, HL75426 and HL94567